Method and Compositions for Detection and Enumeration of Genetic Variations
Many areas of biomedical research depend on the analysis of uncommon variations in individual genes or transcripts. Here we describe a method that can quantify such variation at a scale and ease heretofore unattainable. Each DNA molecule in a collection of such molecules is converted into a single particle to which thousands of copies of DNA identical in sequence to the original are bound. This population of beads then corresponds to a one-to-one representation of the starting DNA molecules. Variation within the original population of DNA molecules can then be simply assessed by counting fluorescently-labeled particles via flow cytometry. Millions of individual DNA molecules can be assessed in this fashion with standard laboratory equipment. Moreover, specific variants can be isolated by flow sorting and employed for further experimentation. This approach can be used for the identification and quantification of rare mutations as well as to study variations in gene sequences or transcripts in specific populations or tissues.
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This application claims the benefit of application Ser. No. 60/485,301 filed Jul. 5, 2003 and 60/525,859, filed Dec. 1, 2003, the contents of both of which are expressly incorporated herein.
The invention disclosed herein was made using funds from the National Institutes of Health grants CA 43460, CA 57345, and CA62924. The United States government therefore retains certain rights in the invention.
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.FIELD OF THE INVENTION
The invention relates to the field of genetic analysis. In particular, it relates to methods and compositions for analyzing variations in individual genes or transcripts and separating variants.BACKGROUND OF THE INVENTION
The study of DNA sequence variation is essential for many areas of research. The study of germ-line variations is essential for assessing the role of inheritance in normal and abnormal physiologic states (1). Other variations, developed somatically, are responsible for neoplasia (2). The identification of such mutations in urine, sputum, and stool can therefore be used for the detection of presymptomatic cancers (3-5). Similarly, the detection of somatic mutations in lymph nodes, blood, or bone marrow can provide data about the stage of disease, prognosis, and appropriateness of various therapies (5). Somatic mutations in non-neoplastic cells also occur and appear to accumulate as humans age or are exposed to environmental hazards (6). Such mutations occur in only a small fraction of the cells in a tissue, thereby complicating their analysis.
Central to the investigation of many of these issues is the detection and quantification of sequence variants within a population of DNA molecules. The number of molecules in each such collection is finite and therefore countable. Consider, for example, a collection of red and green balls. Counting these balls is simple in principle but subject to basic probability theory. If there is only one red ball for every 500 green balls, then it is necessary to count several thousand balls to get an accurate estimate of the proportion of red balls. If it is difficult to count enough balls to make a reliable estimate, one can elute the paint off all the balls and measure the color of the resultant paint mix.
In analogous fashion, small numbers of DNA molecules that vary by subtle changes (single base pair substitutions or small deletions or insertions) can be directly counted by amplifying individual DNA molecules (single molecule PCR) (7-12). Such digital techniques have been shown to be extremely useful for measuring variation in genes or their transcripts. But digital technologies have so far been limited to counting tens to thousands of molecules, either in the wells of microtiter plates, on microscope slides, or after electrophoresis of individual PCR products. Analog techniques, analogous to the elution of paint from the balls described above, are generally easier to implement and can assess millions of molecules simultaneously (13). However, their accuracy and sensitivity is limited by instrumental and experimental noise. There is a continuing need in the art for methods which are accurate and sensitive for measuring variation in genes or their transcripts.BRIEF SUMMARY OF THE INVENTION
In a first embodiment of the invention a composition is provided. The composition comprises a plurality of beads. Each of the plurality of beads comprises a plurality of bound polynucleotides. The polynucleotides in the composition are heterogeneous; however, on at least 1% of said beads the plurality of bound polynucleotides is homogeneous.
In a second embodiment of the invention a liquid composition is provided. The liquid composition comprises a plurality of microemulsions forming aqueous compartments. At least a portion of said aqueous compartments comprise a bead, a polynucleotide template, and oligonucleotide primers for amplifying the template. At least a portion of the oligonucleotide primers is bound to the bead.
A third embodiment of the invention provides a method for analyzing nucleotide sequence variations. Microemulsions comprising one or more species of analyte DNA molecules are formed. The analyte DNA molecules in the microemulsions are amplified in the presence of reagent beads which are bound to a plurality of molecules of a primer for amplifying the analyte DNA molecules. Product beads are formed that are bound to a plurality of copies of a single species of analyte DNA molecule. The product beads are separated from analyte DNA molecules which are not bound to product beads. A sequence feature of the single species of analyte DNA molecule that is bound to the product beads is determined.
A fourth embodiment of the invention is a probe for use in hybridization to a polynucleotide that is bound to a solid support. The probe comprises an oligonucleotide with a stem-loop structure. At one of the 5′ or 3′ ends there is a photoluminescent dye. The oligonucleotide does not comprise a quenching agent at the opposite 5′ or 3′ end.
A fifth embodiment of the invention is a pair of molecular probes. The first and second probes each comprise an oligonucleotide with a stem-loop structure having a first photoluminescent dye at one of the 5′ or 3′ ends, and not comprising a quenching agent at the opposite 5′ or 3′ end. The first oligonucleotide hybridizes to a wild-type selected genetic sequence better than to a mutant selected genetic sequence and the second oligonucleotide hybridizes to the mutant selected genetic sequence better than to the wild-type selected genetic sequence. The first and the second photoluminescent dyes are distinct.
In a sixth embodiment of the invention a method is provided for isolating nucleotide sequence variants. Microemulsions comprising one or more species of analyte DNA molecules are formed. Analyte DNA molecules in the microemulsions are amplified in the presence of reagent beads. The reagent beads are bound to a plurality of molecules of a primer for amplifying the analyte DNA molecules. Product beads are formed which are bound to a plurality of copies of one species of analyte DNA molecule. The product beads are separated from analyte DNA molecules which are not bound to product beads. The product beads which are bound to a plurality of copies of a first species of analyte DNA molecule are isolated from product beads which are bound to a plurality of copies of a second species of analyte DNA molecule.
These and other embodiments of the invention, which will be apparent from the entire description of the invention, provide the art with the ability to quantify genetic variations at a scale and ease heretofore unattainable.
The inventors describe herein a digital technology, called BEAMing, that has the power to assess millions of molecules and can be generally applied to the study of genetic variation. The technology involves conversion of single DNA molecules to single beads each containing thousands of copies of the sequence of the original DNA molecule. The number of variant DNA molecules in the population can then be assessed, for example, by staining the beads with fluorescent probes and counting them using flow cytometry. Beads representing specific variants can be optionally recovered through flow sorting and used for subsequent confirmation and experimentation.
Beads according to the present invention are also known as microspheres or microparticles. Particle sizes can vary between about 0.1 and 10 microns in diameter. Typically beads are made of a polymeric material, such as polystyrene, although nonpolymeric materials such as silica can also be used. Other materials which can be used include styrene copolymers, methyl methacrylate, functionalized polystyrene, glass, silicon, and carboxylate. Optionally the particles are superparamagnetic, which facilitates their purification after being used in reactions.
Beads can be modified by covalent or non-covalent interactions with other materials, either to alter gross surface properties, such as hydrophobicity or hydrophilicity, or to attach molecules that impart binding specificity. Such molecules include without limitation, antibodies, ligands, members of a specific-binding protein pair, receptors, nucleic acids. Specific-binding protein pairs include avidin-biotin, streptavidin-biotin, and Factor VII-Tissue Factor.
Beads, after being prepared according to the present invention as product beads, have more than one copy of the same nucleic acid molecule bound to them. Preferably each bead is bound to at least 10, 50, 100, 500, or 1000 molecules of the same nucleic acid sequence. In some circumstances some of the product beads are bound to more than one type of nucleic acid molecule. These product beads are generally less useful in the analysis of ratios of genetic sequences in a population of genetic sequences. Such product beads can be readily discriminated and so will not distort the analysis.
A population of product beads will often comprise two or more types of nucleic acids. Such a population is heterogeneous with respect to the nucleic acids. Desirably, a substantial proportion of the product beads comprise only one type of nucleic acid per bead. A substantial proportion can be for example, at least 1%, at least 5%, at least 10%, or at least 50%. A product bead with only one type of nucleic acid per bead is termed homogeneous. Homogeneous beads with only one type of nucleic acid per bead include those with nucleic acids containing errors due to errors in polymerase chain reaction. A product bead with two types of nucleic acid per bead is termed heterogeneous. Although not wishing to be bound by any particular theory, heterogeneous product beads are thought to result from aqueous compartments which have more than two molecules of template of non-identical sequence. A population of product beads can be heterogeneous as a population but contain individual product beads that are homogeneous
Individual product beads preferably comprise more than one copy of template analyte molecule. Each bead may comprise at least 10, at least 50, at least 100, at least 500, or at least 1000 copies of template analyte. If the bead is homogeneous, each of those copies will be identical.
Populations of product beads can be maintained in a liquid suspension. Alternatively they can be sedimented and dried or frozen. The latter alternatives may be beneficial for storage stability.
Analysis of populations of product beads can be useful for distinguishing between many kinds of genetic variants. Polynucleotides can be distinguished which differ by as little as a single nucleotide polymorphism (SNP), by the presence or absence of a mutation, by the presence or absence of an insertion or deletion, by the presence or absence of a non-single nucleotide polymorphism. Thus populations of product beads may be heterogeneous with regard to these genetic variations.
One very convenient way for distinguishing genetic variants, i.e., determining a sequence feature of the analyte, is by differentially labeling the variants with fluorescent dyes. Such labeling can be accomplished by hybridization of a fluorescently labeled oligonucleotide probe to one species of polynucleotide. Alternatively, a fluorescently labeled antibody can be used to specifically attach to one oligonucleotide probe that hybridizes to a particular genetic variant. Such antibody binding can be, for example, mediated by a protein or polypeptide which is attached to an oligonucleotide hybridization probe. Of course, other means of labeling polynucleotides as are known in the art can be used without limitation. Another means of labeling different polynucleotide species is by primer extension. Primers can be extended using labeled deoxyribonucleotides, such as fluorescently labeled deoxyribonucleotides.
Populations of product beads can be used as templates. Template analyte molecules on the product beads can be analyzed to assess DNA sequence variations by hybridization, primer-extension methods, mass spectroscopy, and other methods commonly used in the art. Template analyte molecules on product beads can be employed for solid phase sequencing. In one solid phase sequencing technique, product beads are arrayed by placing them on slides spotted with complementary oligonucleotides. In another solid phase sequencing technique, product beads are placed into individual wells. In still another solid phase sequencing technique product beads are incorporated into acrylamide matrices (with or without subsequent polony formation). Sequencing reactions can be performed with any solid phase sequencing method, such as those using unlabeled nucleotide precursors (e.g., pyrosequencing, as described in Ronaghi et al. Anal. Biochem. 267: 65-71, 1999) or labeled nucleotides (e.g., photocleavable reagents described by Mitra et al. Anal. Biochem. 320:55-65, 2003). Product beads can thus be used for and facilitate multiple parallel sequencing. Product beads can also be used in sequencing employing Type IIS restriction endonucleases. Product beads can also be used to provide templates for conventional dideoxynucleotide sequencing. To obtain useful data upon sequence analysis, a homogeneous template population is desirable. To provide a homogenous template population, product beads can be diluted, separated, or otherwise isolated so that each sequencing reaction contains a single product bead. Alternatively, product beads can be sorted to provide populations of beads with a single species of template.
Oligonucleotide primers can be bound to beads by any means known in the art. They can be bound covalently or non-covalently. They can be bound via an intermediary, such as via a protein-protein interaction, such as an antibody-antigen interaction or a biotin-avidin interaction. Other specific binding pairs as are known in the art can be used as well. To achieve optimum amplification, primers bound to the bead may be longer than necessary in a homogeneous, liquid phase reaction. Oligonucleotide primers may be at least 12, at least 15, at least 18, at least 25, at least 35, or at least 45 nucleotides in length. The length of the oligonucleotide primers which are bound to the beads need not be identical to that of the primers that are in the liquid phase. Primers can be used in any type of amplification reaction known in the art, including without limitation, polymerase chain reaction, isothermal amplification, rolling circle amplification, self-sustaining sequence replication (3SR), nucleic acid sequence-based amplification (NASBA), transcription-mediated amplification (TMA), strand-displacement amplification (SDA), and ligase chain reaction (LCR).
Microemulsions are made by stirring or agitation of oil, aqueous phase, and detergent. The microemulsions form small aqueous compartments which have an average diameter of 0.5 to 50 microns. The compartments may be from 1 to 10 microns, inclusive, from 11 to 100 microns, inclusive, or about 5 microns, on average. All such compartments need not comprise a bead. Desirably, at least one in 10,000 of said aqueous compartments comprise a bead. Typically from 1/100 to 1/1 or from 1/50 to 1/1 of said aqueous compartments comprise a bead. In order to maximize the proportion of beads which are homogeneous with respect to oligonucleotide, it is desirable that on average, each aqueous compartment contains less than 1 template molecule. Aqueous compartments will also desirably contain whatever reagents and enzymes are necessary to carry out amplification. For example, for polymerase chain reaction (PCR) the compartments will desirably contain a DNA polymerase and deoxyribonucleotides. For rolling circle amplification a DNA polymerase and a generic DNA circle may be present.
Emulsions can be “broken” or disrupted by any means known in the art. One particularly simple way to break the emulsions is to add more detergent. Detergents which can be used include, but are not limited to Triton X100, Laureth 4, Nonidet.
Sample DNA for amplification and analysis according to the present invention can be genomic DNA, cDNA, PCR products of genomic DNA, or PCR products of cDNA, for example. Samples can be derived from a single individual, for example, from a body sample such as urine, blood, sputum, stool, tissue or saliva. Samples can also be derived from a population of individuals. The individuals can be humans, but can be any organism, plant or animal, eukaryotic or prokaryotic, viral or non-viral.
Any type of probe can be used for specific hybridization to the amplified polynucleotides which are bound to the beads. Fluorescently labeled probes are useful because their analysis can be automated and can achieve high throughput. Fluorescence activated cell sorting (FACS) permits both the analysis and the isolation of different populations of beads. One type of fluorescently labeled probe that can be used is a modified molecular beacon probe. These probes have stem-loop structures and an attached fluorescent moiety on the probe, typically on one end of the probe, sometimes attached through a linker. Unlike standard molecular beacon probes, modified molecular beacon probes do not have a quenching moiety. The modified molecular beacon probe can have the fluorescent moiety attached on either end of the probe, 5′ or 3′. One such probe will hybridize better to a wild-type sequence than to a mutant. Another such probe will hybridize better to a mutant sequence than to the wild type. Still other probes will preferably hybridize to one polymorphic variant over another.
The method of the present invention provides a reliable and sensitive assay for measuring variations in genes and transcripts. It requires no instrumentation other than machines that are widely available. There are several other advantages of this approach. First, the sensitivity can be increased to meet the particular specifications of an assay simply by analyzing more beads. Such sensitivity is limited only by the error rate of the polymerases used for amplification. Second, the data obtained can be used not only to demonstrate that a variant is present in a particular population of DNA molecules, but also quantifies the fraction of variant DNA molecules in that population (
Several modifications of the basic principles described here can be envisioned that will further simplify the technology or widen its applications. For example, microemulsions were made by stirring water/oil/detergent mixes. The sizes of the resultant aqueous compartments were somewhat heterogeneous, as illustrated in
Though flow cytometry requires only seconds to minutes per sample, multiple parallel analyses could facilitate throughput. Novel particle counting designs may prove useful for this purpose. Another way to increase throughput would be to physically separate the beads that contained PCR products prior to flow cytometry. This could be accomplished with proteins such as antibodies or streptavidin that bind to modified nucleotides incorporated into the PCR product during amplification.
The methods of the invention can be applied to genes or transcripts of any organism or population of organisms. These include without limitation: humans, rodents, ungulates, mammals, primates, cows, goats, pigs, rats, mice, yeast, poultry, fish, shellfish, digs, cats, zebrafish, worms, algae. It can also be used to quantify epigenetic alterations, such as methylation, if DNA is first treated with bisulfite to convert methylated cytosine residues to thymidine. Beads generated from random fragments of whole genomes (24), rather than from individual genes as described above, could be used to identify gene segments that bind to specific DNA-binding proteins (25). And if the product beads are used in compartmentalized in vitro transcription-translation reactions, variant proteins can be bound to beads containing the corresponding variant DNA sequences (23). This could allow facile flow cytometric evaluation of rare mutations using antibodies that distinguished between wild type and mutant gene products (26).
While the invention has been described with respect to specific examples including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques that fall within the spirit and scope of the invention as set forth in the appended claims.EXAMPLES Example 1 Materials and Methods
Step 1—Coupling oligonucleotides to beads. Superparamagnetic beads of 1.05+/−0. 1 um in diameter, covalently bound to streptavidin, were purchased from Dynal Biotech, Inc. (650.01, Lake Success, N.Y.). Beads were washed once with 1×PCR buffer (53286, Invitrogen, Carlsbad, Calif.) then suspended in Bind and Wash Buffer (BWB) (5 mM Tris-HCl, 0.5 mM EDTA, 1.0 M NaCl, pH 7.5). Beads were incubated in BWB for 30 min at room temperature in the presence of 10 uM oligonucleotides (
Step 2—Preparing microemulsions. Microemulsions for PCR were prepared by slight modifications of previously described methods (14) (15). The oil phase was composed of 4.5% Span 80 (S6760, Sigma, St. Louis, Mo.), 0.40% Tween 80 (Sigma S-8074), and 0.05% Triton X-100 (Sigma T-9284) in mineral oil (Sigma M-3516). The oil phase was freshly prepared each day. The aqueous phase consisted of 67 mM Tris-HCl (pH 8.8), 16.6 mM NH4SO4, 6.7 mM MgCl2, 10 mM β-mercaptoethanol, 1 mM dATP, 1 mM dCTP, 1 mM dGTP, 1 mM dTTP, 0.05 uM forward primer, 25 uM reverse primer, 45 units Platinum Taq (Invitrogen 10966-034), various amounts of template DNA (see results), and ˜108 oligonucleotide-coupled beads in a total volume of 300 ul. The forward primer was an oligonucleotide whose sequence was identical to the 3′ 20-22 nt of that described in step 1 and was not modified with biotin.
Water-in-oil microemulsions were prepared by drop wise addition of 200 microliters of the aqueous phase to 400 microliters of the oil phase previously placed in a 2 ml round bottom cryogenic vial (430661, Corning, Corning, N.Y.). The drop wise addition was performed over ˜ one minute while the mixture was being stirred at 1400 RPM with a magnetic microstir bar (58948-353, VWR, Plainfield, N.J.) on a VWR model 565 Magnetic Stirrer. After the addition of the aqueous phase, the mixture continued to be stirred for a total time of 30 minutes. Two emulsions were made at once by placing two tubes in a rack placed at the center of the magnetic stirrer.
Step 3—PCR cycling. The emulsions were aliquotted into five wells of a 96 well PCR plate, each containing 100 ul. PCR was carried out under the following cycling conditions: 94° C. for 2 minutes; 40 cycles of 94° C. for 15 seconds, 57° C. for 30 seconds, 70° C. for 30 seconds. The PCR products analyzed in this study ranged from 189 to 239 bp.
Step 4—Magnetic capture of beads. After PCR cycling, the microemulsion from five wells of a PCR plate were pooled and broken by the addition 800 microliters of NX buffer (100 mM NaCl containing 1% Triton X-100, 10 mM Tris-HCl, pH 7.5, 1 mM EDTA) in a 1.5 ml tube (Corning 430909). After vortexing for ˜20 sec. the beads were pelleted by centrifugation in a microcentrifuge at 8000 rpm (5000 g) for 90 seconds. The top oil phase and all but ˜300 microliters of the aqueous phase was removed from the tube and 600 microliters of NX buffer was added. After vortexing for 20 sec. and centrifugation for 90 sec., the top oil phase and all but ˜300 microliters of the aqueous phase was removed. The addition of 600 microliters NX buffer, vortexing, and centrifugation was repeated once more and the top oil portion and all but ˜300 microliters of the aqueous phase was removed. The tube was then placed on a magnet (Dynal MPC-S) and the rest of the supernatant was carefully pipetted off. The beads were washed an additional 3 times with 1×PCR buffer using magnetic separation rather than centrifugation and finally resuspended in 100 microliters of 1×PCR buffer.
Step 5—Sequence differentiation. Two oligonucleotide probes were used for each reaction. One was 5′-labeled with 6-carboxyfluorescein (6-FAM) and was specific for one allele while the second was 5′-labeled with biotin and was specific for the other allele. Probes were synthesized by IDT. The 30 microliters hybridization reactions contained 10 uM of each probe and 5-25 million beads in 1×PCR buffer. Reactions were performed in PCR plates on a thermal cycler by heating to 94° C. for 30 seconds then cooling to 75° C. at a rate of 0.5° C. per second, cooling to 45° C. at 0.2° C. per second, and finally cooled to 30° C. at 1° C. per second. All subsequent steps were performed at room temperature. The reactions were transferred to a 96 well Costar plate (Corning 3797) and placed on a 96 well magnet. Beads were collected magnetically by exposing them to the magnet for 2 minutes. The supernatant was removed and the beads washed 3 times with 1×PCR buffer by pipetting them and collecting for two minutes. They were finally resuspended in 100 microliters B-PCR buffer (1 mg/mL BSA in 1×PCR buffer). The beads were then incubated for 10 minutes in a total volume of 100 microliters B-PCR buffer containing 3 ug of Alexa-488 rabbit anti-fluorescein antibody (Molecular Probes A-11090, Eugene, Oreg.) and 3 ug of Nutravidin labeled with R-phycoerythrin (Molecular Probes A-2660) in B-PCR buffer. The beads were washed three times and resuspended in B-PCR buffer as described above. They were then incubated for ten minutes in a total volume of 100 microliters B-PCR buffer containing 6 ug of Alexa 488-conjugated chicken anti-rabbit antibody (Molecular Probes A-21441) and 3 ug of biotinylated goat anti-avidin antibody (BA-0300, Vector Laboratories, Burlingame, Calif.). The beads were washed three times and resuspended in B-PCR buffer as described above. They were then incubated for ten minutes in a total volume of 100 microliters B-PCR buffer containing 3 ug of an Alexa 488-conjugated goat anti-chicken antibody (Molecular Probes A-11039) and 3 micrograms of R-phycoerythrin-labeled streptavidin (Molecular Probes S-866). This solution was then washed an additional 3 times with 1×PCR buffer and resuspended in 20 microliters of 1×PCR buffer.
Step 6—Flow Cytometry. The bead suspension was diluted to a concentration of ˜106-107 beads per ml in 10 mM Tris-HCl, 1 mM EDTA (351-010-131, Quality Biological, Inc., Gaithersburg, Md.) and analyzed using a LSR instrument (BD Biosciences, Franklin Lakes, N.J.). The instrument was set up for standard two-color analysis using an argon laser and optical filters that distinguished between the two fluorescent dyes. No spectral deconvolution was required as the major bead populations were well-separated. In some cases, scanning was performed with FACScan or FACSCalibur instruments (BD Biosciences), yielding equivalent results. Sorting was carried out with a FACS Vantage SE instrument (BD Biosciences).
Template preparation and sequence analyses. Human genomic DNA was purified with DNeasy (69504, Qiagen, Valencia, Calif.). RNA was purified with Quickprep (27-9255-01, Amersham Biosciences, Piscataway, N.J.). Reverse transcription of RNA was performed using Superscript II reverse transcriptase (Invitrogen 18064014) according to the manufacturer's instructions. PCR using genomic DNA or reverse transcripts as templates was performed as described (7). PCR products to be used as templates for BEAMing or for sequencing were purified with QIAquick (Qiagen 28104). Sequencing reactions were performed using Big Dye v3.0 reagents (Applied Biosystems, Foster City, Calif.) and analyzed by capillary electrophoresis (Spectrumedix 9600, State College, Pa.).Example 2 Results
Step 1—Coupling oligonucleotides to beads. We used streptavidin-beads because of the simplicity of coupling biotinylated oligonucleotides to them. Oligonucleotides with just a single 5′ biotin group were found to dissociate from the beads during temperature cycling, while oligonucleotides labeled with dual biotin groups at their 5′ end (separated by a six-carbon linker) were stable to cycling. As determined by fluoroscopic measurements of oligonucleotides doubly labeled with 6-FAM and biotin, ˜105 oligonucleotide molecules were bound to each bead. We found that short oligonucleotides (20 bases) did not work as well for priming as longer ones (41 bp), perhaps because of steric hindrance at the bead surface. It is likely that amino-, sulfhydryl-, or carboxyl-modified oligonucleotides covalently coupled to beads modified with corresponding reactive groups could also function as bead-bound primers for BEAMing.
Step 2—Preparing microemulsions. The size of the individual aqueous compartments ranged from less than 1 micron to >10 microns in diameter (
Step 3—PCR cycling. PCR priming by oligonucleotides coupled to beads was found to be very inefficient compared to the priming by the same oligonucleotides when free in solution. For this reason, a small amount of non-biotinylated forward primer identical in sequence to the biotinylated oligonucleotide coupled to the beads was included in the reactions. This facilitated the first few rounds of amplification of the single template within each aqueous compartment. In the absence of additional primer, no detectable amplification on the beads was generated. Conversely, if too much additional primer was included, no amplification on the beads occurred because of competition with the primers in solution. An excess of the reverse primer was included in the aqueous compartment to maximize the probability that bead-bound oligonucleotides extended by polymerase would serve as templates for further amplification cycles.
Step 4—Magnetic capture of beads. There are several ways to break water-in-oil emulsions, including extraction with organics (14). We found that simply adding non-ionic detergents produced phase separations without any detectable modification of the beads or DNA molecules bound to them. By measuring the amount of DNA that could be released from the beads following restriction endonuclease digestion, we estimate that >10,000 extended PCR products were present, on average, per bead.
Step 5—Sequence differentiation. Most fluorescence-based methods for distinguishing alleles in homogeneous or two-phase assays can be used to assess allelic variation captured on beads. These methods include single nucleotide extension, allele specific priming, or hybridization. We generally employed hybridization of fluorescein-conjugated or biotin-conjugated oligonucleotides for discrimination. As shown in
Step 6—Flow Cytometry. Optimum results in flow cytometry depend on high fluorescent signals on the beads. We generally enhanced the fluorescence emanating from the hybridization probes with secondary reagents. For example, Alexa 488—labeled antibodies were used to enhance the signals emanating from fluorescein-coupled oligonucleotide probes. Similarly, R-phycoerythrin-labeled streptavidin was used to generate a signal from biotin-labeled oligonucleotide probes. Flow cytometers equipped with two or three lasers and appropriate filters have the capacity to distinguish multi-allelic loci and to perform multiplex analysis of several genes simultaneously. The newest generation of flow cytometers can also analyze >70,000 events per second. In addition to the analytical power of flow cytometry, FACS instruments can separate specific populations of beads for further analysis.Example 3 Characteristics of Microemulsions
Pilot experiments demonstrated that simply stirring the water-oil mixtures described in Materials and Methods produced very stable microemulsions of a size compatible with that of the beads. In the experiment shown in
Example 5 PCR Products, Genomic DNA or cDNA as Templates
The results shown in
In some situations it might be useful to use genomic DNA rather than PCR products as templates for BEAMing. The data in
BEAMing could also be used to analyze variations in expression from the two alleles of a heterozygous individual. Heritable variations in the expression from individual alleles of the same gene have been shown to occur often in humans (18) and mice (19) and can have significant phenotypic effects (20). The results shown in
The analysis of uncommon variations is ideally suited for analysis via BEAMing because of the large number of molecules that can be independently analyzed while retaining a high signal-to-noise ratio.
The rare beads representing the mutant alleles could not only be quantified but could also be purified for subsequent analysis. As a demonstration, samples of the beads enumerated in
A 100 bp product was amplified on beads as described in Example 1, steps 1 through 4. Two FAM-labeled oligonucleotides (50 and 20 bases in length) were annealed to the 100 bp product on the beads. The beads were then embedded in an acrylamide gel (using conventional Tris-Borate-EDTA electrophoresis buffer) in an oval shaped configuration. An electric field (250 V) was applied under denaturing conditions for 3 minutes. The labeled oligonucleotides migrated off the beads and migrated a distance related to their sizes. See
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1. A method for sequencing nucleic acids comprising:
- (a) fragmenting genomic nucleic acid molecules to generate a plurality of fragmented nucleic acids;
- (b) delivering the fragmented nucleic acids into aqueous microreactors in a water-in-oil emulsion such that a plurality of aqueous microreactors comprise a single copy of a fragmented nucleic acid, a single bead capable of hybridizing to the fragmented nucleic acid, and amplification reaction solution containing reagents necessary to perform nucleic acid amplification;
- (c) amplifying the fragmented nucleic acids in the microreactors to form amplified copies of said nucleic acids and hybridizing the amplified copies to beads in the microreactors;
- (d) delivering the beads to an array of reaction chambers, wherein a plurality of the reaction chambers comprise no more than a single nucleic acid bound bead; and
- (e) performing a sequencing reaction simultaneously on a plurality of the reaction chambers.
2. The method of claim 1 wherein step (c) is accomplished using polymerase chain reaction.
3. The method of claim 1 wherein the sequencing reaction is a pyrophosphate-based sequencing reaction.
4. The method of claim 1 wherein the sequencing reaction comprises the steps of:
- (a) annealing an effective amount of a sequencing primer to the amplified copies of the nucleic acid and extending the sequencing primer with a polymerase and a predetermined nucleotide triphosphate to yield a sequencing product and, if the predetermined nucleotide triphosphate is incorporated onto a 3′ end of said sequencing primer, a sequencing reaction byproduct; and (b) identifying the sequencing reaction byproduct, thereby determining the sequence of the nucleic acid in a plurality of the reaction chambers.
Filed: Jan 29, 2009
Publication Date: Nov 19, 2009
Applicant: The Johns Hopkins University (Baltimore, MD)
Inventors: Devin DRESSMAN (Baltimore, MD), Hai Yan (Chapel Hill, NC), Kenneth Kinzler (Baltimore, MD), Bert Volgelstein (Baltimore, MD)
Application Number: 12/361,690
International Classification: C40B 20/00 (20060101);