Mechanisms for control of spontaneous mutagenesis as revealed by the use gene-targeted mice

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Oxygen radicals, which can be produced through normal cellular metabolism, are thought to play an important role in mutagenesis and tumorigenesis. Among various classes of oxidative DNA damage, 8-oxo-7, 8-dihydroguanine(8-oxoG) is most important because of its abundance and mutagenicity. The MTH1 gene encodes an enzyme that hydrolyzes 8-oxo-dGTP to monophosphate in the nucleotide pool, thereby preventing occurrence of transversion mutations. By means of gene targeting, we have established MTH1 gene-Bknockout cell lines and mice. When examined 18 months after birth, a greater number of tumors were formed in the Rmgs, livers, and stomachs of MTH1-deficient mice, as compared with wild-type mice. The MTH1-deficient mouse will provide a useful model for investigating the role of the MH1 protein in normal conditions and under oxidative stress. Alkylation of DNA at the o^6-position of guanine is one of the most critical events leading to mutation, cancer, and cell death. The enzyme o^6-methylguanine-DNA methyltransferase repairs o^6-methylguanine as well 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 now show that these dual effects of an alkylating agent can be dissociated by introduction of an additional defect in mismatch repair. Mice with mutations in both alleles of the MGMT gene and one of the mismatch repair genes, MLH1, are as resistant to methylnitrosourea (MNU) as are wild-type mice, in terms of survival, but do have numerous tumors after receiving MNU. In contrast to MGMT^<-/-> MLH1^<+/+> mice with decrease in size of the thymus and hypocellular bone marrow after MNU administration no conspicuous change was found in MGMT^<-/-> MLH1^<+/+> mice treated in the same manner. Thus, killing and tumorigenic effects of an alkylating agent can be dissociated by preventing mismatch repair pathways.