? DESCRIPTION (provided by applicant): CD24, a GPI-linked plasma membrane protein, is a major yet understudied biomarker of poor patient outcomes in many cancer types. CD24 was long believed to lack the ability to drive aggressive tumor phenotypes. We challenged this view by showing CD24 drives bladder tumor growth and metastasis, revealing CD24 as a therapeutic target. However, anti-CD24 monoclonal antibody (mAb) therapy only led to moderate reduction in tumor growth and lung metastasis. Interestingly, we found shRNA and mAb both depleted cell surface CD24 similarly, yet shRNA reduced tumor growth and metastasis and intracellular CD24 levels more profoundly. We also discovered CD24 in the nucleus (nCD24). nCD24 is chromatin bound, and its level is reduced more by shRNA than by mAb. nCD24 drives bladder cancer cell growth and gene expression, while high patient tumor levels are associated with poor outcome. Since the moderate impact of mAb therapy may be due to an inability to deplete nCD24, we searched for ways to reduce total cellular CD24 and discovered that androgen receptor (AR) stimulation induces CD24 expression. Concurrent work defining the mutational landscape of bladder cancer found CBP and its paralog p300 are commonly mutated in tumors of low stage, grade and CD24 expression. Importantly, in their wild type form these bind AR and enhance its activity while depletion of CBP or p300 reduces bladder cancer cell CD24 expression and in vitro growth. Here we propose the Hypothesis that WT CBP/p300 and androgen drive metastasis via induction of CD24, which in turn regulates effectors of this process; and thus enhancing CD24 depletion therapy will improve tumor control. Three Specific Aims will test this hypothesis: 1) Investigate the relationship of CBP/p300 to CD24, tumor growth and patient outcome: The biological and prognostic relevance of CBP/p300 mutations or expression is unknown in any cancer. Using human tumor specimens and human bladder cancer cells lacking CBP/p300, we test the hypothesis that WT CBP/p300 expression is a poor prognostic marker for patients and, using animal models, that this is driven in part by CD24 and AR. 2) Determine the effectors of CD24. Finding nCD24 drives tumor growth and gene expression is a paradigm-shifting explanation of how CD24 can impact cancer behavior without an intracellular domain and why mAb therapy is suboptimal. We use ChIP-seq, transcriptional profiling, animal models and human specimens to test the hypothesis that nCD24 induces genes that drive bladder tumor progression. 3) Enhance the effectiveness of CD24 depletion therapy. CD24 depletion by si/shRNA or mAb leads to tumor regrowth in vivo despite continued CD24 suppression. Here we test the hypothesis that acquired and intrinsic resistance of cancer cells to CD24 depletion is due to specific targetable genes. We apply a synthetic lethal screen to identify those responsible for intrinsic resistance and study RCHY1, a gene we found induced in CD24 shRNA depleted bladder cancer cells as a promising candidate driver of acquired resistance. Impact: Our work will provide clinically relevant biomarkers and therapeutic targets while informing CBP/p300 and CD24 biology fields.