Hepatitis delta virus ribonucleoproteins shuttle between the nucleus and the cytoplasm

Hepatitis Delta Virus and Hepatocellular Carcinoma

The hepatitis D virus (HDV) is a compact, enveloped, circular RNA virus that relies on hepatitis B virus (HBV) envelope proteins to initiate a primary infection in hepatocytes, assemble, and secrete new virions. Globally, HDV infection affects an estimated 12 million to 72 million people, carrying a significantly elevated risk of developing cirrhosis, liver failure, and hepatocellular carcinoma (HCC) compared to an HBV mono-infection. Furthermore, HDV-associated HCC often manifests at a younger age and exhibits more aggressive characteristics. The intricate mechanisms driving the synergistic carcinogenicity of the HDV and HBV are not fully elucidated but are believed to involve chronic inflammation, immune dysregulation, and the direct oncogenic effects of the HDV. Indeed, recent data highlight that the molecular profile of HCC associated with HDV is unique and distinct from that of HBV-induced HCC. However, the question of whether the HDV is an oncogenic virus remains unanswered. In this review, we comprehensively examined several crucial aspects of the HDV, encompassing its epidemiology, molecular biology, immunology, and the associated risks of liver disease progression and HCC development.

Comparative analysis of human, rodent and snake deltavirus replication

The recent discovery of Hepatitis D (HDV)-like viruses across a wide range of taxa led to the establishment of the Kolmioviridae family. Recent studies suggest that kolmiovirids can be satellites of viruses other than Hepatitis B virus (HBV), challenging the strict HBV/HDV-association dogma. Studying whether kolmiovirids are able to replicate in any animal cell they enter is essential to assess their zoonotic potential. Here, we compared replication of three kolmiovirids: HDV, rodent (RDeV) and snake (SDeV) deltavirus in vitro and in vivo. We show that SDeV has the narrowest and RDeV the broadest host cell range. High resolution imaging of cells persistently replicating these viruses revealed nuclear viral hubs with a peculiar RNA-protein organization. Finally, in vivo hydrodynamic delivery of viral replicons showed that both HDV and RDeV, but not SDeV, efficiently replicate in mouse liver, forming massive nuclear viral hubs. Our comparative analysis lays the foundation for the discovery of specific host factors controlling Kolmioviridae host-shifting.

Cellular Factors Involved in the Hepatitis D Virus Life Cycle

Hepatitis D virus (HDV) is a defective RNA virus with a negative-strand RNA genome encompassing less than 1700 nucleotides. The HDV genome encodes only for one protein, the hepatitis delta antigen (HDAg), which exists in two forms acting as nucleoproteins. HDV depends on the envelope proteins of the hepatitis B virus as a helper virus for packaging its ribonucleoprotein complex (RNP). HDV is considered the causative agent for the most severe form of viral hepatitis leading to liver fibrosis/cirrhosis and hepatocellular carcinoma. Many steps of the life cycle of HDV are still enigmatic. This review gives an overview of the complete life cycle of HDV and identifies gaps in knowledge. The focus is on the description of cellular factors being involved in the life cycle of HDV and the deregulation of cellular pathways by HDV with respect to their relevance for viral replication, morphogenesis and HDV-associated pathogenesis. Moreover, recent progress in antiviral strategies targeting cellular structures is summarized in this article.

Diagnosis of HDV: From virology to non-invasive markers of fibrosis

Hepatitis D viral infection in humans is a disease that requires the establishment of hepatitis B, relying on hepatitis B surface Ag and host cellular machinery to replicate and propagate the infection. Since its discovery in 1977, substantial progress has been made to better understand the hepatitis D viral life cycle, pathogenesis and modes of transmission along with expanding on clinical knowledge related to prevention, diagnosis, monitoring and treatment. The availability of serologic diagnostic assays for hepatitis D infection has evolved over time with current widespread availability, improved detection and standardized reporting. With human migration, the epidemiology of hepatitis D infection has changed over time. Thus, the ability to use diagnostic assays remains essential to monitor the global impact of hepatitis D infection. Separately, while liver biopsy remains the gold standard for the staging of this rapidly progressive and severe form of chronic viral hepatitis, there is an unmet need for clinical monitoring of chronic hepatitis D infection for management of progressive disease. Thus, exploration of the utility of non-invasive fibrosis markers in hepatitis D is ongoing. In this review, we discuss the virology, the evolution of diagnostics and the development of non-invasive markers for the detection and monitoring of fibrosis in patients with hepatitis D infection.

Cell Culture Systems for Studying Hepatitis B and Hepatitis D Virus Infections

The hepatitis B virus (HBV) and hepatitis D virus (HDV) infections cause liver disease, including hepatitis, cirrhosis, and hepatocellular carcinoma (HCC). HBV infection remains a major global health problem. In 2019, 296 million people were living with chronic hepatitis B and about 5% of them were co-infected with HDV. In vitro cell culture systems are instrumental in the development of therapeutic targets. Cell culture systems contribute to identifying molecular mechanisms for HBV and HDV propagation, finding drug targets for antiviral therapies, and testing antiviral agents. Current HBV therapeutics, such as nucleoside analogs, effectively suppress viral replication but are not curative. Additionally, no effective treatment for HDV infection is currently available. Therefore, there is an urgent need to develop therapies to treat both viral infections. A robust in vitro cell culture system supporting HBV and HDV infections (HBV/HDV) is a critical prerequisite to studying HBV/HDV pathogenesis, the complete life cycle of HBV/HDV infections, and consequently identifying new therapeutics. However, the lack of an efficient cell culture system hampers the development of novel antiviral strategies for HBV/HDV infections. In vitro cell culture models have evolved with significant improvements over several decades. Recently, the development of the HepG2-NTCP sec+ cell line, expressing the sodium taurocholate co-transporting polypeptide receptor (NTCP) and self-assembling co-cultured primary human hepatocytes (SACC-PHHs) has opened new perspectives for a better understanding of HBV and HDV lifecycles and the development of specific antiviral drug targets against HBV/HDV infections. We address various cell culture systems along with different cell lines and how these cell culture systems can be used to provide better tools for HBV and HDV studies.

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.

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.

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.

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.

Interplay between Hepatitis D Virus and the Interferon Response

Chronic hepatitis D (CHD) is the most severe form of viral hepatitis, with rapid progression of liver-related diseases and high rates of development of hepatocellular carcinoma. The causative agent, hepatitis D virus (HDV), contains a small (approximately 1.7 kb) highly self-pairing single-strand circular RNA genome that assembles with the HDV antigen to form a ribonucleoprotein (RNP) complex. HDV depends on hepatitis B virus (HBV) envelope proteins for envelopment and de novo hepatocyte entry; however, its intracellular RNA replication is autonomous. In addition, HDV can amplify HBV independently through cell division. Cellular innate immune responses, mainly interferon (IFN) response, are crucial for controlling invading viruses, while viruses counteract these responses to favor their propagation. In contrast to HBV, HDV activates profound IFN response through the melanoma differentiation antigen 5 (MDA5) pathway. This cellular response efficiently suppresses cell-division-mediated HDV spread and, to some extent, early stages of HDV de novo infection, but only marginally impairs RNA replication in resting hepatocytes. In this review, we summarize the current knowledge on HDV structure, replication, and persistence and subsequently focus on the interplay between HDV and IFN response, including IFN activation, sensing, antiviral effects, and viral countermeasures. Finally, we discuss crosstalk with HBV.

Hepatitis delta virus interacts with splicing factor SF3B155 and alters pre-mRNA splicing of cell cycle control genes

Hepatitis delta virus (HDV) is the agent responsible for the most severe form of human viral hepatitis. The HDV genome consists of a single-stranded circular RNA molecule that encodes for one single protein, the delta antigen. Given its simplicity, HDV must make use of several host cellular proteins to accomplish its life cycle processes, including transcription, replication, post-transcriptional, and post-translational modifications. Consequently, identification of the interactions established between HDV components and host proteins assumes a pivotal interest in the search of novel therapeutic targets. Here, we used the yeast three-hybrid system to screen a human liver cDNA library to identify host proteins that interact with the HDV genomic RNA. One of the identified proteins corresponded to the splicing factor SF3B155, a component of the U2snRNP complex that is essential for the early recognition of 3' splice sites in the pre-mRNAs of human genes. We show that the interaction between the HDV genomic RNA and SF3B155 occurs in vivo and that the expression of HDV promotes changes in splicing of human genes whose alternative splicing is SF3B155-dependent. We further show that expression of HDV triggers alterations in several constitutive and alternative splicing events in the tumor suppressor RBM5 transcript, with consequent reduction of its protein levels. This is the first description that HDV expression promotes changes in the splicing of human genes, and we suggest that the HDV-induced alternative splicing changes, through SF3B155 sequester, may contribute for the early progression to hepatocellular carcinoma characteristic of HDV-infected patients.

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.

Snake Deltavirus Utilizes Envelope Proteins of Different Viruses To Generate Infectious Particles

Satellite viruses, most commonly found in plants, rely on helper viruses to complete their replication cycle. The only known example of a human satellite virus is the hepatitis D virus (HDV), and it is generally thought to require hepatitis B virus (HBV) to form infectious particles. Until 2018, HDV was the sole representative of the genus Deltavirus and was thought to have evolved in humans, the only known HDV host. The subsequent identification of HDV-like agents in birds, snakes, fish, amphibians, and invertebrates indicated that the evolutionary history of deltaviruses is likely much longer than previously hypothesized. Interestingly, none of the HDV-like agents were found in coinfection with an HBV-like agent, suggesting that these viruses use different helper virus(es). Here we show, using snake deltavirus (SDeV), that HBV and hepadnaviruses represent only one example of helper viruses for deltaviruses. We cloned the SDeV genome into a mammalian expression plasmid, and by transfection could initiate SDeV replication in cultured snake and mammalian cell lines. By superinfecting persistently SDeV-infected cells with reptarenaviruses and hartmaniviruses, or by transfecting their surface proteins, we could induce production of infectious SDeV particles. Our findings indicate that deltaviruses can likely use a multitude of helper viruses or even viral glycoproteins to form infectious particles. This suggests that persistent infections, such as those caused by arenaviruses and orthohantaviruses used in this study, and recurrent infections would be beneficial for the spread of deltaviruses. It seems plausible that further human or animal disease associations with deltavirus infections will be identified in the future.IMPORTANCE Deltaviruses need a coinfecting enveloped virus to produce infectious particles necessary for transmission to a new host. Hepatitis D virus (HDV), the only known deltavirus until 2018, has been found only in humans, and its coinfection with hepatitis B virus (HBV) is linked with fulminant hepatitis. The recent discovery of deltaviruses without a coinfecting HBV-like agent in several different taxa suggested that deltaviruses could employ coinfection by other enveloped viruses to complete their life cycle. In this report, we show that snake deltavirus (SDeV) efficiently utilizes coinfecting reptarena- and hartmaniviruses to form infectious particles. Furthermore, we demonstrate that cells expressing the envelope proteins of arenaviruses and orthohantaviruses produce infectious SDeV particles. As the envelope proteins are responsible for binding and infecting new host cells, our findings indicate that deltaviruses are likely not restricted in their tissue tropism, implying that they could be linked to animal or human diseases other than hepatitis.

Chronic hepatitis delta: A state-of-the-art review and new therapies

Chronic delta hepatitis is the most severe form of viral hepatitis affecting nearly 65 million people worldwide. Individuals with this devastating illness are at higher risk for developing cirrhosis and hepatocellular carcinoma. Delta virus is a defective RNA virus that requires hepatitis B surface antigen for propagation in humans. Infection can occur in the form of a co-infection with hepatitis B, which can be self-limiting, vs superinfection in a patient with established hepatitis B infection, which often leads to chronicity in majority of cases. Current noninvasive tools to assess for advanced liver disease have limited utility in delta hepatitis. Guidelines recommend treatment with pegylated interferon, but this is limited to patients with compensated disease and is efficacious in about 30% of those treated. Due to limited treatment options, novel agents are being investigated and include entry, assembly and export inhibitors of viral particles in addition to stimulators of the host immune response. Future clinical trials should take into consideration the interaction of hepatitis B and hepatitis D as suppression of one virus can lead to the activation of the other. Also, surrogate markers of treatment efficacy have been proposed.

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.

Relative Abundance of Integrant-Derived Viral RNAs in Infected Tissues Harvested from Chronic Hepatitis B Virus Carriers

Five matching sets of nonmalignant liver tissues and hepatocellular carcinoma (HCC) samples from individuals chronically infected with hepatitis B virus (HBV) were examined. The HBV genomic sequences were determined by using overlapping PCR amplicons covering the entire viral genome. Four pairs of tissues were infected with HBV genotype C, while one pair was infected with HBV genotype B. HBV replication markers were found in all tissues. In the majority of HCC samples, the levels of pregenomic/precore RNA (pgRNA) and covalently closed circular DNA (cccDNA) were lower than those in liver tissue counterparts. Regardless of the presence of HBV replication markers, (i) integrant-derived HBV RNAs (id-RNAs) were found in all tissues by reverse transcription-PCR (RT-PCR) analysis and were considerably abundant or predominant in 6/10 tissue samples (2 liver and 4 HCC samples), (ii) RNAs that were polyadenylated using the cryptic HBV polyadenylation signal and therefore could be produced by HBV replication or derived from integrated HBV DNA were found in 5/10 samples (3 liver and 2 HCC samples) and were considerably abundant species in 3/10 tissues (2 livers and 1 HCC), and (iii) cccDNA-transcribed RNAs polyadenylated near position 1931 were not abundant in 7/10 tissues (2 liver and 5 HCC samples) and were predominant in only two liver samples. Subsequent RNA sequencing analysis of selected liver/HCC samples also showed relative abundance of id-RNAs in most of the examined tissues. Our findings suggesting that id-RNAs could represent a significant source of HBV envelope proteins, which is independent of viral replication, are discussed in the context of the possible contribution of id-RNAs to the HBV life cycle.IMPORTANCE The relative abundance of integrant-derived HBV RNAs (id-RNAs) in chronically infected tissues suggest that id-RNAs coding for the envelope proteins may facilitate the production of a considerable fraction of surface antigens (HBsAg) in infected cells bearing HBV integrants. If the same cells support HBV replication, then a significant fraction of assembled HBV virions could bear id-RNA-derived HBsAg as a major component of their envelopes. Therefore, the infectivity of these HBV virions and their ability to facilitate virus cell-to-cell spread could be determined mainly by the properties of id-RNA-derived envelope proteins and not by the properties of replication-derived HBsAg. These interpretations suggest that id-RNAs may play a role in the maintenance of chronic HBV infection and therefore contribute to the HBV life cycle. Furthermore, the production of HBsAg from id-RNAs independently of viral replication may explain at least in part why treatment with interferon or nucleos(t)ides in most cases fails to achieve a loss of serum HBsAg.

Hepatitis delta: virological and clinical aspects

There are an estimated 400 million chronic carriers of HBV worldwide; between 15 and 20 million have serological evidence of exposure to HDV. Traditionally, regions with high rates of endemicity are central and northern Africa, the Amazon Basin, eastern Europe and the Mediterranean, the Middle East and parts of Asia. There are two types of HDV/HBV infection which are differentiated by the previous status infection by HBV for the individual. Individuals with acute HBV infection contaminated by HDV is an HDV/HBV co-infection, while individuals with chronic HBV infection contaminated by HDV represent an HDV/HBV super-infection. The appropriate treatment for chronic hepatitis delta is still widely discussed since it does not have an effective drug. Alpha interferon is currently the only licensed therapy for the treatment of chronic hepatitis D. The most widely used drug is pegylated interferon but only approximately 25% of patients maintain a sustained viral response after 1 year of treatment. The best marker of therapeutic success would be the clearance of HBsAg, but this data is rare in clinical practice. Therefore, the best way to predict a sustained virologic response is the maintenance of undetectable HDV RNA levels.

Hepatitis Delta Virus: Replication Strategy and Upcoming Therapeutic Options for a Neglected Human Pathogen

The human Hepatitis Delta Virus (HDV) is unique among all viral pathogens. Encoding only one protein (Hepatitis Delta Antigen; HDAg) within its viroid-like self-complementary RNA, HDV constitutes the smallest known virus in the animal kingdom. To disseminate in its host, HDV depends on a helper virus, the human Hepatitis B virus (HBV), which provides the envelope proteins required for HDV assembly. HDV affects an estimated 15-20 million out of the 240 million chronic HBV-carriers and disperses unequally in disparate geographical regions of the world. The disease it causes (chronic Hepatitis D) presents as the most severe form of viral hepatitis, leading to accelerated progression of liver dysfunction including cirrhosis and hepatocellular carcinoma and a high mortality rate. The lack of approved drugs interfering with specific steps of HDV replication poses a high burden for gaining insights into the molecular biology of the virus and, consequently, the development of specific novel medications that resiliently control HDV replication or, in the best case, functionally cure HDV infection or HBV/HDV co-infection. This review summarizes our current knowledge of HBV molecular biology, presents an update on novel cell culture and animal models to study the virus and provides updates on the clinical development of the three developmental drugs Lonafarnib, REP2139-Ca and Myrcludex B.

Hepatitis delta virus: insights into a peculiar pathogen and novel treatment options

Chronic hepatitis D is the most severe form of viral hepatitis, affecting ∼20 million HBV-infected people worldwide. The causative agent, hepatitis delta virus (HDV), is a unique human pathogen: it is the smallest known virus; it depends on HBV to disseminate its viroid-like RNA; it encodes only one protein (HDAg), which has both structural and regulatory functions; and it replicates using predominantly host proteins. The failure of HBV-specific nucleoside analogues to suppress the HBV helper function, and the limitations of experimental systems to study the HDV life cycle, have impeded the development of HDV-specific drugs. Thus, the only clinical regimen for HDV is IFNα, which shows some efficacy but long-term virological responses are rare. Insights into the receptor-mediated entry of HDV, and the observation that HDV assembly requires farnesyltransferase, have enabled novel therapeutic strategies to be developed. Interference with entry, for example through blockade of the HBV-HDV-specific receptor sodium/taurocholate cotransporting polypeptide NTCP by Myrcludex B, and inhibition of assembly by blockade of farnesyltransferase using lonafarnib or nucleic acid polymers such as REP 2139-Ca, have shown promising results in phase II studies. In this Review, we summarize our knowledge of HDV epidemiology, pathogenesis and molecular biology, with a particular emphasis on possible future developments.

Infectious long non-coding RNAs

Long non protein coding RNAs (lncRNAs) constitute a large category of the RNA world, able to regulate different biological processes. In this review we are focusing on infectious lncRNAs, their classification, pathogenesis and impact on the infected organisms. Here they are presented in two separate groups: 'dependent lncRNAs' (comprising satellites RNA, Hepatitis D virus and lncRNAs of viral origin) which need a helper virus and 'independent lncRNAs' (viroids) that can self-replicate. Even though these lncRNA do not encode any protein, their structure and/or sequence comprise all the necessary information to drive specific interactions with host factors and regulate several cellular functions. These new data that have emerged during the last few years concerning lncRNAs modify the way we understand molecular biology's 'central dogma' and give new perspectives for applications and potential therapeutic strategies.

Envelope proteins derived from naturally integrated hepatitis B virus DNA support assembly and release of infectious hepatitis delta virus particles

A natural subviral agent of human hepatitis B virus (HBV), hepatitis delta virus (HDV), requires only the envelope proteins from HBV in order to maintain persistent infection. HBV surface antigens (HBsAgs) can be produced either by HBV replication or from integrated HBV DNA regardless of replication. The functional properties of the integrant-generated HBsAgs were examined using two human hepatocellular carcinoma-derived cell lines, Hep3B and PLC/PRF/5, that contain HBV integrants but do not produce HBV virions and have no signs of HBV replication. Both cell lines were able to support HDV replication and assembly/egress of HDV virions. Neither of the cell lines was able to produce substantial amounts of the pre-S1-containing HDV particles. HDV virions assembled in PLC/PRF/5 cells were able to infect primary human hepatocytes, while Hep3B-derived HDV appeared to be noninfectious. These results correlate with the findings that the entire open reading frame (ORF) for the large (L) envelope protein that is essential for infectivity is present on HBV RNAs from PLC/PRF/5 cells, while an L protein ORF that was truncated and fused to inverted precore sequences was found using RNAs from Hep3B cells. This study demonstrates for the first time that at least some of the HBV DNA sequence naturally integrated during infection can produce functional small and large envelope proteins capable of the formation of infectious HDV virions. Our data indicate that in vivo chronic HDV infection can persist in the absence of HBV replication (or when HBV replication is profoundly suppressed) if functional envelope proteins are supplied from HBV integrants.

IMPORTANCE

The study addresses the unique mechanism of HDV persistence in the absence of ongoing HBV replication, advances our understanding of HDV-HBV interactions, and supports the implementation of treatments directly targeting HDV for HDV/HBV-infected individuals.

Hepatitis delta virus: a peculiar virus

The hepatitis delta virus (HDV) is distributed worldwide and related to the most severe form of viral hepatitis. HDV is a satellite RNA virus dependent on hepatitis B surface antigens to assemble its envelope and thus form new virions and propagate infection. HDV has a small 1.7 Kb genome making it the smallest known human virus. This deceivingly simple virus has unique biological features and many aspects of its life cycle remain elusive. The present review endeavors to gather the available information on HDV epidemiology and clinical features as well as HDV biology.

Searching for nuclear export elements in hepatitis D virus RNA

AIM: To search for the presence of cis elements in hepatitis D virus (HDV) genomic and antigenomic RNA capable of promoting nuclear export.

METHODS: We made use of a well characterized chloramphenicol acetyl-transferase reporter system based on plasmid pDM138. Twenty cDNA fragments corresponding to different HDV genomic and antigenomic RNA sequences were inserted in plasmid pDM138, and used in transfection experiments in Huh7 cells. The relative amounts of HDV RNA in nuclear and cytoplasmic fractions were then determined by real-time polymerase chain reaction and Northern blotting. The secondary structure of the RNA sequences that displayed nuclear export ability was further predicted using a web interface. Finally, the sensitivity to leptomycin B was assessed in order to investigate possible cellular pathways involved in HDV RNA nuclear export.

RESULTS: Analysis of genomic RNA sequences did not allow identifying an unequivocal nuclear export element. However, two regions were found to promote the export of reporter mRNAs with efficiency higher than the negative controls albeit lower than the positive control. These regions correspond to nucleotides 266-489 and 584-920, respectively. In addition, when analyzing antigenomic RNA sequences a nuclear export element was found in positions 214-417. Export mediated by the nuclear export element of HDV antigenomic RNA is sensitive to leptomycin B suggesting a possible role of CRM1 in this transport pathway.

CONCLUSION: A cis-acting nuclear export element is present in nucleotides 214-417 of HDV antigenomic RNA.

Proteomic changes in HEK-293 cells induced by hepatitis delta virus replication

Hepatitis delta virus (HDV) infection greatly increases the risk of hepatocellular carcinoma in hepatitis B virus chronically infected patients. HDV is highly dependent on host factors for accomplishment of the replication cycle. However, these factors are largely unknown and the mechanisms involved in the pathogenicity of the virus still remain elusive. Here, we made use of the HEK-293 cell line, which was engineered in order to mimic HDV replication. Five different proteomes were analyzed and compared using a MS-based quantitative proteomics approach by (18)O/(16)O stable isotope labeling. About 3000 proteins were quantified and 89 found to be differentially expressed as a consequence HDV RNA replication. The down-regulation of p53 , HSPE, and ELAV as well as the up-regulation of Transportin 1 , EIF3D, and Cofilin 1 were validated by Western blot. A systems biology approach was additionally used to analyze altered pathways and networks. The G2/M DNA damage checkpoint and pyruvate metabolism were among the most affected pathways, and Cancer was the most likely disease associated to HDV replication. Western blot analysis allowed identifying 14-3-3 σ interactor as down-regulated protein acting in the G2/M cell cycle control checkpoint. This evidence supports deregulation of G2/M checkpoint as a possible mechanism involved in the promotion of HDV associated hepatocellular carcinoma.

BIOLOGICAL SIGNIFICANCE

This manuscript provides a description of changes observed in the cellular proteome that arise as result of expression of the hepatitis delta virus (HDV) antigen as well as virus genome replication. Using a systems biology approach cancer was found to be the most probable disease associated with HDV replication. Additionally, results show that HDV alters the regulation of G2/M cell cycle control checkpoint. Taken together, our data provide new insights into probable mechanisms associated with the increased incidence of hepatocellular carcinoma observed in HDV infected patients.

Lysine-71 in the large delta antigen of hepatitis delta virus clade 3 modulates its localization and secretion

Hepatitis delta virus (HDV) is an RNA virus and eight clades of HDV have been identified. HDV clade 3 (HDV-3) is isolated only in the northern area of South America. The outcome of HDV-3 infection is associated with severe fulminant hepatitis. Variations in the large delta antigen (LDAg) between HDV clade 1 (HDV-1) and HDV-3 have been proposed to contribute to differences in viral secretion efficiency, but which changes might be relevant remains unclear. The control of subcellular localization of LDAg has been reported to be associated with post-translational modifications, such as phosphorylation and isoprenylation. We have observed evidence for acetylation on the LDAg of HDV-3 (LDAg-3) and LDAg of HDV-1 (LDAg-1). Green fluorescent protein-fused LDAg-3 (GFP-LD3) was used to investigate the cellular distribution and secretion of the protein. Sequence alignment of LDAg amino acids suggested that lysine-71 of LDAg-3 could be an acetylation site. Expression of a mutant form of LDAg-3 with an arginine-substitution at lysine-71 (GFP-LD3K71R) showed a distribution of the protein predominantly in the cytoplasm instead of the nucleus. Western blot analyses of secreted empty viral particles (EVPs) revealed a higher amount of secreted GFP-LD3K71R compared to GFP-LD3. Furthermore, the ectopic expression of p300, a histone acetyltransferase, led to a reduction of GFP-LD3 in EVPs. By contrast, expression of three histone deacetylases (HDAC-4, -5, and -6) facilitated the secretion of GFP-LD3. Combined, our observations support the hypothesis that the acetylation status of LDAg-3 plays a role in regulating LDAg-3's localization inside the nucleus or cytoplasm, and its secretion.

Hepatitis delta virus infects the cells of hepadnavirus-induced hepatocellular carcinoma in woodchucks

Hepatitis delta virus (HDV) is a natural subviral agent of human hepatitis B virus (HBV). HDV enhances liver damage during concomitant infection with HBV. The molecular pathogenesis of HDV infection remains poorly understood. To advance our understanding of the relationship between HDV infection and liver cancer, it was determined whether HDV could infect in vivo the cells of hepadnavirus-induced hepatocellular carcinoma (HCC). Woodchucks (Marmota monax) that were chronically infected with HBV-related woodchuck hepatitis virus (WHV) and already developed HCCs were used as an experimental model. The locations of HCCs within the livers were determined using ultrasound imaging followed by open surgery. One week after surgery the WHV carrier woodchucks were superinfected with WHV-enveloped HDV (wHDV). Six weeks later the animals were sacrificed and HDV replication in normal liver tissues and in center masses of HCCs was evidenced by Northern analysis, real-time polymerase chain reaction assay, and immunohistochemistry. Based on accumulation levels of HDV RNAs and numbers of infected cells, the efficiency of wHDV infection appears to be comparable in most HCCs and normal liver tissues.

CONCLUSION

Cells of WHV-induced HCCs are susceptible to HDV infection in vivo, and therefore express functional putative WHV receptors and support the steps of the attachment/entry governed by the hepadnavirus envelope proteins. Because others previously hypothesized that hepadnavirus-induced HCCs are resistant to reinfection with a hepadnavirus in vivo, our data suggest that if such a resistance exists it likely occurs via a block at the post-entry step. The demonstrated ability of HDV to infect already formed HCCs may facilitate development of novel strategies further dissecting the mechanism of liver pathogenesis associated with HDV infection.

The heterogeneous ribonuclear protein C interacts with the hepatitis delta virus small antigen

BACKGROUND

Hepatitis delta virus (HDV) is considered to be a satellite virus of the Hepatitis B virus. The genome consists of a 1679 nt ssRNA molecule in which a single ORF was identified. This ORF codes for a unique protein, the Delta antigen (HDAg). During transcription, two forms, small (S-HDAg; p24) and large (L-HDAg; p27) of this antigen are derived as a result of an editing mechanism catalyzed by cellular adenosine deaminase 1. Despite its simplicity, little is still known about the host factors that interact with the virus RNA and antigens being to modulate virus replication.

METHODS

A yeast two-hybrid screening of a human liver cDNA library, using the hepatitis delta virus (HDV) small antigen (S-HDAg) as bait, was performed. Blot overlay and co-immunoprecipitation assays were used in an attempt to confirm the interaction of hnRNPC and S-HDAg. siRNA knockdown assays of hnRNPC were performed to assess the effect on HDV antigen expression.

RESULTS

Thirty known proteins were identified as S-HDAg interactors in the yeast two-hybrid screening. One of the identified proteins, hnRNPC, was found to interact with S-HDAg in vitro and in vivo in human liver cells. The interaction of the two proteins is mediated by the C-terminal half of the S-HDAg which contains a RNA-binding domain (aa 98-195). HDV RNA, S-HDAg, and hnRNPC, were also found to co-localize in the nucleus of human liver cells. Knockdown of hnRNPC mRNA using siRNAs resulted in a marked decreased expression of HDV antigens.

CONCLUSIONS

S-HDAg was found to interact with human liver proteins previously assigned to different functional categories. Among those involved in nucleic acid metabolism, hnRNPC was found to interact in vitro and in vivo in human liver cells. Similar to other RNA viruses, it seems plausible that hnRNPC may also be involved in HDV replication. However, further investigation is mandatory to clarify this question.

In Vivo Interaction of the Hepatitis Delta Virus Small Antigen with the ELAV-Like Protein HuR

The small and large delta antigens (S-HDAg and L-HDAg, respectively) represent two forms of the only protein encoded by the hepatitis delta virus (HDV) RNA genome. Consequently, HDV relies, at a large extent, on the host cell machinery for replication and transcription. Until now, only a limited number of cellular proteins were identified as S-HDAg or L-HDAg partners being involved in the modulation of the virus life cycle. In an attempt to identify cellular S-HDAg-binding proteins we made use of a yeast two-hybrid approach to screen a human liver cDNA library. We were able to identify HuR, a ubiquitously expressed protein involved in RNA stabilization, as an S-HDAg partner both in vitro and in vivo. HuR was found to be overexpressed and colocalize with HDAg in human hepatoma cells. siRNA knockdown of HuR mRNA resulted in inhibition of S-HDAg and L-HDAg expression.

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.

Chapter 3. Replication of the hepatitis delta virus RNA genome

Hepatitis delta virus (HDV) is a subviral agent dependent upon hepatitis B virus (HBV). HDV uses the envelope proteins of HBV to achieve assembly and infection of target cells. Otherwise, the replication of the RNA genome of HDV is totally different from that of its helper virus, and involves redirection of host polymerase activity. This chapter is concerned with recent developments in our understanding of the genome replication process.

Capped small RNAs and MOV10 in human hepatitis delta virus replication

The evolutionary origin of human Hepatitis Delta Virus (HDV) replication by RNA-directed transcription is unclear. Here we identify two species of 5′ capped, ∼18-25 nucleotide small RNAs. One was of antigenomic polarity corresponding to the 5′ end of Hepatitis Delta Antigen (HDAg) mRNA and interacted with HDAg and RNA POLYMERASE II (POL II), while the other mapped to a structurally analogous region on the genomic RNA hairpin. An HDAg-interaction screen uncovered MOV10, the human homologue of the A. thaliana RNA amplification factor SDE3 and D. melanogaster RISC-maturation factor Armitage. MOV10 knockdown inhibited HDV replication, but not HDAg mRNA translation supporting a role for MOV10 in RNA-directed transcription. Together, our studies define RNA hairpins as critical elements for the initiation of HDV-related RNA-directed transcription. The identification of capped small RNAs and the involvement of MOV10 in HDV replication further suggest a conserved mechanism related to RNA-directed transcription in lower eukaryotes.

Nucleolar targeting of hepatitis delta antigen abolishes its ability to initiate viral antigenomic RNA replication

Hepatitis delta virus (HDV) is a small RNA virus that contains one 1.7-kb single-stranded circular RNA of negative polarity. The HDV particle also contains two isoforms of hepatitis delta antigen (HDAg), small (SHDAg) and large HDAg. SHDAg is required for the replication of HDV, which is presumably carried out by host RNA-dependent RNA polymerases. The localization and the HDAg and host RNA polymerase responsible for HDV replication remain important issues to be addressed. In this study, using recombinant SHDAg fused with a heterologous nucleolar localization sequence (NoLS) to confine its subcellular localization in nucleoli, we aimed to study the effect of SHDAg subcellular localization on HDV RNA replication. The initiation of genomic RNA synthesis from antigenomic template was hardly detectable when SHDAg was fused with the NoLS motif and localized mainly in nucleoli. In contrast, the initiation of antigenomic RNA synthesis was not affected. Drug treatment to release a SHDAg-NoLS mutant from nucleoli could partially restore the replication of HDV genomic RNA from antigenomic RNA. This also recovered the cointeraction between SHDAg and RNA polymerase II. These data strongly suggest that nuclear polymerase (RNA polymerase II) is involved in the synthesis of genomic RNA and that the synthesis of antigenomic RNA can occur in nucleoli. Our results support the idea that the replication of HDV genomic RNA or antigenomic RNA is likely to be carried out by different machineries in different subcellular localizations.

Characterization of the nuclear localization signal of the hepatitis delta virus antigen

The delta antigen (HDAg) is the only protein encoded by the hepatitis delta virus (HDV) RNA genome. The HDAg contains an RNA binding domain, a dimerization domain, and a nuclear localization signal (NLS). The nuclear import of HDV RNPs is thought to be one of the first tasks of the HDAg during the HDV replication cycle. Using c-myc-PK fusions with several regions of the HDAg in transfection assays in Huh7 cells, we found that the HDAg NLS consists of a single stretch of 10 amino acids, EGAPPAKRAR, located in positions 66-75. Deletion and mutation analysis of this region showed that both the acidic glutamic acid residue at position 66 and the basic arginine residue at position 75 are essential for promoting nuclear import.

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.

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.

Post-translational modification of delta antigen of hepatitis D virus

The hepatitis delta virus (HDV) genome has only one open reading frame, which encodes the viral small delta antigen. After RNA editing, the same open reading frame is extended 19 amino acids at the carboxyl terminus and encodes the large delta antigen. These two viral proteins escort the HDV genome through different cellular compartments for the complicated phases of replication, transcription and, eventually, the formation of progeny virions. To orchestrate these events, the delta antigens have to take distinct cues to traffic to the right compartments and make correct molecular contacts. In eukaryotes, post-translational modification (PTM) is a major mechanism of dictating the multiple functions of a single protein. Multiple PTMs, including phosphorylation, isoprenylation, acetylation, and methylation, have been identified on hepatitis delta antigens. In this chapter we review these PTMs and discuss their functions in regulating and coordinating the life cycle of HDV.

Novel nuclear export signal-interacting protein, NESI, critical for the assembly of hepatitis delta virus

The process of host factor-mediated nucleocytoplasmic transport is critical for diverse cellular events in eukaryotes and the life cycle of viruses. We have previously identified a chromosome region maintenance 1-independent nuclear export signal (NES) at the C terminus of the large form of hepatitis delta antigen (HDAg), designated NES(HDAg-L) that is required for the assembly of hepatitis delta virus (HDV) (C.-H. Lee et al., J. Biol. Chem. 276:8142-8148, 2001). To look for interacting proteins of the NES(HDAg-L), yeast two-hybrid screening was applied using the GAL4-binding domain fused to the NES(HDAg-L) as bait. Among the positive clones, one encodes a protein, designated NESI [NES(HDAg-L) interacting protein] that specifically interacted with the wild-type NES(HDAg-L) but not with the export/package-defective HDAg-L mutant, NES*(HDAg-L), in which Pro-205 has been replaced by Ala. Northern blot analysis revealed NESI as the gene product of a 1.9-kb endogenous mRNA transcript that is present predominantly in human liver tissue. NESI consists of 467 amino acid residues and bears a putative actin-binding site and a bipartite nuclear localization signal. Specific interaction between HDAg-L and NESI was further confirmed by coimmunoprecipitation and immunofluorescence staining. Overexpression of antisense NESI RNAs inhibited the expression of NESI and abolished HDAg-L-mediated nuclear export and assembly of HDV genomic RNA. These data indicate a critical role of NESI in the assembly of HDV through interaction with HDAg-L.

Pathologic, immunohistochemical, and electron microscopic findings in naturally occurring virulent systemic feline calicivirus infection in cats

Infection with feline calicivirus (FCV) is a common cause of upper respiratory and oral disease in cats. FCV infection is rarely fatal, however, virulent, systemic strains of FCV (VS-FCV) that cause alopecia, cutaneous ulcers, subcutaneous edema, and high mortality in affected cats have recently been described. Seven cats with natural VS-FCV infection all had subcutaneous edema and ulceration of the oral cavity, with variable ulceration of the pinnae, pawpads, nares, and skin. Other lesions that were present in some affected cats included bronchointerstitial pneumonia, and pancreatic, hepatic, and splenic necrosis. Viral antigen was present within endothelial and epithelial cells in affected tissues as determined by immunohistochemical staining with a monoclonal antibody to FCV. Mature intranuclear and intracytoplasmic virions in necrotic epithelial cells were identified by transmission electron microscopy. VS-FCV infection causes epithelial cell cytolysis and systemic vascular compromise in susceptible cats, leading to cutaneous ulceration, severe edema, and high mortality.

Nucleic acid binding properties of the nucleic acid chaperone domain of hepatitis delta antigen

The N terminal region of hepatitis delta antigen (HDAg), referred to here as NdAg, has a nucleic acid chaperone activity that modulates the ribozyme activity of hepatitis delta virus (HDV) RNA and stimulates hammerhead ribozyme catalysis. We characterized the nucleic acid binding properties of NdAg, identified the structural and sequence domains important for nucleic acid binding, and studied the correlation between the nucleic acid binding ability and the nucleic acid chaperone activity. NdAg does not recognize the catalytic core of HDV ribozyme specifically. Instead, NdAg interacts with a variety of nucleic acids and has higher affinities to longer nucleic acids. The studies with RNA homopolymers reveal that the binding site size of NdAg is around nine nucleotides long. The extreme N terminal portion of NdAg, the following coiled-coil domain and the basic amino acid clusters in these regions are important for nucleic acid binding. The nucleic acid-NdAg complex is stabilized largely by electrostatic interactions. The formation of RNA-protein complex appears to be a prerequisite for facilitating hammerhead ribozyme catalysis of NdAg and its derivatives. Mutations that reduce the RNA binding activity or high ionic strength that destabilizes the RNA-protein complex, reduce the nucleic acid chaperone activity of NdAg.

The small delta antigen of hepatitis delta virus is an acetylated protein and acetylation of lysine 72 may influence its cellular localization and viral RNA synthesis

Hepatitis delta virus (HDV) is a single-stranded RNA virus that encodes two viral nucleocapsid proteins named small and large form hepatitis delta antigen (S-HDAg and L-HDAg). The S-HDAg is essential for viral RNA replication while the L-HDAg is required for viral assembly. In this study, we demonstrated that HDAg are acetylated proteins. Metabolic labeling with [(3)H]acetate revealed that both forms of HDAg could be acetylated in vivo. The histone acetyltransferase (HAT) domain of cellular acetyltransferase p300 could acetylate the full-length and the N-terminal 88 amino acids of S-HDAg in vitro. By mass spectrometric analysis of the modified protein, Lys-72 of S-HDAg was identified as one of the acetylation sites. Substitution of Lys-72 to Arg caused the mutant S-HDAg to redistribute from the nucleus to the cytoplasm. The mutant reduced viral RNA accumulation and resulted in the earlier appearance of L-HDAg. These results demonstrated that HDAg is an acetylated protein and mutation of HDAg at Lys-72 modulates HDAg subcellular localization and may participate in viral RNA nucleocytoplasmic shuttling and replication.

Resistance of human hepatitis delta virus RNAs to dicer activity

The endonuclease dicer cleaves RNAs that are 100% double stranded and certain RNAs with extensive but <100% pairing to release approximately 21-nucleotide (nt) fragments. Circular 1,679-nt genomic and antigenomic RNAs of human hepatitis delta virus (HDV) can fold into a rod-like structure with 74% pairing. However, during HDV replication in hepatocytes of human, woodchuck, and mouse origin, no approximately 21-nt RNAs were detected. Likewise, in vitro, purified recombinant dicer gave <0.2% cleavage of unit-length HDV RNAs. Similarly, rod-like RNAs of potato spindle tuber viroid (PSTVd) and avocado sunblotch viroid (ASBVd) were only 0.5% cleaved. Furthermore, when a 66-nt hairpin RNA with 79% pairing, the putative precursor to miR-122, which is an abundant liver micro-RNA, replaced one end of HDV genomic RNA, it was poorly cleaved, both in vivo and in vitro. In contrast, this 66-nt hairpin, in the absence of appended HDV sequences, was >80% cleaved in vitro. Other 66-nt hairpins derived from one end of genomic HDV, PSTVd, or ASBVd RNAs were also cleaved. Apparently, for unit-length RNAs of HDV, PSTVd, and ASBVd, it is the extended structure with <100% base pairing that confers significant resistance to dicer action.

Replication of human hepatitis delta virus: recent developments

In a natural setting, hepatitis delta virus (HDV) is only found in patients that are also infected with hepatitis B virus (HBV). In hepatocytes infected with these two viruses, HDV RNA genomes are assembled using the envelope proteins of HBV. Since 1986, we have known that HDV has a small single-stranded RNA genome with a unique circular conformation that is replicated using a host RNA polymerase. These and other features make HDV and its replication unique, at least among agents that infect animals. This mini-review focuses on advances gained over the last 2-3 years, together with an evaluation of HDV questions that are either unsolved or not yet solved satisfactorily.

Replicating hepatitis delta virus RNA is edited in the nucleus by the small form of ADAR1

Hepatitis delta virus (HDV) uses a host-encoded RNA-editing activity to express its two essential proteins from the same coding sequence. Adenosine deaminase that acts on RNA (ADAR)1 and ADAR2 are enzymes that catalyze such reactions, and each, when overexpressed, are capable of editing HDV RNA in vivo. However, the enzyme responsible for editing HDV RNA during replication has not been determined. Mammalian cells express two forms of ADAR1, a large form (ADAR1-L) that mainly localizes to the cytoplasm and a small form (ADAR1-S) that resides in the nucleus. Recently, we found that the specific activity of ADAR1-L within cells is much higher than that of ADAR1-S but only when the substrate can be edited in the cytoplasm. Here we observed that although both ADAR1-S and ADAR1-L were expressed throughout HDV replication, no ADAR2 could be observed at any time. Using expression vectors that individually overexpress either form of ADAR1, we found that ADAR1-S could stimulate editing during replication more efficiently. We next reduced ADAR1 levels during HDV replication. After transfection of an ADAR1-L-specific small interfering RNA (siRNA), we observed a significant loss of that protein and its associated cytoplasmic editing activity while the level of ADAR1-S remained unchanged. Transfection of this siRNA, however, did not reduce editing during HDV replication. In contrast, transfection of an siRNA that targets both forms of ADAR1 greatly reduced the expression of both proteins and potently inhibited editing during replication. We conclude that ADAR1-S edits HDV RNA during replication and that editing occurs in the nucleus.