Quantitative and molecular analyses of ethyl methanesulfonate- and ICR 191-induced mutation in AS52 cells.

A pSV2gpt-transformed Chinese hamster ovary (CHO) cell line has been used to study mutation at the molecular level. This cell line, designated AS52, was constructed from a hypoxanthine-guanine phosphoribosyl transferase (HPRT)-deficient CHO cell line, and has been previously shown to contain a single, functional copy of the E. coli xanthine-guanine phosphoribosyl transferase (XPRT) gene (gpt) stably integrated into the Chinese hamster genome. In this study, conditions for its use in the study of mammalian cell mutagenesis have been stringently defined. The spontaneous mutation rate (2 X 10(-6)/cell division) and phenotypic expression time (7 days) of the gpt locus compare favorably with those of the hprt locus in wild-type CHO-K1-BH4 cells. While both cell lines exhibit similar cytotoxic responses to ethyl methanesulfonate (EMSO and ICR 191, significant differences in mutation induction were observed. Ratios of XPRT to HPRT mutants induced per unit dose of EMS and ICR 191 are 0.70 and 1.6, respectively. Southern blot hybridization analyses revealed that most XPRT mutant cell lines which arose following treatment with EMS (20/22) or ICR 191 (20/24) exhibited no alterations of the gpt locus detectable by this technique. Similar observations were made for the hprt locus in EMS-(21/21) and ICR 191-induced (22/22) HPRT mutants. In contrast, most spontaneous gpt mutants (14/23) contained deletions, while most spontaneous hprt mutants (18/23) exhibited no detectable alterations. Results of this study indicate that the AS52 cell line promises to be useful for future study of mutation in mammalian cells at the DNA sequence level.