Homologous recombination rescues mismatch-repair-dependent cytotoxicity of SN1-type methylating agents in S-cerevisiae
Homologous recombination rescues mismatch-repair-dependent cytotoxicity of SN1-type methylating agents in S-cerevisiae
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DOI：
10.1016/j.cub.2005.07.032
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发表时间：
2005-08-09
期刊：
影响因子：
9.2
通讯作者：
Jiricny, J
中科院分区：
文献类型：
--
作者：
Cejka, P;Mojas, N;Jiricny, J
哺乳动物细胞对S(N)1型甲基化剂（如N-甲基-N-硝基-N-亚硝基胍（MNNG））的抗性通常是通过甲基鸟嘌呤甲基转移酶（MGMT）的表达增加而产生的，MGMT可将细胞毒性o- 6-甲基鸟嘌呤（(Me)G）还原为鸟嘌呤，或者通过失活错配修复（MMR）系统，当MGMT能力超过时，DNA复制过程中产生的（Me）G/T错对会被异常处理，从而引发细胞死亡(1)。考虑到MMR和meg解毒蛋白在进化过程中在功能上是保守的，而且缺乏MMR的大肠杆菌dam（-）菌株也对MNNG[3]具有抗性，MMR状态不影响酿酒酵母对MNNG[3]的敏感性的发现是出乎意料的。由于DNA中的MeG残基触发同源重组（homologous recombination， HR）[4-7]，我们想知道酿酒酵母中高效的HR是否可以减轻（Me）G加工的细胞毒性作用。我们现在表明，HR失活使葡萄球菌对MNNG敏感，并且与人类细胞一样，MMR基因MLH1和MSH2的缺陷挽救了这种敏感性。EXO1基因编码迄今为止唯一与MMR相关的外切酶[8,9]，其失活未能挽救这种超敏反应，这意味着scExo1不参与酿酒葡萄球菌MMR系统对（Me）G残基的加工。
Resistance of mammalian cells to S(N)1-type methylating agents such as N-methyl-N-nitro-N-nitrosoguanidine (MNNG) generally arises through increased expression of methylguanine methyltransferase (MGMT), which reverts the cytotoxic O-6-methylguanine ((Me)G) to guanine, or through inactivation of the mismatch repair (MMR) system, which triggers cell death through aberrant processing of (Me)G/T mispairs generated during DNA replication when MGMT capacity is exceeded (1]. Given that MMR and MeG-detoxifying proteins are functionally conserved through evolution, and that MMR-deficient Escherichia coli dam(-) strains are also resistant to MNNG [2], the finding that MMR status did not affect the sensitivity of Saccharomyces cerevisiae to MNNG [3] was unexpected. Because MeG residues in DNA trigger homologous recombination (HR) [4-7], we wondered whether the efficient HR in S. cerevisiae might alleviate the cytotoxic effects of (Me)G processing. We now show that HR inactivation sensitizes S. cerevisiae to MNNG and that, as in human cells, defects in the MMR genes MLH1 and MSH2 rescue this sensitivity. Inactivation of the EXO1 gene, which encodes the only exonuclease implicated in MMR to date [8, 9], failed to rescue the hypersensitivity, which implies that scExo1 is not involved in the processing of (Me)G residues by the S. cerevisiae MMR system.