LENGTH DETERMINATION OF NUCLEIC ACID REPEAT SEQUENCES BY DISCONTINUOUS PRIMER EXTENSION

Abstract: Disclosed is a method for determining the number of repeat units in a repeat region of a target nucleic acid. The method comprises a discontinuous primer extension reaction wherein a primer is extended in discrete increments corresponding to a single repeat unit. Following each increment of primer extension, a detection step is performed in which a modulation in a signal is detected when the primer has been extended by an amount equal to the total length of a repeat region. By counting the number of increments of discrete primer extension required to cause a modulation in the signal, the number of repeat units making up the repeat region is determined. In the method, a plurality of different-sequence primers are contacted with a polynucleotide sample under conditions effective for the primers to anneal to primer-complementary regions in one or more target polynucleotides, to form one or more target-primer hybrid(s), wherein either (1) each different-sequence primer contains (i) a target binding segment and (ii) a tag segment having a nucleotide sequence that uniquely identifies the target binding segment, or (2) one or more polynucleotides in the sample are tagged polynucleotides that contain a tag segment having a nucleotide sequence that uniquely identifies the attached polynucleotide. The tagged moieties can conveniently be immobilized on an array of immobilized tag complements for analysis.

Description: LENGTH DETERMINATION OF NUCLEIC ACID REPEAT SEQUENCES BY DISCONTINUOUS PRIMER EXTENSION FIELD OF THE INVENTION This invention relates to methods and kits useful for determining the length of nucleic acid repeat sequences. More specifically, this invention relates to methods and kits useful for determining the length of nucleic acid repeat sequences by employing a discontinuous primer extension reaction. REFERENCES Ausubel et al. eds.. Current Protocols in Molecular Biology Volume 1, Chapter 2, Section I, John Wiley & Sons, New York (1993). Barany et al., International Patent Application PCT/US91/06103. Beattie et al., Clinical Chemistry, 39:719-722 (1993). Beaucage and Iyer, Tetrahedron, 48: 2223-2311 (1992). Boom et al., U.S. Patent No. 5,234,809. Brenner, PCT Publications No. WO 96/12014 and WO 96/41011. Breslauer et al., Proc. Natl. Acad. Sci. 83:3746-3750 (1986). Bronstein, et al., J. Biolumin. Chemilumin., 4: 99-11 1 (1990). Cantor et al, U.S. Patent No. 5,482,836. 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Pi oc Natl Acad Sci 93 4913 (1996) BACKGROUND Methods for the analysis of genetic polymorphism have found wide utility in basic research, clinical diagnostics, forensics, and other areas One particularly useful method of detecting genetic polymorphism is based on vaπations in the length of repeat sequences, such methods being vaπously referred to as short tandem repeat analysis (STR), vaπable number of tandem repeat analysis (VNTR), minisatelhte analysis, and microsatelhte analysis Detection of length polymorphisms m nucleic acid repeat sequences has up to now relied on gel electrophoresis for the determination of the length of the repeat sequence However, gel electrophoresis has several important drawbacks in the context of repeat sequence length polymorphism analysis First, molecular length measurements based on electrophoretic mobility are inherently imprecise due to a complicated relationship between molecular size and electrophoretic mobility Second, the degree to which the electrophoretic process can be multiplexed is limited by the number of electrophoresis lanes and by the size of different loci run in a single lane, l e , loci must be selected which do not electrophoretically co-migrate SUMMARY The method of the present invention compπses a discontinuous pπmer extension reaction wherein a pπmer is extended in discrete increments such that in each increment of pπmer extension the pπmer is extended by an amount corresponding to a single repeat unit Following each increment of discrete pπmer extension, a detection step is performed in which a modulation in a signal is detected when the pπmer has been extended by an amount equal to the total length of a repeat region Thus, by counting the number of increments of discrete pπmer extension required to cause a modulation in the signal, the number of repeat units making up the repeat region is determined It is an object of the present invention to provide a precise and reproducible method for determining the number of repeat units making up a repeat region of a nucleic acid repeat sequence It is another object of the present invention to provide a method for determining the number of repeat units making up a repeat region of a nucleic acid repeat sequence which can perform a large number of measurements m parallel It is yet an additional object of the present invention to provide a method for determining the number of repeat units making up a repeat region of a nucleic acid repeat sequence which does not require an electrophoretic separation It is an object of the present invention to provide kits and reagents useful for practicing a method for determining the number of repeat units making up a repeat region of a nucleic acid repeat sequence having the above descπbed characteπstics In a first aspect, the foregoing and other objects of the invention are achieved by a method for determining the number of repeat units in a repeat region of a target nucleic acid compπsing annealing a pπmer-complementary portion of a target nucleic acid to a pπmer thereby forming a target-pπmer hybπd, performing a first pπmer extension reaction using a first pπmer extension reagent, separating the target-pπmer hybπd and unreacted first pπmer extension reagent, performing a second pπmer extension reaction using a second pπmer extension reagent, wherein at least one of the first or second pπmer extension reagents includes an extendible nucleotide having a label attached thereto, separating the target-pπmer hybπd from unreacted second pπmer extension reagent, measunng a signal produced by the label, treating the label so as to render the label undetectable, and repeating the above steps until the signal is substantially less than a signal detected m a previous cycle In one preferred embodiment of the first aspect of the inv ention, the step of performing a second pπmer extension reaction further includes reacting the target-pπmer hybπd with a pπmer termination reagent In yet another preferred embodiment of the first aspect of the invention, the label is a fluorescent or chemiluminescent molecule. In another prefeπed embodiment of the first aspect of the invention, the label is attached to the extendible nucleotide through a cleavable linker. In an additional preferred embodiment of the first aspect of the invention, the target nucleic acid is amplified prior to analysis. Preferably such amplification is achieved using a PCR. In another preferred embodiment of the first aspect of the invention, the step of treating the label so as to render the label undetectable includes either cleaving the label from the labeled extendible nucleotide or destroying a signal producing property of the label. In another preferred embodiment of the first aspect of the invention, the target-primer hybrid is attached to a solid support. Preferably, one of the primer or the target nucleic acid is attached to the solid support. In a novel, prefeπed embodiment employing a solid support, the invention includes a method for determining the number of repeat units in a repeat region of a target nucleic acid comprising the steps of: (A) contacting a plurality of different-sequence primers with a polynucleotide sample under conditions effective for the primers to anneal to primer-complementary regions in one or more target polynucleotides, to form one or more target-primer hybrid(s), wherein either (1) each different-sequence primer contains (i) a target binding segment and (ii) a tag segment having a nucleotide sequence that uniquely identifies the target binding segment, or (2) one or more polynucleotides in the sample are tagged polynucleotides that contain a tag segment having a nucleotide sequence that uniquely identifies the attached polynucleotide, (B) performing a first primer extension reaction on the hybrid(s) using a first primer extension reagent; (C) separating the target-primer hybrid(s) and unreacted first primer extension reagent; (D) performing a second primer extension reaction on the hybrid(s) using a second primer extension reagent, wherein at least one of the first or second primer extension reagents includes an extendible nucleotide having a label attached thereto; (E) separating the target-primer hybrid(s) from unreacted second primer extension reagent; (F) measuring a signal produced by the label; (G) treating the label so as to render the label undetectable; and (H) repeating a cycle of steps (A) through (G) until the signal detected in the target- primer hybrid(s) is substantially less than a signal detected in a previous cycle, wherein (I) prior to step (F), at least an aliquot of either (1 ) the different-sequence primers or (2) the tagged sample polynucleotides are contacted with an addressable array of immobilized, different tag complements, and each different tag complement contains a sequence that is complementary to one of the tag segments, under conditions effective to hybridize the tag segments to corresponding tag complements on the support. In one embodiment, the contacting in step (I) is performed prior to step (A). In another embodiment, the contacting in step (I) is performed after step (A), and/or before any one of steps (B), (C), (D), (E), and (F). In yet another embodiment, steps (A) through (H) are performed on at least two replicate arrays, and one of the replicate arrays is subjected to at least one more cycle of steps (A) through (G) than is a second replicate aπay. In a second aspect, the foregoing and other objects of the invention are achieved by a method for determining the number of repeat units in a repeat region of a target nucleic acid comprising annealing a primer-complementary portion of a target nucleic acid to a primer thereby forming a target-primer hybrid; performing a first primer extension reaction using a first primer-extension reagent; separating the target-primer hybrid from unreacted first primer extension reagent; performing a second primer extension reaction using a second primer extension reagent and with a primer termination reagent, the primer termination reagent including a nucleotide terminator having a label attached thereto; separating the target-primer hybrid from unreacted second primer extension reagent and unreacted primer termination reagent; measuring a signal produced by the label; and repeating the above steps until a signal is detected indicating incorporation of the labeled nucleotide terminator into the primer extension product. In yet another preferred embodiment of the second aspect of the invention, the label is selected from the group consisting of fluorescent and chemiluminescent molecules. In an additional preferred embodiment of the second aspect of the invention, the target nucleic acid is amplified prior to analysis. Preferably such amplification is achieved using a PCR. In a prefeπed embodiment of the second aspect of the invention, the target-primer hybrid is attached to a solid support. Preferably, one of the primer or the target nucleic acid is attached to the solid support. In a novel, prefeπed embodiment,the invention includes a method for determining the number of repeat units in a repeat region of a target nucleic acid comprising the steps of: (A) contacting a plurality of different-sequence primers with a polynucleotide sample under conditions effective for the primers to anneal to primer-complementary regions in one or more target polynucleotides. to form one or more target-primer hybrid(s), wherein either (1) each different-sequence primer contains (i) a target binding segment and (ii) a tag segment having a nucleotide sequence that uniquely identifies the target binding segment, or (2) one or more polynucleotides in the sample are tagged polynucleotides that contain a tag segment having a nucleotide sequence that uniquely identifies the attached polynucleotide, (B) performing a first primer extension reaction on the hybrid(s) using a first primer- extension reagent; (C) separating the target-primer hybrid(s) from unreacted first primer extension reagent; (D) performing a second primer extension reaction on the hybrid(s) using a second primer extension reagent and with a primer termination reagent, the primer termination reagent including a nucleotide terminator having a label attached thereto; (E) separating the target-primer hybrid(s) from unreacted second primer extension reagent and unreacted primer termination reagent; (F) measuring a signal produced by the label; and (G) repeating a cycle of steps (A) through (F) until a signal is detected indicating incorporation of the nucleotide terminator, wherein (H) prior to step (F), at least an aliquot of either (1 ) the different-sequence primers or (2) the tagged sample polynucleotides, are contacted with an addressable array of immobilized, different tag complements, and each different tag complement contains a sequence that is complementary to one of the tag segments, under conditions effective to hybridize the tag segments to corresponding tag complements on the support. In one embodiment, the contacting in step (H) is performed prior to step (A). In another embodiment, the contacting in step (H) is performed after step (A), and/or before any one of steps (B), (C), (D), (E), and (F). In yet another embodiment, steps (A) through (G) are performed on at least two replicate arrays, and one of the replicate aπays is subjected to at least one more cycle of steps (A) through (F) than is a second replicate aπay. In a third aspect, the foregoing and other objects of the invention are achieved by a kit useful for determining the number of repeat units in a repeat region of a target nucleic acid comprising a primer having a sequence complementary to a primer-complementary portion of a target nucleic acid; a first primer extension reagent; and a second primer extension reagent, wherein at least one of the first or second primer extension reagents includes an extendible nucleotide having a label attached thereto. In a prefeπed embodiment of the third aspect of the invention, the primer is attached to a solid support. In an additional preferred embodiment of the second aspect of the invention, the label is selected from the group consisting of fluorescent and chemiluminescent molecules. In another prefeπed embodiment of the second aspect of the invention, the label is attached to the extendible nucleotide through a cleavable linker. These and other objects, features, and advantages of the present invention will become better understood with reference to the following description, drawings, and appended claims. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic depiction of a target nucleic acid. FIGS. 2A-C show a first aspect of the method of the invention wherein an extendible nucleotide is labeled and the label is rendered undetectable subsequent to each discrete increment of primer extension. FIGS. 3A-B show a second aspect of the method of the invention wherein a nucleotide terminator is labeled. FIGS 4 and 5 show exemplary schemes for practicing the invention using a solid phase support containing an aπay of tag complements for obtaining sequence repeat information for a plurality of different samples in parallel, using tagged primers or tagged sample polynucleotides. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to the prefeπed embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the prefeπed embodiments, it will be understood that they are not intended to limit the invention to those embodiments. On the contrary, the invention is intended to cover alternatives, modifications, and equivalents, which may be included within the invention as defined by the appended claims. I. DEFINITIONS Unless stated otherwise, the following terms and phrases as used herein are intended to have the following meanings: "Nucleoside" refers to a compound consisting of a purine, deazapurine, or pyrimidine nucleoside base, e.g., adenine, guanine, cytosine, uracil, thymine, deazaadenine, deazaguanosine, and the like, linked to a pentose at the 1 ' position, including 2'-deoxy and 2'- hydroxyl forms (Stryer). The term "nucleotide" as used herein refers to a phosphate ester of a nucleoside, e.g., a triphosphate ester, wherein the most common site of esterification is the hydroxyl group attached at the C-5 position of the pentose. Many times in the present disclosure the term nucleoside will be intended to include both nucleosides and nucleotides. The terms nucleotide and nucleoside as used herein are intended to include synthetic analogs having modified nucleoside base moieties, modified sugar moieties, and/or modified phosphate ester moieties, e.g., as described elsewhere (Scheit; Eckstein). "Polynucleotide" or "oligonucleotide" refer to linear polymers of nucleotide monomers, including single, double and triple stranded deoxyribonucleotides, ribonucleotides. α-anomeric forms thereof, and the like. Usually the nucleoside monomers are linked by phosphodiester linkages, where as used herein, the term "phosphodiester linkage" refers to phosphodiester bonds or bonds including phosphate analogs thereof wherein the phosphorous atom is in the +5 oxidation state and one or more of the oxygen atoms is replaced with a non-oxygen moiety. Exemplary phosphate analogs include phosphorothioate, phosphorodithioate, phosphoroselenoate, phosphorodiselenoate, phosphoroanilothioate, phosphoranilidate, phosphoramidate, boronophosphates, and the like, including associated counterions, e.g., H , NH 4 , Na , and the like if such counterions are present. Alternatively, polynucleotides may comprise polymers of non-nucleotidic monomers, linked through phosphodiester linkages or other linkages, which are capable of forming sequence-specific hybrids with a target nucleic acid, e.g., peptide nucleic acid polymers (PNAs, e.g., see Knudsen, 1996). Polynucleotides typically range in size from a few monomeric units, e.g. 8-40, to several thousands of monomeric units. Whenever a polynucleotide is represented by a sequence of letters, such as "ATGCCTG," it will be understood that the nucleotides are in 5'->3' order from left to right and that "A" denotes deoxyadenosine, "C" denotes deoxycytidine, "G" denotes deoxyguanosine, and "T" denotes thymidine, unless otherwise noted. "Extendible nucleotide" means any nucleotide that when incorporated into a primer extension product during a primer extension reaction allows for the further extension of such primer extension product. Exemplary extendible nucleotides include 2'-deoxynucleotide triphosphates, e.g., 2'-deoxyuridine-5'-triphosphate, 2'-deoxyguanosine-5'-triphosphate, 2'- deoxy-7-deazadeoxyguanosine-5'-triphosphate, 2'-deoxyadenosine-5'-triphosphate, 2'- deoxythymidine-5'-triphosphate, and 2'-deoxycytidine-5'-triphosphate. Optionally, one or more of the extendible nucleotides includes a label. "Nucleotide terminator" means any nucleotide that when incorporated into a primer extension product prevents the further extension of such primer extension product. One requirement of a nucleotide terminator is that when the nucleotide terminator includes a ribofuranose sugar portion, the 3 '-position must not have a hydroxy group capable of being subsequently used by a polymerase to incorporate additional nucleotides. Alternatively, a ribofuranose analog could be used, such as arabinose. Exemplary nucleotide terminators include 2',3'-dideoxy-β-D-ribofuranosyl, β-D-arabinofuranosyl, 3'-deoxy-β-D- arabinofuranosyl. 3'-amino-2',3'-dideoxy-β-D-ribofuranosyl, and 2',3'-dideoxy-3'-fluoro-β- D-ribofuranosyl (Chidgeavadze, 1984, 1985). Nucleotide terminators also include reversible nucleotide terminators (Metzker), and 3'-deoxy substituents such as hydrogen, 3'-fluoro, 3'- amino, and 3'-azido, for example (Mikhailopulo et al., 1989; Krayevski et al, 1984; Chidgeavadze, 1986). "Polymerase" means an enzyme or other catalyst capable of catalyzing a reaction leading to a target-sequence dependent incorporation of a nucleotide onto a 3 '-end of a primer or primer extension product when such primer or primer extension product is annealed to a target nucleic acid. Exemplary polymerases include but are not limited to Pfu DNA polymerase, E.Coli Polymerase I, T-7 polymerase, reverse transcriptase, Taq DNA polymerase, and the like (Kornberg and Baker). "Label" means any moiety that, when attached to a nucleotide or polynucleotide of the invention, render such nucleotide or polynucleotide detectable using known detection means. Labels may be direct labels which themselves are detectable or indirect labels which are detectable in combination with other agents. Exemplary direct labels include but are not limited to fluorophores. chromophores, radioisotopes, spin-labels, chemiluminescent labels, and the like. Exemplary indirect labels include enzymes which catalyze a signal-producing event, and ligands such as an antigen or biotin which can bind specifically with high affinity to a detectable anti-ligand, such as a labeled antibody or avidin. "Primer extension reaction" means a reaction between a target-primer hybrid and a nucleotide which results in the addition of the nucleotide to an end of the primer, usually the 3 '-end, such that the added nucleotide is complementary to the coπesponding nucleotide of the target nucleic acid. "Pπmer-extension reagent" means a reagent including components necessary to effect a pπmer extension reaction Pπmer extension reagents typically include (1) a polymerase enzyme, (n) a buffer, and (in) one or more extendible nucleotides "Specific binding pair" refers to a pair of molecules that specifically bind to one another to form a binding complex Examples of specific binding pairs include, but are not limited to antibody-antigen (or hapten) pairs, gand-receptor pairs, enzyme-substrate pairs, biotin-avidin pairs, polynucleotides having complementary base pairs, and the like "Pπmer" is a polynucleotide capable of selectively annealing to a specified target sequence and thereafter serve as a point of initiation of a pπmer extension reaction wherein the pπmer is extended in the 3 '— > 5' a 5'— > 3' direction, typically the latter II MATERIALS USED IN THE METHOD OF THE INVENTION A Target Nucleic Acid The target nucleic acids for use with the invention may be deπved from any living or once living organisms, including but not limited to prokaryotes. eukaryotes, plants, animals, and viruses, as well as synthetic nucleic acids The target nucleic acids may ongmate from any of a wide vaπety of sample types, such as cell nuclei (e g , genomic DNA) and extranuclear nucleic acids e g , plasmids, mitrochondπal nucleic acids, and the like The target nucleic acids can include DNA or RNA, and are usually DNA Many methods are available for the isolation and puπfication of a target nucleic acid for use in the present invention The prefeπed puπfication method should provide target nucleic acid sufficiently free of protein to allow efficient pπmer extension and nucleic acid amplification Prefeπed puπfication methods include (I) organic extraction followed by ethanol precipitation, e g , using a phenol/chloroform organic reagent (Ausubel), preferably using an automated DNA extractor, e g , the Model 341 DNA Extractor available from PE Applied Biosystems (Foster City, CA), (n) solid phase adsorption methods (Walsh, Boom), and (in) salt-induced DNA precipitation methods (Miller), such methods being typically refeπed to as "salting-out" methods Optimally, each of the above puπfication methods is preceded by an enzyme digestion step to help eliminate protein from the sample, e.g., digestion with proteinase K, or other like proteases. To increase sensitivity, preferably the target nucleic acid is amplified prior to performing the method using a suitable nucleic acid amplification procedure. Such amplification may be linear or exponential. In a prefeπed embodiment, amplification of the target nucleic acid is accomplished using the polymerase chain reaction (PCR) (Mullis). Generally, the PCR consists of an initial denaturation step which separates the strands of a double stranded nucleic acid sample, followed by the repetition of (i) an annealing step which allows amplification primers to anneal specifically to positions flanking a target sequence; (ii) an extension step which extends the primers in a 5'— > 3' direction thereby forming an amplicon nucleic acid complementary to the target sequence, and (iii) a denaturation step which causes the separation of the amplicon and the target sequence. Each of the above steps may be conducted at a different temperature, where the temperature changes may be accomplished using a thermocycler (PE Applied Biosystems, Foster City, CA). The generalized structure of a target nucleic acid for use in the present invention is shown in FIG. 1 where the target nucleic acid 5 includes a 5 '-flanking portion 10 including a primer complementary portion 15, a 3'-flanking portion 25, and a repeat region 20 located between the 5 '-flanking portion and the and the 3 '-flanking portion. The repeat region 20 of the target nucleic acid comprises multiple repeat units (R) n 21 where R indicates a repeat unit and n designates the number of repeat units making up the repeat region. The repeat unit R may be any type of repeat motif, for example, but not limited to a microsatellite repeat (Webber and May; Smeets; Williamson), a minisatellite repeat (Jeffreys, Caskey), or an α- satellite repeat (Jabs). The repeat region may be made up of multiple types of repeat units or repeat units which are themselves polymorphic. B. Primer Primers for use in the present invention are designed to obtain a balance between specificity of primer annealing, i.e., the frequency with which an undesired target sequence participates in a primer extension reaction, and efficiency of primer extension, i.e., the extent to which a desired target sequence participates in the primer extension reaction. Specificity of primer annealing is generally controlled by the length of the primer and the temperature of the annealing reaction. Polynucleotides between about 18 and 24 bases are prefeπed because such polynucleotides tend to be very sequence specific when the annealing temperature is set within a few degrees of a primer melting temperature (Dieffenbach). To facilitate primer extension, a 3'-end of the primer includes an — OH group or other moiety which allows incorporation of a nucleotide onto the 3 '-end of the primer. There exist a number of computer programs to facilitate primer selection in different contexts (Osborne; Montpetit). In a prefeπed embodiment, the sequence of the primer is selected such that the primer anneals to the primer complementary portion of the 5 '-flanking portion of the target nucleic acid. Preferably, the primer anneals such that a 3 '-end of the primer is adjacent to a 5 '-end of a repeat region of a target nucleic acid. However, the primer may also anneal to a segment of the repeat region of the target nucleic acid so long as it is at least partially anchored to the 5'- flanking portion of the target. For embodiments of the invention which employ sample identifier tags and aπays of tag complements, the invention utilizes a plurality of extendable, different-sequence primers for detecting target sequences of interests. In one embodiment, the tagged primer includes a target binding segment, a tag segment, and an extendable primer end (5' or 3'). The target binding segment includes a polynucleotide sequence which is selected to bind to a selected target sequence. The tag segment contains a unique polynucleotide sequence that allows identification of the target binding segment to which the tag segment is attached. The tag segment can be directly attached to the distal end of the target binding segment, or is optionally linked to the tag segment by an intervening spacer group. In another embodiment, the tag segment is linked to an internal site within the target binding segment. Thus, the tag can be linked to an intersubunit linking group, or to a nucleotide base, within the target binding segment. Preferably, the tag is attached to an end of the target binding segment that is distal with respect to the extendable end of the primer. The sequence of each target binding segment is selected to hybridize to a selected complementary target which contains a potential polymorphism or mutation, preferably such that the 3'-end of the primer is adjacent to a 5 '-end of a repeat region of a target nucleic acid (for extension in the 5' to 3' direction). However, the primer may also anneal to a segment of the repeat region of the target nucleic acid so long as it is at least partially anchored to the 5'- flanking portion of the target. The length of the target binding segment in each tagged primer is selected to ensure specific hybridization of the primer to the desired target, without significant cross- hybridization to non-target nucleic acids in the sample. Also, to enhance primer specificity, it is prefeπed that the melting temperatures of the target binding segments are within a few degrees of each other. Preferably, the melting temperatures of the target binding segments fall within a ΔTm range (Tmax - Tmin) of 10°C or less, and preferably 5°C or less. This can be accomplished by suitable choice of binding segment lengths based on known methods for predicting primer melting temperatures (Breslauer, 1986; Rychlik, 1989 and 1990; Wetmur, 1991 ; Osborne. 1991 ; Montpetit, 1992) for example. As above, target binding segments between about 18 and 24 bases in length are prefeπed. The tag segment in each tagged primer is designed to contain a sequence that uniquely identifies the attached target binding segment. Thus, the tag sequences should be selected to minimize (1) internal, self-hybridization, (2) hybridization with other same-sequence tags, (3) hybridization with other, different sequence tag complements. (4) and hybridization with the sample polynucleotides. Also, it is prefeπed that each tag can specifically recognize and hybridize to its coπesponding tag complement under the same conditions for all tags in the primers. Tag sequences can be selected by any suitable method. For example, computer algorithms for selected non-crosshybridizing sets of tags are described in Brenner (1996) and Shoemaker (1997). Preferably, the tag sequences have strands that are within a preselected temperature range, as discussed above with respect to the extendable primers. Preferably, the melting temperatures of the target binding segments fall within a ΔTm range (Tmax - Tmin) of 10°C or less, and preferably within 5°C or less, as calculated using any of the methods above (e.g., Breslauer). Preferably, the tag segments are at least 12 bases in length to facilitate specific hybridization to coπesponding tag complements. Typically, tag segments are from 12 to 60 bases in length, and typically from 15 to 30 bases in length. Tags and tag complements may be single or double stranded, such that sequence specific hybridization forms either duplexes by Watson and Crick base-pairing, or triplexes by forward or reverse Hoogsteen bonding. In embodiments where specific hybridization occurs via triplex formation, coding of tag sequences follows the same principles as for duplex - forming tags; however, there are further constraints on the selection of word sequences. Generally, third strand association via Hoogsteen type of binding is most stable along homopyrimidine-homopurine tracks in a double stranded target. Usually, base triplets form in T-A*T or C-G*C motifs (where "-" indicates Watson-Crick pairing and "*" indicates Hoogsteen type of binding); however, other motifs are also possible. For example, Hoogsteen base pairing permits parallel and antiparallel orientations between the third strand (the Hoogsteen strand) and the purine-rich strand of the duplex to which the third strand binds, depending on conditions and the composition of the strands. There is extensive guidance in the literature for selecting appropriate sequences, orientation, conditions, nucleoside type (e.g. whether ribose or deoxyribose nucleosides are employed), base modifications (e.g. methylated cytosine, and the like in order to maximize, or otherwise regulate, triplex stability as desired in particular embodiments, e.g., Brenner (supra). More generally, conditions for annealing single-stranded or duplex tags to single-stranded or duplex sequence complements are well known, e.g. Brenner (supra), Ji et al. (1993), Cantor et al. (supra), Wetmur (1991), Breslauer et al. (1986), Schena (1995), and the like. Preferably, polynucleotides such as primers are synthesized conventionally on an automated DNA synthesizer, e.g., PE Applied Biosystems (Foster City, CA) model 392 or 394 DNNRNA Synthesizer, using standard chemistries, e.g., phosphoramidite chemistry (Beaucage). In an alternative method, primers can be isolated from a biological source. C. Solid Phase Supports In a prefeπed embodiment of the method of the invention, a target-primer hybrid is attached to a solid phase support during a separating step. Such attachment may be through either the target nucleic acid or the primer polynucleotide. Solid phase supports for use with the invention may have a wide variety of forms, including microparticles, beads, membranes, slides, plates, micromachined chips, and the like. In addition, solid phase supports of the invention may comprise a wide variety of compositions, including glass, plastic, silicon, alkanethiolate-derivatized gold, GaAs, copper, germanium, cellulose, low cross-linked and high cross-linked polystyrene, crosslinked polyacrylamide matrices, silica gel, polyamide, membranes such as nylon, polyvinylidine difluoride (PVDF), or poly-tetrafluoroethylene, and the like. Where attachment of the target-primer hybrid is through the primer, primers may be used with a solid phase support on which they were synthesized, or they may be separately synthesized and attached to a solid phase support for use during or before the separation step of the method. When primers are synthesized on and used with the same solid phase support, such support may comprise a variety of forms and include a variety of linking moieties. Such supports may comprise microparticles or planar aπays, or matrices of regions having substantially uniform populations of primers. A wide variety of microparticle synthesis supports may be used with the invention, including microparticles made of controlled pore glass (CPG), highly cross-linked polystyrene, acrylic copoiymers, cellulose, nylon, dextran, latex, polyacrolein, and the like. Microparticle supports further include commercially available nucleoside-derivatized CPG and polystyrene beads (e.g. available from Applied Biosystems, Foster City, CA); derivatized magnetic beads; polystyrene grafted with polyethylene glycol (e.g., TentaGel, Rapp Polymere, Tubingen Germany); and the like. Selection of the support characteristics, such as material, porosity, size, shape, and the like, and the type of linking moiety employed depends on the conditions under which the primers are used. For example, in the present invention, supports and linkers that minimize steric hindrance of the polymerase enzymes and that facilitate access to nucleotide substrate are prefeπed. Other important factors to be considered in selecting the most appropriate microparticle support include size uniformity, efficiency as a synthesis support, degree to which surface area known, and optical properties, e.g., clear smooth beads provide instrumentational advantages when handling large numbers of beads on a surface. As mentioned above, primers may also be synthesized on a single (or a few) solid phase supports to form an aπay of regions uniformly coated with primers. That is, within each region in such an aπay the same primer is synthesized. Techniques for synthesizing such aπays are disclosed elsewhere (Pease; Southern). When primers are separately synthesized, and subsequently attached to a solid phase support for use. the primer may be attached to the support through a covalent linkage or a non- covalent linkage. When the primer is attached to the solid support through a non-covalent linkage, the primer includes one member of specific binding pair, e.g., biotin, the other member of the pair being attached to the solid support, e.g., avidin. Several methods are available for covalently linking polynucleotides to solid supports, e.g., through reaction of a 5'- amino polynucleotide with an isothiocyanate-functionalized glass support (Guo). A wide range of exemplary linking moieties for attaching primers onto solid supports either covalently or non-covalently are disclosed elsewhere. (Pon; Webb; Barany; Damha; Beattie; Maskos and Southern). Where attachment of the primer-template hybrid is through the template nucleic acid, and the template nucleic acid is a PCR amplicon, the means for covalent or non-covalent attachment may be incorporated into a PCR primer used to effect the PCR. Thus, the PCR primer may contain a member of a specific binding pair, e.g., biotin, or a reactive moiety which can react with a functionalized solid support to form a covalent linkage, e.g., a 5'-amino group which reacts with a an isothiocyanate-functionalized glass support. As noted above, the invention also utilizes a set of tag complements which are complementary to coπesponding tag sequences in the tagged primers. The tag complements are provided as an addressable aπay, according to the design choice of the user. By "addressable aπay" is meant that the sequence of the target binding segment of each primer is known or can be determined from the position of hybridization of each primer on the aπay. Preferably, the tag complements are immobilized in discrete regions on a planar surface, such that each discrete region contains only tag complements having a particular sequence, and such that the sequence of the tag complement at each different discrete region is known. Conveniently, the tag complements are distributed as a periodic two-dimensional aπay of discrete tag complement regions which can be indexed via X and Y coordinates, or any equivalent thereof. Tag complements can be attached to appropriate solid phase support materials following the same considerations as for attachment of primers, discussed above. To reduce the amounts of assay reagents used for tag detection, and to facilitate the sequencing of large numbers of fragment sequences, the aπays are preferably formed as microaπays having tag complement region densities of greater than 100 regions/cm , 300 regions/cm 2 , 10 J regions/cm " , 3 x 10 3 regions/cm 2 , 10 4 regions/ cm , 10 " regions/cm " , or 10° regions/cm 2 . In addition, the number of different sequence tag complements in each aπay is preferably equal to or greater than 10, 20, 50, 100, 200, 500. 1000, 3000, 10,000, 30,000, 100,000, or 300.000. D. Labeled Nucleotides In the methods of the present invention, one or more extendible nucleotides and/or nucleotide terminators include a label. The label is attached to the nucleotide in such a way that the label does not substantially interfere with polymerase-mediated incorporation of the labeled nucleotide in a primer extension reaction. Many alternative methods are available for labeling nucleotides in a manner suitable for use with the present invention (Kricka). In a prefeπed class of labeling methods, a nucleoside base of the nucleotide is modified to include a label, i.e., the N-6 position of a purine base or the C-5 position of a pyrimidine base. A particularly prefeπed class of labeled nucleotides are the propargylethoxyamino nucleotides (Khan). In one prefeπed embodiment of the invention, a labeled extendible nucleotide is capable of being rendered undetectable, e.g., upon treatment with a suitable reagent or electromagnetic radiation. In this embodiment, the labeled extendible nucleotide may be rendered undetectable by either removing the label from the nucleotide or by destroying the signal-producing properties of the label. Several methods are available for attaching a label to an extendible nucleotide such that the label may be easily removed. Exemplary cleavable linkers for linking a label to a nucleotide include but are not limited to (N-[4-(p-azidosalicylamido)butyl]-3'-[2'- pyridyldithiojpropionamide (APDP), (bis[2-(succinimidooxycarbonyloxyl)ethyl]sulfone (BSOCOES), disuccininimdyl tartarate (DST), and ethylene glycobis-fsuccinimidylsuccinate] (EGS), and the like (Pierce Catalog). Prefeπed labels whose signal producing properties may be destroyed upon treatment with a suitable reagent or electromagnetic radiation include fluorescent labels whose fluorescent properties may be destroyed by photodestruction through exposure to high intensity light or by chemical degradation through treatment with oxidizing chemicals, e.g., oxygen, sodium hypochlorite, permanginate and the like. Alternative prefeπed classes of labels include enzymes, which may be rendered undetectable by reaction w ith an lπeversable enzyme inhibitor or by denaturation, and chemiluminescent labels which can undergo only a single light-producing transformation and are thus autodestructing E First and Second Pnmer-Extension Reagents The present invention includes first and second pπmer extension reagents that, when used according to methods of the invention, enable the extension of a pπmer to proceed in discrete increments of single repeat units, 1 e , the pπmer is extended only by one repeat unit per discrete pnmer extension reaction cycle The first pπmer extension reagent of the invention includes a set of extendible nucleotides which allow a pπmer extension reaction to proceed only to the extent that a pπmer is extended by an amount less than a full repeat unit Thus, depending on the particular sequence of the repeat unit, the first pπmer extension reagent may include a vaπety of possible extendible nucleotide combinations For example, if the sequence of the repeat unit is AGCT, the first pπmer extension reagent could include extendible nucleotides T (complementary to A), T and C (complementary to A and G), or T and C and G (complementary to A and G and C) However, to prevent uncontrolled continuous pπmer extension, the first pπmer extension reagent should not contain extendible nucleotides T, C, G and A together In certain situations, it is desirable to sub-divide the first primer extension reagent into separate sub-reagents such that each sub-reagent includes extendible nucleotides sufficient to allow extension of a primer only to the extent that a sub-portion of the repeat sequence is formed For example, if the repeat unit is ATGCCGT, one sub reagent of the first pπmer extension reagent could include extendible nucleotides T, A, and C, while another sub-reagent of the first pπmer extension reagent could include extendible nucleotides G and C The second pπmer extension reagent of the invention includes a set of extendible nucleotides w hich allow a pπmer extension reaction to proceed only to the extent that the portion of a repeat unit not synthesized by the first pπmer extension reagent is synthesized Thus, depending on the particular sequence of the repeat unit and the composition of the first pπmer reagent, the second pπmer extension reagent may include a vaπety of possible nucleotide combinations Continuing the example discussed above, if the sequence of the repeat unit is AGCT and the first primer extension reagent includes extendible nucleotides T and C, the second primer extension reagent may include extendible nucleotides G and A. F. Primer Termination Reagent The present invention includes a primer termination reagent for causing the termination of primer extension such that once a primer extension product has reacted with the primer termination reagent, no further primer extension may be accomplished. The primer termination reagent of the invention includes one or more nucleotide terminators, and optionally, a set of extendible nucleotides which allow a primer extension reaction to proceed only to the extent that a primer is extended by an amount less than a full repeat unit in a primer extension reaction. Thus, depending on the particular sequence of the repeat unit, the sequence of the 3 '-flanking portion of the target nucleic acid, and the composition of the first and second primer extension reagents, the primer termination reagent may include a variety of possible extendible nucleotide and nucleotide terminator combinations. For example, if the sequence of the repeat unit is AGCT, and the first nucleotide of the 3 '-flanking portion is G, and the first and second primer extension reagents include the extendible nucleotides T, C, G and A, the primer termination reagent would include only the nucleotide terminator C, such nucleotide terminator being complementary to the G located in the 3'-flanking portion. Alternatively, if the first and second primer extension reagents only include the extendible nucleotides T and C, the primer termination reagent would include extendible nucleotides G and A and nucleotide terminator C. In certain aspects of the present invention, the primer termination reagent includes a labeled nucleotide terminator. The labeling of the nucleotide terminator is accomplished essentially as described above in Section D. III. THE METHOD A prefeπed embodiment of a first aspect of the method of the invention is schematically depicted in FIGS. 2A-C. In the figure, the method is applied to a target nucleic acid 5 having a repeat region 20 made up of two copies of a two-base repeat having the sequence "AC" and a 3 '-flanking portion 25 having a G nucleotide abutting the repeat region. In this prefeπed embodiment of the first aspect, a primer 200 is annealed to a primer- complementary portion 15 of the target nucleic acid 5 thereby forming a target-primer hybrid. The target-pπmer hybπd is then reacted with a first pπmer-extension reagent including a labeled extendible nucleotide T, resulting in the incorporation of the labeled T nucleotide into the 3'-end of a pπmer extension product 210 Following reaction with the first pπmer extension reagent, the first pπmer extension reagent is separated from the target-pπmer hybπd and the target-pπmer hybπd is reacted with a second pπmer-extension reagent including an extendible G nucleotide, resulting in the addition of the G nucleotide into a 3 '-end of the pπmer extension product 215 Next the unreacted second pπmer extension reagent is separated from the target-pπmer hybπd and a measurement is performed to determine the amount of labeled extendible nucleotide incorporated into the pπmer extension product As indicated by the histogram in the figure, at this point m the process, a large signal is detected, indicating the presence of the incorporated labeled T nucleotide Finally, in order to prepare the target-pπmer hybrid for a subsequent discrete pπmer extension reaction cycle, the label attached to the incorporated extendible nucleotide is rendered undetectable In the example the above descπbed process steps are repeated two more times as shown in FIGS 2 B and 2C In the third cycle shown m FIG 2C, the intensity of the measured signal is substantially reduced as compared to the signal intensities seen in the first two cycles because the labeled extendible nucleotide T can not be incorporated into the pπmer extension product at this point Thus, this reduction in the measured signal seen in the third cycle indicates that the repeat region only contains two copies of the AC repeat unit A prefeπed embodiment of a second aspect of the method of the invention is shown in FIGS 3A-B As before, the method is applied to a target nucleic acid having a repeat region made up of two copies of a two-base repeat having the sequence "AC" and a 3 '-flanking portion having a G nucleotide abutting the repeat region In this prefeπed embodiment of the second aspect, as before, a pnmer 200 is annealed to a pπmer-complementary portion 15 of a target nucleic acid 5 thereby forming a target-pπmer hybπd The target-pπmer hybπd is then reacted with a first pπmer-extension reagent including an unlabeled extendible nucleotide T, resulting in the incorporation of the unlabeled T nucleotide into the 3 '-end of a pπmer extension product 310 Following reaction with the first pπmer extension reagent, the first pπmer extension reagent is separated from the target-pπmer hybπd and the target-pπmer hybπd is reacted with a second pπmer-extension reagent, including an extendible G nucleotide, and a pπmer termination reagent including a labeled C nucleotide terminator, resulting in the addition of only the G extendible nucleotide into the 3 '-end of the pπmer extension product 315 Next the unreacted second pπmer extension reagent and pπmer termination reagent are separated from the target-primer hybrid and a measurement is performed to determine the amount of labeled nucleotide terminator incorporated into the primer extension product. As indicated in the figure, at this point in the process, no signal is detected, indicating that the labeled nucleotide terminator is not incorporated into the primer extension product during this cycle of the process. In FIG. 3B. the above described process step is repeated. In this second cycle shown in FIG. 3B, the intensity of the measured signal is substantially increased as compared to the nominally zero signal intensity seen in the first step of the process because during the second step, the labeled nucleotide C terminator is incorporated into the primer extension product. The following discussion provides a more detailed description of the above-described method steps of the first and second aspects of the invention. A. Primer Annealing The annealing reaction is performed under conditions which are stringent enough to guarantee sequence specificity yet sufficiently permissive to allow formation of stable hybrids at an acceptable rate. The temperature and length of time required for primer annealing depend upon several factors including the base composition, length and concentration of the primer, and the nature of the solvent used, e.g., the concentration of cosolvents such as DMSO, formamide. or glycerol, and counter ions such as magnesium. Typically, hybridization with synthetic polynucleotides is carried out at a temperature that is approximately 5 to 10°C below the melting temperature of the target-primer hybrid in the annealing solvent. Preferably, the annealing temperature is in the range of 55 to 75°C and the primer concentration is approximately 0.2 μM. Under these prefeπed conditions, the annealing reaction will be complete in only a few seconds. B. Primer Extension Reaction The time required to effect a primer extension reaction depends upon the length and concentration of the target sequence and upon the reaction temperature. Estimates for the rate of nucleotide addition under typical conditions vary from 35 to 100 nucleotides per second depending upon the buffer, pH, salt concentration, polymerase enzyme, and the nature of the DNA template. In order to achieve a pπmer extension reaction which proceeds in discrete increments of single repeat units, according to the method of the invention, the pπmer extension reaction is divided into two independent steps: a first pπmer extension reaction and a second pπmer extension reaction In the first pπmer extension reaction, the pπmer is extend by an amount less than the length of a single repeat unit, where control of the extent of pπmer extension is effected by the composition of a first pπmer extension reagent. In the second pπmer extension reaction, the pπmer is extended by an amount such that, in combination with the first pπmer extension reaction, the pπmer is extended by an amount equal to the length of a single repeat unit. According to the first aspect of the method of the invention, one of the first or second pπmer extension reagents includes a labeled extendible nucleotide Also according to the first aspect of the method of the invention, in a vaπant of the above-descπbed two-step discrete pπmer extension reaction, the second pπmer extension reagent includes a pπmer teπnination reagent. Thus, if after a second pnmer extension reaction the pnmer has been extended to the end of the repeat region of the target nucleic acid, a nucleotide terminator will be incorporated into the pnmer extension product thus prohibiting any further extension of that pnmer extension product This is advantageous because it will remove the possibility that any spurious pnmer extension beyond the repeat region of the target nucleic acid will take place This embodiment of the invention is particularly prefeπed where two alleles of a repeat sequence are being investigated simultaneously According to the second aspect of the invention, a pnmer termination reagent including a labeled nucleotide terminator is included in the second pnmer extension reaction Preferably, the labeled nucleotide terminator is selected to be complementary to the nucleotide at the 3'- end of the S'-flankmg portion of the target nucleic acid which abuts the repeat region of such target nucleic acid. C Separation of Pnmer and Pnmer Extension Reagents Between the first and second pnmer extension reactions, a separation step is performed to prevent the mixing of first and second pnmer extension reagents and thereby prevent uncontrolled pnmer extension. The means used to effect the separation step may be any means capable of separating the target-pπmer hybrid from the first and/or second pπmer extension reagents. Exemplary separation methods include but are not limited to HPLC, electrophoresis, liquid-liquid extraction, solid-liquid extraction, adsorption, and the like. In a prefeπed embodiment, the target-primer hybrid is attached to a solid support during the separation step such that the primer extension reagents may be separated from the target-primer hybrid simply by washing the solid support. According to this embodiment, the primer may be attached to the solid support before or after performing the first primer extension reaction. The washing conditions are such that the target-primer hybrid is not substantially disrupted and nonspecific adsorption of the primer extension reagents is minimized. D. Measuring a Signal Subsequent to either the first or second primer extension reaction, a detection step is performed wherein the amount of intact label which has been incorporated into a primer extension product is determined. Any detection method may be used which is suitable to the type of label employed. Thus, possible detection methods include radioactive detection, optical absorbance detection, e.g., UV-visible absorbance detection, optical emission detection, e.g., fluorescence or chemiluminescence. The measuring step can take place at various points in the process depending upon the aspect of the invention being practiced and the composition of the first and second primer extension reagents. In the first aspect, preferably the measuring step takes place after the primer extension reagent including the labeled extendible nucleotide has been reacted with and separated from the target-primer hybrid. In the second aspect, the measuring step takes place after the primer termination reagent including the labeled nucleotide terminator has been reacted with and separated from the target-primer hybrid If the target-primer hybrid(s) are immobilized on a solid support for analysis, extended primers can be detected in an addressable aπay by scanning all or portions of each aπay simultaneously or serially, depending on the scanning method used. For fluorescence labeling, selected regions on an aπay may be serially scanned one-by-one or row-by-row using a fluorescence microscope apparatus, such as described in Fodor (1995) and Mathies et al. (1992). Hybridization patterns may also be scanned using a CCD camera (e.g., Model TE/CCD512SF, Princeton Instruments, Trenton, NJ) with suitable optics (Ploem, 1993), such as descπbed in Yershov et al (1996), or may be imaged by TV monitonng (Khrapko, 1991 ) For radioactive signals (e g , ~ P), a phosphonmager device can be used (Johnston et al , 1990, Drmanac et al . 1992, 1993) Other commercial suppliers of imaging instruments include General Scanning Inc , (Watertown, MA, www genscan com), Genix Technologies (Waterloo, Ontaπo, Canada, www confocal com), and Applied Precision Inc Such detection methods are particularly useful to achiev e simultaneous scanning of multiple tag complement regions E Rendeπng A Label Undetectable According to the first aspect of the method of the invention, once a signal from a label is detected, pπor to performing a subsequent cycle of discrete pnmer extension, the label is rendered undetectable In one prefeπed embodiment, the label is rendered undetectable by cleaving the label off of the labeled extendible nucleotide incorporated in the pnmer extension product The method used for cleaving the label from the nucleotide depends on the type of cleavable linkage used to link the label and the nucleotide See above Exemplary cleavage reagents include thiol, base, penodate, hydroxylamme and the like In one prefeπed method, the label is attached to a base-portion of a uracil nucleotide and subsequent to detection the label is cleaved off of the labeled extendible nucleotide by treatment with the enzyme uracil N- glycosylase (UNG) In a second prefeπed embodiment, the label is rendered undetectable by destroying the signal-producing properties of the label itself Depending on the type of label used, there are several methods which be employed for destroying the signal-producing properties of the label For example, if the label is a fluorescent dye, the label may be rendered undetectable by photobleaching the dye using an intense light source in an oxygen-nch environment If the label is an enzvme, the label may be rendered undetectable by reacting the label with an lπeversible enzyme inhibitor which renders the enzyme incapable of catalyzing a signal producing reaction If the label is a chemiluminescent agent which can undergo only a single light-producing transformation, the label is autodistructing and thus does not require a separate reaction to render the label undetectable (Bronstein) F Tag Complement Aπays As noted above, the invention also includes embodiments that utilize pπmers or sample polynucleotides that contain tag sequences that uniquely identify the attached pπmers or sample polynucleotides, which can be immobilized on addressable aπays for analysis of multiple sample polynucleotides in parallel A plurality of different-sequence pπmers are contacted with a polynucleotide sample under conditions effective for the primers to anneal to pnmer-complementary regions in one or more target polynucleotides, to form one or more target-primer hybnd(s), wherein either (1) each different-sequence pπmer contains (I) a target binding segment and (n) a tag segment having a nucleotide sequence that uniquely identifies the target binding segment, or (2) one or more polynucleotides in the sample are tagged polynucleotides that contain a tag segment having a nucleotide sequence that uniquely identifies the attached polynucleotide One or more pπmer extension cycles are performed as descπbed abov e and the appearance or loss of signal is determined at the appropπate times to measure repeat lengths Depending on the preferences of the user, the tagged primers or tagged sample polynucleotides can be contacted with an addressable aπay of immobilized, different-sequence tag complements which each contain a sequence that is complementary to one of the tag segments, under conditions effective to hybndize the tag segments to coπesponding tag complements on the support By way of illustration, embodiments using either tagged pπmers or tagged sample polynucleotides are shown in Figs 4 and 5, respectively, after the tag segments have been hybndized to a solid phase support, and a target-pnmer hybπd has been formed to yield tertiary polynucleotide complexes Fig 4 shows a tertiary complex wherein a tag complement ohgonucleotide 410 immobilized on a solid phase support 400 is specifically hybndized to a tagged pπmer 420 Tagged pnmer 420 contains a target binding segment 422 and a tag segment 424 The vertical lines between the tag complement ohgonucleotide 410 and tag segment 424 indicate complementary base-paiπng Also shown is a sample poKnucleotide 440 compπsing a sequence 442 which hybπdizes to target binding segment 422, and which, in the embodiment shown, terminates at a nucleotide immediately adjacent to a sample repeat region 444 Sample polynucleotide 440 optionally includes flanking sequences 446 and 448 which do not hybndize to the tagged pnmer Once polynucleotide 440 and tagged pπmer 420 have formed a target- pnmer hybnd, the hybnd can be treated as discussed in preceding sections to extend the extendable end of the pπmer into the repeat region of the sample polynucleotide Extension cycles are repeated until the end of the repeat region has been reached, or the desired number of repeats have been counted Fig 5 shows an alternative tertiary complex wherein a tag complement ohgonucleotide 510 immobilized on a solid phase support 500 is specifically hybndized to a tagged sample polynucleotide 520 Polynucleotide 520 includes a tag segment 524 which is hybndized to ohgonucleotide 510, and which is connected to a sample sequence 522 Sequence 522 includes a repeat region 528 which is flanked on either side by first and second sample segments 526 and 530 An extendable pπmer 540 is annealed to one of sample segments 526 and 530 to form a target-pnmer hybπd such that the extendable end is adjacent to (or protrudes into) the sample repeat region 528 The hybπd can be treated as above to extend the extendable end of the pπmer into the repeat region of the sample polynucleotide, until the end of the repeat region has been reached Tagged sample polynucleotides can be formed by any suitable method Conveniently, tagged samples can be formed by PCR amplification using pnmer pairs compnsing first and second target-specific pπmers which flank each sample sequence of interest, wherein one of the probes contains an identifier tag segment In one embodiment, the tagged pnmer is constructed so that the tag segment is not copied dunng amplification, either due to the presence of non-standard mternucleoside linkages (e g , PNA linkages), or due to the presence of a non-polynucleotide linker region which separates the tag segment from the target-binding segment, as can be prepared by standard synthetic methods After the tag segment is hybndized to a tag complement on an addressable aπay, the non-tagged strand can be removed from the tagged strand, e g , by elevated temperature and/or reducing salt concentration to destabilize DNA-DNA duplex structure, particularly if either the tag melts from the tag complement at a higher temperature than the melting temperature between the target-binding segment of the pnmer and the sample polynucleotide Many other approaches for obtaining a selected strand from duplex nucleic acids are known in the art, and include for example, (1 ) the use of a biotinylated primer in PCR to enable capture and separation of the non-tagged strand from the tagged strand under denatunng conditions, (2) the use of a PCR pnmer (non-tagged) that contains a short RNA segment which can be degraded with RNAse after PCR is complete, followed by destruction of the second RNAse degraded strand with an enzyme that has an exonuclease activity (e.g., an appropriate DNAse), or an asymmetric PCR method which favors amplification of the tagged primer strand. Although Figs. 4 and 5 show particular embodiments, it will be apparent that other variations can be used in accordance with the methods of the invention. For example, with reference to Fig. 5, extendable primer 540 can be designed to bind instead to sample segment 526, between the tag segment and the repeat region, such that the extendable end of the primer is again adjacent to sample repeat region 528. The tagged primers or tagged sample polynucleotides can be hybridized to a tag complement aπay at any appropriate time during the primer extension process. For example, the tagged primers of sample polynucleotides can be immobilized on the support prior to the first contacting step in which the different-sequence primers and sample polynucleotides are annealed to form target-primer hybrids. Alternatively, immobilization can be performed after the first primer extension reaction, or after the second primer extension reaction. In the latter case, a small aliquot of the extension reaction mixture can be removed after each extension cycle and hybridized to replicate aπays, such that each replicate aπay provides counting information after each cycle. IV. KITS FOR PRACTICING THE METHOD The present invention includes kits for caπying out the various aspects and embodiments of the methods of the invention. In a first aspect, kits of the invention include a primer, a first primer extension reagent, and a second primer extension reagent, wherein at least one of the first or second primer extension reagents includes an extendible nucleotide having a label attached thereto. Preferably, the label attached to the extendible nucleotide may be rendered undetectable following a treatment step effective to cleave the label from a primer extension product and/or destroy the signal producing properties of the label. Preferably, the primer is bound to a solid support, or, is capable of being bound to a solid support through a specific binding pair or through a covalent linking moiety. In another prefeπed embodiment, this aspect of the invention includes a solid-phase support for attachment of a target-primer hybrid through the primer or the target nucleic acid. Optionally, the kits of this first aspect of the invention include a primer termination reagent. In a second aspect, the kits of the invention include a pnmer, a first pnmer extension reagent, a second pπmer extension reagent, and a pπmer termination reagent, wherein the pπmer teπmnation reagent includes a nucleotide terminator having a label attached thereto The second pπmer extension reagent and the pπmer termination reagent may be packaged either separately or together as a mixture Preferably, the pπmer is bound to a solid support, or, is capable of being bound to a solid support through a specific binding pair or through a covalent linking moiety In another prefeπed embodiment, this aspect of the invention includes a solid-phase support for attachment of a target-pnmer hybnd through the pnmer or the target nucleic acid In another embodiment, the kits can include (A) a plurality of different-sequence pπmers, each containing (1) a target binding segment and (n) a tag segment having a nucleotide sequence that uniquely identifies the target binding segment, a first pπmer extension reagent, and a second pnmer extension reagent, wherein at least one of the first or second pπmer extension reagents includes an extendible nucleotide having a label attached thereto, and/or (B) an addressable aπay of immobilized, different tag complements, wherein each different tag complement contains a sequence that is complementary to one of the pnmer tag segments, under conditions effectiv e to hybndize the tag segments to coπesponding tag complements on the support The invention mav be further understood in light of the following examples, which are not intended to limit the invention Example The following study illustrates exemplary sample hybndization conditions and extension cycles for counting repeats in four different sample sequences using an aπay of target-specific capture o gonucleotides immobilized on a solid support A Support Glass microscope slides were immersed in 1 N HN0 3 for 1 to 2 hours, followed by nnsmg with deionized water Optionally, slides were soaked overnight m 5% HCl to improve long-term stability of subsequent functionahzization Slides were then sonicated sequentially in the following three solvents for 10 minutes each hexane, acetone, and ethanol, followed by drying in air A solution of 2% (v/v) aminopropyltriethoxy silane (0.8 mL in 40 mL) in 95% acetone:5% water was prepared in a disposable plastic Falcon tube. This amount of solution was usually sufficient to coat at least three slides. The solution was allowed to stand for 5 to 20 minutes to hydrolyze the ethoxy groups to hydroxyl groups. Air-dried slides were dipped in the solution for 2 minutes each, and then rinsed by dipping in three or more successive acetone baths. The slides were cured in 100°C oven for 45 minutes. Cured slides were treated for 2 hours with a 0.2% solution of 1 ,4-phenylene diisothiocyanate (PDITC) in 10% pyridine:dimethyl formamide, followed by washes in baths of methanol and acetone, and air- drying. The slides may be stored under vacuum with a dessicant at 4°C. Stacking the slides also helps preserve the functionalization and keeps the functinalized surfaces free from particulates. (In an alternative embodiment, instead of 1 ,4-phenylene diisothiocyanate, the cured slides can be treated with a 1 mM solution of EMCS (N-(e-maleimido caproxyl) succinimide in methanokdimethylsulfoxide (80:20) for 2 hours.) B. Immobilization of Capture Oligos. Synthetic capture ohgonucleotides were prepared by standard phosphoramidite synthetic methods. The capture ohgonucleotides had the following general structure: 5'-amino-[(PEO) 2 ]-[capture oligo (24 nt)], where PEO = -O- (CH 2 CH 2 0) 6 added via DMT-0-(CH 2 CH 2 O) 6 -phosphoramidite. The capture ohgonucleotides had polynucleotide sequences complementary to the double-underlined sequences in the four sample sequences shown in section C below. The capture oligos were spotted in a rectangular aπay pattern onto slides prepared as above using an ASYMTEC robotic loader (Asymtec, Carlsbad, CA, Model No. C-708), with ohgonucleotide concentrations of about 50 μM (in 100 mM Na 2 C0 3 , pH 9.0) and spotting volumes of about 0.2 to 1 nL. The pattern included a separate set of 8 spots (2 x 4 rows) for each different capture oligo to provide redundancy. The spots had diameters of about 200 micrometers and were spaced apart by 320 micrometers center-to-center. The robotic loader included a slide holder to hold the slides during ohgonucleotide deposition, and a blotting area for cleaning dispenser tips. After spotting, the capture oligos were fixed onto the slides by incubating the slides in a humidity chamber at room temperature for 60 minutes, followed by soaking in 1% NH 4 OH (aq) for 20 minutes and in 0.2% casein in TBS (50 mM Tris. 150 mM NaCl. pH 8) for 20 minutes. The slides were washed in deionized water and stored at 4°C. C. Sample Hybridization and Analysis. Four different sample oligos were prepared to represent four different microsatellite alleles. In brief, the sample polynucleotides included the following sequences, where single-underlining indicates repeat regions, and double underlining indicates segments for hybridizing to complementary capture oligos on the support: Sample Oligo 2260-2G (SEQ ID NO:l) 5'-GTCAGGACACAAAGTGATTTGATGTAGATTTTGA-3' Sample Oligo 141 1-1 G (SEQ ID NO:2) 5 '-GTCAGG ACTGATAA AGTGTAAAAGTGTATGAT-3 ' Sample Oligo 3149-2T fSEO ID NO:3) 5'-GTCATTACACTGTATGATAAAGGATTTTGATTGA-3' Sample Oligo 4223-4T (SEQ ID NO:4) 5'-GTCATTACACACACGTATTGATTTGATTGATTGAGATT-3' A mixture of the four sample oligos (0.33 μM each in IX PCR buffer containing 1.5 mM MgCl 2 , 50 mM KC1, 1.5 mM MgCl 2 , 10 mM Tris-HCl, pH 8.3, and 0.001% (w/v) gelatin) was placed over the capture oligos on the aπay and allowed to incubate for 1 hour at room temperature. After the incubation, unbound oligos were removed with IX TE (10 mM Tris, pH 8, 1 mM EDTA) containing 50 mM NaCl. Replicate slides were subjected to different numbers of extension cycles (one, two, three or four cycles). Each extension cycle (also called a "reagent cycle") involved the following steps: 1) Incubate slide for 4 minutes at 37°C in an extension mixture containing: 500 μM dGTP 20 μM Big Dye R6G ddATP (Perkin-Elmer, Foster City, CA, Emax = 560 nm) 20 μM Big Dye dRox ddCTP (Perkin-Elmer, Foster City, CA, Emax = 615 nm) 1 U/μL AMPLITAQ FS (plus 0 125 U/μL pyrophosphatase) IX buffer (80 mM Tπs, pH 9 0, 2 5 mM MgCl 2 ) 2) Rinse slide with IX TE containing 50 mM NaCl. then wash 3 times by immersion in same solution 3) Incubate slide for 4 minutes at 37°C in an extension mixture containing 500 μM dATP 20 μM Big Dye R6G ddATP (Perkin-Elmer, Foster City, CA, Emax = 560 nm) 20 μM Big Dye dRox ddCTP (Perkm-Elmer, Foster City, CA, Emax = 615 nm) 1 U/μL AMPLITAQ FS (plus 0 125 U/μL pyrophosphatase) IX buffer (80 mM Tπs, pH 9 0, 2 5 mM MgCl 2 ) 4) Rinse slide with IX TE containing 50 mM NaCl, then wash 3 times by immersion in same solution After 4 extension cycles had been completed (each cycle included steps 1 through 4), each slide was overlaid with a v iewing buffer containing 50% (w v) urea and IX TBE (0 09 M Tns-borate, 2 mM EDTA, pH ~ 8 3) to provide an alkaline pH environment to enhance the fluorescence emissions of the dye labels The slides were each viewed using an imaging device set for fluorescence emission detection at 560 nm and 610 nm The device was an imaging fiuonmeter that produces a two- dimensional image aπay of emission intensities (electronic image) Each point in the image aπay coπesponds to a physical location on the sample slide The excitation source is a 40 mW argon ion laser The excitation wavelength can be selected from either of two natural laser lines (488 nm and/or 514 nm) The selection of laser wavelengths is accomplished by passing the beam which contains light of both wavelengths through one of three filters The selected filter will pass either 488 nm only, 514 nm only, or both 488 and 514 nm The filters are mounted on a moter dnven platfoπn so that the selection can be performed under computer control The beam containing either or both wavelengths passes through a classical beam expander The colhmated expanded beam is then directed into a commercially available assembly (Scanlab, Puchhein, Geπnany, Model SKI 020) containing two orthogonal "galvo miπors" This is a mechanical device designed to rotate each of the two minors quickly and precisely over a small angle The axes of rotation are orthoganal and independent so that the beam can be rastered over a rectangular pattern, also under computer control The scanned beam is focused by an f-theta lens which forms a scanned laser spot at one focal length The spot size is 20 μm diameter Typically, it is stepped over an area of 20 mm x 20 mm in increments of 20 μm, thereby forming a 1000 x 1000 aπay of pixels The focused laser spot is formed after passing through a dichroic beam splitter which is designed to reflected the laser excitation wavelengths but transmit the fluorophore emission As the laser beam steps across a fluorescent region, the fraction of fluorescence radiation withm the solid angle of collection of the emission optics is transmitted by the dichroic miπor and is colhmated by a lens having a 150 mm focal length The colhmated beam is then filtered by a long pass interference filter which further rejects laser light. A second lens of 75 mm focal length focuses the beam onto the photocathode of a red sensitive photomultipher tube (PMT) The PMT photocathode does not need to be carefully adjusted since the focus is not cntical. A filter wheel in front of the PMT allows only a small wavelength band (10 nm) to reach the detector The filter wheel can be adjusted to one of six positions to allow for emission of multiple fluorophores to be discnminated The electncal output of the PMT is proportional to the intensity of the light reaching the photocathode The computer system is capable of stonng the signals at each of an aπay of miπor positions to form the electronic image The results were as expected Specifically, for oligo 1 (SEQ ID NO 1), no signal was observed until completion of the second cycle, after which tw o GT dimers and a fluorescence- labeled ddC terminator had been appended to the immobilized capture oligo, which also served as the extendable pnmer Similarly, for oligos 2 through 4 (SEQ ID NO 2 through 4), the appropπate fluorescent signal was observed after 1, 2 and 4 extension cycles, respectively All publications and patent applications refered to in this disclosure are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference Although only a few embodiments have been described in detail above, those having ordinary skill in the molecular biology art will clearly understand that many modifications are possible in the prefeπed embodiments without departing from the teachings thereof. Accordingly, all such modifications are intended to be encompassed within the scope of the following claims.

Claims: WE CLAIM: 1. A method for determining the number of repeat units in a repeat region of a target nucleic acid comprising the steps of: (A) contacting a plurality of different-sequence primers with a polynucleotide sample under conditions effective for said primers to anneal to primer-complementary regions in one or more target polynucleotides, to form one or more target-primer hybrid(s), wherein either (1) each different-sequence primer contains (i) a target binding segment and (ii) a tag segment having a nucleotide sequence that uniquely identifies the target binding segment, or (2) one or more polynucleotides in the sample are tagged polynucleotides that contain a tag segment having a nucleotide sequence that uniquely identifies the attached polynucleotide, (B) performing a first primer extension reaction on said hybrid(s) using a first primer extension reagent; (C) separating the target-primer hybrid(s) and unreacted first primer extension reagent; (D) performing a second primer extension reaction on said hybrid(s) using a second primer extension reagent, wherein at least one of the first or second primer extension reagents includes an extendible nucleotide having a label attached thereto; (E) separating the target-primer hybrid(s) from unreacted second primer extension reagent; (F) measuring a signal produced by the label; (G) treating the label so as to render the label undetectable; and (H) repeating a cycle of steps (A) through (G) until the signal detected in the target- primer hybrid(s) is substantially less than a signal detected in a previous cycle, wherein (I) prior to step (F), at least an aliquot of either (1 ) said different-sequence primers or (2) said tagged sample polynucleotides are contacted with an addressable aπay of immobilized, different tag complements, and each different tag complement contains a sequence that is complementary to one of said tag segments, under conditions effective to hybridize the tag segments to coπesponding tag complements on the support. 2. The method of claim 1 wherein step (D) further includes reacting the target-primer hybrid(s) with a primer termination reagent. 3 The method of claim 1 wherein each different-sequence pnmer contains (l) a target binding segment and (n) a tag segment having a nucleotide sequence that uniquely identifies the target binding segment 4. The method of claim 3 wherein the contacting in step (I) is performed pnor to step (A). 5. The method of claim 3 wherein the contacting in step (I) is performed after step (A). 6. The method of claim 3 wherein said method is performed on at least two replicate aπays, and one of the replicate aπays is subjected to at least one more cycle of steps (A) through (G) than is a second replicate aπay. 7. The method of claim 1 wherein one or more polynucleotides in the sample are tagged polynucleotides that contain a tag segment having a nucleotide sequence that uniquely identifies the attached polynucleotide. 8. The method of claim 7 wherein the contacting in step (I) is performed pnor to step (A). 9. The method of claim 7 wherein the contacting m step (I) is perfonned after step (A) 10. The method of claim 7 wherein said method is performed on at least two replicate aπays, and one of the replicate aπays is subjected to at least one more cycle of steps (A) through (G) than is a second replicate aπay 1 1. The method of claim 1 wherein the label is selected from the group consisting of fluorescent and chemiluminescent molecules. 12 The method of claim 1 wherein the label is attached to the extendible nucleotide through a cleavable linker 13. The method of claim 1 wherein the target nucleic acid is amplified pnor to analysis. 14. The method of claim 13 wherein amplification is achieved using a PCR. 15. The method of claim 1 wherein the step of treating the label so as to render the label undetectable includes cleaving the label from the labeled extendible nucleotide. 16. The method of claim 1 wherein the step of treating the label so as to render the label undetectable includes destroying a signal-producing property of the label. 17. A method for determining the number of repeat units in a repeat region of a target nucleic acid comprising the steps of: (A) contacting a plurality of different-sequence primers with a polynucleotide sample under conditions effective for said primers to anneal to primer-complementary regions in one or more target polynucleotides, to form one or more target-primer hybrid(s), wherein either (1) each different-sequence primer contains (i) a target binding segment and (ii) a tag segment having a nucleotide sequence that uniquely identifies the target binding segment, or (2) one or more polynucleotides in the sample are tagged polynucleotides that contain a tag segment having a nucleotide sequence that uniquely identifies the attached polynucleotide, (B) performing a first primer extension reaction on said hybrid(s) using a first primer- extension reagent; (C) separating the target-primer hybrid(s) from unreacted first primer extension reagent; (D) performing a second primer extension reaction on said hybrid(s) using a second primer extension reagent and with a primer termination reagent, the primer termination reagent including a nucleotide terminator having a label attached thereto; (E) separating the target-primer hybrid(s) from unreacted second primer extension reagent and unreacted primer termination reagent; (F) measuring a signal produced by the label; and (G) repeating a cycle of steps (A) through (F) until a signal is detected indicating incorporation of the nucleotide terminator, wherein (H) prior to step (F), at least an aliquot of either (1) said different-sequence primers or (2) said tagged sample polynucleotides are contacted with an addressable aπay of immobilized, different tag complements, and each different tag complement contains a sequence that is complementary to one of said tag segments, under conditions effective to hybridize the tag segments to coπesponding tag complements on the support. 18. The method of claim 17 wherein each different-sequence primer contains (i) a target binding segment and (ii) a tag segment having a nucleotide sequence that uniquely identifies the target binding segment. 19. The method of claim 18 wherein the contacting in step (H) is performed prior to step (A). 20. The method of claim 18 wherein the contacting in step (H) is performed after step (A). 21. The method of claim 18 wherein said method is performed on at least two replicate aπays, and one of the replicate aπays is subjected to at least one more cycle of steps (A) through (F) than is a second replicate aπay. 22. The method of claim 17 wherein one or more polynucleotides in the sample are tagged polynucleotides that contain a tag segment having a nucleotide sequence that uniquely identifies the attached polynucleotide. 23. The method of claim 22 wherein the contacting in step (H) is performed prior to step (A). 24. The method of claim 22 wherein the contacting in step (H) is performed after step (A). 25. The method of claim 22 wherein said method is performed on at least two replicate aπays, and one of the replicate aπays is subjected to at least one more cycle of steps (A) through (F) than is a second replicate aπay. 26. The method of claim 17 wherein the label is selected from the group consisting of fluorescent and chemiluminescent molecules. 27. The method of claim 17 wherein the target nucleic acid is amplified prior to analysis. 28. The method of claim 27 wherein amplification is achieved using a PCR. 29. A kit useful for determining the number of repeat units in a repeat region of a target nucleic acid comprising: a plurality of different-sequence primers, each containing (i) a target binding segment and (ii) a tag segment having a nucleotide sequence that uniquely identifies the target binding segment, a first primer extension reagent; and a second primer extension reagent, wherein at least one of the first or second primer extension reagents includes an extendible nucleotide having a label attached thereto. 30. The kit of claim 29, which further includes an addressable aπay of immobilized, different tag complements, wherein each different tag complement contains a sequence that is complementary to one of said primer tag segments, under conditions effective to hybridize the tag segments to coπesponding tag complements on the support. 31. The kit of claim 29 wherein the label is selected from the group consisting of fluorescent and chemiluminescent molecules. 32. The kit of claim 29 wherein the label is attached to the extendible nucleotide through a cleavable linker.

Inventors: BLASBAND ANDREW J, LIVAK KENNETH J, LOWE ADAM L, BLASBAND, ANDREW, J., LIVAK, KENNETH, J., LOWE, ADAM, L.
Applicants: PERKIN ELMER CORP, THE PERKIN-ELMER CORPORATION
Application year: 1999
Main classification: C12Q1
IPC codes: C12N15, C12Q1, G01N33