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Family of viruses

Hepadnaviridae is a family of viruses. Humans, apes, and birds serve as natural hosts. There are currently 18 species in this family, divided among 5 genera. Its best known member is hepatitis B virus. Diseases associated with this family include: liver infections, such as hepatitis, hepatocellular carcinomas (chronic infections), and cirrhosis. It is the sole accepted family in the order Blubervirales.

Although liver diseases transmissible among human populations were identified early in the history of medicine, the first known hepatitis with a viral etiological agent was Hepatitis A, in the picornaviridae family. Hepatitis B Virus (HBV) was identified as an infection distinct from Hepatitis A through its contamination of yellow fever vaccine. The vaccine contained human serum as a stabilizing agent which was HBV infected. HBV was identified as a new DNA virus in the 1960s, followed a couple of 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.

Hepadnaviruses have very small genomes of partially double stranded, partially single stranded circular DNA (pdsDNA). 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, the viral pdsDNA is converted in the host cell nucleus to covalently closed circular DNA (cccDNA) by the viral polymerase. Replication involves an RNA intermediate, as in viruses belonging to group VII of Baltimore classification. 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.

Members of the family 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).

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 subunits 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.

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. Birds may be the original hosts of the Hepadnaviridae with mammals becoming infected after a bird (see host switch). Endogenous hepatitis B virus genomes have been described in crocodilian, snake and turtle genomes. This suggests that these viruses have infected vertebrates for over
Hepadnaviruses have been described in fish and amphibians also.

A proposed family of viruses – the Nackednaviridae – has been isolated from fish. This family has a similar genomic organisation to that of members of the family Hepadnaviridae. These two families separated over , suggesting an ancient origin for the family Hepadnaviridae.

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.