Hepadnaviridae

Hepadnaviridae
TEM micrograph showing hepatitis B virions
Virus classification
Group:	Group VII (dsDNA-RT)
Order:	Unassigned
Family:	Hepadnaviridae
Genera
Orthohepadnavirus Avihepadnavirus

Hepadnaviridae^{[lower-alpha 1]} is a family of viruses. Humans, apes, and birds serve as natural hosts. There are currently seven species in this family, divided among 2 genera. Its most well-known member is the Hepatitis B virus. Diseases associated with this family include: liver infections, such as hepatitis, hepatocellular carcinomas (chronic infections), and cirrhosis.^[1][2]

Taxonomy

Group: dsDNA-RT

^[2]

A new virus has been described in fish - White sucker hepatitis B virus.^[3] This is the first hepadnavirus described from fish. While clearly a hepadnavirus it appears only to be distantly related to the previously described genera and will almost certainly be placed in a new genus.

Several other viruses have been described from fish and from a frog: Bluegill hepadnavirus (BGHB), African cichlid hepadnavirus (ACHBV) and Tibetan frog hepadnavirus.^[4] It seems likely that new genera in this family will need to be created.

History and Discovery

Although liver diseases transmissible among human populations were identified early in documented medical history, the first known hepatitis with a viral etiological agent was Hepatitis A in the Picornaviridae family. Hepatits B Virus (HBV) was identified as an infection distinct from Hepatitis A through its contamination of measles, mumps, and yellow fever vaccines in the 1930s and 1940s. These vaccines contained HBV-infected human serum as a stabilizing agent. HBV was identified as a new DNA virus in the 1960s, followed only a few decades later by the discovery of the Flavivirus Hepatitis C. HBV was first identified in the lab as the "Australia agent" by Blumberg and colleagues in the blood of an Aboriginal transfusion patient. This work earned Blumberg the 1976 Nobel Prize in Medicine.

Genome

Hepadnaviruses have very small genomes of partially double-stranded, partially single stranded circular DNA. The genome consists of two strands, a longer negative-sense strand and a shorter and positive-sense strand of variable length. In the virion these strands are arranged such that the two ends of the long strand meet but are not covalently bonded together. The shorter strand overlaps this divide and is connected to the longer strand on either side of the split through a direct repeat (DR) segment that pairs the two strands together. In replication, this pds genome is converted in the host cell nucleus to covalently-closed-circular DNA (cccDNA) by the viral polymerase.

As it is a group 7 virus, replication involves an RNA intermediate. Four main open reading frames are encoded (ORFs) and the virus has four known genes which encode seven proteins: the core capsid protein, the viral polymerase, surface antigens—preS1, preS2, and S, the X protein and HBeAg. The X protein is thought to be non-structural. Its function and significance are poorly understood but it is suspected to be associated with host gene expression modulation.

Viral Polymerase

Hepadnaviridae encode their own polymerase, rather than co-opting host machinery as some other viruses do. This enzyme is unique among viral polymerases in that it has reverse transcriptase activity to convert RNA into DNA to replicate the genome (the only other human-pathogenic virus family encoding a polymerase with this capability is Retroviridae), RNAse activity (used when the DNA genome is synthesized from pgRNA that was packaged in virions for replication to destroy the RNA template and produce the pdsDNA genome), and DNA-dependent-DNA-polymerase activity (used to create cccDNA from pdsDNA in the first step of the replication cycle).

Envelope Proteins

The hepatitis envelope proteins are composed of subunits made from the viral preS1, preS2, and S genes. The L (for "large") envelope protein contains all three subunits. The M (for "medium") protein contains only preS2 and S. The S (for "small") protein contains only S. The genome portions encoding these envelope protein subuntis share both the same frame and the same stop codon (generating nested transcripts on a single open reading frame. The pre-S1 is encoded first (closest to the 5' end), followed directly by the pre-S2 and the S. When a transcript is made from the beginning of the pre-S1 region, all three genes are included in the transcript and the L protein is produced. When the transcript starts after the pro-S1 at the beginning of the pre-S2 the final protein contains the pre-S2 and S subunits only and therefore is an M protein. The smallest envelope protein containing just the S subunit is made most because it is encoded closest to the 3' end and comes from the shortest transcript. These envelope proteins can assemble independently of the viral capsid and genome into non-infectious virus-like particles that give the virus a pleomorphic appearance and promote a strong immune response in hosts.

Replication

Hepadnaviruses 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.^[5] Most hepadnaviruses 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 viral polymerase to form a complete circular dsDNA genome (called covalently-closed-circular DNA or cccDNA). The genome then undergoes transcription by the host cell RNA polymerase and the pregenomicRNA (pgRNA) is sent out of the nucleus. The pgRNA is inserted into an assembled viral capsid containing the viral polymerase. Inside this capsid the genome is converted from RNA to pdsDNA through activity of the polymerase as an RNA-dependent-DNA-polymerase and subsequently as an RNAse to eliminate the pgRNA transcript. These new virions either leave the cell to infect others or are immediately dismantled so the new viral genomes can enter the nucleus and magnify the infection. The virions that leave the cell egress through budding.

Structure

Viruses in Hepadnaviridae are enveloped, with spherical geometries, and T=4 symmetry. The diameter is around 42 nm. Genomes are circular, around 3.2kb in length. The genome codes for 7 proteins.^[1]

Evolution

Based on the presence of viral genomes in bird DNA it appears that the Hepatoviruses evolved >82 million years ago.^[6] Birds may be the original hosts of the Hepatovirus with mammals becoming infected after a bird -> mammal host switch.

Cell Tropism

Hepadnaviruses, as their "hepa" name implies, infect liver cells and cause hepatitis. This is true not only of the human pathogen Hepatitis B Virus but also the hepadnaviruses that infect other organisms. The "adhesion" step of the dynamic phase—in which an exterior viral protein stably interacts with a host cell protein—determines cell tropism. In the case of HBV the host receptor is human sodium taurocholate receptor (NTCP), a mediator of bile acid uptake, and the virus anti-receptor is the abundant HB-AgS envelope protein.^[7]

Notes

References

External links

• Viralzone: Hepadnaviridae
• ICTV
• "Hepadnaviridae". NCBI Taxonomy Browser. 10404.