title: Hepatitis B virus X protein stimulates gene expression selectively from extrachromosomal DNA templates.
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.
  
  
  CONCLUSIONS
  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.
authors: Department of Microbiology and Molecular Medicine, University Medical Centre (C.M.U.), Geneva, Switzerland.
authors: Pieter C
authors: PC
authors: van Breugel
authors: Eva I
authors: EI
authors: Robert
authors: Henrik
authors: H
authors: Mueller
authors: Adrien
authors: A
authors: Decorsière
authors: Fabien
authors: F
authors: Zoulim
authors: Olivier
authors: O
authors: Hantz
authors: Michel
authors: M
authors: Strubin
mesh_terms: D018899:CpG Islands; D019175:DNA Methylation; D004270:DNA, Circular; D004279:DNA, Viral / Q000235:genetics*; D004268:DNA-Binding Proteins / Q000378:metabolism; D015967:Gene Expression Regulation, Viral; D017930:Genes, Reporter / Q000235:genetics; D056945:Hep G2 Cells; D017919:Hepatitis B Virus, Woodchuck / Q000235:genetics / Q000378:metabolism; D006515:Hepatitis B virus / Q000235:genetics* / Q000378:metabolism; D019694:Hepatitis B, Chronic; D006801:Humans; D008156:Luciferases / Q000235:genetics; D010957:Plasmids; D015534:Trans-Activators / Q000378:metabolism*; D014162:Transfection; D044767:Ubiquitin-Protein Ligases / Q000378:metabolism; D015854:Up-Regulation / Q000235:genetics; D054334:Viral Regulatory and Accessory Proteins
chemical_list: C091435:DDB1 protein, human; D004270:DNA, Circular; D004279:DNA, Viral; D004268:DNA-Binding Proteins; D015534:Trans-Activators; D054334:Viral Regulatory and Accessory Proteins; C057628:hepatitis B virus X protein; D008156:Luciferases; D044767:Ubiquitin-Protein Ligases