Assembly of hepatitis delta virus particles

Adapting the rhizome concept to an extended definition of viral quasispecies and the implications for molecular evolution

The rhizome concept proposed by Gilles Deleuze and Félix Guattari offers a novel perspective on the organization and interdependence of complex constellations of heterogeneous entities, their mapping and their ruptures. The emphasis of the present study is placed on the dynamics of contacts and communication among such entities that arise from experimentation, without any favored hierarchy or origin. When applied to biological evolution, the rhizome concept integrates all types of heterogeneity resulting from "symbiotic" relationships among living beings (or their genomic material), horizontal genetic transfer, recombination and mutation, and breaks away from the approach that gives rise to the phylogenetic tree of life. It has already been applied to describe the dynamics and evolution of RNA viruses. Thus, here we introduce a novel framework for the interpretation the viral quasispecies concept, which explains the evolution of RNA virus populations as the result of dynamic interconnections and multifaceted interdependence between highly heterogeneous viral sequences and its inherently heterogeneous host cells. The rhizome network perspective underlines even further the medical implications of the broad mutant spectra of viruses that are in constant flow, given the multiple pathways they have available for fitness loss and gain.

Analysis of Replication, Cell Division-Mediated Spread, and HBV Envelope Protein-Dependent Pseudotyping of Three Mammalian Delta-like Agents

The human hepatitis delta virus (HDV) is a satellite RNA virus that depends on hepatitis B virus (HBV) surface proteins (HBsAg) to assemble into infectious virions targeting the same organ (liver) as HBV. Until recently, the evolutionary origin of HDV remained largely unknown. The application of bioinformatics on whole sequence databases lead to discoveries of HDV-like agents (DLA) and shed light on HDV's evolution, expanding our understanding of HDV biology. DLA were identified in heterogeneous groups of vertebrates and invertebrates, highlighting that the evolution of HDV, represented by eight distinct genotypes, is broader and more complex than previously foreseen. In this study, we focused on the characterization of three mammalian DLA discovered in woodchuck (Marmota monax), white-tailed deer (Odocoileus virginianus), and lesser dog-like bat (Peropteryx macrotis) in terms of replication, cell-type permissiveness, and spreading pathways. We generated replication-competent constructs expressing 1.1-fold over-length antigenomic RNA of each DLA. Replication was initiated by transfecting the cDNAs into human (HuH7, HeLa, HEK293T, A549) and non-human (Vero E6, CHO, PaKi, LMH) cell lines. Upon transfection and replication establishment, none of the DLA expressed a large delta antigen. A cell division-mediated viral amplification assay demonstrated the capability of non-human DLA to replicate and propagate in hepatic and non-hepatic tissues, without the requirement of envelope proteins from a helper virus. Remarkably L-HDAg but not S-HDAg from HDV can artificially mediate envelopment of WoDV and DeDV ribonucleoproteins (RNPs) by HBsAg to form infectious particles, as demonstrated by co-transfection of HuH7 cells with the respective DLA expression constructs and a plasmid encoding HBV envelope proteins. These chimeric viruses are sensitive to HDV entry inhibitors and allow synchronized infections for comparative replication studies. Our results provide a more detailed understanding of the molecular biology, evolution, and virus-host interaction of this unique group of animal viroid-like agents in relation to HDV.

Exosomes as Conduits: Facilitating Hepatitis B Virus-Independent Hepatitis D Virus Transmission and Propagation in Hepatocytes

A number of research studies, including ours, have spotlighted exosomes as critical facilitators of viral dissemination. While hepatitis B virus (HBV) transmission through exosomes has been studied, the focus on its satellite virus, the hepatitis delta virus (HDV), has been unexplored in this context. HDV, although being a defective virus, can replicate its genome autonomously within hepatocytes, independently of HBV. Investigations on Huh7 cells revealed an intriguing phenomenon: the HDV proteins, S-HDAg and L-HDAg, are transmitted between cells without a complete viral structure. Detailed analysis further revealed that the expression of these proteins not only bolstered exosome secretion but also ensured their enrichment within these vesicles. Our experimental approach utilized transfection of various plasmids to examine the role of HDV RNA and proteins in the process. One salient finding was the differential propagation of the HDV proteins S-HDAg and L-HDAg, suggesting intricate molecular mechanisms behind their transmission. Notably, the purity of our exosome preparations was monitored using markers such as TSG101 and CD81. Importantly, these exosomes were found to carry both HDV RNA and proteins, highlighting their role in HDV dissemination. This novel study underscores the role of exosomes in mediating the transmission of HDV components between hepatocytes independent of HBV. These revelations about the exosomal pathway of HDV transmission provide a foundation for the development of innovative therapeutic strategies against HDV infections.

Medical Advances in Hepatitis D Therapy: Molecular Targets

An approximate number of 250 million people worldwide are chronically infected with hepatitis B virus, making them susceptible to a coinfection with hepatitis D virus. The superinfection causes the most severe form of a viral hepatitis and thus drastically worsens the course of the disease. Until recently, the only available therapy consisted of interferon-α, only eligible for a minority of patients. In July 2020, the EMA granted Hepcludex conditional marketing authorization throughout the European Union. This first-in-class entry inhibitor offers the promise to prevent the spread in order to gain control and eventually participate in curing hepatitis B and D. Hepcludex is an example of how understanding the viral lifecycle can give rise to new therapy options. Sodium taurocholate co-transporting polypeptide, the virus receptor and the target of Hepcludex, and other targets of hepatitis D therapy currently researched are reviewed in this work. Farnesyltransferase inhibitors such as Lonafarnib, targeting another essential molecule in the HDV life cycle, represent a promising target for hepatitis D therapy. Farnesyltransferase attaches a farnesyl (isoprenyl) group to proteins carrying a C-terminal Ca1a2X (C: cysteine, a: aliphatic amino acid, X: C-terminal amino acid) motif like the large hepatitis D virus antigen. This modification enables the interaction of the HBV/HDV particle and the virus envelope proteins. Lonafarnib, which prevents this envelopment, has been tested in clinical trials. Targeting the lifecycle of the hepatitis B virus needs to be considered in hepatitis D therapy in order to cure a patient from both coexisting infections. Nucleic acid polymers target the hepatitis B lifecycle in a manner that is not yet understood. Understanding the possible targets of the hepatitis D virus therapy is inevitable for the improvement and development of a sufficient therapy that HDV patients are desperately in need of.

Comprehensive Analysis of Hepatitis Delta Virus Assembly Determinants According to Genotypes: Lessons From a Study of 526 Hepatitis Delta Virus Clinical Strains

Human hepatitis Delta virus (HDV) infection is associated to the most severe viral hepatic disease, including severe acute liver decompensation and progression to cirrhosis, and hepatocellular carcinoma. HDV is a satellite of hepatitis B virus (HBV) that requires the HBV envelope proteins for assembly of HDV virions. HDV and HBV exhibit a large genetic diversity that extends, respectively to eight (HDV-1 to -8) and to ten (HBV/A to/J) genotypes. Molecular determinants of HDV virion assembly consist of a C-terminal Proline-rich domain in the large Hepatitis Delta Antigen (HDAg) protein, also known as the Delta packaging domain (DPD) and of a Tryptophan-rich domain, the HDV matrix domain (HMD) in the C-terminal region of the HBV envelope proteins. In this study, we performed a systematic genotyping of HBV and HDV in a cohort 1,590 HDV-RNA-positive serum samples collected between 2001 to 2014, from patients originated from diverse parts of the world, thus reflecting a large genetic diversity. Among these samples, 526 HBV (HBV/A, B, C, D, E, and G) and HDV (HDV-1, 2, 3, and 5 to -8) genotype couples could be obtained. We provide results of a comprehensive analysis of the amino-acid sequence conservation within the HMD and structural and functional features of the DPD that may account for the yet optimal interactions between HDV and its helper HBV.

Constraint of Base Pairing on HDV Genome Evolution

The hepatitis delta virus is a single-stranded circular RNA virus, which is characterized by high self-complementarity. About 70% of the genome sequences can form base-pairs with internal nucleotides. There are many studies on the evolution of the hepatitis delta virus. However, the secondary structure has not been taken into account in these studies. In this study, we developed a method to examine the effect of base pairing as a constraint on the nucleotide substitutions during the evolution of the hepatitis delta virus. The method revealed that the base pairing can reduce the evolutionary rate in the non-coding region of the virus. In addition, it is suggested that the non-coding nucleotides without base pairing may be under some constraint, and that the intensity of the constraint is weaker than that by the base pairing but stronger than that on the synonymous site.

Proteomic Analysis of Nuclear HBV rcDNA Associated Proteins Identifies UV-DDB as a Host Factor Involved in cccDNA Formation

Hepatitis B virus (HBV) utilizes host DNA repair mechanisms to convert viral relaxed circular DNA (rcDNA) into a persistent viral genome, the covalently closed circular DNA (cccDNA). To identify said host factors involved in cccDNA formation, we developed an unbiased approach to discover proteins involved in cccDNA formation by precipitating nuclear rcDNA from induced HepAD38 cells and identifying the co-precipitated proteins by mass spectrometry. The DNA damage binding protein 1 (DDB1) surfaced as a hit, coinciding with our previously reported shRNA screen in which shRNA-DDB1 in HepDES19 cells reduced cccDNA production. DDB1 binding to nuclear rcDNA was confirmed in HepAD38 cells via ChIP-qPCR. DDB1 and DNA damage binding protein 2 (DDB2) form the UV-DDB complex and the latter senses DNA damage to initiate the global genome nucleotide excision repair (GG-NER) pathway. To investigate the role of DDB complex in cccDNA formation, DDB2 was knocked out in HepAD38 and HepG2-NTCP cells. In both knockout cell lines, cccDNA formation was stunted significantly, and in HepG2-NTCP-DDB2 knockout cells, downstream indicators of cccDNA such as HBV RNA, HBcAg, and HBeAg were similarly reduced. Knockdown of DDB2 in HBV-infected HepG2-NTCP cells and primary human hepatocytes (PHH) also resulted in cccDNA reduction. Trans-complementation of wild type DDB2 in HepG2-NTCP-DDB2 knockout cells rescued cccDNA formation and its downstream indicators. However, ectopic expression of DDB2 mutants deficient in DNA-binding, DDB1-binding, or ubiquitination failed to rescue cccDNA formation. Our study thus suggests an integral role of UV-DDB, specifically DDB2, in the formation of HBV cccDNA. IMPORTANCE Serving as a key viral factor for chronic hepatitis B virus (HBV) infection, HBV covalently closed circular DNA (cccDNA) is formed in the cell nucleus from viral relaxed circular DNA (rcDNA) by hijacking host DNA repair machinery. Previous studies have identified a handful of host DNA repair factors involved in cccDNA formation through hypothesis-driven research with some help from RNAi screening and/or biochemistry approaches. To enrich the landscape of tools for discovering host factors responsible for rcDNA-to-cccDNA conversion, we developed an rcDNA immunoprecipitation paired mass spectrometry assay, which allowed us to pull down nuclear rcDNA in its transitional state to cccDNA and observe the associated host factors. From this assay we discovered a novel relationship between the UV-DDB complex and cccDNA formation, hence, providing a proof-of-concept for a more direct discovery of novel HBV DNA-host interactions that can be exploited to develop new cccDNA-targeting antivirals.

Hepatitis Delta Virus (HDV) and Delta-Like Agents: Insights Into Their Origin

Hepatitis delta virus (HDV) is a human pathogen, and the only known species in the genus Deltavirus. HDV is a satellite virus and depends on the hepatitis B virus (HBV) for packaging, release, and transmission. Extracellular HDV virions contain the genomic HDV RNA, a single-stranded negative-sense and covalently closed circular RNA molecule, which is associated with the HDV-encoded delta antigen forming a ribonucleoprotein complex, and enveloped by the HBV surface antigens. Replication occurs in the nucleus and is mediated by host enzymes and assisted by cis-acting ribozymes allowing the formation of monomer length molecules which are ligated by host ligases to form unbranched rod-like circles. Recently, meta-transcriptomic studies investigating various vertebrate and invertebrate samples identified RNA species with similarities to HDV RNA. The delta-like agents may be representatives of novel subviral agents or satellite viruses which share with HDV, the self-complementarity of the circular RNA genome, the ability to encode a protein, and the presence of ribozyme sequences. The widespread distribution of delta-like agents across different taxa with considerable phylogenetic distances may be instrumental in comprehending their evolutionary history by elucidating the transition from transcriptome to cellular circular RNAs to infectious subviral agents.

Vitamin D deficiency in patients with chronic hepatitis D viral infection

Aim of the study

Vitamin D deficiency is known to be associated with disease severity, unresponsiveness to treatment, and morbidity among patients with chronic viral hepatitis B and C, autoimmune hepatitis, and alcoholic hepatitis. This study aims to research vitamin D levels in patients suffering from cirrhotic and non-cirrhotic phases of hepatitis D.

Material and methods

170 individuals in total were included in the study in the form of two groups: the first group of 100 patients with chronic hepatitis D (CHD), 30 of whom had cirrhosis, and the second control group of 70 individuals with similar characteristics to those of the first group in terms of age, type, and seasonal sampling. Levels of 25-hydroxy vitamin D [25(OH)D] were measured in the serum collected from patients and the control group.

Results

The lowest 25(OH)D levels were identified in patients with cirrhotic CHD. When these levels were compared with those of the control group, they were found to be significant (15.30 ±6.92 and 18.90 ±8.30 ng/ml, respectively, p = 0.04). 25(OH)D deficiency (< 10 ng/ml) was detected at significantly higher rates in patients with both cirrhotic and non-cirrhotic CHD compared to the healthy controls (30%, 25%, and 8.5%, respectively, p = 0.01). A significant correlation was established between 25(OH)D levels and bilirubin in patients with CHD (r = 0.252, p = 0.012). Multivariate analysis showed that chronic hepatitis D (odds ratio [OR] = 3.608, 95% confidence interval [CI]: 1.31-9.89, p = 0.013) and age (OR = 1.04, 95% CI: 1.00-1.08, p = 0.033) were associated with vitamin D deficiency.

Conclusions

Frequency of 25(OH)D vitamin deficiency is higher in patients with CHD. The identification of vitamin D levels and the replacement of any deficiency may create a positive effect on disease progression, morbidity, and mortality levels.

Innate immunity in hepatitis B and D virus infection: consequences for viral persistence, inflammation, and T cell recognition

Chronic infections with human hepatitis viruses continue to be a major health burden worldwide. Despite the availability of an effective prophylactic vaccine against the hepatitis B virus (HBV) and of antiviral agents efficiently suppressing HBV replication, more than 250 million people are currently chronically infected with this hepatotropic DNA virus, and resolution of chronic hepatitis B (CHB) is rarely achieved. Moreover, coinfection with the hepatitis D virus (HDV), a human RNA satellite virus requiring the envelope proteins of HBV for productive viral spreading, substantially aggravates the disease course of CHB. The molecular mechanisms by which these viruses interact with each other and with the intrinsic innate responses of the hepatocytes are not fully understood. While HBV appears to avoid innate immune recognition, HDV elicits a strong enhancement of innate responses. Notwithstanding, such induction does not hamper HDV replication but contributes to liver inflammation and pathogenesis. Intriguingly, HDV appears to influence the ability of T cells to recognize infected hepatocytes by boosting antigen presentation. This review focuses on current knowledge regarding how these viruses can shape and counteract the intrinsic innate responses of the hepatocytes, thus affecting the immune system and pathogenesis. Understanding the distinct strategies of persistence that HBV and HDV have evolved is central for advancing the development of curative therapies.

Inosine in Biology and Disease

The nucleoside inosine plays an important role in purine biosynthesis, gene translation, and modulation of the fate of RNAs. The editing of adenosine to inosine is a widespread post-transcriptional modification in transfer RNAs (tRNAs) and messenger RNAs (mRNAs). At the wobble position of tRNA anticodons, inosine profoundly modifies codon recognition, while in mRNA, inosines can modify the sequence of the translated polypeptide or modulate the stability, localization, and splicing of transcripts. Inosine is also found in non-coding and exogenous RNAs, where it plays key structural and functional roles. In addition, molecular inosine is an important secondary metabolite in purine metabolism that also acts as a molecular messenger in cell signaling pathways. Here, we review the functional roles of inosine in biology and their connections to human health.

Identification of novel avian and mammalian deltaviruses provides new insights into deltavirus evolution

Hepatitis delta virus (HDV) is a satellite virus that requires hepadnavirus envelope proteins for its transmission. Although recent studies identified HDV-related deltaviruses in certain animals, the evolution of deltaviruses, such as the origin of HDV and the mechanism of its coevolution with its helper viruses, is unknown, mainly because of the phylogenetic gaps among deltaviruses. Here, we identified novel deltaviruses of passerine birds, woodchucks, and white-tailed deer by extensive database searches and molecular surveillance. Phylogenetic and molecular epidemiological analyses suggest that HDV originated from mammalian deltaviruses and the past interspecies transmission of mammalian and passerine deltaviruses. Further, metaviromic and experimental analyses suggest that the satellite-helper relationship between HDV and hepadnavirus was established after the divergence of the HDV lineage from non-HDV mammalian deltaviruses. Our findings enhance our understanding of deltavirus evolution, diversity, and transmission, indicating the importance of further surveillance for deltaviruses.

The evolution and clinical impact of hepatitis B virus genome diversity

The global burden of hepatitis B virus (HBV) is enormous, with 257 million persons chronically infected, resulting in more than 880,000 deaths per year worldwide. HBV exists as nine different genotypes, which differ in disease progression, natural history and response to therapy. HBV is an ancient virus, with the latest reports greatly expanding the host range of the Hepadnaviridae (to include fish and reptiles) and casting new light on the origins and evolution of this viral family. Although there is an effective preventive vaccine, there is no cure for chronic hepatitis B, largely owing to the persistence of a viral minichromosome that is not targeted by current therapies. HBV persistence is also facilitated through aberrant host immune responses, possibly due to the diverse intra-host viral populations that can respond to host-mounted and therapeutic selection pressures. This Review summarizes current knowledge on the influence of HBV diversity on disease progression and treatment response and the potential effect on new HBV therapies in the pipeline. The mechanisms by which HBV diversity can occur both within the individual host and at a population level are also discussed.

Hepatitis Delta Virus Alters the Autophagy Process To Promote Its Genome Replication

A substantial number of viruses have been demonstrated to subvert autophagy to promote their own replication. Recent publications have reported the proviral effect of autophagy induction on hepatitis B virus (HBV) replication. Hepatitis delta virus (HDV) is a defective virus and an occasional obligate satellite of HBV. However, no previous work has studied the relationship between autophagy and HDV. In this article, we analyze the impact of HBV and HDV replication on autophagy as well as the involvement of the autophagy machinery in the HDV life cycle when produced alone and in combination with HBV. We prove that HBxAg and HBsAg can induce early steps of autophagy but ultimately block flux. It is worth noting that the two isoforms of the HDV protein, the small HDAg (S-HDAg) and large HDAg (L-HDAg) isoforms, can also efficiently promote autophagosome accumulation and disturb autophagic flux. Using CRISPR-Cas9 technology to generate specific knockouts, we demonstrate that the autophagy machinery, specifically the proteins implicated in the elongation step (ATG7, ATG5, and LC3), is important for the release of HBV without affecting the level of intracellular HBV genomes. Surprisingly, the knockout of ATG5 and ATG7 decreased the intracellular HDV RNA level in both Huh7 and HepG2.2.15 cells without an additional effect on HDV secretion. Therefore, we conclude that HBV and HDV have evolved to utilize the autophagy machinery so as to assist at different steps of their life cycle.IMPORTANCE Hepatitis delta virus is a defective RNA virus that requires hepatitis B virus envelope proteins (HBsAg) to fulfill its life cycle. Thus, HDV can only infect individuals at the same time as HBV (coinfection) or superinfect individuals who are already chronic carriers of HBV. The presence of HDV in the liver accelerates the progression of infection to fibrosis and to hepatic cancer. Since current treatments against HBV are ineffective against HDV, it is of paramount importance to study the interaction between HBV, HDV, and host factors. This will help unravel new targets whereby a therapy that is capable of simultaneously impeding both viruses could be developed. In this research paper, we evidence that the autophagy machinery promotes the replication of HBV and HDV at different steps of their life cycle. Notwithstanding their contribution to HBV release, autophagy proteins seem to assist HDV intracellular replication but not its secretion.

Epitranscriptomic marks: Emerging modulators of RNA virus gene expression

Epitranscriptomics, the study of posttranscriptional chemical moieties placed on RNA, has blossomed in recent years. This is due in part to the emergence of high‐throughput detection methods as well as the burst of discoveries showing biological function of select chemical marks. RNA modifications have been shown to affect RNA structure, localization, and functions such as alternative splicing, stabilizing transcripts, nuclear export, cap‐dependent and cap‐independent translation, microRNA biogenesis and binding, RNA degradation, and immune regulation. As such, the deposition of chemical marks on RNA has the unique capability to spatially and temporally regulate gene expression. The goal of this article is to present the exciting convergence of the epitranscriptomic and virology fields, specifically the deposition and biological impact of N7‐methylguanosine, ribose 2′‐O‐methylation, pseudouridine, inosine, N6‐methyladenosine, and 5‐methylcytosine epitranscriptomic marks on gene expression of RNA viruses.

This article is categorized under:

RNA in Disease and Development > RNA in Disease

RNA Interactions with Proteins and Other Molecules > Protein–RNA Interactions: Functional Implications

Structural Pattern Differences in Unbranched Rod-like RNA of Hepatitis Delta Virus affect RNA Editing

Hepatitis delta virus (HDV) RNA forms an unbranched rod-like structure and complexes with the delta antigen (HDAg). Host ADAR1-catalyzed RNA editing at the amber/W site of the small HDAg leads to the production of the large HDAg, which inhibits replication and is required for virion assembly. For HDV genotype 1, amber/W editing is controlled by HDAg and the RNA structure immediate vicinity and downstream of the editing site. Here, the effects of 20 mutants carrying an increased length of consecutive base-pairing at various sites in HDV RNA on amber/W site editing were examined. All nine mutants carrying genomic regions that formed up to 15 consecutive base pairs, which is also the maximum length observed in 41 naturally occurring HDV genomes, showed normal editing rate. However, mutants carrying a 16 or 17 consecutive base-paired antigenomic segment located as far as 114 nt upstream could increase editing efficiency, possibly by interfering with HDAg binding. These data show for the first time that extended base-pairing upstream of the amber/W site could increase HDV RNA editing efficiency. Furthermore, it appears that the naturally occurring HDV RNA structures have been selected for suboptimal amber/W RNA editing, which favors the HDV replication cycle.

Common viral infections in kidney transplant recipients

Infectious complications have been considered as a major cause of morbidity and mortality after kidney transplantation, especially in the Asian population. Therefore, prevention, early detection, and prompt treatment of such infections are crucial in kidney transplant recipients. Among all infectious complications, viruses are considered to be the most common agents because of their abundance, infectivity, and latency ability. Herpes simplex virus, varicella zoster virus, Epstein-Barr virus, cytomegalovirus, hepatitis B virus, BK polyomavirus, and adenovirus are well-known etiologic agents of viral infections in kidney transplant patients worldwide because of their wide range of distribution. As DNA viruses, they are able to reactivate after affected patients receive immunosuppressive agents. These DNA viruses can cause systemic diseases or allograft dysfunction, especially in the first six months after transplantation. Pretransplant evaluation and immunization as well as appropriate prophylaxis and preemptive approaches after transplant have been established in the guidelines and are used effectively to reduce the incidence of these viral infections. This review will describe the etiology, diagnosis, prevention, and treatment of viral infections that commonly affect kidney transplant recipients.

The Hepatitis Delta Virus accumulation requires paraspeckle components and affects NEAT1 level and PSP1 localization

The Hepatitis Delta Virus (HDV) relies mainly on host proteins for its replication. We previously identified that PSF and p54nrb associate with the HDV RNA genome during viral replication. Together with PSP1, these proteins are part of paraspeckles, which are subnuclear bodies nucleated by the long non-coding RNA NEAT1. In this work, we established the requirement for PSF, p54nrb and PSP1 in HDV replication using RNAi-mediated knockdown in HEK-293 cells replicating the HDV RNA genome. We determined that HDV replication induces the delocalization of PSP1 to cytoplasmic foci containing PABP and increases NEAT1 level causing an enlargement of NEAT1 foci. Overall, our data support a role for the main paraspeckles proteins in HDV life cycle and indicate that HDV replication causes a cellular stress and induces both a delocalization of the PSP1 to the cytoplasm and a disruption of paraspeckles.

Molecular characterization of the full-length genome sequences of HDV strains circulating in Tunisia

While Tunisia is endemic for hepatitis B virus (HBV), a recent large-scale retrospective study, revealed a very low prevalence (2%) of hepatitis Delta virus (HDV) (Yacoubi et al. in J Clin Virol 72:126-132, 2015). All strains were classified within the genotype 1 (HDV-1) as assessed by nucleotide sequencing of the so-called 'R0' region of the genome described previously. In this study, we aimed to determine the full-length genome sequence of HDV isolates in order to fully characterize the HDV strains spreading in Tunisia. Eleven HDV antibody and RNA positive samples were obtained from the 1615 clinical samples previously studied. The whole genome sequence was obtained for 5 strains by sequencing and realignment of four overlapping regions covering the entire genome, followed by extensive phylogenetic analyses. Tunisian sequences segregated together with Turkish and African sequences and showed 60% GC content. Alignment with an HDV-1 consensus sequence revealed that they exhibited several point mutations in different functional domains of the delta proteins that, according to previous studies, might possibly affect their properties. In conclusion, the first full-length genome sequences of Tunisian HDV isolates are provided, isolates which are closely related to Turkish and Sub-Saharan Africa strains, supporting the hypothesis for the spread of HDV-1-strains from Africa via Tunisia to Turkey, before spread to the rest of the world.

The hepatitis delta virus: Replication and pathogenesis

Hepatitis delta virus (HDV) is a defective virus and a satellite of the hepatitis B virus (HBV). Its RNA genome is unique among animal viruses, but it shares common features with some plant viroids, including a replication mechanism that uses a host RNA polymerase. In infected cells, HDV genome replication and formation of a nucleocapsid-like ribonucleoprotein (RNP) are independent of HBV. But the RNP cannot exit, and therefore propagate, in the absence of HBV, as the latter supplies the propagation mechanism, from coating the HDV RNP with the HBV envelope proteins for cell egress to delivery of the HDV virions to the human hepatocyte target. HDV is therefore an obligate satellite of HBV; it infects humans either concomitantly with HBV or after HBV infection. HDV affects an estimated 15 to 20 million individuals worldwide, and the clinical significance of HDV infection is more severe forms of viral hepatitis--acute or chronic--, and a higher risk of developing cirrhosis and hepatocellular carcinoma in comparison to HBV monoinfection. This review covers molecular aspects of HDV replication cycle, including its interaction with the helper HBV and the pathogenesis of infection in humans.

Therapeutic Strategies and New Intervention Points in Chronic Hepatitis Delta Virus Infection

Chronic hepatitis delta virus infection (CHD) is a condition arising from super-infection of hepatitis B virus (HBV)-infected patients, resulting in a more rapid advance in liver pathology and hepatocellular carcinoma than is observed for HBV mono-infection. Although hepatitis delta virus (HDV) is structurally simple, its life cycle involves the complex participation of host enzymes, HBV-derived surface antigen (HBsAg), and HDV-auto-ribozyme and hepatitis delta antigen (HDAg) activities. Unsatisfactory clinical trial results with interferon-based therapies are motivating researchers to adjust and redirect the approach to CHD drug development. This new effort will likely require additional structural and functional studies of the viral and cellular/host components involved in the HDV replication cycle. This review highlights recent work aimed at new drug interventions for CHD, with interpretation of key pre-clinical- and clinical trial outcomes and a discussion of promising new technological approaches to antiviral drug design.

Tumour necrosis factor-alpha, interleukin-10, interferon-gamma and vitamin D receptor gene polymorphisms in patients with chronic hepatitis delta

No data exist to assess certain polymorphisms that have a potential effect on the immune response in patients with chronic hepatitis delta (CHD). The aim of this study was to investigate polymorphisms in 6 polymorphic sites: IL-10 -1082 (rs1800896), IL-10 -627 (rs1800872), IFN-γ +874 (rs62559044), TNF-α -308 (rs1800629), vitamin D receptor (VDR) FokI (rs2228570) and VDR TaqI (rs731236). The genotypes of 67 patients with CHD and 119 patients with chronic hepatitis B (CHB) were compared. In addition, 56 individuals with resolved hepatitis B virus (HBV) infection were used as a control group for patients with CHB. Polymorphisms in TNF-α, IL-10, and VDR genes were analysed using polymerase chain reaction/restriction fragment length polymorphism methods. The IFN-γ gene polymorphism was detected by allele-specific polymerase chain reaction (PCR). Patients with CDH were more likely to have advanced liver disease compared with patients with CHB (P < 0.0001). IL-10 -1082 and VDR TaqI polymorphisms showed significant differences between patients with CHD and CHB. The high secretory IL-10 -1082 genotype GG was less frequent in CHD compared with patients with CHB and resolved HBV (17.7%, 37.4% and 47.1%, respectively (P < 0.05 for CHD vs CHB and resolved HBV). The frequency of the high secretory VDR TaqI TT genotype was 86.6% in patients with CHD, 62.7% in patients with CHB and 62.5% in resolved HBV individuals (CHD vs CHB: P < 0.05). None of the polymorphisms analysed had an effect on HBV persistence. IL-10 -1082 and VDR TaqI polymorphisms may contribute to the more severe liver disease associated with CHD compared with CHB.

Hepatitis delta virus genotype-1 alone cocirculates with hepatitis B virus genotypes A and D in Pakistan

INTRODUCTION

Hepatitis delta virus (HDV) and hepatitis B virus (HBV) have been identified as major causes of morbidity and mortality in Pakistan because HDV causes infection only in the presence of HBV. Coinfection with both hepatitis viruses can lead to a more severe acute form of disease and to an increased risk of fulminant hepatitis. HDV infection differs in its distribution and severity depending on the geographical distribution, and several genotypes of HDV have been identified so far.

OBJECTIVES

The aim of the present study was to establish the HDV and HBV genotypes in chronically infected Pakistani patients and to determine whether there is any correlation between HDV and HBV genotypes.

PATIENTS AND METHODS

We studied samples from a total of 46 chronically infected HBV and HDV patients for HBV and HDV genotype analysis out of a total of 75 chronic HBV carriers enrolled. HBV and HDV genotypes were determined using type-specific PCR, followed by sequencing of PCR amplified products.

RESULTS

The results of HBV genotyping showed that 33 of 46 (71.7%) patients had genotype D, five (10.9%) had A+D mixed genotypes, whereas eight (17.3) samples were untypable. We could detect only one HDV genotype (HDV-1) prevalent in the Pakistani population. The HDV-1 genotype isolate was associated with HBV genotype D alone or in combination with A (HBV-A+D).

CONCLUSION

The present study concludes that HDV/HBV coinfection is very high in the Pakistani population and was previously underestimated. The most prevalent circulating genotypes of HBV and HDV are HDV-1 and HBV-D, respectively, in the studied area. There is no specific interaction between HBV and HDV genotypes as suggested by HDV-1/HBV-D or HDV-1/HBV-A+D coinfection. Coinfection of HDV-1 and HBV-D simply reflects the most frequent genotypes circulating in this specific geographical region of the world.

Construction and eukaryotic expression of recombinant large hepatitis delta antigen

BACKGROUND

Hepatitis delta virus (HDV) is a subviral human pathogen that exploits host RNA editing activity to produce two essential forms of the sole viral protein, hepatitis delta antigen (HDAg). Editing at the amber/W site of HDV antigenomic RNA leads to the production of the large form (L-HDAg), which is required for RNA packaging.

METHODS

In this study, PCR-based site-directed mutagenesis by the overlap extension method was used to create the point mutation converting the small-HDAg (S-HDAg) stop codon to a tryptophan codon through three stages.

RESULTS

Sequencing confirmed the desirable mutation and integrity of the L-HDAg open reading frame. The amplicon was ligated into pcDNA3.1 and transfected to Huh7 and HEK 293 cell lines. Western blot analysis using enhanced chemiluminescence confirmed L-HDAg expression. The recombinant L-HDAg localized within the nuclei of cells as determined by immunofluorescence and confocal microscopy.

CONCLUSION

Because L-HDAg requires extensive post-translational modifications, the recombinant protein expressed in a mammalian system might be fully functional and applicable as a tool in HDV molecular studies, as well as in future vaccine research.

An update on HDV: virology, pathogenesis and treatment

Hepatitis delta is an inflammatory liver disease caused by infection with HDV. HDV is a single-stranded circular RNA pathogen with a diameter of 36 nm. HDV is classified in the genus Deltavirus and is still awaiting a final taxonomic classification up to the family level. HDV shares similarities with satellite RNA and viroids including a small circular single-stranded RNA with secondary structure that replicates through the 'double rolling circle' mechanism. The HDV RNA genome is capable of self-cleavage through a ribozyme and encodes only one structural protein, the hepatitis delta antigen (HDAg), from the antigenomic RNA. There are two forms of HDAg, a shorter (S; 22 kDa) and a longer (L; 24 kDa) form, the latter generated from an RNA editing mechanism. The S form is essential for viral genomic replication. The L form participates in the assembly and formation of HDV. For complete replication and transmission, HDV requires the hepatitis B surface antigen (HBsAg). Thus, HDV infection only occurs in HBsAg-positive individuals, either as acute coinfection in treatment-naive HBV-infected persons, or as superinfection in patients with pre-existing chronic hepatitis B (CHB). HDV is found throughout the world, but its prevalence, incidence, clinical features and epidemiological characteristics vary by geographic region. There are eight genotypes (1 to 8) distributed over different geographic areas: HDV-1 is distributed worldwide, whereas HDV-2 to 8 are seen more regionally. Levels of HDV viraemia change over the course of HDV infection, being significantly higher in patients with early chronic hepatitis than in cirrhosis. Chronic HDV infection leads to more severe liver disease than chronic HBV monoinfection with an accelerated course of fibrosis progression, an increased risk of hepatocellular carcinoma and early decompensation in the setting of established cirrhosis. Current treatments include pegylated interferon-α and liver transplantation; the latter of which can be curative. Further studies are needed to develop better treatment strategies for this challenging disease.

Identification of a binding site for ASF/SF2 on an RNA fragment derived from the hepatitis delta virus genome

The hepatitis delta virus (HDV) is a small (∼1700 nucleotides) RNA pathogen which encodes only one open reading frame. Consequently, HDV is dependent on host proteins to replicate its RNA genome. Recently, we reported that ASF/SF2 binds directly and specifically to an HDV-derived RNA fragment which has RNA polymerase II promoter activity. Here, we localized the binding site of ASF/SF2 on the HDV RNA fragment by performing binding experiments using purified recombinant ASF/SF2 combined with deletion analysis and site-directed mutagenesis. In addition, we investigated the requirement of ASF/SF2 for HDV RNA replication using RNAi-mediated knock-down of ASF/SF2 in 293 cells replicating HDV RNA. Overall, our results indicate that ASF/SF2 binds to a purine-rich region distant from both the previously published initiation site of HDV mRNA transcription and binding site of RNAP II, and suggest that this protein is not involved in HDV replication in the cellular system used.

A new Ebola virus nonstructural glycoprotein expressed through RNA editing

Ebola virus (EBOV), an enveloped, single-stranded, negative-sense RNA virus, causes severe hemorrhagic fever in humans and nonhuman primates. The EBOV glycoprotein (GP) gene encodes the nonstructural soluble glycoprotein (sGP) but also produces the transmembrane glycoprotein (GP₁,₂) through transcriptional editing. A third GP gene product, a small soluble glycoprotein (ssGP), has long been postulated to be produced also as a result of transcriptional editing. To identify and characterize the expression of this new EBOV protein, we first analyzed the relative ratio of GP gene-derived transcripts produced during infection in vitro (in Vero E6 cells or Huh7 cells) and in vivo (in mice). The average percentages of transcripts encoding sGP, GP₁,₂, and ssGP were approximately 70, 25, and 5%, respectively, indicating that ssGP transcripts are indeed produced via transcriptional editing. N-terminal sequence similarity with sGP, the absence of distinguishing antibodies, and the abundance of sGP made it difficult to identify ssGP through conventional methodology. Optimized 2-dimensional (2D) gel electrophoresis analyses finally verified the expression and secretion of ssGP in tissue culture during EBOV infection. Biochemical analysis of recombinant ssGP characterized this protein as a disulfide-linked homodimer that was exclusively N glycosylated. In conclusion, we have identified and characterized a new EBOV nonstructural glycoprotein, which is expressed as a result of transcriptional editing of the GP gene. While ssGP appears to share similar structural properties with sGP, it does not appear to have the same anti-inflammatory function on endothelial cells as sGP.

Interaction of host cellular proteins with components of the hepatitis delta virus

The hepatitis delta virus (HDV) is the smallest known RNA pathogen capable of propagation in the human host and causes substantial global morbidity and mortality. Due to its small size and limited protein coding capacity, HDV is exquisitely reliant upon host cellular proteins to facilitate its transcription and replication. Remarkably, HDV does not encode an RNA-dependent RNA polymerase which is traditionally required to catalyze RNA-templated RNA synthesis. Furthermore, HDV lacks enzymes responsible for post-transcriptional and -translational modification, processes which are integral to the HDV life cycle. This review summarizes the known HDV-interacting proteins and discusses their significance in HDV biology.

Hepatitis delta virus RNA replication

Hepatitis delta virus (HDV) is a distant relative of plant viroids in the animal world. Similar to plant viroids, HDV replicates its circular RNA genome using a double rolling-circle mechanism. Nevertheless, the production of hepatitis delta antigen (HDAg), which is indispensible for HDV replication, is a unique feature distinct from plant viroids, which do not encode any protein. Here the HDV RNA replication cycle is reviewed, with emphasis on the function of HDAg in modulating RNA replication and the nature of the enzyme involved.

Hepatitis delta virus large antigen sensitizes to TNF-alpha-induced NF-kappaB signaling

Hepatitis delta virus (HDV) infection causes fulminant hepatitis and liver cirrhosis. To elucidate the molecular mechanism of HDV pathogenesis, we examined the effects of HDV viral proteins, the small hepatitis delta antigen (SHDAg) and the large hepatitis delta antigen (LHDAg), on NF-kappaB signaling pathway. In this study, we demonstrated that TNF-alpha-induced NF-kappaB transcriptional activation was increased by LHDAg but not by SHDAg in both HEK293 and Huh7 cells. Furthermore, LHDAg promoted TRAF2-induced NF-kappaB activation. Using coimmunoprecipitation assays, we demonstrated that both SHDAg and LHDAg interacted with TRAF2 protein. We showed that isoprenylation of LHDAg was not required for the increase of NF-kappaB activity. We further showed that only LHDAg but not SHDAg increased the TNF-alpha-mediated nuclear translocation of p65. This was accomplished by activation of IkappaBalpha degradation by LHDAg. Finally, we demonstrated that LHDAg augmented the COX-2 expression level in Huh7 cells. These data suggest that LHDAg modulates NF-kappaB signaling pathway and may contribute to HDV pathogenesis.

Hepatitis delta virus inhibits alpha interferon signaling

Hepatitis delta virus (HDV) can cause severe acute and chronic liver disease in patients infected with hepatitis B virus. Interferon-alpha (IFN-alpha) is the only treatment reported to be effective in chronic hepatitis delta, albeit in a minority of patients. The molecular mechanisms underlying resistance to therapy are unclear. IFN-alpha-induced activation of the Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling cascade is essential for the induction of an antiviral state. Interference of HDV with the JAK-STAT pathway could be responsible for the IFN-alpha resistance in chronic hepatitis delta patients. We analyzed IFN-alpha-induced signal transduction through the JAK-STAT pathway in human hepatoma cells transfected with the complete HDV genome. The expression of IFN-alpha-stimulated genes was investigated with reverse transcription real-time polymerase chain reaction (PCR). STATs and JAKs activations were examined by immunofluorescence and immunoblot. The IFN-alpha-stimulated genes coding for the antiviral proteins myxovirus resistance A, double-stranded RNA (dsRNA)-activated protein kinase and 2',5'-oligoadenylate synthetase were down-regulated in HDV-transfected hepatoma cells in response to IFN-alpha treatment. HDV severely impaired the phosphorylation of both STAT1 and STAT2, thus preventing their accumulation in the nucleus. Furthermore, HDV blocked the IFN-alpha-stimulated tyrosine phosphorylation of IFN receptor-associated JAK kinase Tyk2, without affecting either the tyrosine phosphorylation of Jak1 or the expression of type I IFN receptor subunits.

CONCLUSIONS

IFN-alpha-induced intracellular signaling is impaired in HDV-transfected human hepatoma cells. HDV subverts the effect of IFN-alpha by blocking Tyk2 activation, thereby resulting in selective impairment of activation and translocation to the nucleus of STAT1 and STAT2. Interference of HDV with IFN-alpha signaling could represent an important mechanism of viral persistence and treatment resistance.

Primary human hepatocytes are susceptible to infection by hepatitis delta virus assembled with envelope proteins of woodchuck hepatitis virus

Hepatitis B virus (HBV) and hepatitis delta virus (HDV) share the HBV envelope proteins. When woodchucks chronically infected with woodchuck hepatitis virus (WHV) are superinfected with HDV, they produce HDV with a WHV envelope, wHDV. Several lines of evidence are provided that wHDV infects not only cultured primary woodchuck hepatocytes (PWH) but also primary human hepatocytes (PHH). Surprisingly, HBV-enveloped HDV (hHDV) and wHDV infected PHH with comparable efficiencies; however, hHDV did not infect PWH. The basis for these host range specificities was investigated using as inhibitors peptides bearing species-specific pre-S (where S is the small envelope protein) sequences. It was found that pre-S1 contributed to the ability of wHDV to infect both PHH and PWH. In addition, the inability of hHDV to infect PWH was not overcome using a chimeric form of hHDV containing WHV S protein, again supporting the essential role of pre-S1 in infection of target cells. One interpretation of these data is that host range specificity of HDV is determined entirely by pre-S1 and that the WHV and HBV pre-S1 proteins recognize different receptors on PHH.

Histone H1e interacts with small hepatitis delta antigen and affects hepatitis delta virus replication

Hepatitis delta virus (HDV) encodes two isoforms of delta antigens (HDAgs). The small form of HDAg (SHDAg) is required for HDV RNA replication, while the large form of HDAg (LHDAg) is required for viral assembly. Using tandem affinity purification method combined with mass spectrometry, we found that linker histone H1e bound to SHDAg. The binding domain of SHDAg to histone H1e was mapped to the N-terminal 67 amino acids. Oligomerization of SHDAg was required for its interaction with histone H1e. LHDAg barely bound to histone H1e and was masked at N-terminus. The binding domain of histone H1e to SHDAg was mapped to its central globular domain. HDV replication was inhibited by N- or C-terminal deletion mutants of histone H1e and was rescued by wild-type histone H1e. We conclude that histone H1e plays a significant role in HDV replication through forming protein complex with SHDAg.

Assembly of hepatitis delta virus: particle characterization, including the ability to infect primary human hepatocytes

Efficient assembly of hepatitis delta virus (HDV) was achieved by cotransfection of Huh7 cells with two plasmids: one to provide expression of the large, middle, and small envelope proteins of hepatitis B virus (HBV), the natural helper of HDV, and another to initiate replication of the HDV RNA genome. HDV released into the media was assayed for HDV RNA and HBV envelope proteins and characterized by rate-zonal sedimentation, immunoaffinity purification, electron microscopy, and the ability to infect primary human hepatocytes. Among the novel findings were that (i) immunostaining for delta antigen 6 days after infection with 300 genome equivalents (GE) per cell showed only 1% of cells as infected, but this was increased to 16% when 5% polyethylene glycol was present during infection; (ii) uninfected cells did not differ from infected cells in terms of albumin accumulation or the presence of E-cadherin at cell junctions; and (iii) sensitive quantitative real-time PCR assays detected HDV replication even when the multiplicity of infection was 0.2 GE/cell. In the future, this HDV assembly and infection system can be further developed to better understand the mechanisms shared by HBV and HDV for attachment and entry into host cells.

Large hepatitis delta antigen modulates transforming growth factor-beta signaling cascades: implication of hepatitis delta virus-induced liver fibrosis

BACKGROUND & AIMS

Transforming growth factor-beta (TGF-beta) has been implicated in the pathogenesis of liver disease. TGF-beta is involved in liver regeneration and in the fibrotic and cirrhotic transformation with hepatitis viral infection. Hepatitis delta virus (HDV) infection causes fulminant hepatitis and liver cirrhosis. To elucidate the molecular mechanism of HDV pathogenesis, we examined the effects of HDV-encoded-only protein, the small hepatitis delta antigen (SHDAg), and the large hepatitis delta antigen (LHDAg), on TGF-beta- and c-Jun-induced signaling cascades.

METHODS

The effects of either SHDAg or LHDAg on TGF-beta- and c-Jun-induced signaling cascades in Huh7 and Cos7 cells were investigated by luciferase reporter gene assay, immunoprecipitation assay, electrophoretic mobility shift assay, Western blot analysis, and confocal microscopy analysis.

RESULTS

The LHDAg, but not the SHDAg, potentiated TGF-beta- and c-Jun-induced signal activation, and the isoprenylation of LHDAg played a major role in signaling cascades. LHDAg synergistically activated hepatitis B virus X protein-mediated TGF-beta and AP-1 signaling cascades. In addition, LHDAg enhanced the protein expression level of TGF-beta-induced plasminogen activator inhibitor-1.

CONCLUSIONS

LHDAg may induce liver fibrosis through the regulation of TGF-beta-induced signal transductions. This regulation of TGF-beta-mediated signaling is accomplished by the isoprenylation of LHDAg, which is a novel mechanism involved in HDV pathogenesis.

Large hepatitis delta antigen modulates transforming growth factor-beta signaling cascades: implication of hepatitis delta virus-induced liver fibrosis

BACKGROUND & AIMS

Transforming growth factor-beta (TGF-beta) has been implicated in the pathogenesis of liver disease. TGF-beta is involved in liver regeneration and in the fibrotic and cirrhotic transformation with hepatitis viral infection. Hepatitis delta virus (HDV) infection causes fulminant hepatitis and liver cirrhosis. To elucidate the molecular mechanism of HDV pathogenesis, we examined the effects of HDV-encoded-only protein, the small hepatitis delta antigen (SHDAg), and the large hepatitis delta antigen (LHDAg), on TGF-beta- and c-Jun-induced signaling cascades.

METHODS

The effects of either SHDAg or LHDAg on TGF-beta- and c-Jun-induced signaling cascades in Huh7 and Cos7 cells were investigated by luciferase reporter gene assay, immunoprecipitation assay, electrophoretic mobility shift assay, Western blot analysis, and confocal microscopy analysis.

RESULTS

The LHDAg, but not the SHDAg, potentiated TGF-beta- and c-Jun-induced signal activation, and the isoprenylation of LHDAg played a major role in signaling cascades. LHDAg synergistically activated hepatitis B virus X protein-mediated TGF-beta and AP-1 signaling cascades. In addition, LHDAg enhanced the protein expression level of TGF-beta-induced plasminogen activator inhibitor-1.

CONCLUSIONS

LHDAg may induce liver fibrosis through the regulation of TGF-beta-induced signal transductions. This regulation of TGF-beta-mediated signaling is accomplished by the isoprenylation of LHDAg, which is a novel mechanism involved in HDV pathogenesis.

The human RNA polymerase II interacts with the terminal stem-loop regions of the hepatitis delta virus RNA genome

The hepatitis delta virus (HDV) is an RNA virus that depends on DNA-dependent RNA polymerase (RNAP) for its transcription and replication. While it is generally accepted that RNAP II is involved in HDV replication, its interaction with HDV RNA requires confirmation. A monoclonal antibody specific to the carboxy terminal domain of the largest subunit of RNAP II was used to establish the association of RNAP II with both polarities of HDV RNA in HeLa cells. Co-immunoprecipitations using HeLa nuclear extract revealed that RNAP II interacts with HDV-derived RNAs at sites located within the terminal stem-loop domains of both polarities of HDV RNA. Analysis of these regions revealed a strong selection to maintain a rod-like conformation and demonstrated several conserved features. These results provide the first direct evidence of an association between human RNAP II and HDV RNA and suggest two transcription start sites on both polarities of HDV RNA.

RNA editing in hepatitis delta virus

Hepatitis delta virus (HDV) relies heavily on host functions and on structural features of the viral RNA. A good example of this reliance is found in the process known as HDV RNA editing, which requires particular structural features in the HDV antigenome, and a host RNA editing enzyme, ADAR1. During replication, the adenosine at the amber/W site in the HDV antigenome is edited to inosine. As a result, the amber stop codon in the hepatitis delta antigen (HDAg) open reading frame is changed to a tryptophan codon and the reading frame is extended by 19 or 20 codons. Because these extra amino acids alter the functional properties of HDAg, this change serves a critical purpose in the HDV replication cycle. Analysis of the RNA secondary structures and regulation of editing in HDV genotypes I and III has indicated that although editing is essential for both genotypes, there are substantial differences. This review covers the mechanisms of RNA editing in the HDV replication cycle and the regulatory mechanisms by which HDV controls editing.

The role of the HBV envelope proteins in the HDV replication cycle

The hepatitis delta virus (HDV) is a subviral agent that utilizes the envelope proteins of the hepatitis B virus (HBV) for propagation. When introduced into permissive cells, the HDV RNA genome replicates and associates with multiple copies of the HDV-encoded proteins to assemble a ribonucleoprotein (RNP) complex. The mechanism necessary to export the RNP from the cell is provided by the HBV envelope proteins, which have the capacity to assemble lipoprotein vesicles that bud into the lumen of a pre-Golgi compartment before being secreted. In addition to allowing the release of the HDV RNP, the HBV envelope proteins also provide a means for its targeting to an uninfected cell, thereby ensuring the spread of HDV. This chapter covers the molecular aspects of the HBV envelope protein functions in the HDV replication cycle, in particular the activity of the small envelope protein in RNP export and the function of the large envelope protein at viral entry.

Structure and replication of hepatitis delta virus RNA

While this volume covers many different aspects of hepatitis delta virus (HDV) replication, the focus in this chapter is on studies of the structure and replication of the HDV RNA genome. An evaluation of such studies is not only an integral part of our understanding of HDV infections but it also sheds new light on some important aspects of cell biology, such as the fidelity of RNA transcription by a host RNA polymerase and on various forms of post-transcriptional RNA processing. Representations of the replication of the RNA genome are frequently simplified to a form of rolling-circle model, analogous to what have been described for plant viroids. One theme of this review is that such models, even after some revision, deceptively simplify the complexity of HDV replication and can fail to make clear major questions yet to be solved.

The Woodchuck Model of HDV Infection

The Eastern woodchuck, Marmota monax, has been a useful model system for the study of the natural history of hepadnavirus infection and for the development and preclinical testing of antiviral therapies. The model has also been used for hepatitis delta virus (HDV). In this chapter several new applications of the woodchuck model of HDV infection are presented and discussed. The development of a woodchuck HDV inoculum derived from a molecular clone has facilitated the analysis of viral genetic changes occurring during acute and chronic infection. This analysis has provided insights into one of the more important aspects of the natural history of HDV infection-whether a superinfection becomes chronic. These results could renew interest in further vaccine development. An effective therapy for chronic HDV infection remains an important clinical goal for this agent, particularly because of the severity of the disease and the inability of current hepadnaviral therapies to ameliorate it. The recent application of the woodchuck model of chronic HDV infection to therapeutic development has yielded promising results which indicate that targeting the hepadnavirus surface protein may be a successful therapeutic strategy for HDV.

Genetic changes in hepatitis delta virus from acutely and chronically infected woodchucks

A woodchuck-derived hepatitis delta virus (HDV) inoculum was created by transfection of a genotype I HDV cDNA clone directly into the liver of a woodchuck that was chronically infected with woodchuck hepatitis virus. All woodchucks receiving this inoculum became positive for HDV RNA in serum, and 67% became chronically infected, similar to the rate of chronic HDV infection in humans. Analysis of HDV sequences obtained at 73 weeks postinfection indicated that changes had occurred at a rate of 0.5% per year; many of these modifications were consistent with editing by host RNA adenosine deaminase. The appearance of sequence changes, which were not evenly distributed on the genome, was correlated with the course of HDV infection. A limited number of modifications occurred in the consensus sequence of the viral genome that altered the sequence of the hepatitis delta antigen (HDAg). All chronically infected animals examined exhibited these changes 73 weeks following infection, but at earlier times, only one of the HDV carriers exhibited consensus sequence substitutions. On the other hand, sequence modifications in animals that eventually recovered from HDV infection were apparent after 27 weeks. The data are consistent with a model in which HDV sequence changes are selected by host immune responses. Chronic HDV infection in woodchucks may result from a delayed and weak immune response that is limited to a small number of epitopes on HDAg.

The role of a metastable RNA secondary structure in hepatitis delta virus genotype III RNA editing

RNA editing plays a critical role in the life cycle of hepatitis delta virus (HDV). The host editing enzyme ADAR1 recognizes specific RNA secondary structure features around the amber/W site in the HDV antigenome and deaminates the amber/W adenosine. A previous report suggested that a branched secondary structure is necessary for editing in HDV genotype III. This branched structure, which is distinct from the characteristic unbranched rod structure required for HDV replication, was only partially characterized, and knowledge concerning its formation and stability was limited. Here, we examine the secondary structures, conformational dynamics, and amber/W site editing of HDV genotype III RNA using a miniaturized HDV genotype III RNA in vitro. Computational analysis of this RNA using the MPGAfold algorithm indicated that the RNA has a tendency to form both metastable and stable unbranched secondary structures. Moreover, native polyacrylamide gel electrophoresis demonstrated that this RNA forms both branched and unbranched rod structures when transcribed in vitro. As predicted, the branched structure is a metastable structure that converts readily to the unbranched rod structure. Only branched RNA was edited at the amber/W site by ADAR1 in vitro. The structural heterogeneity of HDV genotype III RNA is significant because not only are both conformations of the RNA functionally important for viral replication, but the ratio of the two forms could modulate editing by determining the amount of substrate RNA available for modification.