A critical unmet medical need is to understand why some patients diagnosed with hormone receptor (HR+, estrogen- and progesterone-receptor positive; Her2 negative) breast cancer experience recurrences with distant metastatic disease whereas others do not, despite similar treatment. This is a significant issue considering that the majority of patients diagnosed with breast cancer and that experience metastatic relapse have HR+ tumors. Metastatic spread occurs early, often prior to the detection of the primary mass, and is facilitated by crosstalk between the tissue and developing tumor. This gives rise to the possibility that differences in the normal tissue environment exist which enhance the risk for metastatic disease. However, the mechanisms through which changes arise in the mammary tissue environment and their contributions to metastatic breast cancer remain largely undefined. We recently established gut commensal dysbiosis, defined as an inflammatory microbiome with low biodiversity, enhances metastasis of HR+ tumors. Establishing dysbiosis prior to tumor initiation significantly enhanced dissemination of tumor cells into the blood, distal lymph nodes, and lungs, with no impact upon primary tumor growth. Dysbiosis enhanced the accumulation of mast cells into the normal mammary tissue of non-tumor-bearing mice, whereas inhibition of mast cell degranulation significantly reduced metastatic dissemination. These results indicated an important role for tissue-associated mast cells in the promotion of breast tumor dissemination. Mechanistically, we demonstrate that blockade of CCL2 prior to tumor initiation is sufficient to reduce mast cell accumulation in the mammary tissue and to diminish dysbiosis-induced dissemination of HR+ tumor cells. The goal of this proposal is to define how gut commensal dysbiosis distally orchestrates cellular and molecular changes in normal mammary tissues of non-tumor-bearing mice that enhance susceptibility to HR+ tumor dissemination. Aim 1 will define how commensal dysbiosis distally changes the normal (non-tumor-bearing) mammary tissue immune environment to favor early HR+ tumor dissemination. We will integrate a variety of approaches to investigate the link between the gut microbiome, host metabolism, and metastatic breast cancer. Aim 2 will establish how mast cells promote early dissemination of HR+ tumors. The goal of this aim will be to define how mast cells are activated in mammary tissues of mice with dysbiosis and establish how mast cells initiate metastasis of HR+ tumors through interactions with tissue fibroblasts and macrophages. These mechanistic studies will have the potential to unveil novel therapeutic targets for the prevention of metastatic breast cancer, targets that could be identifiable prior to a tumor diagnosis. Considering the role of the gut commensal dysbiosis, modification of microbiome or targeting of commensal-associated metabolites emerges as a potential low cost therapeutic to prevent or reduce the incidence of metastatic breast cancer.

PROJECT NARRATIVE Defining early ‘triggers’ that exist within the mammary tissue of non-tumor bearing individuals and establishing how these triggers enhance long-term risk for metastatic hormone receptor positive breast cancer would have a substantial impact on reducing the mortality associated with this disease. We have identified that gut commensal dysbiosis is an early trigger that induces specific immune changes in the normal mammary tissue that lead to enhanced long-term risk for metastatic breast cancer. This project will establish how the gut microbiome distally regulates normal mammary tissues to enhance early tumor dissemination, the impact of which could lead to therapies that modify the gut microbiome, host metabolism, or tissue microenvironment for the long-term prevention of metastatic breast cancer.