Polyomaviridae

Polyomaviridae is a family of viruses whose natural hosts are primarily mammals and birds. As of 2020, there are six recognized genera and 117 species, five of which are unassigned to a genus. 14 species are known to infect humans, while others, such as Simian Virus 40, have been identified in humans to a lesser extent. Most of these viruses are very common and typically asymptomatic in most human populations studied. BK virus is associated with nephropathy in renal transplant and non-renal solid organ transplant patients, JC virus with progressive multifocal leukoencephalopathy, and Merkel cell virus with Merkel cell cancer. Polyomaviruses are non-enveloped double-stranded DNA viruses with circular genomes of around 5000 base pairs. The genome is packaged in a viral capsid of about 40-50 nanometers in diameter, which is icosahedral in shape (T=7 symmetry). The capsid is composed of 72 pentameric capsomeres of a protein called VP1, which is capable of self-assembly into a closed icosahedron; each pentamer of VP1 is associated with one molecule of one of the other two capsid proteins, VP2 or VP3. The genome of a typical polyomavirus codes for between 5 and 9 proteins, divided into two transcriptional regions called the early and late regions due to the time during infection in which they are transcribed. Each region is transcribed by the host cell's RNA polymerase II as a single pre-messenger RNA containing multiple genes. The early region usually codes for two proteins, the small and large tumor antigens, produced by alternative splicing. The late region contains the three capsid structural proteins VP1, VP2, and VP3, produced by alternative translational start sites. Additional genes and other variations on this theme are present in some viruses: for example, rodent polyomaviruses have a third protein called middle tumor antigen in the early region, which is extremely efficient at inducing cellular transformation; SV40 has an additional capsid protein VP4; some examples have an additional regulatory protein called agnoprotein expressed from the late region.