STK11 p.G270W: A Novel Mutation Detected in a Case of MSI High Mixed Medullary–Mucinous Carcinoma of the Transverse Colon

Background/Aim: To report a case of mixed medullary/mucinous adenocarcinoma with unusual mutational gene profile. Patients and Methods: A 79-year-old female was diagnosed with a colorectal carcinoma of the transverse colon. The diagnostic work-up of this case included thorough clinicopathological evaluation, immunohistochemistry and next generation Sequencing. Results: The clinicopathological evaluation showed a tumor with morphological features of both medullary and mucinous colorectal carcinoma. Immunohistochemistry revealed the loss of DNA mismatch repair proteins. NGS showed that the medullary component of this tumor had a novel STK11 p.G270W mutation, which was not present in the mucinous component. Both the medullary and mucinous components also had BRAF V600E and AKT1 (pE17K) mutations. Conclusion: We report a novel mutation STK11 (p.G270W), in medullary carcinoma of the colon with an associated mucinous component