Distinct Wnt Signaling Thresholds Required for Polyposis in the Proximal and Distal Stomach

Abstract

ObjectiveThe Wnt pathway is critical for gastrointestinal stem cell proliferation. Patients with Familial Adenomatous Polyposis (FAP) have germline mutations in the adenomatous polyposis coli (APC)gene, resulting in dysregulated Wnt signaling and a polyposis syndrome with stark regional differences in disease severity. In the stomach, the proximal corpus contains numerous polyps, while few polyps are located in the distal antrum. Here, we tested the hypothesis that there are regional differences in sensitivity to Wnt signaling along the gastric proximal‐distal axis, and that a ‘just‐right’ level of Wnt signaling is required for gastric polyposis.MethodsTo model gastric polyp formation in FAP patients, we used mice with an inducible Apcloss‐of‐function mutation. Gastric tissue responses were studied one month after maximal Wnt activation in homozygous Sox2‐CreERT2; Apc580flox/flox(APC‐/‐) and submaximal Wnt activation in heterozygous Sox2‐CreERT2; Apc580flox/+ (APC+/‐) mice. To gain insight into regional differences in Wnt response, RNA sequencing was performed on corpus and antral organoids treated with the Wnt inhibitor XAV939.ResultsIn the distal stomach, we observed hyperproliferation and tissue expansion in homozygous APC‐/‐ mice with no change in heterozygous APC+/‐mice. In contrast, the proximal stomach did not show an effect in homozygous APC‐/‐ mice but showed a modest increase in proliferation in heterozygous mice. PCR analysis of APC‐/‐ tissue confirmed APC gene deletion in the antrum, but not corpus, suggesting regional differences in maintenance of homozygous mutant stem cells. Following acute Wnt inhibition, corpus and antral organoids shared many Wnt‐regulated genes. However, Gene Ontology Enrichment analysis showed genes associated with cell differentiation, cell population proliferation, and cell cycle regulation were upregulated in antral organoids but downregulated in corpus organoids. KEGG analysis indicated downregulation of gastric cancer genes in both tissue organoids; however, only antral organoids showed downregulation of signaling pathways regulating pluripotency.ConclusionThe corpus and antrum exhibit differences in the Wnt signaling threshold required to initiate polyposis. Whereas submaximal levels of Wnt signaling are sufficient to initiate growth in the corpus, maximal levels are needed for antral hyperproliferation. Notably, ‘high‐Wnt’ stem cells are retained in the antrum and lost from the corpus, suggesting regional differences in Wnt regulation of gastric stem cells. Our findings in mouse highlight how heterozygous APC mutations may be sufficient for proximal stomach polyposis—a clinically relevant observation, as the vast majority of FAP patient gastric polyps are benign sessile growths located in the proximal stomach, with less frequent antral adenomatous polyps more closely associated with gastric cancer.