Mutations accumulate in the genome of every cell of the body throughout life, causing cancer and other genetic diseases. Almost all of these mosaic mutations begin as nucleotide mismatches or damage in only one of the two strands of the DNA prior to becoming double-strand mutations if unrepaired or misrepaired. However, current DNA sequencing technologies cannot resolve these initial single-strand events. Here, we developed a single-molecule, long-read sequencing method that achieves single-molecule fidelity for single-base substitutions when present in either one or both strands of the DNA. It also detects single-strand cytosine deamination events, a common type of DNA damage. We profiled 110 samples from diverse tissues, including from individuals with cancer-predisposition syndromes, and define the first single-strand mismatch and damage signatures. We find correspondences between these single-strand signatures and known double-strand mutational signatures, which resolves the identity of the initiating lesions. Tumors deficient in both mismatch repair and replicative polymerase proofreading show distinct single-strand mismatch patterns compared to samples deficient in only polymerase proofreading. In the mitochondrial genome, our findings support a mutagenic mechanism occurring primarily during replication. Since the double-strand DNA mutations interrogated by prior studies are only the endpoint of the mutation process, our approach to detect the initiating single-strand events at single-molecule resolution will enable new studies of how mutations arise in a variety of contexts, especially in cancer and aging.
Numerous applications in molecular biology and genomics require characterization of mutant DNA molecules present at low levels within a larger sample of non‐mutant DNA. This is often achieved either by selectively amplifying mutant DNA, or by sequencing all the DNA followed by computational identification of the mutant DNA. However, selective amplification is challenging for insertions and deletions (indels). Additionally, sequencing all the DNA in a sample may not be cost effective when only the presence of a mutation needs to be ascertained rather than its allelic fraction. The MutS protein evolved to detect DNA heteroduplexes in which the two DNA strands are mismatched. Prior methods have utilized MutS to enrich mutant DNA by hybridizing mutant to non‐mutant DNA to create heteroduplexes. However, the purity of heteroduplex DNA these methods achieve is limited because they can only feasibly perform one or two enrichment cycles. We developed a MutS‐magnetic bead system that enables rapid serial enrichment cycles. With six cycles, we achieve complete purification of heteroduplex indel DNA originally present at a 5% fraction and over 40‐fold enrichment of heteroduplex DNA originally present at a 1% fraction. This system may enable novel approaches for enriching mutant DNA for targeted sequencing.
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