Indexing based deep DNA sequencing to identify rare sequences
The invention pertains to an assay that is capable of detecting a mutant polynucleotide in a plurality of polynucleotides. In one embodiment, the assay of the invention is capable of detecting one copy of a mutant polynucleotide in about 50,000 to about 100,000 copies of polynucleotides. The assay of the invention can be used to identify a mutant viral quasispecies or a mutant mRNA encoding an oncogenic protein from a tumor sample. The assay of the invention involves producing the single stranded complements of each of a plurality of polynucleotides containing the target sequence, wherein each of the single stranded complements contain a unique tag sequence and amplifying the single stranded complements by PCR using several sets of primers designed to introduce the sequences appropriate for a paired-end sequencing analysis of the amplified polynucleotides. The invention also pertains to kits for carrying out the assays of the invention.
1. An assay to identify, from a plurality of polynucleotides, a polynucleotide having a mutation within a target sequence, the assay comprising the steps of:
a) producing a single-stranded complement of each of the plurality of polynucleotides containing the target sequence by conducting one cycle of PCR using a plurality of first primers, wherein each of the plurality of first primers comprises, from the 5′ end:
i) an outer PCR primer motif,
ii) an inner PCR primer motif,
iii) a tag comprising a sequence unique for each of the first primers, wherein the unique sequence comprises about 4-20 nucleotides, and
iv) a 3′ target sequence which has a sequence that hybridizes to the sequence at the 3′ end of the target sequence,
wherein each of the single-stranded complements of each of the plurality of polynucleotides produced in this step comprises, from the 5′ end:
i) the outer PCR primer motif,
ii) the inner PCR primer motif,
iii) the tag comprising a unique sequence of about 4-20 nucleotides, and
iv) the 3′ target sequence,
b) optionally, isolating the single-stranded complements produced in step a),
c) PCR amplifying the single-stranded complements produced in step a) or isolated in step b) using a first primer set comprising an outer PCR primer and a first 5′ target primer to produce multiple double-stranded copies of each of the single-stranded complements produced in step a), wherein the outer PCR primer has a sequence that corresponds to the outer PCR primer motif portion of the first primers and the first 5′ target primer has a sequence that corresponds to the sequence at the 5′ end of the target sequence,
d) optionally, isolating the double-stranded copies produced in step c),
e) PCR amplifying the double-stranded copies produced in step c) or isolated in step d) using a second primer set comprising:
i) a first barcode primer comprising, from the 5′ end: a sequence corresponding to a 3′ portion of a first sequencing primer, a first barcode and an inner PCR primer sequence, wherein the inner PCR primer sequence corresponds to the inner PCR primer motif portion of the first primer, and
ii) a second barcode primer comprising, from the 5′ end: a sequence corresponding to a 3′ portion of a second sequencing primer, a second barcode and a second 5′ target sequence, wherein the second 5′ target sequence corresponds to the portion of the target sequence that is at the 3′ end of the sequence corresponding to the first 5′ target primer,
f) optionally, isolating the amplified double-stranded copies produced in step e),
g) PCR amplifying the double-stranded copies produced in step e) or isolated in step f) using a third primer set comprising a first sequencing primer and a second sequencing primer, wherein the first sequencing primer has a sequence corresponding to a first paired-end sequencing primer and the second sequencing primer has a sequence corresponding to a second paired-end sequencing primer,
h) optionally, isolating the amplified double-stranded copies produced in step g), and
i) subjecting the double-stranded copies produced in step g) or isolated in step h) to paired-end sequencing using the first paired-end sequencing primer and the second paired-end sequencing primer.
2. The assay of claim 1 , wherein the target sequence is a gene of interest and the plurality of polynucleotides is a sample of polynucleotides obtained from a subject.
3. The assay of claim 2 , wherein the gene of interest is a drug-resistant gene of an RNA virus and the sample of polynucleotides obtained from the subject is a viral RNA sample from the subject.
4. The assay of claim 3 , wherein the RNA virus is human immunodeficiency virus or hepatitis C virus.
5. The assay of claim 1 , wherein the target sequence is an mRNA of interest and the plurality of polynucleotides is a sample of mRNA obtained from a subject.
6. The assay of claim 5 , wherein the mRNA of interest encodes an oncogenic protein of interest and the sample of mRNA is obtained from a tumor from the subject.
7. The assay of claim 1 , wherein the tag is about 12 nucleotides in length.
8. The assay of claim 1 , wherein each of the outer PCR primer motif, the inner PCR primer motif, the 3′ target sequence, the outer PCR primer sequence, the first 5′ target primer, the inner PCR primer sequence, the sequence corresponding to the 3′ portion of the first sequencing primer, the sequence corresponding to the 3′ portion of the second sequencing primer, the first sequencing primer, and the second sequencing primer are each about 15 nucleotides in length.
9. The assay of claim 1 , wherein the first and the second barcodes are four to eight nucleotides in length.
10. The assay of claim 1 , wherein the isolating in steps d), f) and h) is performed by gel extraction.