Alan McLachlan

Professor

Ph.D. University of Aberdeen (Scotland), 1980

Room: 8091 COMRB, Tel: 312-355-0211

Email: mclach@uic.edu

Regulation of Hepatitis B Virus (HBV) Transcription and Biosynthesis

Introduction

HBV infection is a worldwide health problem and is endemic in many regions of Asia and Africa. The clinical consequences of HBV infection can be acute or chronic and range from the subclinical to fatal forms of the disease, including fulminant hepatitis where the patient dies shortly after infection. Although HBV infection can generally be prevented by vaccination with hepatitis B surface antigen (HBsAg), chronic HBV infection remains a major clinical problem. It is estimated that there are 200 to 500 million HBV chronic carriers in the world for whom, to date, there is no reliable treatment. The consequences of chronic HBV infection can include debilitating chronic active hepatitis, and liver cirrhosis, which is a major cause of mortality. Chronic carrier status has additional clinical implications. The estimated relative risk of primary hepatocellular carcinoma (PHC) in chronic HBV carriers is approximately 100-times greater than in uninfected individuals. Therefore, effective treatments for chronic HBV infection are required. Understanding the viral life cycle in detail may reveal potential targets for antiviral therapy. HBV replicates by reverse transcription of the viral pregenomic RNA encoded by the HBV genome. Consequently, transcription of the viral genome and the regulation of pregenomic RNA synthesis, in particular, are essential steps in virus replication.

Transcriptional regulation of HBV by nuclear hormone receptors is a critical determinant of viral tropism

We have developed a new viral replication system using non-hepatoma cells where the effect of one or more liver-enriched transcription factors on HBV RNA synthesis and replication can be examined. Using this approach, it has been possible to demonstrate that the nuclear hormone receptors, hepatocyte nuclear factor 4 (HNF4) or retinoid X receptor α (RXRα) plus peroxisome proliferator-activated receptor α (PPARα), are the liver-enriched transcription factors that are essential for pregenomic RNA synthesis and viral replication. Hepatocyte nuclear factor 3 (HNF3) antagonizes the nuclear hormone receptor-mediated viral replication by inhibiting pregenomic RNA synthesis. Therefore, this novel replication system has identified new roles for these liver-enriched transcription factors in controlling viral transcription and replication. Surprisingly, nuclear hormone receptors are the only essential liver-enriched transcription factors critical to pregenomic RNA synthesis and viral replication, indicating a previously unknown importance of these factors in the HBV life cycle and tissue-specific tropism of the virus. In addition, this analysis suggests nuclear hormone receptors and HNF3 may represent suitable targets of the development of antiviral therapies.

Nuclear covalently closed circular (CCC) viral genomic DNA in the liver of hepatocyte nuclear factor 1α-null hepatitis B virus transgenic mice

The role of hepatocyte nuclear factor 1α (HNF1α) in regulating viral transcription and replication was examined in an HBV transgenic mouse model system. In transient transfection analysis, it has been demonstrated previously that HNF1α regulates the level of transcription from the large surface antigen promoter. This observation predicts that the loss of HNF1α might be associated with a reduction in the level of the 2.4-kb HBV RNA and the large surface antigen polypeptide it encodes. As the large surface antigen polypeptide is essential for viral biosynthesis, the loss of HNF1α might be expected to limit viral biosynthesis and lead to an increased abundance of mature capsids in the cytoplasm of the cell. In turn, these mature capsids may deliver their viral genomes to the nucleus to amplify the pool of CCC DNA, as is observed in duck hepatitis B virus infection. To examine whether this occurs in vivo, the viral replication intermediates present in the liver of HNF1α-null HBV transgenic mice were examined. The levels of the HBV RNAs, including the 2.4-kb viral transcript, in the HNF1α-null HBV transgenic mice were similar to the levels present in HBV transgenic mice expressing HNF1α. However, nuclear HBV CCC DNA was present in the hepatocytes of the HNF1α-null HBV transgenic mice. This suggests that subtle alterations in the levels of the HBV RNAs resulting from the absence of HNF1α may have resulted in the translocation of HBV genomic DNA into the nucleus of the hepatocytes. Alternatively, the absence of HNF1α may alter the physiological properties of the hepatocytes in a manner that favors the translocation of HBV genomic DNA into the nucleus. In either case, it is apparent that cycling of encapsidated HBV DNA from the cytoplasm into the nucleus can occur in the HNF1α-null HBV transgenic mouse model and represents a system where the molecular events regulating this aspect of the HBV life cycle can be analyzed in detail. In addition, the presence of nuclear HBV CCC DNA in these mice permits their role in HBV-mediated hepatocellular carcinoma to be examined.

Interferon a therapy is a standard treatment for chronic HBV infection. Using the HNF1a-null HBV transgenic mouse model, the stability of HBV CCC DNA in the hepatocytes of these mice has been examined in response to the induction of an interferon a/b response. Induction of an interferon a/b response in HBV transgenic mice by polyinosinic-polycytidylic acid treatment dramatically reduces the level of cytoplasmic HBV replication intermediates.  Therefore it was of interest to determine if induction of an interferon a/b response in HNF1a-null HBV transgenic mice might reduce the level of cytoplasmic and nuclear viral replication intermediates to a similar or different extent.  From this analysis, it is apparent that cytoplasmic replication intermediates are much more sensitive to elimination by interferon a/b induction than nuclear HBV CCC DNA.  These observations support the contention that elimination of nuclear HBV CCC DNA is the major problem in resolving chronic HBV infection.  In addition, these findings help to explain why interferon a therapy may reduce patients’ viral load without necessarily preventing the recurrence of viral biosynthesis after the completion of therapy.