Hepatocyte-specific gene expression from integrated lentiviral vectors

BACKGROUND: For many applications, efficient gene therapy will require long-term, organ-specific therapeutic gene expression. Lentiviral vectors based on HIV-1 are promising gene delivery vehicles due to their ability to integrate transgenes into non-dividing cells. Many experimental vectors express transgenes under the control of the cytomegalovirus (CMV) immediate-early gene promoter. Although this promoter directs strong gene expression in vitro, it may be shut off rapidly in vivo. This study explores the potential of HIV-1-based vectors to transduce hepatocytes and compares gene expression from different promoters in integrated vectors. METHODS: HIV-1-based vector plasmids expressing the green fluorescent protein (GFP) under the control of the CMV promoter, the alpha-1 antitrypsin gene promoter or promoters derived from the hepatitis B virus (HBV) genome were used to compare expression in transfected and transduced cell lines. RESULTS: Hepatocyte cell lines differed strikingly in their transfectability. Transduction with replication-deficient HIV-1-based vector particles incorporating the different promoter elements was uniformly effective in hepatocyte and non-hepatocyte lines. However, in hepatocytes, only the CMV, alpha-1 antitrypsin and HBV core but not HBV surface promoters were able to produce GFP expression. Addition of the HBV enhancer 2 element improved the transducing ability of the HBV surface promoter and suppressed expression in non-hepatocytes increasing specificity for hepatocytes. CONCLUSIONS: Integrated lentiviral vectors can be used to direct transgene expression in liver cells both promiscuously and specifically. Promoters derived from the alpha-1 antitrypsin gene or HBV are alternatives to the CMV promoter. Inclusion of the HBV enhancer 2 permits strong liver-specific gene expression in vitro.     

Silencing of fat-1 transgene expression in sheep may result from hypermethylation of its driven cytomegalovirus (CMV) promoter

The fat-1 gene was isolated from roundworm Caenorhabditis elegans, and built into pIRES2-EGFP expression vectors driven by cytomegalovirus (CMV) promoter or cytomegalovirus enhancer and chickenβ-actin (CAG) promoter. Both CMV- and CAG-driven expression vectors were transfected to sheep fetal fibroblast cells. Positive transfected cells were used as donors for somatic cell nuclear transfer (SCNT) and the cloned embryos were transferred into the oviducts of synchronized recipient sheep. Two lambs derived from CMV vector and three lambs derived from CAG vector developed to term. Although Southern analyses using tissues from the two lambs derived from CMV vectors indicated integration of fat-1 gene into the genome, fat-1 mRNAs were not detected by RT-PCR. However, there was fat-1 expression (detected by RT-PCR) in tissues from transgenic lambs driven by CAG vectors. To investigate potential mechanisms involved in the two transgene models, methylation state of the vector promoters were examined. In CMV-driven transgenics, CMV promoters had almost no methylation in transfected cells and the resultant cloned embryos, whereas high methylations were detected in tissues and organs in transgenic lambs. In the CAG-driven transgenics, there were almost no methylations in transgenic cells and transgenic cloned embryos, and cloned lambs expressed fat-1 mRNA (detected by RT-PCR). Moreover, although SV40 promoters which drove neo/kan marker gene in CMV vectors were highly methylated in tissues from transgenic lambs, they were without methylation in cells and embryos. Therefore, we concluded that highly methylated CMV promoters induced the silence of fat-1 transgene expression in sheep. Furthermore, CAG promoter, but not CMV promoter was suitable for generation of fat-1 transgenic sheep.     

Performance of AAV8 vectors expressing human factor IX from a hepatic-selective promoter following intravenous injection into rats

The AAV9 capsid displays a high natural affinity for the heart following a single intravenous (IV) administration in both newborn and adult mice. It also results in substantial albeit relatively lower expression levels in many other tissues. To increase the overall safety of this gene delivery method we sought to identify which one of a group of promoters is able to confer the highest level of cardiac specific expression and concurrently, which is able to provide a broad biodistribution of expression across both cardiac and skeletal muscle. The in vivo behavior of five different promoters was compared: CMV, desmin (Des), alpha-myosin heavy chain (α-MHC), myosin light chain 2 (MLC-2) and cardiac troponin C (cTnC). Following IV administration to newborn mice, LacZ expression was measured by enzyme activity assays. Results showed that rAAV2/9-mediated gene delivery using the α-MHC promoter is effective for focal transgene expression in the heart and the Des promoter is highly suitable for achieving gene expression in cardiac and skeletal muscle following systemic vector administration. Importantly, these promoters provide an added layer of control over transgene activity following systemic gene delivery.     

Comparison of promoter region constructs for in vivo intramuscular expression

BACKGROUND: High transgene expression is generally expected after gene transfer. However, different level, kinetics and localization of expression might be needed for relevant therapeutic applications. Former studies have compared various promoter regions driving gene expression leading to conflicting results. In the present work, two promoter families have been compared using the efficient in vivo intramuscular electrotransfer technique. METHODS: Three promoter regions were constructed by associating the strong ubiquitous cytomegalovirus (CMV) enhancer-promoter to its homologous intron A or to a heterologous intron, or to a hybrid intron. Promoter regions derived from the muscle creatine kinase (MCK) promoter were also studied. The expression of the same transgene (SeAP or neurotrophin-3) under control of these different promoters was compared after plasmid electrotransfer in mouse tibialis-cranialis skeletal muscle. RESULTS: Heterologous intron association to the CMV promoter did not modify gene expression kinetics nor increase gene expression level. Usefulness of intron A or hybrid intron association to the CMV promoter depended on the gene. The various MCK promoters drove efficient gene expression but lower than that obtained with the CMV promoter. Furthermore, peak value was reached earlier with MCK promoter regions (14 days). CONCLUSION: For applications of gene transfer restricted to skeletal muscle, the MCK promoter or a MCK promoter variant would be a promising alternative to the CMV promoter. Indeed, it has been demonstrated that the use of MCK promoter limits humoral and cell-mediated immune responses. Furthermore, the MCK promoter decreases the initial expression peak that may be detrimental, drives a sustained gene expression, and improves gene transfer safety.     

Characterization of promoter function and cell-type-specific expression from viral vectors in the nervous system

Viral vectors have become important tools to effectively transfer genes into terminally differentiated cells, including neurons. However, the rational for selection of the promoter for use in viral vectors remains poorly understood. Comparison of promoters has been complicated by the use of different viral backgrounds, transgenes, and target tissues. Adenoviral vectors were constructed in the same vector background to directly compare three viral promoters, the human cytomegalovirus (CMV) immediate-early promoter, the Rous sarcoma virus (RSV) long terminal repeat, and the adenoviral E1A promoter, driving expression of the Escherichia coli lacZ gene or the gene for the enhanced green fluorescent protein. The temporal patterns, levels of expression, and cytotoxicity from the vectors were analyzed. In sensory neuronal cultures, the CMV promoter produced the highest levels of expression, the RSV promoter produced lower levels, and the E1A promoter produced limited expression. There was no evidence of cytotoxicity produced by the viral vectors. In vivo analyses following stereotaxic injection of the vector into the rat hippocampus demonstrated differences in the cell-type-specific expression from the CMV promoter versus the RSV promoter. In acutely prepared hippocampal brain slices, marked differences in the cell type specificity of expression from the promoters were confirmed. The CMV promoter produced expression in hilar regions and pyramidal neurons, with minimal expression in the dentate gyrus. The RSV promoter produced expression in dentate gyrus neurons. These results demonstrate that the selection of the promoter is critical for the success of the viral vector to express a transgene in specific cell types.     

Rescue and production of vaccine and therapeutic adenovirus vectors expressing inhibitory transgenes

Expression of certain transgenes from an adenovirus vector can be deleterious to its own replication. This can result in the inhibition of virus rescue, reduced viral yields, or, in the worst case, make it impossible to construct a vector expressing the inhibiting transgene product. A gene regulation system based on the tet operon was used to allow the rescue and efficient growth of adenovectors that express transgenes to high levels. A key advantage to this system is that repression of transgene expression is mediated by the packaging cell line, thus, expression of regulatory products from the adenovector are not required. This provides a simple, broadly applicable system wherein transgene repression is constitutive during vector rescue and growth and there is no effect on adenovector-mediated expression of gene products in transduced cells. Several high-level expression vectors based on first- and second-generation adenovectors were rescued and produced to high titer that otherwise could not be grown. Yields of adenovectors expressing inhibitory transgene products were increased, and the overgrowth of cultures by adenovectors with nonfunctional expression cassettes was prevented. The gene regulation system is a significant advancement for the development of adenovirus vectors for vaccine and other gene transfer applications.     

The EF‐1α promoter maintains high‐level transgene expression from episomal vectors in transfected CHO‐K1 cells

In our previous study, we demonstrated that episomal vectors based on the characteristic sequence of matrix attachment regions (MARs) and containing the cytomegalovirus (CMV) promoter allow transgenes to be maintained episomally in Chinese hamster ovary (CHO) cells. However, the transgene expression was unstable and the number of copies was low. In this study, we focused on enhancers, various promoters and promoter variants that could improve the transgene expression stability, expression magnitude (level) and the copy number of a MAR‐based episomal vector in CHO‐K1 cells. In comparison with the CMV promoter, the eukaryotic translation elongation factor 1 α (EF‐1α, gene symbol EEF1A1) promoter increased the transfection efficiency, the transgene expression, the proportion of expression‐positive clones and the copy number of the episomal vector in long‐term culture. By contrast, no significant positive effects were observed with an enhancer, CMV promoter variants or CAG promoter in the episomal vector in long‐term culture. Moreover, the high‐expression clones harbouring the EF‐1α promoter tended to be more stable in long‐term culture, even in the absence of selection pressure. According to these findings, we concluded that the EF‐1α promoter is a potent regulatory sequence for episomal vectors because it maintains high transgene expression, transgene stability and copy number. These results provide valuable information on improvement of transgene stability and the copy number of episomal vectors.     

Recombinant AAV-mediated gene delivery to the central nervous system

Various regions of the brain have been successfully transduced by recombinant adeno-associated virus (rAAV) vectors with no detected toxicity. When using the cytomegalovirus immediate early (CMV) promoter, a gradual decline in the number of transduced cells has been described. In contrast, the use of cellular promoters such as the neuron-specific enolase promoter or hybrid promoters such as the chicken beta-actin/CMV promoter resulted in sustained transgene expression. The cellular tropism of rAAV-mediated gene transfer in the central nervous system (CNS) varies depending on the serotype used. Serotype 2 vectors preferentially transduce neurons whereas rAAV5 and rAAV1 transduce both neurons and glial cells. Recombinant AAV4-mediated gene transfer was inefficient in neurons and glial cells of the striatum (the only structure tested so far) but efficient in ependymal cells. No inflammatory response has been described following rAAV2 administration to the brain. In contrast, antibodies to AAV2 capsid and transgene product were elicited but no reduction of transgene expression was observed and readministration of vector without loss of efficiency was possible from 3 months after the first injection. Based on the success of pioneer work performed with marker genes, various strategies for therapeutic gene delivery were designed. These include enzyme replacement in lysosomal storage diseases, Canavan disease and Parkinson's disease; delivery of neuroprotective factors in Parkinson's disease, Huntington disease, Alzheimer's disease, amyotrophic lateral sclerosis, ischemia and spinal cord injury; as well as modulation of neurotransmission in epilepsy and Parkinson's disease. Several of these strategies have demonstrated promising results in relevant animal models. However, their implementation in the clinics will probably require a tight regulation and a specific targeting of therapeutic gene expression which still demands further developments of the vectors.     

Secretion of a TNFR:Fc fusion protein following pulmonary administration of pseudotyped adeno-associated virus vectors

This study evaluated and compared delivery of the tumor necrosis factor alpha receptor (TNFR)-immunoglobulin G1 (IgG1) Fc fusion (TNFR:Fc) gene to the lung by single and repeat administrations of multiple pseudotyped adeno-associated virus (AAV) vectors as a means for achieving systemic distribution of the soluble TNFR:Fc protein. A single endotracheal administration of AAV[2/5]cytomegalovirus (CMV)-TNFR:Fc vector (containing the AAV2 inverted terminal repeats and AAV5 capsid) to the rat lung resulted in long-term, high levels of serum TNFR:Fc protein that gradually declined over a period of 8 months. Endotracheal delivery of AAV[2/1]CMV-TNFR:Fc resulted in serum TNFR:Fc protein levels that were detectable for at least 4 months but were 10-fold lower than that of the AAV[2/5] vector. In contrast, secretion of the TNFR:Fc protein following pulmonary delivery of AAV[2/2]CMV-TNFR:Fc vector was very inefficient, and the protein was detected in the blood only when an airway epithelial cell-specific promoter, CC10, was substituted for the CMV enhancer/promoter to control transgene expression. In the context of AAV[2/5], the CC10 promoter was as efficient as CMV enhancer/promoter in generating similar levels of systemic TNFR:Fc protein, suggesting that this protein is secreted primarily from the airway epithelium. In mice, comparable long-term secretion of TNFR:Fc protein was demonstrated after AAV[2/2] and AAV[2/5] delivery, although the kinetics of transduction appeared to be different. All pseudotyped AAV vectors elicited serum anti-AAV capsid-neutralizing antibody responses, but these did not prevent lung transduction and efficient secretion of TNFR:Fc protein to the circulation following readministration with AAV[2/5]. These results highlight the potential utility of AAV vectors containing serotype 5 capsid to deliver and redeliver genes of secreted proteins to the lung to achieve long-term systemic protein expression.     

Improved neuronal transgene expression from an AAV-2 vector with a hybrid CMV enhancer/PDGF-beta promoter

BACKGROUND: Adeno-associated virus type 2 (AAV-2) vectors are highly promising tools for gene therapy of neurological disorders. After accommodating a cellular promoter, AAV-2 vectors are able to drive sustained expression of transgene in the brain. This study aimed to develop AAV-2 vectors that also facilitate a high level of neuronal expression by enhancing the strength of a neuron-specific promoter, the human platelet-derived growth factor beta-chain (PDGF) promoter. METHODS AND RESULTS: A hybrid promoter approach was adopted to fuse the enhancer of human cytomegalovirus immediately early (CMV) promoter to the PDGF promoter. In cultured cortex neurons, AAV-2 vectors containing the hybrid promoter augmented transgene expression up to 20-fold over that mediated by titer-matched AAV-2 vectors with the PDGF promoter alone and 4-fold over the CMV enhancer/promoter. Injection of AAV-2 vectors with the hybrid promoter into the rat striatum resulted in neuron-specific transgene expression, the level of which was about 10-fold higher than those provided by the two control AAV-2 expression cassettes at 4 weeks post-injection and maintained for at least 12 weeks. Gene expression in the substantia nigra through possible retrograde transport of the AAV-2 vectors injected into the striatum was not obvious. After direct injection of AAV-2 vectors into the substantia nigra, transgene expression driven by the hybrid promoter was observed specifically in dopaminergic neurons and its level was about 3 and 17 times higher than that provided by the PDGF promoter alone and the CMV enhancer/promoter, respectively. CONCLUSIONS: Enhanced transgene capacity plus neuron-specificity of the AAV-2 vectors developed in this study might prove valuable for gene therapy of Parkinson's disease.     

Utility of intraperitoneal administration as a route of AAV serotype 5 vector-mediated neonatal gene transfer

BACKGROUND: Gene transfer into a fetus or neonate can be a fundamental approach for treating genetic diseases, particularly disorders that have irreversible manifestations in adulthood. Although the potential utility of this technique has been suggested, the advantages of neonatal gene transfer have not been widely investigated. Here, we tested the usefulness of neonatal gene transfer using adeno-associated virus (AAV) vectors by comparing the administration routes and vector doses. METHODS: To determine the optimal administration route, neonates were subjected to intravenous (i.v.) or intraperitoneal (i.p.) injections of AAV5-based vectors encoding the human coagulation factor IX (hfIX) gene, and the dose response was examined. To determine the distribution of transgene expression, vectors encoding lacZ or luciferase (luc) genes were used and assessed by X-gal staining and in vivo imaging, respectively. After the observation period, the vector distribution across tissues was quantified. RESULTS: The factor IX concentration was higher in i.p.-injected mice than in i.v.-injected mice. All transgenes administered by i.p. injection were more efficiently expressed in neonates than in adults. The expression was confined to the peritoneal tissue. Interestingly, a sex-related difference was observed in transgene expression in adults, whereas this difference was not apparent in neonates. CONCLUSIONS: AAV vector administration to neonates using the i.p. route was clearly advantageous in obtaining robust transgene expression. Vector genomes and transgene expression were observed mainly in the peritoneal tissue. These findings indicate the advantages of neonatal gene therapy and would help in designing strategies for gene therapy using AAV vectors.     

Immune responses following salivary gland administration of recombinant adeno-associated virus serotype 2 vectors

BACKGROUND: Gene transfer to salivary glands (SGs) can be accomplished in a minimally invasive manner, resulting in stable, long-term secretion of the transgene product. Therefore, SGs provide a novel target site for several potentially useful clinical gene therapeutics applications. Previous studies have indicated that intravenous, intramuscular and intranasal administration of recombinant adeno-associated virus serotype 2 (rAAV2) vectors induce host immune responses. There are no reported studies on immune responsiveness of rAAV2 vector administration to SGs. MATERIAL AND METHODS: Vectors were administered by retrograde infusion to the SGs of Balb/c mice in various combinations. Thereafter, transgene expression was determined, and evaluations of host innate and adaptive immune responsiveness performed over a 56-day period. RESULTS: Histological examination of SGs from vector-treated mice showed no significant changes in appearance from controls, including the frequency of activated macrophage detection. There were also no differences in salivary flow rates among experimental groups. In vitro stimulation of splenocytes from mice administered rAAV2 showed elevated interferon-gamma levels in culture media. Significant titers of neutralizing antibodies to rAAV2 were detected in serum of mice following rAAV2 vector administration. While SGs could be transduced with low doses of vector it was not possible to repeat the administration and detect transduction with the same serotype at low doses. However, repeat administration was possible with an alternative serotype (rAAV4). CONCLUSIONS: Following a single administration of rAAV2 vectors to SGs there is no significant innate immune response. However, rAAV2 vector administration to SGs results in both cellular and humoral immune responses. The latter may interfere with the efficacy of repeated rAAV2 vector administration.     

Development of photoreceptor-specific promoters and their utility to investigate EIAV lentiviral vector mediated gene transfer to photoreceptors

BACKGROUND: We wanted to investigate the ability of recombinant equine infectious anemia virus (EIAV) vectors to transduce photoreceptor cells by developing a series of photoreceptor-specific promoters that drive strong gene expression in photoreceptor cells. METHODS: Promoter fragments derived from the rhodopsin (RHO), the beta phosphodiesterase (PDE) and the retinitis pigmentosa (RP1) genes were cloned in combination with an enhancer element, derived from the interphotoreceptor retinoid-binding protein gene (IRBP), into luciferase reporter plasmids. An in vitro transient reporter assay was carried out in the human Y-79 retinoblastoma cell line. The optimal promoters from this screen were then cloned into the recombinant EIAV vector for evaluation in vivo following subretinal delivery into mice. RESULTS: All promoters maintained a photoreceptor-specific expression profile in vitro and the gene expression was further enhanced in combination with the IRBP enhancer. The use of IRBP-combined RHO or PDE promoters showed modest but exclusive expression in photoreceptors following subretinal delivery to mice. By contrast an EIAV vector containing the cytomegalovirus (CMV) promoter drove reporter gene expression in both photoreceptors and retinal pigment epithelium. CONCLUSIONS: It may be possible to use recombinant EIAV vectors containing photoreceptor-specific promoters to drive therapeutic gene expression to treat a range of retinal degenerative diseases where the photoreceptor cell is the primary disease target.     

Stable and efficient intraocular gene transfer using pseudotyped EIAV lentiviral vectors

BACKGROUND: We have developed minimal non-primate lentiviral vectors based on the equine infectious anaemia virus (EIAV). We evaluated the in vivo expression profiles of these vectors delivered regionally to ocular tissues to define their potential utility in ocular gene therapy. METHODS: EIAV vectors pseudotyped with VSV-G or rabies-G envelope proteins were delivered subretinally, intravitreally or into the anterior chambers (intracameral administration) in mice. Reporter gene (eGFP) expression was analysed using in vivo retinal imaging or histological examination of eyes and brains at intervals between 3 days and 16 months. We investigated the effects of vector titre, pseudotype, genome configuration, site of intraocular administration, intentional retinal trauma and the degree of retinal maturation on the spatial and temporal expression profiles of these vectors. RESULTS: Subretinal vector delivery resulted in efficient and stable transduction of retinal pigment epithelial (RPE) cells and variable transduction of photoreceptors up to 16 months post-injection. Retinal trauma facilitated the local transduction of neurosensory retinal cells. Intracameral administration of VSV-G- but not rabies-G-pseudotyped vectors produced stable eGFP expression in corneal endothelial cells and trabecular meshwork. CONCLUSIONS: The cellular tropism and expression kinetics of optimised EIAV vectors after intraocular administration make them attractive vehicles for delivering therapeutic genes in the management of inherited and acquired retinal and anterior segment disorders.     

Development of multiple cloning site cis-vectors for recombinant adeno-associated virus production

Recombinant adeno-associated virus (rAAV) has become a very popular gene therapy vector in the past several years. A cis-plasmid is used to generate the rAAV stocks. In this plasmid, the entire expression cassette is incorporated between two AAV inverted terminal repeats. The construction of cis-plasmid has been problematic because of the high-frequency recombination of the viral inverted terminal repeats. Here we describe the design and construction of several multiple cloning site cis-plasmids that are driven by five different promoters, including the ubiquitous cytomegalovirus enhancer/chicken beta-actin (CAG), cytomegalovirus (CMV), rous sarcoma virus (RSV), simian virus 40 (SV40), and a muscle-specific promoter (CK6). The application of these multiple cloning site cis-plasmids improves the cloning efficiency. As an example of the utilization of these multiple cloning site vectors, the prokaryotic beta-galactosidase cDNA was cloned in the multiple cloning site cis-plasmids. High-level rAAV-mediated beta-galactosidase expression was achieved in HeLa cells from CAG, CMV, RSV and SV40 promoters, respectively, but notfrom the CK6 promoter. In vivo application in the adult mdx mouse (mouse model for Duchenne muscular dystrophy) muscle revealed efficient transgene expression from CMV and CK6 promoters, followed by CAG and RSV promoters. The SV40 promoter was the least efficient.