Triple negative breast cancers (TNBCs) do not express estrogen receptor-α (ERα), progesterone receptor (PR), or human epidermal growth factor receptor 2 (HER2), and therefore, none of the targeted drugs currently in use for breast cancer are effective against them. Approximately 60-80% of TNBCs express estrogen receptor-β (ERβ). However, pro- versus anti-tumorigenic capabilities of ERβ remains controversial. Another key molecular characteristic of TNBC is the high frequency (80%) of p53 mutation. In addition to losing tumor suppressor properties and exerting dominant-negative regulation over any remaining wild type p53 (WTp53), mutant p53 also acquires oncogenic gain-of-function. Increasing evidence suggests that not all mutant p53s function similarly. Although ERβ and p53 have been implicated in TNBC pathology, whether p53 has a role in the pro- versus anti-proliferative functional duality of ERβ remains an open question. The long-term goal is to understand and exploit the role of ERβ-p53 crosstalk in breast cancer for the development of better therapeutic strategies. The objective is to study how specific mutations in p53 impinges upon ERβ function in TNBC, with the prediction that specific p53 mutation will determine its role in the ERβ-mutant p53-p73 signaling axis impacting multiple aspects of tumor progression and metastasis. The hypothesis is that ERβ binds to and inhibits both WTp53 and mutant p53, leading to opposite effects on progression and therapeutic response of TNBC to agents such as Tamoxifen (Tam). The rationale for the proposed research is that understanding how ERβ elicits opposite functions in a p53 status-dependent manner will be critical to stratify TNBC patients to repurpose established therapeutic agents such as Tam to treat large percentage of TNBC patients. The specific aims are: (1 Determine the interaction of different p53 mutants with p73 and ERβ in TNBC cells; (2) Analyze the differential effects of p53 mutants on tumor progression, metastasis and therapeutic response in vivo; and (3) Evaluate the clinical significance of the ERβ-p53-p73 signaling axis. In specific aim 1, Isogenic TNBC cells expressing different combinations of ERβ and WT and p53 mutants generated using CRISPR technology and shRNA-mediated conditional knockdown will be used for analyzing the mechanisms underlying the interaction and its impact on cellular functions in vitro and tumor progression in vivo. In specific aim 2, the effect of different p53 mutations on tumor growth and metastasis will be analyzed in vivo. The clinical relevance of these studies will be evaluated using well-characterized patient derived xenografts (PDXs); patient tumor-derived organoids (PDOs); and patient tumor tissues with linked clinical database (specific aim 3). The contribution of this research is expected to be better understanding of the mechanisms by which ERβ-p53-p73 axis in the context of different p53 mutations affects the disease progression and therapeutic res...