Point mutations upstream of hepatitis B virus core gene affect DNA replication at the step of core protein expression

Impact of core protein naturally selected mutants associated with HBeAg-negative status in HBV biosynthesis

Hepatitis B e antigen (HBeAg) loss represents a late stage of chronic hepatitis B virus (HBV) infection associated with a drastic decrease in HBV-DNA, a lower risk of disease progression, and the occurrence of several mutations in the preCore/core region. However, the underlying mechanisms supporting the downregulation of viral replication have yet to be elucidated. In the present study, the analysis of the frequency of subgenotype D1 core protein (HBc) mutations associated with HBeAg status revealed a higher mutation rate in HBeAg-negative sequences compared to HBeAg-positive ones. Particularly, 22 amino acids exhibited a higher frequency of mutation in HBeAg-negative sequences, while the remaining residues showed a high degree of conservation. Subsequently, the assessment of HBc mutants derived from HBeAg-negative patients in viral structure and replicative capacity revealed that HBc mutations have the ability to modulate the subcellular localization of the protein (either when the protein was expressed alone or in the context of viral replication), capsid assembly, and, depending on specific mutation patterns, alter covalently closed circular DNA (cccDNA) recycling and up- or downregulate viral replication. In conclusion, HBc mutations associated with HBeAg-negative status impact on various stages of the HBV life cycle modulating viral replication during the HBeAg-negative stage of infection.

In vitro expression of precore proteins of hepatitis B virus subgenotype A1 is affected by HBcAg, and can affect HBsAg secretion

HBeAg, a non-particulate protein of hepatitis B virus (HBV), is translated from the precore/core region as a precursor, which is post-translationally modified. Subgenotype A1 of HBV, which is a risk factor for hepatocellular carcinoma (HCC), has unique molecular characteristics in the basic core promoter/precore regions. Carriers of A1 exhibit early HBeAg loss. We sought to further characterize the precore proteins of A1 in vitro. HuH-7 cells were transfected with subgenomic constructs expressing individual precore proteins. Western blot analysis using DAKO anti-core antibody showed the expected sizes and a 1 kDa larger band for P22, P20 and P17. Using confocal microscopy, a cytoplasmic accumulation of HBeAg and precursors was observed with P25-expressing plasmid, whereas P22 localized both in the cytoplasm and nucleus. P20 and P17, which lack the carboxy end of P22 showed strong nuclear accumulation, implicating a nuclear localization signal in the N-terminal 10 amino acids. G1862T, unique to subgenotype A1, is frequently found in HBV from HCC patients. P25 with G1862T showed delayed and reduced HBeAg expression/secretion. Knock-out of core in the replication competent clones led to precore protein accumulation in the cytoplasm/perinuclear region, and decreased HBeAg secretion. Knock-out of precore proteins increased HBsAg secretion but intracellular HBsAg expression was unaffected. Over-expression of precore proteins in trans led to decreased HBsAg expression and secretion. Intracellular trafficking of HBV A1 precore proteins was followed. This was unaffected by the CMV promoter and different cell types. In the viral context, precore protein expression was affected by absence of core, and affected HBsAg expression, suggesting an interrelationship between precore proteins, HBcAg and HBsAg. This modulatory role of HBeAg and its precursors may be important in viral persistence and ultimate development of HCC.

Case Report: Application of hepatitis B virus (HBV) deep sequencing to distinguish between acute and chronic infection

Deep sequencing of the full-length hepatitis B virus (HBV) genome provides the opportunity to determine the extent to which viral diversity, genotype, polymorphisms, insertions and deletions may influence presentation and outcomes of disease. Increasing experience with analysis of HBV genomic data opens up the potential for using these data to inform insights into pathophysiology of infection and to underpin decision making in clinical practice. We here set out to undertake whole genome HBV sequencing from an adult who presented acutely unwell with a new diagnosis of HBV infection, and tested positive for both HBV anti-core IgM and IgG, possibly representing either acute hepatitis B infection (AHB) or chronic hepatitis B with an acute reactivation (CHB-AR). The distinction between these two scenarios may be important in predicting prognosis and underpinning treatment decisions, but can be challenging based on routine laboratory tests. Through application of deep whole-genome sequencing we typed the isolate as genotype-D1, and identified several minority variants including G1764A and G1986A substitutions in the pre-core promoter and pre-core regions, which support CHB-AR rather than AHB. In the longer term, enhanced deep sequencing data for HBV may provide improved evidence to distinguish between acute and chronic infection, to predict outcomes and to stratify treatment.

Case Report: Application of hepatitis B virus (HBV) deep sequencing to distinguish between acute and chronic infection

Deep sequencing of the full-length hepatitis B virus (HBV) genome provides the opportunity to determine the extent to which viral diversity, genotype, polymorphisms, insertions and deletions may influence presentation and outcomes of disease. Increasing experience with analysis of HBV genomic data opens up the potential for using these data to inform insights into pathophysiology of infection and to underpin decision making in clinical practice. We here set out to undertake whole genome HBV sequencing from an adult who presented acutely unwell with a new diagnosis of HBV infection, and tested positive for both HBV anti-core IgM and IgG, possibly representing either acute hepatitis B infection (AHB) or chronic hepatitis B with an acute reactivation (CHB-AR). The distinction between these two scenarios may be important in predicting prognosis and underpinning treatment decisions, but can be challenging based on routine laboratory tests. Through application of deep whole-genome sequencing we typed the isolate as genotype-D1, and identified several minority variants including G1764A and G1986A substitutions in the pre-core promoter and pre-core regions, which support CHB-AR rather than AHB. In the longer term, enhanced deep sequencing data for HBV may provide improved evidence to distinguish between acute and chronic infection, to predict outcomes and to stratify treatment.

An antiviral drug-resistant mutant of hepatitis B virus with high replication capacity in association with a large in-frame deletion in the preS1 region of viral surface gene

We amplified a full-length hepatitis B virus (HBV) genome from the serum of a chronic hepatitis B patient who experienced virological breakthrough with high HBV DNA titer following adefovir (ADV) therapy. The PCR product was cloned and sequencing of the six clones revealed an isolate of C2 subgenotype. Mutation(s) in the polymerase gene responsible for ADV resistance included rtA181T (all clones) and rtN236T (four clones). The rtA181T mutation caused the W172* nonsense mutation in the overlapping S gene. In addition, all the clones harbored another nonsense mutation in the S gene (C69*) and a 207nt in-frame deletion in the preS1 region. These clones were converted to a 1.1mer construct for transient transfection of Huh7 cells. All the clones were deficient in hepatitis B surface antigen production. Three clones had similar levels of DNA replication. Comparison with a wild-type clone of the same genotype revealed a higher intracellular level of replicative DNA for clone c4, which was reduced by putting back the deleted 207nt, but not by co-transfection with an expression construct for the three surface proteins to rescue virion production. The HBcAg expression of the c4 and c4+207nt clones was mainly in the nucleus. Co-transfection with the L/M/S proteins expression construct did not alter the distribution of core. Clone c4 showed a significantly decreased susceptibility to ADV, a mild reduction in susceptibility to lamivudine and tenofovir, but remained sensitive to entecavir. In conclusion, this is an unusual ADV-resistant HBV isolate harboring two nonsense mutations in the S gene and a large in-frame deletion in the preS1 region, but still retains a high replication phenotype, which can provide a platform for recombinant vector construction.

Expression of wild-type or G1862T mutant HBe antigen of subgenotype A1 of hepatitis B virus and the unfolded protein response in Huh7 cells

The G1862T mutation, which occurs most frequently in subgenotype A1 of the hepatitis B virus (HBV), results in a valine to phenylalanine substitution at the -3 position of the signal peptide cleavage site at the amino end of the precore/core (preC/C) precursor protein. The objective of this study was to functionally characterize the G1862T mutation relative to its wild-type counterpart in subgenotype A1. Huh7 cells were transfected with subgenotype A1 replication-competent plasmids, with and without G1862T. Secretion of HBsAg and HBeAg, preC/C/HBeAg expression in the secretory pathway, activation of the unfolded protein response (UPR) and subsequent activation of apoptosis were monitored. The introduction of G1862T did not affect HBsAg expression. Cells transfected with the G1862T subgenotype A1 plasmid showed decreased expression of intracellular HBcAg and of nuclear preC/C/HBeAg and extracellular HBeAg, when compared to cells transfected with its wild-type counterpart as a result of the accumulation of the mutant protein in the endoplasmic reticulum (ER) and ER-Golgi intermediate compartment (ERGIC) . This accumulation of preC/C/HBeAg protein in the ER led to the earlier activation of the three UPR pathways, but not to an increase in apoptosis. Therefore, it is evident that the presence of G1862T in subgenotype A1 does not completely abolish HBeAg expression, but affects the rate of HBeAg maturation, its passage through the secretory pathway and activation of the UPR. Increase in ER stress can result in liver damage, which has been shown to be a contributing factor to hepatocarcinogenesis and may explain why G1862T is frequently found in subgenotype A1 from liver disease patients.

Characterization of contrasting features between hepatitis B virus genotype A and genotype D in small envelope protein expression and surface antigen secretion

Hepatitis B virus (HBV) genotypes A and D are prevalent in many parts of the world and show overlapping geographic distributions. We amplified the entire HBV genome from sera of patients with genotypes A and D and generated overlength constructs for transient transfection into Huh7 or HepG2 cells. Genotype D clones were associated with less HBsAg in culture supernatant and even less intracellular HBsAg. They produced less 2.1-kb RNA due to a weaker SPII promoter. Chimeric promoter constructs identified three divergent positions as most critical, and their exchange reversed extracellular HBsAg phenotype. The S protein of genotype D was more efficient at secretion, while its L protein possessed greater inhibitory effect. Swapping the S gene diminished genotypic difference in intracellular S protein but widened the difference in secreted HBsAg. In conclusion, HBV genotypes A and D differ in S protein expression, secretion and modulation by L protein.

LUC7L3/CROP inhibits replication of hepatitis B virus via suppressing enhancer II/basal core promoter activity

The core promoter of hepatitis B virus (HBV) genome is a critical region for transcriptional initiation of 3.5 kb, pregenome and precore RNAs and for the viral replication. Although a number of host-cell factors that potentially regulate the viral promoter activities have been identified, the molecular mechanisms of the viral gene expression, in particular, regulatory mechanisms of the transcriptional repression remain elusive. In this study, we identified LUC7 like 3 pre-mRNA splicing factor (LUC7L3, also known as hLuc7A or CROP) as a novel interacting partner of HBV enhancer II and basal core promoter (ENII/BCP), key elements within the core promoter, through the proteomic screening and found that LUC7L3 functions as a negative regulator of ENII/BCP. Gene silencing of LUC7L3 significantly increased expression of the viral genes and antigens as well as the activities of ENII/BCP and core promoter. In contrast, overexpression of LUC7L3 inhibited their activities and HBV replication. In addition, LUC7L3 possibly contributes to promotion of the splicing of 3.5 kb RNA, which may also be involved in negative regulation of the pregenome RNA level. This is the first to demonstrate the involvement of LUC7L3 in regulation of gene transcription and in viral replication.

Functional characterization of hepatitis B virus core promoter mutants revealed transcriptional interference among co-terminal viral mRNAs

Hepatitis B virus (HBV) has a 3.2 kb circular DNA genome. It employs four promoters in conjunction with a single polyadenylation signal to generate 3.5, 2.4, 2.1 and 0.7 kb co-terminal RNAs. The 3.5 kb RNA is subdivided into the precore RNA for e-antigen expression and pregenomic RNA for genome replication. When introduced to a genotype A clone, several core promoter mutations markedly enhanced HBV genome replication, but suppressed e-antigen expression through up-regulation of pregenomic RNA at the expense of precore RNA. In this study, we found such mutations also diminished envelope proteins and hepatitis B surface antigen, products of the 2.1 and 2.4 kb subgenomic RNAs. Indeed, Northern blot analysis revealed overall increase in 3.5 kb RNA, but reduction in all subgenomic RNAs. To validate transcriptional interference, we subcloned 1.1×, 0.7× and 0.6× HBV genome, respectively, to a vector with or without a cytomegalovirus (CMV) promoter at the 5' end, so as to produce the pregenomic RNA, 2.4 kb RNA, and 2.1 kb RNA in large excess or not at all. Parallel transfection of the three pairs of constructs into a human hepatoma cell line confirmed the ability of pregenomic RNA to suppress all subgenomic transcripts and established the ability of the 2.4 and 2.1 kb RNAs to suppress the 0.7 kb RNA. Consistent with our findings, pregenomic RNA of the related duck HBV has been reported to interfere with transcription of the subgenomic RNAs. Transcriptional interference might explain why HBV produces so little 0.7 kb RNA and HBx protein despite a strong X promoter.

Precore/core region mutations of hepatitis B virus related to clinical severity

Despite the availability of an effective vaccine, hepatitis B virus (HBV) infection remains a major health problem, with more than 350 million chronically infected people worldwide and over 1 million annual deaths due to cirrhosis and liver cancer. HBV mutations are primarily generated due both to a lack of proofreading capacity by HBV polymerase and to host immune pressure, which is a very important factor for predicting disease progression and therapeutic outcomes. Several types of HBV precore/core (preC/C) mutations have been described to date. The host immune response against T cells drives mutation in the preC/C region. Specifically, preC/C mutations in the MHC class II restricted region are more common than in other regions and are significantly related to hepatocellular carcinoma. Certain mutations, including preC G1896A, are also significantly related to HBeAg-negative chronic infection. This review article mainly focuses on the HBV preC/C mutations that are related to disease severity and on the HBeAg serostatus of chronically infected patients.

Hepatitis B virus genotypes and mutations in the basal core promoter and pre-core/core in chronically infected patients in southern Brazil: a cross-sectional study of HBV genotypes and mutations in chronic carriers

INTRODUCTION

In Brazil, little data exist regarding the distribution of genotypes in relation to basal core promoter (BCP) and precore/core mutations among chronic hepatitis B virus (HBV) carriers from different regions of the country. The aim of this study was to identify HBV genotypes and the frequency of mutations at the BCP and precore/core region among the prevalent genotypes in chronic carriers from southern Brazil.

METHODS

Nested-polymerase chain reaction (nested-PCR) products amplified from the S-polymerase gene, BCP and precore/core region from 54 samples were sequenced and analyzed.

RESULTS

Phylogenetic analysis of the S-polymerase gene sequences showed that 66.7% (36/54) of the patients were infected with genotype D (D1, D2, D3), 25.9% (14/54) with genotype A (A1, A2), 5.6% (3/54) with subgenotype C2, and 2% (1/54) with genotype E. A comparison of virological characteristics showed significant differences between genotypes A, C and D. The comparison between HBeAg status and the G1896A stop codon mutation in patients with genotype D revealed a relationship between HBV G1896A precore mutants and genotype D and hepatitis B e antigen (HBeAg) seroconversion. Genotype D had a higher prevalence of the G1896A mutation and the presence of a thymine at position 1858. Genotype A was associated with a higher prevalence of the G1862T mutation and the presence of a cytosine at position 1858.

CONCLUSIONS

HBV genotype D (D3) is predominant in HBV chronic carriers from southern Brazil. The presence of mutations in the BCP and precore/core region was correlated with the HBV genotype and HBeAg negative status.

Hepatitis B virus genotype distribution and genotype-specific BCP/preCore substitutions in acute and chronic infections in Argentina

AIM

In order to assess Hepatitis B Virus genotype (g) and subgenotype (sg) implications in the course of infection, 234 HBsAg positive patients in different infection stages were characterized (66 acute infections, 63 HBeAg positive chronic infections and 105 anti-HBe positive chronic infections).

RESULTS

Overall, sgA2 (17.9%), gD (20.9%), sgF1b (34.2%) and sgF4 (19.7%) were the most prevalent. Subgenotype F1b was overrepresented in acute and chronic HBeAg infections (56.1%), whereas gD was the most frequent (40.0%) in anti-HBe positive chronic infections. Among chronic infections, HBeAg positivity rates were 50.0, 12.5, 62.8 and 35.3% for sgA2, gD, sgF1b and sgF4, respectively (p <0.05). A bias toward BCP/preCore mutations was observed among genotypes. In anti-HBe positive chronic infections, sgF1b was more prone to have A1762T/G1764A mutation than sgA2, sgF4 and gD (75.0, 40.0, 33.3 and 31.8%, p<0.005), whereas in the pC region, gD and sgF4 were more likely to have G1896A than sgA2 and sgF1b (81.0, 72.7, 0.0 and 31.3%, p <0.001). The unexpected low frequency of the G1896A mutation in the sgF1b (despite carrying 1858T) prompted us to perform a further analysis in order to identify genotype-specific features that could justify the pattern mutations observed. A region encompassing nucleotides 1720 to 1920 showed the higher dissimilarity between sgF1b and sgF4. Genotypes and subgenotypes carrying the 1727G, 1740C and 1773T polymorphisms were prevented to mutate position 1896.

DISCUSSION

HBeAg seroconversion is a critical event in the natural history of HBV infection. Differences in the HBeAg positivity rate might be relevant since different studies have observed that delayed HBeAg seroconversion is associated with a more severe clinical course of infection, highlighting the critical role that genotypes/subgenotypes might play in the progression of HBV infection. Polymorphisms in the regions 1720 to 1920 could be involved in the molecular mechanisms underlying seroconversion of each genotype/subgenotype.

Correlation Between Hepatitis B G1896A Precore Mutations and HBeAg in Chronic HBV Patients

Background:

Hepatitis B virus (HBV) infection is an important health concern worldwide, with critical outcomes. Hepatitis B e antigen (HBeAg) negative chronic hepatitis B is frequently caused by a mutation (G1896A) in the hepatitis B virus (HBV) precore (PC) reading frame, which creates a stop codon, causing premature termination of the HBe protein.

Objectives:

This study aimed to investigate the G1896A PC mutation and its effect on HBeAg detection in chronic HBV patients.

Patients and Methods:

In this study, 120 chronic HBV patients neither vaccinated or who had benefited from immunoglobulin therapy, were recruited. The HBV-DNA was extracted from plasma and polymerase chain reaction (PCR) was performed. Positive PCR products were subjected to automated sequencing. The HBV serological markers [hepatitis B s antigen (HBsAg), HBeAg] were tested.

Results:

One hundred out of 120 (83.3%) patients were HBeAg negative and 100% were HBsAg positive. The comparison of nucleotide sequences with the reference sequence (Accession number: AB033559) in HBeAg negative patients showed that there was a high rate of mutations in G1896A (93.18%).

Conclusions:

This study indicates that the rate of G1896A mutation at the PC region among HBeAg negative patients, in the Golestan province of Iran, was similar to the average rate encountered in other parts of Iran. The PC stop codon mutation was detected in 93.18% of HBeAg negative patients. Further studies with larger sample sizes are required to elucidate the exact role of these mutations in the clinical course of chronic HBV infection.

Construction of replication competent plasmids of hepatitis B virus subgenotypes A1, A2 and D3 with authentic endogenous promoters

Hepatitis B virus (HBV) is hyperendemic to southern Africa, with genotype A of HBV being the predominant genotype, and subgenotype A1 prevailing. Infection with this subgenotype is associated with rapid disease progression, and high frequency of hepatocellular carcinoma development. The objectives of our study was to construct recombinant 1.28 mer replication competent HBV DNA plasmids of subgenotypes A1, A2 and D3 containing authentic endogenous HBV promoters and to follow their replication in vitro after transfection of Huh7 cells. We found that subgenotype D3 replicated at a lower level, as measured by HBsAg and HBV DNA levels, when compared to cells transfected with genotype A. There was no difference in the intracellular and extracellular HBsAg between cells transfected with subgenotypes A1 or A2. Cells transfected with subgenotype A1 had higher levels of intracellular replicative intermediates and HBcAg, and lower extracellular expression of HBeAg from days 1 to 3, when compared to cells transfected with subgenotype A2. In conclusion, the generation of these replication competent clones is an important step in the functional characterization of subgenotypes of HBV circulating in Africa and their comparison to strains circulating in other geographical regions of the world.

Variability in the precore and core promoter region of the hepatitis B virus genome

There is increasing evidence that hepatitis B virus (HBV) infections with different genotypes and subgenotypes differ in response to treatment and long-term prognosis. The differences emerge from variability within the genomes that leads to structural deviations at the pregenomic level and to changes at the translational level. Naturally occurring HBV strains covering the four major genotypes A-D were obtained from 393 patients and part of the genome was amplified using polymerase chain reaction (PCR), sequenced, and analyzed for mutational differences in the precore and core promoter regions. The study confirmed that core promoter and precore mutations occur at key positions (A1762T, G1764A, G1896A, and G1899A), and that the proportions of strains with seroconvertion in patients differ between the four HBV genotypes. A rare double mutation (C1857T together with G1897A) was observed, and C1856T was found together with the emerging G1898A mutation, which itself was found to be more widespread geographically than previously described. We found a novel mutation (T1850C), never before observed in human HBV strains but known from woodchuck hepatitis virus (WHV). A novel association of mutation C1773T with G1764T, C1766A, and G1757A was also found within a site already suggested to be a putative binding site for HNF-3. This novel association is proposed by us to be of importance for additional binding of HNH-2 to this site and is a better indicator of the emergence of the double mutation G1764T and C1766A than the G1757A mutation proposed previously.

Quasispecies structure, cornerstone of hepatitis B virus infection: Mass sequencing approach

Hepatitis B virus (HBV) is a DNA virus with complex replication, and high replication and mutation rates, leading to a heterogeneous viral population. The population is comprised of genomes that are closely related, but not identical; hence, HBV is considered a viral quasispecies. Quasispecies variability may be somewhat limited by the high degree of overlapping between the HBV coding regions, which is especially important in the P and S gene overlapping regions, but is less significant in the X and preCore/Core genes. Despite this restriction, several clinically and pathologically relevant variants have been characterized along the viral genome. Next-generation sequencing (NGS) approaches enable high-throughput analysis of thousands of clonally amplified regions and are powerful tools for characterizing genetic diversity in viral strains. In the present review, we update the information regarding HBV variability and present a summary of the various NGS approaches available for research in this virus. In addition, we provide an analysis of the clinical implications of HBV variants and their study by NGS.

Human cytidine deaminases facilitate hepatitis B virus evolution and link inflammation and hepatocellular carcinoma

During hepatitis B virus (HBV)-induced hepatocarcinogenesis, chronic inflammation facilitates the evolution of hepatocellular carcinoma (HCC)-promoting HBV mutants. Cytidine deaminases, whose expression is stimulated by inflammatory cytokines and/or chemokines, play an important role in bridging inflammation and HCC. Through G-to-A hypermutation, cytidine deaminases inhibit HBV replication and facilitate the generation of HCC-promoting HBV mutants including C-terminal-truncated HBx. Cytidine deaminases also promote cancer-related somatic mutations including TP53 mutations. Their editing efficiency is counteracted by uracil-DNA glycosylase. Understanding the effects of cytidine deaminases in HBV-induced hepatocarcinogenesis and HCC progression will aid in developing efficient prophylactic and therapeutic strategies against HCC in HBV-infected population.

Short hairpin RNAs with a 2- or 3-base mismatch inhibit HBV expression and replication in HepG2 cells

PURPOSE

To assess the functions of mismatched short hairpin RNAs (shRNAs) that inhibit replication and the expression of hepatitis B virus (HBV), two shRNAs possessing a 2- or 3-base mismatch that targeted HBV were studied.

METHODS

shRNAs and pHY106-HBV were cotransfected into HepG2 cells. The culture supernatants were collected and used in hepatitis B surface antigen (HBsAg) and hepatitis B e antigen (HBeAg) assays. The levels of HBsAg and HBcAg mRNA were detected by reverse-transcriptase PCR (RT-PCR). HBV DNA replication intermediates were extracted for Southern blot hybridization.

RESULTS

The results demonstrate that mismatched shRNA-458 and shRNA-635 can significantly inhibit HBsAg and HBeAg protein expression, and the maximal inhibition ratio for both proteins was found at 72 h after cotransfection: 80 and 50 %, respectively. Similar inhibitory effects were found on HBsAg and HBcAg mRNA levels and HBV DNA replication intermediates at 72 h after cotransfection, and the inhibition ratio was found to be approximately 70 and 90 %, respectively.

CONCLUSIONS

Despite the 2- or 3-base mismatch between the shRNAs and the HBV target sequences, shRNA-458 and shRNA-635 exerted a significant inhibitory effect on HBsAg and HBeAg expression and HBV replication. This indicates that mismatched shRNAs could be a promising therapy for HBV.

Effects of hepatitis B virus mutations on its replication and liver disease severity

Hepatitis B virus (HBV), nowadays, is one of the major human pathogens worldwide. Approximately, 400 million people worldwide have chronic HBV infection. Only 5% of persons infected during adulthood develop chronic infection. The reverse is true for those infected at birth or in early childhood, i.e. more than 90% of these persons progress to chronic infection. Currently, eight different genotypes o f HBV have been identified, differing in nucleotide sequence by greater than 8%. In addition, numerous subgenotypes have a l s o been recognized based on the nucleotide sequence variability of 4- 8%. It has invariably been found that these genotypes and mutations play a pivotal role in the liver disease aggravation and virus replication. The precore mutations (G1896A) and the double mutation (T1762/A1764) in the basal core promoter are important mutations that alter expression of the hepatitis B e antigen (HBeAg). The HBeAg is important for establishing viral persistence. The precore G1896A mutation abrogates the expression of HBeAg. Numerous other mutations alter the disease severity and progression. It is predictive that the infected patient has high risk of hepatocellular carcinoma if the genotype C is incriminated or if HBV possesses basal core promoter double mutation. Association of the remaining genotypes have been noted but with less degree than genotype C. Phenotypic assays of the different HBV protein markers with different molecular techniques illustrate the replication efficiency of the virus in cell lines. This review will discuss various mutations into their association with liver disease severity and progression as well as virus replication.

Ultra-deep pyrosequencing analysis of the hepatitis B virus preCore region and main catalytic motif of the viral polymerase in the same viral genome

Hepatitis B virus (HBV) pregenomic RNA contains a hairpin structure (ε) located in the preCore region, essential for viral replication. ε stability is enhanced by the presence of preCore variants and ε is recognized by the HBV polymerase (Pol). Mutations in the retrotranscriptase domain (YMDD) of Pol are associated with treatment resistance. The aim of this study was to analyze the preCore region and YMDD motif by ultra-deep pyrosequencing (UDPS). To evaluate the UDPS error rate, an internal control sequence was inserted in the amplicon. A newly developed technique enabled simultaneous analysis of the preCore region and Pol in the same viral genome, as well as the conserved sequence of the internal control. Nucleotide errors in HindIII yielded a UDPS error rate <0.05%. UDPS study confirmed the possibility of simultaneous detection of preCore and YMDD mutations, and demonstrated the complexity of the HBV quasispecies and cooperation between viruses. Thermodynamic stability of the ε signal was found to be the main constraint for selecting main preCore mutations. Analysis of ε-signal variability suggested the essential nature of the ε structural motif and that certain nucleotides may be involved in ε signal functions.

Hepatitis B virus genotype C isolates with wild-type core promoter sequence replicate less efficiently than genotype B isolates but possess higher virion secretion capacity

Infection by hepatitis B virus (HBV) genotype C is associated with a prolonged viremic phase, delayed hepatitis B e antigen (HBeAg) seroconversion, and an increased incidence of liver cirrhosis and hepatocellular carcinoma compared with genotype B infection. Genotype C is also associated with the more frequent emergence of core promoter mutations, which increase genome replication and are independently associated with poor clinical outcomes. We amplified full-length HBV genomes from serum samples from Chinese and U. S. patients with chronic HBV infection and transfected circularized genome pools or dimeric constructs of individual clones into Huh7 cells. The two genotypes could be differentiated by Western blot analysis due to the reactivities of M and L proteins toward a monoclonal pre-S2 antibody and slightly different S-protein mobilities. Great variability in replication capacity was observed for both genotypes. The A1762T/G1764A core promoter mutations were prevalent in genotype C isolates and correlated with increased replication capacity, while the A1752G/T mutation frequently found in genotype B isolates correlated with a low replication capacity. Importantly, most genotype C isolates with wild-type core promoter sequence replicated less efficiently than the corresponding genotype B isolates due to less efficient transcription of the 3.5-kb RNA. However, genotype C isolates often displayed more efficient virion secretion. We propose that the low intracellular levels of viral DNA and core protein of wild-type genotype C delay immune clearance and trigger the subsequent emergence of A1762T/G1764A core promoter mutations to upregulate replication; efficient virion secretion compensates for the low replication capacity to ensure the establishment of persistent infection by genotype C.

Characterization of occult hepatitis B virus infection from blood donors in China

Prevalence and characteristics of occult hepatitis B virus (HBV) infection (OBI) of genotypes B and C prevalent in China have not been extensively explored. Characterization of OBI strains obtained from Chinese blood donors was based on clinical and serological analyses, follow-up testing, and sequence analyses. Twenty-eight samples from 165,371 HBV surface antigen (HBsAg)-negative plasmas were confirmed HBsAg negative and DNA positive(HBsAg(-)/DNA(+)), of which 22 were classified as OBIs and 6 as window period infections. The OBI incidence was 1:7,517 in blood donors, whose ages ranged between 20 and 45 years (median, 28 years). OBI donors had normal alanine aminotransferase (ALT) levels and low viral loads ranging between unquantifiable amounts and 178 IU/ml (median, 14 IU/ml). Sequences from 21 basic core promoter/precore (BCP/PC) regions, five whole genomes, and two additional pre-S/S regions from OBI strains were compared to genotypes B and C HBsAg(+) reference strains. Eighty-six percent (6/7) of OBI strains were genotype C. Deletions, insertions, stop codons, and substitutions were detected in 15/21 (71%) core regulatory elements of OBI strains. Critical mutations were found in the core proteins of 5/5 OBI strains in parallel with random substitutions in pre-S/S proteins from 6/7 (86%) OBI strains. Critical mutations in core regulatory elements and core proteins might affect OBI genotype B and C strain replication. That there were few S protein substitutions suggests a minor role of the host immune defenses in OBI occurrence.

Improved method for rapid and efficient determination of genome replication and protein expression of clinical hepatitis B virus isolates

Different hepatitis B virus (HBV) genotypes and variants are associated with different clinical outcomes and/or response to antiviral therapy, yet the comparison of the in vitro replication capacity of a large number of clinical isolates remains technically challenging and time-consuming. Although the full-length HBV genome can be amplified from high-titer blood samples by PCR using High Fidelity(plus) DNA polymerase and primers targeting the conserved precore region, the HBV clones thus generated are replication deficient due to the inability to generate the terminally redundant pregenomic RNA essential for genome replication. The transfection experiment is further complicated by PCR errors and the presence of viral quasispecies. A previous study found that the precise removal of non-HBV sequence by SapI digestion led to HBV replication in transfected cells, possibly due to low-level genome circularization by a cellular enzyme. We released HBV genome from the cloning vector using BspQI, an inexpensive isoschizomer of SapI, and increased the efficiency of genome replication by an extra step of in vitro DNA ligation. The uncut plasmid DNA can be used for transfection if the sole purpose is to study envelope protein expression. We found significant PCR errors associated with the High Fidelity(plus) DNA polymerase, which could be greatly diminished using Phusion DNA polymerase or masked by the use of a clone pool. The reduced PCR error and modified enzymatic steps prior to transfection should facilitate a more widespread functional characterization of clinical HBV isolates, while the clone pool approach is useful for samples with significant sequence heterogeneity.

Massive APOBEC3 editing of hepatitis B viral DNA in cirrhosis

DNA viruses, retroviruses and hepadnaviruses, such as hepatitis B virus (HBV), are vulnerable to genetic editing of single stranded DNA by host cell APOBEC3 (A3) cytidine deaminases. At least three A3 genes are up regulated by interferon-α in human hepatocytes while ectopic expression of activation induced deaminase (AICDA), an A3 paralog, has been noted in a variety of chronic inflammatory syndromes including hepatitis C virus infection. Yet virtually all studies of HBV editing have confined themselves to analyses of virions from culture supernatants or serum where the frequency of edited genomes is generally low (≤10⁻²). We decided to look at the nature and frequency of HBV editing in cirrhotic samples taken during removal of a primary hepatocellular carcinoma. Forty-one cirrhotic tissue samples (10 alcoholic, 10 HBV⁺, 11 HBV⁺HCV⁺ and 10 HCV⁺) as well as 4 normal livers were studied. Compared to normal liver, 5/7 APOBEC3 genes were significantly up regulated in the order: HCV±HBV>HBV>alcoholic cirrhosis. A3C and A3D were up regulated for all groups while the interferon inducible A3G was over expressed in virus associated cirrhosis, as was AICDA in ∼50% of these HBV/HCV samples. While AICDA can indeed edit HBV DNA ex vivo, A3G is the dominant deaminase in vivo with up to 35% of HBV genomes being edited. Despite these highly deleterious mutant spectra, a small fraction of genomes survive and contribute to loss of HBeAg antigenemia and possibly HBsAg immune escape. In conclusion, the cytokine storm associated with chronic inflammatory responses to HBV and HCV clearly up regulates a number of A3 genes with A3G clearly being a major restriction factor for HBV. Although the mutant spectrum resulting from A3 editing is highly deleterious, a very small part, notably the lightly edited genomes, might help the virus evolve and even escape immune responses.

Retroviruses and hepadnaviruses such as hepatitis B virus (HBV) are vulnerable to mutation by host cell single stranded DNA cytidine deaminases. The result is hypermutated viral peppered with uracil residues. While there are potentially 11 such human enzymes, the major players belong to the 7 gene APOBEC3 cluster on chromosome 22, some of which can be activated by anti-viral interferons. We investigated the nature and frequency of HBV editing in 41 cirrhotic samples following surgical removal of primary hepatocellular carcinoma. Numerous APOBEC3 genes were activated in the decreasing order HCV±HBV>HBV>alcoholic cirrhosis. We observed that APOBEC3G was the dominant restricting factor in vivo with up to 35% of HBV edited genomes. Among the HBV mutants generated by APOBEC3 editing, we found a small fraction of lightly APOBEC3G edited genomes that can impact HBV replication in vivo and possibly immune escape.

APOBEC3G directly binds Hepatitis B virus core protein in cell and cell free systems

APOBEC3G (A3G) is an intrinsic antiretroviral factor which can inhibit Hepatitis B virus (HBV) replication. This antiviral activity mainly depends on A3G incorporation into viral particles. However, the mechanisms of A3G packaging into HBV particles have not been well characterized. In this paper, we demonstrated that A3G interacted with the HBV core protein (HBc) directly in co-transfected HepG2 cells using the fluorescence resonance energy transfer (FRET) approach. In addition, we further found that this interaction did not require other factors in vitro using surface plasmon resonance (SPR) technology on BIAcore 3000. While cellular RNA or viral RNA was added to A3G protein solution before flow through the BIAcore chip, the interaction was not affected. In conclusion, these results suggest the possibility that A3G is incorporated into HBV viral particles via direct binding with HBc protein.

Identification and characterization of clevudine-resistant mutants of hepatitis B virus isolated from chronic hepatitis B patients

Clevudine (CLV) is a nucleoside analog with potent antiviral activity against chronic hepatitis B virus (HBV) infection. Viral resistance to CLV in patients receiving CLV therapy has not been reported. The aim of this study was to characterize CLV-resistant HBV in patients with viral breakthrough (BT) during long-term CLV therapy. The gene encoding HBV reverse transcriptase (RT) was analyzed from chronic hepatitis B patients with viral BT during CLV therapy. Sera collected from the patients at baseline and at the time of viral BT were studied. To characterize the mutations of HBV isolated from the patients, we subjected the HBV mutants to in vitro drug susceptibility assays. Several conserved mutations were identified in the RT domain during viral BT, with M204I being the most common. In vitro phenotypic analysis showed that the mutation M204I was predominantly associated with CLV resistance, whereas L229V was a compensatory mutation for the impaired replication of the M204I mutant. A quadruple mutant (L129M, V173L, M204I, and H337N) was identified that conferred greater replicative ability and strong resistance to both CLV and lamivudine. All of the CLV-resistant clones were lamivudine resistant. They were susceptible to adefovir, entecavir, and tenofovir, except for one mutant clone. In conclusion, the mutation M204I in HBV RT plays a major role in CLV resistance and leads to viral BT during long-term CLV treatment. Several conserved mutations may have a compensatory role in replication. Drug susceptibility assays reveal that adefovir and tenofovir are the most effective compounds against CLV-resistant mutants. These data may provide additional therapeutic options for CLV-resistant patients.

Enhanced intracellular retention of a hepatitis B virus strain associated with fulminant hepatitis

A plasmid carrying 1.3-fold HBV genome was constructed from a HBV strain that caused five consecutive cases of fulminant hepatitis (pBFH2), and HepG2 cells were transfected with pBFH2 or its variants. The pBFH2 construct with A1762T/G1764A, G1862T, and G1896A showed the largest amount of core particle-associated intracellular HBV DNA, but no significant increase of extracellular HBV DNA in comparison with the wild construct, suggesting that these mutations might work together for retention of the replicative intermediates in the cells. The retention might relate to the localization of hepatitis B core antigen (HBcAg) in the nucleus of HepG2, which was observed by confocal fluorescence microscopy. HBcAg immunohistochemical examination of liver tissue samples obtained from the consecutive fulminant hepatitis patients showed stronger staining in the nucleus than acute hepatitis patients. In conclusion, the fulminant HBV strain caused retention of the core particles and the core particle-associated HBV DNA in the cells.

Subgenotypes of hepatitis B virus genotype D (D1, D2, D3 and D5) in India: differential pattern of mutations, liver injury and occult HBV infection

Hepatitis B genotype D (HBV/D) is the most widespread genotype and exists as at least five subgenotypes (HBV/D1-D5). However, little is known about the association of virological characteristics with clinical differences among HBV/D subgenotypes. To investigate the virological characteristics of these subgenotypes and their clinical implications, we selected a cohort of 109 genotype D infected individuals from the state of West Bengal, India, including 68 HBsAg positive patients and 41 with occult HBV infection. Among the HBsAg positive subjects 28 had chronic hepatitis B virus infection, 40 were asymptomatic carriers based on clinical examination, liver function test and ultrasonograph results. Overall, HBV/D1 was found in 17%, HBV/D2 in 29%, HBV/D3 in 34% and HBV/D5 in 20% of the cases. HBV/D1 was significantly associated with chronic liver disease (P = 0.01), and in this subgenotype A1896 (PreC mutations) were most common. Although BCP mutations (A/C1753 and T1762/A1764) were found to be frequently associated with HBV/D2 (33% and 33%) and D5 (47% and 59%), no apparent clinical correlation was observed. On the other hand, occult HBV infection was significantly associated with HBV/D3 infection, along with low level of BCP and PreC mutations and several non-synonymous substitutions in the catalytic reverse transcriptase (RT) domain of polymerase gene. Similar nucleotide substitutions in the surface (S) gene region were observed from both northern and eastern Indian HBV/D3 isolates. In conclusion, HBV/D subgenotypes differ in their mutational patterns in the S, polymerase and the BCP/PreC regions that may influence their clinical outcomes.

The first transmembrane domain of the hepatitis B virus large envelope protein is crucial for infectivity

The early steps of the hepatitis B virus (HBV) life cycle are still poorly understood. Indeed, neither the virus receptor at the cell surface nor the mechanism by which nucleocapsids are delivered to the cytosol of infected cells has been identified. Extensive mutagenesis studies in pre-S1, pre-S2, and most of the S domain of envelope proteins revealed the presence of two regions essential for HBV infectivity: the 77 first residues of the pre-S1 domain and a conformational motif in the antigenic loop of the S domain. In addition, at the N-terminal extremity of the S domain, a putative fusion peptide, partially overlapping the first transmembrane (TM1) domain and preceded by a PEST sequence likely containing several proteolytic cleavage sites, was identified. Since no mutational analysis of these two motifs potentially implicated in the fusion process was performed, we decided to investigate the ability of viruses bearing contiguous deletions or substitutions in the putative fusion peptide and PEST sequence to infect HepaRG cells. By introducing the mutations either in the L and M proteins or in the S protein, we demonstrated the following: (i) that in the TM1 domain of the L protein, three hydrophobic clusters of four residues were necessary for infectivity; (ii) that the same clusters were critical for S protein expression; and, finally, (iii) that the PEST sequence was dispensable for both assembly and infection processes.

Drastic reduction in the production of subviral particles does not impair hepatitis B virus virion secretion

Hepatitis B virus (HBV) contains three coterminal envelope proteins on the virion surface: large (L), middle (M), and small (S). The M and S proteins are also secreted as empty "subviral particles," which exceed virions by at least 1,000-fold. The S protein serves as the morphogenic factor for both types of particles, while the L protein is required only for virion formation. We found that cotransfecting replication constructs with a small dose of the expression construct for the missing L, M, and S proteins reconstituted efficient virion secretion but only 5 to 10% of subviral particles. The L protein inhibited secretion of subviral particles in a dose-dependent manner, whereas a too-high or too-low L/S protein ratio inhibited virion secretion. Consistent with the results of cotransfection experiments, a point mutation at the -3 position of the S gene AUG codon reduced HBsAg secretion by 60 to 70% but maintained efficient virion secretion. Surprisingly, ablating M protein expression reduced virion secretion but markedly increased the maturity of virion-associated genomes, which could be reversed by providing in trans both L and M proteins but not just M protein. M protein stability was dependent on the coexpression of S protein. Our findings suggest that efficient HBV virion secretion could be maintained despite drastic reduction in subviral particle production, which supports the recent demonstration of separate secretion pathways adopted by the two types of particles. The M protein appears to facilitate core particle envelopment, thus shortening the window of plus strand DNA elongation.

Chimeric constructs between two hepatitis B virus genomes confirm transcriptional impact of core promoter mutations and reveal multiple effects of core gene mutations

Hepatitis B virus (HBV) clone 4B replicated much more efficiently than clone 2A of the same genotype. Introduction of its T1753C, A1762T, G1764A, and C1766T core promoter mutations into the 2A genome greatly enhanced genome replication and suppressed HBeAg expression. Here we show that these effects are mediated by transcriptional up regulation of pregenomic RNA and suppression of precore RNA. Analysis of chimeric constructs suggested that the 5' end of the 2A core gene conferred higher level of pregenomic RNA, but less core protein and genome replication relative to the 4B sequence. Genome maturity of secreted virions was reduced by mutations present in the core protein of the 2A genome but enhanced by mutations found in the 4B core protein. The 4B core protein migrated faster than that of clone 2A. The possible links among the various phenotypes and the responsible mutations remain to be established.

Adaptive mutations in the signal peptide of the type 1 fimbrial adhesin of uropathogenic Escherichia coli

Signal peptides (SPs) are critical for protein transport across cellular membranes, have a highly conserved structure, and are cleaved from the mature protein upon translocation. Here, we report that naturally occurring mutations in the SP of the adhesive, tip-associated subunit of type 1 fimbriae (FimH) are positively selected in uropathogenic Escherichia coli. On the one hand, these mutations have a detrimental effect, with reduced FimH transport across the inner membrane, fewer FimH and fimbriae expressed on the bacterial surface, and decreased bacterial adhesion under flow conditions. On the other hand, the fimbriae expressed by the mutants are significantly longer on average, with many fimbriae able to stretch to >20 microm in length. More surprisingly, the SP mutant bacteria display an increased ability to resist detachment from the surface upon a switch from high to low flow. This functional effect of longer fimbriae highlights the importance of the nonadhesive fimbrial rod for adhesive function. Also, whereas bacterial adhesion to bladder epithelial cells was preserved in most mutants, binding to and killing by human neutrophils was decreased, providing an additional reason the SP mutations are relatively common among uropathogenic strains. Thus, this study demonstrates how mutations in an SP, while decreasing transport function and not affecting the final structure of the translocated protein, can lead to functional gains of the extracellular organelles that incorporate the protein and overall adaptive changes in the organism's fitness.

A valine to phenylalanine mutation in the precore region of hepatitis B virus causes intracellular retention and impaired secretion of HBe-antigen

AIM

Hepatitis B virus (HBV) e antigen (HBeAg) is translated from precore mRNA as a precore/core protein, which is post-translationally modified to give rise to the protein that is secreted into the serum. The G1862T mutation in HBV occurs in the bulge of the encapsidation signal within the pregenomic RNA. When the precore mRNA is translated, this mutation results in a valine to phenylalanine substitution at the -3 position to the signal peptide cleavage site at the amino end of the precursor protein. The aim of this study was to determine whether this mutation could affect HBV replication and/or HBeAg expression.

METHODS

Following transfection of Huh 7 cells, HBV replication was followed using real time polymerase reaction (PCR) and expression of HBeAg expression was monitored using confocal microscopy.

RESULTS

HBV replication was reduced when this mutation was introduced into genotype D but not into genotype A replication-competent constructs. Using mutant HBeAg-expressing plasmids, we demonstrated a 54% reduction in HBeAg secretion relative to the wild type. Confocal microscopy demonstrated that the mutant HBeAg accumulated in the endoplasmic reticulum, endoplasmic reticulum intermediate compartment and Golgi. These aggregates of mutant protein increased in size following treatment of the cells with a proteasome inhibitor, MG132, and had the hallmark features of aggresomes. They attracted ubiquitin, heat shock proteins and proteasomes and were isolated from the cytosol by the intermediate filaments, vimentin and cytokeratin.

CONCLUSION

The formation of aggresomes, as a result of the G1862T mutation, may play a contributory role in HBV-induced liver disease.

Genotyping and genomic sequencing in clinical practice

The global prevalence of chronic hepatitis B and its associated serious sequelae demand technologically advanced techniques of management. Nucleic acid testing (NAT) plays a key role in the diagnosis, surveillance, and treatment of chronic hepatitis B. NAT includes quantitative PCR-based HBV DNA assays, HBV genotyping, tests for mutations associated with resistance to antiviral medications, and assays to detect precore and core promoter mutations. This article reviews the uses of NAT in the diagnosis and management of chronic hepatitis B.

Critical role of the 36-nucleotide insertion in hepatitis B virus genotype G in core protein expression, genome replication, and virion secretion

Frequent coinfection of hepatitis B virus genotype G with genotype A suggests that genotype G may require genotype A for replication or transmission. In this regard, genotype G is unique in having a 12-amino-acid extension in the core protein due to a 36-nucleotide insertion near the core gene translation initiation codon. The insertion alters base pairing in the lower stem of the pregenome encapsidation signal, which harbors the core gene initiator, and thus has the potential to affect both core protein translation and pregenomic RNA encapsidation. Genotype G is also unusual for possessing two nonsense mutations in the precore region, which together with the core gene encode a secreted nonstructural protein called hepatitis B e antigen (HBeAg). We found that genotype G clones were indeed incapable of HBeAg expression but were competent in RNA transcription, genome replication, and virion secretion. Interestingly, the 36-nucleotide insertion markedly increased the level of core protein, which was achieved at the level of protein translation but did not involve alteration in the mRNA level. Consequently, the variant core protein was readily detectable in patient blood. The 12-amino-acid insertion also enhanced the genome maturity of secreted virus particles, possibly through less efficient envelopment of core particles. Cotransfection of genotypes G and A did not lead to mutual interference of genome replication or virion secretion. Considering that HBeAg is an immunotolerogen required for the establishment of persistent infection, its lack of expression rather than a replication defect could be the primary determinant for the rare occurrence of genotype G monoinfection.

Molecular characterization of subgenotype A1 (subgroup Aa) of hepatitis B virus

Subgenotypes of hepatitis B virus (HBV) were first recognized after a unique segment of genotype A was identified when sequencing the preS2/S region of southern African HBV isolates. Originally named subgroup A', subsequently called subgroup Aa (for Africa) or subgenotype A1, this subgenotype is found in South Africa, Malawi, Uganda, Tanzania, Somalia, Yemen, India, Nepal, the Philippines and Brazil. The relatively higher mean nucleotide divergence of subgenotype A1 suggests that it has been endemic and has a long evolutionary history in the populations where it prevails. Distinctive sequence characteristics could account for the high hepatitis B e-antigen (HBeAg) negativity and low HBV DNA levels in carriers of this subgenotype. Substitutions or mutations can reduce HBeAg expression at three levels: (i) 1762T1764A atthe transcriptional level; (ii) substitutions at nt 1809-1812 at the translational level; and (iii) 1862T at the post-translational level. Co-existence of 1762T1764A and nt 1809-1812 mutations reduces HBeAg expression in an additive manner. In addition, subgenotype A1 has unique sequence alterations in the transcriptional regulatory elements and the polymerase coding region. The distinct sequence characteristics of subgenotype A1 may contribute to the 4.5-fold increased risk of heptocellular carcinoma in HBV carriers infected with genotype A, which is entirely attributable to subgenotype A1.

G1862T mutation among hepatitis B virus-infected individuals: association with viral genotypes and disease outcome in Kolkata, Eastern India

OBJECTIVE

To study the prevalence of G1862T mutation in hepatitis B virus (HBV) isolates among Eastern Indian patients and its relationship with genotypes, HBeAg status and disease manifestation.

METHODS

HBV DNA was isolated from patients, amplified by nested PCR and sequenced directly.

RESULTS

Of the 102 patients, 32 were HBeAg positive and 70 HBeAg negative; 55, 24 and 23 isolates were infected with genotypes D, A and C, respectively. G1862T was detected in 18 samples, 15 (83%) of them belonged to genotype A (subgenotype HBV/A1), 3 (17%) to genotype D. This mutation was more frequent in HBeAg-negative than in HBeAg-positive patients (21 vs. 9%), whereas in HBV/A1 it was as common in HBeAg-positive as in HBeAg-negative patients and significantly associated with T1762/A1764 mutation. The mean viral load was lower in patients with G1862T mutation. Furthermore, this mutation was common in various clinical outcomes.

CONCLUSION

In our community, G1862T mutation was predominantly found in HBV/A1 isolates irrespective of HBeAg status. Moreover this mutation could not be correlated to the clinical outcome. These findings indicate that the G1862T mutation is probably a part of the natural variability of HBV/A1.

Hepatitis B virus replication

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