Poxviridae

Structure

Poxviridae viral particles (virions) are generally enveloped (external enveloped virion- EEV), though the intracellular mature virion (IMV) form of the virus, which contains different envelope, is also infectious. They vary in their shape depending upon the species but are generally shaped like a brick or as an oval form similar to a rounded brick because they are wrapped by the endoplasmic reticulum. The virion is exceptionally large, its size is around 200 nm in diameter and 300 nm in length and carries its genome in a single, linear, double-stranded segment of DNA. By comparison, Rhinovirus is 1/10 as large as a typical Poxviridae virion.

Replication

Replication of the poxvirus involves several stages. The first thing the virus does is to bind to a receptor on the host cell surface; the receptors for the poxvirus are thought to be glycosaminoglycans (GAGs). After binding to the receptor, the virus enters the cell where it uncoats. Uncoating of the virus is a two step process. Firstly the outer membrane is removed as the particle enters the cell; secondly the virus particle (without the outer membrane) fuses with the cellular membrane to release the core into the cytoplasm. The pox viral genes are expressed in two phases. The early genes encode the non-structural protein, including proteins necessary for replication of the viral genome, and are expressed before the genome is replicated. The late genes are expressed after the genome has been replicated and encode the structural proteins to make the virus particle. The assembly of the virus particle occurs in five stages of maturation that lead to the final exocytosis of the new enveloped virion. After the genome has been replicated, the immature virion (IV) assembles the A5 protein to create the intracellular mature virion (IMV). The protein aligns and the brick-shaped envelope of the intracellular enveloped virion (IEV). These IEV particles are then fused to the cell plasma to form the cell-associated enveloped virion (CEV). Finally, the CEV encounters the microtubules and the virion prepares to exit the cell as an extracellular enveloped virion (EEV). The assembly of the virus particle occurs in the cytoplasm of the cell and is a complex process that is currently being researched to understand each stage in more depth. Considering the fact that this virus is large and complex, replication is relatively quick taking approximately 12 hours until the host cell dies by the release of viruses.

The replication of poxvirus is unusual for a virus with double-stranded DNA genome (dsDNA) because it occurs in the cytoplasm, 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 dsDNA viruses require the host cell's DNA-dependent RNA polymerase to perform transcription. These host DNA are found in the nucleus, and therefore most dsDNA viruses carry out a part of their infection cycle within the host cell's nucleus.

Evolution

The ancestor of the poxviruses is not known but structural studies suggest it may have been an adenovirus or a species related to both the poxviruses and the adenoviruses.

Based on the genome organisation and DNA replication mechanism it seems that 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 these genomes has been estimated to be 0.9-1.2 x 10⁻⁶ substitutions per site per year. A second estimate puts this rate at 0.5-7 × 10⁻⁶ nucleotide substitutions per site per year. A third estimate places the rate at 4-6 × 10⁻⁶.

The last common ancestor of the extant poxviruses that infect vertebrates existed 0.5 million years ago. 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 0.3 million years ago. A second estimate of this divergence time places this event at 166,000 ± 43,000 years ago. The division of the Orthopox into the extant genera occurred ~14,000 years ago. The genus Leporipoxvirus diverged ~137,000 ± 35,000 years ago. This was followed by the ancestor of the genus Yatapoxvirus. The last common ancestor of the Capripoxvirus and Suipoxvirus diverged 111,000 ± 29,000 years ago.

An isolate from a fish - Salmon Gill Poxvirus - appears to be the earliest branch in the Chordopoxvirinae.

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. This is consistent with archaeological and historical evidence regarding the appearance of smallpox as a human disease which suggests a relatively recent origin. However, if the mutation rate is assumed to be similar to that of the herpesviruses the divergence date between variola from Taterapox has been estimated to be 50,000 years ago. While this is consistent with the other published estimates it suggests that the archaeological and historical evidence is very incomplete. Better estimates of mutation rates in these viruses are needed.

Taxonomy

Group: dsDNA

The name of the family, Poxviridae, is a legacy of the original grouping of viruses associated with diseases that produced poxes in the skin. Modern viral classification is based on phenotypic characteristics; morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. The smallpox virus remains as the most notable member of the family.

The species in the subfamily Chordopoxvirinae infect vertebrates and those in the subfamily Entomopoxvirinae infect insects. There are 10 recognised genera in the Chordopoxvirinae and 3 in the Entomopoxvirinae. Both subfamiles also contain a number of unclassified species for which new genera may be created in the future. Cotia virus is an unusual virus that may belong to a new genus.

Vaccinia virus

The prototypial poxvirus is vaccinia virus, known for its role as the active agent 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 IMV contained within an envelope membrane. The exact origin of the envelope membrane is still unknown. The IMV is then transported to the Golgi apparatus where it is wrapped with an additional two membranes, becoming the Intracellular Enveloped Virus (IEV). The IEV is transported along cytoskeletal microtubules to reach the cell periphery, where it fuses with the plasma membrane to become the Cell-associated Enveloped Virus (CEV). This triggers actin tails on cell surfaces or is released as EEV.

History

Diseases caused by pox viruses, especially smallpox, have been known about for centuries. One of the earliest suspected cases is that of Egyptian pharaoh Ramses V who is thought to have died from smallpox circa 1150 years BCE. Smallpox was thought to have been transferred to Europe around the early 8th century and then to the Americas in the early 16th century. It is widely accepted that the main defeat of the Aztecs was due to a smallpox epidemic and within two years over 3.2 million Aztecs died. This death toll can be attributed to the American population's complete lack of exposure to the virus over millennia. A century after Edward Jenner showed that the less potent cow pox 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. Post September 11, 2001 the American and UK governments have had increased concern over the use of smallpox, or a smallpox like disease, in bio-terrorism.