Christoph Seeger

Hepatitis B Virus Biology

SUMMARY Hepadnaviruses (hepatitis B viruses) cause transient and chronic infections of the liver. Transient infections run a course of several months, and chronic infections are often lifelong. Chronic infections can lead to liver failure with cirrhosis and hepatocellular carcinoma. The replication strategy of these viruses has been described in great detail, but virus-host interactions leading to acute and chronic disease are still poorly understood. Studies on how the virus evades the immune response to cause prolonged transient infections with high-titer viremia and lifelong infections with an ongoing inflammation of the liver are still at an early stage, and the role of the virus in liver cancer is still elusive. The state of knowledge in this very active field is therefore reviewed with an emphasis on past accomplishments as well as goals for the future.

Effect of Alpha Interferon on the Hepatitis C Virus Replicon

ABSTRACT Chronic hepatitis C virus (HCV) infections can be cured only in a fraction of patients treated with alpha interferon (IFN-α) and ribavirin combination therapy. The mechanism of the IFN-α response against HCV is not understood, but evidence for a role for viral nonstructural protein 5A (NS5A) in IFN resistance has been provided. To elucidate the mechanism by which NS5A and possibly other viral proteins inhibit the cellular antiviral program, we have constructed a subgenomic replicon from a known infectious HCV clone and demonstrated that it has an approximately 1,000-fold-higher transduction efficiency than previously used subgenomes. We found that IFN-α reduced replication of HCV subgenomic replicons approximately 10-fold. The estimated half-life of viral RNA in the presence of the cytokine was about 12 h. HCV replication was sensitive to IFN-α independently of whether the replicon expressed an NS5A protein associated with sensitivity or resistance to the cytokine. Furthermore, our results indicated that HCV replicons can persist in Huh7 cells in the presence of high concentrations of IFN-α. Finally, under our conditions, selection for IFN-α-resistant variants did not occur.

The reverse transcriptase of hepatitis B virus acts as a protein primer for viral DNA synthesis.

Hepatitis B viruses (hepadnaviruses) replicate their DNA genomes by reverse transcription of an RNA intermediate. Efforts to examine the biochemical mechanism for viral DNA synthesis have been hampered by the failure to solubilize the reverse transcriptase from virions and to express the polymerase in heterologous systems in an enzymatically active form. Here, we demonstrate that the polymerase of a hepadnavirus synthesized in an in vitro translation reaction exhibits reverse transcriptase activity. Furthermore, our results show that the polymerase acts as a primer for DNA synthesis and remains covalently linked to nascent DNA, a feature that is not known to exist in any other RNA-directed DNA polymerases. Priming of DNA synthesis requires viral RNA but occurs independently of other viral components. The ability to express the hepadnavirus reverse transcriptase in an enzymatically active form will allow detailed biochemical and functional analyses of this complex enzyme, and may facilitate the identification of inhibitors required for antiviral therapy.

Hepadnavirus assembly and reverse transcription require a multi-component chaperone complex which is incorporated into nucleocapsids

Assembly of hepadnaviruses depends on the formation of a ribonucleoprotein (RNP) complex comprising the viral polymerase polypeptide and an RNA segment, ϵ, present on pregenomic RNA. This interaction, in turn, activates the reverse transcription reaction, which is primed by a tyrosine residue on the polymerase. We have shown recently that the formation of this RNP complex in an avian hepadnavirus, the duck hepatitis B virus, depends on cellular factors that include the heat shock protein 90 (Hsp90). We now report that RNP formation also requires ATP hydrolysis and the function of p23, a recently identified chaperone partner for Hsp90. Furthermore, we also provide evidence that the chaperone complex is incorporated into the viral nucleocapsids in a polymerase‐dependent reaction. Based on these findings, we propose a model for hepadnavirus assembly and priming of viral DNA synthesis where a dynamic, energy‐driven process, mediated by a multi‐component chaperone complex consisting of Hsp90, p23 and, potentially, additional factors, maintains the reverse transcriptase in a specific conformation that is competent for RNA packaging and protein priming of viral DNA synthesis.

Replication of Hepatitis C Virus Subgenomes in Nonhepatic Epithelial and Mouse Hepatoma Cells

ABSTRACT The hepatitis C virus (HCV) pandemic affects the health of more than 170 million people and is the major indication for orthotopic liver transplantations. Although the human liver is the primary site for HCV replication, it is not known whether extrahepatic tissues are also infected by the virus and whether nonprimate cells are permissive for RNA replication. Because HCV exists as a quasispecies, it is conceivable that a viral population may include variants that can replicate in different cell types and in other species. We have tested this hypothesis and found that subgenomic HCV RNAs can replicate in mouse hepatoma and nonhepatic human epithelial cells. Replicons isolated from these cell lines carry new mutations that could be involved in the control of tropism of the virus. Our results demonstrated that translation and RNA-directed RNA replication of HCV do not depend on hepatocyte or primate-specific factors. Moreover, our results could open the path for the development of animal models for HCV infection.

Molecular biology of hepatitis B virus infection.

Human hepatitis B virus (HBV) is the prototype of a family of small DNA viruses that productively infect hepatocytes, the major cell of the liver, and replicate by reverse transcription of a terminally redundant viral RNA, the pregenome. Upon infection, the circular, partially double-stranded virion DNA is converted in the nucleus to a covalently closed circular DNA (cccDNA) that assembles into a minichromosome, the template for viral mRNA synthesis. Infection of hepatocytes is non-cytopathic. Infection of the liver may be either transient (<6 months) or chronic and lifelong, depending on the ability of the host immune response to clear the infection. Chronic infections can cause immune-mediated liver damage progressing to cirrhosis and hepatocellular carcinoma (HCC). The mechanisms of carcinogenesis are unclear. Antiviral therapies with nucleoside analog inhibitors of viral DNA synthesis delay sequelae, but cannot cure HBV infections due to the persistence of cccDNA in hepatocytes.

West Nile Virus Inhibits the Signal Transduction Pathway of Alpha Interferon

ABSTRACT West Nile virus (WNV) is a human pathogen that can cause neurological disorders, including meningoencephalitis. Experiments with mice and mammalian cell cultures revealed that WNV exhibited resistance to the innate immune program induced by alpha interferon (IFN-α). We have investigated the nature of this inhibition and have found that WNV replication inhibited the activation of many known IFN-inducible genes, because it prevented the phosphorylation and activation of the Janus kinases JAK1 and Tyk2. As a consequence, activation of the transcription factors STAT1 and STAT2 did not occur in WNV-infected cells. Moreover, we demonstrated that the viral nonstructural proteins are responsible for this effect. Thus, our results provided an explanation for the observed resistance of WNV to IFN-α in cells of vertebrate origin.

Apoptosis and Regeneration of Hepatocytes during Recovery from Transient Hepadnavirus Infections

ABSTRACT It is well known that hepatitis B virus infections can be transient or chronic, but the basis for this dichotomy is not known. To gain insight into the mechanism responsible for the clearance of hepadnavirus infections, we have performed a molecular and histologic analysis of liver tissues obtained from transiently infected woodchucks during the critical phase of the recovery period. We found as expected that clearance from transient infections occurred subsequent to the appearance of CD4+ and CD8+ T cells and the production of interferon gamma and tumor necrosis factor alpha in the infected liver. These events were accompanied by a significant increase in apoptosis and regeneration of hepatocytes. Surprisingly, however, accumulation of virus-free hepatocytes was delayed for several weeks following this initial influx of lymphocytes. In addition, we observed that chronically infected animals can exhibit levels of T-cell accumulation, cytokine expression, and apoptosis that are comparable with those observed during the initial phase of transient infections. Our results are most consistent with a model for recovery predicting replacement of infected hepatocytes with regenerated cells, which by unknown mechanisms remain protected from reinfection in animals that can be cured.

Genome-Wide Analyses of Avian Sarcoma Virus Integration Sites

ABSTRACT The chromosomal features that influence retroviral integration site selection are not well understood. Here, we report the mapping of 226 avian sarcoma virus (ASV) integration sites in the human genome. The results show that the sites are distributed over all chromosomes, and no global bias for integration site selection was detected. However, RNA polymerase II transcription units (protein-encoding genes) appear to be favored targets of ASV integration. The integration frequency within genes is similar to that previously described for murine leukemia virus but distinct from the higher frequency observed with human immunodeficiency virus type 1. We found no evidence for preferred ASV integration sites over the length of genes and immediate flanking regions. Microarray analysis of uninfected HeLa cells revealed that the expression levels of ASV target genes were similar to the median level for all genes represented in the array. Although expressed genes were targets for integration, we found no preference for integration into highly expressed genes. Our results provide a more detailed description of the chromosomal features that may influence ASV integration and support the idea that distinct, virus-specific mechanisms mediate integration site selection. Such differences may be relevant to viral pathogenesis and provide utility in retroviral vector design.

Targeting Hepatitis B Virus With CRISPR/Cas9

Hepatitis B virus persistence in infected hepatocytes is due to the presence of covalently closed circular DNA (cccDNA), the template for the transcription of viral RNAs. Antiviral therapies with nucleoside analogues inhibit replication of HBV DNA in capsids present in the cytoplasm of infected cells, but do not reduce or destroy nuclear cccDNA. To investigate whether cccDNA derived from infectious HBV could be directly targeted for destruction, we used the CRISPR/Cas9 system in HepG2 cells expressing the HBV receptor sodium taurocholate cotransporting polypeptide (NTCP). We tested different HBV-specific guide RNAs and demonstrated that they could inhibit HBV infections up to eightfold. Inhibition was due to mutations and deletions in cccDNA similar to those observed with chromosomal DNA cleaved by Cas9 and repaired by nonhomologous end joining (NHEJ). Interferon alpha (IFN-α) did not have a measurable effect on the antiviral activity of the CRISPR/Cas9 system, suggesting that Cas9 and NHEJ activities are not affected by induction of an innate immune response with the cytokine. Taken together, our results demonstrated that Cas9 can be recruited to cccDNA, opening the possibility for the development of future antiviral strategies aimed at targeting cccDNA for endonucleolytic cleavage with small molecules.