Genomic Instability from Loss of XPG, a BRCA1/2 Partner: Role in Ovarian Cancer?

描述(申请人提供)：卵巢癌是美国女性癌症死亡的第五大原因，主要来自晚期高级别浆液性卵巢腺癌(HGS-OvCa)。BRCA1和BRCA2的遗传突变在同源重组修复(HRR)中发挥作用，导致卵巢癌风险的大幅增加。癌症基因组图谱(TCGA)项目最近发现异常高的基因组不稳定性是HGS-OvCa肿瘤的显著特征，并估计约50%有HRR缺陷。由于这一频率明显高于BRCA突变，因此可能与遗传性易感综合征无关的HRR基因的改变在散发性卵巢癌的病因中很重要。该项目将探索高度多功能的DNA修复蛋白XPG在卵巢癌发生中的新作用。最近令人惊讶的发现表明，XPG通过与HRR途径的几个成员直接相互作用而发挥作用，特别是包括BRCA蛋白和RAD51重组酶，并且XPG的缺失对BRCA1和BRCA2功能的损害具有重要后果。XPG的敲除会导致HRR降低，无法重新启动停滞的复制分叉，并显著增加基因组的不稳定性。相反，在核苷酸切除修复(NER)中需要XPG的酶功能来清除许多化疗药物形成的加合物，而对顺铂和卡铂等一线治疗的获得性耐药与包括XPG在内的NER蛋白的过度表达有关。对于开发新的治疗方法具有重要意义的是，由于XPG低表达而导致HRR降低的肿瘤可能对PARP抑制剂特别敏感。建议的探索性研究将检验的假设是，XPG是一种新的卵巢癌发生的肿瘤抑制因子，它的缺失或下调是卵巢癌病因中以前未确定的因素，以及 了解卵巢癌细胞中XPG蛋白的调节和活性具有直接的治疗意义。这一假设将通过两个具体目标进行检验。(1)XPG缺失对永生化的非致癌卵巢表面上皮(OSE)细胞和输卵管分泌上皮细胞(FTSEC)基因组不稳定性和细胞转化的影响，这两种细胞都被认为是卵巢癌发生的前体细胞类型。(2)XPG蛋白的数量、功能和调控将在一组卵巢癌细胞系中进行检测，这些细胞系已被证明与原发HGS-OvCa肿瘤最相似，包括几个已知具有XPG突变的肿瘤。如果新近发现的一种重要的HRR蛋白XPG在HGS-OvCa的启动过程中起作用的假设是正确的，这些结果有可能为卵巢癌的易感性提供一个新的标记物，新的治疗策略，并改善对卵巢癌发生机制的理解。

DESCRIPTION (provided by applicant): Ovarian cancer is the fifth leading cause of cancer death among U.S. women, primarily from advanced high-grade serous ovarian adenocarcinoma (HGS-OvCa). Inherited mutation of BRCA1 and BRCA2, which function in homologous recombination repair (HRR), leads to a substantial increase in ovarian cancer risk. The Cancer Genome Atlas (TCGA) project recently identified unusually high genomic instability as a prominent feature of HGS-OvCa tumors and estimated that ~50% have HRR defects. Since this frequency significantly exceeds that of BRCA mutations, it is likely that changes in HRR genes not known to be associated with inherited predisposition syndromes are important in the etiology of sporadic ovarian cancer. This project will explore a novel role for the highly multifunctional DNA repair protein XPG as a driver in ovarian carcinogenesis. Surprising recent findings have established that XPG functions in HRR through direct interaction with several members of the HRR pathway, notably including both BRCA proteins as well as the RAD51 recombinase, and that loss of XPG has significant consequences for impairment of BRCA1 and BRCA2 functions. Knockdown of XPG leads to reduced HRR, inability to restart stalled replication forks, and a dramatic increase in genomic instability. Conversely, the enzymatic function of XPG is required in nucleotide excision repair (NER) to remove adducts formed by many chemotherapeutic agents, and acquired resistance to first-line therapies such as cisplatin and carboplatin has been linked to over-expression of NER proteins including XPG. Importantly for development of new therapeutic approaches, tumors with diminished HRR due to low expression of XPG may be particularly sensitive to PARP inhibitors. The hypothesis to be tested by the proposed exploratory studies is that XPG is a novel tumor suppressor for ovarian carcinogenesis that its loss or down- regulation is a previously unidentified factor in the etiology of ovarian cancer, and that understanding XPG protein regulation and activity in ovarian cancer cells has direct therapeutic implications. This hypothesis will be tested through two specific aims. (1) The effect of XPG loss on genomic instability and cellular transformation in immortalized, non-tumorigenic ovarian surface epithelial (OSE) cells and fallopian tube secretory epithelial cells (FTSEC), which have each been proposed to be the precursor cell types for ovarian carcinogenesis, will be determined. (2) XPG protein amounts, function, and regulation will be examined in a panel of ovarian cancer cell lines that have been shown to most closely resemble primary HGS-OvCa tumors, including several that are known to have XPG mutations. If the hypothesis is correct that XPG, newly identified as an important HRR protein, plays a role in the initiating events in HGS-OvCa, these results have the potential to provide a new marker for ovarian cancer susceptibility, new therapeutic strategies, and improved understanding of mechanisms involved in ovarian carcinogenesis.