An appendix carcinoid tumor in a patient with hereditary nonpolyposis colorectal cancer.

Gastrointestinal carcinoid tumors are often associated with other tumors, particularly colon adenocarcinomas; but the association between carcinoid tumors and hereditary nonpolyposis colorectal cancer (HNPCC) syndrome has not yet been explored. We report an unusual case of a 28-year-old woman with HNPCC who underwent surgery for a transverse colon adenocarcinoma in whom an appendix carcinoid tumor was incidentally found. To assess whether the carcinoid tumor displayed the characteristic molecular features of HNPCC tumors, we investigated the expression of mismatch-repair (MMR) proteins and microsatellite instability (MSI) status in both tumors. Both tumors demonstrated normal expression of the MMR proteins hMLH1, hMSH2, hMSH6, and hPMS2. Interestingly, the adenocarcinoma exhibited an MSI phenotype but the carcinoid tumor did not, indicating that these 2 tumors arose through different molecular pathways.