Errors introduced by DNA polymerase ε (POLE) during DNA replication contribute significantly to the occurrence of C-to-T mutations at methylated CpG dinucleotides. A study published in Nature Genetics reports how researchers revealed this molecular mechanism of CpG>CpT mutagenesis using a novel sequencing-based approach.

Cytosine-to-thymine substitution at CpG dinucleotides is one of the most common mutations and frequently found in tumor and also observed in normal cells. In a previous study the researchers had reported that C-to-T mutations at CpG dinucleotides are particularly frequent in tumor cells with point mutations in POLE. DNA polymerase ε is an enzyme that synthesizes and proofreads DNA and repairs errors during replication. The catalytic subunit of the enzyme is encoded by the POLE gene.

The researchers developed a novel method, polymerase error rate sequencing (PER-seq), that allows to determine the frequency, type and the context of errors caused by DNA polymerases. In addition, they generated a mouse embryonic cell line with a p.P286R POLE mutation, a common mutation in human cancers, and they compared PER-seq signatures with mutation patterns in tumor cells of mice and humans. They discovered that during DNA replication both the mutated tumor-associated DNA polymerase ε and the wild-type enzyme produce C-to-T mutations, especially at methylated CpG dinucleotides.