Repair of oxidative DNA damage in Mammalian cells

在大肠杆菌中由n编码的核酸内切酶III可以去除胸腺嘧啶二醇，这是一种有毒的DNA氧化损伤。为了确定这种修复在哺乳动物中的生物学意义，我们通过基因靶向建立了一只nth的同源物mNth1突变的小鼠。到目前为止，纯合子mNth1突变小鼠未显示出可检测到的表型异常。携带野生型mNth1或不携带野生型mNth1的胚胎细胞对甲萘醌和过氧化氢的敏感性没有差异。x射线照射突变小鼠肝脏DNA产生的胸腺嘧啶二醇随着时间的推移而消失，但比野生型小鼠慢。在突变小鼠肝脏的提取物中，我们发现至少有两种新的DNA糖基酶活性，而不是mNTH1活性。突变小鼠线粒体中的一种活性明显高于野生型小鼠，而另一种是胸腺嘧啶二醇的另一种核糖基化酶。这些结果强调了胸腺嘧啶二醇的碱基切除修复在哺乳动物细胞核和线粒体中的重要性。

Endonuclease III, encoded by nth in Escherichia coli, removes thymine glycols, a toxic oxidative DNA lesion. To determine the biological significance of this repair in mammals, we established a mouse mutated in mNth1, a homologue of nth, by gene targeting. The homozygous mNth1 mutant mouse showed no detectable phenotypic abnormality so far analyzed. Embryonic cells with or without wild-type mNth1 showed no difference in sensitivity to menadione or hydrogen peroxide. Thymine glycols produced in the mutant mouse liver DNA by X-ray irradiation disappeared with time, though more slowly than in wild-type mouse. In extracts from mutant mouse liver, we found, instead of mNTH1 activity, at least two novel DNA glycosylase activities against thymine glycols. One activity is significantly higher in the mutant than in wild-type mouse in mitochondria, while the other is another nuclear glycosylase, for thymine glycols. These results underscore the importance of base excision repair of thymine glycols both in the nuclei and in the mitochondria in mammals.