Oxidative DNA damage-induced tumorigenesis and its avoidance mechanism

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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 the most abundant, and appears to play important roles in mutagenesis and carcinogenesis. Studies with Escherichia coli mutator mutants revealed that organisms possess elaborate mechanisms that prevent mutations caused by oxidation of the guanine base, in both DNA and free nucleotide forms. Enzymatic activities which may be responsible for preventing 8-oxoG-evoked mutations were identified in mammalian cells. We have focused on following the two enzymes. MTH1 (Mthl) protein is the mammalian counterpart of E. coli MutT protein, which hydrolyzes 8-oxo-dGTP to monophosphate in the nucleotide pool, thereby preventing occurrence of transversion mutations. On the other hand, MUTYH (Mutyh) protein, a counterpart of E. coli MutY protein, having adenine/2-hydroxyadeni … More ne 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 Mthl 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.When examined 18 months after birth, a greater number of tumors were formed in the lungs, livers of Mthl-deficient mice, as compared with wild-type mice (Tsuzuki, T. et al., 2001). Mutation frequencies on the rpsL transgene in spleen samples recovered at the age of 4 and 24 weeks, were determined. The spontaneous mutation frequency observed in spleen samples from Mthl-deficient mice showed no dramatic increase compared to the value of the one in wild-type mice. The site distribution of the mutations occurred on rpsL gene was slightly different between these to Mthl genotypes in spleen samples. In Mthl-deficient mice, there are 1-basepair frameshift mutations at the mononucleotide repeats those were not found in wild-type mice (Egashira, A. et al., 2002). When examined 18 months after birth, a greater number of tumors had formed in various tissues of Mutyh-deficient mice, as compared with wild-type mice. Especially, more small intestinal tumors were formed in Mutyh-deficient mice than in wild-type mice (in preparation). Mutation frequency observed in spleen samples from the Mutyh-deficient mice, at the age of 24 weeks, showed no apparent increase compared to the value of samples from wild-type mice. However, the site distribution of the mutations that occurred in the rpsL gene was significantly different between these two Mutyh genotypes ; an increase in frequency of G : C to T : A transversions was evident in Mutyh nullizygous mice (in preparation).Thus, both the Mthl-and Mutyh-deficient mice will provide useful models for investigating the roles of oxidative stress in human health. Less