PROJECT SUMMARY/ABSTRACT Breast cancer is the most common cancer and the second most common cause of cancer death in women. Most breast cancers are estrogen receptor (ER) positive. The primary cause of treatment failure and death in patients with ER+ breast cancer is resistance to endocrine therapy (ET). Novel therapeutics are needed to improve outcomes for patients with resistant tumors. Several findings demonstrate that the RET receptor tyrosine kinase alters sensitivity to ET. RET expression in ER+ breast cancer is associated with worse outcome and recurrent tumors that are resistant to ET express RET at higher levels than primary tumors. Preliminary data show that RET is overexpressed over time in breast cancer cells treated with ET and that inhibiting RET reduces ERK/MAPK activity and sensitizes ER+ cell lines and xenografts to ET. We have established and validated ER+ breast cancer organoids which we will use to dissect precise mechanisms of increased RET expression, how RET impacts sensitivity to ET, and how RET directs kinase signaling with ET treatment. We hypothesize that high expression of RET in ET resistant ER+ breast cancers drives tamoxifen resistance and is a therapeutic target to overcome resistance. To test this hypothesis, in Aim1 we will define reprogramming of regulatory elements at the RET promoter over time with tamoxifen leading to increased expression of RET. In Aim2, we will determine the efficacy of targeting RET to enhance response to ET in cell lines, organoids, and patient derived xenografts. Finally, in Aim3 we will determine how RET alters kinase signaling adaptation in response to tamoxifen using a functional inhibitor bead capture assay coupled with mass spectroscopy. Cumulatively, these studies will form a scientific basis to develop clinical strategies and select patients for RET inhibitor therapy. In addition to advancing scientific knowledge, this proposal provides training to a physician-scientist. Dr Spanheimer is a practicing surgical oncologist specializing in breast cancer with a background in molecular biology. His long-term goal is to combine his research and clinical expertise to develop an independently funded research program focused on therapeutic vulnerabilities of altered gene regulation in response to therapy. He benefits from established mentors with a strong track record of training independent scientists and an extremely supportive research environment. This proposal includes a structured career development plan and training in: 1) molecular biology of transcriptional regulation, 2) targeted therapeutics and translationally relevant breast cancer models, and 3) functional proteomics. Training will also include development of expertise in increasingly complex hypothesis driven experimental design, execution, and analysis. The proposal includes mentored experiential learning, course work and conference participation, frequent mentor meetings and a graded increase in research independence. Cumul...