Merkel Cell Polyomavirus (MCPyV) and Cancers: Emergency Bell or False Alarm?

SIMPLE SUMMARY: Merkel cell polyomavirus is a widespread pathogen, strongly associated with the highly aggressive Merkel cell carcinoma. In this review paper we aim to examine a possible association between Merkel cell polyomavirus and the emergence of different types of cancer. Evidently, Merkel cell polyomavirus is the major oncogenic factor for Merkel cell carcinomas, but no establishing results can be derived about the virus’ prevalence and role in the pathogenesis of other malignant diseases. Most certainly, it is imperative that the virus’ biology, infection cycle, as well as its interactions with the host, are further investigated. Identifying a link between a virus and tumorigenesis is crucial and could lead to new virus-targeted therapeutic approaches with one final target: cancer elimination. ABSTRACT: Merkel cell polyomavirus (MCPyV), the sole member of Polyomavirus associated with oncogenesis in humans, is the major causative factor of Merkel cell carcinoma (MCC), a rare, neuroendocrine neoplasia of the skin. Many aspects of MCPyV biology and oncogenic mechanisms remain poorly understood. However, it has been established that oncogenic transformation is the outcome of the integration of the viral genome into the host DNA. The high prevalence of MCPyV in the population, along with the detection of the virus in various human tissue samples and the strong association of MCPyV with the emergence of MCC, have prompted researchers to further investigate the role of MCPyV in malignancies other than MCC. MCPyV DNA has been detected in several different non-MCC tumour tissues but with significantly lower prevalence, viral load and protein expression. Moreover, the two hallmarks of MCPyV MCC have rarely been investigated and the studies have produced generally inconsistent results. Therefore, the outcomes of the studies are inadequate and unable to clearly demonstrate a direct correlation between cellular transformation and MCPyV. This review aims to present a comprehensive recapitulation of the available literature regarding the association of MCPyV with oncogenesis (MCC and non-MCC tumours).