African Americans (AA) have the greatest burden of colorectal cancer (CRC) in the US, and biological reasons for this disparity remain incompletely understood. Interactions between host and environmental factors, including chemopreventive treatments, are known to modify CRC risk, and there is emerging evidence of differences in treatment effects between AA and European Americans (EA) for the two most promising chemopreventive agents, vitamin D and aspirin. Our broad objective is to model and understand how interethnic differences in treatment responses could contribute to CRC disparities. Our laboratory has optimized stem cell-derived human organoid cultures to study cellular responses between individuals that have not been feasible using traditional models. In this proposal, we use colonic organoids to test the central hypothesis that transcriptional, chromatin accessibility and cellular responses to vitamin D and/or aspirin differ between AA and EA, and that these inter-ethnic differences could impact CRC risk and clinical treatment response. We previously treated ex vivo primary colon tissue from AA and EA with active vitamin D (1,25D) and identified several genes with inter-ethnic response differences. The success of finding genes with inter-ethnic response differences provides rationale for extending our genome-wide approach to 1,25D, aspirin and combination treatments in a larger sample size of colonic organoids (80 AA & 80 EA) to achieve greater power to identify inter-ethnic differences in transcriptional networks as well as chromatin accessibility. We will replicate observed differences in an independent cohort of organoids as well as test for cancer-relevant cellular treatment phenotypes in a subset of treated organoids (50 AA & 50 EA) (Aim 1). Further, using RNA-seq data obtained in Aim 1, we will test for a genetic contribution to response differences between individuals and ethnicities using allele specific expression. The response genes and SNPs identified will be tested for enrichment among genes and SNPs from NIH-funded CRC GWAS and chemoprevention trials as well as tested mechanistically using functional assays (Aim 2). The outcomes of our innovative study will: i) elucidate underlying biology and genetic architecture of responses to vitamin D and aspirin in the colon and how they differ by ethnicity, ii) connect cellular response with CRC risk and response to chemoprevention, and iii) identify genes and SNPs for mechanistic studies as well as for possible development as novel biomarkers for personalized CRC prevention in order to, ultimately, reduce CRC disparities.