Track: Case Reports

Abstract:

Merkel cell carcinoma (MCC) is an uncommon, highly aggressive primary cutaneous neuroendocrine carcinoma with a high mortality rate, often requiring multimodal therapy and driving ongoing efforts to develop new treatments. An 80-year-old man with a prior history of multiple basal and squamous cell carcinomas, status post prostatectomy for prostate carcinoma, with no personal or family history of colorectal cancer, presented with a dime-sized pink plaque on the right superior medial malar cheek and a pink papule on the right anterior earlobe. Histopathological evaluation revealed sebaceous adenoma on the right anterior earlobe and MCC in the right superior malar cheek. This unusual presentation prompted immunohistochemistry studies on both specimens to include mismatch repair (MMR) proteins MSH2, MSH6, MLH1, and PMS2, with loss of staining for MLH1 and PMS2 in both tumors. High MMR protein expression is observed in the vast majority of MCC, with low-level expression occurring in a subset of MCC. Outright loss of MMR proteins as demonstrated in this case, is rare. MCCs demonstrating reduced or loss of MMR expression may represent a potential biomarker for further investigation into their response to immune checkpoint inhibitors (ICI), especially given the relevance of MMR deficiency in other cancers and their positive response to ICIs.

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Roy King