PROJECT SUMMARY/ABSTRACT Obesity is a well-established risk factor for breast cancer. Obese women who carry a mutation in the DNA repair enzymes BRCA1 and BRCA2 are at a greater risk of developing breast cancer compared with lean BRCA mutation carriers. Molecular mechanisms that explain the increased penetrance of breast cancer in obese BRCA mutation carriers are unknown. We have found that obesity is positively associated with DNA damage in breast epithelial cells of BRCA mutation carriers. We also found that conditioned media (CM) from obese breast adipose tissue stimulates DNA damage in association with elevated reactive oxygen species (ROS) in breast epithelial cells. Furthermore, important for DNA repair genes are downregulated in breast epithelial cells from obese women compared with lean women. This proposal will test the hypothesis that elevation in obesity-induced DNA damage in breast epithelial cells is mediated by local adipose-derived factors which 1) stimulate DNA damage via genotoxic effects of ROS and/or 2) reduce capacity for DNA repair. Furthermore, carrying a BRCA mutation enhances this effect due to intrinsic defective DNA repair leading to increased tumor penetrance. Therapies aimed at reducing adiposity may decrease DNA damage and consequently decrease tumor burden. To test this hypothesis, the first aim of this proposal will identify the adipose-derived factors that are responsible for driving DNA damage in breast epithelial cells and will determine whether they act through mitochondrial ROS. The second aim will determine whether obesity is associated with a defect in DNA repair. Finally, in the third aim, caloric restriction will be utilized to determine whether reducing adiposity is sufficient to attenuate obesity-induced elevation in mammary gland DNA damage leading to decreased tumor penetrance in obese mice carrying a Brca mutation. The identification of factors responsible for causing DNA damage in BRCA mutation carriers and the molecular mechanisms involved will highlight targets for therapeutic intervention in this at-risk population who are currently given few treatment options beyond surgical intervention. This project will be undertaken in the laboratory of sponsor Dr. Kristy Brown, a recognized expert in the field of obesity-related breast cancer, with the support of co-sponsor Dr. Lewis Cantley, world expert in PI3K in the context of cell metabolism and cancer, including in BRCA mutation carriers. The rich research environment at Weill Cornell Medical College and neighboring Rockefeller University and Memorial Sloan Kettering Cancer Center, where cancer genetics and genomics expertise can be found at every corner, is highly conducive to the exchange of ideas that will push this project forward.