Deng Yibin, Guo Xiaolan, Ferguson David O, Chang Sandy
Department of Genetics, Box 1010, The M.D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, Texas 77030, USA.
Nature. 2009 Aug 13;460(7257):914-8. doi: 10.1038/nature08196. Epub 2009 Jul 26.
Progressive telomere attrition or uncapping of the shelterin complex elicits a DNA damage response as a result of a cell's inability to distinguish dysfunctional telomeric ends from DNA double-strand breaks. Telomere deprotection activates both ataxia telangiectasia mutated (ATM) and telangiectasia and Rad3-related (ATR) kinase-dependent DNA damage response pathways, and promotes efficient non-homologous end-joining (NHEJ) of dysfunctional telomeres. The mammalian MRE11-RAD50-NBS1 (MRN; NBS1 is also known as NBN) complex interacts with ATM to sense chromosomal double-strand breaks and coordinate global DNA damage responses. Although the MRN complex accumulates at dysfunctional telomeres, it is not known whether mammalian MRN promotes repair at these sites. Here we address this question by using mouse alleles that either inactivate the entire MRN complex or eliminate only the nuclease activities of MRE11 (ref. 8). We show that cells lacking MRN do not activate ATM when telomeric repeat binding factor 2 (TRF2) is removed from telomeres, and ligase 4 (LIG4)-dependent chromosome end-to-end fusions are markedly reduced. Residual chromatid fusions involve only telomeres generated by leading strand synthesis. Notably, although cells deficient for MRE11 nuclease activity efficiently activate ATM and recruit 53BP1 (also known as TP53BP1) to deprotected telomeres, the 3' telomeric overhang persists to prevent NHEJ-mediated chromosomal fusions. Removal of shelterin proteins that protect the 3' overhang in the setting of MRE11 nuclease deficiency restores LIG4-dependent chromosome fusions. Our data indicate a critical role for the MRN complex in sensing dysfunctional telomeres, and show that in the absence of TRF2, MRE11 nuclease activity removes the 3' telomeric overhang to promote chromosome fusions. MRE11 can also protect newly replicated leading strand telomeres from NHEJ by promoting 5' strand resection to generate POT1a-TPP1-bound 3' overhangs.
由于细胞无法区分功能失调的端粒末端与DNA双链断裂，端粒的渐进性损耗或保护复合物的解帽会引发DNA损伤反应。端粒去保护激活了共济失调毛细血管扩张突变（ATM）和毛细血管扩张症及Rad3相关（ATR）激酶依赖性DNA损伤反应途径，并促进功能失调端粒的高效非同源末端连接（NHEJ）。哺乳动物的MRE11-RAD50-NBS1（MRN；NBS1也称为NBN）复合物与ATM相互作用，以感知染色体双链断裂并协调全局DNA损伤反应。尽管MRN复合物在功能失调的端粒处积累，但尚不清楚哺乳动物的MRN是否促进这些位点的修复。在这里，我们通过使用使整个MRN复合物失活或仅消除MRE11核酸酶活性的小鼠等位基因来解决这个问题。我们表明，当从端粒中去除端粒重复结合因子2（TRF2）时，缺乏MRN的细胞不会激活ATM，并且连接酶4（LIG4）依赖性的染色体端对端融合明显减少。残留的染色单体融合仅涉及前导链合成产生的端粒。值得注意的是，尽管缺乏MRE11核酸酶活性的细胞有效地激活了ATM并将53BP1（也称为TP53BP1）募集到去保护的端粒，但3'端粒悬突持续存在以防止NHEJ介导的染色体融合。在MRE11核酸酶缺乏的情况下，去除保护3'悬突的保护复合物蛋白可恢复LIG4依赖性染色体融合。我们的数据表明MRN复合物在感知功能失调的端粒中起关键作用，并表明在没有TRF2的情况下，MRE11核酸酶活性去除3'端粒悬突以促进染色体融合。MRE11还可以通过促进5'链切除以产生与POT1a-TPP1结合的3'悬突来保护新复制的前导链端粒免受NHEJ的影响。
