Cancersusceptibility disease Nijmegen Breakage Syndrome and the function of underlying gene.

奈梅金氏断裂综合征(NBS)是一种人类遗传性疾病，具有对辐射高度敏感、染色体不稳定和易患癌症等特点，其潜在基因NBS1揭示了一种编码修复蛋白的基因，该蛋白既缺乏核酸酶活性，也缺乏DNA结合区。我们发现NBS1在C末端与Mre11/Ras50复合体结合，Mre11/Ras50复合体是同源重组的关键核酸酶。此外，根据组蛋白H_2AX在照射后立即被磷酸化的证据，我们发现NBS1通过N-末端的FHA/BRCT结构域与磷酸化的组蛋白结合。IP-Western和体外结合实验都证实了这种结合。因此，我们提出了一个损伤反应模型，在该模型中，NBS1识别损伤部位，并通过招募Mre11/Rad50核酸酶启动同源重组。事实上，使用SCneo报告基因对同源重组的分析显示，患者细胞和小鼠NBS细胞中的同源重组显著减少。此外，在缺少NBS1的C-端和N-端的克隆中，粘附素Smc1与Nbs1结合，从而抑制了Smc1的磷酸化，这是对S检查点的一种非必要的修饰。同样，在Werner综合征中，NBS1在照射后显示形成包括BRCA1和WRN在内的多蛋白复合体。由于复合体形成的破坏会导致染色体不稳定，因此它必须参与修复保真度的维持以及与检查点的串扰。

Nijmegen breakage syndrome (NBS) is a human hereditary disease, characterized by high sensitivity to radiation, chromosomal instability and predisposition to cancer, and the underlying gene, NBS1, reveals to code a repair protein, which lacks both nuclease activity and DNA-binding region. We presented here that NBS1 binds Mre11/Ras50 complex, a crucial nuclease for homologous recombination, at the C-terminus. Moreover, based on the evidence that histon H2AX is phosphorylated immediately after irradiation, we showed NBS1 binds to the phosphorylated histon through FHA/BRCT domains at the N-terminus. This binding was confirmed by both IP-western and in vitro binding assay. As a result, we proposed a damage response model, in which NBS1 recognizes damaged sites and initiates homologous recombination by recruitment of Mre11/Rad50 nuclease. In fact, analysis of homologous recombination using SCneo reporter gene showed significantly decreased homologous recombination in patient cells and mouse NBS cells. Furthermore, cohesin SMC1 binding to NBS1 and consequently SMC1 phosphorylation, a dispensable modification for S-checkpoint, were repressed in the clone lacking the C-terminus and N-terminus of NBS1. Similarly, NBS1 revealed to form the multi-protein complex including BRCA1 and WRN, mutated in Werner Syndrome, after irradiation. Since the disruption of complex formation results in chromosomal instability, it must be involved in maintenance of repair fidelity and cross-talk with checkpoint.