Abstract 4757: CREBBP drives tumorigenicity via aberrant IL-1α signaling in EP300 altered bladder cancer

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Abstract EP300 and CREBBP encode p300 and CBP respectively, which are H3K27 acetylases. Both EP300 and CREBBP mutated in a subset of urothelial cancers (∼15% for EP300 or CREBBP). The EP300 and CREBBP genes have ∼60% sequence similarity and thus have been presumed to have largely overlapping functional roles in cell homeostasis and cancer pathogenesis. To characterize the role of EP300 mutations in urothelial cancer pathogenesis and to identify non-redundant roles of these paralogues, we generated EP300 and CREBBP isogenic knockout urothelial cancer cell lines and characterized low passage mutant and wild type for EP300 and CREBBP urothelial cancer-derived patient derived organoids. EP300 KO and loss-of-function mutation was associated with enhanced cell growth in soft agar, increased invasive potential in vitro and altered cellular metabolism. These gain-of-function phenotypes were mediated by enhanced JAK-STAT3 activation resulting from IL-6 trans-signaling, the proximal driver of which was increased transcription and production of IL-1á. Notably, isogenic BLCA cells with CREBBP knockout did not confer IL-1á hypersecretion or hyperactivation of the IL-6/JAK1/STAT3 signaling axis indicating that this is a phenomenon was specific to EP300 loss-of-function. Transcriptomic analysis of Parental, EP300 KO, and CREBBP KO RT112 clones revealed that CREBBP KO significantly depressed IL1A transcript levels which EP300 KO significantly elevated. Additionally, CREBBP inactivated BLCA lines could not upregulate IL1A expression following genotoxic stress in contrast to parental and EP300 null cell lines. Using an inducible short hairpin RNA construct targeting CREBBP, we also find that CREBBP knockdown rescued IL1A upregulation in EP300 ko clones that coincided with a significant growth defect. Pharmacologic inhibition via the p300/CBP specific inhibitor A485 also abolished IL1A upregulation and caused significant growth defect in EP300 KO cells. EP300 null clones were also significantly more sensitive to A485 treatment than either CREBBP null urothelial cell or parental cell lines. In sum, our results identify regulation of IL-1á-JAK-STAT3 signaling as a novel non-redundancy between EP300 and CREBBP that could be exploited therapeutically in patient with EP300 loss-of-function mutations. Citation Format: James August Rodrigues, Hikmat A. Al-Ahmadie, Sizhi P. Gao, Jiaqian Luo, Jacob Tallman, Fengshen Kuo, Merve Basar, Cansu Yol, Jordan Eichholz, Alejandra Lopez Rojas, Ecenur Turkay, Jonathan E. Rosenberg, Gopa Iyer, Eugene J. Pietzak, Dan Li, Dana Schoeps, Shipra Shukla, Zoe Jacobs, Chen Khuan Wong, Woo Hyun Cho, Ping Chi, David B. Solit, Yu Chen, . CREBBP drives tumorigenicity via aberrant IL-1α signaling in EP300 altered bladder cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4757.

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