Diagnostic and biological implications of Mel-CAM expression in mesenchymal neoplasms

Mel-CAM (previously MUC18) is an integral membrane glycoprotein involved in heterophilic intercellular adhesions. Mel-CAM is expressed specifically in certain normal mesenchymal tissues, including smooth muscle, endothelium, and Schwann cells. As a member of the immunoglobulin supergene family of cell adhesion molecules (CAMs), Mel-CAM may play a pivotal role in the normal differentiation and functional activity of these tissues. To determine the distribution of Mel-CAM in mesenchymal neoplasms and to investigate its potential role as a factor in tumor progression, we evaluated a spectrum of mesenchymal neoplasms by immunohistochemistry using a Mel-CAM-specific polyclonal antibody on formalin-fixed tissues. Mel-CAM positivity was observed in 5 (100%) of 5 leiomyomas, 29 (91%) of 32 leiomyosarcomas, 5 (100%) of 5 hemangiomas, 5 (100%) of 5 angiosarcomas, 3 (100%) of 3 Kaposi's sarcomas, 8 (100%) of 8 schwannomas, 10 (100%) of 10 neurofibromas, 0 (0%) of 8 malignant peripheral nerve sheath tumors, 2 (15%) of 13 malignant fibrous histiocytomas, 0 (0%) of 8 fibrosarcomas, 0 (0%) of 7 synovial sarcomas, and 0 (0%) of 6 liposarcomas. These results show that Mel-CAM is expressed consistently in neoplasms of smooth muscle and vascular origin, and that immunostaining for Mel-CAM may serve as a useful adjunct in differentiating leiomyosarcomas, angiosarcomas, and Kaposi's sarcomas from other spindle cell neoplasms. Furthermore, the observation that Mel-CAM is expressed consistently in schwannomas and neurofibromas but not in malignant peripheral nerve sheath tumors implicates Mel-CAM as a potential modulator of malignant transformation in peripheral nerve tumors.