Hepatitis B virus X protein stimulates gene expression selectively from extrachromosomal DNA templates

Authors

  • Pieter C. van Breugel,

    1. Department of Microbiology and Molecular Medicine, University Medical Centre (C.M.U.), Geneva, Switzerland
    Current affiliation:
    1. Netherlands Cancer Institute, Division of Gene Regulation, Plesmanlaan 121, 1066CX, Amsterdam, The Netherlands
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  • Eva I. Robert,

    1. Department of Microbiology and Molecular Medicine, University Medical Centre (C.M.U.), Geneva, Switzerland
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    • These authors contributed equally to this work.

  • Henrik Mueller,

    1. Department of Microbiology and Molecular Medicine, University Medical Centre (C.M.U.), Geneva, Switzerland
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    • These authors contributed equally to this work.

  • Adrien Decorsière,

    1. Department of Microbiology and Molecular Medicine, University Medical Centre (C.M.U.), Geneva, Switzerland
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  • Fabien Zoulim,

    1. CRCL, INSERM U1052, CNRS 5286, Université de Lyon, Lyon, France
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  • Olivier Hantz,

    1. CRCL, INSERM U1052, CNRS 5286, Université de Lyon, Lyon, France
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  • Michel Strubin

    Corresponding author
    1. Department of Microbiology and Molecular Medicine, University Medical Centre (C.M.U.), Geneva, Switzerland
    • Department of Microbiology & Molecular Medicine, University Medical Centre (C.M.U.), Rue Michel-Servet 1, CH-1211 Geneva 4, Switzerland

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    • fax: (41-22) 379 5702


  • Potential conflict of interest: Nothing to report.

Abstract

Chronic hepatitis B virus (HBV) infection is a major risk factor for liver cancer development. HBV encodes the hepatitis B virus X (HBx) protein that promotes transcription of the viral episomal DNA genome by the host cell RNA polymerase II. Here we provide evidence that HBx accomplishes this task by a conserved and unusual mechanism. Thus, HBx strongly stimulates expression of transiently transfected reporter constructs, regardless of the enhancer and promoter sequences. This activity invariably requires HBx binding to the cellular UV-damaged DDB1 E3 ubiquitin ligase, suggesting a common mechanism. Unexpectedly, none of the reporters tested is stimulated by HBx when integrated into the chromosome, despite remaining responsive to their cognate activators. Likewise, HBx promotes gene expression from the natural HBV episomal template but not from a chromosomally integrated HBV construct. The same was observed with the HBx protein of woodchuck HBV. HBx does not affect nuclear plasmid copy number and functions independently of CpG dinucleotide methylation. Conclusion: We propose that HBx supports HBV gene expression by a conserved mechanism that acts specifically on episomal DNA templates independently of the nature of the cis-regulatory sequences. Because of its uncommon property and key role in viral transcription, HBx represents an attractive target for new antiviral therapies. (HEPATOLOGY 2012;56:2116–2124)

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