Transformation of distinct mammary epithelial cell populations influences breast cancer phenotype

The mammary gland is a branched, secretory organ that that is composed of diverse cells types. The populations that comprise the mammary epithelium include mammary stem cells, lineage-restricted progenitors, and differentiated luminal and basal cells. Breast cancer develops when one or a subset of these epithelial cells undergoes transformation, resulting in disease that can vary broadly in pathology and prognosis. For example, gene expression analysis of a large cohort of breast cancers has classified tumors into distinct subtypes, including: HER2-positive, Basal-Like, Luminal A, and Luminal B. Furthermore, hormone receptor status is also used in the clinic to classify tumors and define the course of treatment. Factors that may contribute to this broad breast cancer diversity are still unclear; however, we hypothesize that breast cancer phenotype may be influenced by intrinsic properties of the originating cell. Therefore, to determine whether specific cells types influence breast cancer pathology, the goal of this dissertation has been to develop a series of mouse models of breast cancer that can be used to target oncogene expression to distinct mammary epithelial cell (MEC) lineages. One such model was generated by infecting mouse MECs with a lentivirus expressing the polyoma virus middle T antigen oncogene, then cells were enriched for specific cell lineages using fluorescence activated cell sorting, and were orthotopically transplanted to generate tumors. Resulting tumors were classified by histology, estrogen receptor status, molecular subtype, and metastatic propensity. Our results demonstrated that each MEC population we evaluated could give rise to diverse tumor types; however, we also determined that MEC populations exhibited differences in their propensity for tumor histology, molecular subtype, and metastatic potential. Furthermore, we developed an additional approach of targeting specific gene expression to distinct MEC populations. In this system, lentiviral constructs were designed to express a specific gene after Cre-mediated recombination. Then, MECs isolated from mice that express Cre iv through cell lineage-restricted promoters were infected with the lentivirus and orthotopically transplanted or analyzed by three-dimensional culture. We validated this approach by targeting fluorescent Tomato reporter and polyoma virus middle T antigen oncogene expression to specific MEC populations in vitro and in vivo.