Department of Molecular Medicine/Institute of Biotechnology, The Barshop Institute for Longevity and Aging Studies, The University of Texas Health Science Center at San Antonio, San Antonio, Texas 78245-3207, USA.
Nature. 2013 Sep 26;501(7468):569-72. doi: 10.1038/nature12500. Epub 2013 Sep 8.
Replication fork maintenance pathways preserve chromosomes, but their faulty application at nonallelic repeats could generate rearrangements causing cancer, genomic disorders and speciation. Potential causal mechanisms are homologous recombination and error-free postreplication repair (EF-PRR). Homologous recombination repairs damage-induced DNA double-strand breaks (DSBs) and single-ended DSBs within replication. To facilitate homologous recombination, the recombinase RAD51 and mediator BRCA2 form a filament on the 3' DNA strand at a break to enable annealing to the complementary sister chromatid while the RecQ helicase, BLM (Bloom syndrome mutated) suppresses crossing over to prevent recombination. Homologous recombination also stabilizes and restarts replication forks without a DSB. EF-PRR bypasses DNA incongruities that impede replication by ubiquitinating PCNA (proliferating cell nuclear antigen) using the RAD6-RAD18 and UBC13-MMS2-RAD5 ubiquitin ligase complexes. Some components are common to both homologous recombination and EF-PRR such as RAD51 and RAD18. Here we delineate two pathways that spontaneously fuse inverted repeats to generate unstable chromosomal rearrangements in wild-type mouse embryonic stem (ES) cells. Gamma-radiation induced a BLM-regulated pathway that selectively fused identical, but not mismatched, repeats. By contrast, ultraviolet light induced a RAD18-dependent pathway that efficiently fused mismatched repeats. Furthermore, TREX2 (a 3'→5' exonuclease) suppressed identical repeat fusion but enhanced mismatched repeat fusion, clearly separating these pathways. TREX2 associated with UBC13 and enhanced PCNA ubiquitination in response to ultraviolet light, consistent with it being a novel member of EF-PRR. RAD18 and TREX2 also suppressed replication fork stalling in response to nucleotide depletion. Interestingly, replication fork stalling induced fusion for identical and mismatched repeats, implicating faulty replication as a causal mechanism for both pathways.
复制叉维护途径可以保护染色体，但它们在非等位重复序列上的错误应用可能会导致染色体重排，从而引发癌症、基因组紊乱和物种形成。潜在的因果机制是同源重组和无差错复制后修复（EF-PRR）。同源重组修复诱导的 DNA 双链断裂（DSBs）和复制过程中的单链 DNA 断裂。为了促进同源重组，重组酶 RAD51 和介质 BRCA2 在断裂处的 3'DNA 链上形成细丝，以便与互补的姐妹染色单体退火，而 RecQ 解旋酶 BLM（Bloom 综合征突变）抑制交叉以防止重组。同源重组还可以稳定和重新启动没有 DSB 的复制叉。EF-PRR 通过使用 RAD6-RAD18 和 UBC13-MMS2-RAD5 泛素连接酶复合物泛素化 PCNA（增殖细胞核抗原）来绕过阻碍复制的 DNA 不连续性。一些组件是同源重组和 EF-PRR 的共同组件，例如 RAD51 和 RAD18。在这里，我们描述了两种自发融合反转重复序列以在野生型小鼠胚胎干细胞（ES）中产生不稳定染色体重排的途径。γ辐射诱导 BLM 调节的途径选择性地融合相同但不匹配的重复序列。相比之下，紫外线诱导 RAD18 依赖性途径有效地融合不匹配的重复序列。此外，TREX2（一种 3'→5' 外切酶）抑制相同重复序列的融合，但增强不匹配重复序列的融合，清楚地将这些途径分开。TREX2 与 UBC13 结合并增强 PCNA 泛素化对紫外线的反应，这与它是 EF-PRR 的一个新成员一致。RAD18 和 TREX2 还抑制核苷酸耗竭时复制叉的停滞。有趣的是，复制叉停滞诱导相同和不匹配重复序列的融合，这表明错误复制是这两种途径的因果机制。
