Department of Biochemistry and Molecular Biology, The Pennsylvania State University College of Medicine, Hershey, PA, 17033, USA.
Nat Commun. 2022 Sep 9;13(1):5323. doi: 10.1038/s41467-022-33028-y.
The inability to protect stalled replication forks from nucleolytic degradation drives genome instability and underlies chemosensitivity in BRCA-deficient tumors. An emerging hallmark of BRCA-deficiency is the inability to suppress replication-associated single-stranded DNA (ssDNA) gaps. Here, we report that lagging strand ssDNA gaps interfere with the ASF1-CAF-1 nucleosome assembly pathway, and drive fork degradation in BRCA-deficient cells. We show that CAF-1 function at replication forks is lost in BRCA-deficient cells, due to defects in its recycling during replication stress. This CAF-1 recycling defect is caused by lagging strand gaps which preclude PCNA unloading, causing sequestration of PCNA-CAF-1 complexes on chromatin. Importantly, correcting PCNA unloading defects in BRCA-deficient cells restores CAF-1-dependent fork stability. We further show that the activation of a HIRA-dependent compensatory histone deposition pathway restores fork stability to BRCA-deficient cells. We thus define lagging strand gap suppression and nucleosome assembly as critical enablers of BRCA-mediated fork stability.
无法保护停滞的复制叉免受核酸酶降解会导致基因组不稳定，并构成 BRCA 缺陷型肿瘤对化疗药物敏感的基础。BRCA 缺陷型的一个新出现的特征是无法抑制与复制相关的单链 DNA（ssDNA）缺口。在这里，我们报告说，滞后链 ssDNA 缺口会干扰 ASF1-CAF-1 核小体组装途径，并在 BRCA 缺陷型细胞中驱动叉降解。我们表明，由于复制应激过程中其回收的缺陷，BRCA 缺陷型细胞中的 CAF-1 在复制叉处的功能丧失。这种 CAF-1 回收缺陷是由滞后链缺口引起的，这些缺口阻止了 PCNA 的卸载，从而将 PCNA-CAF-1 复合物隔离在染色质上。重要的是，纠正 BRCA 缺陷型细胞中 PCNA 卸载缺陷可恢复 CAF-1 依赖性叉稳定性。我们进一步表明，HIRA 依赖性补偿性组蛋白沉积途径的激活可恢复 BRCA 缺陷型细胞的叉稳定性。因此，我们将滞后链缺口抑制和核小体组装定义为 BRCA 介导的叉稳定性的关键促成因素。
