Oxygen radicals are produced through normal cellular metabolism, and formation of such radicals is further enhanced by ionizing radiation and by various chemicals. Among various classes of oxidative DNA damage, 8-oxo-7,8-dihydroguanine (8-oxoG) is the most abundant, and appears to play important roles in mutagenesis and carcinogenesis. Enzymatic activities that may be responsible for preventing 8-oxoG-evoked mutations were identified in mammalian cells. We have focused on following the two enzymes. MTH1 (Mth1) protein is the mammalian counterpart of E. coli MutT protein, which hydrolyzes 8-oxo-dGTP to its monophosphate form in the nucleotide pool, thereby preventing the incorporation of the mutagenic substrate into DNA. On the other hand, MUTYH (Mutyh) protein, a counterpart of E. coli MutY protein, having adenine/2-hydroxyadenine DNA glycosylase activity, is expected to prevent G:C to T:A transversions, by excising adenine from G:A mismatches induced by 8-oxoG and 2-OH-A. To analyze the function of the mammalian Mth1 and Mutyh proteins in vivo, we established gene-knockout mice for these two enzymes by gene targeting, and investigated spontaneous tumorigenesis as well as mutagenesis. Here we discuss our recent progress on spontaneous and oxidative stress-induced mutagenesis with these mutant mouse lines.
