“
“Netherlands Cancer Institute, Division of Gene Regulation, Plesmanlaan 121, 1066CX, Amsterdam, The Netherlands Chronic hepatitis B virus (HBV) infection is a major buy Sotrastaurin 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 selleck products 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) Chronic infection by hepatitis B virus (HBV) affects close to 400 million people worldwide and is a leading cause of hepatocellular carcinoma, one of the most common human cancers.1 HBV belongs to the hepadnavirus family of DNA viruses that also includes woodchuck hepatitis virus and ground

squirrel hepatitis virus.2 HBV replicates its genome in a manner very analogous to retroviruses, by reverse transcription (RT) of an RNA intermediate into double-stranded DNA that serves as template for transcription by the host cell RNA Polymerase II (Pol II) (reviewed3). A distinctive feature of HBV, those however, is that the viral DNA genome does not integrate into the chromosome of the newly infected cell but instead is maintained as a covalently closed circular DNA (cccDNA) molecule. The cccDNA is transcribed into four major RNA species encoding the viral proteins, including a more than full-length transcript termed the pregenomic RNA. The pregenomic RNA is then reverse-transcribed in the cytoplasm within newly formed viral particles. As the cccDNA does not replicate, a pool of 10-100 copies of the cccDNA is maintained by recycling of a small proportion of the newly synthesized viral DNA genomes into the nucleus. HBV encodes the nonstructural hepatitis B virus X (HBx) protein that is conserved among mammalian hepadnaviruses, suggesting an important function.