METHODS AND COMPOSITIONS FOR MULTIPLE DISPLACEMENT AMPLIFICATION OF NUCLEIC ACIDS
Disclosed are methods for multiple displacement amplification of a nucleic acid sequence in a sample. The nucleic acid is contacted with a reaction mixture that includes a set of oligonucleotide primers and a plurality of polymerase enzymes. The reaction mixture is subjected to conditions under which the nucleic acid sequence is amplified to produce an amplification product in a multiple displacement amplification reaction. Also disclosed are kits containing a set of oligonucleotide primers with random sequences having lengths of 6 to 8 nucleobases. At least some of the individual members of the primers have one or more ribose modifications that stabilize or lock the ribose ring in a 3′-endo conformation. At least some of the primers have one or more universal nucleobases.
This invention was made with United States Government support under FBI contract J-FBI-05-027, FBI contract J-FBI-03-134, and HSARPA contract W81XWH-05-C-0116. The United States Government has certain rights in the invention.
SUPPORT FIELD OF THE INVENTIONThe methods disclosed herein relate to methods and compositions for amplifying nucleic acid sequences.
BACKGROUND OF THE INVENTIONIn many fields of research such as genetic diagnosis or forensic investigations, the scarcity of genomic DNA can be a severely limiting factor on the type and quantity of genetic tests that can be performed on a sample. One approach designed to overcome this problem is whole genome amplification. The objective is to amplify a limited DNA sample in a non-specific manner in order to generate a new sample that is indistinguishable from the original but with a higher DNA concentration. The aim of a typical whole genome amplification technique would be to amplify a sample up to a microgram level while respecting the original sequence representation.
The first whole genome amplification methods were described in 1992, and were based on the principles of the polymerase chain reaction (PCR). Zhang and coworkers (Zhang et al. Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 5847-5851) developed the primer extension PCR technique (PEP) and Telenius and collaborators (Telenius et al., Genomics 1992, 13, 718-725) designed the degenerate oligonucleotide-primed PCR method (DOP-PCR).
PEP involves a high number of PCR cycles; using Taq polymerase and 15 base random primers that anneal at a low stringency temperature. Although the PEP protocol has been improved in different ways, it still results in incomplete genome coverage, failing to amplify certain sequences such as repeats. Failure to prime and amplify regions containing repeats may lead to incomplete representation of a whole genome because consistent primer coverage across the length of the genome provides for optimal representation of the genome. This method also has limited efficiency on very small samples (such as single cells). Moreover, the use of Taq polymerase implies that the maximal product length is about 3 kb.
DOP-PCR is a method which uses Taq polymerase and semi-degenerate oligonucleotides that bind at a low annealing temperature at approximately one million sites within the human genome. The first cycles are followed by a large number of cycles with a higher annealing temperature, allowing only for the amplification of the fragments that were tagged in the first step. This leads to incomplete representation of a whole genome. DOP-PCR generates, like PEP, fragments that are in average 400-500 bp, with a maximum size of 3 kb, although fragments up to 10 kb have been reported. On the other hand, as noted for PEP, a low input of genomic DNA (less than 1 ng) decreases the fidelity and the genome coverage (Kittler et al. Anal. Biochem. 2002, 300, 237-244).
Multiple displacement amplification (MDA, also known as strand displacement amplification; SDA) is a non-PCR-based isothermal method based on the annealing of random hexamers to denatured DNA, followed by strand-displacement synthesis at constant temperature (Blanco et al. J. Biol. Chem. 1989, 264, 8935-8940). It has been applied to small genomic DNA samples, leading to the synthesis of high molecular weight DNA with limited sequence representation bias (Lizardi et al. Nature Genetics 1998, 19, 225-232; Dean et al., Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5261-5266). As DNA is synthesized by strand displacement, a gradually increasing number of priming events occur, forming a network of hyper-branched DNA structures. The reaction can be catalyzed by the Phi29 DNA polymerase or by the large fragment of the Bst DNA polymerase. The Phi29 DNA polymerase possesses a proofreading activity resulting in error rates 100 times lower than the Taq polymerase (Lasken et al. Trends Biotech. 2003, 21, 531-535).
The methods described above generally do not successfully amplify DNA samples when the quantity of template DNA being amplified is below the level of one 1 nanogram (ng). Problems encountered during such amplification attempts include, for example, poor representation of the original template DNA in the amplified product (Dean et al. Proc. Natl. Acad. Sci U.S.A. 2002, 99, 5261-5266) and competing amplification of non-template DNA (Lage et al. Genome Research 2003, 13, 294-307).
There remains a long felt need for methods and kits for performing whole genome amplification reactions on small quantities of DNA. The present invention satisfies this need.
SUMMARY OF THE INVENTIONDisclosed are methods for multiple displacement amplification of a nucleic acid sequence in a sample. The nucleic acid is contacted with a reaction mixture that includes a set of oligonucleotide primers and a plurality of polymerase enzymes. The reaction mixture is subjected to conditions under which the nucleic acid sequence is amplified to produce an amplification product in a multiple displacement amplification reaction.
Also disclosed are reaction mixtures containing a plurality of polymerase enzymes, a set of natural deoxynucleotide triphosphates and a plurality of compatible solutes.
Also disclosed are kits containing a set of oligonucleotide primers with random sequences having lengths of 6 to 8 nucleobases. At least some of the individual members of the primers have one or more ribose modifications that stabilize or lock the ribose ring in a 3′-endo conformation. At least some of the primers have one or more universal nucleobases.
To facilitate an understanding of the methods disclosed herein, a number of terms and phrases are defined below:
The term “allele” as used herein, is any one of a number of viable DNA codings occupying a given locus (position) on a chromosome. Usually alleles are DNA (deoxyribonucleic acid) sequences that code for a gene, but sometimes the term is used to refer to a non-gene sequence. An individual's genotype for that gene is the set of alleles it happens to possess. In a diploid organism, one that has two copies of each chromosome, two alleles make up the individual's genotype.
The term “allelic balance” as used herein, refers to the ratio of the quantity of the minor allele to the quantity of the major allele.
The term “allele call” as used herein, refers to successful characterization of an allele by a given analysis method. If the analysis provides successful characterization of both alleles of a gene locus of a DNA sample, it is said that two allele calls are made. If one allele is characterized while the other allele is not characterized, it is said that one allele call is made. If neither of the two alleles is successfully characterized, no allele calls are made.
The term “amplification,” as used herein, refers to a process of multiplying an original quantity of a nucleic acid template in order to obtain greater quantities of the original nucleic acid.
The term “compatible solute” as used herein, refers to a class of compounds that stabilize cells and cellular components. Compatible solutes include, for example, amino acids and their derivatives, and carbohydrates.
The term “genome,” as used herein, generally refers to the complete set of genetic information in the form of one or more nucleic acid sequences, including text or in silico versions thereof. A genome may include either DNA or RNA, depending upon its organism of origin. Most organisms have DNA genomes while some viruses have RNA genomes. As used herein, the term “genome” need not comprise the complete set of genetic information.
The term “hexamer” as used herein refers to a polymer composed of six units. More specifically, the term hexamer is used to describe an oligonucleotide primer having six nucleotide residues.
The term “heptamer” as used herein refers to a polymer composed of seven units. More specifically, the term heptamer is used to describe an oligonucleotide primer having seven nucleotide residues.
The term “hybridization,” as used herein refers to the process of joining two complementary strands of DNA or one each of DNA and RNA to form a double-stranded molecule through Watson and Crick base-pairing or pairing of a universal nucleobase with one of the four natural nucleobases of DNA (adenine, guanine, thymine and cytosine).
The term “locked nucleic acid” (LNA), refers to a modified RNA nucleotide. The ribose moiety of a locked nucleotide is modified with an extra bridge connecting 2′ and 4′ carbons. The ribose structure with this bridge is a bicyclic structure. The bridge “locks” the ribose in a 3′-endo structural conformation, which is often found in the A-form of DNA or RNA. LNA nucleotides can be mixed with DNA or RNA bases in an oligonucleotide. The locked ribose conformation enhances base stacking and backbone pre-organization and has the effect of significantly increasing the thermal stability (melting temperature) of a DNA duplex. Two examples of LNAs are the classic LNA which has a single carbon (methylene) bridge between the ribose 2′ and 4′ carbons and another type of LNA known as ENA, which has an ethylene bridge between the ribose 2′ and 4′ carbons. An individual LNA nucleotide is considered to be an example of a modified nucleotide which can be incorporated into DNA and oligonucleotide primers.
The term “multiple displacement amplification” as used herein, refers to a non-PCR-based isothermal method based on the annealing of random hexamers to denatured DNA, followed by strand-displacement synthesis at constant temperature. It has been applied to small genomic DNA samples, leading to the synthesis of high molecular weight DNA with limited sequence representation bias. As DNA is synthesized by strand displacement, a gradually increasing number of priming events occur, forming a network of hyper-branched DNA structures. The reaction can be catalyzed by the Phi29 DNA polymerase or by the large fragment of the Bst DNA polymerase.
The term “nucleic acid” as used herein, refers to a high-molecular-weight biochemical macromolecule composed of nucleotide chains that convey genetic information. The most common nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The monomers from which nucleic acids are constructed are called nucleotides. Each nucleotide consists of three components: a nitrogenous heterocyclic base, either a purine or a pyrimidine (also known as a nucleobase); and a pentose sugar. Different nucleic acid types differ in the structure of the sugar in their nucleotides; DNA contains 2-deoxyribose while RNA contains ribose.
The term “nucleobase” as used herein, refers to a nitrogenous heterocyclic base, either a purine or a pyrimidine in a nucleotide residue or within a nucleic acid. The term nucleobase is used herein to describe the length of a given oligonucleotide primer according to the number of nucleotide residues included in the oligonucleotide primer.
The term “octamer” as used herein refers to a polymer composed of eight units. More specifically, the term octamer is used herein to describe a primer having eight nucleotide residues.
The term “polymerase” as used herein, refers to an enzyme that catalyzes the process of replication of nucleic acids. More specifically, DNA polymerase catalyzes the polymerization of deoxyribonucleotides alongside a DNA strand, which the DNA polymerase “reads” and uses as a template. The newly-polymerized molecule is complementary to the template strand and identical to the template's partner strand.
The term “primer,” as used herein refers to an isolated oligonucleotide which is capable of acting as a point of initiation of synthesis when placed under conditions in which synthesis of a primer extension product which is complementary to a nucleic acid strand is induced, (i.e., in the presence of nucleotides and an inducing agent such as DNA polymerase and at a suitable temperature and pH). The primer is preferably single stranded for maximum efficiency in amplification, but may alternatively be double stranded. If double stranded, the primer is first treated to separate its strands before being used to prepare extension products. Preferably, the primer is an oligodeoxyribonucleotide. The primer must be sufficiently long to prime the synthesis of extension products in the presence of the inducing agent. The exact lengths of the primers will depend on many factors, including temperature, source of primer, use of the method, and the parameters used for primer design, as disclosed herein.
The term “processivity,” as used herein, refers to the ability of an enzyme to repetitively continue its catalytic function without dissociating from its substrate. For example, Phi29 polymerase is a highly processive polymerase due to its tight binding of the template DNA substrate.
The term “Profiler” as used herein, is a generic term that refers to an assay used to characterize a group of genetic loci. Any group of genetic loci may be analyzed. One of the more common groups of loci generally includes a core group of about 13 STR marker loci known as the CODIS group (Combined DNA Index system). In a given Profiler assay, the STRs are characterized by a capillary electrophoresis method that detects fluorescently tagged STR amplification products. An example of a specific profiler assay is the AmpFlSTR® Profiler Plus™ assay (Applied Biosystems, Foster City, Calif.).
The term “quality of amplification” refers collectively to the yield (or fold amplification) of amplified nucleic acid, the specificity of amplification with respect to non-template nucleic acids, and the performance of the resulting amplified products in the Profiler™ assay.
The term “reaction mixture” as used herein refers to a mixture containing sufficient components to carry out an amplification reaction.
The term “representation” as used herein, refers to a measure of retaining the original characteristics of the template DNA being amplified in the multiple displacement amplification reaction. For example, if the template DNA is from an individual who has an allelic balance of 1.5 to 1.0 at a particular genetic locus, and the amplified DNA indicates that the allelic balance is 2.0 to 1.0 at that same locus, one would conclude that the amplification reaction resulted in poor representation relative to a different amplification reaction producing an allelic balance of 1.6 to 1.0 for the same template DNA sample.
The term “sensitivity” as used herein refers to a measure of the ability of a given reaction mixture to amplify very low quantities of DNA such as, for example, quantities in the picogram range. For example, a given reaction mixture that produces a useful quantity of a amplified DNA in an amplification reaction starting from a given quantity of template DNA is more sensitive than another given reaction mixture which cannot produce a useful quantity of DNA from the same quantity of DNA.
The term “set of oligonucleotide primers” as used herein, refers to a plurality of oligonucleotide primers whose members are not matched up in forward and reverse pairs as they generally are for a typical polymerase chain reaction. As used herein a “set of oligonucleotide primers” is a plurality of oligonucleotide primers having generally random sequences and ranging in length from about six to about eight nucleobases.
The term “short tandem repeat” or “STR” as used herein refers to a class of polymorphisms that occurs when a pattern of two or more nucleotides are repeated and the repeated sequences are directly adjacent to each other. The pattern can range in length from 2 to 10 base pairs (bp) (for example (CATG)n in a genomic region) and is typically in the non-coding intron region. By examining several STR loci and counting how many repeats of a specific STR sequence there are at a given locus, it is possible to create a unique genetic profile of an individual.
The term “template nucleic acid” or “template DNA” as used herein, refers to the strand or strands of DNA that are replicated in an amplification reaction catalyzed by a polymerase enzyme. More specifically, a template nucleic acid represents the target nucleic acid added to the amplification reaction. The target nucleic acid refers to the object of analysis. A sample containing the template nucleic acid may contain other nucleic acid contaminants which would not be equally considered as template nucleic acids. For example, if the objective of analysis is to identify an individual from his or her DNA, a sample containing this DNA would be obtained and amplified. This DNA is the target of analysis and represents the template. Contaminating nucleic acids may be present but are not considered as template DNA because they are not the object of analysis.
The term “universal nucleobase” as used herein, refers to a nucleobase that is capable of forming a base pair with any of the four natural nucleobases of DNA; adenine, guanine, thymine or cytosine.
The term “whole genome amplification” or “WGA” as used herein generally refers to a method for amplification of a limited DNA sample in a non-specific manner, in order to generate a new sample that is indistinguishable from the original but with a higher DNA concentration. The ideal whole genome amplification technique would amplify a sample up to a microgram level while maintaining the original sequence representation. The DNA of the sample may include an entire genome or a portion thereof. Degenerate oligonucleotide-primed PCR (DOP), primer extension PCR technique (PEP) and multiple displacement amplification (MDA), are examples of whole genome amplification methods.
Description of Embodiments OverviewDisclosed herein are methods, reaction mixtures, kits and primer compositions for multiple displacement amplification reactions appropriate for amplifying small quantities of DNA. The amplified DNA is particularly useful for carrying out human forensics testing and is also useful for clinical testing or for identification of pathogens in environmental samples.
Methods and Reaction MixturesThe methods for multiple displacement amplification include the steps of preparing a reaction mixture that enhances the sensitivity, allelic balance and quality of the template DNA being amplified.
Sensitivity is a measure of the ability of the reaction mixture to amplify the smallest quantities of DNA.
Allelic balance refers to the ratio of the quantities of two forms of a given allele. If representation of the original DNA is maintained in a given multiple displacement amplification reaction, the allelic balance should be maintained. Poor representation of the original template DNA in the amplified product is a common problem associated with whole genome amplification methods (Dean et al. Proc. Natl. Acad. Sci U.S.A. 2002, 99, 5261-5266).
Quality is a general measure of the extent of amplification obtained and the percentage of the amplification products from the template (or “target”) DNA. A further measure of quality is the performance of the amplification product in a DNA profiler™ assay where specific human DNA markers are measured for the purpose of identifying human individuals.
In some embodiments, the reaction mixtures employed in the multiple displacement amplification reactions are compounds known as compatible solutes. These compounds stabilize cells and cellular components when exposed to extreme conditions. In bacteria, the uptake or synthesis of compatible solutes renders the cells and their enzymatic machinery more resistant to stress-inducing environmental conditions such as high osmolarity or high temperatures. These protective effects can be extended to amplification reactions by inclusion as components of an amplification reaction mixture. The compatible solute betaine (N,N,N-trimethylglycine), is an amino acid that acts as an osmoprotectant which increases the resistance of polymerase enzymes to denaturation and also allows amplification reactions to overcome low levels of contaminants that often result in low-quality amplification reactions (Weissensteiner et al. Biotechniques 1996, 21, 1102-1108). The compatible solute trehalose is a non-reducing disaccharide in which two D-glucose units are linked by an alpha-alpha-1,1-glycosidic bond. Trehalose been identified as an enhancer of the polymerase chain reaction (PCR) acting to lower the melting temperature of DNA and increasing the thermal stability of Taq polymerase (Speiss et al. Clin. Chem. 2004, 50, 1256-1259).
In some embodiments, the compatible solutes are betaine, trehalose, or a combination thereof. In some embodiments, the concentrations of betaine included in the reaction mixtures is in a range between about 0.25 M to about 1.5 M or any fractional concentration therebetween. In some embodiments, the concentration of betaine is between about 0.75 M to about 1.0 M and preferably about 0.8 M. In some embodiments, the concentration of trehalose included in the reaction mixtures is in a range between about 0.2 M to about 1.0 M or any fractional concentration therebetween. In some embodiments, the concentration of betaine is between about 0.6 M to about 1.0 M and preferably about 0.8 M.
In some embodiments, the reaction mixtures employed for the multiple displacement amplification include a plurality of polymerase enzymes. In some embodiments, the catalytic activities include 5′→3′ DNA polymerase activity, 3′→5′ exonuclease proofreading activity, and DNA repair activities such as, for example, 5′→3′ excision repair activity. Examples of various polymerase enzymes include, but are not limited to, the following: Phi29, Klenow fragment, T4 polymerase, T7 polymerase, BstE polymerase, E. coli Pol I, Vent, Deep Vent, Vent exo-, Deep Vent exo-, KOD HiFi, Pfu ultra, Pfu turbo, Pfu native, Pfu exo-, Pfu exo-Cx, Pfu cloned, Proofstart (Qiagen), rTth, Tgo and Tfu Qbio. These polymerases are known and most are commercially available. In a preferred embodiment, said plurality of polymerases is Phi29 and at least one more polymerase enzyme. More preferably, said plurality of polymerases is Phi29 and at least one Pol I polymerase. Most preferably, said plurality of polymerases is Phi29 and E. coli Pol I.
In embodiments wherein said plurality of polymerases in the reaction mixture is Phi29 polymerase, and E coli Polymerase I (also known as Pol I), the enzymes include 5′→3′ DNA polymerase activity, 3′→5′ exonuclease proofreading activity, and 5′→3′ excision repair activity. In some embodiments, Phi29 is the major polymerase while E. coli DNA Polymerase I is present at lower activity levels. In some embodiments, about 10 units of Phi29 polymerase is present in the reaction mixture while about 2.0 units of E. coli DNA Polymerase I is present in the reaction mixture.
In other embodiments, other non-polymerase enzymes or accessory proteins are included in the reaction mixtures such as, for example, helicase, gyrase, T4G32 and SSBP for example. These accessory proteins are known and most are commercially available.
In some embodiments, the reaction mixture further includes pyrophosphatase which serves to convert pyrophosphate to phosphate. Pyrophosphate accumulates in the reaction mixture as a result of the amplification reaction (one equivalent of pyrophosphate is generated from each incorporated deoxynucleotide triphosphate added and is known to inhibit the amplification reaction). In some embodiments about 0.004 units of pyrophosphate is added to the reaction mixture.
In some embodiments, it is preferable that bovine serum albumin (BSA) is not included in the reaction mixture because commercially obtained lots of BSA often are contaminated with bovine DNA which represents a significant contaminant that may be co-amplified with the template DNA in the amplification reaction.
In some embodiments, the amplification reaction is an isothermal amplification reaction, meaning that it is carried out at a constant temperature. In some embodiments, the reaction conditions include thermal cycling where the temperature of the reaction mixture is successively raised and lowered to pre-determined temperatures in order to melt and anneal the two strands of DNA. In some embodiments, it may be appropriate to perform an isothermal amplification if, for example, representation is maintained by amplification with Phi29 polymerase. In other embodiments, a greater contribution of enzymatic activity originating from different polymerase (such as Pol I, for example) may be advantageous, in which case thermal cycling may be included in the reaction conditions.
In some embodiments, the amplification reaction results in the amplification of template DNA of a whole genome, or a substantial portion thereof.
In some embodiments, the multiple displacement amplification reaction produces an amplification product from a template DNA at a constant ratio relative to production of amplification products of other extraneous DNAs in a given sample. This preserves the representation of the original template DNA.
In some embodiments, the total quantity of the template DNA added to the reaction mixture for amplification is at least about 2 picograms.
Primer SetsThe primer sets used in the amplification reactions disclosed herein are generally defined as a plurality of oligonucleotide primers. In some embodiments, the primers have random sequences that hybridize randomly to the template nucleic acid at positions of the template that substantially base-pair with the primers. The members of the primer sets may be random hexamers (primers with six nucleotide residues), random heptamers (primers with seven nucleotide residues), or random octomers (primers with eight nucleotide residues). The syntheses of such hexamers, heptamers and octomers with random sequences are accomplished by known procedures.
In some embodiments, the primers include modifications that increase their affinity for the template nucleic acid. In certain embodiments, the modifications include substituents on the ribose ring of a given nucleotide residue of a given primer, which stabilize or lock the ribose ring in the 3′-endo conformation which provides for a higher affinity of the nucleotide residue for a pairing nucleotide residue on a template nucleic acid.
The conformation of the ribose sugar of a nucleotide residue within a primer is influenced by various factors including substitution at the 2′-, 3′- or 4′-positions of the pentofuranosyl sugar. Electronegative substituents generally prefer the axial positions, while sterically demanding substituents generally prefer the equatorial positions (Principles of Nucleic Acid Structure, Wolfgang Sanger, 1984, Springer-Verlag.) Modification of the 2′ position to favor the 3′-endo conformation can be achieved while maintaining the 2′-OH as a recognition element (Gallo et al. Tetrahedron 2001, 57, 5707-5713; Harry-O'kuru et al., J. Org. Chem., 1997, 62), 1754-1759; and Tang et al., J. Org. Chem. 1999, 64, 747-754). Alternatively, preference for the 3′-endo conformation can be achieved by deletion of the 2′-OH as exemplified by 2′-deoxy-2′-F-nucleosides (Kawasaki et al., J. Med. Chem. 1993, 36, 831-841), which adopts the 3′-endo conformation positioning the electronegative fluorine atom in the axial position. Other modifications of the ribose ring, for example substitution at the 4′-position to give 4′-F modified nucleosides (Guillerm et al., Bioorg. Medicinal Chem. Lett. 1995, 5, 1455-1460 and Owen et al., J. Org. Chem. 1976, 41, 3010-3017), or for example modification to yield methanocarba nucleoside analogs (Jacobson et al., J. Med. Chem. Lett. 2000, 43, 2196-2203 and Lee et al., Bioorg. Med. Chem. Lett. 2001, 11, 1333-1337) also induce preference for the 3′-endo conformation.
The most common locked nucleic acid modification is a 2′ to 4′ methylene bridge which locks the ribose ring in the 3′-endo conformation. This modification is often abbreviated as “LNA,” meaning “locked nucleic acid. Another type of locked nucleic acid is referred to as ENA (ethylene-bridged nucleic acid). This modification includes a 2′ to 4′ ethylene bridge. The synthesis and preparation of the 2′ to 4′ bridged monomers adenine, cytosine, guanine, 5-methyl-cytosine, thymine and uracil, along with their oligomerization, and nucleic acid recognition properties have been described (Koshkin et al., Tetrahedron, 1998, 54, 3607-3630). The 2′ to 4′ bridged monomers and preparation thereof are also described in WO 98/39352 and WO 99/14226. The first analogs of 2′ to 4′ bridged nucleic acids, phosphorothioate-LNA and 2′-thio-LNAs, have also been prepared (Kumar et al., Bioorg. Med. Chem. Lett., 1998, 8, 2219-2222). Preparation of oligodeoxyribonucleotide duplexes with 2′ to 4′ bridged nucleoside analogs as substrates for nucleic acid polymerases has also been described (WO 99/14226). Furthermore, the synthesis of 2′-amino-LNA, a novel conformationally restricted high-affinity oligonucleotide analog with a handle has been described in the art (Singh et al., J. Org. Chem. 1998, 63, 10035-10039). In addition, 2′-amino- and 2′-methylamino-LNAs have been prepared and the thermal stability of their duplexes with complementary RNA and DNA strands has been previously reported.
In some embodiments, at least some of the primers contain at least one 2′ to 4′ bridged nucleotide residues or at least two 2′ to 4′ bridged nucleotide residues. In other embodiments, 2′ to 4′ bridged nucleotide residues are located at the 2′ and 5th positions of the oligonucleotide primers. These embodiments of the primer sets are hexamers, heptamers or octamers or any combination thereof.
In some embodiments, the primers have random sequences with the exception of having specifically located universal nucleobases such as inosine for example. The specific locations of the inosine nucleobases are preferably at the two final terminal nucleobases of a given inosine-containing primer.
In some embodiments, one or more phosphorothioate linkages are incorporated into the primers at the 3′ end of a given primer for the purpose of making the primer more resistant to nuclease activity.
Primer KitsSome embodiments also provide kits comprising the primers disclosed herein.
In some embodiments, the kits comprise a sufficient quantity of a polymerase enzyme having high processivity. In some embodiments, the high processivity polymerase is Phi29 polymerase or Taq polymerase. In other embodiments, the high processivity polymerase is a genetically engineered polymerase whose processivity is increased relative to the native polymerase from which it was constructed.
In some embodiments, the kits comprise a sufficient quantity of an additional polymerase in addition to a high processivity polymerase, for improvement of the characteristics of the amplification reaction. In some embodiments, the additional polymerase is E. coli Pol I polymerase.
In some embodiments, the kits comprise a sufficient quantity of pyrophosphatase which catalyzes conversion of pyrophosphate to two equivalents of phosphate. Pyrophosphate is known to inhibit polymerase reactions.
In some embodiments, the kits further comprise deoxynucleotide triphosphates, buffers, and buffer additives such as compatible solutes including trehalose and betaine at concentrations optimized for multiple displacement amplification.
In some embodiments, the kits further comprise instructions for carrying out targeted whole genome amplification reactions.
EXAMPLES Example 1 Effects of Compatible Solutes on Sensitivity of Amplification ReactionThe purpose of this series of experiments was to investigate the effects of the compatible solutes trehalose and betaine on enhancement of the sensitivity of the amplification reaction where sensitivity reflects the ability to successfully amplify the target DNA at low concentrations below the 1 nanogram level.
Initial investigations indicated that the concentration of trehalose that provides an optimal increase in sensitivity of the amplification reaction was approximately 0.8 M, as indicated in an amplification reaction of 50 picograms of human DNA, followed by a calculation of the percentage of alleles detected in analysis of the amplified DNA obtained in the reaction. Likewise, the optimal concentration of betaine was found to be approximately 0.8 M.
In order to investigate the effects of compatible solutes on the sensitivity of the amplification reaction, amplification reaction mixtures were developed as indicated in Table 1.
1) 30° C. for 4 minutes;
2) 15° C. for 15 seconds;
3) repeat steps 1 and 2 for a total of 150 times;
4) 90° C. for 3 minutes; and
5) hold at 4° C.
The resulting amplified DNA was analyzed for identification of alleles using a mass spectrometry-based analysis method wherein specific primer pairs are then used to obtain additional specific amplification products via PCR of loci. These specific amplification products have lengths up to about 140 nucleobases which are appropriate for base composition analysis by mass spectrometry in a manner similar to that disclosed in Jiang et al. Clin Chem. 2007, 53, 195-203.
In another experiment, the resulting amplified DNA was analyzed using the Profiler™ fluorescence procedure, for determination of a series of short tandem repeat (STR) alleles known as the CODIS (Combined DNA Index System) group. Specific primer pairs were then used to obtain additional specific amplification products via PCR of the loci of interest. These amplification products were then subjected to the Profiler™ fluorescence procedure. In Tables 2 and 3, ten of the thirteen CODIS core STR (short tandem repeat) loci are included (abbreviations for the loci column labels are as follows: D3S=D3S1358; D8S=D8S1179; D21S=D21S11; D185=D18551; D5S=D5S8181; D135=D135317; and D7S=D7S820, while vWA indicates the von Willebrand factor gene; TPDX indicates the thyroid peroxidase gene; FGA indicates the fibrinogen alpha-chain gene; and AMEL indicates the amelogenin gene). The leftmost column indicates the quantity of DNA used in the amplification reactions. The numbers appearing under the loci indicate the number of allele calls made in the analysis according to the Profiler™ method. The objective is to make two allele calls for as many loci as possible at the lowest possible quantity of DNA prior to amplification because the ability to do so would provide the ability to obtain useful forensic DNA samples from small quantities of tissues.
Tables 2 and 3 clearly indicate that mixture 3 comprising added solutes substantially improves sensitivity over that achieved using non-solute mixture 1. Mixture 3, therefore, generates product from trace nucleic acid samples as low as 2 picograms and provides 2 or more allele calls for trace samples as low as 4 picograms.
Example 2 Effects of Compatible Solutes on Maintaining Allelic Balance in the Amplification ReactionAllelic balance is a measure that indicates the ratio of quantity of detection of the minor allele vs. the major allele. It is desirable to maintain the allelic balance of a given sample of DNA as it is amplified in order to provide an accurate representation of the allelic balance in the original sample.
A human DNA sample designated SC35495 was amplified according to conditions described in Example 1. In this example, the amounts of the alleles detected were quantified using the commercially available kit Quantifiler™ (Applied Biosystems). The quantity values were converted to Log2 to provide a more intuitive measure of balance. These values are shown in
The quality of the amplification reaction can be described in terms of providing a combined measure of optimal fold amplification, a high percentage of amplification of the target nucleic acid being analyzed, and optimal performance in the Profiler™ assay.
Shown in
To assess the effect of augmenting the action of Phi29 polymerase, additional polymerase enzymes were individually added to the 3 amplification mixture along with 15 picograms of template DNA of human sample SC35495. The samples were amplified as indicated in Example 1. The resulting amplified DNA was analyzed in the Profiler™ assay and allele calls were made and tallied. Table 4 shows the results and indicates that the addition of Pol I polymerase results in an average of four additional allele calls in the experiment and also indicates that addition of Pol I polymerase is a favorable modification of the amplification mixture.
The addition of Pol I polymerase further increases the yield of amplified DNA and also enhances the genotyping of trace amounts of DNA. Addition of a further enzyme, pyrophosphatase is useful because accumulation of pyrophosphate during the amplification process is known to inhibit polymerase reactions.
Example 5 Design and Testing of Individual Oligonucleotide Primer ModificationsA series of primer motifs were designed for improvement of the quality, sensitivity and balance of the amplification reaction. The modifications included inclusion of inosine nucleobases at specific positions within the hexamer, heptamer and octomer primers. Phosphorothioate modified linkages were incorporated into these primers at the two 3′-most terminal linkages. The most effective placement of inosine nucleobases was found to be at the fifth and sixth positions of the hexamer primers, sixth and seventh positions of the heptamer primers and seventh and eighth positions of the octomer primers. These primers containing inosine nucleobases produced less amplified product than the corresponding primers that did not contain inosine. However, the total amplified product was found to represent a greater proportion of the template DNA, indicating that inclusion of inosines in the primers improves the quality of amplification.
The position of the LNA modified nucleotide residues of the heptamer primers was examined in detail by systematically changing the position of one or two LNA modifications (L) in random heptamers as indicated in Table 5. The symbols in Table 5 are as follows: N=A, T, C or G; I=inosine; NI=nitroindole; L=LNA (locked versions of A, C, T or G). In this experiment, improvements in fold amplification are achieved for primers having LNA substituted in position 2, position 4, position 5, positions 1 and 4, and positions 2 and 5. LNA substitutions were well tolerated at all positions, with only the most 3′ position showing a slight negative effect (LNA-7). Primers bearing LNA substituted residues at two positions have a higher fold amplification increase as compared to those having only a single LNA substituted residue. Moreover, substituting inosine residues at the most 3′ positions of a position 2, position 5 LNA substituted primer further improved fold amplification (LNA-11 and LNA-12). It is notable that the primer containing the nitroindole universal base (NI) did not perform well in the amplification reaction.
The preceding examples illustrate that the use of a plurality of polymerases, inclusion of compatible solutes, and modifications of primers individually and collectively improve the sensitivity of amplification while preserving representation of the original nucleic acid sample and producing high quality amplification products.
Various modifications of the invention, in addition to those described herein, will be apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fall within the scope of the appended claims. Each reference (including, but not limited to, journal articles, U.S. and non-U.S. patents, patent application publications, international patent application publications, gene bank accession numbers, internet web sites, and the like) cited in the present application is incorporated herein by reference in its entirety. Those skilled in the art will appreciate that numerous changes and modifications may be made to the embodiments of the invention and that such changes and modifications may be made without departing from the spirit of the invention. It is therefore intended that the appended claims cover all such equivalent variations as fall within the true scope of the invention.
Claims
1. A method for multiple displacement amplification of a nucleic acid sequence in a sample comprising the steps of:
- contacting said nucleic acid with a reaction mixture, wherein said reaction mixture includes a set of oligonucleotide primers and a plurality of polymerase enzymes; and subjecting said reaction mixture to conditions under which said nucleic acid sequence is amplified to produce an amplified product in a multiple displacement reaction.
2. The method of claim 1 wherein said sample is a forensic sample comprising human DNA.
3. The method of claim 1 wherein said plurality of polymerase enzymes have 5′→3′ DNA polymerase activity, 3′→5′ exonuclease activity, and 5′→3′ excision repair activity.
4. The method of claim 1 wherein members of said set of oligonucleotide primers have random sequences.
5. The method of claim 4 wherein said oligonucleotide primers have lengths of 6, 7 or 8 nucleotide residues.
6. The method of claim 4 wherein said oligonucleotide primers include at least one universal nucleobase.
7. The method of claim 6 wherein said universal nucleobase is inosine.
8. The method of claim 6 wherein said universal nucleobase is located at the 3′-terminal end of said oligonucleotide primers.
9. The method of claim 1 wherein said set of oligonucleotide primers includes primers having one or more modifications of the ribose ring that favor a 3′-endo conformation of said ribose ring or lock said ribose ring in said 3′-endo conformation.
10. The method of claim 9 wherein said modifications comprise a 2′ to 4′ ribose bridge.
11. The method of claim 10 wherein said modifications are located at the 2nd and 5th positions of said primers.
12. The method of claim 1 wherein said one or more enzymes include phi29 polymerase and pol I polymerase.
13. The method of claim 1 wherein said one or more enzymes further includes pyrophosphatase.
14. The method of claim 1 including the proviso that bovine serum albumin is not included in said reaction mixture.
15. The method of claim 1 wherein said reaction mixture comprises betaine and trehalose.
16. The method of claim 1 wherein said conditions comprise thermal cycling of said reaction mixture.
17. The method of claim 1 wherein said multiple displacement amplification results in whole genome amplification.
18. The method of claim 1 wherein said multiple displacement amplification reaction produces said amplification product at a constant ratio relative to production of other amplification products of other nucleic acids present in said sample.
19. The method of claim 1 wherein the total quantity of said nucleic acid sequence is between about 2 picograms to about 1000 picograms.
Type: Application
Filed: Mar 20, 2017
Publication Date: Sep 7, 2017
Inventors: David D. Duncan (Encinitas, CA), Mark W. Eshoo (Solana Beach, CA)
Application Number: 15/463,846
