Merkel cell polyomavirus: a newly discovered human virus with oncogenic potential

Abstract

A marked escalation in the rate of discovery of new types of human polyomavirus has occurred over the last five years largely owing to recent technological advances in their detection. Among the newly discovered viruses, Merkel Cell Polyomavirus (MCPyV or MCV) has gained the most attention due to its link with a rare human cancer. Infection with MCPyV is common in the human population, and the virus is detected in several anatomical locations, but most frequently in skin. Study of MCPyV molecular virology has been complicated by the lack of straightforward cell culture models, but recent in vitro studies are making strides towards understanding the virus life cycle, its cellular tropism, and mode of transmission. While MCPyV shares several traditional traits with other human polyomaviruses, the burst of research since its discovery reveals insight into a virus with many unique genetic and mechanistic features. The evidence for a causal link between MCPyV and the rare neuroendocrine cancer, Merkel Cell Carcinoma (MCC), is compelling. A majority of MCCs contain clonally integrated viral DNA, express viral T antigen transcripts and protein, and exhibit an addiction to the viral large T and small t antigen oncoproteins. The MCPyV large T antigen contains MCC tumor-specific mutations that ablate its replication capacity but preserve its oncogenic functions, and the small t antigen promotes an environment favorable for cap-dependent translation. The mechanisms of MCPyV-induced transformation have not been fully elucidated, but the likely etiological role of this new polyomavirus in human cancer provides a strong opportunity to expand knowledge of virus-host interactions and viral oncology.

Figure 1. Merkel cell location and immunoreactivity

A) Cutaneous Merkel cells located in the basal layer of the epidermis associated with the terminal ends of nerves, and also enriched in the bulge region of hair follicles, as depicted in a section of skin tissue. B) Whisker pad tissue was harvested from mice, paraffin-embedded, and representative sections analyzed by indirect immunofluorescence using Merkel cell-specific markers. Top panel: Images of a murine vibrissae follicle co-stained with antibodies specific to the Merkel cell markers cytokeratin 8 (CK8; red Alexa-Fluor 647) and cytokeratin 20 (CK20; green Alexa-Fluor 488). Bottom panel: A vibrissae follicle co-stained with the Merkel cell-specific markers CK8 (red; Alexa-Fluor 647) and neuron-specific enolase (NSE; green Alexa-Fluor 488). In each panel, representative cells staining positive for both Merkel cell markers are indicated with white arrowheads. High background staining with the Alexa-Fluor 488 secondary antibody is evident in cells surrounding the follicle (white arrows). This background staining was also seen in no primary antibody controls (data not shown). Both immunostaining analyses highlight the hair/whisker follicle as a prominent location of Merkel cells.

Figure 2. The Merkel Cell Polyomavirus (MCPyV) genome and capsids

A) Schematic of the MCPyV prototype MCV350. The double-stranded DNA genome is approximately 5.4 kilobases in length. The bi-directional origin of replication (white) promotes temporal expression of discrete regions of the genome. The early region encodes three viral T antigens: large T antigen (LT; red), 57 kT antigen (57 kT; orange) and small t antigen (sT; yellow). The late region is transcribed in the opposite direction from the origin and encodes the viral structural proteins: the major capsid protein Virus Protein 1 (VP1; green), and the minor capsid proteins Virus Protein 2 and Virus Protein 3 (VP2; blue, VP3; gray). B) Transmission electron micrograph of MCPyV virus-like particles composed of VP1 and VP2 proteins. The VP1 and VP2 proteins self-assemble into ~50 nm icosahedral particles. (Scale bar=100 nm)

Figure 3. The MCPyV T antigen locus

The MCPyV T antigen locus is located in the early region of the genome and encodes three T antigen isoforms that arise by alternative splicing. The large T antigen (LT) shares many traditional domains found in other polyomaviruses: CR1 (LXXLL; black), DnaJ (HPDKGG; pink), pRb-binding (LXCXE; red), a nuclear localization signal (NLS; orange), origin-binding domain (OBD; purple), and a helicase domain (green). LT also contains unique features, such as a binding site for Vam6p (blue) and a 200-amino acid “MCV-Unique Region” (MUR). The location of reported tumor-specific mutations in the 3′ region of LT is depicted with a red bar. The site of MCPyV microRNA (MCV-mIR-M1-5p) complementarity in LT is denoted (black arrow). The small t antigen (sT) shares the first exon with LT, but due to alternative splicing also contains a protein phosphatase 2A-binding site (PP2A; yellow). The 57 kT antigen is identical to LT up to the NLS motif, at which point differential splicing precludes the inclusion of complete OBD and helicase domains.