Isolation and characterization of a hepatitis B virus endemic in herons

Identification and characterization of a novel replicative intermediate of heron hepatitis B virus

We have identified and characterized a novel intracellular DNA replicative intermediate that is synthesized by heron hepatitis B virus (HHBV) and not by other avian hepadnaviruses. The new DNA form is synthesized in all host cells tested. The HHBV nucleic acid template, and not HHBV proteins, is responsible for the formation of the new form. The new form is comprised of a full-length minus-strand DNA and an incomplete plus-strand DNA whose 5' ends are mapped to DR2, predominantly. The 3' ends of its plus-strand are located between nucleotides 946 and 1046. Genetic analysis indicates that the sequences responsible for the formation of the new form lie between nucleotides 910 and 1364. The endogenous polymerase activity of capsids isolated from cells converted the new form into RC DNA. Intracellular capsids containing the new form are secreted inefficiently as virions, in comparison to RC- and DL DNA-containing capsids. Our analysis suggests that the new form is an incomplete RC DNA molecule that is due to a specific block or pause in the synthesis of plus-strand DNA. Our analysis also suggests that capsids become competent for efficient secretion sometime after the synthesis of 1500 nucleotides of plus-strand DNA.

Inhibition of duck hepatitis B virus infection by a myristoylated pre-S peptide of the large viral surface protein

We have used the duck hepatitis B virus (DHBV) model to study the interference with infection by a myristoylated peptide representing an N-terminal pre-S subdomain of the large viral envelope protein. Although lacking the essential part of the carboxypeptidase D (formerly called gp180) receptor binding site, the peptide binds hepatocytes and subsequently blocks DHBV infection. Since its activity requires an amino acid sequence involved in host discrimination between DHBV and the related heron HBV (T. Ishikawa and D. Ganem, Proc. Natl. Acad. Sci. USA 92:6259-6263, 1995), we suggest that it is related to the postulated host-discriminating cofactor of infection.

Identification and analysis of a new hepadnavirus in white storks

We identified, cloned, and functionally characterized a new avian hepadnavirus infecting storks (STHBV). STHBV has the largest DNA genome of all avian hepadnaviruses and, based on sequence and phylogenetic analysis, is most closely related to, but distinct from, heron hepatitis B virus (HHBV). Unique for STHBV among the other avian hepadnaviruses is a potential HNF1 binding site in the preS promoter. In common only with HHBV, STHBV has a myristylation signal on the S and not the preS protein, two C terminally located glycosylation sites on the precore/core proteins and lacks the phosphorylation site essential for the transcriptional transactivation activity of duck-HBV preS protein. The cloned STHBV genomes were competent in gene expression, replication, and viral particle secretion. STHBV infected primary duck hepatocytes very inefficiently suggesting a restricted host range, similar to other hepadnaviruses. This discovery of stork infections unravels novel evolutionary aspects of hepadnaviruses and provides new opportunities for hepadnavirus research.

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.

Sequence comparison of an Australian duck hepatitis B virus strain with other avian hepadnaviruses

The genome of an Australian strain of duck hepatitis B virus (AusDHBV) was cloned from a pool of congenitally DHBV-infected-duck serum, fully sequenced and found by phylogenetic analyses to belong to the 'Chinese' DHBV branch of the avian hepadnaviruses. Sequencing of the Pre-S/S gene of four additional AusDHBV clones demonstrated that the original clone (pBL4.8) was representative of the virus present in the pool, and a head-to-tail dimer of the clone was infectious when inoculated into newly hatched ducks. When the published sequences of 20 avian hepadnaviruses were compared, substitutions or deletions in the polymerase (POL) gene were most frequent in the 500 nt segment encoding the 'spacer' domain that overlaps with the Pre-S domain of the Pre-S/S gene in a different reading frame. In contrast, substitutions and deletions were rare within the adjacent segment that encodes the reverse transcriptase domain of the POL protein and the S domain of the envelope protein, presumably because they are more often deleterious.

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.

Hepadnavirus infection in captive gibbons

The recent isolation of a nonhuman primate hepadnavirus from woolly monkeys prompted an examination of other primates for potentially new hepadnaviruses. A serological analysis of 30 captive gibbons revealed that 47% were positive for at least one marker of ongoing or previous infection with a hepatitis B virus (HBV). The amino acid sequences of the core and surface genes of human and gibbon virus isolates were very similar. Phylogenetic analysis indicated that the gibbon isolates lie within the human HBV family, indicating that these HBV isolates most likely stem from infection of gibbons from a human source.

Hepatitis B virus biology

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.

Identification of hepatitis B virus indigenous to chimpanzees

Hepatitis B viruses (HBV) and related viruses, classified in the Hepadnaviridae family, are found in a wide variety of mammals and birds. Although the chimpanzee has been the primary experimental model of HBV infection, this species has not been considered a natural host for the virus. Retrospective analysis of 13 predominantly wild-caught chimpanzees with chronic HBV infection identified a unique chimpanzee HBV strain in 11 animals. Nucleotide and derived amino acid analysis of the complete HBV genome and the gene coding for the hepatitis B surface antigen (S gene) identified sequence patterns that could be used to reliably identify chimpanzee HBV. This analysis indicated that chimpanzee HBV is distinct from known human HBV genotypes and is closely related to HBVs previously isolated from a chimpanzee, gibbons, gorillas, and orangutans.

Species-specific TT viruses and cross-species infection in nonhuman primates

Viruses resembling human TT virus (TTV) were searched for in sera from nonhuman primates by PCR with primers deduced from well-conserved areas in the untranslated region. TTV DNA was detected in 102 (98%) of 104 chimpanzees, 9 (90%) of 10 Japanese macaques, 4 (100%) of 4 red-bellied tamarins, 5 (83%) of 6 cotton-top tamarins, and 5 (100%) of 5 douroucoulis tested. Analysis of the amplification products of 90 to 106 nucleotides revealed TTV DNA sequences specific for each species, with a decreasing similarity to human TTV in the order of chimpanzee, Japanese macaque, and tamarin/douroucouli TTVs. Full-length viral sequences were amplified by PCR with inverted nested primers deduced from the untranslated region of TTV DNA from each species. All animal TTVs were found to be circular with a genomic length at 3.5 to 3.8 kb, which was comparable to or slightly shorter than human TTV. Sequences closely similar to human TTV were determined by PCR with primers deduced from a coding region (N22 region) and were detected in 49 (47%) of the 104 chimpanzees; they were not found in any animals of the other species. Sequence analysis of the N22 region (222 to 225 nucleotides) of chimpanzee TTV DNAs disclosed four genetic groups that differed by 36.1 to 50.2% from one another; they were 35.0 to 52.8% divergent from any of the 16 genotypes of human TTV. Of the 104 chimpanzees, only 1 was viremic with human TTV of genotype 1a. It was among the 53 chimpanzees which had been used in transmission experiments with human hepatitis viruses. Antibody to TTV of genotype 1a was detected significantly more frequently in the chimpanzees that had been used in transmission experiments than in those that had not (8 of 28 [29%] and 3 of 35 [9%], respectively; P = 0.038). These results indicate that species-specific TTVs are prevalent in nonhuman primates and that human TTV can cross-infect chimpanzees.

Therapy of chronic hepatitis B virus infection: inhibition of the viral polymerase and other antiviral strategies

Chronic hepatitis B infection remains a major public health problem worldwide. The hepatitis B virus belongs to the family of hepadnaviruses that replicate their DNA genome via a reverse transcription pathway. The chronicity of infection in infected hepatocytes is maintained by the persistence of the viral covalently closed circular DNA. The main strategies to combat chronic HBV infection rely on the stimulation of the specific antiviral immune response and on the inhibition of viral replication. While the prolonged administration of reverse transcriptase inhibitors is most often associated with a control of viral replication rather than eradication, it may select for resistant mutants. The search for new viral targets is therefore mandatory to design combination strategies to prevent the emergence of resistant mutants and eventually clear viral infection.

A new avian hepadnavirus infecting snow geese (Anser caerulescens) produces a significant fraction of virions containing single-stranded DNA

We describe the identification and functional analysis of an evolutionary distinct new avian hepadnavirus. Infection of snow geese (Anser caerulescens) with a duck hepatitis B virus (DHBV)-related virus, designated SGHBV, was demonstrated by detection of envelope proteins in sera with anti-DHBV preS and S antibodies. Comparative sequence analysis of the PCR-amplified SGHBV genomes revealed unique SGHBV sequence features compared with other avian hepadnaviruses. Unlike DHBV, SGHBV shows an open reading frame in an analogous position to orthohepadnavirus X genes. Four of five cloned genomes were competent in replication, gene expression, and virus particle secretion in chicken hepatoma cells. Primary duck hepatocytes were permissive for infection with SGHBV, suggesting a similar or identical host range. SGHBV was found to secrete a significant fraction of virion-like particles containing single-stranded viral DNA. This was observed both in cell culture medium of SGHBV DNA-transfected LMH cells and in viremic sera of several birds, suggesting that it is a stable trait of SGHBV. Taken together, SGHBV has several unique features that expand the knowledge of the functional and evolutionary diversity of hepadnaviruses and offers new experimental opportunities for studies on the life cycle of hepadnaviruses.

Enrichment of a precore-minus mutant of duck hepatitis B virus in experimental mixed infections

A precore-deficient mutant of duck hepatitis B virus (DHBV) produced by site-directed mutagenesis was tested for its ability to compete with wild-type virus in a mixed infection of 3-day-old ducklings. The mutation was shown to produce a cis-acting defect, resulting in a replication rate that was about one-half that of wild-type virus. Accordingly, wild-type virus was rapidly selected during the spread of infection. During the chronic phase of the infection, however, two selection patterns were seen. In 4 of 10 ducks, the wild-type virus slowly replaced the precore mutant. In another four ducks, the precore mutant virus slowly replaced the wild-type virus. In the remaining two ducklings, ratios of wild-type and precore mutant virus fluctuated, with wild-type virus slowly predominating. The replacement of wild-type virus was not due to the emergence of a rapidly replicating variant of the precore mutant, since genomes cloned from the infected ducks retained their original replication defect. Replacement of wild-type virus, however, correlated with elevated anti-core antibody titers, which continued to increase with time. The selection of a precore-negative strain of DHBV may be analogous to the selection for precore mutants of HBV during chronic hepatitis in humans.

Host cell-virus cross talk: phosphorylation of a hepatitis B virus envelope protein mediates intracellular signaling

Phosphorylation of cytosolic pre-S domains of the duck hepatitis B virus (DHBV) large envelope protein (L) was identified as a regulatory modification involved in intracellular signaling. By using biochemical and mass spectrometric analyses of phosphopeptides obtained from metabolically radiolabeled L protein, a single phosphorylation site was identified at serine 118 as part of a PX(S/T)P motif, which is strongly preferred by ERK-type mitogen-activated protein kinases (MAP kinases). ERK2 specifically phosphorylated L at serine 118 in vitro, and L phosphorylation was inhibited by a coexpressed MAP kinase-specific phosphatase. Furthermore, L phosphorylation and ERK activation were shown to be induced in parallel by various stimuli. Functional analysis with transfected cells showed that DHBV L possesses the ability to activate gene expression in trans and, by using mutations eliminating (S-->A) or mimicking (S-->D) serine phosphorylation, that this function correlates with L phosphorylation. These mutations had, however, no major effects on virus production in cell culture and in vivo, indicating that L phosphorylation and transactivation are not essential for hepadnavirus replication and morphogenesis. Together, these data suggest a role of the L protein in intracellular host-virus cross talk by varying the levels of pre-S phosphorylation in response to the state of the cell.

Competition in vivo between a cytopathic variant and a wild-type duck hepatitis B virus

Several examples of human hepatitis B virus strains with enhanced replication in vitro have been described. To understand whether this characteristic could be a cause of liver disease, we have studied a variant of the closely related duck hepatitis B virus (DHBV) that had enhanced levels of cccDNA accumulation, previously shown to be cytopathic in vitro, as a model for the pathogenesis of analogous viruses in humans. In vivo liver damage caused by this variant (G133E) occurred only during the first 2 weeks p.i., after which time cccDNA levels and liver histology returned to near normal despite continued virus replication. To determine whether recovery was due to the emergence of noncytopathic revertant, we tested whether wild-type virus would have a selective advantage in competition with the cytopathic mutant in a fully infected liver. In a mixed infection of ducklings with G133E and a small amount of wild-type virus, the wild-type virus was detected as the predominant genotype after recovery of normal liver histology. Two candidate revertant viral genomes were cloned directly from the serum virus of G133E-infected birds after recovery and tested for (i) control of cccDNA levels in primary hepatocyte cultures and (ii) their ability to compete with wild-type virus in a mixed infection. At least one noncytopathic revertant was identified by these two criteria. The results support the conclusion that the recovery from liver damage in G133E-infected ducklings was due to the emergence of spontaneous noncytopathic revertants rather than to host suppression of virus cytotoxicity. The results indicate that acute liver injury may result from infection with a cytopathic hepadnavirus but that such viruses may be rapidly replaced by noncytopathic variants during persistent infection.

Formation of a functional hepatitis B virus replication initiation complex involves a major structural alteration in the RNA template

The DNA genome of a hepatitis B virus is generated by reverse transcription of the RNA pregenome. Replication initiation does not involve a nucleic acid primer; instead, the hepadnavirus P protein binds to the structured RNA encapsidation signal epsilon, from which it copies a short DNA primer that becomes covalently linked to the enzyme. Using in vitro-translated duck hepatitis B virus (DHBV) P protein, we probed the secondary structure of the protein-bound DHBV epsilon RNA (Depsilon) and observed a marked conformational change compared to free Depsilon RNA. Several initiation-competent mutant RNAs with a different free-state structure were similarly altered, whereas a binding-competent but initiation-deficient variant was not, indicating the importance of the rearrangement for replication initiation and suggesting a mechanistic coupling to encapsidation.

Hepatitis B viruses and cancerogenesis

Hepatitis B virus (HBV) is a small, enveloped DNA virus which primarily infects liver cells and causes acute and persistent liver disease. Chronic HBV infection, frequently associated with cirrhosis and eventually hepatocellular carcinoma (HCC), represents a major health problem in the world. HBV is the prototype member of the hepadnavirus family, which includes several related mammalian viruses also implicated in liver carcinogenesis in the host. Although epidemiological evidence has clearly linked HBV infection with HCC development, the precise role of the virus and the molecular mechanisms of liver cell transformation remain elusive. Here we discuss potential oncogenic strategies of HBV, ranging from indirect mechanisms related to chronic necroinflammatory disease and to the effects of viral gene products on cell proliferation and apoptosis, to direct insertional activation of cellular (onco)genes. Presently, vaccination of high risk populations represents a major way to prevent the development of HBV-related liver cancer.

Avian hepatitis B virus infection is initiated by the interaction of a distinct pre-S subdomain with the cellular receptor gp180

Functionally relevant hepadnavirus-cell surface interactions were investigated with the duck hepatitis B virus (DHBV) animal model by using an in vitro infection competition assay. Recombinant DHBV pre-S polypeptides, produced in Escherichia coli, were shown to inhibit DHBV infection in a dose-dependent manner, indicating that monomeric pre-S chains were capable of interfering with virus-receptor interaction. Particle-associated pre-S was, however, 30-fold more active, suggesting that cooperative interactions enhance particle binding. An 85-amino-acid pre-S sequence, spanning about half of the DHBV pre-S chain, was characterized by deletion analysis as essential for maximal inhibition. Pre-S polypeptides from heron hepatitis B virus (HHBV) competed DHBV infection equally well despite a 50% difference in amino acid sequence and a much-reduced infectivity of HHBV for duck hepatocytes. These observations are taken to indicate (i) that the functionality of the DHBV pre-S subdomain, which interacts with the cellular receptor, is determined predominantly by a defined three-dimensional structure rather than by primary sequence elements; (ii) that cellular uptake of hepadnaviruses is a multistep process involving more than a single cellular receptor component; and (iii) that gp180, a cellular receptor candidate unable to discriminate between DHBV and HHBV, is a common component of the cellular receptor complex for avian hepadnaviruses.

Carboxypeptidase D (gp180), a Golgi-resident protein, functions in the attachment and entry of avian hepatitis B viruses

Carboxypeptidase D (gp180), one of many candidate receptors proposed for hepatitis B viruses (HBVs), was examined and found to be the actual cellular receptor for avian HBVs. This conclusion was based on the following observations: (i) gp180 was the only host protein that bound with high affinity to the pre-S ectodomain of the large duck hepatitis B virus (DHBV) envelope protein, which is known to be essential for virus infection; (ii) a pre-S subdomain which determines physical binding to gp180 was found to coincide with a domain functionally defined in infection competition experiments as a receptor binding domain; (iii) soluble gp180, lacking the membrane anchor, efficiently inhibited DHBV infection; (iv) efficient interspecies gp180-pre-S interaction was limited to the natural hosts of avian hepadnaviruses; and (v) expression of gp180 in a heterologous hepatoma cell line mediated cellular attachment and subsequent internalization of fluorescently labeled viral particles into vesicular structures. However, gp180 expression did not render transfected heterologous cells permissive for productive infection, suggesting that a species-specific coreceptor is required for fusion to complete viral entry. In contrast to the case for known virus receptors, gp180 was not detected on the hepatocyte cell surface but was found to be concentrated in the Golgi apparatus, from where it functions by cycling to and from the plasma membrane.

Isolation of a hepadnavirus from the woolly monkey, a New World primate

Hepatitis B virus (HBV) infections are a major worldwide health problem with chronic infections leading to cirrhosis and liver cancer. Viruses related to human HBV have been isolated from birds and rodents, but despite efforts to find hepadnaviruses that infect species intermediate in evolution between rodents and humans, none have been described. We recently isolated a hepadnavirus from a woolly monkey (Lagothrix lagotricha) that was suffering from fulminant hepatitis. Phylogenetic analysis of the nucleotide sequences of the core and surface genes indicated that the virus was distinct from the human HBV family, and because it is basal (ancestral) to the human monophyletic group, it probably represents a progenitor of the human viruses. This virus was designated woolly monkey hepatitis B virus (WMHBV). Analysis of woolly monkey colonies at five zoos indicated that WMHBV infections occurred in most of the animals at the Louisville zoo but not at four other zoos in the United States. The host range of WMHBV was examined by inoculation of one chimpanzee and two black-handed spider monkeys (Ateles geoffroyi), the closest nonendangered relative of the woolly monkey. The data suggest that spider monkeys are susceptible to infection with WMHBV and that minimal replication was observed in a chimpanzee. Thus, we have isolated a hepadnavirus with a host intermediate between humans and rodents and establishes a new animal model for evaluation of antiviral therapies for treating HBV chronic infections.

Antisense therapy of hepatitis B virus infection

Chronic infection with the hepatitis B virus (HBV) is a major health problem worldwide. The only established therapy is interferon-a with an efficacy of only 30-40% in highly selected patients. The discovery of animal viruses closely related to the HBV has contributed to active research on antiviral therapy of chronic hepatitis B. The animal model tested and described in this article are Peking ducks infected with the duck hepatitis B virus (DHBV). Molecular therapeutic strategies aimed at blocking gene expression include antisense DNA. An antisense oligodeoxynucleotide directed against the 5'-region of the preS gene of DHBV inhibited viral replication and gene expression in vitro in primary duck hepatocytes and in vivo in Peking ducks. These results demonstrate the potential clinical use of antisense DNA as antiviral therapeutics.

Hepatitis viruses: genetic variants and clinical significance

Variants of hepatitis B, C, and delta virus have been identified in patients both with acute and chronic infections. In the hepatitis B virus genome, naturally occurring mutations have been found in all viral genes, most notably in the genes coding for the structural envelope and nucleocapsid proteins. In the hepatitis C virus genome, the regions coding for the structural envelope proteins E1 and E2, as well as the 3'-contiguous non-structural region NS1, were found to be hypervariable. Viral variants may be associated with a specific clinical course of the infection, e.g., acute, fulminant or chronic hepatitis. Specific mutations may reduce viral clearance by immune mechanisms ('vaccine escape' and 'immune escape'), response to antiviral therapy ('therapy escape'), as well as detection ('diagnosis escape'). The exact contribution, however, of specific mutations to the pathogenesis and natural course of hepatitis B, C, or delta virus infection, including hepatocellular carcinoma development, and the response to antiviral treatment remains to be established.

Previously unsuspected cis-acting sequences for DNA replication revealed by characterization of a chimeric heron/duck hepatitis B virus

Heron hepatitis B virus (HHBV) is an avian hepadnavirus that is closely related to duck hepatitis B virus (DHBV). To learn more about the mechanism of hepadnavirus replication, we characterized a clone of HHBV that contains a substitution of DHBV sequence from nucleotide coordinates 403 to 1364. This clone, named HDE1, expresses a chimeric pregenomic RNA, a chimeric polymerase (P) protein, and a core (C) protein with a one-amino-acid substitution at its carboxy terminus. We have shown that HDE1 is defective for minus-strand DNA synthesis, resulting in an overall reduction of viral DNA. HDE1 was also defective for plus-strand DNA synthesis, resulting in aberrant ratios of replication intermediates. Genetic complementation assays indicated that HDE1 replication proteins, C and P, are functional for replication and wild-type HHBV proteins do not rescue either defect. These findings indicate that the HDE1 substitution mutation acts primarily in cis. By restoring nucleotides 403 to 902 to the HHBV sequence, we showed that cis-acting sequences for plus-strand DNA synthesis are located in the 5' half of the HDE1 chimeric region. These data indicate the presence of one or more formerly unrecognized cis-acting sequences for DNA synthesis within the chimeric region (nucleotides 403 to 1364). These cis-acting sequences in the middle of the genome might interact directly or indirectly with known cis elements that are located near the ends of the genome. Our findings suggest that a specific higher-order template structure is involved in the mechanism of hepadnavirus DNA replication.

Mutations within DR2 independently reduce the amount of both minus- and plus-strand DNA synthesized during duck hepatitis B virus replication

The initial aim of this study was to examine the role of complementarity between the plus-strand primer and the minus-strand DNA template for translocation of the plus-strand primer in hepadnaviral replication. We show that when a 5-nucleotide substitution was placed in either DR1 or DR2, translocation of the primer at a detectable level did not occur. Placing the mutation in both DR1 and DR2 did not restore primer translocation, which indicates that complementarity is not the sole determinant for primer translocation. These mutants, in which primer translocation has been inhibited, have been additionally informative. The mutation in DR1 led to efficient synthesis of plus-strand DNA, albeit primed in situ. In contrast, the mutation in DR2 resulted in a reduction in the amount of plus-strand DNA synthesized per unit of minus-strand DNA. These findings were interpreted as indicating that a mutation at DR2, the primer acceptor site, can inhibit both primer translocation and in situ priming. Lastly, we show that mutations within DR2 can result in a reduction in the synthesis of minus-strand DNA and that this reduction is occurring at an early phase of the process. We speculate that this reduction in the amount of minus-strand DNA synthesized could be due to an inhibition of the template switch during minus-strand DNA synthesis.

Duck hepatitis B virus polymerase acts as a suppressor of core protein translation

Nucleocapsid assembly in hepadnavirus replication requires selective encapsidation of the pregenomic RNA template and the viral polymerase by the core proteins. It has been shown that an encapsidation signal located at the 5' end of the pregenomic RNA is responsible for its interaction with the polymerase. In the present study, we have shown that a region located at the 3' periphery of the core open reading frame may interact with the viral polymerase in duck hepatitis B virus. By using an in vitro rabbit reticulocyte lysate translation system, we found that interaction of the polymerase with this region resulted in selective suppression of core mRNA translation. Insertion of this putative inhibitory sequence into the CD4 gene also led to a selective inhibition of CD4 mRNA translation in the presence of polymerase. Specific inhibition of core protein synthesis was observed in a chicken hepatoma cell line (LMH) cotransfected with core and polymerase plasmid DNA.

Recent advances in the molecular biology of hepatitis B virus

Hepatitis B virus (HBV) is an enveloped hepatotropic DNA virus. Acute and chronic HBV infection causes significant liver diseases such as acute hepatis, fulminant hepatitis and chronic active hepatitis that may lead to liver cirrhosis and the development of hepatocellular carcinoma. The use of molecular biological techniques has substantially improved our understanding of the HBV life cycle. In this review, we discuss recent advances that have contributed to a better understanding of HBV biology. Recent studies in the understanding of the life cycle of HBV such as viral entry, replication, transcriptional regulation, viral regulatory proteins, viral assembly and secretion, and nucleic acid based approaches to antiviral therapy will be emphasized. These advances in molecular biology and relationship to clinical disease will be instrumental in developing effective therapeutic approaches for the estimated 300 million individuals worldwide chronically infected with HBV.

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.

A splice hepadnavirus RNA that is essential for virus replication

According to the current model of hepadnavirus gene expression, the viral envelope proteins are produced from unspliced subgenomic RNAs, in contrast to the retroviral mechanism, where the subgenomic env RNA is generated by RNA splicing. We now describe and characterize a novel duck hepatitis B virus RNA species which is derived from the RNA pregenome by loss of a 1.15 kb intron. This RNA (termed spliced L RNA) codes for the large surface protein (L protein), as does the previously described unspliced mRNA (the preS RNA); however, it differs in 5' leader sequence and promoter control. Mutational analysis indicates that the spliced L RNA is functionally important for virus replication in infected hepatocytes and ducks, but not for virus formation from transfected DNA genomes. This suggests that the newly discovered second pathway for L protein synthesis plays a distinct role in an early step in the viral life cycle.

Interaction between duck hepatitis B virus and a 170-kilodalton cellular protein is mediated through a neutralizing epitope of the pre-S region and occurs during viral infection

Identification of cell surface viral binding proteins is important for understanding viral attachment and internalization. We have fused the pre-S domain of the duck hepatitis B virus (DHBV) large envelope protein to glutathione S-transferase and demonstrated a 170-kDa binding protein (p170) in [35S]methionine-labeled duck hepatocyte lysates. This glycoprotein was found abundantly in all extrahepatic tissues infectible with DHBV and in some noninfectible tissues, though it is not secreted into the blood. The interaction of pre-S fusion protein with p170 was competitively inhibited by wild-type DHBV in a dose-dependent manner. In addition, infection of hepatocytes with DHBV blocked the binding of pre-S fusion protein to p170, which suggests a biological role for p170 during natural infection. The p170 binding site was mapped to a conserved sequence of 16 amino acid residues (positions 87 to 102) by using 24 pre-S deletion mutants; this binding domain coincides with a major virus-neutralizing antibody epitope. Furthermore, site-directed mutagenesis revealed that an arginine residue at position 97 is critical for p170 binding. p170 was purified by a combination of ion-exchange and affinity chromatographies, and four peptide sequences were obtained. Two peptides showed significant similarities to human and animal carboxypeptides H, M, and N. Taken together, these results raise the possibility that the p170 binding protein is important during the replication cycle of DHBV.

The pre-S domain of the large viral envelope protein determines host range in avian hepatitis B viruses

In addition to their well-recognized hepatotropism, all hepatitis B viruses (HBVs) display marked species specificity, growing poorly or not at all in species other than those closely related to their natural hosts. We have examined the molecular basis for this narrow host range, using duck HBV (DHBV) and heron HBV (HHBV) as a model system. HHBV virions will not infect ducks in vivo and infect cultured duck hepatocytes extremely inefficiently in vitro. Mutant HHBV genomes lacking all viral envelope proteins (HHBV env-) can be complemented in trans with DHBV envelope proteins; the resulting pseudotyped virions can efficiently infect duck hepatocytes. Further complementation analysis reveals that of the two viral surface proteins (L and S), it is the L protein that determines host range. Pseudotyping of HHBV env- with DHBV/HHBV chimeric envelope proteins reveals that replacement of as few as 69 amino acids of the pre-S domain of the HHBV L protein by their DHBV counterparts is sufficient to permit infection of duck hepatocytes. These studies indicate that the species-specificity of hepadnaviral infection is determined at the level of virus entry and is governed by the pre-S domain of the viral L protein.

Role of RNA in enzymatic activity of the reverse transcriptase of hepatitis B viruses

The hepadnavirus reverse transcriptase is a multifunction enzyme. In addition to its role in DNA synthesis, the polymerase is required for RNA packaging and also functions as the primer for minus-strand DNA synthesis. Previously, we demonstrated that the protein-priming activity of the polymerase requires a viral RNA segment, termed epsilon, which serves as a template for the synthesis of a short DNA oligomer that is covalently attached to the reverse transcriptase (G.-H. Wang and C. Seeger, J. Virol. 67:6507-6512, 1993). We now report that epsilon is sufficient for activation of the reverse transcriptase to prime DNA synthesis through the formation of a stable RNA-protein (RNP) complex. We also demonstrate that the binding reaction depends on sequence-specific determinants on epsilon. Moreover, our results indicate that two genetically separated domains of the reverse transcriptase are required for formation of the RNP complex. Finally, we show that the polymerase has a DNA polymerase activity in the absence of epsilon which does not depend on the protein-priming mechanism.

Molecular therapeutic strategies in hepatitis B virus infection

Chronic infection with the hepatitis B virus is a major health problem worldwide. The only established therapy is interferon-alpha, with an efficacy of only 30-40% in highly selected patients. Nucleoside analogues do not show a significant clinical benefit. Molecular therapeutic strategies aimed at blocking gene expression include antisense DNA/RNA and ribozymes acting at the posttranscriptional level and triple helix formation blocking at the transcriptional level. In vitro, antisense oligodeoxynucleotides inhibit viral replication and gene expression in human hepatoma cell lines. In vivo, an antisense oligodeoxynucleotide directed against the 5'-region of the pre-S gene of the duck hepatitis B virus inhibited viral replication and gene expression in ducks. In vitro, ribozymes accurately cleave HBV substrate RNA. Triple helix formation is another very promising molecular approach. Results in hepadnaviral infection are not yet available, however.

pet, a small sequence distal to the pregenome cap site, is required for expression of the duck hepatitis B virus pregenome

We have found that transcription of the pregenome of an avian hepadnavirus, duck hepatitis B virus (DHBV), is dependent on the presence of a small element in the 5' transcribed region of the pregenome-encoding sequence. This element, which we have named pet (positive effector of transcription), exerts its effect in cis in a position and orientation-dependent manner, suggesting that it may function as part of the nascent pregenome transcript. The requirement for pet depends on the presence in the transcription unit of a region of the DHBV genome located upstream of the envelope promoters, which specifically suppresses transcription of templates lacking pet. In the presence of this region, deletion of pet activates transcription from downstream promoters, suggesting that pregenome transcription complexes fail to reach the downstream promoters. In vitro transcription experiments support the model that pet is required for transcription elongation on the DHBV template. We speculate that pet is required to suppress transcription termination during the first passage of pregenome transcription complexes through a viral termination region on the circular viral DNA.

Molecular events in the pathogenesis of hepadnavirus-associated hepatocellular carcinoma

Chronic hepadnavirus infection is associated with hepatocellular carcinoma (HCC) in natural hosts such as humans, woodchucks, and Beechey ground squirrels. Several possible oncogenic mechanisms have been identified, including a potential role of the hepadnavirus x (hbx) gene, which transactivates transcription regulated by certain cis-acting sequences, e.g. regulatory sequences of the hepatitis B virus (HBV) and heterologous regulatory sequences of other viruses and cellular genes. The oncogenic potential of hbx is suggested by the observation of HCCs in hbx transgenic mice, the oncogenic transformation of cells expressing hbx in culture, and the transactivation of oncogenes c-myc and c-jun by hbx. Cis-activation of cellular oncogenes N-myc and c-myc by viral promoter insertion has been a common finding in woodchuck hepatitis virus (WHV)-associated HCCs of woodchucks. No such cis-activation of any cellular gene has been shown in virus-associated HCCs of ground squirrels or humans. Amplification and overexpression of the c-myc gene has been a common finding in HCCs of ground squirrels, and is rare in woodchuck or human HCCs. Point mutations in the p53 gene and allelic deletion of p53 have been common findings in human HCCs, but have not been found in HCCs in woodchucks and have been found rarely in ground squirrels. How each of these genetic changes in the different hosts contributes to HCC remains to be determined, but apparently different changes in different HCCs of hepadnavirus-infected hosts suggest that several separate genetic events may contribute to the development of HCC. These events may differ in each host, and some may not result from a direct virus-specific mechanism. Chronic hepadnavirus infection is often associated with chronic necroinflammatory liver disease and cirrhosis, a pathologic process common to several other risk factors for HCC. This suggests that this pathologic process (necroinflammatory disease) may be hepatocarcinogenic regardless of the inciting agent. Thus hepadnavirus infection may play an important role in the development of HCC by causing chronic hepatitis and HCC with the same mechanisms by which other risk factors for HCC cause chronic necroinflammatory liver disease and HCC.

The carbocyclic analog of 2'-deoxyguanosine induces a prolonged inhibition of duck hepatitis B virus DNA synthesis in primary hepatocyte cultures and in the liver

The carbocyclic analog of 2'-deoxyguanosine (2'-CDG) is a strong inhibitor of hepatitis B virus (HBV) DNA synthesis in HepG2 cells (P.M. Price, R. Banerjee, and G. Acs, Proc. Natl. Acad. USA 86:8543-8544, 1989). We now report that 2'-CDG inhibited duck hepatitis B virus (DHBV) DNA synthesis in primary cultures of duck hepatocytes and in experimentally infected ducks. Like foscarnet (phosphonoformic acid [PFA]) and 2'-,3'-dideoxycytidine (ddC), 2'-CDG blocked viral DNA replication in primary hepatocyte cultures when present during an infection but failed to inhibit the DNA repair reaction that occurs during the initiation of infection to convert virion relaxed circular DNA to covalently closed circular DNA, the template for viral mRNA transcription. Moreover, as for PFA and ddC, viral RNA synthesis was detected when infection was initiated in the presence 2'-CDG. In another respect, however, 2'-CDG exhibited antiviral activity unlike that of ddC or PFA: a single 1-day treatment of hepatocytes with 2'-CDG blocked initiation of viral DNA synthesis for at least 8 days, irrespective of whether DHBV infection was carried out at the time of drug treatment or several days later. Furthermore, orally administered 2'-CDG was long-acting against DHBV in experimentally infected ducklings. Virus replication was delayed by up to 4 days in ducklings infected after administration of 2'-CDG. These observations of long-lasting efficacy in vitro and in vivo even after oral administration suggest that this inhibitor or a nucleoside with similar pharmacological properties may be ideal for reducing virus replication in patients with chronic HBV infection.

Current pathogenetic and molecular concepts in viral liver carcinogenesis

Hepatocellular carcinoma (HCC) is one of the most frequent malignancies in humans and in most cases a consequence of chronic infection of the liver by hepatotropic viruses (Hepatitis B Virus (HBV) and possibly Hepatitis C Virus (HCV)). Formation of HCC results from a stepwise process involving different preneoplastic lesions that reflect multiple genetic events, like protooncogene activation, tumor suppressor gene inactivation, and growth factor over- or reexpression. Recent investigations have gained new insights into how these factors are activated and may interact. In addition, improved knowledge of the molecular biology of HBV has led to better understanding of its pleiotropic effects on induction and progression in hepatocarcinogenesis.

Hepadnavirus infection requires interaction between the viral pre-S domain and a specific hepatocellular receptor

To better define the molecules involved in the initial interaction between hepadnaviruses and hepatocytes, we performed binding and infectivity studies with the duck hepatitis B virus (DHBV) and cultured primary duck hepatocytes. In competition experiments with naturally occurring subviral particles containing DHBV surface proteins, these DNA-free particles were found to interfere with viral infectivity if used at sufficiently high concentrations. In direct binding saturation experiments with radiolabelled subviral particles, a biphasic titration curve containing a saturable component was obtained. Quantitative evaluation of both the binding and the infectivity data indicates that the duck hepatocyte presents about 10(4) high-affinity binding sites for viral and subviral particles. Binding to these productive sites may be preceded by reversible virus attachment to a large number of less specific, nonsaturable primary binding sites. To identify which of the viral envelope proteins is responsible for hepatocyte-specific attachment, subviral particles containing only one of the two DHBV surface proteins were produced in Saccharomyces cerevisiae. In infectivity competition experiments, only particles containing the large pre-S/S protein were found to markedly reduce the efficiency of DHBV infection, while particles containing the small S protein had only a minor effect. Similarly, physical binding of radiolabelled serum-derived subviral particles to primary duck hepatocytes was inhibited well only by the yeast-derived pre-S/S particles. Together, these results strongly support the notion that hepadnaviral infection is initiated by specific attachment of the pre-S domain of the large DHBV envelope protein to a limited number of hepatocellular binding sites.

Mutational analysis of the cysteine residues in the hepatitis B virus small envelope protein

The small envelope protein of hepatitis B virus is the major component of the viral coat and is also secreted from cells as a 20-nm subviral particle, even in the absence of other viral proteins. Such empty envelope particles are composed of approximately 100 copies of this polypeptide and host-derived lipids and are stabilized by extensive intermolecular disulfide cross-linking. To study the contribution of disulfide bonds to assembly and secretion of the viral envelope, single and multiple mutants involving all 14 cysteines in HepG2 and COS-7 cells were analyzed. Of the six cysteines located outside the region carrying the surface antigen, Cys-48, Cys-65, and Cys-69 were each found to be essential for secretion of 20-nm particles, whereas Cys-76, Cys-90, and Cys-221 were dispensable. By introduction of an additional cysteine substituting serine 58, the yield of secreted particles was increased. Of four mutants involving the eight cysteines located in the antigenic region, only the double mutant lacking Cys-121 and Cys-124 was secreted with wild-type efficiency. Secretion-competent envelope proteins were intracellularly retained by secretion-deficient cysteine mutants. According to alkylation studies, both intracellular and secreted envelope proteins contained free sulfhydryl groups. Disulfide-linked oligomers were studied by gel electrophoresis under nonreducing conditions.

Evaluation of Phyllanthus amarus and Phyllanthus maderaspatensis as agents for postexposure prophylaxis in neonatal duck hepatitis B virus infection

The therapeutic potential of plant extracts of Phyllanthus amarus and Phyllanthus maderas patensis for postexposure prophylaxis against infection by Hepadnaviruses was studied in ducklings infected by the duck hepatitis B virus (DHBV). Forty-four Pekin ducklings were inoculated intraperitoneally with DHBV at 24 hr post-hatch. They were treated by intraperitoneal injection of Phyllanthus amarus (aqueous extract) (100 mg/kg body weight) or Phyllanthus mad eraspatensis (alcoholic extract) (100 mg/kg body weight) for a period of 4 weeks. Infected ducklings treated with saline served as controls. Weekly serum samples obtained before, during, and after treatment were analysed for the presence of DHBV DNA in serum by dot blot hybridisation using alpha 32P-labelled probes. Liver tissue was collected after killing the ducks at various time intervals and was studied for replicative status of the viral DNA and liver histopathology; 17 of 21 ducks were viraemic on completion of treatment with Phyllanthus amarus. At 16 week posttreatment follow-up four of seven animals remained viraemic. Similar results were obtained with Phyllanthus maderaspatensis. There was no alteration in DHBV replication in the liver. No toxicity was observed with this treatment. These observations suggest that Phyllanthus amarus and Phyllanthus maderaspatensis are not useful as therapeutic agents for postexposure prophylaxis against DHBV infection.

The woodchuck hepatitis virus X gene is important for establishment of virus infection in woodchucks

All mammalian hepadnaviruses possess a gene, termed X, that encodes a protein capable of transactivating virus gene expression. The X gene overlaps the polymerase and precore genes as well as two newly identified open reading frames (ORFs) termed ORF5 and ORF6. In this investigation, we examined whether ORF5, ORF6, and the X gene were important for the replication of woodchuck hepatitis virus (WHV) in susceptible woodchucks. First, we investigated whether proteins were produced from ORF5 and ORF6 by in vitro translation of appropriate viral transcripts, searched for antibodies against the putative proteins in the sera of animals infected with wild-type virus, and looked for an antisense WHV transcript, necessary for expression of a protein from ORF6, in the livers of acutely or chronically infected woodchucks. All such experiments yielded negative results. Next, we used oligonucleotide-directed mutagenesis to introduce termination codons into ORF5 and ORF6 at two locations within each ORF. Adult woodchucks in groups of three were transfected with one of the four mutant genomes. All of these woodchucks developed WHV infections that were indistinguishable from those of animals transfected with the wild-type WHV recombinant. Polymerase chain reaction amplification and direct DNA sequencing confirmed that reversion of the mutants to a wild-type genotype did not occur. Taken together, these data indicate that ORF5 and ORF6 are not essential for virus replication and are unlikely to represent authentic genes. Finally, we generated five WHV X-gene mutants that either removed the initiation codon for protein synthesis or truncated the carboxyl terminus of the protein by 3, 16, 31, or 52 amino acids. Groups of three adult woodchucks were transfected with one of the five X-gene mutants. Only the mutant that possessed an X gene lacking 3 amino acids from the carboxyl terminus was capable of replication within the 6-month time frame of the experiment. In contrast, all seven woodchucks transfected with wild-type WHV DNA developed markers consistent with viral infection. Thus, it is likely (P < 0.01) that the WHV X gene is important for virus replication in the natural host.

Complete nucleotide sequences of six hepatitis B viral genomes encoding the surface antigen subtypes ayw4, adw4q-, and adrq- and their phylogenetic classification

The complete nucleotide sequences of six hepatitis B viral (HBV) genomes were determined by dideoxy chain termination sequencing of ten overlapping nucleotide fragments obtained by the polymerase chain reaction. Four of the genomes belonged to the two genomic groups E and F of HBV which have been previously identified by us on the basis of sequence divergences within the S gene. Genomic group E encodes the HBsAg subtype ayw4, group F adw4q-. The other two genomes were of Pacific origin within group C and encoded adrq-. The relationship of these complete human HBV genomes to 21 that have been previously published, together with one chimpanzee virus and four rodent hepadnaviral genomes, was investigated by constructing a phylogenetic tree utilizing a combination of distance matrix and approximate parsimonious methods. Thereby, the previously demonstrated segregation of human HBV strains into six genomic groups was confirmed. Both of the representatives of the groups E and F were found to differ by 8.1-13.6% and by 12.8-15.5% from the genomes of the other genomic groups and by 1.5 and 3.7% from each other. Since they differed by more than 8% from the genomes in the other groups, the limit originally used to define HBV, genomic groups their status as new genomic groups was confirmed. The two Pacific group C strains were found to differ by 2.7% from each other and by 4.1 to 5.4% from other group C genomes, suggesting that they diverged early from the other group C genomes. According to both the overall similarity and the phylogenetic dendrogram the F strains formed the most divergent cluster of HBV genomes favoring the concept that they represented the original HBV strains of the New World. The next split in the dendrogram segregated the A, D, E and the chimpanzee strains from the Asian B and C strains. Information on the nucleotide sequences and their encoded products of HBV strains of different genomic groups will provide a basis to understand biological variations of the HBV infection in different parts of the world.

The role of hepatitis B virus in the development of primary hepatocellular carcinoma: Part I

Chronic infections with hepatitis B virus (HBV) of humans and animal hepadnavirus infections in their natural hosts are strongly associated with primary hepatocellular carcinoma (HCC). Although viral integrations are found in cells of many HCC, no general viral-specific hepatocarcinogenic mechanism for hepadnaviruses has been identified. In approximately one half of HCC in woodchuck hepatitis virus (WHV) infected woodchucks, viral integrations near the c-myc or N-myc genes have been reported which result in enhanced expression of the respective gene. Such host gene-specific insertional mutagenesis has not been found in HCC of other hepadnavirus infected hosts. Thus in humans, ground squirrels and ducks hepadnaviral integrations appear to be at different host chromosomal DNA sites in each HCC and few integrations have been found within or near any cellular gene. Other possible hepadnavirus-specific carcinogenic mechanisms that are being investigated include transactivation of cellular gene expression by an hepadnavirus gene product (e.g. the X-gene), and mutation of host genes by unknown hepadnavirus-specific mechanisms. It should be noted, however, that chronic hepadnavirus infection is associated with chronic necroinflammatory liver disease with hepatocellular necrosis and regeneration (sometimes leading to cirrhosis in humans), a pathological process that is common to numerous other risk factors for HCC. This suggests the possibility that this pathological process is hepatocarcinogenic irrespective of the inciting agent and the role of hepadnavirus infection is no different from that of other risk factors in causing chronic necroinflammatory liver disease.

Evidence for involvement of a ribosomal leaky scanning mechanism in the translation of the hepatitis B virus pol gene from the viral pregenome RNA

In retroviruses, the pol gene is expressed in the form of a gag-pol fusion protein by the mechanism of ribosomal frameshifting. In studies of the possible mechanism of hepadnaviral pol protein synthesis, recent results have ruled out core-pol fusion protein synthesis by ribosomal frameshifting. In this study, an in vitro transcription and translation coupling system was used to demonstrate that the HBV core and pol proteins could be synthesized independently using the pregenome RNA template. The result has led us to design experiments to distinguish between the involvement of a termination-reinitiation, internal initiation, or leaky scanning mechanism in the pol protein synthesis. In vitro experiments were then carried out to measure the amount of pol proteins being synthesized from (i) the preC mRNA, which contained an extra AUG and seven more nucleotides at the 5'-end in comparison with the pregenome RNA; (ii) the pregenome RNA in the presence of various amounts of antisense RNA annealing to the 5'-end of the pregenome RNA; and (iii) the pregenome RNA with an additional hairpin structure located upstream of the C gene. Results indicated that the synthesis of both core and pol proteins was concomitantly reduced in these three conditions, which suggested that leaky scanning is the most probable mechanism for pol protein synthesis in vitro. To further verify the mechanism in vivo, experiments were performed to assay the activity of DNA polymerase in virions, which were obtained from hepatoma cells transfected by plasmids containing either a wild-type sequence (5'-GGCATGG-3') or an optimal initiation context (5'-ACCATGG-3') of the C gene. Transfection results showed that the plasmid-containing mutations of the C gene significantly decreased the DNA polymerase activity in virions. This observation supports our hypothesis that the leaky scanning model is involved in the synthesis of pol protein.

Inhibition of hepatitis B virus production by modified 2',3'-dideoxy-thymidine and 2',3'-dideoxy-5-methylcytidine derivatives. In vitro and in vivo studies

The effect of analogues of both 2',3'-dideoxy-3'-fluorothymidine (FddThd) [2',3'-dideoxy-3'-fluorouridine (FddUrd), 2',3'-dideoxy-3'-fluoro-5-chlorouridine (FddClUrd), 2',3'-dideoxy-3'- fluoro-5-bromouridine (FddBrUrd) and 2',3'-dideoxy-3'-fluoro-5-bromovinyluridine (FddBVUrd)] and 2',3'-dideoxy-3'-fluorocytidine (FddCyt) [2',3'-dideoxy-3'-fluoro-5-fluorocytidine (FddFCyt), 2',3'-dideoxy-3'-fluoro-5-chlorocytidine (FddClCyt), 2',3'-dideoxy-3'-fluoro-5-methylcytidine (FddMeCyt), 2',3'-dideoxy-3'-fluoro-5-ethylcytidine (FddEtCyt), 2',3'-dideoxy-3'-chloro-5-methylcytidine (ClddMeCyt), 2',3'-dideoxy-3'-amino-5-methylcytidine (AmddMeCyt), 2',3'-dideoxy-3'-azido-5- methylcytidine (AzddMeCyt) and arabinosyl-5-methylcytosine (AraMeCyt)] were tested for their potential antiviral activity in vitro using the human hepatoblastoma cell line, Hep G2 2.2.15, which was transfected with a vector containing hepatitis B virus (HBV). It was found that FddThd, FddMeCyt, FddEtCyt, ClddMeCyt, AmddMeCyt and AraMeCyt display cytostatic activity at concentrations (CD50 values) between 0.54 (FddMeCyt) and 3.93 microM (FddEtCyt), while FddUrd, FddClUrd, FddBrUrd, FddBVUrd, FddCyt, FddFCyt, FddClCyt and AzddMeCyt do not affect cell growth at concentrations of up to 25 microM. Among the thymidine analogues tested, FddThd is the most effective antiviral agent: at a concentration of 0.03 microM a more than 90% reduction of HBV DNA synthesis was measured. On the other hand, the antiviral indexes displayed by FddClUrd, FddBrUrd and FddBVUrd are higher than tht of FddThd; FddUrd was completely inactive. The most powerful antiviral agents in the group of cytidine analogues tested in vitro were FddMeCyt (more than 90% reduction of HBVDNA synthesis at 0.10 microM) and ClddMeCyt (0.10 microM); FddClCyt, FddEtCyt, AmddMeCyt and AraMeCyt were of intermediate activity. None of the negligible antiviral activity was determined for FddUrd, FddCyt, FddFCyt and AzddMeCyt. FddThd and FddMeCyt displayed in vivo an antiviral effect in the duck/duck HBV (DHBV) animal system. Administration of 10 or 20 mg/kg (total daily dose) of FddThd and 5 or 10 mg/kg of FddMeCyt (i.m. daily) to ducks infected with DHBV for 12 days blocked virus production. Termination of treatment with FddThd of infected animals led to reappearance of the virus in the serum though at lower levels. The in vitro and the in vivo data suggest that FddThd and FddMeCyt might be promising antiviral agents for the treatment of infection caused by HBV in humans.