New approaches to patient care and better understanding of disease mechanisms at the molecular level have brought considerable benefit to breast cancer patients and women at risk of the disease. Surgery has become less extensive and better tolerated for most patients, diagnostic imaging approaches have improved, and adjuvant chemotherapy and hormonal therapy have decreased a woman's risk of breast cancer recurrence. More recently, the introduction of the anti-HER2 targeted therapy (trastuzumab and lapatinib), aromatase inhibitors, and other targeted therapies have further improved disease outlook. Clinical experience and molecular analysis of breast tumor tissue revealed that breast cancer is a heterogeneous disease with at least three major subtypes, ER+/HER2- (luminal A-type); HER2+; and ER-/PR-/HER2- (basal-like breast cancer). Certain mutations activating specific signal pathways that are required for the development and maintenance of breast tumors are now known. Knowledge of the molecular properties of two of the three disease subtypes has allowed the development of rationale targeted therapy that is more specific and less toxic: hormonal interventions for ER+ disease and anti-HER2-directed therapy for HER2+ tumors. Unfortunately, currently there is no proven targeted therapy for basal-like tumors. Most of these tumors carry mutations in the TP53 tumor suppressor gene and display distinctive gene expression signatures resembling mammary epithelial stem cells. In part due to these properties, basal-like breast tumors have a higher risk of distant metastasis to visceral organs such as lung and brain than ER+ tumors. Basal-like breast cancer constitutes nearly 15% of sporadic breast cancer cases in the United States and ~40% of breast cancer cases in premenopausal African-American women. Because these tumors are often resistant to chemotherapy and occur at higher frequency in younger women than other breast cancer subtypes, the impact on breast cancer mortality is disproportionate to the incidence, accounting for an even higher proportion of breast cancer mortality than the incidence would imply.

Novel and systematic approaches to target identification in breast cancer are feasible, timely, and urgently needed. These approaches are particularly important to apply to a treatment-resistant subset of breast cancers, such as a fraction of basal-like breast tumors. The goal of this proposal is to use a novel somatic cell genetics approach combined with comprehensive unbiased screening strategies for the identification of hereditary molecular alterations in basal-like breast cancer cells that are responsible for the stem cell-like phenotype as associated with poor clinical outcome of patients diagnosed with this tumor type. In some ways, this approach is similar to the family linkage studies that were used to identify the BRCA1 and BRCA2 tumor suppressor genes, except that we are analyzing the "progeny" (basal-luminal cell hetero-fusion) of luminal and basal-like breast cancer cell "parents." Based on our preliminary data, we hypothesize that basal-like breast cancer cells have a genetic and/or epigenetic alteration that prevents the luminal epithelial differentiation of these cells and maintains them in a stem cell-like state. This hypothesis is based on our observation that somatic cell fusions of luminal and basal-like breast cancer cells resulted in basal-like phenotype, indicating the dominance of this trait over the more differentiated hormone receptor positive luminal epithelial cells. We will analyze the genome-wide genetic and epigenetic profiles of these fusion cells and characterize their functional properties such as their response to chemotherapy and ability to form tumors and metastasize. Using these unbiased comprehensive approaches, we will identify hereditary (e.g., genetic and/or epigenetic) events that inhibit luminal epithelial cell differentiation in these fusions and maintain them in basal-like state. Findings obtained in these cellular models will be validated in primary human tumor samples. In particular, we will determine if these events also characterize basal-like breast tumors.

Better understanding of the molecular mechanism responsible for the block of differentiation in basal-like breast cancer cells will aid the clinical management of patients diagnosed with this disease and may even lead to its prevention. Differentiation therapy has been successfully used in some tumor types that were characterized by block of cellular differentiation due to tumor-specific transforming events. Some of the alterations we will identify during the course of the proposed project may be amenable to therapeutic or preventive interventions that could results in decreased incidence and/or better outcome of patients diagnosed with basal-like breast cancer. Thus, the proposed project is highly innovative and relevant to breast cancer biology and the clinical management of breast cancer patients diagnosed with basal-like tumors. Depending on the specifics of the alterations we find, the results of this study more or less rapidly can be translated into clinical practice and improve the management of patients diagnosed with basal-like breast cancer.