Insight into the Wnt Pathway in Sporadic Small Bowel Adenocarcinoma

Background/Objectives: The Wnt signaling pathway is pivotal in the adenoma–carcinoma sequence; however, its role in small bowel adenocarcinoma (SBA) remains insufficiently characterized. We analyzed the clinicopathological significance of Wnt pathway-related gene mutations and the expression of downstream or associated proteins in SBA. Methods: Immunohistochemical staining for β-catenin, cyclin D1, c-Myc, E-cadherin, and Wnt5a was performed in 75 primary SBA surgical specimens. Targeted next-generation sequencing was conducted in 48 of these cases. Results: The genomic alterations in the Wnt pathway were identified as APC (14.6%) and CTNNB1 (8.3%), with no overlap between the two mutations. Aberrant (reduced membranous and/or nuclear) expression of β-catenin was observed in 37% of cases. Cyclin D1 and c-Myc were expressed in 60% and 41% of cases, respectively. Aberrant expression of β-catenin and/or Wnt5a was present in 60% of cases and was correlated with cyclin D1 and c-Myc expression. Mutations in APC and CTNNB1 were found in intestinal- and gastrointestinal-type SBAs, but were absent in gastric-type SBA. In intestinal-type SBA, the mutation frequency of APC and CTNNB1 was 39%, closely aligning with the 45% aberrant expression of β-catenin. Aberrant expression of β-catenin and/or Wnt5a, a ligand of the noncanonical Wnt pathway, was detected in 60% of cases and showed a correlation with both cyclin D1 and c-Myc expression. Conclusions: These findings suggest that both canonical and noncanonical Wnt pathway-related proteins are involved in SBA carcinogenesis and progression. Notably, the canonical Wnt pathway appears to play a predominant role in intestinal-type SBA.