Department of Pathology and Cell Biology, Institute for Cancer Genetics, College of Physicians and Surgeons, Columbia University, New York, NY 10032, USA.
Proc Natl Acad Sci U S A. 2012 Mar 6;109(10):3903-8. doi: 10.1073/pnas.1120160109. Epub 2012 Feb 21.
Nonhomologous end joining (NHEJ), a major pathway of DNA double-strand break (DSB) repair, is required during lymphocyte development to resolve the programmed DSBs generated during Variable, Diverse, and Joining [V(D)J] recombination. XRCC4-like factor (XLF) (also called Cernunnos or NHEJ1) is a unique component of the NHEJ pathway. Although germ-line mutations of other NHEJ factors abrogate lymphocyte development and lead to severe combined immunodeficiency (SCID), XLF mutations cause a progressive lymphocytopenia that is generally less severe than SCID. Accordingly, XLF-deficient murine lymphocytes show no measurable defects in V(D)J recombination. We reported earlier that ATM kinase and its substrate histone H2AX are both essential for V(D)J recombination in XLF-deficient lymphocytes, despite moderate role in V(D)J recombination in WT cells. p53-binding protein 1 (53BP1) is another substrate of ATM. 53BP1 deficiency led to small reduction of peripheral lymphocyte number by compromising both synapse and end-joining at modest level during V(D)J recombination. Here, we report that 53BP1/XLF double deficiency blocks lymphocyte development at early progenitor stages, owing to severe defects in end joining during chromosomal V(D)J recombination. The unrepaired DNA ends are rapidly degraded in 53BP1(-/-)XLF(-/-) cells, as reported for H2AX(-/-)XLF(-/-) cells, revealing an end protection role for 53BP1 reminiscent of H2AX. In contrast to the early embryonic lethality of H2AX(-/-)XLF(-/-) mice, 53BP1(-/-)XLF(-/-) mice are born alive and develop thymic lymphomas with translocations involving the T-cell receptor loci. Together, our findings identify a unique function for 53BP1 in end-joining and tumor suppression.
非同源末端连接（NHEJ）是 DNA 双链断裂（DSB）修复的主要途径，在淋巴细胞发育过程中需要该途径来解决 V(D)J 重组过程中产生的程序性 DSB。XRCC4 样因子（XLF）（也称为 Cernunnos 或 NHEJ1）是 NHEJ 途径的独特组成部分。尽管其他 NHEJ 因子的种系突变会使淋巴细胞发育受阻并导致严重联合免疫缺陷（SCID），但 XLF 突变会导致淋巴细胞减少症，其严重程度通常低于 SCID。因此，XLF 缺陷的小鼠淋巴细胞在 V(D)J 重组中没有可测量的缺陷。我们之前报道过，尽管 ATM 激酶及其底物组蛋白 H2AX 在 XLF 缺陷的淋巴细胞中 V(D)J 重组中起着至关重要的作用，但在 WT 细胞中 V(D)J 重组的作用适度。p53 结合蛋白 1（53BP1）是 ATM 的另一种底物。53BP1 缺陷导致外周淋巴细胞数量略有减少，这是由于 V(D)J 重组过程中适度的突触和末端连接缺陷所致。在这里，我们报告称，53BP1/XLF 双重缺陷会导致淋巴细胞发育在早期祖细胞阶段受阻，这是由于染色体 V(D)J 重组过程中末端连接严重缺陷所致。未修复的 DNA 末端在 53BP1(-/-)XLF(-/-)细胞中迅速降解，正如 H2AX(-/-)XLF(-/)细胞中所报道的那样，这揭示了 53BP1 对 DNA 末端的保护作用类似于 H2AX。与 H2AX(-/-)XLF(-/-)小鼠的早期胚胎致死性相反，53BP1(-/-)XLF(-/-)小鼠出生后存活，并发展为涉及 T 细胞受体基因座的胸腺淋巴瘤。总之，我们的研究结果确定了 53BP1 在末端连接和肿瘤抑制中的独特功能。
