Divergent roles of HMGCS2 in intestinal stemness and colonic tumorigenesis Chia-Wei Cheng1*, Omer H. Yilmaz2* 1Department of Genetics and Development at Columbia University; 2Koch Institute at MIT *Corresponding author Little is known how an endogenous metabolite functions as a cell-fate determinant and by which to influence tumor development. We previously reported that the ketone body-producing enzyme HMGCS2 (HMG-CoA synthase 2), distinguishes self-renewing Lgr5+ intestinal stem cells (ISCs) from differentiated cell types in the small intestine. ISCs rely on the HMGCS2-derived ketone body βOHB to regulate stem cell self-renewal and lineage commitment through HDAC inhibition and Notch signaling. Here, we investigate the role of HMGCS2 and the ketone body signaling in colon and colon cancer. In humans and mice, bimodal HMGCS2 expression distinguishes between proximal and distal colon. In humans, the anatomically relevant HMGCS2 expression pattern is established during development and maintained in adulthood but becomes less evident in tumors. Notably, TCGA data for colon adenocarcinoma indicates a poor overall survival in patients with HMGCS2low tumors. In mice, proximal and distal colonic crypts display distinct organoid forming capacities and cellular lineage compositions, with HMGCS2 exclusively expressed in the absorptive lineage. Loss of HMGCS2 in the APChet tumor mouse model reduces tumor lesions in the small intestine while exacerbating tumor progression in the distal colon, a novel phenotype previously thought to be an unsolved mouse versus human conundrum. These findings highlight the divergent roles of Hmgcs2 in intestinal stemness and colonic tumorigenesis and raise the possibility that dysregulation of metabolic cell-fate determinant may destabilize the lineage commitment and contribute to oncogenic transformation.