Oxidative damage to DNA has been proposed to have a role in cancer and ageing1. Oxygen-free radicals formed during normal aerobic cellular metabolism attack bases in DNA, and 7,8-dihydro-8-oxoguanine (8-oxoG) is one of the adducts formed2,3. Eukaryotic replicative DNA polymerases replicate DNA containing 8-oxoG by inserting an adenine opposite the lesion4; consequently, 8-oxoG is highly mutagenic and causes G:C to T:A transversions5. Genetic studies in yeast have indicated a role for mismatch repair in minimizing the incidence of these mutations. In Saccharomyces cerevisiae, deletion of OGG1, encoding a DNA glycosylase that functions in the removal of 8-oxoG when paired with C, causes an increase in the rate of G:C to T:A transversions6. The ogg1Δ msh2Δ double mutant displays a higher rate of CAN1s to can1r forward mutations than the ogg1Δ or msh2Δ single mutants, and this enhanced mutagenesis is primarily due to G:C to T:A transversions7. The gene RAD30 of S. cerevisiae encodes a DNA polymerase, Polη, that efficiently replicates DNA containing a cis-syn thymine-thymine (T-T) dimer by inserting two adenines across from the dimer8. In humans, mutations in the yeast RAD30 counterpart, POLH, cause the variant form of xeroderma pigmentosum9.10 (XP-V), and XP-V individuals suffer from a high incidence of sunlight-induced skin cancers. Here we show that yeast and human POLη replicate DNA containing 8-oxoG efficiently and accurately by inserting a cytosine across from the lesion and by proficiently extending from this base pair. Consistent with these biochemical studies, a synergistic increase in the rate of spontaneous mutations occurs in the absence of POLη in the yeast ogg1Δ mutant. Our results suggest an additional role for Polη in the prevention of internal cancers in humans that would otherwise result from the mutagenic replication of 8-oxoG in DNA.
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