Mutations in APC (~85%) or β-catenin (~5%) initiate >90% of sporadic colorectal cancers. These mutations produce β-catenin accumulation and nuclear translocation, activating Tcf which drives transcriptional programs underlying tumorigenesis. GUCY2C is the intestinal receptor for the hormone guanylin expressed in colorectum. In the healthy state, guanylin-GUCY2C signaling regulates homeostatic processes that organize the crypt-surface axis, in part through an unknown mechanism which blocks β-catenin accumulation. Of significance, guanylin is the most commonly lost mRNA transcript in colorectal cancer. Hormone loss, which silences GUCY2C, occurs at the earliest stages of transformation through an unknown mechanism conserved in mice and humans. Preliminary studies reveal that guanylin loss is mediated by mutant APC-β-catenin-Tcf signaling. Moreover, transgenic guanylin expression eliminated carcinogen-induced tumor formation in mice. Based on these observations, we propose that mutant APC-β-catenin-Tcf signaling transcriptionally silences guanylin as an obligatory step in tumorigenesis because guanylin-GUCY2C signaling blocks β-catenin accumulation required for transformation. The correlative chemoprevention hypothesis suggests that guanylin replacement should eliminate tumorigenesis driven by APC-β-catenin signaling. Here, the Pathophysiologic Aim will identify the transcriptional mechanisms mediating guanylin loss by mutant APC-β-catenin-Tcf. These studies will establish a novel molecular link between mutant APC-β-catenin-Tcf signaling, guanylin suppression, and GUCY2C silencing which is obligatory for tumorigenesis. The Mechanistic Aim tests the hypothesis that guanylin is lost because GUCY2C blocks β-catenin accumulation required for tumorigenesis. These studies will demonstrate that the guanylin-GUCY2C axis restricts β-catenin accumulation by blocking protein translation (synthesis) and inducing proteosomal degradation. The Prevention Aim tests the hypothesis that colorectal cancer reflects suppression of guanylin expression and GUCY2C signaling which can be overcome by hormone replacement. These studies will demonstrate the ability of transgenic guanylin expression to repair oncogenic β-catenin signaling, reconstitute endogenous GUCY2C hormone expression, and prevent tumorigenesis following biallelic APC inactivation in mice. Together, these studies will reveal a previously unanticipated required step in colorectal cancer initiation involving guanylin loss and GUCY2C silencing. They will shift the pathophysiological paradigm for colorectal cancer from a disease of irreversible gene mutations to one of reversible hormone insufficiency. Mechanistically, they will identify a unique vulnerability at the apex of the oncogenic cascade in which β-catenin-dependent events required for tumorigenesis can be overcome by GUCY2C signaling. Moreover, they will establish the proof of principle that colorectal cancer can be prevented by hormone replacement. The potential for immediate translation is underscored by the approval of the oral GUCY2C ligand linaclotide to treat chronic constipation.