The repair of DNA double-stranded breaks (DSBs) is an essential function performed by the Classical Non-Homologous End-Joining (C-NHEJ) pathway in higher eukaryotes. C-NHEJ, in fact, does double duty as it is also required for the repair of the intermediates formed during lymphoid B- and T-cell recombination. Consequently, the failure to properly repair DSBs leads to both genomic instability and immunodeficiency. A critical DSB protein required for C-NHEJ is the DNA Ligase IV (LIGIV) accessory factor, X-Ray Cross Complementing 4 (XRCC4). XRCC4 is believed to stabilize LIGIV, participate in LIGIV activation, and to help tether the broken DSB ends together. XRCC4's role in these processes has been muddied by the identification of two additional XRCC4 paralogs, XRCC4-Like Factor (XLF), and Paralog of XRCC4 and XLF (PAXX). The roles that these paralogs play in C-NHEJ is partially understood, but, in turn, has itself been obscured by species-specific differences observed in the absence of one or the other paralogs. In order to investigate the role(s) that XRCC4 may play, with or without XLF and/or PAXX, in lymphoid variable(diversity)joining [V(D)J] recombination as well as in DNA DSB repair in human somatic cells, we utilized gene targeting to inactivate the XRCC4 gene in both parental and XLF- HCT116 cells and then inactivated PAXX in those same cell lines. The loss of XRCC4 expression by itself led, as anticipated, to increased sensitivity to DNA damaging agents as well as an increased dependence on microhomology-mediated DNA repair whether in the context of DSB repair or during V(D)J recombination. The additional loss of XLF in these cell lines sensitized the cells even more whereas the presence or absence of PAXX was scarcely negligible. These studies demonstrate that, of the three LIG4 accessory factor paralogs, the absence of XRCC4 influences DNA repair and recombination the most in human cells.

中文翻译：

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人类细胞中XRCC4及其旁系同源物的缺失揭示了DNA修复和V（D）J重组的结果差异。

DNA双链断裂（DSB）的修复是经典的非同源末端连接（C-NHEJ）途径在高等真核生物中执行的一项基本功能。实际上，C-NHEJ具有双重作用，因为它也是修复淋巴B细胞和T细胞重组过程中形成的中间体所必需的。因此，无法正确修复DSB会导致基因组不稳定和免疫缺陷。C-NHEJ所需的关键DSB蛋白是DNA连接酶IV（LIGIV）辅助因子X射线交叉互补4（XRCC4）。据信XRCC4可以稳定LIGIV，参与LIGIV激活，并有助于将断裂的DSB连接在一起。通过识别另外两个XRCC4旁系同源物，如XRCC4-Like Factor（XLF）和XRCC4和XLF旁系同源物（PAXX），使XRCC4在这些过程中的作用变得模糊。这些旁系同源物在C-NHEJ中的作用已被部分理解，但反过来，由于缺乏一种或另一种旁系同源物而观察到的物种特异性差异，其本身也被掩盖了。为了研究在有或没有XLF和/或PAXX的情况下，XRCC4在连接[V（D）J]的淋巴样变量（多样性）以及人体细胞中DNA DSB修复中可能发挥的作用，我们利用基因靶向使亲本和XLF-HCT116细胞中的XRCC4基因失活，然后使相同细胞系中的PAXX失活。如预期的那样，XRCC4表达的丧失本身导致对DNA损伤剂的敏感性增加，以及对微同源性介导的DNA修复的依赖性增加，无论是在DSB修复还是在V（D）J重组过程中。这些细胞系中XLF的额外损失使细胞更加敏感，而PAXX的存在或缺乏几乎可以忽略不计。这些研究表明，在三个LIG4辅助因子旁系同源物中，XRCC4的缺失对人类细胞中DNA修复和重组的影响最大。