SEPARATION OF KILLING AND TUMORIGENIC EFFECTS OF AN ALKYLATING AGENT IN MICE DEFECTIVE IN 2 OF THE DNA-REPAIR GENES

Alkylation of DNA at the O-6-position of guanine is one of the most cr itical events leading to mutation, cancer, and cell death. The enzyme O-6-methylguanine-DNA methyltransferase repairs O-6-methylguanine as w ell as a minor methylated base, O-4-methylthymine, in DNA. Mouse lines deficient in the methyltransferase (MGMT) gene are hypersensitive to both the killing and to the tumorigenic effects of alkylating agents. We mow show that these dual effects of an alkylating agent can be diss ociated by introduction of an additional defect in mismatch repair. Mi ce with mutations in both alleles of the MGMT gene and one of the mism atch repair genes, MLH1, are as resistant to methylnitrosourea (MNU) a s are wild-type mice, in terms of survival, but do have numerous tumor s after receiving MNU. in contrast to MGMT(-/-)MLH1(+) mice with decre ase in size of the thymus and hypocellular bone marrow after MNU admin istration, no conspicuous change was found in MGMT(-/-)MLH1(-/-) mice treated in the same manner. Thus, killing and tumorigenic effects of a n alkylating agent can be dissociated by preventing mismatch repair pa thways.