DNase I hypersensitive site maps to the HBV enhancer

Methylation regulates hepatitis B viral protein expression

BACKGROUND

Hepatitis B virus (HBV) DNA has been shown to contain CpG islands that are methylated in human tissue, which suggests a role for methylation in regulating viral protein production. However, data are lacking about whether methylation regulates viral gene expression.

METHODS

To investigate the hypothesis that methylation of viral DNA regulates viral gene expression, unmethylated, partially methylated, and fully methylated viral DNA was transfected into HepG2 cells. In addition, a new assay was designed that specifically identifies methylated covalently closed circular DNA (cccDNA) in human liver tissue.

RESULTS

Transfection of methylated HBV DNA led to reduced HBV mRNA levels in HepG2 cells, decreased surface and core protein expression in these cells, and decreased secretion of HBV viral proteins into the cell supernatant. These data provide direct evidence that CpG islands regulate gene transcription of HBV. Furthermore, methylated cccDNA was found in tumor and nonneoplastic human liver tissues. Finally, an in vitro equivalent of cccDNA showed decreased viral protein production in HepG2 cells after DNA methylation.

CONCLUSION

Taken together, these data demonstrate that methylation of viral CpG islands can regulate viral protein production.

Hepatitis B viral DNA is methylated in liver tissues

The mechanisms that regulate hepatitis B virus (HBV) replication within the liver are poorly understood. Given that methylation of CpG islands regulates gene expression in human tissues, we sought to identify CpG islands in HBV-DNA and to determine if they are methylated in human tissues. In silico analysis demonstrated three CpG islands in HBV genotype A sequences, two of which were of particular interest because of their proximity to the HBV surface gene start codon (island 1) and to the enhancer 1/X gene promoter region (island 2). Human sera with intact virions that were largely unmethylated were used to transfect HepG2 cells and HBV-DNA became partially methylated at both islands 1 and 2 by day 6 following exposure of HepG2 to virus. Examination of three additional human sera and 10 liver tissues showed no methylation in sera but tissues showed methylation of island 1 in six of 10 cases and of island 2 in five of 10 cases. The cell line Hep3B, with integrated HBV, showed complete methylation of island 1 but no methylation of island 2. In conclusion, HBV-DNA can be methylated in human tissues and methylation may play an important role in regulation of HBV gene expression.

Identification of proximal and distal regulatory elements of the rat connexin32 gene

We have investigated the genetic basis of the transcriptional regulation of the rat connexin32 gene which encodes the major gap junction protein in rat liver. Primer extension analysis of RNA isolated from adult rat liver identified multiple initiation sites clustered between -110 bp and -50 bp upstream from the translation start codon. An approx. 760 bp genomic DNA fragment upstream of the first exon which included the mRNA start sites was cloned 5' to the luciferase reporter cassette in p19LUC to yield pCx32-800/-33-LUC. Transfection of pCx32-800/-33-LUC resulted in a 200-fold increase in luciferase activity above p19LUC in the human hepatoma cell line HuH-7. Using a series of vectors containing 5' deletions of the 760 bp fragment, a basal promoter was localized between -179 bp and -134 bp. Three DNA:protein complexes were identified with the basal promoter fragment by DNA mobility shift assay using nuclear extracts from HuH-7 cells. Two of the DNA-binding complexes appeared to be related to the transcription factor Sp1. In addition, three DNase hypersensitive (HS) sites were identified within the genomic locus of connexin32 in adult rat liver. Two of the DNase HS regions behaved as silencer elements with both the native promoter and a heterologous promoter in HuH-7 cells. These data demonstrate that (1) the active promoter responsible for rat connexin32 mRNA transcription is located upstream of the first exon, (2) a basal promoter region was localized to a 50 bp region which formed multiple DNA:protein complexes, and (3) multiple proximal and distant regulatory elements are involved in the expression of connexin32.

Aberrant tissue specific expression of the transgene in transgenic mice that carry the hepatitis B virus genome defective in the X gene

The control mechanisms for the transgene expression in mice that carry the hepatitis B virus genome defective in the polymerase and X genes were analyzed. Ten lines of transgenic mouse were established, and in seven lines the surface and e antigens were detected in the serum. In transgenic mice from five lines examined, the transgene was markedly expressed in a broad spectrum of tissues including the kidney, heart, brain, muscle and intestine, but only poorly in the liver. In the kidney and heart the 3.5 kb and 2.1 kb mRNAs were expressed, whereas only the 0.8 kb and 4.0 kb mRNAs were detected in the testis and brain, respectively, suggesting that each of the mRNAs was transcribed through a different control mechanism. The surface, e and core antigens accumulated in the kidney and heart. DNA was hypomethylated at a region closely downstream of the enhancer in the liver, kidney and heart, and a DNase I hypersensitive site was detected upstream of the enhancer in these tissues. In the testis, however, the whole transgene was hypomethylated and the DNase I hypersensitive site was closer to the enhancer. These differences may be relevant to the preferential expression of the 0.8 kb mRNA in the testis, but cannot explain the inefficiency of transgene expression in the liver. Our observations suggest that the X protein is required for efficient expression of the viral gene in the liver but not in other tissues.