PROJECT SUMMARY The rate of early onset colorectal cancer (EOCRC; <50 years of age at diagnosis) continues to increase, even as CRC rates for individuals over 50 have been declining, largely as a result of prevention by enhanced colonoscopic screening. Cancers diagnosed in younger patients tend to be more distal/rectal, mucinous, poorly differentiated and diagnosed at an advanced stage, suggesting a rapid disease progression. Although sporadic EOCRCs are less likely to show aneuploidy, BRAF mutations or CIMP, they are otherwise similar at a molecular level to cancers in individuals greater than 50 years of age. Given the early formation and rapid progression of EOCRC, it is likely that strong promotional factors are at play. Fibroblasts are a key cell type in establishing a microenvironment conducive to cancer progression as they coordinate the activities of epithelial, endothelial and immune cells in the tissue. Fibroblasts can exist in a number of distinct states with dramatically different activities. Resident fibroblast in healthy tissue are non-dividing cells that help establish tissue architecture and crypt cell dynamics. However, when adjacent to cancerous cells, fibroblasts can become persistently activated as cancer-associated fibroblasts (CAFs) that promote tumor growth and angiogenesis, while suppressing immune responses. CAFs can also become senescent and acquire an irreversible senescence associated secretory phenotype (SASP) that establishes a “permanent” cancer promoting microenvironment. We hypothesize that the underlying stroma advances rapidly in EOCRC to drive early disease pathogenesis. Specifically, we propose that environmental and/or life-style factors cause aberrant fibroblast activation that negatively impacts the normal function of immunoregulatory cells within the stroma, while promoting epithelial cell division. The exploratory experiments proposed here will assess fibroblast proliferation, activation and senescence at different stages of cancer development in young patients. Understanding fibroblast dysregulation in individuals at risk of EOCRC could provide important information for understanding the factors responsible for the increasing incidence of EOCRC and ultimately point to approaches that reduce this risk. We will study fibroblast populations in colonic lesions from patients under 45 and over 60 years old. Using a combination of laser-capture microdissection combined with targeted RNA expression analysis and Imaging Mass CytometryTM, we will define the distinguishing set of molecular alterations characterize define EOCRC cases. Our study population will include normal mucosa, preneoplastic tissue with activated fibroblasts, advanced adenomas and CRCs. Overall, these studies will determine how a hyper-responsive “hot” stromal microenvironment established by activated and/or senescent fibroblasts relates to other stromal events that contribute to the rapid advancement of EOCRC. Once details of the activated fib...