Specificity of mutations induced by methyl methanesulfonate in mismatch repair-deficient human cancer cell lines

Recently, we showed that the cytotoxic and mutagenic response in human cell s to the model S(N)2 alkylating agent methyl methanesulfonate (MMS) can be modulated by the mismatch repair (MMR) pathway. That is, human cancer cell lines defective in MMR are more resistant to the cytotoxic effects of MMS e xposure and suffer more induced mutations at the HPRT locus than MMR-profic ient cell lines. Since MMS produces little O-6-methylguanine (O-6-meG), the observed hypermutability and resistance to cytotoxicity in MMR-defective c ells likely results from lesions other than O-6-meG. MMS produces a high yi eld of N7-methylguanine (N7-meG) and N3-methyladenine (N3-meA), which can l ead to the formation of promutagenic abasic sites, and these lesions may be responsible for the observed cytotoxic and/or mutagenic effects of MMS. To further investigate the mechanism of MMS mutagenesis, two MMR-defective hu man cancer cell lines were treated with MMS and the frequency and the types of mutations produced at the HPRT locus were determined. MMS treatment (1. 5 mM) produced a 1.6- and a 2.2-fold increase in mutations above spontaneou s levels in HCT116 and DLD-1 cell lines, respectively. An average 3.7-fold increase in transversion mutations was observed, which accounted for greate r than one-third of all induced mutations in both cell lines. In contrast, an average 1.6-fold increase was seen among transition mutations (the class expected from O-alkylation products). Since transversion mutations are not produced by O-6-meG, these findings suggest that abasic sites may be the l esion responsible for a large proportion of MMS mutagenicity in MMR-defecti ve cells. Furthermore, these data suggest the MMS-induced damage, either ab asic site-inducing base alterations (i.e., N7-meG and N3-meA) or the result ing abasic sites themselves, may be substrates for recognition and/or repai r by MMR proteins. (C) 1999 Elsevier Science B.V. All rights reserved.