METHODS FOR VARIANT DETECTION
The invention can be used to provide a more efficient and less error-prone method of detecting variants in DNA, such as SNPs and indels. The invention also provides a method for performing inexpensive multiplex assays. The invention also provides methods for detection of DNA sequences altered after cleavage by a targetable endonuclease, such as the CRISPR Cas9 protein from the bacterium Streptococcus pyogenes.
This application is a continuation-in-part of U.S. application Ser. No. 15/487,401, filed Apr. 13, 2017, which is a continuation-in-part of U.S. application Ser. No. 15/361,280, filed Nov. 25, 2016, which claims the benefit of U.S. Provisional Application No. 62/339,317, filed May 20, 2016, and also claims the benefit of U.S. Provisional Application No. 62/259,913, filed Nov. 25, 2015, the disclosures of all of which are hereby incorporated by reference in their entireties.
FIELD OF THE INVENTIONThe invention can be used to provide a more efficient and less error-prone method of detecting variants in DNA, such as single nucleotide polymorphisms (SNPs), multi-nucleotide polymorphisms (MNPs), and indels. The invention also provides a method for performing inexpensive multi-color assays, and provides methods for visualizing multiple allele results in a two-dimensional plot. The invention also provides methods for detection of DNA sequences altered after cleavage by a targetable endonuclease, such as the CRISPR Cas9 protein from the bacterium Streptococcus pyogenes.
BACKGROUND OF THE INVENTIONRNase H2-dependent PCR (rhPCR) (see U.S. Patent Application Publication No. US 2009/0325169 A1, incorporated by reference herein in its entirety) and standard allele-specific PCR (ASPCR) can both be utilized for mutation detection. In ASPCR, the DNA polymerase performs the mismatch discrimination by detection of a mismatch at or near the 3′ end of the primer. While ASPCR is sometimes successful in mismatch detection, the discrimination can be limited, due to the low mismatch detection ability of wild-type DNA polymerases.
In contrast with ASPCR, the mismatch sensitivity of the RNase H2 enzyme in rhPCR allows for both sensitive detection of DNA mutations, and elimination of primer-dimer artifacts from the reaction. When attempting to detect DNA mutations with rhPCR, however, placement of the mismatch within the primer is important. The nearer to the cleavable RNA the mismatch is located, the more discrimination is observed from the RNase H2 enzyme, and the greater the discrimination of the resulting rhPCR assay. Given the fact that most common wild-type DNA polymerases such as Taq often display low levels of mismatch detection, the polymerase cannot be solely relied upon to perform this discrimination after RNase H2 cleavage. Coupled with the repeated interrogation desired from every cycle of standard rhPCR, placing the mismatch anywhere other than immediately opposite the RNA is undesirable when utilizing these polymerases.
There is thus a need for assays with improved mismatch sensitivity.
In addition, there is a need for improved methods for detection of mutations altered after cleavage by targetable endonucleases, such as the CRISPR Cas9 protein. A commonly used method to detect mutations introduced into genomic DNA following repair of dsDNA cleavage events is the enzymatic mismatch cleavage assay (EMCA). EMCA assays cleave at sites where base mismatches are present in dsDNA. For EMCA detection of the mutations introduced into DNA following Cas9 cleavage and repair, genomic DNA from cells is harvested and the regions around the dsDNA cut site is amplified by PCR using primers that flank the cut site. Typically 100-1000 base amplicons are used for this purpose. Following completion of amplification, heteroduplexes are formed by heating the reaction products and allowing them to re-anneal, which leads to formation of homoduplex WT/WT, Mut/Mut or heteroduplex WT/Mut or Mut1/Mut2 variants. The dsDNAs are then subjected to cleavage by a mismatch endonuclease (such as T7EI, Surveyor, etc.). Heteroduplexes are cleaved and the presence of shorter fragments is detected by gel electrophoresis, capillary electrophoresis, or any of a number of methods known to those of skill in the art. Although such an assay is fast and inexpensive, it often does not accurately reflect the changes that are actually generated from the CRISPR mutagenesis process. If the same mutation is introduced a large number of times, Mut/Mut homodimers form, which are not detected. Further, the mismatch endonuclease enzymes often fail to cleave single-base events, leading to yet another class of mutations that are undetected. Thus an EMCA assay will almost always underestimate the extent of genome editing that occurred after Cas9 dsDNA cleavage and repair.
An alternative method of analysis involves large scale DNA sequencing using “Next-Gen” sequencing (NGS) methods of the modified DNA, which is highly accurate, but is slow and costly. Other methods are available to assess the mutation outcome following CRISPR/Cas9 cleavage and repair. For example, Sanger sequencing results can be analyzed using sequence trace decomposition (“TIDE” analysis); fluorescent-labeled primer-extension on an amplicon spanning the Cas9 cut site can be used to map indels using Indel Detection by Amplicon Analysis (IDAA); or high resolution melt analysis (HRM) can be applied to PCR amplicons that span the Cas9 cut site. However, none of these methods approaches the accuracy of NGS analysis, while all are more costly and slower to perform than EMCA methods. Thus, improved methods are needed to assess the frequency of mutations that arise from genome editing experiments that are rapid and low cost.
BRIEF SUMMARY OF THE INVENTIONThe disclosure provides assays making use of high discrimination polymerase mutants or other high mismatch discrimination polymerases to create a new assay design that can utilize mismatches located 5′ of the RNA, 3′ of the RNA, overlapping with the RNA, and/or in the same position as the RNA.
The invention can be used to provide a more efficient and less error-prone method of detecting mutations in DNA, such as SNPs and indels. The invention also provides a method for performing inexpensive multi-color assays. The invention also provides methods for detection of DNA sequences altered after cleavage by a targetable endonuclease, such as the CRISPR Cas9 protein from the bacterium Streptococcus pyogenes.
These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
The invention pertains to a methods of single-nucleotide polymorphism (SNP) discrimination utilizing blocked-cleavable rhPCR primers (see U.S. Patent Application Publication No. US 2009/0325169 A1, incorporated by reference herein in its entirety) and a DNA polymerase with high levels of mismatch discrimination. In one embodiment, the mismatch is placed at a location other than opposite the RNA base. In these situations, the majority of the discrimination comes not from the RNase H2, but from the high discrimination polymerase. The use of blocked-cleavable primers with RNase H2 acts to reduce or eliminate primer-dimers and provide some increased amount of SNP or indel (insertion/deletion) discrimination (
For the purposes of this invention, high discrimination is defined as any amount of discrimination over the average discrimination of WT Thermus aquaticus (Taq) polymerase. Examples include KlenTaq® DNA polymerase (Wayne Barnes), and mutant polymerases described in U.S. Patent Application Publication No. US 2015/0191707 (incorporated by reference herein in its entirety) such as H784M, H784S, H784A and H784Q mutants.
In a further embodiment a universal detection sequence(s) is added to the 5′-end of the blocked-cleavable primers. The detection sequence includes a binding site for a probe, and a binding site for a universal amplification primer. The primer binding site is positioned at or near the 5′-end of the final oligonucleotide and the probe binding site is positioned internally between the universal primer site and the SNP-detection primer domain. Use of more than one such chimeric probe in a detection reaction wherein distinct probe binding sites are employed allows primers to be multiplexed and further allows for multiple color detection of SNPs or other genomic features. Blocked-cleavable rhPCR primers reduce or eliminate primer-dimers. Primer-dimers are a major problem for use of “universal” primer designs in SNP detection assays, and that limits their utility (
Previously, the best preferred embodiment for rhPCR SNP discrimination employed blocked-cleavable primers having the mismatch (SNP site) positioned opposite the single RNA base (cleavage site). While this works for many SNP targets, there are base match/mismatch pairings where sufficient discrimination is not obtained for robust base calling. Moreover, due to the high level of differential SNP discrimination observed with rhPCR, end-point detection can be difficult, especially with heterozygous target DNAs. In the proposed method, the RNA base is identical in both discriminating primers, eliminating this issue.
Placing the mismatch at either a position 5′ or 3′ of the RNA is also useful in some embodiments. For example, a mutation located 5′ of the RNA allows for the use of a highly discriminatory DNA polymerase as a secondary selection step during PCR amplification following primer cleavage, although this configuration restricts the mismatch discrimination potential of the RNase H2 to the first cleavage event, and subsequent priming events on daughter amplicons will bind to whatever sequence was present in the primer. Placement of the mismatch 3′ of the RNA reduces or eliminates the potential for primer conversion to the opposite allele.
Thus, in some aspects, the disclosure provides methods and compositions comprising mutant RNase H2 enzymes that show enhanced mismatch discrimination during rhPCR when the mutations are positioned 5′- or 3′-relative to the RNA residue. For example, and without limiting the scope of the disclosed aspects and embodiments of the disclosure, mutations located at the twelfth, thirteenth, and 169th amino acid positions in the RNase H2 enzyme from Pyrococcus abyssi (P.a.) have been identified that have this unexpected and useful property. Similar mutations in RNase H2 enzymes from other species that support rhPCR may likewise show similar improved properties relative to their WT forms.
In other embodiments, the methods of the disclosure comprise the use of blocked-cleavable primers wherein the mismatch is placed 1-2 bases 5′ of the RNA. In a further embodiment, the method involves the use of blocked-cleavable primers with three or more DNA bases 3′ of an RNA residue, and the primers are designed such that the mismatch is placed immediately 5′ of the RNA.
Following cleavage by RNase H2, the remaining primer has a DNA residue positioned at the 3′-end exactly at the SNP site, effectively creating an ASPCR primer. In this configuration, a high-specificity DNA polymerase can discriminate between match and mismatch with the template strand (
In another embodiment, the invention may utilize a “tail” domain added to the 5′ end of the primer, containing a universal forward primer binding site sequence and optionally a universal probe sequence. This tail would not be complementary to the template of interest, and when a probe is used, the tail would allow for inexpensive fluorescent signal detection, which could be multiplexed to allow for multiple color signal detection in qPCR (
In another embodiment, a forward primer is optionally used with a reverse primer, and a tail domain is added to the 5′ end of one or both of a forward and reverse primer set. The tail domain comprises a universal forward primer binding site. The primers can be used to hybridize and amplify a target such as a genomic sample of interest. The primers would add universal priming sites to the target, and further cycles of amplification can be performed using universal primers that contain adapter sequences that enable further processing of the sample, such as the addition of P5/P7 flow cell binding sites and associated index or barcoding sequences useful in adapters for next-generation sequencing (see
As noted in U.S. Patent Application Publication No. US 2009/0325169 (incorporated by reference herein in its entirety), RNase H2 can cleave at positions containing one or more RNA bases, at 2′-modified nucleosides such as 2′-fluoronucleosides. The primers can also contain nuclease resistant linkages such as phosphorothioate, phosphorodithioate, or methylphosphonate.
Further aspects of the disclosure pertain to detection of DNA sequences altered after cleavage by a targetable endonuclease, such as the CRISPR Cas9 protein from the bacterium Streptococcus pyogenes. This protein and similar ones have successfully been used for targeted genomic modification in the well documented Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) system.
In a further embodiment, the tailed primers detailed above could be used to detect editing events for genome editing technology. For example, CRISPR/Cas9 is a revolutionary strategy in genome editing that enables generation of targeted, double-stranded breaks (DSBs) in genomic DNA. Methods to achieve DSBs by CRISPR/Cas9—a bacterial immune defense system comprised of an endonuclease that is targeted to double-stranded DNA by a guide RNA—are widely used in gene disruption, gene knockout, gene insertion, etc. In mammalian cells, the endonuclease activity is followed by an endogenous repair process that leads to some frequency of insertions/deletions/substitutions in wild-type DNA at the target locus which gives the resultant genome editing.
RNase H-cleavable primers have been designed to flank edited loci in order to 1) generate locus-specific amplicons with universal tails, and 2) be subsequently amplified with indexed P5/P7 universal primers for next-generation sequencing. In pilot experiments, this strategy resulted in reliable, locus-specific amplification which captures CRISPR/Cas9 editing events in a high-throughput and reproducible manner. The key finding is that the overall targeted editing by this NGS-based method was determined to be 95%; whereas, previous enzymatic strategies suggested overall editing from the same samples was approximately 55% at the intended target site. Further, primers were designed to amplify off-target locations of genomic editing based on in silico predictions by internal bioinformatics tools.
These assays would be pooled for amplification of a single genomic DNA sample in order to capture the on-target as well as >100 potential sites for off-target genome editing mediated by sequence homology to the guide RNA. The results from this experiment would allow for 1) identification of CRISPR/Cas9 off-target sites and provide an assay for comparing strategies to reduce those effects, 2) improved design of the CRISPR/Cas9 off-target prediction algorithm, and 3) improved design of primer sets.
Thus, in further aspects, the disclosure provides methods that employ the above-described universal rhPCR assay system to detect mutations generated by a targetable endonuclease such as Cas9 or Cpf1. The rhPCR assays according to these aspects of the disclosure utilizes a thermostable RNase H2 enzyme, and optionally a DNA polymerase with enhanced mismatch discrimination. The RNase H2 cleaves at the single RNA residue only when the primer oligonucleotide is duplexed with a target nucleic acid, which removes a 3′-blocking group and activates the primer. The DNA polymerase uses the primer to initiate DNA synthesis and, in multiple cycles, supports PCR. Discrimination of mutations is achieved by the action of the RNase H2 or the combined action of both the RNase H2 and the DNA polymerase, wherein the RNase H2 has reduced de-blocking activity when a mismatch is present and the DNA polymerase has reduced priming/DNA synthesis activity when a mismatch is present. In one embodiment, the primers comprise multiple functional domains including (from the 5′-end): a universal primer domain, a universal probe binding domain, a target-specific primer domain, a single RNA residue (cleavable linkage), a short 3′-extension domain, and a 3′-blocking group that prevents the oligonucleotide from priming DNA synthesis. Cleavage by RNase H2 removes the RNA residue, 3′-extension domain, and 3′-blocking group.
In some embodiments, a second assay is present in the reaction and runs as a 2-color multiplex, targeting the RNase P gene or some other control gene. This second assay allows for normalization to an internal control gene that was not targeted by the CRISPR genome editing reaction. This control assay may be performed as either a standard three-oligonucleotide 5′ nuclease assay, or as a second rhPCR-based universal assay.
In another embodiment, the primers lack the universal 5′ domain, but still retain the 3′ removable blocking group. In this alternative embodiment, a standard 5′ nuclease fluorescence-quenched probe is placed between the forward and reverse primers. The probe is positioned within the amplicon such that it lies outside of any region that may be altered by the genome editing event.
In each experiment, relative position of the discriminatory (i.e., mutation interrogating) primer on the sequence is important. RNase H2 cleaves 5′ of an RNA residue. Placement of the primer so that the RNA residue binds two nucleotides after the most common cleavage site is important for recognition of the mutagenized samples. A diagram of this principle is shown in
Thus, in one aspect, the disclosure provides blocked-cleavable primers for rhPCR, the primers comprising: 5′-A-B-C-D-E-3′, wherein A is optional and is a tail extension that is not complementary to a target; B is a sequence domain that is complementary to a target; C is a discrimination domain; D is a cleavable domain that, when hybridized to the target, is cleavable by RNase H2; and E is a blocking domain that prevents extension of the primer.
In some embodiments, D is comprised of 1-3 RNA bases. In some embodiments, D is comprised of 1 RNA base. In some embodiments, the cleavage domain comprises one or more of the following moieties: a DNA residue, an abasic residue, a modified nucleoside, or a modified phosphate internucleotide linkage. In some embodiments, a sequence flanking the cleavage site contains one or more internucleoside linkages resistant to nuclease cleavage. In some embodiments, the nuclease resistant linkage is a phosphorothioate. In some embodiments, the 3′ oxygen atom of at least one of the RNA residues is substituted with an amino group, thiol group, or a methylene group. In some embodiments, the blocking group is attached to the 3′-terminal nucleotide of the primer. In some embodiments, A is comprised of a region that is identical to a universal forward primer and optionally a probe binding domain.
In another aspect, the disclosure provides methods of detecting a variation in a target DNA sequence, the method comprising: (a) providing a reaction mixture comprising (i) an oligonucleotide primer having a cleavage domain positioned 5′ of a blocking group and 3′ of a position of variation, the blocking group linked at or near the end of the 3′-end of the oligonucleotide primer wherein the blocking group prevents primer extension and/or inhibits the primer from serving as a template for DNA synthesis, (ii) a sample nucleic acid that may or may not have the target sequence, and where the target sequence may or may not have the variation (iii) a cleaving enzyme and (iv) a polymerase; (b) hybridizing the primer to the target DNA sequence to form a double-stranded substrate; (c) cleaving the hybridized primer, if the primer is complementary at the variation, with the cleaving enzyme at a point within or adjacent to the cleavage domain to remove the blocking group from the primer; and (d) extending the primer with the polymerase.
In some embodiments, the cleaving enzyme is a hot start cleaving enzyme which is thermostable and has reduced activity at lower temperatures. In some embodiments, the cleaving enzyme is a chemically modified hot start cleaving enzyme which is thermostable and has reduced activity at lower temperatures. In some embodiments, the hot start cleaving enzyme is a chemically modified Pyrococcus abyssi RNase H2. In some embodiments, the cleaving enzyme is a hot start cleaving enzyme that is reversibly inactivated through interaction with an antibody at lower temperatures which is thermostable and has reduced activity at lower temperatures. In some embodiments, the cleaving domain is comprised of at least one RNA base, and the cleaving enzyme cleaves between the position complementary to the variation and the RNA base. In some embodiments, the cleaving domain is comprised of one or more 2′-modified nucleosides, and the cleaving enzyme cleaves between the position complementary to the variation and the one or more modified nucleosides. In some embodiments, the one or more modified nucleosides are 2′-fluoronucleosides. In some embodiments, the polymerase is a high-discrimination polymerase. In some embodiments, the polymerase is a mutant H784Q Taq polymerase. In some embodiments, the mutant H784Q Taq polymerase is reversibly inactivated via chemical, aptamer or antibody modification. In some embodiments, the primer contains a 5′ tail sequence that comprises a universal primer sequence and optionally a universal probe sequence, wherein the tail is non-complementary to the target DNA sequence. In some embodiments, the target DNA sequence is a sample that has been treated with a gene editing enzyme. In some embodiments, the target DNA sequence is a sample that has been treated with a CRISPR enzyme. In some embodiments, the target DNA sequence is a sample that has been treated with a Cas9 or Cpf1 enzyme.
In another aspect, the disclosure provides oligonucleotide compositions for genotyping a target sample, wherein the composition comprises, from 5′ to 3′: (a) a tailing portion that is non-complementary to the target sample but contains (5′ to 3′) a first sequence that is identical to a universal forward primer and a second sequence that is identical to a reporter probe sequence that corresponds to an allele; (b) a region complementary to a target; (c) an allele-specific domain; and (d) a 3′ terminal region containing a blocking domain. In some embodiments, the allele-specific domain is capable of being cleaved by an RNase H enzyme when hybridized to the target sample. In some embodiments, the allele-specific domain is 5′-D-R-3′, wherein D is a DNA base that aligns with a SNP position of interest and R is an RNA base. In some embodiments, the oligonucleotide is about 15-30 bases long. In some embodiments, the blocking domain is comprised of 0-5 DNA bases. In some embodiments, the blocking domain is comprised of at least two of the flowing: DNA, RNA, 1,3-propanediol, mismatched DNA or RNA, and a labeling moiety. In some embodiments, the allele-specific domain is comprised of 2′-fluoro analogues.
In another aspect, the disclosure provides kits for genotyping assays, the kits comprising: (a) a first allele oligonucleotide comprised as in the previous aspect; (b) a second allele oligonucleotide comprised as in the previous aspect; (c) a locus-specific reverse primer; (d) a universal forward primer; (e) a polymerase; (f) a first probe corresponding to a first allele; (g) a second probe corresponding to a second allele; and (h) an RNase H enzyme.
In another aspect, the disclosure provides methods of visualization of multiple different fluorescent signals from allelic amplification plots, the methods comprising: (a) using three fluorescent signals from multiple fluorescent dye signals in a single reaction well, subtracting a lowest fluorescence Dye3 from fluorescence signals from Dye1 and Dye2; (b) calculating the distance of data from an origin and an angle from one of the axis with an equation
Distance from origin=√{square root over ((ΔRnDye1)2+(ΔRnDye2)2)}
Angle=tan−1(ΔRnDye1÷ΔRnDye2)×120/90; and
(c) plotting on a circle plot with three axis, one for each dye or allele, the resulting distance.
In another aspect, the disclosure provides methods of target enrichment comprising: (a) providing a reaction mixture comprising (i) a first oligonucleotide primer having a tail domain that is not complementary to a target sequence, the tail domain comprising a first universal primer sequence; a cleavage domain positioned 5′ of a blocking group and 3′ of a position of variation, the blocking group linked at or near the end of the 3′-end of the first oligonucleotide primer wherein the blocking group prevents primer extension and/or inhibits the first primer from serving as a template for DNA synthesis, (ii) a sample nucleic acid that may or may not have the target sequence, (iii) a cleaving enzyme and (iv) a polymerase; (b) hybridizing the first primer to the target DNA sequence to form a double-stranded substrate; (c) cleaving the hybridized first primer, if the first primer is complementary to the target, with the cleaving enzyme at a point within or adjacent to the cleavage domain to remove the blocking group from the first primer; and (d) extending the first primer with the polymerase.
In some embodiments, the methods further comprise a second primer in reverse orientation to support priming and extension of the first primer extension product. In some embodiments, the second primer further comprises a tail domain comprising a second universal primer sequence. In some embodiments, steps b-d are performed 1-10 times. In some embodiments, the methods further comprise removing unextended primers from the reaction and hybridizing universal primers to the extension product to form a second extension product. In some embodiments, the universal primers further comprise tailed sequences for addition of adapter sequences to the second extension product. In some embodiments, sequencing is performed on the second extension product to determine the sequence of the target. In some embodiments, the target DNA sequence is a sample that has been treated with a gene editing enzyme. In some embodiments, the target DNA sequence is a sample that has been treated with a CRISPR enzyme. In some embodiments, the target DNA sequence is a sample that has been treated with a Cas9 or Cpf1 enzyme. In some embodiments, the cleaving enzyme is a hot start cleaving enzyme which is thermostable and has reduced activity at lower temperatures. In some embodiments, the cleaving enzyme is a chemically modified hot start cleaving enzyme which is thermostable and has reduced activity at lower temperatures. In some embodiments, the hot start cleaving enzyme is a chemically modified Pyrococcus abyssi RNase H2. In some embodiments, the cleaving enzyme is a hot start cleaving enzyme that is reversibly inactivated through interaction with an antibody at lower temperatures which is thermostable and has reduced activity at lower temperatures. In some embodiments, the cleaving domain is comprised of at least one RNA base, and the cleaving enzyme cleaves between the position complementary to the variation and the RNA base. In some embodiments, the cleaving domain is comprised of one or more 2′-modified nucleosides, and the cleaving enzyme cleaves between the position complementary to the variation and the one or more modified nucleosides. In some embodiments, the one or more modified nucleosides are 2′-fluoronucleosides. In some embodiments, the polymerase is a high-discrimination polymerase. In some embodiments, the polymerase is a mutant H784Q Taq polymerase. In some embodiment the mutant H784Q Taq polymerase is reversibly inactivated via chemical, aptamer or antibody modification.
In another aspect, the disclosure provides methods of detecting variations in target DNA sequences that have been altered with a gene editing enzyme, the methods comprising: (a) providing a reaction mixture comprising: (i) an oligonucleotide primer having a cleavage domain positioned 5′ of a blocking group and 3′ of a position of variation, the blocking group linked at or near the end of the 3′-end of the oligonucleotide primer wherein the blocking group prevents primer extension and/or inhibits the primer from serving as a template for DNA synthesis; (ii) a sample nucleic acid that may or may not have the target sequence, and where the target sequence may or may not have the variation; (iii) a cleaving enzyme; and (iv) a polymerase; (b) hybridizing the primer to the target DNA sequence to form a double-stranded substrate; (c) cleaving the hybridized primer, if the primer is complementary at the variation, with the cleaving enzyme at a point within or adjacent to the cleavage domain to remove the blocking group from the primer; and (d) extending the primer with the polymerase.
In some embodiments, the target DNA sequence has been treated with a CRISPR enzyme. In some embodiments, the target DNA sequence has been treated with a Cas9 or Cpf1 enzyme. In some embodiments, the cleaving enzyme is a hot start cleaving enzyme which is thermostable and has reduced activity at lower temperatures. In some embodiments, the cleaving enzyme is an RNase H2 enzyme. In some embodiments, the cleaving enzyme is Pyrococcus abyssi RNase H2 enzyme. In some embodiments, the cleaving enzyme is a chemically modified hot start cleaving enzyme which is thermostable and has reduced activity at lower temperatures. In some embodiments, the hot start cleaving enzyme is a chemically modified Pyrococcus abyssi RNase H2. In some embodiments, the cleaving enzyme is a hot start cleaving enzyme that is reversibly inactivated through interaction with an antibody at lower temperatures.
In some embodiments, the cleavage domain comprises at least one RNA base, and the cleaving enzyme cleaves between the position complementary to the variation and the RNA base. In some embodiments, the cleavage domain comprises at least one RNA base located 3′ of the position of variation, and comprises one DNA base between the position of variation and the RNA base. In some embodiments, there are no DNA bases between the position of variation and the RNA base. In other embodiments, the RNA base is located within the position of variation. In some embodiments, the cleavage domain comprises one or more 2′-modified nucleosides, and the cleaving enzyme cleaves between the position complementary to the variation and the one or more modified nucleosides. In some embodiments, the one or more modified nucleosides are 2′-fluoronucleosides.
In some embodiments, the polymerase is a high-discrimination polymerase. In some embodiments, the polymerase is a mutant H784Q Taq polymerase. In some embodiments, the mutant H784Q Taq polymerase is reversibly inactivated via chemical, aptamer, or antibody modification. In some embodiments, the primer contains a 5′ tail sequence that comprises a universal primer sequence and optionally a universal probe sequence, wherein the tail is non-complementary to the target DNA sequence.
In some embodiments, the methods of this aspect of the disclosure further comprise (e) detection of an internal control gene not targeted by the gene editing enzyme; and (f) normalization of the results of steps (a)-(d) to the results of step (e). In some embodiments, the internal control gene not targeted by the gene editing enzyme is the RNase P gene. In some embodiments, the reaction mixture further comprises a control oligonucleotide primer specific for the internal control gene not targeted by the gene editing enzyme, wherein the control oligonucleotide primer comprises a cleavage domain positioned 5′ of a blocking group and 3′ of a position of variation, the blocking group linked at or near the end of the 3′-end of the oligonucleotide primer wherein the blocking group prevents primer extension and/or inhibits the primer from serving as a template for DNA synthesis. In some embodiments, the internal control gene not targeted by the gene editing enzyme is detected using a three-oligonucleotide 5′ nuclease assay.
In another aspect, the disclosure provides methods of target enrichment comprising: (a) providing a reaction mixture comprising: (i) a first oligonucleotide primer having a tail domain that is not complementary to a target sequence, the tail domain comprising a first universal primer sequence; a cleavage domain positioned 5′ of a blocking group and 3′ of a position of variation, the blocking group linked at or near the end of the 3′-end of the first oligonucleotide primer wherein the blocking group prevents primer extension and/or inhibits the first primer from serving as a template for DNA synthesis; (ii) a sample nucleic acid that has been treated with a gene editing enzyme, which may or may not have the target sequence; (iii) a cleaving enzyme; and (iv) a polymerase; (b) hybridizing the first primer to the target DNA sequence to form a double-stranded substrate; (c) cleaving the hybridized first primer, if the first primer is complementary to the target, with the cleaving enzyme at a point within or adjacent to the cleavage domain to remove the blocking group from the first primer; and (d) extending the first primer with the polymerase.
In some embodiments, the target DNA sequence is a sample that has been treated with a CRISPR enzyme. In some embodiments, the target DNA sequence is a sample that has been treated with a Cas9 or Cpf1 enzyme. In some embodiments, the methods further comprise a second primer in reverse orientation to support priming and extension of the first primer extension product. In some embodiments, the second primer further comprises a tail domain comprising a second universal primer sequence. In some embodiments, steps (b)-(d) are performed 1-10 times. In some embodiments, the methods further comprise removing unextended primers from the reaction and hybridizing universal primers to the extension product to form a second extension product. In some embodiments, the universal primers further comprise tailed sequences for addition of adapter sequences to the second extension product. In some embodiments, sequencing is performed on the second extension product to determine the sequence of the target.
In some embodiments, the cleaving enzyme is a hot start cleaving enzyme which is thermostable and has reduced activity at lower temperatures. In some embodiments, the cleaving enzyme is an RNase H2 enzyme. In some embodiments, the cleaving enzyme is Pyrococcus abyssi RNase H2 enzyme. In some embodiments, the cleaving enzyme is a chemically modified hot start cleaving enzyme which is thermostable and has reduced activity at lower temperatures. In some embodiments, the hot start cleaving enzyme is a chemically modified Pyrococcus abyssi RNase H2. In some embodiments, the cleaving enzyme is a hot start cleaving enzyme that is reversibly inactivated through interaction with an antibody at lower temperatures which is thermostable and has reduced activity at lower temperatures. In some embodiments, the cleavage domain comprises at least one RNA base, and the cleaving enzyme cleaves between the position complementary to the variation and the RNA base.
In some embodiments, the cleavage domain comprises at least one RNA base located 3′ of the position of variation, and comprises one DNA base between the position of variation and the RNA base. In some embodiments, there are no DNA bases between the position of variation and the RNA base. In other embodiments, the RNA base is located within the position of variation. In some embodiments, the cleavage domain comprises one or more 2′-modified nucleosides, and the cleaving enzyme cleaves between the position complementary to the variation and the one or more modified nucleosides. In some embodiments, the one or more modified nucleosides are 2′-fluoronucleosides. In some embodiments, the polymerase is a high-discrimination polymerase. In some embodiments, the polymerase is a mutant H784Q Taq polymerase. In some embodiments, the mutant H784Q Taq polymerase is reversibly inactivated via chemical, aptamer or antibody modification.
In another aspect, the disclosure provides blocked-cleavable primers for rhPCR, comprising: 5′-A-B-C-D-E-3′, wherein A is optional and is a tail extension that is not complementary to a target; B is a sequence domain that is complementary to a target; C is a discrimination domain; D is a cleavage domain that, when hybridized to the target, is cleavable by RNase H2, and which comprises an RNA base; and E is a blocking domain that prevents extension of the primer.
In some embodiments, D is a cleavage domain that, when hybridized to the target, is cleavable by RNase H2, and which comprises an RNA base that is: separated from the discrimination domain by one base position, within the discrimination domain, or adjacent to the discrimination domain. In some embodiments, the RNA base is separated from the discrimination domain by one base position. In some embodiments, the RNA base is within the discrimination domain. In some embodiments, the RNA base is adjacent to the discrimination domain. For example, when the RNA base is adjacent to the discrimination domain, no intervening DNA residue is present between the RNA base and the discrimination domain.
In some embodiments, D comprises 1-3 RNA bases. In some embodiments, the cleavage domain comprises one or more of the following moieties: a DNA residue, an abasic residue, a modified nucleoside, or a modified phosphate internucleotide linkage. In some embodiments, a sequence flanking the cleavage site contains one or more internucleoside linkages resistant to nuclease cleavage. In some embodiments, the nuclease resistant linkage is a phosphorothioate. In some embodiments, the 3′ oxygen atom of at least one of the RNA residues is substituted with an amino group, thiol group, or a methylene group. In some embodiments, the blocking group is attached to the 3′-terminal nucleotide of the primer. In some embodiments, A is comprised of a region that is identical to a universal forward primer and optionally a probe binding domain.
In some embodiments, the discrimination domain C does not comprise or overlap with the cleavage domain D. In some other embodiments, the discrimination domain C comprises the cleavage domain D. In some other embodiments, the discrimination domain C overlaps with the cleavage domain D.
In another aspect, the disclosure provides blocked-cleavable primers for rhPCR, the primers comprising:
5′-A-B-Z-E-3′
wherein A is optional and is a tail extension that is not complementary to a target; B is a sequence domain that is complementary to a target; Z comprises: C, a discrimination domain, and D, a cleavage domain that, when hybridized to the target, is cleavable by RNase H2; and E is a blocking domain that prevents extension of the primer.
In some embodiments, the discrimination domain C is located 5′ of the cleavage domain D. In some embodiments, the discrimination domain C is located 3′ of the cleavage domain D. In some embodiments, the discrimination domain C overlaps with the cleavage domain D. In some embodiments, C comprises 1-3 RNA bases. In some embodiments, C comprises 1 RNA base. In some embodiments, the discrimination domain C consists of the 1 RNA base. In some embodiments, the cleavage domain comprises one or more of the following moieties: a DNA residue, an abasic residue, a modified nucleoside, or a modified phosphate internucleotide linkage. In some embodiments, a sequence flanking the cleavage site contains one or more internucleoside linkages resistant to nuclease cleavage. In some embodiments, the nuclease resistant linkage is a phosphorothioate. In some embodiments, the 3′ oxygen atom of at least one of the RNA residues is substituted with an amino group, thiol group, or a methylene group. In some embodiments, the blocking group is attached to the 3′-terminal nucleotide of the primer. In some embodiments, A comprises a region that is identical to a universal forward primer and optionally a probe binding domain.
In another aspect, the disclosure provides methods of detecting a variation in a target DNA sequence, the methods comprising: (a) providing a reaction mixture comprising (i) an oligonucleotide primer having a cleavage domain positioned 5′ of a blocking group, the blocking group linked at or near the end of the 3′-end of the oligonucleotide primer wherein the blocking group prevents primer extension and/or inhibits the primer from serving as a template for DNA synthesis, (ii) a sample nucleic acid that may or may not have the target sequence, and where the target sequence may or may not have the variation, (iii) a cleaving enzyme, and (iv) a polymerase; (b) hybridizing the primer to the target DNA sequence to form a double-stranded substrate; (c) cleaving the hybridized primer, if the primer is complementary at the variation, with the cleaving enzyme at a point within or adjacent to the cleavage domain to remove the blocking group from the primer; and (d) extending the primer with the polymerase.
In some embodiments, the cleaving enzyme is a hot start cleaving enzyme comprising at least one amino acid substitution as compared to its wild type, which is thermostable and has reduced activity at lower temperatures, and which exhibits enhanced mismatch discrimination during rhPCR as compared to its wild type. In some embodiments, the hot start cleaving enzyme is Pyrococcus abyssi RNase H2 comprising (a) a G12A amino acid substitution; (b) a P13T amino acid substitution; (c) a G169A amino acid substitution; or (d) a combination thereof. In some embodiments, the cleaving enzyme is chemically modified. In some embodiments, the cleaving enzyme is reversibly inactivated through interaction with an antibody at lower temperatures. In some embodiments, the cleavage domain comprises at least one RNA base. In some embodiments, the cleavage domain is located 3′ of the position of variation. In some embodiments, the cleavage domain is located 5′ of the position of variation. In some embodiments, the cleavage domain overlaps with the position of variation. In some embodiments, the cleavage domain comprises one or more 2′-modified nucleosides, and the cleaving enzyme cleaves between the position complementary to the variation and the one or more modified nucleosides. In some embodiments, the one or more modified nucleosides are 2′-fluoronucleosides. In some embodiments, the polymerase is a high-discrimination polymerase. In some embodiments, the polymerase is a mutant H784Q Taq polymerase. In some embodiments, the mutant H784Q Taq polymerase is reversibly inactivated via chemical, aptamer or antibody modification. In some embodiments, the primer contains a 5′ tail sequence that comprises a universal primer sequence and optionally a universal probe sequence, wherein the tail is non-complementary to the target DNA sequence. In some embodiments, the target DNA sequence is a sample that has been treated with a gene editing enzyme. In some embodiments, the target DNA sequence is a sample that has been treated with a CRISPR enzyme. In some embodiments, the target DNA sequence is a sample that has been treated with a Cas9 or Cpf1 enzyme.
In another aspect, the disclosure provides methods of target enrichment comprising: (a) providing a reaction mixture comprising (i) a first oligonucleotide primer having a tail domain that is not complementary to a target sequence, the tail domain comprising a first universal primer sequence; a cleavage domain positioned 5′ of a blocking group, the blocking group linked at or near the end of the 3′-end of the first oligonucleotide primer wherein the blocking group prevents primer extension and/or inhibits the first primer from serving as a template for DNA synthesis, (ii) a sample nucleic acid that may or may not have the target sequence, (iii) a cleaving enzyme, and (iv) a polymerase; (b) hybridizing the first primer to the target DNA sequence to form a double-stranded substrate; (c) cleaving the hybridized first primer, if the first primer is complementary to the target, with the cleaving enzyme at a point within or adjacent to the cleavage domain to remove the blocking group from the first primer; and (d) extending the first primer with the polymerase.
In some embodiments, the methods further comprise a second primer in reverse orientation to support priming and extension of the first primer extension product. In some embodiments, the second primer further comprises a tail domain comprising a second universal primer sequence. In some embodiments, steps (b)-(d) are performed 1-10 times. In some embodiments, the methods further comprise removing unextended primers from the reaction and hybridizing universal primers to the extension product to form a second extension product. In some embodiments, the universal primers further comprise tailed sequences for addition of adapter sequences to the second extension product. In some embodiments, sequencing is performed on the second extension product to determine the sequence of the target. In some embodiments, the target DNA sequence is a sample that has been treated with a gene editing enzyme. In some embodiments, the target DNA sequence is a sample that has been treated with a CRISPR enzyme. In some embodiments, the target DNA sequence is a sample that has been treated with a Cas9 or Cpf1 enzyme. In some embodiments, the cleaving enzyme is a hot start cleaving enzyme comprising at least one amino acid substitution as compared to its wild type, which is thermostable and has reduced activity at lower temperatures, and which exhibits enhanced mismatch discrimination during rhPCR as compared to its wild type. In some embodiments, the hot start cleaving enzyme is Pyrococcus abyssi RNase H2 comprising (a) a G12A amino acid substitution; (b) a P13T amino acid substitution; or (c) a G169A amino acid substitution; or (d) a combination thereof. In some embodiments, the cleaving enzyme is chemically modified. In some embodiments, the cleaving enzyme is a hot start cleaving enzyme that is reversibly inactivated through interaction with an antibody at lower temperatures. In some embodiments, the cleaving domain comprises at least one RNA base. In some embodiments, the cleavage domain is located 3′ of the position of variation. In some embodiments, the cleavage domain is located 5′ of the position of variation. In some embodiments, the cleavage domain overlaps with the position of variation. In some embodiments, the cleaving domain comprises one or more 2′-modified nucleosides, and the cleaving enzyme cleaves between the position complementary to the variation and the one or more modified nucleosides. In some embodiments, the one or more modified nucleosides are 2′-fluoronucleosides. In some embodiments, the polymerase is a high-discrimination polymerase. In some embodiments, the polymerase is a mutant H784Q Taq polymerase. In some embodiments, the mutant H784Q Taq polymerase is reversibly inactivated via chemical, aptamer or antibody modification.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. In case of conflict, the present document, including definitions, will control. Preferred methods and materials are described below, although methods and materials similar or equivalent to those described herein can be used in practice or testing of the present invention.
“Complement” or “complementary” as used herein means a nucleic acid, and can mean Watson-Crick (e.g., A-T/U and C-G) or Hoogsteen base pairing between nucleotides or nucleotide analogs of nucleic acid molecules.
“Fluorophore” or “fluorescent label” refers to compounds with a fluorescent emission maximum between about 350 and 900 nm.
“Hybridization” as used herein, refers to the formation of a duplex structure by two single-stranded nucleic acids due to complementary base pairing. Hybridization can occur between fully complementary nucleic acid strands or between “substantially complementary” nucleic acid strands that contain minor regions of mismatch. “Identical” sequences refers to sequences of the exact same sequence or sequences similar enough to act in the same manner for the purpose of signal generation or hybridizing to complementary nucleic acid sequences. “Primer dimers” refers to the hybridization of two oligonucleotide primers. “Stringent hybridization conditions” as used herein means conditions under which hybridization of fully complementary nucleic acid strands is strongly preferred. Under stringent hybridization conditions, a first nucleic acid sequence (for example, a primer) will hybridize to a second nucleic acid sequence (for example, a target sequence), such as in a complex mixture of nucleic acids. Stringent conditions are sequence-dependent and will be different in different circumstances. Stringent conditions can be selected to be about 5-10° C. lower than the thermal melting point (Tm) for the specific sequence at a defined ionic strength pH. The Tm can be the temperature (under defined ionic strength, pH, and nucleic concentration) at which 50% of an oligonucleotide complementary to a target hybridize to the target sequence at equilibrium (as the target sequences are present in excess, at Tm, 50% of the probes are occupied at equilibrium). Stringent conditions can be those in which the salt concentration is less than about 1.0 M sodium ion, such as about 0.01-1.0 M sodium ion concentration (or other salts) at pH 7.0 to 8.3 and the temperature is at least about 30° C. for short probes (e.g., about 10-50 nucleotides) and at least about 60° C. for long probes (e.g., greater than about 50 nucleotides). Stringent conditions can also be achieved with the addition of destabilizing agents such as formamide. For selective or specific hybridization, a positive signal can be at least 2 to 10 times background hybridization. Exemplary stringent hybridization conditions include the following: 50% formamide, 5×SSC, and 1% SDS, incubating at 42° C., or, 5×SSC, 1% SDS, incubating at 65° C., with wash in 0.2×SSC, and 0.1% SDS at 65° C.
The terms “nucleic acid,” “oligonucleotide,” or “polynucleotide,” as used herein, refer to at least two nucleotides covalently linked together. The depiction of a single strand also defines the sequence of the complementary strand. Thus, a nucleic acid also encompasses the complementary strand of a depicted single strand. Many variants of a nucleic acid can be used for the same purpose as a given nucleic acid. Thus, a nucleic acid also encompasses substantially identical nucleic acids and complements thereof. A single strand provides a probe that can hybridize to a target sequence under stringent hybridization conditions. Thus, a nucleic acid also encompasses a probe that hybridizes under stringent hybridization conditions.
Nucleic acids can be single stranded or double stranded, or can contain portions of both double stranded and single stranded sequences. The nucleic acid can be DNA, both genomic and cDNA, RNA, or a hybrid, where the nucleic acid can contain combinations of deoxyribo- and ribonucleotides, and combinations of bases including uracil, adenine, thymine, cytosine, guanine, inosine, xanthine hypoxanthine, isocytosine and isoguanine. Nucleic acids can be obtained by chemical synthesis methods or by recombinant methods. A particular nucleic acid sequence can encompass conservatively modified variants thereof (e.g., codon substitutions), alleles, orthologs, single nucleotide polymorphisms (SNPs), and complementary sequences as well as the sequence explicitly indicated.
“Polymerase Chain Reaction (PCR)” refers to the enzymatic reaction in which DNA fragments are synthesized and amplified from a substrate DNA in vitro. The reaction typically involves the use of two synthetic oligonucleotide primers, which are complementary to nucleotide sequences in the substrate DNA which are separated by a short distance of a few hundred to a few thousand base pairs, and the use of a thermostable DNA polymerase. The chain reaction consists of a series of 10 to 40 cycles. In each cycle, the substrate DNA is first denatured at high temperature. After cooling down, synthetic primers which are present in vast excess, hybridize to the substrate DNA to form double-stranded structures along complementary nucleotide sequences. The primer-substrate DNA complexes will then serve as initiation sites for a DNA synthesis reaction catalyzed by a DNA polymerase, resulting in the synthesis of a new DNA strand complementary to the substrate DNA strand. The synthesis process is repeated with each additional cycle, creating an amplified product of the substrate DNA.
“Primer,” as used herein, refers to an oligonucleotide capable of acting as a point of initiation for DNA synthesis under suitable conditions. Suitable conditions include those in which hybridization of the oligonucleotide to a template nucleic acid occurs, and synthesis or amplification of the target sequence occurs, in the presence of four different nucleoside triphosphates and an agent for extension (e.g., a DNA polymerase) in an appropriate buffer and at a suitable temperature.
“Probe” and “fluorescent generation probe” are synonymous and refer to either a) a sequence-specific oligonucleotide having an attached fluorophore and/or a quencher, and optionally a minor groove binder or b) a DNA binding reagent, such as, but not limited to, SYBR® Green dye.
“Quencher” refers to a molecule or part of a compound, which is capable of reducing the emission from a fluorescent donor when attached to or in proximity to the donor. Quenching may occur by any of several mechanisms including fluorescence resonance energy transfer, photo-induced electron transfer, paramagnetic enhancement of intersystem crossing, Dexter exchange coupling, and exciton coupling such as the formation of dark complexes.
The term “RNase H PCR (rhPCR)” refers to a PCR reaction which utilizes “blocked” oligonucleotide primers and an RNase H enzyme. “Blocked” primers contain at least one chemical moiety (such as, but not limited to, a ribonucleic acid residue) bound to the primer or other oligonucleotide, such that hybridization of the blocked primer to the template nucleic acid occurs, without amplification of the nucleic acid by the DNA polymerase. Once the blocked primer hybridizes to the template or target nucleic acid, the chemical moiety is removed by cleavage by an RNase H enzyme, which is activated at a high temperature (e.g., 50° C. or greater). Following RNase H cleavage, amplification of the target DNA can occur.
The term “discrimination domain” can be the same or different as the cleavage domain. The discrimination domain is the position of the potential mutation site, and the enzyme will only cleave at the cleavage site if the criteria at the discrimination domain are met. For example, in one embodiment RNase H2 will cleave or not cleave a double-stranded target at the RNA residue (cleavage domain), depending on whether a mutation exists at the discrimination domain.
In one embodiment, the 3′ end of a blocked primer can comprise the moiety rDDDDMx, wherein relative to the target nucleic acid sequence, “r” is an RNA residue, “D” is a complementary DNA residue, “M” is a mismatched DNA residue, and “x” is a C3 spacer. A C3 spacer is a short 3-carbon chain attached to the terminal 3′ hydroxyl group of the oligonucleotide, which further inhibits the DNA polymerase from binding before cleavage of the RNA residue.
The methods described herein can be performed using any suitable RNase H enzyme that is derived or obtained from any organism. Typically, RNase H-dependent PCR reactions are performed using an RNase H enzyme obtained or derived from the hyperthermophilic archaeon Pyrococcus abyssi (P.a.), such as RNase H2. Thus, in one embodiment, the RNase H enzyme employed in the methods described herein desirably is obtained or derived from Pyrococcus abyssi, preferably an RNase H2 obtained or derived from Pyrococcus abyssi. In other embodiments, the RNase H enzyme employed in the methods described herein can be obtained or derived from other species, for example, Pyrococcus furiosis, Pyrococcus horikoshii, Thermococcus kodakarensis, or Thermococcus litoralis.
The following examples further illustrate the invention but should not be construed as in any way limiting its scope.
Example 1This example demonstrates an enhanced rhPCR assay that utilizes a highly discriminatory DNA polymerase and RNase H2 for discrimination
To demonstrate the utility of these new assay designs, rhPrimers and standard allele-specific primers were designed against rs113488022, the V600E mutation in the human BRAF gene. These primers were tested in PCR and rhPCR with WT or H784Q mutant Taq polymerase. Primers utilized in these assays were as shown in Table 1 (SEQ ID NOs: 1-7).
Nucleic acid sequences are shown 5′-3′. DNA is uppercase, RNA is lowercase. Location of potential mismatch is underlined. ZEN=internal ZEN™ quencher (IDT, Coralville, Iowa), FAM=6-carboxyfluorescein, IBFQ=Iowa Black® FQ (fluorescence quencher, IDT, Coralville, Iowa), and x=C3 propanediol spacer block
10 μL reaction volumes were used in these assays. To perform the reaction, 5 μL of 2× Integrated DNA Technologies (IDT) (Coralville, Iowa) rhPCR genotyping master mix (containing dNTPs, H784Q mutant or WT Taq DNA polymerase, stabilizers, and MgCl2) was combined with 200 nM (2 pmol) of either of the allelic primers. 200 nM (2 pmol) of the probe, as well as 200 nM (5 pmol) of the reverse primer were also added. Additionally, 2.5 mU (5.25 fmol/0.53 nM) of P.a. RNase H2 and 1000 copies of synthetic gBlock™ (Integrated DNA Technologies, Coralville, Iowa) template (1000 copies Allele 1, 500 copies allele 1+500 copies allele 2 (heterozygote), or 1000 copies Allele 2 (for gBlock™ sequences, see Table 2, SEQ ID NOs: 8-9) were added to the reaction mix. The reaction was thermocycled on a Bio-Rad™ CFX384™ Real-time system. Cycling conditions were as follows: 953:00−(950:10−650:30)×65 cycles. Each reaction was performed in triplicate.
Nucleic acid sequences are shown 5′-3′. Location of SNPs are shown bold and underlined.
Cq Results of the experiment are shown in Table 3. This data shows that the mismatch discrimination of the assay system increases with rhPCR over ASPCR with WT Taq polymerase, and that the discrimination is enhanced by the use of the H784Q Taq polymerase.
All numbers in this table represent Cq values obtained from the CFX384™ instrument (Bio-Rad™, Hercules, Calif.).
Example 2The following example demonstrates an enhanced rhPCR assay that utilizes a highly discriminatory DNA polymerase and RNase H2 for discrimination.
In order to demonstrate that this new assay design could function, rhPrimers and standard allele-specific primers were designed against rs113488022, the V600E mutation in the human BRAF gene. These primers were tested in PCR and rhPCR with H784Q mutant Taq polymerase. Primers utilized in these assays were as shown in Table 4 (SEQ ID NOs: 1, 4 and 10-12).
Nucleic acid sequences are shown 5′-3′. DNA is uppercase, RNA is lowercase. Location of potential mismatch is underlined. ZEN=internal Zen™ fluorescent quencher (IDT, Coralville, Iowa). FAM=6-carboxyfluorescein, IBFQ=Iowa Black FQ (fluorescence quencher), and x=C3 propanediol spacer.
10 μL reaction volumes were used in these assays. To perform the reaction, 5 μL of 2× Integrated DNA Technologies (IDT) (Coralville, Iowa) rhPCR genotyping master mix (containing dNTPs, H784Q mutant DNA polymerase, stabilizers, and MgCl2) was combined with 200 nM (2 pmol) of either of the allelic primers. 200 nM (2 pmol) of the probe, as well as 200 nM (5 pmol) of the reverse primer were also added. Additionally, 7.5 mU (15.75 fmol/1.58 nM), 50 mU (105 fmol/10.5 nM) or 200 mU (420 fmol/42 nM) of P.a. RNase H2 and 5e4 copies of synthetic gBlock™ (Integrated DNA Technologies, Coralville, Iowa) template (1e5 copies Allele 1, 5e4 copies allele 1+5e4 copies allele 2 (heterozygote), or 1e5 copies Allele 2 (for gBlock™ sequences, see Table 2, SEQ ID NOs: 8-9) were added to the reaction mix. The reaction was thermocycled on a Bio-Rad™ CFX384™ Real-time system. Cycling conditions were as follows: 953:00−(950:10−650:30)×65 cycles. Each reaction was performed in triplicate.
Cq Results of the experiment are shown in Table 5. This data shows that the mismatch discrimination of the assay system increases with rhPCR over ASPCR with WT Taq polymerase, and that the discrimination is enhanced by the use of the H784Q Taq polymerase.
All numbers in this table represent Cq values obtained from the CFX384™ instrument (Bio-Rad™, Hercules, Calif.).
The delta Cq values were significantly higher than the ones obtained with the Gen 1 versions of these primers, indicating that there is an advantage to this primer design, as seen before in rhPCR.
Example 3The following example illustrates the heightened reliability of universal assays using a DNA polymerase with a high mismatch discrimination.
To demonstrate that the disclosed assays can function in a universal format and that they are significantly improved with a polymerase with high mismatch discrimination, “universal” assay primers were designed against rs351855, the G338R mutation in the human FGFR4 gene. This “universal” assay design has numerous advantages, including the ability to multiplex the allele-specific rhPrimers and obtain multiple-color readouts. Primers utilized in this assay were as shown in Table 6 (SEQ ID NOs: 13-18).
Nucleic acid sequences are shown 5′-3′. DNA is uppercase, RNA is lowercase. Location of potential mismatch is underlined. LNA residues are designated with a +. FAM=6-carboxyfluorescein, HEX=6-carboxy-2′,4,4′,5′,7,7′-hexachlorofluorescein, IBFQ=Iowa Black FQ (fluorescence quencher), and x=C3 propanediol spacer block.
10 μL reaction volumes were used in these assays. To perform the reaction, 5 μL of 2× Integrated DNA Technologies (IDT) (Coralville, Iowa) rhPCR genotyping master mix (containing dNTPs, mutant or WT Taq DNA polymerase, stabilizers, and MgCl2) was combined with 50 nM (500 fmol) of each of the two allelic primers. 250 nM (2.5 pmol) of each of the two probes, as well as 500 nM (5 pmol) of the Universal Forward primer and 500 nM (5 pmol) of the reverse primer were also added. Additionally, 2.5 mU (5.25 fmol/0.53 nM) of P.a. RNase H2 and 1000 copies of synthetic gBlock™ (Integrated DNA Technologies, Coralville, Iowa) template (1000 copies Allele 1, 500 copies allele 1+500 copies allele 2 (heterozygote), or 1000 copies Allele 2 (for gBlock™ sequences, see Table 7, SEQ ID NOs: 19-20) were added to the reaction mix. The reaction was thermocycled on a Bio-Rad™ CFX384™ Real-time system. Cycling conditions were as follows: 953:00−(950:10−600:30)×3 cycles−(950:10−650:30)×65 cycles. Each reaction was performed in triplicate. Fluorescence reads were taken after a total of 50 cycles were completed. Fluorescence values were plotted on the FAM and HEX axis, and results are shown in
Nucleic acid sequences are shown 5′-3′. Location of SNPs are shown bold and underlined.
The results illustrate that the mismatch discrimination between homozygotes is sufficient with both polymerases, although the resulting data using the WT Taq demonstrate that it is more difficult to make an allelic call. Importantly, however, the WT Taq polymerase cannot efficiently discriminate heterozygotes from homozygotes, and places them too close to the allele 1 and 2 signals (
The importance of the mutant Taq can be further understood when examining the Cq values from this example (Table 8). The data show that not only does the H784Q Taq mutant increase mismatch discrimination dramatically, but the Cqs of the NTCs decrease from the low-to-mid 50s, to greater than the number tested in the assay (>65). From this experiment, it is shown that allele identity can be determined from Cq values as well as end-point fluorescence.
All numbers in this table represent Cq and delta Cq values obtained from the CFX384 instrument (Bio-Rad™, Hercules, Calif.).
The following example illustrates the detection of rare allelic variants with the assay designs of the present invention. To demonstrate the utility of these new assay designs to detect rare allelic variants, previously described second generation rhPrimers (rdxxdm) were utilized against rs113488022, the V600E mutation in the human BRAF gene (see Table 4; SEQ ID NOs: 1, 4 and 10-12).
10 μL reaction volumes were used in these assays. To perform the reaction, 5 μL of 2× Integrated DNA Technologies (IDT) (Coralville, Iowa) rhPCR genotyping master mix (containing dNTPs, H784Q mutant or WT Taq DNA polymerase, stabilizers, and MgCl2) was combined with 200 nM (2 pmol) of either of the allelic primers, or the non-discriminatory forward primer. 200 nM (2 pmol) of the probe, as well as 200 nM (5 pmol) of the reverse primer were also added. Additionally, 50 mU (105 fmol/10.5 nM) of P.a. RNase H2 and 50,000 copies of synthetic gBlock™ (Integrated DNA Technologies, Coralville, Iowa) match template, was combined with either 0, 50, or 500 copies of the opposite allele (for gBlock™ sequences, see Table 6, SEQ ID NOs: 16-17) were added to the reaction mix. The reaction was thermocycled on a Bio-Rad™ CFX384™ Real-time system. Cycling conditions were as follows: 953:00−(950:10−600:30)×65 cycles. Each reaction was performed in triplicate.
Data for the WT polymerase is shown in Table 9, and for the H784Q mutant Taq polymerase in Table 10. One of the advantages of this system for rare allele detection over “conventional” rhPCR is the ability to utilize a single amount of RNase H2 for both alleles. This advantage halves the potential requirement for determining the enzyme amount required for cleavage.
All numbers in this table represent Cq and delta Cq values obtained from the CFX384 instrument (Bio-Rad™, Hercules, Calif.). DNA is uppercase, RNA is lowercase. Location of potential mismatch is underlined. x=internal C3 propanediol spacer block.
All numbers in this table represent Cq and delta Cq values obtained from the CFX384 instrument (Bio-Rad™, Hercules, Calif.). DNA is uppercase, RNA is lowercase. Location of potential mismatch is underlined. x=internal C3 propanediol spacer block.
The data clearly shows that the H784Q DNA polymerase allows for detection of 50 copies of target in a 50,000 copies of background DNA (a 1:1000 discrimination level) for the mutant A allele of rs113488022, with a delta Cq of over 11 cycles. While only slightly more than 3 cycles was observed for the T allele in this assay, this was a significant improvement over the WT Taq polymerase, which did not show any discrimination for the T allele, and only a delta of 3 cycles for the A allele.
Example 5This example demonstrates successful allelic discrimination with the use of a universal rhPCR genotyping assay and Integrated DNA Technologies (IDT) (Coralville, Iowa) rhPCR genotyping master mix, and the robust stability of the reaction components. To demonstrate the robust nature of the assay design and mixture components, universal primers were designed against rs4657751, a SNP located on the human Chromosome 1 (See Table 11, SEQ ID NOs: 14, 21-25).
Identical universal rhPCR genotyping reactions were set up in two white Hard-Shell® 384-well skirted PCR plates (Bio-Rad, Hercules, Calif.) on the Bio-Rad CFX384 Touch™ Real-Time PCR Detection System with 10 μL final volume. Each well contained the rhPCR assay primers (150 nM of rs4657751 Allele Specific Primer 1 (SEQ ID NO: 23), 150 nM of rs4657751 Allele Specific Primer 2 (SEQ ID NO: 24), and 500 nM rs4657751 Locus Specific Primer (SEQ ID NO: 25). Reactions contained universal reporter oligos at the following concentrations: 250 nM of universal FAM probe (SEQ ID NO: 14), 450 nM of universal Yakima Yellow® (SEQ ID NO: 22) probe, and 1000 nM of universal forward primer (SEQ ID NO: 21), and 5 μL of 2× Integrated DNA Technologies (IDT) (Coralville, Iowa) rhPCR genotyping master mix (containing dNTPs, a mutant H784Q Taq polymerase (see Behlke, et al. U.S. 2015/0191707), chemically modified Pyrococcus abyssi RNase H2 (See Walder et al. UA20130288245A1), stabilizers, and MgCl2).
gBlocks® Gene Fragments (Integrated DNA Technologies, Inc., Coralville, Iowa) containing either allele of the rs4657751 SNP were utilized as the source of template DNA (See Table 12, SEQ ID NOs: 26 and 27). Each well contained template representing one of three possible genotypes: allele 1 homozygote (1000 copies rs4657751 Allele 1 gBlock® template (SEQ ID NO: 26)), allele 2 homozygote (1000 copies rs4657751 Allele 2 gBlock® template (SEQ ID NO: 27)), or heterozygote (mix of 500 copies of rs4657751 Allele 1 gBlock® template (SEQ ID NO: 26) and 500 copies of rs4657751 Allele 2 gBlock® template (SEQ ID NO: 27)). Template or water for the no template control (NTC) reactions were added into three replicate wells of two individual plates. The reactions underwent the following cycling protocol: 95° C. for 10 minutes, then 45 cycles of 95° C. for 10 seconds and 60° C. for 45 seconds.
Nucleic acid sequences are shown 5′-3′. DNA is uppercase, RNA is lowercase. Location of potential mismatch is underlined. LNA residues are designated with a +. FAM=6-carboxyfluorescein, Yak=Yakima Yellow (3-(5,6,4′,7′-tetrachloro-5′-methyl-3′,6′-dipivaloylfluorescein-2-yl)), IBFQ=Iowa Black FQ (fluorescence quencher), and x=C3 propanediol spacer block.
Nucleic acid sequences are shown 5′-3′. DNA is uppercase. The location of the SNP is underlined.
One reaction plate was cycled immediately (0 hr benchtop hold) and one reaction plate was held at room temperature for 2 days (48 hr benchtop hold) to demonstrate reaction stability over time. Allelic discrimination analysis was performed using Bio-Rad CFX Manager 3.1 software (Bio-Rad, Hercules, Calif.). FAM and Yakima Yellow fluorophores were detected in each well. For both fluorophores the baseline cycles were set to begin at cycle 10 and end at cycle 25. Fluorescence signal (RFU) in each well at the end of 45 cycles was used to generate an allelic discrimination plot and genotypes were determined with auto-call features of the analysis software. Identical performance was obtained with the immediate run (
The following example compares the performance of the genotyping methods of the present invention versus traditional 5′ nuclease genotyping assay methods (Taqman™).
The rs1799865 SNP in the CCR2 gene was selected, and rhPCR genotyping primers as well as an rs1799865 5′ nuclease assay (Thermo-Fisher (Waltham, Mass.)), were designed and obtained. Sequences for the rs1799865 rhPCR genomic SNP assay are shown in Table 14 (SEQ ID NOs: 14, 21, 22, and 28-30). Thermo-Fisher 5′ nuclease primer/probe (Taqman™) sequences are not published, and therefore are not included in this document.
Reactions were performed in 10 μL volumes, containing 10 ng Coriell genomic DNA (Camden, N.J.), 250 nM of universal FAM probe (SEQ ID NO: 14), 450 nM of universal Yakima Yellow® (SEQ ID NO: 22) probe, 1000 nM of universal forward primer (SEQ ID NO: 21), 150 nM of the two allele-specific forward primers (SEQ ID NOs: 28 and 29), 500 nM of the reverse primer (SEQ ID NO: 30), and 5 μL of 2× Integrated DNA Technologies (IDT) (Coralville, Iowa) rhPCR genotyping master mix (containing dNTPs, a mutant H784Q Taq polymerase (see Behlke, et al. U.S. 2015/0191707), chemically modified Pyrococcus abyssi RNase H2 (See Walder et al. UA20130288245A1), stabilizers, and MgCl2).
PCR was performed on Life Technologies (Carlsbad, Calif.) QuantStudio™ 7 Flex real-time PCR instrument using the following cycling conditions: 10 mins at 95° C. followed by 50 cycles of 95° C. for 10 seconds and 60° C. for 45 seconds. End-point analysis of each of the plates was performed after 45 cycles with the QuantStudio™ Real-Time PCR Software v1.3 (Carlsbad, Calif.).
Nucleic acid sequences are shown 5′-3′. DNA is uppercase, RNA is lowercase. LNA residues are designated with a +. Location of potential mismatch is underlined. FAM=6-carboxyfluorescein, Yak=Yakima Yellow (3-(5,6,4′,7′-tetrachloro-5′-methyl-3′,6′-dipivaloylfluorescein-2-yl)), IBFQ=Iowa Black FQ (fluorescence quencher), and x=C3 propanediol spacer block.
The following example illustrates the present methods allowing for detection and analysis of tri-allelic SNP. The rs72558195 SNP is present in the CYP2C8 gene, and has three potential genotypes. This SNP was selected for analysis with the rhPCR genotyping system.
Conventional workflow of interrogating tri-allelic SNP, as illustrated in
To demonstrate that such a system can function with the universal rhPCR genotyping system, reactions were set up in a white Hard-Shell® 384-well skirted PCR plates (Bio-Rad, Hercules, Calif.) on the Life Technologies (Carlsbad, Calif.) QuantStudio™ 7 Flex real-time PCR with 10 μL final volume. Each well contained the rhPCR assay primers (See Table 16, SEQ ID NOs: 14, 21, 22, 31-33). Specifically, 150 nM of rs72558195 G:A Allele Specific Primer 1 (SEQ ID NO: 31) and 150 nM of rs72558195 G:A Allele Specific Primer 2 (SEQ ID NO: 32), or 150 nM of rs72558195 G:A Allele Specific Primer 1 (SEQ ID NO: 31) and 150 nM of rs72558195 G:C Allele Specific Primer 3 (SEQ ID NO: 33) as well as 500 nM rs72558195 Locus Specific Primer (SEQ ID NO: 34) were included in the reactions.
Reactions contained universal reporter oligos at the following concentrations: 250 nM of universal FAM probe (SEQ ID NO: 14), 450 nM of universal Yakima Yellow® (SEQ ID NO: 22) probe, and 1000 nM of universal forward primer (SEQ ID NO: 21), 50 nM ROX internal standard, and 5 μL of 2× Integrated DNA Technologies (IDT) (Coralville, Iowa) rhPCR genotyping master mix (containing dNTPs, a mutant H784Q Taq polymerase (see Behlke, et al. U.S. 2015/0191707), chemically modified Pyrococcus abyssi RNase H2 (See Walder et al. UA20130288245A1), stabilizers, and MgCl2).
Nucleic acid sequences are shown 5′-3′. DNA is uppercase, RNA is lowercase. LNA residues are designated with a +. Location of potential mismatch is underlined. FAM=6-carboxyfluorescein, Yak=Yakima Yellow (3-(5,6,4′,7′-tetrachloro-5′-methyl-3′,6′-dipivaloylfluorescein-2-yl)), IBFQ=Iowa Black FQ (fluorescence quencher), and x=C3 propanediol spacer block.
gBlocks® Gene Fragments (Integrated DNA Technologies, Inc., Coralville, Iowa) containing alleles of the rs72558195 SNP were utilized as the source of template DNA (See Table 17, SEQ ID NOs: 35, 36 and 37). Each well contained template representing one of six possible genotypes: allele 1 homozygote (1000 copies rs72558195 Allele 1 gBlock® template (SEQ ID NO: 35)), allele 2 homozygote (1000 copies rs72558195 Allele 2 gBlock® template (SEQ ID NO: 36)), allele 3 homozygote (1000 copies rs72558195 Allele 2 gBlock® template (SEQ ID NO: 37)), heterozygote (mix of 500 copies of rs72558195 Allele 1 gBlock® template (SEQ ID NO: 35) and 500 copies of rs72558195 Allele 2 gBlock® template (SEQ ID NO: 36). heterozygote (mix of 500 copies of rs72558195 Allele 1 gBlock® template (SEQ ID NO: 35) and 500 copies of rs72558195 Allele 3 gBlock® template (SEQ ID NO: 37)). Template or water for the no template control (NTC) reactions were added into three replicate wells of two individual plates. The reactions underwent the following cycling protocol: 95° C. for 10 minutes, then 45 cycles of 95° C. for 10 seconds and 60° C. for 45 seconds.
Nucleic acid sequences are shown 5′-3′. DNA is uppercase. The location of the SNP is underlined.
The results are shown in
A Tri-allelic AD 360 plot was designed for illustrating allelic discrimination. Fluorescence signal (ΔRn) from the last PCR cycle of each dye was normalized across the three dyes from the same well. Angle and distance of data point from the origin is calculated using formula below:
Angle=tan−1(ΔRnDye1÷ΔRnDye2)×120/90
Distance from origin=√{square root over ((ΔRnDye1)2+(ΔRnDye2)2)}
A 360 plot could be implemented for tetra-allelic, penta-allelic or hexa-allelic visualization. Therefore, visualization is possible for positions that could have multiple bases as well as potential deletions. The distance from origin remains unchanged for each calculation, and the angle formulas would be:
tetra-allelic (4 alleles): Angle=tan−1(ΔRnDye1÷ΔRnDye2)
penta-allelic (5 alleles): Angle=tan−1(ΔRnDye1÷ΔRnDye2)×72/90
hexa-allelic (6 alleles): Angle=tan−1(ΔRnDye1÷ΔRnDye2)×60/90
The following example illustrates the capability of the methods of the present invention to provide quantitative SNP genotyping, allowing for determination of the copy numbers of different alleles. To demonstrate this, an assay was designed against rs1135840, a SNP in the human CYP2D6 gene. This gene can be present in multiple copies, and the number of copies with the rs1135840 SNP appears to affect drug metabolism (rapid metabolism of the drug Debrisoquine).
To demonstrate that the assay system can detect small differences in allele rations, a standard curve for analysis was created. Two gBlock™ (IDT, Coralville, Iowa) gene fragments were synthesized (Allele 1 and Allele 2, representing the two allelic variants (G>C) of the rs1135840 SNP) and then mixed at different ratios. Reactions were performed in 10 μL volumes, containing a total of 1500 copies of template at the ratios shown (10:0, 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9, and 0:10), 250 nM of universal FAM probe (SEQ ID NO: 14), 450 nM of universal Yakima Yellow® (SEQ ID NO: 22) probe, 1000 nM of universal forward primer (SEQ ID NO: 21), 150 nM of the two allele-specific forward primers, 500 nM of the reverse primer, and 5 μL of 2× Integrated DNA Technologies (IDT) (Coralville, Iowa) rhPCR genotyping master mix (containing dNTPs, a mutant H784Q Taq polymerase (see Behlke, et al. U.S. 2015/0191707), chemically modified Pyrococcus abyssi RNase H2 (See Walder et al. UA20130288245A1), stabilizers, and MgCl2).
PCR was performed on Life Technologies (Carlsbad, Calif.) QuantStudio™ 7 Flex real-time PCR instrument using the following cycling conditions: 10 mins at 95° C. followed by 45 cycles of 95° C. for 10 seconds and 60° C. for 45 seconds. End-point analysis of each of the plates was performed after 45 cycles with software provided by the respective companies (Bio-Rad CFX Manager 3.1 software (Bio-Rad, Hercules, Calif.) and QuantStudio™ Real-Time PCR Software v1.3 (Carlsbad, Calif.)).
Nucleic acid sequences are shown 5′-3′. DNA is uppercase, RNA is lowercase. LNA residues are designated with a +. Location of potential mismatch is underlined. FAM=6-carboxyfluorescein, Yak=Yakima Yellow (3-(5,6,4′,7′-tetrachloro-5′-methyl-3′,6′-dipivaloylfluorescein-2-yl)), IBFQ=Iowa Black FQ (fluorescence quencher), and x=C3 propanediol spacer block.
The resulting data is illustrated in
After demonstration of the required amount of separation of allelic quantities, it is possible to determine the number of copies present of each allele in an experimental sample. To test this, the previously described assay designed against rs1135840, was utilized to test thirteen Coriell genomic DNA (Camden, N.J.) samples with varying CYP2D6 copy numbers with varying rs1135840 genotypes. These samples have known defined copy numbers and rs1135840 genotypes which could be verified after testing with the universal rhPCR genotyping mix. From this, these samples can also be categorized as being homozygotes for either allele, or heterozygotes.
To calculate the copy number from the data, two duplex reactions were run for each sample. Reactions were performed in 10 μL volumes, containing 3 ng of one of the following genomic DNAs: NA17123, NA17131, NA17132, NA17149, NA17104, NA17113, NA17144, NA17213, NA17221, NA17114, NA17235, or NA17241. Each individual assay also contained 50 nM ROX normalizer oligo, 250 nM of universal FAM probe (SEQ ID NO: 14), 450 nM of universal Yakima Yellow® (SEQ ID NO: 22) probe, 1000 nM of universal forward primer (SEQ ID NO: 21), 150 nM of the two allele-specific forward primers (SEQ ID NO: 38 and 39), 500 nM of the reverse primer (SEQ ID NO: 40), and 5 μL of 2× Integrated DNA Technologies (IDT) (Coralville, Iowa) rhPCR genotyping master mix (containing dNTPs, a mutant H784Q Taq polymerase (see Behlke, et al. U.S. 2015/0191707), chemically modified Pyrococcus abyssi RNase H2 (See Walder et al. UA20130288245A1), stabilizers, and MgCl2). Assays also contained a separate RNase P assay (See table 17, SEQ ID NOs: 41-43)) for normalization of the template concentration.
Nucleic acid sequences are shown 5′-3′. DNA is uppercase. FAM=6-carboxyfluorescein, Yak=Yakima Yellow (3-(5,6,4′,7′-tetrachloro-5′-methyl-3′,6′-dipivaloylfluorescein-2-yl)), IBFQ=Iowa Black FQ (fluorescence quencher).
Quantitative PCR was performed on Life Technologies (Carlsbad, Calif.) QuantStudio™ 7 Flex real-time PCR instrument using the following cycling conditions: 10 mins at 95° C. followed by 45 cycles of 95° C. for 10 seconds and 60° C. for 45 seconds. End-point analysis of each of the plates was performed after 45 cycles with the QuantStudio™ Real-Time PCR Software v1.3 (Carlsbad, Calif.) software provided by the company.
Copy number was determined by the following method. For each sample shown to be a homozygote, ΔCq (RNase P Cq-rs1135840 assay Cq) was calculated for each sample. For samples shown to be heterozygotes, ΔCq was calculated for both alleles (RNase P Cq-rs1135840 assay 1 Cq and RNase P Cq-rs1135840 assay 2 Cq). Next, ΔΔCq (ΔCq−mean ΔCq for known 2 copy control DNA samples) was calculated for each allele. This correction allowed for normalization against amplification differences between the SNP assay and the RNase P assay. Finally, the following equation was used to calculate copy number for each allele:
Copy number of allele=2*(2̂(ΔΔCq))
The resulting end-point data is shown in
The following example demonstrates that a variation of an rhPCR probe can be used for multiplexed rhPCR.
The assay schematic is provided in
As illustrated in
The first round of PCR reactions contained the 96 plex at 10 nM of each blocked target specific primer, 10 ng of NA12878 human genomic DNA (Coriell Institute for Medical Research, Camden, N.J.), 200 mU of chemically modified Pyrococcus abyssi RNase H2 (See Walder et al. UA20130288245A1) (IDT, Coralville, Iowa) and 1×KAPA 2G HotStart Fast Ready Mix™ (Kapa Biosystems, Wilmington, Mass.). The thermal cycling profile was 10 mins at 95° C. followed by 8 cycles of 95° C. for 15 seconds and 60° C. for 4 minutes, and a final 99° C. finishing step for 15 minutes. Reactions were cleaned up with a 2×AMPure™ XP beads (Beckman Coulter, Brea, Calif.). Briefly, 100 μL AMPure™ SPRI beads were added to each PCR well, incubated for 5 minutes at room temperature and collected for 5 minutes at room temperature on plate magnet (DynaMag™ (Thermo-Fisher, (Watherham, Mass.) 96-well plate side-magnet). Beads were washed twice with 80% ethanol, and allowed to dry for 3 minutes at room temperature. Samples were eluted in 22 μL of TE at pH 8.0.
The second round of PCR was set up using 20 μL of the cleaned up first round PCR products, universal PCR-50F and PCR-47R primers (See table 18, SEQ ID NOs: 44 and 45) at 2 uM and 1×KAPA 2G HotStart Fast Ready Mix™ (KAPA Biosystems, Wilmington, Mass.). Reactions were cycled for 45 seconds at 98° C. followed by 20 cycles of 98° C. for 15 seconds, 60° C. for 30 seconds, and 72° C. for 30 seconds. A final 1 minute 72° C. polishing step finished the reaction. Samples were cleaned up again with 0.8× AMPure™ beads. Briefly, 40 AMPure™ SPRI beads were added the second PCR wells, incubated for 5 minutes at room temperature and collected for 5 minutes at room temperature on plate magnet (DynaMag™ (Thermo-Fisher, (Watherham, Mass.) 96-well plate side-magnet). Beads were washed twice with 80% ethanol, and allowed to dry for 3 minutes at room temperature. Samples were eluted in 22 μL of TE at pH 8.0, and 20 μL was transferred to a new tube.
2 μL of the samples were analyzed using the Agilent® High Sensitivity D1000™ Screen Tape™ on the Agilent® 2200 Tape Station™ (Agilent Technologies®, Santa Clara, Calif.). Quantification was performed using the KAPA Library Quantification Kit (KAPA Biosystems, Wilmington, Mass.) for Illumina® Platforms, according to the manufacturer's protocol. Replicate samples were pooled to a final concentration of 10 pM, and 1% PhiX bacteriophage sequencing control was added. Samples were run with a V2 300 cycle MiSeq™ kit on an Illumina® (San Diego, Calif.) MiSeq™ platform, using standard protocols from the manufacturer.
Nucleic acid sequences are shown 5′-3′. DNA is uppercase.
This example demonstrates enhanced sensitivity and accuracy of assay systems of the disclosure as compared to standard T7 endonuclease cleavage assays.
A total of 36 sites modified with the CRISPR/Cas9 protein system were chosen to be comparatively analyzed by T7 endonuclease, next-generation sequencing (NGS), or a qPCR “Genie” assay system of the disclosure.
To mutate the chosen genomic sites, a HEK293 cell line was generated with stable expression of S pyogenes Cas9. AltR™ guide RNAs were reverse transfected into the cells in 96-well plates (40,000 cells/well) using 0.75 μL RNAiMAX (Thermo) per well. Briefly, AltR™ crRNAs were designed from the IDT CRISPR2.0 design engine to target exon 1 or 2 of selected human genes. Prior to transfection, guide RNAs (crRNA/tracrRNA with AltR™ chemistry) were duplexed in an equimolar ratio at 3 μM final concentration of the complex in IDT duplex buffer (Integrated DNA Technologies, Coralville, Iowa). Complexes were heated to 95° C. for 5 min and cooled to room temperature. Genomic DNA was isolated after 48 hrs with 50 μL Quick Extract buffer (Epicentre), using standard techniques described by the manufacturer. DNA solutions were further diluted with 100 μL water before further analysis was performed.
Analysis by T7 endonuclease digestion was done as follows. CRISPR-Cas9-treated cells were washed with 100 μL of PBS. Cells were lysed by adding 50 μL of QuickExtract™ DNA Extraction Solution (Integrated DNA Technologies). Cell lysates were then transferred to appropriate PCR tubes or plate, then vortexed and heated in a thermal cycler at 65° C. for 10 min, followed by 98° C. for 5 min, after which 100 μL of Nuclease-Free Water was added to dilute the genomic DNA. The samples were then vortexed and spun down. PCR was set up using template, primers, and components of the Alt-R Genome Editing Detection Kit and KAPA HiFi HotStart PCR Kit as follows. Sample: 4 (˜40 ng) genomic DNA, 300 nM forward primer, 300 nM reverse primer, 5 (1×) of KAPA HiFi Fidelity Buffer (5×), 1.2 mM (0.3 mM each) dNTPs, 0.5 U KAPA HiFi Hotstart DNA Polymerase (1 U/μL), for a total volume of 25 μL. Alt-R™ Control A: 1 μL Alt-R™ Control A template/primer mix, 5 (1×) of KAPA HiFi Fidelity Buffer (5×), 1.2 mM (0.3 mM each) dNTPs, 0.5 U KAPA HiFi Hotstart DNA Polymerase (1 U/μt), for a total volume of 25 μL. Alt-R™ Control B: 1 μL Alt-R™ Control B template/primer mix, 5 μL (1×) of KAPA HiFi Fidelity Buffer (5×), 1.2 mM (0.3 mM each) dNTPs, 0.5 U KAPA HiFi Hotstart DNA Polymerase (1 U/μt), for a total volume of 25 μL. PCR was run using the following conditions: denature at 95° C. for 5 min; 30 cycles of: denature at 98° C. for 20 sec, anneal between 64-67° C. (depending on polymerase) for 15 sec, extend at 72° C. for 30 sec; then extend at 72° C. for 2 minutes. Heteroduplexes for T7EI digestion were formed as follows. 2 μL T7EI Reaction Buffer (10×) and 6 μL Nuclease-Free Water was combined with 10 μL experimental target or Alt-R™ HPRT control from the PCR, 10 μL Control A PCR component (homoduplex control), or 5 μL Control A and 5 μL Control B (heteroduplex control). The PCR products were then placed in a thermal cycler with 95° C. denaturation for 10 min, ramp from 95-85° C. at a ramp rate of −2° C./sec, then ramp from 85-25° C. at a ramp rate of −0.3° C./sec. 18 μL of PCR heteroduplexes from the previous step were combined with 2 μL T7 endonuclease I (1 U/μL), then the T7EI reaction was incubated at 37° C. for 60 min. T7EI mismatch detection result were visualized on a Fragment Analyzer™ system with Mutation Discovery Kit according to the manufacturer's instructions (Integrated DNA Technologies). After amplification and cleavage, amplicons were sized on the Fragment Analyzer™ (Advanced Analytical, Inc, Ames, Iowa) capillary electrophoresis system.
NGS sequencing analysis of the mutated samples employed locus-specific primers positioned approximately 75-bp flanking the Cas9 cleavage site. Primers contained universal 5′-tails that allowed for secondary amplification that added Illumina™ TruSeq™ i5 and i7 adapters with sample-specific barcodes to the amplicons. The locus-specific primers were designed as RNase H2-cleavable primers with the 4DMX blocking modification at the 3′-end (where the 4DMX nomenclature indicates 4 DNA bases, a mismatched DNA base and a propanediol C3-spacer 3′ of the RNA base). Using a master-mix containing a hot-start Taq polymerase and hot-start RNaseH2, the genomic DNAs were amplified using the following cycling conditions: 95° C.5:00+(95° C.0:15+60° C.1:00)×8 cycles+99° C.15:00. Samples were purified using SPRI beads (1.5× Agencourt™ Ampure® XP beads, Beckman Coulter) per the manufacturer's protocol. The second PCR incorporated the Illumina adapters and was run under the following conditions: 95° C.5:00+(95° C.0:15+60° C.3:00+72° C.0:30)×18 cycles+99° C.15:00.
The resultant amplicons underwent 1×SPRI™ clean-up and were quantified via the KAPA™ library qPCR quantitation (KAPA Biosystems, Wilmington, Mass.) kit per the manufacturer's recommended protocol. In addition, amplicons were sized on the Fragment Analyzer™ (Advanced Analytical, Inc., Ames, Iowa) capillary electrophoresis system.
DNA sequencing was carried out on an Illumina™ MiSeq® using a MiSeq® Nano cartridge (v2, 300 cycles). Data was de-multiplexed via an in-house bioinformatics processing pipeline. Analysis for specific editing events relative to a reference amplicon was performed with CRISPResso™ using methods described.
Quantitative PCR assay primers for analysis according to methods of the disclosure were designed so that the RNA nucleotide was located two bases after the primary Cas9 cleavage site, allowing for maximal discrimination from both the RNase H2 enzyme and the DNA polymerase (
DNA is uppercase, RNA is lowercase. FAM=6-Fluorescein fluorescent dye (IDT, Coralville, Iowa). IBFQ=Iowa Black™ fluorescent quencher (IDT, Coralville, Iowa). UniFor-UniPro=universal forward primer, and universal probe binding site. X=propanediol (C3) spacer blocking group.
DNA is uppercase.
Although the reverse primers were not cleaved by RNase H2 in these assays, the results suggest that blocked-cleavable reverse primers could also be utilized in these assays.
Analysis of the amplification data was performed using a ΔΔCq method. Briefly, the ΔCq was calculated between each of the target Cqs and the corresponding reference (RNase P) Cqs. A conversion was then done with the calculated ΔCq, where ΔCq experimental was calculated as being equal to 2̂−ΔCq. ΔΔCq was then calculated by normalization against the ΔCq calculated from the WT (un-mutated) control.
Experimental results are shown in
This example demonstrates the effect of certain mutations on the activity of P.a. RNase H2 enzyme. In particular, the mutation G12A has limited effect on mismatch discrimination when the mismatch is located opposite the RNA, while the mutation P13T has no effect.
Towards identifying mutant RNase H2 enzymes with the desired properties, a collection of P.a. RNase H2 mutants were developed. Among these, mutations G12A and P13T were tested and found to confer increased specificity. Similar mutations in RNase H2 enzymes from other species that support rhPCR may likewise show similar improved properties relative to their WT forms.
6×-His-tagged-mutant and WT P.a. RNase H2 proteins were generated by standard site-directed mutagenesis (SDM) techniques (see, e.g., Weiner M. et al., Gene, 151:119-123 (1994)). The primers used for SDM of the P.a. RNase H2 enzyme are shown in Table 21. The mutagenized proteins were sequence verified and expressed in E. coli using standard methods. Purification was done by affinity purification over a charged Ni2+ column, as described in Dobosy et al. (2011) and U.S. Pat. No. 8,911,948 B2). Final amino acid sequences for the mutagenized proteins are shown in Table 22. After expression and purification, specific activity and unit definitions were determined using previously described techniques (U.S. Pat. No. 8,911,948 B2).
All bases are DNA. Characters shown in bold are the mutagenic nucleotides.
Location of mutations are shown in bold and underlined typeface.
To determine whether these mutant RNase H2 enzymes increase mismatch discrimination when the mismatch is opposite the RNA, 1st generation quantitative rhPCR assays were utilized against the rs4939827 SNP present in SMAD7. This SNP has been utilized in the past (Dobosy et al, 2011, and U.S. Pat. No. 8,911,948 B2) to characterize rhPCR efficiency and specificity, and its response under differing conditions is well understood. The primers utilized in these assays are shown in Table 23, SEQ ID NOs: 200-203. Assays were done in 10 μL reaction volumes. Thermal cycling and data collection was performed with a CFX384® Real Time System (Bio-Rad®, Hercules, Calif.). Briefly, 1× of the iQ™ SYBR® Green Supermix® was utilized in combination with 200 nM (2 pmol) of either the blocked forward primer (SEQ ID NOs: 202 or 203) and 200 nM (2 pmol) of the unblocked reverse primer (SEQ ID NO: 201), or 200 nM (2 pmol) of unblocked control forward primer (SEQ ID NO: 200) and 200 nM (2 pmol) of unblocked reverse primer (SEQ ID NO: 201). 5 mU of either WT (SEQ ID NO: 197) or each mutant RNase H2 enzyme (SEQ ID NOs: 198 or 199) was added to each reaction. 10 ng of genomic cell line DNA (cell lines NA1852 and NA18537, Coriell Institute for Medical Research, Camden, N.J.), representing the two homozygous genotypes at the rs4939827 SNP, was included in each reaction. Reactions were cycled under the following conditions 95° C.3:00→(95° C.0:10→60° C.0:30)×85. Fluorescence data was collected after each extension time point. After the reaction was completed, data was analyzed, and the average Cq values for each point was calculated. The results are presented in Table 24.
DNA is uppercase, RNA is lowercase. x=C3 spacer (propanediol) blocker group.
These data show that these mutations do not significantly increase mismatch discrimination when the SNP is located immediately opposite the RNA residue in the rhPCR primer. The possible exception of the G12A mutation is noted, but insufficient amount of RNase H2 was utilized in this experiment resulting in inefficient cleavage of primers and a severely delayed amplification compared to the match template.
Example 12This example demonstrates the effect of certain mutations on the activity of P.a. RNase H2 enzyme. In particular, mutations G12A and P13T increase mismatch discrimination when the mismatch is placed 5′ of the RNA nucleotide.
To determine whether mismatch discrimination can be increased with the G12A and P13T mutant RNase H2 enzymes when the mismatch is located 5′ of the RNA nucleotide, assays targeting rs113488022, the V600E SNP in the human BRAF gene were designed with the mismatch located 5′ of the RNA. The primers utilized in these assays are shown in Table 25, SEQ ID NOs: 204-207. Assays were performed in 10 μL reaction volumes. Thermal cycling and data collection were carried out with a CFX384® Real Time System (Bio-Rad®, Hercules, Calif.). Briefly, 1× of the iQ™ SYBR® Green Supermix® was utilized in combination with 200 nM (2 pmol) of either the blocked forward primer (SEQ ID NOs: 206 or 207) and 200 nM (2 pmol) of the unblocked reverse primer (SEQ ID NO: 205), or 200 nM (2 pmol) of unblocked control forward primer (SEQ ID NO: 204) and 200 nM (2 pmol) of unblocked reverse primer (SEQ ID NO: 205). 10 mU or 25 mU of either WT (SEQ ID NO: 197) or each mutant RNase H2 enzyme (SEQ ID NOs: 198 or 199) was added to each reaction. The 2,000 copies of synthetic double-stranded gBlock® (IDT, Coralville, Iowa) template, representing the two homozygous genotypes at the rs113488022 SNP (SEQ ID NOs: 208 or 209), were also added to each reaction. Samples were cycled under the following conditions 95° C.3:00→(95° C.0:10→60° C.0:30)×85. Fluorescence intensity was collected after each 60° C. extension step. The Cq values were averaged from 3 replicates. The results are shown in Table 26.
DNA is uppercase, RNA is lowercase. X=C3 spacer (propanediol) blocker group. Location of the mismatch is shown in bold and underlined in the gBlocks®.
These data show that the P13T RNase H2 mutant improved the ΔCq quantification of the mismatch discrimination with the TrG primer from 2.3 cycles with WT to 4.3 cycles (with 10 mU RNase H2). The P13T RNase H2 also improved mismatch discrimination with the ArG primer, increasing the ΔCq from 10.4 to 12.0 cycles. While modest, these changes can be significant in sensitive assays. The data also shows that the G12A RNase H2 mutant improved the ΔCq quantification of the mismatch discrimination with the TrG primer from 2.3 cycles with WT (with 10 mU RNase H2) to 6.2 cycles (with 25 mU RNase H2). The G12A RNase H2 also improved mismatch discrimination with the ArG primer, increasing the ΔCq from 10.4 to 11.4 cycles. The change with the ArG primer is less important here, as it already was effective. Both of these mutations therefore improve mismatch discrimination when the mismatch is located 5′ of the RNA nucleotide.
Example 13This example demonstrates the effect of certain mutations on the activity of P.a. RNase H2 enzyme. In particular, mutations G12A and P13T increase mismatch discrimination when the mismatch is placed 3′ of the RNA.
To determine whether mismatch discrimination can be increased with the G12A and P13T mutant RNase H2 enzymes when the mismatch is located 3′ of the RNA, assays targeting rs113488022, the V600E SNP in the human BRAF gene were designed with the mismatch located 3′ of the RNA. The primers utilized in these assays are shown in Table 27, SEQ ID NOs: 204, 205, 210, and 211. Assays were conducted in 10 μL reaction volumes. Thermal cycling and data collection was performed with a CFX384® Real Time System (Bio-Rad®, Hercules, Calif.). Briefly, 1× of the iQ™ SYBR® Green Supermix® was utilized in combination with 200 nM (2 pmol) of either the blocked forward primer (SEQ ID NOs: 210 or 211) and 200 nM (2 pmol) of the unblocked reverse primer (SEQ ID NO: 205), or 200 nM (2 pmol) of unblocked control forward primer (SEQ ID NO: 204) and 200 nM (2 pmol) of unblocked reverse primer (SEQ ID NO: 205). 5 mU of either WT (SEQ ID NO: 197) or each mutant RNase H2 enzyme (SEQ ID NOs: 198 or 199) was added to each reaction. The 2,000 copies of synthetic double-stranded gBlock® (IDT, Coralville, Iowa) template, representing the two homozygous genotypes at the rs113488022 SNP (SEQ ID NOs: 208 or 209), were also added to each reaction. Reactions were cycled under the following conditions 95° C.3:00→(95° C.0:10→60° C.0:30)×85. Fluorescence intensity was collected after each extension step. The average Cq values were averaged from 3 replicates. The results are shown in Table 28.
DNA is uppercase, RNA is lowercase. X=C3 spacer (propanediol) blocker group.
These data show that the P13T RNase H2 mutant improved the ΔCq discrimination with the rGT primer from 0.2 cycles (seen for WT) to 4.5 cycles (seen for mutant at 10 mU RNase H2). The P13T RNase H2 also improved mismatch discrimination with the rGA primer, increasing the ΔCq from 7.3 to 15.0 cycles. These improvements turned a failed assay into successful one, in particular when the rGT primer was employed. Furthermore, the G12A RNase H2 mutant improved the ΔCq of the mismatch discrimination with the rGT primer from 0.2 cycles to 8.6 cycles. The G12A RNase H2 also improved mismatch discrimination with the rGA primer, increasing the ΔCq from 7.3 to 22.5 cycles. This change came at the cost of some amplification efficiency, but this would likely be overcome with the addition of more G12A mutant RNase H2 enzyme. In conclusion, both of these mutations improve mismatch discrimination when the mismatch is located 3′ of the RNA nucleotide.
Example 14This example demonstrates the effect of certain mutations on the activity of P.a. RNase H2 enzyme. In particular, mutation G169A of P.a. RNase H2 has no effect on mismatch discrimination when the mismatch is located opposite the RNA.
Towards identifying mutant RNase H2 enzymes with the desired properties, an additional P.a. RNase H2 mutant was developed with by substituting glycine with alanine at position 169 (G169A). Similar mutations in RNase H2 enzymes from other species that support rhPCR may likewise show similar improved properties relative to their WT forms.
6×-His-tagged-mutant and WT P.a. RNase H2 proteins were generated by standard site-directed mutagenesis (SDM) techniques (see, e.g., Weiner M. et al., Gene, 151:119-123 (1994)). The primers used for SDM of the P.a. RNase H2 enzyme are shown in Table 29. The mutagenized proteins were sequence verified and expressed in E. coli using standard methods. Purification was done by affinity purification over a charged Ni2+ column, as described in Dobosy et al. (2011) and U.S. Pat. No. 8,911,948 B2). Final amino acid sequences for the mutagenized proteins are shown in Table 30. After expression and purification, specific activity and unit definitions were determined using previously described techniques (U.S. Pat. No. 8,911,948 B2).
All bases are DNA. Characters shown in bold are the mutagenic nucleotides.
Location of mutations are shown in bold and underlined typeface.
To determine whether this mutant RNase H2 enzyme increases mismatch discrimination when the mismatch is opposite the RNA, quantitative rhPCR assays were utilized against the rs4939827 SNP present in SMAD7. This SNP has been utilized in the past (Dobosy et al, 2011, and U.S. Pat. No. 8,911,948 B2) to characterize rhPCR efficiency and specificity, and its response under differing conditions is well understood. The primers utilized in these assays are shown in Table 31, SEQ ID NOs: 215-218. Assays were done in 10 reaction volumes. Thermal cycling and data collection was performed with a CFX384® Real Time System (Bio-Rad®, Hercules, Calif.). Briefly, 1× of the iQ™ SYBR® Green Supermix® was utilized in combination with 200 nM (2 pmol) of either the blocked forward primer (SEQ ID NOs: 217 or 218) and 200 nM (2 pmol) of the unblocked reverse primer (SEQ ID NO: 216), or 200 nM (2 pmol) of unblocked control forward primer (SEQ ID NO: 215) and 200 nM (2 pmol) of unblocked reverse primer (SEQ ID NO: 216). 5 mU of either WT (SEQ ID NO: 197) or G169 mutant RNase H2 enzyme (SEQ ID NO: 214) was added to each reaction. 10 ng of genomic cell line DNA (cell lines NA1852 and NA18537, Coriell Institute for Medical Research, Camden, N.J.), representing the two homozygous genotypes at the rs4939827 SNP, was included in each reaction. Reactions were cycled under the following conditions 95° C.3:00→(95° C.0:10→60° C.0:30)×85. Fluorescence data was collected after each extension time point. After the reaction was completed, data was analyzed, and the average Cq values for each point was calculated. The results are presented in Table 24.
DNA is uppercase, RNA is lowercase. x=C3 spacer (propanediol) blocker group.
These data show that these mutations do not significantly increase mismatch discrimination when the SNP is located immediately opposite the RNA residue in the rhPCR primer.
Example 15This example demonstrates the effect of certain mutations on the activity of P.a. RNase H2 enzyme. In particular, mutation of P.a. RNase H2 at G169A increases mismatch discrimination when the mismatch is placed 5′ of the RNA nucleotide.
To determine whether mismatch discrimination can be increased with the G169A mutant RNase H2 enzyme when the mismatch is located 5′ of the RNA nucleotide, assays targeting rs113488022, the V600E SNP in the human BRAF gene were designed with the mismatch located 5′ of the RNA. The primers utilized in these assays are shown in Table 27, SEQ ID NOs: 204-207. Assays were performed in 10 μL reaction volumes. Thermal cycling and data collection were carried out with a CFX384® Real Time System (Bio-Rad®, Hercules, Calif.). Briefly, 1× of the iQ™ SYBR® Green Supermix® was utilized in combination with 200 nM (2 pmol) of either the blocked forward primer (SEQ ID NOs: 206 or 207) and 200 nM (2 pmol) of the unblocked reverse primer (SEQ ID NO: 205), or 200 nM (2 pmol) of unblocked control forward primer (SEQ ID NO: 204) and 200 nM (2 pmol) of unblocked reverse primer (SEQ ID NO: 205). 25 mU of either WT (SEQ ID NO: 197) or G169 mutant RNase H2 enzyme (SEQ ID NO: 214) was added to each reaction. The 2,000 copies of synthetic double-stranded gBlock® (IDT, Coralville, Iowa) template, representing the two homozygous genotypes at the rs113488022 SNP (SEQ ID NOs: 208 or 209), were also added to each reaction. Reactions were cycled under the following conditions 95° C.3:00→(95° C.0:10→60° C.0:30)×85. Fluorescence intensity was collected after each extension step. The average Cq values were averaged from 3 replicates. The results are shown in Table 33.
These data show that the G169A RNase H2 mutant improved the ΔCq discrimination with the rGT primer from 0.0 cycles (seen for WT) to 10.1 cycles (seen for mutant at 5 mU RNase H2). The G169A RNase H2 also slightly improved mismatch discrimination with the rGA primer, increasing the ΔCq from 14.2 to 16.8 cycles. These improvements turned failed assay into successful one, in particular when the rGT primer was employed. In conclusion, this mutation improves mismatch discrimination when the mismatch is located 3′ of the RNA nucleotide.
The RNase H2 mutations examined in Examples 11-15 enhanced mismatch discrimination more when the mutation was located 5′ or 3′ of the RNA than when located opposite of the RNA nucleotide. Together, these results suggest that limitations of mutation discrimination of rhPCR are based not only on the ability of the RNase H2 to recognize the mismatch and the ability of a polymerase to extend the cleaved primer, but also from the conversion of the mismatched template to the primer sequence. This conversion masks the ability of the RNase H2 enzyme to recognize the mismatch when the mismatch is located at the RNA nucleotide. This could happen in several ways. Most (if not all) conversions spring from a cleavage occurring on the 3′ side of the mismatch. Once this type of cleavage occurs, it can be extended by the DNA polymerase, after which the newly generated template fully matches the primers. The converted template then contaminates the sample because it is expected to be amplified in subsequent cycles.
RNase H2 usually cleaves strand at 5′ side of RNA nucleotide. Experiments (not shown) have indicated that the rare cleavage on the 3′ side of RNA nucleotide occurs at a rate of approximately 1 per 1,000 cleavage reactions. If the mismatch is located at the RNA site, such rare cleavage will cause template conversion. Introducing the mismatch 5′ of the RNA nucleotide makes conversion of the template to the primer sequence occur in the first cycle of replication. In this case, mismatch discrimination depends solely on the ability of the RNase H2 to recognize the mismatch and the ability of polymerase to extend the mismatched primer, since the polymerase is expected to be inhibited by a mismatch at the 3′ end of a primer.
On the other hand, placing the mismatch 3′ of the RNA nucleotide reduces (or completely eliminates) the template conversion. Again, in this position, mismatch discrimination depends solely on the ability of RNase H2 to recognize the mismatch and the ability of the polymerase to extend the mismatched primer. However, the polymerase is not expected to be inhibited in this case since the mismatch site is cleaved out of the primer. The results may also have arisen because the mutations revealed here are more susceptible to mutations present 5′ and 3′ of the RNA than they are opposite the RNA. These hypotheses are not intended to limit the scope of the invention.
Example 16This example demonstrates the effect of certain mutations on the activity of P.a. RNase H2 enzyme. In particular, mutations G12A and P13T increase mismatch discrimination when the mismatch is placed 3′ of the RNA with multiple SNPs.
To determine whether mismatch discrimination can be increased with the G12A and P13T mutant RNase H2 enzymes when the mismatch is located 3′ of the RNA, assays targeting rs12744607, rs60118785, rs60607502, rs7992897, rs3117947, rs7583169, rs4853850, rs28520334, rs13316719, rs61514082, rs12908737, rs6572257, rs2242527, rs1076939, rs35593667, and rs6046375 were designed with the mismatch located 3′ of the RNA. Assays were conducted in 10 μL reaction volumes. Thermal cycling and data collection was performed with a CFX384™ Real-Time System (Bio-Rad®, Hercules, Calif.). Briefly, 1× of the iQ™ SYBR® Green Supermix® was utilized in combination with 200 nM (2 pmol) of either the blocked forward primer and 200 nM (2 pmol) of the unblocked reverse primer. 10 mU of either WT (SEQ ID NO: 197) or 20 mU P13T mutant RNase H2 enzyme (SEQ ID NOs: 198 or 199) or 50 mU G12A mutant RNase H2 enzyme (SEQ ID NO: 199) was added to each reaction. The 10 ng of NA12878 or NA24385 genomic cell line DNA, representing the two homozygous genotypes at each of these SNPs, were also added to each reaction. Reactions were cycled under the following conditions 95° C.3:00→(95° C.0:10→60° C.0:30)×85 cycles. Fluorescence intensity was collected after each extension step. The average Cq values were averaged from 3 replicates. The results are shown in Table 34.
These data show that the P13T RNase H2 mutant improved the ΔCq discrimination at multiple SNPs. An excellent example is rs61514082, where the ΔCq increased from 2.5 cycles (seen for WT) to 26.7 cycles (seen for mutant at 20 mU RNase H2). The P13T RNase H2 also improved mismatch discrimination at the rs60607502 SNP, increasing the ΔCq from 0.0 to 8.9 cycles. Other SNPs also improved their specificity. The two SNPs that did not improve ΔCq (rs13316719 and rs28520334) show that this mutation can be further improved, and does not solve all issues with all assays, despite a wide-ranging improvement being observed. These improvements turned many failed assays into successful ones, showing the broad improvements in ΔCq that this mutation confers.
The G12A RNase H2 mutant also improved the ΔCq of the mismatch discrimination at multiple SNPs. The rs61514082 SNP increased in ΔCq from 2.5 cycles to 10.5 cycles. The G12A RNase H2 also improved mismatch discrimination with the rs60607502 SNP, increasing the ΔCq from 0.0 to 14.1 cycles. In conclusion, both mutations improve mismatch discrimination when the mismatch is located 3′ of the RNA nucleotide.
All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.
The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.
Claims
1. A blocked-cleavable primer for rhPCR, the primer comprising: wherein
- 5′-A-B-Z-E-3′
- A is optional and is a tail extension that is not complementary to a target;
- B is a sequence domain that is complementary to a target;
- Z comprises: C, a discrimination domain, and D, a cleavage domain that, when hybridized to the target, is cleavable by RNase H2; and
- E is a blocking domain that prevents extension of the primer.
2. The primer of claim 1, wherein the discrimination domain C is located 5′ of the cleavage domain D.
3. The primer of claim 1, wherein the discrimination domain C is located 3′ of the cleavage domain D.
4. The primer of claim 1, wherein the discrimination domain C overlaps with the cleavage domain D.
5. The primer of claim 1, wherein C comprises 1-3 RNA bases.
6. The primer of claim 1, wherein C comprises 1 RNA base.
7. The primer of claim 6, wherein the discrimination domain C consists of the 1 RNA base.
8. The primer of claim 1, wherein the cleavage domain comprises one or more of the following moieties: a DNA residue, an abasic residue, a modified nucleoside, or a modified phosphate internucleotide linkage.
9. The primer of claim 8, wherein a sequence flanking the cleavage site contains one or more internucleoside linkages resistant to nuclease cleavage.
10. The primer of claim 9, wherein the nuclease resistant linkage is a phosphorothioate.
11. The primer of claim 5, wherein the 3′ oxygen atom of at least one of the RNA residues is substituted with an amino group, thiol group, or a methylene group.
12. The primer of claim 1, wherein the blocking group is attached to the 3′-terminal nucleotide of the primer.
13. The primer of claim 1, wherein A comprises a region that is identical to a universal forward primer and optionally a probe binding domain.
14. A method of detecting a variation in a target DNA sequence, the method comprising:
- (a) providing a reaction mixture comprising (i) an oligonucleotide primer having a cleavage domain positioned 5′ of a blocking group, the blocking group linked at or near the end of the 3′-end of the oligonucleotide primer wherein the blocking group prevents primer extension and/or inhibits the primer from serving as a template for DNA synthesis, (ii) a sample nucleic acid that may or may not have the target sequence, and where the target sequence may or may not have the variation (iii) a cleaving enzyme and (iv) a polymerase;
- (b) hybridizing the primer to the target DNA sequence to form a double-stranded substrate;
- (c) cleaving the hybridized primer, if the primer is complementary at the variation, with the cleaving enzyme at a point within or adjacent to the cleavage domain to remove the blocking group from the primer; and
- (d) extending the primer with the polymerase.
15. The method of claim 14, wherein the cleaving enzyme is a hot start cleaving enzyme comprising at least one amino acid substitution as compared to its wild type, which is thermostable and has reduced activity at lower temperatures, and which exhibits enhanced mismatch discrimination during rhPCR as compared to its wild type.
16. The method of claim 15, wherein the hot start cleaving enzyme is Pyrococcus abyssi RNase H2 comprising (a) a G12A amino acid substitution; (b) a P13T amino acid substitution; (c) a G169A amino acid substitution; or (d) a combination thereof.
17. The method of claim 15, wherein the cleaving enzyme is chemically modified.
18. The method of claim 15, wherein the cleaving enzyme is reversibly inactivated through interaction with an antibody at lower temperatures.
19. The method of claim 14, wherein the cleavage domain comprises at least one RNA base.
20. The method of claim 19, wherein the cleavage domain is located 3′ of the position of variation.
21. The method of claim 19, wherein the cleavage domain is located 5′ of the position of variation.
22. The method of claim 19, wherein the cleavage domain overlaps with the position of variation.
23. The method of claim 14, wherein the cleavage domain comprises one or more 2′-modified nucleosides, and the cleaving enzyme cleaves between the position complementary to the variation and the one or more modified nucleosides.
24. The method of claim 23, wherein the one or more modified nucleosides are 2′-fluoronucleosides.
25. The method of claim 14, wherein the polymerase is a high-discrimination polymerase.
26. The method of claim 14, wherein the polymerase is a mutant H784Q Taq polymerase.
27. The method of claim 26, wherein the mutant H784Q Taq polymerase is reversibly inactivated via chemical, aptamer or antibody modification.
28. The method of claim 14, wherein the primer contains a 5′ tail sequence that comprises a universal primer sequence and optionally a universal probe sequence, wherein the tail is non-complementary to the target DNA sequence.
29. The method of claim 14, wherein the target DNA sequence is a sample that has been treated with a gene editing enzyme.
30. The method of claim 14, wherein the target DNA sequence is a sample that has been treated with a CRISPR enzyme.
31. The method of claim 14, wherein the target DNA sequence is a sample that has been treated with a Cas9 or Cpf1 enzyme.
32. A method of target enrichment comprising:
- (a) providing a reaction mixture comprising (i) a first oligonucleotide primer having a tail domain that is not complementary to a target sequence, the tail domain comprising a first universal primer sequence; a cleavage domain positioned 5′ of a blocking group, the blocking group linked at or near the end of the 3′-end of the first oligonucleotide primer wherein the blocking group prevents primer extension and/or inhibits the first primer from serving as a template for DNA synthesis, (ii) a sample nucleic acid that may or may not have the target sequence, (iii) a cleaving enzyme and (iv) a polymerase;
- (b) hybridizing the first primer to the target DNA sequence to form a double-stranded substrate;
- (c) cleaving the hybridized first primer, if the first primer is complementary to the target, with the cleaving enzyme at a point within or adjacent to the cleavage domain to remove the blocking group from the first primer; and
- (d) extending the first primer with the polymerase.
33. The method of claim 32, the method further comprising a second primer in reverse orientation to support priming and extension of the first primer extension product.
34. The method of claim 33, wherein the second primer further comprises a tail domain comprising a second universal primer sequence.
35. The method of claim 34, wherein steps (b)-(d) are performed 1-10 times.
36. The method of claim 35, further comprising removing unextended primers from the reaction and hybridizing universal primers to the extension product to form a second extension product.
37. The method of claim 36, wherein the universal primers further comprise tailed sequences for addition of adapter sequences to the second extension product.
38. The method of claim 37, wherein sequencing is performed on the second extension product to determine the sequence of the target.
39. The method of claim 32, wherein the target DNA sequence is a sample that has been treated with a gene editing enzyme.
40. The method of claim 32, wherein the target DNA sequence is a sample that has been treated with a CRISPR enzyme.
41. The method of claim 32, wherein the target DNA sequence is a sample that has been treated with a Cas9 or Cpf1 enzyme.
42. The method of claim 32, wherein the cleaving enzyme is a hot start cleaving enzyme comprising at least one amino acid substitution as compared to its wild type, which is thermostable and has reduced activity at lower temperatures, and which exhibits enhanced mismatch discrimination during rhPCR as compared to its wild type.
43. The method of claim 42, wherein the hot start cleaving enzyme is Pyrococcus abyssi RNase H2 comprising (a) a G12A amino acid substitution; (b) a P13T amino acid substitution; (c) a G169A amino acid substitution; or (d) a combination thereof.
44. The method of claim 42, wherein the cleaving enzyme is chemically modified.
45. The method of claim 32, wherein the cleaving enzyme is a hot start cleaving enzyme that is reversibly inactivated through interaction with an antibody at lower temperatures.
46. The method of claim 32, wherein the cleaving domain comprises at least one RNA base.
47. The method of claim 46, wherein the cleavage domain is located 3′ of the position of variation.
48. The method of claim 46, wherein the cleavage domain is located 5′ of the position of variation.
49. The method of claim 46, wherein the cleavage domain overlaps with the position of variation.
50. The method of claim 32, wherein the cleaving domain comprises one or more 2′-modified nucleosides, and the cleaving enzyme cleaves between the position complementary to the variation and the one or more modified nucleosides.
51. The method of claim 50, wherein the one or more modified nucleosides are 2′-fluoronucleosides.
52. The method of claim 32, wherein the polymerase is a high-discrimination polymerase.
53. The method of claim 32, wherein the polymerase is a mutant H784Q Taq polymerase.
54. The method of claim 53, wherein the mutant H784Q Taq polymerase is reversibly inactivated via chemical, aptamer or antibody modification.
Type: Application
Filed: May 24, 2017
Publication Date: Sep 14, 2017
Inventors: Joseph Dobosy (Coralville, IA), Richard Owczarzy (Coralville, IA), Mark Aaron Behlke (Coralville, IA)
Application Number: 15/604,204
