Detection and characterization of cytoplasmic hepatitis B virus reverse transcriptase

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.

Preparation and functional evaluation of monoclonal antibodies targeting Hepatitis B Virus Polymerase

HBV pol plays a critical role in the replication of hepatitis B virus (HBV). Previous studies conducted on HBV pol have produced limited evidence on HBV pol expression due to the lack of effective detection methods. The present study used the HBV pol (159–406 aa) protein as a target to screen for specific monoclonal antibodies that recognize HBV pol and subsequently evaluate their diagnostic and therapeutic value. Four antibodies (P3, P5, P12, P20) against HBV pol were obtained. Among them, the P20 antibody indicated optimal binding with HBV pol as demonstrated by Western blotting (WB) in a cell model transfected with the HBV genome. We also expressed P5 and P12 antibodies in mouse liver cells by transfection and the results indicated significant antiviral effects caused by these two antibodies especially P12. In summary, the present study established an antibody which was denoted P20. This antibody can be used to detect HBV pol expression by four HBV genomes via WB analysis. In addition, the antibody denoted P12 could exert antiviral effects via intracellular expression, which may provide a promising approach for the treatment of chronic hepatitis B.

Nuclear Import of Hepatitis B Virus Capsids and Genome

Hepatitis B virus (HBV) is an enveloped pararetrovirus with a DNA genome, which is found in an up to 36 nm-measuring capsid. Replication of the genome occurs via an RNA intermediate, which is synthesized in the nucleus. The virus must have thus ways of transporting its DNA genome into this compartment. This review summarizes the data on hepatitis B virus genome transport and correlates the finding to those from other viruses.

Hepatitis B Virus Polymerase Localizes to the Mitochondria, and Its Terminal Protein Domain Contains the Mitochondrial Targeting Signal

To understand subcellular sites of hepatitis B virus (HBV) replication, we visualized core (Cp), polymerase (Pol), and pregenomic RNA (pgRNA) in infected cells. Interestingly, we found that the majority of Pol localized to the mitochondria in cells undergoing viral replication. The mitochondrial localization of Pol was independent of both the cell type and other viral components, indicating that Pol contains an intrinsic mitochondrial targeting signal (MTS). Neither Cp nor pgRNA localized to the mitochondria during active replication, suggesting a role other than DNA synthesis for Pol at the mitochondria. The Pol of duck hepatitis B virus (DHBV) also localized to the mitochondria. This result indicates that localization of Pol to mitochondria is likely a feature of all hepadnaviruses. To map the MTS within HBV Pol, we generated a series of Pol-green fluorescent protein (Pol-GFP) fusions and found that a stretch spanning amino acids (aa) 141 to 160 of Pol was sufficient to target GFP to the mitochondria. Surprisingly, deleting aa 141 to 160 in full-length Pol did not fully ablate Pol's mitochondrial localization, suggesting that additional sequences are involved in mitochondrial targeting. Only by deleting the N-terminal 160 amino acids in full-length Pol was mitochondrial localization ablated. Crucial residues for pgRNA packaging are contained within aa 141 to 160, indicating a multifunctional role of this region of Pol in the viral life cycle. Our studies show an unexpected Pol trafficking behavior that is uncoupled from its role in viral DNA synthesis.

IMPORTANCE

Chronic infection by HBV is a serious health concern. Existing therapies for chronically infected individuals are not curative, underscoring the need for a better understanding of the viral life cycle to develop better antiviral therapies. To date, the most thoroughly studied function of Pol is to package the pgRNA and reverse transcribe it to double-stranded DNA within capsids. This study provides evidence for mitochondrial localization of Pol and defines the MTS. Recent findings have implicated a non-reverse transcription role for Pol in evading host innate immune responses. Mitochondria play an important role in controlling cellular metabolism, apoptosis, and innate immunity. Pol may alter one or more of these host mitochondrial functions to gain a replicative advantage and persist in chronically infected individuals.

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.

A novel method for nucleos(t)ide analogues susceptibility assay of hepatitis B virus by viral polymerase transcomplementation

Nucleos(t)ide analogues (NUCs) susceptibility assay is important for the study of hepatitis B virus (HBV) drug resistance. The purpose of susceptibility assay is to test the sensitivity of a specific HBV variant to NUCs in vitro, by which assesses if and to what extent the mutant virus is resistant to a specific NUC. Among the existing susceptibility assay methods, stable cell line expressing the specific variant is one of the commonly used assessment systems based on its high repeatability. However, establishment of stable cell lines expressing individual variant is laborious and time-consuming. In the present study, we developed a novel strategy for rapidly establishing HBV replicating stable cell lines. We first established an acceptor cell line stably transfected with a polymerase-null HBV 1.1mer genome DNA, then lentiviruses expressing different mutant HBV polymerases were transduced into the acceptor cell line respectively. Stable cell lines replicating HBV DNA with the trans-complemented HBV polymerases were established by antibiotics selection. Lamivudine and entecavir susceptibility data from these polymerase-complementing cell lines were validated by comparing with other assays. Taken together, this transcomplementation strategy for establishment of stable cell lines replicating HBV DNA with clinically isolated HBV polymerase provides a new tool for NUC susceptibility assay of HBV.

Hepatitis B virus polymerase disrupts K63-linked ubiquitination of STING to block innate cytosolic DNA-sensing pathways

The cellular innate immune system recognizing pathogen infection is essential for host defense against viruses. In parallel, viruses have developed a variety of strategies to evade the innate immunity. The hepatitis B virus (HBV), a DNA virus that causes chronic hepatitis, has been shown to inhibit RNA helicase RIG-I-mediated interferon (IFN) induction. However, it is still unknown whether HBV could affect the host DNA-sensing pathways. Here we report that in transiently HBV-transfected Huh7 cells, the stably HBV-producing cell line HepAD38, and HBV-infected HepaRG cells and primary human hepatocytes, HBV markedly interfered with IFN-β induction and antiviral immunity mediated by the stimulator of interferon genes (STING), which has been identified as a central factor in foreign DNA recognition and antiviral innate immunity. Screening analysis demonstrated that the viral polymerase (Pol), but not other HBV-encoded proteins, was able to inhibit STING-stimulated interferon regulatory factor 3 (IRF3) activation and IFN-β induction. Moreover, the reverse transcriptase (RT) and the RNase H (RH) domains of Pol were identified to be responsible for the inhibitory effects. Furthermore, Pol was shown to physically associate with STING and dramatically decrease the K63-linked polyubiquitination of STING via its RT domain without altering the expression level of STING. Taken together, these observations suggest that besides its inherent catalytic function, Pol has a role in suppression of IFN-β production by direct interaction with STING and subsequent disruption of its K63-linked ubiquitination, providing a new mechanism for HBV to counteract the innate DNA-sensing pathways.

IMPORTANCE

Although whether and how HBV infection induces the innate immune responses are still controversial, it has become increasingly clear that HBV has developed strategies to counteract the pattern recognition receptor-mediated signaling pathways. Previous studies have shown that type I IFN induction activated by the host RNA sensors could be inhibited by HBV. However, it remains unknown whether HBV as a DNA virus utilizes evasion mechanisms against foreign DNA-elicited antiviral signaling. In recent years, the cytosolic DNA sensor and key adaptor STING has been demonstrated to be essential in multiple foreign DNA-elicited innate immune signalings. Here, for the first time, we report STING as a new target of HBV to antagonize IFN induction and identify the viral polymerase responsible for the inhibitory effect, thus providing an additional molecular mechanism by which HBV evades the innate immunity; this implies that in addition to its inherent catalytic function, HBV polymerase is a multifunctional immunomodulatory protein.

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.

Expression of viral polymerase and phosphorylation of core protein determine core and capsid localization of the human hepatitis B virus

Biopsies from patients show that hepadnaviral core proteins and capsids - collectively called core - are found in the nucleus and cytoplasm of infected hepatocytes. In the majority of studies, cytoplasmic core localization is related to low viraemia while nuclear core localization is associated with high viral loads. In order to better understand the molecular interactions leading to core localization, we analysed transfected hepatoma cells using immune fluorescence microscopy. We observed that expression of core protein in the absence of other viral proteins led to nuclear localization of core protein and capsids, while expression of core in the context of the other viral proteins resulted in a predominantly cytoplasmic localization. Analysis of which viral partner was responsible for cytoplasmic retention indicated that the HBx, surface proteins and HBeAg had no impact but that the viral polymerase was the major determinant. Further analysis revealed that ϵ, an RNA structure to which the viral polymerase binds, was essential for cytoplasmic retention. Furthermore, we showed that core protein phosphorylation at Ser 164 was essential for the cytoplasmic core localization phenotype, which is likely to explain differences observed between individual cells.

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.

Inhibition of hepatitis B virus replication by cIAP2 involves accelerating the ubiquitin-proteasome-mediated destruction of polymerase

Cellular inhibitor of apoptosis protein 2 (cIAP2) is a potent suppressor of apoptotic cell death. We have shown previously that cIAP2 is involved in the tumor necrosis factor alpha (TNF-α)-induced anti-hepatitis B virus (HBV) response; however, the mechanism for this antiviral effect remains unclear. In the present study, we demonstrate that cIAP2 can significantly reduce the levels of HBV DNA replication intermediates but not the total viral RNA or core protein levels. Domain-mapping analysis revealed that the carboxy-terminal domains of cIAP2 were indispensable for this anti-HBV ability and that an E3 ligase-deficient mutant of cIAP2 (termed cIAP2*) completely lost its antiviral activity. We further identified HBV polymerase as the target of cIAP2. Overexpression of cIAP2 but not cIAP2* reduced polymerase protein levels, while cIAP2 knockdown increased polymerase expression. In addition, we observed that cIAP2 promoted the degradation of the viral polymerase through a proteasome-dependent pathway. Further experiments demonstrated that cIAP2 can bind to polymerase and promote its polyubiquitylation. Finally, we found that cIAP2 downregulated the encapsidation of HBV pregenomic RNA. Taken together, these data reveal a novel mechanism for the inhibition of HBV replication by cIAP2 via acceleration of the ubiquitin-proteasome-mediated decay of polymerase and reduction of the encapsidation of HBV pregenomic RNA, making this mechanism a novel strategy for HBV therapy.

Generation of covalently closed circular DNA of hepatitis B viruses via intracellular recycling is regulated in a virus specific manner

Persistence of hepatitis B virus (HBV) infection requires covalently closed circular (ccc)DNA formation and amplification, which can occur via intracellular recycling of the viral polymerase-linked relaxed circular (rc) DNA genomes present in virions. Here we reveal a fundamental difference between HBV and the related duck hepatitis B virus (DHBV) in the recycling mechanism. Direct comparison of HBV and DHBV cccDNA amplification in cross-species transfection experiments showed that, in the same human cell background, DHBV but not HBV rcDNA converts efficiently into cccDNA. By characterizing the distinct forms of HBV and DHBV rcDNA accumulating in the cells we find that nuclear import, complete versus partial release from the capsid and complete versus partial removal of the covalently bound polymerase contribute to limiting HBV cccDNA formation; particularly, we identify genome region-selectively opened nuclear capsids as a putative novel HBV uncoating intermediate. However, the presence in the nucleus of around 40% of completely uncoated rcDNA that lacks most if not all of the covalently bound protein strongly suggests a major block further downstream that operates in the HBV but not DHBV recycling pathway. In summary, our results uncover an unexpected contribution of the virus to cccDNA formation that might help to better understand the persistence of HBV infection. Moreover, efficient DHBV cccDNA formation in human hepatoma cells should greatly facilitate experimental identification, and possibly inhibition, of the human cell factors involved in the process.

Persistent infection with hepatitis B virus (HBV) causes chronic hepatitis B which frequently progresses to hepatocellular carcinoma, a leading cause of cancer-mediated mortality worldwide. Persistence requires formation and amplification of covalently closed circular (ccc)DNA, an episomal form of the viral genome that is not targeted by current drugs and thus is responsible for the notorious difficulties in therapeutic elimination of infection. Initial generation of cccDNA occurs upon nuclear import of the virion-borne relaxed circular (rc) DNA to which the viral polymerase is covalently linked; amplification occurs via intracellular recycling. The underlying molecular pathway is poorly understood. Because HBV infects only primates, in vivo studies are extremely restricted; in vitro, select hepatoma cell lines transfected with HBV support viral replication, however with little if any cccDNA formation. Here, we compared intracellular recycling of HBV and DHBV, a model hepatitis B virus from ducks, in cross-species transfections. Surprisingly, the major contribution to cccDNA formation comes from the virus rather than the cell as DHBV but not HBV rcDNA converted efficiently into cccDNA in the same human cell background. This unexpected difference might help to better understand persistence of HBV infection; efficient DHBV cccDNA formation in human cells provides a new tool to facilitate identification, and possibly targeting, of the human cell factors involved.

Hepatitis B virus polymerase blocks pattern recognition receptor signaling via interaction with DDX3: implications for immune evasion

Viral infection leads to induction of pattern-recognition receptor signaling, which leads to interferon regulatory factor (IRF) activation and ultimately interferon (IFN) production. To establish infection, many viruses have strategies to evade the innate immunity. For the hepatitis B virus (HBV), which causes chronic infection in the liver, the evasion strategy remains uncertain. We now show that HBV polymerase (Pol) blocks IRF signaling, indicating that HBV Pol is the viral molecule that effectively counteracts host innate immune response. In particular, HBV Pol inhibits TANK-binding kinase 1 (TBK1)/IkappaB kinase-epsilon (IKKepsilon), the effector kinases of IRF signaling. Intriguingly, HBV Pol inhibits TBK1/IKKepsilon activity by disrupting the interaction between IKKepsilon and DDX3 DEAD box RNA helicase, which was recently shown to augment TBK1/IKKepsilon activity. This unexpected role of HBV Pol may explain how HBV evades innate immune response in the early phase of the infection. A therapeutic implication of this work is that a strategy to interfere with the HBV Pol-DDX3 interaction might lead to the resolution of life-long persistent infection.

Hepatitis B virus core interacts with the host cell nucleolar protein, nucleophosmin 1

Hepatitis B virus (HBV) genome replication requires the packaging of viral factors (pregenomic RNA and polymerase) as well as host factors, including heat shock proteins and protein kinase C. Previous reports have suggested that there are several unidentified host factors that affect this encapsidation step. In this study, we identified a new host factor, nucleophosmin (B23) that interacts with the HBV core protein 149 (Cpl49). We analyzed this factor using NHS-activated sepharose resin and MALDI-TOF MS. Using the BIAcore analysis system, we were also able to deduce that the B23.1 residues 259-294 were required for the interaction between Cpl49 and B23.1 in vitro.

Four conserved cysteine residues of the hepatitis B virus polymerase are critical for RNA pregenome encapsidation

Hepadnaviruses replicate via reverse transcription of an RNA template, the pregenomic RNA (pgRNA). Although hepadnaviral polymerase (Pol) and retroviral reverse transcriptase are distantly related, some of their features are distinct. In particular, Pol contains two additional N-terminal subdomains, the terminal protein and spacer subdomains. Since much of the spacer subdomain can be deleted without detrimental effects to hepatitis B virus (HBV) replication, this subdomain was previously thought to serve only as a spacer that links the terminal protein and reverse transcriptase subdomains. Unexpectedly, we found that the C terminus of the spacer subdomain is indispensable for the encapsidation of pgRNA. Alanine-scanning mutagenesis revealed that four conserved cysteine residues, three at the C terminus of the spacer subdomain and one at the N terminus of the reverse transcriptase subdomain, are critical for encapsidation. The inability of the mutant Pol proteins to incorporate into nucleocapsid particles, together with other evidence, argued that the four conserved cysteine residues are critical for RNA binding. One implication is that these four cysteine residues might form a putative zinc finger motif. Based on these findings, we speculate that the RNA binding activity of HBV Pol may be mediated by this newly identified putative zinc finger motif.

DDX3 DEAD-Box RNA helicase inhibits hepatitis B virus reverse transcription by incorporation into nucleocapsids

Viruses utilize host factors in many steps of their life cycles. Yet, little is known about host factors that contribute to the life cycle of hepatitis B virus (HBV), which replicates its genome by reverse transcription. To identify host factors that contribute to viral reverse transcription, we sought to identify cellular proteins that interact with HBV polymerase (Pol) by using affinity purification coupled with mass spectrometry. One of the HBV Pol-interacting host factors identified was DDX3 DEAD-box RNA helicase, which unwinds RNA in an ATPase-dependent manner. Recently, it was shown that DDX3 is essential for both human immunodeficiency virus and hepatitis C virus infection. In contrast, we found that the ectopic expression of DDX3 led to significantly reduced viral DNA synthesis. The DDX3-mediated inhibition of viral DNA synthesis did not affect RNA encapsidation, a step prior to reverse transcription, and indicated that DDX3 inhibits HBV reverse transcription. Mutational analysis revealed that mutant DDX3 with an inactive ATPase motif, but not that with an inactive RNA helicase motif, failed to inhibit viral DNA synthesis. Our interpretation is that DDX3 inhibits viral DNA synthesis at a step following ATP hydrolysis but prior to RNA unwinding. Finally, OptiPrep density gradient analysis revealed that DDX3 was incorporated into nucleocapsids, suggesting that DDX3 inhibits viral reverse transcription following nucleocapsid assembly. Thus, DDX3 represents a novel host restriction factor that limits HBV infection.

Hepatitis B virus polymerase inhibits the interferon-inducible MyD88 promoter by blocking nuclear translocation of Stat1

Previous studies have suggested that hepatitis B virus (HBV) blocks expression of the alpha interferon (IFN-alpha)-inducible myeloid differential primary response protein (MyD88) gene. To study the molecular mechanism(s) of the inhibition of MyD88 expression by HBV, MyD88 promoter reporter plasmids and vectors expressing different HBV viral proteins were constructed. Co-transfection experiments showed that IFN-induced MyD88 promoter activity was inhibited by HBV polymerase expression in a dose-dependent manner and that the terminal protein (TP) domain of HBV polymerase was responsible for this antagonistic activity. Analysis of site mutants showed that the region targeted by the polymerase protein contained the signal transducer and activator of transcription (Stat) binding site. Chromatin immunoprecipitation analysis showed that the IFN-induced DNA-binding activity of Stat1 was affected. Further study demonstrated that the HBV polymerase protein inhibited the Stat1 nuclear translocation induced by IFN-alpha, but did not induce Stat1 degradation nor interfere with its phosphorylation. In addition, HBV polymerase could inhibit the transcriptional activity of other IFN-stimulated response element-driven promoters and the expression of interferon-stimulated genes (ISGs), such as Stat1 and ISG15. In summary, these results indicate that HBV polymerase is a general inhibitor of IFN signalling and can inhibit IFN-inducible MyD88 expression by inhibiting the activity of the MyD88 promoter through blocking the nuclear translocation of Stat1.

Hepatitis B virus replication

Hepadnaviruses, including human hepatitis B virus (HBV), replicate through reverse transcription of an RNA intermediate, the pregenomic RNA (pgRNA). Despite this kinship to retroviruses, there are fundamental differences beyond the fact that hepadnavirions contain DNA instead of RNA. Most peculiar is the initiation of reverse transcription: it occurs by protein-priming, is strictly committed to using an RNA hairpin on the pgRNA, ε, as template, and depends on cellular chaperones; moreover, proper replication can apparently occur only in the specialized environment of intact nucleocapsids. This complexity has hampered an in-depth mechanistic understanding. The recent successful reconstitution in the test tube of active replication initiation complexes from purified components, for duck HBV (DHBV), now allows for the analysis of the biochemistry of hepadnaviral replication at the molecular level. Here we review the current state of knowledge at all steps of the hepadnaviral genome replication cycle, with emphasis on new insights that turned up by the use of such cell-free systems. At this time, they can, unfortunately, not be complemented by three-dimensional structural information on the involved components. However, at least for the ε RNA element such information is emerging, raising expectations that combining biophysics with biochemistry and genetics will soon provide a powerful integrated approach for solving the many outstanding questions. The ultimate, though most challenging goal, will be to visualize the hepadnaviral reverse transcriptase in the act of synthesizing DNA, which will also have strong implications for drug development.

Suppression of mRNA accumulation by the duck hepatitis B virus reverse transcriptase

Hepadnaviruses establish chronic liver infections, but the mechanisms of persistence and immune evasion are poorly understood. We previously found that the duck hepatitis B virus (DHBV) and hepatitis B virus reverse transcriptases (P protein) unexpectedly accumulate in the cytoplasm where they could affect function(s) beyond viral DNA synthesis, such as gene expression. Therefore, we measured effects of DHBV P on gene expression from reporter constructs and the viral genome. P reduced reporter expression at the mRNA level to approximately 30-40%, independent of reporter tested. Accumulation of the viral pregenomic RNA from its native promoter was suppressed three-to four-fold by P, and accumulation of the capsid protein and intracellular core particles was similarly suppressed because the pregenomic RNA encodes the capsid protein. Therefore, suppression of the pregenomic RNA by DHBV P creates a negative feedback loop to limit viral antigen accumulation and replication, possibly contributing to maintenance of chronic infection.