Study of the G1-phase specific novel repair pathway for X-ray-induced DNA damage

电离辐射(IR)可引起多种DNA损伤。最重要的损伤是DNA双链断裂(DSB)，它通过同源重组或非同源末端连接(NHEJ)途径修复。由于我们先前证明了IR反应蛋白53BP1特异性地增强DNA连接酶IV的活性，DNA连接酶IV是NHEJ所需的DNA连接酶，因此我们研究了53BP1缺陷的鸡DT40细胞对IR的反应。53BP1基因缺陷的细胞在G1期对X射线的敏感性增加。尽管S细胞内检查点和G2/M检查点完好无损，但照射后53BP1基因缺陷细胞的同色单体型染色体畸变率升高。此外，在53BP1基因缺陷的细胞中，X射线诱导的γ-H_2AX焦点(DNADSB的标志)的消失时间延长。因此，G1期细胞对X射线的敏感性升高可归因于IR诱导的DNA损伤的修复缺陷。上位性分析表明，53BP1在不同于Ku依赖和Artemis依赖的NHEJ途径中发挥作用，但需要DNA连接酶IV。显著的是，53BP1基因的破坏和Wortmannin对磷脂酰肌醇3-激酶家族的抑制完全取消了G1期照射细胞的克隆形成。这些结果表明，在细胞周期的G1期，依赖于53BP1的修复通路对于IR照射的细胞的存活是重要的。

Ionizing radiation (IR) induces a variety of DNA lesions. The most significant lesion is a DNA double-strand break (DSB), which is repaired by homologous recombination or nonhomologous end joining (NHEJ) pathway. Since we previously demonstrated that IR-responsive protein 53BP1 specifically enhances activity of DNA ligase IV, a DNA ligase required for NHEJ, we investigated responses of 53BP1-deficient chicken DT40 cells to IR. 53BP1-deficient cells showed increased sensitivity to X-rays during G1 phase. Although intra-S and G2/M checkpoints were intact, a frequency of isochromatid-type chromosomal aberrations was elevated after irradiation in 53BP1-deficient cells. Furthermore, disappearance of X-ray-induced γ-H2AX foci, a marker of DNA DSBs, was prolonged in 53BP1-deficient cells. Thus, the elevated X-ray sensitivity in G1 phase cells was attributable to repair defect for IR-induced DNA-damage. Epistasis analysis revealed that 53BP1 plays a role in a pathway distinct from the Ku-dependent and Artemis-dependent NHEJ pathways, but requires DNA ligase IV. Strikingly, disruption of the 53BP1 gene together with inhibition of phosphatidylinositol 3-kinase family by wortmannin completely abolished colony formation by cells irradiated during G1 phase. These results demonstrate that the 53BP1-dependent repair pathway is important for survival of cells irradiated with IR during the G1 phase of the cell cycle.