MOC-31/Ep-CAM immunoreactivity in Merkel cells and Merkel cell carcinomas

Aims: To evaluate the monoclonal antibody MOC-31in Merkel cell carcinomas and normal Merkel cells.Merkel cell carcinoma is a rare and aggressive tumourthat occurs mainly in elderly individuals. The histolog-ical diagnosis of Merkel cell carcinoma can be difficultbecause it looks similar to other small blue celltumours, particularly skin metastases of small-cell lungcarcinomas. This antibody recognizes the epithelial celladhesion molecule (Ep-CAM), that has been assigned tothe small cell lung cancer cluster 2 of antibodies. To thebest of our knowledge, immunostaining for MOC-31 ⁄Ep-CAM has not been previously described in Merkelcells or Merkel cell carcinomas.Methods and results: Thirty-one cases of Merkel cellcarcinoma and three samples of normal human finger-tip were selected to analyse the expression of MOC-31 ⁄Ep-CAM by immunohistochemistry. A high numberof Merkel cell carcinomas (21 ⁄ 31, 67.7%) showedintense and readily interpretable positivity. Immuno-staining was diffuse or focal and always localized to theplasma membrane. Normal Merkel cells of humanfingertip also showed plasma membrane immunoreac-tivity for MOC-31 ⁄ Ep-CAM.Conclusion: The demonstration of positivity for MOC-31/Ep-CAM in Merkel cell carcinomas precludes the useof this immunohistochemical marker to distinguishbetween tumours and skin metastases of small-cell lungcarcinoma.Keywords: MOC-31, Ep-CAM, EGP-2, Merkel cell carcinoma, immunohistochemistry