Mechanisms for control of spontaneous mutagenesis as revealed by the use gene-targeted mice

氧自由基可以通过正常的细胞代谢产生，被认为在诱变和肿瘤发生中起重要作用。在各种类型的氧化性DNA损伤中，8-氧代-7，8-二氢鸟嘌呤（8-oxoG）由于其丰富性和致突变性而最重要。MTH 1基因编码将8-氧代-dGTP水解为核苷酸库中的单磷酸的酶，从而防止颠换突变的发生。通过基因打靶的方法，我们建立了MTH 1基因敲除的细胞系和小鼠。出生后18个月检查时，与野生型小鼠相比，MTH 1缺陷小鼠的RMG，肝脏和胃中形成了更多的肿瘤。MTH 1缺陷小鼠将为研究MH 1蛋白在正常条件下和氧化应激下的作用提供有用的模型。鸟嘌呤o^6位的DNA烷基化是导致突变、癌症和细胞死亡的最关键事件之一。o^6-甲基鸟嘌呤-DNA甲基转移酶修复DNA中的o^6-甲基鸟嘌呤以及一个较小的甲基化碱基o^4-甲基胸腺嘧啶。甲基转移酶（MGMT）基因缺陷的小鼠品系对烷化剂的杀伤作用和致瘤作用都非常敏感。我们现在表明，这些双重效应的烷化剂可以解离引入一个额外的错配修复缺陷。在MGMT基因和错配修复基因MLH 1的两个等位基因中具有突变的小鼠在存活方面与野生型小鼠一样对甲基亚硝基脲（MNU）具有抗性，但在接受MNU后确实有许多肿瘤。与MGMT^<-/-> MLH 1 ^<+/+>小鼠相比，MNU给药后胸腺和骨髓细胞减少，但以相同方式给药的MGMT^<-/-> MLH 1 ^<+/+>小鼠中未发现明显变化。因此，可以通过阻止错配修复途径来分离烷化剂的杀伤和致瘤作用。

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 most important because of its abundance and mutagenicity. The MTH1 gene encodes an enzyme that hydrolyzes 8-oxo-dGTP to monophosphate in the nucleotide pool, thereby preventing occurrence of transversion mutations. By means of gene targeting, we have established MTH1 gene-Bknockout cell lines and mice. When examined 18 months after birth, a greater number of tumors were formed in the Rmgs, livers, and stomachs of MTH1-deficient mice, as compared with wild-type mice. The MTH1-deficient mouse will provide a useful model for investigating the role of the MH1 protein in normal conditions and under oxidative stress. Alkylation of DNA at the o^6-position of guanine is one of the most critical events leading to mutation, cancer, and cell death. The enzyme o^6-methylguanine-DNA methyltransferase repairs o^6-methylguanine as well as a minor methylated base, o^4-methylthymine, in DNA. Mouse lines deficient in the methyltransferase (MGMT) gene are hypersensitive to both the killing and to the tumorigenic effects of alkylating agents. We now show that these dual effects of an alkylating agent can be dissociated by introduction of an additional defect in mismatch repair. Mice with mutations in both alleles of the MGMT gene and one of the mismatch repair genes, MLH1, are as resistant to methylnitrosourea (MNU) as are wild-type mice, in terms of survival, but do have numerous tumors after receiving MNU. In contrast to MGMT^<-/-> MLH1^<+/+> mice with decrease in size of the thymus and hypocellular bone marrow after MNU administration no conspicuous change was found in MGMT^<-/-> MLH1^<+/+> mice treated in the same manner. Thus, killing and tumorigenic effects of an alkylating agent can be dissociated by preventing mismatch repair pathways.