UV-LIGHT INDUCES DELAYED MUTATIONS IN CHINESE-HAMSTER CELLS

The possibility was examined that mutational events at the glucose 6-p hosphate dehydrogenase locus can be delayed for significantly more tha n one or two cell divisions following treatment of Chinese hamster cel ls with UV light. To detect these later mutant events, the proportion of G6PD(-) mutant cells in a colony was obtained by replating cells fr om a single colony 5-7 days after UV irradiation and staining the resu lting colonies for G6PD activity. Eight colonies out of a total of 165 7 colonies from the treated population yielded G6PD-negative colonies upon replating, while no mutant clones were obtained from 947 colonies grown from untreated cells. One clone contained only mutant cells sug gesting that the mutation occurred before the first cell division afte r irradiation. A second clone contained a 1:1 ratio of mutant to wild- type cells suggesting that the mutation occurred at the first cell div ision. The remaining six clones contained low numbers of mutant cells and the ratio of mutant to wild-type cells in these clones was in agre ement with the theoretical ratios expected for mutations occurring at the fourth (1:13), sixth (1:63), seventh (1:127), eighth (1:255), nint h (1:511), and eleventh (1:2047) cell division after UV light exposure . G6PD(-) mutant cells deficient in staining activity were isolated fr om five clones and all have significant reductions in G6PD activity co nfirming the genetically stable character of these mutants. These resu lts support the conclusion that UV light induces mutants for up to 11 cell generations after treatment of mammalian cells and suggest that m utagens can induce in mammalian DNA long-term alterations which act to increase the apparently spontaneous mutation frequency.