An effect of cell-cycle position on ultraviolet-light-induced mutagenesis in Chinese hamster ovary cells

Using synchronous populations obtained by selectively detaching mitotic cells from cultures grown in monolayer, we demonstrate here that Chinese hamster ovary (CHO) cells exhibit a differential sensitivity to mutation induction by UV as a function of position in the cell cycle. When mutation induction to 6-thioguanine (TG) resistance is monitored, several maxima and minima are displayed during cell-cycle traverse, with a major maximum occurring in early S phase. Although cells in S phase are more sensitive to UV-mediated cell lethality than those in G₁ or G₂/M phases, there is not a strict correlation with induced mutation frequency. Fluence-response curves obtained at several times during the cell cycle yield D_q values approximating 6 J/m². The primary survival characteristic which varies with cell cycle position is D₀, ranging from 2.5 J/m² at 6 h after mitotic selection to 5.5 J/m² at 11 h afterward. Based on studies with asynchronous, logarithmically growing populations, as well as those mitotically selected to be synchronous, the optimum phenotypic expression time for induced TG resistance is 7–9 days and is essentially independent of both UV fluence and position in the cell cycle. All isolated mutants have altered hypozanthine—guanine phosphoribosyl transferase (HGPRT) activity, and no difference in the residual level of activity was detected among isolated clones receiving UV radiation during G₁, S, or late S/G₂ phases of the cell cycle. Changes in cellular morphology during cell-cycle traverse do not contribute to the differential susceptibility to UV-induced mutagenesis.