Spontaneous mutagenesis in human cells is controlled by REV1-Polymerase ζ and PRIMPOL.  Spontaneous mutagenesis in human cells is controlled by REV1-Polymerase ζ and PRIMPOL.

Translesion DNA synthesis (TLS) facilitates replication over damaged or difficult-to-replicate templates by employing specialized DNA polymerases. We investigate the effect on spontaneous mutagenesis of three main TLS control mechanisms: REV1 and PCNA ubiquitylation that recruit TLS polymerases and PRIMPOL that creates post-replicative gaps. Using whole-genome sequencing of cultured human RPE-1 cell clones, we find that REV1 and Polymerase ζ are wholly responsible for one component of base substitution mutagenesis that resembles homologous recombination deficiency, whereas the remaining component that approximates oxidative mutagenesis is reduced in PRIMPOL^−/− cells. Small deletions in short repeats appear in REV1^−/−PCNA^K164R/K164R double mutants, revealing an alternative TLS mechanism. Also, 500–5,000 bp deletions appear in REV1^−/− and REV3L^−/− mutants, and chromosomal instability is detectable in REV1^−/−PRIMPOL^−/− cells. Our results indicate that TLS protects the genome from deletions and large rearrangements at the expense of being responsible for the majority of spontaneous base substitutions.