Department of Systems Biology, Harvard Medical School, Boston, MA 02115, USA.
Mol Cell. 2012 Jul 27;47(2):320-9. doi: 10.1016/j.molcel.2012.05.052.
DNA double-strand breaks are repaired by two main pathways: nonhomologous end joining (NHEJ) and homologous recombination (HR). The choice between these pathways depends on cell-cycle phase; however the continuous effect of cell cycle on the balance between them is still unclear. We used live cell imaging and fluorescent reporters for 53BP1, Rad52, and cell cycle to quantify the relative contribution of NHEJ and HR at different points of the cell cycle in single cells. We found that NHEJ is the dominant repair pathway in G1 and G2 even when both repair pathways are functional. The shift from NHEJ to HR is gradual, with the highest proportion of breaks repaired by HR in mid S, where the amount of DNA replication is highest. Higher proportions of HR also strongly correlate with slower rates of repair. Our study shows that the choice of repair mechanism is continuously adjusted throughout the cell cycle and suggests that the extent of active replication, rather than the presence of a sister chromatid influences the balance between the two repair pathways in human cells.
DNA 双链断裂通过两种主要途径进行修复：非同源末端连接（NHEJ）和同源重组（HR）。这两种途径的选择取决于细胞周期阶段；然而，细胞周期对它们之间平衡的持续影响仍不清楚。我们使用活细胞成像和用于 53BP1、Rad52 和细胞周期的荧光报告基因，在单细胞中定量分析了不同细胞周期点 NHEJ 和 HR 的相对贡献。我们发现，即使两种修复途径都具有功能，NHEJ 也是 G1 和 G2 期的主要修复途径。从 NHEJ 到 HR 的转变是逐渐的，HR 修复的断裂比例在 S 中期最高，此时 DNA 复制量最高。HR 的比例较高也与修复速度较慢强烈相关。我们的研究表明，修复机制的选择在整个细胞周期中不断调整，并表明活跃复制的程度，而不是姐妹染色单体的存在，会影响人类细胞中两种修复途径之间的平衡。
