Hepadnaviruses: Virological and Clinical Features

Hepadnaviruses are a group of DNA viruses that infect hepatocytes and may cause liver injury and hepatocellular carcinoma (HCC) in mammals and birds. Hepatitis B virus (HBV) is the prototype of the family of hepadnaviruses that causes acute and chronic hepatitis B, cirrhosis, and HCC in human beings. These viruses have small DNA genomes, 3.0–3.3 kilo-base pairs (kb) in length that are replicated by reverse transcription of RNA intermediates. They use overlapping open reading frames (ORFs) to encode envelope (surface) and nucleocapsid (core) proteins, and a polymerase with domains that have priming, reverse transcriptase, and RNase H activities. All of the viruses encode an additional secreted protein, HBeAg that is synthesized from a precursor encoded by the core ORF and seems to act as an immune tolerogen. The host specificity typical of this virus family seems to be attributable to interactions between the large surface protein and the primary receptor on the hepatocyte. HBV infection of humans is preventable by immunization and responds well to treatment with nucleoside and nucleotide analogs.