Hepatitis B Virus X protein is essential to initiate and maintain virus replication after infection

Institute of Virology, Technische Universität München/Helmholtz Zentrum München, Trogerstrasse, 30, 81675 Munich, Germany.
Journal of Hepatology (Impact Factor: 10.4). 03/2011; 55(5):996-1003. DOI: 10.1016/j.jhep.2011.02.015
Source: PubMed

ABSTRACT The molecular biology of hepatitis B virus (HBV) has been extensively studied but the exact role of the hepatitis B X protein (HBx) in the context of natural HBV infections remains unknown.
Primary human hepatocytes and differentiated HepaRG cells allowing conditional trans complementation of HBx were infected with wild type (HBV(wt)) or HBx deficient (HBV(x-)) HBV particles and establishment of HBV replication was followed.
We observed that cells inoculated with HBx-deficient HBV particles (HBV(x-)) did not lead to productive HBV infection contrary to cells inoculated with wild type HBV particles (HBV(wt)). Although equal amounts of nuclear covalently closed circular HBV-DNA (cccDNA) demonstrated comparable uptake and nuclear import, active transcription was only observed from HBV(wt) genomes. Trans-complementation of HBx was able to rescue transcription from the HBV(x-) genome and led to antigen and virion secretion, even weeks after infection. Constant expression of HBx was necessary to maintain HBV antigen expression and replication. Finally, we demonstrated that HBx is not packaged into virions during assembly but is expressed after infection within the new host cell to allow epigenetic control of HBV transcription from cccDNA.
Our results demonstrate that HBx is required to initiate and maintain HBV replication and highlight HBx as the key regulator during the natural infection process.

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    • "Recent study also showed that the truncation of X protein may result in the inhibition of Core protein although the level of cccDNA didn't change by sgRNA targeting HBx protein (Seeger and Sohn, 2014). It is possible because HBx takes an important role in transcription of preC/C RNA from episomal cccDNA templates (Belloni et al., 2009; Lucifora et al., 2011). Considering the highly economical use of HBV genetic material, whether the inhibition effects caused by frame shift of direct mutagenesis or truncated HBx transcriptional inaction need to be further investigated. "
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    • "The cccDNA also functions as an HBV reservoir responsible for persistent replication, and thus is considered to be a reliable marker for HBV infection [Levrero et al., 2009]. Viral HBx has been shown to be essential for the initiation and maintenance of transcription from HBV cccDNA: HBx stimulates the acetylation of histones associated with cccDNA, and is required for transcription in the context of HBV infections [Lucifora et al., 2011]. "
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    • "The role of the X protein was not understood for a long time, because it was not essential for HBV replication in permanent cell cultures. As shown by Ulrike Protzer (Munich) and Massimo Levrero (Rome) and their coworkers, it is however an essential transcription activator for expression of the HBV proteins in differentiated hepatocytes [71]. "
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