Merkel cell carcinoma: evaluation of KIT (CD117) expression and failure to demonstrate activating mutations in the C-KIT proto-oncogene - implications for treatment with imatinib mesylate.

Background: Merkel cell carcinomas (MCCs) express the tyrosine kinase receptor KIT. However, it is not known if MCCs have activating mutations in KIT that would make them responsive to treatment with imatinib mesylate. The purpose of this article is to describe the KIT immunohistological staining pattern (CD117) of MCCs and analyze those MCCs for mutations in areas of KIT where mutations are found in gastrointestinal stromal cell tumors.

Methods: We evaluated KIT immunostaining in nine MCCs from nine patients. In addition, we extracted DNA from the same MCCs, performed PCR amplification of C-KIT exons 9, 11, 13, and 17, and sequenced those gene products for mutations.

Results: Eight of nine (88.8%) MCCs expressed KIT. No mutations were found.

Conclusions: The majority of MCCs express KIT but do not contain activating mutations in exons 9, 11, 13, or 17 of KIT. Imatinib mesylate is unlikely to provide effective therapy in MCC unless activating mutations in other areas of KIT or activating mutations in other related genes can be detected.