Isogawa Asako
Department of Medical Biophysics and Radiation Biology, Graduate School of Medical Sciences, Kyushu University, Fukuoka, Japan.
Fukuoka Igaku Zasshi. 2004 Jan;95(1):17-30.
Oxygen radicals generated through normal cellular metabolism induce a variety of types of oxidative damage into DNA and its precursors. Among such types of oxidative damage, 7, 8-dihydro-8-oxoguanine (8-oxoG), an oxidized form of guanine, is known to be abundant and highly mutagenic. 8-OxoG can pair with adenine as well as cytosine, thus causing G: C to T: A transversions after DNA replication, if not repaired. Organisms are equipped with elaborate systems to avoid such mutations caused by 8-oxoG. In Escherichia coli, two DNA glycosylases have been identified to suppress these mutations. One is MutM, an 8-oxoguanine DNA glycosylase that removes 8-oxoG from 8-oxoG: C base pairs. The other is MutY, an adenine DNA glycosylase that excises adenine from 8-oxoG: A mismatches. Mammals also have such DNA glycosylases; OGG1 (Ogg1) is the functional counterpart of MutM, and MUTYH (Mutyh) is the MutY homologue. In order to investigate the roles of these two enzymes in the avoidance of 8-oxoG-related mutagenesis in mammals, we analyzed spontaneous mutagenesis in the small intestine of Ogg1-deficient (Ogg1-/-) and Ogg1-, Mutyh-double deficient (Ogg1-/-; Mutyh-/-) mice at the age of 4-5 weeks using the prokaryotic rpsL transgene as a reporter. The observed mutation frequency was 1.00 x 10(-5) in both wild type and Ogg1-/- mice, and 1.91 x 10(-5) in Ogg1-/-; Mutyh-/- mice, indicating that the overall spontaneous mutation frequency was increased in Ogg1-/-; Mutyh-/- mice, but not in Ogg1-/- mice. Analysis of the mutation spectrum revealed that the frequency of G: C to T: A transversions were significantly increased in both Ogg1-/- and Ogg1-/-; Mutyh-/- mice; a 5-fold increase in Ogg1-/- mice, and a 41-fold increase in Ogg1-/-; Mutyh-/- mice when compared with wild type mice. A previous study in our laboratory indicated that a defect in Mutyh caused a 4-fold increase in the frequency of G: C to T: A transversions in mice. Combined, these observations suggest that a cooperative function between Ogg1 and Mutyh exists to prevent 8-oxoG-related mutagenesis in mammals.
正常细胞代谢产生的氧自由基会对DNA及其前体造成多种类型的氧化损伤。在这些氧化损伤类型中，鸟嘌呤的氧化形式7, 8-二氢-8-氧代鸟嘌呤（8-氧代鸟嘌呤，8-oxoG）含量丰富且具有高度致突变性。8-氧代鸟嘌呤可与腺嘌呤以及胞嘧啶配对，因此如果不进行修复，在DNA复制后会导致G:C到T:A的颠换。生物体具备复杂的系统来避免由8-氧代鸟嘌呤引起的此类突变。在大肠杆菌中，已鉴定出两种DNA糖基化酶来抑制这些突变。一种是MutM，一种8-氧代鸟嘌呤DNA糖基化酶，可从8-氧代鸟嘌呤：C碱基对中去除8-氧代鸟嘌呤。另一种是MutY，一种腺嘌呤DNA糖基化酶，可从8-氧代鸟嘌呤：A错配中切除腺嘌呤。哺乳动物也有此类DNA糖基化酶；OGG1（Ogg1）是MutM的功能对应物，而MUTYH（Mutyh）是MutY的同源物。为了研究这两种酶在哺乳动物避免8-氧代鸟嘌呤相关诱变中的作用，我们使用原核rpsL转基因作为报告基因，分析了4 - 5周龄的Ogg1缺陷型（Ogg1-/-）和Ogg1、Mutyh双缺陷型（Ogg1-/-; Mutyh-/-）小鼠小肠中的自发诱变情况。野生型和Ogg1-/-小鼠中观察到的突变频率均为1.00×10⁻⁵，而Ogg1-/-; Mutyh-/-小鼠中的突变频率为1.91×10⁻⁵，这表明Ogg1-/-; Mutyh-/-小鼠的总体自发突变频率增加，而Ogg1-/-小鼠中未增加。对突变谱的分析表明，Ogg1-/-和Ogg1-/-; Mutyh-/-小鼠中G:C到T:A颠换的频率均显著增加；与野生型小鼠相比，Ogg1-/-小鼠增加了5倍，Ogg1-/-; Mutyh-/-小鼠增加了41倍。我们实验室之前的一项研究表明，Mutyh缺陷会导致小鼠中G:C到T:A颠换频率增加4倍。综合这些观察结果表明，Ogg1和Mutyh之间存在协同功能，以防止哺乳动物中与8-氧代鸟嘌呤相关的诱变。
