Merkel cell carcinoma update: the case for two tumours

Merkel cell carcinoma (MCC) is an aggressive tumour with neuroendocrine differentiation. Clinically significant differences within the entity we know as MCC are apparent. This review aims to evaluate the evidence for differences in tumours within Merkel cell carcinoma and to stratify these. A literature search of research pertaining to various characteristics MCC was undertaken from 1972, when Merkel cell carcinoma was first described, to 2018, using PubMed and similar search engines. A total of 41 papers were analysed, including clinical trials, laboratory-based research and reviews. A proportion of MCC has Merkel cell polyomavirus genome integrated (MCPyV+) while others do not (MCPyV-). Both types have a different mutation burden. MCPyV+ tumours are likely true neuroendocrine carcinomas, with a dermal origin, probably from fibroblasts which have been transformed by integration of the viral genome. MCPyV-tumours are likely derived from either keratinocytes or epidermal stem cells, are probably squamous cell carcinomas with neuroendocrine differentiation, and are related to sun damage. Prognostic factors (apart from tumour stage) include the MCPyV status, with MCPyV+ tumours having a better prognosis. P63 expression confers a worse prognosis in most studies. CD8+ lymphocytes play an important role, providing a possible target for PD1/PD-L1 blockade treatment. The incidence of MCC varies from country to country. Countries such as Australia have a high rate and a far greater proportion of MCPyV- tumours than places such as the United Kingdom. MCC doubtlessly encompasses two tumours. The two tumours have demonstrated differences in prognosis and management. One is a neuroendocrine carcinoma related to MCPyV integration likely derived from fibroblasts, and the other is a UV-related squamous cell carcinoma with neuroendocrine differentiation, presumptively derived from either keratinocytes or epidermal stem cells. We propose naming the former Merkel type sarcoma and the latter squamous cell carcinoma, Merkel type.