[A case of familial adenomatous polyposis in an adult male with Lynch-like syndrome].

Familial adenomatous polyposis and Lynch syndrome represent two different molecular pathways of colorectal carcinogenesis that are commonly considered mutually exclusive: chromosomal instability and microsatellite instability. Here, we report a rare case of familial adenomatous polyposis in an adult male with Lynch-like syndrome. A 46-year-old male patient was found to have hundreds of adenomatous polyps throughout the whole intestine, and irregular masses in rectum, sigmoid and transverse colon. Genetic test showed that the patient carried pathogenic germline APC (c.423-1G>A) variant and two variants of uncertain significance in MLH1 (p.R725H) and PTCH1 (p.S438N), combined with tumor characteristics of somatic AKT1/PIK3CA/KRAS co-mutations, microsatellite instability and high tumor mutation burden. The patient underwent laparoscopic total colectomy with abdominoperineal resection and end ileostomy, then received 4 cycles adjuvant chemotherapy of oxaliplatin with capecitabine. This patient was followed up to April 2024 and performed well without abnormalities in serum cancer biomarkers and radiological examinations.