Hepatitis B virus replication

Abstract

Hepadnaviruses, including human hepatitis B virus (HBV), replicate through reverse transcription of an RNA intermediate, the pregenomic RNA (pgRNA). Despite this kinship to retroviruses, there are fundamental differences beyond the fact that hepadnavirions contain DNA instead of RNA. Most peculiar is the initiation of reverse transcription: it occurs by protein-priming, is strictly committed to using an RNA hairpin on the pgRNA, ε, as template, and depends on cellular chaperones; moreover, proper replication can apparently occur only in the specialized environment of intact nucleocapsids. This complexity has hampered an in-depth mechanistic understanding. The recent successful reconstitution in the test tube of active replication initiation complexes from purified components, for duck HBV (DHBV), now allows for the analysis of the biochemistry of hepadnaviral replication at the molecular level. Here we review the current state of knowledge at all steps of the hepadnaviral genome replication cycle, with emphasis on new insights that turned up by the use of such cellfree systems. At this time, they can, unfortunately, not be complemented by three-dimensional structural information on the involved components. However, at least for the ε RNA element such information is emerging, raising expectations that combining biophysics with biochemistry and genetics will soon provide a powerful integrated approach for solving the many outstanding questions. The ultimate, though most challenging goal, will be to visualize the hepadnaviral reverse transcriptase in the act of synthesizing DNA, which will also have strong implications for drug development.© 2007 The WJG Press. All rights reserved. Dieter Glebe, PhD, Series EditorPO Box 2345, Beijing 100023, China World J Gastroenterol 2007 January 7; 13(1): 48-64 www.wjgnet.com World Journal of Gastroenterology ISSN 1007-9327 wjg@wjgnet.com © 2007 OVERVIEW OVER THE HEPADNAVIRAL GENOME REPLICATION CYCLEReplication of the hepadnaviral genome can broadly be divided into three phases ( Figure 1): (1) Infectious virions contain in their inner icosahedral core the genome as a partially double-stranded, circular but not covalently closed DNA of about 3.2 kb in length (relaxed circular, or RC-DNA); (2) upon infection, the RC-DNA is converted, inside the host cell nucleus, into a plasmid-like covalently closed circular DNA (cccDNA); (3) from the cccDNA, several genomic and subgenomic RNAs are transcribed by cellular RNA polymerase Ⅱ; of these, the pregenomic RNA (pgRNA) is selectively packaged into progeny capsids and is reverse transcribed by the co-packaged P protein into new RC-DNA genomes. Matured RC-DNA containing-but not immature RNA containingnucleocapsids can be used for intracellular cccDNA amplification, or be enveloped and released from the cell as progeny virions. Below we discuss these genome conversions, with emphasis on the reverse transcription step, and particularly its unique initiation mechanism. RC-DNA TO cccDNA CONVERSIONPersistent viral infections require tha...

Involvement of the host DNA-repair enzyme TDP2 in formation of the covalently closed circular DNA persistence reservoir of hepatitis B viruses

“…The typical hepadnavirus feature of restricting the bulk of reverse transcription to the inside of intact cytoplasmic nucleocapsids which then deliver the RC-DNA to the nuclear pore for release into the nucleoplasm (61) appears to be an appropriate strategy for avoiding such untimely encounters. If so, the most dangerous period would be the period immediately after P protein has bound to e, triggering both pgRNA encapsidation and replication initiation (5,6). The order of these events is unknown, but initiating assembly before reverse transcription would protect the replication-initiation complex from inappropriate TDP2 activity.…”

Involvement of host cellular multivesicular body functions in hepatitis B virus budding

“…In the nuclei of infected cells, HBV virion DNA is converted into covalently closed circular viral DNA and transcribed to produce a terminally redundant pregenomic RNA (pgRNA). HBV core protein encapsidates this pgRNA together with the viral DNA polymerase into nucleocapsids (NCs), within which pgRNA is reverse transcribed into dsDNA (5). Mature, DNA-containing NCs can then be enveloped by the viral surface proteins, yielding progeny virions (3).…”

Viral epigenomes in human tumorigenesis

“…HBV codes for several proteins from four known genes (P, S, X and C). The P gene codes for DNA polymerase; the S gene for three polypeptides (HBs) of different sizes (large, medium and small); X for an HBx protein that is thought to be involved in hepatocarcinogenesis; and C for the core protein (Seeger and Mason, 2000;zur Hausen, 2006;Beck and Nassal, 2007;Lupberger and Hildt, 2007).…”