Poxviridae is a family of double-stranded DNA viruses. Vertebrates and arthropods serve as natural hosts. The family contains 28 genera that are assigned to two subfamilies: Chordopoxvirinae and Entomopoxvirinae. Entomopoxvirinae infect insects and Chordopoxvirinae infect vertebrates. Diseases associated with this family include smallpox.

Four genera of poxviruses can infect humans: Orthopoxvirus, Parapoxvirus, Yatapoxvirus, Molluscipoxvirus. Only 2 are species-specific to humans: variola virus (VARV), and Molluscum contagiosum (MCV). Smallpox was thought to have been transferred to Europe around the early 8th century and then to the Americas in the early 16th century, resulting in the deaths of 3.2 million Aztecs within two years of introduction. This death toll can be attributed to the indigenous population's complete lack of exposure to the virus over millennia.

A century after Edward Jenner (1798) showed that the less potent cowpox, could be used to effectively vaccinate against the more deadly smallpox, a worldwide effort to vaccinate everyone against smallpox began with the ultimate goal to rid the world of the plague-like epidemic. The last case of endemic smallpox occurred in Somalia in 1977. Extensive searches over two years detected no further cases, and in 1979 the World Health Organization (WHO) declared the disease officially eradicated.

In 1986, all virus samples were destroyed or transferred to two approved WHO reference labs: at the headquarters of the federal Centers for Disease Control and Prevention (the C.D.C.) in Atlanta, Georgia (the United States) and at the Institute of Virus Preparations in Moscow. After the September 11 attacks in 2001, the American and UK governments have had increased concern over the use of smallpox, or a smallpox-like disease, in bioterrorism. However, several poxviruses including vaccinia virus, myxoma virus, tanapox virus and raccoon pox virus are currently being investigated for their therapeutic potential in various human cancers in preclinical and clinical studies.

The study of poxviruses within cell cultures, allowed for the discovery of Guanosine-5'-triphosphate 5'-methyl-(GTP) capping, 2'-O-methylation, and 3' polyadenylation; vital molecules in the stability and effective translation of mRNA into proteins.

Microbiology

Structure

thumb|Poxviridae virion

Poxviridae viral particles (virions) are generally enveloped (external enveloped virion), although the intracellular mature virion form of the virus, which contains different envelope, is also infectious. They vary in their shape depending upon the species, but are generally brick- or oval-shaped. The virion is exceptionally large, its size ranges from 220-450nm long, 140-260nm wide, and 140-260nm thick, this is likely due to poxviruses encoding all of the vitally needed proteins for their self-sufficient DNA replication and transcription. The genome is a single, linear, double-stranded segment of DNA. On the outer surface membrane it has randomly arranged tubules.

Genome

Phylogenetic analysis of 26 different Chordopoxvirinae genomes has shown that the central region of the genome is conserved, and contains ~90 genes. The termini in contrast are not conserved between species. Of this group Avipoxvirus is the most divergent. The next most divergent is Molluscipoxvirus. Capripoxvirus, Leporipoxvirus, Suipoxvirus and Yatapoxvirus genera cluster together: Capripoxvirus and Suipoxvirus share a common ancestor and are distinct from the genus Orthopoxvirus. Within the Othopoxvirus genus, Cowpox virus strain Brighton Red, Ectromelia virus and Mpox virus do not group closely with any other member. Variola virus and Camelpox virus form a subgroup. Vaccinia virus is most closely related to CPV-GRI-90.

The GC-content of family member genomes differ considerably. Avipoxvirus, capripoxvirus, cervidpoxvirus, orthopoxvirus, suipoxvirus, yatapoxvirus and one Entomopox genus (Betaentomopoxvirus) along with several other unclassified Entomopoxviruses have a low G+C content while others - Molluscipoxvirus, Orthopoxvirus, Parapoxvirus and some unclassified Chordopoxvirus - have a relatively high G+C content. The reasons for these differences are not known.

The ends of their double-stranded DNA (dsDNA) genomes are closed with a hairpin-like structure which is AT-rich, and incompletely base-paired. This structure can exist in two forms: inverted, and complementary.

Replication

thumb|Poxviridae replication cycle

The replication of poxviruses is unique, as most DNA viruses enter the nucleus of host cells to being viral replication, whereas Poxviruses complete their full replication cycle within the host cells cytoplasm. The replication can be divided into early, intermediate and late phase. Firstly, the virus binds to glycosaminoglycan receptors on the host cells surface, and undergoes membrane fusion. although this is typical of other large DNA viruses. Poxvirus encodes its own machinery for genome transcription, a DNA-dependent RNA polymerase, which makes replication in the cytoplasm possible. Most double-stranded DNA viruses require the host cell's DNA-dependent RNA polymerase to perform transcription. These host polymerases are found in the nucleus, and therefore most double-stranded DNA viruses carry out a part of their infection cycle within the host cell's nucleus.

The intermediate phase of replication is critical because, in this stage, the virus affects the host's normal function, and modifies it more optimally, to itself. For example, the virus can inhibit host apoptosis and block the antiviral state. One early viral protein, B1 kinase, has also been shown to stimulate translation of postreplicative viral mRNAs, through specific phosphorylation of ribosomal subunits.

Based on the genome organisation and DNA replication mechanism a phylogenetic relationships may exist between the rudiviruses (Rudiviridae) and the large eukaryal DNA viruses: the African swine fever virus (Asfarviridae), Chlorella viruses (Phycodnaviridae) and poxviruses (Poxviridae).

The mutation rate in poxvirus genomes has been estimated to be 0.9–1.2 × 10<sup>−6</sup> substitutions per site per year. A second estimate puts this rate at 0.5–7 × 10<sup>−6</sup> nucleotide substitutions per site per year. A third estimate places the rate at 4–6 × 10<sup>−6</sup>.

The last common ancestor of the extant poxviruses that infect vertebrates existed . The genus Avipoxvirus diverged from the ancestor 249 ± 69 thousand years ago. The ancestor of the genus Orthopoxvirus was next to diverge from the other clades at . A second estimate of this divergence time places this event at 166,000 ± 43,000 years ago. A new systematic has been proposed recently after findings of a new squirrel poxvirus in Berlin, Germany.

Smallpox

The date of the appearance of smallpox is not settled. It most likely evolved from a rodent virus between 68,000 and 16,000 years ago. The wide range of dates is due to the different records used to calibrate the molecular clock. One clade was the variola major strains (the more clinically severe form of smallpox), which spread from Asia between 400 and 1,600 years ago. A second clade included both alastrim minor (a phenotypically mild smallpox), described from the American continents and isolates from West Africa, which diverged from an ancestral strain between 1,400, and 6,300 years, before present. This clade further diverged into two subclades, at least 800 years ago.

A second estimate has placed the separation of variola from Taterapox at 3000–4000 years ago.

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Vaccinia virus

The prototypical poxvirus is vaccinia virus, known for its role in the eradication of smallpox. The vaccinia virus is an effective tool for foreign protein expression, as it elicits a strong host immune-response. The vaccinia virus enters cells primarily by cell fusion, although currently the receptor responsible is unknown.

Vaccinia contains three classes of genes: early, intermediate and late. These genes are transcribed by viral RNA polymerase and associated transcription factors. Vaccinia replicates its genome in the cytoplasm of infected cells, and after late-stage gene expression undergoes virion morphogenesis, which produces intracellular mature virions contained within an envelope membrane. The origin of the envelope membrane is still unknown. The intracellular mature virions are then transported to the Golgi apparatus where it is wrapped with an additional two membranes, becoming the intracellular enveloped virus. This is transported along cytoskeletal microtubules to reach the cell periphery, where it fuses with the plasma membrane to become the cell-associated enveloped virus. This triggers actin tails on cell surfaces or is released as external enveloped virion.

See also

  • Quokkapox virus
  • Water warts

References

  • Electron micrographs of Orthopoxvirus and Parapoxvirus Genera, including the smallpox virus, have been collected by the International Committee on Taxonomy of Viruses in their Poxviridae picture gallery.
  • NCBI Taxonomy Page.
  • Poxviridae at the Viral Bioinformatics Resource Center .
  • Viralzone: Poxviridae
  • ICTV
  • Virus Pathogen Database and Analysis Resource (ViPR): Poxviridae