Diagnostic value of microtubule-associated protein-2 in Merkel cell carcinoma

Merkel cell carcinoma (MCC) is a rare, aggressive cutaneous carcinoma with neuroendocrine differentiation and a propensity for early spread to regional lymph nodes. Since surgical resection is the mainstay of treatment of MCC, differentiation of MCC from malignant lymphoma, metastatic small cell carcinoma, basal cell carcinoma, and malignant melanoma is very important and is sometimes challenging with routine histologic examination. Immunohistochemical studies may be required to differentiate MCC from other primary and metastatic skin neoplasms. Previously, the authors reported that microtubule-associated protein-2 (MAP-2) is a sensitive and specific marker for pulmonary neoplasms with neuroendocrine differentiation. Because MCC is also a neuroendocrine carcinoma, the authors hypothesized that MAP-2 may be expressed in MCC and therefore may be a useful marker in establishing an accurate diagnosis. MAP-2 staining was demonstrated in all 14 MCCs with diffuse (10 cases) to focal (4 cases) patterns of immunoreactivity. No MAP-2 immunoreactivity was observed in any lymphoma (14 cases), basal cell carcinoma (20 cases), or squamous cell carcinoma (14 cases). CK20 reactivity was present in 12 of 14 cases with focal (2 cases) to diffuse (10 cases) staining having the characteristic perinuclear dot-like pattern. NSE was positive in 13 of 14 cases, SYN was positive in all 14 cases, CHR was positive in 8 of 14 cases, CK7 was positive in 4 of 14 cases, and CD99 was focally positive in 2 cases and diffusely positive in 3 cases. MAP-2 showed a diffuse or focal staining of MCC with a +1 to +4 intensity in most cases. MAP-2 was positive in two cases of MCC that were negative for CK20 and CHR and negative or only slightly positive for SYN and NSE. Therefore, MAP-2 may be a valuable ancillary study in skin tumors suspicious for neuroendocrine origin with faint or negative staining with the antibodies traditionally used for diagnosing MCC. The authors believe this is the first study to demonstrate the utility of MAP-2 in the immunohistochemical workup of MCC. The authors recommend that MAP-2 be added to immunohistochemical panels to confirm the diagnosis of MCC.