Molecular virology of hepatitis B virus for clinicians

Abstract

This article reviews the molecular biology of the hepatitis B virus in an effort to explain its natural history from a molecular perspective. The life cycle of the virus, with special attention to virus replication, polypeptide production, and morphogenesis, is described. The way in which these steps may influence the natural history of viral pathogenesis, as well as the effectiveness of interventions, receives special consideration.

Figure 1. The HBV virion and genomic structural features

(A) Illustration of the HBV virion (1A), showing envelope, S, M, L, with their S, preS2 and preS1 domains in sub-boxes of size proportional to their polypeptide length. The HBV genome, contained within the capsid, is shown as a partly double stranded, incomplete circle, with the polymerase protein covalently attached to the “minus” strand. (B) The circular map of the HBV genome is shown, with nucleotides numbered from the single EcoR1 site (by convention) and transcripts and their polypeptide products indicated.

Figure 2. A schematic outline of the HBV replication cycle

Major steps in the molecular biology of the HBV replication life cycle are shown, from attachment to translocation of the virion DNA to the nucleus, to conversion of entering viral DNA into cccDNA, followed by transcription of the cccDNA into the viral RNA gene products. Encapsidations in the cytoplasm, virion morphogenesis and secretion is also shown. Note, replicated progeny HBV DNA can return to the nucleus by way of an intracellular pathway, which may result in an “auto” amplification of cccDNA.

Figure 3. Transcript products of HBV, with the proteins they specify (for mRNA) and encapsidation and replications (for pregenomes)

CHO indicates N-linked glycosylation, and is positioned at the appropriate glycosylation sites. cccDNA, represented by the double lined circle on the left, in transcribed into at least 5 RNA gene products that serve as either pregenomes for encapsidation and/or mRNA molecules that are translated into the proteins indicated. In the case of preCore, it is proteolytically cleaved (by presumably host proteases) into HBeAg. Approximate molecular weight of the (unglycosylated) polypeptide products is indicated in kilodaltons (kD).

Figure 4. DNA structures associated with HBV

Relative mobility of the various structures as they appear resolved through agarose gels (upper left), the illustration of their structures and the step in the life cycle with which they are associated.

Figure 5. Mutations within the pol gene can result in changes within the envelope protein

The genome HBV is shown, as in Fig. 1, as a circle, with 1 to 3,182 nucleotides numbered consecutively from the single Eco R1 site. The coding regions for each gene are shown as curved arrows that follow the direction of the transcripts. The polypeptide specified by the pol (polymerase) and env (envelope) genes are expanded as horizontal boxes. The amino acid sequence of the polymerase between amino acids 163 and 210 is the region that contains amino acids that, when mutated, can confer resistance to some polymerase inhibitors. The YMDD amino acid motif (underlined), for example, when mutated (as shown) confers resistance to lamivudine and is provided with the amino acid transition indicated. Since the open reading frames for pol and env overlap, (amino acid 204 of the rt (reverse transcriptase) roughly correspond to amino acid 195/6 of the envelope protein, as indicated) a change in this motif will change the amino acids specified by the env open reading frame, and this is concomitant change is indicated.