Close evolutionary relatedness of the hepatitis B virus and murine leukemia virus polymerase gene sequences

Hepatitis B viral polymerase fusion proteins are biologically active and can interact with the hepatitis C virus core protein in vivo

Hepadnaviruses and retroviruses are evolutionarily related families because they both require a process of reverse transcription for genome replication. However, hepadnaviruses produce polymerase (pol) and core proteins separately, while retroviruses synthesize a gag-pol fusion protein that is subsequently cleaved by a virally encoded protease to release a functional polymerase. To test whether an additional sequence at the N-terminus of pol in hepatitis B virus (HBV) interferes with its function, we created two plasmids expressing core-pol fusion proteins, core144-pol and core31-pol. Secreted particles obtained from HuH-7 cells, which were cotransfected with a core-pol fusion protein-expressing plasmid and a core-expressing plasmid, showed a positive signal of HBV DNA by the endogenous polymerase assay, indicating that the core-pol fusion proteins retain DNA priming, polymerization and RNase H activities. The fusion protein was detected in the cytoplasm of transfected cells and in secreted virions by immunoprecipitation. Furthermore, we found by immunofluorescence staining that the HBV core-pol fusion protein colocalized with the hepatitis C virus (HCV) core protein in cytoplasm and in lipid droplets. Immunoprecipitation studies showed that the anti-HCV core complex contained the HBV core-pol fusion protein while the anti-HBV pol complex contained the HCV core protein, which supports the hypothesis that the HCV core protein can form a complex with the HBV core-pol fusion protein.

Relationships within and between genotypes of hepatitis B virus at points across the genome: footprints of recombination in certain isolates

Hepatitis B virus (HBV) was partitioned into type, subtype and isolate categories and the average evolutionary distances within and between categories was plotted at each of 54 points along the genome. The graphs showed alternating variable and conserved domains within and between HBV subtypes and revealed that some specimens assigned to different groups are more similar across several contiguous intervals than specimens belonging to the same group. Isolates were screened individually to determine their conformation to type and mosaic structure was identified in 14/65 specimens. Two entire clades (six specimens) of genotype B had a B/C sequence switch in the core gene region, whereas six genotype D specimens showed D/A switching in one or more regions of the genome. Genotype E was not separate from genotype D in the X and C subgenomic regions. The nature and distribution of polymorphic sites in mosaic regions was mapped at both the nucleotide and protein levels and the position of the variant fragments was related to mutational hot spots and linear epitopes of HBV. Mosaic structure was demonstrated statistically in 11 isolates using bootstrap resampling and recombination, rather than random change, appeared to be the mechanism responsible. The sequence between and including the two DR regions was represented in all putative recombinants. The distribution of genetic distances over subgenomic regions showed that substitution rates are not constant among the lineages of HBV in the preS regions. Genotype F is the most diverse group. Only genotypes A, C and F partition consistently into subtypes.

The woodchuck hepatitis virus X gene is important for establishment of virus infection in woodchucks

All mammalian hepadnaviruses possess a gene, termed X, that encodes a protein capable of transactivating virus gene expression. The X gene overlaps the polymerase and precore genes as well as two newly identified open reading frames (ORFs) termed ORF5 and ORF6. In this investigation, we examined whether ORF5, ORF6, and the X gene were important for the replication of woodchuck hepatitis virus (WHV) in susceptible woodchucks. First, we investigated whether proteins were produced from ORF5 and ORF6 by in vitro translation of appropriate viral transcripts, searched for antibodies against the putative proteins in the sera of animals infected with wild-type virus, and looked for an antisense WHV transcript, necessary for expression of a protein from ORF6, in the livers of acutely or chronically infected woodchucks. All such experiments yielded negative results. Next, we used oligonucleotide-directed mutagenesis to introduce termination codons into ORF5 and ORF6 at two locations within each ORF. Adult woodchucks in groups of three were transfected with one of the four mutant genomes. All of these woodchucks developed WHV infections that were indistinguishable from those of animals transfected with the wild-type WHV recombinant. Polymerase chain reaction amplification and direct DNA sequencing confirmed that reversion of the mutants to a wild-type genotype did not occur. Taken together, these data indicate that ORF5 and ORF6 are not essential for virus replication and are unlikely to represent authentic genes. Finally, we generated five WHV X-gene mutants that either removed the initiation codon for protein synthesis or truncated the carboxyl terminus of the protein by 3, 16, 31, or 52 amino acids. Groups of three adult woodchucks were transfected with one of the five X-gene mutants. Only the mutant that possessed an X gene lacking 3 amino acids from the carboxyl terminus was capable of replication within the 6-month time frame of the experiment. In contrast, all seven woodchucks transfected with wild-type WHV DNA developed markers consistent with viral infection. Thus, it is likely (P < 0.01) that the WHV X gene is important for virus replication in the natural host.

Proteaselike sequence in hepatitis B virus core antigen is not required for e antigen generation and may not be part of an aspartic acid-type protease

The hepatitis B virus (HBV) C gene directs the synthesis of two major gene products: HBV core antigen (HBcAg[p21c]), which forms the nucleocapsid, and HBV e antigen (HBeAg [p17e]), a secreted antigen that is produced by several processing events during its maturation. These proteins contain an amino acid sequence similar to the active-site residues of aspartic acid and retroviral proteases. On the basis of this sequence similarity, which is highly conserved among mammalian hepadnaviruses, a model has been put forward according to which processing to HBeAg is due to self-cleavage of p21c involving the proteaselike sequence. Using site-directed mutagenesis in conjunction with transient expression of HBV proteins in the human hepatoma cell line HepG2, we tested this hypothesis. Our results with HBV mutants in which one or two of the conserved amino acids have been replaced by others suggest strongly that processing to HBeAg does not depend on the presence of an intact proteaselike sequence in the core protein. Attempts to detect an influence of this sequence on the processing of HBV P gene products into enzymatically active viral polymerase also gave no conclusive evidence for the existence of an HBV protease. Mutations replacing the putatively essential aspartic acid showed little effect on polymerase activity. Additional substitution of the likewise conserved threonine residue by alanine, in contrast, almost abolished the activity of the polymerase. We conclude that an HBV protease, if it exists, is functionally different from aspartic acid and retroviral proteases.

Mutation rate of the hepadnavirus genome

An essential factor for charting the evolution of hepadnaviruses is an estimation of the mutation rate of the virus genome during replication in the host. In order to determine the mutation rate of the hepadnavirus genome under defined experimental conditions, we transfected 10 neonatal woodchucks with an infectious molecular clone of woodchuck hepatitis virus (WHV). By 4 months post-transfection, all 10 animals showed serological evidence for WHV infection. Subsequently, 1 animal became chronically infected and was used for further study. At 16 months post-transfection WHV DNA from serum virions was cloned and the nucleotide sequence of three independent progeny genomes compared directly with that of the input recombinant DNA. Although the consensus nucleotide sequence remained unchanged, we found three differences in individual progeny genomes when compared to the parental genome sequence. Thus, we estimate the mutation rate of the WHV genome to be less than or equal to 2 X 10(-4) base substitutions/site/year. This figure is one to two orders of magnitude lower than the mutation rates previously calculated for the positive- and negative-strand RNA viruses, but is similar to the mutation rate of the gag gene which is the most slowly evolving gene of retroviruses. Therefore, we find that the hepadnavirus genome is relatively stable during replication in host tissues when compared to other viruses that lack polymerase-associated proofreading functions.

The molecular biology of hepatitis B virus

Infection with the hepatitis B virus (HBV) or related hepadnaviruses is associated with a wide spectrum of liver diseases, including hepatocellular carcinoma (HCC). In this article we review the current state of knowledge about the structure, genetic organization and life cycle of HBV. The mechanisms of viral pathogenesis and HCC development remain poorly understood, but new approaches may soon begin to shed light on these areas.