RETROVIRUSES

 

INTRODUCTION:  Retroviruses are unique in the sense that the flow of genetic information is reverse to the universal flow.  In all other organisms, DNA is the store house of genetic information and from here it is transmitted to RNA.  In retroviruses the sequence is RNA to DNA and consequent use of the enzyme reverse transcriptase.  Genetic information is passed on from RNA to DNA through RNA-dependent DNA polymerase (reverse transcriptase).

INVASIVENESS OF RETROVIRUSES:  The viral genetic information in DNA form is called as provirus.  This is capable of integration into the host genome.  On activation of provirus, virus specific proteins are manufactured. 

CLASSIFICATION:  The family Retroviridae comprised of three subfamilies: Oncovirinae, Lentivirinae and Spumavirinae.

HUMAN T CELL LEUKAEMIA VIRUSES:  The virion of human T cell leukaemia virus (HTLV) comprises icosahedral core containing the RNA genome and surrounded by an envelope acquired as the virion buds through the host cell membrane.  Virus specific envelope glycoproteins are inserted within the membrane that surrounds the virus.

Antigens:  Three structural antigens are recognised in HTLV.  These are core antigen, envelope antigen and RT antigen. The RT or in full Reverse Transcriptase antigen is derived from a polypeptide precursor which on being cleaved results into RT, protease and endonuclease.  RT is quite antigenic and sera from infected patients contain antibody to it.

HUMAN IMMUNODEFICIENCY VIRUS:  Emergence of this deadly disease appeared when the first few cases of AIDS were detected in June, 1981 by the US. Centers for Diseases Control and Prevention (CDC).  They reported a cluster of Pneumocystis pneumonia (PCP) caused by a form of Pneumocystis carinii, now recognised as a distinct species Pneumocystis jiroveci, in five homosexual men in Los Angeles.  The disease was originally dubbed GRID, or Gay-Related Immune Deficiency, but health authorities soon realized that nearly half of the people identified with the syndrome were not homosexual men.  In 1981, the CDC introduced the term AIDS to describe the newly recognised syndrome, though it was still casually referred to as GRID.  In 1983, scientists led by Luc Montaginer at the Pasteur Institute in France first discovered the virus that causes AIDS.  They called it lymphadenopathy associated virus (LAV).  A year later a team led by Robert Gallo of the United States confirmed the discovery of the virus, but they renamed it human T lymphotropic virus type III (HTLV-III).  The dual discovery led to considerable scientific disagreement, and it was not until President Mitterrand of France and President Reagan of the USA met that the major issues were resolved.  In 1986, both the French and the US names for the virus itself were dropped in favour of the new term, human immunodeficiency virus (HIV).

Global Scenario HIV infection in humans is now pandemic.  As of January 2006, the Joint United Nations Programmes on HIV/AIDS (UNAIDS) and the World Health Organization (WHO) estimate that AIDS has killed more than 25 million people since it was first recognized on December 1, 1981, making it one of the most destructive pandemics in recorded history.  In 2005 alone, AIDS claimed an estimated 2.4-3.3 million lives, of which more than 570,000 were children.  It is estimated that about 0.6% of the world’s living population is infected with HIV.  A third of these deaths are occurring in sub-Saharan Africa, retarding economic growth and increasing poverty.

Structure and Genome:   HIV is different in structure from other retroviruses.  It is about 120 nm in diameter which is around 60 times smaller than a red blood cell and roughly spherical.  It is composed of two copies of positive single stranded RNA that codes for the virus’s nine genes enclosed by a conical capsid composed of 2,000 copies of the viral protein P24.

RELATED;

1.  HIV/AIDS

2.  VIROLOGY

REFERENCES