BRD7通过调控染色质重塑促进转录激活区域DNA双链断裂同源重组修复的分子机制及功能研究

Chromatin remodeling mediated gene transcription regulation and DNA damage response are tightly coordinated. Recent studies indicate that transcriptionally active chromatin recruits homologous recombination protein at DNA double-strand breaks （DSBs）. During the process, ATM prevents RNA polymerase II elongation-dependent chromatin decompensation at regions distal to DSBs. However, little is known about the mechanism that links DNA damage responses and nascent transcription on a contiguous stretch of chromatin. Therefore, we hypothesized that there is a chromatin remodeling proteins involved in HR repair and transcriptional activity near DSBs. In recently studies, we have found that chromatin remodeling protein BRD7 is a direct substrate of ATM and ATR. Furthermore, we also observed that the depletion of BRD7 resulted in defective transcriptional repression upon DSBs. On the one hand, we demonstrated that BRD7 depletion impaired MRN complex recruitment to DSBs and inhibited HR repair efficiency; on the other hand, chromatin structure is more compact in BRD7-depleted cells. Functional study indicated that BRD7 depletion rendered cells more sensitive toward Etoposide, CPT treatment due to its deficiency of HR repair. Taken together, our aims of this study are further clarifying the molecular mechanism and significance of BRD7-mediated regulation of HR repair in response to DNA damage, and providing strong theoretical basis for reasonable selection of PARP inhibitors in cancer therapy.

染色质重塑与基因转录调控和DNA损伤修复有着密切的联系。同源重组修复（HR）发生时，染色质结构呈松弛状态，同时RNA转录被抑制，但具体机制仍不明确。我们近期研究发现：1) 染色质重塑因子BRD7可结合同源修复感应复合物（MRN）参与HR修复; 2) 在修复过程中，BRD7不仅促进染色质的松弛，而且招募转录抑制复合物（NuRD）抑制HR区域的转录活性；3) 上述功能的发挥依赖于ATM介导的BRD7磷酸化信号；4) BRD7的缺失导致HR修复能力下降，使细胞对化疗药物及PARP 抑制剂更加敏感。由此我们提出科学假设：BRD7通过调控染色质重塑促进转录激活区域DNA双链断裂同源重组修复。本研究将进一步阐明BRD7作为染色质重塑因子参与DNA损伤修复过程中的染色质重塑、DNA修复方式的选择、转录抑制分子机制以及肿瘤学功能，为DNA损伤相关的放化疗方案选择及合理使用PARP1抑制剂提供新的理论依据。

DNA双链断裂（DNA double-strand breaks, DSBs）是最严重的DNA损伤类型之一，如果不能及时修复或修复错误，则会导致基因突变、基因组不稳定及肿瘤相关等重大疾病的发生。为应对内外源性因素诱导的DSBs，机体进化出非常保守的DNA损伤应答反应，包括DNA损伤修复通路的启动、细胞周期检测点激活及基因转录的暂停等效应。当细胞内发生DSBs时，细胞首先在DNA断裂的附近激活蛋白激酶ATM。激活的ATM磷酸化染色体组蛋白H2AX，从而招募下游的MDC1/RNF8/RNF168等信号分子，启动早期的DNA损伤信号传递和修复。最近又发现ATM还具有抑制转录的新功能：通过调节DSBs处组蛋白H2A的单泛素化水平从而调控RNA聚合酶ǁ介导的转录活性，维持基因组-表观组的完整性。然而，ATM是如何兼顾DNA损伤修复与DSBs处的转录抑制活性的分子机制还很不清楚。我们发现其中的关键枢纽分子：染色质重塑复合物PBAF亚基BRD7。本研究发现激酶ATM通过直接结合并磷酸化BRD7，促进DSBs处转录活性的停滞以及进一步增强DNA修复效应蛋白的募集，从而维持基因组-表观组的完整性。功能上发现，BRD7缺失或磷酸化突变导致DSBs处转录抑制的解除，抵抗HR修复通路的启动，提示BRD7基因可作为乳腺癌(BRCA野生型)潜在的放化疗增敏剂和靶向药物作用靶点。