Safe sequencing system
Error rates in massively parallel sequencing instruments are generally too high to allow confident identification of rare variants. An approach that can substantially increase the sensitivity of massively parallel sequencing instruments for this purpose, called “Safe-SeqS” for (Safe-Sequencing System) includes (i) assignment of a unique identifier (UID) to each template molecule; (ii) amplification of each uniquely tagged template molecule to create UID-families; and (iii) redundant sequencing of the amplification products. PCR fragments with the same UID are truly mutant (“super-mutants”) if ≧95% of them contain the identical mutation. We illustrate the utility of this approach for determining the fidelity of a polymerase, the accuracy of oligonucleotides synthesized in vitro, and the prevalence of mutations in the nuclear and mitochondrial genomes of normal cells.
1. A method to identify single base substitution, insertion, and deletion mutations in an analyte nucleic acid fragment, comprising:
attaching a unique identifier sequence (UID) from a pool of UIDs to a first end of each strand of a plurality of analyte DNA fragments using at least two cycles of amplification with first and second primers to form a plurality of uniquely identified analyte DNA fragments, wherein the pool of UIDs are in excess of the analyte DNA fragments during amplification, wherein the first primers comprise:
a first segment complementary to a desired amplicon;
a second segment containing the UID; and
a third segment containing a universal priming site for subsequent amplification;
and wherein the second primers comprise a universal priming site for subsequent amplification; wherein each cycle of amplification attaches one universal priming site to a strand;
amplifying the uniquely identified analyte DNA fragments to form a family of uniquely identified analyte DNA fragments from each uniquely identified analyte DNA fragment; and
determining nucleotide sequences of a plurality of members of the family;
comparing nucleotide sequences of the family of uniquely identified analyte DNA fragments;
identifying a nucleotide sequence as accurately representing an analyte DNA fragment when at least 1% of members of the family contain the sequence and the sequence is found in at least two families; and
identifying a single base substitution, insertion, or deletion mutation in the analyte DNA fragment when the nucleotide sequence that accurately represents the analyte DNA fragment is different from a reference sequence by a single base substitution, insertion, or deletion in the analyte DNA fragment.
2. The method of claim 1 wherein the second primers each comprise a UID.
3. The method of claim 1 wherein the nucleotide sequence is identified as accurately representing an analyte DNA fragment when:
at least 5% of members of the family contain the sequence.
4. The method of claim 1 wherein the UIDs are from 2 to 4000 bases inclusive.
5. The method of claim 1 wherein prior to the step of amplifying the uniquely identified analyte DNA fragments, a single strand-specific exonuclease is used to digest excess primers used to attach the UID the analyte DNA fragments.
6. The method of claim 5 wherein prior to the step of amplifying the single strand-specific exonuclease is inactivated, inhibited, or removed.
7. The method of claim 6 wherein the single strand-specific exonuclease is inactivated by heat treatment.
8. The method of claim 5 wherein primers used in the step of amplifying comprise one or more phosphorothioate linkages.
9. The method of claim 1 wherein the nucleotide sequence is identified as accurately representing an analyte DNA fragment when at least 25% of members of the family contain the sequence.
10. The method of claim 1 wherein the nucleotide sequence is identified as accurately representing an analyte DNA fragment when at least 50% of members of the family contain the sequence.
11. The method of claim 1 wherein the nucleotide sequence is identified as accurately representing an analyte DNA fragment when at least 70% of members of the family contain the sequence.
12. The method of claim 1 wherein the nucleotide sequence is identified as accurately representing an analyte DNA fragment when at least 90% of members of the family contain the sequence.
13. The method of claim 1 wherein the nucleotide sequence is identified as accurately representing an analyte DNA fragment when at least 95% of members of the family contain the sequence.