Cytotoxicity, mutations and SCEs induced by methylating agents are reduced in CHO cells expressing an active mammalian O6-methylguanine-DNA methyltransferase gene.

Alkylation at the O6 position of guanine leading to miscoding during DNA replication has been shown to correlate with mutagenesis both in bacteria and mammalian cells. The widely used Chinese hamster ovary cells (CHO) are unable to remove O6-methylguanine (O6-meG) due to the absence of O6-meG DNA methyltransferase (MT) activity. Recently Ding et al. [Mol. Cell. Biol. (1985) 5, 3293-3296] transfected CHO cells with human liver DNA obtaining a line provided with a function for the repair of O6-meG. We confirmed the presence of MT activity in this particular clone (14,300 molecules/cell). We used this MT-proficient cell line as compared with the original MT-deficient CHO cell line to analyse the relevance of repair of this lesion on cell killing, ouabain resistance (ouar) mutations and sister chromatid exchanges (SCEs) induced by methylating agents. MT-proficient cells were more resistant than MT-deficient ones to the cytotoxic and mutagenic effects of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and N-methyl-N-nitrosourea (MNU). Furthermore a lower number of MNNG-induced SCEs were found in MT-proficient CHO than in MT-deficient cells. Similar ouar mutation frequencies were recorded in the two cell lines after 4-nitroquinoline-1-oxide (4NQO) treatment showing that the differences in cytotoxicity and mutagenesis are restricted to treatment with alkylating agents.