Evaluation of HBV promoters for use in hepatic gene therapy

Evaluation of hepatitis B virus promoters for sustained transgene expression in mice by bioluminescence imaging

To find new liver-specific expression cassettes for long-term expression of therapeutic genes in the context of pDNA, the function and specificity of hepatitis B virus (HBV)' two hepatic enhancers (EnI and EnII), combined with HBV core and X promoters in cultured cells were evaluated. By bioluminescence imaging and hydrodynamic gene transfer technology, the persistence of transgene expression containing these regulatory sequences in the liver of mice was assessed. Our data indicated that both HBV enhancers were able to stimulate HBV core and X promoter activity in cultured cells of hepatic origin. In vivo, HBV core promoter linked to EnI and EnII (EII-EI-Pc) and X promoter linked to EnI and EnII (EI-EII-Px) could direct a constant and high-level gene expression.

Identification of an ovine atadenovirus gene whose product activates the viral E2 promoter: possible involvement of E2F-1

Activation of the adenoviral E2 promoter is an early step in adenovirus gene expression. For members of the mast- and aviadenoviruses, this requires induction of the cellular transcription factor E2F by virally encoded gene products such as E1A, E4orf6/7 and orf22/GAM-1. The newly recognized genus atadenovirus, of which the ovine isolate OAdV is the prototype, lacks any sequence homology to those genes. To find a possible link between E2 promoter activation and OAdV gene expression, we utilized a screening method to search for genes within the OAdV genome that were capable of stimulating the viral E2 promoter. One such gene, E43, was identified within the proposed E4 region toward the right-hand end of the OAdV genome. The E43 gene product was also found to be capable of stimulating E2F-1-dependent gene expression. A closer inspection of the E2 promoter revealed the presence of a non-palindromic E2F binding site within the OAdV E2 promoter. Mutation of this site markedly reduced both E2F-1- and E43-dependent promoter activation. Moreover, a direct protein-protein interaction of the E43 gene product with E2F, but not with the retinoblastoma protein pRb, suggested a possible cooperation between these two proteins in activating the E2 promoter. The importance of the E43 gene product for virus replication is also underlined by the finding that an OAdV recombinant with a functionally inactivated E43 gene showed severely inhibited virus growth.

Construction, rescue, and characterization of vectors derived from ovine atadenovirus

Gene transfer vectors derived from ovine atadenovirus type 7 (OAdV) can efficiently infect a variety of mammalian cells in vitro and in vivo to deliver and express transgenes. However, early OAdV vectors were designed on human mastadenovirus principles prior to the complete characterization of OAdV genes and transcripts. The distinctive arrangement of the OAdV genome has suggested ways to improve OAdV vector design and utility. We therefore developed a cosmid-based approach that allows efficient construction of recombinant ovine atadenovirus genomes in which the transgene is inserted at one of three sites. Viruses were rescued by transfection of viral DNA into a new ovine fetal skin fibroblast producer cell line, HVO156. The suitability of the three insertion sites was compared with respect to virus rescue efficiency, gene expression levels, and genetic stability of the vectors. We found that one vector with a transgene inserted at site 1, between the pVIII and fiber genes, was unstable. Only one vector that carried a transgene at site 2, near the right end of the genome, together with a nearby deletion was rescued. In contrast, several vectors with different transgenes inserted in site 3, between the E4 and RH transcription units, were repeatedly rescued, and these vectors were stable over at least four passages. Transgene orientation in site 3 had only little effect on expression. Finally, a vector carrying a human factor IX cDNA at site 3, when administered intravenously, produced nearly physiological levels of human factor IX in mice. The availability of an efficient method for vector construction and the identification of a new insertion site for virus rescue and gene expression substantially enhance the utility of the OAdV vector system.

In vitro and in vivo comparative study of chimeric liver-specific promoters

Targeting therapeutic genes to the liver is essential to improve gene therapy protocols of hepatic diseases and of some hereditary disorders. Transcriptional targeting can be achieved using liver-specific promoters. In this study we have made chimeric constructs combining promoter and enhancer regions of the albumin, alpha 1-antitrypsin, hepatitis B virus core protein, and hemopexin genes. Tissue specificity, activity, and length of gene expression driven from these chimeric regulatory sequences have been analyzed in cultured cells from hepatic and nonhepatic origin as well as in mice livers and other organs. We have identified a collection of liver-specific promoters whose activities range from twofold to less than 1% of the CMV promoter in human hepatoma cells. We found that the best liver specificity was attained when both enhancer and promoter sequences of hepatic genes were combined. In vivo studies were performed to analyze promoter function during a period of 50 days after gene transfer to the mouse liver. We found that among the various chimeric constructs tested in this work, the alpha1-antitrypsin promoter alone or linked to the albumin or hepatitis B enhancers is the most potent in directing stable gene expression in liver cells.

Transfer of hepatitis B virus genome by adenovirus vectors into cultured cells and mice: crossing the species barrier

For the study of hepatitis B virus infection, no permissive cell line or small animal is available. Stably transfected cell lines and transgenic mice which contain hepadnavirus genomes produce virus, but--unlike in natural infection--from an integrated viral transcription template. To transfer hepadnavirus genomes across the species barrier, we developed adenovirus vectors in which 1.3-fold-overlength human and duck hepatitis B virus genomes were inserted. The adenovirus-mediated genome transfer efficiently initiated hepadnavirus replication from an extrachromosomal template in established cell lines, in primary hepatocytes from various species, and in the livers of mice. Following the transfer, hepatitis B virus proteins, genomic RNA, and all replicative DNA intermediates were detected. Detection of covalently closed circular DNA in hepatoma cell lines and in primary hepatocytes indicated that an intracellular replication cycle independent from the transferred linear viral genome was established. High-titer hepatitis B virions were released into the culture medium of hepatoma cells and the various primary hepatocytes. In addition, infectious virions were secreted into the sera of mice. In conclusion, adenovirus-mediated genome transfer initiated efficient hepatitis B virus replication in cultured liver cells and in the experimental animals from an extrachromosomal template. This will allow development of small-animal systems of hepatitis B virus infection and will facilitate study of pathogenicity of wild-type and mutant viruses as well as of virus-host interaction and new therapeutic approaches.

Time course of gene expression after plasmid DNA gene transfer to the liver

BACKGROUND

High levels of expression in hepatocytes can be achieved after intraportal delivery of plasmid DNA vectors with up to 10% of all liver cells transfected. CMV promoter-driven expression is very high on Day 1 after injection, but is diminished strongly by Day 2. Expression slowly declines after 1 week. We describe experiments aimed at elucidating the reasons for this rapid decline in transgene expression.

METHODS

Histological methods were used to determine the presence and extent of liver damage and hepatocyte proliferation. Viral and liver-specific promoters were tested to study promoter shut-off, Southern blotting was performed to determine the loss of the pDNA vector over time, and several mouse models were used to study the host immunological response.

RESULTS

pDNA is lost rapidly early after injection, but remains at a relatively stable copy number after Day 4. Southern blotting experiments showed that plasmid DNA could be detected for at least 12 weeks after injection (0.2 copies per genome). The early rapid decline of expression is promoter dependent. A liver-specific albumin promoter resulted in similar levels of expression on Days 1 and 7, suggesting that promoter inactivation may be responsible for the instability of CMV promoter-driven expression. The slow decline in expression levels after 1 week appears to be the result of an immune response directed against the expressed transgene. Expression was much prolonged in immunosuppressed, immunodeficient, or antigen-tolerized mice.

CONCLUSION

The present data suggest that if promoter inactivation can be overcome, intravascular delivery of plasmid DNA could be a highly efficient, simple and non-toxic liver gene therapy approach. Intravascular delivery of pDNA allows for the rapid screening of novel expression vectors in vivo.

System for efficient helper-dependent minimal adenovirus construction and rescue

Helper-dependent minimal adenoviral vectors deleted for all viral coding sequences are promising vectors for gene therapy. They retain only the adenovirus cis elements for replication and packaging, can accommodate up to 36 kb of foreign DNA, and exhibit prolonged transgene expression and reduced tissue toxicity as compared with first-generation adenoviral vectors. We have developed a system consisting of a set of cosmid cloning vectors (pMV and pMVX) for simple routine construction and efficient rescue of minimal adenoviral vectors. In the cloning vectors the inverted terminal repeats (ITRs) are flanked by recognition sites for the super rare-cutting endonuclease I-SceI. This allows the release of linear minimal adenovirus genomes for rescue of minimal adenovirus regardless of the sequence of the insert DNA. pMV contains a multiple cloning site for the insertion of 26 to 36 kb of therapeutic DNA. pMVX contains a noncoding human X-chromosomal DNA fragment as a vector backbone, which provides endonuclease restriction sites that allow for complete or partial replacement of the vector backbone by 1 to 26 kb of therapeutic DNA sequences, while retaining a packageable final minimal adenovirus genome size between 27 and 37.5 kb. Both vectors exist in two forms, with or without an Escherichia coli lacZ reporter gene cassette. Several minimal adenoviral vectors with insert sizes ranging from 1.5 to 16 kb were constructed with these cloning vectors. Minimal adenoviruses were efficiently rescued and amplified to high titers, using a Cre/lox-based helper system. Vectors containing the X-chromosomal backbone were stable during amplification. This simple and efficient system facilitates the construction of minimal adenoviruses and should be useful for further improvement of these new vectors.

Factors influencing in vivo transduction by recombinant adeno-associated viral vectors expressing the human factor IX cDNA

Long-term expression of coagulation factor IX (FIX) has been observed in murine and canine models following administration of recombinant adeno-associated viral (rAAV) vectors into either the portal vein or muscle. These studies were designed to evaluate factors that influence rAAV-mediated FIX expression. Stable and persistent human FIX (hFIX) expression (> 22 weeks) was observed from 4 vectors after injection into the portal circulation of immunodeficient mice. The level of expression was dependent on promoter with the highest expression, 10% of physiologic levels, observed with a vector containing the cytomegalovirus (CMV) enhancer/beta-actin promoter complex (CAGG). The kinetics of expression after injection of vector particles into muscle, tail vein, or portal vein were similar with hFIX detectable at 2 weeks and reaching a plateau by 8 weeks. For a given dose, intraportal administration of rAAV CAGG-FIX resulted in a 1.5-fold or 4-fold higher level of hFIX compared to tail vein or intramuscular injections, respectively. Polymerase chain reaction analysis demonstrated predominant localization of the rAAV FIX genome in liver and spleen after tail vein injection with a higher proportion in liver after portal vein injection. Therapeutic levels of hFIX were detected in the majority of immunocompetent mice (21 of 22) following intravenous administration of rAAV vector without the development of anti-hFIX antibodies, but hFIX was not detected in 14 immunocompetent mice following intramuscular administration, irrespective of strain. Instead, neutralizing anti-hFIX antibodies were detected in all the mice. These observations may have important implications for hemophilia B gene therapy with rAAV vectors.

A novel negative cis-regulatory element on the hepatitis B virus S-(+)-strand

Hepatitis B virus (HBV) has a double-stranded DNA genome. The minus-strand contains coding regions for all known HBV proteins and most of the cis-regulatory elements. Little is known about transcription from the S-(+)-strand and its regulation. Thus, the presence of regulatory elements located on the S-(+)-strand was investigated by inserting nt 1038-1783 of HBV in both orientations between the human cytomegalovirus (HCMV) promoter and a luciferase gene. Transfection experiments revealed that the plasmid containing this HBV DNA fragment in an orientation allowing expression from the S-(+)-strand (antisense) led to inhibition of luciferase gene expression compared to the plasmid containing this sequence in an orientation that allows gene expression from the L-(-)-strand (sense). Deletion analyses delimit the sequence essential for the inhibitory effect to a 150 bp region that also carries part of the enhancerII/core promoter complex. However, the possible influence of this regulatory element has been excluded in various experiments. The repressing HBV sequence acts in an orientation- and position-dependent manner; no inhibition was observed when this DNA element was inserted upstream of the HCMV promoter or downstream of the luciferase gene. Northern blot analyses revealed reduced luciferase mRNA steady-state levels in cells transfected with constructs containing the essential HBV sequence in antisense orientation compared to plasmids containing this sequence in sense orientation. Since nuclear run-on experiments showed similar transcription initiation rates with these plasmids, the diminished luciferase mRNA steady-state levels must be due to altered stabilities, suggesting that nt 1783-1638 of HBV encode an RNA-destabilizing element.

Reactivation of the previously silenced cytomegalovirus major immediate-early promoter in the mouse liver: involvement of NFkappaB

The cytomegalovirus (CMV) major immediate-early promoter/enhancer is active in many cell culture systems and is considered to be one of the strongest promoters in vitro. However, when this promoter was used in in vivo approaches to gene therapy, it was silenced within a few weeks in several organs including the liver. In this study, we demonstrated transcriptional inactivation of the CMV promoter in mouse liver. In contrast to the CMV promoter, a hybrid promoter consisting of a minimal CMV promoter and the enhancer II of hepatitis B virus was active for at least 11 weeks in mouse liver. While investigating the reason for the shutdown of the CMV promoter, we did not find evidence for methylation of adenovirus DNA in the region of transgene insertion, but we could show that the silenced CMV promoter was reactivated after lipopolysaccharide treatment of mice or partial hepatectomy. Both stimuli are known to activate the transcription factor NFkappaB, which binds to four sites in the CMV promoter/enhancer. We show that expression from the CMV promoter in hepatocyte-derived cell lines in vitro depends on NFkappaB. In vivo experiments demonstrate that NFkappaB, which is not present in mouse hepatocytes in vivo, is activated after infection with recombinant adenoviruses and that the time course of NFkappaB activation parallels that of CMV promoter-dependent expression. Moreover, adenovirus infection of transgenic mice carrying a CMV promoter-driven lacZ gene leads to strong activation of the expression of this gene in the liver. Thus, NFkappaB is involved in the activation of the CMV promoter in the liver.