EMS and UV-light-induced colony sectoring and delayed mutation in Chinese hamster cells

Purpose: To review studies of mutagen-induced colony sectoring which demons trate that UV light and EMS produce delayed mutational events in Chinese ha mster ovary cells. Methods and results: Since the late 1940s, it has been known that the treat ment of a single bacterial or yeast cell with mutagenic agents produces com plete mutant colonies (pures) and colonies composed of both mutant and non- mutant cell types (mosaics) with various sectored patterns. A similar secto ring phenomenon has been observed in Chinese hamster ovary cells (CHO) usin g the DNA alkylating agent ethyl methane sulphonate (EMS) or ultraviolet li ght. However, unlike bacteria and yeast, a significant fraction of CHO muta nt colonies contained sectors of less than 1/2; i.e. 1/4, 1/8 and 1/16 sect ors, suggesting a delayed production of mutations. Using various colony-rep lating approaches, it was found that these mutagenic agents produced the ra tio of mutant to wild-type cells expected for a delayed mutational process which produces mutant events for at least 12-14 cell divisions following tr eatment. This delayed mutation phenomenon was observed at both the glucose- 6-phosphate dehydrogenase (G6PD) and hypoxanthine guanine phosphoribosyltra nsferase (HGPRT) loci. Various mutational mechanisms for the production of delayed mutations are discussed. Conclusions: These studies suggest that mutagens such as UV light and EMS i nduce long-term alterations in mammalian cells that act to increase the 'ap parent' spontaneous mutation frequency. This delayed mutational decrease in stability of the genome may explain the accumulation over time of the mult iple genetic changes observed in malignant tumours.