DNA mismatch repair mutants do not increase N-methyl-N'-nitro-N-nitrosoguanidine tolerance in O6-methylguanine DNA methyltransferase-deficient yeast cells.

Treatment of cells with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) produces, among other lesions, mutagenic and carcinogenic lesions such as O6-methylguanine (O6MeG) and O4-methylthymine in DNA. An O6MeG DNA methyl-transferase (MTase) specifically and efficiently repairs such lesions. MTase-deficient bacterial, yeast and mammalian cells exhibit increased sensitivity not only to MNNG-induced mutagenesis, but also to MNNG-induced killing, suggesting that O6MeG-type lesions are also lethal to the cells. The lethal effect caused by O6MeG is not clear. Results from several recent experiments indicate that some MNNG-tolerant cell lines exhibit a loss of DNA mismatch binding/repair activity, suggesting that functional mismatch repair is probably responsible for the cellular sensitivity to DNA methylating agents. We tested this abortive O6MeG-T mismatch repair hypothesis in a well-defined lower eukaryote, Saccharomyces cerevisiae. We found that while mgt1-deleted MTase-deficient yeast strains are hypersensitive to MNNG-induced killing, combination of this mutation with any of the mlh1, msh2 or pms1 mutations did not render cells more tolerant to killing. msh3 mutation also did not rescue MNNG-induced genotoxicity. Furthermore, through the isolation and characterization of MNNG-tolerant cell lines from the MTase-deficient mutants we demonstrated that a DNA mismatch repair defect is neither sufficient nor required for this process. Since both DNA repair MTases and mismatch repair proteins are highly conserved between yeast and mammalian cells, our results could suggest alternative mechanisms in the cellular tolerance to O6MeG-induced killing.