REVERSAL OF METHYLATION TOLERANCE BY TRANSFER OF HUMAN-CHROMOSOME-2

Human cell lines resistant to N-methyl-N-nitrosourea (MNU) were previo usly assigned to two complementation groups. Members of group I are de fective in mismatch correction [S. Ceccotti, G. Aquilina, P. Macpherso n, M. Yamada, P. Karran, M. Bignami, Processing of O-6-methylguanine b y mismatch correction in human cell extracts. Current Biol. 6 (1996) 1 528-1531]. To identify the mechanism responsible for the less pronounc ed phenotype of the second complementation group, we characterized the persistence of MNU-induced O-6-methylguanine (O-6-meGua) and mutation induction at the hypoxanthine guanine phosphoribosyl-transferase (HPR T) locus. Group II clones are unable to repair the premutagenic base O -6-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 signific ant instability at mononucleotide repeat microsatellites. Introduction of a single human chromosome 2 (but not of chromosome 3 or 7) into gr oup II cells partially reverts both MNU resistance and the increased s pontaneous mutation rate. The properties of group II variants are cons istent with methylation tolerance and a partially defective mismatch r epair. We propose that members of group II are defective in the chromo some 2-based mismatch correction gene GTBP/hMSH6. (C) 1997 Elsevier Sc ience B.V.