METHODS AND COMPOSITIONS FOR EDITING NUCLEOTIDE SEQUENCES
The present disclosure provides compositions and methods for conducting prime editing of a target DNA molecule (e.g., a genome) that enables the incorporation of a nucleotide change and/or targeted mutagenesis. The nucleotide change can include a single-nucleotide change (e.g., any transition or any transversion), an insertion of one or more nucleotides, or a deletion of one or more nucleotides. More in particular, the disclosure provides fusion proteins comprising nucleic acid programmable DNA binding proteins (napDNAbp) and a polymerase (e.g., reverse transcriptase), which is guided to a specific DNA sequence by a modified guide RNA, named an PEgRNA. The PEgRNA has been altered (relative to a standard guide RNA) to comprise an extended portion that provides a DNA synthesis template sequence which encodes a single strand DNA flap, which is homologous to a strand of the targeted endogenous DNA sequence to be edited, but which contains the desired one or more nucleotide changes and which, following synthesis by the polymerase (e.g., reverse transcriptase), becomes incorporated into the target DNA molecule. Also disclosed herein are various methods that leverage prime editing, including treating trinucleotide repeat contraction diseases, installing targeted peptide tags, treating prion disease through the installation of protection mutations, manipulating RNA-encoding genes for the installation of RNA tags for controlling the function and expression of RNA, using prime editing to construct sophisticated gene libraries, using prime editing to insert immunoepitopes into proteins, use of prime editing to insert inducible dimerization domains into protein targets, and delivery methods, among others.
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This U.S. Provisional application refers to and incorporates by reference the following applications, namely, U.S. Provisional Application No. 62/820,813, filed Mar. 19, 2019 (Attorney Docket No. B1195.70074US00), U.S. Provisional Application No. 62/858,958 (Attorney Docket No. B1195.70074US01), filed Jun. 7, 2019, U.S. Provisional Application No. 62/889,996 (Attorney Docket No. B1195.70074US02), filed Aug. 21, 2019, U.S. Provisional Application No. 62/922,654, filed Aug. 21, 2019 (Attorney Docket No. B1195.70083US00), U.S. Provisional Application No. 62/913,553 (Attorney Docket No. B1195.70074US03), filed Oct. 10, 2019, U.S. Provisional Application No. 62/973,558 (Attorney Docket No. B1195.70083US01), filed Oct. 10, 2019, U.S. Provisional Application No. 62/931,195 (Attorney Docket No. B1195.70074US04), filed Nov. 5, 2019, U.S. Provisional Application No. 62/944,231 (Attorney Docket No. B1195.70074US05), filed Dec. 5, 2019, U.S. Provisional Application No. 62/974,537 (Attorney Docket No. B1195.70083US02), filed Dec. 5, 2019, U.S. Provisional Application No. 62/991,069 (Attorney Docket No. B1195.70074US06), filed Mar. 17, 2020, and U.S. Provisional application No. (serial number not available as of this filing) (Attorney Docket No. B1195.70083US03), filed Mar. 17, 2020.
GOVERNMENT SUPPORTThis invention was made with government support under grant numbers U01AI142756, RM1HG009490, R01EB022376, and R35GM118062 awarded by the National Institutes of Health. The government has certain rights in the invention.
BACKGROUND OF THE INVENTIONPathogenic single nucleotide mutations contribute to approximately 50% of human diseases for which there is a genetic component,7 according to some estimates. Unfortunately, treatment options for patients with these genetic disorders remain extremely limited, despite decades of gene therapy exploration8. Perhaps the most parsimonious solution to this therapeutic challenge is direct correction of single nucleotide mutations in patient genomes, which would address the root cause of disease and would likely provide lasting benefit. Although such a strategy was previously unthinkable, recent improvements in genome editing capabilities brought about by the advent of the CRISPR/Cas system9 have now brought this therapeutic approach within reach. By straightforward design of a guide RNA (gRNA) sequence that contains ˜20 nucleotides complementary to the target DNA sequence, nearly any conceivable genomic site can be specifically accessed by CRISPR associated (Cas) nucleases1,2. To date, several monomeric bacterial Cas nuclease systems have been identified and adapted for genome editing applications10. This natural diversity of Cas nucleases, along with a growing collection of engineered variants11-14, offers fertile ground for developing new genome editing technologies.
While gene disruption with CRISPR is now a mature technique, precision editing of single base pairs in the human genome remains a major challenge3. Homology directed repair (HDR) has long been used in human cells and other organisms to insert, correct, or exchange DNA sequences at sites of double strand breaks (DSBs) using donor DNA repair templates that encode the desired edits15. However, traditional HDR has very low efficiency in most human cell types, particularly in non-dividing cells, and competing non-homologous end joining (NHEJ) leads predominantly to insertion-deletion (indel) byproducts16. Other issues relate to the generation of DSBs, which can give rise to large chromosomal rearrangements and deletions at target loci17, or activate the p53 axis leading to growth arrest and apoptosis18,19.
Several approaches have been explored to address these drawbacks of HDR. For example, repair of single-stranded DNA breaks (nicks) with oligonucleotide donors has been shown to reduce indel formation, but yields of desired repair products remain low20. Other strategies attempt to bias repair toward HDR over NHEJ using small molecule and biologic reagents21-23. However, the effectiveness of these methods is likely cell-type dependent, and perturbation of the normal cell state could lead to undesirable and unforeseeable effects.
Recently, the inventors, led by Prof. David Liu et al., developed base editing as a technology that edits target nucleotides without creating DSBs or relying on HDR4-6,24-27. Direct modification of DNA bases by Cas-fused deaminase enzymes allows for C●G to T●A, or A●T to G●C, base pair conversions in a short target window (˜5-7 bases) with very high efficiency. As a result, base editors have been rapidly adopted by the scientific community. However, the following factors limit their generality for precision genome editing: (1) “bystander editing” of non-target C or A bases within the target window are observed; (2) target nucleotide product mixtures are observed; (3) target bases must be located 15±2 nucleotides upstream of a PAM sequence; and (5) repair of small insertion and deletion mutations is not possible.
Therefore, the development of programmable editors that are flexibly capable of introducing any desired single nucleotide change and/or which could install base pair insertions or deletions (e.g., at least 1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 30, 40, 50, 60, 70, 80, 90, 100, or more base pair insertions or deletions) and/or which could alter or modify the nucleotide sequence at a target site with high specificity and efficiency would substantially expand the scope and therapeutic potential of genome editing technologies based on CRISPR.
SUMMARY OF THE INVENTIONThe present invention describes an entirely new platform for genome editing called “prime editing.” Prime editing is a versatile and precise genome editing method that directly writes new genetic information into a specified DNA site using a nucleic acid programmable DNA binding protein (“napDNAbp”) working in association with a polymerase (i.e., in the form of a fusion protein or otherwise provided in trans with the napDNAbp), wherein the prime editing system is programmed with a prime editing (PE) guide RNA (“PEgRNA”) that both specifies the target site and templates the synthesis of the desired edit in the form of a replacement DNA strand by way of an extension (either DNA or RNA) engineered onto a guide RNA (e.g., at the 5′ or 3′ end, or at an internal portion of a guide RNA). The replacement strand containing the desired edit (e.g., a single nucleobase substitution) shares the same sequence as the endogenous strand of the target site to be edited (with the exception that it includes the desired edit). Through DNA repair and/or replication machinery, the endogenous strand of the target site is replaced by the newly synthesized replacement strand containing the desired edit. In some cases, prime editing may be thought of as a “search-and-replace” genome editing technology since the prime editors, as described herein, not only search and locate the desired target site to be edited, but at the same time, encode a replacement strand containing a desired edit which is installed in place of the corresponding target site endogenous DNA strand.
The prime editors of the present disclosure relate, in part, to the discovery that the mechanism of target-primed reverse transcription (TPRT) or “prime editing” can be leveraged or adapted for conducting precision CRISPR/Cas-based genome editing with high efficiency and genetic flexibility (e.g., as depicted in various embodiments of
The newly synthesized strand (i.e., the replacement DNA strand containing the desired edit) that is formed by the herein disclosed prime editors would be homologous to the genomic target sequence (i.e., have the same sequence as) except for the inclusion of a desired nucleotide change (e.g., a single nucleotide change, a deletion, or an insertion, or a combination thereof). The newly synthesized (or replacement) strand of DNA may also be referred to as a single strand DNA flap, which would compete for hybridization with the complementary homologous endogenous DNA strand, thereby displacing the corresponding endogenous strand. In certain embodiments, the system can be combined with the use of an error-prone reverse transcriptase enzyme (e.g., provided as a fusion protein with the Cas9 domain, or provided in trans to the Cas9 domain). The error-prone reverse transcriptase enzyme can introduce alterations during synthesis of the single strand DNA flap. Thus, in certain embodiments, error-prone reverse transcriptase can be utilized to introduce nucleotide changes to the target DNA. Depending on the error-prone reverse transcriptase that is used with the system, the changes can be random or non-random.
Resolution of the hybridized intermediate (comprising the single strand DNA flap synthesized by the reverse transcriptase hybridized to the endogenous DNA strand) can include removal of the resulting displaced flap of endogenous DNA (e.g., with a 5′ end DNA flap endonuclease, FEN1), ligation of the synthesized single strand DNA flap to the target DNA, and assimilation of the desired nucleotide change as a result of cellular DNA repair and/or replication processes. Because templated DNA synthesis offers single nucleotide precision for the modification of any nucleotide, including insertions and deletions, the scope of this approach is very broad and could foreseeably be used for myriad applications in basic science and therapeutics.
In one aspect, the specification provides a fusion protein comprising a nucleic acid programmable DNA binding protein (napDNAbp) and a reverse transcriptase. In various embodiments, the fusion protein is capable of carrying out genome editing by target-primed reverse transcription in the presence of an extended guide RNA.
In certain embodiments, the napDNAbp has a nickase activity. The napDNAbp may also be a Cas9 protein or functional equivalent thereof, such as a nuclease active Cas9, a nuclease inactive Cas9 (dCas9), or a Cas9 nickase (nCas9).
In certain embodiments, the napDNAbp is selected from the group consisting of: Cas9, Cas12e, Cas12d, Cas12a, Cas12b1, Cas13a, Cas12c, and Argonaute and optionally has a nickase activity.
In other embodiments, the fusion protein when complexed with an extended guide RNA is capable of binding to a target DNA sequence.
In still other embodiments, the target DNA sequence comprises a target strand and a complementary non-target strand.
In other embodiments, the binding of the fusion protein complexed to the extended guide RNA forms an R-loop. The R-loop can comprise (i) an RNA-DNA hybrid comprising the extended guide RNA and the target strand, and (ii) the complementary non-target strand.
In still other embodiments, the complementary non-target strand is nicked to form a reverse transcriptase priming sequence having a free 3′ end.
In various embodiments, the extended guide RNA comprises (a) a guide RNA and (b) an RNA extension at the 5′ or the 3′ end of the guide RNA, or at an intramolecular location in the guide RNA. The RNA extension can comprise (i) a reverse transcription template sequence comprising a desired nucleotide change, (ii) a reverse transcription primer binding site, and (iii) optionally, a linker sequence. In various embodiments, the reverse transcription template sequence may encode a single-strand DNA flap that is complementary to an endogenous DNA sequence adjacent to the nick site, wherein the single-strand DNA flap comprises the desired nucleotide change.
In various embodiments, the RNA extension is at least 5 nucleotides, at least 6 nucleotides, at least 7 nucleotides, at least 8 nucleotides, at least 9 nucleotides, at least 10 nucleotides, at least 11 nucleotides, at least 12 nucleotides, at least 13 nucleotides, at least 14 nucleotides, at least 15 nucleotides, at least 16 nucleotides, at least 17 nucleotides, at least 18 nucleotides, at least 19 nucleotides, at least 20 nucleotides, at least 21 nucleotides, at least 22 nucleotides, at least 23 nucleotides, at least 24 nucleotides, or at least 25 nucleotides in length.
In still other embodiments, the single-strand DNA flap may hybridize to the endogenous DNA sequence adjacent to the nick site, thereby installing the desired nucleotide change. In still other embodiments, the single-stranded DNA flap displaces the endogenous DNA sequence adjacent to the nick site and which has a free 5′ end. In certain embodiments, the displaced endogenous DNA having the 5′ end is excised by the cell.
In various embodiments, the cellular repair of the single-strand DNA flap results in installation of the desired nucleotide change, thereby forming a desired product.
In various other embodiments, the desired nucleotide change is installed in an editing window that is between about −4 to +10 of the PAM sequence.
In still other embodiments, the desired nucleotide change is installed in an editing window that is between about −5 to +5 of the nick site, or between about −10 to +10 of the nick site, or between about −20 to +20 of the nick site, or between about −30 to +30 of the nick site, or between about −40 to +40 of the nick site, or between about −50 to +50 of the nick site, or between about −60 to +60 of the nick site, or between about −70 to +70 of the nick site, or between about −80 to +80 of the nick site, or between about −90 to +90 of the nick site, or between about −100 to +100 of the nick site, or between about −200 to +200 of the nick site.
In various embodiments, the napDNAbp comprises an amino acid sequence of SEQ ID NO: 18. In various other embodiments, the napDNAbp comprises an amino acid sequence that is at least 80%, 85%, 90%, 95%, 98%, or 99% identical to the amino acid sequence of any one of SEQ ID NOs: 26-39, 42-61, 75-76, 126, 130, 137, 141, 147, 153, 157, 445, 460, 467, and 482-487 (Cas9); (SpCas9); SEQ ID NO: 77-86 (CP-Cas9); SEQ ID NO: 18-25 and 87-88 (SpCas9); and SEQ ID NOs: 62-72 (Cas12)
In other embodiments, the reverse transcriptase of the disclosed fusion proteins and/or compositions may comprise any one of the amino acid sequences of SEQ ID NOs: 89-100, 105-122, 128-129, 132, 139, 143, 149, 154, 159, 235, 454, 471, 516, 662, 700-716, 739-742, and 766. In still other embodiments, the reverse transcriptase may comprise an amino acid sequence that is at least 80%, 85%, 90%, 95%, 98%, or 99% identical to the amino acid sequence of any one of SEQ ID NOs: 89-100, 105-122, 128-129, 132, 139, 143, 149, 154, 159, 235, 454, 471, 516, 662, 700-716, 739-742, and 766. These sequences may be naturally occurring reverse transcriptase sequences, e.g., from a retrovirus or a retrotransposon, of the sequences may be recombinant.
In various other embodiments, the fusion proteins herein disclosed may comprise various structural configurations. For example, the fusion proteins may comprise the structure NH2-[napDNAbp]-[reverse transcriptase]-COOH; or NH2-[reverse transcriptase]-[napDNAbp]-COOH, wherein each instance of “]-[” indicates the presence of an optional linker sequence.
In various embodiments, the linker sequence comprises an amino acid sequence of SEQ ID NOs: 127, 165-176, 446, 453, and 767-769, or an amino acid sequence that this at least 80%, 85%, or 90%, or 95%, or 99% identical to any one of the linker amino acid sequence of SEQ ID NOs: 127, 165-176, 446, 453, and 767-769.
In various embodiments, the desired nucleotide change that is incorporated into the target DNA can be a single nucleotide change (e.g., a transition or transversion), an insertion of one or more nucleotides, or a deletion of one or more nucleotides.
In certain cases, the insertion is at least 1, at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, at least 10, at least 11, at least 12, at least 13, at least 14, at least 15, at least 16, at least 17, at least 18, at least 19, at least 20, at least 30, at least 40, at least 50, at least 60, at least 70, at least 80, at least 90, at least 100, at least 200, at least 300, at least 400, or at least 500 nucleotides in length.
In certain other cases, the deletion is at least 1, at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, at least 10, at least 11, at least 12, at least 13, at least 14, at least 15, at least 16, at least 17, at least 18, at least 19, at least 20, at least 30, at least 40, at least 50, at least 60, at least 70, at least 80, at least 90, at least 100, at least 200, at least 300, at least 400, or at least 500 nucleotides in length.
In another aspect, the present disclosure provides an extended guide RNA comprising a guide RNA and at least one RNA extension. The RNA extension can be positioned at the 3′ end of the guide RNA. In other embodiments, the RNA extension can be positioned at the 5′ of the guide RNA. In still other embodiments, the RNA extension can be positioned at an intramolecular position within the guide RNA, however, preferable, the intramolecular positioning of the extended portion does not disrupt the functioning of the protospacer.
In various embodiments, the extended guide RNA is capable of binding to a napDNAbp and directing the napDNAbp to a target DNA sequence. The target DNA sequence can comprise a target strand and a complementary non-target strand, wherein the guide RNA hybridizes to the target strand to form an RNA-DNA hybrid and an R-loop.
In various embodiments of the extended guide RNA, the at least one RNA extension can comprise a reverse transcription template sequence. In various other embodiment, the RNA extension may further comprises a reverse transcription primer binding site. In still further embodiments, the RNA extension may comprise a linker or spacer sequence that joins the RNA extension to the guide RNA.
In various embodiments, the RNA extension can be at least 5 nucleotides, at least 6 nucleotides, at least 7 nucleotides, at least 8 nucleotides, at least 9 nucleotides, at least 10 nucleotides, at least 11 nucleotides, at least 12 nucleotides, at least 13 nucleotides, at least 14 nucleotides, at least 15 nucleotides, at least 16 nucleotides, at least 17 nucleotides, at least 18 nucleotides, at least 19 nucleotides, at least 20 nucleotides, at least 21 nucleotides, at least 22 nucleotides, at least 23 nucleotides, at least 24 nucleotides, at least 25 nucleotides, at least 30 nucleotides, at least 40 nucleotides, at least 50 nucleotides, at least 60 nucleotides, at least 70 nucleotides, at least 80 nucleotides, at least 90 nucleotides, at least 100 nucleotides, at least 150 nucleotides, at least 200 nucleotides, at least 300 nucleotides, at least 400 nucleotides, or at least 500 nucleotides in length.
In other embodiments, the reverse transcription template sequence is at least 3 nucleotides, at least 4 nucleotides, at least 5 nucleotides, at least 6 nucleotides, at least 7 nucleotides, at least 8 nucleotides, at least 9 nucleotides, at least 10 nucleotides, at least 11 nucleotides, at least 12 nucleotides, at least 13 nucleotides, at least 14 nucleotides, at least 15 nucleotides, at least 16 nucleotides, at least 17 nucleotides, at least 18 nucleotides, at least 19 nucleotides, at least 20 nucleotides, at least 30 nucleotides, at least 40 nucleotides, at least 50 nucleotides, at least 60 nucleotides, at least 70 nucleotides, at least 80 nucleotides, at least 90 nucleotides, at least 100 nucleotides, at least 200 nucleotides, at least 300 nucleotides, at least 400 nucleotides, or at least 500 nucleotides in length.
In still other embodiments, wherein the reverse transcription primer binding site sequence is at least 3 nucleotides, at least 4 nucleotides, at least 5 nucleotides, at least 6 nucleotides, at least 7 nucleotides, at least 8 nucleotides, at least 9 nucleotides, at least 10 nucleotides, at least 11 nucleotides, at least 12 nucleotides, at least 13 nucleotides, at least 14 nucleotides, at least 15 nucleotides, at least 16 nucleotides, at least 17 nucleotides, at least 18 nucleotides, at least 19 nucleotides, at least 20 nucleotides, at least 30 nucleotides, at least 40 nucleotides, at least 50 nucleotides, at least 60 nucleotides, at least 70 nucleotides, at least 80 nucleotides, at least 90 nucleotides, at least 100 nucleotides, at least 200 nucleotides, at least 300 nucleotides, at least 400 nucleotides, or at least 500 nucleotides in length.
In other embodiments, the optional linker or spacer sequence is at least 3 nucleotides, at least 4 nucleotides, at least 5 nucleotides, at least 6 nucleotides, at least 7 nucleotides, at least 8 nucleotides, at least 9 nucleotides, at least 10 nucleotides, at least 11 nucleotides, at least 12 nucleotides, at least 13 nucleotides, at least 14 nucleotides, at least 15 nucleotides, at least 16 nucleotides, at least 17 nucleotides, at least 18 nucleotides, at least 19 nucleotides, at least 20 nucleotides, at least 30 nucleotides, at least 40 nucleotides, at least 50 nucleotides, at least 60 nucleotides, at least 70 nucleotides, at least 80 nucleotides, at least 90 nucleotides, at least 100 nucleotides, at least 200 nucleotides, at least 300 nucleotides, at least 400 nucleotides, or at least 500 nucleotides in length.
In various embodiments of the extended guide RNAs, the reverse transcription template sequence may encode a single-strand DNA flap that is complementary to an endogenous DNA sequence adjacent to a nick site, wherein the single-strand DNA flap comprises a desired nucleotide change. The single-stranded DNA flap may displace an endogenous single-strand DNA at the nick site. The displaced endogenous single-strand DNA at the nick site can have a 5′ end and form an endogenous flap, which can be excised by the cell. In various embodiments, excision of the 5′ end endogenous flap can help drive product formation since removing the 5′ end endogenous flap encourages hybridization of the single-strand 3′ DNA flap to the corresponding complementary DNA strand, and the incorporation or assimilation of the desired nucleotide change carried by the single-strand 3′ DNA flap into the target DNA.
In various embodiments of the extended guide RNAs, the cellular repair of the single-strand DNA flap results in installation of the desired nucleotide change, thereby forming a desired product.
In certain embodiments, the PEgRNA comprises the nucleotide sequence of SEQ ID NOs: 131, 222, 394, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442, 641, 642, 643, 644, 645, 646, 647, 648, 649, 678, 679, 680, 681, 682, 683, 684, 685, 686, 687, 688, 689, 690, 691, 692, 738, 2997, 2998, 2999, 3000, 3001, 3002, 3003, 3004, 3005, 3006, 3007, 3008, 3009, 3010, 3011, 3012, 3013, 3014, 3015, 3016, 3017, 3018, 3019, 3020, 3021, 3022, 3023, 3024, 3025, 3026, 3027, 3028, 3029, 3030, 3031, 3032, 3033, 3034, 3035, 3036, 3037, 3038, 3039, 3040, 3041, 3042, 3043, 3044, 3045, 3046, 3047, 3048, 3049, 3050, 3051, 3052, 3053, 3054, 3055, 3056, 3057, 3058, 3059, 3060, 3061, 3062, 3063, 3064, 3065, 3066, 3067, 3068, 3069, 3070, 3071, 3072, 3073, 3074, 3075, 3076, 3077, 3078, 3079, 3080, 3081, 3082, 3083, 3084, 3085, 3086, 3087, 3088, 3089, 3090, 3091, 3092, 3093, 3094, 3095, 3096, 3097, 3098, 3099, 3100, 3101, 3102, 3103, 3113, 3114, 3115, 3116, 3117, 3118, 3119, 3120, 3121, 3305, 3306, 3307, 3308, 3309, 3310, 3311, 3312, 3313, 3314, 3315, 3316, 3317, 3318, 3319, 3320, 3321, 3322, 3323, 3324, 3325, 3326, 3327, 3328, 3329, 3330, 3331, 3332, 3333, 3334, 3335, 3336, 3337, 3338, 3339, 3340, 3341, 3342, 3343, 3344, 3345, 3346, 3347, 3348, 3349, 3350, 3351, 3352, 3353, 3354, 3355, 3356, 3357, 3358, 3359, 3360, 3361, 3362, 3363, 3364, 3365, 3366, 3367, 3368, 3369, 3370, 3371, 3372, 3373, 3374, 3375, 3376, 3377, 3378, 3379, 3380, 3381, 3382, 3383, 3384, 3385, 3386, 3387, 3388, 3389, 3390, 3391, 3392, 3393, 3394, 3395, 3396, 3397, 3398, 3399, 3400, 3401, 3402, 3403, 3404, 3405, 3406, 3407, 3408, 3409, 3410, 3411, 3412, 3413, 3414, 3415, 3416, 3417, 3418, 3419, 3420, 3421, 3422, 3423, 3424, 3425, 3426, 3427, 3428, 3429, 3430, 3431, 3432, 3433, 3434, 3435, 3436, 3437, 3438, 3439, 3440, 3441, 3442, 3443, 3444, 3445, 3446, 3447, 3448, 3449, 3450, 3451, 3452, 3453, 3454, 3455, 3479, 3480, 3481, 3482, 3483, 3484, 3485, 3486, 3487, 3488, 3489, 3490, 3491, 3492, 3493, 3522, 3523, 3524, 3525, 3526, 3527, 3528, 3529, 3530, 3531, 3532, 3533, 3534, 3535, 3536, 3537, 3538, 3539, 3540, 3549, 3550, 3551, 3552, 3553, 3554, 3555, 3556, 3628, 3629, 3630, 3631, 3632, 3633, 3634, 3635, 3636, 3637, 3638, 3639, 3640, 3641, 3642, 3643, 3644, 3645, 3646, 3647, 3648, 3649, 3650, 3651, 3652, 3653, 3654, 3655, 3656, 3657, 3658, 3659, 3660, 3661, 3662, 3663, 3664, 3665, 3666, 3667, 3668, 3669, 3670, 3671, 3672, 3673, 3674, 3675, 3676, 3677, 3678, 3679, 3680, 3681, 3682, 3683, 3684, 3685, 3686, 3687, 3688, 3689, 3690, 3691, 3692, 3693, 3694, 3695, 3696, 3697, 3698, 3755, 3756, 3757, 3758, 3759, 3760, 3761, 3762, 3763, 3764, 3765, 3766, 3767, 3768, 3769, 3770, 3771, 3772, 3773, 3774, 3775, 3776, 3777, 3778, 3779, 3780, 3781, 3782, 3783, 3784, 3785, 3786, 3787, 3788, 3789, 3790, 3791, 3792, 3793, 3794, 3795, 3796, 3797, 3798, 3799, 3800, 3801, 3802, 3803, 3804, 3805, 3806, 3807, 3808, 3809, and 3810, or a nucleotide sequence having at least 85%, or at least 90%, or at least 95%, or at least 98%, or at least 99% sequence identity with any one of SEQ ID NOs: 131, 222, 394, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442, 641, 642, 643, 644, 645, 646, 647, 648, 649, 678, 679, 680, 681, 682, 683, 684, 685, 686, 687, 688, 689, 690, 691, 692, 738, 2997, 2998, 2999, 3000, 3001, 3002, 3003, 3004, 3005, 3006, 3007, 3008, 3009, 3010, 3011, 3012, 3013, 3014, 3015, 3016, 3017, 3018, 3019, 3020, 3021, 3022, 3023, 3024, 3025, 3026, 3027, 3028, 3029, 3030, 3031, 3032, 3033, 3034, 3035, 3036, 3037, 3038, 3039, 3040, 3041, 3042, 3043, 3044, 3045, 3046, 3047, 3048, 3049, 3050, 3051, 3052, 3053, 3054, 3055, 3056, 3057, 3058, 3059, 3060, 3061, 3062, 3063, 3064, 3065, 3066, 3067, 3068, 3069, 3070, 3071, 3072, 3073, 3074, 3075, 3076, 3077, 3078, 3079, 3080, 3081, 3082, 3083, 3084, 3085, 3086, 3087, 3088, 3089, 3090, 3091, 3092, 3093, 3094, 3095, 3096, 3097, 3098, 3099, 3100, 3101, 3102, 3103, 3113, 3114, 3115, 3116, 3117, 3118, 3119, 3120, 3121, 3305, 3306, 3307, 3308, 3309, 3310, 3311, 3312, 3313, 3314, 3315, 3316, 3317, 3318, 3319, 3320, 3321, 3322, 3323, 3324, 3325, 3326, 3327, 3328, 3329, 3330, 3331, 3332, 3333, 3334, 3335, 3336, 3337, 3338, 3339, 3340, 3341, 3342, 3343, 3344, 3345, 3346, 3347, 3348, 3349, 3350, 3351, 3352, 3353, 3354, 3355, 3356, 3357, 3358, 3359, 3360, 3361, 3362, 3363, 3364, 3365, 3366, 3367, 3368, 3369, 3370, 3371, 3372, 3373, 3374, 3375, 3376, 3377, 3378, 3379, 3380, 3381, 3382, 3383, 3384, 3385, 3386, 3387, 3388, 3389, 3390, 3391, 3392, 3393, 3394, 3395, 3396, 3397, 3398, 3399, 3400, 3401, 3402, 3403, 3404, 3405, 3406, 3407, 3408, 3409, 3410, 3411, 3412, 3413, 3414, 3415, 3416, 3417, 3418, 3419, 3420, 3421, 3422, 3423, 3424, 3425, 3426, 3427, 3428, 3429, 3430, 3431, 3432, 3433, 3434, 3435, 3436, 3437, 3438, 3439, 3440, 3441, 3442, 3443, 3444, 3445, 3446, 3447, 3448, 3449, 3450, 3451, 3452, 3453, 3454, 3455, 3479, 3480, 3481, 3482, 3483, 3484, 3485, 3486, 3487, 3488, 3489, 3490, 3491, 3492, 3493, 3522, 3523, 3524, 3525, 3526, 3527, 3528, 3529, 3530, 3531, 3532, 3533, 3534, 3535, 3536, 3537, 3538, 3539, 3540, 3549, 3550, 3551, 3552, 3553, 3554, 3555, 3556, 3628, 3629, 3630, 3631, 3632, 3633, 3634, 3635, 3636, 3637, 3638, 3639, 3640, 3641, 3642, 3643, 3644, 3645, 3646, 3647, 3648, 3649, 3650, 3651, 3652, 3653, 3654, 3655, 3656, 3657, 3658, 3659, 3660, 3661, 3662, 3663, 3664, 3665, 3666, 3667, 3668, 3669, 3670, 3671, 3672, 3673, 3674, 3675, 3676, 3677, 3678, 3679, 3680, 3681, 3682, 3683, 3684, 3685, 3686, 3687, 3688, 3689, 3690, 3691, 3692, 3693, 3694, 3695, 3696, 3697, 3698, 3755, 3756, 3757, 3758, 3759, 3760, 3761, 3762, 3763, 3764, 3765, 3766, 3767, 3768, 3769, 3770, 3771, 3772, 3773, 3774, 3775, 3776, 3777, 3778, 3779, 3780, 3781, 3782, 3783, 3784, 3785, 3786, 3787, 3788, 3789, 3790, 3791, 3792, 3793, 3794, 3795, 3796, 3797, 3798, 3799, 3800, 3801, 3802, 3803, 3804, 3805, 3806, 3807, 3808, 3809, and 3810.
In yet another aspect of the invention, the specification provides for complexes comprising a fusion protein described herein and any extended guide RNA described above.
In still other aspects of the invention, the specification provides a complex comprising a napDNAbp and an extended guide RNA. The napDNAbp can be a Cas9 nickase, or can be an amino acid sequence of SEQ ID NOs: 42-57 (Cas9 nickase) and 65 (AsCas12a nickase), or an amino acid sequence that is at least 80%, 85%, 90%, 95%, 98%, or 99% identical to the amino acid sequence of any one of SEQ ID NOs: 42-57 (Cas9 nickase) and 65 (AsCas12a nickase).
In various embodiments involving a complex, the extended guide RNA is capable of directing the napDNAbp to a target DNA sequence. In various embodiments, a reverse transcriptase may be provided in trans, i.e., provided from a different source than the complex itself. For example, a reverse transcriptase could be provided to the same cell having the complex by introducing a separate vector separately encoding the reverse transcriptase.
In yet another aspect, the specification provides polynucleotides. In certain embodiments, the polynucleotides may encode any of the fusion proteins disclosed herein. In certain other embodiments, the polynucleotides may encode any of the napDNAbps disclosed herein. In still further embodiments, the polynucleotides may encode any of the reverse transcriptases disclosed herein. In yet other embodiments, the polynucleotides may encode any of the extended guide RNAs disclosed herein, any of the reverse transcription template sequences, or any of the reverse transcription primer sites, or any of the optional linker sequences.
In still other aspects, the specification provides vectors comprising the polynucleotides described herein. Thus, in certain embodiments, the vectors comprise polynucleotides for encoding the fusion proteins comprising a napDNAbp and a reverse transcriptase. In other embodiments, the vectors comprise polynucleotides that separately encode a napDNAbp and reverse transcriptase. In still other embodiments, the vectors may comprise polynucleotides that encode the extended guide RNAs. In various embodiments, the vectors may comprise one or more polynucleotides that encode napDNAbps, reverse transcriptase, and extended guide RNAs on the same or separate vectors.
In still other aspects, the specification provides cells comprising a fusion protein as described herein and an extended guide RNA. The cells may be transformed with the vectors comprising the fusion proteins, napDNAbps, reverse transcriptase, and extended guide RNAs. These genetic elements may be comprised on the same vectors or on different vectors.
In yet another aspect, the specification provides pharmaceutical compositions. In certain embodiments, the pharmaceutical compositions comprise one or more of a napDNAbp, a fusion protein, a reverse transcriptase, and an extended guide RNA. In certain embodiments, the fusion protein described herein and a pharmaceutically acceptable excipient. In other embodiments, the pharmaceutical compositions comprise any extend guide RNA described herein and a pharmaceutically acceptable excipient. In still other embodiments, the pharmaceutical compositions comprise any extend guide RNA described herein in combination with any fusion protein described herein and a pharmaceutically acceptable excipient. In yet other embodiments, the pharmaceutical compositions comprise any polynucleotide sequence encoding one or more of a napDNAbp, a fusion protein, a reverse transcriptase, and an extended guide RNA. In still other embodiments, the various components disclosed herein may be separated into one or more pharmaceutical compositions. For example, a first pharmaceutical composition may comprise a fusion protein or a napDNAbp, a second pharmaceutical compositions may comprise a reverse transcriptase, and a third pharmaceutical composition may comprise an extended guide RNA.
In still a further aspect, the present disclosure provides kits. In one embodiment, the kit comprises one or more polynucleotides encoding one or more components, including a fusion protein, a napDNAbp, a reverse transcriptase, and an extended guide RNA. The kits may also comprise vectors, cells, and isolated preparations of polypeptides, including any fusion protein, napDNAbp, or reverse transcriptase disclosed herein.
In yet another aspect, the present disclosure provides for methods of using the disclosed compositions of matter.
In one embodiment, the methods relate to a method for installing a desired nucleotide change in a double-stranded DNA sequence. The method first comprises contacting the double-stranded DNA sequence with a complex comprising a fusion protein and an extended guide RNA, wherein the fusion protein comprises a napDNAbp and a reverse transcriptase and wherein the extended guide RNA comprises a reverse transcription template sequence comprising the desired nucleotide change. Next, the method involves nicking the double-stranded DNA sequence on the non-target strand, thereby generating a free single-strand DNA having a 3′ end. The method then involves hybridizing the 3′ end of the free single-strand DNA to the reverse transcription template sequence, thereby priming the reverse transcriptase domain. The method then involves polymerizing a strand of DNA from the 3′ end, thereby generating a single-strand DNA flap comprising the desired nucleotide change. Then, the method involves replacing an endogenous DNA strand adjacent the cut site with the single-strand DNA flap, thereby installing the desired nucleotide change in the double-stranded DNA sequence.
In other embodiments, the disclosure provides for a method for introducing one or more changes in the nucleotide sequence of a DNA molecule at a target locus, comprising contacting the DNA molecule with a nucleic acid programmable DNA binding protein (napDNAbp) and a guide RNA which targets the napDNAbp to the target locus, wherein the guide RNA comprises a reverse transcriptase (RT) template sequence comprising at least one desired nucleotide change. Next, the method involves forming an exposed 3′ end in a DNA strand at the target locus and then hybridizing the exposed 3′ end to the RT template sequence to prime reverse transcription. Next, a single strand DNA flap comprising the at least one desired nucleotide change based on the RT template sequence is synthesized or polymerized by reverse transcriptase. Lastly, the at least one desired nucleotide change is incorporated into the corresponding endogenous DNA, thereby introducing one or more changes in the nucleotide sequence of the DNA molecule at the target locus.
In still other embodiments, the disclosure provides a method for introducing one or more changes in the nucleotide sequence of a DNA molecule at a target locus by target-primed reverse transcription, the method comprising: (a) contacting the DNA molecule at the target locus with a (i) fusion protein comprising a nucleic acid programmable DNA binding protein (napDNAbp) and a reverse transcriptase and (ii) a guide RNA comprising an RT template comprising a desired nucleotide change; (b) conducting target-primed reverse transcription of the RT template to generate a single strand DNA comprising the desired nucleotide change; and (c) incorporating the desired nucleotide change into the DNA molecule at the target locus through a DNA repair and/or replication process.
In certain embodiments, the step of replacing the endogenous DNA strand comprises: (i) hybridizing the single-strand DNA flap to the endogenous DNA strand adjacent the cut site to create a sequence mismatch; (ii) excising the endogenous DNA strand; and (iii) repairing the mismatch to form the desired product comprising the desired nucleotide change in both strands of DNA.
In various embodiments, the desired nucleotide change can be a single nucleotide substitution (e.g., and transition or a transversion change), a deletion, or an insertion. For example, the desired nucleotide change can be (1) a G to T substitution, (2) a G to A substitution, (3) a G to C substitution, (4) a T to G substitution, (5) a T to A substitution, (6) a T to C substitution, (7) a C to G substitution, (8) a C to T substitution, (9) a C to A substitution, (10) an A to T substitution, (11) an A to G substitution, or (12) an A to C substitution.
In other embodiments, the desired nucleoid change can convert (1) a G:C basepair to a T:A basepair, (2) a G:C basepair to an A:T basepair, (3) a G:C basepair to C:G basepair, (4) a T:A basepair to a G:C basepair, (5) a T:A basepair to an A:T basepair, (6) a T:A basepair to a C:G basepair, (7) a C:G basepair to a G:C basepair, (8) a C:G basepair to a T:A basepair, (9) a C:G basepair to an A:T basepair, (10) an A:T basepair to a T:A basepair, (11) an A:T basepair to a G:C basepair, or (12) an A:T basepair to a C:G basepair.
In still other embodiments, the method introduces a desired nucleotide change that is an insertion. In certain cases, the insertion is at least 1, at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, at least 10, at least 11, at least 12, at least 13, at least 14, at least 15, at least 16, at least 17, at least 18, at least 19, at least 20, at least 30, at least 40, at least 50, at least 60, at least 70, at least 80, at least 90, at least 100, at least 200, at least 300, at least 400, or at least 500 nucleotides in length.
In other embodiments, the method introduces a desired nucleotide change that is a deletion. In certain other cases, the deletion is at least 1, at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, at least 10, at least 11, at least 12, at least 13, at least 14, at least 15, at least 16, at least 17, at least 18, at least 19, at least 20, at least 30, at least 40, at least 50, at least 60, at least 70, at least 80, at least 90, at least 100, at least 200, at least 300, at least 400, or at least 500 nucleotides in length.
In various embodiments, the desired nucleotide change corrects a disease-associated gene. The disease-associated gene can be associated with a monogenetic disorder selected from the group consisting of: Adenosine Deaminase (ADA) Deficiency; Alpha-1 Antitrypsin Deficiency; Cystic Fibrosis; Duchenne Muscular Dystrophy; Galactosemia; Hemochromatosis; Huntington's Disease; Maple Syrup Urine Disease; Marfan Syndrome; Neurofibromatosis Type 1; Pachyonychia Congenita; Phenylkeotnuria; Severe Combined Immunodeficiency; Sickle Cell Disease; Smith-Lemli-Opitz Syndrome; and Tay-Sachs Disease. In other embodiments, the disease-associated gene can be associated with a polygenic disorder selected from the group consisting of: heart disease; high blood pressure; Alzheimer's disease; arthritis; diabetes; cancer; and obesity.
The methods disclosed herein may involve fusion proteins having a napDNAbp that is a nuclease dead Cas9 (dCas9), a Cas9 nickase (nCas9), or a nuclease active Cas9. In other embodiments, a napDNAbp and reverse transcriptase are not encoded as a single fusion protein, but rather can be provided in separate constructs. Thus, in some embodiments, the reverse transcriptase can be provided in trans relative to the napDNAbp (rather than by way of a fusion protein).
In various embodiments involving methods, the napDNAbp may comprise an amino acid sequence of SEQ ID NOs: 26-61, 75-76, 126, 130, 137, 141, 147, 153, 157, 445, 460, 467, and 482-487 (Cas9); (SpCas9); SEQ ID NO: 77-86 (CP-Cas9); SEQ ID NO: 18-25 and 87-88 (SpCas9); and SEQ ID NOs: 62-72 (Cas12). The napDNAbp may also comprise an amino acid sequence that is at least 80%, 85%, 90%, 95%, 98%, or 99% identical to the amino acid sequence of any one of SEQ ID NOs: 26-61, 75-76, 126, 130, 137, 141, 147, 153, 157, 445, 460, 467, and 482-487 (Cas9); (SpCas9); SEQ ID NO: 77-86 (CP-Cas9); SEQ ID NO: 18-25 and 87-88 (SpCas9); and SEQ ID NOs: 62-72 (Cas12).
In various embodiments involving methods, the reverse transcriptase may comprise any one of the amino acid sequences of SEQ ID NOs: 89-100, 105-122, 128-129, 132, 139, 143, 149, 154, 159, 235, 454, 471, 516, 662, 700-716, 739-742, and 766. The reverse transcriptase may also comprise an amino acid sequence that is at least 80%, 85%, 90%, 95%, 98%, or 99% identical to the amino acid sequence of any one of SEQ ID NOs: 89-100, 105-122, 128-129, 132, 139, 143, 149, 154, 159, 235, 454, 471, 516, 662, 700-716, 739-742, and 766.
The methods may involve the use of a PEgRNA comprising a nucleotide sequence of SEQ ID NOs: 131, 222, 394, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442, 641, 642, 643, 644, 645, 646, 647, 648, 649, 678, 679, 680, 681, 682, 683, 684, 685, 686, 687, 688, 689, 690, 691, 692, 738, 2997, 2998, 2999, 3000, 3001, 3002, 3003, 3004, 3005, 3006, 3007, 3008, 3009, 3010, 3011, 3012, 3013, 3014, 3015, 3016, 3017, 3018, 3019, 3020, 3021, 3022, 3023, 3024, 3025, 3026, 3027, 3028, 3029, 3030, 3031, 3032, 3033, 3034, 3035, 3036, 3037, 3038, 3039, 3040, 3041, 3042, 3043, 3044, 3045, 3046, 3047, 3048, 3049, 3050, 3051, 3052, 3053, 3054, 3055, 3056, 3057, 3058, 3059, 3060, 3061, 3062, 3063, 3064, 3065, 3066, 3067, 3068, 3069, 3070, 3071, 3072, 3073, 3074, 3075, 3076, 3077, 3078, 3079, 3080, 3081, 3082, 3083, 3084, 3085, 3086, 3087, 3088, 3089, 3090, 3091, 3092, 3093, 3094, 3095, 3096, 3097, 3098, 3099, 3100, 3101, 3102, 3103, 3113, 3114, 3115, 3116, 3117, 3118, 3119, 3120, 3121, 3305, 3306, 3307, 3308, 3309, 3310, 3311, 3312, 3313, 3314, 3315, 3316, 3317, 3318, 3319, 3320, 3321, 3322, 3323, 3324, 3325, 3326, 3327, 3328, 3329, 3330, 3331, 3332, 3333, 3334, 3335, 3336, 3337, 3338, 3339, 3340, 3341, 3342, 3343, 3344, 3345, 3346, 3347, 3348, 3349, 3350, 3351, 3352, 3353, 3354, 3355, 3356, 3357, 3358, 3359, 3360, 3361, 3362, 3363, 3364, 3365, 3366, 3367, 3368, 3369, 3370, 3371, 3372, 3373, 3374, 3375, 3376, 3377, 3378, 3379, 3380, 3381, 3382, 3383, 3384, 3385, 3386, 3387, 3388, 3389, 3390, 3391, 3392, 3393, 3394, 3395, 3396, 3397, 3398, 3399, 3400, 3401, 3402, 3403, 3404, 3405, 3406, 3407, 3408, 3409, 3410, 3411, 3412, 3413, 3414, 3415, 3416, 3417, 3418, 3419, 3420, 3421, 3422, 3423, 3424, 3425, 3426, 3427, 3428, 3429, 3430, 3431, 3432, 3433, 3434, 3435, 3436, 3437, 3438, 3439, 3440, 3441, 3442, 3443, 3444, 3445, 3446, 3447, 3448, 3449, 3450, 3451, 3452, 3453, 3454, 3455, 3479, 3480, 3481, 3482, 3483, 3484, 3485, 3486, 3487, 3488, 3489, 3490, 3491, 3492, 3493, 3522, 3523, 3524, 3525, 3526, 3527, 3528, 3529, 3530, 3531, 3532, 3533, 3534, 3535, 3536, 3537, 3538, 3539, 3540, 3549, 3550, 3551, 3552, 3553, 3554, 3555, 3556, 3628, 3629, 3630, 3631, 3632, 3633, 3634, 3635, 3636, 3637, 3638, 3639, 3640, 3641, 3642, 3643, 3644, 3645, 3646, 3647, 3648, 3649, 3650, 3651, 3652, 3653, 3654, 3655, 3656, 3657, 3658, 3659, 3660, 3661, 3662, 3663, 3664, 3665, 3666, 3667, 3668, 3669, 3670, 3671, 3672, 3673, 3674, 3675, 3676, 3677, 3678, 3679, 3680, 3681, 3682, 3683, 3684, 3685, 3686, 3687, 3688, 3689, 3690, 3691, 3692, 3693, 3694, 3695, 3696, 3697, 3698, 3755, 3756, 3757, 3758, 3759, 3760, 3761, 3762, 3763, 3764, 3765, 3766, 3767, 3768, 3769, 3770, 3771, 3772, 3773, 3774, 3775, 3776, 3777, 3778, 3779, 3780, 3781, 3782, 3783, 3784, 3785, 3786, 3787, 3788, 3789, 3790, 3791, 3792, 3793, 3794, 3795, 3796, 3797, 3798, 3799, 3800, 3801, 3802, 3803, 3804, 3805, 3806, 3807, 3808, 3809, and 3810, or a nucleotide sequence having at least 80%, or at least 85%, or at least 90%, or at least 95%, or at least 99% sequence identity thereto The methods may comprise the use of extended guide RNAs that comprise an RNA extension at the 3′ end, wherein the RNA extension comprises the reverse transcription template sequence.
The methods may comprise the use of extended guide RNAs that comprise an RNA extension at the 5′ end, wherein the RNA extension comprises the reverse transcription template sequence.
The methods may comprise the use of extended guide RNAs that comprise an RNA extension at an intramolecular location in the guide RNA, wherein the RNA extension comprises the reverse transcription template sequence.
The methods may comprise the use of extended guide RNAs having one or more RNA extensions that are at least 1, at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, at least 10, at least 11, at least 12, at least 13, at least 14, at least 15, at least 16, at least 17, at least 18, at least 19, at least 20, at least 30, at least 40, at least 50, at least 60, at least 70, at least 80, at least 90, at least 100, at least 200, at least 300, at least 400, or at least 500 nucleotides in length.
It should be appreciated that the foregoing concepts, and additional concepts discussed below, may be arranged in any suitable combination, as the present disclosure is not limited in this respect. Further, other advantages and novel features of the present disclosure will become apparent from the following detailed description of various non-limiting embodiments when considered in conjunction with the accompanying figures.
The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present disclosure, which can be better understood by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein.
The embodiment of
The schematic of
9 (first residues of the C-terminal extein) which are important for the intein trans splicing reaction. ‘SMP’ are the native residues, which we also mutated to the ‘CFN’ consensus splicing sequence. The consensus sequence is shown to yield the highest reconstitution as measured by prime editing percentage.
Unless defined otherwise, all technical and scientific terms used herein have the meaning commonly understood by a person skilled in the art to which this invention belongs. The following references provide one of skill with a general definition of many of the terms used in this invention: Singleton et al., Dictionary of Microbiology and Molecular Biology (2nd ed. 1994); The Cambridge Dictionary of Science and Technology (Walker ed., 1988); The Glossary of Genetics, 5th Ed., R. Rieger et al. (eds.), Springer Verlag (1991); and Hale & Marham, The Harper Collins Dictionary of Biology (1991). As used herein, the following terms have the meanings ascribed to them unless specified otherwise.
Antisense StrandIn genetics, the “antisense” strand of a segment within double-stranded DNA is the template strand, and which is considered to run in the 3′ to 5′ orientation. By contrast, the “sense” strand is the segment within double-stranded DNA that runs from 5′ to 3′, and which is complementary to the antisense strand of DNA, or template strand, which runs from 3′ to 5′. In the case of a DNA segment that encodes a protein, the sense strand is the strand of DNA that has the same sequence as the mRNA, which takes the antisense strand as its template during transcription, and eventually undergoes (typically, not always) translation into a protein. The antisense strand is thus responsible for the RNA that is later translated to protein, while the sense strand possesses a nearly identical makeup to that of the mRNA. Note that for each segment of dsDNA, there will possibly be two sets of sense and antisense, depending on which direction one reads (since sense and antisense is relative to perspective). It is ultimately the gene product, or mRNA, that dictates which strand of one segment of dsDNA is referred to as sense or antisense.
Bi-Specific LigandThe term “bi-specific ligand” or “bi-specific moiety,” as used herein, refers to a ligand that binds to two different ligand-binding domains. In certain embodiments, the ligand is a small molecule compound, or a peptide, or a polypeptide. In other embodiments, ligand-binding domain is a “dimerization domain,” which can be install as a peptide tag onto a protein. In various embodiments, two proteins each comprising the same or different dimerization domains can be induced to dimerize through the binding of each dimerization domain to the bi-specific ligand. As used herein, “bi-specific ligands” may be equivalently refer to “chemical inducers of dimerization” or “CIDs”.
Cas9
The term “Cas9” or “Cas9 nuclease” refers to an RNA-guided nuclease comprising a Cas9 domain, or a fragment thereof (e.g., a protein comprising an active or inactive DNA cleavage domain of Cas9, and/or the gRNA binding domain of Cas9). A “Cas9 domain” as used herein, is a protein fragment comprising an active or inactive cleavage domain of Cas9 and/or the gRNA binding domain of Cas9. A “Cas9 protein” is a full length Cas9 protein. A Cas9 nuclease is also referred to sometimes as a casn1 nuclease or a CRISPR (Clustered Regularly Interspaced Short Palindromic Repeat)-associated nuclease. CRISPR is an adaptive immune system that provides protection against mobile genetic elements (viruses, transposable elements, and conjugative plasmids). CRISPR clusters contain spacers, sequences complementary to antecedent mobile elements, and target invading nucleic acids. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA). In type II CRISPR systems correct processing of pre-crRNA requires a trans-encoded small RNA (tracrRNA), endogenous ribonuclease 3 (rnc) and a Cas9 domain. The tracrRNA serves as a guide for ribonuclease 3-aided processing of pre-crRNA. Subsequently, Cas9/crRNA/tracrRNA endonucleolytically cleaves linear or circular dsDNA target complementary to the spacer. The target strand not complementary to crRNA is first cut endonucleolytically, then trimmed 3′-5′ exonucleolytically. In nature, DNA-binding and cleavage typically requires protein and both RNAs. However, single guide RNAs (“sgRNA”, or simply “gNRA”) can be engineered so as to incorporate aspects of both the crRNA and tracrRNA into a single RNA species. See, e.g., Jinek M., Chylinski K., Fonfara I., Hauer M., Doudna J. A., Charpentier E. Science 337:816-821(2012), the entire contents of which are hereby incorporated by reference. Cas9 recognizes a short motif in the CRISPR repeat sequences (the PAM or protospacer adjacent motif) to help distinguish self versus non-self. Cas9 nuclease sequences and structures are well known to those of skill in the art (see, e.g., “Complete genome sequence of an M1 strain of Streptococcus pyogenes.” Ferretti et al., J. J., McShan W. M., Ajdic D. J., Savic D. J., Savic G., Lyon K., Primeaux C., Sezate S., Suvorov A. N., Kenton S., Lai H. S., Lin S. P., Qian Y., Jia H. G., Najar F. Z., Ren Q., Zhu H., Song L., White J., Yuan X., Clifton S. W., Roe B. A., McLaughlin R. E., Proc. Natl. Acad. Sci. U.S.A. 98:4658-4663(2001); “CRISPR RNA maturation by trans-encoded small RNA and host factor RNase III.” Deltcheva E., Chylinski K., Sharma C. M., Gonzales K., Chao Y., Pirzada Z. A., Eckert M. R., Vogel J., Charpentier E., Nature 471:602-607(2011); and “A programmable dual-RNA-guided DNA endonuclease in adaptive bacterial immunity.” Jinek M., Chylinski K., Fonfara I., Hauer M., Doudna J. A., Charpentier E. Science 337:816-821(2012), the entire contents of each of which are incorporated herein by reference). Cas9 orthologs have been described in various species, including, but not limited to, S. pyogenes and S. thermophilus. Additional suitable Cas9 nucleases and sequences will be apparent to those of skill in the art based on this disclosure, and such Cas9 nucleases and sequences include Cas9 sequences from the organisms and loci disclosed in Chylinski, Rhun, and Charpentier, “The tracrRNA and Cas9 families of type II CRISPR-Cas immunity systems” (2013) RNA Biology 10:5, 726-737; the entire contents of which are incorporated herein by reference. In some embodiments, a Cas9 nuclease comprises one or more mutations that partially impair or inactivate the DNA cleavage domain.
A nuclease-inactivated Cas9 domain may interchangeably be referred to as a “dCas9” protein (for nuclease-“dead” Cas9). Methods for generating a Cas9 domain (or a fragment thereof) having an inactive DNA cleavage domain are known (see, e.g., Jinek et al., Science. 337:816-821(2012); Qi et al., “Repurposing CRISPR as an RNA-Guided Platform for Sequence-Specific Control of Gene Expression” (2013) Cell. 28; 152(5):1173-83, the entire contents of each of which are incorporated herein by reference). For example, the DNA cleavage domain of Cas9 is known to include two subdomains, the HNH nuclease subdomain and the RuvC1 subdomain. The HNH subdomain cleaves the strand complementary to the gRNA, whereas the RuvC1 subdomain cleaves the non-complementary strand. Mutations within these subdomains can silence the nuclease activity of Cas9. For example, the mutations D10A and H840A completely inactivate the nuclease activity of S. pyogenes Cas9 (Jinek et al., Science. 337:816-821(2012); Qi et al., Cell. 28; 152(5):1173-83 (2013)). In some embodiments, proteins comprising fragments of Cas9 are provided. For example, in some embodiments, a protein comprises one of two Cas9 domains: (1) the gRNA binding domain of Cas9; or (2) the DNA cleavage domain of Cas9. In some embodiments, proteins comprising Cas9 or fragments thereof are referred to as “Cas9 variants.” A Cas9 variant shares homology to Cas9, or a fragment thereof. For example, a Cas9 variant is at least about 70% identical, at least about 80% identical, at least about 90% identical, at least about 95% identical, at least about 96% identical, at least about 97% identical, at least about 98% identical, at least about 99% identical, at least about 99.5% identical, at least about 99.8% identical, or at least about 99.9% identical to wild type Cas9 (e.g., SpCas9 of SEQ ID NO: 18). In some embodiments, the Cas9 variant may have 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 21, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, or more amino acid changes compared to wild type Cas9 (e.g., SpCas9 of SEQ ID NO: 18). In some embodiments, the Cas9 variant comprises a fragment of SEQ ID NO: 18 Cas9 (e.g., a gRNA binding domain or a DNA-cleavage domain), such that the fragment is at least about 70% identical, at least about 80% identical, at least about 90% identical, at least about 95% identical, at least about 96% identical, at least about 97% identical, at least about 98% identical, at least about 99% identical, at least about 99.5% identical, or at least about 99.9% identical to the corresponding fragment of wild type Cas9 (e.g., SpCas9 of SEQ ID NO: 18). In some embodiments, the fragment is at least 30%, at least 35%, at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95% identical, at least 96%, at least 97%, at least 98%, at least 99%, or at least 99.5% of the amino acid length of a corresponding wild type Cas9 (e.g., SpCas9 of SEQ ID NO: 18).
cDNA
The term “cDNA” refers to a strand of DNA copied from an RNA template. cDNA is complementary to the RNA template.
Circular PermutantAs used herein, the term “circular permutant” refers to a protein or polypeptide (e.g., a Cas9) comprising a circular permutation, which is change in the protein's structural configuration involving a change in order of amino acids appearing in the protein's amino acid sequence. In other words, circular permutants are proteins that have altered N- and C-termini as compared to a wild-type counterpart, e.g., the wild-type C-terminal half of a protein becomes the new N-terminal half. Circular permutation (or CP) is essentially the topological rearrangement of a protein's primary sequence, connecting its N- and C-terminus, often with a peptide linker, while concurrently splitting its sequence at a different position to create new, adjacent N- and C-termini. The result is a protein structure with different connectivity, but which often can have the same overall similar three-dimensional (3D) shape, and possibly include improved or altered characteristics, including, reduced proteolytic susceptibility, improved catalytic activity, altered substrate or ligand binding, and/or improved thermostability. Circular permutant proteins can occur in nature (e.g., concanavalin A and lectin). In addition, circular permutation can occur as a result of posttranslational modifications or may be engineered using recombinant techniques.
Circularly Permuted Cas9The term “circularly permuted Cas9” refers to any Cas9 protein, or variant thereof, that has been occurs as a circular permutant, whereby its N- and C-termini have been topically rearranged. Such circularly permuted Cas9 proteins (“CP-Cas9”), or variants thereof, retain the ability to bind DNA when complexed with a guide RNA (gRNA). See, Oakes et al., “Protein Engineering of Cas9 for enhanced function,” Methods Enzymol, 2014, 546: 491-511 and Oakes et al., “CRISPR-Cas9 Circular Permutants as Programmable Scaffolds for Genome Modification,” Cell, Jan. 10, 2019, 176: 254-267, each of are incorporated herein by reference. The instant disclosure contemplates any previously known CP-Cas9 or use a new CP-Cas9 so long as the resulting circularly permuted protein retains the ability to bind DNA when complexed with a guide RNA (gRNA). Exemplary CP-Cas9 proteins are SEQ ID NOs: 77-86.
CRISPRCRISPR is a family of DNA sequences (i.e., CRISPR clusters) in bacteria and archaea that represent snippets of prior infections by a virus that have invaded the prokaryote. The snippets of DNA are used by the prokaryotic cell to detect and destroy DNA from subsequent attacks by similar viruses and effectively compose, along with an array of CRISPR-associated proteins (including Cas9 and homologs thereof) and CRISPR-associated RNA, a prokaryotic immune defense system. In nature, CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA). In certain types of CRISPR systems (e.g., type II CRISPR systems), correct processing of pre-crRNA requires a trans-encoded small RNA (tracrRNA), endogenous ribonuclease 3 (rnc) and a Cas9 protein. The tracrRNA serves as a guide for ribonuclease 3-aided processing of pre-crRNA. Subsequently, Cas9/crRNA/tracrRNA endonucleolytically cleaves linear or circular dsDNA target complementary to the RNA. Specifically, the target strand not complementary to crRNA is first cut endonucleolytically, then trimmed 3′-5′ exonucleolytically. In nature, DNA-binding and cleavage typically requires protein and both RNAs. However, single guide RNAs (“sgRNA”, or simply “gNRA”) can be engineered so as to incorporate aspects of both the crRNA and tracrRNA into a single RNA species—the guide RNA. See, e.g., Jinek M., Chylinski K., Fonfara I., Hauer M., Doudna J. A., Charpentier E. Science 337:816-821(2012), the entire contents of which is hereby incorporated by reference. Cas9 recognizes a short motif in the CRISPR repeat sequences (the PAM or protospacer adjacent motif) to help distinguish self versus non-self. CRISPR biology, as well as Cas9 nuclease sequences and structures are well known to those of skill in the art (see, e.g., “Complete genome sequence of an M1 strain of Streptococcus pyogenes.” Ferretti et al., J. J., McShan W. M., Ajdic D. J., Savic D. J., Savic G., Lyon K., Primeaux C., Sezate S., Suvorov A. N., Kenton S., Lai H. S., Lin S. P., Qian Y., Jia H. G., Najar F. Z., Ren Q., Zhu H., Song L., White J., Yuan X., Clifton S. W., Roe B. A., McLaughlin R. E., Proc. Natl. Acad. Sci. U.S.A. 98:4658-4663(2001); “CRISPR RNA maturation by trans-encoded small RNA and host factor RNase III.” Deltcheva E., Chylinski K., Sharma C. M., Gonzales K., Chao Y., Pirzada Z. A., Eckert M. R., Vogel J., Charpentier E., Nature 471:602-607(2011); and “A programmable dual-RNA-guided DNA endonuclease in adaptive bacterial immunity.” Jinek M., Chylinski K., Fonfara I., Hauer M., Doudna J. A., Charpentier E. Science 337:816-821(2012), the entire contents of each of which are incorporated herein by reference). Cas9 orthologs have been described in various species, including, but not limited to, S. pyogenes and S. thermophilus. Additional suitable Cas9 nucleases and sequences will be apparent to those of skill in the art based on this disclosure, and such Cas9 nucleases and sequences include Cas9 sequences from the organisms and loci disclosed in Chylinski, Rhun, and Charpentier, “The tracrRNA and Cas9 families of type II CRISPR-Cas immunity systems” (2013) RNA Biology 10:5, 726-737; the entire contents of which are incorporated herein by reference.
In certain types of CRISPR systems (e.g., type II CRISPR systems), correct processing of pre-crRNA requires a trans-encoded small RNA (tracrRNA), endogenous ribonuclease 3 (rnc), and a Cas9 protein. The tracrRNA serves as a guide for ribonuclease 3-aided processing of pre-crRNA. Subsequently, Cas9/crRNA/tracrRNA endonucleolytically cleaves linear or circular nucleic acid target complementary to the RNA. Specifically, the target strand not complementary to crRNA is first cut endonucleolytically, then trimmed 3′-5′ exonucleolytically. In nature, DNA-binding and cleavage typically requires protein and both RNAs. However, single guide RNAs (“sgRNA”, or simply “gRNA”) can be engineered so as to incorporate embodiments of both the crRNA and tracrRNA into a single RNA species—the guide RNA.
In general, a “CRISPR system” refers collectively to transcripts and other elements involved in the expression of or directing the activity of CRISPR-associated (“Cas”) genes, including sequences encoding a Cas gene, a tracr (trans-activating CRISPR) sequence (e.g. tracrRNA or an active partial tracrRNA), a tracr mate sequence (encompassing a “direct repeat” and a tracrRNA-processed partial direct repeat in the context of an endogenous CRISPR system), a guide sequence (also referred to as a “spacer” in the context of an endogenous CRISPR system), or other sequences and transcripts from a CRISPR locus. The tracrRNA of the system is complementary (fully or partially) to the tracr mate sequence present on the guide RNA.
DNA Synthesis TemplateAs used herein, the term “DNA synthesis template” refers to the region or portion of the extension arm of a PEgRNA that is utilized as a template strand by a polymerase of a prime editor to encode a 3′ single-strand DNA flap that contains the desired edit and which then, through the mechanism of prime editing, replaces the corresponding endogenous strand of DNA at the target site. In various embodiments, the DNA synthesis template is shown in
In the case of trans prime editing (e.g.,
The term “dimerization domain” refers to a ligand-binding domain that binds to a binding moiety of a bi-specific ligand. A “first” dimerization domain binds to a first binding moiety of a bi-specific ligand and a “second” dimerization domain binds to a second binding moiety of the same bi-specific ligand. When the first dimerization domain is fused to a first protein (e.g., via PE, as discussed herein) and the second dimerization domain (e.g., via PE, as discussed herein) is fused to a second protein, the first and second protein dimerize in the presence of a bi-specific ligand, wherein the bi-specific ligand has at least one moiety that binds to the first dimerization domain and at least another moiety that binds to the second dimerization domain.
DownstreamAs used herein, the terms “upstream” and “downstream” are terms of relativity that define the linear position of at least two elements located in a nucleic acid molecule (whether single or double-stranded) that is orientated in a 5′-to-3′ direction. In particular, a first element is upstream of a second element in a nucleic acid molecule where the first element is positioned somewhere that is 5′ to the second element. For example, a SNP is upstream of a Cas9-induced nick site if the SNP is on the 5′ side of the nick site. Conversely, a first element is downstream of a second element in a nucleic acid molecule where the first element is positioned somewhere that is 3′ to the second element. For example, a SNP is downstream of a Cas9-induced nick site if the SNP is on the 3′ side of the nick site. The nucleic acid molecule can be a DNA (double or single stranded). RNA (double or single stranded), or a hybrid of DNA and RNA. The analysis is the same for single strand nucleic acid molecule and a double strand molecule since the terms upstream and downstream are in reference to only a single strand of a nucleic acid molecule, except that one needs to select which strand of the double stranded molecule is being considered. Often, the strand of a double stranded DNA which can be used to determine the positional relativity of at least two elements is the “sense” or “coding” strand. In genetics, a “sense” strand is the segment within double-stranded DNA that runs from 5′ to 3′, and which is complementary to the antisense strand of DNA, or template strand, which runs from 3′ to 5′. Thus, as an example, a SNP nucleobase is “downstream” of a promoter sequence in a genomic DNA (which is double-stranded) if the SNP nucleobase is on the 3′ side of the promoter on the sense or coding strand.
Edit TemplateThe term “edit template” refers to a portion of the extension arm that encodes the desired edit in the single strand 3′ DNA flap that is synthesized by the polymerase, e.g., a DNA-dependent DNA polymerase, RNA-dependent DNA polymerase (e.g., a reverse transcriptase). Certain embodiments described here (e.g.,
The term “effective amount,” as used herein, refers to an amount of a biologically active agent that is sufficient to elicit a desired biological response. For example, in some embodiments, an effective amount of a prime editor (PE) may refer to the amount of the editor that is sufficient to edit a target site nucleotide sequence, e.g., a genome. In some embodiments, an effective amount of a prime editor (PE) provided herein, e.g., of a fusion protein comprising a nickase Cas9 domain and a reverse transcriptase may refer to the amount of the fusion protein that is sufficient to induce editing of a target site specifically bound and edited by the fusion protein. As will be appreciated by the skilled artisan, the effective amount of an agent, e.g., a fusion protein, a nuclease, a hybrid protein, a protein dimer, a complex of a protein (or protein dimer) and a polynucleotide, or a polynucleotide, may vary depending on various factors as, for example, on the desired biological response, e.g., on the specific allele, genome, or target site to be edited, on the cell or tissue being targeted, and on the agent being used.
Error-Prone Reverse TranscriptaseAs used herein, the term “error-prone” reverse transcriptase (or more broadly, any polymerase) refers to a reverse transcriptase (or more broadly, any polymerase) that occurs naturally or which has been derived from another reverse transcriptase (e.g., a wild type M-MLV reverse transcriptase) which has an error rate that is less than the error rate of wild type M-MLV reverse transcriptase. The error rate of wild type M-MLV reverse transcriptase is reported to be in the range of one error in 15,000 (higher) to 27,000 (lower). An error rate of 1 in 15,000 corresponds with an error rate of 6.7×10−5. An error rate of 1 in 27,000 corresponds with an error rate of 3.7×10−5. See Boutabout et al. (2001) “DNA synthesis fidelity by the reverse transcriptase of the yeast retrotransposon Ty1,” Nucleic Acids Res 29(11):2217-2222, which is incorporated herein by reference. Thus, for purposes of this application, the term “error prone” refers to those RT that have an error rate that is greater than one error in 15,000 nucleobase incorporation (6.7×10−5 or higher), e.g., 1 error in 14,000 nucleobases (7.14×10−5 or higher), 1 error in 13,000 nucleobases or fewer (7.7×10−5 or higher), 1 error in 12,000 nucleobases or fewer (7.7×10−5 or higher), 1 error in 11,000 nucleobases or fewer (9.1×10−5 or higher), 1 error in 10,000 nucleobases or fewer (1×104 or 0.0001 or higher), 1 error in 9,000 nucleobases or fewer (0.00011 or higher), 1 error in 8,000 nucleobases or fewer (0.00013 or higher) 1 error in 7,000 nucleobases or fewer (0.00014 or higher), 1 error in 6,000 nucleobases or fewer (0.00016 or higher), 1 error in 5,000 nucleobases or fewer (0.0002 or higher), 1 error in 4,000 nucleobases or fewer (0.00025 or higher), 1 error in 3,000 nucleobases or fewer (0.00033 or higher), 1 error in 2,000 nucleobase or fewer (0.00050 or higher), or 1 error in 1,000 nucleobases or fewer (0.001 or higher), or 1 error in 500 nucleobases or fewer (0.002 or higher), or 1 error in 250 nucleobases or fewer (0.004 or higher).
ExteinThe term “extein,” as used herein, refers to an polypeptide sequence that is flanked by an intein and is ligated to another extein during the process of protein splicing to form a mature, spliced protein. Typically, an intein is flanked by two extein sequences that are ligated together when the intein catalyzes its own excision. Exteins, accordingly, are the protein analog to exons found in mRNA. For example, a polypeptide comprising an intein may be of the structure extein(N)-intein-extein(C). After excision of the intein and splicing of the two exteins, the resulting structures are extein(N)-extein(C) and a free intein. In various configurations, the exteins may be separate proteins (e.g., half of a Cas9 or PE fusion protein), each fused to a split-intein, wherein the excision of the split inteins causes the splicing together of the extein sequences.
Extension ArmThe term “extension arm” refers to a nucleotide sequence component of a PEgRNA which provides several functions, including a primer binding site and an edit template for reverse transcriptase. In some embodiments, e.g.,
The extension arm may also be described as comprising generally two regions: a primer binding site (PBS) and a DNA synthesis template, as shown in
As used herein, the term “flap endonuclease” refers to an enzyme that catalyzes the removal of 5′ single strand DNA flaps. These are naturally occurring enzymes that process the removal of 5′ flaps formed during cellular processes, including DNA replication. The prime editing methods herein described may utilize endogenously supplied flap endonucleases or those provided in trans to remove the 5′ flap of endogenous DNA formed at the target site during prime editing. Flap endonucleases are known in the art and can be found described in Patel et al., “Flap endonucleases pass 5′-flaps through a flexible arch using a disorder-thread-order mechanism to confer specificity for free 5′-ends,” Nucleic Acids Research, 2012, 40(10): 4507-4519, Tsutakawa et al., “Human flap endonuclease structures, DNA double-base flipping, and a unified understanding of the FEN1 superfamily,” Cell, 2011, 145(2): 198-211, and Balakrishnan et al., “Flap Endonuclease 1,” Annu Rev Biochem, 2013, Vol 82: 119-138 (each of which are incorporated herein by reference). An exemplary flap endonuclease is FEN1, which can be represented by the following amino acid sequence:
The term “functional equivalent” refers to a second biomolecule that is equivalent in function, but not necessarily equivalent in structure to a first biomolecule. For example, a “Cas9 equivalent” refers to a protein that has the same or substantially the same functions as Cas9, but not necessarily the same amino acid sequence. In the context of the disclosure, the specification refers throughout to “a protein X, or a functional equivalent thereof.” In this context, a “functional equivalent” of protein X embraces any homolog, paralog, fragment, naturally occurring, engineered, mutated, or synthetic version of protein X which bears an equivalent function.
Fusion ProteinThe term “fusion protein” as used herein refers to a hybrid polypeptide which comprises protein domains from at least two different proteins. One protein may be located at the amino-terminal (N-terminal) portion of the fusion protein or at the carboxy-terminal (C-terminal) protein thus forming an “amino-terminal fusion protein” or a “carboxy-terminal fusion protein,” respectively. A protein may comprise different domains, for example, a nucleic acid binding domain (e.g., the gRNA binding domain of Cas9 that directs the binding of the protein to a target site) and a nucleic acid cleavage domain or a catalytic domain of a nucleic-acid editing protein. Another example includes a Cas9 or equivalent thereof to a reverse transcriptase. Any of the proteins provided herein may be produced by any method known in the art. For example, the proteins provided herein may be produced via recombinant protein expression and purification, which is especially suited for fusion proteins comprising a peptide linker. Methods for recombinant protein expression and purification are well known, and include those described by Green and Sambrook, Molecular Cloning: A Laboratory Manual (4th ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (2012)), the entire contents of which are incorporated herein by reference.
Gene of Interest (GOI)The term “gene of interest” or “GOT” refers to a gene that encodes a biomolecule of interest (e.g., a protein or an RNA molecule). A protein of interest can include any intracellular protein, membrane protein, or extracellular protein, e.g., a nuclear protein, transcription factor, nuclear membrane transporter, intracellular organelle associated protein, a membrane receptor, a catalytic protein, and enzyme, a therapeutic protein, a membrane protein, a membrane transport protein, a signal transduction protein, or an immunological protein (e.g., an IgG or other antibody protein), etc. The gene of interest may also encode an RNA molecule, including, but not limited to, messenger RNA (mRNA), transfer RNA (tRNA), ribosomal RNA (rRNA), small nuclear RNA (snRNA), antisense RNA, guide RNA, microRNA (miRNA), small interfering RNA (siRNA), and cell-free RNA (cfRNA).
Guide RNA (“gRNA”)
As used herein, the term “guide RNA” is a particular type of guide nucleic acid which is mostly commonly associated with a Cas protein of a CRISPR-Cas9 and which associates with Cas9, directing the Cas9 protein to a specific sequence in a DNA molecule that includes complementarity to protospacer sequence of the guide RNA. However, this term also embraces the equivalent guide nucleic acid molecules that associate with Cas9 equivalents, homologs, orthologs, or paralogs, whether naturally occurring or non-naturally occurring (e.g., engineered or recombinant), and which otherwise program the Cas9 equivalent to localize to a specific target nucleotide sequence. The Cas9 equivalents may include other napDNAbp from any type of CRISPR system (e.g., type II, V, VI), including Cpf1 (a type-V CRISPR-Cas systems), C2c1 (a type V CRISPR-Cas system), C2c2 (a type VI CRISPR-Cas system) and C2c3 (a type V CRISPR-Cas system). Further Cas-equivalents are described in Makarova et al., “C2c2 is a single-component programmable RNA-guided RNA-targeting CRISPR effector,” Science 2016; 353(6299), the contents of which are incorporated herein by reference. Exemplary sequences are and structures of guide RNAs are provided herein. In addition, methods for designing appropriate guide RNA sequences are provided herein. As used herein, the “guide RNA” may also be referred to as a “traditional guide RNA” to contrast it with the modified forms of guide RNA termed “prime editing guide RNAs” (or “PEgRNAs”) which have been invented for the prime editing methods and composition disclosed herein.
Guide RNAs or PEgRNAs may comprise various structural elements that include, but are not limited to:
Spacer sequence—the sequence in the guide RNA or PEgRNA (having about 20 nts in length) which binds to the protospacer in the target DNA.
gRNA core (or gRNA scaffold or backbone sequence)—refers to the sequence within the gRNA that is responsible for Cas9 binding, it does not include the 20 bp spacer/targeting sequence that is used to guide Cas9 to target DNA.
Extension arm—a single strand extension at the 3′ end or the 5′ end of the PEgRNA which comprises a primer binding site and a DNA synthesis template sequence that encodes via a polymerase (e.g., a reverse transcriptase) a single stranded DNA flap containing the genetic change of interest, which then integrates into the endogenous DNA by replacing the corresponding endogenous strand, thereby installing the desired genetic change.
Transcription terminator—the guide RNA or PEgRNA may comprise a transcriptional termination sequence at the 3′ of the molecule.
Homology ArmThe term “homology arm” refers to a portion of the extension arm that encodes a portion of the resulting reverse transcriptase-encoded single strand DNA flap that is to be integrated into the target DNA site by replacing the endogenous strand. The portion of the single strand DNA flap encoded by the homology arm is complementary to the non-edited strand of the target DNA sequence, which facilitates the displacement of the endogenous strand and annealing of the single strand DNA flap in its place, thereby installing the edit. This component is further defined elsewhere. The homology arm is part of the DNA synthesis template since it is by definition encoded by the polymerase of the prime editors described herein.
Host CellThe term “host cell,” as used herein, refers to a cell that can host, replicate, and express a vector described herein, e.g., a vector comprising a nucleic acid molecule encoding a fusion protein comprising a Cas9 or Cas9 equivalent and a reverse transcriptase.
InteinsAs used herein, the term “intein” refers to auto-processing polypeptide domains found in organisms from all domains of life. An intein (intervening protein) carries out a unique auto-processing event known as protein splicing in which it excises itself out from a larger precursor polypeptide through the cleavage of two peptide bonds and, in the process, ligates the flanking extein (external protein) sequences through the formation of a new peptide bond. This rearrangement occurs post-translationally (or possibly co-translationally), as intein genes are found embedded in frame within other protein-coding genes. Furthermore, intein-mediated protein splicing is spontaneous; it requires no external factor or energy source, only the folding of the intein domain. This process is also known as cis-protein splicing, as opposed to the natural process of trans-protein splicing with “split inteins.” Inteins are the protein equivalent of the self-splicing RNA introns (see Perler et al., Nucleic Acids Res. 22:1125-1127 (1994)), which catalyze their own excision from a precursor protein with the concomitant fusion of the flanking protein sequences, known as exteins (reviewed in Perler et al., Curr. Opin. Chem. Biol. 1:292-299 (1997); Perler, F. B. Cell 92(1):1-4 (1998); Xu et al., EMBO J. 15(19):5146-5153 (1996)).
As used herein, the term “protein splicing” refers to a process in which an interior region of a precursor protein (an intein) is excised and the flanking regions of the protein (exteins) are ligated to form the mature protein. This natural process has been observed in numerous proteins from both prokaryotes and eukaryotes (Perler, F. B., Xu, M. Q., Paulus, H. Current Opinion in Chemical Biology 1997, 1, 292-299; Perler, F. B. Nucleic Acids Research 1999, 27, 346-347). The intein unit contains the necessary components needed to catalyze protein splicing and often contains an endonuclease domain that participates in intein mobility (Perler, F. B., Davis, E. O., Dean, G. E., Gimble, F. S., Jack, W. E., Neff, N., Noren, C. J., Thomer, J., Belfort, M. Nucleic Acids Research 1994, 22, 1127-1127). The resulting proteins are linked, however, not expressed as separate proteins. Protein splicing may also be conducted in trans with split inteins expressed on separate polypeptides spontaneously combine to form a single intein which then undergoes the protein splicing process to join to separate proteins.
The elucidation of the mechanism of protein splicing has led to a number of intein-based applications (Comb, et al., U.S. Pat. No. 5,496,714; Comb, et al., U.S. Pat. No. 5,834,247; Camarero and Muir, J. Amer. Chem. Soc., 121:5597-5598 (1999); Chong, et al., Gene, 192:271-281 (1997), Chong, et al., Nucleic Acids Res., 26:5109-5115 (1998); Chong, et al., J. Biol. Chem., 273:10567-10577 (1998); Cotton, et al. J. Am. Chem. Soc., 121:1100-1101 (1999); Evans, et al., J. Biol. Chem., 274:18359-18363 (1999); Evans, et al., J. Biol. Chem., 274:3923-3926 (1999); Evans, et al., Protein Sci., 7:2256-2264 (1998); Evans, et al., J. Biol. Chem., 275:9091-9094 (2000); Iwai and Pluckthun, FEBS Lett. 459:166-172 (1999); Mathys, et al., Gene, 231:1-13 (1999); Mills, et al., Proc. Natl. Acad. Sci. USA 95:3543-3548 (1998); Muir, et al., Proc. Natl. Acad. Sci. USA 95:6705-6710 (1998); Otomo, et al., Biochemistry 38:16040-16044 (1999); Otomo, et al., J. Biolmol. NMR 14:105-114 (1999); Scott, et al., Proc. Natl. Acad. Sci. USA 96:13638-13643 (1999); Severinov and Muir, J. Biol. Chem., 273:16205-16209 (1998); Shingledecker, et al., Gene, 207:187-195 (1998); Southworth, et al., EMBO J. 17:918-926 (1998); Southworth, et al., Biotechniques, 27:110-120 (1999); Wood, et al., Nat. Biotechnol., 17:889-892 (1999); Wu, et al., Proc. Natl. Acad. Sci. USA 95:9226-9231 (1998a); Wu, et al., Biochim Biophys Acta 1387:422-432 (1998b); Xu, et al., Proc. Natl. Acad. Sci. USA 96:388-393 (1999); Yamazaki, et al., J. Am. Chem. Soc., 120:5591-5592 (1998)). Each reference is incorporated herein by reference.
Ligand-Dependent InteinThe term “ligand-dependent intein,” as used herein refers to an intein that comprises a ligand-binding domain. Typically, the ligand-binding domain is inserted into the amino acid sequence of the intein, resulting in a structure intein (N)-ligand-binding domain-intein (C). Typically, ligand-dependent inteins exhibit no or only minimal protein splicing activity in the absence of an appropriate ligand, and a marked increase of protein splicing activity in the presence of the ligand. In some embodiments, the ligand-dependent intein does not exhibit observable splicing activity in the absence of ligand but does exhibit splicing activity in the presence of the ligand. In some embodiments, the ligand-dependent intein exhibits an observable protein splicing activity in the absence of the ligand, and a protein splicing activity in the presence of an appropriate ligand that is at least 5 times, at least 10 times, at least 50 times, at least 100 times, at least 150 times, at least 200 times, at least 250 times, at least 500 times, at least 1000 times, at least 1500 times, at least 2000 times, at least 2500 times, at least 5000 times, at least 10000 times, at least 20000 times, at least 25000 times, at least 50000 times, at least 100000 times, at least 500000 times, or at least 1000000 times greater than the activity observed in the absence of the ligand. In some embodiments, the increase in activity is dose dependent over at least 1 order of magnitude, at least 2 orders of magnitude, at least 3 orders of magnitude, at least 4 orders of magnitude, or at least 5 orders of magnitude, allowing for fine-tuning of intein activity by adjusting the concentration of the ligand. Suitable ligand-dependent inteins are known in the art, and in include those provided below and those described in published U.S. Patent Application U.S. 2014/0065711 A1; Mootz et al., “Protein splicing triggered by a small molecule.” J. Am. Chem. Soc. 2002; 124, 9044-9045; Mootz et al., “Conditional protein splicing: a new tool to control protein structure and function in vitro and in vivo.” J. Am. Chem. Soc. 2003; 125, 10561-10569; Buskirk et al., Proc. Natl. Acad. Sci. USA. 2004; 101, 10505-10510); Skretas & Wood, “Regulation of protein activity with small-molecule-controlled inteins.” Protein Sci. 2005; 14, 523-532; Schwartz, et al., “Post-translational enzyme activation in an animal via optimized conditional protein splicing.” Nat. Chem. Biol. 2007; 3, 50-54; Peck et al., Chem. Biol. 2011; 18 (5), 619-630; the entire contents of each are hereby incorporated by reference. Exemplary sequences are as follows:
The term “linker,” as used herein, refers to a molecule linking two other molecules or moieties. The linker can be an amino acid sequence in the case of a linker joining two fusion proteins. For example, a Cas9 can be fused to a reverse transcriptase by an amino acid linker sequence. The linker can also be a nucleotide sequence in the case of joining two nucleotide sequences together. For example, in the instant case, the traditional guide RNA is linked via a spacer or linker nucleotide sequence to the RNA extension of a prime editing guide RNA which may comprise a RT template sequence and an RT primer binding site. In other embodiments, the linker is an organic molecule, group, polymer, or chemical moiety. In some embodiments, the linker is 5-100 amino acids in length, for example, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 30-35, 35-40, 40-45, 45-50, 50-60, 60-70, 70-80, 80-90, 90-100, 100-150, or 150-200 amino acids in length. Longer or shorter linkers are also contemplated.
Isolated“Isolated” means altered or removed from the natural state. For example, a nucleic 20 acid or a peptide naturally present in a living animal is not “isolated,” but the same nucleic acid or peptide partially or completely separated from the coexisting materials of its natural state is “isolated.” An isolated nucleic acid or protein can exist in substantially purified form, or can exist in a non-native environment such as, for example, a host cell.
In some embodiments, a gene of interest is encoded by an isolated nucleic acid. As used herein, the term “isolated,” refers to the characteristic of a material as provided herein being removed from its original or native environment (e.g., the natural environment if it is naturally occurring). Therefore, a naturally-occurring polynucleotide or protein or polypeptide present in a living animal is not isolated, but the same polynucleotide or polypeptide, separated by human intervention from some or all of the coexisting materials in the natural system, is isolated. An artificial or engineered material, for example, a non-naturally occurring nucleic acid construct, such as the expression constructs and vectors described herein, are, accordingly, also referred to as isolated. A material does not have to be purified in order to be isolated. Accordingly, a material may be part of a vector and/or part of a composition, and still be isolated in that such vector or composition is not part of the environment in which the material is found in nature.
MS2 Tagging TechniqueIn various embodiments (e.g., as depicted in the embodiments of
The prime editors described herein may incorporate as an aspect any known RNA-protein interaction domain to recruit or “co-localize” specific functions of interest to a prime editor complex. A review of other modular RNA-protein interaction domains are described in the art, for example, in Johansson et al., “RNA recognition by the MS2 phage coat protein,” Sem Virol., 1997, Vol. 8(3): 176-185; Delebecque et al., “Organization of intracellular reactions with rationally designed RNA assemblies,” Science, 2011, Vol. 333: 470-474; Mali et al., “Cas9 transcriptional activators for target specificity screening and paired nickases for cooperative genome engineering,” Nat. Biotechnol., 2013, Vol. 31: 833-838; and Zalatan et al., “Engineering complex synthetic transcriptional programs with CRISPR RNA scaffolds,” Cell, 2015, Vol. 160: 339-350, each of which are incorporated herein by reference in their entireties. Other systems include the PP7 hairpin, which specifically recruits the PCP protein, and the “corn” hairpin, which specifically recruits the Com protein. See Zalatan et al.
The nucleotide sequence of the MS2 hairpin (or equivalently referred to as the “MS2 aptamer”) is:
The amino acid sequence of the MCP or MS2cp is:
The MS2 hairpin (or “MS2 aptamer”) may also be referred to as a type of “RNA effector recruitment domain” (or equivalently as “RNA-binding protein recruitment domain” or simply as “recruitment domain”) since it is a physical structure (e.g., a hairpin) that is installed into a PEgRNA or tPERT that effectively recruits other effector functions (e.g., RNA-binding proteins having various functions, such as DNA polymerases or other DNA-modifying enzymes) to the PEgRNA or rPERT that is so modified, and thus, co-localizing effector functions in trans to the prime editing machinery. This application is not intended to be limited in any way to any particular RNA effector recruitment domains and may include any available such domain, including the MS2 hairpin. Example 19 and
napDNAbp
As used herein, the term “nucleic acid programmable DNA binding protein” or “napDNAbp,” of which Cas9 is an example, refer to a proteins which use RNA:DNA hybridization to target and bind to specific sequences in a DNA molecule. Each napDNAbp is associated with at least one guide nucleic acid (e.g., guide RNA), which localizes the napDNAbp to a DNA sequence that comprises a DNA strand (i.e., a target strand) that is complementary to the guide nucleic acid, or a portion thereof (e.g., the protospacer of a guide RNA). In other words, the guide nucleic-acid “programs” the napDNAbp (e.g., Cas9 or equivalent) to localize and bind to a complementary sequence.
Without being bound by theory, the binding mechanism of a napDNAbp-guide RNA complex, in general, includes the step of forming an R-loop whereby the napDNAbp induces the unwinding of a double-strand DNA target, thereby separating the strands in the region bound by the napDNAbp. The guide RNA protospacer then hybridizes to the “target strand.” This displaces a “non-target strand” that is complementary to the target strand, which forms the single strand region of the R-loop. In some embodiments, the napDNAbp includes one or more nuclease activities, which then cut the DNA leaving various types of lesions. For example, the napDNAbp may comprises a nuclease activity that cuts the non-target strand at a first location, and/or cuts the target strand at a second location. Depending on the nuclease activity, the target DNA can be cut to form a “double-stranded break” whereby both strands are cut. In other embodiments, the target DNA can be cut at only a single site, i.e., the DNA is “nicked” on one strand. Exemplary napDNAbp with different nuclease activities include “Cas9 nickase” (“nCas9”) and a deactivated Cas9 having no nuclease activities (“dead Cas9” or “dCas9”). Exemplary sequences for these and other napDNAbp are provided herein.
NickaseThe term “nickase” refers to a Cas9 with one of the two nuclease domains inactivated. This enzyme is capable of cleaving only one strand of a target DNA.
Nuclear Localization Sequence (NLS)The term “nuclear localization sequence” or “NLS” refers to an amino acid sequence that promotes import of a protein into the cell nucleus, for example, by nuclear transport. Nuclear localization sequences are known in the art and would be apparent to the skilled artisan. For example, NLS sequences are described in Plank et al., international PCT application, PCT/EP2000/011690, filed Nov. 23, 2000, published as WO/2001/038547 on May 31, 2001, the contents of which are incorporated herein by reference for its disclosure of exemplary nuclear localization sequences. In some embodiments, a NLS comprises the amino acid sequence
The term “nucleic acid,” as used herein, refers to a polymer of nucleotides. The polymer may include natural nucleosides (i.e., adenosine, thymidine, guano sine, cytidine, uridine, deoxyadenosine, deoxythymidine, deoxyguanosine, and deoxycytidine), nucleoside analogs (e.g., 2-aminoadenosine, 2-thiothymidine, inosine, pyrrolo-pyrimidine, 3-methyl adenosine, 5-methylcytidine, C5 bromouridine, C5 fluorouridine, C5 iodouridine, C5 propynyl uridine, C5 propynyl cytidine, C5 methylcytidine, 7 deazaadenosine, 7 deazaguanosine, 8 oxoadenosine, 8 oxoguanosine, O(6) methylguanine, 4-acetylcytidine, 5-(carboxyhydroxymethyl)uridine, dihydrouridine, methylpseudouridine, 1-methyl adenosine, 1-methyl guanosine, N6-methyl adenosine, and 2-thiocytidine), chemically modified bases, biologically modified bases (e.g., methylated bases), intercalated bases, modified sugars (e.g., 2′-fluororibose, ribose, 2′-deoxyribose, 2′-O-methylcytidine, arabinose, and hexose), or modified phosphate groups (e.g., phosphorothioates and 5′ N phosphoramidite linkages).
PEgRNAAs used herein, the terms “prime editing guide RNA” or “PEgRNA” or “extended guide RNA” refers to a specialized form of a guide RNA that has been modified to include one or more additional sequences for implementing the prime editing methods and compositions described herein. As described herein, the prime editing guide RNA comprise one or more “extended regions” of nucleic acid sequence. The extended regions may comprise, but are not limited to, single-stranded RNA or DNA. Further, the extended regions may occur at the 3′ end of a traditional guide RNA. In other arrangements, the extended regions may occur at the 5′ end of a traditional guide RNA. In still other arrangements, the extended region may occur at an intramolecular region of the traditional guide RNA, for example, in the gRNA core region which associates and/or binds to the napDNAbp. The extended region comprises a “DNA synthesis template” which encodes (by the polymerase of the prime editor) a single-stranded DNA which, in turn, has been designed to be (a) homologous with the endogenous target DNA to be edited, and (b) which comprises at least one desired nucleotide change (e.g., a transition, a transversion, a deletion, or an insertion) to be introduced or integrated into the endogenous target DNA. The extended region may also comprise other functional sequence elements, such as, but not limited to, a “primer binding site” and a “spacer or linker” sequence, or other structural elements, such as, but not limited to aptamers, stem loops, hairpins, toe loops (e.g., a 3′ toeloop), or an RNA-protein recruitment domain (e.g., MS2 hairpin). As used herein the “primer binding site” comprises a sequence that hybridizes to a single-strand DNA sequence having a 3′ end generated from the nicked DNA of the R-loop.
In certain embodiments, the PEgRNAs are represented by
In certain other embodiments, the PEgRNAs are represented by
In still other embodiments, the PEgRNAs are represented by
In still other embodiments, the PEgRNAs are represented by
As used herein, “PE1” refers to a PE complex comprising a fusion protein comprising Cas9(H840A) and a wild type MMLV RT having the following structure: [NLS]-[Cas9(H840A)]-[linker]-[MMLV_RT(wt)]+a desired PEgRNA, wherein the PE fusion has the amino acid sequence of SEQ ID NO: 123, which is shown as follows;
As used herein, “PE2” refers to a PE complex comprising a fusion protein comprising Cas9(H840A) and a variant MMLV RT having the following structure: [NLS]-[Cas9(H840A)]-[linker]-[MMLV_RT(D200N)(T330P)(L603W)(T306K)(W313F)]+a desired PEgRNA, wherein the PE fusion has the amino acid sequence of SEQ ID NO: 134, which is shown as follows:
As used herein, “PE3” refers to PE2 plus a second-strand nicking guide RNA that complexes with the PE2 and introduces a nick in the non-edited DNA strand in order to induce preferential replacement of the edited strand.
PE3bAs used herein, “PE3b” refers to PE3 but wherein the second-strand nicking guide RNA is designed for temporal control such that the second strand nick is not introduced until after the installation of the desired edit. This is achieved by designing a gRNA with a spacer sequence that matches only the edited strand, but not the original allele. Using this strategy, referred to hereafter as PE3b, mismatches between the protospacer and the unedited allele should disfavor nicking by the sgRNA until after the editing event on the PAM strand takes place.
PE-ShortAs used herein, “PE-short” refers to a PE construct that is fused to a C-terminally truncated reverse transcriptase, and has the following amino acid sequence:
The term “peptide tag” refers to a peptide amino acid sequence that is genetically fused to a protein sequence to impart one or more functions onto the proteins that facilitate the manipulation of the protein for various purposes, such as, visualization, purification, solubilization, and separation, etc. Peptide tags can include various types of tags categorized by purpose or function, which may include “affinity tags” (to facilitate protein purification), “solubilization tags” (to assist in proper folding of proteins), “chromatography tags” (to alter chromatographic properties of proteins), “epitope tags” (to bind to high affinity antibodies), “fluorescence tags” (to facilitate visualization of proteins in a cell or in vitro).
PolymeraseAs used herein, the term “polymerase” refers to an enzyme that synthesizes a nucleotide strand and which may be used in connection with the prime editor systems described herein. The polymerase can be a “template-dependent” polymerase (i.e., a polymerase which synthesizes a nucleotide strand based on the order of nucleotide bases of a template strand). The polymerase can also be a “template-independent” polymerase (i.e., a polymerase which synthesizes a nucleotide strand without the requirement of a template strand). A polymerase may also be further categorized as a “DNA polymerase” or an “RNA polymerase.” In various embodiments, the prime editor system comprises a DNA polymerase. In various embodiments, the DNA polymerase can be a “DNA-dependent DNA polymerase” (i.e., whereby the template molecule is a strand of DNA). In such cases, the DNA template molecule can be a PEgRNA, wherein the extension arm comprises a strand of DNA. In such cases, the PEgRNA may be referred to as a chimeric or hybrid PEgRNA which comprises an RNA portion (i.e., the guide RNA components, including the spacer and the gRNA core) and a DNA portion (i.e., the extension arm). In various other embodiments, the DNA polymerase can be an “RNA-dependent DNA polymerase” (i.e., whereby the template molecule is a strand of RNA). In such cases, the PEgRNA is RNA, i.e., including an RNA extension. The term “polymerase” may also refer to an enzyme that catalyzes the polymerization of nucleotide (i.e., the polymerase activity). Generally, the enzyme will initiate synthesis at the 3′-end of a primer annealed to a polynucleotide template sequence (e.g., such as a primer sequence annealed to the primer binding site of a PEgRNA), and will proceed toward the 5′ end of the template strand. A “DNA polymerase” catalyzes the polymerization of deoxynucleotides. As used herein in reference to a DNA polymerase, the term DNA polymerase includes a “functional fragment thereof”. A “functional fragment thereof” refers to any portion of a wild-type or mutant DNA polymerase that encompasses less than the entire amino acid sequence of the polymerase and which retains the ability, under at least one set of conditions, to catalyze the polymerization of a polynucleotide. Such a functional fragment may exist as a separate entity, or it may be a constituent of a larger polypeptide, such as a fusion protein.
Prime EditingAs used herein, the term “prime editing” refers to a novel approach for gene editing using napDNAbps, a polymerase (e.g., a reverse transcriptase), and specialized guide RNAs that include a DNA synthesis template for encoding desired new genetic information (or deleting genetic information) that is then incorporated into a target DNA sequence. Certain embodiments of prime editing are described in the embodiments of
Prime editing represents an entirely new platform for genome editing that is a versatile and precise genome editing method that directly writes new genetic information into a specified DNA site using a nucleic acid programmable DNA binding protein (“napDNAbp”) working in association with a polymerase (i.e., in the form of a fusion protein or otherwise provided in trans with the napDNAbp), wherein the prime editing system is programmed with a prime editing (PE) guide RNA (“PEgRNA”) that both specifies the target site and templates the synthesis of the desired edit in the form of a replacement DNA strand by way of an extension (either DNA or RNA) engineered onto a guide RNA (e.g., at the 5′ or 3′ end, or at an internal portion of a guide RNA). The replacement strand containing the desired edit (e.g., a single nucleobase substitution) shares the same (or is homologous to) sequence as the endogenous strand (immediately downstream of the nick site) of the target site to be edited (with the exception that it includes the desired edit). Through DNA repair and/or replication machinery, the endogenous strand downstream of the nick site is replaced by the newly synthesized replacement strand containing the desired edit. In some cases, prime editing may be thought of as a “search-and-replace” genome editing technology since the prime editors, as described herein, not only search and locate the desired target site to be edited, but at the same time, encode a replacement strand containing a desired edit which is installed in place of the corresponding target site endogenous DNA strand. The prime editors of the present disclosure relate, in part, to the discovery that the mechanism of target-primed reverse transcription (TPRT) or “prime editing” can be leveraged or adapted for conducting precision CRISPR/Cas-based genome editing with high efficiency and genetic flexibility (e.g., as depicted in various embodiments of
In various embodiments, prime editing operates by contacting a target DNA molecule (for which a change in the nucleotide sequence is desired to be introduced) with a nucleic acid programmable DNA binding protein (napDNAbp) complexed with a prime editing guide RNA (PEgRNA). In reference to
The term “prime editor (PE) system” or “prime editor (PE)” or “PE system” or “PE editing system” refers the compositions involved in the method of genome editing using target-primed reverse transcription (TPRT) describe herein, including, but not limited to the napDNAbps, reverse transcriptases, fusion proteins (e.g., comprising napDNAbps and reverse transcriptases), prime editing guide RNAs, and complexes comprising fusion proteins and prime editing guide RNAs, as well as accessory elements, such as second strand nicking components (e.g., second strand sgRNAs) and 5′ endogenous DNA flap removal endonucleases (e.g., FEN1) for helping to drive the prime editing process towards the edited product formation.
Although in the embodiments described thus far the PEgRNA constitutes a single molecule comprising a guide RNA (which itself comprises a spacer sequence and a gRNA core or scaffold) and a 5′ or 3′ extension arm comprising the primer binding site and a DNA synthesis template (e.g., see
The term “prime editor” refers to the herein described fusion constructs comprising a napDNAbp (e.g., Cas9 nickase) and a reverse transcriptase and is capable of carrying out prime editing on a target nucleotide sequence in the presence of a PEgRNA (or “extended guide RNA”). The term “prime editor” may refer to the fusion protein or to the fusion protein complexed with a PEgRNA, and/or further complexed with a second-strand nicking sgRNA. In some embodiments, the prime editor may also refer to the complex comprising a fusion protein (reverse transcriptase fused to a napDNAbp), a PEgRNA, and a regular guide RNA capable of directing the second-site nicking step of the non-edited strand as described herein. In other embodiments, the reverse transcriptase component of the “primer editor” may be provided in trans.
Primer Binding SiteThe term “primer binding site” or “the PBS” refers to the nucleotide sequence located on a PEgRNA as component of the extension arm (typically at the 3′ end of the extension arm) and serves to bind to the primer sequence that is formed after Cas9 nicking of the target sequence by the prime editor. As detailed elsewhere, when the Cas9 nickase component of a prime editor nicks one strand of the target DNA sequence, a 3′-ended ssDNA flap is formed, which serves a primer sequence that anneals to the primer binding site on the PEgRNA to prime reverse transcription.
The term “promoter” is art-recognized and refers to a nucleic acid molecule with a sequence recognized by the cellular transcription machinery and able to initiate transcription of a downstream gene. A promoter can be constitutively active, meaning that the promoter is always active in a given cellular context, or conditionally active, meaning that the promoter is only active in the presence of a specific condition. For example, a conditional promoter may only be active in the presence of a specific protein that connects a protein associated with a regulatory element in the promoter to the basic transcriptional machinery, or only in the absence of an inhibitory molecule. A subclass of conditionally active promoters are inducible promoters that require the presence of a small molecule “inducer” for activity. Examples of inducible promoters include, but are not limited to, arabinose-inducible promoters, Tet-on promoters, and tamoxifen-inducible promoters. A variety of constitutive, conditional, and inducible promoters are well known to the skilled artisan, and the skilled artisan will be able to ascertain a variety of such promoters useful in carrying out the instant invention, which is not limited in this respect.
ProtospacerAs used herein, the term “protospacer” refers to the sequence (˜20 bp) in DNA adjacent to the PAM (protospacer adjacent motif) sequence. The protospacer shares the same sequence as the spacer sequence of the guide RNA. The guide RNA anneals to the complement of the protospacer sequence on the target DNA (specifically, one strand thereof, i.e., the “target strand” versus the “non-target strand” of the target DNA sequence). In order for Cas9 to function it also requires a specific protospacer adjacent motif (PAM) that varies depending on the bacterial species of the Cas9 gene. The most commonly used Cas9 nuclease, derived from S. pyogenes, recognizes a PAM sequence of NGG that is found directly downstream of the target sequence in the genomic DNA, on the non-target strand. The skilled person will appreciate that the literature in the state of the art sometimes refers to the “protospacer” as the ˜20-nt target-specific guide sequence on the guide RNA itself, rather than referring to it as a “spacer.” Thus, in some cases, the term “protospacer” as used herein may be used interchangeably with the term “spacer.” The context of the description surrounding the appearance of either “protospacer” or “spacer” will help inform the reader as to whether the term is in reference to the gRNA or the DNA target.
Protospacer Adjacent Motif (PAM)As used herein, the term “protospacer adjacent sequence” or “PAM” refers to an approximately 2-6 base pair DNA sequence that is an important targeting component of a Cas9 nuclease. Typically, the PAM sequence is on either strand, and is downstream in the 5′ to 3′ direction of Cas9 cut site. The canonical PAM sequence (i.e., the PAM sequence that is associated with the Cas9 nuclease of Streptococcus pyogenes or SpCas9) is 5′-NGG-3′ wherein “N” is any nucleobase followed by two guanine (“G”) nucleobases. Different PAM sequences can be associated with different Cas9 nucleases or equivalent proteins from different organisms. In addition, any given Cas9 nuclease, e.g., SpCas9, may be modified to alter the PAM specificity of the nuclease such that the nuclease recognizes alternative PAM sequence.
For example, with reference to the canonical SpCas9 amino acid sequence is SEQ ID NO: 18, the PAM sequence can be modified by introducing one or more mutations, including (a) D1135V, R1335Q, and T1337R “the VQR variant”, which alters the PAM specificity to NGAN or NGNG, (b) D1135E, R1335Q, and T1337R “the EQR variant”, which alters the PAM specificity to NGAG, and (c) D1135V, G1218R, R1335E, and T1337R “the VRER variant”, which alters the PAM specificity to NGCG. In addition, the D1135E variant of canonical SpCas9 still recognizes NGG, but it is more selective compared to the wild type SpCas9 protein.
It will also be appreciated that Cas9 enzymes from different bacterial species (i.e., Cas9 orthologs) can have varying PAM specificities. For example, Cas9 from Staphylococcus aureus (SaCas9) recognizes NGRRT or NGRRN. In addition, Cas9 from Neisseria meningitis (NmCas) recognizes NNNNGATT. In another example, Cas9 from Streptococcus thermophilus (StCas9) recognizes NNAGAAW. In still another example, Cas9 from Treponema denticola (TdCas) recognizes NAAAAC. These are example are not meant to be limiting. It will be further appreciated that non-SpCas9s bind a variety of PAM sequences, which makes them useful when no suitable SpCas9 PAM sequence is present at the desired target cut site. Furthermore, non-SpCas9s may have other characteristics that make them more useful than SpCas9. For example, Cas9 from Staphylococcus aureus (SaCas9) is about 1 kilobase smaller than SpCas9, so it can be packaged into adeno-associated virus (AAV). Further reference may be made to Shah et al., “Protospacer recognition motifs: mixed identities and functional diversity,” RNA Biology, 10(5): 891-899 (which is incorporated herein by reference).
RecombinaseThe term “recombinase,” as used herein, refers to a site-specific enzyme that mediates the recombination of DNA between recombinase recognition sequences, which results in the excision, integration, inversion, or exchange (e.g., translocation) of DNA fragments between the recombinase recognition sequences. Recombinases can be classified into two distinct families: serine recombinases (e.g., resolvases and invertases) and tyrosine recombinases (e.g., integrases). Examples of serine recombinases include, without limitation, Hin, Gin, Tn3, β-six, CinH, ParA, γδ, Bxb1, φC31, TP901, TG1, φBT1, R4, φRV1, φFC1, MR11, A118, U153, and gp29. Examples of tyrosine recombinases include, without limitation, Cre, FLP, R, Lambda, HK101, HK022, and pSAM2. The serine and tyrosine recombinase names stem from the conserved nucleophilic amino acid residue that the recombinase uses to attack the DNA and which becomes covalently linked to the DNA during strand exchange. Recombinases have numerous applications, including the creation of gene knockouts/knock-ins and gene therapy applications. See, e.g., Brown et al., “Serine recombinases as tools for genome engineering.” Methods. 2011; 53(4):372-9; Hirano et al., “Site-specific recombinases as tools for heterologous gene integration.” Appl. Microbiol. Biotechnol. 2011; 92(2):227-39; Chavez and Calos, “Therapeutic applications of the ΦC31 integrase system.” Curr. Gene Ther. 2011; 11(5):375-81; Turan and Bode, “Site-specific recombinases: from tag-and-target-to tag-and-exchange-based genomic modifications.” FASEB J. 2011; 25(12):4088-107; Venken and Bellen, “Genome-wide manipulations of Drosophila melanogaster with transposons, Flp recombinase, and (I)C31 integrase.” Methods Mol. Biol. 2012; 859:203-28; Murphy, “Phage recombinases and their applications.” Adv. Virus Res. 2012; 83:367-414; Zhang et al., “Conditional gene manipulation: Cre-ating a new biological era.” J. Zhejiang Univ. Sci. B. 2012; 13(7):511-24; Karpenshif and Bernstein, “From yeast to mammals: recent advances in genetic control of homologous recombination.” DNA Repair (Amst). 2012; 1; 11(10):781-8; the entire contents of each are hereby incorporated by reference in their entirety. The recombinases provided herein are not meant to be exclusive examples of recombinases that can be used in embodiments of the invention. The methods and compositions of the invention can be expanded by mining databases for new orthogonal recombinases or designing synthetic recombinases with defined DNA specificities (See, e.g., Groth et al., “Phage integrases: biology and applications.” J. Mol. Biol. 2004; 335, 667-678; Gordley et al., “Synthesis of programmable integrases.” Proc. Natl. Acad. Sci. USA. 2009; 106, 5053-5058; the entire contents of each are hereby incorporated by reference in their entirety). Other examples of recombinases that are useful in the methods and compositions described herein are known to those of skill in the art, and any new recombinase that is discovered or generated is expected to be able to be used in the different embodiments of the invention. In some embodiments, the catalytic domains of a recombinase are fused to a nuclease-inactivated RNA-programmable nuclease (e.g., dCas9, or a fragment thereof), such that the recombinase domain does not comprise a nucleic acid binding domain or is unable to bind to a target nucleic acid (e.g., the recombinase domain is engineered such that it does not have specific DNA binding activity). Recombinases lacking DNA binding activity and methods for engineering such are known, and include those described by Klippel et al., “Isolation and characterisation of unusual gin mutants.” EMBO J. 1988; 7: 3983-3989: Burke et al., “Activating mutations of Tn3 resolvase marking interfaces important in recombination catalysis and its regulation. Mol Microbiol. 2004; 51: 937-948; Olorunniji et al., “Synapsis and catalysis by activated Tn3 resolvase mutants.” Nucleic Acids Res. 2008; 36: 7181-7191; Rowland et al., “Regulatory mutations in Sin recombinase support a structure-based model of the synaptosome.” Mol Microbiol. 2009; 74: 282-298; Akopian et al., “Chimeric recombinases with designed DNA sequence recognition.” Proc Natl Acad Sci USA. 2003; 100: 8688-8691; Gordley et al., “Evolution of programmable zinc finger-recombinases with activity in human cells. J Mol Biol. 2007; 367: 802-813; Gordley et al., “Synthesis of programmable integrases.” Proc Natl Acad Sci USA. 2009; 106: 5053-5058; Arnold et al., “Mutants of Tn3 resolvase which do not require accessory binding sites for recombination activity.” EMBO J. 1999; 18: 1407-1414; Gaj et al., “Structure-guided reprogramming of serine recombinase DNA sequence specificity.” Proc Natl Acad Sci USA. 2011; 108(2):498-503; and Proudfoot et al., “Zinc finger recombinases with adaptable DNA sequence specificity.” PLoS One. 2011; 6(4):e19537; the entire contents of each are hereby incorporated by reference. For example, serine recombinases of the resolvase-invertase group, e.g., Tn3 and γ6 resolvases and the Hin and Gin invertases, have modular structures with autonomous catalytic and DNA-binding domains (See, e.g., Grindley et al., “Mechanism of site-specific recombination.” Ann Rev Biochem. 2006; 75: 567-605, the entire contents of which are incorporated by reference). The catalytic domains of these recombinases are thus amenable to being recombined with nuclease-inactivated RNA-programmable nucleases (e.g., dCas9, or a fragment thereof) as described herein, e.g., following the isolation of ‘activated’ recombinase mutants which do not require any accessory factors (e.g., DNA binding activities) (See, e.g., Klippel et al., “Isolation and characterisation of unusual gin mutants.” EMBO J. 1988; 7: 3983-3989: Burke et al., “Activating mutations of Tn3 resolvase marking interfaces important in recombination catalysis and its regulation. Mol Microbiol. 2004; 51: 937-948; Olorunniji et al., “Synapsis and catalysis by activated Tn3 resolvase mutants.” Nucleic Acids Res. 2008; 36: 7181-7191; Rowland et al., “Regulatory mutations in Sin recombinase support a structure-based model of the synaptosome.” Mol Microbiol. 2009; 74: 282-298; Akopian et al., “Chimeric recombinases with designed DNA sequence recognition.” Proc Natl Acad Sci USA. 2003; 100: 8688-8691). Additionally, many other natural serine recombinases having an N-terminal catalytic domain and a C-terminal DNA binding domain are known (e.g., phiC31 integrase, TnpX transposase, IS607 transposase), and their catalytic domains can be co-opted to engineer programmable site-specific recombinases as described herein (See, e.g., Smith et al., “Diversity in the serine recombinases.” Mol Microbiol. 2002; 44: 299-307, the entire contents of which are incorporated by reference). Similarly, the core catalytic domains of tyrosine recombinases (e.g., Cre, λ integrase) are known, and can be similarly co-opted to engineer programmable site-specific recombinases as described herein (See, e.g., Guo et al., “Structure of Cre recombinase complexed with DNA in a site-specific recombination synapse.” Nature. 1997; 389:40-46; Hartung et al., “Cre mutants with altered DNA binding properties.” J Biol Chem 1998; 273:22884-22891; Shaikh et al., “Chimeras of the Flp and Cre recombinases: Tests of the mode of cleavage by Flp and Cre. J Mol Biol. 2000; 302:27-48; Rongrong et al., “Effect of deletion mutation on the recombination activity of Cre recombinase.” Acta Biochim Pol. 2005; 52:541-544; Kilbride et al., “Determinants of product topology in a hybrid Cre-Tn3 resolvase site-specific recombination system.” J Mol Biol. 2006; 355:185-195; Warren et al., “A chimeric cre recombinase with regulated directionality.” Proc Natl Acad Sci USA. 2008 105:18278-18283; Van Duyne, “Teaching Cre to follow directions.” Proc Natl Acad Sci USA. 2009 Jan. 6; 106(1):4-5; Numrych et al., “A comparison of the effects of single-base and triple-base changes in the integrase arm-type binding sites on the site-specific recombination of bacteriophage λ.” Nucleic Acids Res. 1990; 18:3953-3959; Tirumalai et al., “The recognition of core-type DNA sites by λ integrase.” J Mol Biol. 1998; 279:513-527; Aihara et al., “A conformational switch controls the DNA cleavage activity of λ integrase.” Mol Cell. 2003; 12:187-198; Biswas et al., “A structural basis for allosteric control of DNA recombination by λ integrase.” Nature. 2005; 435:1059-1066; and Warren et al., “Mutations in the amino-terminal domain of λ-integrase have differential effects on integrative and excisive recombination.” Mol Microbiol. 2005; 55:1104-1112; the entire contents of each are incorporated by reference).
Recombinase Recognition SequenceThe term “recombinase recognition sequence”, or equivalently as “RRS” or “recombinase target sequence”, as used herein, refers to a nucleotide sequence target recognized by a recombinase and which undergoes strand exchange with another DNA molecule having a the RRS that results in excision, integration, inversion, or exchange of DNA fragments between the recombinase recognition sequences.
Recombine or RecombinationThe term “recombine,” or “recombination,” in the context of a nucleic acid modification (e.g., a genomic modification), is used to refer to the process by which two or more nucleic acid molecules, or two or more regions of a single nucleic acid molecule, are modified by the action of a recombinase protein (e.g., an inventive recombinase fusion protein provided herein). Recombination can result in, inter alia, the insertion, inversion, excision, or translocation of nucleic acids, e.g., in or between one or more nucleic acid molecules. recombinase recognition sequences
Reverse TranscriptaseThe term “reverse transcriptase” describes a class of polymerases characterized as RNA-dependent DNA polymerases. All known reverse transcriptases require a primer to synthesize a DNA transcript from an RNA template. Historically, reverse transcriptase has been used primarily to transcribe mRNA into cDNA which can then be cloned into a vector for further manipulation. Avian myoblastosis virus (AMV) reverse transcriptase was the first widely used RNA-dependent DNA polymerase (Verma, Biochim. Biophys. Acta 473:1 (1977)). The enzyme has 5′-3′ RNA-directed DNA polymerase activity, 5′-3′ DNA-directed DNA polymerase activity, and RNase H activity. RNase H is a processive 5′ and 3′ ribonuclease specific for the RNA strand for RNA-DNA hybrids (Perbal, A Practical Guide to Molecular Cloning, New York: Wiley & Sons (1984)). Errors in transcription cannot be corrected by reverse transcriptase because known viral reverse transcriptases lack the 3′-5′ exonuclease activity necessary for proofreading (Saunders and Saunders, Microbial Genetics Applied to Biotechnology, London: Croom Helm (1987)). A detailed study of the activity of AMV reverse transcriptase and its associated RNase H activity has been presented by Berger et al., Biochemistry 22:2365-2372 (1983). Another reverse transcriptase which is used extensively in molecular biology is reverse transcriptase originating from Moloney murine leukemia virus (M-MLV). See, e.g., Gerard, G. R., DNA 5:271-279 (1986) and Kotewicz, M. L., et al., Gene 35:249-258 (1985). M-MLV reverse transcriptase substantially lacking in RNase H activity has also been described. See, e.g., U.S. Pat. No. 5,244,797. The invention contemplates the use of any such reverse transcriptases, or variants or mutants thereof.
In addition, the invention contemplates the use of reverse transcriptases which are error-prone, i.e., which may be referred to as error-prone reverse transcriptases or reverse transcriptases which do not support high fidelity incorporation of nucleotides during polymerization. During synthesis of the single-strand DNA flap based on the RT template integrated with the guide RNA, the error-prone reverse transcriptase can introduce one or more nucleotides which are mismatched with the RT template sequence, thereby introducing changes to the nucleotide sequence through erroneous polymerization of the single-strand DNA flap. These errors introduced during synthesis of the single strand DNA flap then become integrated into the double strand molecule through hybridization to the corresponding endogenous target strand, removal of the endogenous displaced strand, ligation, and then through one more round of endogenous DNA repair and/or sequencing processes.
Reverse TranscriptionAs used herein, the term “reverse transcription” indicates the capability of enzyme to synthesize DNA strand (that is, complementary DNA or cDNA) using RNA as a template. In some embodiments, the reverse transcription can be “error-prone reverse transcription,” which refers to the properties of certain reverse transcriptase enzymes which are error-prone in their DNA polymerization activity.
PACEThe term “phage-assisted continuous evolution (PACE),” as used herein, refers to continuous evolution that employs phage as viral vectors. The general concept of PACE technology has been described, for example, in International PCT Application, PCT/US2009/056194, filed Sep. 8, 2009, published as WO 2010/028347 on Mar. 11, 2010; International PCT Application, PCT/US2011/066747, filed Dec. 22, 2011, published as WO 2012/088381 on Jun. 28, 2012; U.S. application, U.S. Pat. No. 9,023,594, issued May 5, 2015, International PCT Application, PCT/US2015/012022, filed Jan. 20, 2015, published as WO 2015/134121 on Sep. 11, 2015, and International PCT Application, PCT/US2016/027795, filed Apr. 15, 2016, published as WO 2016/168631 on Oct. 20, 2016, the entire contents of each of which are incorporated herein by reference.
PhageThe term “phage,” as used herein interchangeably with the term “bacteriophage,” refers to a virus that infects bacterial cells. Typically, phages consist of an outer protein capsid enclosing genetic material. The genetic material can be ssRNA, dsRNA, ssDNA, or dsDNA, in either linear or circular form. Phages and phage vectors are well known to those of skill in the art and non-limiting examples of phages that are useful for carrying out the PACE methods provided herein are λ (Lysogen), T2, T4, T7, T12, R17, M13, MS2, G4, P1, P2, P4, Phi X174, N4, Φ6, and Φ29. In certain embodiments, the phage utilized in the present invention is M13. Additional suitable phages and host cells will be apparent to those of skill in the art and the invention is not limited in this aspect. For an exemplary description of additional suitable phages and host cells, see Elizabeth Kutter and Alexander Sulakvelidze: Bacteriophages: Biology and Applications. CRC Press; 1st edition (December 2004), ISBN: 0849313368; Martha R. J. Clokie and Andrew M. Kropinski: Bacteriophages: Methods and Protocols, Volume 1: Isolation, Characterization, and Interactions (Methods in Molecular Biology) Humana Press; 1st edition (December, 2008), ISBN: 1588296822; Martha R. J. Clokie and Andrew M. Kropinski: Bacteriophages: Methods and Protocols, Volume 2: Molecular and Applied Aspects (Methods in Molecular Biology) Humana Press; 1st edition (December 2008), ISBN: 1603275649; all of which are incorporated herein in their entirety by reference for disclosure of suitable phages and host cells as well as methods and protocols for isolation, culture, and manipulation of such phages).
Protein, Peptide, and PolypeptideThe terms “protein,” “peptide,” and “polypeptide” are used interchangeably herein, and refer to a polymer of amino acid residues linked together by peptide (amide) bonds. The terms refer to a protein, peptide, or polypeptide of any size, structure, or function. Typically, a protein, peptide, or polypeptide will be at least three amino acids long. A protein, peptide, or polypeptide may refer to an individual protein or a collection of proteins. One or more of the amino acids in a protein, peptide, or polypeptide may be modified, for example, by the addition of a chemical entity such as a carbohydrate group, a hydroxyl group, a phosphate group, a farnesyl group, an isofarnesyl group, a fatty acid group, a linker for conjugation, functionalization, or other modification, etc. A protein, peptide, or polypeptide may also be a single molecule or may be a multi-molecular complex. A protein, peptide, or polypeptide may be just a fragment of a naturally occurring protein or peptide. A protein, peptide, or polypeptide may be naturally occurring, recombinant, or synthetic, or any combination thereof. Any of the proteins provided herein may be produced by any method known in the art. For example, the proteins provided herein may be produced via recombinant protein expression and purification, which is especially suited for fusion proteins comprising a peptide linker. Methods for recombinant protein expression and purification are well known, and include those described by Green and Sambrook, Molecular Cloning: A Laboratory Manual (4th ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (2012)), the entire contents of which are incorporated herein by reference.
Protein SplicingThe term “protein splicing,” as used herein, refers to a process in which a sequence, an intein (or split inteins, as the case may be), is excised from within an amino acid sequence, and the remaining fragments of the amino acid sequence, the exteins, are ligated via an amide bond to form a continuous amino acid sequence. The term “trans” protein splicing refers to the specific case where the inteins are split inteins and they are located on different proteins.
Second-Strand NickingThe resolution of heteroduplex DNA (i.e., containing one edited and one non-edited strand) formed as a result of prime editing determines long-term editing outcomes. In words, a goal of prime editing is to resolve the heteroduplex DNA (the edited strand paired with the endogenous non-edited strand) formed as an intermediate of PE by permanently integrating the edited strand into the complement, endogenous strand. The approach of “second-strand nicking” can be used herein to help drive the resolution of heteroduplex DNA in favor of permanent integration of the edited strand into the DNA molecule. As used herein, the concept of “second-strand nicking” refers to the introduction of a second nick at a location downstream of the first nick (i.e., the initial nick site that provides the free 3′ end for use in priming of the reverse transcriptase on the extended portion of the guide RNA), preferably on the unedited strand. In certain embodiments, the first nick and the second nick are on opposite strands. In other embodiments, the first nick and the second nick are on opposite strands. In yet another embodiment, the first nick is on the non-target strand (i.e., the strand that forms the single strand portion of the R-loop), and the second nick is on the target strand. In still other embodiments, the first nick is on the edited strand, and the second nick is on the unedited strand. The second nick can be positioned at least 5 nucleotides downstream of the first nick, or at least 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, or 150 or more nucleotides downstream of the first nick. The second nick, in certain embodiments, can be introduced between about 5-150 nucleotides on the unedited strand away from the site of the PEgRNA-induced nick, or between about 5-140, or between about 5-130, or between about 5-120, or between about 5-110, or between about 5-100, or between about 5-90, or between about 5-80, or between about 5-70, or between about 5-60, or between about 5-50, or between about 5-40, or between about 5-30, or between about 5-20, or between about 5-10. In one embodiment, the second nick is introduced between 14-116 nucleotides away from the PEgRNA-induced nick. Without being bound by theory, the second nick induces the cell's endogenous DNA repair and replication processes towards replacement or editing of the unedited strand, thereby permanently installing the edited sequence on both strands and resolving the heteroduplex that is formed as a result of PE. In some embodiments, the edited strand is the non-target strand and the unedited strand is the target strand. In other embodiments, the edited strand is the target strand, and the unedited strand is the non-target strand.
Sense StrandIn genetics, a “sense” strand is the segment within double-stranded DNA that runs from 5′ to 3′, and which is complementary to the antisense strand of DNA, or template strand, which runs from 3′ to 5′. In the case of a DNA segment that encodes a protein, the sense strand is the strand of DNA that has the same sequence as the mRNA, which takes the antisense strand as its template during transcription, and eventually undergoes (typically, not always) translation into a protein. The antisense strand is thus responsible for the RNA that is later translated to protein, while the sense strand possesses a nearly identical makeup to that of the mRNA. Note that for each segment of dsDNA, there will possibly be two sets of sense and antisense, depending on which direction one reads (since sense and antisense is relative to perspective). It is ultimately the gene product, or mRNA, that dictates which strand of one segment of dsDNA is referred to as sense or antisense.
In the context of a PEgRNA, the first step is the synthesis of a single-strand complementary DNA (i.e., the 3′ ssDNA flap, which becomes incorporated) oriented in the 5′ to 3′ direction which is templated off of the PEgRNA extension arm. Whether the 3′ ssDNA flap should be regarded as a sense or antisense strand depends on the direction of transcription since it well accepted that both strands of DNA may serve as a template for transcription (but not at the same time). Thus, in some embodiments, the 3′ ssDNA flap (which overall runs in the 5′ to 3′ direction) will serve as the sense strand because it is the coding strand. In other embodiments, the 3′ ssDNA flap (which overall runs in the 5′ to 3′ direction) will serve as the antisense strand and thus, the template for transcription.
Spacer SequenceAs used herein, the term “spacer sequence” in connection with a guide RNA or a PEgRNA refers to the portion of the guide RNA or PEgRNA of about 20 nucleotides which contains a nucleotide sequence that is complementary to the protospacer sequence in the target DNA sequence. The spacer sequence anneals to the protospacer sequence to form a ssRNA/ssDNA hybrid structure at the target site and a corresponding R loop ssDNA structure of the endogenous DNA strand that is complementary to the protospacer sequence.
SubjectThe term “subject,” as used herein, refers to an individual organism, for example, an individual mammal. In some embodiments, the subject is a human. In some embodiments, the subject is a non-human mammal. In some embodiments, the subject is a non-human primate. In some embodiments, the subject is a rodent. In some embodiments, the subject is a sheep, a goat, a cattle, a cat, or a dog. In some embodiments, the subject is a vertebrate, an amphibian, a reptile, a fish, an insect, a fly, or a nematode. In some embodiments, the subject is a research animal. In some embodiments, the subject is genetically engineered, e.g., a genetically engineered non-human subject. The subject may be of either sex and at any stage of development.
Split InteinAlthough inteins are most frequently found as a contiguous domain, some exist in a naturally split form. In this case, the two fragments are expressed as separate polypeptides and must associate before splicing takes place, so-called protein trans-splicing.
An exemplary split intein is the Ssp DnaE intein, which comprises two subunits, namely, DnaE-N and DnaE-C. The two different subunits are encoded by separate genes, namely dnaE-n and dnaE-c, which encode the DnaE-N and DnaE-C subunits, respectively. DnaE is a naturally occurring split intein in Synechocytis sp. PCC6803 and is capable of directing trans-splicing of two separate proteins, each comprising a fusion with either DnaE-N or DnaE-C.
Additional naturally occurring or engineered split-intein sequences are known in the or can be made from whole-intein sequences described herein or those available in the art. Examples of split-intein sequences can be found in Stevens et al., “A promiscuous split intein with expanded protein engineering applications,” PNAS, 2017, Vol. 114: 8538-8543; Iwai et al., “Highly efficient protein trans-splicing by a naturally split DnaE intein from Nostoc punctiforme, FEBS Lett, 580: 1853-1858, each of which are incorporated herein by reference. Additional split intein sequences can be found, for example, in WO 2013/045632, WO 2014/055782, WO 2016/069774, and EP2877490, the contents each of which are incorporated herein by reference.
In addition, protein splicing in trans has been described in vivo and in vitro (Shingledecker, et al., Gene 207:187 (1998), Southworth, et al., EMBO J. 17:918 (1998); Mills, et al., Proc. Natl. Acad. Sci. USA, 95:3543-3548 (1998); Lew, et al., J. Biol. Chem., 273:15887-15890 (1998); Wu, et al., Biochim. Biophys. Acta 35732:1 (1998b), Yamazaki, et al., J. Am. Chem. Soc. 120:5591 (1998), Evans, et al., J. Biol. Chem. 275:9091 (2000); Otomo, et al., Biochemistry 38:16040-16044 (1999); Otomo, et al., J. Biolmol. NMR 14:105-114 (1999); Scott, et al., Proc. Natl. Acad. Sci. USA 96:13638-13643 (1999)) and provides the opportunity to express a protein as to two inactive fragments that subsequently undergo ligation to form a functional product, e.g., as shown in
The term “target site” refers to a sequence within a nucleic acid molecule that is edited by a prime editor (PE) disclosed herein. The target site further refers to the sequence within a nucleic acid molecule to which a complex of the prime editor (PE) and gRNA binds.
tPERT
See definition for “trans prime editor RNA template (tPERT).”
Temporal Second-Strand NickingAs used herein, the term “temporal second-strand nicking” refers to a variant of second strand nicking whereby the installation of the second nick in the unedited strand occurs only after the desired edit is installed in the edited strand. This avoids concurrent nicks on both strands that could lead to double-stranded DNA breaks. The second-strand nicking guide RNA is designed for temporal control such that the second strand nick is not introduced until after the installation of the desired edit. This is achieved by designing a gRNA with a spacer sequence that matches only the edited strand, but not the original allele. Using this strategy, mismatches between the protospacer and the unedited allele should disfavor nicking by the sgRNA until after the editing event on the PAM strand takes place.
Trans Prime EditingAs used herein, the term “trans prime editing” refers to a modified form of prime editing that utilizes a split PEgRNA, i.e., wherein the PEgRNA is separated into two separate molecules: an sgRNA and a trans prime editing RNA template (tPERT). The sgRNA serves to target the prime editor (or more generally, to target the napDNAbp component of the prime editor) to the desired genomic target site, while the tPERT is used by the polymerase (e.g., a reverse transcriptase) to write new DNA sequence into the target locus once the tPERT is recruited in trans to the prime editor by the interaction of binding domains located on the prime editor and on the tPERT. In one embodiment, the binding domains can include RNA-protein recruitment moieties, such as a MS2 aptamer located on the tPERT and an MS2cp protein fused to the prime editor. An advantage of trans prime editing is that by separating the DNA synthesis template from the guide RNA, one can potentially use longer length templates.
An embodiment of trans prime editing is shown in
While the tPERT is shown in
Trans Prime Editor RNA Template (tPERT)
As used herein, a “trans prime editor RNA template (tPERT)” refers to a component used in trans prime editing, a modified version of prime editing which operates by separating the PEgRNA into two distinct molecules: a guide RNA and a tPERT molecule. The tPERT molecule is programmed to co-localize with the prime editor complex at a target DNA site, bringing the primer binding site and the DNA synthesis template to the prime editor in trans. For example, see
As used herein, “transitions” refer to the interchange of purine nucleobases (A↔G) or the interchange of pyrimidine nucleobases (C↔T). This class of interchanges involves nucleobases of similar shape. The compositions and methods disclosed herein are capable of inducing one or more transitions in a target DNA molecule. The compositions and methods disclosed herein are also capable of inducing both transitions and transversion in the same target DNA molecule. These changes involve A↔G, G↔A, C↔T, or T↔C. In the context of a double-strand DNA with Watson-Crick paired nucleobases, transversions refer to the following base pair exchanges: A:T↔G:C, G:G↔A:T, C:G↔T:A, or T:A↔C:G. The compositions and methods disclosed herein are capable of inducing one or more transitions in a target DNA molecule. The compositions and methods disclosed herein are also capable of inducing both transitions and transversion in the same target DNA molecule, as well as other nucleotide changes, including deletions and insertions.
TransversionsAs used herein, “transversions” refer to the interchange of purine nucleobases for pyrimidine nucleobases, or in the reverse and thus, involve the interchange of nucleobases with dissimilar shape. These changes involve T↔A, T↔G, C↔G, C↔A, A↔T, A↔C, G↔C, and G↔T. In the context of a double-strand DNA with Watson-Crick paired nucleobases, transversions refer to the following base pair exchanges: T:A↔A:T, T:A↔G:C, C:G↔G:C, C:G↔A:T, A:T↔T:A, A:T↔C:G, G:C↔C:G, and G:C↔T:A. The compositions and methods disclosed herein are capable of inducing one or more transversions in a target DNA molecule. The compositions and methods disclosed herein are also capable of inducing both transitions and transversion in the same target DNA molecule, as well as other nucleotide changes, including deletions and insertions.
TreatmentThe terms “treatment,” “treat,” and “treating,” refer to a clinical intervention aimed to reverse, alleviate, delay the onset of, or inhibit the progress of a disease or disorder, or one or more symptoms thereof, as described herein. As used herein, the terms “treatment,” “treat,” and “treating” refer to a clinical intervention aimed to reverse, alleviate, delay the onset of, or inhibit the progress of a disease or disorder, or one or more symptoms thereof, as described herein. In some embodiments, treatment may be administered after one or more symptoms have developed and/or after a disease has been diagnosed. In other embodiments, treatment may be administered in the absence of symptoms, e.g., to prevent or delay onset of a symptom or inhibit onset or progression of a disease. For example, treatment may be administered to a susceptible individual prior to the onset of symptoms (e.g., in light of a history of symptoms and/or in light of genetic or other susceptibility factors). Treatment may also be continued after symptoms have resolved, for example, to prevent or delay their recurrence.
Trinucleotide Repeat DisorderAs used herein, a “trinucleotide repeat disorder” (or alternatively, “expansion repeat disorder” or “repeat expansion disorder”) refers to a set of genetic disorders which are cause by “trinucleotide repeat expansion,” which is a kind of mutation where a certain trinucleotide repeats in certain genes or introns. Trinucleotide repeats were once thought to be commonplace iterations in the genome, but the 1990s clarified these disorders. These apparently ‘benign’ stretches of DNA can sometimes expand and cause disease. Several defining features are shared amongst disorders caused by trinucleotide repeat expansions. First, the mutant repeats show both somatic and germline instability and, more frequently, they expand rather than contract in successive transmissions. Secondly, an earlier age of onset and increasing severity of phenotype in subsequent generations (anticipation) generally are correlated with larger repeat length. Finally, the parental origin of the disease allele can often influence anticipation, with paternal transmissions carrying a greater risk of expansion for many of these disorders.
Triplet expansion is thought to be caused by slippage during DNA replication. Due to the repetitive nature of the DNA sequence in these regions ‘loop out’ structures may form during DNA replication while maintaining complementary base pairing between the parent strand and daughter strand being synthesized. If the loop out structure is formed from sequence on the daughter strand this will result in an increase in the number of repeats. However, if the loop out structure is formed on the parent strand a decrease in the number of repeats occurs. It appears that expansion of these repeats is more common than reduction. Generally the larger the expansion the more likely they are to cause disease or increase the severity of disease. This property results in the characteristic of anticipation seen in trinucleotide repeat disorders. Anticipation describes the tendency of age of onset to decrease and severity of symptoms to increase through successive generations of an affected family due to the expansion of these repeats.
Nucleotide repeat disorders may include those in which the triplet repeat occurs in a non-coding region (i.e., a non-coding trinucleotide repeat disorder) or in a coding region
The prime editor (PE) system described herein may use to treat nucleotide repeat disorders, which may include fragile X syndrome (FRAXA), fragile XE MR (FRAXE), Freidreich ataxia (FRDA), myotonic dystrophy (DM), spinocerebellar ataxia type 8 (SCA8), and spinocerebellar ataxia type 12 (SCA12), among others.
UpstreamAs used herein, the terms “upstream” and “downstream” are terms of relativity that define the linear position of at least two elements located in a nucleic acid molecule (whether single or double-stranded) that is orientated in a 5′-to-3′ direction. In particular, a first element is upstream of a second element in a nucleic acid molecule where the first element is positioned somewhere that is 5′ to the second element. For example, a SNP is upstream of a Cas9-induced nick site if the SNP is on the 5′ side of the nick site. Conversely, a first element is downstream of a second element in a nucleic acid molecule where the first element is positioned somewhere that is 3′ to the second element. For example, a SNP is downstream of a Cas9-induced nick site if the SNP is on the 3′ side of the nick site. The nucleic acid molecule can be a DNA (double or single stranded). RNA (double or single stranded), or a hybrid of DNA and RNA. The analysis is the same for single strand nucleic acid molecule and a double strand molecule since the terms upstream and downstream are in reference to only a single strand of a nucleic acid molecule, except that one needs to select which strand of the double stranded molecule is being considered. Often, the strand of a double stranded DNA which can be used to determine the positional relativity of at least two elements is the “sense” or “coding” strand. In genetics, a “sense” strand is the segment within double-stranded DNA that runs from 5′ to 3′, and which is complementary to the antisense strand of DNA, or template strand, which runs from 3′ to 5′. Thus, as an example, a SNP nucleobase is “downstream” of a promoter sequence in a genomic DNA (which is double-stranded) if the SNP nucleobase is on the 3′ side of the promoter on the sense or coding strand.
VariantAs used herein the term “variant” should be taken to mean the exhibition of qualities that have a pattern that deviates from what occurs in nature, e.g., a variant Cas9 is a Cas9 comprising one or more changes in amino acid residues as compared to a wild type Cas9 amino acid sequence. The term “variant” encompasses homologous proteins having at least 75%, or at least 80%, or at least 85%, or at least 90%, or at least 95%, or at least 99% percent identity with a reference sequence and having the same or substantially the same functional activity or activities as the reference sequence. The term also encompasses mutants, truncations, or domains of a reference sequence, and which display the same or substantially the same functional activity or activities as the reference sequence.
VectorThe term “vector,” as used herein, refers to a nucleic acid that can be modified to encode a gene of interest and that is able to enter into a host cell, mutate and replicate within the host cell, and then transfer a replicated form of the vector into another host cell. Exemplary suitable vectors include viral vectors, such as retroviral vectors or bacteriophages and filamentous phage, and conjugative plasmids. Additional suitable vectors will be apparent to those of skill in the art based on the instant disclosure.
Wild TypeAs used herein the term “wild type” is a term of the art understood by skilled persons and means the typical form of an organism, strain, gene or characteristic as it occurs in nature as distinguished from mutant or variant forms.
5′ Endogenous DNA FlapAs used herein, the term “5′ endogenous DNA flap” refers to the strand of DNA situated immediately downstream of the PE-induced nick site in the target DNA. The nicking of the target DNA strand by PE exposes a 3′ hydroxyl group on the upstream side of the nick site and a 5′ hydroxyl group on the downstream side of the nick site. The endogenous strand ending in the 3′ hydroxyl group is used to prime the DNA polymerase of the prime editor (e.g., wherein the DNA polymerase is a reverse transcriptase). The endogenous strand on the downstream side of the nick site and which begins with the exposed 5′ hydroxyl group is referred to as the “5′ endogenous DNA flap” and is ultimately removed and replaced by the newly synthesized replacement strand (i.e., “3′ replacement DNA flap”) the encoded by the extension of the PEgRNA.
5′ Endogenous DNA Flap RemovalAs used herein, the term “5′ endogenous DNA flap removal” or “5′ flap removal” refers to the removal of the 5′ endogenous DNA flap that forms when the RT-synthesized single-strand DNA flap competitively invades and hybridizes to the endogenous DNA, displacing the endogenous strand in the process. Removing this endogenous displaced strand can drive the reaction towards the formation of the desired product comprising the desired nucleotide change. The cell's own DNA repair enzymes may catalyze the removal or excision of the 5′ endogenous flap (e.g., a flap endonuclease, such as EXO1 or FEN1). Also, host cells may be transformed to express one or more enzymes that catalyze the removal of said 5′ endogenous flaps, thereby driving the process toward product formation (e.g., a flap endonuclease). Flap endonucleases are known in the art and can be found described in Patel et al., “Flap endonucleases pass 5′-flaps through a flexible arch using a disorder-thread-order mechanism to confer specificity for free 5′-ends,” Nucleic Acids Research, 2012, 40(10): 4507-4519 and Tsutakawa et al., “Human flap endonuclease structures, DNA double-base flipping, and a unified understanding of the FEN1 superfamily,” Cell, 2011, 145(2): 198-211 (each of which are incorporated herein by reference).
3′ Replacement DNA FlapAs used herein, the term “3′ replacement DNA flap” or simply, “replacement DNA flap,” refers to the strand of DNA that is synthesized by the prime editor and which is encoded by the extension arm of the prime editor PEgRNA. More in particular, the 3′ replacement DNA flap is encoded by the polymerase template of the PEgRNA. The 3′ replacement DNA flap comprises the same sequence as the 5′ endogenous DNA flap except that it also contains the edited sequence (e.g., single nucleotide change). The 3′ replacement DNA flap anneals to the target DNA, displacing or replacing the 5′ endogenous DNA flap (which can be excised, for example, by a 5′ flap endonuclease, such as FEN1 or EXO1) and then is ligated to join the 3′ end of the 3′ replacement DNA flap to the exposed 5′ hydroxyl end of endogenous DNA (exposed after excision of the 5′ endogenous DNA flap, thereby reforming a phosphodiester bond and installing the 3′ replacement DNA flap to form a heteroduplex DNA containing one edited strand and one unedited strand. DNA repair processes resolve the heteroduplex by copying the information in the edited strand to the complementary strand permanently installs the edit in to the DNA. This resolution process can be driven further to completion by nicking the unedited strand, i.e., by way of “second-strand nicking,” as described herein.
DETAILED DESCRIPTION OF CERTAIN EMBODIMENTSAdoption of the clustered regularly interspaced short palindromic repeat (CRISPR) system for genome editing has revolutionized the life sciences1-3. Although gene disruption using CRISPR is now routine, the precise installation of single nucleotide edits remains a major challenge, despite being necessary for studying or correcting a large number of disease-causative mutations. Homology directed repair (HDR) is capable of achieving such edits, but suffers from low efficiency (often <5%), a requirement for donor DNA repair templates, and deleterious effects of double-stranded DNA break (DSB) formation. Recently, Prof. David Liu et al.'s laboratory developed base editing, which achieves efficient single nucleotide editing without DSBs. Base editors (BEs) combine the CRISPR system with base-modifying deaminase enzymes to convert target C●G or A●T base pairs to A●T or G●C, respectively4-6. Although already widely used by researchers worldwide, current BEs enable only four of the twelve possible base pair conversions and are unable to correct small insertions or deletions. Moreover, the targeting scope of base editing is limited by the editing of non-target C or A bases adjacent to the target base (“bystander editing”) and by the requirement that a PAM sequence exist 15±2 bp from the target base. Overcoming these limitations would therefore greatly broaden the basic research and therapeutic applications of genome editing.
The present disclosure proposes a new precision editing approach that offers many of the benefits of base editing—namely, avoidance of double strand breaks and donor DNA repair templates—while overcoming its major limitations. The proposed approach described herein achieves the direct installation of edited DNA strands at target genomic sites using target-primed reverse transcription (TPRT). In the design discussed herein, CRISPR guide RNA (gRNA) will be engineered to carry a reverse transcriptase (RT) template sequence encoding a single-stranded DNA comprising a desired nucleotide change. The CRISPR nuclease (Cas9)-nicked target site DNA will serve as the primer for reverse transcription of the template sequence on the modified gRNA, allowing for direct incorporation of any desired nucleotide edit.
Accordingly, the present invention relates in part to the discovery that the mechanism of target-primed reverse transcription (TPRT) can be leveraged or adapted for conducting precision CRISPR/Cas-based genome editing with high efficiency and genetic flexibility (e.g., as depicted in various embodiments of
[1] napDNAbp
The prime editors and trans prime editors described herein may comprise a nucleic acid programmable DNA binding protein (napDNAbp).
In one aspect, a napDNAbp can be associated with or complexed with at least one guide nucleic acid (e.g., guide RNA or a PEgRNA), which localizes the napDNAbp to a DNA sequence that comprises a DNA strand (i.e., a target strand) that is complementary to the guide nucleic acid, or a portion thereof (e.g., the spacer of a guide RNA which anneals to the protospacer of the DNA target). In other words, the guide nucleic-acid “programs” the napDNAbp (e.g., Cas9 or equivalent) to localize and bind to complementary sequence of the protospacer in the DNA.
Any suitable napDNAbp may be used in the prime editors described herein. In various embodiments, the napDNAbp may be any Class 2 CRISPR-Cas system, including any type II, type V, or type VI CRISPR-Cas enzyme. Given the rapid development of CRISPR-Cas as a tool for genome editing, there have been constant developments in the nomenclature used to describe and/or identify CRISPR-Cas enzymes, such as Cas9 and Cas9 orthologs. This application references CRISPR-Cas enzymes with nomenclature that may be old and/or new. The skilled person will be able to identify the specific CRISPR-Cas enzyme being referenced in this Application based on the nomenclature that is used, whether it is old (i.e., “legacy”) or new nomenclature. CRISPR-Cas nomenclature is extensively discussed in Makarova et al., “Classification and Nomenclature of CRISPR-Cas Systems: Where from Here?,” The CRISPR Journal, Vol. 1. No. 5, 2018, the entire contents of which are incorporated herein by reference. The particular CRISPR-Cas nomenclature used in any given instance in this Application is not limiting in any way and the skilled person will be able to identify which CRISPR-Cas enzyme is being referenced.
For example, the following type II, type V, and type VI Class 2 CRISPR-Cas enzymes have the following art-recognized old (i.e., legacy) and new names. Each of these enzymes, and/or variants thereof, may be used with the prime editors described herein:
Without being bound by theory, the mechanism of action of certain napDNAbp contemplated herein includes the step of forming an R-loop whereby the napDNAbp induces the unwinding of a double-strand DNA target, thereby separating the strands in the region bound by the napDNAbp. The guide RNA spacer then hybridizes to the “target strand” at the protospacer sequence. This displaces a “non-target strand” that is complementary to the target strand, which forms the single strand region of the R-loop. In some embodiments, the napDNAbp includes one or more nuclease activities, which then cut the DNA leaving various types of lesions. For example, the napDNAbp may comprises a nuclease activity that cuts the non-target strand at a first location, and/or cuts the target strand at a second location. Depending on the nuclease activity, the target DNA can be cut to form a “double-stranded break” whereby both strands are cut. In other embodiments, the target DNA can be cut at only a single site, i.e., the DNA is “nicked” on one strand. Exemplary napDNAbp with different nuclease activities include “Cas9 nickase” (“nCas9”) and a deactivated Cas9 having no nuclease activities (“dead Cas9” or “dCas9”).
The below description of various napDNAbps which can be used in connection with the presently disclose prime editors is not meant to be limiting in any way. The prime editors may comprise the canonical SpCas9, or any ortholog Cas9 protein, or any variant Cas9 protein—including any naturally occurring variant, mutant, or otherwise engineered version of Cas9—that is known or which can be made or evolved through a directed evolutionary or otherwise mutagenic process. In various embodiments, the Cas9 or Cas9 variants have a nickase activity, i.e., only cleave of strand of the target DNA sequence. In other embodiments, the Cas9 or Cas9 variants have inactive nucleases, i.e., are “dead” Cas9 proteins. Other variant Cas9 proteins that may be used are those having a smaller molecular weight than the canonical SpCas9 (e.g., for easier delivery) or having modified or rearranged primary amino acid structure (e.g., the circular permutant formats).
The prime editors described herein may also comprise Cas9 equivalents, including Cas12a (Cpf1) and Cas12b1 proteins which are the result of convergent evolution. The napDNAbps used herein (e.g., SpCas9, Cas9 variant, or Cas9 equivalents) may also may also contain various modifications that alter/enhance their PAM specifities. Lastly, the application contemplates any Cas9, Cas9 variant, or Cas9 equivalent which has at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 99.9% sequence identity to a reference Cas9 sequence, such as a references SpCas9 canonical sequence or a reference Cas9 equivalent (e.g., Cas12a (Cpf1)).
The napDNAbp can be a CRISPR (clustered regularly interspaced short palindromic repeat)-associated nuclease. As outlined above, CRISPR is an adaptive immune system that provides protection against mobile genetic elements (viruses, transposable elements and conjugative plasmids). CRISPR clusters contain spacers, sequences complementary to antecedent mobile elements, and target invading nucleic acids. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA). In type II CRISPR systems correct processing of pre-crRNA requires a trans-encoded small RNA (tracrRNA), endogenous ribonuclease 3 (rnc) and a Cas9 protein. The tracrRNA serves as a guide for ribonuclease 3-aided processing of pre-crRNA. Subsequently, Cas9/crRNA/tracrRNA endonucleolytically cleaves linear or circular dsDNA target complementary to the spacer. The target strand not complementary to crRNA is first cut endonucleolytically, then trimmed 3′-5′ exonucleolytically. In nature, DNA-binding and cleavage typically requires protein and both RNAs. However, single guide RNAs (“sgRNA”, or simply “gRNA”) can be engineered so as to incorporate aspects of both the crRNA and tracrRNA into a single RNA species. See, e.g., Jinek M. et al., Science 337:816-821(2012), the entire contents of which is hereby incorporated by reference.
In some embodiments, the napDNAbp directs cleavage of one or both strands at the location of a target sequence, such as within the target sequence and/or within the complement of the target sequence. In some embodiments, the napDNAbp directs cleavage of one or both strands within about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 50, 100, 200, 500, or more base pairs from the first or last nucleotide of a target sequence. In some embodiments, a vector encodes a napDNAbp that is mutated to with respect to a corresponding wild-type enzyme such that the mutated napDNAbp lacks the ability to cleave one or both strands of a target polynucleotide containing a target sequence. For example, an aspartate-to-alanine substitution (D10A) in the RuvC I catalytic domain of Cas9 from S. pyogenes converts Cas9 from a nuclease that cleaves both strands to a nickase (cleaves a single strand). Other examples of mutations that render Cas9 a nickase include, without limitation, H840A, N854A, and N863A in reference to the canonical SpCas9 sequence, or to equivalent amino acid positions in other Cas9 variants or Cas9 equivalents.
As used herein, the term “Cas protein” refers to a full-length Cas protein obtained from nature, a recombinant Cas protein having a sequences that differs from a naturally occurring Cas protein, or any fragment of a Cas protein that nevertheless retains all or a significant amount of the requisite basic functions needed for the disclosed methods, i.e., (i) possession of nucleic-acid programmable binding of the Cas protein to a target DNA, and (ii) ability to nick the target DNA sequence on one strand. The Cas proteins contemplated herein embrace CRISPR Cas 9 proteins, as well as Cas9 equivalents, variants (e.g., Cas9 nickase (nCas9) or nuclease inactive Cas9 (dCas9)) homologs, orthologs, or paralogs, whether naturally occurring or non-naturally occurring (e.g., engineered or recombinant), and may include a Cas9 equivalent from any Class 2 CRISPR system (e.g., type II, V, VI), including Cas12a (Cpf1), Cas12e (CasX), Cas12b1 (C2c1), Cas12b2, Cas12c (C2c3), C2c4, C2c8, C2c5, C2c10, C2c9 Cas13a (C2c2), Cas13d, Cas13c (C2c7), Cas13b (C2c6), and Cas13b. Further Cas-equivalents are described in Makarova et al., “C2c2 is a single-component programmable RNA-guided RNA-targeting CRISPR effector,” Science 2016; 353(6299) and Makarova et al., “Classification and Nomenclature of CRISPR-Cas Systems: Where from Here?,” The CRISPR Journal, Vol. 1. No. 5, 2018, the contents of which are incorporated herein by reference.
The terms “Cas9” or “Cas9 nuclease” or “Cas9 moiety” or “Cas9 domain” embrace any naturally occurring Cas9 from any organism, any naturally-occurring Cas9 equivalent or functional fragment thereof, any Cas9 homolog, ortholog, or paralog from any organism, and any mutant or variant of a Cas9, naturally-occurring or engineered. The term Cas9 is not meant to be particularly limiting and may be referred to as a “Cas9 or equivalent.” Exemplary Cas9 proteins are further described herein and/or are described in the art and are incorporated herein by reference. The present disclosure is unlimited with regard to the particular Cas9 that is employed in the prime editor (PE) of the invention.
As noted herein, Cas9 nuclease sequences and structures are well known to those of skill in the art (see, e.g., “Complete genome sequence of an M1 strain of Streptococcus pyogenes.” Ferretti et al., J. J., McShan W. M., Ajdic D. J., Savic D. J., Savic G., Lyon K., Primeaux C., Sezate S., Suvorov A. N., Kenton S., Lai H. S., Lin S. P., Qian Y., Jia H. G., Najar F. Z., Ren Q., Zhu H., Song L., White J., Yuan X., Clifton S. W., Roe B. A., McLaughlin R. E., Proc. Natl. Acad. Sci. U.S.A. 98:4658-4663(2001); “CRISPR RNA maturation by trans-encoded small RNA and host factor RNase III.” Deltcheva E., Chylinski K., Sharma C. M., Gonzales K., Chao Y., Pirzada Z. A., Eckert M. R., Vogel J., Charpentier E., Nature 471:602-607(2011); and “A programmable dual-RNA-guided DNA endonuclease in adaptive bacterial immunity.” Jinek M., Chylinski K., Fonfara I., Hauer M., Doudna J. A., Charpentier E. Science 337:816-821(2012), the entire contents of each of which are incorporated herein by reference).
Examples of Cas9 and Cas9 equivalents are provided as follows; however, these specific examples are not meant to be limiting. The primer editor of the present disclosure may use any suitable napDNAbp, including any suitable Cas9 or Cas9 equivalent.
A. Wild Type Canonical SpCas9
In one embodiment, the primer editor constructs described herein may comprise the “canonical SpCas9” nuclease from S. pyogenes, which has been widely used as a tool for genome engineering and is categorized as the type II subgroup of enzymes of the Class 2 CRISPR-Cas systems. This Cas9 protein is a large, multi-domain protein containing two distinct nuclease domains. Point mutations can be introduced into Cas9 to abolish one or both nuclease activities, resulting in a nickase Cas9 (nCas9) or dead Cas9 (dCas9), respectively, that still retains its ability to bind DNA in a sgRNA-programmed manner. In principle, when fused to another protein or domain, Cas9 or variant thereof (e.g., nCas9) can target that protein to virtually any DNA sequence simply by co-expression with an appropriate sgRNA. As used herein, the canonical SpCas9 protein refers to the wild type protein from Streptococcus pyogenes having the following amino acid sequence:
The prime editors described herein may include canonical SpCas9, or any variant thereof having at least 80%, at least 85%, at least 90%, at least 95%, or at least 99% sequence identity with a wild type Cas9 sequence provided above. These variants may include SpCas9 variants containing one or more mutations, including any known mutation reported with the SwissProt Accession No. Q99ZW2 (SEQ ID NO: 18) entry, which include:
Other wild type SpCas9 sequences that may be used in the present disclosure, include:
The prime editors described herein may include any of the above SpCas9 sequences, or any variant thereof having at least 80%, at least 85%, at least 90%, at least 95%, or at least 99% sequence identity thereto.
B. Wild Type Cas9 Orthologs
In other embodiments, the Cas9 protein can be a wild type Cas9 ortholog from another bacterial species different from the canonical Cas9 from S. pyogenes. For example, the following Cas9 orthologs can be used in connection with the prime editor constructs described in this specification. In addition, any variant Cas9 orthologs having at least 80%, at least 85%, at least 90%, at least 95%, or at least 99% sequence identity to any of the below orthologs may also be used with the present prime editors.
The prime editors described herein may include any of the above Cas9 ortholog sequences, or any variants thereof having at least 80%, at least 85%, at least 90%, at least 95%, or at least 99% sequence identity thereto.
The napDNAbp may include any suitable homologs and/or orthologs or naturally occurring enzymes, such as, Cas9. Cas9 homologs and/or orthologs have been described in various species, including, but not limited to, S. pyogenes and S. thermophilus. Preferably, the Cas moiety is configured (e.g, mutagenized, recombinantly engineered, or otherwise obtained from nature) as a nickase, i.e., capable of cleaving only a single strand of the target doubpdditional suitable Cas9 nucleases and sequences will be apparent to those of skill in the art based on this disclosure, and such Cas9 nucleases and sequences include Cas9 sequences from the organisms and loci disclosed in Chylinski, Rhun, and Charpentier, “The tracrRNA and Cas9 families of type II CRISPR-Cas immunity systems” (2013) RNA Biology 10:5, 726-737; the entire contents of which are incorporated herein by reference. In some embodiments, a Cas9 nuclease has an inactive (e.g., an inactivated) DNA cleavage domain, that is, the Cas9 is a nickase. In some embodiments, the Cas9 protein comprises an amino acid sequence that is at least 80% identical to the amino acid sequence of a Cas9 protein as provided by any one of the variants of Table 3. In some embodiments, the Cas9 protein comprises an amino acid sequence that is at least 85%, at least 90%, at least 92%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 99.5% identical to the amino acid sequence of a Cas9 protein as provided by any one of the Cas9 orthologs in the above tables.
C. Dead Cas9 Variant
In certain embodiments, the prime editors described herein may include a dead Cas9, e.g., dead SpCas9, which has no nuclease activity due to one or more mutations that inactive both nuclease domains of Cas9, namely the RuvC domain (which cleaves the non-protospacer DNA strand) and HNH domain (which cleaves the protospacer DNA strand). The nuclease inactivation may be due to one or mutations that result in one or more substitutions and/or deletions in the amino acid sequence of the encoded protein, or any variants thereof having at least 80%, at least 85%, at least 90%, at least 95%, or at least 99% sequence identity thereto.
As used herein, the term “dCas9” refers to a nuclease-inactive Cas9 or nuclease-dead Cas9, or a functional fragment thereof, and embraces any naturally occurring dCas9 from any organism, any naturally-occurring dCas9 equivalent or functional fragment thereof, any dCas9 homolog, ortholog, or paralog from any organism, and any mutant or variant of a dCas9, naturally-occurring or engineered. The term dCas9 is not meant to be particularly limiting and may be referred to as a “dCas9 or equivalent.” Exemplary dCas9 proteins and method for making dCas9 proteins are further described herein and/or are described in the art and are incorporated herein by reference.
In other embodiments, dCas9 corresponds to, or comprises in part or in whole, a Cas9 amino acid sequence having one or more mutations that inactivate the Cas9 nuclease activity. In other embodiments, Cas9 variants having mutations other than D10A and H840A are provided which may result in the full or partial inactivate of the endogenous Cas9 nuclease activity (e.g., nCas9 or dCas9, respectively). Such mutations, by way of example, include other amino acid substitutions at D10 and H820, or other substitutions within the nuclease domains of Cas9 (e.g., substitutions in the HNH nuclease subdomain and/or the RuvC1 subdomain) with reference to a wild type sequence such as Cas9 from Streptococcus pyogenes (NCBI Reference Sequence: NC_017053.1). In some embodiments, variants or homologues of Cas9 (e.g., variants of Cas9 from Streptococcus pyogenes (NCBI Reference Sequence: NC_017053.1 (SEQ ID NO: 20))) are provided which are at least about 70% identical, at least about 80% identical, at least about 90% identical, at least about 95% identical, at least about 98% identical, at least about 99% identical, at least about 99.5% identical, or at least about 99.9% identical to NCBI Reference Sequence: NC_017053.1. In some embodiments, variants of dCas9 (e.g., variants of NCBI Reference Sequence: NC_017053.1 (SEQ ID NO: 20)) are provided having amino acid sequences which are shorter, or longer than NC_017053.1 (SEQ ID NO: 20) by about 5 amino acids, by about 10 amino acids, by about 15 amino acids, by about 20 amino acids, by about 25 amino acids, by about 30 amino acids, by about 40 amino acids, by about 50 amino acids, by about 75 amino acids, by about 100 amino acids or more.
In one embodiment, the dead Cas9 may be based on the canonical SpCas9 sequence of Q99ZW2 and may have the following sequence, which comprises a D10X and an H810X, wherein X may be any amino acid, substitutions (underlined and bolded), or a variant be variant of SEQ ID NO: 40 having at least 80%, at least 85%, at least 90%, at least 95%, or at least 99% sequence identity thereto.
In one embodiment, the dead Cas9 may be based on the canonical SpCas9 sequence of Q99ZW2 and may have the following sequence, which comprises a D10A and an H810A substitutions (underlined and bolded), or be a variant of SEQ ID NO: 41 having at least 80%, at least 85%, at least 90%, at least 95%, or at least 99% sequence identity thereto.
D. Cas9 Nickase Variant
In one embodiment, the prime editors described herein comprise a Cas9 nickase. The term “Cas9 nickase” of “nCas9” refers to a variant of Cas9 which is capable of introducing a single-strand break in a double strand DNA molecule target. In some embodiments, the Cas9 nickase comprises only a single functioning nuclease domain. The wild type Cas9 (e.g., the canonical SpCas9) comprises two separate nuclease domains, namely, the RuvC domain (which cleaves the non-protospacer DNA strand) and HNH domain (which cleaves the protospacer DNA strand). In one embodiment, the Cas9 nickase comprises a mutation in the RuvC domain which inactivates the RuvC nuclease activity. For example, mutations in aspartate (D) 10, histidine (H) 983, aspartate (D) 986, or glutamate (E) 762, have been reported as loss-of-function mutations of the RuvC nuclease domain and the creation of a functional Cas9 nickase (e.g., Nishimasu et al., “Crystal structure of Cas9 in complex with guide RNA and target DNA,” Cell 156(5), 935-949, which is incorporated herein by reference). Thus, nickase mutations in the RuvC domain could include D10X, H983X, D986X, or E762X, wherein X is any amino acid other than the wild type amino acid. In certain embodiments, the nickase could be D10A, of H983A, or D986A, or E762A, or a combination thereof.
In various embodiments, the Cas9 nickase can having a mutation in the RuvC nuclease domain and have one of the following amino acid sequences, or a variant thereof having an amino acid sequence that has at least 80%, at least 85%, at least 90%, at least 95%, or at least 99% sequence identity thereto.
In another embodiment, the Cas9 nickase comprises a mutation in the HNH domain which inactivates the HNH nuclease activity. For example, mutations in histidine (H) 840 or asparagine (R) 863 have been reported as loss-of-function mutations of the HNH nuclease domain and the creation of a functional Cas9 nickase (e.g., Nishimasu et al., “Crystal structure of Cas9 in complex with guide RNA and target DNA,” Cell 156(5), 935-949, which is incorporated herein by reference). Thus, nickase mutations in the HNH domain could include H840X and R863X, wherein X is any amino acid other than the wild type amino acid. In certain embodiments, the nickase could be H840A or R863A or a combination thereof.
In various embodiments, the Cas9 nickase can have a mutation in the HNH nuclease domain and have one of the following amino acid sequences, or a variant thereof having an amino acid sequence that has at least 80%, at least 85%, at least 90%, at least 95%, or at least 99% sequence identity thereto.
In some embodiments, the N-terminal methionine is removed from a Cas9 nickase, or from any Cas9 variant, ortholog, or equivalent disclosed or contemplated herein. For example, methionine-minus Cas9 nickases include the following sequences, or a variant thereof having an amino acid sequence that has at least 80%, at least 85%, at least 90%, at least 95%, or at least 99% sequence identity thereto.
E. Other Cas9 Variants
Besides dead Cas9 and Cas9 nickase variants, the Cas9 proteins used herein may also include other “Cas9 variants” having at least about 70% identical, at least about 80% identical, at least about 90% identical, at least about 95% identical, at least about 96% identical, at least about 97% identical, at least about 98% identical, at least about 99% identical, at least about 99.5% identical, or at least about 99.9% identical to any reference Cas9 protein, including any wild type Cas9, or mutant Cas9 (e.g., a dead Cas9 or Cas9 nickase), or fragment Cas9, or circular permutant Cas9, or other variant of Cas9 disclosed herein or known in the art. In some embodiments, a Cas9 variant may have 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 21, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50 or more amino acid changes compared to a reference Cas9. In some embodiments, the Cas9 variant comprises a fragment of a reference Cas9 (e.g., a gRNA binding domain or a DNA-cleavage domain), such that the fragment is at least about 70% identical, at least about 80% identical, at least about 90% identical, at least about 95% identical, at least about 96% identical, at least about 97% identical, at least about 98% identical, at least about 99% identical, at least about 99.5% identical, or at least about 99.9% identical to the corresponding fragment of wild type Cas9. In some embodiments, the fragment is at least 30%, at least 35%, at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95% identical, at least 96%, at least 97%, at least 98%, at least 99%, or at least 99.5% of the amino acid length of a corresponding wild type Cas9 (e.g., SEQ ID NO: 18).
In some embodiments, the disclosure also may utilize Cas9 fragments which retain their functionality and which are fragments of any herein disclosed Cas9 protein. In some embodiments, the Cas9 fragment is at least 100 amino acids in length. In some embodiments, the fragment is at least 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1050, 1100, 1150, 1200, 1250, or at least 1300 amino acids in length.
In various embodiments, the prime editors disclosed herein may comprise one of the Cas9 variants described as follows, or a Cas9 variant thereof having at least about 70% identical, at least about 80% identical, at least about 90% identical, at least about 95% identical, at least about 96% identical, at least about 97% identical, at least about 98% identical, at least about 99% identical, at least about 99.5% identical, or at least about 99.9% identical to any reference Cas9 variants.
F. Small-Sized Cas9 Variants
In some embodiments, the prime editors contemplated herein can include a Cas9 protein that is of smaller molecular weight than the canonical SpCas9 sequence. In some embodiments, the smaller-sized Cas9 variants may facilitate delivery to cells, e.g., by an expression vector, nanoparticle, or other means of delivery. In certain embodiments, the smaller-sized Cas9 variants can include enzymes categorized as type II enzymes of the Class 2 CRISPR-Cas systems. In some embodiments, the smaller-sized Cas9 variants can include enzymes categorized as type V enzymes of the Class 2 CRISPR-Cas systems. In other embodiments, the smaller-sized Cas9 variants can include enzymes categorized as type VI enzymes of the Class 2 CRISPR-Cas systems.
The canonical SpCas9 protein is 1368 amino acids in length and has a predicted molecular weight of 158 kilodaltons. The term “small-sized Cas9 variant”, as used herein, refers to any Cas9 variant—naturally occurring, engineered, or otherwise—that is less than at least 1300 amino acids, or at least less than 1290 amino acids, or than less than 1280 amino acids, or less than 1270 amino acid, or less than 1260 amino acid, or less than 1250 amino acids, or less than 1240 amino acids, or less than 1230 amino acids, or less than 1220 amino acids, or less than 1210 amino acids, or less than 1200 amino acids, or less than 1190 amino acids, or less than 1180 amino acids, or less than 1170 amino acids, or less than 1160 amino acids, or less than 1150 amino acids, or less than 1140 amino acids, or less than 1130 amino acids, or less than 1120 amino acids, or less than 1110 amino acids, or less than 1100 amino acids, or less than 1050 amino acids, or less than 1000 amino acids, or less than 950 amino acids, or less than 900 amino acids, or less than 850 amino acids, or less than 800 amino acids, or less than 750 amino acids, or less than 700 amino acids, or less than 650 amino acids, or less than 600 amino acids, or less than 550 amino acids, or less than 500 amino acids, but at least larger than about 400 amino acids and retaining the required functions of the Cas9 protein. The Cas9 variants can include those categorized as type II, type V, or type VI enzymes of the Class 2 CRISPR-Cas system.
In various embodiments, the prime editors disclosed herein may comprise one of the small-sized Cas9 variants described as follows, or a Cas9 variant thereof having at least about 70% identical, at least about 80% identical, at least about 90% identical, at least about 95% identical, at least about 96% identical, at least about 97% identical, at least about 98% identical, at least about 99% identical, at least about 99.5% identical, or at least about 99.9% identical to any reference small-sized Cas9 protein.
G. Cas9 Equivalents
In some embodiments, the prime editors described herein can include any Cas9 equivalent. As used herein, the term “Cas9 equivalent” is a broad term that encompasses any napDNAbp protein that serves the same function as Cas9 in the present prime editors despite that its amino acid primary sequence and/or its three-dimensional structure may be different and/or unrelated from an evolutionary standpoint. Thus, while Cas9 equivalents include any Cas9 ortholog, homolog, mutant, or variant described or embraced herein that are evolutionarily related, the Cas9 equivalents also embrace proteins that may have evolved through convergent evolution processes to have the same or similar function as Cas9, but which do not necessarily have any similarity with regard to amino acid sequence and/or three dimensional structure. The prime editors described here embrace any Cas9 equivalent that would provide the same or similar function as Cas9 despite that the Cas9 equivalent may be based on a protein that arose through convergent evolution. For instance, if Cas9 refers to a type II enzyme of the CRISPR-Cas system, a Cas9 equivalent can refer to a type V or type VI enzyme of the CRISPR-Cas system.
For example, Cas12e (CasX) is a Cas9 equivalent that reportedly has the same function as Cas9 but which evolved through convergent evolution. Thus, the Cas12e (CasX) protein described in Liu et al., “CasX enzymes comprises a distinct family of RNA-guided genome editors,” Nature, 2019, Vol. 566: 218-223, is contemplated to be used with the prime editors described herein. In addition, any variant or modification of Cas12e (CasX) is conceivable and within the scope of the present disclosure.
Cas9 is a bacterial enzyme that evolved in a wide variety of species. However, the Cas9 equivalents contemplated herein may also be obtained from archaea, which constitute a domain and kingdom of single-celled prokaryotic microbes different from bacteria.
In some embodiments, Cas9 equivalents may refer to Cas12e (CasX) or Cas12d (CasY), which have been described in, for example, Burstein et al., “New CRISPR-Cas systems from uncultivated microbes.” Cell Res. 2017 Feb. 21. doi: 10.1038/cr.2017.21, the entire contents of which is hereby incorporated by reference. Using genome-resolved metagenomics, a number of CRISPR-Cas systems were identified, including the first reported Cas9 in the archaeal domain of life. This divergent Cas9 protein was found in little-studied nanoarchaea as part of an active CRISPR-Cas system. In bacteria, two previously unknown systems were discovered, CRISPR-Cas12e and CRISPR-Cas12d, which are among the most compact systems yet discovered. In some embodiments, Cas9 refers to Cas12e, or a variant of Cas12e. In some embodiments, Cas9 refers to a Cas12d, or a variant of Cas12d. It should be appreciated that other RNA-guided DNA binding proteins may be used as a nucleic acid programmable DNA binding protein (napDNAbp), and are within the scope of this disclosure. Also see Liu et al., “CasX enzymes comprises a distinct family of RNA-guided genome editors,” Nature, 2019, Vol. 566: 218-223. Any of these Cas9 equivalents are contemplated.
In some embodiments, the Cas9 equivalent comprises an amino acid sequence that is at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 99.5% identical to a naturally-occurring Cas12e (CasX) or Cas12d (CasY) protein. In some embodiments, the napDNAbp is a naturally-occurring Cas12e (CasX) or Cas12d (CasY) protein. In some embodiments, the napDNAbp comprises an amino acid sequence that is at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 99.5% identical to a wild-type Cas moiety or any Cas moiety provided herein.
In various embodiments, the nucleic acid programmable DNA binding proteins include, without limitation, Cas9 (e.g., dCas9 and nCas9), Cas12e (CasX), Cas12d (CasY), Cas12a (Cpf1), Cas12b1 (C2c1), Cas13a (C2c2), Cas12c (C2c3), Argonaute, and Cas12b1. One example of a nucleic acid programmable DNA-binding protein that has different PAM specificity than Cas9 is Clustered Regularly Interspaced Short Palindromic Repeats from Prevotella and Francisella 1 (i.e, Cas12a (Cpf1)). Similar to Cas9, Cas12a (Cpf1) is also a Class 2 CRISPR effector, but it is a member of type V subgroup of enzymes, rather than the type II subgroup. It has been shown that Cas12a (Cpf1) mediates robust DNA interference with features distinct from Cas9. Cas12a (Cpf1) is a single RNA-guided endonuclease lacking tracrRNA, and it utilizes a T-rich protospacer-adjacent motif (TTN, TTTN, or YTN). Moreover, Cpf1 cleaves DNA via a staggered DNA double-stranded break. Out of 16 Cpf1-family proteins, two enzymes from Acidaminococcus and Lachnospiraceae are shown to have efficient genome-editing activity in human cells. Cpf1 proteins are known in the art and have been described previously, for example Yamano et al., “Crystal structure of Cpf1 in complex with guide RNA and target DNA.” Cell (165) 2016, p. 949-962; the entire contents of which is hereby incorporated by reference.
In still other embodiments, the Cas protein may include any CRISPR associated protein, including but not limited to, Cas12a, Cas12b1, Cas1, Cas1B, Cas2, Cas3, Cas4, Cas5, Cas6, Cas7, Cas8, Cas9 (also known as Csn1 and Csx12), Cas10, Csy1, Csy2, Csy3, Cse1, Cse2, Csc1, Csc2, Csa5, Csn2, Csm2, Csm3, Csm4, Csm5, Csm6, Cmr1, Cmr3, Cmr4, Cmr5, Cmr6, Csb1, Csb2, Csb3, Csx17, Csx14, Csx10, Csx16, CsaX, Csx3, Csx1, Csx15, Csf1, Csf2, Csf3, Csf4, homologs thereof, or modified versions thereof, and preferably comprising a nickase mutation (e.g., a mutation corresponding to the D10A mutation of the wild type Cas9 polypeptide of SEQ ID NO: 18).
In various other embodiments, the napDNAbp can be any of the following proteins: a Cas9, a Cas12a (Cpf1), a Cas12e (CasX), a Cas12d (CasY), a Cas12b1 (C2c1), a Cas13a (C2c2), a Cas12c (C2c3), a GeoCas9, a CjCas9, a Cas12g, a Cas12h, a Cas12i, a Cas13b, a Cas13c, a Cas13d, a Cas14, a Csn2, an xCas9, an SpCas9-NG, a circularly permuted Cas9, or an Argonaute (Ago) domain, or a variant thereof.
Exemplary Cas9 equivalent protein sequences can include the following:
The prime editors described herein may also comprise Cas12a (Cpf1) (dCpf1) variants that may be used as a guide nucleotide sequence-programmable DNA-binding protein domain. The Cas12a (Cpf1) protein has a RuvC-like endonuclease domain that is similar to the RuvC domain of Cas9 but does not have a HNH endonuclease domain, and the N-terminal of Cas12a (Cpf1) does not have the alfa-helical recognition lobe of Cas9. It was shown in Zetsche et al., Cell, 163,759-771,2015 (which is incorporated herein by reference) that, the RuvC-like domain of Cas12a (Cpf1) is responsible for cleaving both DNA strands and inactivation of the RuvC-like domain inactivates Cas12a (Cpf1) nuclease activity.
In some embodiments, the napDNAbp is a single effector of a microbial CRISPR-Cas system. Single effectors of microbial CRISPR-Cas systems include, without limitation, Cas9, Cas12a (Cpf1), Cas12b1 (C2c1), Cas13a (C2c2), and Cas12c (C2c3). Typically, microbial CRISPR-Cas systems are divided into Class 1 and Class 2 systems. Class 1 systems have multisubunit effector complexes, while Class 2 systems have a single protein effector. For example, Cas9 and Cas12a (Cpf1) are Class 2 effectors. In addition to Cas9 and Cas12a (Cpf1), three distinct Class 2 CRISPR-Cas systems (Cas12b1, Cas13a, and Cas12c) have been described by Shmakov et al., “Discovery and Functional Characterization of Diverse Class 2 CRISPR Cas Systems”, Mol. Cell, 2015 Nov. 5; 60(3): 385-397, the entire contents of which are hereby incorporated by reference.
Effectors of two of the systems, Cas12b1 and Cas12c, contain RuvC-like endonuclease domains related to Cas12a. A third system, Cas13a contains an effector with two predicated HEPN RNase domains. Production of mature CRISPR RNA is tracrRNA-independent, unlike production of CRISPR RNA by Cas12b1. Cas12b1 depends on both CRISPR RNA and tracrRNA for DNA cleavage. Bacterial Cas13a has been shown to possess a unique RNase activity for CRISPR RNA maturation distinct from its RNA-activated single-stranded RNA degradation activity. These RNase functions are different from each other and from the CRISPR RNA-processing behavior of Cas12a. See, e.g., East-Seletsky, et al., “Two distinct RNase activities of CRISPR-Cas13a enable guide-RNA processing and RNA detection”, Nature, 2016 Oct. 13; 538(7624):270-273, the entire contents of which are hereby incorporated by reference. In vitro biochemical analysis of Cas13a in Leptotrichia shahii has shown that Cas13a is guided by a single CRISPR RNA and can be programed to cleave ssRNA targets carrying complementary protospacers. Catalytic residues in the two conserved HEPN domains mediate cleavage. Mutations in the catalytic residues generate catalytically inactive RNA-binding proteins. See e.g., Abudayyeh et al., “C2c2 is a single-component programmable RNA-guided RNA-targeting CRISPR effector”, Science, 2016 Aug. 5; 353(6299), the entire contents of which are hereby incorporated by reference.
The crystal structure of Alicyclobacillus acidoterrastris Cas12b1 (AacC2c1) has been reported in complex with a chimeric single-molecule guide RNA (sgRNA). See e.g., Liu et al., “C2c1-sgRNA Complex Structure Reveals RNA-Guided DNA Cleavage Mechanism”, Mol. Cell, 2017 Jan. 19; 65(2):310-322, the entire contents of which are hereby incorporated by reference. The crystal structure has also been reported in Alicyclobacillus acidoterrestris C2c1 bound to target DNAs as ternary complexes. See e.g., Yang et al., “PAM-dependent Target DNA Recognition and Cleavage by C2C1 CRISPR-Cas endonuclease”, Cell, 2016 Dec. 15; 167(7):1814-1828, the entire contents of which are hereby incorporated by reference. Catalytically competent conformations of AacC2c1, both with target and non-target DNA strands, have been captured independently positioned within a single RuvC catalytic pocket, with C2c1-mediated cleavage resulting in a staggered seven-nucleotide break of target DNA. Structural comparisons between C2c1 ternary complexes and previously identified Cas9 and Cpf1 counterparts demonstrate the diversity of mechanisms used by CRISPR-Cas9 systems.
In some embodiments, the napDNAbp may be a C2c1, a C2c2, or a C2c3 protein. In some embodiments, the napDNAbp is a C2c1 protein. In some embodiments, the napDNAbp is a Cas13a protein. In some embodiments, the napDNAbp is a Cas12c protein. In some embodiments, the napDNAbp comprises an amino acid sequence that is at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 99.5% identical to a naturally-occurring Cas12b1 (C2c1), Cas13a (C2c2), or Cas12c (C2c3) protein. In some embodiments, the napDNAbp is a naturally-occurring Cas12b1 (C2c1), Cas13a (C2c2), or Cas12c (C2c3) protein.
H. Cas9 Circular Permutants
In various embodiments, the prime editors disclosed herein may comprise a circular permutant of Cas9.
The term “circularly permuted Cas9” or “circular permutant” of Cas9 or “CP-Cas9”) refers to any Cas9 protein, or variant thereof, that occurs or has been modify to engineered as a circular permutant variant, which means the N-terminus and the C-terminus of a Cas9 protein (e.g., a wild type Cas9 protein) have been topically rearranged. Such circularly permuted Cas9 proteins, or variants thereof, retain the ability to bind DNA when complexed with a guide RNA (gRNA). See, Oakes et al., “Protein Engineering of Cas9 for enhanced function,” Methods Enzymol, 2014, 546: 491-511 and Oakes et al., “CRISPR-Cas9 Circular Permutants as Programmable Scaffolds for Genome Modification,” Cell, Jan. 10, 2019, 176: 254-267, each of are incorporated herein by reference. The instant disclosure contemplates any previously known CP-Cas9 or use a new CP-Cas9 so long as the resulting circularly permuted protein retains the ability to bind DNA when complexed with a guide RNA (gRNA).
Any of the Cas9 proteins described herein, including any variant, ortholog, or naturally occurring Cas9 or equivalent thereof, may be reconfigured as a circular permutant variant.
In various embodiments, the circular permutants of Cas9 may have the following structure:
N-terminus-[original C-terminus]-[optional linker]-[original N-terminus]-C-terminus.
As an example, the present disclosure contemplates the following circular permutants of canonical S. pyogenes Cas9 (1368 amino acids of UniProtKB—Q99ZW2 (CAS9_STRP1) (numbering is based on the amino acid position in SEQ ID NO: 18)):
N-terminus-[1268-1368]-[optional linker]-[1-1267]-C-terminus;
N-terminus-[1168-1368]-[optional linker]-[1-1167]-C-terminus;
N-terminus-[1068-1368]-[optional linker]-[1-1067]-C-terminus;
N-terminus-[968-1368]-[optional linker]-[1-967]-C-terminus;
N-terminus-[868-1368]-[optional linker]-[1-867]-C-terminus;
N-terminus-[768-1368]-[optional linker]-[1-767]-C-terminus;
N-terminus-[668-1368]-[optional linker]-[1-667]-C-terminus;
N-terminus-[568-1368]-[optional linker]-[1-567]-C-terminus;
N-terminus-[468-1368]-[optional linker]-[1-467]-C-terminus;
N-terminus-[368-1368]-[optional linker]-[1-367]-C-terminus;
N-terminus-[268-1368]-[optional linker]-[1-267]-C-terminus;
N-terminus-[168-1368]-[optional linker]-[1-167]-C-terminus;
N-terminus-[68-1368]-[optional linker]-[1-67]-C-terminus; or
N-terminus-[10-1368]-[optional linker]-[1-9]-C-terminus, or the corresponding circular permutants of other Cas9 proteins (including other Cas9 orthologs, variants, etc).
In particular embodiments, the circular permutant Cas9 has the following structure (based on S. pyogenes Cas9 (1368 amino acids of UniProtKB—Q99ZW2 (CAS9_STRP1) (numbering is based on the amino acid position in SEQ ID NO: 18):
N-terminus-[102-1368]-[optional linker]-[1-101]-C-terminus;
N-terminus-[1028-1368]-[optional linker]-[1-1027]-C-terminus;
N-terminus-[1041-1368]-[optional linker]-[1-1043]-C-terminus;
N-terminus-[1249-1368]-[optional linker]-[1-1248]-C-terminus; or
N-terminus-[1300-1368]-[optional linker]-[1-1299]-C-terminus, or the corresponding circular permutants of other Cas9 proteins (including other Cas9 orthologs, variants, etc).
In still other embodiments, the circular permutant Cas9 has the following structure (based on S. pyogenes Cas9 (1368 amino acids of UniProtKB—Q99ZW2 (CAS9_STRP1) (numbering is based on the amino acid position in SEQ ID NO: 18):
N-terminus-[103-1368]-[optional linker]-[1-102]-C-terminus;
N-terminus-[1029-1368]-[optional linker]-[1-1028]-C-terminus;
N-terminus-[1042-1368]-[optional linker]-[1-1041]-C-terminus;
N-terminus-[1250-1368]-[optional linker]-[1-1249]-C-terminus; or
N-terminus-[1301-1368]-[optional linker]-[1-1300]-C-terminus, or the corresponding circular permutants of other Cas9 proteins (including other Cas9 orthologs, variants, etc).
In some embodiments, the circular permutant can be formed by linking a C-terminal fragment of a Cas9 to an N-terminal fragment of a Cas9, either directly or by using a linker, such as an amino acid linker. In some embodiments, The C-terminal fragment may correspond to the C-terminal 95% or more of the amino acids of a Cas9 (e.g., amino acids about 1300-1368), or the C-terminal 90%, 85%, 80%, 75%, 70%, 65%, 60%, 55%, 50%, 45%, 40%, 35%, 30%, 25%, 20%, 15%, 10%, or 5% or more of a Cas9 (e.g., any one of SEQ ID NOs: 77-86). The N-terminal portion may correspond to the N-terminal 95% or more of the amino acids of a Cas9 (e.g., amino acids about 1-1300), or the N-terminal 90%, 85%, 80%, 75%, 70%, 65%, 60%, 55%, 50%, 45%, 40%, 35%, 30%, 25%, 20%, 15%, 10%, or 5% or more of a Cas9 (e.g., of SEQ ID NO: 18).
In some embodiments, the circular permutant can be formed by linking a C-terminal fragment of a Cas9 to an N-terminal fragment of a Cas9, either directly or by using a linker, such as an amino acid linker. In some embodiments, the C-terminal fragment that is rearranged to the N-terminus, includes or corresponds to the C-terminal 30% or less of the amino acids of a Cas9 (e.g., amino acids 1012-1368 of SEQ ID NO: 18). In some embodiments, the C-terminal fragment that is rearranged to the N-terminus, includes or corresponds to the C-terminal 30%, 29%, 28%, 27%, 26%, 25%, 24%, 23%, 22%, 21%, 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1% of the amino acids of a Cas9 (e.g., the Cas9 of SEQ ID NO: 18). In some embodiments, the C-terminal fragment that is rearranged to the N-terminus, includes or corresponds to the C-terminal 410 residues or less of a Cas9 (e.g., the Cas9 of SEQ ID NO: 18). In some embodiments, the C-terminal portion that is rearranged to the N-terminus, includes or corresponds to the C-terminal 410, 400, 390, 380, 370, 360, 350, 340, 330, 320, 310, 300, 290, 280, 270, 260, 250, 240, 230, 220, 210, 200, 190, 180, 170, 160, 150, 140, 130, 120, 110, 100, 90, 80, 70, 60, 50, 40, 30, 20, or 10 residues of a Cas9 (e.g., the Cas9 of SEQ ID NO: 18). In some embodiments, the C-terminal portion that is rearranged to the N-terminus, includes or corresponds to the C-terminal 357, 341, 328, 120, or 69 residues of a Cas9 (e.g., the Cas9 of SEQ ID NO: 18).
In other embodiments, circular permutant Cas9 variants may be defined as a topological rearrangement of a Cas9 primary structure based on the following method, which is based on S. pyogenes Cas9 of SEQ ID NO: 18: (a) selecting a circular permutant (CP) site corresponding to an internal amino acid residue of the Cas9 primary structure, which dissects the original protein into two halves: an N-terminal region and a C-terminal region; (b) modifying the Cas9 protein sequence (e.g., by genetic engineering techniques) by moving the original C-terminal region (comprising the CP site amino acid) to precede the original N-terminal region, thereby forming a new N-terminus of the Cas9 protein that now begins with the CP site amino acid residue. The CP site can be located in any domain of the Cas9 protein, including, for example, the helical-II domain, the RuvCIII domain, or the CTD domain. For example, the CP site may be located (relative the S. pyogenes Cas9 of SEQ ID NO: 18) at original amino acid residue 181, 199, 230, 270, 310, 1010, 1016, 1023, 1029, 1041, 1247, 1249, or 1282. Thus, once relocated to the N-terminus, original amino acid 181, 199, 230, 270, 310, 1010, 1016, 1023, 1029, 1041, 1247, 1249, or 1282 would become the new N-terminal amino acid. Nomenclature of these CP-Cas9 proteins may be referred to as Cas9-CP181, Cas9-CP199, Cas9-CP230, Cas9-CP270, Cas9-CP310, Cas9-CP1010, Cas9-CP1016, Cas9-CP1023, Cas9-CP1029, Cas9_CP1041, Cas9-CP1247, Cas9_CP1249, and Cas9-CP1282, respectively. This description is not meant to be limited to making CP variants from SEQ ID NO: 18, but may be implemented to make CP variants in any Cas9 sequence, either at CP sites that correspond to these positions, or at other CP sites entirely. This description is not meant to limit the specific CP sites in any way. Virtually any CP site may be used to form a CP-Cas9 variant.
Exemplary CP-Cas9 amino acid sequences, based on the Cas9 of SEQ ID NO: 18, are provided below in which linker sequences are indicated by underlining and optional methionine (M) residues are indicated in bold. It should be appreciated that the disclosure provides CP-Cas9 sequences that do not include a linker sequence or that include different linker sequences. It should be appreciated that CP-Cas9 sequences may be based on Cas9 sequences other than that of SEQ ID NO: 18 and any examples provided herein are not meant to be limiting. Exemplary CP-Cas9 sequences are as follows:
The Cas9 circular permutants that may be useful in the prime editing constructs described herein. Exemplary C-terminal fragments of Cas9, based on the Cas9 of SEQ ID NO: 18, which may be rearranged to an N-terminus of Cas9, are provided below. It should be appreciated that such C-terminal fragments of Cas9 are exemplary and are not meant to be limiting. These exemplary CP-Cas9 fragments have the following sequences:
I. Cas9 Variants with Modified PAM Specificities
The prime editors of the present disclosure may also comprise Cas9 variants with modified PAM specificities. Some aspects of this disclosure provide Cas9 proteins that exhibit activity on a target sequence that does not comprise the canonical PAM (5′-NGG-3′, where N is A, C, G, or T) at its 3′-end. In some embodiments, the Cas9 protein exhibits activity on a target sequence comprising a 5′-NGG-3′ PAM sequence at its 3′-end. In some embodiments, the Cas9 protein exhibits activity on a target sequence comprising a 5′-NNG-3′ PAM sequence at its 3′-end. In some embodiments, the Cas9 protein exhibits activity on a target sequence comprising a 5′-NNA-3′ PAM sequence at its 3′-end. In some embodiments, the Cas9 protein exhibits activity on a target sequence comprising a 5′-NNC-3′ PAM sequence at its 3′-end. In some embodiments, the Cas9 protein exhibits activity on a target sequence comprising a 5′-NNT-3′ PAM sequence at its 3′-end. In some embodiments, the Cas9 protein exhibits activity on a target sequence comprising a 5′-NGT-3′ PAM sequence at its 3′-end. In some embodiments, the Cas9 protein exhibits activity on a target sequence comprising a 5′-NGA-3′ PAM sequence at its 3′-end. In some embodiments, the Cas9 protein exhibits activity on a target sequence comprising a 5′-NGC-3′ PAM sequence at its 3′-end. In some embodiments, the Cas9 protein exhibits activity on a target sequence comprising a 5′-NAA-3′ PAM sequence at its 3′-end. In some embodiments, the Cas9 protein exhibits activity on a target sequence comprising a 5′-NAC-3′ PAM sequence at its 3′-end. In some embodiments, the Cas9 protein exhibits activity on a target sequence comprising a 5′-NAT-3′ PAM sequence at its 3′-end. In still other embodiments, the Cas9 protein exhibits activity on a target sequence comprising a 5′-NAG-3′ PAM sequence at its 3′-end.
It should be appreciated that any of the amino acid mutations described herein, (e.g., A262T) from a first amino acid residue (e.g., A) to a second amino acid residue (e.g., T) may also include mutations from the first amino acid residue to an amino acid residue that is similar to (e.g., conserved) the second amino acid residue. For example, mutation of an amino acid with a hydrophobic side chain (e.g., alanine, valine, isoleucine, leucine, methionine, phenylalanine, tyrosine, or tryptophan) may be a mutation to a second amino acid with a different hydrophobic side chain (e.g., alanine, valine, isoleucine, leucine, methionine, phenylalanine, tyrosine, or tryptophan). For example, a mutation of an alanine to a threonine (e.g., a A262T mutation) may also be a mutation from an alanine to an amino acid that is similar in size and chemical properties to a threonine, for example, serine. As another example, mutation of an amino acid with a positively charged side chain (e.g., arginine, histidine, or lysine) may be a mutation to a second amino acid with a different positively charged side chain (e.g., arginine, histidine, or lysine). As another example, mutation of an amino acid with a polar side chain (e.g., serine, threonine, asparagine, or glutamine) may be a mutation to a second amino acid with a different polar side chain (e.g., serine, threonine, asparagine, or glutamine). Additional similar amino acid pairs include, but are not limited to, the following: phenylalanine and tyrosine; asparagine and glutamine; methionine and cysteine; aspartic acid and glutamic acid; and arginine and lysine. The skilled artisan would recognize that such conservative amino acid substitutions will likely have minor effects on protein structure and are likely to be well tolerated without compromising function. In some embodiments, any amino of the amino acid mutations provided herein from one amino acid to a threonine may be an amino acid mutation to a serine. In some embodiments, any amino of the amino acid mutations provided herein from one amino acid to an arginine may be an amino acid mutation to a lysine. In some embodiments, any amino of the amino acid mutations provided herein from one amino acid to an isoleucine, may be an amino acid mutation to an alanine, valine, methionine, or leucine. In some embodiments, any amino of the amino acid mutations provided herein from one amino acid to a lysine may be an amino acid mutation to an arginine. In some embodiments, any amino of the amino acid mutations provided herein from one amino acid to an aspartic acid may be an amino acid mutation to a glutamic acid or asparagine. In some embodiments, any amino of the amino acid mutations provided herein from one amino acid to a valine may be an amino acid mutation to an alanine, isoleucine, methionine, or leucine. In some embodiments, any amino of the amino acid mutations provided herein from one amino acid to a glycine may be an amino acid mutation to an alanine. It should be appreciated, however, that additional conserved amino acid residues would be recognized by the skilled artisan and any of the amino acid mutations to other conserved amino acid residues are also within the scope of this disclosure.
In some embodiments, the Cas9 protein comprises a combination of mutations that exhibit activity on a target sequence comprising a 5′-NAA-3′ PAM sequence at its 3′-end. In some embodiments, the combination of mutations are present in any one of the clones listed in Table 1. In some embodiments, the combination of mutations are conservative mutations of the clones listed in Table 1. In some embodiments, the Cas9 protein comprises the combination of mutations of any one of the Cas9 clones listed in Table 1.
In some embodiments, the Cas9 protein comprises an amino acid sequence that is at least 80% identical to the amino acid sequence of a Cas9 protein as provided by any one of the variants of Table 1. In some embodiments, the Cas9 protein comprises an amino acid sequence that is at least 85%, at least 90%, at least 92%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 99.5% identical to the amino acid sequence of a Cas9 protein as provided by any one of the variants of Table 1.
In some embodiments, the Cas9 protein exhibits an increased activity on a target sequence that does not comprise the canonical PAM (5′-NGG-3′) at its 3′ end as compared to Streptococcus pyogenes Cas9 as provided by SEQ ID NO: 18. In some embodiments, the Cas9 protein exhibits an activity on a target sequence having a 3′ end that is not directly adjacent to the canonical PAM sequence (5′-NGG-3′) that is at least 5-fold increased as compared to the activity of Streptococcus pyogenes Cas9 as provided by SEQ ID NO: 18 on the same target sequence. In some embodiments, the Cas9 protein exhibits an activity on a target sequence that is not directly adjacent to the canonical PAM sequence (5′-NGG-3′) that is at least 10-fold, at least 50-fold, at least 100-fold, at least 500-fold, at least 1,000-fold, at least 5,000-fold, at least 10,000-fold, at least 50,000-fold, at least 100,000-fold, at least 500,000-fold, or at least 1,000,000-fold increased as compared to the activity of Streptococcus pyogenes as provided by SEQ ID NO: 2 on the same target sequence. In some embodiments, the 3′ end of the target sequence is directly adjacent to an AAA, GAA, CAA, or TAA sequence. In some embodiments, the Cas9 protein comprises a combination of mutations that exhibit activity on a target sequence comprising a 5′-NAC-3′ PAM sequence at its 3′-end. In some embodiments, the combination of mutations are present in any one of the clones listed in Table 2. In some embodiments, the combination of mutations are conservative mutations of the clones listed in Table 2. In some embodiments, the Cas9 protein comprises the combination of mutations of any one of the Cas9 clones listed in Table 2.
In some embodiments, the Cas9 protein comprises an amino acid sequence that is at least 80% identical to the amino acid sequence of a Cas9 protein as provided by any one of the variants of Table 2. In some embodiments, the Cas9 protein comprises an amino acid sequence that is at least 85%, at least 90%, at least 92%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 99.5% identical to the amino acid sequence of a Cas9 protein as provided by any one of the variants of Table 2.
In some embodiments, the Cas9 protein exhibits an increased activity on a target sequence that does not comprise the canonical PAM (5′-NGG-3′) at its 3′ end as compared to Streptococcus pyogenes Cas9 as provided by SEQ ID NO: 18. In some embodiments, the Cas9 protein exhibits an activity on a target sequence having a 3′ end that is not directly adjacent to the canonical PAM sequence (5′-NGG-3′) that is at least 5-fold increased as compared to the activity of Streptococcus pyogenes Cas9 as provided by SEQ ID NO: 18 on the same target sequence. In some embodiments, the Cas9 protein exhibits an activity on a target sequence that is not directly adjacent to the canonical PAM sequence (5′-NGG-3′) that is at least 10-fold, at least 50-fold, at least 100-fold, at least 500-fold, at least 1,000-fold, at least 5,000-fold, at least 10,000-fold, at least 50,000-fold, at least 100,000-fold, at least 500,000-fold, or at least 1,000,000-fold increased as compared to the activity of Streptococcus pyogenes as provided by SEQ ID NO: 18 on the same target sequence. In some embodiments, the 3′ end of the target sequence is directly adjacent to an AAC, GAC, CAC, or TAC sequence.
In some embodiments, the Cas9 protein comprises a combination of mutations that exhibit activity on a target sequence comprising a 5′-NAT-3′ PAM sequence at its 3′-end. In some embodiments, the combination of mutations are present in any one of the clones listed in Table 3. In some embodiments, the combination of mutations are conservative mutations of the clones listed in Table 3. In some embodiments, the Cas9 protein comprises the combination of mutations of any one of the Cas9 clones listed in Table 3.
The above description of various napDNAbps which can be used in connection with the presently disclose prime editors is not meant to be limiting in any way. The prime editors may comprise the canonical SpCas9, or any ortholog Cas9 protein, or any variant Cas9 protein—including any naturally occurring variant, mutant, or otherwise engineered version of Cas9—that is known or which can be made or evolved through a directed evolutionary or otherwise mutagenic process. In various embodiments, the Cas9 or Cas9 variants have a nickase activity, i.e., only cleave of strand of the target DNA sequence. In other embodiments, the Cas9 or Cas9 variants have inactive nucleases, i.e., are “dead” Cas9 proteins. Other variant Cas9 proteins that may be used are those having a smaller molecular weight than the canonical SpCas9 (e.g., for easier delivery) or having modified or rearranged primary amino acid structure (e.g., the circular permutant formats). The prime editors described herein may also comprise Cas9 equivalents, including Cas12a/Cpf1 and Cas12b proteins which are the result of convergent evolution. The napDNAbps used herein (e.g., SpCas9, Cas9 variant, or Cas9 equivalents) may also may also contain various modifications that alter/enhance their PAM specifities. Lastly, the application contemplates any Cas9, Cas9 variant, or Cas9 equivalent which has at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 99.9% sequence identity to a reference Cas9 sequence, such as a references SpCas9 canonical sequences or a reference Cas9 equivalent (e.g., Cas12a/Cpf1).
In a particular embodiment, the Cas9 variant having expanded PAM capabilities is SpCas9 (H840A) VRQR (SEQ ID NO: 87), which has the following amino acid sequence (with the V, R, Q, R substitutions relative to the SpCas9 (H840A) of SEQ ID NO: 51 being show in bold underline. In addition, the methionine residue in SpCas9 (H840) was removed for SpCas9 (H840A) VRQR):
In another particular embodiment, the Cas9 variant having expanded PAM capabilities is SpCas9 (H840A) VRER, which has the following amino acid sequence (with the V, R, E, R substitutions relative to the SpCas9 (H840A) of SEQ ID NO: 51 being shown in bold underline. In addition, the methionine residue in SpCas9 (H840) was removed for SpCas9 (H840A) VRER):
In some embodiments, the napDNAbp that functions with a non-canonical PAM sequence is an Argonaute protein. One example of such a nucleic acid programmable DNA binding protein is an Argonaute protein from Natronobacterium gregoryi (NgAgo). NgAgo is a ssDNA-guided endonuclease. NgAgo binds 5′ phosphorylated ssDNA of ˜24 nucleotides (gDNA) to guide it to its target site and will make DNA double-strand breaks at the gDNA site. In contrast to Cas9, the NgAgo-gDNA system does not require a protospacer-adjacent motif (PAM). Using a nuclease inactive NgAgo (dNgAgo) can greatly expand the bases that may be targeted. The characterization and use of NgAgo have been described in Gao et al., Nat Biotechnol., 2016 July; 34(7):768-73. PubMed PMID: 27136078; Swarts et al., Nature. 507(7491) (2014):258-61; and Swarts et al., Nucleic Acids Res. 43(10) (2015):5120-9, each of which is incorporated herein by reference.
In some embodiments, the napDNAbp is a prokaryotic homolog of an Argonaute protein. Prokaryotic homologs of Argonaute proteins are known and have been described, for example, in Makarova K., et al., “Prokaryotic homologs of Argonaute proteins are predicted to function as key components of a novel system of defense against mobile genetic elements”, Biol Direct. 2009 Aug. 25; 4:29. doi: 10.1186/1745-6150-4-29, the entire contents of which is hereby incorporated by reference. In some embodiments, the napDNAbp is a Marinitoga piezophila Argunaute (MpAgo) protein. The CRISPR-associated Marinitoga piezophila Argunaute (MpAgo) protein cleaves single-stranded target sequences using 5′-phosphorylated guides. The 5′ guides are used by all known Argonautes. The crystal structure of an MpAgo-RNA complex shows a guide strand binding site comprising residues that block 5′ phosphate interactions. This data suggests the evolution of an Argonaute subclass with noncanonical specificity for a 5′-hydroxylated guide. See, e.g., Kaya et al., “A bacterial Argonaute with noncanonical guide RNA specificity”, Proc Natl Acad Sci USA. 2016 Apr. 12; 113(15):4057-62, the entire contents of which are hereby incorporated by reference). It should be appreciated that other argonaute proteins may be used, and are within the scope of this disclosure.
Some aspects of the disclosure provide Cas9 domains that have different PAM specificities. Typically, Cas9 proteins, such as Cas9 from S. pyogenes (spCas9), require a canonical NGG PAM sequence to bind a particular nucleic acid region. This may limit the ability to edit desired bases within a genome. In some embodiments, the base editing fusion proteins provided herein may need to be placed at a precise location, for example where a target base is placed within a 4 base region (e.g., a “editing window”), which is approximately 15 bases upstream of the PAM. See Komor, A. C., et al., “Programmable editing of a target base in genomic DNA without double-stranded DNA cleavage” Nature 533, 420-424 (2016), the entire contents of which are hereby incorporated by reference. Accordingly, in some embodiments, any of the fusion proteins provided herein may contain a Cas9 domain that is capable of binding a nucleotide sequence that does not contain a canonical (e.g., NGG) PAM sequence. Cas9 domains that bind to non-canonical PAM sequences have been described in the art and would be apparent to the skilled artisan. For example, Cas9 domains that bind non-canonical PAM sequences have been described in Kleinstiver, B. P., et al., “Engineered CRISPR-Cas9 nucleases with altered PAM specificities” Nature 523, 481-485 (2015); and Kleinstiver, B. P., et al., “Broadening the targeting range of Staphylococcus aureus CRISPR-Cas9 by modifying PAM recognition” Nature Biotechnology 33, 1293-1298 (2015); the entire contents of each are hereby incorporated by reference.
For example, a napDNAbp domain with altered PAM specificity, such as a domain with at least 80%, at least 85%, at least 90%, at least 95%, or at least 99% sequence identity with wild type Francisella novicida Cpf1 (D917, E1006, and D1255) (SEQ ID NO: 74), which has the following amino acid sequence:
An additional napDNAbp domain with altered PAM specificity, such as a domain having at least 80%, at least 85%, at least 90%, at least 95%, or at least 99% sequence identity with wild type Geobacillus thermodenitrificans Cas9 (SEQ ID NO: 75), which has the following amino acid sequence:
In some embodiments, the nucleic acid programmable DNA binding protein (napDNAbp) is a nucleic acid programmable DNA binding protein that does not require a canonical (NGG) PAM sequence. In some embodiments, the napDNAbp is an argonaute protein. One example of such a nucleic acid programmable DNA binding protein is an Argonaute protein from Natronobacterium gregoryi (NgAgo). NgAgo is a ssDNA-guided endonuclease. NgAgo binds 5′ phosphorylated ssDNA of ˜24 nucleotides (gDNA) to guide it to its target site and will make DNA double-strand breaks at the gDNA site. In contrast to Cas9, the NgAgo-gDNA system does not require a protospacer-adjacent motif (PAM). Using a nuclease inactive NgAgo (dNgAgo) can greatly expand the bases that may be targeted. The characterization and use of NgAgo have been described in Gao et al., Nat Biotechnol., 34(7): 768-73 (2016), PubMed PMID: 27136078; Swarts et al., Nature, 507(7491): 258-61 (2014); and Swarts et al., Nucleic Acids Res. 43(10) (2015): 5120-9, each of which is incorporated herein by reference. The sequence of Natronobacterium gregoryi Argonaute is provided in SEQ ID NO: 76.
The disclosed fusion proteins may comprise a napDNAbp domain having at least 80%, at least 85%, at least 90%, at least 95%, or at least 99% sequence identity with wild type Natronobacterium gregoryi Argonaute (SEQ ID NO: 76), which has the following amino acid sequence:
In addition, any available methods may be utilized to obtain or construct a variant or mutant Cas9 protein. The term “mutation,” as used herein, refers to a substitution of a residue within a sequence, e.g., a nucleic acid or amino acid sequence, with another residue, or a deletion or insertion of one or more residues within a sequence. Mutations are typically described herein by identifying the original residue followed by the position of the residue within the sequence and by the identity of the newly substituted residue. Various methods for making the amino acid substitutions (mutations) provided herein are well known in the art, and are provided by, for example, Green and Sambrook, Molecular Cloning: A Laboratory Manual (4th ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (2012)). Mutations can include a variety of categories, such as single base polymorphisms, microduplication regions, indel, and inversions, and is not meant to be limiting in any way. Mutations can include “loss-of-function” mutations which is the normal result of a mutation that reduces or abolishes a protein activity. Most loss-of-function mutations are recessive, because in a heterozygote the second chromosome copy carries an unmutated version of the gene coding for a fully functional protein whose presence compensates for the effect of the mutation. Mutations also embrace “gain-of-function” mutations, which is one which confers an abnormal activity on a protein or cell that is otherwise not present in a normal condition. Many gain-of-function mutations are in regulatory sequences rather than in coding regions, and can therefore have a number of consequences. For example, a mutation might lead to one or more genes being expressed in the wrong tissues, these tissues gaining functions that they normally lack. Because of their nature, gain-of-function mutations are usually dominant.
Mutations can be introduced into a reference Cas9 protein using site-directed mutagenesis. Older methods of site-directed mutagenesis known in the art rely on sub-cloning of the sequence to be mutated into a vector, such as an M13 bacteriophage vector, that allows the isolation of single-stranded DNA template. In these methods, one anneals a mutagenic primer (i.e., a primer capable of annealing to the site to be mutated but bearing one or more mismatched nucleotides at the site to be mutated) to the single-stranded template and then polymerizes the complement of the template starting from the 3′ end of the mutagenic primer. The resulting duplexes are then transformed into host bacteria and plaques are screened for the desired mutation. More recently, site-directed mutagenesis has employed PCR methodologies, which have the advantage of not requiring a single-stranded template. In addition, methods have been developed that do not require sub-cloning. Several issues must be considered when PCR-based site-directed mutagenesis is performed. First, in these methods it is desirable to reduce the number of PCR cycles to prevent expansion of undesired mutations introduced by the polymerase. Second, a selection must be employed in order to reduce the number of non-mutated parental molecules persisting in the reaction. Third, an extended-length PCR method is preferred in order to allow the use of a single PCR primer set. And fourth, because of the non-template-dependent terminal extension activity of some thermostable polymerases it is often necessary to incorporate an end-polishing step into the procedure prior to blunt-end ligation of the PCR-generated mutant product.
Mutations may also be introduced by directed evolution processes, such as phage-assisted continuous evolution (PACE) or phage-assisted noncontinuous evolution (PANCE). The term “phage-assisted continuous evolution (PACE),” as used herein, refers to continuous evolution that employs phage as viral vectors. The general concept of PACE technology has been described, for example, in International PCT Application, PCT/US2009/056194, filed Sep. 8, 2009, published as WO 2010/028347 on Mar. 11, 2010; International PCT Application, PCT/US2011/066747, filed Dec. 22, 2011, published as WO 2012/088381 on Jun. 28, 2012; U.S. application, U.S. Pat. No. 9,023,594, issued May 5, 2015, International PCT Application, PCT/US2015/012022, filed Jan. 20, 2015, published as WO 2015/134121 on Sep. 11, 2015, and International PCT Application, PCT/US2016/027795, filed Apr. 15, 2016, published as WO 2016/168631 on Oct. 20, 2016, the entire contents of each of which are incorporated herein by reference. Variant Cas9s may also be obtain by phage-assisted non-continuous evolution (PANCE),” which as used herein, refers to non-continuous evolution that employs phage as viral vectors. PANCE is a simplified technique for rapid in vivo directed evolution using serial flask transfers of evolving ‘selection phage’ (SP), which contain a gene of interest to be evolved, across fresh E. coli host cells, thereby allowing genes inside the host E. coli to be held constant while genes contained in the SP continuously evolve. Serial flask transfers have long served as a widely-accessible approach for laboratory evolution of microbes, and, more recently, analogous approaches have been developed for bacteriophage evolution. The PANCE system features lower stringency than the PACE system.
Any of the references noted above which relate to Cas9 or Cas9 equivalents are hereby incorporated by reference in their entireties, if not already stated so.
J. Divided napDNAbp Domains for Split PE Delivery
In various embodiments, the prime editors described herein may be delivered to cells as two or more fragments which become assembled inside the cell (either by passive assembly, or by active assembly, such as using split intein sequences) into a reconstituted prime editor. In some cases, the self assembly may be passive whereby the two or more prime editor fragments associate inside the cell covalently or non-covalently to reconstitute the prime editor. In other cases, the self-assembly may be catalyzed by dimerization domains installed on each of the fragments. Examples of dimerization domains are described herein. In still other cases, the self-assembly may be catalyzed by split intein sequences installed on each of the prime editor fragments.
Split PE delivery may be advantageous to address various size constraints of different delivery approaches. For example, delivery approaches may include virus-based delivery methods, messenger RNA-based delivery methods, or RNP-based delivery (ribonucleoprotein-based delivery). And, each of these methods of delivery may be more efficient and/or effective by dividing up the prime editor into smaller pieces. Once inside the cell, the smaller pieces can assemble into a functional prime editor. Depending on the means of splitting, the divided prime editor fragments can be reassembled in a non-covalent manner or a covalent manner to reform the prime editor. In one embodiment, the prime editor can be split at one or more split sites into two or more fragments. The fragments can be unmodified (other than being split). Once the fragments are delivered to the cell (e.g., by direct delivery of a ribonucleoprotein complex or by nucleic delivery—e.g., mRNA delivery or virus vector based delivery), the fragments can reassociate covalently or non-covalently to reconstitute the prime editor. In another embodiment, the prime editor can be split at one or more split sites into two or more fragments. Each of the fragments can be modified to comprise a dimerization domain, whereby each fragment that is formed is coupled to a dimerization domain. Once delivered or expressed within a cell, the dimerization domains of the different fragments associate and bind to one another, bringing the different prime editor fragments together to reform a functional prime editor. In yet another embodiment, the prime editor fragment may be modified to comprise a split intein. Once delivered or expressed within a cell, the split intein domains of the different fragments associate and bind to one another, and then undergo trans-splicing, which results in the excision of the split-intein domains from each of the fragments, and a concomitant formation of a peptide bond between the fragments, thereby restoring the prime editor.
In one embodiment, the prime editor can be delivered using a split-intein approach.
The location of the split site can be positioned between any one or more pair of residues in the prime editor and in any domains therein, including within the napDNAbp domain, the polymerase domain (e.g., RT domain), linker domain that joins the napDNAbp domain and the polymerase domain.
In one embodiment, depicted in
In certain embodiments, the napDNAbp is a canonical SpCas9 polypeptide of SEQ ID NO: 18, as follows:
In certain embodiments, the SpCas9 is split into two fragments at a split site located between residues 1 and 2, or 2 and 3, or 3 and 4, or 4 and 5, or 5 and 6, or 6 and 7, or 7 and 8, or 8 and 9, or 9 and 10, or between any two pair of residues located anywhere between residues 1-10, 10-20, 20-30, 30-40, 40-50, 50-60, 60-70, 70-80, 80-90, 90-100, 100-200, 200-300, 300-400, 400-500, 500-600, 600-700, 700-800, 800-900, 1000-1100, 1100-1200, 1200-1300, or 1300-1368 of canonical SpCas9 of SEQ ID NO: 18.
In certain embodiments, a napDNAbp is split into two fragments at a split site that is located at a pair of residue that corresponds to any two pair of residues located anywhere between positions 1-10, 10-20, 20-30, 30-40, 40-50, 50-60, 60-70, 70-80, 80-90, 90-100, 100-200, 200-300, 300-400, 400-500, 500-600, 600-700, 700-800, 800-900, 1000-1100, 1100-1200, 1200-1300, or 1300-1368 of canonical SpCas9 of SEQ ID NO: 18.
In certain embodiments, the SpCas9 is split into two fragments at a split site located between residues 1 and 2, or 2 and 3, or 3 and 4, or 4 and 5, or 5 and 6, or 6 and 7, or 7 and 8, or 8 and 9, or 9 and 10, or between any two pair of residues located anywhere between residues 1-10, 10-20, 20-30, 30-40, 40-50, 50-60, 60-70, 70-80, 80-90, 90-100, 100-200, 200-300, 300-400, 400-500, 500-600, 600-700, 700-800, 800-900, 1000-1100, 1100-1200, 1200-1300, or 1300-1368 of canonical SpCas9 of SEQ ID NO: 18. In certain embodiments, the split site is located one or more polypeptide bond sites (i.e., a “split site or split-intein split site”), fused to a split intein, and then delivered to cells as separately-encoded fusion proteins. Once the split-intein fusion proteins (i.e., protein halves) are expressed within a cell, the proteins undergo trans-splicing to form a complete or whole PE with the concomitant removal of the joined split-intein sequences.
For example, as shown in
To take advantage of a split-PE delivery strategy using split-inteins, the prime editor needs to be divided at one or more split sites to create at least two separate halves of a prime editor, each of which may be rejoined inside a cell if each half is fused to a split-intein sequence.
In certain embodiments, the prime editor is split at a single split site. In certain other embodiments, the prime editor is split at two split sites, or three split sites, or four split sites, or more.
In a preferred embodiment, the prime editor is split at a single split site to create two separate halves of a prime editor, each of which can be fused to a split intein sequence
An exemplary split intein is the Ssp DnaE intein, which comprises two subunits, namely, DnaE-N and DnaE-C. The two different subunits are encoded by separate genes, namely dnaE-n and dnaE-c, which encode the DnaE-N and DnaE-C subunits, respectively. DnaE is a naturally occurring split intein in Synechocytis sp. PCC6803 and is capable of directing trans-splicing of two separate proteins, each comprising a fusion with either DnaE-N or DnaE-C.
Additional naturally occurring or engineered split-intein sequences are known in the or can be made from whole-intein sequences described herein or those available in the art. Examples of split-intein sequences can be found in Stevens et al., “A promiscuous split intein with expanded protein engineering applications,” PNAS, 2017, Vol. 114: 8538-8543; Iwai et al., “Highly efficient protein trans-splicing by a naturally split DnaE intein from Nostoc punctiforme, FEBS Lett, 580: 1853-1858, each of which are incorporated herein by reference. Additional split intein sequences can be found, for example, in WO 2013/045632, WO 2014/055782, WO 2016/069774, and EP2877490, the contents each of which are incorporated herein by reference.
In addition, protein splicing in trans has been described in vivo and in vitro (Shingledecker, et al., Gene 207:187 (1998), Southworth, et al., EMBO J. 17:918 (1998); Mills, et al., Proc. Natl. Acad. Sci. USA, 95:3543-3548 (1998); Lew, et al., J. Biol. Chem., 273:15887-15890 (1998); Wu, et al., Biochim. Biophys. Acta 35732:1 (1998b), Yamazaki, et al., J. Am. Chem. Soc. 120:5591 (1998), Evans, et al., J. Biol. Chem. 275:9091 (2000); Otomo, et al., Biochemistry 38:16040-16044 (1999); Otomo, et al., J. Biolmol. NMR 14:105-114 (1999); Scott, et al., Proc. Natl. Acad. Sci. USA 96:13638-13643 (1999)) and provides the opportunity to express a protein as to two inactive fragments that subsequently undergo ligation to form a functional product, e.g., as shown in
In various embodiments described herein, the continuous evolution methods (e.g., PACE) may be used to evolve a first portion of a base editor. A first portion could include a single component or domain, e.g., a Cas9 domain, a deaminase domain, or a UGI domain. The separately evolved component or domain can be then fused to the remaining portions of the base editor within a cell by separately express both the evolved portion and the remaining non-evolved portions with split-intein polypeptide domains. The first portion could more broadly include any first amino acid portion of a base editor that is desired to be evolved using a continuous evolution method described herein. The second portion would in this embodiment refer to the remaining amino acid portion of the base editor that is not evolved using the herein methods. The evolved first portion and the second portion of the base editor could each be expressed with split-intein polypeptide domains in a cell. The natural protein splicing mechanisms of the cell would reassemble the evolved first portion and the non-evolved second portion to form a single fusion protein evolved base editor. The evolved first portion may comprise either the N- or C-terminal part of the single fusion protein. In an analogous manner, use of a second orthogonal trans-splicing intein pair could allow the evolved first portion to comprise an internal part of the single fusion protein.
Thus, any of the evolved and non-evolved components of the base editors herein described may be expressed with split-intein tags in order to facilitate the formation of a complete base editor comprising the evolved and non-evolved component within a cell.
The mechanism of the protein splicing process has been studied in great detail (Chong, et al., J. Biol. Chem. 1996, 271, 22159-22168; Xu, M-Q & Perler, F. B. EMBO Journal, 1996, 15, 5146-5153) and conserved amino acids have been found at the intein and extein splicing points (Xu, et al., EMBO Journal, 1994, 13 5517-522). The constructs described herein contain an intein sequence fused to the 5′-terminus of the first gene (e.g., the evolved portion of the base editor). Suitable intein sequences can be selected from any of the proteins known to contain protein splicing elements. A database containing all known inteins can be found on the World Wide Web (Perler, F. B. Nucleic Acids Research, 1999, 27, 346-347). The intein sequence is fused at the 3′ end to the 5′ end of a second gene. For targeting of this gene to a certain organelle, a peptide signal can be fused to the coding sequence of the gene. After the second gene, the intein-gene sequence can be repeated as often as desired for expression of multiple proteins in the same cell. For multi-intein containing constructs, it may be useful to use intein elements from different sources. After the sequence of the last gene to be expressed, a transcription termination sequence must be inserted. In one embodiment, a modified intein splicing unit is designed so that it can both catalyze excision of the exteins from the inteins as well as prevent ligation of the exteins. Mutagenesis of the C-terminal extein junction in the Pyrococcus species GB-D DNA polymerase was found to produce an altered splicing element that induces cleavage of exteins and inteins but prevents subsequent ligation of the exteins (Xu, M-Q & Perler, F. B. EMBO Journal, 1996, 15, 5146-5153). Mutation of serine 538 to either an alanine or glycine induced cleavage but prevented ligation. Mutation of equivalent residues in other intein splicing units should also prevent extein ligation due to the conservation of amino acids at the C-terminal extein junction to the intein. A preferred intein not containing an endonuclease domain is the Mycobacterium xenopi GyrA protein (Telenti, et al. J. Bacteriol. 1997, 179, 6378-6382). Others have been found in nature or have been created artificially by removing the endonuclease domains from endonuclease containing inteins (Chong, et al. J. Biol. Chem. 1997, 272, 15587-15590). In a preferred embodiment, the intein is selected so that it consists of the minimal number of amino acids needed to perform the splicing function, such as the intein from the Mycobacterium xenopi GyrA protein (Telenti, A., et al., J. Bacteriol. 1997, 179, 6378-6382). In an alternative embodiment, an intein without endonuclease activity is selected, such as the intein from the Mycobacterium xenopi GyrA protein or the Saccharomyces cerevisiae VMA intein that has been modified to remove endonuclease domains (Chong, 1997). Further modification of the intein splicing unit may allow the reaction rate of the cleavage reaction to be altered allowing protein dosage to be controlled by simply modifying the gene sequence of the splicing unit.
Inteins can also exist as two fragments encoded by two separately transcribed and translated genes. These so-called split inteins self-associate and catalyze protein-splicing activity in trans. Split inteins have been identified in diverse cyanobacteria and archaea (Caspi et al, Mol Microbiol. 50: 1569-1577 (2003); Choi J. et al, J Mol Biol. 556: 1093-1106 (2006.); Dassa B. et al, Biochemistry. 46:322-330 (2007.); Liu X. and Yang J., J Biol Chem. 275:26315-26318 (2003); Wu H. et al.
Proc Natl Acad Sci USA. £5:9226-9231 (1998.); and Zettler J. et al, FEBS Letters. 553:909-914 (2009)), but have not been found in eukaryotes thus far. Recently, a bioinformatic analysis of environmental metagenomic data revealed 26 different loci with a novel genomic arrangement. At each locus, a conserved enzyme coding region is interrupted by a split intein, with a freestanding endonuclease gene inserted between the sections coding for intein subdomains. Among them, five loci were completely assembled: DNA helicases (gp41-1, gp41-8); Inosine-5′-monophosphate dehydrogenase (IMPDH-1); and Ribonucleotide reductase catalytic subunits (NrdA-2 and NrdJ-1). This fractured gene organization appears to be present mainly in phages (Dassa et al, Nucleic Acids Research. 57:2560-2573 (2009)).
The split intein Npu DnaE was characterized as having the highest rate reported for the protein trans-splicing reaction. In addition, the Npu DnaE protein splicing reaction is considered robust and high-yielding with respect to different extein sequences, temperatures from 6 to 37° C., and the presence of up to 6M Urea (Zettler J. et al, FEBS Letters. 553:909-914 (2009); Iwai I. et al, FEBS Letters 550: 1853-1858 (2006)). As expected, when the Cys1 Ala mutation at the N-domain of these inteins was introduced, the initial N to S-acyl shift and therefore protein splicing was blocked. Unfortunately, the C-terminal cleavage reaction was also almost completely inhibited. The dependence of the asparagine cyclization at the C-terminal splice junction on the acyl shift at the N-terminal scissile peptide bond seems to be a unique property common to the naturally split DnaE intein alleles (Zettler J. et al. FEBS Letters. 555:909-914 (2009)).
The mechanism of protein splicing typically has four steps [29-30]: 1) an N—S or N—O acyl shift at the intein N-terminus, which breaks the upstream peptide bond and forms an ester bond between the N-extein and the side chain of the intein's first amino acid (Cys or Ser); 2) a transesterification relocating the N-extein to the intein C-terminus, forming a new ester bond linking the N-extein to the side chain of the C-extein's first amino acid (Cys, Ser, or Thr); 3) Asn cyclization breaking the peptide bond between the intein and the C-extein; and 4) a S—N or O—N acyl shift that replaces the ester bond with a peptide bond between the N-extein and C-extein.
Protein trans-splicing, catalyzed by split inteins, provides an entirely enzymatic method for protein ligation [31]. A split-intein is essentially a contiguous intein (e.g. a mini-intein) split into two pieces named N-intein and C-intein, respectively. The N-intein and C-intein of a split intein can associate non-covalently to form an active intein and catalyze the splicing reaction essentially in same way as a contiguous intein does. Split inteins have been found in nature and also engineered in laboratories [31-35]. As used herein, the term “split intein” refers to any intein in which one or more peptide bond breaks exists between the N-terminal and C-terminal amino acid sequences such that the N-terminal and C-terminal sequences become separate molecules that can non-covalently reassociate, or reconstitute, into an intein that is functional for trans-splicing reactions. Any catalytically active intein, or fragment thereof, may be used to derive a split intein for use in the methods of the invention. For example, in one aspect the split intein may be derived from a eukaryotic intein. In another aspect, the split intein may be derived from a bacterial intein. In another aspect, the split intein may be derived from an archaeal intein. Preferably, the split intein so-derived will possess only the amino acid sequences essential for catalyzing trans-splicing reactions.
As used herein, the “N-terminal split intein (In)” refers to any intein sequence that comprises an N-terminal amino acid sequence that is functional for trans-splicing reactions. An In thus also comprises a sequence that is spliced out when trans-splicing occurs. An In can comprise a sequence that is a modification of the N-terminal portion of a naturally occurring intein sequence. For example, an In can comprise additional amino acid residues and/or mutated residues so long as the inclusion of such additional and/or mutated residues does not render the In non-functional in trans-splicing. Preferably, the inclusion of the additional and/or mutated residues improves or enhances the trans-splicing activity of the In.
As used herein, the “C-terminal split intein (Ic)” refers to any intein sequence that comprises a C-terminal amino acid sequence that is functional for trans-splicing reactions. In one aspect, the Ic comprises 4 to 7 contiguous amino acid residues, at least 4 amino acids of which are from the last β-strand of the intein from which it was derived. An Ic thus also comprises a sequence that is spliced out when trans-splicing occurs. An Ic can comprise a sequence that is a modification of the C-terminal portion of a naturally occurring intein sequence. For example, an Ic can comprise additional amino acid residues and/or mutated residues so long as the inclusion of such additional and/or mutated residues does not render the In non-functional in trans-splicing. Preferably, the inclusion of the additional and/or mutated residues improves or enhances the trans-splicing activity of the Ic.
In some embodiments of the invention, a peptide linked to an Ic or an In can comprise an additional chemical moiety including, among others, fluorescence groups, biotin, polyethylene glycol (PEG), amino acid analogs, unnatural amino acids, phosphate groups, glycosyl groups, radioisotope labels, and pharmaceutical molecules. In other embodiments, a peptide linked to an Ic can comprise one or more chemically reactive groups including, among others, ketone, aldehyde, Cys residues and Lys residues. The N-intein and C-intein of a split intein can associate non-covalently to form an active intein and catalyze the splicing reaction when an “intein-splicing polypeptide (ISP)” is present. As used herein, “intein-splicing polypeptide (ISP)” refers to the portion of the amino acid sequence of a split intein that remains when the Ic, In, or both, are removed from the split intein. In certain embodiments, the In comprises the ISP. In another embodiment, the Ic comprises the ISP. In yet another embodiment, the ISP is a separate peptide that is not covalently linked to In nor to Ic.
Split inteins may be created from contiguous inteins by engineering one or more split sites in the unstructured loop or intervening amino acid sequence between the −12 conserved beta-strands found in the structure of mini-inteins [25-28]. Some flexibility in the position of the split site within regions between the beta-strands may exist, provided that creation of the split will not disrupt the structure of the intein, the structured beta-strands in particular, to a sufficient degree that protein splicing activity is lost.
In protein trans-splicing, one precursor protein consists of an N-extein part followed by the N-intein, another precursor protein consists of the C-intein followed by a C-extein part, and a trans-splicing reaction (catalyzed by the N- and C-inteins together) excises the two intein sequences and links the two extein sequences with a peptide bond. Protein trans-splicing, being an enzymatic reaction, can work with very low (e.g. micromolar) concentrations of proteins and can be carried out under physiological conditions.
[2] Other Programmable NucleasesIn various embodiments described herein, the prime editors comprise a napDNAbp, such as a Cas9 protein. These proteins are “programmable” by way of their becoming complexed with a guide RNA (or a PEgRNA, as the case may be), which guides the Cas9 protein to a target site on the DNA which possess a sequence that is complementary to the spacer portion of the gRNA (or PEgRNA) and also which possesses the required PAM sequence. However, in certain embodiment envisioned here, the napDNAbp may be substituted with a different type of programmable protein, such as a zinc finger nuclease or a transcription activator-like effector nuclease (TALEN).
Suitable alternative programmable nucleases are well known in the art which may be used in place of a napDNAbp:gRNA complex to construct an alternative prime editor system that can be programmed to selectively bind a target site of DNA, and which can be further modified in the manner described above to co-localize a polymerase and an RNA or DNA extension arm comprising a primer binding site and a DNA synthesis template to specific nick site. For example, and as represented in
As represented in
In various embodiments, the prime editor (PE) system disclosed herein includes a polymerase (e.g., DNA-dependent DNA polymerase or RNA-dependent DNA polymerase, such as, reverse transcriptase), or a variant thereof, which can be provided as a fusion protein with a napDNAbp or other programmable nuclease, or provide in trans.
Any polymerase may be used in the prime editors disclosed herein. The polymerases may be wild type polymerases, functional fragments, mutants, variants, or truncated variants, and the like. The polymerases may include wild type polymerases from eukaryotic, prokaryotic, archaeal, or viral organisms, and/or the polymerases may be modified by genetic engineering, mutagenesis, directed evolution-based processes. The polymerases may include T7 DNA polymerase, T5 DNA polymerase, T4 DNA polymerase, Klenow fragment DNA polymerase, DNA polymerase III and the like. The polymerases may also be thermostable, and may include Taq, Tne, Tma, Pfu, Tfl, Tth, Stoffel fragment, VENT® and DEEPVENT® DNA polymerases, KOD, Tgo, JDF3, and mutants, variants and derivatives thereof (see U.S. Pat. Nos. 5,436,149; 4,889,818; 4,965,185; 5,079,352; 5,614,365; 5,374,553; 5,270,179; 5,047,342; 5,512,462; WO 92/06188; WO 92/06200; WO 96/10640; Barnes, W. M., Gene 112:29-35 (1992); Lawyer, F. C., et al., PCR Meth. Appl. 2:275-287 (1993); Flaman, J.-M, et al., Nuc. Acids Res. 22(15):3259-3260 (1994), each of which are incorporated by reference). For synthesis of longer nucleic acid molecules (e.g, nucleic acid molecules longer than about 3-5 Kb in length), at least two DNA polymerases can be employed. In certain embodiments, one of the polymerases can be substantially lacking a 3′ exonuclease activity and the other may have a 3′ exonuclease activity. Such pairings may include polymerases that are the same or different. Examples of DNA polymerases substantially lacking in 3′ exonuclease activity include, but are not limited to, Taq, Tne(exo-), Tma(exo-), Pfu(exo-), Pwo(exo-), exo-KOD and Tth DNA polymerases, and mutants, variants and derivatives thereof.
Preferably, the polymerase usable in the prime editors disclosed herein are “template-dependent” polymerase (since the polymerases are intended to rely on the DNA synthesis template to specify the sequence of the DNA strand under synthesis during prime editing. As used herein, the term “template DNA molecule” refers to that strand of a nucleic acid from which a complementary nucleic acid strand is synthesized by a DNA polymerase, for example, in a primer extension reaction of the DNA synthesis template of a PEgRNA.
As used herein, the term “template dependent manner” is intended to refer to a process that involves the template dependent extension of a primer molecule (e.g., DNA synthesis by DNA polymerase). The term “template dependent manner” refers to polynucleotide synthesis of RNA or DNA wherein the sequence of the newly synthesized strand of polynucleotide is dictated by the well-known rules of complementary base pairing (see, for example, Watson, J. D. et al., In: Molecular Biology of the Gene, 4th Ed., W. A. Benjamin, Inc., Menlo Park, Calif. (1987)). The term “complementary” refers to the broad concept of sequence complementarity between regions of two polynucleotide strands or between two nucleotides through base-pairing. It is known that an adenine nucleotide is capable of forming specific hydrogen bonds (“base pairing”) with a nucleotide which is thymine or uracil. Similarly, it is known that a cytosine nucleotide is capable of base pairing with a guanine nucleotide. As such, in the case of prime editing, it can be said that the single strand of DNA synthesized by the polymerase of the prime editor against the DNA synthesis template is said to be “complementary” to the sequence of the DNA synthesis template.
A. Exemplary Polymerases
In various embodiments, the prime editors described herein comprise a polymerase. The disclosure contemplates any wild type polymerase obtained from any naturally-occurring organism or virus, or obtained from a commercial or non-commercial source. In addition, the polymerases usable in the prime editors of the disclosure can include any naturally-occurring mutant polymerase, engineered mutant polymerase, or other variant polymerase, including truncated variants that retain function. The polymerases usable herein may also be engineered to contain specific amino acid substitutions, such as those specifically disclosed herein. In certain preferred embodiments, the polymerases usable in the prime editors of the disclosure are template-based polymerases, i.e., they synthesize nucleotide sequences in a template-dependent manner.
A polymerase is an enzyme that synthesizes a nucleotide strand and which may be used in connection with the prime editor systems described herein. The polymerases are preferrably “template-dependent” polymerases (i.e., a polymerase which synthesizes a nucleotide strand based on the order of nucleotide bases of a template strand). In certain configurations, the polymerases can also be a “template-independent” (i.e., a polymerase which synthesizes a nucleotide strand without the requirement of a template strand). A polymerase may also be further categorized as a “DNA polymerase” or an “RNA polymerase.” In various embodiments, the prime editor system comprises a DNA polymerase. In various embodiments, the DNA polymerase can be a “DNA-dependent DNA polymerase” (i.e., whereby the template molecule is a strand of DNA). In such cases, the DNA template molecule can be a PEgRNA, wherein the extension arm comprises a strand of DNA. In such cases, the PEgRNA may be referred to as a chimeric or hybrid PEgRNA which comprises an RNA portion (i.e., the guide RNA components, including the spacer and the gRNA core) and a DNA portion (i.e., the extension arm). In various other embodiments, the DNA polymerase can be an “RNA-dependent DNA polymerase” (i.e., whereby the template molecule is a strand of RNA). In such cases, the PEgRNA is RNA, i.e., including an RNA extension. The term “polymerase” may also refer to an enzyme that catalyzes the polymerization of nucleotide (i.e., the polymerase activity). Generally, the enzyme will initiate synthesis at the 3′-end of a primer annealed to a polynucleotide template sequence (e.g., such as a primer sequence annealed to the primer binding site of a PEgRNA), and will proceed toward the 5′ end of the template strand. A “DNA polymerase” catalyzes the polymerization of deoxynucleotides. As used herein in reference to a DNA polymerase, the term DNA polymerase includes a “functional fragment thereof”. A “functional fragment thereof” refers to any portion of a wild-type or mutant DNA polymerase that encompasses less than the entire amino acid sequence of the polymerase and which retains the ability, under at least one set of conditions, to catalyze the polymerization of a polynucleotide. Such a functional fragment may exist as a separate entity, or it may be a constituent of a larger polypeptide, such as a fusion protein.
In some embodiments, the polymerases can be from bacteriophage. Bacteriophage DNA polymerases are generally devoid of 5′ to 3′ exonuclease activity, as this activity is encoded by a separate polypeptide. Examples of suitable DNA polymerases are T4, T7, and phi29 DNA polymerase. The enzymes available commercially are: T4 (available from many sources e.g., Epicentre) and T7 (available from many sources, e.g. Epicentre for unmodified and USB for 3′ to 5′ exo T7 “Sequenase” DNA polymerase).
The other embodiments, the polymerases are archaeal polymerases. There are 2 different classes of DNA polymerases which have been identified in archaea: 1. Family B/pol I type (homologs of Pfu from Pyrococcus furiosus) and 2. pol II type (homologs of P. furiosus DP1/DP2 2-subunit polymerase). DNA polymerases from both classes have been shown to naturally lack an associated 5′ to 3′ exonuclease activity and to possess 3′ to 5′ exonuclease (proofreading) activity. Suitable DNA polymerases (pol I or pol II) can be derived from archaea with optimal growth temperatures that are similar to the desired assay temperatures.
Thermostable archaeal DNA polymerases are isolated from Pyrococcus species (furiosus, species GB-D, woesii, abysii, horikoshii), Thermococcus species (kodakaraensis KOD1, litoralis, species 9 degrees North-7, species JDF-3, gorgonarius), Pyrodictium occultum, and Archaeoglobus fulgidus.
Polymerases may also be from eubacterial species. There are 3 classes of eubacterial DNA polymerases, pol I, II, and III. Enzymes in the Pol I DNA polymerase family possess 5′ to 3′ exonuclease activity, and certain members also exhibit 3′ to 5′ exonuclease activity. Pol II DNA polymerases naturally lack 5′ to 3′ exonuclease activity, but do exhibit 3′ to 5′ exonuclease activity. Pol III DNA polymerases represent the major replicative DNA polymerase of the cell and are composed of multiple subunits. The pol III catalytic subunit lacks 5′ to 3′ exonuclease activity, but in some cases 3′ to 5′ exonuclease activity is located in the same polypeptide.
There are a variety of commercially available Pol I DNA polymerases, some of which have been modified to reduce or abolish 5′ to 3′ exonuclease activity.
Suitable thermostable pol I DNA polymerases can be isolated from a variety of thermophilic eubacteria, including Thermus species and Thermotoga maritima such as Thermus aquaticus (Taq), Thermus thermophilus (Tth) and Thermotoga maritima (Tma UlTma).
Additional eubacteria related to those listed above are described in Thermophilic Bacteria (Kristjansson, J. K., ed.) CRC Press, Inc., Boca Raton, Fla., 1992.
The invention further provides for chimeric or non-chimeric DNA polymerases that are chemically modified according to methods disclosed in U.S. Pat. Nos. 5,677,152, 6,479,264 and 6,183,998, the contents of which are hereby incorporated by reference in their entirety.
Additional archaea DNA polymerases related to those listed above are described in the following references: Archaea: A Laboratory Manual (Robb, F. T. and Place, A. R., eds.), Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1995 and Thermophilic Bacteria (Kristjansson, J. K., ed.) CRC Press, Inc., Boca Raton, Fla., 1992.
B. Exemplarily Reverse Transcriptases
In various embodiments, the prime editors described herein comprise a reverse transcriptase as the polymerase. The disclosure contemplates any wild type reverse transcriptase obtained from any naturally-occurring organism or virus, or obtained from a commercial or non-commercial source. In addition, the reverse transcriptases usable in the prime editors of the disclosure can include any naturally-occurring mutant RT, engineered mutant RT, or other variant RT, including truncated variants that retain function. The RTs may also be engineered to contain specific amino acid substitutions, such as those specifically disclosed herein.
Reverse transcriptases are multi-functional enzymes typically with three enzymatic activities including RNA- and DNA-dependent DNA polymerization activity, and an RNaseH activity that catalyzes the cleavage of RNA in RNA-DNA hybrids. Some mutants of reverse transcriptases have disabled the RNaseH moiety to prevent unintended damage to the mRNA. These enzymes that synthesize complementary DNA (cDNA) using mRNA as a template were first identified in RNA viruses. Subsequently, reverse transcriptases were isolated and purified directly from virus particles, cells or tissues. (e.g., see Kacian et al., 1971, Biochim. Biophys. Acta 46: 365-83; Yang et al., 1972, Biochem. Biophys. Res. Comm. 47: 505-11; Gerard et al., 1975, J. Virol. 15: 785-97; Liu et al., 1977, Arch. Virol. 55 187-200; Kato et al., 1984, J. Virol. Methods 9: 325-39; Luke et al., 1990, Biochem. 29: 1764-69 and Le Grice et al., 1991, J. Virol. 65: 7004-07, each of which are incorporated by reference). More recently, mutants and fusion proteins have been created in the quest for improved properties such as thermostability, fidelity and activity. Any of the wild type, variant, and/or mutant forms of reverse transcriptase which are known in the art or which can be made using methods known in the art are contemplated herein.
The reverse transcriptase (RT) gene (or the genetic information contained therein) can be obtained from a number of different sources. For instance, the gene may be obtained from eukaryotic cells which are infected with retrovirus, or from a number of plasmids which contain either a portion of or the entire retrovirus genome. In addition, messenger RNA-like RNA which contains the RT gene can be obtained from retroviruses. Examples of sources for RT include, but are not limited to, Moloney murine leukemia virus (M-MLV or MLVRT); human T-cell leukemia virus type 1 (HTLV-1); bovine leukemia virus (BLV); Rous Sarcoma Virus (RSV); human immunodeficiency virus (HIV); yeast, including Saccharomyces, Neurospora, Drosophila; primates; and rodents. See, for example, Weiss, et al., U.S. Pat. No. 4,663,290 (1987); Gerard, G. R., DNA: 271-79 (1986); Kotewicz, M. L., et al., Gene 35:249-58 (1985); Tanese, N., et al., Proc. Natl. Acad. Sci. (USA): 4944-48 (1985); Roth, M. J., at al., J. Biol. Chem. 260:9326-35 (1985); Michel, F., et al., Nature 316:641-43 (1985); Akins, R. A., et al., Cell 47:505-16 (1986), EMBO J. 4:1267-75 (1985); and Fawcett, D. F., Cell 47:1007-15 (1986) (each of which are incorporated herein by reference in their entireties).
Wild type RTs
Exemplary enzymes for use with the herein disclosed prime editors can include, but are not limited to, M-MLV reverse transcriptase and RSV reverse transcriptase. Enzymes having reverse transcriptase activity are commercially available. In certain embodiments, the reverse transcriptase provided in trans to the other components of the prime editor (PE) system. That is, the reverse transcriptase is expressed or otherwise provided as an individual component, i.e., not as a fusion protein with a napDNAbp.
A person of ordinary skill in the art will recognize that wild type reverse transcriptases, including but not limited to, Moloney Murine Leukemia Virus (M-MLV); Human Immunodeficiency Virus (HIV) reverse transcriptase and avian Sarcoma-Leukosis Virus (ASLV) reverse transcriptase, which includes but is not limited to Rous Sarcoma Virus (RSV) reverse transcriptase, Avian Myeloblastosis Virus (AMV) reverse transcriptase, Avian Erythroblastosis Virus (AEV) Helper Virus MCAV reverse transcriptase, Avian Myelocytomatosis Virus MC29 Helper Virus MCAV reverse transcriptase, Avian Reticuloendotheliosis Virus (REV-T) Helper Virus REV-A reverse transcriptase, Avian Sarcoma Virus UR2 Helper Virus UR2AV reverse transcriptase, Avian Sarcoma Virus Y73 Helper Virus YAV reverse transcriptase, Rous Associated Virus (RAV) reverse transcriptase, and Myeloblastosis Associated Virus (MAV) reverse transcriptase may be suitably used in the subject methods and composition described herein.
Exemplary wild type RT enzymes are as follows:
Reverse transcriptases are essential for synthesizing complementary DNA (cDNA) strands from RNA templates. Reverse transcriptases are enzymes composed of distinct domains that exhibit different biochemical activities. The enzymes catalyze the synthesis of DNA from an RNA template, as follows: In the presence of an annealed primer, reverse transcriptase binds to an RNA template and initiates the polymerization reaction. RNA-dependent DNA polymerase activity synthesizes the complementary DNA (cDNA) strand, incorporating dNTPs. RNase H activity degrades the RNA template of the DNA:RNA complex. Thus, reverse transcriptases comprise (a) a binding activity that recognizes and binds to a RNA/DNA hybrid, (b) an RNA-dependent DNA polymerase activity, and (c) an RNase H activity. In addition, reverse transcriptases generally are regarded as having various attributes, including their thermostability, processivity (rate of dNTP incorporation), and fidelity (or error-rate). The reverse transcriptase variants contemplated herein may include any mutations to reverse transcriptase that impacts or changes any one or more of these enzymatic activities (e.g., RNA-dependent DNA polymerase activity, RNase H activity, or DNA/RNA hybrid-binding activity) or enzyme properties (e.g., thermostability, processivity, or fidelity). Such variants may be available in the art in the public domain, available commercially, or may be made using known methods of mutagenesis, including directed evolutionary processes (e.g., PACE or PANCE).
In various embodiments, the reverse transcriptase may be a variant reverse transcriptase. As used herein, a “variant reverse transcriptase” includes any naturally occurring or genetically engineered variant comprising one or more mutations (including singular mutations, inversions, deletions, insertions, and rearrangements) relative to a reference sequences (e.g., a reference wild type sequence). RT naturally have several activities, including an RNA-dependent DNA polymerase activity, ribonuclease H activity, and DNA-dependent DNA polymerase activity. Collectively, these activities enable the enzyme to convert single-stranded RNA into double-stranded cDNA. In retroviruses and retrotransposons, this cDNA can then integrate into the host genome, from which new RNA copies can be made via host-cell transcription. Variant RT's may comprise a mutation which impacts one or more of these activities (either which reduces or increases these activities, or which eliminates these activities all together). In addition, variant RTs may comprise one or more mutations which render the RT more or less stable, less prone to aggregation, and facilitates purification and/or detection, and/or other the modification of properties or characteristics.
A person of ordinary skill in the art will recognize that variant reverse transcriptases derived from other reverse transcriptases, including but not limited to Moloney Murine Leukemia Virus (M-MLV); Human Immunodeficiency Virus (HIV) reverse transcriptase and avian Sarcoma-Leukosis Virus (ASLV) reverse transcriptase, which includes but is not limited to Rous Sarcoma Virus (RSV) reverse transcriptase, Avian Myeloblastosis Virus (AMV) reverse transcriptase, Avian Erythroblastosis Virus (AEV) Helper Virus MCAV reverse transcriptase, Avian Myelocytomatosis Virus MC29 Helper Virus MCAV reverse transcriptase, Avian Reticuloendotheliosis Virus (REV-T) Helper Virus REV-A reverse transcriptase, Avian Sarcoma Virus UR2 Helper Virus UR2AV reverse transcriptase, Avian Sarcoma Virus Y73 Helper Virus YAV reverse transcriptase, Rous Associated Virus (RAV) reverse transcriptase, and Myeloblastosis Associated Virus (MAV) reverse transcriptase may be suitably used in the subject methods and composition described herein.
One method of preparing variant RTs is by genetic modification (e.g., by modifying the DNA sequence of a wild-type reverse transcriptase). A number of methods are known in the art that permit the random as well as targeted mutation of DNA sequences (see for example, Ausubel et. al. Short Protocols in Molecular Biology (1995) 3.sup.rd Ed. John Wiley & Sons, Inc.). In addition, there are a number of commercially available kits for site-directed mutagenesis, including both conventional and PCR-based methods. Examples include the QuikChange Site-Directed Mutagenesis Kits (AGILENT®), the Q5® Site-Directed Mutagenesis Kit (NEW ENGLAND BIOLABS®), and GeneArt™ Site-Directed Mutagenesis System (THERMOFISHER SCIENTIFIC®).
In addition, mutant reverse transcriptases may be generated by insertional mutation or truncation (N-terminal, internal, or C-terminal insertions or truncations) according to methodologies known to one skilled in the art. The term “mutation,” as used herein, refers to a substitution of a residue within a sequence, e.g., a nucleic acid or amino acid sequence, with another residue, or a deletion or insertion of one or more residues within a sequence. Mutations are typically described herein by identifying the original residue followed by the position of the residue within the sequence and by the identity of the newly substituted residue. Various methods for making the amino acid substitutions (mutations) provided herein are well known in the art, and are provided by, for example, Green and Sambrook, Molecular Cloning: A Laboratory Manual (4th ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (2012)). Mutations can include a variety of categories, such as single base polymorphisms, microduplication regions, indel, and inversions, and is not meant to be limiting in any way. Mutations can include “loss-of-function” mutations which is the normal result of a mutation that reduces or abolishes a protein activity. Most loss-of-function mutations are recessive, because in a heterozygote the second chromosome copy carries an unmutated version of the gene coding for a fully functional protein whose presence compensates for the effect of the mutation. Mutations also embrace “gain-of-function” mutations, which is one which confers an abnormal activity on a protein or cell that is otherwise not present in a normal condition. Many gain-of-function mutations are in regulatory sequences rather than in coding regions, and can therefore have a number of consequences. For example, a mutation might lead to one or more genes being expressed in the wrong tissues, these tissues gaining functions that they normally lack. Because of their nature, gain-of-function mutations are usually dominant.
Older methods of site-directed mutagenesis known in the art rely on sub-cloning of the sequence to be mutated into a vector, such as an M13 bacteriophage vector, that allows the isolation of single-stranded DNA template. In these methods, one anneals a mutagenic primer (i.e., a primer capable of annealing to the site to be mutated but bearing one or more mismatched nucleotides at the site to be mutated) to the single-stranded template and then polymerizes the complement of the template starting from the 3′ end of the mutagenic primer. The resulting duplexes are then transformed into host bacteria and plaques are screened for the desired mutation.
More recently, site-directed mutagenesis has employed PCR methodologies, which have the advantage of not requiring a single-stranded template. In addition, methods have been developed that do not require sub-cloning. Several issues must be considered when PCR-based site-directed mutagenesis is performed. First, in these methods it is desirable to reduce the number of PCR cycles to prevent expansion of undesired mutations introduced by the polymerase. Second, a selection must be employed in order to reduce the number of non-mutated parental molecules persisting in the reaction. Third, an extended-length PCR method is preferred in order to allow the use of a single PCR primer set. And fourth, because of the non-template-dependent terminal extension activity of some thermostable polymerases it is often necessary to incorporate an end-polishing step into the procedure prior to blunt-end ligation of the PCR-generated mutant product.
Methods of random mutagenesis, which will result in a panel of mutants bearing one or more randomly situated mutations, exist in the art. Such a panel of mutants may then be screened for those exhibiting the desired properties, for example, increased stability, relative to a wild-type reverse transcriptase.
An example of a method for random mutagenesis is the so-called “error-prone PCR method.” As the name implies, the method amplifies a given sequence under conditions in which the DNA polymerase does not support high fidelity incorporation. Although the conditions encouraging error-prone incorporation for different DNA polymerases vary, one skilled in the art may determine such conditions for a given enzyme. A key variable for many DNA polymerases in the fidelity of amplification is, for example, the type and concentration of divalent metal ion in the buffer. The use of manganese ion and/or variation of the magnesium or manganese ion concentration may therefore be applied to influence the error rate of the polymerase.
In various aspects, the RT of the prime editors may be an “error-prone” reverse transcriptase variant. Error-prone reverse transcriptases that are known and/or available in the art may be used. It will be appreciated that reverse transcriptases naturally do not have any proofreading function; thus the error rate of reverse transcriptase is generally higher than DNA polymerases comprising a proofreading activity. The error-rate of any particular reverse transcriptase is a property of the enzyme's “fidelity,” which represents the accuracy of template-directed polymerization of DNA against its RNA template. An RT with high fidelity has a low-error rate. Conversely, an RT with low fidelity has a high-error rate. The fidelity of M-MLV-based reverse transcriptases are reported to have an error rate in the range of one error in 15,000 to 27,000 nucleotides synthesized. See Boutabout et al., “DNA synthesis fidelity by the reverse transcriptase of the yeast retrotransposon Ty1,” Nucleic Acids Res, 2001, 29: 2217-2222, which is incorporated by reference. Thus, for purposes of this application, those reverse transcriptases considered to be “error-prone” or which are considered to have an “error-prone fidelity” are those having an error rate that is less than one error in 15,000 nucleotides synthesized.
Error-prone reverse transcriptase also may be created through mutagenesis of a starting RT enzyme (e.g., a wild type M-MLV RT). The method of mutagenesis is not limited and may include directed evolution processes, such as phage-assisted continuous evolution (PACE) or phage-assisted noncontinuous evolution (PANCE). The term “phage-assisted continuous evolution (PACE),” as used herein, refers to continuous evolution that employs phage as viral vectors. The general concept of PACE technology has been described, for example, in International PCT Application, PCT/US2009/056194, filed Sep. 8, 2009, published as WO 2010/028347 on Mar. 11, 2010; International PCT Application, PCT/US2011/066747, filed Dec. 22, 2011, published as WO 2012/088381 on Jun. 28, 2012; U.S. application, U.S. Pat. No. 9,023,594, issued May 5, 2015, International PCT Application, PCT/US2015/012022, filed Jan. 20, 2015, published as WO 2015/134121 on Sep. 11, 2015, and International PCT Application, PCT/US2016/027795, filed Apr. 15, 2016, published as WO 2016/168631 on Oct. 20, 2016, the entire contents of each of which are incorporated herein by reference.
Error-prone reverse transcriptases may also be obtain by phage-assisted non-continuous evolution (PANCE),” which as used herein, refers to non-continuous evolution that employs phage as viral vectors. PANCE is a simplified technique for rapid in vivo directed evolution using serial flask transfers of evolving ‘selection phage’ (SP), which contain a gene of interest to be evolved, across fresh E. coli host cells, thereby allowing genes inside the host E. coli to be held constant while genes contained in the SP continuously evolve. Serial flask transfers have long served as a widely-accessible approach for laboratory evolution of microbes, and, more recently, analogous approaches have been developed for bacteriophage evolution. The PANCE system features lower stringency than the PACE system.
Other error-prone reverse transcriptases have been described in the literature, each of which are contemplated for use in the herein methods and compositions. For example, error-prone reverse transcriptases have been described in Bebenek et al., “Error-prone Polymerization by HIV-1 Reverse Transcriptase,” J Biol Chem, 1993, Vol. 268: 10324-10334 and Sebastian-Martin et al., “Transcriptional inaccuracy threshold attenuates differences in RNA-dependent DNA synthesis fidelity between retroviral reverse transcriptases,” Scientific Reports, 2018, Vol. 8: 627, each of which are incorporated by reference. Still further, reverse transcriptases, including error-prone reverse transcriptases can be obtained from a commercial supplier, including ProtoScript® (II) Reverse Transcriptase, AMV Reverse Transcriptase, WarmStart® Reverse Transcriptase, and M-MuLV Reverse Transcriptase, all from NEW ENGLAND BIOLABS®, or AMV Reverse Transcriptase XL, SMARTScribe Reverse Transcriptase, GPR ultra-pure MMLV Reverse Transcriptase, all from TAKARA BIO USA, INC. (formerly CLONTECH).
The herein disclosure also contemplates reverse transcriptases having mutations in RNaseH domain. As mentioned above, one of the intrinsic properties of reverse transcriptases is the RNase H activity, which cleaves the RNA template of the RNA:cDNA hybrid concurrently with polymerization. The RNase H activity can be undesirable for synthesis of long cDNAs because the RNA template may be degraded before completion of full-length reverse transcription. The RNase H activity may also lower reverse transcription efficiency, presumably due to its competition with the polymerase activity of the enzyme. Thus, the present disclosure contemplates any reverse transcriptase variants that comprise a modified RNaseH activity.
The herein disclosure also contemplates reverse transcriptases having mutations in the RNA-dependent DNA polymerase domain. As mentioned above, one of the intrinsic properties of reverse transcriptases is the RNA-dependent DNA polymerase activity, which incorporates the nucleobases into the nascent cDNA strand as coded by the template RNA strand of the RNA:cDNA hybrid. The RNA-dependent DNA polymerase activity can be increased or decreased (i.e., in terms of its rate of incorporation) to either increase or decrease the processivity of the enzyme. Thus, the present disclosure contemplates any reverse transcriptase variants that comprise a modified RNA-dependent DNA polymerase activity such that the processivity of the enzyme of either increased or decreased relative to an unmodified version.
Also contemplated herein are reverse transcriptase variants that have altered thermostability characteristics. The ability of a reverse transcriptase to withstand high temperatures is an important aspect of cDNA synthesis. Elevated reaction temperatures help denature RNA with strong secondary structures and/or high GC content, allowing reverse transcriptases to read through the sequence. As a result, reverse transcription at higher temperatures enables full-length cDNA synthesis and higher yields, which can lead to an improved generation of the 3′ flap ssDNA as a result of the prime editing process. Wild type M-MLV reverse transcriptase typically has an optimal temperature in the range of 37-48° C.; however, mutations may be introduced that allow for the reverse transcription activity at higher temperatures of over 48° C., including 49° C., 50° C., 51° C., 52° C., 53° C., 54° C., 55° C., 56° C., 57° C., 58° C., 59° C., 60° C., 61° C., 62° C., 63° C., 64° C., 65° C., 66° C., and higher.
The variant reverse transcriptases contemplated herein, including error-prone RTs, thermostable RTs, increase-processivity RTs, can be engineered by various routine strategies, including mutagenesis or evolutionary processes. In some cases, the variants can be produced by introducing a single mutation. In other cases, the variants may require more than one mutation. For those mutants comprising more than one mutation, the effect of a given mutation may be evaluated by introduction of the identified mutation to the wild-type gene by site-directed mutagenesis in isolation from the other mutations borne by the particular mutant. Screening assays of the single mutant thus produced will then allow the determination of the effect of that mutation alone.
Variant RT enzymes used herein may also include other “RT variants” having at least about 70% identical, at least about 80% identical, at least about 90% identical, at least about 95% identical, at least about 96% identical, at least about 97% identical, at least about 98% identical, at least about 99% identical, at least about 99.5% identical, or at least about 99.9% identical to any reference RT protein, including any wild type RT, or mutant RT, or fragment RT, or other variant of RT disclosed or contemplated herein or known in the art.
In some embodiments, an RT variant may have 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 21, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, or up to 100, or up to 200, or up to 300, or up to 400, or up to 500 or more amino acid changes compared to a reference RT. In some embodiments, the RT variant comprises a fragment of a reference RT, such that the fragment is at least about 70% identical, at least about 80% identical, at least about 90% identical, at least about 95% identical, at least about 96% identical, at least about 97% identical, at least about 98% identical, at least about 99% identical, at least about 99.5% identical, or at least about 99.9% identical to the corresponding fragment of the reference RT. In some embodiments, the fragment is at least 30%, at least 35%, at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95% identical, at least 96%, at least 97%, at least 98%, at least 99%, or at least 99.5% of the amino acid length of a corresponding wild type RT (M-MLV reverse transcriptase) (e.g., SEQ ID NO: 89) or to any of the reverse transcriptases of SEQ ID NOs: 90-100.
In some embodiments, the disclosure also may utilize RT fragments which retain their functionality and which are fragments of any herein disclosed RT proteins. In some embodiments, the RT fragment is at least 100 amino acids in length. In some embodiments, the fragment is at least 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, or up to 600 or more amino acids in length.
In still other embodiments, the disclosure also may utilize RT variants which are truncated at the N-terminus or the C-terminus, or both, by a certain number of amino acids which results in a truncated variant which still retains sufficient polymerase function. In some embodiments, the RT truncated variant has a truncation of at least 1, at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, at least 10, at least 11, at least 12, at least 13, at least 14, at least 15, at least 16, at least 17, at least 18, at least 19, at least 20, at least 21, at least 22, at least 23, at least 24, at least 25, at least 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, or 250 amino acids at the N-terminal end of the protein. In other embodiments, the RT truncated variant has a truncation of at least 1, at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, at least 10, at least 11, at least 12, at least 13, at least 14, at least 15, at least 16, at least 17, at least 18, at least 19, at least 20, at least 21, at least 22, at least 23, at least 24, at least 25, at least 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, or 250 amino acids at the C-terminal end of the protein. In still other embodiments, the RT truncated variant has a truncation at the N-terminal and the C-terminal end which are the same or different lengths.
For example, the prime editors disclosed herein may include a truncated version of M-MLV reverse transcriptase. In this embodiment, the reverse transcriptase contains 4 mutations (D200N, T306K, W313F, T330P; noting that the L603W mutation present in PE2 is no longer present due to the truncation). The DNA sequence encoding this truncated editor is 522 bp smaller than PE2, and therefore makes its potentially useful for applications where delivery of the DNA sequence is challenging due to its size (i.e., adeno-associated virus and lentivirus delivery). This embodiment is referred to as MMLV-RT(trunc) and has the following amino acid sequence:
In various embodiments, the prime editors disclosed herein may comprise one of the RT variants described herein, or a RT variant thereof having at least about 70% identical, at least about 80% identical, at least about 90% identical, at least about 95% identical, at least about 96% identical, at least about 97% identical, at least about 98% identical, at least about 99% identical, at least about 99.5% identical, or at least about 99.9% identical to any reference Cas9 variants.
In still other embodiments, the present methods and compositions may utilize a DNA polymerase that has been evolved into a reverse transcriptase, as described in Effefson et al., “Synthetic evolutionary origin of a proofreading reverse transcriptase,” Science, Jun. 24, 2016, Vol. 352: 1590-1593, the contents of which are incorporated herein by reference.
In certain other embodiments, the reverse transcriptase is provided as a component of a fusion protein also comprising a napDNAbp. In other words, in some embodiments, the reverse transcriptase is fused to a napDNAbp as a fusion protein.
In various embodiments, variant reverse transcriptases can be engineered from wild type M-MLV reverse transcriptase as represented by SEQ ID NO: 89.
In various embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising one or more of the following mutations: P51L, S67K, E69K, L139P, T197A, D200N, H204R, F209N, E302K, E302R, T306K, F309N, W313F, T330P, L345G, L435G, N454K, D524G, E562Q, D583N, H594Q, L603W, E607K, or D653N in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence.
Some exemplary reverse transcriptases that can be fused to napDNAbp proteins or provided as individual proteins according to various embodiments of this disclosure are provided below. Exemplary reverse transcriptases include variants with at least 80%, at least 85%, at least 90%, at least 95%, or at least 99% sequence identity to the following wild-type enzymes or partial enzymes:
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising one or more of the following mutations: P51X, S67X, E69X, L139X, T197X, D200X, H204X, F209X, E302X, T306X, F309X, W313X, T330X, L345X, L435X, N454X, D524X, E562X, D583X, H594X, L603X, E607X, or D653X in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a P51X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is L.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a S67X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is K.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a E69X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is K.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a L139X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is P.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a T197X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is A.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a D200X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is N.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a H204X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is R.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a F209X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is N.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a E302X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is K.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a E302X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is R.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a T306X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is K.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a F309X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is N.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a W313X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is F.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a T330X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is P.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a L345X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is G.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a L435X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is G.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a N454X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is K.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a D524X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is G.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a E562X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is Q.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a D583X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is N.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a H594X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is Q.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a L603X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is W.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a E607X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is K.
In various other embodiments, the prime editors described herein (with RT provided as either a fusion partner or in trans) can include a variant RT comprising a D653X mutation in the wild type M-MLV RT of SEQ ID NO: 89 or at a corresponding amino acid position in another wild type RT polypeptide sequence, wherein “X” can be any amino acid. In certain embodiments, X is N.
Some exemplary reverse transcriptases that can be fused to napDNAbp proteins or provided as individual proteins according to various embodiments of this disclosure are provided below. Exemplary reverse transcriptases include variants with at least 80%, at least 85%, at least 90%, at least 95% or at least 99% sequence identity to the following wild-type enzymes or partial enzymes:
The prime editor (PE) system described here contemplates any publicly-available reverse transcriptase described or disclosed in any of the following U.S. patents (each of which are incorporated by reference in their entireties): U.S. Pat. Nos. 10,202,658; 10,189,831; 10,150,955; 9,932,567; 9,783,791; 9,580,698; 9,534,201; and 9,458,484, and any variant thereof that can be made using known methods for installing mutations, or known methods for evolving proteins. The following references describe reverse transcriptases in art. Each of their disclosures are incorporated herein by reference in their entireties.
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Luan, D. D., Korman, M. H., Jakubczak, J. L. & Eickbush, T. H. Reverse transcription of R2Bm RNA is primed by a nick at the chromosomal target site: a mechanism for non-LTR retrotransposition. Cell 72, 595-605 (1993).
Nottingham, R. M. et al. RNA-seq of human reference RNA samples using a thermostable group II intron reverse transcriptase. RNA 22, 597-613 (2016).
Telesnitsky, A. & Goff, S. P. RNase H domain mutations affect the interaction between Moloney murine leukemia virus reverse transcriptase and its primer-template. Proc. Natl. Acad. Sci. U.S.A. 90, 1276-1280 (1993).
Halvas, E. K., Svarovskaia, E. S. & Pathak, V. K. Role of Murine Leukemia Virus Reverse Transcriptase Deoxyribonucleoside Triphosphate-Binding Site in Retroviral Replication and In Vivo Fidelity. Journal of Virology 74, 10349-10358 (2000).
Nowak, E. et al. Structural analysis of monomeric retroviral reverse transcriptase in complex with an RNA/DNA hybrid. Nucleic Acids Res 41, 3874-3887 (2013).
Stamos, J. L., Lentzsch, A. M. & Lambowitz, A. M. Structure of a Thermostable Group II Intron Reverse Transcriptase with Template-Primer and Its Functional and Evolutionary Implications. Molecular Cell 68, 926-939.e4 (2017).
Das, D. & Georgiadis, M. M. The Crystal Structure of the Monomeric Reverse Transcriptase from Moloney Murine Leukemia Virus. Structure 12, 819-829 (2004).
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Any of the references noted above which relate to reverse transcriptases are hereby incorporated by reference in their entireties, if not already stated so.
[4] PE Fusion ProteinsThe prime editor (PE) system described herein contemplate fusion proteins comprising a napDNAbp and a polymerase (e.g., DNA-dependent DNA polymerase or RNA-dependent DNA polymerase, such as, reverse transcriptase), and optionally joined by a linker. The application contemplates any suitable napDNAbp and polymerase (e.g., DNA-dependent DNA polymerase or RNA-dependent DNA polymerase, such as, reverse transcriptase) to be combined in a single fusion protein. Examples of napDNAbps and polymerases (e.g., DNA-dependent DNA polymerase or RNA-dependent DNA polymerase, such as, reverse transcriptase) are each defined herein. Since polymerases are well-known in the art, and the amino acid sequences are readily available, this disclosure is not meant in any way to be limited to those specific polymerases identified herein.
In various embodiments, the fusion proteins may comprise any suitable structural configuration. For example, the fusion protein may comprise from the N-terminus to the C-terminus direction, a napDNAbp fused to a polymerase (e.g., DNA-dependent DNA polymerase or RNA-dependent DNA polymerase, such as, reverse transcriptase). In other embodiments, the fusion protein may comprise from the N-terminus to the C-terminus direction, a polymerase (e.g., a reverse transcriptase) fused to a napDNAbp. The fused domain may optionally be joined by a linker, e.g., an amino acid sequence. In other embodiments, the fusion proteins may comprise the structure NH2-[napDNAbp]-[polymerase]-COOH; or NH2-[polymerase]-[napDNAbp]-COOH, wherein each instance of “]-[” indicates the presence of an optional linker sequence. In embodiments wherein the polymerase is a reverse transcriptase, the fusion proteins may comprise the structure NH2-[napDNAbp]-[RT]-COOH; or NH2-[RT]-[napDNAbp]-COOH, wherein each instance of “]-[” indicates the presence of an optional linker sequence.
An exemplary fusion protein is depicted in
In various embodiments, the prime editor fusion protein may have the following amino acid sequence (referred to herein as “PE1”), which includes a Cas9 variant comprising an H840A mutation (i.e., a Cas9 nickase) and an M-MLV RT wild type, as well as an N-terminal NLS sequence (19 amino acids) and an amino acid linker (32 amino acids) that joins the C-terminus of the Cas9 nickase domain to the N-terminus of the RT domain. The PE1 fusion protein has the following structure: [NLS]-[Cas9(H840A)]-[linker]-[MMLV_RT(wt)]. The amino acid sequence of PE1 and its individual components are as follows:
In another embodiment, the prime editor fusion protein may have the following amino acid sequence (referred to herein as “PE2”), which includes a Cas9 variant comprising an H840A mutation (i.e., a Cas9 nickase) and an M-MLV RT comprising mutations D200N, T330P, L603W, T306K, and W313F, as well as an N-terminal NLS sequence (19 amino acids) and an amino acid linker (33 amino acids) that joins the C-terminus of the Cas9 nickase domain to the N-terminus of the RT domain. The PE2 fusion protein has the following structure: [NLS]-[Cas9(H840A)]-[linker]-[MMLV_RT(D200N)(T330P)(L603W)(T306K)(W313F)]. The amino acid sequence of PE2 is as follows:
In still other embodiments, the prime editor fusion protein may have the following amino acid sequences:
In other embodiments, the prime editor fusion proteins can be based on SaCas9 or on SpCas9 nickases with altered PAM specificities, such as the following exemplary sequences:
In yet other embodiments, the prime editor fusion proteins contemplated herein may include a Cas9 nickase (e.g., Cas9 (H840A)) fused to a truncated version of M-MLV reverse transcriptase. In this embodiment, the reverse transcriptase also contains 4 mutations (D200N, T306K, W313F, T330P; noting that the L603W mutation present in PE2 is no longer present due to the truncation). The DNA sequence encoding this truncated editor is 522 bp smaller than PE2, and therefore makes its potentially useful for applications where delivery of the DNA sequence is challenging due to its size (i.e. adeno-associated virus and lentivirus delivery). This embodiment is referred to as Cas9(H840A)-MMLV-RT(trunc) or “PE2-short” or “PE2-trunc” and has the following amino acid sequence:
See
In various embodiments, the prime editor fusion proteins contemplated herein may also include any variants of the above-disclosed sequences having an amino acid sequence that is at least about 70% identical, at least about 80% identical, at least about 90% identical, at least about 95% identical, at least about 96% identical, at least about 97% identical, at least about 98% identical, at least about 99% identical, at least about 99.5% identical, or at least about 99.9% identical to PE1, PE2, or any of the above indicated prime editor fusion sequences.
In certain embodiments, linkers may be used to link any of the peptides or peptide domains or moieties of the invention (e.g., a napDNAbp linked or fused to a reverse transcriptase).
[5] Linkers and Other DomainsThe PE fusion proteins may comprise various other domains besides the napDNAbp (e.g., Cas9 domain) and the polymerase domain (e.g., RT domain). For example, in the case where the napDNAbp is a Cas9 and the polymerase is a RT, the PE fusion proteins may comprise one or more linkers that join the Cas9 domain with the RT domain. The linkers may also join other functional domains, such as nuclear localization sequences (NLS) or a FEN1 (or other flap endonuclease) to the PE fusion proteins or a domain thereof.
In addition, in embodiments involving trans prime editing, linkers may be used to link tPERT recruitment protein to a prime editor, e.g., between the tPERt recruitment protein and the napDNAbp. See e.g.,
A. Linkers
As defined above, the term “linker,” as used herein, refers to a chemical group or a molecule linking two molecules or moieties, e.g., a binding domain and a cleavage domain of a nuclease. In some embodiments, a linker joins a gRNA binding domain of an RNA-programmable nuclease and the catalytic domain of a polymerase (e.g., a reverse transcriptase). In some embodiments, a linker joins a dCas9 and reverse transcriptase. Typically, the linker is positioned between, or flanked by, two groups, molecules, or other moieties and connected to each one via a covalent bond, thus connecting the two. In some embodiments, the linker is an amino acid or a plurality of amino acids (e.g., a peptide or protein). In some embodiments, the linker is an organic molecule, group, polymer, or chemical moiety. In some embodiments, the linker is 5-100 amino acids in length, for example, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 30-35, 35-40, 40-45, 45-50, 50-60, 60-70, 70-80, 80-90, 90-100, 100-150, or 150-200 amino acids in length. Longer or shorter linkers are also contemplated.
The linker may be as simple as a covalent bond, or it may be a polymeric linker many atoms in length. In certain embodiments, the linker is a polypeptide or based on amino acids. In other embodiments, the linker is not peptide-like. In certain embodiments, the linker is a covalent bond (e.g., a carbon-carbon bond, disulfide bond, carbon-heteroatom bond, etc.). In certain embodiments, the linker is a carbon-nitrogen bond of an amide linkage. In certain embodiments, the linker is a cyclic or acyclic, substituted or unsubstituted, branched or unbranched aliphatic or heteroaliphatic linker. In certain embodiments, the linker is polymeric (e.g., polyethylene, polyethylene glycol, polyamide, polyester, etc.). In certain embodiments, the linker comprises a monomer, dimer, or polymer of aminoalkanoic acid. In certain embodiments, the linker comprises an aminoalkanoic acid (e.g., glycine, ethanoic acid, alanine, beta-alanine, 3-aminopropanoic acid, 4-aminobutanoic acid, 5-pentanoic acid, etc.). In certain embodiments, the linker comprises a monomer, dimer, or polymer of aminohexanoic acid (Ahx). In certain embodiments, the linker is based on a carbocyclic moiety (e.g., cyclopentane, cyclohexane). In other embodiments, the linker comprises a polyethylene glycol moiety (PEG). In other embodiments, the linker comprises amino acids. In certain embodiments, the linker comprises a peptide. In certain embodiments, the linker comprises an aryl or heteroaryl moiety. In certain embodiments, the linker is based on a phenyl ring. The linker may included funtionalized moieties to facilitate attachment of a nucleophile (e.g., thiol, amino) from the peptide to the linker. Any electrophile may be used as part of the linker. Exemplary electrophiles include, but are not limited to, activated esters, activated amides, Michael acceptors, alkyl halides, aryl halides, acyl halides, and isothiocyanates.
In some other embodiments, the linker comprises the amino acid sequence (GGGGS)n (SEQ ID NO: 165), (G)n (SEQ ID NO: 166), (EAAAK)n (SEQ ID NO: 167), (GGS)n (SEQ ID NO: 168), (SGGS)n (SEQ ID NO: 169), (XP)n (SEQ ID NO: 170), or any combination thereof, wherein n is independently an integer between 1 and 30, and wherein X is any amino acid. In some embodiments, the linker comprises the amino acid sequence (GGS)N(SEQ ID NO: 176), wherein n is 1, 3, or 7. In some embodiments, the linker comprises the amino acid sequence SGSETPGTSESATPES (SEQ ID NO: 171). In some embodiments, the linker comprises the amino acid sequence SGGSSGGSSGSETPGTSESATPESSGGSSGGS (SEQ ID NO: 172). In some embodiments, the linker comprises the amino acid sequence SGGSGGSGGS (SEQ ID NO: 173). In some embodiments, the linker comprises the amino acid sequence SGGS (SEQ ID NO: 174). In other embodiments, the linker comprises the amino acid sequence
In certain embodiments, linkers may be used to link any of the peptides or peptide domains or moieties of the invention (e.g., a napDNAbp linked or fused to a reverse transcriptase).
As defined above, the term “linker,” as used herein, refers to a chemical group or a molecule linking two molecules or moieties, e.g., a binding domain and a cleavage domain of a nuclease. In some embodiments, a linker joins a gRNA binding domain of an RNA-programmable nuclease and the catalytic domain of a recombinase. In some embodiments, a linker joins a dCas9 and reverse transcriptase. Typically, the linker is positioned between, or flanked by, two groups, molecules, or other moieties and connected to each one via a covalent bond, thus connecting the two. In some embodiments, the linker is an amino acid or a plurality of amino acids (e.g., a peptide or protein). In some embodiments, the linker is an organic molecule, group, polymer, or chemical moiety. In some embodiments, the linker is 5-100 amino acids in length, for example, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 30-35, 35-40, 40-45, 45-50, 50-60, 60-70, 70-80, 80-90, 90-100, 100-150, or 150-200 amino acids in length. Longer or shorter linkers are also contemplated.
The linker may be as simple as a covalent bond, or it may be a polymeric linker many atoms in length. In certain embodiments, the linker is a polypeptide or based on amino acids. In other embodiments, the linker is not peptide-like. In certain embodiments, the linker is a covalent bond (e.g., a carbon-carbon bond, disulfide bond, carbon-heteroatom bond, etc.). In certain embodiments, the linker is a carbon-nitrogen bond of an amide linkage. In certain embodiments, the linker is a cyclic or acyclic, substituted or unsubstituted, branched or unbranched aliphatic or heteroaliphatic linker. In certain embodiments, the linker is polymeric (e.g., polyethylene, polyethylene glycol, polyamide, polyester, etc.). In certain embodiments, the linker comprises a monomer, dimer, or polymer of aminoalkanoic acid. In certain embodiments, the linker comprises an aminoalkanoic acid (e.g., glycine, ethanoic acid, alanine, beta-alanine, 3-aminopropanoic acid, 4-aminobutanoic acid, 5-pentanoic acid, etc.). In certain embodiments, the linker comprises a monomer, dimer, or polymer of aminoHEXAnoic acid (Ahx). In certain embodiments, the linker is based on a carbocyclic moiety (e.g., cyclopentane, cycloHEXAne). In other embodiments, the linker comprises a polyethylene glycol moiety (PEG). In other embodiments, the linker comprises amino acids. In certain embodiments, the linker comprises a peptide. In certain embodiments, the linker comprises an aryl or heteroaryl moiety. In certain embodiments, the linker is based on a phenyl ring. The linker may included funtionalized moieties to facilitate attachment of a nucleophile (e.g., thiol, amino) from the peptide to the linker. Any electrophile may be used as part of the linker. Exemplary electrophiles include, but are not limited to, activated esters, activated amides, Michael acceptors, alkyl halides, aryl halides, acyl halides, and isothiocyanates.
In some other embodiments, the linker comprises the amino acid sequence (GGGGS)n (SEQ ID NO: 165), (G)n (SEQ ID NO: 166), (EAAAK)n (SEQ ID NO: 167), (GGS)n (SEQ ID NO: 168), (SGGS)n (SEQ ID NO: 169), (XP)n (SEQ ID NO: 170), or any combination thereof, wherein n is independently an integer between 1 and 30, and wherein X is any amino acid. In some embodiments, the linker comprises the amino acid sequence (GGS)N(SEQ ID NO: 176), wherein n is 1, 3, or 7. In some embodiments, the linker comprises the amino acid sequence SGSETPGTSESATPES (SEQ ID NO: 171). In some embodiments, the linker comprises the amino acid sequence SGGSSGGSSGSETPGTSESATPESSGGSSGGS (SEQ ID NO: 172). In some embodiments, the linker comprises the amino acid sequence SGGSGGSGGS (SEQ ID NO: 173). In some embodiments, the linker comprises the amino acid sequence SGGS (SEQ ID NO: 174).
In particular, the following linkers can be used in various embodiments to join prime editor domains with one another:
B. Nuclear Localization Sequence (NLS)
In various embodiments, the PE fusion proteins may comprise one or more nuclear localization sequences (NLS), which help promote translocation of a protein into the cell nucleus. Such sequences are well-known in the art and can include the following examples:
The NLS examples above are non-limiting. The PE fusion proteins may comprise any known NLS sequence, including any of those described in Cokol et al., “Finding nuclear localization signals,” EMBO Rep., 2000, 1(5): 411-415 and Freitas et al., “Mechanisms and Signals for the Nuclear Import of Proteins,” Current Genomics, 2009, 10(8): 550-7, each of which are incorporated herein by reference.
In various embodiments, the prime editors and constructs encoding the prime editors disclosed herein further comprise one or more, preferably, at least two nuclear localization signals. In certain embodiments, the prime editors comprise at least two NLSs. In embodiments with at least two NLSs, the NLSs can be the same NLSs or they can be different NLSs. In addition, the NLSs may be expressed as part of a fusion protein with the remaining portions of the prime editors. In some embodiments, one or more of the NLSs are bipartite NLSs (“bpNLS”). In certain embodiments, the disclosed fusion proteins comprise two bipartite NLSs. In some embodiments, the disclosed fusion proteins comprise more than two bipartite NLSs.
The location of the NLS fusion can be at the N-terminus, the C-terminus, or within a sequence of a prime editor (e.g., inserted between the encoded napDNAbp component (e.g., Cas9) and a polymerase domain (e.g., a reverse transcriptase domain).
The NLSs may be any known NLS sequence in the art. The NLSs may also be any future-discovered NLSs for nuclear localization. The NLSs also may be any naturally-occurring NLS, or any non-naturally occurring NLS (e.g., an NLS with one or more desired mutations).
The term “nuclear localization sequence” or “NLS” refers to an amino acid sequence that promotes import of a protein into the cell nucleus, for example, by nuclear transport. Nuclear localization sequences are known in the art and would be apparent to the skilled artisan. For example, NLS sequences are described in Plank et al., International PCT application PCT/EP2000/011690, filed Nov. 23, 2000, published as WO/2001/038547 on May 31, 2001, the contents of which are incorporated herein by reference. In some embodiments, an NLS comprises the amino acid sequence PKKKRKV (SEQ ID NO: 16), MDSLLMNRRKFLYQFKNVRWAKGRRETYLC (SEQ ID NO: 17), KRTADGSEFESPKKKRKV (SEQ ID NO: 3864), or KRTADGSEFEPKKKRKV (SEQ ID NO: 13). In other embodiments, NLS comprises the amino acid sequences
In one aspect of the disclosure, a prime editor may be modified with one or more nuclear localization signals (NLS), preferably at least two NLSs. In certain embodiments, the prime editors are modified with two or more NLSs. The disclosure contemplates the use of any nuclear localization signal known in the art at the time of the disclosure, or any nuclear localization signal that is identified or otherwise made available in the state of the art after the time of the instant filing. A representative nuclear localization signal is a peptide sequence that directs the protein to the nucleus of the cell in which the sequence is expressed. A nuclear localization signal is predominantly basic, can be positioned almost anywhere in a protein's amino acid sequence, generally comprises a short sequence of four amino acids (Autieri & Agrawal, (1998) J. Biol. Chem. 273: 14731-37, incorporated herein by reference) to eight amino acids, and is typically rich in lysine and arginine residues (Magin et al., (2000) Virology 274: 11-16, incorporated herein by reference). Nuclear localization signals often comprise proline residues. A variety of nuclear localization signals have been identified and have been used to effect transport of biological molecules from the cytoplasm to the nucleus of a cell. See, e.g., Tinland et al., (1992) Proc. Natl. Acad. Sci. U.S.A. 89:7442-46; Moede et al., (1999) FEBS Lett. 461:229-34, which is incorporated by reference. Translocation is currently thought to involve nuclear pore proteins.
Most NLSs can be classified in three general groups: (i) a monopartite NLS exemplified by the SV40 large T antigen NLS (PKKKRKV (SEQ ID NO: 16)); (ii) a bipartite motif consisting of two basic domains separated by a variable number of spacer amino acids and exemplified by the Xenopus nucleoplasmin NLS (KRXXXXXXXXXXKKKL (SEQ ID NO: 3868)); and (iii) noncanonical sequences such as M9 of the hnRNP A1 protein, the influenza virus nucleoprotein NLS, and the yeast Gal4 protein NLS (Dingwall and Laskey 1991).
Nuclear localization signals appear at various points in the amino acid sequences of proteins. NLS's have been identified at the N-terminus, the C-terminus and in the central region of proteins. Thus, the disclosure provides prime editors that may be modified with one or more NLSs at the C-terminus, the N-terminus, as well as at in internal region of the prime editor. The residues of a longer sequence that do not function as component NLS residues should be selected so as not to interfere, for example tonically or sterically, with the nuclear localization signal itself. Therefore, although there are no strict limits on the composition of an NLS-comprising sequence, in practice, such a sequence can be functionally limited in length and composition.
The present disclosure contemplates any suitable means by which to modify a prime editor to include one or more NLSs. In one aspect, the prime editors may be engineered to express a prime editor protein that is translationally fused at its N-terminus or its C-terminus (or both) to one or more NLSs, i.e., to form a prime editor-NLS fusion construct. In other embodiments, the prime editor-encoding nucleotide sequence may be genetically modified to incorporate a reading frame that encodes one or more NLSs in an internal region of the encoded prime editor. In addition, the NLSs may include various amino acid linkers or spacer regions encoded between the prime editor and the N-terminally, C-terminally, or internally-attached NLS amino acid sequence, e.g, and in the central region of proteins. Thus, the present disclosure also provides for nucleotide constructs, vectors, and host cells for expressing fusion proteins that comprise a prime editor and one or more NLSs.
The prime editors described herein may also comprise nuclear localization signals which are linked to a prime editor through one or more linkers, e.g., and polymeric, amino acid, nucleic acid, polysaccharide, chemical, or nucleic acid linker element. The linkers within the contemplated scope of the disclosure are not indented to have any limitations and can be any suitable type of molecule (e.g., polymer, amino acid, polysaccharide, nucleic acid, lipid, or any synthetic chemical linker domain) and be joined to the prime editor by any suitable strategy that effectuates forming a bond (e.g., covalent linkage, hydrogen bonding) between the prime editor and the one or more NLS s.
C. Flap Endonucleases (e.g., FEN1)
In various embodiments, the PE fusion proteins may comprise one or more flap endonucleases (e.g., FEN1), which refers to an enzyme that catalyzes the removal of 5′ single strand DNA flaps. These are naturally occurring enzymes that process the removal of 5′ flaps formed during cellular processes, including DNA replication. The prime editing methods herein described may utilize endogenously supplied flap endonucleases or those provided in trans to remove the 5′ flap of endogenous DNA formed at the target site during prime editing. Flap endonucleases are known in the art and can be found described in Patel et al., “Flap endonucleases pass 5′-flaps through a flexible arch using a disorder-thread-order mechanism to confer specificity for free 5′-ends,” Nucleic Acids Research, 2012, 40(10): 4507-4519 and Tsutakawa et al., “Human flap endonuclease structures, DNA double-base flipping, and a unified understanding of the FEN1 superfamily,” Cell, 2011, 145(2): 198-211 (each of which are incorporated herein by reference). An exemplary flap endonuclease is FEN1, which can be represented by the following amino acid sequence:
The flap endonucleases may also include any FEN1 variant, mutant, or other flap endonuclease ortholog, homolog, or variant. Non-limiting FEN1 variant examples are as follows:
In various embodiments, the prime editor fusion proteins contemplated herein may include any flap endonuclease variant of the above-disclosed sequences having an amino acid sequence that is at least about 70% identical, at least about 80% identical, at least about 90% identical, at least about 95% identical, at least about 96% identical, at least about 97% identical, at least about 98% identical, at least about 99% identical, at least about 99.5% identical, or at least about 99.9% identical to any of the above sequences.
Other endonucleases that may be utilized by the instant methods to facilitate removal of the 5′ end single strand DNA flap include, but are not limited to (1) trex 2, (2) exol endonuclease (e.g., Keijzers et al., Biosci Rep. 2015, 35(3): e00206)
Human exonuclease 1 (EXO1) has been implicated in many different DNA metabolic processes, including DNA mismatch repair (MMR), micro-mediated end-joining, homologous recombination (HR), and replication. Human EXO1 belongs to a family of eukaryotic nucleases, Rad2/XPG, which also include FEN1 and GEN1. The Rad2/XPG family is conserved in the nuclease domain through species from phage to human. The EXO1 gene product exhibits both 5′ exonuclease and 5′ flap activity. Additionally, EXO1 contains an intrinsic 5′ RNase H activity. Human EXO1 has a high affinity for processing double stranded DNA (dsDNA), nicks, gaps, pseudo Y structures and can resolve Holliday junctions using its inherit flap activity. Human EXO1 is implicated in MMR and contain conserved binding domains interacting directly with MLH1 and MSH2. EXO1 nucleolytic activity is positively stimulated by PCNA, MutSa (MSH2/MSH6 complex), 14-3-3, MRN and 9-1-1 complex.
D. Inteins and Split-Inteins
It will be understood that in some embodiments (e.g., delivery of a prime editor in vivo using AAV particles), it may be advantageous to split a polypeptide (e.g., a deaminase or a napDNAbp) or a fusion protein (e.g., a prime editor) into an N-terminal half and a C-terminal half, delivery them separately, and then allow their colocalization to reform the complete protein (or fusion protein as the case may be) within the cell. Separate halves of a protein or a fusion protein may each comprise a split-intein tag to facilitate the reformation of the complete protein or fusion protein by the mechanism of protein trans splicing.
Protein trans-splicing, catalyzed by split inteins, provides an entirely enzymatic method for protein ligation. A split-intein is essentially a contiguous intein (e.g. a mini-intein) split into two pieces named N-intein and C-intein, respectively. The N-intein and C-intein of a split intein can associate non-covalently to form an active intein and catalyze the splicing reaction essentially in same way as a contiguous intein does. Split inteins have been found in nature and also engineered in laboratories. As used herein, the term “split intein” refers to any intein in which one or more peptide bond breaks exists between the N-terminal and C-terminal amino acid sequences such that the N-terminal and C-terminal sequences become separate molecules that can non-covalently reassociate, or reconstitute, into an intein that is functional for trans-splicing reactions. Any catalytically active intein, or fragment thereof, may be used to derive a split intein for use in the methods of the invention. For example, in one aspect the split intein may be derived from a eukaryotic intein. In another aspect, the split intein may be derived from a bacterial intein. In another aspect, the split intein may be derived from an archaeal intein. Preferably, the split intein so-derived will possess only the amino acid sequences essential for catalyzing trans-splicing reactions.
As used herein, the “N-terminal split intein (In)” refers to any intein sequence that comprises an N-terminal amino acid sequence that is functional for trans-splicing reactions. An In thus also comprises a sequence that is spliced out when trans-splicing occurs. An In can comprise a sequence that is a modification of the N-terminal portion of a naturally occurring intein sequence. For example, an In can comprise additional amino acid residues and/or mutated residues so long as the inclusion of such additional and/or mutated residues does not render the In non-functional in trans-splicing. Preferably, the inclusion of the additional and/or mutated residues improves or enhances the trans-splicing activity of the In.
As used herein, the “C-terminal split intein (Ic)” refers to any intein sequence that comprises a C-terminal amino acid sequence that is functional for trans-splicing reactions. In one aspect, the Ic comprises 4 to 7 contiguous amino acid residues, at least 4 amino acids of which are from the last β-strand of the intein from which it was derived. An Ic thus also comprises a sequence that is spliced out when trans-splicing occurs. An Ic can comprise a sequence that is a modification of the C-terminal portion of a naturally occurring intein sequence. For example, an Ic can comprise additional amino acid residues and/or mutated residues so long as the inclusion of such additional and/or mutated residues does not render the In non-functional in trans-splicing. Preferably, the inclusion of the additional and/or mutated residues improves or enhances the trans-splicing activity of the Ic.
In some embodiments of the invention, a peptide linked to an Ic or an In can comprise an additional chemical moiety including, among others, fluorescence groups, biotin, polyethylene glycol (PEG), amino acid analogs, unnatural amino acids, phosphate groups, glycosyl groups, radioisotope labels, and pharmaceutical molecules. In other embodiments, a peptide linked to an Ic can comprise one or more chemically reactive groups including, among others, ketone, aldehyde, Cys residues and Lys residues. The N-intein and C-intein of a split intein can associate non-covalently to form an active intein and catalyze the splicing reaction when an “intein-splicing polypeptide (ISP)” is present. As used herein, “intein-splicing polypeptide (ISP)” refers to the portion of the amino acid sequence of a split intein that remains when the Ic, In, or both, are removed from the split intein. In certain embodiments, the In comprises the ISP. In another embodiment, the Ic comprises the ISP. In yet another embodiment, the ISP is a separate peptide that is not covalently linked to In nor to Ic.
Split inteins may be created from contiguous inteins by engineering one or more split sites in the unstructured loop or intervening amino acid sequence between the −12 conserved beta-strands found in the structure of mini-inteins. Some flexibility in the position of the split site within regions between the beta-strands may exist, provided that creation of the split will not disrupt the structure of the intein, the structured beta-strands in particular, to a sufficient degree that protein splicing activity is lost.
In protein trans-splicing, one precursor protein consists of an N-extein part followed by the N-intein, another precursor protein consists of the C-intein followed by a C-extein part, and a trans-splicing reaction (catalyzed by the N- and C-inteins together) excises the two intein sequences and links the two extein sequences with a peptide bond. Protein trans-splicing, being an enzymatic reaction, can work with very low (e.g. micromolar) concentrations of proteins and can be carried out under physiological conditions.
Exemplary sequences are as follows:
Although inteins are most frequently found as a contiguous domain, some exist in a naturally split form. In this case, the two fragments are expressed as separate polypeptides and must associate before splicing takes place, so-called protein trans-splicing.
An exemplary split intein is the Ssp DnaE intein, which comprises two subunits, namely, DnaE-N and DnaE-C. The two different subunits are encoded by separate genes, namely dnaE-n and dnaE-c, which encode the DnaE-N and DnaE-C subunits, respectively. DnaE is a naturally occurring split intein in Synechocytis sp. PCC6803 and is capable of directing trans-splicing of two separate proteins, each comprising a fusion with either DnaE-N or DnaE-C.
Additional naturally occurring or engineered split-intein sequences are known in the or can be made from whole-intein sequences described herein or those available in the art. Examples of split-intein sequences can be found in Stevens et al., “A promiscuous split intein with expanded protein engineering applications,” PNAS, 2017, Vol. 114: 8538-8543; Iwai et al., “Highly efficient protein trans-splicing by a naturally split DnaE intein from Nostoc punctiforme, FEBS Lett, 580: 1853-1858, each of which are incorporated herein by reference. Additional split intein sequences can be found, for example, in WO 2013/045632, WO 2014/055782, WO 2016/069774, and EP2877490, the contents each of which are incorporated herein by reference.
In addition, protein splicing in trans has been described in vivo and in vitro (Shingledecker, et al., Gene 207:187 (1998), Southworth, et al., EMBO J. 17:918 (1998); Mills, et al., Proc. Natl. Acad. Sci. USA, 95:3543-3548 (1998); Lew, et al., J. Biol. Chem., 273:15887-15890 (1998); Wu, et al., Biochim. Biophys. Acta 35732:1 (1998b), Yamazaki, et al., J. Am. Chem. Soc. 120:5591 (1998), Evans, et al., J. Biol. Chem. 275:9091 (2000); Otomo, et al., Biochemistry 38:16040-16044 (1999); Otomo, et al., J. Biolmol. NMR 14:105-114 (1999); Scott, et al., Proc. Natl. Acad. Sci. USA 96:13638-13643 (1999)) and provides the opportunity to express a protein as to two inactive fragments that subsequently undergo ligation to form a functional product, e.g., as shown in
E. RNA-Protein Interaction Domain
In various embodiments, two separate protein domains (e.g., a Cas9 domain and a polymerase domain) may be colocalized to one another to form a functional complex (akin to the function of a fusion protein comprising the two separate protein domains) by using an “RNA-protein recruitment system,” such as the “MS2 tagging technique.” Such systems generally tag one protein domain with an “RNA-protein interaction domain” (aka “RNA-protein recruitment domain”) and the other with an “RNA-binding protein” that specifically recognizes and binds to the RNA-protein interaction domain, e.g., a specific hairpin structure. These types of systems can be leveraged to colocalize the domains of a prime editor, as well as to recruitment additional functionalities to a prime editor, such as a UGI domain. In one example, the MS2 tagging technique is based on the natural interaction of the MS2 bacteriophage coat protein (“MCP” or “MS2cp”) with a stem-loop or hairpin structure present in the genome of the phage, i.e., the “MS2 hairpin.” In the case of the MS2 hairpin, it is recognized and bound by the MS2 bacteriophage coat protein (MCP). Thus, in one exemplarily scenario a deaminase-MS2 fusion can recruit a Cas9-MCP fusion.
A review of other modular RNA-protein interaction domains are described in the art, for example, in Johansson et al., “RNA recognition by the MS2 phage coat protein,” Sem Virol., 1997, Vol. 8(3): 176-185; Delebecque et al., “Organization of intracellular reactions with rationally designed RNA assemblies,” Science, 2011, Vol. 333: 470-474; Mali et al., “Cas9 transcriptional activators for target specificity screening and paired nickases for cooperative genome engineering,” Nat. Biotechnol., 2013, Vol. 31: 833-838; and Zalatan et al., “Engineering complex synthetic transcriptional programs with CRISPR RNA scaffolds,” Cell, 2015, Vol. 160: 339-350, each of which are incorporated herein by reference in their entireties. Other systems include the PP7 hairpin, which specifically recruits the PCP protein, and the “com” hairpin, which specifically recruits the Com protein. See Zalatan et al.
The nucleotide sequence of the MS2 hairpin (or equivalently referred to as the “MS2 aptamer”) is
The amino acid sequence of the MCP or MS2cp is:
F. UGI Domain
In other embodiments, the prime editors described herein may comprise one or more uracil glycosylase inhibitor domains. The term “uracil glycosylase inhibitor (UGI)” or “UGI domain,” as used herein, refers to a protein that is capable of inhibiting a uracil-DNA glycosylase base-excision repair enzyme. In some embodiments, a UGI domain comprises a wild-type UGI or a UGI as set forth in SEQ ID NO: 3873. In some embodiments, the UGI proteins provided herein include fragments of UGI and proteins homologous to a UGI or a UGI fragment. For example, in some embodiments, a UGI domain comprises a fragment of the amino acid sequence set forth in SEQ ID NO: 3873. In some embodiments, a UGI fragment comprises an amino acid sequence that comprises at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 99.5% of the amino acid sequence as set forth in SEQ ID NO: 3873. In some embodiments, a UGI comprises an amino acid sequence homologous to the amino acid sequence set forth in SEQ ID NO: 3873, or an amino acid sequence homologous to a fragment of the amino acid sequence set forth in SEQ ID NO: 3873. In some embodiments, proteins comprising UGI or fragments of UGI or homologs of UGI or UGI fragments are referred to as “UGI variants.” A UGI variant shares homology to UGI, or a fragment thereof. For example a UGI variant is at least 70% identical, at least 75% identical, at least 80% identical, at least 85% identical, at least 90% identical, at least 95% identical, at least 96% identical, at least 97% identical, at least 98% identical, at least 99% identical, at least 99.5% identical, or at least 99.9% identical to a wild type UGI or a UGI as set forth in SEQ ID NO: 3873. In some embodiments, the UGI variant comprises a fragment of UGI, such that the fragment is at least 70% identical, at least 80% identical, at least 90% identical, at least 95% identical, at least 96% identical, at least 97% identical, at least 98% identical, at least 99% identical, at least 99.5% identical, or at least 99.9% to the corresponding fragment of wild-type UGI or a UGI as set forth in SEQ ID NO: 3873. In some embodiments, the UGI comprises the following amino acid sequence:
Uracil-DNA glycosylase inhibitor:
The prime editors described herein may comprise more than one UGI domain, which may be separated by one or more linkers as described herein.
G. Additional PE Elements
In certain embodiments, the prime editors described herein may comprise an inhibitor of base repair. The term “inhibitor of base repair” or “IBR” refers to a protein that is capable in inhibiting the activity of a nucleic acid repair enzyme, for example a base excision repair enzyme. In some embodiments, the IBR is an inhibitor of OGG base excision repair. In some embodiments, the IBR is an inhibitor of base excision repair (“iBER”). Exemplary inhibitors of base excision repair include inhibitors of APE1, Endo III, Endo IV, Endo V, Endo VIII, Fpg, hOGG1, hNEIL1, T7 EndoI, T4PDG, UDG, hSMUG1, and hAAG. In some embodiments, the IBR is an inhibitor of Endo V or hAAG. In some embodiments, the IBR is an iBER that may be a catalytically inactive glycosylase or catalytically inactive dioxygenase or a small molecule or peptide inhibitor of an oxidase, or variants thereof. In some embodiments, the IBR is an iBER that may be a TDG inhibitor, MBD4 inhibitor or an inhibitor of an AlkBH enzyme. In some embodiments, the IBR is an iBER that comprises a catalytically inactive TDG or catalytically inactive MBD4. An exemplary catalytically inactive TDG is an N140A mutant of SEQ ID NO: 3872 (human TDG).
Some exemplary glycosylases are provided below. The catalytically inactivated variants of any of these glycosylase domains are iBERs that may be fused to the napDNAbp or polymerase domain of the prime editors provided in this disclosure.
In some embodiments, the fusion proteins described herein may comprise one or more heterologous protein domains (e.g., about or more than about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more domains in addition to the prime editor components). A fusion protein may comprise any additional protein sequence, and optionally a linker sequence between any two domains. Other exemplary features that may be present are localization sequences, such as cytoplasmic localization sequences, export sequences, such as nuclear export sequences, or other localization sequences, as well as sequence tags that are useful for solubilization, purification, or detection of the fusion proteins.
Examples of protein domains that may be fused to a prime editor or component thereof (e.g., the napDNAbp domain, the polymerase domain, or the NLS domain) include, without limitation, epitope tags, and reporter gene sequences. Non-limiting examples of epitope tags include histidine (His) tags, V5 tags, FLAG tags, influenza hemagglutinin (HA) tags, Myc tags, VSV-G tags, and thioredoxin (Trx) tags. Examples of reporter genes include, but are not limited to, glutathione-5-transferase (GST), horseradish peroxidase (HRP), chloramphenicol acetyltransferase (CAT), beta-galactosidase, beta-glucuronidase, luciferase, green fluorescent protein (GFP), HcRed, DsRed, cyan fluorescent protein (CFP), yellow fluorescent protein (YFP), and autofluorescent proteins including blue fluorescent protein (BFP). A prime editor may be fused to a gene sequence encoding a protein or a fragment of a protein that bind DNA molecules or bind other cellular molecules, including, but not limited to, maltose binding protein (MBP), S-tag, Lex A DNA binding domain (DBD) fusions, GAL4 DNA binding domain fusions, and herpes simplex virus (HSV) BP16 protein fusions. Additional domains that may form part of a prime editor are described in US Patent Publication No. 2011/0059502, published Mar. 10, 2011 and incorporated herein by reference in its entirety.
In an aspect of the disclosure, a reporter gene which includes, but is not limited to, glutathione-5-transferase (GST), horseradish peroxidase (HRP), chloramphenicol acetyltransferase (CAT) beta-galactosidase, beta-glucuronidase, luciferase, green fluorescent protein (GFP), HcRed, DsRed, cyan fluorescent protein (CFP), yellow fluorescent protein (YFP), and autofluorescent proteins including blue fluorescent protein (BFP), may be introduced into a cell to encode a gene product which serves as a marker by which to measure the alteration or modification of expression of the gene product. In certain embodiments of the disclosure the gene product is luciferase. In a further embodiment of the disclosure the expression of the gene product is decreased.
Suitable protein tags provided herein include, but are not limited to, biotin carboxylase carrier protein (BCCP) tags, myc-tags, calmodulin-tags, FLAG-tags, hemagglutinin (HA)-tags, polyhistidine tags, also referred to as histidine tags or His-tags, maltose binding protein (MBP)-tags, nus-tags, glutathione-S-transferase (GST)-tags, green fluorescent protein (GFP)-tags, thioredoxin-tags, S-tags, Softags (e.g., Softag 1, Softag 3), strep-tags, biotin ligase tags, FlAsH tags, V5 tags, and SBP-tags. Additional suitable sequences will be apparent to those of skill in the art. In some embodiments, the fusion protein comprises one or more His tags.
In some embodiments of the present disclosure, the activity of the prime editing system may be temporally regulated by adjusting the residence time, the amount, and/or the activity of the expressed components of the PE system. For example, as described herein, the PE may be fused with a protein domain that is capable of modifying the intracellular half-life of the PE. In certain embodiments involving two or more vectors (e.g., a vector system in which the components described herein are encoded on two or more separate vectors), the activity of the PE system may be temporally regulated by controlling the timing in which the vectors are delivered. For example, in some embodiments a vector encoding the nuclease system may deliver the PE prior to the vector encoding the template. In other embodiments, the vector encoding the PEgRNA may deliver the guide prior to the vector encoding the PE system. In some embodiments, the vectors encoding the PE system and PEgRNA are delivered simultaneously. In certain embodiments, the simultaneously delivered vectors temporally deliver, e.g., the PE, PEgRNA, and/or second strand guide RNA components. In further embodiments, the RNA (such as, e.g., the nuclease transcript) transcribed from the coding sequence on the vectors may further comprise at least one element that is capable of modifying the intracellular half-life of the RNA and/or modulating translational control. In some embodiments, the half-life of the RNA may be increased. In some embodiments, the half-life of the RNA may be decreased. In some embodiments, the element may be capable of increasing the stability of the RNA. In some embodiments, the element may be capable of decreasing the stability of the RNA. In some embodiments, the element may be within the 3′ UTR of the RNA. In some embodiments, the element may include a polyadenylation signal (PA). In some embodiments, the element may include a cap, e.g., an upstream mRNA or PEgRNA end. In some embodiments, the RNA may comprise no PA such that it is subject to quicker degradation in the cell after transcription. In some embodiments, the element may include at least one AU-rich element (ARE). The AREs may be bound by ARE binding proteins (ARE-BPs) in a manner that is dependent upon tissue type, cell type, timing, cellular localization, and environment. In some embodiments the destabilizing element may promote RNA decay, affect RNA stability, or activate translation. In some embodiments, the ARE may comprise 50 to 150 nucleotides in length. In some embodiments, the ARE may comprise at least one copy of the sequence AUUUA. In some embodiments, at least one ARE may be added to the 3′ UTR of the RNA. In some embodiments, the element may be a Woodchuck Hepatitis Virus (WHP).
Posttranscriptional Regulatory Element (WPRE), which creates a tertiary structure to enhance expression from the transcript. In further embodiments, the element is a modified and/or truncated WPRE sequence that is capable of enhancing expression from the transcript, as described, for example in Zufferey et al., J Virol, 73(4): 2886-92 (1999) and Flajolet et al., J Virol, 72(7): 6175-80 (1998). In some embodiments, the WPRE or equivalent may be added to the 3′ UTR of the RNA. In some embodiments, the element may be selected from other RNA sequence motifs that are enriched in either fast- or slow-decaying transcripts.
In some embodiments, the vector encoding the PE or the PEgRNA may be self-destroyed via cleavage of a target sequence present on the vector by the PE system. The cleavage may prevent continued transcription of a PE or a PEgRNA from the vector. Although transcription may occur on the linearized vector for some amount of time, the expressed transcripts or proteins subject to intracellular degradation will have less time to produce off-target effects without continued supply from expression of the encoding vectors.
[6] PEgRNAsThe prime editing system described herein contemplates the use of any suitable PEgRNAs. The inventors have discovered that the mechanism of target-primed reverse transcription (TPRT) can be leveraged or adapted for conducting precision and versatile CRISPR/Cas-based genome editing through the use of a specially configured guide RNA comprising a reverse transcription (RT) template sequence that codes for the desired nucleotide change. The application refers to this specially configured guide RNA as an “extended guide RNA” or a “PEgRNA” since the RT template sequence can be provided as an extension of a standard or traditional guide RNA molecule. The application contemplates any suitable configuration or arrangement for the extended guide RNA.
PEgRNA Architecture
In one embodiment, the position of the intermolecular RNA extension is not in the protospacer sequence of the guide RNA. In another embodiment, the position of the intermolecular RNA extension in the gRNA core. In still another embodiment, the position of the intermolecular RNA extension is any with the guide RNA molecule except within the protospacer sequence, or at a position which disrupts the protospacer sequence.
In one embodiment, the intermolecular RNA extension is inserted downstream from the 3′ end of the protospacer sequence. In another embodiment, the intermolecular RNA extension is inserted at least 1 nucleotide, at least 2 nucleotides, at least 3 nucleotides, at least 4 nucleotides, at least 5 nucleotides, at least 6 nucleotides, at least 7 nucleotides, at least 8 nucleotides, at least 9 nucleotides, at least 10 nucleotides, at least 11 nucleotides, at least 12 nucleotides, at least 13 nucleotides, at least 14 nucleotides, at least 15 nucleotides, at least 16 nucleotides, at least 17 nucleotides, at least 18 nucleotides, at least 19 nucleotides, at least 20 nucleotides, at least 21 nucleotides, at least 22 nucleotides, at least 23 nucleotides, at least 24 nucleotides, at least 25 nucleotides downstream of the 3′ end of the protospacer sequence.
In other embodiments, the intermolecular RNA extension is inserted into the gRNA, which refers to the portion of the guide RNA corresponding or comprising the tracrRNA, which binds and/or interacts with the Cas9 protein or equivalent thereof (i.e, a different napDNAbp). Preferably the insertion of the intermolecular RNA extension does not disrupt or minimally disrupts the interaction between the tracrRNA portion and the napDNAbp.
The length of the RNA extension can be any useful length. In various embodiments, the RNA extension is at least 5 nucleotides, at least 6 nucleotides, at least 7 nucleotides, at least 8 nucleotides, at least 9 nucleotides, at least 10 nucleotides, at least 11 nucleotides, at least 12 nucleotides, at least 13 nucleotides, at least 14 nucleotides, at least 15 nucleotides, at least 16 nucleotides, at least 17 nucleotides, at least 18 nucleotides, at least 19 nucleotides, at least 20 nucleotides, at least 21 nucleotides, at least 22 nucleotides, at least 23 nucleotides, at least 24 nucleotides, at least 25 nucleotides, at least 30 nucleotides, at least 40 nucleotides, at least 50 nucleotides, at least 60 nucleotides, at least 70 nucleotides, at least 80 nucleotides, at least 90 nucleotides, at least 100 nucleotides, at least 200 nucleotides, at least 300 nucleotides, at least 400 nucleotides, or at least 500 nucleotides in length.
The RT template sequence can also be any suitable length. For example, the RT template sequence can be at least 3 nucleotides, at least 4 nucleotides, at least 5 nucleotides, at least 6 nucleotides, at least 7 nucleotides, at least 8 nucleotides, at least 9 nucleotides, at least 10 nucleotides, at least 11 nucleotides, at least 12 nucleotides, at least 13 nucleotides, at least 14 nucleotides, at least 15 nucleotides, at least 16 nucleotides, at least 17 nucleotides, at least 18 nucleotides, at least 19 nucleotides, at least 20 nucleotides, at least 30 nucleotides, at least 40 nucleotides, at least 50 nucleotides, at least 60 nucleotides, at least 70 nucleotides, at least 80 nucleotides, at least 90 nucleotides, at least 100 nucleotides, at least 200 nucleotides, at least 300 nucleotides, at least 400 nucleotides, or at least 500 nucleotides in length.
In still other embodiments, wherein the reverse transcription primer binding site sequence is at least 3 nucleotides, at least 4 nucleotides, at least 5 nucleotides, at least 6 nucleotides, at least 7 nucleotides, at least 8 nucleotides, at least 9 nucleotides, at least 10 nucleotides, at least 11 nucleotides, at least 12 nucleotides, at least 13 nucleotides, at least 14 nucleotides, at least 15 nucleotides, at least 16 nucleotides, at least 17 nucleotides, at least 18 nucleotides, at least 19 nucleotides, at least 20 nucleotides, at least 30 nucleotides, at least 40 nucleotides, at least 50 nucleotides, at least 60 nucleotides, at least 70 nucleotides, at least 80 nucleotides, at least 90 nucleotides, at least 100 nucleotides, at least 200 nucleotides, at least 300 nucleotides, at least 400 nucleotides, or at least 500 nucleotides in length.
In other embodiments, the optional linker or spacer sequence is at least 3 nucleotides, at least 4 nucleotides, at least 5 nucleotides, at least 6 nucleotides, at least 7 nucleotides, at least 8 nucleotides, at least 9 nucleotides, at least 10 nucleotides, at least 11 nucleotides, at least 12 nucleotides, at least 13 nucleotides, at least 14 nucleotides, at least 15 nucleotides, at least 16 nucleotides, at least 17 nucleotides, at least 18 nucleotides, at least 19 nucleotides, at least 20 nucleotides, at least 30 nucleotides, at least 40 nucleotides, at least 50 nucleotides, at least 60 nucleotides, at least 70 nucleotides, at least 80 nucleotides, at least 90 nucleotides, at least 100 nucleotides, at least 200 nucleotides, at least 300 nucleotides, at least 400 nucleotides, or at least 500 nucleotides in length.
The RT template sequence, in certain embodiments, encodes a single-stranded DNA molecule which is homologous to the non-target strand (and thus, complementary to the corresponding site of the target strand) but includes one or more nucleotide changes. The least one nucleotide change may include one or more single-base nucleotide changes, one or more deletions, and one or more insertions.
As depicted in
In various embodiments, the nucleotide sequence of the RT template sequence corresponds to the nucleotide sequence of the non-target strand which becomes displaced as the 5′ flap species and which overlaps with the site to be edited.
In various embodiments of the extended guide RNAs, the reverse transcription template sequence may encode a single-strand DNA flap that is complementary to an endogenous DNA sequence adjacent to a nick site, wherein the single-strand DNA flap comprises a desired nucleotide change. The single-stranded DNA flap may displace an endogenous single-strand DNA at the nick site. The displaced endogenous single-strand DNA at the nick site can have a 5′ end and form an endogenous flap, which can be excised by the cell. In various embodiments, excision of the 5′ end endogenous flap can help drive product formation since removing the 5′ end endogenous flap encourages hybridization of the single-strand 3′ DNA flap to the corresponding complementary DNA strand, and the incorporation or assimilation of the desired nucleotide change carried by the single-strand 3′ DNA flap into the target DNA.
In various embodiments of the extended guide RNAs, the cellular repair of the single-strand DNA flap results in installation of the desired nucleotide change, thereby forming a desired product.
In still other embodiments, the desired nucleotide change is installed in an editing window that is between about −5 to +5 of the nick site, or between about −10 to +10 of the nick site, or between about −20 to +20 of the nick site, or between about −30 to +30 of the nick site, or between about −40 to +40 of the nick site, or between about −50 to +50 of the nick site, or between about −60 to +60 of the nick site, or between about −70 to +70 of the nick site, or between about −80 to +80 of the nick site, or between about −90 to +90 of the nick site, or between about −100 to +100 of the nick site, or between about −200 to +200 of the nick site.
In other embodiments, the desired nucleotide change is installed in an editing window that is between about +1 to +2 from the nick site, or about +1 to +3, +1 to +4, +1 to +5, +1 to +6, +1 to +7, +1 to +8, +1 to +9, +1 to +10, +1 to +11, +1 to +12, +1 to +13, +1 to +14, +1 to +15, +1 to +16, +1 to +17, +1 to +18, +1 to +19, +1 to +20, +1 to +21, +1 to +22, +1 to +23, +1 to +24, +1 to +25, +1 to +26, +1 to +27, +1 to +28, +1 to +29, +1 to +30, +1 to +31, +1 to +32, +1 to +33, +1 to +34, +1 to +35, +1 to +36, +1 to +37, +1 to +38, +1 to +39, +1 to +40, +1 to +41, +1 to +42, +1 to +43, +1 to +44, +1 to +45, +1 to +46, +1 to +47, +1 to +48, +1 to +49, +1 to +50, +1 to +51, +1 to +52, +1 to +53, +1 to +54, +1 to +55, +1 to +56, +1 to +57, +1 to +58, +1 to +59, +1 to +60, +1 to +61, +1 to +62, +1 to +63, +1 to +64, +1 to +65, +1 to +66, +1 to +67, +1 to +68, +1 to +69, +1 to +70, +1 to +71, +1 to +72, +1 to +73, +1 to +74, +1 to +75, +1 to +76, +1 to +77, +1 to +78, +1 to +79, +1 to +80, +1 to +81, +1 to +82, +1 to +83, +1 to +84, +1 to +85, +1 to +86, +1 to +87, +1 to +88, +1 to +89, +1 to +90, +1 to +90, +1 to +91, +1 to +92, +1 to +93, +1 to +94, +1 to +95, +1 to +96, +1 to +97, +1 to +98, +1 to +99, +1 to +100, +1 to +101, +1 to +102, +1 to +103, +1 to +104, +1 to +105, +1 to +106, +1 to +107, +1 to +108, +1 to +109, +1 to +110, +1 to +111, +1 to +112, +1 to +113, +1 to +114, +1 to +115, +1 to +116, +1 to +117, +1 to +118, +1 to +119, +1 to +120, +1 to +121, +1 to +122, +1 to +123, +1 to +124, or +1 to +125 from the nick site.
In still other embodiments, the desired nucleotide change is installed in an editing window that is between about +1 to +2 from the nick site, or about +1 to +5, +1 to +10, +1 to +15, +1 to +20, +1 to +25, +1 to +30, +1 to +35, +1 to +40, +1 to +45, +1 to +50, +1 to +55, +1 to +100, +1 to +105, +1 to +110, +1 to +115, +1 to +120, +1 to +125, +1 to +130, +1 to +135, +1 to +140, +1 to +145, +1 to +150, +1 to +155, +1 to +160, +1 to +165, +1 to +170, +1 to +175, +1 to +180, +1 to +185, +1 to +190, +1 to +195, or +1 to +200, from the nick site.
In various aspects, the extended guide RNAs are modified versions of a guide RNA. Guide RNAs maybe naturally occurring, expressed from an encoding nucleic acid, or synthesized chemically. Methods are well known in the art for obtaining or otherwise synthesizing guide RNAs and for determining the appropriate sequence of the guide RNA, including the protospacer sequence which interacts and hybridizes with the target strand of a genomic target site of interest.
In various embodiments, the particular design aspects of a guide RNA sequence will depend upon the nucleotide sequence of a genomic target site of interest (i.e., the desired site to be edited) and the type of napDNAbp (e.g., Cas9 protein) present in prime editing systems described herein, among other factors, such as PAM sequence locations, percent G/C content in the target sequence, the degree of microhomology regions, secondary structures, etc.
In general, a guide sequence is any polynucleotide sequence having sufficient complementarity with a target polynucleotide sequence to hybridize with the target sequence and direct sequence-specific binding of a napDNAbp (e.g., a Cas9, Cas9 homolog, or Cas9 variant) to the target sequence. In some embodiments, the degree of complementarity between a guide sequence and its corresponding target sequence, when optimally aligned using a suitable alignment algorithm, is about or more than about 50%, 60%, 75%, 80%, 85%, 90%, 95%, 97.5%, 99%, or more. Optimal alignment may be determined with the use of any suitable algorithm for aligning sequences, non-limiting example of which include the Smith-Waterman algorithm, the Needleman-Wunsch algorithm, algorithms based on the Burrows-Wheeler Transform (e.g., the Burrows Wheeler Aligner), ClustalW, Clustal X, BLAT, Novoalign (Novocraft Technologies, ELAND (Illumina, San Diego, Calif.), SOAP (available at soap.genomics.org.cn), and Maq (available at maq.sourceforge.net). In some embodiments, a guide sequence is about or more than about 5, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 35, 40, 45, 50, 75, or more nucleotides in length.
In some embodiments, a guide sequence is less than about 75, 50, 45, 40, 35, 30, 25, 20, 15, 12, or fewer nucleotides in length. The ability of a guide sequence to direct sequence-specific binding of a prime editor (PE) to a target sequence may be assessed by any suitable assay. For example, the components of a prime editor (PE), including the guide sequence to be tested, may be provided to a host cell having the corresponding target sequence, such as by transfection with vectors encoding the components of a prime editor (PE) disclosed herein, followed by an assessment of preferential cleavage within the target sequence, such as by Surveyor assay as described herein. Similarly, cleavage of a target polynucleotide sequence may be evaluated in a test tube by providing the target sequence, components of a prime editor (PE), including the guide sequence to be tested and a control guide sequence different from the test guide sequence, and comparing binding or rate of cleavage at the target sequence between the test and control guide sequence reactions. Other assays are possible, and will occur to those skilled in the art.
A guide sequence may be selected to target any target sequence. In some embodiments, the target sequence is a sequence within a genome of a cell. Exemplary target sequences include those that are unique in the target genome. For example, for the S. pyogenes Cas9, a unique target sequence in a genome may include a Cas9 target site of the form MMMMMMMMNNNNNNNNNNNNXGG (SEQ ID NO: 204) where NNNNNNNNNNNNXGG (SEQ ID NO: 205) (N is A, G, T, or C; and X can be anything). A unique target sequence in a genome may include an S. pyogenes Cas9 target site of the form MMMMMMMMMNNNNNNNNNNNXGG (SEQ ID NO: 206) where NNNNNNNNNNNXGG (SEQ ID NO: 207) (N is A, G, T, or C; and X can be anything). For the S. thermophilus CRISPR1Cas9, a unique target sequence in a genome may include a Cas9 target site of the form MMMMMMMMNNNNNNNNNNNNXXAGAAW (SEQ ID NO: 208) where NNNNNNNNNNNNXXAGAAW (SEQ ID NO: 209) (N is A, G, T, or C; X can be anything; and W is A or T). A unique target sequence in a genome may include an S. thermophilus CRISPR 1 Cas9 target site of the form MMMMMMMMMNNNNNNNNNNNXXAGAAW (SEQ ID NO: 210) where NNNNNNNNNNNXXAGAAW (SEQ ID NO: 211) (N is A, G, T, or C; X can be anything; and W is A or T). For the S. pyogenes Cas9, a unique target sequence in a genome may include a Cas9 target site of the form MMMMMMMMNNNNNNNNNNNNXGGXG (SEQ ID NO: 212) where NNNNNNNNNNNNXGGXG (SEQ ID NO: 213) (N is A, G, T, or C; and X can be anything). A unique target sequence in a genome may include an S. pyogenes Cas9 target site of the form MMMMMMMMMNNNNNNNNNNNXGGXG (SEQ ID NO: 214) where NNNNNNNNNNNXGGXG (SEQ ID NO: 215) (N is A, G, T, or C; and X can be anything). In each of these sequences “M” may be A, G, T, or C, and need not be considered in identifying a sequence as unique.
In some embodiments, a guide sequence is selected to reduce the degree of secondary structure within the guide sequence. Secondary structure may be determined by any suitable polynucleotide folding algorithm. Some programs are based on calculating the minimal Gibbs free energy. An example of one such algorithm is mFold, as described by Zuker and Stiegler (Nucleic Acids Res. 9 (1981), 133-148). Another example folding algorithm is the online webserver RNAfold, developed at Institute for Theoretical Chemistry at the University of Vienna, using the centroid structure prediction algorithm (see e.g. A. R. Gruber et al., 2008, Cell 106(1): 23-24; and PA Carr and GM Church, 2009, Nature Biotechnology 27(12): 1151-62). Further algorithms may be found in U.S. application Ser. No. 61/836,080; Broad Reference BI-2013/004A); incorporated herein by reference.
In general, a tracr mate sequence includes any sequence that has sufficient complementarity with a tracr sequence to promote one or more of: (1) excision of a guide sequence flanked by tracr mate sequences in a cell containing the corresponding tracr sequence; and (2) formation of a complex at a target sequence, wherein the complex comprises the tracr mate sequence hybridized to the tracr sequence. In general, degree of complementarity is with reference to the optimal alignment of the tracr mate sequence and tracr sequence, along the length of the shorter of the two sequences. Optimal alignment may be determined by any suitable alignment algorithm, and may further account for secondary structures, such as self-complementarity within either the tracr sequence or tracr mate sequence. In some embodiments, the degree of complementarity between the tracr sequence and tracr mate sequence along the length of the shorter of the two when optimally aligned is about or more than about 25%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, 97.5%, 99%, or higher. In some embodiments, the tracr sequence is about or more than about 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 40, 50, or more nucleotides in length. In some embodiments, the tracr sequence and tracr mate sequence are contained within a single transcript, such that hybridization between the two produces a transcript having a secondary structure, such as a hairpin. Preferred loop forming sequences for use in hairpin structures are four nucleotides in length, and most preferably have the sequence GAAA. However, longer or shorter loop sequences may be used, as may alternative sequences. The sequences preferably include a nucleotide triplet (for example, AAA), and an additional nucleotide (for example C or G). Examples of loop forming sequences include CAAA and AAAG. In an embodiment of the invention, the transcript or transcribed polynucleotide sequence has at least two or more hairpins. In preferred embodiments, the transcript has two, three, four or five hairpins. In a further embodiment of the invention, the transcript has at most five hairpins. In some embodiments, the single transcript further includes a transcription termination sequence; preferably this is a polyT sequence, for example six T nucleotides. Further non-limiting examples of single polynucleotides comprising a guide sequence, a tracr mate sequence, and a tracr sequence are as follows (listed 5′ to 3′), where “N” represents a base of a guide sequence, the first block of lower case letters represent the tracr mate sequence, and the second block of lower case letters represent the tracr sequence, and the final poly-T sequence represents the transcription terminator:
In some embodiments, sequences (1) to (3) are used in combination with Cas9 from S. thermophilus CRISPR1. In some embodiments, sequences (4) to (6) are used in combination with Cas9 from S. pyogenes. In some embodiments, the tracr sequence is a separate transcript from a transcript comprising the tracr mate sequence.
It will be apparent to those of skill in the art that in order to target any of the fusion proteins comprising a Cas9 domain and a single-stranded DNA binding protein, as disclosed herein, to a target site, e.g., a site comprising a point mutation to be edited, it is typically necessary to co-express the fusion protein together with a guide RNA, e.g., an sgRNA. As explained in more detail elsewhere herein, a guide RNA typically comprises a tracrRNA framework allowing for Cas9 binding, and a guide sequence, which confers sequence specificity to the Cas9:nucleic acid editing enzyme/domain fusion protein.
In some embodiments, the guide RNA comprises a structure 5′-[guide sequence]-GUUUUAGAGCUAGAAAUAGCAAGUUAAAAUAAAGGCUAGUCCGUUAUCAACU UGAAAAAGUGGCACCGAGUCGGUGCUUUUU-3′ (SEQ ID NO: 222), wherein the guide sequence comprises a sequence that is complementary to the target sequence. The guide sequence is typically 20 nucleotides long. The sequences of suitable guide RNAs for targeting Cas9:nucleic acid editing enzyme/domain fusion proteins to specific genomic target sites will be apparent to those of skill in the art based on the instant disclosure. Such suitable guide RNA sequences typically comprise guide sequences that are complementary to a nucleic sequence within 50 nucleotides upstream or downstream of the target nucleotide to be edited. Some exemplary guide RNA sequences suitable for targeting any of the provided fusion proteins to specific target sequences are provided herein. Additional guide sequences are well known in the art and can be used with the prime editor (PE) described herein.
In other embodiments, the PEgRNAs include those depicted in
In still other embodiments, the PEgRNAs may include those depicted in
PEgRNA Improvements
The PEgRNAs may also include additional design improvements that may modify the properties and/or characteristics of PEgRNAs thereby improving the efficacy of prime editing. In various embodiments, these improvements may belong to one or more of a number of different categories, including but not limited to: (1) designs to enable efficient expression of functional PEgRNAs from non-polymerase III (pol III) promoters, which would enable the expression of longer PEgRNAs without burdensome sequence requirements; (2) improvements to the core, Cas9-binding PEgRNA scaffold, which could improve efficacy; (3) modifications to the PEgRNA to improve RT processivity, enabling the insertion of longer sequences at targeted genomic loci; and (4) addition of RNA motifs to the 5′ or 3′ termini of the PEgRNA that improve PEgRNA stability, enhance RT processivity, prevent misfolding of the PEgRNA, or recruit additional factors important for genome editing.
In one embodiment, PEgRNA could be designed with polIII promoters to improve the expression of longer-length PEgRNA with larger extension arms. sgRNAs are typically expressed from the U6 snRNA promoter. This promoter recruits pol III to express the associated RNA and is useful for expression of short RNAs that are retained within the nucleus. However, pol III is not highly processive and is unable to express RNAs longer than a few hundred nucleotides in length at the levels required for efficient genome editing. Additionally, pol III can stall or terminate at stretches of U's, potentially limiting the sequence diversity that could be inserted using a PEgRNA. Other promoters that recruit polymerase II (such as pCMV) or polymerase I (such as the U1 snRNA promoter) have been examined for their ability to express longer sgRNAs. However, these promoters are typically partially transcribed, which would result in extra sequence 5′ of the spacer in the expressed PEgRNA, which has been shown to result in markedly reduced Cas9:sgRNA activity in a site-dependent manner. Additionally, while pol III-transcribed PEgRNAs can simply terminate in a run of 6-7 U's, PEgRNAs transcribed from pol II or pol I would require a different termination signal. Often such signals also result in polyadenylation, which would result in undesired transport of the PEgRNA from the nucleus. Similarly, RNAs expressed from pol II promoters such as pCMV are typically 5′-capped, also resulting in their nuclear export.
Previously, Rinn and coworkers screened a variety of expression platforms for the production of long-noncoding RNA-(lncRNA) tagged sgRNAs183. These platforms include RNAs expressed from pCMV and that terminate in the ENE element from the MALAT1 ncRNA from humans184, the PAN ENE element from KSHV185, or the 3′ box from U1 snRNA186. Notably, the MALAT1 ncRNA and PAN ENEs form triple helices protecting the polyA-tail184, 187. These constructs could also enhance RNA stability. It is contemplated that these expression systems will also enable the expression of longer PEgRNAs.
In addition, a series of methods have been designed for the cleavage of the portion of the pol II promoter that would be transcribed as part of the PEgRNA, adding either a self-cleaving ribozyme such as the hammerhead188, pistol189, hatchet189, hairpin190, VS191, twister192, or twister sister192 ribozymes, or other self-cleaving elements to process the transcribed guide, or a hairpin that is recognized by Csy4193 and also leads to processing of the guide. Also, it is hypothesized that incorporation of multiple ENE motifs could lead to improved PEgRNA expression and stability, as previously demonstrated for the KSHV PAN RNA and element185. It is also anticipated that circularizing the PEgRNA in the form of a circular intronic RNA (ciRNA) could also lead to enhanced RNA expression and stability, as well as nuclear localization194.
In various embodiments, the PEgRNA may include various above elements, as exemplified by the following sequence.
In various other embodiments, the PEgRNA may be improved by introducing improvements to the scaffold or core sequences. This can be done by introducing known
The core, Cas9-binding PEgRNA scaffold can likely be improved to enhance PE activity. Several such approaches have already been demonstrated. For instance, the first pairing element of the scaffold (P1) contains a GTTTT-AAAAC pairing element. Such runs of Ts have been shown to result in pol III pausing and premature termination of the RNA transcript. Rational mutation of one of the T-A pairs to a G-C pair in this portion of P1 has been shown to enhance sgRNA activity, suggesting this approach would also be feasible for PEgRNAs195. Additionally, increasing the length of P1 has also been shown to enhance sgRNA folding and lead to improved activity195, suggesting it as another avenue for the improvement of PEgRNA activity. Example improvements to the core can include: