MIC2 GENE-EXPRESSION IN CUTANEOUS NEUROENDOCRINE CARCINOMA (MERKEL CELL-CARCINOMA)

The distinction between small round-cell tumors in the skin is difficu lt and is dependent on clinical characteristics, morphology, immunophe notype, and genotype. We attempted to characterize further the immunop henotype of cutaneous neuroendocrine carcinomas (CNCs; Merkel cell tum ors) by using CD99, which recognizes the product of the pseudoautosoma l gene, MIC2. We studied the staining pattern of 12E7, a CD99 monoclon al antibody, in 21 examples of CNCs and O13, another CD99 monoclonal a ntibody, in a subset of cases for which formalin-fixed, paraffin-embed ded tissue was available in our archives. An additional neuroendocrine carcinoma from the endometrium also was studied. Nineteen cutaneous c ases did not display any staining, whereas two cases demonstrated conv incing cytoplasmic reactivity (two of 21). The endometrial neuroendocr ine carcinoma was also CD99(+). Regardless of these rare positive case s, the results contrast with the almost uniformly positive staining pa ttern reported in a subset of other small round-cell tumors, including Ewing's sarcoma, primitive neuroectodermal tumors, and many acute lym phoblastic leukemias and lymphoblastic lymphomas. Despite the rare CNC that may express CD99, a negative reaction with 12E7 or O13 or both f avors the diagnosis of CNC over other small round-cell tumors, given t he appropriate clinical, morphologic, and immunologic findings.