Ali R B, Teo A K, Oh H K, Chuang L S, Ayi T C, Li B F
Chemical Carcinogenesis Laboratory, Institute of Molecular and Cell Biology, National University of Singapore, Republic of Singapore.
Mol Cell Biol. 1998 Mar;18(3):1660-9. doi: 10.1128/MCB.18.3.1660.
DNA lesions that halt RNA polymerase during transcription are preferentially repaired by the nucleotide excision repair pathway. This transcription-coupled repair is initiated by the arrested RNA polymerase at the DNA lesion. However, the mutagenic O6-methylguanine (6MG) lesion which is bypassed by RNA polymerase is also preferentially repaired at the transcriptionally active DNA. We report here a plausible explanation for this observation: the human 6MG repair enzyme O6-methylguanine-DNA methyltransferase (MGMT) is present as speckles concentrated at active transcription sites (as revealed by polyclonal antibodies specific for its N and C termini). Upon treatment of cells with low dosages of N-methylnitrosourea, which produces 6MG lesions in the DNA, these speckles rapidly disappear, accompanied by the formation of active-site methylated MGMT (the repair product of 6MG by MGMT). The ability of MGMT to target itself to active transcription sites, thus providing an effective means of repairing 6MG lesions, possibly at transcriptionally active DNA, indicates its crucial role in human cancer and chemotherapy by alkylating agents.
在转录过程中使RNA聚合酶停滞的DNA损伤优先通过核苷酸切除修复途径进行修复。这种转录偶联修复由DNA损伤处停滞的RNA聚合酶启动。然而，RNA聚合酶能绕过的诱变O6-甲基鸟嘌呤（6MG）损伤在转录活跃的DNA处也优先得到修复。我们在此报告对这一观察结果的一种合理的解释：人类6MG修复酶O6-甲基鸟嘌呤-DNA甲基转移酶（MGMT）以斑点形式存在，集中在活跃转录位点（由针对其N端和C端的多克隆抗体揭示）。在用低剂量N-甲基亚硝基脲处理细胞后，该物质会在DNA中产生6MG损伤，这些斑点迅速消失，同时伴随着活性位点甲基化的MGMT（MGMT对6MG的修复产物）的形成。MGMT将自身靶向活跃转录位点的能力，从而提供了一种可能在转录活跃的DNA处修复6MG损伤的有效手段，表明了其在人类癌症以及烷基化剂化疗中的关键作用。
