Small Intestinal Involvement and Genotype-Phenotype Correlation in Familial Adenomatous Polyposis

Background and Aims: Small intestinal involvement in familial adenomatous polyposis (FAP) remains unclarified. We performed capsule endoscopy (CE) and balloon-assisted (BA) endoscopy to analyze small intestinal polyps and their genotype-phenotype correlations in a large cohort of FAP patients. Methods: In this retrospective study, we performed CE in all 149 FAP patients and BA endoscopy in 74 patients with larger polyps. The prevalence of small intestinal adenoma with high-grade dysplasia and cancer was investigated. Correlation between APC variant and polyp phenotype was analyzed. Results: Median patient age was 44 years and 69% were male. Pathogenic germline APC variants were found in 117/136 (86.1%). Overall prevalence and median number of small intestinal polyps were 85.2% (127/149) and 17 (IQR 6-36), respectively. A total of 5318 polyps were detected by CE and all 433 polyps histologically examined were confirmed as adenoma. The number of polyps significantly decreased from the proximal-to-distal tracts. Adenoma with high-grade dysplasia was detected in 26.2% (39/149), and its incidence among the polyps was approximately 1.4% (74/5318). Of these, 14 were intramucosal carcinomas with a prevalence of 5.4% (8/149). The germline APC variant in codon 1251-1580 was significantly correlated with a high number of small intestinal polyps. Multivariate analysis revealed APC variant in codon 1251-1580 and a high number of small intestinal polyps as independent risk factors for adenoma with high-grade dysplasia. Spigelman stage was significantly associated with number of jejunal/ileal polyps and their high-grade dysplasia. Conclusion: Among FAP patients, small intestinal adenoma with high-grade dysplasia was detected in 26.2% and cancer in 5.4%. FAP patients, particularly those with APC variant in codon 1251-1580 and/or high Spigelman stage may require surveillance for small intestinal polyps.