Hepadnaviridae

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? Hepadnaviruses
Virus classification
Group: Group VII (dsDNA-RT) Family: Hepadnaviridae
Genera
Orthohepadnavirus Avihepadnavirus

Hepadnaviruses are the viruses in the family Hepadnaviridae. Hepadnaviruses cause liver infections in humans and animals. There are two genera included here:

• Genus Orthohepadnavirus; type species: Hepatitis B virus
• Genus Avihepadnavirus; type species: Duck hepatitis B virus

Hepadnaviruses have very small genomes of partially double-stranded, partially single stranded non-circular DNA. The genome consists of two uneven strands of DNA of which one is in a negative sense orientation and the other longer strand is in a positive sense orientation. As it is a group 7 virus, replication involves a RNA intermediate. Three main open reading frames are encoded (ORFs) and the virus has four known genes which encode the core protein, the virus polymerase, surface antigens (preS1, preS2, and S) and the X protein. The X protein is thought to be non structural; however its function and significance is poorly understood.

[edit] Hepadnavirus Replication and Treatment

They have a peculiar mode of replication; they replicate through an RNA intermediate (which they transcribe back into cDNA using reverse transcriptase). The reverse transcriptase becomes covalently linked to a short 3- or 4-nucleotide primer^[1]. Most hepadanaviruses will only replicate in specific hosts and this makes experiments using in vitro methods very difficult.

The virus binds to specific receptors on cells and the core particle enters the cell cytoplasm. This is then translocated to the nucleus where the partially double stranded DNA is 'repaired' by the cell to form a complete circle of DNA. This then undergoes transcription by the host cell RNA polymerase and the transcript is translated by host cell ribosomes. New virus particles are formed which acquire lipid from the endoplasmic reticulum of the host cell and the genome is packaged within these particles which then bud off from the cell.

Hepadnavirus infected cells translate the protein known as the virus surface antigen many times until there is too much protein to coat the virions formed these proteins then aggregate to form rod shapes and it is this Australian antigen or the hepatitis B surface antigen which are released from the cell this then leads to a very strong immune response from the host. It is thought that most of the people that come into contact with the virus are able to clear the infection alone however there are some that cannot and this leads to fulminant Hepatitis this causes severe liver damage and in very rare cases may also lead to Primary Hepatocellular Carcinoma[1]. The treatment for hepatitis B includes a course of alpha interferon; this is a very expensive treatment which lasts for around 12-15 weeks. The treatment makes the patient very sick. It seems that the interferon treatment aims at kick starting the host immune response to clear the infection; it is not the drugs that clear the infection. There are reverse transcriptase inhibitors available as treatment these drugs target the virus replication strategy by as the name suggests inhibiting reverse transcription. There is also a way of preventing Hepatitis B infection with a recombinant Hepatitis B surface antigen vaccine.

[edit] External links

1 http://www.microbiologybytes.com/virology/HBV.html

[edit] References