Polyposis Caused by Low APC Mosaicism

Purpose: To present a patient with familial adenomatous polyposis (FAP) caused by a low level of somatic mosaicism. Case description: A twenty-one year old female presented with rectal bleeding and abdominal pain. She underwent a colonoscopy and esophagogastroduodenoscopy which revealed extensive polyposis. There was no family history of polyps or early onset colon cancer in her family. Methodology: Next-generation sequencing (NGS) analysis was performed using the ColoSeqTM panel on DNA extracted from both peripheral blood lymphocytes and colonic polyps. RESULTS: Molecular analysis detected the p.E1408X deleterious mutation in the APC gene in in 12 of 276 (4%) reads of the DNA in the peripheral blood leukocytes and in 30% of the DNA from colonic polyps. Conclusion: We report that low level of 4% APC mosaicism led to florid polyposis. Our report highlights the power of deep next-generation sequencing to identify mosaic mutations that are missed by traditional approaches. Though somatic APC mosaicism has previously been reported to cause polyposis syndrome in a few cases, it has been underestimated as a cause of polyposis coli. This case should reinforce the need to search for mosaicism in all patients with a personal history of polyposis and no family history.