[A Case of Lynch Syndrome Diagnosed with MSI-H Ulcerative Colitis-Associated Colon Cancer].

More than 90% of Lynch syndrome(LS)associated colorectal cancers show high-frequency microsatellite instability(MSI-H). It is known that ulcerative colitis(UC)is associated with a high incidence of colorectal cancer due to chronic inflammatory changes in the intestinal mucosa. MSI-H is also found in 2.6-40% of ulcerative colitis associated neoplasia(UCAN), and much neoplasia occur at a young age. Our case was a 35-year-old man who had developed ulcerative colitis at the age of 20 and was undergoing treatment. At the age of 35, he complained of right lower abdominal pain and was diagnosed with ascending colon cancer by colonoscopy. As the first stage of a 3-stage operation, laparoscopic total colectomy with ileostomy was performed. The histological type was poorly differentiated adenocarcinoma, and the progression was pT4a pN1bcM0, pStage Ⅲb. Before the start of adjuvant chemotherapy, MSI testing was performed, and the result was MSI-H. Immunohistochemistry revealed loss of expression of MSH2 and MSH6. Although there was no strong family history suggestive of LS, the patient's clinical characteristics, such as early age onset, location in the right-sided colon, and poorly differentiated adenocarcinoma as the primary histological type, were consistent with LS. As he requested genetic testing, Sanger sequencing of MSH2 was performed using peripheral blood, which revealed a likely pathogenic variant in MSH2 (NM_000251.3 c.1045C>T)and led to a diagnosis of LS. MSI-H may be observed in UCAN, and there are many similarities between the clinical and histopathological findings of UCAN and LS. Although the coexistence of UC and LS is rare, it is important to keep this possibility in mind when managing clinical cases.