Dominant negative mutants of the duck hepatitis B virus core protein interfere with RNA pregenome packaging and viral DNA synthesis

Gene therapeutic approaches to inhibit hepatitis B virus replication

Acute and chronic hepatitis B virus (HBV) infections remain to present a major global health problem. The infection can be associated with acute symptomatic or asymptomatic hepatitis which can cause chronic inflammation of the liver and over years this can lead to cirrhosis and the development of hepatocellular carcinomas. Currently available therapeutics for chronically infected individuals aim at reducing viral replication and to slow down or stop the progression of the disease. Therefore, novel treatment options are needed to efficiently combat and eradicate this disease. Here we provide a state of the art overview of gene therapeutic approaches to inhibit HBV replication. We discuss non-viral and viral approaches which were explored to deliver therapeutic nucleic acids aiming at reducing HBV replication. Types of delivered therapeutic nucleic acids which were studied since many years include antisense oligodeoxynucleotides and antisense RNA, ribozymes and DNAzymes, RNA interference, and external guide sequences. More recently designer nucleases gained increased attention and were exploited to destroy the HBV genome. In addition we mention other strategies to reduce HBV replication based on delivery of DNA encoding dominant negative mutants and DNA vaccination. In combination with available cell culture and animal models for HBV infection, in vitro and in vivo studies can be performed to test efficacy of gene therapeutic approaches. Recent progress but also challenges will be specified and future perspectives will be discussed. This is an exciting time to explore such approaches because recent successes of gene therapeutic strategies in the clinic to treat genetic diseases raise hope to find alternative treatment options for patients chronically infected with HBV.

Inhibition of hepatitis B virus replication by the internal fragment of hepatitis B core protein

The nucleocapsids formation is a pivotal step of hepatitis B virus (HBV) life cycle. The inhibition of HBV nucleocapsids assembly is a promising strategy for the anti-HBV treatment. HBc78-117 is an internal fragment of hepatitis B core protein (HBc). In this study, we used lentiviral vector to deliver HBc78-117 cDNA sequence into HepG2.2.15 cells and examined the effect of HBc78-117 on HBV replication. We confirmed by immunoprecipitation analysis that HBc78-117 interacted with full-length HBc in HepG2.2.15 cells. The nucleocapsids and HBV DNA replication intermediates were markedly reduced in the cells expressing HBc78-117, although HBV pregenome RNA was not affected. The level of HBV DNA was also significantly reduced in culture supernatant. These suggest that HBc78-117 can inhibit HBV DNA replication by interfering with nucleocapsids assembly.

The phenylpropenamide derivative AT-130 blocks HBV replication at the level of viral RNA packaging

Nucleos(t)ide analogue antiviral therapy for chronic hepatitis B has proven to be effective in the short term but the frequent development of resistance limits its clinical utility. Agents targeting other stages of viral replication are needed in order to develop improved combination therapies. The phenylpropenamide derivatives AT-61 and AT-130 have been shown to inhibit HBV replication in vitro, but the mechanism of action of these compounds remains undefined. The aim of this study was to determine the mechanism of action of AT-130, a non-nucleoside inhibitor of HBV in several in vitro models of replication. These studies found that AT-130 inhibited HBV DNA replication in hepatoma cells but had no effect on viral DNA polymerase activity or core protein translation. Total HBV RNA production was also unaffected in the presence of the drug whilst the amount of encapsidated RNA was significantly reduced, thereby inhibiting subsequent viral reverse transcription. These studies have established that the inhibition of HBV genome replication by a non-nucleoside analogue acting at the level of viral encapsidation and packaging is a potentially useful strategy for future therapeutic drug development in the management of chronic hepatitis B.

VP22 fusion protein-based dominant negative mutant can inhibit hepatitis B virus replication

AIM: To investigate the inhibitory effect of VP22 fusion protein-based dominant negative (DN) mutant on Hepatitis Bvrus (HBV) replication.

METHODS: Full-length or truncated fragment of VP22 was fused to C terminal of HBV core protein (HBc), and subcloned into pcDNA3.1 (-) vector, yielding eukaryotic expression plasmids of DN mutant. After transfection into HepG2.2.15 cells, the expression of DN mutant was identified by immunofluorescence staining. The inhibitory effect of DN mutant on HBV replication was indexed as the supernatant HBsAg concentration determined by RIA and HBV-DNA content by fluorescent quantification-PCR (FQ-PCR). Meanwhile, metabolism of HepG2.2.15 cells was evaluated by MTT colorimetry.

RESULTS: VP22-based DN mutants and its truncated fragment were expressed in HepG2.2.15 cells, and had no toxic effect on host cells. DN mutants could inhibit HBV replication and the transduction ability of mutant-bearing protein had a stronger inhibitory effect on HBV replication. DN mutants with full length of VP22 had the strongest inhibitory effect on HBV replication, reducing the HBsAg concentration by 81.94%, and the HBV-DNA content by 72.30%. MTT assay suggested that there were no significant differences in cell metabolic activity between the groups.

CONCLUSION: VP22-based DN mutant can inhibit HBV replication effectively.

Interfering with capsid formation: a practicable antiviral strategy against hepatitis B virus?

Chronic hepatitis B virus (HBV) infection is a major cause of liver disease. Only interferon-alpha and the nucleosidic inhibitors of the viral polymerase, 3TC and adefovir, are approved for therapy. However, these therapies are limited by the side effects of interferon and the substantial resistance of the virus to nucleosidic inhibitors. Potent new antiviral compounds suitable for monotherapy or combination therapy are highly desired. We describe nonnucleosidic inhibitors of HBV nucleocapsid maturation that possess in vitro and in vivo antiviral activity. These inhibitors have potential for future therapeutic regimens to combat chronic HBV infection.

Relevance of hepatitis B core gene deletions in patients after kidney transplantation

BACKGROUND & AIMS

Hepatitis B virus (HBV) infection is a major cause of death in the long-term follow-up after organ transplantation and immunosuppressive therapy. The selection pressure on the HBV genome in these patients is reduced. The aim was to analyze and characterize variations in the HBV core gene after organ transplantation and their impact for prognosis.

METHODS

In patients with chronic HBV infection after organ transplantation (liver, n = 60; heart, n = 50; kidney, n = 30) the HBV core gene was amplified by polymerase chain reaction (PCR). Core gene deletions were cloned into replication competent and expression vectors. The impact of these mutations on HBV replication and capsid formation was analyzed and correlated with disease progression.

RESULTS

Central core gene deletions only were detected in patients after kidney transplantation. Two types of core gene deletions--small and large--were found. Large core gene deletions showed no capsid formation and HBV replication, which resulted in nuclear core expression. The occurrence of large core gene deletions was associated with a severe course of liver disease.

CONCLUSIONS

Core gene deletions occur specifically after kidney transplantation. Only large core gene deletions resulted in impaired capsid formation and nuclear localization of the core protein. The presence of large core gene deletions was associated with progressive disease.

Antiviral therapy for hepatitis B virus infections: new targets and technical challenges

There are presently only two licensed therapies for treating liver disease caused by infection with the hepatitis B virus (HBV). These are interferon-alpha and lamivudine. Neither agent was specifically developed as an antiviral compound for treating patients infected with HBV. Both therapies are limited in the clinic by a low response rate and in the case of lamivudine, selection of drug-resistant mutants, whilst troublesome side effects limit the use of interferon-alpha. Several promising nucleoside/nucleotide analogues are undergoing clinical trials, including adefovir dipivoxil and entecavir, both of which appear to be active against lamivudine- resistant HBV. In addition to these nucleoside/nucleotide analogues, it will be important to develop new agents with different modes of action, which can be added to the antiviral cocktails that will be required to adequately suppress and hopefully eliminate HBV replication.

Viral determinants and host immune responses in the pathogenesis of HBV infection

Hepatitis B virus (HBV) is a virus that infects about 350,000,000 people worldwide with a clinical spectrum of acute hepatitis, the healthy carrier state, cirrhosis and hepatocellular carcinoma (HCC). The outcome of HBV infection is the result of complicated viral-host interactions. As in other infections with non-cythopatic viruses, the immune response is thought to play a crucial role in disease pathogenesis but there is increasing evidence that a variety of viral mechanisms, some depending on the function of virally encoded proteins, have a profound impact on the infected hepatocytes, the liver microenvironment, and host anti-viral responses. Indeed, the virus has evolved multiple mechanisms to ensure its success in infecting a susceptible host. The essential aspects of the life cycle of HBV and the host immune response are reviewed and recent new developments in the molecular virology of HBV, including experimental animal models, in the role of accessory viral proteins in disease pathogenesis and HCC development and in the characterisation of the T cell response in the control of HBV infection, are highlighted.

Cis-preferential recruitment of duck hepatitis B virus core protein to the RNA/polymerase preassembly complex

Hepadnaviral replication requires the concerted action of the polymerase and core proteins to ensure selective packaging of the RNA pregenome into nucleocapsids. Virus assembly is initiated by cis-preferential binding of polymerase to the encapsidation signal straightepsilon, present on pregenomic RNA. Using the duck hepatitis B virus (DHBV) model, we analyzed how core protein is recruited to the RNA/polymerase preassembly complex. Two sets of trans-complementation assays were performed in cotransfected hepatoma cells. First, a replication-competent DHBV construct was tested for its ability to rescue replication of genomes bearing mutations within the core region. Self-packaging of wild-type pregenomes was more efficient than cross-packaging of core-deficient pregenomes, and this bias was strongly enhanced if mutant pregenomes coded for self-assembly-competent, but packaging-deficient, core proteins. Second, the site of wild-type core protein translation, i.e., pregenomic RNA (cis) or separate messenger RNA (trans), was analyzed for its effect on the phenotype of a previously described dominant-negative (DN) DHBV core protein mutant. This mutant forms chimeric nucleocapsids with wild-type core proteins and blocks reverse transcription within most, but not all, mixed particles. Strikingly, suppression of viral DNA synthesis by the mutant increased 100-fold when wild-type core protein was provided in trans. Our results suggest that recruitment of core protein to the DHBV preassembly complex occurs in a cis-preferential manner. This mechanism may account for the leakiness of DN DHBV core protein mutants targeting reverse transcription.

Peptide aptamers targeting the hepatitis B virus core protein: a new class of molecules with antiviral activity

A substantial proportion of the worldwide liver cancer incidence is associated with chronic hepatitis B virus (HBV) infection. The therapeutic management of HBV infections is still problematic and novel antiviral strategies are urgently required. Using the peptide aptamer screening system, we aimed to isolate new molecules, which can block viral replication by interfering with capsid formation. Eight peptide aptamers were isolated from a randomized expression library, which specifically bound to the HBV core protein under intracellular conditions. One of them, named C1-1, efficiently inhibited viral capsid formation and, consequently, HBV replication and virion production. Hence, C1-1 is a novel model compound for inhibiting HBV replication by blocking capsid formation and provides a new basis for the development of therapeutic molecules with specific antiviral potential against HBV infections.

Significant interference with hepatitis B virus replication by a core-nuclease fusion protein

Hepatitis B virus (HBV), a small DNA containing virus that replicates via reverse transcription, causes acute and chronic B-type hepatitis in humans. The limited success of current therapies for chronic infection has prompted exploration of alternative strategies. Capsid-targeted viral inactivation is a conceptually powerful approach that exploits virion structural proteins to target a degradative enzyme specifically into viral particles. Its principal feasibility has been demonstrated in retroviral model systems but not yet for a medically relevant virus outside the retrovirus family. Recently, we found that C proximal fusion to the HBV capsid protein of the Ca(2+)-dependent nuclease (SN) from Staphylococcus aureus yields a chimeric protein, coreSN, that in Escherichia coli coassembles with the wild-type capsid protein into particles with internal SN domains. Here we show that, in HBV co-transfected human hepatoma cells, less than 1 coreSN protein per 10 wild-type core protein subunits reduced titers of enveloped DNA containing virions by more than 95%. The antiviral effect depends on both an enzymatically active SN and on the core domain. CoreSN does not block assembly of RNA containing nucleocapsids but interferes with proper synthesis of viral DNA inside the capsid, or leads to rapid DNA degradation. Our data suggest an intracellular nuclease activation that, owing to the characteristics of HBV morphogenesis, is nonetheless highly virus specific. HBV may therefore be particularly vulnerable to the capsid-targeted viral inactivation approach.

Interaction of wild-type and naturally occurring deleted variants of hepatitis B virus core polypeptides leads to formation of mosaic particles

The simultaneous presence of hepatitis B virus (HBV) genomes carrying wild-type (wt) and in-frame deleted variants of the HBV core gene has been identified as a typical feature of HBV-infected renal transplant patients with severe liver disease. To investigate possible interactions of wt and deleted core polypeptides a two-vector Escherichia coli expression system ensuring their concomitant synthesis has been developed. Co-expression of wt and a mutant core lacking 17 amino acid residues (77-93) within the immunodominant region led to the formation of mosaic particles, whereas the mutant alone was incapable of self-assembly.

Core protein phosphorylation modulates pregenomic RNA encapsidation to different extents in human and duck hepatitis B viruses

To clarify the role of core protein phosphorylation in pregenomic-RNA encapsidation of human and duck hepatitis B viruses (HBV and DHBV, respectively), we have examined the phosphorylation states of different forms of intracellular HBV core protein and the phenotypic effects of mutations in the phosphorylation sites of HBV and DHBV core proteins. We show that HBV core protein is phosphorylated to similar extents in the form of protein dimers and after further assembly in pregenomic RNA-containing capsids. Individual and multiple substitutions of alanine and aspartic acid for serine in the phosphorylation sites of HBV core protein resulted in site-specific and synergistic effects on RNA encapsidation, ranging from 2-fold enhancement to more than 10-fold inhibition. Core protein variants with mutations in all phosphorylation sites exhibited dominant-negative effects on RNA encapsidation by wild-type protein. The results suggest that the presence of phosphoserine at position 162 of HBV core protein is required for pregenomic-RNA encapsidation, whereas phosphoserine at position 170 optimizes the process and serine might be preferable in position 155. Examination of the pregenomic-RNA-encapsidating capacities of DHBV core protein variants, in which four phosphorylation sites were jointly mutated to alanine or aspartic acid, suggests that phosphorylation of DHBV core protein at these sites may optimize pregenomic-RNA encapsidation but that its impact is much less profound than in the case of HBV. The possible mechanisms by which RNA encapsidation may be modulated by core protein phosphorylation are discussed in the context of the observed differences between the two viruses.