Nucleotide sequence of a cloned woodchuck hepatitis virus genome: evolutional relationship between hepadnaviruses

In silico molecular study of hepatitis B virus X protein as a therapeutic target

The Hepatitis B virus is a leading cause of liver cirrhosis and hepatocellular carcinoma. HBx viral protein is considered a contributor to pathogenesis and hepatocarcinogenesis. This study aimed to screen the effect of some antiviral compounds to target HBx protein for inhibition of its function. Here, molecular docking, molcular dynsmic simulation, MM/GBSA and T-SNE methods were applied to study the complex stability and to cluster the conformations that generated in the simulation. Among the 179 compounds screened in this study, three antiviral agents (SC75741, Punicalagin, and Ledipasvir) exhibited the lowest docking energy and best interaction. Among these compounds, SC75741 was identified as a potent inhibitor of HBx that showed the best and most stable interaction during molecular dynamic simulation, and blocking a region near to HBx helix resides (aa 88-100) that is associated with cell invasion. The analysis of relative binding free energy through MM/GBSA for molecular dynamic simulation results revealed binding energy -9.9 kcal/mol for SC75741, -11 kcal/mol for Punicalagin, and -10.1 kcal/mol for Ledipasvir. These results elucidate the possible use of these compounds in the research for targeting HBx.Communicated by Ramaswamy H. Sarma.

Prevalence and Genomic Sequence Analysis of Domestic Cat Hepadnavirus in the United States

Hepadnaviruses are partially double-stranded DNA viruses that infect a variety of species. The prototypical virus in this family is the human hepatitis B virus, which chronically infects approximately 400 million people worldwide and is a risk factor for progressive liver disease and liver cancer. The first hepadnavirus isolated from carnivores was a domestic cat hepadnavirus (DCH), initially identified in Australia and subsequently detected in cats in Europe and Asia. As with all characterized hepadnaviruses so far, DCH infection has been associated with hepatic disease in its host. Prevalence of this infection in the United States has not been explored broadly. Thus, we utilized conventional and quantitative PCR to screen several populations of domestic cats to estimate DCH prevalence in the United States. We detected DCH DNA in 1 out of 496 animals (0.2%) in the U.S. cohort. In contrast, we detected circulating DCH DNA in 7 positive animals from a cohort of 67 domestic cats from Australia (10.4%), consistent with previous studies. The complete consensus genome of the U.S. DCH isolate was sequenced by Sanger sequencing with overlapping PCR products. An in-frame deletion of 157 bp was identified in the N-terminus of the core open reading frame. The deletion begins at the direct repeat 1 sequence (i.e., the 5′ end of the expected double-stranded linear DNA form), consistent with covalently closed circular DNA resultant from illegitimate recombination described in other hepadnaviruses. Comparative genome sequence analysis indicated that the closest described relatives of the U.S. DCH isolate are those previously isolated in Italy. Motif analysis supports DCH using NTCP as an entry receptor, similar to human HBV. Our work indicates that chronic DCH prevalence in the U.S. is likely low compared to other countries.

Distinct Viral Lineages from Fish and Amphibians Reveal the Complex Evolutionary History of Hepadnaviruses

Hepadnaviruses (hepatitis B viruses [HBVs]) are the only animal viruses that replicate their DNA by reverse transcription of an RNA intermediate. Until recently, the known host range of hepadnaviruses was limited to mammals and birds. We obtained and analyzed the first amphibian HBV genome, as well as several prototype fish HBVs, which allow the first comprehensive comparative genomic analysis of hepadnaviruses from four classes of vertebrates. Bluegill hepadnavirus (BGHBV) was characterized from in-house viral metagenomic sequencing. The African cichlid hepadnavirus (ACHBV) and the Tibetan frog hepadnavirus (TFHBV) were discovered using in silico analyses of the whole-genome shotgun and transcriptome shotgun assembly databases. Residues in the hydrophobic base of the capsid (core) proteins, designated motifs I, II, and III, are highly conserved, suggesting that structural constraints for proper capsid folding are key to capsid protein evolution. Surface proteins in all vertebrate HBVs contain similar predicted membrane topologies, characterized by three transmembrane domains. Most striking was the fact that BGHBV, ACHBV, and the previously described white sucker hepadnavirus did not form a fish-specific monophyletic group in the phylogenetic analysis of all three hepadnaviral genes. Notably, BGHBV was more closely related to the mammalian hepadnaviruses, indicating that cross-species transmission events have played a major role in viral evolution. Evidence of cross-species transmission was also observed with TFHBV. Hence, these data indicate that the evolutionary history of the hepadnaviruses is more complex than previously realized and combines both virus-host codivergence over millions of years and host species jumping.

IMPORTANCE

Hepadnaviruses are responsible for significant disease in humans (hepatitis B virus) and have been reported from a diverse range of vertebrates as both exogenous and endogenous viruses. We report the full-length genome of a novel hepadnavirus from a fish and the first hepadnavirus genome from an amphibian. The novel fish hepadnavirus, sampled from bluegills, was more closely related to mammalian hepadnaviruses than to other fish viruses. This phylogenetic pattern reveals that, although hepadnaviruses have likely been associated with vertebrates for hundreds of millions of years, they have also been characterized by species jumping across wide phylogenetic distances.

Hepatitis B virus X protein accelerates the development of hepatoma

The chronic infection of hepatitis B virus (HBV) is closely related to the occurrence and development of hepatocellular carcinoma (HCC). Accumulated evidence has shown that HBV X protein (HBx protein) is a multifunctional regulator with a crucial role in hepatocarcinogenesis. However, information on the mechanism by which HBV induces HCC is lacking. This review focuses on the pathological functions of HBx in HBV-induced hepatocarcinogenesis. As a transactivator, HBx can modulate nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and transcription factor AP-2. Moreover, HBx can affect regulatory non-coding RNAs (ncRNAs) including microRNAs and long ncRNAs (lncRNAs), such as miRNA-205 and highly upregulated in liver cancer (HULC), respectively. HBx is also involved in epigenetic modification, including methylation and acetylation. HBx interacts with various signal-transduction pathways, such as protein kinase B/Akt, Wnt/β-catenin, signal transducer and activator of transcription, and NF-κB pathways. Moreover, HBx affects cellular fate by shifting the balance toward cell survival. HBx may lead to the loss of apoptotic functions or directly contributes to oncogenesis by achieving transforming functions, which induce hepatocarcinogenesis. Additionally, HBx can modulate apoptosis and immune response by direct or indirect interaction with host factors. We conclude that HBx hastens the development of hepatoma.

HBx Gene Mutations in Hepatitis B Virus and Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is one of the most prevalent cancers which are found in many Asian and African countries. There are several risk factors that may develop to HCC. Along with several other factors contributing to HCC, hepatitis B virus (HBV) infection also accounts for a major cause. HBV infection represents a major health problem worldwide. Among all of HBV genes, HBx is believed to play a prominent role in carcinogenesis, although the actual mechanism is not yet fully understood. The HBx gene of HBV is the most common open reading frame that may undergo mutations and may develop into HCC. This review summarizes the current knowledge about the most important roles of HBx gene that may lead to the development of HCC.

Dating the origin and dispersal of hepatitis B virus infection in humans and primates

The origin of hepatitis B virus (HBV) infection in humans and other primates remains largely unresolved. Understanding the origin of HBV is crucial because it provides a framework for studying the burden, and subsequently the evolution, of HBV pathogenicity with respect to changes in human population size and life expectancy. To investigate this controversy we examined the relationship between HBV phylogeny and genetic diversity of modern humans, investigated the timescale of global HBV dispersal, and tested the hypothesis of HBV-human co-divergence. We find that the global distribution of HBV genotypes and subgenotypes are consistent with the major prehistoric modern human migrations. We calibrate the HBV molecular clock using the divergence times of different indigenous human populations based on archaeological and genetic evidence and show that HBV jumped into humans around 33,600 years ago; 95% higher posterior density (HPD): 22,000-47,100 years ago (estimated substitution rate: 2.2 × 10(-6) ; 95% HPD: 1.5-3.0 × 10(-6) substitutions/site/year). This coincides with the origin of modern non-African humans. Crucially, the most pronounced increase in the HBV pandemic correlates with the global population increase over the last 5,000 years. We also show that the non-human HBV clades in orangutans and gibbons resulted from cross-species transmission events from humans that occurred no earlier than 6,100 years ago.

CONCLUSION

Our study provides, for the first time, an estimated timescale for the HBV epidemic that closely coincides with dates of human dispersals, supporting the hypothesis that HBV has been co-expanding and co-migrating with human populations for the last 40,000 years. (HEPATOLOGY 2013).

C-terminal substitution of HBV core proteins with those from DHBV reveals that arginine-rich 167RRRSQSPRR175 domain is critical for HBV replication

To investigate the contributions of carboxyl-terminal nucleic acid binding domain of HBV core (C) protein for hepatitis B virus (HBV) replication, chimeric HBV C proteins were generated by substituting varying lengths of the carboxyl-terminus of duck hepatitis B virus (DHBV) C protein for the corresponding regions of HBV C protein. All chimeric C proteins formed core particles. A chimeric C protein with 221–262 amino acids of DHBV C protein, in place of 146–185 amino acids of the HBV C protein, supported HBV pregenomic RNA (pgRNA) encapsidation and DNA synthesis: 40% amino acid sequence identity or 45% homology in the nucleic-acid binding domain of HBV C protein was sufficient for pgRNA encapsidation and DNA synthesis, although we predominantly detected spliced DNA. A chimeric C protein with 221–241 and 251–262 amino acids of DHBV C, in place of HBV C 146–166 and 176–185 amino acids, respectively, could rescue full-length DNA synthesis. However, a reciprocal C chimera with 242–250 of DHBV C (²⁴²RAGSPLPRS²⁵⁰) introduced in place of 167–175 of HBV C (¹⁶⁷RRRSQSPRR¹⁷⁵) significantly decreased pgRNA encapsidation and DNA synthesis, and full-length DNA was not detected, demonstrating that the arginine-rich ¹⁶⁷RRRSQSPRR¹⁷⁵ domain may be critical for efficient viral replication. Five amino acids differing between viral species (underlined above) were tested for replication rescue; R169 and R175 were found to be important.

Genomic analysis of hepatitis B virus reveals antigen state and genotype as sources of evolutionary rate variation

Hepatitis B virus (HBV) genomes are small, semi-double-stranded DNA circular genomes that contain alternating overlapping reading frames and replicate through an RNA intermediary phase. This complex biology has presented a challenge to estimating an evolutionary rate for HBV, leading to difficulties resolving the evolutionary and epidemiological history of the virus. Here, we re-examine rates of HBV evolution using a novel data set of 112 within-host, transmission history (pedigree) and among-host genomes isolated over 20 years from the indigenous peoples of the South Pacific, combined with 313 previously published HBV genomes. We employ Bayesian phylogenetic approaches to examine several potential causes and consequences of evolutionary rate variation in HBV. Our results reveal rate variation both between genotypes and across the genome, as well as strikingly slower rates when genomes are sampled in the Hepatitis B e antigen positive state, compared to the e antigen negative state. This Hepatitis B e antigen rate variation was found to be largely attributable to changes during the course of infection in the preCore and Core genes and their regulatory elements.

The C-terminal region of the hepatitis B virus X protein is essential for its stability and function

More than 350 million people worldwide are chronically infected with the human hepatitis B virus (HBV). Chronic HBV infections are associated with the development of hepatocellular carcinoma. While the mechanism of HBV-associated carcinoma remains undefined, it is thought to involve a combination of a continuous inflammatory response to HBV-infected hepatocytes and activities of HBV proteins such as the HBV X protein (HBx). HBx stimulates HBV replication; however, the mechanism by which HBx stimulates HBV replication remains incompletely understood. Studies performed with the woodchuck hepatitis virus (WHV) in woodchucks demonstrated that a C-terminally truncated mutant of the WHV X protein could not stimulate WHV replication. However, whether the C-terminus of HBx is important for HBx-stimulation of HBV replication is unclear. We have constructed C-terminal truncation mutants of HBx and have demonstrated that the C-terminus of HBx impacts HBx stability, HBx activation of transcription, and HBx stimulation of HBV replication. These observations highlight the impact of the HBx C-terminus on HBx activities and the importance of directly analyzing HBx expression and functions in HBV-associated tumors that contain chromosomal integrants of HBV with truncations of the HBx gene.

Identification of CD4+ and CD8+ T cell epitopes of woodchuck hepatitis virus core and surface antigens in BALB/c mice

A therapeutic vaccine against chronic hepatitis B virus (HBV) infection requires the development of a strong and multispecific Th1 cell immune response. Woodchucks chronically infected with the woodchuck hepatitis virus (WHV) closely resemble HBV infection and represent the best animal model for this hepadnavirus-induced disease. Using the BIMAS "HLA Peptide Binding Predictions" program, we have identified and further characterized novel H-2 d-restricted CD8+ epitopes within the WHV core (peptides C#12-21, C#18-32, C#19-27, C#61-69) and surface antigens (peptides preS2#10-18, preS2#27-35, S#76-84, S#133-140 and S#257-265), respectively. These peptides bind to H-2 d with high efficiency and upon immunization of mice with peptide and Freund's adjuvant they induce the development of IFN-gamma producing T cells. More importantly, WHV core peptides C#19-27 and C#61-69 and WHV surface peptides S#133-140 and S#257-265 were also recognized by CD8+ T cells after immunization of mice with DNA/PEI nanoparticles. Direct stimulation of splenocytes obtained from such DNA-immunized mice with peptides C#18-32, S#76-84, and S#257-265 resulted in significant production of IFN-gamma. Thus, we have identified T cell determinants in mice from WHV core and surface antigens that have important value for designing and evaluating an effective vaccine against hepadnavirus infection.

Hepatitis B virus: origin and evolution

The pathogenesis of hepatitis B virus (HBV) is complex and it appears that molecular variants play a role in this process. HBV undergoes numerous rounds of error prone production within an infected host. The resulting quasispecies are heterogeneous and in the absence of archaeological records of past infection, the evolution of HBV can only be inferred indirectly from its epidemiology and by genetic analysis. This review gathered the controversies about the HBV origin and factors influencing its quasispecies. Also, it provided some evidence on how HBV genotypes correlated with human history and patterns of migration. It is our belief that this topic deserves further attention and thus it is likely that more critical research work will be performed to elucidate the unknown mechanisms and processes in this area.

Four conserved cysteine residues of the hepatitis B virus polymerase are critical for RNA pregenome encapsidation

Hepadnaviruses replicate via reverse transcription of an RNA template, the pregenomic RNA (pgRNA). Although hepadnaviral polymerase (Pol) and retroviral reverse transcriptase are distantly related, some of their features are distinct. In particular, Pol contains two additional N-terminal subdomains, the terminal protein and spacer subdomains. Since much of the spacer subdomain can be deleted without detrimental effects to hepatitis B virus (HBV) replication, this subdomain was previously thought to serve only as a spacer that links the terminal protein and reverse transcriptase subdomains. Unexpectedly, we found that the C terminus of the spacer subdomain is indispensable for the encapsidation of pgRNA. Alanine-scanning mutagenesis revealed that four conserved cysteine residues, three at the C terminus of the spacer subdomain and one at the N terminus of the reverse transcriptase subdomain, are critical for encapsidation. The inability of the mutant Pol proteins to incorporate into nucleocapsid particles, together with other evidence, argued that the four conserved cysteine residues are critical for RNA binding. One implication is that these four cysteine residues might form a putative zinc finger motif. Based on these findings, we speculate that the RNA binding activity of HBV Pol may be mediated by this newly identified putative zinc finger motif.

The woodchuck as an animal model for pathogenesis and therapy of chronic hepatitis B virus infection

This review describes the woodchuck and the woodchuck hepatitis virus (WHV) as an animal model for pathogenesis and therapy of chronic hepatitis B virus (HBV) infection and disease in humans. The establishment of woodchuck breeding colonies, and use of laboratory-reared woodchucks infected with defined WHV inocula, have enhanced our understanding of the virology and immunology of HBV infection and disease pathogenesis, including major sequelae like chronic hepatitis and hepatocellular carcinoma. The role of persistent WHV infection and of viral load on the natural history of infection and disease progression has been firmly established along the way. More recently, the model has shed new light on the role of host immune responses in these natural processes, and on how the immune system of the chronic carrier can be manipulated therapeutically to reduce or delay serious disease sequelae through induction of the recovery phenotype. The woodchuck is an outbred species and is not well defined immunologically due to a limitation of available host markers. However, the recent development of several key host response assays for woodchucks provides experimental opportunities for further mechanistic studies of outcome predictors in neonatal- and adult-acquired infections. Understanding the virological and immunological mechanisms responsible for resolution of self-limited infection, and for the onset and maintenance of chronic infection, will greatly facilitate the development of successful strategies for the therapeutic eradication of established chronic HBV infection. Likewise, the results of drug efficacy and toxicity studies in the chronic carrier woodchucks are predictive for responses of patients chronically infected with HBV. Therefore, chronic WHV carrier woodchucks provide a well-characterized mammalian model for preclinical evaluation of the safety and efficacy of drug candidates, experimental therapeutic vaccines, and immunomodulators for the treatment and prevention of HBV disease sequelae.

A TaqMan PCR assay using degenerate primers for the quantitative detection of woodchuck hepatitis virus DNA of multiple genotypes

Woodchuck hepatitis virus (WHV) is a valuable animal model system for studies of hepatitis B virus infection and accurate assessments of WHV viral load are necessary in these studies. Wild-captured woodchucks that are naturally infected with WHV are sometimes used in these studies, however, the sequence variation in WHV isolates generally precludes the use of TaqMan PCR. To facilitate this, we have created a real-time TaqMan PCR assay for WHV using degenerate primers with inosine residues employed at the locations of known sequence heterogeneity. This TaqMan assay has a dynamic range of 10-10(8) genomic equivalents (ge) of WHV DNA per reaction and the assay is robust and reproducible in the 10(2)-10(7) ge WHV DNA per reaction range (intra-assay coefficient of variation (CV) <2.1%, inter-assay CV <2.9%). During our assay validation, we cloned and analyzed a series of six naturally occurring virus variants that contained sequence heterogeneity in the TaqMan primer sequence region. We showed that the presence of some of these sequence variations prevented the PCR amplification of the target when regular primer sequences were used, while degenerate primer sequences were able to efficiently amplify all tested sequences equally well.

Sandwich type ELISA and a fluorescent cytometric microbead assay for quantitative determination of hepatitis B virus X antigen level in human sera

The hepatitis B virus X protein (HBxAg) is responsible for severe complications of HBV infections including primary hepatocellular carcinoma. A sandwich type ELISA and a flow cytometric microbead assay for quantitative determination of serum levels of Hbx-Ag are introduced. We have previously developed monoclonal antibody families against well-conserved epitopes on HbxAg, characterized by different immunohistochemical and immunoserological techniques. Special selection of the antibody pairs provided highly sensitive and highly specific tools for quantitative immunoassay development. The resulting assays were tested on human sera (208 samples) collected from patients suffering from different clinical forms of HBV infection. The sensitivity range of the sandwich type ELISA was between 4 and 2000 ng/ml as measured on both the recombinant antigen and the sera of chronic hepatitis patients. A further flow cytometric microbead assay was established and tested in parallel with the ELISA. The quantitative results of these two immunoserological techniques were in strong correlation and they were found to be highly specific and sensitive on clinical samples. The HBxAg ELISA technique is applicable for routine clinical laboratory measurements, and our HBxAg microbead technique is recommended for complex multiparametric measurements combined with other markers.

cDNA microarray analysis of early gene expression profiles associated with hepatitis B virus X protein-mediated hepatocarcinogenesis

Chronic hepatitis B virus (HBV) infection is one of the major causes of hepatocellular carcinoma. HBV encodes an oncogenic hepatitis B virus X protein (HBx), which can transactivate host cell transcriptional machinery and mediate cellular transformation. To disclose the early genetic response in HBx-mediated transformation process, we constructed a conditional HBx-expressing hepatocyte cell line, which allows us to compare the gene expression profiles under controllable HBx induction. A cDNA microarray containing more than 8700 mouse genes and ESTs was utilized to examine the gene expression profiles. We identified 260 candidate genes and 259 ESTs which have shown aberrant expression under HBx induction. Most of them are involved in signal transduction pathway, cell cycle control, metastasis, transcriptional regulation, immune response, and metabolism. These results provide additional insight into early cellular targets of HBx, which could give us a better understanding of the function of HBx and their progressive changes during HBx-mediated hepatocarcinogenesis.

Effect of a combination of clevudine and emtricitabine with adenovirus-mediated delivery of gamma interferon in the woodchuck model of hepatitis B virus infection

Our aim was to evaluate the antiviral effect of a combination of two nucleoside reverse transcriptase inhibitors, emtricitabine (FTC) and clevudine (L-FMAU), with the addition of an adenovirus-driven delivery of recombinant gamma interferon (IFN-gamma) in the woodchuck model of hepatitis B virus infection. Six woodchuck hepatitis virus (WHV)-infected woodchucks received L-FMAU (10 mg/kg) plus FTC (30 mg/kg) intraperitoneally for 8 weeks; six other animals received in addition an intravenous injection of a recombinant adenovirus vector expressing woodchuck IFN-gamma (Ad-IFN) at weeks 4 and 8. In the control group, two animals received Ad-IFN alone, two received adenovirus vector expressing the green fluorescent protein reporter gene, and one remained untreated. In less than 2 weeks, all woodchucks that received L-FMAU plus FTC showed a rapid and marked inhibition of viral replication, with a 4-log(10) drop in serum WHV DNA. In two animals, viremia remained suppressed for several months after the end of treatment. Similarly, a dramatic decrease in intrahepatic replicative intermediates of viral DNA was observed in the L-FMAU/FTC-treated groups. The additional administration of Ad-IFN led to increased inflammation in the liver but did not enhance the antiviral effect of the L-FMAU/FTC combination. In conclusion, therapies combining L-FMAU and FTC in WHV-infected woodchucks resulted in a potent and sustained antihepadnaviral effect both in the liver and in the blood circulation. However, no extra benefit of adding IFN-gamma gene transduction to the L-FMAU/FTC combination could be detected.

Low doses of hepadnavirus induce infection of the lymphatic system that does not engage the liver

Woodchuck hepatitis virus (WHV), which is closely related to human hepatitis B virus and is considered to be principally hepatotropic, invades the host's lymphatic system and persists in lymphoid cells independently of whether the infection is symptomatic and serologically evident or concealed. In this study, we show, with the woodchuck model of hepatitis B, that hepadnavirus can establish an infection that engages the lymphatic system, but not the liver, and persists in the absence of virus serological markers, including antiviral antibodies. This primary occult infection is caused by wild-type virus invading the host at a quantity usually not greater than 10(3) virions. It is characterized by trace virus replication progressing in lymphatic organs and peripheral lymphoid cells that, with time, may also spread to the liver. The infection is transmissible to virus-naive hosts as an asymptomatic, indefinitely long, occult carriage of small amounts of biologically competent virus. In contrast to residual silent WHV persistence, which normally endures after the resolution of viral hepatitis and involves the liver, primary occult infection restricted to the lymphatic system does not protect against reinfection with a large, liver-pathogenic WHV dose; however, the occult infection is associated with a swift recovery from hepatitis caused by the superinfection. Our study documents that the lymphatic system is the primary target of WHV infection when small quantities of virions invade a susceptible host.

Quantitative detection of hepadnavirus-infected lymphoid cells by in situ PCR combined with flow cytometry: implications for the study of occult virus persistence

The detection of small amounts of viral pathogens in infected cells by classical PCR is hampered by a partial loss of virus nucleic acid due to extraction and by difficulties in discrimination between truly intracellular virus genome material and that possibly adhered to the cell surface. These impediments limit reliable identification of virus traces within infected cells, which are typically encountered in latent and persistent occult infections. In this study, hepadnavirus-specific in situ PCR combined with the enzymatic elimination of extracellular virus and flow cytometry permitted detection of viral genomes in lymphoid cells without nucleic acid isolation and allowed quantification of infected cells during the course of persistent infection with woodchuck hepatitis virus (WHV). The validity of the procedure was confirmed by hybridization analysis of the in situ-amplified viral sequences. The results showed that hepadnavirus can be directly detected within lymphoid cells not only in serologically accountable infection, but also years after recovery from viral hepatitis and in the course of primary occult virus carriage. Percentages of infected peripheral lymphoid cells in symptomatic WHV hepatitis fluctuate between 3.4 and 20.4% (mean +/- standard error of the mean, 9.6% +/- 1.7%), whereas those in persistent, serologically mute WHV infection range from 1.1 to 14.6% (mean +/- standard error of the mean, 4.8% +/- 0.8%) (P = 0.005). The data obtained provide further evidence that WHV infection continues indefinitely in the lymphatic system independently of whether it is symptomatic or concealed. They document that hepadnavirus can be detected in a significant proportion of circulating lymphoid cells in both immunovirologically apparent as well as occult persistent infection.

A pregenomic RNA sequence adjacent to DR1 and complementary to epsilon influences hepatitis B virus replication efficiency

Hepatitis B virus (HBV) possesses a 3.2-kb partially double-stranded DNA genome that is generated inside the nucleocapsid by the reverse transcription of the 3.5-kb pregenomic viral transcript. The initial steps in viral replication involve the recognition of an encapsidation signal termed epsilon (epsilon) at the 5'-end of the pregenomic RNA by the HBV polymerase. The polymerase-bound pregenomic RNA is subsequently incorporated into an immature nucleocapsid particle and minus-strand HBV DNA synthesis is initiated utilizing the bulge region of epsilon as a template and a tyrosine residue in the amino-terminal region of the polymerase as a primer. Three nucleotides complementary to the 3'-end of the bulge region of epsilon are synthesized and subsequently translocated with the polymerase molecule to the acceptor site located in the DR1 sequence present at the 3'-end of the pregenomic RNA. Using mutagenesis analysis, a sequence element designated phi (phi) located upstream of the 3' DR1 sequence has been identified that is complementary to epsilon and is important for efficient viral replication. This element may bring the 3' DR1 sequence into proximity with the three nucleotide primer synthesized at the bulge of epsilon and facilitate primer translocation to the 3' DR1 acceptor sequence. Sequence elements with similar proximity to the 3' DR1 sequences and complementarity to epsilon are present in the woodchuck hepatitis virus (WHV) and duck hepatitis B virus (DHBV), suggesting the phi regulatory element may be phylogenetically conserved due to its functional importance in hepadnavirus minus-strand DNA synthesis.

Avian and Mammalian hepadnaviruses have distinct transcription factor requirements for viral replication

Hepadnavirus replication occurs in hepatocytes in vivo and in hepatoma cell lines in cell culture. Hepatitis B virus (HBV) replication can occur in nonhepatoma cells when pregenomic RNA synthesis from viral DNA is activated by the expression of the nuclear hormone receptors hepatocyte nuclear factor 4 (HNF4) and the retinoid X receptor alpha (RXR alpha) plus peroxisome proliferator-activated receptor alpha (PPAR alpha) heterodimer. Nuclear hormone receptor-dependent HBV replication is inhibited by hepatocyte nuclear factor 3 (HNF3). In contrast, HNF3 and HNF4 support duck hepatitis B virus (DHBV) replication in nonhepatoma cells, whereas the RXR alpha-PPAR alpha heterodimer inhibits HNF4-dependent DHBV replication. HNF3 and HNF4 synergistically activate DHBV pregenomic RNA synthesis and viral replication. The conditions that support HBV or DHBV replication in nonhepatoma cells are not able to support woodchuck hepatitis virus replication. These observations indicate that avian and mammalian hepadnaviruses have distinct transcription factor requirements for viral replication.

Mutations of the woodchuck hepatitis virus polymerase gene that confer resistance to lamivudine and 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil

Administration of either lamivudine (2'-deoxy-3'-thiacytidine) or L-FMAU (2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil) to woodchucks chronically infected with woodchuck hepatitis virus (WHV) induces a transient decline in virus titers. However, within 6 to 12 months, virus titers begin to increase towards pretreatment levels. This is associated with the emergence of virus strains with mutations of the B and C regions of the viral DNA polymerase (T. Zhou et al., Antimicrob. Agents Chemother. 43:1947-1954, 1999; Y. Zhu et al., J. Virol. 75:311-322, 2001). The present study was carried out to determine which of the mutants that we have identified conferred resistance to lamivudine and/or to L-FMAU. When inserted into a laboratory strain of WHV, each of the mutations, or combinations of mutations, of regions B and C produced a DNA replication-competent virus and typically conferred resistance to both nucleoside analogs in cell culture. Sequencing of the polymerase active site also occasionally revealed other mutations, but these did not appear to contribute to drug resistance. Moreover, in transfected cells, most of the mutants synthesized viral DNA nearly as efficiently as wild-type WHV. Computational models suggested that persistence of several of the WHV mutants as prevalent species in the serum and, by inference, liver for up to 6 months following drug withdrawal required a replication efficiency of at least 10 to 30% of that of the wild type. However, their delayed emergence during therapy suggested replication efficiency in the presence of the drug that was still well below that of wild-type WHV in the absence of the drug.

Deficiencies in the acute-phase cell-mediated immune response to viral antigens are associated with development of chronic woodchuck hepatitis virus infection following neonatal inoculation

In vitro proliferation of peripheral blood mononuclear cells was used to measure virus-specific cell-mediated immunity (vCMI) following neonatal woodchuck hepatitis virus (WHV) infection. Fifteen neonates were inoculated with the W8 strain of WHV. In 11, infection was resolved, and 4 became chronic carriers. Nineteen neonates were inoculated with the W7 strain and all became chronic carriers. Seven age-matched uninfected woodchucks served as controls. Virologic and vCMI profiles among the W8 and W7 infections were compared and related to the outcome of infection. Resolving woodchucks had robust, acute-phase vCMI to WHV antigens (core, surface, and x) and to several nonoverlapping core peptides. The acute-phase vCMI was associated temporally with the clearance of viral DNA and of surface antigen from serum at 14 to 22 weeks postinfection. In contrast, in approximately half of the W8 and W7 infections that progressed to chronicity, no significant acute-phase vCMI was detected. In the remaining carriers, acute-phase vCMI was observed, but it was less frequent and incomplete compared to that of resolved woodchucks. Serum viral load developed less rapidly in those carriers that had evidence of acute-phase vCMI, but it was still increased compared to that of resolving woodchucks. Thus, vigorous and multispecific acute-phase vCMI was associated with resolution of neonatal WHV infection. Absent or incomplete acute-phase vCMI was associated with the progression to chronic infection. By analogy, these results suggest that the onset of chronic hepatitis B virus (HBV) infection in humans may be associated with deficiencies in the primary T-cell response to acute HBV infection.

Out-of-frame versus in-frame core internal deletion variants of human and woodchuck hepatitis B viruses

Human hepatitis B virus (HBV) variants containing in-frame core internal deletion (CID) have been demonstrated to contain all the functional features of defective interfering (DI) particles (Yuan, T. T.-T., M.-H. Lin, D. S. Chen, and C. Shih, 1998, J. Virol. 72, 578-584). Here, we report that out-of-frame HBV CID variants exhibit defective interfering property similar to in-frame CID variants characterized previously. This result raises the possibility that it may be the deleted pregenomic RNA product, rather than the deleted core protein product, that is responsible for interference. Furthermore, a genomic deletion elsewhere does not cause interference since preS2 deletion variants exhibit no influence on wild-type HBV replication. Consistent with the natural occurrence of HBV CID variants, we recently identified CID variants of woodchuck hepatitis virus (WHV) in natural infection. However, unlike HBV CID variants, functional characterization of WHV CID variants using a human hepatoma cell line has not revealed any interference in tissue culture. In summary, defective interference is a general phenomenon for both in-frame and out-of-frame HBV CID variants.

Protection against woodchuck hepatitis virus (WHV) infection by gene gun coimmunization with WHV core and interleukin-12

Woodchuck hepatitis virus (WHV) and hepatitis B virus (HBV) are closely similar with respect to genomic organization, host antiviral responses, and pathobiology of the infection. T-cell immunity against viral nucleocapsid (HBcAg or WHcAg) has been shown to play a critical role in viral clearance and protection against infection. Here we show that vaccination of healthy woodchucks by gene gun bombardment with a plasmid coding for WHcAg (pCw) stimulates proliferation of WHcAg-specific T cells but that these cells do not produce significant levels of gamma interferon (IFN-gamma) upon antigen stimulation. In addition, animals vaccinated with pCw alone were not protected against WHV inoculation. In order to induce a Th1 cytokine response, another group of woodchucks was immunized with pCw together with another plasmid coding for woodchuck interleukin-12 (IL-12). These animals exhibited WHcAg-specific T-cell proliferation with high IFN-gamma production and were protected against challenge with WHV, showing no viremia or low-level transient viremia after WHV inoculation. In conclusion, gene gun immunization with WHV core generates a non-Th1 type of response which does not protect against experimental infection. However, steering the immune response to a Th1 cytokine profile by IL-12 coadministration achieves protective immunity. These data demonstrate a crucial role of Th1 responses in the control of hepadnavirus replication and suggest new approaches to inducing protection against HBV infection.

The woodchuck model of hepatitis B virus infection

The woodchuck hepatitis virus (WHV) was the first of the mammalian and avian hepadnaviruses described after discovery of the virus of hepatitis B (HBV). Woodchucks chronically infected with WHV develop progressively severe hepatitis and hepatocellular carcinoma, which present as lesions that are remarkably similar to those associated with HBV infection in humans. The initial virological studies and studies of pathogenesis utilized woodchucks that had been trapped in the wild and had acquired WHV infection naturally. Research with wild woodchucks was complicated by lack of knowledge of their backgrounds (e.g., dietary history, exposure to parasites or environmental toxins, and source and duration of WHV infection). Breeding colonies of woodchucks have been established and maintained in laboratory animal facilities, and laboratory-reared woodchucks are superior for experimental studies of pathogenesis or hepatocarcinogenesis. It is possible to infect neonatal woodchucks born in the laboratory with standardized inocula and produce a high rate of chronic WHV carriers that are useful for controlled investigations. WHV has been shown experimentally to cause hepatocellular carcinoma, supporting conclusions based on epidemiological and molecular virological studies that HBV is an important etiological factor in human hepatocarcinogenesis. Chronic WHV carrier woodchucks have become a valuable animal model for the preclinical evaluation of antiviral therapy for HBV infection, providing useful pharmacokinetic and pharmacodynamic results in a relevant animal disease model. It also has been shown that the pattern of toxicity and hepatic injury observed in woodchucks treated with certain fluorinated pyrimidines is remarkably similar to that observed in humans that were treated with the same drugs, suggesting the woodchuck has significant potential for the preclincial assessment of antiviral drug toxicity.

Antiviral activity of beta-L-2',3'-dideoxy-2',3'-didehydro-5-fluorocytidine in woodchucks chronically infected with woodchuck hepatitis virus

The L-nucleoside analog beta-L-2',3'-dideoxy-2',3'-didehydro-5-fluorocytidine (beta-L-Fd4C) was first shown to exhibit potent activity against hepatitis B virus (HBV) in tissue culture and then to significantly inhibit viral spread during acute infection in the duck HBV model (F. Le Guerhier et al., Antimicrob. Agents Chemother. 44:111-122, 2000). We have therefore examined its antiviral activity in a mammalian model of chronic HBV infection, the woodchuck chronically infected with woodchuck hepatitis virus (WHV). Side-by-side comparison of beta-L-Fd4C and lamivudine administered intraperitoneally during short-term and long-term protocols demonstrated a more profound inhibition of viremia in beta-L-Fd4C-treated groups. Moreover, beta-L-Fd4C induced a marked inhibition of intrahepatic viral DNA synthesis compared with that induced by lamivudine. Nevertheless, covalently closed circular (CCC) DNA persistence explained the lack of clearance of infected hepatocytes expressing viral antigens and the relapse of WHV replication after drug withdrawal. Liver histology showed a decrease in the inflammatory activity of chronic hepatitis in woodchucks receiving beta-L-Fd4C. An electron microscopy study showed the absence of ultrastructural changes of hepatic mitochondria, biliary canaliculi, and bile ducts. However, a loss of weight was observed in all animals, whatever the treatment, as was a transient skin pigmentation in all woodchucks during beta-L-Fd4C treatment. There was no evidence that lamivudine or beta-L-Fd4C could prevent the development of hepatocellular carcinoma with the protocols used. These results indicate that beta-L-Fd4C exhibits a more potent antiviral effect than lamivudine in the WHV model but was not able to eradicate CCC DNA and infected cells from the liver at the dosage and with the protocol used.

In vitro and in vivo infectivity and pathogenicity of the lymphoid cell-derived woodchuck hepatitis virus

Woodchuck hepatitis virus (WHV) and human hepatitis B virus are closely related, highly hepatotropic mammalian DNA viruses that also replicate in the lymphatic system. The infectivity and pathogenicity of hepadnaviruses propagating in lymphoid cells are under debate. In this study, hepato- and lymphotropism of WHV produced by naturally infected lymphoid cells was examined in specifically established woodchuck hepatocyte and lymphoid cell cultures and coculture systems, and virus pathogenicity was tested in susceptible animals. Applying PCR-based assays discriminating between the total pool of WHV genomes and covalently closed circular DNA (cccDNA), combined with enzymatic elimination of extracellular viral sequences potentially associated with the cell surface, our study documents that virus replicating in woodchuck lymphoid cells is infectious to homologous hepatocytes and lymphoid cells in vitro. The productive replication of WHV from lymphoid cells in cultured hepatocytes was evidenced by the appearance of virus-specific DNA, cccDNA, and antigens, transmissibility of the virus through multiple passages in hepatocyte cultures, and the ability of the passaged virus to infect virus-naive animals. The data also revealed that WHV from lymphoid cells can initiate classical acute viral hepatitis in susceptible animals, albeit small quantities (approximately 10(3) virions) caused immunovirologically undetectable (occult) WHV infection that engaged the lymphatic system but not the liver. Our results provide direct in vitro and in vivo evidence that lymphoid cells in the infected host support propagation of infectious hepadnavirus that has the potential to induce hepatitis. They also emphasize a principal role of the lymphatic system in the maintenance and dissemination of hepadnavirus infection, particularly when infection is induced by low virus doses.

Animal models of hepadnavirus-associated hepatocellular carcinoma

Animal models of hepatitis B virus infection have been valuable for determining the mechanisms of hepadnavirus replication, for studies of pathogenesis, and for investigations of viral hepatocarcinogenesis. The woodchuck model also seems to be useful in the discovery and development of antiviral drugs to treat HBV infection and for testing new forms of immunotherapy. In particular, the woodchuck seems to be ideal for studying the effect of antiviral treatment and immunotherapy on the outcome of hepadnavirus infection and on survival. The median life expectancy of experimentally infected, chronic WHV carriers is approximately 29 months, and almost all develop HCC. New types of prophylaxis or therapy can be evaluated under controlled experimental conditions, in a relevant animal model, and within a reasonable time frame.

Kinetics of hepadnavirus loss from the liver during inhibition of viral DNA synthesis

Hepadnaviruses replicate by reverse transcription, which takes place in the cytoplasm of the infected hepatocyte. Viral RNAs, including the pregenome, are transcribed from a covalently closed circular (ccc) viral DNA that is found in the nucleus. Inhibitors of the viral reverse transcriptase can block new DNA synthesis but have no direct effect on the up to 50 or more copies of cccDNA that maintain the infected state. Thus, during antiviral therapy, the rates of loss of cccDNA, infected hepatocytes (1 or more molecules of cccDNA), and replicating DNAs may be quite different. In the present study, we asked how these losses compared when woodchucks chronically infected with woodchuck hepatitis virus were treated with L-FMAU [1-(2-fluoro-5-methyl-beta-L-arabinofuranosyl) uracil], an inhibitor of viral DNA synthesis. Viremia was suppressed for at least 8 months, after which drug-resistant virus began replicating to high titers. In addition, replicating viral DNAs were virtually absent from the liver after 6 weeks of treatment. In contrast, cccDNA declined more slowly, consistent with a half-life of approximately 33 to 50 days. The loss of cccDNA was comparable to that expected from the estimated death rate of hepatocytes in these woodchucks, suggesting that death of infected cells was one of the major routes for elimination of cccDNA. However, the decline in the actual number of infected hepatocytes lagged behind the decline in cccDNA, so that the average cccDNA copy number in infected cells dropped during the early phase of therapy. This observation was consistent with the possibility that some fraction of cccDNA was distributed to daughter cells in those infected hepatocytes that passed through mitosis.

Limited proteolysis induces woodchuck hepatitis virus infectivity for human HepG2 cells

Previous work from our laboratory has shown that digestion of hepatitis B virus (HBV) with V8 protease rendered the virus infectious for human hepatoblastoma cell line (HepG2). It was hypothesized that the cleavage exposes a 16 amino acid region that includes a consensus 'fusion' motif necessary to mediate infectivity. Since woodchuck hepatitis virus (WHV) and HBV possess significant homology in this region of their envelope proteins, including the V8 protease cleavage site, the possibility that WHV infectivity for HepG2 cells could be induced by V8 digestion was explored. WHV isolated from the serum of chronically infected woodchucks, digested with V8 protease, was shown to loose its preS domain. V8 digested WHV eluted from gel filtration columns with a size similar to that of undigested virus, suggesting that digestion with V8 protease did not cause significant changes in virion size. The amount of progeny virus secreted into the culture medium following infection of HepG2 cells with V8 digested WHV reached 2.5 pg/ml, after 8 days. Moreover, WHV DNA replicative intermediates could be detected in the cells following infection with protease digested, but not undigested, viruses. These data suggest that protease modification of WHV, a non-human virus, induced infectivity for human tissue culture cells. These results are consistent with the hypothesis that exposure of an amino acid region of the envelope polypeptide that contains a consensus fusion motif is important in Hepadnavirus entry.

Effects of age and viral determinants on chronicity as an outcome of experimental woodchuck hepatitis virus infection

Acute hepadnavirus infections either resolve or progress to chronicity. Factors that influence chronicity as an outcome of hepatitis B virus (HBV) infection in humans can be studied experimentally in the woodchuck model. Accordingly, several woodchuck hepatitis virus (WHV) inocula were characterized. Representative inocula had high titers of infectious virus (approximately 10(7.7)-10(9.5) woodchuck 50% infectious doses per milliliter [WID(50%)/mL] by subcutaneous inoculation), with 1 WID(50%) ranging between 21 and 357 physical virion particles. WHV7P1 (standard high dose, 5 x 10(6) WID(50%)) produced a 72% chronicity rate (i.e., percent chronic of total infected) in neonatal woodchucks (1-3 days old). Comparable doses of WHV8P1 resulted in a lower chronicity rate in neonates (34% chronic) indicating that it represented a strain different from WHV7P1. Neonatal woodchucks were more susceptible to chronic infection by high doses of WHV7P1 (range, 65%-75% chronic) compared with 8-week-old weanlings (33% chronic) and adult woodchucks (0% chronic; i.e., all resolved). High doses of cloned wild-type viruses also induced high rates of chronicity in neonates (70%-80% chronic). Chronicity rates in neonates were decreased for low doses of WHV7P1 (500 WID(50%), 9% chronic) and for high doses of a precore WHeAg-minus mutant WHV8 clone (17% chronic). Thus, both age and viral determinants can influence chronicity as an outcome of experimental WHV infection. Standardized inocula will enable the study of mechanisms that initiate and maintain chronic hepadnavirus infection and also provide a means for developing WHV carriers for therapeutic studies.

Hepatitis B virus in hepatocarcinogenesis

Hepatitis B virus (HBV) is an important etiologic agent of chronic hepatitis, cirrhosis, and hepatocellular carcinoma (HCC). Although the mechanism whereby HBV causes HCC is not fully understood, it is likely that there are many relevant molecular pathways that contribute to the development of HBV-associated HCC. This review provides an overview of some of these proposed pathways and their relative importance. It also raises questions on basic and translational research that will signficantly contribute to the better understanding of underlying mechanisms, prevention, and treatment of this tumor type.

Emergence of drug-resistant populations of woodchuck hepatitis virus in woodchucks treated with the antiviral nucleoside lamivudine

Lamivudine [(-)-beta-L-2',3'-dideoxy-3'-thiacytidine] reduces woodchuck hepatitis virus (WHV) titers in the sera of chronically infected woodchucks by inhibiting viral DNA synthesis. However, after 6 to 12 months, WHV titers begin to increase toward pretreatment levels. Three WHV variants with mutations in the active site of the DNA polymerase gene are present at this time (W. S. Mason et al., Virology 245:18-32, 1998). We have asked if these mutant viruses were responsible for the lamivudine resistance and if their emergence caused an immediate rise in virus titers. Cell cultures studies implied that the mutants were resistant to lamivudine. Emergence of mutant WHV was not always associated, however, with an immediate rise in virus titers in the serum. One of the three types of mutant viruses became prominent in serum up to 7 months before titers in serum actually began to increase, at a time when wild-type virus was still predominant in the liver. The two other mutants did not show this behavior but were detected in serum and liver later, just at the time that virus titers began to rise. A factor linking all three mutants was that a similar duration of drug administration preceded the rise in titers, irrespective of which mutant ultimately prevailed. A simple explanation for these results is that the increase in virus titers following emergence of drug-resistant mutants can occur only as the preexisting wild-type virus is cleared from the hepatocyte population, allowing spread of the mutants. Thus, prolonged suppression of virus titers in the serum may sometimes be a measure of the stability of hepatocyte infection rather than of a successful therapeutic outcome.

Persistence of infectious hepadnavirus in the offspring of woodchuck mothers recovered from viral hepatitis

Mother-to-child transmission is an important route for hepatitis B virus (HBV) dissemination. It has been established that HBV traces persist for years after complete clinical recovery from hepatitis B. Similarly, resolution of hepatitis caused by HBV-related woodchuck hepatitis virus (WHV) is followed by occult lifelong carriage of pathogenic virus. In this study, we documented that WHV persisting after termination of acute hepatitis is transmittable to newborns as an asymptomatic long-term infection. All 11 offspring from 4 dams studied carried transcriptionally active WHV genomes for 3.5 years after birth without immunovirological markers of infection. WHV genomes and mRNA were detected both in the liver and lymphoid tissue in the majority of offspring; WHV covalently closed circular DNA was detected in some samples. In 4 offspring, however, the virus was restricted to the lymphatic system. In the circulation, WHV DNA-reactive particles were DNase resistant and of comparable size and density to complete virions. Importantly, the virus in offspring with or without hepatic WHV DNA expression was infectious to WHV-naive woodchucks. Finally, offspring challenged with WHV were not protected against reinfection. These findings show that mothers with occult hepadnaviral carriage transmit pathogenic virus to their offspring, inducing a persistent infection invariably within the lymphatic system but not always in the liver.

Occult lifelong persistence of infectious hepadnavirus and residual liver inflammation in woodchucks convalescent from acute viral hepatitis

Traces of hepatitis B virus (HBV) genome can persist for years following recovery from hepatitis B. To determine overall duration, molecular characteristics, and pathological implications of this serologically undetectable form of hepadnaviral carriage, we have analyzed the expression of transcriptionally active virus genomes, their infectivity, and examined liver alterations during the natural lifespan of woodchucks convalescent from acute infection with HBV- related woodchuck hepatitis virus (WHV). In this study, we document lifelong persistence of scanty amounts of replicating virus both in the liver and lymphatic system after spontaneous resolution of an episode of experimental hepadnaviral hepatitis. Antibodies to virus nucleocapsid (core) were found to be the most reliable immunovirological marker coexisting with occult infection. In the majority of convalescent woodchucks, serial liver biopsies showed protracted minimal to mild necroinflammation with periods of normal morphology; however, hepatocellular carcinoma (HCC) ultimately developed in 2 of 9 animals studied. Inocula derived from lymphoid cells of convalescent animals induced classical acute hepatitis in virus-naive woodchucks that progressed to chronic hepatitis and HCC in 1 of the animals, demonstrating infectivity and pathogenic competence of the carried virus. Our results reveal that low levels of infectious WHV and residual hepatic inflammation usually continue for life after resolution of hepatitis and that this recovery does not avert HCC development. They also demonstrate that, in addition to the liver, the lymphatic system is the site of the occult lifelong maintenance of replicating hepadnavirus.

In vivo effects of mutations in woodchuck hepatitis virus enhancer II

Woodchuck hepatitis virus (WHV) enhancer II (EnII) is located upstream of the major pregenomic RNA promoter and is thought to play an important role in the insertional activation of the N-myc2 gene during WHV hepatocarcinogenesis. WHV EnII is recognized by at least three host transcription factors: HNF-1, HNF-4, and Oct-1. Here, the roles of these EnII-binding factors in viral transcription and replication have been further examined. In HepG2 cells transiently transfected with a chloramphenicol acetyltransferase (CAT) gene whose expression is dependent upon EnII, mutations in either the HNF-1 or the HNF-4 site strongly reduced CAT activity, while ablation of the Oct-1 site decreased CAT expression only twofold. Mutations in more than one site completely abolished reporter expression. These same mutations were also tested in an overlength WHV genome for their impact on viral replication and gene expression. In transfected HepG2 cells, lesions in the HNF-1 site inactivated pregenomic RNA expression and viral reverse transcription, with only minimal effects on the expression of other viral mRNAs. By contrast, Oct-1 site lesions had no effect on either viral RNA synthesis or DNA replication, and HNF-4 site lesions produced a modest reduction of pregenomic RNA but had no impact on viral DNA synthesis. Testing of the mutants in susceptible woodchucks revealed that, as expected, viruses with lesions in the HNF-1 site were nearly noninfectious, while mutants with lesions at the Oct-1 site were fully replication competent. HNF-4 site mutants were replication competent but may display reduced levels of replication in the intact animal host. We conclude that (i) EnII is primarily devoted to the regulation of pregenomic RNA in WHV, (ii) HNF-1 is essential for EnII function in vivo, and (iii) HNF-4 plays a demonstrable but adjunctive role in EnII function.

Lamivudine therapy of WHV-infected woodchucks

Hepatitis B viruses establish a chronic, productive, and noncytopathic infection of hepatocytes. Viral products are produced by transcription from multiple copies (5-50) of covalently closed circular (ccc) viral DNA. This cccDNA does not replicate, but can be replaced by DNA precursors that are synthesized in the cytoplasm. The present study was carried out to determine if long-term treatment with an inhibitor of viral DNA synthesis would lead to loss of virus products, including cccDNA, from the liver of woodchucks chronically infected with woodchuck hepatitis virus. Viral DNA synthesis was inhibited with the nucleoside analog, lamivudine (2'-deoxy-3'-thiacytidine). Lamivudine treatment produced a slow but progressive decline in viral titers in serum, to about 0.3% or less of the initial level. However, even after maintenance of drug therapy for 3-12 months, > 95% of the hepatocytes in most animals were still infected. Significant declines in the percentage of infected hepatocytes and of intrahepatic cccDNA levels were observed in only three woodchucks, two in the group receiving lamivudine and one in the placebo control group. Moreover, virus titers eventually rose in woodchucks receiving lamivudine, suggesting that drug-resistant viruses began to spread through the liver starting at least as early as 9-12 months of treatment. Three types of mutation that may be associated with drug resistance were found at this time, in a region upstream of the YMDD motif in the active site of the viral reverse transcriptase. The YMDD motif itself remained unchanged. Not unexpectedly, the lamivudine therapy did not have a impact on development of liver cancer.

Lack of effect of antiviral therapy in nondividing hepatocyte cultures on the closed circular DNA of woodchuck hepatitis virus

The template for synthesis of hepadnaviral RNAs is a covalently closed circular (ccc) DNA located in the nucleus of the infected hepatocyte. Hepatocytes are normally long-lived and nondividing, and antiviral therapies in chronically infected individuals face the problem of eliminating not only the replicative forms of viral DNA found in the cytoplasm but also the cccDNA from the nucleus. Because cccDNA does not replicate semiconservatively, it is not an obvious target for antiviral therapy. However, elimination of cccDNA might be facilitated if its half-life were short in comparison to the generation time of hepatocytes and if new cccDNA formation were effectively blocked. We have therefore measured cccDNA levels in woodchuck hepatocyte cultures following in vitro infection with woodchuck hepatitis virus and treatment with inhibitors of viral DNA synthesis. The viral reverse transcriptase inhibitors lamivudine (3TC) [(-)-beta-L-2',3'-dideoxy-3'-thiacytidine), FTC (5-fluoro-2',3'-dideoxy-3'-thiacytidine) and ddC (2',3'-dideoxycytidine) were added to the cultures beginning at 4 days postinfection. Treatment for up to 36 days with 3TC reduced the amount of cccDNA in the cultures not more than twofold compared to that of an untreated control. Treatment with ddC for 36 days and with FTC for 12 days resulted in effects similar to that of treatment with 3TC. Moreover, the declines in cccDNA appeared to reflect the loss of hepatocytes from the cultures rather than of cccDNA from hepatocytes. These results emphasize the important role of the longevity of the infected hepatocytes in the persistence of an infection.

A novel transcriptional element in circular DNA monomers of the duck hepatitis B virus

We report the presence of two elements, pet and net, that are required for proper transcription of the duck hepatitis B virus (DHBV). These regions were previously identified by using plasmid clones of the virus in transient expression assays (M. Huang and J. Summers, J. Virol. 68:1564-1572, 1994). In this study, we further analyzed these regions by using in vitro-synthesized circular DHBV DNA monomers to mimic the authentic transcriptional template. We observed that pet was required for pregenome transcription from circular viral monomers, and in the absence of pet-dependent transcription, expression of the viral envelope genes was increased. We found that deletion of net in circularized DNA monomers led to the production of abnormally long transcripts due to a failure to form 3' ends during transcription. In addition, we report the presence of a net-like region in the mammalian hepadnavirus woodchuck hepatitis virus. These results are consistent with a model that net is a region involved in transcription termination and that in DHBV, pet is required for transcription complexes to read through this region during the first pass through net.

Human hepatitis B virus enhancer 1 is responsive to human interleukin-6

Serum levels of interleukin-6 (IL-6) are elevated in acute and chronic hepatitis B patients. The effect of IL-6 and its transcription factor of NF-IL6 (a nuclear factor for IL-6) on hepatitis B virus (HBV) enhancer 1 (Enh1), which controls HBV X expression, were investigated in HepG2 cells. Twenty ng/ml of IL-6 increased 4-fold the enhancer activity of Enh1 according to the CAT assay. The IL-6 stimulation was abolished by introducing a mutation either in an AP-1-related site or a C-stretch sequence in the Enh1 sequence, demonstrating that the cis-elements are necessary for the IL-6 response. Co-transfection of NF-IL6 expression plasmid similarly increased the enhancer activity of Enh1 through both binding sites. Further, a specific complex formation of the Enh1 was detected using HepG2 nuclear lysates by electromobility shift assays, and the complex formation was increased in the lysates of cells treated with IL-6 and NF-IL6-transfection. In competition assays, one half of the complex formed was found to remain in the presence of 500-times excess competitor DNA fragment harboring NF-IL2 binding site, suggesting indirect binding of NF-IL6 to the Enh1 sequence. These results indicate that IL-6 increased the enhancer activity of HBV Enh1 through signal transduction pathways, indirectly involving NF-IL6, and may control HBV X expression and viral replication in HBV infected liver.

Major differences between WHV and HBV in the regulation of transcription

Studies were carried out to further characterize enhancer and promoter elements on the woodchuck hepatitis virus (WHV) genome. We were able to confirm the existence of WHV promoters analogous to the major promoters of the related human hepatitis B virus (HBV) and of an enhancer analogous to the recently described WHV E2 element (Ueda, K., Wei, Y., and Ganem, D., Virology 217, 413, 1996). However, we were unable to identity an enhancer analogous to the E1 element of (HBV), despite the fact that these two viruses share a high degree of sequence homology and genetic organization. Some factor binding sites in the E1 region appeared to be conserved between the two viruses and may be required for the activity of the overlapping X gene promoter of WHV. Others did not appear to be essential for WHV X gene promoter activity, and their functional activity, if any, was not revealed. Our failure to detect a functional enhancer element in the region of WHV homologous to the HBV E1 enhancer may indicate that (i) fundamental differences exist in transcriptional regulation of the small circular genomes of WHV and HBV; (ii) WHV contains an E1 element which is functional in the context of the intact viral genome, but which is unable to function in the context of the various expression constructs used in our experiments; or (iii) correct regulation of WHV transcription via an E1 element is dependent upon transcription factors which are not expressed in the liver-specific cell lines used in our experiments.

Constrained evolution with respect to gene overlap of hepatitis B virus

With the aim of elucidating the evolution of a hepadnavirus family, we constructed molecular phylogenetic trees for 27 strains of hepatitis B virus (HBV) using both the unweighted pair-grouping and neighbor-joining methods. All five gene regions, P, C, S, X, and preS, were used to construct the phylogenetic trees. Using the phylogenetic trees obtained, we classified these strains into five major groups in which the strains were closely related to each other. Our classification reinforced our previous view that genetic classification is not always compatible with conventional classification determined by serological subtypes. Moreover, constraints on the evolutionary process of HBV were analyzed for amino-acid-altering (nonsynonymous) and silent (synonymous) substitutions, because two-thirds of the open reading frame (ORF), P, contains alternating overlapping ORFs. In our unique analysis of this interesting gene structure of HBV, the most frequent synonymous substitutions were observed in the nonoverlapped parts of the P and C genes. On the other hand, the number of synonymous substitutions per nucleotide site for the S gene was quite low and appeared a strongly constrained evolution. Because the P gene overlaps the S gene in a different frame, the low rate of synonymous substitution for the S gene can be explained by the evolutionary constraints which are imposed on the overlapping gene region. In other words, synonymous substitutions in the S gene can cause amino acid changes in its overlapping region in a different frame. Thus, the evolution of HBV is constrained evolutionarily by the overlapping genes. We propose calling this mode of viral evolution "constrained evolution." The evolution of HBV represents a typical constrained evolution.

Covalently closed circular viral DNA formed from two types of linear DNA in woodchuck hepatitis virus-infected liver

We found that livers from woodchucks chronically infected with woodchuck hepatitis virus (WHV) contained covalently closed circular DNA (cccDNA) molecules with deletions and insertions indicative of their formation from linear viral DNA by nonhomologous recombination, as we previously described for the duck hepatitis B virus (W. Yang and J. Summers, J. Virol. 69:4029-4036, 1995). However, evidence for two different types of linear precursors was obtained by analysis of the recombination joints in WHV cccDNA. Type 1 linear precursors possessed the structural properties that correspond to those of in situ-primed linear DNA molecules, which constitute between 7 and 20% of all viral DNA replicative intermediates synthesized in the liver. Type 2 linear precursors are hypothetical species of linear DNAs with a terminal duplication of the cohesive-end region, between DR1 and DR2. This type of linear DNA has not been previously described and was not detected among the DNA species present in nucleocapsids. A fraction of cccDNAs formed from both type 1 and type 2 linear DNAs are predicted to be functional for further DNA synthesis, and some evidence for the formation of two or more generations of cccDNA from linear DNA was observed.

Cellular factors controlling the activity of woodchuck hepatitis virus enhancer II

Woodchuck hepatitis virus (WHV) efficiently induces hepatocellular carcinoma in chronically infected hosts. A key step in hepatocarcinogenesis by WHV is insertional activation of the cellular N-myc gene by integrated viral DNA. WHV enhancer II (En II) is the major cis-acting element involved in this activation. Here we characterize this viral enhancer element and define the cellular factors involved in its activity. WHV En II activity is strongly liver specific and maps to an 88-nucleotide DNA segment (nucleotides 1772 to 1859) located 5' to the pregenomic RNA start site. Genetic analyses and electrophoretic mobility shift assays indicate that the enhancer contains three subregions important to its activity. The core elements of the enhancer are recognition sites for the liver-enriched factors HNF1 and HNF4; together, these signals account for the bulk of En II activity as well as its strong liver specificity. Multimerization of either recognition site produced strong activity even in the absence of other En II sequences. 5' to these elements is a binding site for the ubiquitous Oct-1 transcription factor, which further augments enhancer activity ca. twofold.

A new hepadnavirus endemic in arctic ground squirrels in Alaska

We present evidence for a novel member of the hepadnavirus family that is endemic in wild arctic ground squirrels (Spermophylus parryi kennicotti) in Alaska. This virus, designated arctic squirrel hepatitis virus (ASHV), was initially detected in the livers of animals bearing large hepatic nodules by nucleic acid hybridization with hepadnavirus probes and in plasma by cross-reactivity with antibodies to hepadnavirus surface and core antigens. The complete nucleotide sequence of the 3,302-bp-long ASHV genome was determined and compared with those of ground squirrel hepatitis virus (GSHV) and woodchuck hepatitis virus (WHV); all sequences were organized into four open reading frames, designated pre-C/C, pre-S/S, pol, and X. Despite roughly equivalent variability among the three rodent hepadnaviruses (around 16% base and 19% amino acid exchanges), ASHV appeared to be more closely related to GSHV than to WHV in phylogenetic analysis. Accordingly, preliminary studies of the pathology of ASHV infection suggested that ASHV may be a less efficient oncogenic agent than WHV. About one-third of aged animals maintained in captivity, including virus-infected as well as uninfected squirrels, developed large liver nodules, consisting of hepatocellular adenomas or carcinomas or nonmalignant lesions characterized by drastic microvesicular steatosis. ASHV-infected arctic ground squirrels may serve as a new model with which to analyze the contribution of hepadnavirus- and host-specific determinants to liver pathology and tumorigenesis.

Apoptosis is induced by N-myc expression in hepatocytes, a frequent event in hepadnavirus oncogenesis, and is blocked by insulin-like growth factor II

Induction of hepatocellular carcinoma in woodchucks by woodchuck hepatitis virus is associated with the activation of N-myc gene expression, usually by viral DNA integration in cis to the N-myc locus. We have examined the consequences of N-myc up-regulation in rodent hepatic cells in culture. Mouse alpha ML hepatocytes infected with a retroviral vector overexpressing the woodchuck N-myc2 gene display a higher proliferation rate than parental alpha ML cells but are morphologically unchanged and do not form colonies in soft agar. However, they display an increased propensity to undergo apoptosis, an effect that is markedly augmented by serum deprivation. Expression of the woodchuck hepatitis virus X gene in alpha ML cells does not alter the growth phenotype of the cells and has no effect upon N-myc-dependent apoptosis. However, apoptosis in N-myc2-expressing alpha ML cells is strongly inhibited by insulin-like growth factor II (IGF II). IGF II gene expression is also strongly up-regulated during hepatic carcinogenesis in vivo in virally infected animals and has been speculated to be part of an autocrine growth-stimulatory pathway. Our results suggest that IGF II may play another role in the development of virus-induced hepatoma: the prevention of programmed cell death triggered by deregulated N-myc expression.