A mutation in the MSH5 gene results in alkylation tolerance.

DNA methylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) are potent carcinogens; their carcinogenic effect is mainly due to the effect of production of O6-methylguanine (O6 MeG) on DNA. O6 MeG is not only mutagenic but also toxic to the cell because Mer-/Mex- cells unable to remove O6 MeG are very sensitive to killing by MNNG. It has been proposed that repeated futile mismatch correction of O6 MeG-containing bp is responsible for the genotoxicity of the O6 MeG lesion and that loss of mismatch repair activity results in cellular tolerance to O6 MeG, but the hypothesis has not been proved. We used yeast as a model to test this hypothesis and found that chromosome deletion of any known nuclear mitotic mismatch repair genes, including MLH1, MSH2, MSH3, MSH6, and PMS1, did not rescue mgt1delta O6 MeG DNA repair methyltransferase-deficient cells from killing by MNNG. A large number of mgt1delta, MNNG-tolerant revertants were isolated, among which one cell line, XS-14, has been found to carry a mutated allele of the MSH5 gene. The mutation also affected spore survival but did not increase the spontaneous mutation rate. We further demonstrated that a mutated form of the MSH5 gene, msh5-14, not the msh5delta-null mutation, is responsible for the cellular tolerance to MNNG in XS-14 cells. This observation offers an alternative model that may reconcile seemingly contradictory observations of yeast and mammalian cells.