Project Summary The most aggressive cases of estrogen receptor-positive (ER+) breast cancers (BC) are diagnosed in young women (<50 years-of-age). This project is focused on an understudied, aggressive, and therapy-resistant form of ER+ BC that is characterized by predominantly ER+ luminal cancer cells interspersed with rare clusters of ER-negative stem/progenitor cells that express the basal cytokeratin-5 (CK5). Patients with this mixed ‘lumino- basal breast cancer’ (LBBC) phenotype represent a previously unrecognized population with unfavorable prognosis. These tumors are aggressive due to their inherent ER-independent endocrine escape pathway, which involves rapid expansion of the ER-/CK5+ cancer cell population in response to antiestrogen therapies. Our data reveal that LBBC shares the younger age distribution and aggressive features of basal-like BC, a BC subtype that is associated with pregnancy at early ages without breastfeeding – factors that are more prevalent among Black and poorer women and thus may help explain disparities in BC. Intriguingly, the basal-like features of LBBC are promoted by the pregnancy hormone progesterone and suppressed by the lactogenic hormone prolactin. Compelling pilot data indicates that progestins or anti-estrogens expand the therapy- resistant CK5+ stem/progenitor cell population in LBBC through a WNT11-PLZF-BCL6-dependent signaling network and is reversed by prolactin. The proposed studies will mechanistically test the general requirement of this WNT11-PLZF-BCL6 signaling network required for antiestrogen and progesterone-driven expansion of the CK5+ cell population and LBBC progression (Aim 1). Because the CK5+ cell population is often rare in untreated tumors, more robust diagnostic classifiers for LBBC than CK5 alone will be developed (Aim 2). Finally, efforts will explore the efficacy of targeted agents to disrupt this signaling network in vivo in preclinical models to provide rationale for new therapeutic strategies for LBBC (Aim 3). In sum, we will mechanistically validate a newly discovered pathway that promotes the therapy-resistant LBBC phenotype, develop better diagnostic methods, and explore therapeutic strategies to overcome endocrine resistance of these tumors. The project holds potential for high impact progress that can help reduce breast cancer mortality and morbidity.