The below mentioned article provides notes on retroviridae.
The family Retroviridae falls under the Group VI (ssRNA-reverse transcribing viruses). The virions are enveloped, helical or isometric having two linear (+) sense ssRNA, reverse transcriptase and integrase. Examples are MLV [Murine leukemia virus and related virus e.g. Spumavirus (human foamy virus)], Lentivirus (human immunodeficiency virus or HIV), Visna virus, Deltaretrovints, Alpharetrovims.
Out of the retroviruses known, HIV has drawn maximum attention due to spread of very dangerous disease called AIDS (acquired immuno-deficiency syndrome). AIDS is a condition in humans in which the immune system gradually fails, leading to opportunistic infections that threaten the life.
There has been controversy regarding the discovery of HIV. Luc Montagnier (France) claimed that he discovered HIV first in 1983. Similarly, Robert Gallo (America) also claimed that he discovered HIV first in 1984. In 1991 a study confirmed that the samples in Gallo’s laboratory had originated in Montagnier’s.
Therefore, the Karolinska Institute awarded half of the 2008 Nobel Prize in Physiology or Medicine to Montagnier and his colleague Francoise Barre-Sinoussi for their discovery of ‘human immunodeficiency virus’. The other half went to Harald zur Hausen (Germany) for unrelated work on ‘human papilloma virus causing cervical cancer’.
Retroviruses are also known as oncornaviruses because they are oncogenic (cancer causing). Most of the retroviruses infect vertebrates including fish, reptiles, birds, mammals, etc.
They are known to cause leukemia and sarcoma in chickens and lymphoma and mammary carcinoma in mice. So far it is not clear whether they cause cancer in humans or not. They are known as retroviruses because their genome synthesised DNA molecule which is a reverse phenomenon.
There are two strains of HIV that infect humans, HIV-1 and HIV-2. HIV-1 was known as human T-lymphotropic virus-III (HTLV-III), lymphadenopathy-associated virus (LAV), and AIDS- associated retrovirus (ARV). HIV-1 is more virulent, relatively easily transmitted, and is the global cause of majority of HIV infections.
HIV-2 is less transmit- table and is largely confined to West Africa. It is believed that both of the species originated in West- Central Africa, and jumped from a non-human primate to humans. During the 20th century HIV-1 as thought to have originated was in southern Cameroon after jumping from wild chimpanzees to humans.
The retrovirus particles are enveloped, icosahedral measuring about 100-120 nm. Virions consist of a nucleocapsid which is derived from the plasma membrane of host cell. The envelope encloses the virion. On the outer surface of virion several spikes are present which contain glycoproteins. The glycoprotein recognises the specific receptor site present on host cell.
The core capsid is composed of three major internal proteins (gp30) inside which are reverse transcriptase and RNA. Molecular weight of glycoprotein varies according to virus particle. Hence, the nomenclature of structural protein is done accordingly.
However, the major internal protein is known as gp30 (mol weight 30,000) and the glycoprotein as gp70 (mol weight 70,000). The structural proteins act as antigens and thus they induce antibodies. The retroviruses has been reviewed by Bishop (1978).
Genomes of all retroviruses consist of two molecules of RNA which are single-stranded, (+) sense and have 5′ cap and 3′ poly-(A) (similar to mRNA). The genome size varies from ~8 to 11 kb.
Retrovirus genomes have four unique features:
(i) They are the only viruses which are truly diploid,
(ii) They are the only RNA viruses whose genome is produced by cellular transcriptional machinery (without any participation by a virus-encoded polymerase),
(iii) They are the only viruses whose genome requires a specific cellular RNA (tRNA) for replication, and
(iv) They are the only (+)sense RNA viruses whose genome does not serve directly as mRNA immediately after infection.
Replication of retroviruses is very interesting. They carry their own enzyme reverse transcriptase. This enzyme was discovered by Temin and coworkers. Temin (1976) has given an excellent account of this enzyme. Varmus (1987) has discussed the action of reverse transcriptase and its role in retroviruses. Reverse transcriptase uses the viral RNA as template and synthesizes a DNA strand and forms RNA-DNA complex.
In turn the ssDNA replicates to forms a dsDNA which is transcribed back into RNA. The RNA acts as mRNA for viral protein synthesis (Fig. 17.40) and incorporated into the new virions. Before transcription, the viral DNA must be integrated into the host chromosome. This integrated DNA is called provirus that never comes out of the chromosome.