Ethylnitrosourea-induced mutagenesis in asynchronous and synchronous Chinese hamster ovary cells

The toxic and mutagenic activity of the alkylating carcinogen N-ethyl-N-nitrosourea (ENU) was studied in Chinese hamster ovary (CHO) cells. Cell killing and induction of 6-thioguanine-resistant (TGr) and ouabain-resistant (OUAr) mutants were determined as a function of ENU dose and treatment time in asynchronous cell populations. A dose-dependent induction of mutants was observed. The mutation frequency did not increase with longer than 30-min treatment times, implying that ENU breaks down rapidly in the cell. When synchronous populations of CHO cells obtained by mitotic detachment were treated with ENU at various times during the cell cycle, ENU-induced reproductive death was strongly dependent on the position in the cell cycle at the time of treatment, the time of highest sensitivity being the beginning of the S period. The pattern of mutation induction by ENU over the cell cycle was quite different from the pattern for cell killing. The induction of TGr mutants seemed to be independent of cell-cycle time. The induction of OUAr mutants was also independent of cell-cycle time after a low ENU dose; however, after a high ENU dose the frequency of OUAr mutants varied during the cell cycle, with a slight enhancement in B1 and a decrease in the early S period. There was no sign of enhanced mutation induction at the growing point for the two genetic markers tested.