Retroviruses

Retorviruses are a large and diverse group of pathogenic and non-pathogenic viruses infecting animals and man. They can be transmitted between individuals of the same species or to other species (interspecies or trans-species transmission). They infect their specific target cells, transcribe their RNA into DNA using the enzyme reverse transcriptase (RT) and integrate as proviruses into the cellular genome of the target cells. Infections of germ cells result in the presence of theses viruses in the genome of all cells of the organism and transmission of these sequences to the offspring.

Retroviruses are so called because they go "backwards" in genetic information flow, that is, they synthesize DNA from an RNA template. This step occurs early in the replication cycle of retroviruses.5 Once the double-stranded DNA genome is formed in the "preintegration complex" of the virus; it migrates to the nucleus and becomes inserted into the DNA in the chromosomes of the host. Insertion of the viral genome into the chromosomal DNA is called integration. It results in the viral genome, now called the DNA provirus, being contiguous with the cellular DNA. This means that if the infected cell subsequently undergoes DNA synthesis and division, the proviral DNA will be replicated as part of the cellular DNA in the chromosomes.5

Until the discovery of these viruses it had been dogma that the transfer of the genetic information always occurs in the direction of DNA to RNA, so finding that some viruses carry out "transcription backwards" ("reverse transcription") caused something of a revolution. I tis now known that reverse transcription is carried out, not only by these RNA viruses, but also by some DNA viruses and by uninfected cells.4



This capacity of genomic integration results in the capability of retroviruses to cause lifelong infection, evade the usual mechanisms of immune system clearance, and produce chronic diseases in the host that become manifested after a long period that may last years and even decades.3

Endogenous Retroviruses

Endogenous retroviruses are genetic elements representing the result of retrovirus infections and integration of the proviruses into the germline of vertebrates including humans. Endogenous retroviruses behave like normal genes such as albumin or hemoglobin. The transmission from parent to child is called vertical transmission. Transmission from one individual to another is called horizontal transmission. It is important to note that endogenous retroviruses releasing infectious particles may also be transmitted horizontally.

There are no mechanisms to eliminate these viruses from the genome. However, parts of the virus may be mutated or transformed through various mechanisms thus making the virus defective and unable to replicate. Only a few endogenous retroviruses are able to replicate and produce infectious particles, and these are mainly found in mice, pigs and koalas.

Role of Endogenous Retroviruses in Animal and Human Health

Since trans-species transmission of retrovirus is very common, endogenous retrovirus may also be important for the health of other species. For example, pig cells can release endogenous retroviruses that infect human cells and therefore represent a risk for transplantation pig cells or organs.

Exogenous Retroviruses

Endogenous retroviruses may exist as complete infectious particles and in this case thay are called exogenous retroviruses. They comprise three distinct families:

  1. Oncovirinae (RNA tumor virus group). These are further subdivided into:
    • intracellular A-particles
    • B-type retroviruses (mouse mammary tumor virus
    • C-type retroviruses (leukemia viruses HTLV-1 and 2)
    • D-type retroviruses (cause tumors in primates)
  2. Lentivirinae (slow virus group). Lentiviruses do not cause cancer; they cause chronic, inflammatory disorders in the host. HIV-1 and 2 are related to this group.
  3. Spumavirinae (foamy virus group). Spumavirus has been isolated from many species, including man, and does not cause disease.2

References

  1. Retroviruses. Reinhard Kurth, Norbert Bannert
  2. A practical guide to clinical virology. John Ridley Pattison, Richard J. Whitley
  3. Textbook of pediatric infectious diseases, Volume 2 By Ralph D. Feigin
  4. Virology: principles and applications. John B. Carter, Venetia A. Saunders
  5. Infectious causes of cancer: targets for intervention edited by James J. Goedert





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