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Development of Replicative and Nonreplicative Hepatitis B Virus Vectors

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Development of Replicative and Nonreplicative Hepatitis B Virus Vectors Gene Therapy (1997) 4, 1330–1340 1997 Stockton Press All rights reserved 0969-7128/97 $12.00 Development of replicative and nonreplicative hepatitis B virus vectors S Chaisomchit, DLJ Tyrrell and L-J Chang Department of Medical Microbiology and Immunology and Glaxo Wellcome Heritage Research Institute, University of Alberta, Edmonton, Alberta, Canada To investigate the possibility of using hepatitis B virus transactivation activity of the HBVtat recombinant since a (HBV) as a vector, the tat gene from human immunodefi- frameshift mutation in the pol gene did not affect the ciency virus type 1 (HIV-1) was inserted into the full-length recombinant tat function. The functional tat protein, there- HBV genome in-frame with the polymerase (pol) open fore, was most likely expressed as a Tat-Pol fusion pro- reading frame in the tether region and downstream of the duct. Endogenous polymerase assays showed that the pol preS1 promoter. We demonstrated that the tat gene was protein expressed from the HBVtat recombinant was still expressed with full activity in transactivating the HIV-1 long active although at a reduced level. Hepatitis B surface anti- terminal repeat (LTR). The expression of the tat gene in gens and e antigen produced from this recombinant were the context of the HBV genome in chicken hepatoma and detected at similar levels as those produced from the wild human cervical carcinoma cells, however, was not as type. Notably, the capability of forming complete HBV par- efficient as that in human hepatoblastoma cells, which ticles was still retained. These studies indicate the potential reflects the cellular and species specificity of promoters of of constructing HBV as a replicative vector. We also hepadnaviruses. Detection of RNA expressed from this showed that manipulation of a nonreplicative HBV vector HBVtat recombinant revealed transcription of the tat gene was possible. Expression of the HBV polymerase could be by two promoters: the core/pol promoter and the preS1 completely eliminated and replication of the nonreplicative promoter. A Pol-Tat fusion protein expressed by the HBV recombinant could be supported by Pol trans- core/pol promoter did not seem to contribute to the tat complementation. Keywords: HBV; tat; vector; gene therapy; liver Introduction The genomic organization of these viruses is extremely compact and efficiently organized with overlapping open As we are faced with a number of diseases involving the reading frames (ORFs).13,14 Hepatitis B virus (HBV), the liver, particularly inherited single gene defects and viral prototype of hepadnaviruses and causative agent for diseases, a novel therapeutic approach using targeted human hepatitis, carries four major overlapping ORFs: gene transfer to this organ is of particular interest, preS1/preS2/S (collectively known as the envelope or especially strategies of using human viruses as vectors. surface gene), preC/C, X and P. The envelope gene, In vitro protocols for transferring the low density lipopro- encompassing the preS1, preS2 and S regions as delin- tein (LDL) receptor gene into hepatocytes using a retrovi- eated by three in-frame initiation codons, codes for three 1,2 ral vector have been established. Adenoviral vectors envelope proteins: large (L), middle (M) and major (S). have also been used to deliver therapeutic genes, such as The preC/C gene, including the preC and C regions as 3 4,5 6,7 the genes for factors VIII and IX and LDL receptor delineated by two in-frame initiation codons, codes for into liver cells. However, these viral vectors infect a wide secreted HBV e antigen (HBeAg) and capsid or core pro- range of tissues, not specifically targeting to the liver. tein (HBcAg). The X gene codes for a transactivating pro- Expression of the transferred gene by adenoviral vectors, tein which has activity on HBV enhancers and other for example, is detected in different tissues after systemic cellular genes.15 The P or polymerase (pol) gene has the 6,8,9 7,10,11 administration or via portal or splenic vein. longest ORF. It encompasses about 80% of the entire viral Therefore, the use of hepadnaviruses which are hepato- genome and overlaps with the C-terminus of the preC/C tropic and possess strong liver-specific promoter and gene, the entire envelope gene and the N-terminus of the 12 enhancer elements, as vector systems, may provide a X gene. The C-terminus of the X gene also overlaps with more efficient means for gene delivery to the liver. the N-terminus of the preC/C gene. The protein product Hepadnaviruses are among the smallest DNA viruses (Pol) encoded by the pol gene can be divided into three known, carrying only 3200 base pairs in their genome. major functional domains: the terminal protein domain at the N-terminus, the reverse transcriptase–DNA poly- merase in the central domain and the RNase H domain 16,17 Correspondence: L-J Chang, 611 HMRC, University of Alberta, Edmon- at the C-terminus. The terminal protein and reverse ton, Alberta T6G 2S2, Canada transcriptase–DNA polymerase domains are separated Received 16 April 1997; accepted 7 August 1997 by a spacer or tether region. Four promoter elements, the HBV vectors S Chaisomchit et al 1331 preS1, preS2/S, X and core/pol promoters,18 which regu- late transcription of pregenomic and subgenomic mess- engers for expression of the corresponding genes, have been identified on the HBV genome. Almost all nucleo- tides are included in coding sequences and are therefore indispensable. Only the spacer or tether region may be nonessential for the pol gene function or HBV repli- cation.17,19 To our knowledge, HBV or other hepadnaviruses have not yet been engineered and used as gene transfer tools. The unusually efficient genome of HBV is a factor that limits its genetic manipulation. Mutations, insertions or deletions in many regions of the HBV genome have del- eterious effects on viral gene expression and repli- cation.17,20–24 The tether region of the pol gene, however, seems to be dispensable for HBV replication and can be manipulated. Mutational and computer sequence analy- ses show that this region starts upstream of the preS1 gene and overlaps with the preS1 and preS2 regions.17,21 Part of the tether region, however, does not overlap with any other HBV genes. A mutational analysis of the pol gene of HBV has demonstrated that up to 90 codons of the intervening tether sequence can be deleted without significant loss of the endogenous polymerase activity.17 It has also been shown that such a deletion has no effect on the RNA encapsidation process.25 Mutants of HBV containing deletions in the preS1 region which overlaps the tether region are capable of replication.23 The duck hepatitis B virus (DHBV) genome carrying the gene for protein A (123 amino acids) inserted in the tether region also retains the capability of expressing an active endogenous polymerase.19 This region, moreover, toler- Figure 1 Schematic representation of HBV constructs and mutants. (a) ates many mutations resulting in amino acid changes.26,27 Construction of HBVtat. The HIV-1 tat gene (267 bp) was inserted into The tether region, therefore, seems to be the most suitable the unique BstEII site in-frame with the pol gene and between the pro- site for engineering the HBV genome as a vector. moter (2784 nt) and the initiation codon of the preS1 gene. All the ORFs encoded on the EcoRI–EcoRI monomer of the HBV genome (3221 bp) are We report here that the HBV genome can be manipu- shown with the positions of all initiation codons according to the adw2 lated to accommodate a foreign gene whose functional subtype. The ORFs start from the blunt end and stop at the arrow end. activity can be demonstrated in the context of the full The four domains of the pol gene corresponding to the functional activities length HBV genome in tissue culture cells. We con- of the Pol protein are indicated. The solid bar is the preS1 promoter and structed a recombinant HBV carrying the HIV-1 tat gene the transcription initiation site of the preS1 RNA (2.4 kb) is indicated by in the tether region. Transient expression in hepatoma an arrow. The NcoI site at the initiation codon of the X gene and the BspEI site downstream of the initiation codon of the pol gene are also and cervical carcinoma cells showed that the tat gene was shown. RT/Pol, reverse transcriptase and DNA polymerase; TP, terminal expressed with functional activity. In addition, the protein. (b) Linear map of the HBVtat replication-competent plasmid HBVtat recombinant exhibited polymerase activity, albeit (pTHBVT-d) (9859 bp) with two EcoRI–EcoRI monomers in a head to at a reduced level compared to the wild-type HBV. tail tandem configuration subcloned into the pT7T318U vector. All ORFs Remarkably, intact viral particles were still produced are depicted by solid bars. The locations of the insertion are indicated from human hepatoma cells transfected with the HBVtat by hatched boxes. T3, T3 promoter; T7, T7 promoter; AmpR, ampicillin resistance. (c) Diagrammatic representation of HBVtat mutations. (i) Site- recombinant. We further established a nonreplicative directed mutagenesis of the X gene at the initiation codon (1376 nt) with HBV vector by inserting a full length Zeocin-resistant an additional stop codon at 1397 nt. (ii) Frameshift mutation in the pol gene in-frame with the pol gene which totally ablated the ORF by digestion of the BspEI site and filling in at 2332 nt to 2336 nt. pol gene expression. Production of this nonreplicative The mutated or additional nucleotides are shown as boldface letters. recombinant HBV vector was successfully complemented by the Pol protein in trans. not interfere with any other HBV ORFs.
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