A MUTATOR PHENOTYPE CHARACTERIZES ONE OF 2 COMPLEMENTATION GROUPS IN HUMAN-CELLS TOLERANT TO METHYLATION DAMAGE

Sixty % of clones isolated from HeLa cells treated with toxic concentr ations of a methylating carcinogen showed increased resistance to the cytotoxicity of N-methyl-N-nitrosourea. D-37 values were 6- to 100-fol d higher than in the parental cell population. The absence of detectab le levels of the repair enzyme O-6-methylguanine-DNA methyltransferase indicated that the resistant clones were able to tolerate the presenc e of O-6-methylguanine in their DNA. Analysis of N-methyl-N-nitrosoure a survival in the hybrids between tolerant clones and HeLa cells showe d that tolerance can be either recessive or codominant. Fusion between tolerant clones indicated two complementation groups. We measured spo ntaneous mutation rates at microsatellites and at the hypoxanthine-gua nine phosphoribosyl transferase (hprt) locus in several tolerant clone s. All the clones of Complementation Group I showed unstable microsate llites and 4-8-fold increases in mutation rates at hprt. No significan t alterations in spontaneous mutation rates sere found in clones of Co mplementation Group II. The data indicate that tolerance to methylatio n damage can be conferred by alterations in at least two different gen e products and that one of the two groups has the mutator phenotype ty pical of mismatch correction defective cells.