Mechanisms of transition from pre-malignancy to cancer in the Brca1-mutant breast

SUMMARY – Project 3 Women with deleterious mutations in the breast cancer susceptibility gene BRCA1 have a high risk of developing breast cancer. Standard of care is risk-reducing mastectomy which is a painful procedure that often results in significant clinical complications, therefore the development of targeted therapy approaches for cancer prevention in these patients are desperately needed. BRCA1 encodes a protein with pleiotropic functions and most of its tumor-inducing role has been associated with its DNA repair function. Early studies suggested that genome instability is the main cause of tumor initiation in BRCA1 mutation carriers, but clinical observations and functional studies in animal models indicate that non-genetic mechanisms are also critical for cancer initiation. To identify potential non-genetic drivers, in preliminary studies we used a combination of single cell RNA sequencing and spatial genomics approaches to perform detailed characterization of the cellular and molecular changes that occur during tumor progression in a Brca1/p53 deficient mouse model of the disease. Our most striking finding is the presence of epithelial cells with a hybrid phenotype that co-express markers of both basal and luminal cells (termed basal-luminal intermediate, BLI cells). These cells appear in pre-malignant tissues and expand to constitute tumors. Our single cell analysis also revealed alterations in activin signaling, an important regulator of epithelial proliferation in the mammary gland, as well as alterations in the immune microenvironment in pre-malignant tissues. Utilizing mathematical modeling, we found that if these events are combined, they should be sufficient to induce tumor initiation at very low probability, which is consistent with observations in mice and humans that tumor initiation is a rare event that occurs after a prolonged latency period. This type of model is called “collective stochastic escape from growth control”. In the proposed studies, we will investigate mechanisms regulating the origin and fate of BLI cells in an effort to identify new ways to block expansion of these cells for potential cancer prevention. We will perform functional experiments to evaluate whether targeting activin signaling or the immune microenvironment attenuate cancer initiation, and represent actionable targets for therapeutic development.