SRAD在DNA双链断裂损伤修复途径选择中的作用及其机制

Replication-associated one-ended DNA double-strand breaks (DSBs) are almost exclusively repaired by homologous recombination (HR), but the underlying molecular mechanisms remain poorly understood. Here we identify SRAD, a largely uncharacterized protein, as the key determinant of DSB repair pathway choice. SRAD physically interacts with CtIP and is required for efficient CtIP accumulation at DSBs. SRAD possesses intrinsic DNA binding ability with a strong preference for DNA substrates that mimic structures generated at stalled replication forks. Consequently, loss of SRAD impairs CtIP-dependent DNA end resection and compromises HR repair. We propose that, through enhancing the tethering of CtIP at perturbed replication forks, SRAD directs replication-associated one-ended DSBs towards the HR repair pathway. A successful completion of this study will provide new insights into the molecular mechanisms that regulate DSB repair pathway choice.

DSBs(DNA double-strand breaks)根据它末端性质可以分为双末端DSBs(Two-ended DSBs)和复制相关的单末端DSBs(Replication-associated one-ended DSBs)。复制相关的单末端DSBs只能通过同源重组(homologous recombination，HR)完成修复；如果通过非同源末端连接(non-homologous end-joining，NHEJ)完成修复，则会导致不同染色体之间的连接，进而引起translocation的发生以及基因组不稳定。但是，细胞如何区分这两种不同类型的 DSBs从而保证复制相关的单末端DSBs通过HR完成修复还不清楚。我们发现新蛋白SRAD可能通过区分不同类型的DNA底物从而调控DSBs修复途径选择。该项目的完成将使我们进一步认识正确的DSBs修复途径选择在维持基因组稳定中的重要作用。

DSBs是最具威胁的DNA损伤类型之一，如果不能及时修复或发生错误修复，则会导致基因突变、基因组不稳定以及肿瘤等重大疾病的发生。DSBs根据它末端的性质可以分为两种类型：双末端DSBs (Two-ended DSBs)和复制相关的单末端DSBs (Replication-associated one-ended DSBs)。DSBs主要通过非同源末端连接(non-homologous end-joining，NHEJ)和同源重组(homologous recombination，HR)这两种修复途径完成修复。研究表明，Two-ended DSBs主要通过NHEJ完成修复；而Replication-associated one-ended DSBs则只能通过HR完成修复。如果Replication-associated one-ended DSBs通过NHEJ完成修复，则会导致不同染色体之间的连接，进而引起translocation的发生以及基因组不稳定。然而，细胞如何识别并区分这两种不同类型的DSBs从而选择正确的DSBs修复途径还很不清楚。. 我们发现SRAD/AUNIP蛋白是一个重要的DSBs的识别因子。一方面，AUNIP通过优先结合Replication-associated one-ended DSBs从而区分Two-ended DSBs和Replication-associated one-ended DSBs。另一方面，SRAD/AUNIP通过与HR关键起始蛋白CtIP直接相互作用从而优先招募CtIP蛋白到Replication-associated one-ended DSBs上，使得Replication-associated one-ended DSBs只能通过HR完成修复。与此同时，由于SRAD/AUNIP蛋白结合Two-ended DSBs能力较弱，导致CtIP蛋白不能够被有效的招募到Two-ended DSBs位点，进而使得Two-ended DSBs主要通过NHEJ完成修复。相关研究结果分别发表于Nature Communications (2篇)、Molecular Cell (2篇)、PNAS以及Nucleic Acids Research等杂志。

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