A role for Pms2 in the prevention of tandem CC → TT substitutions induced by ultraviolet radiation and oxidative stress

DNA mismatch repair (MMR) is important for preventing base-pair substitutions caused by spontaneous or damage-related DNA polymerase errors. We have used a reversion assay based on mouse Aprt to investigate the role of MMR in preventing ultraviolet radiation (UV) and oxidative stress induced tandem CC → TT base pair substitutions in cultured mammalian cells. The reversion construct used for this assay can detect both C → T and CC → TT mutational events. Most spontaneous mutations in Pms2-deficient cells were single C → T substitutions (88%), with the remainder being tandem CC → TT substitutions (12%). The percentage of tandem CC → TT substitutions rose to 64% and 94% for Pms2-deficient cells exposed to UV and a mixture of hydrogen peroxide and metals (Cu/Fe), respectively. Exposure to hydrogen peroxide alone or metals alone did not induce the tandem substitutions, nor did treatment of the cells with the alkylating agent ethylmethane sulfonate, which induces G → A substitutions on the opposite strand. Tandem CC → TT substitutions were also induced by UV irradiation and the hydrogen peroxide/metal mixture in Pms2-proficient cells, but at frequencies significantly lower than those observed in the Pms2-deficient cells. We conclude that mismatch repair plays an important role in preventing tandem CC → TT substitutions induced by certain genotoxin exposures.