Mechanisms os spontaneous mutation and its control

显示自发突变频率增加的突变子突变导致了自发突变的多个路径波的阐明。8-氧代-dGTP(8-氧-7，8-二氢脱氧鸟苷三磷酸)是在细胞正常代谢过程中在细胞的核苷酸池中形成的，当它被掺入DNA中时会引起突变。大肠杆菌的MUT蛋白和相关的哺乳动物酶能特异性地将8-oxo-dGTP降解为8-oxo-dGMP，从而防止颠换突变的发生。编码人类酶的基因被命名为MTH1(MUT同源物1)，映射到染色体7p22。为了阐明8-oxo-dGPTase在肿瘤发生中的作用，有必要建立一个酶活性水平改变的动物模型。确定8-oxo-dGTPase基因缺陷的小鼠是否会增加肿瘤的发生频率是有意义的。为此，我们分离了小鼠8-oxo-dGTPase Peotein的基因组序列，鉴定了该基因的外显子/内含子区域，并鉴定了与基因表达调控有关的启动子。DNA在鸟氨酸O^6-位的烷基化是导致突变和致癌的最关键事件之一。O^6-甲基鸟嘌呤-DNA甲基转移酶修复DNA中的这种和相关的损伤。通过基因打靶的方法，我们建立了甲基转移酶基因缺陷的小鼠株系，这些小鼠的组织中没有甲基转移酶活性。给这些基因靶向的小鼠注射甲基亚硝脲会导致早期死亡，同样处理的正常小鼠没有表现出不良反应。在接受甲基亚硝脲治疗的小鼠中，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繁殖能力受损。甲基转移酶明显保护这些小鼠免受烷化剂引起的白细胞减少。

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.