Hepatitis B virus reverse transcriptase and epsilon RNA sequences required for specific interaction in vitro

HBV polymerase recruits the phosphatase PP1 to dephosphorylate HBc-Ser170 to complete encapsidation

The HBV core (HBc) protein contains an N-terminal domain (NTD) for capsid assembly and an arginine-rich C-terminal domain (CTD) for pregenomic RNA (pgRNA) encapsidation. Phosphorylation of the HBc CTD, especially at Ser162 and Ser170, is essential for nucleation with the polymerase (Pol) to initiate pgRNA encapsidation. As capsids mature, the HBc CTD undergoes dephosphorylation, suggesting the involvement of a phosphatase in the late stage of encapsidation, which remains to be determined. Using a C-S170 antibody specific for non-phosphorylated HBc-Ser170, we observed a transition from a phosphorylated to a dephosphorylated state during pgRNA packaging. The Pol-dependent dephosphorylation of HBc-Ser170 was confirmed by the substitution of one single amino acid at Val782 in the RNase H domain, which abolished the dephosphorylation of HBc-Ser170. Immunoprecipitation, mass spectrometry analyses, and the protein structural analyses showed that the recruitment of the host phosphatase PP1 is dependent on the Pol-Val782 domain. This recruitment does not require HBc but does require Pol via epsilon RNA signal, suggesting that the Pol-pgRNA complex plays a key role in PP1 recruitment. Pol-pgRNA-PP1-mediated dephosphorylation of HBc-Ser170 is essential for the completion of pgRNA encapsidation and appears to be associated with late endosomes/multivesicular bodies (MVBs). Therefore, HBV Pol may play a dual role by initially bringing pgRNA to phosphorylated HBc and recruiting PP1 for later completion of RNA packaging into the capsids. These findings not only decipher the mechanism by which Pol-mediated dephosphorylation of HBc regulates pgRNA encapsulation, but also reveal the possibility of PP1 as a potential target for antiviral development.

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.

The Predictive Role of Hepatitis B Biomarkers on HBV Reactivation following Direct-Acting Antiviral Therapy in HBV/HCV Coinfected Patients

Hepatitis B and C (HBV/HCV) coinfected patients have a potential risk of hepatitis B reactivation (HBVr) after direct-acting antivirals (DAAs) treatment. The study intends to investigate the predictive role of HBV biomarkers in HBVr. Forty-six HBV/HCV coinfected patients receiving DAAs were enrolled. All patients completed treatment and follow-up to the 12th-week post-DAA treatment (P12). Blood samples were measured for HBV biomarkers, including hepatitis B surface antigen (HBsAg), hepatitis B core-related antigen (HBcrAg), and HBV pregenomic RNA (HBV pgRNA). The predictive factors for HBVr after DAA treatment were analyzed. Among 31 patients without nucleot(s)ide analogue (NA) treatment, seven (22.5%, 7/31) developed HBVr without hepatitis flare-up. Patients with HBVr had higher HBsAg titers than those without HBVr from baseline to P12 (p = 0.008, 0.009, 0.004, and 0.006 at baseline, week 4, end of treatment, and P12, respectively). The baseline HBsAg level was the only predictive factor associated with HBVr (HR, 2.303; 95% CI, 1.086−4.882; p = 0.030). In predicting HBVr, a baseline HBsAg titer > 20 IU/mL had a sensitivity, specificity, positive predictive value, and negative predictive value of 85.7%, 75.0%, 50%, and 94.7%, respectively. No patient had HBVr if the baseline HBsAg titer was <8 IU/mL. Serum HBcrAg and HBV pgRNA levels had no role in predicting HBVr. In conclusion, HBV/HCV coinfected patients are at risk of HBVr after DAA treatment. The baseline HBsAg level was the predictive factor associated with HBVr. Patients with a baseline HBsAg titer < 8 IU/mL can be considered as not having HBVr.

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.

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.

N6-methyladenosine modification of the 5' epsilon structure of the HBV pregenome RNA regulates its encapsidation by the viral core protein

HBV infections are the leading cause of chronic hepatitis and carry the risk of liver cirrhosis and cancer. The HBV life cycle is perpetuated by an RNA intermediate termed pregenomic RNA (pgRNA), which is encapsidated by the viral core protein. The pgRNA packaging process is an essential step in viral replication. Here, we investigated the role of N6-methyladenosine (m⁶A) modification in the recognition of pgRNA by the core protein during encapsidation. m⁶A modification of 5′ epsilon structural motifs serves as the recognition signal for the core protein interaction, as evidenced by the failure of 5′ epsilon m⁶A mutant to encapsidate pgRNA. This study identifies the structural role of m⁶A modification in pgRNA encapsidation and provides an avenue in RNA–protein complex interactions.

Hepatitis B virus (HBV) contains a partially double-stranded DNA genome. During infection, its replication is mediated by reverse transcription (RT) of an RNA intermediate termed pregenomic RNA (pgRNA) within core particles in the cytoplasm. An epsilon structural element located in the 5′ end of the pgRNA primes the RT activity. We have previously identified the N6-methyladenosine (m⁶A)–modified DRACH motif at 1905 to 1909 nucleotides in the epsilon structure that affects myriad functions of the viral life cycle. In this study, we investigated the functional role of m⁶A modification of the 5′ ε (epsilon) structural element of the HBV pgRNA in the nucleocapsid assembly. Using the m⁶A site mutant in the HBV 5′ epsilon, we present evidence that m⁶A methylation of 5′ epsilon is necessary for its encapsidation. The m⁶A modification of 5′ epsilon increased the efficiency of viral RNA packaging, whereas the m⁶A of 3′ epsilon is dispensable for encapsidation. Similarly, depletion of methyltransferases (METTL3/14) decreased pgRNA and viral DNA levels within the core particles. Furthermore, the m⁶A modification at 5′ epsilon of HBV pgRNA promoted the interaction with core proteins, whereas the 5′ epsilon m⁶A site–mutated pgRNA failed to interact. HBV polymerase interaction with 5′ epsilon was independent of m⁶A modification of 5′ epsilon. This study highlights yet another pivotal role of m⁶A modification in dictating the key events of the HBV life cycle and provides avenues for investigating RNA–protein interactions in various biological processes, including viral RNA genome encapsidation in the context of m⁶A modification.

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.

The hepatitis B virus polymerase

Hepatitis B virus (HBV) is a hepatotropic, partially double-stranded DNA virus that replicates by reverse transcription and is a major cause of chronic liver disease and hepatocellular carcinoma. Reverse transcription is catalyzed by the four-domain multifunctional HBV polymerase (P) protein that has protein-priming, RNA- and DNA-dependent DNA synthesis (i.e., reverse transcriptase), and ribonuclease H activities. P also likely promotes the three strand transfers that occur during reverse transcription, and it may participate in immune evasion by HBV. Reverse transcription is primed by a tyrosine residue in the amino-terminal domain of P, and P remains covalently attached to the product DNA throughout reverse transcription. The reverse transcriptase activity of P is the target for the nucleos(t)ide analog drugs that dominate HBV treatment, and P is the target of ongoing efforts to develop new drugs against both the reverse transcriptase and ribonuclease H activities. Despite the unusual reverse transcription pathway catalyzed by P and the importance of P to HBV therapy, understanding the enzymology and structure of HBV P severely lags that of the retroviral reverse transcriptases due to substantial technical challenges to studying the enzyme. Obtaining a better understanding of P will broaden our appreciation of the diversity among reverse transcribing elements in nature, and will help improve treatment for people chronically infected with HBV.

Recent Advances in Hepatitis B Treatment

Hepatitis B virus infection affects over 250 million chronic carriers, causing more than 800,000 deaths annually, although a safe and effective vaccine is available. Currently used antiviral agents, pegylated interferon and nucleos(t)ide analogues, have major drawbacks and fail to completely eradicate the virus from infected cells. Thus, achieving a "functional cure" of the infection remains a real challenge. Recent findings concerning the viral replication cycle have led to development of novel therapeutic approaches including viral entry inhibitors, epigenetic control of cccDNA, immune modulators, RNA interference techniques, ribonuclease H inhibitors, and capsid assembly modulators. Promising preclinical results have been obtained, and the leading molecules under development have entered clinical evaluation. This review summarizes the key steps of the HBV life cycle, examines the currently approved anti-HBV drugs, and analyzes novel HBV treatment regimens.

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.

In vitro transcribed mRNA for expression of designer nucleases: Advantages as a novel therapeutic for the management of chronic HBV infection

Chronic infection with the hepatitis B virus (HBV) remains a significant worldwide medical problem. While diseases caused by HIV infection, tuberculosis and malaria are on the decline, new cases of chronic hepatitis B are on the rise. Because often fatal complications of cirrhosis and hepatocellular carcinoma are associated with chronic hepatitis B, the need for a cure is as urgent as ever. Currently licensed therapeutics fail to eradicate the virus and this is attributable to persistence of the viral replication intermediate comprising covalently closed circular DNA (cccDNA). Elimination or inactivation of the viral cccDNA is thus a goal of research aimed at hepatitis B cure. The ability to engineer nucleases that are capable of specific cleavage of a DNA sequence now provides the means to disable cccDNA permanently. The scientific literature is replete with many examples of using designer zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and RNA-guided endonucleases (RGENs) to inactivate HBV. However, important concerns about safety, dose control and efficient delivery need to be addressed before the technology is employed in a clinical setting. Use of in vitro transcribed mRNA to express therapeutic gene editors goes some way to overcoming these concerns. The labile nature of RNA limits off-target effects and enables dose control. Compatibility with hepatotropic non-viral vectors is convenient for the large scale preparation that will be required for advancing gene editing as a mode of curing chronic hepatitis B.

Insights into Hepatitis B Virus DNA Integration-55 Years after Virus Discovery

Hepatitis B virus (HBV), which was discovered in 1965, is a threat to global public health. HBV infects human hepatocytes and leads to acute and chronic liver diseases, and there is no cure. In cells infected by HBV, viral DNA can be integrated into the cellular genome. HBV DNA integration is a complicated process during the HBV life cycle. Although HBV integration normally results in replication-incompetent transcripts, it can still act as a template for viral protein expression. Of note, it is a primary driver of hepatocellular carcinoma (HCC). Recently, with the development of detection methods and research models, the molecular biology and the pathogenicity of HBV DNA integration have been better revealed. Here, we review the advances in the research of HBV DNA integration, including molecular mechanisms, detection methods, research models, the effects on host and viral gene expression, the role of HBV integrations in the pathogenesis of HCC, and potential treatment strategies. Finally, we discuss possible future research prospects of HBV DNA integration.

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.

Molecular, Evolutionary, and Structural Analysis of the Terminal Protein Domain of Hepatitis B Virus Polymerase, a Potential Drug Target

Approximately 250 million people are living with chronic hepatitis B virus (HBV) infections, which claim nearly a million lives annually. The target of all current HBV drug therapies (except interferon) is the viral polymerase; specifically, the reverse transcriptase domain. Although no high-resolution structure exists for the HBV polymerase, several recent advances have helped to map its functions to specific domains. The terminal protein (TP) domain, unique to hepadnaviruses such as HBV, has been implicated in the binding and packaging of the viral RNA, as well as the initial priming of and downstream synthesis of viral DNA—all of which make the TP domain an attractive novel drug target. This review encompasses three types of analysis: sequence conservation analysis, secondary structure prediction, and the results from mutational studies. It is concluded that the TP domain of HBV polymerase is comprised of seven subdomains (three unstructured loops and four helical regions) and that all three loop subdomains and Helix 5 are the major determinants of HBV function within the TP domain. Further studies, such as modeling inhibitors of these critical TP subdomains, will advance the TP domain of HBV polymerase as a therapeutic drug target in the progression towards a cure.

Discovery and Selection of Hepatitis B Virus-Derived T Cell Epitopes for Global Immunotherapy Based on Viral Indispensability, Conservation, and HLA-Binding Strength

Multiple HBV-derived T cell epitopes have been reported, which can be useful in a therapeutic vaccination strategy. However, these epitopes are largely restricted to HLA-A*02, which is not dominantly expressed in populations with high HBV prevalence. Thus, current epitopes are falling short in the development of a global immunotherapeutic approach. Therefore, we aimed to identify novel epitopes for 6 HLA supertypes most prevalent in the infected population. Moreover, established epitopes might not all be equally effective as they can be subject to different levels of immune escape. It is therefore important to identify targets that are crucial in viral replication and conserved in the majority of the infected population. Here, we applied a stringent selection procedure to compose a combined overview of existing and novel HBV-derived T cell epitopes most promising for viral eradication. This set of T cell epitopes now lays the basis for the development of globally effective HBV antigen-specific immunotherapies.

Immunotherapy represents an attractive option for the treatment of chronic hepatitis B virus (HBV) infection. The HBV proteins polymerase (Pol) and HBx are of special interest for antigen-specific immunotherapy because they are essential for viral replication and have been associated with viral control (Pol) or are still expressed upon viral DNA integration (HBx). Here, we scored all currently described HBx- and Pol-derived epitope sequences for viral indispensability and conservation across all HBV genotypes. This yielded 7 HBx-derived and 26 Pol-derived reported epitopes with functional association and high conservation. We subsequently predicted novel HLA-binding peptides for 6 HLA supertypes prevalent in HBV-infected patients. Potential epitopes expected to be the least prone to immune escape were subjected to a state-of-the-art in vitro assay to validate their HLA-binding capacity. Using this method, a total of 13 HLA binders derived from HBx and 33 binders from Pol were identified across HLA types. Subsequently, we demonstrated interferon gamma (IFN-γ) production in response to 5 of the novel HBx-derived binders and 17 of the novel Pol-derived binders. In addition, we validated several infrequently described epitopes. Collectively, these results specify a set of highly potent T cell epitopes that represent a valuable resource for future HBV immunotherapy design.

IMPORTANCE Multiple HBV-derived T cell epitopes have been reported, which can be useful in a therapeutic vaccination strategy. However, these epitopes are largely restricted to HLA-A*02, which is not dominantly expressed in populations with high HBV prevalence. Thus, current epitopes are falling short in the development of a global immunotherapeutic approach. Therefore, we aimed to identify novel epitopes for 6 HLA supertypes most prevalent in the infected population. Moreover, established epitopes might not all be equally effective as they can be subject to different levels of immune escape. It is therefore important to identify targets that are crucial in viral replication and conserved in the majority of the infected population. Here, we applied a stringent selection procedure to compose a combined overview of existing and novel HBV-derived T cell epitopes most promising for viral eradication. This set of T cell epitopes now lays the basis for the development of globally effective HBV antigen-specific immunotherapies.

Fluorescence-based biochemical analysis of human hepatitis B virus reverse transcriptase activity

Although the unique mechanism by which hepatitis B virus (HBV) polymerase primes reverse transcription is now well-characterized, the subsequent elongation process remains poorly understood. Reverse transcriptase (RT)-RNase H sequences from polymerase amino acid 304 (the C-terminal part of spacer domain) to 843 were expressed in Escherichia coli and purified partially. RT elongation activity was investigated using the fluorescent-tagged primer and homopolymeric RNA templates. RT elongation activity depended on both Mg²⁺ and Mn²⁺, and had low affinity for purine deoxynucleotides, which may be related with the success of adefovir, tenofovir, and entecavir. However, the polymerization rate was lower than that of human immunodeficiency virus RT. All HBV genotypes displayed similar RT activity, except for genotype B, which demonstrated increased elongation activity.

Coevolution analysis of amino-acids reveals diversified drug-resistance solutions in viral sequences: a case study of hepatitis B virus

The study of mutational landscapes of viral proteins is fundamental for the understanding of the mechanisms of cross-resistance to drugs and the design of effective therapeutic strategies based on several drugs. Antiviral therapy with nucleos(t)ide analogues targeting the hepatitis B virus (HBV) polymerase protein (Pol) can inhibit disease progression by suppression of HBV replication and makes it an important case study. In HBV, treatment may fail due to the emergence of drug-resistant mutants. Primary and compensatory mutations have been associated with lamivudine resistance, whereas more complex mutational patterns are responsible for resistance to other HBV antiviral drugs. So far, all known drug-resistance mutations are located in one of the four Pol domains, called reverse transcriptase. We demonstrate that sequence covariation identifies drug-resistance mutations in viral sequences. A new algorithmic strategy, BIS2TreeAnalyzer, is designed to apply the coevolution analysis method BIS2, successfully used in the past on small sets of conserved sequences, to large sets of evolutionary related sequences. When applied to HBV, BIS2TreeAnalyzer highlights diversified viral solutions by discovering thirty-seven positions coevolving with residues known to be associated with drug resistance and located on the four Pol domains. These results suggest a sequential mechanism of emergence for some mutational patterns. They reveal complex combinations of positions involved in HBV drug resistance and contribute with new information to the landscape of HBV evolutionary solutions. The computational approach is general and can be applied to other viral sequences when compensatory mutations are presumed.

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.

Development of Direct-acting Antiviral and Host-targeting Agents for Treatment of Hepatitis B Virus Infection

Hepatitis B virus (HBV) infection affects approximately 300 million people worldwide. Although antiviral therapies have improved the long-term outcomes, patients often require life-long treatment and there is no cure for HBV infection. New technologies can help us learn more about the pathogenesis of HBV infection and develop therapeutic agents to reduce its burden. We review recent advances in development of direct-acting antiviral and host-targeting agents, some of which have entered clinical trials. We also discuss strategies for unbiased high-throughput screens to identify compounds that inhibit HBV and for repurposing existing drugs.

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.

PRMT5 restricts hepatitis B virus replication through epigenetic repression of covalently closed circular DNA transcription and interference with pregenomic RNA encapsidation

Chronic hepatitis B virus (HBV) infection remains a major health problem worldwide. The covalently closed circular DNA (cccDNA) minichromosome, which serves as the template for the transcription of viral RNAs, plays a key role in viral persistence. While accumulating evidence suggests that cccDNA transcription is regulated by epigenetic machinery, particularly the acetylation of cccDNA-bound histone 3 (H3) and H4, the potential contributions of histone methylation and related host factors remain obscure. Here, by screening a series of methyltransferases and demethylases, we identified protein arginine methyltransferase 5 (PRMT5) as an effective restrictor of HBV transcription and replication. In cell culture-based models for HBV infection and in liver tissues of patients with chronic HBV infection, we found that symmetric dimethylation of arginine 3 on H4 on cccDNA was a repressive marker of cccDNA transcription and was regulated by PRMT5 depending on its methyltransferase domain. Moreover, PRMT5-triggered symmetric dimethylation of arginine 3 on H4 on the cccDNA minichromosome involved an interaction with the HBV core protein and the Brg1-based human SWI/SNF chromatin remodeler, which resulted in down-regulation of the binding of RNA polymerase II to cccDNA. In addition to the inhibitory effect on cccDNA transcription, PRMT5 inhibited HBV core particle DNA production independently of its methyltransferase activity. Further study revealed that PRMT5 interfered with pregenomic RNA encapsidation by preventing its interaction with viral polymerase protein through binding to the reverse transcriptase-ribonuclease H region of polymerase, which is crucial for the polymerase-pregenomic RNA interaction.

CONCLUSION

PRMT5 restricts HBV replication through a two-part mechanism including epigenetic suppression of cccDNA transcription and interference with pregenomic RNA encapsidation; these findings improve the understanding of epigenetic regulation of HBV transcription and host-HBV interaction, thus providing new insights into targeted therapeutic intervention. (Hepatology 2017;66:398-415).

Interferon-inducible ribonuclease ISG20 inhibits hepatitis B virus replication through directly binding to the epsilon stem-loop structure of viral RNA

Hepatitis B virus (HBV) replicates its DNA genome through reverse transcription of a viral RNA pregenome. We report herein that the interferon (IFN) stimulated exoribonuclease gene of 20 KD (ISG20) inhibits HBV replication through degradation of HBV RNA. ISG20 expression was observed at basal level and was highly upregulated upon IFN treatment in hepatocytes, and knock down of ISG20 resulted in elevation of HBV replication and attenuation of IFN-mediated antiviral effect. The sequence element conferring the susceptibility of HBV RNA to ISG20-mediated RNA degradation was mapped at the HBV RNA terminal redundant region containing epsilon (ε) stem-loop. Furthermore, ISG20-induced HBV RNA degradation relies on its ribonuclease activity, as the enzymatic inactive form ISG20^D94G was unable to promote HBV RNA decay. Interestingly, ISG20^D94G retained antiviral activity against HBV DNA replication by preventing pgRNA encapsidation, resulting from a consequence of ISG20-ε interaction. This interaction was further characterized by in vitro electrophoretic mobility shift assay (EMSA) and ISG20 was able to bind HBV ε directly in absence of any other cellular proteins, indicating a direct ε RNA binding capability of ISG20; however, cofactor(s) may be required for ISG20 to efficiently degrade ε. In addition, the lower stem portion of ε is the major ISG20 binding site, and the removal of 4 base pairs from the bottom portion of ε abrogated the sensitivity of HBV RNA to ISG20, suggesting that the specificity of ISG20-ε interaction relies on both RNA structure and sequence. Furthermore, the C-terminal Exonuclease III (ExoIII) domain of ISG20 was determined to be responsible for interacting with ε, as the deletion of ExoIII abolished in vitro ISG20-ε binding and intracellular HBV RNA degradation. Taken together, our study sheds light on the underlying mechanisms of IFN-mediated HBV inhibition and the antiviral mechanism of ISG20 in general.

HBV is a DNA virus but replicates its DNA via retrotranscription of a viral RNA pregenome. ISG20, an antiviral RNase induced by interferons, inhibits the replication of many RNA viruses but the underlying molecular antiviral mechanism remains elusive. Since all the known viruses, except for prions, have RNA products in their life cycles, ISG20 can be a broad spectrum antiviral protein; but in order to distinguish viral RNA from host RNA, ISG20 may have evolved to recognize virus-specific signals as its antiviral target. We demonstrated herein that ISG20 selectively binds to a unique stem-loop structure called epsilon (ε) in all HBV RNA species and degrades viral RNA to inhibit HBV replication. Because ε is the HBV pregenomic RNA packaging signal and reverse transcription priming site, the binding of ISG20 to ε, even in the absence of ribonuclease activity, results in antiviral effect to prevent DNA replication due to preventing viral polymerase binding to pgRNA. We also determined the structure and sequence requirements of ε RNA and ISG20 protein for ISG20-ε binding and antiviral activity. Such information will aid the function study of ISG20 against viral pathogens in host innate defense, and ISG20 has potentials to be developed into a therapeutic agent for viral diseases including hepatitis B.

The Envelope Gene of Hepatitis B Virus Is Implicated in Both Differential Virion Secretion and Genome Replication Capacities between Genotype B and Genotype C Isolates

Chronic infection by hepatitis B virus (HBV) genotype C is associated with a prolonged replicative phase and an increased risk of liver cancer, compared with genotype B infection. We previously found lower replication capacity but more efficient virion secretion by genotype C than genotype B isolates. Virion secretion requires interaction between core particles and ENVELOPE proteins. In the present study, chimeric constructs between genotype B and genotype C clones were generated to identify the structural basis for differential virion secretion. In addition to dimeric constructs, we also employed 1.1mer constructs, where the cytomegalovirus (CMV) promoter drove pregenomic RNA transcription. Through transient transfection experiments in Huh7 cells, we found that exchanging the entire envelope gene or just its S region could enhance virion secretion by genotype B clones while diminishing virion secretion by genotype C. Site-directed mutagenesis established the contribution of genotype-specific divergence at codons 108 and 115 in the preS1 region, as well as codon 126 in the S region, to differential virion secretion. Surprisingly, exchanging the envelope gene or just its S region, but not the core gene or 3′ S region, could markedly increase intracellular replicative DNA for genotype C clones but diminish that for genotype B, although the underlying mechanism remains to be clarified.

Mapping of Functional Subdomains in the Terminal Protein Domain of Hepatitis B Virus Polymerase

Hepatitis B virus (HBV) encodes a multifunction reverse transcriptase or polymerase (P), which is composed of several domains. The terminal protein (TP) domain is unique to HBV and related hepadnaviruses and is required for specifically binding to the viral pregenomic RNA (pgRNA). Subsequently, the TP domain is necessary for pgRNA packaging into viral nucleocapsids and the initiation of viral reverse transcription for conversion of the pgRNA to viral DNA. Uniquely, the HBV P protein initiates reverse transcription via a protein priming mechanism using the TP domain as a primer. No structural homologs or high-resolution structure exists for the TP domain. Secondary structure prediction identified three disordered loops in TP with highly conserved sequences. A meta-analysis of mutagenesis studies indicated these predicted loops are almost exclusively where functionally important residues are located. Newly constructed TP mutations revealed a priming loop in TP which plays a specific role in protein-primed DNA synthesis beyond simply harboring the site of priming. Substitutions of potential sites of phosphorylation surrounding the priming site demonstrated that these residues are involved in interactions critical for priming but are unlikely to be phosphorylated during viral replication. Furthermore, the first 13 and 66 TP residues were shown to be dispensable for protein priming and pgRNA binding, respectively. Combining current and previous mutagenesis work with sequence analysis has increased our understanding of TP structure and functions by mapping specific functions to distinct predicted secondary structures and will facilitate antiviral targeting of this unique domain.

IMPORTANCE

HBV is a major cause of viral hepatitis, liver cirrhosis, and hepatocellular carcinoma. One important feature of this virus is its polymerase, the enzyme used to create the DNA genome from a specific viral RNA by reverse transcription. One region of this polymerase, the TP domain, is required for association with the viral RNA and production of the DNA genome. Targeting the TP domain for antiviral development is difficult due to the lack of homology to other proteins and high-resolution structure. This study mapped the TP functions according to predicted secondary structure, where it folds into alpha helices or unstructured loops. Three predicted loops were found to be the most important regions functionally and the most conserved evolutionarily. Identification of these functional subdomains in TP will facilitate its targeting for antiviral development.

In Vitro Assays for RNA Binding and Protein Priming of Hepatitis B Virus Polymerase

The hepatitis B virus (HBV) polymerase synthesizes the viral DNA genome from the pre-genomic RNA (pgRNA) template through reverse transcription. Initiation of viral DNA synthesis is accomplished via a novel protein priming mechanism, so named because the polymerase itself acts as a primer, whereby the initiating nucleotide becomes covalently linked to a tyrosine residue on the viral polymerase. Protein priming, in turn, depends on specific recognition of the packaging signal on pgRNA called epsilon. These early events in viral DNA synthesis can now be dissected in vitro as described here.The polymerase is expressed in mammalian cells and purified by immunoprecipitation. The purified protein is associated with host cell factors, is enzymatically active, and its priming activity is epsilon dependent. A minimal epsilon RNA construct from pgRNA is co-expressed with the polymerase in cells. This RNA binds to and co-immunoprecipitates with the polymerase. Modifications can be made to either the epsilon RNA or the polymerase protein by manipulating the expression plasmids. Also, the priming reaction itself can be modified to assay for the initiation or subsequent DNA synthesis during protein priming, the susceptibility of the polymerase to chemical inhibitors, and the precise identification of the DNA products upon their release from the polymerase. The identity of associated host factors can also be evaluated. This protocol closely mirrors our current understanding of the RNA binding and protein priming steps of the HBV replication cycle, and it is amenable to modification. It should therefore facilitate both basic research and drug discovery.

Purification and enzymatic characterization of the hepatitis B virus ribonuclease H, a new target for antiviral inhibitors

Hepatitis B virus (HBV) reverse transcription requires coordinated function of the reverse transcriptase and ribonuclease H (RNaseH) activities of the viral polymerase protein. The reverse transcriptase has been biochemically characterized, but technical difficulties have prevented both assessment of the RNaseH and development of high throughput inhibitor screens against the RNaseH. Expressing the HBV RNaseH domain with both maltose binding protein and hexahistidine tags led to stable, high-level accumulation of the RNaseH in bacteria. Nickel-affinity purification in the presence of Mg(2+) and ATP removed co-purifying bacterial chaperones and yielded nearly pure monomeric recombinant enzyme. The endonucleolytic RNaseH activity required an DNA:RNA duplex ≥14 nt, could not tolerate a stem-loop in either the RNA or DNA strands, and could tolerate a nick in the DNA strand but not a gap. The RNaseH had no obvious sequence specificity or positional dependence within the RNA, and it cut the RNA at multiple positions even within the minimal 14 nt duplex. The RNaseH also possesses a processive 3'-5' exoribonuclease activity that is slower than the endonucleolytic reaction. These results are consistent with the HBV reverse transcription mechanism that features an initial endoribonucleolytic cut, 3'-5' degradation of RNA, and a sequence-independent terminal RNA cleavage. These data provide support for ongoing anti-RNaseH drug discovery efforts.

Assembly and Release of Hepatitis B Virus

The hepatitis B virus (HBV) core protein is a dynamic and versatile protein that directs many viral processes. During capsid assembly, core protein allosteric changes ensure efficient formation of a stable capsid that assembles while packaging viral RNA-polymerase complex. Reverse transcription of the RNA genome as well as transport of the capsid to multiple cellular compartments are directed by dynamic phosphorylation and structural changes of core protein. Subsequently, interactions of the capsid with the surface proteins and/or host proteins trigger envelopment and release of the viral capsids or the transport to the nucleus. Held together by many weak protein-protein interactions, the viral capsid is an extraordinary metastable machine that is stable enough to persist in the cellular and extracellular environment but dissociates to allow release of the viral genome at the right time during infection.

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.

New insights into hepatitis B virus biology and implications for novel antiviral strategies

Hepatitis B virus (HBV), a small DNA virus with a unique replication mode, can cause chronic hepatitis (CHB), which is characterized by the persistence of the viral covalently closed circular DNA that serves as the template for HBV replication and the production of large amounts of secreted HBV surface antigen (HBsAg) that is present in excess of the levels of infectious virus. Despite the success of currently approved antiviral treatments for CHB patients, including interferon and nucleotide analogs, which suppress HBV replication and reduce the risk of CHB-related liver diseases, these therapies fail to eradicate the virus in most of the patients. With the development of the cell and animal models for HBV study, a better understanding of the HBV life cycle has been achieved and a series of novel antiviral strategies that target different stages of HBV replication have been designed to overcome the viral factors that contribute to HBV persistence. Such basic HBV research advancements and therapeutic developments are the subject of this review.

Sequences in the terminal protein and reverse transcriptase domains of the hepatitis B virus polymerase contribute to RNA binding and encapsidation

Hepatitis B virus (HBV) antiviral therapy is plagued by limited efficacy and resistance to most nucleos(t)ide analog drugs. We have proposed that the complex RNA binding mechanism of the HBV reverse transcriptase (P) may be a novel target for antivirals. We previously found that RNA binds to the duck HBV (DHBV) P through interactions with the T3 and RT1 motifs in the viral terminal protein and reverse transcriptase domains, respectively. Here, we extended these studies to HBV P. HBV T3 and RT1 synthetic peptides bound RNA in a similar manner as did analogous DHBV peptides. The HBV T3 motif could partially substitute for DHBV T3 during RNA binding and DNA priming by DHBV P, whereas replacing RT1 supported substantial RNA binding but not priming. Substituting both the HBV T3 and RT1 motifs restored near wild-type levels of RNA binding but supported very little priming. Alanine-scanning mutations to the HBV T3 and RT1 motifs blocked HBV ε RNA binding in vitro and pgRNA encapsidation in cells. These data indicate that both the HBV T3 and RT1 motifs contain sequences essential for HBV ε RNA binding and encapsidation of the RNA pregenome, which is similar to their functions in DHBV. Small molecules that bind to T3 and/or RT1 would therefore inhibit encapsidation of the viral RNA and block genomic replication. Such drugs would target a novel viral function and would be good candidates for use in combination with the nucleoside analogs to improve efficacy of antiviral therapy.

Large-scale production and structural and biophysical characterizations of the human hepatitis B virus polymerase

Hepatitis B virus (HBV) is a major human pathogen that causes serious liver disease and 600,000 deaths annually. Approved therapies for treating chronic HBV infections usually target the multifunctional viral polymerase (hPOL). Unfortunately, these therapies--broad-spectrum antivirals--are not general cures, have side effects, and cause viral resistance. While hPOL remains an attractive therapeutic target, it is notoriously difficult to express and purify in a soluble form at yields appropriate for structural studies. Thus, no empirical structural data exist for hPOL, and this impedes medicinal chemistry and rational lead discovery efforts targeting HBV. Here, we present an efficient strategy to overexpress recombinant hPOL domains in Escherichia coli, purifying them at high yield and solving their known aggregation tendencies. This allowed us to perform the first structural and biophysical characterizations of hPOL domains. Apo-hPOL domains adopt mainly α-helical structures with small amounts of β-sheet structures. Our recombinant material exhibited metal-dependent, reverse transcriptase activity in vitro, with metal binding modulating the hPOL structure. Calcomine orange 2RS, a small molecule that inhibits duck HBV POL activity, also inhibited the in vitro priming activity of recombinant hPOL. Our work paves the way for structural and biophysical characterizations of hPOL and should facilitate high-throughput lead discovery for HBV.

IMPORTANCE

The viral polymerase from human hepatitis B virus (hPOL) is a well-validated therapeutic target. However, recombinant hPOL has a well-deserved reputation for being extremely difficult to express in a soluble, active form in yields appropriate to the structural studies that usually play an important role in drug discovery programs. This has hindered the development of much-needed new antivirals for HBV. However, we have solved this problem and report here procedures for expressing recombinant hPOL domains in Escherichia coli and also methods for purifying them in soluble forms that have activity in vitro. We also present the first structural and biophysical characterizations of hPOL. Our work paves the way for new insights into hPOL structure and function, which should assist the discovery of novel antivirals for HBV.

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.

A modified murine model based on hydrodynamic injection for the analysis of chronic human hepatitis B virus infection

Hepatitis B virus (HBV) is a persistent pathogen that causes acute and chronic necroinflammatory liver disease and is attributable to ~1 million deaths per year. In the present study, a conventional murine model was introduced based on the hydrodynamic injection of engineered replication‑competent HBV DNA into the tail veins of C57BL/6 mice. In a previous study, nine in‑frame ATG (start) codons in the S open reading frame (S1‑S9) were analyzed. The highly conserved ATG S5 was mutated to ACG by T378C, which led to the substitution sM75T and inhibition of the production and secretion of the hepatitis B surface antigen (HBsAg), and subsequent inhibition of HBV replication. In the present study, T378C was introduced into the pAAV‑HBV1.3 plasmid and was confirmed to affect HBsAg production and secretion, and HBV replication in vivo, which was in agreement with the previous in vitro results. Furthermore, the murine model was improved by co‑injection of the replication‑competent HBV plasmid DNA with Lipofectamine 2000 (LP). In this model, LP not only significantly enhanced HBV replication in mice, but also upregulated the expression of HBsAg and the hepatitis B core antigen. The current modified murine model was superior to the conventional murine HBV model based on HBV challenge by hydrodynamic injection.

Hepatitis B virus reverse transcriptase: diverse functions as classical and emerging targets for antiviral intervention

Hepatitis B virus (HBV) infection remains a global health problem with over 350 million chronically infected, causing an increased risk of cirrhosis and hepatocellular carcinoma. Current antiviral chemotherapy for HBV infection include five nucleos(t)ide analog reverse transcriptase inhibitors (NRTIs) that all target one enzymatic activity, DNA strand elongation, of the HBV polymerase (HP), a specialized reverse transcriptase (RT). NRTIs are not curative and long-term treatment is associated with toxicity and the emergence of drug resistant viral mutations, which can also result in vaccine escape. Recent studies on the multiple functions of HP have provided important mechanistic insights into its diverse roles during different stages of viral replication, including interactions with viral pregenomic RNA, RNA packaging into nucleocapsids, protein priming, minus- and plus-strand viral DNA synthesis, RNase H-mediated degradation of viral RNA, as well as critical host interactions that regulate the multiple HP functions. These diverse functions provide ample opportunities to develop novel HP-targeted antiviral treatments that should contribute to curing chronic HBV infection.

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.

Sulfamoylbenzamide derivatives inhibit the assembly of hepatitis B virus nucleocapsids

Chronic hepatitis B virus (HBV) infection, a serious public health problem leading to cirrhosis and hepatocellular carcinoma, is currently treated with either pegylated alpha interferon (pegIFN-α) or one of the five nucleos(t)ide analogue viral DNA polymerase inhibitors. However, neither pegIFN-α nor nucleos(t)ide analogues are capable of reliably curing the viral infection. In order to develop novel antiviral drugs against HBV, we established a cell-based screening assay by using an immortalized mouse hepatocyte-derived stable cell line supporting a high level of HBV replication in a tetracycline-inducible manner. Screening of a library consisting of 26,900 small molecules led to the discovery of a series of sulfamoylbenzamide (SBA) derivatives that significantly reduced the amount of cytoplasmic HBV DNA. Structure-activity relationship studies have thus far identified a group of fluorine-substituted SBAs with submicromolar antiviral activity against HBV in human hepatoma cells. Mechanistic analyses reveal that the compounds dose dependently inhibit the formation of pregenomic RNA (pgRNA)-containing nucleocapsids of HBV but not other animal hepadnaviruses, such as woodchuck hepatitis virus (WHV) and duck hepatitis B virus (DHBV). Moreover, heterologous genetic complementation studies of capsid protein, DNA polymerase, and pgRNA between HBV and WHV suggest that HBV capsid protein confers sensitivity to the SBAs. In summary, SBAs represent a novel chemical entity with superior activity and a unique antiviral mechanism and are thus warranted for further development as novel antiviral therapeutics for the treatment of chronic hepatitis B.

Computational evolutionary analysis of the overlapped surface (S) and polymerase (P) region in hepatitis B virus indicates the spacer domain in P is crucial for survival

Introduction

The Hepatitis B Virus (HBV) genome contains four ORFs, S (surface), P (polymerase), C (core) and X. S is completely overlapped by P and as a consequence the overlapping region is subject to distinctive evolutionary constraints compared to the remainder of the genome. Specifically, a non-synonymous substitution in one coding frame may produce a synonymous substitution in the alternative frame, suggesting a possible conflict between requirements for diversifying and purifying forces. To examine how these contrasting requirements are balanced within this region, we investigated the relationship amongst positive selection sites, conserved regions, epitopes and elements of protein structure to consider how HBV balances the contrasting evolutionary pressures.

Methodology/Results

323 HBV genotype D genome sequences were collected and analyzed to identify sites under positive selection and highly conserved regions. Epitopes sequences were retrieved from previously published experimental studies stored in the Immune Epitope Database. Predicted secondary structures were used to investigate the association between structure and conservation. Entropy was used as a measure of conservation and bivariate logistic regression was used to investigate the relationship between positive selection/conserved sites and epitope/secondary structure regions. Our results indicate: (i) conservation in S is primarily dictated by α-helix elements in the protein structure, (ii) variable residues are mainly located in PreS, the major hydrophilic region (MHR) and the C-terminus, (iii) epitopes in S, which are directly targeted by the host immune system, are significantly associated with sites under positive selection.

Conclusions

The highly variable spacer domain in P, which corresponds to PreS in S, appears to act as a harbor for the accumulation of mutations that can provide flexibility for conformational changes and responding to immune pressure.

Novel recombinant hepatitis B virus vectors efficiently deliver protein and RNA encoding genes into primary hepatocytes

Hepatitis B virus (HBV) has extremely restricted host and hepatocyte tropism. HBV-based vectors could form the basis of novel therapies for chronic hepatitis B and other liver diseases and would also be invaluable for the study of HBV infection. Previous attempts at developing HBV-based vectors encountered low yields of recombinant viruses and/or lack of sufficient infectivity/cargo gene expression in primary hepatocytes, which hampered follow-up applications. In this work, we constructed a novel vector based on a naturally occurring, highly replicative HBV mutant with a 207-bp deletion in the preS1/polymerase spacer region. By applying a novel insertion strategy that preserves the continuity of the polymerase open reading frame (ORF), recombinant HBV (rHBV) carrying protein or small interfering RNA (siRNA) genes were obtained that replicated and were packaged efficiently in cultured hepatocytes. We demonstrated that rHBV expressing a fluorescent reporter (DsRed) is highly infective in primary tree shrew hepatocytes, and rHBV expressing HBV-targeting siRNA successfully inhibited antigen expression from coinfected wild-type HBV. This novel HBV vector will be a powerful tool for hepatocyte-targeting gene delivery, as well as the study of HBV infection.

Effects of hepatitis B virus mutations on its replication and liver disease severity

Hepatitis B virus (HBV), nowadays, is one of the major human pathogens worldwide. Approximately, 400 million people worldwide have chronic HBV infection. Only 5% of persons infected during adulthood develop chronic infection. The reverse is true for those infected at birth or in early childhood, i.e. more than 90% of these persons progress to chronic infection. Currently, eight different genotypes o f HBV have been identified, differing in nucleotide sequence by greater than 8%. In addition, numerous subgenotypes have a l s o been recognized based on the nucleotide sequence variability of 4- 8%. It has invariably been found that these genotypes and mutations play a pivotal role in the liver disease aggravation and virus replication. The precore mutations (G1896A) and the double mutation (T1762/A1764) in the basal core promoter are important mutations that alter expression of the hepatitis B e antigen (HBeAg). The HBeAg is important for establishing viral persistence. The precore G1896A mutation abrogates the expression of HBeAg. Numerous other mutations alter the disease severity and progression. It is predictive that the infected patient has high risk of hepatocellular carcinoma if the genotype C is incriminated or if HBV possesses basal core promoter double mutation. Association of the remaining genotypes have been noted but with less degree than genotype C. Phenotypic assays of the different HBV protein markers with different molecular techniques illustrate the replication efficiency of the virus in cell lines. This review will discuss various mutations into their association with liver disease severity and progression as well as virus replication.

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.

Carbonyl J acid derivatives block protein priming of hepadnaviral P protein and DNA-dependent DNA synthesis activity of hepadnaviral nucleocapsids

Current treatments for chronic hepatitis B are effective in only a fraction of patients. All approved directly antiviral agents are nucleos(t)ide analogs (NAs) that target the DNA polymerase activity of the hepatitis B virus (HBV) P protein; resistance and cross-resistance may limit their long-term applicability. P protein is an unusual reverse transcriptase that initiates reverse transcription by protein priming, by which a Tyr residue in the unique terminal protein domain acts as an acceptor of the first DNA nucleotide. Priming requires P protein binding to the ε stem-loop on the pregenomic RNA (pgRNA) template. This interaction also mediates pgRNA encapsidation and thus provides a particularly attractive target for intervention. Exploiting in vitro priming systems available for duck HBV (DHBV) but not HBV, we demonstrate that naphthylureas of the carbonyl J acid family, in particular KM-1, potently suppress protein priming by targeting P protein and interfering with the formation of P-DHBV ε initiation complexes. Quantitative evaluation revealed a significant increase in complex stability during maturation, yet even primed complexes remained sensitive to KM-1 concentrations below 10 μM. Furthermore, KM-1 inhibited the DNA-dependent DNA polymerase activity of both DHBV and HBV nucleocapsids, including from a lamivudine-resistant variant, directly demonstrating the sensitivity of human HBV to the compound. Activity against viral replication in cells was low, likely due to low intracellular availability. KM-1 is thus not yet a drug candidate, but its distinct mechanism of action suggests that it is a highly useful lead for developing improved, therapeutically applicable derivatives.

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.

Distinct families of cis-acting RNA replication elements epsilon from hepatitis B viruses

The hepadnavirus encapsidation signal, epsilon (ε), is an RNA structure located at the 5′ end of the viral pregenomic RNA. It is essential for viral replication and functions in polymerase protein binding and priming. This structure could also have potential regulatory roles in controlling the expression of viral replicative proteins. In addition to its structure, the primary sequence of this RNA element has crucial functional roles in the viral lifecycle. Although the ε elements in hepadnaviruses share common critical functions, there are some significant differences in mammalian and avian hepadnaviruses, which include both sequence and structural variations.

 

Here we present several covariance models for ε elements from the Hepadnaviridae. The model building included experimentally determined data from previous studies using chemical probing and NMR analysis. These models have sufficient similarity to comprise a clan. The clan has in common a highly conserved overall structure consisting of a lower-stem, bulge, upper-stem and apical-loop.

The models differ in functionally critical regions—notably the two types of avian ε elements have a tetra-loop (UGUU) including a non-canonical UU base pair, while the hepatitis B virus (HBV) epsilon has a tri-loop (UGU). The avian epsilon elements have a less stable dynamic structure in the upper stem. Comparisons between these models and all other Rfam models, and searches of genomes, showed these structures are specific to the Hepadnaviridae. Two family models and the clan are available from the Rfam database.

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.