Mismatch repair is required for O-6-methylguanine-induced homologous recombination in human fibroblasts

O-6-methylguanine is responsible for homologous recombination induced by N- methyl-N'-nitro-N-nitrosoguanidine (MNNG) [H.Zhang et al, (1996) Carcinogen esis, 17, 2229], To test the hypothesis that mismatch repair is causally in volved in this process, we generated mismatch repair-deficient strains from a human fibroblast line containing a substrate for detecting intrachromoso mal homologous recombination, The four strains selected for study exhibited greatly increased resistance to the cytotoxic effects of MNNG, which was n ot affected by depletion of O-6-alkylguanine-DNA alkyltransferase, and grea tly increased sensitivity to the mutagenic effect of MNNG, suggesting that the mutagenic base modifications induced in these four cell strains by MNNG persist in their genomic DNA, Tests showed that their extracts are deficie nt in the repair of G:T mismatches. The frequency of homologous recombinati on induced by MNNG in three of these strains was significantly (5-7-fold) l ower than that induced in the parental cell strain. This was not the result of a generalized defect in recombination, because when (+/-)-7 beta,8 alph a-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene was us ed to induce recombination, all three lines responded with a normal or even a somewhat higher frequency than that observed in the parental strain. The lack of recombination displayed by the fourth strain was shown to result f rom the loss of part of the recombination substrate. The results strongly s uggest that functional mismatch repair is required for MNNG-induced homolog ous recombination.