Safe sequencing system
Though massively parallel sequencing instruments are in principle well-suited to the task of identification of mutations present in a small fraction of DNA templates, the error rates in such 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 is called “Safe-SeqS” for (Safe-Sequencing System) and 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.
1. A method of massively parallel sequencing to differentiate between an accurate sequence and a sequencing artifact in an analyte DNA nucleic acid sequence, the method comprising:
providing fragmented analyte DNA, the fragmented analyte DNA generated by random shearing and ranging from 30 to 2000 bases, inclusive, and the fragments having end sequences that are endogenous unique identifier (UID) sequences;
ligating sequencing adapter oligonucleotides to both ends of the fragments to form a set of adapted fragments;
capturing adapted fragments of interest on a solid phase containing a plurality of oligonucleotides complementary to a plurality of sequences of interest;
performing PCR amplification to form amplicons of the adapted fragments of interest;
performing redundant massively parallel sequencing including a bridge PCR step to determine the nucleic acid sequence of each member of each amplicon without reducing the number of duplicates;
grouping the nucleic acid sequences into endogenous UID families based on sequence identity of the UID sequences of each amplicon;
comparing the nucleotide sequences within each endogenous UID family that includes 2 or more members;
identifying a nucleotide sequence as accurately representing an analyte DNA fragment when at least 95% of the members of the UID family contain the sequence; and
identifying a nucleotide sequence as containing a sequencing artifact when less than 95% of the members of the UID family contain the sequence.
2. The method of claim 1 , wherein prior to the PCR amplification to form amplicons of the adapted fragments of interest, the analyte DNA is treated with bisulfite to convert unmethylated cytosine bases to uracil bases.
3. The method of claim 1 , wherein the redundant massively parallel sequencing is performed on a Illumina GA IIx system.
4. The method of claim 1 , wherein the first and last three bases of the endogenous UID are excluded from the step of grouping.
5. The method of claim 1 , wherein the nucleotide sequence accurately represents an analyte DNA fragment when at least 95% of the members of at least two UID families contains the sequence.
6. The method of claim 1 , wherein each endogenous UID family includes more than 2, more than 5, or more than 10 members.