Mutant BRCA1 and BRCA2 DNA repair enzymes are causally linked to an increased risk of breast and ovarian cancers. Recent evidence suggests that cancers occur at these hormone-sensitive sites, at least in part, due to the pro-proliferative and mutagenic effects of estrogens. We reported that women who are obese have elevated levels of aromatase, the rate-limiting enzyme for estrogen production, in inflamed breast adipose tissue. This would be predicted to lead to increased local production of estrogen and may explain why obese postmenopausal women are at increased risk of developing hormone receptor-positive breast cancer. Obesity has been reported to increase the penetrance of breast cancer in BRCA1/2 mutation carriers. A major and potentially transformative research challenge is determining whether our discovery of the obesity-inflammation-aromatase link is important in the pathogenesis of breast cancer in BRCA1/2 mutation carriers. We now have possibly fieldchanging preliminary data which suggest that acquired characteristics, such as obesity and adipose inflammation, are associated with increased expression of aromatase in breast adipose stromal cells (ASCs) and an associated increase in DNA damage in the normal breast epithelium of BRCA mutation carriers. In this proposal, we will test the hypothesis that obesity, and associated breast white adipose tissue inflammation (WATi), will increase breast cancer penetrance in BRCA mutation carriers via local and systemic effects, including higher estrogen and insulin levels, that will lead to DNA damage in the breast epithelium and increase tumor burden. This hypothesis will be tested by first investigating whether an association exists between obesity, breast WATi and increased levels of systemic factors (e.g., estrogens, insulin, leptin, IL-6) in BRCA1/2 mutation carriers (Aim 1). Then, we will assess correlations between body mass index (BMI), breast WATi, estrogens and DNA damage in normal appearing breast epithelium (Aim 2). Given the importance of estrogens in the pathogenesis of hereditary breast cancer, we will next characterize mechanisms of aromatase regulation in breast adipose stromal cells of BRCA mutation carriers (Aim 3). Finally, we will utilize mouse models to determine whether suppressing estrogen biosynthesis can reduce mammary gland DNA damage and increase tumor latency, while also exploring whether a lifestyle intervention, i.e. reduction in caloric intake, or pharmacological intervention, i.e. use of the anti-diabetic drug metformin, can prevent these cancer-promoting changes (Aim 4). By focusing on the potential link between estrogen, DNA damage and breast cancer, this study promises to provide insights into why obesity increases the penetrance of breast cancer in BRCA1/2 mutation carriers. Importantly, results in this high risk population could prove relevant for understanding the mechanisms underlying the obesity-cancer connection for sporadic breast cancer. Finally, our findings should strengthen the rationale for evidence-based risk reduction strategies.