PROJECT SUMMARY Endometrial cancer is the most common gynecological cancer in the United States and causes more than 12,500 deaths per year with rising incidence and mortality as well as declining 5-year survival rates. Type I (endometrioid) tumors make up 85% of all endometrial cancer cases and the vast majority of these tumors express estrogen receptor alpha (ER). Almost every major risk factor for endometrial cancer involves excess or unopposed estrogen, indicating that estrogen signaling through ER is a critical driver of endometrial cancer progression. Despite the clear importance of estrogen signaling in endometrial cancer, animal models indicate that excess estrogens are not sufficient in isolation to cause endometrial cancer and 98% of human endometrial tumors harbor at least one somatic mutation in a commonly mutated gene. ARID1A is one of those commonly mutated genes, harboring mostly truncating mutations in the majority of endometrioid endometrial tumors at a rate that is higher in endometrial cancer than any other cancer type. ARID1A is a member of the canonical BAF nucleosome remodeling complex, a complex that interacts with ER based on the literature and our data. The loss of ARID1A in endometrial cancer cells could have several effects on estrogen signaling and gene regulation in general, including an increased importance of ARID1B or non-canonical BAF complex variants. These changes may create distinct vulnerabilities in ARID1A altered estrogen-responsive endometrial tumors. In this proposal, we will explore the molecular and phenotypic crosstalk between estrogen signaling and ARID1A alterations while taking advantage of our unique resources, including purified complete BAF complex and a collection of endometrial cancer and normal endometrial models. • In specific aim 1, we will probe the biochemical consequences of ARID1A loss on BAF activity and recruitment by ER. • In specific aim 2, we will analyze the genome-wide gene regulation consequences of combined ARID1A alterations and active estrogen signaling. • In specific aim 3, we will look for unique vulnerabilities in ARID1A mutant and ER active endometrial cancer cells using genetic and pharmaceutical approaches. The successful completion of this project will greatly enhance our understanding of how ARID1A alterations work together with estrogen signaling to promote endometrial cancer and has the potential to uncover novel treatment strategies that are desperately needed for this disease.