Analysis of a novel back-up repair system for UV-induced DNA damage in human cells.

我们建立了一种检测生物剂量紫外线诱导的环丁烷嘧啶二聚体(CPDS)和(6-4)光产物(6-4pps)的超灵敏检测方法，提示人类细胞可能有一种新的修复系统，主要针对6-4pps，而不是核苷酸切除修复，这被认为是迄今为止人类对这些损伤唯一的修复系统。在本研究中，我们获得了以下发现：1.即使在xpa(-/-)或xpg(-/-)基因敲除小鼠的胚胎成纤维细胞中也观察到6-4pps的去除(24小时后30-50%)。2.稳定表达AlwNI型突变体XPA的HeLa细胞没有表现出核苷酸切除修复活性的任何损害，表明该突变的XPA蛋白不具有显性-负性效应。3.我们进一步敏化了CPDS的检测方法，使我们能够确定CPDS在低至0.1J/m^2的非致死剂量下的修复动力学。当人体细胞暴露在0.2J/m^2时，我们可以观察到1J/m^2的紫外线照射后修复效率有一些微弱的增强。然而，我们还需要更广泛的实验来判断后备修复系统是否受适应性反应的控制。综合这些结果，我们得出结论，哺乳动物细胞有一定的后备修复系统，它不依赖于XPA和XPG，主要清除6-4PP，但也有效率较低的CPD。

We have established a super-sensitive detection method for measuring cyclobutane pyrimidine dimers (CPDs) and (6-4)photoproducts (6-4PPs) induced by biological doses of ultraviolet (UV) light and suggested that human cells might have a novel repair system mainly for 6-4PPs other than nucleotide excision repair which has been thought to be only repair system for those lesions in humans so far. In this study, we have obtained the following findings.1.The removal of 6-4PPs was observed even in the embryonic fibroblasts derived from xpa(-/-) or xpg(-/-) knock-out mice (30-50% after 24 hr). In addition, the removal of CPDs was also detected in those cells with a less efficiency (20-30% after 48 hr).2.HeLa cells stably expressing AlwNI-type mutant XPA did not show any impairment of nucleotide excision repair activity, suggesting that this mutant XPA protein does not have a dominant-negative effect.3.We have further sensitized the detection method for CPDs, enabling us to determine the repair kinetics of CPDs in cells exposed to non-killing doses of Was low as 0.1 J/m^2. Under this condition, we have found that xeroderma pimentosum cells significantly remove CPDs (〜40% after 48hr).4.We could observe some weak enhancement of repair efficiency after UV irradiation of 1 J/m^2 when the human cells had been pie-exposed to 0.2 J/m^2. However, we need more extensive experiments to conclude whether the back-up repair system is under the control of adaptive responses.Taken together with those results, we concluded that mammalian cells have a certain back-up repair system, independent of XPA and XPG, which removes mainly 6-4PP but also CPDs with a less efficiency.