The large delta antigen of hepatitis delta virus potently inhibits genomic but not antigenomic RNA synthesis: a mechanism enabling initiation of viral replication

Diversity and evolution of viroids and viroid-like agents with circular RNA genomes revealed by metatranscriptome mining

Viroids, the agents of several plant diseases, are the smallest and simplest known replicators that consist of covalently closed circular (ccc) RNA molecules between 200 and 400 nucleotides in size. Viroids encode no proteins and rely on host RNA polymerases for replication, but some contain ribozymes involved in replication intermediate processing. Although other viroid-like agents with cccRNAs genomes, such as satellite RNAs, ribozyviruses and retrozymes, have been discovered, until recently, the spread of these agents in the biosphere appeared narrow, and their actual diversity and evolution remained poorly understood. Extensive, targeted metatranscriptome mining dramatically expanded the known diversity of cccRNAs genomes. These searches identified numerous, diverse viroid-like cccRNAs, many found in environments devoid of plant and animal material, suggesting replication in unicellular eukaryotic and/or prokaryotic hosts. Several cccRNAs are targeted by CRISPR systems, supporting their association with bacteria. In addition to small cccRNAs in the viroid size range, a broad variety of ribozyviruses and novel viruses with cccRNAs genomes, with genomes reaching nearly 5 kilobases, were discovered. Thus, metatranscriptome mining shows that the diversity of viroid-like cccRNAs genomes is far greater than previously suspected, prompting reassessment of the relevance of these replicators for understanding the primordial RNA world.

Frequency of hepatitis D virus with different hepatitis B virus serological markers and coinfections in hospital patients from Argentina: synchronous testing of anti-HDV antibodies and HDV RNA

BACKGROUND

Hepatitis D virus (HDV) RNA-positive cases with total anti-HDV antibodies nonreactive were documented. Moreover, HDV infection was observed in subjects with occult hepatitis B virus infection. The prevalence of HDV infection in Argentina is low; however, further research in different populations is needed.

OBJECTIVE

This study aimed to perform synchronous HDV detection in reactive hepatitis B virus patients treated in a public hospital in the province of Buenos Aires, Argentina, some of whom were coinfected with hepatitis C virus and/or HIV. A total of 189 hepatitis B virus-reactive serum samples with or without hepatitis C virus and/or HIV coinfection were synchronously analyzed for anti-HDV antibodies and HDV RNA.

RESULTS

HDV prevalence was 4.2% with HDV RNA found in 61 samples, most of which were nonreactive to anti-HDV antibodies and hepatitis B surface antigen. Genotype 1 was identified in all HDV sequences. Moreover, triple and quadruple infections were observed, showing a high frequency of HDV infection in hospitalized patients not following the recommended diagnostic algorithm.

CONCLUSIONS

This study is evidence that the synchronous testing of anti-HDV antibodies and HDV RNA is necessary for the diagnosis of HDV infection in Argentina. Finally, further research is necessary to identify high-risk populations and improve prevention and control strategies for triple and quadruple infections and their potential consequences.

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.

Hepatocyte Intrinsic Innate Antiviral Immunity against Hepatitis Delta Virus Infection: The Voices of Bona Fide Human Hepatocytes

The pathogenesis of viral infection is attributed to two folds: intrinsic cell death pathway activation due to the viral cytopathic effect, and immune-mediated extrinsic cellular injuries. The immune system, encompassing both innate and adaptive immunity, therefore acts as a double-edged sword in viral infection. Insufficient potency permits pathogens to establish lifelong persistent infection and its consequences, while excessive activation leads to organ damage beyond its mission to control viral pathogens. The innate immune response serves as the front line of defense against viral infection, which is triggered through the recognition of viral products, referred to as pathogen-associated molecular patterns (PAMPs), by host cell pattern recognition receptors (PRRs). The PRRs-PAMPs interaction results in the induction of interferon-stimulated genes (ISGs) in infected cells, as well as the secretion of interferons (IFNs), to establish a tissue-wide antiviral state in an autocrine and paracrine manner. Cumulative evidence suggests significant variability in the expression patterns of PRRs, the induction potency of ISGs and IFNs, and the IFN response across different cell types and species. Hence, in our understanding of viral hepatitis pathogenesis, insights gained through hepatoma cell lines or murine-based experimental systems are uncertain in precisely recapitulating the innate antiviral response of genuine human hepatocytes. Accordingly, this review article aims to extract and summarize evidence made possible with bona fide human hepatocytes-based study tools, along with their clinical relevance and implications, as well as to identify the remaining gaps in knowledge for future investigations.

Hepatitis Delta Virus-Host Protein Interactions: From Entry to Egress

Hepatitis delta virus (HDV) is the smallest known human virus and causes the most severe form of human viral hepatitis, yet it is still not fully understood how the virus replicates and how it interacts with many host proteins during replication. This review aims to provide a systematic review of all the host factors currently known to interact with HDV and their mechanistic involvement in all steps of the HDV replication cycle. Finally, we discuss implications for therapeutic development based on our current knowledge of HDV-host protein interactions.

Persistent hepatic IFN system activation in HBV-HDV infection determines viral replication dynamics and therapeutic response

Hepatitis delta virus (HDV), a satellite virus of HBV, is regarded as the most severe type of hepatitis virus because of the substantial morbidity and mortality. The IFN system is the first line of defense against viral infections and an essential element of antiviral immunity; however, the role of the hepatic IFN system in controlling HBV-HDV infection remains poorly understood. Herein, we showed that HDV infection of human hepatocytes induced a potent and persistent activation of the IFN system whereas HBV was inert in triggering hepatic antiviral response. Moreover, we demonstrated that HDV-induced constitutive activation of the hepatic IFN system resulted in a potent suppression of HBV while modestly inhibiting HDV. Thus, these pathogens are equipped with distinctive immunogenicity and varying sensitivity to the antiviral effectors of IFN, leading to the establishment of a paradoxical mode of viral interference wherein HDV, the superinfectant, outcompetes HBV, the primary pathogen. Furthermore, our study revealed that HDV-induced constitutive IFN system activation led to a state of IFN refractoriness, rendering therapeutic IFNs ineffective. The present study provides potentially novel insights into the role of the hepatic IFN system in regulating HBV-HDV infection dynamics and its therapeutic implications through elucidating the molecular basis underlying the inefficacy of IFN-based antiviral strategies against HBV-HDV infection.

Hepatitis D virus interferes with hepatitis B virus RNA production via interferon-dependent and -independent mechanisms

BACKGROUND & AIMS

Chronic coinfection with HBV and HDV leads to the most aggressive form of chronic viral hepatitis. Herein, we aimed to elucidate the molecular mechanisms underlying the widely reported observation that HDV interferes with HBV in most coinfected patients.

METHODS

Patient liver tissues, primary human hepatocytes, HepaRG cells and human liver chimeric mice were used to analyze the effect of HDV on HBV using virological and RNA-sequencing analyses, as well as RNA synthesis, stability and association assays.

RESULTS

Transcriptomic analyses in cell culture and mouse models of coinfection enabled us to define an HDV-induced signature, mainly composed of interferon (IFN)-stimulated genes (ISGs). We also provide evidence that ISGs are upregulated in chronically HDV/HBV-coinfected patients but not in cells that only express HDV antigen (HDAg). Inhibition of the hepatocyte IFN response partially rescued the levels of HBV parameters. We observed less HBV RNA synthesis upon HDV infection or HDV protein expression. Additionally, HDV infection or expression of HDAg alone specifically accelerated the decay of HBV RNA, and HDAg was associated with HBV RNAs. On the contrary, HDAg expression did not affect other viruses such as HCV or SARS-CoV-2.

CONCLUSIONS

Our data indicate that HDV interferes with HBV through both IFN-dependent and IFN-independent mechanisms. Specifically, we uncover a new viral interference mechanism in which proteins of a satellite virus affect the RNA production of its helper virus. Exploiting these findings could pave the way to the development of new therapeutic strategies against HBV.

IMPACT AND IMPLICATIONS

Although the molecular mechanisms remained unexplored, it has long been known that despite its dependency, HDV decreases HBV viremia in patients. Herein, using in vitro and in vivo models, we showed that HDV interferes with HBV through both IFN-dependent and IFN-independent mechanisms affecting HBV RNA metabolism, and we defined the HDV-induced modulation signature. The mechanisms we uncovered could pave the way for the development of new therapeutic strategies against HBV by mimicking and/or increasing the effect of HDAg on HBV RNA. Additionally, the HDV-induced modulation signature could potentially be correlated with responsiveness to IFN-α treatment, thereby helping to guide management of HBV/HDV-coinfected patients.

Hepatitis D virus: Improving virological knowledge to develop new treatments

Hepatitis delta virus (HDV), a satellite of hepatitis B virus (HBV), possesses the smallest viral genome known to infect animals. HDV needs HBV surface protein for secretion and entry into target liver cells. However, HBV is dispensable for HDV genome amplification, as it relies almost exclusively on cellular host factors for replication. HBV/HDV co-infections affect over 12 million people worldwide and constitute the most severe form of viral hepatitis. Co-infected individuals are at higher risk of developing liver cirrhosis and hepatocellular carcinoma compared to HBV mono-infected patients. Bulevirtide, an entry inhibitor, was conditionally approved in July 2020 in the European Union for adult patients with chronic hepatitis delta (CHD) and compensated liver disease. There are several drugs in development, including lonafarnib and interferon lambda, with different modes of action. In this review, we detail our current fundamental knowledge of HDV lifecycle and review antiviral treatments under development against this virus, outlining their respective mechanisms-of-action. Finally, we describe the antiviral effect these compounds are showing in ongoing clinical trials, discussing their promise and potential pitfalls for managing HDV infected patients.

A stable hepatitis D virus-producing cell line for host target and drug discovery

Chronic hepatitis D is the most aggressive form of chronic viral hepatitis. It is caused by super-infection of hepatitis B virus (HBV)-infected hepatocytes with hepatitis D virus (HDV). While the recent conditional approval of bulevirtide for HDV treatment offers a new therapeutic modality in Europe, there is an unmet medical need to further improve therapy. A more detailed characterization of virus-host interactions is needed for the identification of novel therapeutic targets. Addressing this need, we engineered a new stably-transformed cell line, named HuH7-2C8D, producing high titer recombinant HDV and allowing the study of viral particles morphogenesis and infectivity. Using this culture system, where viral propagation by re-infection is limited, we observed an increased accumulation of edited version of the viral genomes within secreted HDV viral particles over time that is accompanied with a decrease in viral particle infectivity. We confirmed the interaction of HDV proteins with a previously described host factor in HuH7-2C8D cells and additionally showed that these cells are suitable for co-culture assays with other cell types such as macrophages. Finally, the use of HuH7-2C8D cells allowed to confirm the dual antiviral activity of farnesyl transferase inhibitors, including the clinical candidate lonafarnib, against HDV. In conclusion, we have established an easy-to-handle cell culture model to investigate HDV replication, morphogenesis, and host interactions. HuH7-2C8D cells are also suitable for high-throughput antiviral screening assays for the development of new therapeutic strategies.

Effect of variants in LGP2 on MDA5-mediated activation of interferon response and suppression of hepatitis D virus replication

BACKGROUND & AIMS

Retinoic acid inducible gene I (RIG-I)-like receptors (RLRs), including RIG-I, melanoma differentiation-associated protein 5 (MDA5), and laboratory of genetics and physiology 2 (LGP2), sense viral RNA to induce the antiviral interferon (IFN) response. LGP2, unable to activate the IFN response itself, modulates RIG-I and MDA5 signalling. HDV, a small RNA virus causing the most severe form of viral hepatitis, is sensed by MDA5. The mechanism underlying IFN induction and its effect on HDV replication is unclear. Here, we aimed to unveil the role of LGP2 and clinically relevant variants thereof in these processes.

METHODS

RLRs were depleted in HDV susceptible HepaRG^NTCP cells and primary human hepatocytes. Cells were reconstituted to express different LGP2 versions. HDV and IFN markers were quantified in a time-resolved manner. Interaction studies among LGP2, MDA5, and RNA were performed by pull-down assays.

RESULTS

LGP2 is essential for the MDA5-mediated IFN response induced upon HDV infection. This induction requires both RNA binding and ATPase activities of LGP2. The IFN response only moderately reduced HDV replication in resting cells but profoundly suppressed cell division-mediated HDV spread. An LGP2 variant (Q425R), predominating in Africans who develop less severe chronic hepatitis D, mediated detectably higher basal and faster HDV-induced IFN response as well as stronger HDV suppression. Mechanistically, LGP2 RNA binding was a prerequisite for the formation of stable MDA5-RNA complexes. MDA5 binding to RNA was enhanced by the Q425R LGP2 variant.

CONCLUSIONS

LGP2 is essential to mount an antiviral IFN response induced by HDV and stabilises MDA5-RNA interaction required for downstream signalling. The natural Q425R LGP2 is a gain-of-function variant and might contribute to an attenuated course of hepatitis D.

IMPACT AND IMPLICATIONS

HDV is the causative pathogen of chronic hepatitis D, a severe form of viral hepatitis that can lead to cirrhosis and hepatocellular carcinoma. Upon infection, the human immune system senses HDV and mounts an antiviral interferon (IFN) response. Here, we demonstrate that the immune sensor LGP2 cooperates with MDA5 to mount an IFN response that represses HDV replication. We mapped LGP2 determinants required for IFN system activation and characterised several natural genetic variants of LGP2. One of them reported to predominate in sub-Saharan Africans can accelerate HDV-induced IFN responses, arguing that genetic determinants, possibly including LGP2, might contribute to slower disease progression in this population. Our results will hopefully prompt further studies on genetic variations in LGP2 and other components of the innate immune sensing system, including assessments of their possible impact on the course of viral infection.

Role of RNA Polymerase II Promoter-Proximal Pausing in Viral Transcription

The promoter-proximal pause induced by the binding of the DRB sensitivity-inducing factor (DSIF) and the negative elongation factor (NELF) to RNAP II is a key step in the regulation of metazoan gene expression. It helps maintain a permissive chromatin landscape and ensures a quick transcriptional response from stimulus-responsive pathways such as the innate immune response. It is also involved in the biology of several RNA viruses such as the human immunodeficiency virus (HIV), the influenza A virus (IAV) and the hepatitis delta virus (HDV). HIV uses the pause as one of its mechanisms to enter and maintain latency, leading to the creation of viral reservoirs resistant to antiretrovirals. IAV, on the other hand, uses the pause to acquire the capped primers necessary to initiate viral transcription through cap-snatching. Finally, the HDV RNA genome is transcribed directly by RNAP II and requires the small hepatitis delta antigen to displace NELF from the polymerase and overcome the transcriptional block caused by RNAP II promoter-proximal pausing. In this review, we will discuss the RNAP II promoter-proximal pause and the roles it plays in the life cycle of RNA viruses such as HIV, IAV and HDV.

Future anti-HDV treatment strategies, including those aimed at HBV functional cure

HDV is a defective virus that uses the HBV surface antigen to enter hepatocytes. It is associated with an accelerated course of liver fibrosis progression and an increased risk of hepatocellular carcinoma. Negative HDV RNA 24 weeks after the end of therapy has been proposed as an endpoint but late relapses make this endpoint suboptimal, hence HBsAg loss appears to be more appropriate. Current HBV antiviral agents have poor activity against HDV hence the search for improved therapy. Drugs only active against HDV, such as lonafarnib, have shown efficacy in combination with nucleoside analogues and peginterferon, but do not lead to HBsAg loss. HBsAg loss sustained 24 weeks after the end of therapy with negative HBV DNA is termed functional cure. Agents that are being investigated for functional cure include those that inhibit replication such as entry inhibitors, polymerase inhibitors and capsid assembly modulators but seldom lead to functional cure. Agents that reduce HBV antigen load such as RNA interference and inhibitors of HBsAg secretion are promising. Immunomodulators on their own seldom achieve functional cure, hence these agents in combination to assess the optimal combination are being investigated. Consequently, agents leading to functional cure of HBV are ideal for both HBV and HDV.

Multiple Regions Drive Hepatitis Delta Virus Proliferation and Are Therapeutic Targets

Hepatitis Delta Virus (HDV) is the smallest mammalian single-stranded RNA virus. It requires host cells and hepatitis B virus (HBV) to complete its unique life cycle. The present review summarizes the specific regions on hepatitis D antigen (HDAg) and hepatitis B surface antigen (HBsAg) that drive HDV to utilize host cell machinery system to produce three types of RNA and two forms of HDAg, and hijack HBsAg for its secretion and de novo entry. Previously, interferon-α was the only recommended therapy for HDV infection. In recent years, some new therapies targeting these regions, such as Bulevirtide, Lonafarnib, Nucleic acid polymers have appeared, with better curative effects and fewer adverse reactions.

HDV-Like Viruses

Hepatitis delta virus (HDV) is a defective human virus that lacks the ability to produce its own envelope proteins and is thus dependent on the presence of a helper virus, which provides its surface proteins to produce infectious particles. Hepatitis B virus (HBV) was so far thought to be the only helper virus described to be associated with HDV. However, recent studies showed that divergent HDV-like viruses could be detected in fishes, birds, amphibians, and invertebrates, without evidence of any HBV-like agent supporting infection. Another recent study demonstrated that HDV can be transmitted and propagated in experimental infections ex vivo and in vivo by different enveloped viruses unrelated to HBV, including hepatitis C virus (HCV) and flaviviruses such as Dengue and West Nile virus. All this new evidence, in addition to the identification of novel virus species within a large range of hosts in absence of HBV, suggests that deltaviruses may take advantage of a large spectrum of helper viruses and raises questions about HDV origins and evolution.

Changing epidemiology and viral interplay of hepatitis B, C and D among injecting drug user-dominant prisoners in Taiwan

The spreading of viral hepatitis among injecting drug users (IDU) is an emerging public health concern. This study explored the prevalence and the risks of hepatitis B virus (HBV), hepatitis C virus (HCV) and hepatitis D virus (HDV) among IDU-dominant prisoners in Taiwan. HBV surface antigen (HBsAg), antibodies to HCV (anti-HCV) and HDV (anti-HDV), viral load and HCV genotypes were measured in 1137(67.0%) of 1697 prisoners. 89.2% of participants were IDUs and none had HIV infection. The prevalence of HBsAg, anti-HCV, dual HBsAg/anti-HCV, HBsAg/anti-HDV, and triple HBsAg/anti-HCV/anti-HDV was 13.6%, 34.8%, 4.9%, 3.4%, and 2.8%, respectively. HBV viremia rate was significantly lower in HBV/HCV-coinfected than HBV mono-infected subjects (66.1% versus 89.9%, adjusted odds ratio/95% confidence intervals [aOR/CI] = 0.27/0.10-0.73). 47.5% anti-HCV-seropositive subjects (n = 396) were non-viremic, including 23.2% subjects were antivirals-induced. The predominant HCV genotypes were genotype 6(40.9%), 1a(24.0%) and 3(11.1%). HBsAg seropositivity was negatively correlated with HCV viremia among the treatment naïve HCV subjects (44.7% versus 72.4%, aOR/CI = 0.27/0.13-0.58). Anti-HCV seropositivity significantly increased the risk of anti-HDV-seropositivity among HBsAg carriers (57.1% versus 7.1%, aOR/CI = 15.73/6.04-40.96). In conclusion, IUDs remain as reservoirs for multiple hepatitis viruses infection among HIV-uninfected prisoners in Taiwan. HCV infection increased the risk of HDV infection but suppressed HBV replication in HBsAg carriers. An effective strategy is mandatory to control the epidemic in this high-risk group.

Murine hepatocytes do not support persistence of Hepatitis D virus mono-infection in vivo

BACKGROUNDS & AIMS

As a result of the limited availability of in vivo models for hepatitis D virus (HDV), treatment options for HDV chronically infected patients are still scant. The discovery of sodium taurocholate cotransporting polypeptide (NTCP) as HDV entry receptor has enabled the development of new infection models.

AIM

To comparatively assess the efficacy and persistence of HDV mono-infection in murine and human hepatocytes in vivo.

METHODS

Mice with humanized NTCP (hNTCP^ed84-87 mice) were generated by editing amino acid residues 84-87 of murine NTCP in C57BL/6J mice. HDV infection was assessed in hNTCP^ed84-87 mice and in immune deficient uPA/SCID/beige (USB) mice, whose livers were reconstituted with human or murine (hNTCP^ed84-87 ) hepatocytes. Livers were analysed between 5 and 42 days post-HDV inoculation by qRT-PCR, immunofluorescence and RNA in situ hybridization (ISH).

RESULTS

hNTCP^ed84-87 mice could be infected with HDV genotype 1 or 3. ISH analysis demonstrated the presence of antigenomic HDV RNA positive murine hepatocytes with both genotypes, proving initiation of HDV replication. Strikingly, murine hepatocytes cleared HDV within 21 days both in immunocompetent hNTCP^ed84-87 mice and in immunodeficient USB mice xenografted with murine hepatocytes. In contrast, HDV infection remained stable for at least 42 days in human hepatocytes. Intrinsic innate responses were not enhanced in any of the HDV mono-infected cells and livers.

CONCLUSION

These findings suggest that in addition to NTCP, further species-specific factors limit HDV infection efficacy and persistence in murine hepatocytes. Identifying such species barriers may be crucial to develop novel potential therapeutic targets of HDV.

Current knowledge on Hepatitis Delta Virus replication

Hepatitis B Virus (HBV) that infects liver parenchymal cells is responsible for severe liver diseases and co-infection with Hepatitis Delta Virus (HDV) leads to the most aggressive form of viral hepatitis. Even tough being different for their viral genome (relaxed circular partially double stranded DNA for HBV and circular RNA for HDV), HBV and HDV are both maintained as episomes in the nucleus of infected cells and use the cellular machinery for the transcription of their viral RNAs. We propose here an update on the current knowledge on HDV replication cycle that may eventually help to identify new antiviral targets.

A review on hepatitis D: From virology to new therapies

Hepatitis delta virus (HDV) is a defective virus that requires the hepatitis B virus (HBV) to complete its life cycle in human hepatocytes. HDV virions contain an envelope incorporating HBV surface antigen protein and a ribonucleoprotein containing the viral circular single-stranded RNA genome associated with both forms of hepatitis delta antigen, the only viral encoded protein. Replication is mediated by the host cell DNA-dependent RNA polymerases. HDV infects up to72 million people worldwide and is associated with an increased risk of severe and rapidly progressive liver disease. Pegylated interferon-alpha is still the only available treatment for chronic hepatitis D, with poor tolerance and dismal success rate. Although the development of antivirals inhibiting the viral replication is challenging, as HDV does not possess its own polymerase, several antiviral molecules targeting other steps of the viral life cycle are currently under clinical development: Myrcludex B, which blocks HDV entry into hepatocytes, lonafarnib, a prenylation inhibitor that prevents virion assembly, and finally REP 2139, which is thought to inhibit HBsAg release from hepatocytes and interact with hepatitis delta antigen. This review updates the epidemiology, virology and management of HDV infection.

HDV RNA replication is associated with HBV repression and interferon-stimulated genes induction in super-infected hepatocytes

Hepatitis D virus (HDV) super-infection of Hepatitis B virus (HBV)-infected patients is the most aggressive form of viral hepatitis. HDV infection is not susceptible to direct anti-HBV drugs, and only suboptimal antiviral responses are obtained with interferon (IFN)-alpha-based therapy. To get insights on HDV replication and interplay with HBV in physiologically relevant hepatocytes, differentiated HepaRG (dHepaRG) cells, previously infected or not with HBV, were infected with HDV, and viral markers were extensively analyzed. Innate and IFN responses to HDV were monitored by measuring pro-inflammatory and interferon-stimulated gene (ISG) expression. Both mono- and super-infected dHepaRG cells supported a strong HDV intracellular replication, which was accompanied by a strong secretion of infectious HDV virions only in the super-infection setting and despite the low number of co-infected cells. Upon HDV super-infection, HBV replication markers including HBeAg, total HBV-DNA and pregenomic RNA were significantly decreased, confirming the interference of HDV on HBV. Yet, no decrease of circular covalently closed HBV DNA (cccDNA) and HBsAg levels was evidenced. At the peak of HDV-RNA accumulation and onset of interference on HBV replication, a strong type-I IFN response was observed, with interferon stimulated genes, RSAD2 (Viperin) and IFI78 (MxA) being highly induced. We established a cellular model to characterize in more detail the direct interference of HBV and HDV, and the indirect interplay between the two viruses via innate immune responses. This model will be instrumental to assess molecular and immunological mechanisms of this viral interference.

Molecular epidemiology of hepatitis D virus circulating in Southwestern Nigeria

BACKGROUND

Hepatitis B virus (HBV) and hepatitis D virus (HDV) infections are major public health problems in sub-Saharan Africa. Whereas it is known that HBV infection is endemic in Nigeria, there is only little data about HDV prevalence available. Here, we assessed the HDV seroprevalence and determined the HDV and HBV genotypes distribution among HBsAg positive individuals in Southwestern Nigeria.

METHODS

This cross-sectional study involved 188 serum samples from HBsAg positive outpatients recruited at four tertiary hospitals in Southwestern Nigeria. Anti-HDV antibodies were detected by ELISA while HDV-RNA was detected by RT-PCR. Sequencing followed by phylogenetic analyses and HBV genotype-specific PCR were used to characterize HDV and HBV genotypes, respectively.

RESULTS

Out of 188 HBsAg positive serum samples, 17 (9 %) showed detectable HDV-RNA. Anti-HDV antibodies test was possible from 103 samples and were observed in 4.9 % (5/103) patients. There was no significant difference in HDV prevalence between four main cities across the country. 64.7 % of HDV-RNA positive samples were from males and 35.3 % from females (P < 0.05). No significant associations were observed with regard to HDV seroprevalence and available demographic factors. Phylogenetic analyses demonstrated a predominance of HDV genotype 1 and HBV genotype E among the HDV-RNA/HBsAg positive patients.

CONCLUSIONS

In conclusion, our study showed a high prevalence of HDV infection in HBsAg carriers and the predominance of HDV genotype 1 infection in Nigerian HBV endemic region. The findings contribute to a better understanding of the relevance of HDV/HBV co-infection and circulating genotypes.

Dynamics of in vivo hepatitis D virus infection

Hepatitis-D virus (HDV) is a satellite virus of hepatitis-B virus (HBV) whose intracellular products are required for the completion of the HDV life cycle. HDV can replicate in a cell without the presence of HBV but needs hepatitis B surface antigen (HBsAg) to complete virus assembly and packaging. In order to better understand HDV dynamics, we developed a mathematical model and successfully simulated HBV and HDV data under a range of scenarios. Compared to HBV mono-infection, dual HDV infection resulted in lower chronic HBV DNA levels, with more marked suppression for coinfection (1 logs HBV DNA copies/ml lower) compared to superinfection (0.6 logs HBV DNA copies/ml). Although they have no effect on HBV, prenylation inhibitors may provide the best therapy for reducing HDV viremia irrespective of the stage in which they are commenced. We found that highly effective long term pegylated interferon (IFN) therapy (99.99%) eliminates HBV and HDV viremia while less effective long term IFN therapy (99%) will only produce approximately 2.03 logs and no decrease in HBV and HDV viremia respectively in both coinfection and superinfection settings. Our study also suggests that there is a substantial difference in the outcome of therapies depending upon the time of commencement.

CONCLUSION

Mathematical modeling of HDV infection can describe the complex interplay between this virus and HBV. Simulations suggest that HDV impacts on the feedback mechanisms that maintain cccDNA levels and that targeting these mechanisms may result in new therapeutic agents for both viruses.

Hepatitis B and Delta Virus: Advances on Studies about Interactions between the Two Viruses and the Infected Hepatocyte

The mechanisms determining persistence of hepatitis B virus (HBV) infection and long-term pathogenesis of HBV-associated liver disease appear to be multifactorial. Although viral replication can be efficiently suppressed by the antiviral treatments currently available, viral clearance is generally not achieved since HBV has developed unique replication strategies, enabling persistence of its genome within the infected hepatocytes. Moreover, no direct antiviral therapy exists for the more than 15 million people worldwide that are also coinfected with the hepatitis delta virus (HDV), a defective virus that needs the HBV envelope proteins for propagation. The limited availability of robust HBV and HDV infection systems has hindered the understanding of the complex network of virus-virus and virus-host interactions that are established in the course of infection and slowed down progress in drug development. Since chronic HBV/HDV coinfection leads to the most severe form of chronic viral hepatitis, elucidation of the molecular mechanisms regulating virus-host interplay and pathogenesis are urgently needed. This article summarizes the current knowledge regarding the interactions among HBV, HDV, and the infected target cell and discusses the dependence of HDV on HBV activity and possible future therapeutic approaches.

High prevalence of hepatitis delta virus among patients with chronic hepatitis B virus infection and HIV-1 in an intermediate hepatitis B virus endemic region

We conducted a study to investigate HIV and hepatitis delta virus (HDV) coinfection among patients with chronic hepatitis B virus (HBV) infection and the triple infection's (HIV/HBV/HDV) clinical implications in India, an intermediate HBV endemic region, with an estimated HIV-positive population of 2.5 million. A total of 450 patients (men: 270; women: 180) with chronic HBV infections and 135 healthy volunteers were screened for HIV and HDV. The incidence of the triple infection was low (4 [0.8%]) compared with dual infections of HIV-1/HBV (7 [1.5%]) and HBV/HDV (22[4.8%]). Among 21- to 40-year-olds, HBV/HDV coinfection (45.8%) and HBV/HDV/HIV-1 triple infection was predominant (75%). Among 11 patients coinfected with HIV-1/HBV, 4 (36%) were tri-infected and were also associated with chronic hepatitis and cirrhosis. The HDV coinfection was higher among patients coinfected with HBV/HIV-1, despite the declining trend in HDV infection among HIV-negative patients, as previously reported. Thus, it is important to assess the impact of HIV, chronic HBV, and HDV tri-infection in India.

Multiple genomic sequences of hepatitis delta virus are associated with cDNA promoter activity and RNA double rolling-circle replication

To understand how DNA-dependent RNA polymerase II (pol II) recognizes hepatitis delta virus (HDV) RNA as a template, it is first necessary to identify the HDV sequence that acts as a promoter of pol II-initiated RNA synthesis. Therefore, we isolated the pol II-response element from HDV cDNA and examined the regulation by hepatitis delta antigens (HDAgs). Two HDV cDNA fragments containing bidirectional promoter activity were identified. One was located at nt 1582–1683 (transcription-promoter region 1, TR-P1) and the other at nt 1223–1363 (transcription-internal region 5, TR-I5). The promoter activities of these two regions were enhanced by HDAgs to differing degrees. Next, the role of these sequences in an HDV cDNA-free RNA replication system was characterized by site-directed mutagenesis. Our data showed that: (i) the AUG codon at the HDAg ORF of HDV RNA (nt 1599–1601) that mutates to UAG (amber stop codon) results in loss of dimeric but not monomeric HDV RNA synthesis. (ii) A 5 nt mutation of TR-P1 (P1-m5, nt 1670–1674) abolishes RNA replication completely. Two-nucleotide-mutated RNA (P1-m2, nt 1662–1663) is able to synthesize short RNAs but not monomeric HDV RNA. (iii) A mutation in 5 nt at the TR-I5 region (I5-m5, nt 1351–1355) also abolishes HDV replication. Mutants with 2 nt mutations (I5-m2, nt 1351–1352) or 3 nt mutations (I5-m3, nt 1353–1355) inhibit HDV dimeric but not monomeric RNA synthesis. Furthermore, large HDAg is expressed in cells transfected with I5-m3 and I5-m2 RNAs and that demonstrate the RNA-editing event in the monomeric HDV RNA. These results provide further understanding of the double rolling-circle mechanism in HDV RNA replication.

An overview of triple infection with hepatitis B, C and D viruses

Viral hepatitis is one of the major health problems worldwide, particularly in South East Asian countries including Pakistan where hepatitis C virus (HCV) and hepatitis B virus (HBV) infections are highly endemic. Hepatitis delta virus (HDV) is also not uncommon world-wide. HCV, HBV, and HDV share parallel routes of transmission due to which dual or triple viral infection can occur in a proportion of patients at the same time. HBV and HCV are important factors in the development of liver cirrhosis (LC) and hepatocellular carcinoma (HCC). In addition to LC and HCC, chronic HDV infection also plays an important role in liver damage with oncogenic potential.

The current article reviews the available literature about the epidemiology, pathogenesis, transmission, symptoms, diagnosis, replication, disease outcome, treatment and preventive measures of triple hepatitis infection by using key words; epidemiology of triple infection, risk factors, awareness status, treatment and replication cycle in PubMed, PakMediNet, Directory of Open Access Journals (DOAJ) and Google Scholar. Total data from 74 different studies published from 1983 to 2010 on triple hepatitis infections were reviewed and included in this study. The present article briefly describes triple infection with HCV, HBV and HDV.

Molecular epidemiology of hepatitis D virus infection among injecting drug users with and without human immunodeficiency virus infection in Taiwan

An outbreak of human immunodeficiency virus (HIV) infection occurred among injecting drug users (IDU) in Taiwan between 2003 and 2006, when an extremely high prevalence of hepatitis C virus (HCV) infection was also detected. To determine whether clusters of hepatitis D virus (HDV) infection occurred in this outbreak, 4 groups of subjects were studied: group 1, HIV-infected IDU (n = 904); group 2, HIV-infected non-IDU (n = 880); group 3, HIV-uninfected IDU (n = 211); and group 4, HIV-uninfected non-IDU (n = 1,928). The seroprevalence of hepatitis B virus (HBV) was 19.8%, 18.4%, 17.1%, and 6.7%, and HDV seroprevalence among HBV carriers was 75.4%, 9.3%, 66.7%, and 2.3%, for groups 1, 2, 3, and 4, respectively. Ninety-nine of 151 (65.6%) HDV-seropositive IDU had HDV viremia: 5 were infected with HDV genotype I, 41 with genotype II, 51 with genotype IV, and 2 with genotypes II and IV. In the phylogenetic analysis, only one cluster of 4 strains within the HDV genotype II was identified. Among patients with HCV viremia, a unique cluster within genotype 1a was observed; yet, patients within this cluster did not overlap with those observed in the HDV phylogenetic analysis. In summary, although IDU had a significantly higher HDV seroprevalence, molecular epidemiologic investigations did not support that HDV was introduced at the same time as HCV among IDU.

Role of hepatitis B virus, hepatitis D virus and other determinants on suppression of hepatitis C viraemia in HIV infected patients with chronic HCV infection: a longitudinal evaluation

The role of hepatitis B virus (HBV) or hepatitis D virus (HDV) coinfections as determinants of hepatitis C virus (HCV) suppression in the setting of HIV-HCV coinfection are poorly understood. Our aim was to assess whether HCV viral replication may be affected by HBV or HDV coinfection in the setting of immunodeficiency driven by HIV.Among the 138 enrolled patients 28(20.3%) tested HCV RNA negative and 110 (79.7%) tested HCV RNA negative. The HCV RNA negative patients showed an higher rate of HBsAg positivity compared with those tested HCVRNA positive [12/28 (42.9%) and 5/110 (4.6%), respectively]. Patients with HCV-HBV-HDV coinfection had the highest chance of having an undetectable HCV RNA (adjusted odds ratio (AOR): 92.0, 95% confidence interval (CI) 5.7-1483.5, p<0.0001). Furthermore, HBV coinfection per se was also found to be independently associated with negative HCV viraemia (AOR: 18.5, 95% CI 2.4-143.5, p<0.0001). HBsAg-positive patients with negative HCV viraemia maintained undetectable levels over time. Our results support a direct role of HBV and HDV coinfections in suppressing HCV viraemia in HIV infected patients. This effect is durable over time, and is not influenced by HAART including anti-HBV drugs.

Transcription of subgenomic mRNA of hepatitis delta virus requires a modified hepatitis delta antigen that is distinct from antigenomic RNA synthesis

Hepatitis delta virus (HDV) contains a viroid-like, 1.7-kb circular RNA genome, which replicates via a double-rolling-circle model. However, the exact mechanism involved in HDV genome RNA replication and subgenomic mRNA transcription is still unclear. Our previous studies have shown that the replications of genomic and antigenomic HDV RNA strands have different sensitivities to alpha-amanitin and are associated with different nuclear bodies, suggesting that these two strands are synthesized in different transcription machineries in the cells. In this study, we developed a unique quantitative reverse transcription-PCR (qRT-PCR) procedure for detection of various HDV RNA species from an RNA transfection system. Using this qRT-PCR procedure and a series of HDV mutants, we demonstrated that Arg-13 methylation, Lys-72 acetylation, and Ser-177 phosphorylation of small hepatitis delta antigen (S-HDAg) are important for HDV mRNA transcription. In addition, these three S-HDAg modifications are dispensable for antigenomic RNA synthesis but are required for genomic RNA synthesis. Furthermore, the three RNA species had different sensitivities to acetylation and deacetylation inhibitors, showing that the metabolic requirements for the synthesis of HDV antigenomic RNA are different from those for the synthesis of genomic RNA and mRNA. In sum, our data support the hypothesis that the cellular machinery involved in the synthesis of HDV antigenomic RNA is different from that of genomic RNA synthesis and mRNA transcription, even though the antigenomic RNA and the mRNA are made from the same RNA template. We propose that acetylation and deacetylation of HDAg may provide a molecular switch for the synthesis of the different HDV RNA species.

Impact of hepatitis D virus infection on the long-term outcomes of patients with hepatitis B virus and HIV coinfection in the era of highly active antiretroviral therapy: a matched cohort study

BACKGROUND

Triple infection with human immunodeficiency virus (HIV), hepatitis B virus (HBV), and hepatitis D virus (HDV) is rare. The influence of HDV infection on the responses to highly active antiretroviral therapy and hepatic complications in patients with HBV-HIV coinfection remains uncertain.

METHODS

Twenty-six HDV-infected case patients and 78 HDV-uninfected matched control subjects were identified between 1 January 1995 and 30 June 2003. Clinical and immunologic outcomes were noted, and HBV and HIV loads and genotypic resistance of HBV to lamivudine were determined.

RESULTS

Case patients had a higher rate of injection drug use (7.7% vs. 1.3%; P=.05) and lower serum levels of HBV DNA (median level, 4.04 vs. 5.75 log10 copies/mL; P=.07) than control subjects. During a median observation period of 54.7 months, HDV infection did not have an adverse impact on clinical, virological, or immunologic responses to highly active antiretroviral therapy. However, case patients had higher rates of hepatitis flares (57.7% vs. 23.1%; P=.002), hyperbilirubinemia (34.6% vs. 14.1%; P=.04), liver cirrhosis (26.9% vs. 5.1%; P=.009), hepatic decompensation (23.1% vs. 5.1%; P=.007), and death (adjusted hazard ratio, 5.41; 95% confidence interval, 1.39-23.85; P=.02), although these patients had a lower risk of genotypic resistance to lamivudine (0% vs. 57.1%; P=.003).

CONCLUSIONS

HDV infection did not affect clinical, virological, or immunologic responses to highly active antiretroviral therapy in patients with HBV-HIV coinfection. HDV infection increased risk of hepatitis flares, liver cirrhosis, hepatic decompensation, and death in patients with HBV-HIV coinfection.

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.

HDV RNA replication: ancient relic or primer?

HDV replicates its circular RNA genome using a double rolling-circle mechanism and transcribes a hepatitis delta antigen-encodeing mRNA from the same RNA template during its life cycle. Both processes are carried out by RNA-dependent RNA synthesis despite the fact that HDV does not encode an RNA-dependent RNA polymerase (RdRP). Cellular RNA polymerase II has long been implicated in these processes. Recent findings, however, have shown that the syntheses of genomic and antigenomic RNA strands have different metabolic requirements, including sensitives to alpha-amanitin and the site of synthesis. Evidence is summarized here for the involvement of other cellular polymerases, probably pol I, in the synthesis of antigenomic RNA strand. The ability of mammalian cells to replicate HDV RNA implies that RNA-dependent RNA synthesis was preserved throughout evolution.

High prevalence of dual or triple infection of hepatitis B, C, and delta viruses among patients with chronic liver disease in Mongolia

Mongolia is known for its high endemicity for hepatitis B virus (HBV), hepatitis C virus (HCV), and hepatitis delta virus (HDV) infections among apparently healthy individuals. However, there are little or no data on the prevalence and genotype distribution of HBV, HCV, and HDV among patients with chronic liver disease in Mongolia. Therefore, serum samples obtained in 2004 from 207 patients (age, mean+/-standard deviation, 51.0+/-11.9 years) including those with chronic hepatitis (n=90), liver cirrhosis (n=41), and hepatocellular carcinoma (n=76) were tested for serological and molecular markers of HBV, HCV, and HDV infections. Of the 207 patients, 144 (69.6%), 106 (51.2%), and 117 (56.5%) tested positive for hepatitis B surface antigen (HBsAg) and/or HBV DNA, HCV RNA, and HDV RNA, respectively. Collectively, 172 patients (83.1%) were viremic for one or more of these viruses, including dual viremia of HBV/HDV (26.6%) or HBV/HCV (7.7%) and triple HBV/HCV/HDV viremia (30.0%). Of note, triple ongoing infection was significantly more frequent among patients with hepatocellular carcinoma than among those with chronic hepatitis (63.2% vs. 14.4%, P<0.0001). One hundred sixty patients (77.3%) had a history of blood transfusion and/or surgery. The distribution of HBV genotypes among the 116 HBV-viremic patients was: A (0.9%), B (0.9%), C (6.0%), D (88.8%), and C plus D (3.4%). All 117 HDV isolates were classified into genotype I. The 106 HCV RNA-positive samples were typed as genotype 1b (92.5%), 2a (0.9%), or 1b plus 2a (6.6%); mixed infection of two distinct HCV genotypes was found exclusively in the patients with hepatocellular carcinoma.

Single (B or C), dual (BC or BD) and triple (BCD) viral hepatitis in HIV-infected patients in Madrid, Spain

INTRODUCTION

There are very limited data about the prevalence of multiple hepatitis virus infections in HIV infected individuals. In HIV uninfected individuals with triple BCD hepatitis, hepatitis D virus (HDV) appears to be the dominant virus. However, in HIV infected patients with triple hepatitis it is not known if HDV replication inhibits hepatitis B virus (HBV) and/or hepatitis C virus (HCV) replication.

METHODS

We calculated the prevalence of single (B or C), dual (BC) and triple (BCD) hepatitis in 423 HIV-infected patients with positive HCV serum antibodies and/or positive serum HBsAg. In patients with multiple infections we performed an evaluation of serum markers of HBV, HCV and HDV replication.

RESULTS

The prevalence of multiple hepatitis was 4.7% (95% confidence interval, 2.7-6.7%). Multiple hepatitis occurred only among patients who acquired HIV through injection drug use. The most common multiple hepatitis was triple BCD. Patients with hepatitis BC and past or chronic hepatitis D were significantly more likely to have cirrhosis and a negative serum HBeAg and HCV PCR than patients with single hepatitis B or hepatitis C. Patients with chronic hepatitis D showed uniform suppression of HBV and HCV replication markers.

CONCLUSIONS

In our geographic area approximately 5% of HIV infected patients with hepatitis suffer multiple hepatitis virus infection. In patients with triple hepatitis BCD virus infection, HDV appears to be the dominant virus causing inhibition of both HBV and HCV replication.

High prevalence of hepatitis B, C and delta virus infections among blood donors in Mongolia

Serum samples obtained from 289 first-time and 114 repeat donors at the Blood Center of Mongolia (MBC) were tested for serological and molecular markers of hepatitis B virus (HBV), hepatitis C virus (HCV), and hepatitis delta virus (HDV) infections. Among the 403 blood donors, 33 (8.2%), 21 (5.2%), and 27 (6.7%) tested positive for hepatitis B surface antigen (HBsAg) and/or HBV DNA, HCV RNA, and HDV RNA, respectively. Collectively, 55 donors were viremic for one or more of these viruses, and included 54 first-time donors (18.7%) and 1 repeat donor (0.9%) (P < 0.0001). One discrepant case with HBsAg detectable only at MBC was negative for HBsAg, HBV DNA and anti-HBc in this study. Four donors who were HCV-viremic in this study were negative for anti-HCV by the MBC method. Further efforts to increase the sensitivity and specificity of the currently-used tests are urgently required in Mongolia. Three donors who were positive for anti-HBc and anti-HDV but negative for HBsAg, had both HBV DNA and HDV RNA. This suggests that introduction of a new anti-HDV serological test is useful for not only HDV screening but also HBV screening of anti-HBc-positive, HBsAg negative donors, considering a possibility of viral interference by coexisting HDV.

High prevalence of hepatitis delta virus infection detectable by enzyme immunoassay among apparently healthy individuals in Mongolia

A previous study revealed a high prevalence of hepatitis B surface antigen (HBsAg) and hepatitis delta virus (HDV) RNA among 249 apparently healthy individuals (mean+/-standard deviation age, 48.4+/-13.9 years; 126 males and 123 females) in Ulaanbaatar, Mongolia. To investigate further the prevalence of HDV infection there, the same serum samples obtained from the cohort were tested for the presence of immunoglobulin G (IgG) class antibody to HDV (anti-HDV) by a newly developed enzyme-linked immunosorbent assay using recombinant hepatitis delta antigen protein expressed in the pupae of silkworm as the antigen probe. Anti-HDV was detected in 42 persons (16.9%), among whom 22 (52.4%) were positive for HBsAg and 20 (47.6%) had detectable HDV RNA. Among 170 persons with anti-HBc in the absence of HBsAg, 20 (11.8%) tested positive for anti-HDV, and 1 of the 20 subjects was positive for HDV RNA. Of note, none of 55 anti-HBc-negative persons had anti-HDV, supporting the specificity of the anti-HDV assay. The optical density (OD) value of anti-HDV was significantly higher among HDV RNA-positive subjects (n=21) than among HDV RNA-negative subjects (n=21) (2.513+/-0.514 vs. 0.836+/-0.550, P<0.0001). The present study confirmed the extremely high prevalence of HDV infection in Mongolia, and identified a person who was positive for both anti-HDV and HDV RNA despite negativity for HBsAg and HBV DNA probably due to viral interference. The anti-HDV assay may be useful for further epidemiological studies on HDV infection in larger cohorts in urban and rural areas of Mongolia, where elucidation of the transmission route of HDV is required urgently.

Hepatitis delta virus antigen is methylated at arginine residues, and methylation regulates subcellular localization and RNA replication

Hepatitis delta virus (HDV) contains a circular RNA which encodes a single protein, hepatitis delta antigen (HDAg). HDAg exists in two forms, a small form (S-HDAg) and a large form (L-HDAg). S-HDAg can transactivate HDV RNA replication. Recent studies have shown that posttranslational modifications, such as phosphorylation and acetylation, of S-HDAg can modulate HDV RNA replication. Here we show that S-HDAg can be methylated by protein arginine methyltransferase (PRMT1) in vitro and in vivo. The major methylation site is at arginine-13 (R13), which is in the RGGR motif of an RNA-binding domain. The methylation of S-HDAg is essential for HDV RNA replication, especially for replication of the antigenomic RNA strand to form the genomic RNA strand. An R13A mutation in S-HDAg inhibited HDV RNA replication. The presence of a methylation inhibitor, S-adenosyl-homocysteine, also inhibited HDV RNA replication. We further found that the methylation of S-HDAg affected its subcellular localization. Methylation-defective HDAg lost the ability to form a speckled structure in the nucleus and also permeated into the cytoplasm. These results thus revealed a novel posttranslational modification of HDAg and indicated its importance for HDV RNA replication. This and other results further showed that, unlike replication of the HDV genomic RNA strand, replication of the antigenomic RNA strand requires multiple types of posttranslational modification, including the phosphorylation and methylation of HDAg.

By inhibiting replication, the large hepatitis delta antigen can indirectly regulate amber/W editing and its own expression

Hepatitis delta virus (HDV) expresses two essential proteins with distinct functions. The small hepatitis delta antigen (HDAg-S) is expressed throughout replication and is needed to promote that process. The large form (HDAg-L) is farnesylated, is expressed only at later times via RNA editing of the amber/W site, and is required for virion assembly. When HDAg-L is artificially expressed at the onset of replication, it strongly inhibits replication. However, there is controversy concerning whether HDAg-L expressed naturally at later times as a consequence of editing and replication can similarly inhibit replication. Here, by stabilizing the predicted secondary structure downstream from the amber/W site, a replication-competent HDV mutant that exhibited levels of editing higher than those of the wild type was created. This mutant expressed elevated levels of HDAg-L early during replication, and at later times, its replication aborted prematurely. No further increase in amber/W editing was observed following the cessation of replication, indicating that editing was coupled to replication. A mutation in HDAg-L and a farnesyl transferase inhibitor were both used to abolish the ability of HDAg-L to inhibit replication. Such treatments rescued the replication defect of the overediting mutant, and even higher levels of amber/W editing resulted. It was concluded that when expressed naturally during replication, HDAg-L is able to inhibit replication and thereby inhibit amber/W editing and its own synthesis. In addition, the structure adjacent to the amber/W site is suboptimal for editing, and this creates a window of time in which replication can occur in the absence of HDAg-L.

Hepatitis B virus infection: co-infection with hepatitis C virus, hepatitis D virus, and human immunodeficiency virus

Hepatitis B virus (HBV) shares routes of transmission, namely exchange of infected body fluids, sharing of contaminated needles, and blood transfusion, with other hepatotropic viruses, such as hepatitis C virus (HCV) and hepatitis D virus (HDV) and with systemic retroviral infections, such as the human immunodeficiency virus (HIV). Thus, many HBV infected patients are co-infected with other viral pathogens. Co-infection appears to increase the risk of progression of liver disease and may have important ramifications on choice of antiviral medication and treatment regimen. This article reviews the current knowledge of co-infection of HBV with HCV, HDV, and HIV.

Interaction and replication activation of genotype I and II hepatitis delta antigens

The nucleotide sequences of hepatitis D viruses (HDV) vary 5 to 14% among isolates of the same genotype and 23 to 34% among different genotypes. The only viral-genome-encoded antigen, hepatitis delta antigen (HDAg), has two forms that differ in size. The small HDAg (HDAg-S) trans-activates viral replication, while the large form (HDAg-L) is essential for viral assembly. Previously, it has been shown that the packaging efficiency of HDAg-L is higher for genotype I than for genotype II. In this study, the question of whether other functional properties of the HDAgs are affected by genotype differences is addressed. By coexpression of the two antigens in HuH-7 cells followed by specific antibody precipitation, it was found that HDAgs of different origins interacted without genotypic discrimination. Moreover, in the presence of hepatitis B virus surface antigen, HDAg-S was incorporated into virion-like particles through interaction with HDAg-L without genotype restriction. As to the differences in replication activation of genotype I HDV RNA, all HDAg-S clones tested had some trans-activation activity, and this activity varied greatly among isolates. As to the support of HDV genotype II replication, only clones of HDAg-S from genotype II showed trans-activation activity, and this activity also varied among isolates. In conclusion, genotype has no effect on HDAg interaction and genotype per se only partly predicts how much the HDAg-S of an HDV isolate affects the replication of a second HDV isolate.

Hepatitis delta virus RNA encoding the large delta antigen cannot sustain replication due to rapid accumulation of mutations associated with RNA editing

Hepatitis delta virus (HDV) contains two RNA species (HDV-S and HDV-L), which encode the small and large forms of hepatitis delta antigens (S- and L-HDAg), respectively. HDV-L RNA is a result of an RNA editing event occurring at an amber/W site of HDV-S RNA. RNA editing must be regulated to prevent premature and excessive accumulation of HDV-L RNA in the viral life cycle. In this study, we used an RNA transfection procedure to study the replication abilities of HDV-L and HDV-S RNA. While HDV-S led to robust RNA replication, HDV-L could not replicate even after 6 days following transfection. The failure of HDV-L to replicate was not due to insufficient amounts of S-HDAg, as identical results were obtained in a cell line that stably overexpresses S-HDAg. Also, it was not due to possible inhibition by L-HDAg, as HDV-S RNA replication was not affected when both HDV-L and HDV-S RNA were cotransfected. Further, when L-HDAg expression from HDV-L RNA was abolished by site-directed mutagenesis, the mutant HDV-L RNA also failed to replicate. Unexpectedly, when the kinetics of RNA replication was examined daily, HDV-L was found to replicate at a low level at the early time points (1 to 2 days posttransfection) but then lose this capability at later time points. Sequence analysis of the replicated HDV-L RNA at day 1 posttransfection showed that it had undergone multiple nucleotide changes, particularly in the region near the putative promoter region of HDV RNA replication. In contrast, very few mutations were found in HDV-S RNA. These results suggest that the editing at the amber/W site triggers a series of additional mutations which rapidly reduce the replication efficiency of the resultant HDV genome and thus help regulate the amount of HDV-L RNA in infected cells. They also explain why L-HDAg is not produced early in HDV infection, despite the fact that HDV-L RNA is present in the virion.

Selective strand annealing and selective strand exchange promoted by the N-terminal domain of hepatitis delta antigen

We have previously shown that the N-terminal domain of hepatitis delta virus (NdAg) has an RNA chaperone activity in vitro (Huang, Z. S., and Wu, H. N. (1998) J. Biol. Chem. 273, 26455-26461). Here we investigate further the basis of the stimulatory effect of NdAg on RNA structural rearrangement: mainly the formation and breakage of base pairs. Duplex dissociation, strand annealing, and exchange of complementary RNA oligonucleotides; the hybridization of yeast U4 and U6 small nuclear RNAs and of hammerhead ribozymes and cognate substrates; and the cis-cleavage reaction of hepatitis delta ribozymes were used to determine directly the role of NdAg in RNA-mediated processes. The results showed that NdAg could accelerate the annealing of complementary sequences in a selective fashion and promote strand exchange for the formation of a more extended duplex. These activities would prohibit NdAg from modifying the structure of a stable RNA, but allow NdAg to facilitate a trans-acting hammerhead ribozyme to find a more extensively matched target in cognate substrate. These and other results suggest that hepatitis delta antigen may have a biological role as an RNA chaperone, modulating the folding of viral RNA for replication and transcription.

Initiation of hepatitis delta virus (HDV) replication: HDV RNA encoding the large delta antigen cannot replicate

The hepatitis delta virus (HDV) nucleocapsid consists of a genomic-length RNA of 1.7 kb and approximately equimolar amounts of the small and large forms of the hepatitis delta antigen (S-HDAg and L-HDAg, respectively). Since HDV RNA particles contain not only a genomic RNA species encoding S-HDAg but also an RNA species encoding L-HDAg, which is produced by an RNA-editing process, the question arises as to whether RNAs encoding either L-HDAg or S-HDAg can initiate replication. To study this, two cDNA-free transfection methods were employed: HDV RNA cotransfected with either the S-HDAg-encoding mRNA species or the ribonucleocapsid protein complex, comprising HDV RNA and recombinant S-HDAg. Results showed that the genomic-sense RNA encoding S-HDAg could promote HDV replication, whereas the L-HDAg-encoding RNA species was unable to replicate under the same conditions. The antigenomic RNA species encoding either S-HDAg or L-HDAg could not replicate by either of these procedures. In addition, L-HDAg alone could not promote replication of the genomic RNA but, by supplementing an equal amount of S-HDAg, replication occurred. These data indicate that L-HDAg-encoding RNA species are probably not involved in the initiation of HDV RNA synthesis; instead, their main function may be to serve as template for producing L-HDAg, which regulates HDV RNA synthesis and virion assembly. These results suggest that the genomic RNA species encoding S-HDAg is the only functional genome for HDV infection and explain why the presence of the edited HDV RNA encoding L-HDAg does not interfere with HDV infection.