Expression of hepatitis B viral core region in mammalian cells

Specific determination of hepatitis B e antigen by antibodies targeting precore unique epitope facilitates clinical diagnosis and drug evaluation against hepatitis B virus infection

Hepatitis B e antigen (HBeAg) is a widely used marker both for chronic hepatitis B (CHB) clinical management and HBV-related basic research. However, due to its high amino acid sequence homology to hepatitis B core antigen (HBcAg), most of available anti-HBe antibodies are cross-reactive with HBcAg resulting in high interference against accurate measurement of the status and level of HBeAg. In the study, we generated several monoclonal antibodies (mAbs) targeting various epitopes on HBeAg and HBcAg. Among these mAbs, a novel mAb 16D9, which recognizes the SKLCLG (aa -10 to -5) motif on the N-terminal residues of HBeAg that is absent on HBcAg, exhibited excellent detection sensitivity and specificity in pairing with another 14A7 mAb targeting the HBeAg C-terminus (STLPETTVVRRRGR, aa141 to 154). Based on these two mAbs, we developed a novel chemiluminescent HBeAg immunoassay (NTR-HBeAg) which could detect HBeAg derived from various HBV genotypes. In contrast to widely used commercial assays, the NTR-HBeAg completely eliminated the cross-reactivity with secreted HBcAg from precore mutant (G1896A) virus in either cell culture or patient sera. The improved specificity of the NTR-HBeAg assay enables its applicability in cccDNA-targeting drug screening in cell culture systems and also provides an accurate tool for clinical HBeAg detection.

The Regulation of HBV Transcription and Replication

Hepatitis B virus (HBV) is a major human pathogen lacking a reliable curative therapy. Current therapeutics target the viral reverse transcriptase/DNA polymerase to inhibit viral replication but generally fail to resolve chronic HBV infections. Due to the limited coding potential of the HBV genome, alternative approaches for the treatment of chronic infections are desperately needed. An alternative approach to the development of antiviral therapeutics is to target cellular gene products that are critical to the viral life cycle. As transcription of the viral genome is an essential step in the viral life cycle, the selective inhibition of viral RNA synthesis is a possible approach for the development of additional therapeutic modalities that might be used in combination with currently available therapies. To address this possibility, a molecular understanding of the relationship between viral transcription and replication is required. The first step is to identify the transcription factors that are the most critical in controlling the levels of HBV RNA synthesis and to determine their in vivo role in viral biosynthesis. Mapping studies in cell culture utilizing reporter gene constructs permitted the identification of both ubiquitous and liver-enriched transcription factors capable of modulating transcription from the four HBV promoters. However, it was challenging to determine their relative importance for viral biosynthesis in the available human hepatoma replication systems. This technical limitation was addressed, in part, by the development of non-hepatoma HBV replication systems where viral biosynthesis was dependent on complementation with exogenously expressed transcription factors. These systems revealed the importance of specific nuclear receptors and hepatocyte nuclear factor 3 (HNF3)/forkhead box A (FoxA) transcription factors for HBV biosynthesis. Furthermore, using the HBV transgenic mouse model of chronic viral infection, the importance of various nuclear receptors and FoxA isoforms could be established in vivo. The availability of this combination of systems now permits a rational approach toward the development of selective host transcription factor inhibitors. This might permit the development of a new class of therapeutics to aid in the treatment and resolution of chronic HBV infections, which currently affects approximately 1 in 30 individuals worldwide and kills up to a million people annually.

Viral deletions among healthy young Chinese adults with occult hepatitis B virus infection

To investigate the mechanism and prognosis of occult hepatitis B virus (HBV) infection (OBI) at a molecular level among healthy young adults, the presence of HBV DNA in 1176 sera samples collected from healthy young people after neonatal vaccination was assessed by nested polymerase chain reaction (PCR) using specific primers designed for the X and S regions of the HBV genome. Full-length HBV DNA from 9 patients with OBI (OB1-OB9) was cloned and sequenced. Deletions in the pre-S, basal core promoter (BCP), core (C) and polymerase (P) regions were observed. The data indicate that there is still a substantial risk of OBI in China despite neonatal vaccination. All deletions that were observed in the pre-S, BCP, C and P regions play a direct or indirect role in OBI. The presence of a deletion mutation in the pre-S1 region was considered to play a pivotal role in hepatocarcinogenesis and was found to increase the risk of hepatocellular carcinoma in the cohorts studied.

Enhanced intracellular retention of a hepatitis B virus strain associated with fulminant hepatitis

A plasmid carrying 1.3-fold HBV genome was constructed from a HBV strain that caused five consecutive cases of fulminant hepatitis (pBFH2), and HepG2 cells were transfected with pBFH2 or its variants. The pBFH2 construct with A1762T/G1764A, G1862T, and G1896A showed the largest amount of core particle-associated intracellular HBV DNA, but no significant increase of extracellular HBV DNA in comparison with the wild construct, suggesting that these mutations might work together for retention of the replicative intermediates in the cells. The retention might relate to the localization of hepatitis B core antigen (HBcAg) in the nucleus of HepG2, which was observed by confocal fluorescence microscopy. HBcAg immunohistochemical examination of liver tissue samples obtained from the consecutive fulminant hepatitis patients showed stronger staining in the nucleus than acute hepatitis patients. In conclusion, the fulminant HBV strain caused retention of the core particles and the core particle-associated HBV DNA in the cells.

Hepatitis B virus variants

HBV replicates through reverse transcription of an RNA intermediate; the inherent lack of proofreading causes a high mutation frequency. Mutations in the precore and core promoter regions that abolish or reduce the production of hepatitis B e antigen occur most commonly. Patients with these HBV variants remain viremic and can develop progressive liver disease. Mutations in the core promoter region are associated with an increased risk of hepatocellular carcinoma. Exogenous selection pressure might favor certain mutations. Mutations in the HBV polymerase that confer resistance to nucleoside and nucleotide analog treatments are a major barrier to the success of therapy for hepatitis B. The development of antiviral drug resistance negates the initial treatment response and can lead to hepatitis flares and hepatic decompensation. Prompt addition of another drug to which the virus is not cross-resistant is required. Mutations in the HBV surface protein that facilitate escape from host immunity are responsible for the failure of immune prophylaxis in infants who received HBV vaccine and in liver transplant recipients who received hepatitis B immune globulin.

Core particles of hepatitis B virus as carrier for foreign epitopes

To be effective as vaccines, most monomeric proteins and peptides either require chemical coupling to high molecular weight carriers or application together with adjuvants. More recently, recombinant DNA techniques have been used to insert foreign epitopes into proteins with inherent multimerization capacity, such as particle-forming viral capsid or envelope proteins. The core protein of hepatitis B virus (HBcAg), because of its unique structural and immunological properties, has gained widespread interest as a potential antigen carrier. Foreign sequences of up to approximately 40 amino acid residues at the N terminus, 50 or 100 amino acids in the central immunodominant c/e 1 epitope region of HBcAg, and up to 100 or even more residues at the C terminus, did not interfere with particle formation. The humoral immunogenicity of inserted epitopes is determined by the immunogenicity of the peptide itself and its surface exposure, and is influenced by the route of application. The probably flexible and surface-exposed c/e1 region emerged as the most promising insertion site. When applied together with adjuvants approved for human and veterinary use, or even without adjuvants, such chimeric particles induced B and T cell immune responses against the inserted epitopes. In some cases neutralizing antibodies, cytotoxic T cells and protection against challenge with the intact pathogen were demonstrated. Major factors for the potentiated immune response against the foreign epitopes are the multimeric structure of chimeric HBcAg that results in a high epitope density per particle, and the provision of T cell help by the carrier moiety. Beyond its use as subunit vaccine, chimeric HBcAg produced in attenuated Salmonella strains may be applicable as live vaccine.

A monoclonal antibody against a hepatitis B e antigen epitope borne by six amino acids encoded by the precore region

Hepatitis B e antigen (HBeAg) polypeptide in the circulation (p17e) is composed of ten amino acids (aa) coded for by the precore region and 149 aa by the core gene of hepatitis B virus. A monoclonal antibody (Y0583A) was raised against the N-terminal ten amino acids (SKLCLGWLWG) encoded by the precore region. The binding of Y0583A with a panel of 203 decapeptides on multipins, which covered the precursor of HBeAg polypeptide made of 212 aa shifting by one aa, recognized an epitope sequenced LGWLWG representing the C-terminal six aa coded for by the precore region. This HBeAg epitope was not readily accessible on HBeAg in serum, but it became exposed and bound with Y0583A by treatment with 0.2 N NaOH. Using Y0583A, an enzyme-linked immunosorbent assay was developed for specific determination of HBeAg. The test sample was incubated with the monoclonal antibody to an HBeAg determinant encoded by the core gene (904) that had been immobilized on a solid support. Captured HBeAg was treated with 0.2 N NaOH, neutralized and released into the fluid phase. The reactant was then tested for a sandwich between monoclonal antibody (C33) to the C-terminus of the HBeAg polypeptide immobilized on a solid support and Y0583A labeled with horseradish peroxidase.

Characterization of a specific region in the hepatitis B virus enhancer I for the efficient expression of X gene in the hepatic cell

Hepatitis B virus (HBV) enhancer I has been shown to consist of several cis-acting sequences for the HBV gene expression efficiently in certain types of cells. Transcriptional regulation of HBV X gene mediated by enhancer I might be one of the mechanisms by which HBV obtains hepatotropism. By mutagenesis analysis of enhancer I function in the enhancer I/X gene promoter complex, we characterized a specific transcriptional regulatory region (designated as a LSR element, nt 989-1030) of enhancer I for the X gene promoter by means of the transient transfection technique using hepatic and nonhepatic cells. Based on the analysis of protein factors interacting with the LSR element, liver-enriched transcriptional factors, HNF3 and HNF4 or retinoid X receptor alpha (RXR alpha), are probably implicated in the activity of enhancer I for the efficient expression of X gene through their interaction with the LSR element in the hepatic cell. Furthermore, the isolated LSR element was demonstrated to function alone as a specific cis-acting element and to be able to activate transcription from the X gene promoter efficiently in the hepatic cell in an orientation-independent manner.

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.

Expression of a human cytomegalovirus gp58 antigenic domain fused to the hepatitis B virus nucleocapsid protein

Hepatitis B virus core antigen (HBcAg) has been used as a carrier for expression and presentation of a variety of heterologous viral epitopes in particulate form. The aim of this study was to produce hybrid antigens comprising HBcAg and an immunogenic epitope of human cytomegalovirus (HCMV). A direct comparison was made of amino and carboxyl terminal fusions in order to investigate the influence of position of the foreign epitope on hybrid core particle formation, antigenicity and immunogenicity. HCMV DNA encoding a neutralising epitope of the surface glycoprotein gp58 was either inserted at the amino terminus or fused to the truncated carboxyl terminus of HBcAg and expressed in Escherichia coli. The carboxyl terminal fusion (HBc3-144-HCMV) was expressed at high levels and assembled into core like particles resembling native HBcAg. Protein with a similar fusion at the amino terminus (HCMV-HBc1-183) could not be purified or characterised immunologically, although it formed core like particles. HBc3-144-HCMV displayed HBc antigenicity but HCMV antigenicity could not be detected by radioimmunoassay or western blotting using anti-HCMV monoclonal antibody 7-17 or an anti-HCMV human polyclonal antiserum. Following immunisation of rabbits with HBc3-144-HCMV, a high titre of anti-HBc specific antibody was produced along with lower titres of HCMV/gp58 specific antibody.

Long-term productive episomal hepatitis B virus replication in primary cultures of adult human hepatocytes infected in vitro

We have previously increased the efficiency of hepatitis B virus (HBV) infection of human hepatocytes in vitro by using polyethylene glycol. After further documenting by neutralization experiments, this in vitro infection, we used this model to define new culture conditions that would maintain stable episomal replication for several weeks. We found that in the presence of 10% porcine serum and 2% dimethyl sulphoxide (DMSO), high-density cultures survived more than 3 months, while addition of hydrocortisone instead of DMSO resulted in survival of less than 1 month. HBV episomal replication was maintained without any evidence of viral integration into the host genome. The maintenance of HBV replication was demonstrated by: first, stability of the covalently-closed-circular DNA in the nucleus and relaxed circular and single-stranded replicative intermediates in the cytoplasm; second, detection of two major transcripts of 3.5 and 2.1-2.4 kb corresponding to the pregenomic and surface genes respectively; and third, continuous secretion of mature viral particles in the supernatant of infected cells. We showed that under these culture conditions, hepatocytes were blocked in the G1 phase of the cell cycle and did not spontaneously proliferate. Upon hepatocyte growth factor (HGF) stimulation, however, the ability of hepatocytes to divide was demonstrated and was compared in infected and non-infected cells. No change in proliferative capacity and no variation in c-myc and c-jun levels could be found. Hepatocyte survival was not modified in infected cells, confirming that HBV is not cytopathic for normal human hepatocytes. These new culture conditions represent substantial progress in the study of HBV-host cell interactions.

Point mutation in precore region of hepatitis B virus: sequential changes from 'wild' to 'mutant'

One point mutation to make a stop codon in the precore (pre-C) region of the hepatitis B virus DNA in anti-HBe-positive patients has been reported recently. This mutation disturbs the formation of the pre-C protein that is processed to make HBeAg. The relationship between the point mutation and HBe antigen antibody status was investigated in B-viral liver diseases. The pre-C region was amplified by a polymerase chain reaction (PCR) method and the nucleotide sequences were determined by a direct sequencing method. In seven cases who were persistently HBeAg-positive, the wild type (no mutation in pre-C region) was detected in all. In 20 cases who were anti-HBeAg-positive at diagnosis, the mutant type (point mutation at nucleotide 1896 in pre-C region, which makes a stop codon) was detected in 16 cases and the wild type in two cases. In HBe seroconversion (SC) cases, the types of virus were investigated in serial blood samples. No mutant type was detected in initial sera during the HBeAg-positive period. In two 'natural' SC cases, the mutant type appeared before anti-HBe formation. However, in three anti-viral 'drug-induced' SC cases, the mutant type appeared after the formation of anti-HBe. In two 'reversed' seroconversion cases only the wild type was detected throughout the follow-up period. These data suggest that the appearance of a pre-C mutant may help to predict seroconversion from HBeAg to anti-HBe and may help distinguish 'natural' and 'drug-induced' seroconversion of HBeAg.

X region deletion mutants associated with surface antigen-positive hepatitis B virus infections

BACKGROUND/AIMS

The finding of antibodies against the polymerase of hepatitis B virus in renal dialysis patients before the incubation phase of infection implies underlying virus replication. Hence, the aim of the study was to test for virus during infection.

METHODS

Viremia was assayed in virus-infected and control patients using the polymerase chain reaction and Southern blotting.

RESULTS

Six months before the appearance of surface antigen, most patients had detectable core region, but few patients were X region positive. Three months after surface antigen appeared, most carriers had detectable core and X products. Three years after surface antigen appeared, 5 of 8 carriers with persistent hepatitis B e antigen and 1 of 8 carriers with corresponding antibody had these products. Cloning and sequencing showed deletions within the X/precore region of viral DNA.

CONCLUSIONS

Infection with X region mutants precedes that of wild-type virus, and they reappear after wild-type virus is eliminated in carriers.

Precore and core mutations in HBV from individuals in India with chronic infection

Ten hepatitis B surface antigen seropositive carriers (5 asymptomatic and 5 with chronic liver disease) were tested for HBeAg/anti-HBe and for HBV-DNA using the polymerase chain reaction. Five were DNA-positive, 2 with HBeAg and 3 with anti-HBe. Nucleotide sequences were determined for these 5 cases. Hepatitis B virus DNA from one cirrhotic carrier with anti-HBe had a mutation in the precore region (nucleotide position 1862) which may affect signal peptide cleavage and HBeAg synthesis. In the other 2 anti-HBe- and DNA-positive cases, a cirrhotic carrier and an asymptomatic case, there was a mutation at nucleotide position 1896 leading to a termination codon in the precore region. In all 5 patients, except for one or two missense mutations, there was no significant variation in the core region.

Hepatitis B virus with mutations in the core promoter for an e antigen-negative phenotype in carriers with antibody to e antigen

Hepatitis B virus (HBV) DNA clones were propagated from 57 carriers with antibody to hepatitis B e antigen (HBeAg) and sequenced within nucleotides (nt) 1685 to 1926 including the core promoter (nt 1742 to 1849) and the pre-C region (nt 1814 to 1900). Mutations in the core promoter or those in the pre-C region, or both, were detected in 328 (97.9%) of 335 clones from them. Five carriers were infected with HBV mutants with mutations in the core promoter alone, while 20 carriers were infected only with those in the pre-C region to abort the translation of HBeAg precursor; the remaining 32 carriers were infected with HBV mutants with mutations in both the core promoter and pre-C region. Some carriers infected with HBV with mutations in the core promoter exclusively had high HBV DNA titers, comparable with those in carriers infected with wild-type HBV, thereby indicating that such mutations would not affect the transcription of the HBV pregenome extensively. Two point mutations in the core promoter, from A to T at nt 1762 and from G to A at nt 1764, were most prevalent. The other mutations included a point mutation at either of the two nucleotides and their deletion. All of these mutations involved the TTAAA sequence (nt 1758 to 1762) at 28 bp upstream of the initiation site for shorter pre-C mRNAs (nt 1790 +/- 1). The ATAAATT sequence (nt 1789 to 1795) at 23 bp upstream of the initiation site for the pregenome RNA (nt 1818), however, remained intact in all 335 HBV DNA clones. HBV mutants with mutations in the core promoter, unaccompanied by pre-C mutations, prevailed and replaced wild-type HBV in two carriers as they seroconverted from HBeAg to the corresponding antibody. These results indicate that HBV mutants with an HBeAg- phenotype would be generated by mutations in the core promoter which might abort the transcription of pre-C mRNA but do not seriously affect that of pregenome RNA.

Differentiated liver cell specificity of the second enhancer of hepatitis B virus

Hepatitis B virus is a hepatotropic virus. Its replication and gene expression are mainly restricted to hepatocytes in the infective process. The viral gene expression thus provides a unique system with which to study the control of tissue-specific gene expression. We have previously reported the identification and characterization of the second enhancer (enhancer II) of hepatitis B virus. In this report, we further demonstrate that the minimal functional constituents of the second enhancer, box alpha and box beta, display liver cell and differentiation state specificity. Moreover, box alpha exhibits the same liver cell and differentiation state specificity when functioning as an upstream regulator for the basal core promoter. Gel shift experiments reveal a unique box alpha-binding protein, protein a, which is present only in differentiated liver cells, where enhancer II is functional. The converse is true for another box alpha-binding protein, protein f, which is present only in poorly differentiated liver cells and nonliver cells. The simplest hypothesis that explains these results is that protein a activates and/or protein f suppresses the enhancer and upstream regulator functions. Although C/EBP is a candidate for a transcription factor that interacts with box alpha or box beta, none of the binding factors identified in the gel shift assays, including protein a and protein f, is likely to be C/EBP because they differ from C/EBP in heat lability and sequence preference.

Frequent and rapid emergence of mutated pre-C sequences in HBV from e-antigen positive carriers who seroconvert to anti-HBe during interferon treatment

Hepatitis B virus (HBV) variants which cannot express e-antigen (HBeAg) are characteristic for many viremic anti-HBe positive chronic carriers who often have particularly severe and fluctuating hepatitis. Whether such variants are selected for and are less amenable to interferon treatment is under dispute. Therefore, by DNA amplification and direct sequencing we have investigated the emergence of HBV pre-C sequence variants in nine e-antigen positive chronic carriers, all of whom seroconverted to anti-HBe or lost HBeAg during interferon treatment, and in three of whom no viral DNA was detectable after interferon treatment. In most, but not all of the patients we found newly emerging pre-C sequences in a subpopulation of the viral genomes that included silent point mutations, amino acid changes, start and stop codon and frameshift mutations. The emergence of these mutations was paralleled by a drastic decrease of viremia during treatment. The observed mutations appeared most frequently during interferon treatment. Some of the mutations appeared or disappeared late after interferon treatment concomitant with anti-HBe antibody development. The appearance or lack of mutations in the pre-C region of a subpopulation of HBV of these patients was independent of successful virus elimination. These data indicate that interferon treatment is frequently associated with the simultaneous fall in titer of viral DNA by several orders of magnitude and the emergence of novel pre-C sequences, some of them preventing HBeAg expression. However, the presumably immune-mediated selection for pre-C mutant viruses and decrease in viremia under interferon treatment appears not to be prognostic for successful or unsuccessful virus elimination.

Transcriptional factor C/EBP binds to and transactivates the enhancer element II of the hepatitis B virus

The human hepatitis B Virus genome (HBV) contains a liver-specific enhancer upstream of the X ORF which has been studied in detail by several investigators. A second liver-specific enhancer element, designated here as enhancer II, has been relatively recently described in the HBV genome, which is located within the core/pregenomic promoter. We have studied the interactions of transcriptional factors with this element and show here that the nuclear factor CCAAT/enhancer binding protein (C/EBP) binds at a unique site within these sequences. Further, using the transient cotransfection scheme of expression with C/EBP encoding vectors and an enhancer II-reporter gene construct, we demonstrate that the enhancer element II responds to increasing amounts of C/EBP by displaying transactivation. Evidence for the functional role of the enhancer element II in transcriptional regulation of the HBV gene expression is presented. A major influence of the enhancer II appears to be on the surface antigen expression.

Foreign epitopes in immunodominant regions of hepatitis B core particles are highly immunogenic and conformationally restricted

The presentation of heterologous amino acid sequences on the surface of hepatitis B core antigen (HBcAg) particles has been studied using a defined linear neutralization site from human rhinovirus (HRV). Previous work has shown that fusion particles, in which the HRV peptide sequence is linked to the amino terminus of the HBcAg protein, induce excellent immune responses in experimental animals. Using predictive models of HBcAg particulate structure and the approximate location of the major immunogenic regions we have designed and constructed bacterial expression vectors which direct synthesis of chimeric particles in which heterologous sequences are presented within an immunodominant area on the particle. Immunological responses to the heterologous peptide sequence are improved by at least tenfold when compared with amino terminal fusions of the same peptide sequence to HBcAg. Moreover, the restriction placed on the heterologous peptide by its linkage at both ends within the HBcAg protein results in a more constrained structure. In the case of the rhinovirus peptide sequence this results in an antigenic conformation more closely resembling that on the native virus particle. Such a system lends itself well as a general approach to the induction of high titre antibodies against defined epitopes.

Mutations in the precore region of hepatitis B virus DNA in patients with fulminant and severe hepatitis

BACKGROUND

The presence of the hepatitis B e antigen (HBeAg) in serum is known to be a marker of a high degree of viral infectivity. However, fulminant hepatitis may occur in persons who are negative for HBeAg. A single point mutation has been reported to produce a stop codon in the precore region of hepatitis B virus DNA and prevent the formation of the precore protein required to make HBeAg. To determine whether a precore-mutant virus is causally related to severe liver injury, we analyzed the entire precore region in viral strains isolated from patients with fatal cases and uncomplicated cases of hepatitis B.

METHODS

Serum was obtained from 9 patients with fatal hepatitis B (5 with fulminant and 4 with severe exacerbations of chronic hepatitis) and 10 patients with acute, self-limited hepatitis B. Serum samples from a sex partner implicated as the source of the virus in one case of fulminant hepatitis were also studied. The 87 nucleotides in the precore region of the hepatitis B virus were amplified by the polymerase chain reaction and then directly sequenced.

RESULTS

Of the nine patients with fatal hepatitis, seven had retrievable hepatitis B DNA: In all seven there was a point mutation from G to A at nucleotide 1896 of the precore region, converting tryptophan (TGG) to a stop codon (TAG). In contrast, this mutation was not found in the 10 patients with acute, self-limited hepatitis B. The hepatitis B DNA from the implicated source contained a sequence with the stop-codon mutation that was identical to the sequence in her partner, who had fulminant hepatitis.

CONCLUSIONS

The presence of a mutant viral strain is associated with and may be involved in the pathogenesis of fulminant hepatitis B and severe exacerbations of chronic hepatitis B.

[Hepatology. New research results in its significance for the understanding of liver diseases]

By two exemplary clinical situations--acute viral hepatitis, acute-phase reaction of the liver--the significance of basic research for the understanding of clinical phenomena and for the development of new diagnostic and therapeutic procedures is demonstrated. The very different phenomena following infection with the hepatitis-B-virus can be explained by the variation in the interactions of virus and liver cell, by the immune reaction of the host, and by mutants of the virus. The reaction of the liver to an extrahepatic infection is mediated by interleukin-6, and characterized by an alteration in protein metabolism. The synthesis of acute-phase proteins is increased. The proteins confine the local injury and establish the homeostasis of the organism.

Mechanism, kinetics, and role of duck hepatitis B virus e-antigen expression in vivo

No duck hepatitis B virus (DHBV) pre-C transcript has been identified so far, and neither the interrelationship of e-antigen (DHBeAg) with the expression of other viral antigens or virus replication nor its function is known. In this study we identified in infected livers a minor transcript from which the precursor protein of DHBeAg could be synthesized. Mutation of the first AUG on this transcript abolished expression of DHBeAg. DHBV genomes containing this mutation were infectious in Pekin ducks, the kinetics of pre-S envelope protein expression and virus secretion were not significantly different from wild-type, and the mutant genomes did not revert to wild-type to a detectable level after several passages. In contrast to pre-S protein, the level of DHBeAg in the serum was independent of the level of viremia, accumulated gradually to a high and constant level after a lag phase, and was also easily detectable in a mixed infection containing less than 0.1% of wild-type in a pre-C mutant virus containing inoculum. These data indicate that precore protein is synthesized from a minor pre-C mRNA with translation initiation at the pre-C AUG codon, and leads to high levels of DHBeAg rather late in infection. High levels of DHBeAg can even be produced efficiently by a very small subpopulation of wild-type virus in a mixed infection with predominantly pre-C mutant virus. Lack of DHBeAg appears to have no effect on DHBV viability and kinetics of virus secretion into the bloodstream when ducklings are infected with the pre-C AUG mutant virus a few days after birth.

In vitro replication competence of a cloned hepatitis B virus variant with a nonsense mutation in the distal pre-C region

Hepatitis B virus (HBV) variants with a nonsense mutation in the distal pre-C region have been detected in patients positive for anti-HBe, and the complete nucleotide sequence of one cloned pre-C variant has been determined. Transfection of this HBV variant clone into the human hepatoma cell line HepG2 resulted in the appearance of major HBV transcripts, replicative forms of viral DNA evidenced by both molecular hybridization and endogenous DNA polymerase assay, as well as the expression and secretion of HBsAg and HBcAg particles. Western blotting revealed only the 21-kDa HBcAg but not the 17-kDa HBeAg. These results demonstrate the replication capacity of the HBV variant with a nonfunctional pre-C region despite its inability to express HBeAg.

Regulation of the hepatitis B virus gene expression by the enhancer element I

Hepatitis B virus (HBV) enhancer is a cis-acting element capable of potentiating transcription with liver cell specificity. To assess the regulatory function of the enhancer during viral gene expression, we employed whole genome transfections in both liver- and non-liver-derived cell lines and compared them with those of enhancerless genome transfections. Using a transient scheme of expression, the effect of enhancer deletion on the transcription of HBV genes was investigated. S1 nuclease analysis showed that deletion of the enhancer sequences did not alter the specificity of the HBV transcripts. Also, the nuclear run-on analysis demonstrated that the influence of enhancer on the HBV gene expression is at the transcriptional level. We further noted that in the absence of the enhancer, synthesis of the pregenomic RNA which originates from the core/pregenomic promoter was markedly affected. This result then implicates an additional role of the enhancer during the replication of HBV.

Expression pattern of the hepatitis B virus genome in transfected mouse fibroblasts

Permanent mouse fibroblast LTK- cells were transfected with dimeric hepatitis B virus (HBV) DNA linked to the simian virus 40 (SV40) early promoter/enhancer. Many clones stably expressed high levels of polyadenylated RNAs encoding hepatitis B surface (HBs) proteins (2.1 kb), HBe protein (3.6 kb), and HBx protein (0.6 kb). Although a chimeric RNA (4.0 kb) probably starting from the SV40 promoter was also synthesized, transcription of viral RNAs was predominantly directed by HBV promoters and its terminator. In contrast to HBV-transfected liver cells, the fibroblasts expressed only pregenomic 3.6-kb transcripts starting 5' to, but not within, the precore sequence. Thus, no normal core protein could be synthesized, but the cells expressed and secreted HBe protein of heterogeneous size. Small and middle HBs proteins were strongly expressed, while large HBs protein was almost absent. HBx mRNA expression was more efficient in mouse fibroblasts than in human hepatoma cells and 18-kDa HBx protein was exclusively detected in purified nuclei. Expression of HBe, small and middle HBs, and HBx proteins apparently does not require hepatic factors. Underexpression of HBc mRNA and large HBs mRNA suggests that activity of their promoters depends on cell-type-specific transcription factors.

Hepatitis B virus core antigen (HBc Ag) accumulation in an HBV nonproducer clone of HepG2-transfected cells is associated with cytopathic effect

Two clones of the hepatoblastoma HepG2 cell line transfected with complete hepatitis B virus deoxyribonucleic acid (HBV DNA) were studied. The kinetics and cytopathic effect of HBV Ag production in these two clones (one of which was an HBV producer) were compared to those of the parent HepG2 cell line. The presence of hepatitis B surface antigen (HBs Ag) and hepatitis B e antigen (HBe Ag) was determined by commercial enzyme-linked immunosorbent assay (ELISA). A hepatitis B core antigen (HBc Ag)-specific ELISA assay was developed, using monoclonal anti-HBc to detect HBc Ag. Amounts of HBs, HBe, and HBC Ags were partially quantified in both intracellular and extracellular compartments. The HBV producer clone excreted high levels of HBc, HBe, and HBs Ags from the beginning of the growth phase, and no cytopathic effect was observed. The HBV nonproducer clone produced high levels of HBs and HBe Ags, but there was no detectable HBc Ag in the supernatant; instead, HBc Ag accumulated in the intracellular compartment. In this clone, significant cell death was observed 4 days after cell confluency, corresponding with notable HBc Ag release into the supernatant. These results suggest a cytopathic effect associated with HBc Ag accumulation in the HBV nonproducer clone, but no cytopathic effect in the HBV producer clone. This suggests that virological factors as well as the host's immune response may be considered in explaining liver injury occurring in hepatitis B.

Defects in the precore region of hepatitis B virus DNA in a plasma pool from carriers seropositive for antibody against e antigen and with infectivity in chimpanzees

A plasma pool from 12 asymptomatic carriers seropositive for antibody against hepatitis B e antigen (anti-HBe) contained hepatitis B virus (HBV) with chimpanzee infectious doses of 1-100/mL, and another pool from 12 carriers positive for hepatitis B e antigen (HBeAg) contained 10(8)/mL doses or more. The HBeAg-positive pool contained 10(6)-fold more HBV DNA than the anti-HBe-positive pool, reflecting the difference in infectivity in chimpanzees. The precore region sequences of HBV DNA in the two plasma pools were amplified by polymerase chain reaction, and separate HBV DNA clones were propagated for determining the nucleotide sequence. Of 114 clones from the anti-HBe-positive pool, 113 displayed a point mutation from guanine to adenine at nucleotide 83 in the precore region, which converted codon 28 for tryptophan (TGG) to a stop codon (TAG), and the remaining clone had a point mutation from adenine to cytosine at the first letter of codon 1 (CTG) to inhibit the translation initiation of the precore region. Precore region defects, in contrast, were observed in only 10 (8%) of 119 clones from the HBeAg-positive pool. These results indicate the infectious capacity of HBV mutants, defective in the precore region and incapable of directing the synthesis and secretion of HBeAg, which prevail in the circulation of hosts after they seroconvert from HBeAg to anti-HBe.

Replication of hepatitis B virus in transfected nonhepatic cells

Permanent murine fibroblasts (LTK-) were transfected with a dimer of hepatitis B virus (HBV) DNA and a neomycin resistance gene which were both linked to the simian virus 40 (SV40) early promoter/enhancer. One of the stably transfected clones, LTK4/36, which secreted HBsAg, HBeAg, and HBV DNA was further analyzed. It contained eight to nine copies of integrated HBV DNA per haploid genome and low amounts of episomal HBV DNA. The secreted viral DNA was covalently linked to protein and was associated with particles which had the characteristic density of natural virions from serum of human viremic carriers. The particles contained an endogenous DNA polymerase, small and middle surface proteins, but in contrast to natural virions very little core protein and large surface protein. Instead of core protein, they contained incompletely processed HBe protein which is colinear to core protein. The fibroblast-derived virions were less stable than virions from human carriers or from transfected hepatoma cells. After several days of storage, their DNA was only partially protected against DNase. Obviously, nonhepatic cells can express HBV-like particles, even if liver-dependent gene products like large surface protein and core protein are missing.

Hepatitis A, B, C, D and E viruses: structure of their genomes and general properties

Hepatitis A virus is an enteric picornavirus. Its genome is a single stranded RNA molecule of positive-strand polarity of 7478 bases. This sequence codes for a polyprotein which is processed to give rise to viral proteins VP-1, VP-2, VP-3 and others. Hepatitis B virus, a major worldwide infectious and cancer promoting agent contains a DNA genome of 3226 base pairs that replicates by a reverse transcriptase via an RNA intermediate. Extensive sequencing and expression experiments have revealed four major genes named surface, core, polymerase and X which are coded in more than one reading frame. Furthermore, within a frame, proteins are expressed from multiple initiation codons resulting in several related products. The viral genome of hepatitis C virus (nonA-nonB), an elusive major infectious agent, has recently been cloned. This genome is a single positive-stranded RNA of at least 10,000 bases which codes for several antigens, some of them associated specifically with nonA-nonB hepatitis infections. The hepatitis D (delta) viral agent, an infectious agent requiring a hepadnarious for propagation, contains a covalently closed circular single-stranded RNA genome of 1167 nucleotides. This genome encodes the protein p24 and p27 that bind specifically to antisera from patients with chronic hepatitis D infections.

Immunoelectron microscopic observation of intrahepatic HBeAg in patients with chronic hepatitis B

Immune light and electron microscopic studies using monoclonal antibodies have been applied to localize HBeAg in liver biopsy specimens of 19 patients with chronic hepatitis B. Under the light microscope, HBeAg was demonstrated in nuclei, cytoplasm and on the cell surface of hepatocytes. The number of HBeAg-positive hepatocytes correlated well with the serum levels of HBeAg (enzyme immunoassay) and DNA-polymerase. Of 11 patients in whom high numbers of HBeAg-positive hepatocytes were found at the light microscopic level, HBeAg was also studied in hepatocytes at the electron microscopic level. The HBeAg in nuclei was either found as aggregates or dispersed diffusely. In the aggregates of HBeAg, the 27-nm core particles were frequently found. In addition, the antigen was found in the cytosol of some hepatocytes as amorphous mass and in some hepatocytes in the cisternae of perinuclear space, endoplasmic reticulum and Golgi saccules. Occasionally the antigen was found on the membranes of the cell organelles and on the plasma membranes that faced the intercellular and the Disse spaces. These findings suggest that cytoplasmic HBeAg in hepatocytes may be ultrastructurally classified into two different patterns by its distribution, mainly in endoplasmic reticulum or in cytosol. The ratio varied between hepatocytes with these two types of patterns. The titer of serum HBeAg tended to be higher when the corresponding liver biopsy specimens contained more hepatocytes with HBeAg in endoplasmic reticulum than those with HBeAg in the cytosol.

Active hepatitis B virus replication in the presence of anti-HBe is associated with viral variants containing an inactive pre-C region

Although rise of anti-HBe immunity in the course of hepatitis B virus (HBV) infection is generally followed by clearance of the infectious virions, ongoing chronic liver disease with circulating virions has been repeatedly observed in a significant number of anti-HBe patients, especially in Mediterranean countries. To investigate the possible role of HBV variants, we cloned HBV DNA from the serum of three such anti-HBe cases. Comparative restriction mapping of HBV clones suggested circulation of different HBV genomes in the three cases. DNA sequencing revealed an inactive pre-C region in all 11 HBV clones derived from the three cases, either as one or two point mutations in the 3' terminus generating an in-frame TAG stop codon, or a 1 nucleotide insertion in the 5' terminus resulting in frameshift mutation. Furthermore, for one clone the complete 3182 nucleotide sequence was determined and no significant mutation was found in the remainder of the genome. We conclude that chronic hepatitis cases positive for anti-HBe are associated with HBV variants containing an inactive pre-C region and hence cannot synthesize pre-C region-derived HBeAg. This finding may provide a molecular explanation for the continued viral replication despite presence of anti-HBe immunity.

Expression of hepatitis B virus core and precore antigens in insect cells and characterization of a core-associated kinase activity

The hepatitis B virus core open reading frame with and without the precore domain was expressed in insect cells using a baculovirus expression system. Precore antigen was not properly processed in insect cells and was present in highly insoluble cytoplasmic aggregates. Core antigen without the precore domain formed core particles with a diameter of 28 nm that were secreted into the medium. Both core and precore antigens were phosphorylated in insect cells. The immune response in mice to both antigens yielded antibodies with a high degree of preferential reactivity for the homologous immunizing polypeptide. A kinase activity that phosphorylated core antigen was associated with highly purified core particles. The kinase activity resembled that previously demonstrated for core particles purified from the cytoplasm of infected hepatocytes and detergent-treated Dane particles. Partial resistance of the phosphate-label to phosphatase treatment suggested that some of the phosphorylated sites are in the interior of the particle. The presence of kinase activity in recombinant core particles demonstrated that this activity is not derived from another hepatitis B virus-encoded polypeptide, and the lack of a kinase consensus sequence in the core open reading frame suggests that the kinase is of cellular origin.

Cell surface expression of hepatitis B surface and core antigens in transfected rat fibroblast cell lines

Hepatocellular necrosis during hepatitis B virus infection is hypothesized to result from host immune responses against either hepatitis B surface antigen or hepatitis B core antigen expressed on the surface membrane of infected hepatocytes. To study the capacity of hepatitis B deoxyribonucleic acid to induce membrane expression of either hepatitis B surface antigen or hepatitis B core antigen in vitro, we assessed transfected rat fibroblast cell lines by indirect immunofluorescence. Rat fibroblasts were transfected with plasmid vectors containing the natural promoters, native enhancer, and uninterrupted sequences of either the Pre S/S gene or core gene. Resulting cell lines produced hepatitis B surface antigen and hepatitis B core antigen/hepatitis B e antigen, respectively. Immunofluorescence microscopy or flow cytometry showed that hepatitis B surface antigen and hepatitis B core antigen were expressed in a granular pattern in the surface membrane of transfected cells. We conclude that surface membrane expression of both hepatitis B surface antigen and hepatitis B core antigen is an intrinsic consequence of expression of either the Pre S/S or core gene.

DNase I hypersensitive site maps to the HBV enhancer

Two lines of HBV transgenic mice (derived from G7 and G26) have been produced, each of which contains a unique locus of HBV DNA and expresses 2.1-kb HBsAg transcripts preferentially in liver and kidney tissues. To investigate the regulation of HBV expression in these mice, we have examined the state of methylation and the chromatin structure in and around the HBV sequences in tissues with and without HBV gene expression. Hypomethylation of HpaII and HhaI sites in and around the HBV sequences strongly correlated with HBV gene expression, although it was clearly not sufficient for HBV expression. Alterations in chromatin configuration were detected by DNase I digestion which identified a major hypersensitive site (HS) in liver and kidney tissue. By restriction enzyme mapping and indirect end-labeling, the HS was localized to the region of the HBV enhancer in both lines of HBV transgenics. The presence of this DNase I hypersensitive site was necessary but not sufficient for HBV expression, since it was also detected in tissues not expressing HBV. An additional DNase I hypersensitive site was mapped to the core promoter region of the G7 transgene in liver and kidney tissue but not in G26 tissues. The identification of a DNase I hypersensitive site mapping to the HBV enhancer region supports the notion that this region can interact with cellular proteins and is involved in the regulation of viral gene transcription in vivo.

Synthesis of hepatitis B virus e antigen in E. coli

Hepatitis B virus core antigen (HBcAg) gene was deleted at some unique restriction enzyme sites, or at random, and inserted into the expression plasmids of E. coli which had the tryptophan promoter. E. coli transformants with the plasmids, synthesized materials with many kinds of antigenicity of HBcAg, HBeAg, or both HBcAg and HBeAg. HBeAg-specific material smaller than native HBeAg was produced in a stable condition.

The molecular biology of hepatitis B virus

Infection with the hepatitis B virus (HBV) or related hepadnaviruses is associated with a wide spectrum of liver diseases, including hepatocellular carcinoma (HCC). In this article we review the current state of knowledge about the structure, genetic organization and life cycle of HBV. The mechanisms of viral pathogenesis and HCC development remain poorly understood, but new approaches may soon begin to shed light on these areas.

Expression mechanism of the hepatitis B virus (HBV) C gene and biosynthesis of HBe antigen

The C gene of the hepatitis B virus, which contains two in-phase initiation codons delimiting the pre-C sequence and the C region, directs the synthesis of the major protein of the capsid (HBcAg) and of a precore protein which upon processing results in the secretion of the HBeAg. We used an adenovirus-based vector to study in the human 293 cell line the C gene products, the intermediates of the precore protein processing and the kind of protease involved in this processing. The synthesis of the 21-kDa HBcAg polypeptide was dependent on the deletion of the pre-C sequence suggesting that a pre-C mRNA is not used for the synthesis of the major capsid protein. With the construct containing the complete C gene, two proteins of 25 and 22 kDa were detected intracellularly, corresponding to the unprocessed and partially processed precore protein, respectively. In addition, a 15-kDa protein (HBeAg) was secreted in the culture medium. Using pepstatin, an inhibitor specific for aspartyl proteinases, reduction of HBeAg secretion and accumulation of the 22-kDa processing intermediate were observed, suggesting the involvement of an aspartyl proteinase in the conversion of the 22-kDa protein into HBeAg.

Negative regulation of the hepatitis B virus pre-S1 promoter by internal DNA sequences

Expression of the surface antigen gene (S gene) of hepatitis B virus is directed by two distinct promoter elements with markedly different activities. The upstream (pre-S1) promoter produces a 2.4-kb transcript at very low levels while the downstream (pre-S2) promoter produces an approximately 2.1-kb transcript in relative abundance. We have constructed a series of internal deletion mutants to analyze differential regulation of the two S gene promoters. We show here that expression directed by the pre-S1 promoter is negatively regulated by DNA sequences containing the downstream pre-S2 promoter region. Nuclear run-on analysis indicates this down-regulation to be at the level of transcription. Furthermore, promoter repression does not appear to be due to products of the S gene region. Deletion mutagenesis studies have permitted the localization of a 61-bp region that may be involved in the apparent down-regulation of the pre-S1 promoter. These results suggest the use of an unusual regulatory mechanism by a dipromoter gene in which an active internal promoter may preclude efficient use of an upstream promoter.

Experimental systems for the study of hepadnavirus and hepatitis delta virus infections

The past decade has seen a dramatic increase in the number of approaches available for the study of hepadnavirus and hepatitis delta virus infections. In this review, we have summarized the recent applications of these approaches to the study of virus replication, tissue specificity, liver injury and hepatocellular carcinogenesis.