Mechanisms os spontaneous mutation and its control

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Mutator mutants that show an increased frequency of spontaneous mutation have led to the elucidation of the multiple pathwavs of spontaneous mutagenesis. 8-Oxo-dGTP (8-oxo-7,8-dihydrodeoxyguanosine triphosphate) is formed in the nucleotide pool of a cell during normal cellular metabolism, and when it is incorporated into DNA causee mutation. MutT protein of Escherichia coli and related mammalian enzymes specifically degrade 8-oxo-dGTP to 8-oxo-dGMP,thereby preventing occurrence of transversion mutation. The gene encoding the human enzyme, designated MTH1 (for mutT homologue 1), maps to chromosome 7p22. To elucidate the roles of 8-oxo-dGPTase in carcinogenesis, it is necessary to construct an animal model with altered levels of the enzyme activity. It is interest to determine whether the frequency of occurrence of tumors would increase in mice defective in the 8-oxo-dGTPase gene. For this, we isolated the genomic sequence for mouse 8-oxo-dGTPase peotein, identified the exon/intron region of the gene, and characterized the promoter in relation to the regulation of expression of the gene.Alkylation of DNA at the O^6-position of guanine is one of the most critical events leading to induction of mutation as well as to cancer. The enzyme O^6-methylguanine-DNA methyltransferase repairs this and related lesions in DNA.By means of gene targeting, we established mouse lines deficient in the methyltransferase gene and tissues from these mice contained no methyltransferase activity. Administration of methylnitrosourea to these gene-targeted mice led to early death, and normal mice treated in the same manner showed no untoward effects. In mice given methylnitrosourea treatment, the bone marrow became hypocellular and there was a drastic decrease in the number of leukocytes and platelets, thereby indicating an impaired reproductive capacity of hematopoietic stem cells. Methyltransferase apparently protected these mice from the pnacytopenia caused by the alkylating agent.