PDF(Pubmed)
Abstract：
Nearly 50 hereditary diseases result from the inheritance of abnormally long repetitive DNA microsatellites. While it was originally believed that the size of inherited repeats is the key factor in disease development, it has become clear that somatic instability of these repeats throughout an individual\'s lifetime strongly contributes to disease onset and progression. Importantly, somatic instability is commonly observed in terminally differentiated, postmitotic cells, such as neurons. To unravel the mechanisms of repeat instability in nondividing cells, we created an experimental system to analyze the mutability of Friedreich\'s ataxia (GAA)n repeats during chronological aging of quiescent Saccharomyces cerevisiae Unexpectedly, we found that the predominant repeat-mediated mutation in nondividing cells is large-scale deletions encompassing parts, or the entirety, of the repeat and adjacent regions. These deletions are caused by breakage at the repeat mediated by mismatch repair (MMR) complexes MutSβ and MutLα and DNA endonuclease Rad1, followed by end-resection by Exo1 and repair of the resulting double-strand breaks (DSBs) via nonhomologous end joining. We also observed repeat-mediated gene conversions as a result of DSB repair via ectopic homologous recombination during chronological aging. Repeat expansions accrue during chronological aging as well-particularly in the absence of MMR-induced DSBs. These expansions depend on the processivity of DNA polymerase δ while being counteracted by Exo1 and MutSβ, implicating nick repair. Altogether, these findings show that the mechanisms and types of (GAA)n repeat instability differ dramatically between dividing and nondividing cells, suggesting that distinct repeat-mediated mutations in terminally differentiated somatic cells might influence Friedreich\'s ataxia pathogenesis.
摘要：
近50种遗传性疾病是由异常长的重复DNA微卫星的遗传引起的。虽然最初认为遗传重复的大小是疾病发展的关键因素，很明显，在个体的整个一生中，这些重复序列的躯体不稳定性强烈地导致了疾病的发生和进展。重要的是,体细胞不稳定性通常在终末分化中观察到，有丝分裂后细胞，如神经元。为了解开非分裂细胞重复不稳定的机制，我们创建了一个实验系统来分析Friedreich的共济失调（GAA）n重复序列在静态酿酒酵母按时间顺序老化过程中的变异性。我们发现非分裂细胞中主要的重复介导的突变是包含部分的大规模缺失，或全部，重复区域和相邻区域。这些缺失是由错配修复（MMR）复合物MutSβ和MutLα以及DNA内切核酸酶Rad1介导的重复序列的断裂引起的，然后通过Exo1进行末端切除并通过非同源末端连接修复所得的双链断裂（DSB）。我们还观察到重复介导的基因转换是在时间老化过程中通过异位同源重组进行DSB修复的结果。重复扩展在时间老化期间也会累积，尤其是在没有MMR诱导的DSB的情况下。这些扩增取决于DNA聚合酶δ的持续合成能力，同时被Exo1和MutSβ抵消，牵涉到尼克修复。总之,这些发现表明（GAA）n重复不稳定的机制和类型在分裂和非分裂的细胞之间存在显著差异，提示终末分化体细胞中不同的重复介导突变可能影响Friedreich的共济失调发病机制。
