Endometrial cancer is one of the most common cancers of the female reproductive system among American women. For patients with localized low-grade endometrioid endometrial carcinoma (EEC), mortality is low. However, for patients with metastatic or recurrent EEC or nonendometrioid carcinomas, such as the aggressive uterine papillary serous carcinoma (UPSC), treatment options are limited, and outcomes are abysmal. Thus, it is critical to identify key gene aberrations in endometrial cancer for development of novel effective targeted therapy. Through analyzing gene sequencing data from The Cancer Genome Atlas, we have found a high rate of mutation of CHD4 in endometrial cancer (15% in EEC and 35% in UPSC). This high CHD4 mutation rate was not seen in other tumor types, suggesting that CHD4 aberrations are a critical driver of endometrial cancer development. In addition, we found that CHD4 expression was significantly reduced in many endometrial cancer cell lines, suggesting that CHD4 may function as a novel tumor suppressor gene and a potential therapeutic target for endometrial cancer. Recently, increasing evidence has shown that an intact DNA damage response (DDR) is essential for maintenance of genomic stability and acts as a critical barrier to cancer development. Interestingly, we have recently identified a critical role of CHD4 in DDR, particularly in homologous recombination (HR) repair. Notably, we demonstrated that CHD4 with mutations seen in endometrial cancer patients lacked HR repair function, suggesting a critical link between the DDR function of CHD4 and endometrial cancer suppression. In our preliminary studies, we also found that CHD4-deficient endometrial cancer cells were more sensitive to PARP (poly[ADP-ribose] polymerase) inhibitors because of their defective HR repair. All of these intriguing results strongly suggest that CHD4 functions as a novel tumor suppressor for endometrial cancer and that targeting CHD4 deficiency using PARP inhibitors constitutes an effective targeted therapy for CHD4-deficient endometrial cancer. Three specific aims are proposed to test this hypothesis: (1) To assess clinical endometrial cancer tissue specimens for CHD4 aberrations and determine whether CHD4 deficiency is correlated with tumor grade, tumor stage, cancer subtype, and patient survival. (2) To determine whether loss of CHD4 contributes to endometrial cancer development using our CHD4 conditional knockout mouse model. These models will provide genetic evidence for CHD4 deficiency as a driving force for endometrial cancer development. (3) To develop a novel treatment for CHD4-deficient endometrial cancer using a synthetic lethality approach. We will assess the response of CHD4-deficient cells to potent PARP inhibitors in cell culture and in the mouse models. We will also combine PARP inhibitors with radiation and with doxorubicin or paclitaxel to establish optimal therapies. In summary, this project will not only help reveal novel molecular therapeutic targets for endometrial cancer but also have immediate clinical impact by establishing a new treatment based on targeting CHD4-deficient endometrial cancer with PARP inhibitors.