N-methyl-N'-nitro-N-nitrosoguanidine-resistant HeLa S3 cells still have little O6-methylguanine-DNA methyltransferase activity and are hypermutable by alkylating agents.

To clarify the involvement of O6-methylguanine (O6-MeG) in mutagenesis, we isolated N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-resistant cells, MR10-1 from HeLa S3 mer- cells. MR10-1 cells were 40 times more resistant to MNNG than the parental cells. MR10-1 cells were also significantly more resistant to N-methyl-N-nitrosourea and slightly more resistant to methyl methanesulfonate and dimethyl sulfate than parental cells. However, we found that MR10-1 cells had still little O6-MeG-DNA methyltransferase activity and were sensitive to 1-(4-amino-2-methyl-5-pyrimidinyl)methyl- 3-(2-chloroethyl)-3-nitrosourea hydrochloride, like HeLa mer- cells, thereby showing that MR10-1 cells are still mer-. When induced 6-thioguanine (6TG)-resistant colonies were plotted as a function of the corresponding percentage survival, the resistant colonies of MR10-1 cells were induced much more frequently than in the case of HeLa mer- cells. However, induction of 6TG-resistant cells in both cell lines did not differ significantly in terms of mutant cells per 0.1 microM MNNG. On the contrary, MR10-1 cells (mer-) and two HeLa S3 mer+ cells lines differed in the induction of mutation as a function of MNNG concentration. The HeLa mer+ cell lines were not mutable, while MR10-1 cells were highly mutable. These above results clearly show that the HeLa mer- cell has at least two defects in the repair of the alkylated adducts which are related to cell killing and mutation, and also suggest that O6-MeG is involved in the induction of mutation.