TTF-1 and PAX5 Are Frequently Expressed in Combined Merkel Cell Carcinoma.

Thyroid transcription factor-1 (TTF-1) is a marker of tumors of pulmonary and thyroid origin, and its expression practically excludes diagnosis of Merkel cell carcinoma (MCC). However, TTF-1 expression in combined MCC was recently reported. PAX5 is a marker of B-cell origin that is also expressed in most classical MCC cases; however, its expression was not described in combined MCC. The authors decided to evaluate the expression of both these markers in a group of 5 combined MCCs (2 with invasive squamous cell carcinoma, 2 with squamous cell carcinoma in situ, and 1 with basal cell carcinoma). Expression of TTF-1 was found in 4 of 5 cases; in 3 cases, the marker was shown in the MCC component (weakly in 2 cases and strongly in 1 case), whereas the non-MCC component presented TTF-1 expression in 2 cases. A weak-to-moderate immunoreactivity for PAX5 was identified in all cases of the MCC component but in none of the non-MCC component. The results show that the expression of TTF-1 is a frequent finding in combined MCC and can be present in the neuroendocrine component, which differs from the conventional MCC. In contrast, PAX5 expression pattern is similar to that of the classical MCC.