Hereditary breast cancers stem from inherited mutations in "susceptibility" genes such as BRCA1, BRCA2 and PALB2, which encode large tumor suppressor proteins that play essential roles in homologous recombination (HR)-mediated DNA double strand break repair and cell cycle checkpoint response. Moreover, these proteins also function in oxidative stress response. As a result of seminal discoveries of the role of the BRCA/PALB2 proteins in DNA repair, the repair defect of the deficient tumor cells is now being targeted using platinum drugs and PARP inhibitors. However, the mutant cells tend to develop resistance to both types of drugs. Thus, for both prevention and better treatment of the cancers, it is imperative to find new avenues to selectively kill the mutant cells, preferably ones that taret different pathways. Using our Palb2 conditional knockout mouse model, we have identified autophagy as a protective mechanism for Palb2-deficient cells during breast cancer development. Our results suggest that, in the face of DNA damage and oxidative stress elicited by PALB2 loss, autophagy opposes a p53-mediated barrier to facilitate tumorigenesis. Based on the similarities between the molecular functions and clinical phenotypes of BRCA1/2 and PALB2, we hypothesize that autophagy inhibition may prevent the development and impede the progression of both PALB2- and BRCA-associated hereditary breast cancers. In this proposal, we will use conditional knockout and inducible knockdown mouse models to test the hypothesis, and deploy a combination of in vivo, ex vivo and in vitro analyses and metabolomics to identify the molecular mechanisms by which autophagy promotes the development of cancers associated with HR deficiency and oxidative stress. In Aim 1, we will define the role of autophagy in mammary epithelial cell fate following PALB2 and BRCA1/2 loss. In Aim 2, we will define the role of autophagy in Palb2- and Brca1/2- associated mammary tumor development and explore the potential of autophagy inhibition for prevention and treatment. In Aim3, we will investigate the mechanisms of autophagy-mediated promotion of Palb2- and Brca1/2-associated mammary tumorigenesis.