Replication advantage and host factor-independent phenotypes attributable to a common naturally occurring capsid mutation (I97L) in human hepatitis B virus

Role of core protein mutations in the development of occult HBV infection

BACKGROUND & AIMS

Occult HBV infection (OBI) is associated with transfusion-transmitted HBV infection and hepatocellular carcinoma. Studies on OBI genesis have concentrated on mutations in the S region and the regulatory elements. Herein, we aimed to determine the role of mutations in the core region on OBIs.

METHODS

An OBI strain (SZA) carrying 9 amino acid (aa) substitutions in the core protein/capsid (Cp) was selected by sequence alignment and Western blot analysis from 26 genotype B OBI samples to extensively explore the impact of Cp mutations on viral antigen production in vitro and in vivo.

RESULTS

A large panel of 30 Cp replicons were generated by a replication-competent pHBV1.3 carrying SZA or wild-type (WT) Cp in a 1.3-fold over-length of HBV genome, in which the various Cp mutants were individually introduced by repairing site mutations of SZA-Cp or creating site mutations of WT-Cp by site-directed mutagenesis. The expression of HBcAg, HBeAg, and HBsAg and viral RNA was quantified from individual SZA and WT Cp mutant replicons in transfected Huh7 cells or infected mice, respectively. An analysis of the effect of Cp mutants on intracellular or extracellular viral protein production indicated that the W62R mutation in Cp had a critical impact on the reduction of HBcAg and HBeAg production during HBV replication, whereas P50H and/or S74G mutations played a limited role in influencing viral protein production invivo.

CONCLUSIONS

W62R and its combination mutations in HBV Cp might massively affect HBcAg and HBeAg production during viral replication, which, in turn, might contribute to the occurrence of OBI.

LAY SUMMARY

Occult hepatitis B virus infections (OBIs) have been found to be associated with amino acid mutations in the S region of the HBV, but the role of mutations in the core protein (Cp) remains unclear. In this study, an OBI strain (SZA) carrying 9 amino acid substitutions in Cp has been examined comprehensively in vitro and in vivo. The W62R mutation in Cp majorly reduces HBcAg and HBeAg production during HBV replication, potentially contributing to the occurrence of OBI.

Virion Secretion of Hepatitis B Virus Naturally Occurring Core Antigen Variants

In natural infection, hepatitis B virus (HBV) core protein (HBc) accumulates frequent mutations. The most frequent HBc variant in chronic hepatitis B patients is mutant 97L, changing from an isoleucine or phenylalanine to a leucine (L) at HBc amino acid 97. One dogma in the HBV research field is that wild type HBV secretes predominantly virions containing mature double-stranded DNA genomes. Immature genomes, containing single-stranded RNA or DNA, do not get efficiently secreted until reaching genome maturity. Interestingly, HBc variant 97L does not follow this dogma in virion secretion. Instead, it exhibits an immature secretion phenotype, which preferentially secretes virions containing immature genomes. Other aberrant behaviors in virion secretion were also observed in different naturally occurring HBc variants. A hydrophobic pocket around amino acid 97 was identified by bioinformatics, genetic analysis, and cryo-EM. We postulated that this hydrophobic pocket could mediate the transduction of the genome maturation signal for envelopment from the capsid interior to its surface. Virion morphogenesis must involve interactions between HBc, envelope proteins (HBsAg) and host factors, such as components of ESCRT (endosomal sorting complex required for transport). Immature secretion can be offset by compensatory mutations, occurring at other positions in HBc or HBsAg. Recently, we demonstrated in mice that the persistence of intrahepatic HBV DNA is related to virion secretion regulated by HBV genome maturity. HBV virion secretion could be an antiviral drug target.

Persistence of Hepatitis B Virus DNA and the Tempos between Virion Secretion and Genome Maturation in a Mouse Model

Chronic infection with human hepatitis B virus (HBV) could lead to cirrhosis and hepatoma. At present, there is no effective treatment to eradicate the virus from patients. HBV in chronic carriers does not exist as a single homogeneous population. The most frequent naturally occurring mutation in HBV core protein occurs at amino acid 97, changing an isoleucine to leucine (I97L). One dogma in the field is that only virions containing a mature genome are preferentially secreted into the medium. Here, we demonstrated that mutant I97L can secrete immature genome in mice. Although viral DNA of mutant I97L with immature genome is less persistent than wild-type HBV in time course experiments, viral DNA of mutant P130T with genome hypermaturation, surprisingly, is more persistent. Therefore, virion secretion regulated by genome maturity could influence viral persistence. It remains an open issue whether virion secretion could be a drug target for HBV therapy.

Hepatitis B virus (HBV) core protein (HBc) accumulates frequent mutations in natural infection. Wild-type HBV is known to secrete predominantly virions containing mature DNA genome. However, a frequent naturally occurring HBc variant, I97L, changing from an isoleucine to a leucine at amino acid 97, exhibited an immature secretion phenotype in culture, which preferentially secretes virions containing immature genomes. In contrast, mutant P130T, changing from a proline to a threonine at amino acid 130, exhibited a hypermaturation phenotype by accumulating an excessive amount of intracellular fully mature DNA genome. Using a hydrodynamic delivery mouse model, we studied the in vivo behaviors of these two mutants, I97L and P130T. We detected no naked core particles in all hydrodynamically injected mice. Mutant I97L in mice exhibited pleiotropic phenotypes: (i) excessive numbers of serum HBV virions containing immature genomes, (ii) significantly reduced numbers of intracellular relaxed-circle and single-stranded DNAs, and (iii) less persistent intrahepatic and secreted HBV DNAs than wild-type HBV. These pleiotropic phenotypes were observed in both immunocompetent and immunodeficient mice. Although mutant P130T also displayed a hypermaturation phenotype in vivo, it cannot efficiently rescue the immature virion secretion of mutant I97L. Unexpectedly, the single mutant P130T exhibited in vivo a novel phenotype in prolonging the persistence of HBV genome in hepatocytes. Taken together, our studies provide a plausible rationale for HBV to regulate envelopment morphogenesis and virion secretion via genome maturity, which is likely to play an important role in the persistence of viral DNA in this mouse model.

IMPORTANCE Chronic infection with human hepatitis B virus (HBV) could lead to cirrhosis and hepatoma. At present, there is no effective treatment to eradicate the virus from patients. HBV in chronic carriers does not exist as a single homogeneous population. The most frequent naturally occurring mutation in HBV core protein occurs at amino acid 97, changing an isoleucine to leucine (I97L). One dogma in the field is that only virions containing a mature genome are preferentially secreted into the medium. Here, we demonstrated that mutant I97L can secrete immature genome in mice. Although viral DNA of mutant I97L with immature genome is less persistent than wild-type HBV in time course experiments, viral DNA of mutant P130T with genome hypermaturation, surprisingly, is more persistent. Therefore, virion secretion regulated by genome maturity could influence viral persistence. It remains an open issue whether virion secretion could be a drug target for HBV therapy.

Naturally occurring core protein mutations compensate for the reduced replication fitness of a lamivudine-resistant HBV isolate

Hepatitis B virus (HBV) replicates its DNA genome through reverse transcription of an RNA intermediate. The lack of proofreading capacity of the viral DNA polymerase results in a high mutation rate of HBV genome. Under the selective pressure created by the nucleos(t)ide analogue (NA) antiviral drugs, viruses with resistance mutations are selected. However, the replication fitness of NA-resistant mutants is markedly reduced compared to wild-type. Compensatory mutations in HBV polymerase, which restore the viral replication capacity, have been reported to arise under continuous treatment with lamivudine (LMV). We have previously identified a highly replicative LMV-resistant HBV isolate from a chronic hepatitis B patient experiencing acute disease exacerbation. Besides the common YMDD drug-resistant mutations, this isolate possesses multiple additional mutations in polymerase and core regions. The transcomplementation assay demonstrated that the enhanced viral replication is due to the mutations of core protein. Further mutagenesis study revealed that the P5T mutation of core protein plays an important role in the enhanced viral replication through increasing the levels of capsid formation and pregenomic RNA encapsidation. However, the LMV-resistant virus harboring compensatory core mutations remains sensitive to capsid assembly modulators (CpAMs). Taken together, our study suggests that the enhanced HBV nucleocapsid formation resulting from core mutations represents an important viral strategy to surmount the antiviral drug pressure and contribute to viral pathogenesis, and CpAMs hold promise for developing the combinational antiviral therapy for hepatitis B.

HBV maintains electrostatic homeostasis by modulating negative charges from phosphoserine and encapsidated nucleic acids

Capsid assembly and stability of hepatitis B virus (HBV) core protein (HBc) particles depend on balanced electrostatic interactions between encapsidated nucleic acids and an arginine-rich domain (ARD) of HBc in the capsid interior. Arginine-deficient ARD mutants preferentially encapsidated spliced viral RNA and shorter DNA, which can be fully or partially rescued by reducing the negative charges from acidic residues or serine phosphorylation of HBc, dose-dependently. Similarly, empty capsids without RNA encapsidation can be generated by ARD hyper-phosphorylation in insect, bacteria, and human hepatocytes. De-phosphorylation of empty capsids by phosphatase induced capsid disassembly. Empty capsids can convert into RNA-containing capsids by increasing HBc serine de-phosphorylation. In an HBV replicon system, we observed a reciprocal relationship between viral and non-viral RNA encapsidation, suggesting both non-viral RNA and serine-phosphorylation could serve as a charge balance buffer in maintaining electrostatic homeostasis. In addition, by comparing the biochemistry assay results between a replicon and a non-replicon system, we observed a correlation between HBc de-phosphorylation and viral replication. Balanced electrostatic interactions may be important to other icosahedral particles in nature.

MicroRNA miR-204 and miR-1236 inhibit hepatitis B virus replication via two different mechanisms

Hepatitis B virus (HBV) is a major human pathogen. In this study, we found that miR-204 and miR-1236 were down-regulated in HBV-producing cells, and each could suppress HBV replication. Using a bioinformatic approach and a reporter assay, we identified miR-1236, which can reduce HBV replication and protein production by directly targeting at HBV specific mRNA. In contrast, miR-204 was identified by a microarray approach, and had no effect on HBV RNA and protein production. Surprisingly, miR-204 could inhibit HBV pregenomic RNA encapsidation and capsid assembly. We further demonstrated that HBV suppressed miR-204 expression via activating a host transcription factor STAT3. We established a positive feed-forward loop between HBV, miR-204 and STAT3. Interestingly, miR-204 has been considered as a tumor suppressor in some literature. Since the risk for hepatocellular carcinoma (HCC) is significantly increased in chronic HBV patients, it is possible that chronic suppression of miR-204 by HBV contributes to HCC incidence. Both miR-204 and miR-1236 might be useful for developing new therapeutics against HBV.

Impact of the rtI187V polymerase substitution of hepatitis B virus on viral replication and antiviral drug susceptibility

A high prevalence of the rtI187V polymerase substitution of hepatitis B virus (HBV) was detected in nucleoside/nucleotide-analogue-naive and -treated chronic hepatitis B (CHB) patients. We aimed at assessing the replicative capacity and susceptibility to lamivudine (LAM) and adefovir (ADV) in vitro of HBV harbouring rtI187V alone or in conjunction with LAM- or ADV-resistant mutations. The reverse transcriptase region of HBV isolates was directly sequenced from a cohort of 300 CHB patients from China. Replication-competent HBV constructs containing rtI187V and combined with LAM-resistant (rtM204I, rtL180M/rtM204V) mutations were generated, and compared with WT, LAM-resistant single (rtM204I) or double (rtL180M/rtM204V) and ADV-resistant (rtN236T) clones. In a Chinese cohort of 300 CHB patients, 8.7 % (26/300) showed substitution of rtI187 with V. Of note, the rtI187V prevalence in HBV genotype B was significantly higher than that in HBV genotype C (95.2 vs 4.8 %). In vitro phenotypic assays showed that the viruses bearing the rtI187V substitution had impaired replication efficacy when compared with the WT and the virus carrying rtI187V combined with LAM-resistant single or double mutations showed even more significantly impaired replicative capacities. Furthermore, rtI187V HBV remained susceptible towards treatment with LAM or ADV in vitro whereas the combination of the rtI187V substitution with LAM-resistant mutations rendered HBV resistant to LAM but still sensitive to ADV. Our study revealed that the rtI187V substitution in the HBV polymerase frequently occurred in CHB patients, particularly those with HBV genotype B. However, the emergence of the rtI187V substitution significantly impaired viral replication but without affecting drug sensitivity in vitro.

Hepatitis B virus core protein with hot-spot mutations inhibit MxA gene transcription but has no effect on inhibition of virus replication by interferon α

It has been reported that hepatitis B virus (HBV) core protein (HBc) can inhibit the transcription of human interferon-induced MxA gene. In this study, we investigated whether HBc protein mutations at hot spots (L60V, S87G and I97L) could still inhibit MxA transcription and the potential significance of this inhibition in virus replication in vitro. Our data indicated that the IFN-induced MxA mRNA expression level and MxA promoter activity was significantly down-regulated by mutant protein of HBc(I97L), compared to WT and the other two mutated HBc proteins(L60V or S87G). However, in Huh7 cells stably expressing WT or the mutated HBc proteins (L60V, S87G or I97L), IFN-α could inhibit the extra- and intracellular HBV DNA level and HBsAg secretion to a similar level compared to that in cells transfected with control plasmids. In conclusion, HBc protein with I97L mutation may play an especial role in suppressing the transcription of MxA gene. Moreover, the inhibitory effect on MxA gene transcription by the WT or mutated HBc proteins (L60V, S87G and I97L) has no impact on inhibition of HBV replication by IFN-α in Huh7 cells. The clinical significance of the inhibitory effect of MxA gene transcription by HBc protein requires further study.

Comparison study on the complete sequence of hepatitis B virus identifies new mutations in core gene associated with hepatocellular carcinoma

BACKGROUND

Mutations in the hepatitis B virus (HBV) genome may influence the activity of liver disease. The aim of this study was to identify new viral variations associated with hepatocellular carcinoma (HCC).

METHODS

We carried out a comparison study on the complete sequence of HBV isolated from 20 HCC and 35 non-HCC patients in Qidong, China, an area with a high incidence of HCC. We compared the HBV sequences in a consecutive series of plasma samples from four HCC cases before and after the occurrence of HCC. In addition, we selected four mutations in the HBV core (C) gene to verify their relationships to HCC in an independent set of 103 HCC cases and 103 sex- and age-matched non-HCC controls.

RESULTS

The pre-S deletion and 12 point mutations, namely, the pre-S2 start codon mutation, T53C in the pre-S2 gene, T766A in the S gene, G1613A, C1653T, A1762T, G1764A in the X gene, and G1899A, C2002T, A2159G, A2189C, and G2203W (A or T) in the pre-C/C gene, showed close associations with HCC. In the validation study, A2159G, A2189C, and G2203W showed consistent associations with HCC by univariate analysis. Multivariate analysis showed that A2189C and G2203W were independent risk factors for HCC. The odds ratios (95% confidence interval) were 3.99 (1.61-9.92) and 9.70 (1.17-80.58), respectively, for A2189C and G2203W.

CONCLUSIONS

These results implicate A2189C and G2203W as new predictive markers for HCC.

IMPACT

The complete genome analysis of HBV provided pilot data for the identification of novel mutations that could serve as markers for HCC.

A mutant hepatitis B virus core protein mimics inhibitors of icosahedral capsid self-assembly

Understanding self-assembly of icosahedral virus capsids is critical to developing assembly directed antiviral approaches and will also contribute to the development of self-assembling nanostructures. One approach to controlling assembly would be through the use of assembly inhibitors. Here we use Cp149, the assembly domain of the hepatitis B virus capsid protein, together with an assembly defective mutant, Cp149-Y132A, to examine the limits of the efficacy of assembly inhibitors. By itself, Cp149-Y132A will not form capsids. However, Cp-Y132A will coassemble with the wild-type protein on the basis of light scattering and size exclusion chromatography. The resulting capsids appear to be indistinguishable from normal capsids. However, coassembled capsids are more fragile, with disassembly observed by chromatography under mildly destabilizing conditions. The relative persistence of capsids assembled under conditions where association energy is weak compared to the fragility of those where association is strong suggests a mechanism of "thermodynamic editing" that allows replacement of defective proteins in a weakly associated complex. There is fine line between weak assembly, where assembly defective protein is edited from a growing capsid, and relatively strong assembly, where assembly defective subunits may dramatically compromise virus stability. Thus, attempts to control virus self-assembly (with small molecules or defective proteins) must take into account the competing process of thermodynamic editing.

In vitro drug susceptibility analysis of hepatitis B virus clinical quasispecies populations

Analysis of the replication and drug resistance of patient serum hepatitis B virus (HBV) populations can contribute to the therapeutic management of chronic hepatitis B. We developed a procedure for cloning serum HBV quasispecies populations and for phenotypic analysis of the cloned populations for in vitro drug susceptibility. Equivalent sequences were compared to the respective serum HBV DNAs of the cloned quasispecies by population sequencing. Analysis of individual clones revealed that each population contained a diversity of HBV quasispecies. Furthermore, secreted HBV in the supernatant following transfection of the quasispecies populations remained mostly unchanged from the respective input populations. HBV obtained from patients who had developed resistance to adefovir or lamivudine, as demonstrated by development of the rtA181V or rtL180M/M204V mutations in HBV polymerase, respectively, were tested. Phenotypic analysis demonstrated that a population containing the HBV rtA181V mutation showed a 2.9-fold increase in the 50% effective concentration (EC(50)) for adefovir compared to the wild-type baseline isolate, while the lamivudine-resistant HBV quasispecies population showed a >1,000-fold increase in the lamivudine EC(50). In summary, a strategy of cloning full genome HBV quasispecies populations from patient sera was developed, which could provide a useful tool in clinical HBV drug resistance phenotyping and studies of the evolution of clinical viral species.

Regulation of hepatitis B virus replication by the phosphatidylinositol 3-kinase-akt signal transduction pathway

The phosphatidylinositol 3-kinase (PI3K)-protein kinase B (Akt) signaling pathway is one of the major oncogenic pathways and is activated in many types of human cancers, including hepatocellular carcinoma. It can also be activated by the hepatitis C virus (HCV) nonstructural 5A (NS5A) protein. In the present study, we set out to determine the regulatory effects of this pathway on the replication of hepatitis B virus (HBV). Our results demonstrate that the expression of a constitutively active Akt1 profoundly inhibited HBV RNA transcription and consequently reduced HBV DNA replication in HepG2 cells. This suppression of HBV gene transcription was apparently mediated by the activation of mTOR, as it was abolished by the mTOR inhibitor rapamycin. Moreover, treatment of HBV-expressing HepG2.2.15 cells with inhibitors of PI3K, Akt, and mTOR increased the transcription of 3.5-kb and 2.4-kb viral RNA as well as the replication of HBV DNA. This observation implies that the basal level activation of this pathway in HepG2 cells regulated HBV replication. Consistent with previous reports showing that the HCV NS5A protein could bind to the p85 subunit of PI3K and activate the PI3K-Akt signal transduction pathway, our results showed that expression of this protein could inhibit HBV RNA transcription and reduce HBV DNA replication in HepG2 cells. Taken together, our results suggest that the activation of the PI3K-Akt pathway during liver oncogenesis may be at least partially responsible for the elimination of HBV replication from tumor cells and may also provide an explanation for the observed suppression of HBV replication by HCV coinfection.

The L80I substitution in the reverse transcriptase domain of the hepatitis B virus polymerase is associated with lamivudine resistance and enhanced viral replication in vitro

Long-term lamivudine (LMV) treatment of chronic hepatitis B almost inevitably engenders viral resistance. Mutations that result in the replacement of the methionine at position 204 of the deoxynucleoside triphosphate-binding site of the hepatitis B virus (HBV) reverse transcriptase (rt) by isoleucine, valine, or (rarely) serine (rtM204I/V/S) confer high-level resistance to LMV but reduce replication efficiency. The subsequent selection or coselection of secondary mutations that partially restore replication efficiency is common and may influence drug resistance. Genotyping has shown that LMV treatment can select for HBV rtL80V/I mutants, but their prevalence and phenotype have not been documented. Analysis of a large sequence database revealed that rtL80V/I occurred almost exclusively in association with LMV resistance, and 85% of these isolates encoded rtL80I. Coselection of rtL80V/I occurred in 46% of isolates in which LMV resistance was attributable to rtM204I but only 9% of those in which resistance was attributable to rtM204V. Moreover, rtL80V/I did not occur in HBV genotype A isolates but occurred at similar frequencies in genotype B, C, and D isolates. In vitro phenotyping showed that although the rtL80I mutant by itself replicated less efficiently and was hypersensitive to LMV compared to the replication efficiency and sensitivity of its wild-type parent, the presence of rtL80I enhanced the replication efficiency of rt204I/V mutants without significantly affecting LMV resistance. Molecular modeling revealed that rt80 does not interact directly with the enzyme's substrates. Collectively, these results suggest that coselection of rtL80V/I and rtM204I/V occurs because the former compensates for the loss of replication efficiency associated with the acquisition of LMV resistance, particularly in the case of rtM204I.

Analysis of the complete hepatitis B virus genome in patients with genotype C chronic hepatitis in relation to HBeAg and anti-HBe

To investigate the relationship between viral factors and the development of chronic hepatitis B, the entire hepatitis B virus (HBV) genome of chronic carriers at different disease stages were analyzed. Eighty genotype C HBV carriers including 12 hepatitis B e antigen (HBeAg) positive asymptomatic carriers (Group A), 49 HBeAg positive patients with chronic liver diseases (Group B) and 19 anti-HBe positive patients with chronic liver diseases (Group C) were studied. HBV nucleic acid from serum samples was sequenced directly and compared with GenBank reference sequences HBV X01587 and M12906. On phylogenetic analysis, 76 cases were genotype C2. Of the 76 genotype C2 cases, the nucleotide and amino acid substitution rates in the precore/core region were significantly higher in Groups B and C than in Group A, also in Group C than in Group B. The nucleotide substitution rates in the full genome and the core promoter region were significantly higher in Group C than in Group A, also in group C than in Group B. The nucleotide and amino acid substitution rates in the X region were significantly higher in Group C than in Group A. The amino acid substitution rate in the pre-S2 region was significantly higher in Group C than in Group B. Deletion mutations were found mainly in Groups B and C. This whole genome analysis of HBV chronic carriers suggested that the nucleotide substitutions and deletions in HBV were closely associated with the pathogenesis of chronic HBV infection.

Potent inhibition of human Hepatitis B virus replication by a host factor Vps4

Vps4 is a host factor known to be involved in cellular vacuolar protein sorting. We report here that HBV replication and secretion can be significantly inhibited by Vps4 dominant negative, ATPase-defective, mutants K173Q and E228Q. In contrast, wild-type Vps4 at low dose can inhibit HBV replication more effectively in human hepatoblastoma cell line HepG2, than in human hepatoma cell line Huh7. To our knowledge, this is the first demonstration of an anti-HBV potential of dominant negative mutants of a protein sorting host factor Vps4.

Reduced secretion of virions and hepatitis B virus (HBV) surface antigen of a naturally occurring HBV variant correlates with the accumulation of the small S envelope protein in the endoplasmic reticulum and Golgi apparatus

We identified two novel naturally occurring mutations (W74L and L77R) in the small S envelope protein of hepatitis B virus (HBV). Mutation L77R alone resulted in >10-fold-reduced secretion of virions. In addition, the 2.8-fold reduction of the extracellular HBV surface antigen (HBsAg) of mutant L77R from transfected Huh7 cells appeared to be correlated with a 1.7-fold reduction of intracellular HBsAg, as measured by enzyme-linked immunosorbent assay (ELISA). Surprisingly, opposite to the ELISA results, Western blot analysis revealed a near-10-fold-increased level of the intracellular mutant small S envelope protein. The discrepancy between ELISA and Western blot data was due to significant accumulation of the mutant L77R HBsAg in the intracellular pellet fraction. In contrast to HBsAg, the secretion of HBeAg was normal in L77R-transfected cells. The wild-type HBsAg was usually more diffuse and evenly distributed in the cytoplasm, often outside the perinuclear endoplasmic reticulum (ER) and Golgi apparatus, as observed by immunofluorescence assay. In contrast, the L77R mutant HBsAg tends to be highly restricted within the ER and Golgi, often accumulated in the Golgi compartments distal from the nucleus. The almost exclusive retention in the ER-Golgi of L77R HBsAg was similar to what was observed when the large envelope protein was overexpressed. These multiple aberrant phenotypes of mutant L77R can be corrected by a second naturally occurring S envelope mutation, W74L. Despite the accumulation of L77R HBsAg in ER-Golgi of transfected Huh7 cells, we detected no increase in Grp78 mRNA and proteins, which are common markers for ER stress response.

Hepatocyte-like cells transdifferentiated from a pancreatic origin can support replication of hepatitis B virus

Recently, a rat pancreatic cell line (AR42J-B13) was shown to transdifferentiate to hepatocyte-like cells upon induction with dexamethasone (Dex). The aim of this study is to determine whether transdifferentiated hepatocytes can indeed function like bona fide liver cells and support replication of hepatotropic hepatitis B virus (HBV). We stably transfected AR42J-B13 cells with HBV DNA and examined the expression of hepatocyte markers and viral activities in control and transdifferentiated cells. A full spectrum of HBV replicative intermediates, including covalently closed circular DNA (cccDNA) and Dane particles, were detected only after induction with Dex and oncostatin M. Strikingly, the small envelope protein and RNA of HBV were increased by 40- to 100-fold upon induction. When HBV RNAs were examined by primer extension analysis, novel core- and precore-specific transcripts were induced by Dex which initiated at nucleotide (nt) 1820 and nt 1789, respectively. Most surprisingly, another species of core-specific RNA, which initiates at nt 1825, is always present at almost equal intensity before and after Dex treatment, a result consistent with Northern blot analysis. The fact that HBV core protein is dramatically produced only after transdifferentiation suggests the possibility of both transcriptional and translational regulation of HBV core antigen in HBV-transfected AR42J-B13 cells. Upon withdrawal of Dex, HBV replication and gene expression decreased rapidly-less than 50% of the cccDNA remained detectable in 1.5 days. Our studies demonstrate that the transdifferentiated AR42J-B13 cells can function like bona fide hepatocytes. This system offers a new opportunity for basic research of virus-host interactions and pancreatic transdifferentiation.

An application of gene comparative image for predicting the effect on replication ratio by HBV virus gene missense mutation

Hepatitis B viruses (HBVs) show instantaneous and high-ratio mutations when they are replicated, some sorts of which significantly affect the efficiency of virus replication through enhancing or depressing the viral replication, while others have no influence at all. The mechanism of gene expression is closely correlated with its gene sequence. With the rapid increase in the number of newly found sequences entering into data banks, it is highly desirable to develop an automated method for simulating the gene regulating function. The establishment of such a predictor will no doubt expedite the process of prioritizing genes and proteins identified by genomics efforts as potential molecular targets for drug design. Based on the power of cellular automata (CA) in treating complex systems with simple rules, a novel method to present HBV gene image has been introduced. The results show that the images thus obtained can very efficiently simulate the effects of the gene missense mutation on the virus replication. It is anticipated that CA may also serve as a useful vehicle for many other studies on complicated biological systems.

Hot-spot mutations in hepatitis B virus core gene: eliciting or evading immune clearance?

The biological implications of substitutions L60V and I97L in the core (c) gene of hepatitis B virus (HBV) were investigated in order to determine whether they could change the immunogenicity of HBcAg or influence the immune response in mice. Three strains of recombinant adenoviruses--AdHBV-WT, AdHBV-L60V and AdHBV-I97L--containing wild-type or mutant HBV genomes were constructed using the AdEasy system and used to infect BALB/c mice intranasally. Infected mice produced anti-HBc efficiently to comparable levels. IgG1 and IgG2a specific for HBcAg were present in mice sera, and the response was dominated by IgG2a. The lymphocyte proliferative response specific for HBcAg was assessed by [3H]-thymidine uptake. We found that AdHBV-WT induced a stronger T-cell proliferation response than AdHBV-L60V and AdHBV-I97L. In conclusion, the L60V and I97L substitutions had no influence on humoral immune responses, but could downregulate T-cell responses to HBcAg, suggesting that L60V and I97L were immune escape mutants.

Exposure of RNA templates and encapsidation of spliced viral RNA are influenced by the arginine-rich domain of human hepatitis B virus core antigen (HBcAg 165-173)

Previously, human hepatitis B virus (HBV) mutant 164, which has a truncation at the C terminus of the HBV core antigen (HBcAg), was speculated to secrete immature genomes. For this study, we further characterized mutant 164 by different approaches. In addition to the 3.5-kb pregenomic RNA (pgRNA), the mutant preferentially encapsidated the 2.2-kb or shorter species of spliced RNA, which can be reverse transcribed into double-stranded DNA before virion secretion. We observed that mutant 164 produced less 2.2-kb spliced RNA than the wild type. Furthermore, it appeared to produce at least two different populations of capsids: one encapsidated a nuclease-sensitive 3.5-kb pgRNA while the other encapsidated a nuclease-resistant 2.2-kb spliced RNA. In contrast, the wild-type core-associated RNA appeared to be resistant to nuclease. When arginines and serines were systematically restored at the truncated C terminus, the core-associated DNA and nuclease-resistant RNA gradually increased in both size and signal intensity. Full protection of encapsidated pgRNA from nuclease was observed for HBcAg 1-171. A full-length positive-strand DNA phenotype requires positive charges at amino acids 172 and 173. Phosphorylation at serine 170 is required for optimal RNA encapsidation and a full-length positive-strand DNA phenotype. RNAs encapsidated in Escherichia coli by capsids of HBcAg 154, 164, and 167, but not HBcAg 183, exhibited nuclease sensitivity; however, capsid instability after nuclease treatment was observed only for HBcAg 164 and 167. A new hypothesis is proposed here to highlight the importance of a balanced charge density for capsid stability and intracapsid anchoring of RNA templates.

Nucleolar localization of human hepatitis B virus capsid protein

Wild-type human hepatitis B virus (HBV) exhibits selective export of virions containing mature genomes. In contrast, changing an isoleucine to a leucine at amino acid 97 (I97L) of the HBV core antigen (HBcAg) causes it to release immature genomes. To elucidate the structure-function relationship of HBcAg at amino acid 97, we systematically replaced the isoleucine residue at this position with 18 other amino acids via mutagenesis. Twelve of the 18 mutants exhibited no significant phenotype, while five new mutants displayed strong phenotypes. The I97D mutant had a near lethal phenotype, the I97P mutant exhibited a significantly reduced level of virion secretion, and the I97G mutant lacked the full-length relaxed circular form of viral DNA. The tip of the spike of the capsid particle is known to contain a predominant B-cell epitope. However, the recognition of this exposed epitope by an anti-HBc antibody appeared to be affected by the I97E mutation or by histidine tagging at the C terminus of mutant HBcAg, which is presumably in the capsid interior. Surprisingly, the nuclear HBcAg of mutants I97E and I97W, produced from either a replicon or an expression vector, was found to be colocalized with nucleolin and B23 at a frequency of nearly 100% by confocal immunofluorescence microscopy. In contrast, this colocalization occurred with wild-type HBcAg only to a limited extent. We also noted that nucleolin-colocalizing cells were often binucleated or apoptotic, suggesting that the presence of HBcAg in the nucleolus may perturb cytokinesis. The mechanism of this phenomenon and its potential involvement in liver pathogenesis are discussed. To our knowledge, this is the first report of nucleolar HBcAg in culture.

Hepatitis B Virus Capsid Assembly Is Enhanced by Naturally Occurring Mutation F97L

In chronic hepatitis B virus (HBV) infections, one of the most common mutations to the virus occurs at amino acid 97 of the core protein, where leucine replaces either phenylalanine or isoleucine, depending on strain. This mutation correlates with changes in viral nucleic acid metabolism and/or secretion. We hypothesize that this phenotype is due in part to altered core assembly, a process required for DNA synthesis. We examined in vitro assembly of empty HBV capsids from wild-type and F97L core protein assembly domains. The mutation enhanced both the rate and extent of assembly relative to those for the wild-type protein. The difference between the two proteins was most obvious in the temperature dependence of assembly, which was dramatically stronger for the mutant protein, indicating a much more positive enthalpy. Since the structures of the mutant and wild-type capsids are essentially the same and the mutation is not involved in the contact between dimers, we suggest that the F97L mutation affects the dynamic behavior of dimer and capsid.

Structural and functional analysis of full-length hepatitis B virus genomes in patients: implications in pathogenesis

The structural analysis, replicative efficiency and immunogenicity of hepatitis B virus (HBV) full-length genomes isolated from different patients or asymptomatic carriers are presented in the present review. Data indicate the importance of viral genome-based studies in elucidating the pathogenesis of HBV infections. Comparison of structural and functional characteristics of viral genomes isolated from various geographical regions might contribute to explaining the differences in HBV clinical manifestation and prognosis in different geographical regions.

The 113th and 117th charged amino acids in the 5th alpha-helix of the HBV core protein are necessary for pgRNA encapsidation

Although the structure-function of hepatitis B virus (HBV) core protein has been investigated by numerous HBV core mutants, functions of many regions in the core protein are still remained to be identified. In this report, it was found that point mutations in the 113th and 117th negative-charged amino acids in the 5th helix region of the HBV core strongly affect pregenomic RNA (pgRNA) encapsidation. These mutations were introduced by site-directed mutagenesis. The following results were obtained from analyses of the mutants. First, endogenous polymerase activity (EPA) was assayed and activity was not detected only in the two mutants, E113K and E117K. Second, the pgRNA encapsidation level of each mutant related to a change in charge of two amino acid sites was evaluated. Mutations in the 113th and 117th amino acids into uncharged amino acids reduced pgRNA encapsidation levels. Moreover, changes of the two amino acids into positive-charged amino acids almost completely reduced pgRNA encapsidation levels. To test whether the mutant core proteins assembled into normal capsid particles, the assembly of the mutant core proteins was seen. However, none of the changes in the 113th and 117th amino acids affected capsid formation. From this data, it can be inferred that the above two amino acids in the 5th alpha-helix in the HBV core protein are important for pgRNA encapsidation.

Stability and morphology comparisons of self-assembled virus-like particles from wild-type and mutant human hepatitis B virus capsid proteins

Instead of displaying the wild-type selective export of virions containing mature genomes, human hepatitis B virus (HBV) mutant I97L, changing from an isoleucine to a leucine at amino acid 97 of HBV core antigen (HBcAg), lost the high stringency of selectivity in genome maturity during virion export. To understand the structural basis of this so-called "immature secretion" phenomenon, we compared the stability and morphology of self-assembled capsid particles from the wild-type and mutant I97L HBV, in either full-length (HBcAg1-183) or truncated core protein contexts (HBcAg1-149 and HBcAg1-140). Using negative staining and electron microscopy, full-length particles appear as "thick-walled" spherical particles with little interior space, whereas truncated particles appear as "thin-walled" spherical particles with a much larger inner space. We found no significant differences in capsid stability between wild-type and mutant I97L particles under denaturing pH and temperature in either full-length or truncated core protein contexts. In general, HBV capsid particles (HBcAg1-183, HBcAg1-149, and HBcAg1-140) are very robust but will dissociate at pH 2 or 14, at temperatures higher than 75 degrees C, or in 0.1% sodium dodecyl sulfate (SDS). An unexpected upshift banding pattern of the SDS-treated full-length particles during agarose gel electrophoresis is most likely caused by disulfide bonding of the last cysteine of HBcAg. HBV capsids are known to exist in natural infection as dimorphic T=3 or T=4 icosahedral particles. No difference in the ratio between T=3 (78%) and T=4 particles (20.3%) are found between wild-type HBV and mutant I97L in the context of HBcAg1-140. In addition, we found no difference in capsid stability between T=3 and T=4 particles successfully separated by using a novel agarose gel electrophoresis procedure.

Coexistence of two distinct secretion mutations (P5T and I97L) in hepatitis B virus core produces a wild-type pattern of secretion

Unlike a Tokyo isolate of hepatitis B virus variants, we found a Shanghai isolate that secretes few virions with an immature genome despite its core I97L mutation. Core mutations P5T and I97L were found to be mutually compensatory in offsetting their respective distinct effects on virion secretion.

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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.