Program in Genetics, Sackler School of Biomedical Sciences, Tufts University, Boston, Massachusetts 02111, USA.
Genetics. 2010 Jan;184(1):65-77. doi: 10.1534/genetics.109.111039. Epub 2009 Nov 9.
Trinucleotide repeats can form secondary structures, whose inappropriate repair or replication can lead to repeat expansions. There are multiple loci within the human genome where expansion of trinucleotide repeats leads to disease. Although it is known that expanded repeats accumulate double-strand breaks (DSBs), it is not known which DSB repair pathways act on such lesions and whether inaccurate DSB repair pathways contribute to repeat expansions. Using Saccharomyces cerevisiae, we found that CAG/CTG tracts of 70 or 155 repeats exhibited significantly elevated levels of breakage and expansions in strains lacking MRE11, implicating the Mre11/Rad50/Xrs2 complex in repairing lesions at structure-forming repeats. About two-thirds of the expansions that occurred in the absence of MRE11 were dependent on RAD52, implicating aberrant homologous recombination as a mechanism for generating expansions. Expansions were also elevated in a sae2 deletion background and these were not dependent on RAD52, supporting an additional role for Mre11 in facilitating Sae2-dependent hairpin processing at the repeat. Mre11 nuclease activity and Tel1-dependent checkpoint functions were largely dispensable for repeat maintenance. In addition, we found that intact homologous recombination and nonhomologous end-joining pathways of DSB repair are needed to prevent repeat fragility and that both pathways also protect against repeat instability. We conclude that failure of principal DSB repair pathways to repair breaks that occur within the repeats can result in the accumulation of atypical intermediates, whose aberrant resolution will then lead to CAG expansions, contractions, and repeat-mediated chromosomal fragility.
三核苷酸重复可以形成二级结构，其不当修复或复制可能导致重复扩展。在人类基因组的多个位置，三核苷酸重复的扩展导致疾病。虽然已知扩展的重复积累双链断裂（DSB），但尚不清楚哪些 DSB 修复途径作用于这些病变，以及不准确的 DSB 修复途径是否导致重复扩展。使用酿酒酵母，我们发现 70 或 155 个重复的 CAG/CTG 片段在缺乏 MRE11 的菌株中表现出明显升高的断裂和扩展水平，这表明 Mre11/Rad50/Xrs2 复合物参与修复形成结构的重复中的损伤。在缺乏 MRE11 的情况下发生的大约三分之二的扩展依赖于 RAD52，这表明异常的同源重组是产生扩展的机制。在 sae2 缺失背景下，扩展也升高，并且这些扩展不依赖于 RAD52，支持 Mre11 在促进重复处发夹加工方面的额外作用。Mre11 核酸酶活性和 Tel1 依赖性检查点功能在很大程度上可忽略不计重复维持。此外，我们发现，完整的同源重组和非同源末端连接 DSB 修复途径都需要防止重复脆弱性，并且这两种途径也可以防止重复不稳定性。我们得出结论，主要 DSB 修复途径未能修复重复内发生的断裂可能导致异常中间产物的积累，其异常解决将导致 CAG 扩展、收缩和重复介导的染色体脆弱性。
