Mechanisms os spontaneous mutation and its control

显示自发突变频率增加的突变子突变体已经导致自发突变的多个路径的阐明。8-oxo-dGTP（8-oxo-7，8-dihydrodeoxyguanosine triphosphate，8-氧代-7，8-二氢脱氧鸟苷三磷酸）是在正常细胞代谢过程中在细胞的核苷酸库中形成的，当它掺入DNA中时会引起突变。大肠杆菌的MutT蛋白和相关的哺乳动物酶特异性地将8-oxo-dGTP降解为8-oxo-dGMP，从而防止颠换突变的发生。编码人类酶的基因，命名为MTH 1（mutT同源物1），定位于染色体7 p22。为了阐明8-氧代-dGPTase在致癌作用中的作用，有必要构建具有改变的酶活性水平的动物模型。确定8-oxo-dGTdR基因缺陷小鼠中肿瘤发生频率是否会增加是有意义的。为此，我们分离了小鼠8-oxo-dGT-peotein的基因组序列，鉴定了该基因的外显子/内含子区域，并表征了与该基因表达调控相关的启动子。O^6-甲基鸟嘌呤-DNA甲基转移酶可以修复这种损伤和相关的DNA损伤，通过基因打靶，我们建立了甲基转移酶基因缺陷的小鼠品系，这些小鼠的组织不含甲基转移酶活性。对这些基因靶向小鼠施用甲基亚硝基脲导致早期死亡，以同样方式治疗的正常小鼠没有显示出不良反应。在接受甲基亚硝基脲治疗的小鼠中，骨髓细胞减少，白细胞和血小板数量急剧减少，从而表明造血干细胞的生殖能力受损。甲基转移酶明显保护这些小鼠免受由烷化剂引起的白细胞减少症。

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.