Frequent expression of glypican-3 in Merkel cell carcinoma: an immunohistochemical study of 55 cases.

The pathologic diagnosis of Merkel cell carcinoma (MCC) is mainly based on routine morphology and a characteristic cytokeratin 20 (CK20) immunohistochemical staining pattern. However, about 10% to 15% of MCCs are reportedly CK20 negative and about 10% to 20% of MCCs have only focal CK20 positivity. We aimed to study glypican-3 (GPC3) expression in MCC and to compare it with CK20 expression in MCC. Immunohistochemical expression of GPC-3, CK20, and CD23 were studied in 55 cases of MCC and 21 cases of noncutaneous small cell neuroendocrine carcinoma (SCNC), including 10 cases of pulmonary SCNC and 11 cases of nonpulmonary SCNC. The results of GPC3 expression in these tumors were then compared with that of CK20. Of the 55 cases, 39 were positive for GPC3 (70.9%). Seventy-seven percent of the positive MCCs (30/39) showed GPC3 immunoreactivity in > or = 30% tumor cells. Of the 55 cases, 53 were positive for CK20 (96%); 49 of them showed CK20 immunoreactivity in > or = 30% tumor cells. All MCC cases were either positive for both GPC3 and CK20 (37 cases) or at least 1 of the markers (18 cases, 16 CK20 only and 2 GPC3 only). In addition, we studied GPC3 and CK20 expression in 10 cases of pulmonary SCNC and 11 cases of nonpulmonary SCNC. Three of ten pulmonary SCNC and 5 of 11 nonpulmonary SCNC expressed GPC3. All pulmonary SCNC and 9 of 11 nonpulmonary SCNC were negative for CK20. All cases of MCC and SCNC were negative for CD23. In conclusion, GPC3 is frequently expressed in SCNC of various origins, in particular in MCC, which, in combination with CK20, may represent another useful marker in the diagnosis of MCC.