Precore and X region mutants in hepatitis B virus infections among renal dialysis patients

Mutations and methods of analysis of mutations in Hepatitis B virus

Immunization programmes against hepatitis-B are being carried out since more than three decades but still HBV is a major public health problem. Hepatitis B virus (HBV) genome consists of circular and partial double stranded DNA. Due to partial double stranded DNA, it uses an RNA intermediate during replication. This replicative strategy of HBV and lack of polymerase proofreading activity give rise to error occurrences comparable to retroviruses. The low fidelity of polymerase, overlapping reading frames and high replication rate produces many non-identical variants at every cycle of replication. Therefore, HBV spreads with mutations and variations. The mutations have been reported both in non-structural as well as structural genes of HBV genome. Recent advances in molecular biology have made easier to analyse these mutations. Hepatitis B antiviral therapy and immunization are all influenced by genetic variability. The analysis and understanding of these mutations are important for therapy against hepatitis B and updating of diagnostic tools. The present review discusses about mutations occurring in whole HBV genome. The mutation occurring both in structural and non-structural genes and non-coding regions have been described in details. It is much more informative because most of literature available, covers only individual gene or DNA regions of HBV.

Recent developments in the diagnosis and monitoring of HBV infection and role of the genetic variability of the S gene

Recent developments in the laboratory diagnosis of hepatitis B virus infection include the optimization of key serologic markers, including hepatitis B virus surface antigen and antihepatitis B virus core antibody, as well as the development of automated nucleic acid amplification assays. There is still a lack of standardization for nucleic acid amplification assays that are used for the monitoring of antiviral therapy and follow-up of chronic infection and the clinical significance of hepatitis B virus DNA levels need to be clarified. Although highly sensitive automated nucleic acid amplification assays for blood donor screening are available, their implementation is still subject to discussion and certain countries rejected hepatitis B virus DNA testing for blood donation due to poor cost effectiveness. Genetic variability of hepatitis B virus constitutes a major challenge for diagnosis of hepatitis B virus infection, particularly with regard to hepatitis B virus surface antigen detection, antihepatitis B virus surface antigen quantification and nucleic acid amplification assays. The performances of hepatitis B virus surface antigen enzyme immunoassays in regard to genotype and surface antigen variability need to be further improved. Polyclonal antibody-based hepatitis B virus surface antigen enzyme immunoassays, although they cannot guarantee 100% sensitivity, demonstrate superior S gene mutant recognition to assays using monoclonal capture and tracer antibodies. Isolated antihepatitis B virus core reactivity is an unusual but frequent result, which requires a test algorithm for resolution and hepatitis B virus DNA detection with sensitive nucleic acid amplification assays in order to exclude occult hepatitis B virus infection.

Characterization of nonconventional hepatitis B viruses lacking the core promoter

The core gene (C-gene) promoter and regulatory sequences play a central role in the hepatitis B virus (HBV) life cycle. They are essential for the synthesis of the pregenomic and precore mRNA. The pregenomic RNA is the template required for replication and also the template for the synthesis of the core protein and polymerase. Here, we report the in vivo existence and functional characterization of HBV variants that lack the C-gene promoter region and the regulatory sequences located therein. HBV promoter fragments were isolated by PCR from sera of chronic carriers and characterized. Truncated promoter elements were identified, and then tested in the context of wild-type genomes in the HuH-7 cell line. The expression of the recombinant HBV genome resulted in the synthesis of surface proteins, and low level of core protein as well as a transcript pattern similar to, but smaller in size to wild-type virus. The recombinant HBV genome with the truncated promoter region produced pregenomic RNA-like transcripts. These transcripts were encapsidated and reverse transcribed when complemented by sufficient core and polymerase protein. These date provide an explanation as to why such deletion mutants of HBV can be produced at all, they highlight the functional potentials of viral sequences activated by mutations and may be of relevance for viral evolution and persistence.

Molecular, immunological and clinical properties of mutated hepatitis B viruses

Hepatitis B virus (HBV) is at the origin of severe liver diseases like chronic active hepatitis, liver cirrhosis and hepatocellular carcinoma. There are some groups of patients with high risk of generation of HBV mutants: infected infants, immunosuppressed individuals (including hemodialysis patients), patients treated with interferon and lamivudine for chronic HBV infection. These groups are the target for molecular investigations reviewed in this paper. The emergence of lamivudine- or other antiviral-resistant variants, rises concern regarding long term use of these drugs. Infection or immunization with one HBV subtype confers immunity to all subtypes. However, reinfection or reactivation of latent HBV infection with HBV mutants have been reported in patients undergoing transplant and those infected with HIV. Mutations of the viral genome which are not replicative incompetent can be selected in further course of infection or under prolonged antiviral treatment and might maintain the liver disease. Four open reading frames (ORF) which are called S-gene, C-gene, X-gene and P-gene were identified within the HBV genome. Mutations may affect each of the ORFs. Mutated S-genes were described to be responsible for HBV-infections in successfully vaccinated persons, mutated C-genes were found to provoke severe chronic liver diseases, mutated X-genes could cause serious medical problems in blood donors by escaping the conventional test systems and mutated P-genes were considered to be the reason for chemotherapeutic drug resistance. This paper reviews molecular, immunological and clinical aspects of the HBV mutants.

Hepatitis B virus markers in anti-HBc only positive individuals

Isolated reactivity to hepatitis B virus (HBV) core antigen (anti-HBc) is observed relatively frequently in immunocompromised individuals, intravenous drug abusers (IVDA), and in the presence of HCV infection. The reason for the lack of HBsAg is not clear. The aim of the present study was to investigate which factors (genetic variability of S gene, low-level HBsAg, and immune complexes may be responsible for the failure of HBsAg detection with commercial HBsAg screening assays. Dilution series of two recombinant HBsAg escape mutants and dilutions of serum samples from chronic HBV carriers with multiple insertions in the a determinant and different HBsAg subtypes were tested with a highly sensitive assay that detects wild-type HBsAg (Elecsys HBsAg, Roche Diagnostics, Penzberg, Germany) and two assays that detect HBV wild-type and escape mutants (Murex HBsAg Version 3, Murex and Enzygnost HBsAg 5.0, Dade Behring, Marburg, Germany). Elecsys HBsAg showed in comparison to Murex HBsAg Version 3 and Enzygnost HBsAg 5.0 a reduced sensitivity for escape mutant detection. On the other hand, the best performance for HBsAg subtype detection was obtained with Elecsys HBsAg. In the second part of the study, a selected panel of isolated anti-HBc reactive (n = 104) serum samples (AxSYM Core) was submitted to testing by Elecsys HBsAg, Murex HBsAg Version 3, Enzygnost HBsAg 5.0, and HBsAg detection after immune complex dissociation (ICD) and anti-HBs determination with two different assays (AxSYM Ausab and Elecsys Anti-HBs). To assess the specificity of anti-HBc test results, all the samples were tested by a second anti-HBc assay (Elecsys Anti-HBc). Quantitative HBV DNA detection was undertaken with a commercially available HBV PCR assay (Amplicor HBV Monitor). HCV infection was present in 65.4% of anti-HBc only reactive individuals. Five AxSYM Core positive samples were negative by Elecsys Anti-HBc. Overall, 15 (14.4%) AxSYM Ausab negative samples gave positive results with Elecsys Anti-HBs (median value: 21 IU/ml). No low-level HBsAg carrier was detected among the isolated anti-HBc reactive individuals with Elecsys HBsAg. There was no evidence for the presence of immune complexes. Only one sample was repeatedly reactive by the Murex HBsAg, suggesting that the a mutant form of HBsAg was responsible for the isolated anti-HBc reactivity, however neutralisation assay was not interpretable and HBV DNA PCR was negative. Fifteen (14.4%) anti-HBc only positive individuals were HBV DNA carriers with concentrations ranging from 800 to more than >4,000,000 copies of viral DNA/ml. In conclusion, the most probable explanations for isolated anti-HBc reactivity in our study group are a possible interference of HBsAg synthesis by HCV infection (65.4%) and divergence of results of anti-HBs assays (14.4%). There is no evidence for the presence of low-level HBsAg carriers and immune complexes. HBsAg mutants cannot be excluded definitively by the test strategy used in the present evaluation.

The core promoter of hepatitis B virus

The core promoter (CP) of hepatitis B virus (HBV) plays a central role in HBV replication and morphogenesis, directing the transcription of both species of 3.5 kb mRNA: pregenomic (pg) RNA and precore (pre-C) mRNA. The CP overlaps the 3' end of the X open-reading frame (ORF) and the 5' end of the pre-C/C ORF. The major functional elements of the CP are the upper regulatory region (URR) and the basic core promoter (BCP). The BCP is sufficient for accurate initiation of both pre-C mRNA and pgRNA transcription. It contains four AT-rich regions and the initiators for pre-C mRNA and pgRNA transcription. The upstream regulatory region consists of the negative regulatory element and the core upstream regulatory sequence. Co-operative interaction of various liver-enriched and ubiquitous transcription factors is necessary for liver-specific expression from the CP. These factors bind to the CP. Sequence conservation within the CP is crucial for maintaining active viral replication, and variation may contribute to the persistence of HBV within the host, leading to chronic infection and, ultimately, hepatocarcinogenesis. The most frequently described mutations within this region are an A to T transversion at position 1762 together with a G to A transition at position 1764. This double mutant is accompanied by a reduced level of hepatitis B e antigen (HBeAg) expression. Deletions, insertions and duplications occur within the CP.

T1762/A1764 variants of the basal core promoter of hepatitis B virus; serological and clinical correlations in Chinese patients

BACKGROUND

A double variant in the basal core promoter, converting nucleotide 1762 from A to T (T1762) and nucleotide 1764 from G to A (A[764), has been described in patients with chronic hepatitis B infection. Its prevalence and significance in Chinese chronic HBV carriers are unknown.

METHODS

We studied 177 Chinese patients with chronic HBV infection (chronic hepatitis/asymptomatic: 89/88; hepatitis B e antigen positive/negative: 84/93). The double variant was detected by mismatched polymerase chain reaction and restriction fragment length polymorphism analysis. The reliability of this method was verified by sequencing in 41 serum samples with 100% matching.

RESULTS

The double variant T1762/ A1764 was found in 52 of 89 patients with chronic hepatitis, but in only 6 of 59 asymptomatic carriers (p<0.001). The prevalence was significantly lower in hepatitis B e antigen positive patients (23/84) than in hepatitis B e antigen negative patients (35/64) (p<0.005). Precore variant, A1896 was detected in 40 individuals; 31 of them suffered from chronic hepatitis and 9 were asymptomatic (p<0.001). A combination of both variants T1762/A1764 and A1896 was seen in 3 of 59 asymptomatic and 22 of 89 patients with chronic hepatitis (p<0.005).

CONCLUSIONS

Mismatched polymerase chain reaction with restriction fragment length polymorphism provides a reliable, easy and fast method for detection of the presence of the T1762/A1764 variant. In Chinese chronic hepatitis B carriers, T1762/A1764 variant was associated with both active liver disease and hepatitis B e antigen negativity.

Natural and iatrogenic variation in hepatitis B virus

The existence of HBV as quasispecies is thought to be favoured by the infidelity of HBV RT, which would account for the emergence of the many natural mutants with point substitutions. RT infidelity may also underlie the hypermutation phenomenon. Indeed, the oft-reported point mutation in the preC gene that leads to failure of HBeAg synthesis may be driven by a hypermutation-related mechanism. The presence of mutants with deletions and insertions involving single nucleotides and oligonucleotides at specific positions in the genome, and of mutants with deletions of even longer stretches particularly in the C gene, suggests that other mutagenic mechanisms operate. Candidates include slippage during mispairing between template and progeny DNA strand, the action of cellular topoisomerase I, and gene splicing using alternative donor and acceptor sites. Natural substitutions, deletions or insertions involving the Cp/ENII locus in the X gene can significantly alter the extent of viral replicative activity. Similar mutations occurring at other locations of Cp/ENII, and at B-cell epitope sites of the S gene are associated with failure to detect serological markers of HBV infection. HBV variation can also arise from recombination between coinfecting strains. S gene mutations that become evident following HBIG administration and HBV vaccination are all point substitutions, as are mutations in functional RT domains of the P gene after treatment with viral RT-inhibitory drugs. Widespread and long-term use of prophylactic and therapeutic agents may potentially generate serologically occult HBV variants that might become difficult to eradicate.

Use of PCR in resolving diagnostic difficulties potentially caused by genetic variation of hepatitis B virus

AIMS

To assess the relevance of genetic variants of hepatitis B virus (HBV) and to demonstrate the usefulness of the polymerase chain reaction (PCR) in cases of HBV diagnostic difficulty.

METHODS

Five serum samples from patients that presented diagnostic difficulty in routine laboratories were sent to a research laboratory for PCR, and if appropriate, S gene sequencing, in vitro expression, and antigenic analysis.

RESULTS

The demonstration of HBV in serum by PCR allowed a definitive diagnosis of current infection. One serum sample with poor reactivity in a diagnostic assay had a minor hepatitis B surface antigen (HBsAg) variant and another with very poor reactivity had multiple variants of HBsAg. Transient HBsAg reactivity was observed in a recently vaccinated patient. A hepatitis Be antigen (HBeAg) false positive reaction was noted in a patient from a well defined risk group for HBV. One patient who was strongly HBsAg/HBeAg positive, but anti-hepatitis B core antibody negative, was viraemic.

CONCLUSIONS

PCR may become the gold standard for the diagnosis of current HBV infection. HBV variants are responsible for a proportion of diagnostically difficult cases. Modification of commercial assays is necessary to increase the sensitivity of detection of such variants.