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Transitions of cytosine to thymine in CpG dinucleotides are the most frequent type of mutations observed in cancer. This increased mutability is commonly explained by the presence of 5-methylcytosine (5mC) and its spontaneous hydrolytic deamination into thymine. Here, we describe observations that question whether spontaneous deamination alone causes the elevated mutagenicity of 5mC. Tumours with somatic mutations in DNA mismatch-repair genes or in the proofreading domain of DNA polymerase ε (Pol ε) exhibit more 5mC to T transitions than would be expected, given the kinetics of hydrolytic deamination. This enrichment is asymmetrical around replication origins with a preference for the leading strand template, in particular in methylated cytosines flanked by guanines (GCG). Notably, GCG to GTG mutations also exhibit strand asymmetry in mismatch-repair and Pol ε wild-type tumours. Together, these findings suggest that mis-incorporation of A opposite 5mC during replication of the leading strand might be a contributing factor in the mutagenesis of methylated cytosine.

Original publication

DOI

10.1016/j.dnarep.2017.11.005

Type

Journal article

Journal

DNA Repair (Amst)

Publication Date

02/2018

Volume

62

Pages

1 - 7

Keywords

Cancer genomics, DNA methylation, DNA replication, Mutagenesis, 5-Methylcytosine, Carcinogenesis, CpG Islands, DNA Mismatch Repair, DNA Polymerase II, DNA Replication, Humans, Mutagenesis, Neoplasms