Reversal of methylation tolerance by transfer of human chromosome 2.

Human cell lines resistant to N-methyl-N-nitrosourea (MNU) were previously assigned to two complementation groups. Members of group I are defective in mismatch correction [S. Ceccotti, G Aquilina, P. Macpherson, M. Yamada, P. Karran, M. Bignami, Processing of O6-methylguanine by mismatch correction in human cell extracts. Current Biol. 6 (1996) 1528-1531]. To identify the mechanism responsible for the less pronounced phenotype of the second complementation group, we characterized the persistence of MNU-induced O6-methylguanine (O6-meGua) and mutation induction at the hypoxanthine guanine phosphoribosyl-transferase (HPRT) locus. Group II clones are unable to repair the premutagenic base O6-meGua and are as mutable by MNU as group I clones and the parental HeLaMR cells. MNU-induced SCE were undetectable in group I clones and drastically reduced in group II in comparison with the parental cells. These observations are consistent with a defective processing of DNA methylation damage by members of both groups. Group II clones exhibit a moderate spontaneous mutator phenotype at the HPRT gene but significant instability at mononucleotide repeat microsatellites. Introduction of a single human chromosome 2 (but not of chromosome 3 or 7) into group II cells partially reverts both MNU resistance and the increased spontaneous mutation rate. The properties of group II variants are consistent with methylation tolerance and a partially defective mismatch repair. We propose that members of group II are defective in the chromosome 2-based mismatch correction gene GTBP/hMSH6.