Merkel Cell Polyomavirus - Classification

Classification

Polyomaviruses are small (~5400 base pair), non-enveloped, double-stranded DNA viruses. MCV is the fifth polyomavirus that infects humans to be discovered. It belongs to the murine polyomavirus group, one of the three main clades of polyomaviruses. (The group is named for murine polyomavirus, the earliest virus of the group to be discovered, and does not imply that MCV is transmitted to humans from rodents.) Although it has been confused with the controversial SV40 virus in some blog postings, it is a completely distinct virus.

MCV is genetically most closely related to the African green monkey lymphotropic polyomavirus (formerly known as African green monkey lymphotropic papovavirus), which is consistent with MCV coevolving with human primates.

The prototype sequence of MCV has a 5387 base pair genome, and encodes characteristic polyomavirus genes including a large T antigen, small T antigen, VP1 and VP2/3 genes . MCV T antigen has similar features to the T antigens of other polyomaviruses, which are known oncoproteins, and is expressed in human tumors. The T antigen is a spliced gene that forms multiple different proteins depending on the splicing pattern. Both large T and small T oncoproteins are probably needed to transform healthy cells into cancer cells, and they act by targeting tumor suppressor proteins, such as retinoblastoma protein. The large T antigen possesses a helicase motif needed for virus replication that is deleted in MCC tumors. Unlike for other polyomaviruses, MCV small T antigen transforms cells in vitro by activating cap-dependent translation.