A bulged region of the hepatitis B virus RNA encapsidation signal contains the replication origin for discontinuous first-strand DNA synthesis

Host 3' flap endonuclease Mus81 plays a critical role in trimming the terminal redundancy of hepatitis B virus relaxed circular DNA during covalently closed circular DNA formation

Hepatitis B virus (HBV) relaxed circular DNA (rcDNA) possesses an 8-9 nucleotide-long terminal redundancy (TR, or r) on the negative (-) strand DNA derived from the reverse transcription of viral pregenomic RNA (pgRNA). It remains unclear whether the TR forms a 5' or 3' flap structure on HBV rcDNA and which TR copy is removed during covalently closed circular DNA (cccDNA) formation. To address these questions, a mutant HBV cell line HepDES-C1822G was established with a C1822G mutation in the pgRNA coding sequence, altering the sequence of 3' TR of (-) strand DNA while the 5' TR remained wild type (wt). The production of HBV rcDNA and cccDNA in HepDES-C1822G cells was comparable to wt levels. Next-generation sequencing (NGS) analysis revealed that the positive (+) strand DNA of rcDNA and both strands of cccDNA predominantly carried the wt nt1822 residue, indicating that the 5' TR of (-) strand DNA serves as the template during rcDNA replication, forming a duplex with the (+) strand DNA, while the 3' TR forms a flap-like structure, which is subsequently removed during cccDNA formation. In a survey of known cellular flap endonucleases using a loss-of-function study, we found that the 3' flap endonuclease Mus81 plays a critical role in cccDNA formation in wild-type HBV replicating cells, alongside the 5' flap endonuclease FEN1. Additionally, we have mapped the potential Mus81 and FEN1 cleavage sites within the TR of nuclear DP-rcDNA by RACE-NGS analyses. The overlapping function between Mus81 and FEN1 in cccDNA formation indicates that the putative 5' and 3' flap formed by TR are dynamically interchangeable on rcDNA precursor. These findings shed light on HBV rcDNA structure and cccDNA formation mechanisms, contributing to our understanding of HBV replication cycle.

Hepatitis B Virus Epsilon (ε) RNA Element: Dynamic Regulator of Viral Replication and Attractive Therapeutic Target

Hepatitis B virus (HBV) chronically infects millions of people worldwide, which underscores the importance of discovering and designing novel anti-HBV therapeutics to complement current treatment strategies. An underexploited but attractive therapeutic target is ε, a cis-acting regulatory stem-loop RNA situated within the HBV pregenomic RNA (pgRNA). The binding of ε to the viral polymerase protein (P) is pivotal, as it triggers the packaging of pgRNA and P, as well as the reverse transcription of the viral genome. Consequently, small molecules capable of disrupting this interaction hold the potential to inhibit the early stages of HBV replication. The rational design of such ligands necessitates high-resolution structural information for the ε-P complex or its individual components. While these data are currently unavailable for P, our recent structural elucidation of ε through solution nuclear magnetic resonance spectroscopy marks a significant advancement in this area. In this review, we provide a brief overview of HBV replication and some of the therapeutic strategies to combat chronic HBV infection. These descriptions are intended to contextualize our recent experimental efforts to characterize ε and identify ε-targeting ligands, with the ultimate goal of developing novel anti-HBV therapeutics.

Virtual Screening of Hepatitis B Virus Pre-Genomic RNA as a Novel Therapeutic Target

The global burden imposed by hepatitis B virus (HBV) infection necessitates the discovery and design of novel antiviral drugs to complement existing treatments. One attractive and underexploited therapeutic target is ε, an ~85-nucleotide (nt) cis-acting regulatory stem-loop RNA located at the 3'- and 5'-ends of the pre-genomic RNA (pgRNA). Binding of the 5'-end ε to the viral polymerase protein (P) triggers two early events in HBV replication: pgRNA and P packaging and reverse transcription. Our recent solution nuclear magnetic resonance spectroscopy structure of ε permits structure-informed drug discovery efforts that are currently lacking for P. Here, we employ a virtual screen against ε using a Food and Drug Administration (FDA)-approved compound library, followed by in vitro binding assays. This approach revealed that the anti-hepatitis C virus drug Daclatasvir is a selective ε-targeting ligand. Additional molecular dynamics simulations demonstrated that Daclatasvir targets ε at its flexible 6-nt priming loop (PL) bulge and modulates its dynamics. Given the functional importance of the PL, our work supports the notion that targeting ε dynamics may be an effective anti-HBV therapeutic strategy.

Review article: novel biomarkers in hepatitis B infection

BACKGROUND

Chronic hepatitis B remains a global health problem with an estimated 296 million people affected worldwide. Individuals are at risk of serious complications such as cirrhosis and hepatocellular carcinoma and accurately predicting these clinical endpoints has proven difficult. However, several viral biomarkers have recently been developed, including quantitative HBV surface antigen (qHBsAg), hepatitis B RNA (HBV RNA) and core-related antigen (HBcrAg), and shown promise in a range of clinical settings.

AIMS

To critically appraise these novel biomarkers, exploring their potential uses, availability of assays and areas for future development.

METHODS

We performed a literature search of PubMed, identifying articles published in the field of hepatitis B biomarkers between 2010 and 2022.

RESULTS

Novel biomarkers such as HBcrAg, HBV RNA and qHBsAg may be useful in predicting treatment outcomes, stratifying the risk of future complications and estimating off-treatment viral reactivation. Furthermore, HBV RNA and HBcrAg titres may accurately reflect cccDNA transcriptional activity, and this is particularly informative in the context of nucleoside analogue therapy. On a cautionary note, most studies have been performed in Caucasian or Asian populations, and methods for detecting HBV RNA lack standardisation.

CONCLUSION

Novel viral biomarkers have the potential to provide additional insights into the natural history of infection and allow a more bespoke, cost-effective framework of care. However, access remains limited, and further efforts are needed to validate their use in ethnically diverse populations, confirm predictive cut-off values, and establish their role in the era of novel antiviral therapies.

Conformational Dynamics of the Hepatitis B Virus Pre-genomic RNA on Multiple Time Scales: Implications for Viral Replication

Human hepatitis B virus (HBV) replication is initiated by the binding of the viral polymerase (P) to epsilon (ε), an ≈85-nucleotide (nt) cis-acting regulatory stem-loop RNA located at the 5'-end of the pre-genomic RNA (pgRNA). This interaction triggers P and pgRNA packaging and protein-primed reverse transcription and is therefore an attractive therapeutic target. Our recent nuclear magnetic resonance (NMR) structure of ε provides a useful starting point toward a detailed understanding of HBV replication, and hints at the functional importance of ε dynamics. Here, we present a detailed description of ε motions on the ps to ns and μs to ms time scales by NMR spin relaxation and relaxation dispersion, respectively. We also carried out molecular dynamics simulations to provide additional insight into ε conformational dynamics. These data outline a series of complex motions on multiple time scales within ε. Moreover, these motions occur in mostly conserved nucleotides from structural regions (i.e., priming loop, pseudo-triloop, and U43 bulge) that biochemical and mutational studies have shown to be essential for P binding, P-pgRNA packaging, protein-priming, and DNA synthesis. Taken together, our work implicates RNA dynamics as an integral feature that governs HBV replication.

Establishment and Characterization of a New Cell Culture System for Hepatitis B Virus Replication and Infection

Hepatitis B virus (HBV) is a primary cause of chronic liver diseases in humans. HBV infection exhibits strict host and tissue tropism. HBV core promoter (Cp) drives transcription of pregenomic RNA (pgRNA) and plays a key role in the viral life cycle. Hepatocyte nuclear factor 4α (HNF4α) acts as a major transcriptional factor that stimulates Cp. In this work, we reported that BEL7404 ​cell line displayed a high efficiency of DNA transfection and high levels of HBV antigen expression after transfection of HBV replicons without prominent viral replication. The introduction of exogenous HNF4α and human sodium taurocholate cotransporting polypeptide (hNTCP) expression into BEL7404 made it permissive for HBV replication and susceptible to HBV infection. BEL7404-derived cell lines with induced HBV permissiveness and susceptibility were constructed by stable co-transfection of hNTCP and Tet-inducible HNF4α followed by limiting dilution cloning. HBV replication in such cells was sensitive to inhibition by nucleotide analog tenofovir, while the infection was inhibited by HBV entry inhibitors. This cell culture system provides a new and additional tool for the study of HBV replication and infection as well as the characterization of antiviral agents.

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BEL7404 ​cells are characterized by a high transfection efficiency, but do not support canonical HBV replication.

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BEL7404 ​cells lack endogenous HNF4α expression, and exogenous HNF4α rescues canonical HBV replication.

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BEL7404 ​cells with stable hNTCP and inducible HNF4α expression support HBV infection and inducible replication.

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BEL7404-derived cell lines supporting HBV infection retain high transfection efficiencies and allow testing of antivirals.

Relaxing the restricted structural dynamics in the human hepatitis B virus RNA encapsidation signal enables replication initiation in vitro

Hepadnaviruses, including hepatitis B virus (HBV) as a major human pathogen, replicate their tiny 3 kb DNA genomes by capsid-internal protein-primed reverse transcription of a pregenomic (pg) RNA. Initiation requires productive binding of the viral polymerase, P protein, to a 5´ proximal bipartite stem-loop, the RNA encapsidation signal ε. Then a residue in the central ε bulge directs the covalent linkage of a complementary dNMP to a Tyr sidechain in P protein´s Terminal Protein (TP) domain. After elongation by two or three nucleotides (nt) the TP-linked DNA oligo is transferred to a 3´ proximal acceptor, enabling full-length minus-strand DNA synthesis. No direct structural data are available on hepadnaviral initiation complexes but their cell-free reconstitution with P protein and ε RNA (Dε) from duck HBV (DHBV) provided crucial mechanistic insights, including on a major conformational rearrangement in the apical Dε part. Analogous cell-free systems for human HBV led at most to P—ε binding but no detectable priming. Here we demonstrate that local relaxation of the highly basepaired ε upper stem, by mutation or via synthetic split RNAs, enables ε-dependent in vitro priming with full-length P protein from eukaryotic translation extract yet also, and without additional macromolecules, with truncated HBV miniP proteins expressed in bacteria. Using selective 2-hydroxyl acylation analyzed by primer extension (SHAPE) we confirm that upper stem destabilization correlates with in vitro priming competence and show that the supposed bulge-closing basepairs are largely unpaired even in wild-type ε. We define the two 3´ proximal nt of this extended bulge as main initiation sites and provide evidence for a Dε-like opening of the apical ε part upon P protein binding. Beyond new HBV-specific basic aspects our novel in vitro priming systems should facilitate the development of high-throughput screens for priming inhibitors targeting this highly virus-specific process.

Chronic hepatitis B virus (HBV) infection puts >250 million people at an increased risk for severe liver disease. Current treatments can control but rarely cure infection. HBV features a 3,200 bp DNA genome, generated by reverse transcription of a pregenomic (pg) RNA. To initiate DNA synthesis the viral polymerase, P protein, employs a stem-loop on pgRNA, ε, to covalently link a defined first nucleotide to its Terminal Protein (TP) domain. This protein-priming is highly virus-specific yet poorly understood. More is known for duck HBV (DHBV) where, different from HBV, protein-priming was successfully reconstituted in vitro years ago. One insight was that gaining priming-competence involves opening of the apical stem in DHBV ε RNA (Dε); in HBV ε the more extensive basepairing might restrict such dynamics. Here we relaxed these constraints by identifying functional but less stably folded, including split, HBV ε variants. Several such variants supported in vitro priming, including in a simple two-component-system employing a shortened recombinant P protein. Amongst other data the new cell-free systems yielded a first view on a major conformational change in HBV ε RNA bound to P protein, highlighting the importance of RNA dynamics for the human virus. Beyond furthering basic understanding our data should facilitate screening for protein-priming inhibitors as new anti-HBV agents.

RNA-Binding motif protein 38 (RBM38) mediates HBV pgRNA packaging into the nucleocapsid

The binding of HBV polymerase (Pol) and the epsilon stem loop (ε) on the 5' terminal region of pgRNA is required for pgRNA packaging and HBV replication. Previous research has demonstrated that RNA binding motif protein 24 (RBM24) is involved in pgRNA packaging by mediating the interaction between HBV polymerase (Pol) and the ε element. Here, we demonstrate that RBM38 interacts with ε, pol, RBM24 and HBV core which mediate pgRNA packaging. RBM38 directly binds to the lower bulge of ε via RNA recognition submotifs (RNPs) and interacts with HBV Pol in an RNA-independent manner. RBM38 interacts with RBM24 and forms heterogeneous oligomers, which mediate Pol-ε binding and the formation of the Pol-RBM38/RBM24-ε complex. More important, RBM38 also binds to the HBV core via the C-terminal region (ARD domain), which facilitates the combination of Pol-ε with the HBV core protein. In conclusion, RBM38 facilitates the Pol-ε interaction and mediates Pol-ε in combining with the HBV core, triggering pgRNA packaging for reverse transcription and DNA synthesis. This study provides new insights into pgRNA encapsidation.

DNA Engineering and Hepatitis B Virus Replication

Recombinant DNA technology is a vital method in human hepatitis B virus (HBV), producing reporter viruses or vectors for gene transferring. Researchers have engineered several genes into the HBV genome for different purposes; however, a systematic analysis of recombinant strategy is lacking. Here, using a 500-bp deletion strategy, we scanned the HBV genome and identified two regions, region I (from nt 2,118 to 2,814) and region II (from nt 99 to 1,198), suitable for engineering. Ten exogenous genes, including puromycin N-acetyl transferase gene (Pac), blasticidin S deaminase gene (BSD), Neomycin-resistance gene (Neo), Gaussia luciferase (Gluc), NanoLuc (Nluc), copGFP, mCherry, UnaG, eGFP, and tTA1, were inserted into these two regions and fused into the open reading frames of hepatitis B core protein (HBC) and hepatitis B surface protein (HBS) via T2A peptide. Recombination of 9 of the 10 genes at region 99-1198 and 5 of the 10 genes at region 2118-2814 supported the formation of relaxed circular (RC) DNA. HBV DNA and HBV RNA assays implied that exogenous genes potentially abrogate RC DNA by inducing the formation of adverse secondary structures. This hypothesis was supported because sequence optimization of the UnaG gene based on HBC sequence rescued RC DNA formation. Findings from this study provide an informative basis and a valuable method for further constructing and optimizing recombinant HBV and imply that DNA sequence might be intrinsically a potential source of selective pressure in the evolution of HBV.

DsRNA Sequencing for RNA Virus Surveillance Using Human Clinical Samples

Although viruses infect various organs and are associated with diseases, there may be many unidentified pathogenic viruses. The recent development of next-generation sequencing technologies has facilitated the establishment of an environmental viral metagenomic approach targeting the intracellular viral genome. However, an efficient method for the detection of a viral genome derived from an RNA virus in animal or human samples has not been established. Here, we established a method for the efficient detection of RNA viruses in human clinical samples. We then tested the efficiency of the method compared to other conventional methods by using tissue samples collected from 57 recipients of living donor liver transplantations performed between June 2017 and February 2019 at Kyushu University Hospital. The viral read ratio in human clinical samples was higher by the new method than by the other conventional methods. In addition, the new method correctly identified viral RNA from liver tissues infected with hepatitis C virus. This new technique will be an effective tool for intracellular RNA virus surveillance in human clinical samples and may be useful for the detection of new RNA viruses associated with diseases.

Conservation of the HBV RNA element epsilon in nackednaviruses reveals ancient origin of protein-primed reverse transcription

Hepadnaviruses, with the human hepatitis B virus as prototype, are small, enveloped hepatotropic DNA viruses which replicate by reverse transcription of an RNA intermediate. Replication is initiated by a unique protein-priming mechanism whereby a hydroxy amino acid side chain of the terminal protein (TP) domain of the viral polymerase (P) is extended into a short DNA oligonucleotide, which subsequently serves as primer for first-strand synthesis. A key component in the priming of reverse transcription is the viral RNA element epsilon, which contains the replication origin and serves as a template for DNA primer synthesis. Here, we show that recently discovered non-enveloped fish viruses, termed nackednaviruses [C. Lauber et al., Cell Host Microbe 22, 387-399 (2017)], employ a fundamentally similar replication mechanism despite their huge phylogenetic distance and major differences in genome organization and viral lifestyle. In vitro cross-priming studies revealed that few strategic nucleotide substitutions in epsilon enable site-specific protein priming by heterologous P proteins, demonstrating that epsilon is functionally conserved since the two virus families diverged more than 400 Mya. In addition, other cis elements crucial for the hepadnavirus-typical replication of pregenomic RNA into relaxed circular double-stranded DNA were identified at conserved positions in the nackednavirus genomes. Hence, the replication mode of both hepadnaviruses and nackednaviruses was already established in their Paleozoic common ancestor, making it a truly ancient and evolutionary robust principle of genome replication that is more widespread than previously thought.

Hepatitis B Virus DNA Integration: In Vitro Models for Investigating Viral Pathogenesis and Persistence

Hepatitis B virus (HBV) is a globally-distributed pathogen and is a major cause of liver disease. HBV (or closely-related animal hepadnaviruses) can integrate into the host genome, but (unlike retroviruses) this integrated form is replication-defective. The specific role(s) of the integrated HBV DNA has been a long-standing topic of debate. Novel in vitro models of HBV infection combined with sensitive molecular assays now enable researchers to investigate this under-characterised phenomenon with greater ease and precision. This review covers the contributions these systems have made to understanding how HBV DNA integration induces liver cancer and facilitates viral persistence. We summarise the current findings into a working model of chronic HBV infection and discuss the clinical implications of this hypothetical framework on the upcoming therapeutic strategies used to curb HBV-associated pathogenesis.

Hepatitis B virus biology and life cycle

Hepatitis B virus (HBV) specifically infects hepatocytes and causes severe liver diseases. The HBV life cycle is unique in that the genomic DNA (relaxed-circular partially double-stranded DNA: rcDNA) is converted to a molecular template DNA (covalently closed circular DNA: cccDNA) to amplify a viral RNA intermediate, which is then reverse-transcribed back to viral DNA. The highly stable characteristics of cccDNA result in chronic infection and a poor rate of cure. This complex life cycle of HBV offers a variety of targets to develop antiviral agents. We provide here an update on the current knowledge of HBV biology and its life cycle, which may help to identify new antiviral targets.

Establishment of a system for finding inhibitors of ε RNA binding with the HBV polymerase

Although several nucleo(s)tide analogs are available for treatment of HBV infection, long‐term treatment with these drugs can lead to the emergence of drug‐resistant viruses. Recent HIV‐1 studies suggest that combination therapies using nucleo(s)tide reverse transcriptase inhibitors (NRTIs) and non‐nucleo(s)tide reverse transcriptase inhibitors (NNRTIs) could drastically inhibit the viral genome replication of NRTI‐resistant viruses. In order to carry out such combinational therapy against HBV, several new NRTIs and NNRTIs should be developed. Here, we aimed to identify novel NNRTIs targeting the HBV polymerase terminal protein (TP)‐reverse transcriptase (RT) (TP‐RT) domain, which is a critical domain for HBV replication. We expressed and purified the HBV TP‐RT with high purity using an Escherichia coli expression system and established an in vitro ε RNA‐binding assay system. Then, we used TP‐RT in cell‐free assays to screen candidate inhibitors from a chemical compound library, and identified two compounds, 6‐hydroxy‐DL‐DOPA and N‐oleoyldopamine, which inhibited the binding of ε RNA with the HBV polymerase. Furthermore, these drugs reduced HBV DNA levels in cell‐based assays as well by inhibiting packaging of pregenome RNA into capsids. The novel screening system developed herein should open a new pathway the discovery of drugs targeting the HBV TP‐RT domain to treat HBV infection.

Hepatitis B Virus RNA Profiles in Liver Biopsies by Digital Polymerase Chain Reaction

Replication of hepatitis B virus (HBV) originates from covalently closed circular DNA (cccDNA) and involves reverse transcription of pregenomic RNA (pgRNA), which is also called core RNA and encodes the capsid protein. The RNA coding for hepatitis B surface antigen (HBsAg) in the envelope of viral or subviral particles is produced from cccDNA or from HBV DNA integrated into the host genome. Because only cccDNA can generate the core and the 3′ redundancy regions of HBV RNA, we aimed to clarify to what extent such HBV integrations are expressed by quantifying the different HBV RNA species in liver tissue. Digital droplet polymerase chain reaction (ddPCR) was employed to quantify six HBV RNA targets in 76 liver biopsies from patients with chronic infection, comprising 14 who were hepatitis B e antigen (HBeAg) positive and 62 who were HBeAg negative. In patients who were HBeAg negative, HBV RNA from the S RNA region was >1.6 log₁₀ units higher than in the core and 3′ redundancy regions (P < 0.0001), indicating that >90% of S RNA was integration derived. HBeAg‐negative samples showed 10 times lower levels of pgRNA (5′ core) compared with core RNA (3′ part of core; P < 0.0001), suggesting that a large proportion of core RNA might have a downstream shift of the transcription starting point. In multiple regression analysis, HBV DNA levels in serum were most strongly dependent on pgRNA. Conclusion: In patients who were HBeAg negative, integration‐derived S RNA seemed to predominate and a large proportion of the core RNA lacked the 5′ part. Because this part comprises the down‐regulator of transcription 1 sequences, which are necessary for virus production (plus strand translocation), the finding might help to explain the low level of HBV DNA in serum that frequently is observed in patients with chronic HBV infection who are HBeAg negative.

Hepatitis B Virus Virions Produced Under Nucleos(t)ide Analogue Treatment Are Mainly Not Infectious Because of Irreversible DNA Chain Termination

Nucleos(t)ide analogues (NAs) have been widely used for the treatment of chronic hepatitis B (CHB). Because viral DNA polymerase lacks proofreading function (3' exonuclease activity), theoretically, the incorporated NAs would irreversibly terminate viral DNA synthesis. This study explored the natures of nascent hepatitis B virus (HBV) DNA and infectivity of progeny virions produced under NA treatment. HBV infectivity was determined by infection of HepG2-NTCP cells and primary human hepatocytes (PHHs). Biochemical properties of HBV DNA in the progeny virions were investigated by qPCR, northern blotting, or Southern blotting hybridization, sucrose gradient centrifugation, and in vitro endogenous DNA polymerase assay. Progeny HBV virions produced under NA treatment were mainly not infectious to HepG2-NTCP cells or PHHs. Biochemical analysis revealed that under NA treatment, HBV DNA in nucleaocapsids or virions were predominantly short minus-strand DNA with irreversible termination. This finding was supported by the observation of first disappearance of relaxed circular DNA and then the proportional decline of HBV-DNA levels corresponding to the regions of PreC/C, S, and X genes in serial sera of patients receiving NA treatment. Conclusion: HBV virions produced under NA treatment are predominantly replication deficient because the viral genomes are truncated and elongation of DNA chains is irreversibly terminated. Clinically, our results suggest that the viral loads of CHB patients under NA therapy vary with the different regions of genome being detected by qPCR assays. Our findings also imply that NA prevention of perinatal and sexual HBV transmission as well as infection of transplanted livers works not only by reducing viral loads, but also by producing noninfectious virions.

Identification and characterization of a novel hepatitis B virus pregenomic RNA encapsidation inhibitor

Currently, therapies to treat chronic hepatitis B (CHB) infection are based on the use of interferon-α or nucleos(t)ide analogs (NAs) to prevent viral DNA synthesis by inhibiting the reverse transcriptase activity of the hepatitis B virus (HBV) polymerase (Pol). However, these therapies are not curative; thus, the development of novel anti-HBV agents is needed. In accordance with this unmet medical need, we devised a new target- and cell-based, high-throughput screening assay to identify novel small molecules that block the initial interaction of the HBV Pol with its replication template the viral pregenomic RNA (pgRNA). We screened approximately 110,000 small molecules for the ability to prevent HBV Pol recognition of the pgRNA 5' epsilon (ε) stem-loop structure, identifying (Z)-2-(allylamino)-4-amino-N'-cyanothiazole-5-carboximidamide (AACC). Viral nucleocapsid-captured quantitative RT-PCR and Western blot results revealed that AACC significantly decreased encapsidated pgRNA levels and blocked capsid assembly without affecting core protein expression in stable HBV-replicating cells. As a result, both intra- and extracellular accumulation of viral DNA was strongly reduced. AACC treatment of HepG2-sodium taurocholate transporting polypeptide (NTCP) cells and primary human hepatocytes infected with cell culture- or patient-derived HBV isolates showed both time- and dose-dependent inhibition of infectious viral progeny and rcDNA production. Furthermore, AACC showed cross-genotypic activity against genotypes B, C, and D. Of note, AACC inhibited the viral replication of lamivudine and a capsid inhibitor-resistant HBV, and showed synergistic effects with NAs and a capsid inhibitor. In conclusion, we identified a novel class of compounds specifically targeting the ε-Pol interaction and thereby preventing the encapsidation of pgRNAs into viral capsids. This promising new HBV inhibitor class potently inhibits HBV amplification with distinct characteristics from existing NAs and other drugs currently under development, promising to add value to existing therapies for CHB.

RNA-Binding Motif Protein 24 (RBM24) Is Involved in Pregenomic RNA Packaging by Mediating Interaction between Hepatitis B Virus Polymerase and the Epsilon Element

Encapsidation of pregenomic RNA (pgRNA) is a crucial step in hepatitis B virus (HBV) replication. Binding by viral polymerase (Pol) to the epsilon stem-loop (ε) on the 5'-terminal region (TR) of pgRNA is required for pgRNA packaging. However, the detailed mechanism is not well understood. RNA-binding motif protein 24 (RBM24) inhibits core translation by binding to the 5'-TR of pgRNA. Here, we demonstrate that RBM24 is also involved in pgRNA packaging. RBM24 directly binds to the lower bulge of ε via RNA recognition submotifs (RNPs). RBM24 also interacts with Pol in an RNA-independent manner. The alanine-rich domain (ARD) of RBM24 and the reverse transcriptase (RT) domain of Pol are essential for binding between RBM24 and Pol. In addition, overexpression of RBM24 increases Pol-ε interaction, whereas RBM24 knockdown decreases the interaction. RBM24 was able to rescue binding between ε and mutant Pol lacking ε-binding activity, further showing that RBM24 mediates the interaction between Pol and ε by forming a Pol-RBM24-ε complex. Finally, RBM24 significantly promotes the packaging efficiency of pgRNA. In conclusion, RBM24 mediates Pol-ε interaction and formation of a Pol-RBM24-ε complex, which inhibits translation of pgRNA and results in pgRNA packing into capsids/virions for reverse transcription and DNA synthesis.IMPORTANCE Hepatitis B virus (HBV) is a ubiquitous human pathogen, and HBV infection is a major global health burden. Chronic HBV infection is associated with the development of liver diseases, including fulminant hepatitis, hepatic fibrosis, cirrhosis, and hepatocellular carcinoma. A currently approved vaccine can prevent HBV infection, and medications are able to reduce viral loads and prevent liver disease progression. However, current treatments rarely achieve a cure for chronic infection. Thus, it is important to gain insight into the mechanisms of HBV replication. In this study, we found that the host factor RBM24 is involved in pregenomic RNA (pgRNA) packaging and regulates HBV replication. These findings highlight a potential target for antiviral therapeutics of HBV infection.

Mouse models for hepatitis B virus research

Hepatitis B virus (HBV) infection remains a major global health problem; indeed, there are 250 million carriers worldwide. The host range of HBV is narrow; therefore, few primates are susceptible to HBV infection. However, ethical constraints, high cost, and large size limit the use of primates as suitable animal models. Thus, in vivo testing of therapies that target HBV has been hampered by the lack of an appropriate in vivo research model. To address this, mouse model systems of HBV are being developed and several are used for studying HBV in vivo. In this review, we summarize the currently available mouse models, including HBV transgenic mice, hydrodynamic injection-mediated HBV replicon delivery systems, adeno-associated virus-mediated HBV replicon delivery systems, and human liver chimeric mouse models. These developed (or being developed) mouse model systems are promising and should be useful tools for studying HBV.

Hepatitis B virus: virology, molecular biology, life cycle and intrahepatic spread

Hepatitis B virus is a member of the Hepadnaviridae family and responsible for causing acute and chronic hepatitis in humans. The current estimates of people chronically infected with the virus are put at 250 million worldwide. Immune-mediated liver damage in these individuals may lead to the development of cirrhosis and hepatocellular carcinoma later in life. This review deals with our current understanding of the virology, molecular biology, life cycle and cell-to-cell spread of this very important pathogen, all of which are considered essential for current and future approaches to antiviral treatment.

Few basepairing-independent motifs in the apical half of the avian HBV ε RNA stem-loop determine site-specific initiation of protein-priming

Hepadnaviruses, including human hepatitis B virus (HBV), replicate their tiny DNA genomes by protein-primed reverse transcription of a pregenomic (pg) RNA. Replication initiation as well as pgRNA encapsidation depend on the interaction of the viral polymerase, P protein, with the ε RNA element, featuring a lower and an upper stem, a central bulge, and an apical loop. The bulge, somehow assisted by the loop, acts as template for a P protein-linked DNA oligo that primes full-length minus-strand DNA synthesis. Phylogenetic conservation and earlier mutational studies suggested the highly based-paired ε structure as crucial for productive interaction with P protein. Using the tractable duck HBV (DHBV) model we here interrogated the entire apical DHBV ε (Dε) half for sequence- and structure-dependent determinants of in vitro priming activity, replication, and, in part, in vivo infectivity. This revealed single-strandedness of the bulge, a following G residue plus the loop subsequence GUUGU as the few key determinants for priming and initiation site selection; unexpectedly, they functioned independently of a specific structure context. These data provide new mechanistic insights into avihepadnaviral replication initiation, and they imply a new concept towards a feasible in vitro priming system for human HBV.

Expression of wild-type or G1862T mutant HBe antigen of subgenotype A1 of hepatitis B virus and the unfolded protein response in Huh7 cells

The G1862T mutation, which occurs most frequently in subgenotype A1 of the hepatitis B virus (HBV), results in a valine to phenylalanine substitution at the -3 position of the signal peptide cleavage site at the amino end of the precore/core (preC/C) precursor protein. The objective of this study was to functionally characterize the G1862T mutation relative to its wild-type counterpart in subgenotype A1. Huh7 cells were transfected with subgenotype A1 replication-competent plasmids, with and without G1862T. Secretion of HBsAg and HBeAg, preC/C/HBeAg expression in the secretory pathway, activation of the unfolded protein response (UPR) and subsequent activation of apoptosis were monitored. The introduction of G1862T did not affect HBsAg expression. Cells transfected with the G1862T subgenotype A1 plasmid showed decreased expression of intracellular HBcAg and of nuclear preC/C/HBeAg and extracellular HBeAg, when compared to cells transfected with its wild-type counterpart as a result of the accumulation of the mutant protein in the endoplasmic reticulum (ER) and ER-Golgi intermediate compartment (ERGIC) . This accumulation of preC/C/HBeAg protein in the ER led to the earlier activation of the three UPR pathways, but not to an increase in apoptosis. Therefore, it is evident that the presence of G1862T in subgenotype A1 does not completely abolish HBeAg expression, but affects the rate of HBeAg maturation, its passage through the secretory pathway and activation of the UPR. Increase in ER stress can result in liver damage, which has been shown to be a contributing factor to hepatocarcinogenesis and may explain why G1862T is frequently found in subgenotype A1 from liver disease patients.

Hepatitis B virus molecular biology and pathogenesis

As obligate intracellular parasites, viruses need a host cell to provide a milieu favorable to viral replication. Consequently, viruses often adopt mechanisms to subvert host cellular signaling processes. While beneficial for the viral replication cycle, virus-induced deregulation of host cellular signaling processes can be detrimental to host cell physiology and can lead to virus-associated pathogenesis, including, for oncogenic viruses, cell transformation and cancer progression. Included among these oncogenic viruses is the hepatitis B virus (HBV). Despite the availability of an HBV vaccine, 350-500 million people worldwide are chronically infected with HBV, and a significant number of these chronically infected individuals will develop hepatocellular carcinoma (HCC). Epidemiological studies indicate that chronic infection with HBV is the leading risk factor for the development of HCC. Globally, HCC is the second highest cause of cancer-associated deaths, underscoring the need for understanding mechanisms that regulate HBV replication and the development of HBV-associated HCC. HBV is the prototype member of the Hepadnaviridae family; members of this family of viruses have a narrow host range and predominately infect hepatocytes in their respective hosts. The extremely small and compact hepadnaviral genome, the unique arrangement of open reading frames, and a replication strategy utilizing reverse transcription of an RNA intermediate to generate the DNA genome are distinguishing features of the Hepadnaviridae. In this review, we provide a comprehensive description of HBV biology, summarize the model systems used for studying HBV infections, and highlight potential mechanisms that link a chronic HBV-infection to the development of HCC. For example, the HBV X protein (HBx), a key regulatory HBV protein that is important for HBV replication, is thought to play a cofactor role in the development of HBV-induced HCC, and we highlight the functions of HBx that may contribute to the development of HBV-associated HCC.

Hepatitis B virus reverse transcriptase - Target of current antiviral therapy and future drug development

Hepatitis B virus (HBV) infections rely on the proper functioning of the viral polymerase enzyme, a specialized reverse transcriptase (RT) with multiple activities. All currently approved antiviral drugs for the treatment of chronic HBV infection, except for interferon, target the RT and belong to the same chemical class - they are all nucleoside analogs. Viral DNA synthesis is carried out by the RT enzyme in several different steps, each with distinct RT conformational requirements. In principle, each stage may be targeted by distinct antiviral drugs. In particular, the HBV RT has the unique ability to initiate viral DNA synthesis using itself as a protein primer in a novel protein priming reaction. In order to help identify RT inhibitors and study their mechanisms of action, a number of experimental systems have been developed, each varying in its ability to dissect the protein priming stage and subsequent stages of viral DNA synthesis at the molecular level. Two of the most effective drugs to date, entecavir and tenofovir, can inhibit both the protein priming and the subsequent DNA elongation stages of HBV DNA synthesis. Interestingly, clevudine, a thymidine analog, can inhibit both protein priming and DNA elongation in a non-competitive manner and without being incorporated into the viral DNA. Thus, a nucleoside RT inhibitor (NRTI) can functionally mimic a non-NRTI (NNRTI) in its inhibition of the HBV RT. Therefore, novel NRTIs as well as NNRTIs may be developed to inhibit the DNA synthesis activity of the HBV RT. Furthermore, additional activities of the RT that are also essential to HBV replication, including specific recognition of the viral RNA and its packaging into viral nucleocapsids, may be exploited for antiviral development. To achieve a more potent inhibition of viral replication and ultimately cure chronic HBV infection, the next generation of anti-HBV therapies will likely need to include NRTIs, NNRTIs, and other agents that target the viral RT as well as other viral and host factors in various combinations. This article forms part of a symposium in Antiviral Research on "An unfinished story: from the discovery of the Australia antigen to the development of new curative therapies for hepatitis B."

Hepadnavirus Genome Replication and Persistence

Hallmarks of the hepadnavirus replication cycle are the formation of covalently closed circular DNA (cccDNA) and the reverse transcription of a pregenomic RNA (pgRNA) in core particles leading to synthesis of the relaxed circular DNA (rcDNA) genome. cccDNA, the template for viral RNA transcription, is the basis for the persistence of these viruses in infected hepatocytes. In this review, we summarize the current state of knowledge on the mechanisms of hepadnavirus reverse transcription and the biochemical and structural properties of the viral reverse transcriptase (RT). We highlight important gaps in knowledge regarding cccDNA biosynthesis and stability. In addition, we discuss the impact of current antiviral therapies on viral persistence, particularly on cccDNA.

Core protein: A pleiotropic keystone in the HBV lifecycle

Hepatitis B Virus (HBV) is a small virus whose genome has only four open reading frames. We argue that the simplicity of the virion correlates with a complexity of functions for viral proteins. We focus on the HBV core protein (Cp), a small (183 residue) protein that self-assembles to form the viral capsid. However, its functions are a little more complicated than that. In an infected cell Cp modulates almost every step of the viral lifecycle. Cp is bound to nuclear viral DNA and affects its epigenetics. Cp correlates with RNA specificity. Cp assembles specifically on a reverse transcriptase-viral RNA complex or, apparently, nothing at all. Indeed Cp has been one of the model systems for investigation of virus self-assembly. Cp participates in regulation of reverse transcription. Cp signals completion of reverse transcription to support virus secretion. Cp carries both nuclear localization signals and HBV surface antigen (HBsAg) binding sites; both of these functions appear to be regulated by contents of the capsid. Cp can be targeted by antivirals - while self-assembly is the most accessible of Cp activities, we argue that it makes sense to engage the broader spectrum of Cp function. This article forms part of a symposium in Antiviral Research on "From the discovery of the Australia antigen to the development of new curative therapies for hepatitis B: an unfinished story."

Entecavir allows an unexpectedly high residual replication of HBV mutants resistant to lamivudine

BACKGROUND

Entecavir is an efficient inhibitor of HBV reverse transcriptase (RT) and widely used for therapy of chronic hepatitis B. Entecavir treatment of HBV patients with lamivudine-resistant viral strains, however, often fails, but the mechanism of cross-resistance development is not fully understood.

METHODS

Using non-linear regression models, dose-response curves of cloned HBV strains from patients pre-treated with RT inhibitors were established in human hepatoma cell lines after transfection with HBV genomes containing HBV polymerase genes from patient isolates. 50% and 90% inhibitory concentrations (IC50 and IC90) and corresponding antiviral resistance factors (RF50 and RF90) were calculated.

RESULTS

The entecavir dose-response curve of lamivudine-resistant HBV RT mutants rtM204 for the replication of HBV decreased less than expected with increasing drug dose. Remarkably, due to the flat dose-response curves, RF90 values against entecavir of samples with rtM204 substitutions were up to 30× higher than their RF50 values.

CONCLUSIONS

The unexpectedly high IC90 indicates a strong residual replication capacity of lamivudine-resistant HBV rtM204 variants under entecavir therapy, although IC50 values are initially within the therapeutic range of entecavir. This characteristic favours rapid selection of additional mutants with overt resistance against entecavir. Thus, the current phenotypic resistance assays should include determination of IC90.

Early mesozoic coexistence of amniotes and hepadnaviridae

Hepadnaviridae are double-stranded DNA viruses that infect some species of birds and mammals. This includes humans, where hepatitis B viruses (HBVs) are prevalent pathogens in considerable parts of the global population. Recently, endogenized sequences of HBVs (eHBVs) have been discovered in bird genomes where they constitute direct evidence for the coexistence of these viruses and their hosts from the late Mesozoic until present. Nevertheless, virtually nothing is known about the ancient host range of this virus family in other animals. Here we report the first eHBVs from crocodilian, snake, and turtle genomes, including a turtle eHBV that endogenized >207 million years ago. This genomic “fossil” is >125 million years older than the oldest avian eHBV and provides the first direct evidence that Hepadnaviridae already existed during the Early Mesozoic. This implies that the Mesozoic fossil record of HBV infection spans three of the five major groups of land vertebrates, namely birds, crocodilians, and turtles. We show that the deep phylogenetic relationships of HBVs are largely congruent with the deep phylogeny of their amniote hosts, which suggests an ancient amniote–HBV coexistence and codivergence, at least since the Early Mesozoic. Notably, the organization of overlapping genes as well as the structure of elements involved in viral replication has remained highly conserved among HBVs along that time span, except for the presence of the X gene. We provide multiple lines of evidence that the tumor-promoting X protein of mammalian HBVs lacks a homolog in all other hepadnaviruses and propose a novel scenario for the emergence of X via segmental duplication and overprinting of pre-existing reading frames in the ancestor of mammalian HBVs. Our study reveals an unforeseen host range of prehistoric HBVs and provides novel insights into the genome evolution of hepadnaviruses throughout their long-lasting association with amniote hosts.

Viruses are not known to leave physical fossil traces, which makes our understanding of their evolutionary prehistory crucially dependent on the detection of endogenous viruses. Ancient endogenous viruses, also known as paleoviruses, are relics of viral genomes or fragments thereof that once infiltrated their host's germline and then remained as molecular “fossils” within the host genome. The massive genome sequencing of recent years has unearthed vast numbers of paleoviruses from various animal genomes, including the first endogenous hepatitis B viruses (eHBVs) in bird genomes. We screened genomes of land vertebrates (amniotes) for the presence of paleoviruses and identified ancient eHBVs in the recently sequenced genomes of crocodilians, snakes, and turtles. We report an eHBV that is >207 million years old, making it the oldest endogenous virus currently known. Furthermore, our results provide direct evidence that the Hepadnaviridae virus family infected birds, crocodilians and turtles during the Mesozoic Era, and suggest a long-lasting coexistence of these viruses and their amniote hosts at least since the Early Mesozoic. We challenge previous views on the origin of the oncogenic X gene and provide an evolutionary explanation as to why only mammalian hepatitis B infection leads to hepatocellular carcinoma.

Involvement of the host DNA-repair enzyme TDP2 in formation of the covalently closed circular DNA persistence reservoir of hepatitis B viruses

Hepatitis B virus (HBV), the causative agent of chronic hepatitis B and prototypic hepadnavirus, is a small DNA virus that replicates by protein-primed reverse transcription. The product is a 3-kb relaxed circular DNA (RC-DNA) in which one strand is linked to the viral polymerase (P protein) through a tyrosyl-DNA phosphodiester bond. Upon infection, the incoming RC-DNA is converted into covalently closed circular (ccc) DNA, which serves as a viral persistence reservoir that is refractory to current anti-HBV treatments. The mechanism of cccDNA formation is unknown, but the release of P protein is one mandatory step. Structural similarities between RC-DNA and cellular topoisomerase-DNA adducts and their known repair by tyrosyl-DNA-phosphodiesterase (TDP) 1 or TDP2 suggested that HBV may usurp these enzymes for its own purpose. Here we demonstrate that human and chicken TDP2, but only the yeast ortholog of TDP1, can specifically cleave the Tyr-DNA bond in virus-adapted model substrates and release P protein from authentic HBV and duck HBV (DHBV) RC-DNA in vitro, without prior proteolysis of the large P proteins. Consistent with TPD2's having a physiological role in cccDNA formation, RNAi-mediated TDP2 depletion in human cells significantly slowed the conversion of RC-DNA to cccDNA. Ectopic TDP2 expression in the same cells restored faster conversion kinetics. These data strongly suggest that TDP2 is a first, although likely not the only, host DNA-repair factor involved in HBV cccDNA biogenesis. In addition to establishing a functional link between hepadnaviruses and DNA repair, our results open new prospects for directly targeting HBV persistence.

Phosphoacceptors threonine 162 and serines 170 and 178 within the carboxyl-terminal RRRS/T motif of the hepatitis B virus core protein make multiple contributions to hepatitis B virus replication

Phosphorylation of serines 157, 164, and 172 within the carboxyl-terminal SPRRR motif of the hepatitis B virus (HBV) core (C) protein modulates HBV replication at multiple stages. Threonine 162 and serines 170 and 178, located within the carboxyl-terminal conserved RRRS/T motif of HBV C protein, have been proposed to be protein kinase A phosphorylation sites. However, in vivo phosphorylation of these residues has never been observed, and their contribution to HBV replication remains unknown. In this study, [(32)P]orthophosphate labeling of cells expressing C proteins followed by immunoprecipitation with anti-HBc antibody revealed that threonine 162 and serines 170 and 178 are phosphoacceptor residues. A triple-alanine-substituted mutant, mimicking dephosphorylation of all three residues, drastically decreased pregenomic RNA (pgRNA) encapsidation, thereby decreasing HBV DNA synthesis. In contrast, a triple-glutamate-substituted mutant, mimicking phosphorylation of these residues, decreased DNA synthesis without significantly decreasing encapsidation. Neither triple mutant affected C protein expression or core particle assembly. Individual alanine substitution of threonine 162 significantly decreased minus-strand, plus-strand, and relaxed-circular DNA synthesis, demonstrating that this residue plays multiple roles in HBV DNA synthesis. Double-alanine substitution of serines 170 and 178 reduced HBV replication at multiple stages, indicating that these residues also contribute to HBV replication. Thus, in addition to serines 157, 164, and 172, threonine 162 and serines 170 and 178 of HBV C protein are also phosphorylated in cells, and phosphorylation and dephosphorylation of these residues play multiple roles in modulation of HBV replication.

IMPORTANCE

Threonine 162, within the carboxyl-terminal end of the hepatitis B virus (HBV adw) core (C) protein, has long been ignored as a phosphoacceptor, even though it is highly conserved among mammalian hepadnaviruses and in the overlapping consensus RxxS/T, RRxS/T, and TP motifs. Here we show, for the first time, that in addition to the well-known phosphoacceptor serines 157, 164, and 172 in SPRRR motifs, threonine 162 and serines 170 and 178 in the RRRS/T motif are phosphorylated in cells. We also show that, like serines 157, 164, and 172, phosphorylated and dephosphorylated threonine 162 and serines 170 and 178 contribute to multiple steps of HBV replication, including pgRNA encapsidation, minus-strand and plus-strand DNA synthesis, and relaxed-circular DNA synthesis. Of these residues, threonine 162 is the most important. Furthermore, we show that phosphorylation of C protein is required for efficient completion of HBV replication.

Nucleic acid chaperone activity associated with the arginine-rich domain of human hepatitis B virus core protein

Hepatitis B virus (HBV) DNA replication occurs within the HBV icosahedral core particles. HBV core protein (HBc) contains an arginine-rich domain (ARD) at its carboxyl terminus. This ARD domain of HBc 149-183 is known to be important for viral replication but not known to have a structure. Recently, nucleocapsid proteins of several viruses have been shown to contain nucleic acid chaperone activity, which can facilitate structural rearrangement of viral genome. Major features of nucleic acid chaperones include highly basic amino acid residues and flexible protein structure. To test the nucleic acid chaperone hypothesis for HBc ARD, we first used the disassembled full-length HBc from Escherichia coli to analyze the nucleic acid annealing and strand displacement activities. To exclude the potential contamination of chaperones from E. coli, we designed synthetic HBc ARD peptides with different lengths and serine phosphorylations. We demonstrated that HBc ARD peptide can behave like a bona fide nucleic acid chaperone and that the chaperone activity depends on basic residues of the ARD domain. The loss of chaperone activity by arginine-to-alanine substitutions in the ARD can be rescued by restoring basic residues in the ARD. Furthermore, the chaperone activity is subject to regulation by phosphorylation and dephosphorylation at the HBc ARD. Interestingly, the HBc ARD can enhance in vitro cleavage activity of RNA substrate by a hammerhead ribozyme. We discuss here the potential significance of the HBc ARD chaperone activity in the context of viral DNA replication, in particular, at the steps of primer translocations and circularization of linear replicative intermediates.

IMPORTANCE

Hepatitis B virus is a major human pathogen. At present, no effective treatment can completely eradicate the virus from patients with chronic hepatitis B. We report here a novel chaperone activity associated with the viral core protein. Our discovery could lead to a new drug design for more effective treatment against hepatitis B virus in the future.

Comparative analysis of hepatitis B virus polymerase sequences required for viral RNA binding, RNA packaging, and protein priming

Hepatitis B virus replicates a DNA genome through reverse transcription of a pregenomic RNA (pgRNA) by using a multifunctional polymerase (HP). A critical function of HP is its specific association with a viral RNA signal, termed ε (Hε), located on pgRNA, which is required for specific packaging of pgRNA into viral nucleocapsids and initiation of viral reverse transcription. HP initiates reverse transcription by using itself as a protein primer (protein priming) and Hε as the obligatory template. HP is made up of four domains, including the terminal protein (TP), the spacer, the reverse transcriptase (RT), and the RNase H domains. A recently developed, Hε-dependent, in vitro protein priming assay was used in this study to demonstrate that almost the entire TP and RT domains and most of the RNase H domain were required for protein priming. Specific residues within TP, RT, and the spacer were identified as being critical for HP-Hε binding and/or protein priming. Comparison of HP sequence requirements for Hε binding, pgRNA packaging, and protein priming allowed the classification of the HP mutants into five groups, each with distinct effects on these complex and related processes. Detailed characterization of HP requirements for these related and essential functions of HP will further elucidate the mechanisms of its multiple functions and aid in the targeting of these functions for antiviral therapy.

Evidence for multiple distinct interactions between hepatitis B virus P protein and its cognate RNA encapsidation signal during initiation of reverse transcription

Replication of hepatitis B virus (HBV) via protein-primed reverse transcription is initiated by binding of the viral P protein to the conserved ε stem-loop on the pregenomic (pg) RNA. This triggers encapsidation of the complex and the ε-templated synthesis of a short P protein-linked DNA oligonucleotide (priming) for subsequent minus-strand DNA extension. ε consists of a lower and upper stem, a bulge containing the priming template, and an apical loop. The nonhelical subelements are considered important for DNA synthesis and pgRNA packaging whereas the role of the upper stem is not well characterized. Priming itself could until recently not be addressed because in vitro generated HBV P - ε complexes showed no activity. Focussing on the four A residues at the base and tip of the upper ε stem and the two U residues in the loop we first investigated the impact of 24 mutations on viral DNA accumulation in transfected cells. While surprisingly many mutations were tolerated, further analyzing the negatively acting mutations, including in a new cell-free priming system, revealed divergent position-related impacts on pgRNA packaging, priming activity and possibly initiation site selection. This genetic separability implies that the ε RNA undergoes multiple distinct interactions with P protein as pgRNA encapsidation and replication initiation progress, and that the strict conservation of ε in nature may reflect its optimal adaptation to comply with all of them. The data further define the most attractive mutants for future studies, including as decoys for interference with HBV replication.

Noncompetitive inhibition of hepatitis B virus reverse transcriptase protein priming and DNA synthesis by the nucleoside analog clevudine

All currently approved antiviral drugs for the treatment of chronic hepatitis B virus (HBV) infection are nucleos(t)ide reverse transcriptase inhibitors (NRTI), which inhibit the DNA synthesis activity of the HBV polymerase. The polymerase is a unique reverse transcriptase (RT) that has a novel protein priming activity in which HP initiates viral DNA synthesis using itself as a protein primer. We have determined the ability of NRTI-triphosphates (TP) to inhibit HBV protein priming and their mechanisms of action. While entecavir-TP (a dGTP analog) inhibited protein priming initiated specifically with dGTP, clevudine-TP (a TTP analog) was able to inhibit protein priming independently of the deoxynucleoside triphosphate (dNTP) substrate and without being incorporated into DNA. We next investigated the effect of NRTIs on the second stage of protein priming, wherein two dAMP nucleotides are added to the initial deoxyguanosine nucleotide. The obtained results indicated that clevudine-TP as well as tenofovir DF-DP strongly inhibited the second stage of protein priming. Tenofovir DF-DP was incorporated into the viral DNA primer, whereas clevudine-TP inhibited the second stage of priming without being incorporated. Finally, kinetic analyses using the HBV endogenous polymerase assay revealed that clevudine-TP inhibited DNA chain elongation by HP in a noncompetitive manner. Thus, clevudine-TP appears to have the unique ability to inhibit HBV RT via binding to and distorting the HP active site, sharing properties with both NRTIs and nonnucleoside RT inhibitors.

Protein-primed terminal transferase activity of hepatitis B virus polymerase

Hepatitis B virus (HBV) replication requires reverse transcription of an RNA pregenome (pgRNA) by a multifunctional polymerase (HP). HP initiates viral DNA synthesis by using itself as a protein primer and an RNA signal on pgRNA, termed epsilon (Hε), as the obligatory template. We discovered a Mn(2+)-dependent transferase activity of HP in vitro that was independent of Hε but also used HP as a protein primer. This protein-primed transferase activity was completely dependent on the HP polymerase active site. The DNA products of the transferase reaction were linked to HP via a phosphotyrosyl bond, and replacement of the Y63 residue of HP, the priming site for templated DNA synthesis, almost completely eliminated DNA synthesis by the transferase activity, suggesting that Y63 also serves as the predominant priming site for the transferase reaction. For this transferase activity, HP could use all four deoxynucleotide substrates, but TTP was clearly favored for extensive polymerization. The transferase activity was highly distributive, leading to the synthesis of DNA homo- and hetero-oligomeric and -polymeric ladders ranging from 1 nucleotide (nt) to >100 nt in length, with single-nt increments. As with Hε-templated DNA synthesis, the protein-primed transferase reaction was characterized by an initial stage that was resistant to the pyrophosphate analog phosphonoformic acid (PFA) followed by PFA-sensitive DNA synthesis, suggestive of an HP conformational change upon the synthesis of a nascent DNA oligomer. These findings have important implications for HBV replication, pathogenesis, and therapy.

Molecular biology of the hepatitis B virus for clinicians

Hepatitis B virus (HBV) infection is one of the major global health problems, especially in economically under-developed or developing countries. HBV infection can lead to a number of clinical outcomes including chronic infection, cirrhosis and liver cancer. It ranks among the top 10 causes of death, being responsible for around 1 million deaths every year. Despite the availability of a highly efficient vaccine and potent antiviral agents, HBV infection still remains a significant clinical problem, particularly in those high endemicity areas where vaccination of large populations has not been possible due to economic reasons. Although HBV is among the smallest viruses in terms of virion and genome size, it has numerous unique features that make it completely distinct from other DNA viruses. It has a partially double stranded DNA with highly complex genome organization, life cycle and natural history. Remarkably distinct from other DNA viruses, it uses an RNA intermediate called pregenomic RNA (pgRNA) and reverse transcriptase for its genome replication. Genome replication is accomplished by a complex mechanism of primer shifting facilitated by direct repeat sequences encoded in the genome. Further, the genome has evolved in such a manner that every single nucleotide of the genome is used for either coding viral proteins or used as regulatory regions or both. Moreover, it utilizes internal in-frame translation initiation codons, as well as different reading frames from the same RNA to generate different proteins with diverse functions. HBV also shows considerable genetic variability which has been related with clinical outcomes, replication potential, therapeutic response etc. This review aims at reviewing fundamental events of the viral life cycle including viral replication, transcription and translation, from the molecular standpoint, as well as, highlights the clinical relevance of genetic variability of HBV.

Extensive mutagenesis of the conserved box E motif in duck hepatitis B virus P protein reveals multiple functions in replication and a common structure with the primer grip in HIV-1 reverse transcriptase

Hepadnaviruses, including the pathogenic hepatitis B virus (HBV), replicate their small DNA genomes through protein-primed reverse transcription, mediated by the terminal protein (TP) domain in their P proteins and an RNA stem-loop, ε, on the pregenomic RNA (pgRNA). No direct structural data are available for P proteins, but their reverse transcriptase (RT) domains contain motifs that are conserved in all RTs (box A to box G), implying a similar architecture; however, experimental support for this notion is limited. Exploiting assays available for duck HBV (DHBV) but not the HBV P protein, we assessed the functional consequences of numerous mutations in box E, which forms the DNA primer grip in human immunodeficiency virus type 1 (HIV-1) RT. This substructure coordinates primer 3'-end positioning and RT subdomain movements during the polymerization cycle and is a prime target for nonnucleosidic RT inhibitors (NNRTIs) of HIV-1 RT. Box E was indeed critical for DHBV replication, with the mutations affecting the folding, ε RNA interactions, and polymerase activity of the P protein in a position- and amino acid side chain-dependent fashion similar to that of HIV-1 RT. Structural similarity to HIV-1 RT was underlined by molecular modeling and was confirmed by the replication activity of chimeric P proteins carrying box E, or even box C to box E, from HIV-1 RT. Hence, box E in the DHBV P protein and likely the HBV P protein forms a primer grip-like structure that may provide a new target for anti-HBV NNRTIs.

TP-RT domain interactions of duck hepatitis B virus reverse transcriptase in cis and in trans during protein-primed initiation of DNA synthesis in vitro

The hepadnavirus reverse transcriptase (RT) has the unique ability to initiate viral DNA synthesis using RT itself as a protein primer. Protein priming requires complex interactions between the N-terminal TP (terminal protein) domain, where the primer (a specific Y residue) resides, and the central RT domain, which harbors the polymerase active site. While it normally utilizes the cis-linked TP to prime DNA synthesis (cis-priming), we found that the duck hepatitis B virus (DHBV) RT domain, in the context of the full-length RT protein or a mini-RT construct containing only truncated TP and RT domains, could additionally use a separate TP or RT domain in trans as a primer (trans-priming). trans interaction could also be demonstrated by the inhibitory effect (trans-inhibition) on cis-priming by TP and RT domain sequences provided in trans. Protein priming was further shown to induce RT conformational changes that resulted in TP-RT domain dissociation, altered priming site selection, and a gain of sensitivity to a pyrophosphate analog inhibitor. trans-priming, trans-inhibition, and trans-complementation, which requires separate TP and RT domains to reconstitute a functional RT protein, were employed to define the sequences in the TP and RT domains that could mediate physical or functional inter- and intradomain interactions. These results provide new insights into TP-RT domain interactions and conformational dynamics during protein priming and suggest novel means to inhibit protein priming by targeting these interactions and the associated conformational transitions.

In vitro epsilon RNA-dependent protein priming activity of human hepatitis B virus polymerase

Hepatitis B virus (HBV) replicates its DNA genome through reverse transcription of a pregenomic RNA (pgRNA) by using a multifunctional polymerase (HP). A critical function of HP is its specific recognition of a viral RNA signal termed ε (Hε) located on pgRNA, which is required for specific packaging of pgRNA into viral nucleocapsids and initiation of viral reverse transcription. HP initiates reverse transcription by using itself as a protein primer (protein priming) and Hε as the obligatory template. We have purified HP from human cells that retained Hε binding activity in vitro. Furthermore, HP purified as a complex with Hε, but not HP alone, displayed in vitro protein priming activity. While the HP-Hε interaction in vitro and in vivo required the Hε internal bulge, but not its apical loop, and was not significantly affected by the cap-Hε distance, protein priming required both the Hε apical loop and internal bulge, as well as a short distance between the cap and Hε, mirroring the requirements for RNA packaging. These studies have thus established new HBV protein priming and RNA binding assays that should greatly facilitate the dissection of the requirements and molecular mechanisms of HP-Hε interactions, RNA packaging, and protein priming.

A SELEX-screened aptamer of human hepatitis B virus RNA encapsidation signal suppresses viral replication

BACKGROUND

The specific interaction between hepatitis B virus (HBV) polymerase (P protein) and the ε RNA stem-loop on pregenomic (pg) RNA is crucial for viral replication. It triggers both pgRNA packaging and reverse transcription and thus represents an attractive antiviral target. RNA decoys mimicking ε in P protein binding but not supporting replication might represent novel HBV inhibitors. However, because generation of recombinant enzymatically active HBV polymerase is notoriously difficult, such decoys have as yet not been identified.

METHODOLOGY/PRINCIPAL FINDINGS

Here we used a SELEX approach, based on a new in vitro reconstitution system exploiting a recombinant truncated HBV P protein (miniP), to identify potential ε decoys in two large ε RNA pools with randomized upper stem. Selection of strongly P protein binding RNAs correlated with an unexpected strong enrichment of A residues. Two aptamers, S6 and S9, displayed particularly high affinity and specificity for miniP in vitro, yet did not support viral replication when part of a complete HBV genome. Introducing S9 RNA into transiently HBV producing HepG2 cells strongly suppressed pgRNA packaging and DNA synthesis, indicating the S9 RNA can indeed act as an ε decoy that competitively inhibits P protein binding to the authentic ε signal on pgRNA.

CONCLUSIONS/SIGNIFICANCE

This study demonstrates the first successful identification of human HBV ε aptamers by an in vitro SELEX approach. Effective suppression of HBV replication by the S9 aptamer provides proof-of-principle for the ability of ε decoy RNAs to interfere with viral P-ε complex formation and suggests that S9-like RNAs may further be developed into useful therapeutics against chronic hepatitis B.

A high level of mutation tolerance in the multifunctional sequence encoding the RNA encapsidation signal of an avian hepatitis B virus and slow evolution rate revealed by in vivo infection

In all hepadnaviruses, protein-primed reverse transcription of the pregenomic RNA (pgRNA) is initiated by binding of the viral polymerase, P protein, to the ε RNA element. Universally, ε consists of a lower stem and an upper stem, separated by a bulge, and an apical loop. Complex formation triggers pgRNA encapsidation and the ε-templated synthesis of a DNA oligonucleotide (priming) that serves to generate minus-strand DNA. In vitro systems for duck hepatitis B virus (DHBV) yielded important insights into the priming mechanism, yet their relevance in infection is largely unexplored. Moreover, additional functions encoded in the DHBV ε (Dε) sequence could affect in vivo fitness. We therefore assessed the in vivo performances of five recombinant DHBVs bearing multiple mutations in the upper Dε stem. Three variants with only modestly reduced in vitro replication competence established chronic infection in ducks. From one variant but not another, three adapted new variants emerged upon passaging, as demonstrated by increased relative fitness in coinfections with wild-type DHBV. All three showed enhanced priming and replication competence in vitro, and in one, DHBV e antigen (DHBeAg) production was restored. Pronounced impacts on other Dε functions were not detected; however, gradual, synergistic contributions to overall performance are suggested by the fact of none of the variants reaching the in vivo fitness of wild-type virus. These data shed more light on the P-Dε interaction, define important criteria for the design of future in vivo evolution experiments, and suggest that the upper Dε stem sequences provided an evolutionary playground for DHBV to optimize in vivo fitness.

A Tyr residue in the reverse transcriptase domain can mimic the protein-priming Tyr residue in the terminal protein domain of a hepadnavirus P protein

Hepadnaviruses are the only known viruses that replicate by protein-primed reverse transcription. Beyond the conserved reverse transcriptase (RT) and RNase H domains, their polymerases (P proteins) carry a unique terminal protein (TP) domain that provides a specific Tyr residue, Tyr96 in duck hepatitis B virus (DHBV), to which the first nucleotide of minus-strand DNA is autocatalytically attached and extended by three more nucleotides. In vitro reconstitution of this priming reaction with DHBV P protein and cellular chaperones had revealed strict requirements for the Dε RNA stem-loop as a template and for catalytic activity of the RT domain plus RNA-binding competence of the TP domain. Chaperone dependence can be obviated by using a truncated P protein (miniP). Here, we found that miniP with a tobacco etch virus (TEV) protease cleavage site between TP and RT (miniP(TEV)) displayed authentic priming activity when supplied with α-(32)P-labeled deoxynucleoside triphosphates; however, protease cleavage revealed, surprisingly, that the RT domain was also labeled. RT labeling had identical requirements as priming at Tyr96 and originated from dNMP transfer to a unique Tyr residue identified as Tyr561 in the presumed RT primer grip motif. Mutating Tyr561 did not affect Tyr96 priming in vitro and only modestly reduced replication competence of an intact DHBV genome; hence, deoxynucleotidylated Tyr561 is not an obligate intermediate in TP priming. However, as a first alternative substrate for the exquisitely complex protein-priming reaction, dNMP transfer to Tyr561 is a novel tool to further clarify the mechanism of hepadnaviral replication initiation and suggests that specific priming inhibitors can be found.

The importance of hepatitis B virus genome diversity in Basal core promoter region

Hepadnaviruses employ an unusual strategy for the production of enormous number of virions during replication which makes rapid and substantial genetic sequence changes and alterations. The pathogenesis and clearance of hepatitis B virus (HBV) infection are engaged by the selection and expression of viral mutants during virus-host interactions. Mutations in regulatory regions such as the basal core promoter (BCP) which is thought to be related to lower production of hepatitis B "e" antigen (HBeAg) directly affects the clinical presentation of liver disease. However, the molecular structure of these mutations in chronic carriers has not been adequately evaluated. In this review we evaluate the molecular aspect and pathologic basis of basal core promoter mutations.

Serine phosphoacceptor sites within the core protein of hepatitis B virus contribute to genome replication pleiotropically

The core protein of hepatitis B virus can be phosphorylated at serines 155, 162, and 170. The contribution of these serine residues to DNA synthesis was investigated. Core protein mutants were generated in which each serine was replaced with either alanine or aspartate. Aspartates can mimic constitutively phosphorylated serines while alanines can mimic constitutively dephosphorylated serines. The ability of these mutants to carry out each step of DNA synthesis was determined. Alanine substitutions decreased the efficiency of minus-strand DNA elongation, primer translocation, circularization, and plus-strand DNA elongation. Aspartate substitutions also reduced the efficiency of these steps, but the magnitude of the reduction was less. Our findings suggest that phosphorylated serines are required for multiple steps during DNA synthesis. It has been proposed that generation of mature DNA requires serine dephosphorylation. Our results suggest that completion of rcDNA synthesis requires phosphorylated serines.

The arginine clusters of the carboxy-terminal domain of the core protein of hepatitis B virus make pleiotropic contributions to genome replication

The carboxy-terminal domain (CTD) of the core protein of hepatitis B virus is not necessary for capsid assembly. However, the CTD does contribute to encapsidation of pregenomic RNA (pgRNA). The contribution of the CTD to DNA synthesis is less clear. This is the case because some mutations within the CTD increase the proportion of spliced RNA to pgRNA that are encapsidated and reverse transcribed. The CTD contains four clusters of consecutive arginine residues. The contributions of the individual arginine clusters to genome replication are unknown. We analyzed core protein variants in which the individual arginine clusters were substituted with either alanine or lysine residues. We developed assays to analyze these variants at specific steps throughout genome replication. We used a replication template that was not spliced in order to study the replication of only pgRNA. We found that alanine substitutions caused defects at both early and late steps in genome replication. Lysine substitutions also caused defects, but primarily during later steps. These findings demonstrate that the CTD contributes to DNA synthesis pleiotropically and that preserving the charge within the CTD is not sufficient to preserve function.

Full-length hepatitis B virus core protein packages viral and heterologous RNA with similarly high levels of cooperativity

A critical feature of a viral life cycle is the ability to selectively package the viral genome. In vivo, phosphorylated hepatitis B virus (HBV) core protein specifically encapsidates a complex of pregenomic RNA (pgRNA) and viral polymerase; it has been suggested that packaging is specific for the complex. Here, we test the hypothesis that core protein has intrinsic specificity for pgRNA, independent of the polymerase. For these studies, we also evaluated the effect of core protein phosphorylation on assembly and RNA binding, using phosphorylated core protein and a phosphorylation mimic in which S155, S162, and S170 were mutated to glutamic acid. We have developed an in vitro system where capsids are disassembled and assembly-active core protein dimer is purified. With this protein, we have reassembled empty capsids and RNA-filled capsids. We found that core protein dimer bound and encapsidated both the HBV pregenomic RNA and heterologous RNA with high levels of cooperativity, irrespective of phosphorylation. In direct competition assays, no specificity for pregenomic RNA was observed. This suggests that another factor, such as the viral polymerase, is required for specific packaging. These results also beg the question of what prevents HBV core protein from assembling on nonviral RNA, preserving the protein for virus production.

Mechanism for CCC DNA synthesis in hepadnaviruses

Hepadnavirus replication requires the synthesis of a covalently closed circular (CCC) DNA from the relaxed circular (RC) viral genome by an unknown mechanism. CCC DNA formation could require enzymatic activities of the viral reverse transcriptase (RT), or cellular DNA repair enzymes, or both. Physical mapping of the 5′ and 3′ ends of RC DNA and sequence analysis of CCC DNA revealed that CCC DNA synthesis requires the removal of the RT and an RNA oligomer from the 5′ ends of minus and plus strand DNA, respectively, removal of sequences from the terminally redundant minus strand, completion of the less than full-length plus strand, and ligation of the ends. Two models have been proposed that could explain CCC DNA formation. The first (model 1) invokes a role for the RT to catalyze a cleavage-ligation reaction leading to the formation of a unit length minus strand in CCC DNA and a DNA repair reaction for the completion and ligation of plus strand DNA; the second (model 2) predicts that CCC DNA formation depends entirely on cellular DNA repair enzymes. To determine which mechanism is utilized, we developed cell lines expressing duck hepatitis B virus genomes carrying mutations permitting us to follow the fate of viral DNA sequences during their conversion from RC to CCC DNA. Our results demonstrated that the oligomer at the 5′ end of minus strand DNA is completely or at least partially removed prior to CCC DNA synthesis. The results indicated that both RC DNA strands undergo DNA repair reactions carried out by the cellular DNA repair machinery as predicted by model 2. Thus, our study provided the basis for the identification of the cellular components required for CCC DNA formation.