PROJECT SUMMARY Breast cancer is a deadly disease and new strategies are needed to fulfill the goals of treatment and eradication. Recent development of epigenetic-based inhibitors offers new avenues of potential therapeutics for breast and other human cancers. Our work using a new mouse model of breast cancer that parallels the aberrant expression of TRIM24 in all breast cancer sub-types will be used to gain a deeper understanding of tumor development and treatment. Epigenetic regulators are frequent targets of aberrant regulation, amplification or mutation in all human cancers. Histone “writers”, “erasers” and “readers” are epigenetic regulators that catalyze addition, removal and/or interaction, respectively, with post-translational modifications (PTMs) of histones or other modified proteins, with subsequent regulatory outcomes for gene expression. Our laboratory discovered Tripartite Motif Protein 24 (TRIM24) as a histone reader and showed that TRIM24: (i) ubiquitinated p53 and siRNA-depletion of TRIM24 led to p53-dependent apoptosis of embryonic stem cells and breast cancer-derived cells (MCF7), (ii) recruited estrogen receptor to chromatin by PHD/bromodomain reading of a unique signature of histone PTMs (H3K4me0; H3K23ac) to co-regulate estrogen-dependent transcription, (iii) induced transformation of immortalized human mammary epithelial cells (iHMECs) by altering metabolism and up-regulating c-Myc expression when ectopically expressed, and (iv) a small molecule inhibitor of the TRIM24 bromodomain disrupts chromatin interactions in vitro. Importantly, we found that aberrant expression of TRIM24 negatively correlates with breast cancer patient survival. TRIM24 is over expressed in all sub-types of breast cancer and is highest in basal breast cancers. We developed a mouse model of TRIM24-expressing breast cancers by conditional over-expression of a Trim24 transgene in mammary epithelia. We saw that aberrant, tissue-specific expression of TRIM24 is sufficient for tumor initiation, development and progression to highly heterogeneous mammary carcinomas. We hypothesize that our proposed, multi-faceted studies, including mouse models, cultured cells and in vitro analyses will uncover how aberrant expression of TRIM24 drives heterogeneous tumor development in mammary/breast epithelia, and that our findings will further development of epigenetic-based therapeutics to treat breast cancers. Our long-term goal is to leverage a deep mechanistic understanding of TRIM24 functions toward innovative therapeutic approaches to treat breast and other cancers in humans.