Improvements in adenoviral vector technology: overcoming barriers for gene therapy

☆DNA assembly technique simplifies the construction of infectious clone of fowl adenovirus

Plasmid bearing adenovirus genome is generally constructed with the method of homologous recombination in E. coli BJ5183 strain. Here, we utilized Gibson gene assembly technique to generate infectious clone of fowl adenovirus 4 (FAdV-4). Primers flanked with partial inverted terminal repeat (ITR) sequence of FAdV-4 were synthesized to amplify a plasmid backbone containing kanamycin-resistant gene and pBR322 origin (KAN-ORI). DNA assembly was carried out by combining the KAN-ORI fragment, virus genomic DNA and DNA assembly master mix. E. coli competent cells were transformed with the assembled product, and plasmids (pKFAV4) were extracted and confirmed to contain viral genome by restriction analysis and sequencing. Virus was successfully rescued from linear pKFAV4-transfected chicken LMH cells. This approach was further verified in cloning of human adenovirus 5 genome. Our results indicated that DNA assembly technique simplified the construction of infectious clone of adenovirus, suggesting its possible application in virus traditional or reverse genetics.

Gene therapy for Mucopolysaccharidoses

Mucopolysaccharidoses (MPS) are a group of lysosomal storage disorders (LSDs) caused by a deficiency of lysosomal enzymes, leading to a wide range of various clinical symptoms depending upon the type of MPS or its severity. Enzyme replacement therapy (ERT), hematopoietic stem cell transplantation (HSCT), substrate reduction therapy (SRT), and various surgical procedures are currently available for patients with MPS. However, there is no curative treatment for this group of disorders. Gene therapy should be a one-time permanent therapy, repairing the cause of enzyme deficiency. Preclinical studies of gene therapy for MPS have been developed over the past three decades. Currently, clinical trials of gene therapy for some types of MPS are ongoing in the United States, some European countries, and Australia. Here, in this review, we summarize the development of gene therapy for MPS in preclinical and clinical trials.

Recent Advancements in Gene Therapy for Hereditary Retinal Dystrophies

Hereditary retinal dystrophies (HRDs) are degenerative diseases of the retina which have marked clinical and genetic heterogeneity. Common presentations among these disorders include night or colour blindness, tunnel vision, and subsequent progression to complete blindness. The known causative disease genes have a variety of developmental and functional roles, with mutations in more than 120 genes shown to be responsible for the phenotypes. In addition, mutations within the same gene have been shown to cause different disease phenotypes, even amongst affected individuals within the same family, highlighting further levels of complexity. The known disease genes encode proteins involved in retinal cellular structures, phototransduction, the visual cycle, and photoreceptor structure or gene regulation. Significant advancements have been made in understanding the genetic pathogenesis of ocular diseases, and gene replacement and gene silencing have been proposed as potentially efficacious therapies. Because of its favorable anatomical and immunological characteristics, the eye has been at the forefront of translational gene therapy. Recent improvements have been made in the safety and specificity of vector-based ocular gene transfer methods. Dozens of promising proofs of concept have been obtained in animal models of HRDs and some of them have been relayed to the clinic. The results from the first clinical trials for a congenital form of blindness have generated great interest and have demonstrated the safety and efficacy of intraocular administrations of viral vectors in humans. This review summarizes the clinical development of retinal gene therapy.

Transient blocking of NK cell function with small molecule inhibitors for helper dependant adenoviral vector-mediated gene delivery

One major challenge in gene therapy is the host immune responses against viral vectors. Previous studies indicate the involvement of NK cells in stunted gene expression in viral vector mediated gene therapy. To understand the problem of the immune responses, we have developed an in-vitro co-culture system with human NK cell line, macrophages and airway epithelial cells. We showed that small molecule blockers, CAPE and ruxolitinib, for NF-κB and JAK-STAT pathways, respectively, significantly inhibited cytokine secretion by macrophages. When NK cells are co-cultured with helper-dependent adenoviral (HD-Ad) vector activated macrophages, IFN-γ cytokine expression by NK cells increased significantly, which was inhibited effectively by ruxolitinib and CAPE, and there was an additive effect when both inhibitors were used. We demonstrated that NK cells activated by cytokines produced by HD-Ad-activated macrophages kill HD-Ad vector transduced bronchial epithelial cells. This cell killing activity was significantly reduced by CAPE and ruxolitinib. Combination of these two inhibitors had an additive effect on inhibiting NK cell mediate killing of gene transduced cells. Transient inhibition of NK cell response at its peak may enhance sustained gene expression. Our data suggest that combination of CAPE and ruxolitinib may help in protecting gene transduced airway epithelial cells to prolong transgene expression.

The adenovirus genome contributes to the structural stability of the virion

Adenovirus (Ad) vectors are currently the most commonly used platform for therapeutic gene delivery in human gene therapy clinical trials. Although these vectors are effective, many researchers seek to further improve the safety and efficacy of Ad-based vectors through detailed characterization of basic Ad biology relevant to its function as a vector system. Most Ad vectors are deleted of key, or all, viral protein coding sequences, which functions to not only prevent virus replication but also increase the cloning capacity of the vector for foreign DNA. However, radical modifications to the genome size significantly decreases virion stability, suggesting that the virus genome plays a role in maintaining the physical stability of the Ad virion. Indeed, a similar relationship between genome size and virion stability has been noted for many viruses. This review discusses the impact of the genome size on Ad virion stability and emphasizes the need to consider this aspect of virus biology in Ad-based vector design.

Lung gene therapy-How to capture illumination from the light already present in the tunnel

Gene therapy has been considered as the most ideal medical intervention for genetic diseases because it is intended to target the cause of diseases instead of disease symptoms. Availability of techniques for identification of genetic mutations and for in vitro manipulation of genes makes it practical and attractive. After the initial hype in 1990s and later disappointments in clinical trials for more than a decade, light has finally come into the tunnel in recent years, especially in the field of eye gene therapy where it has taken big strides. Clinical trials in gene therapy for retinal degenerative diseases such as Leber's congenital amaurosis (LCA) and choroideremia demonstrated clear therapeutic efficacies without apparent side effects. Although these successful examples are still rare and sporadic in the field, they provide the proof of concept for harnessing the power of gene therapy to treat genetic diseases and to modernize our medication. In addition, those success stories illuminate the path for the development of gene therapy treating other genetic diseases. Because of the differences in target organs and cells, distinct barriers to gene delivery exist in gene therapy for each genetic disease. It is not feasible for authors to review the current development in the entire field. Thus, in this article, we will focus on what we can learn from the current success in gene therapy for retinal degenerative diseases to speed up the gene therapy development for lung diseases, such as cystic fibrosis.

Prolonged prophylactic protection from botulism with a single adenovirus treatment promoting serum expression of a VHH-based antitoxin protein

Current therapies for most acute toxin exposures are limited to administration of polyclonal antitoxin serum. We have shown that VHH-based neutralizing agents (VNAs) consisting of two or more linked, toxin-neutralizing heavy-chain-only V_H domains (VHHs), each binding distinct epitopes, can potently protect animals from lethality in several intoxication models including Botulinum neurotoxin serotype A1 (BoNT/A1). Appending a 14 amino acid albumin binding peptide (ABP) to an anti-BoNT/A1 heterodimeric VNA (H7/B5) substantially improved serum stability and resulted in an effective VNA serum half-life of 1 to 2 days. A recombinant, replication-incompetent, adenoviral vector (Ad/VNA-BoNTA) was engineered that induces secretion of biologically active VNA, H7/B5/ABP (VNA-BoNTA), from transduced cells. Mice administered a single dose of Ad/VNA-BoNTA, or a different Ad/VNA, via different administration routes led to a wide range of VNA serum levels measured four days later; generally intravenous > intraperitoneal > intramuscular > subcutaneous. Ad/VNA-BoNTA treated mice were 100% protected from 10 LD₅₀ of BoNT/A1 for more than six weeks and protection positively correlated with serum levels of VNA-BoNTA exceeding about 5 ng/ml. Some mice developed antibodies that inhibited VNA binding to target but these mice displayed no evidence of kidney damage due to deposition of immune complexes. Mice were also successfully protected from 10 LD₅₀ BoNT/A1 when Ad/VNA-BoNTA was administered up to 1.5 hours post-intoxication, demonstrating rapid appearance of the protective VNA in serum following treatment. Genetic delivery of VNAs promises to be an effective method of providing prophylactic protection and/or acute treatments for many toxin-mediated diseases.

The role of chromatin in adenoviral vector function

Vectors based on adenovirus (Ad) are one of the most commonly utilized platforms for gene delivery to cells in molecular biology studies and in gene therapy applications. Ad is also the most popular vector system in human clinical gene therapy trials, largely due to its advantageous characteristics such as high cloning capacity (up to 36 kb), ability to infect a wide variety of cell types and tissues, and relative safety due to it remaining episomal in transduced cells. The latest generation of Ad vectors, helper‑dependent Ad (hdAd), which are devoid of all viral protein coding sequences, can mediate high-level expression of a transgene for years in a variety of species ranging from rodents to non-human primates. Given the importance of histones and chromatin in modulating gene expression within the host cell, it is not surprising that Ad, a nuclear virus, also utilizes these proteins to protect the genome and modulate virus- or vector‑encoded genes. In this review, we will discuss our current understanding of the contribution of chromatin to Ad vector function.

Viral vectors for gene delivery to the central nervous system

The potential benefits of gene therapy for neurological diseases such as Parkinson's, Amyotrophic Lateral Sclerosis (ALS), Epilepsy, and Alzheimer's are enormous. Even a delay in the onset of severe symptoms would be invaluable to patients suffering from these and other diseases. Significant effort has been placed in developing vectors capable of delivering therapeutic genes to the CNS in order to treat neurological disorders. At the forefront of potential vectors, viral systems have evolved to efficiently deliver their genetic material to a cell. The biology of different viruses offers unique solutions to the challenges of gene therapy, such as cell targeting, transgene expression and vector production. It is important to consider the natural biology of a vector when deciding whether it will be the most effective for a specific therapeutic function. In this review, we outline desired features of the ideal vector for gene delivery to the CNS and discuss how well available viral vectors compare to this model. Adeno-associated virus, retrovirus, adenovirus and herpesvirus vectors are covered. Focus is placed on features of the natural biology that have made these viruses effective tools for gene delivery with emphasis on their application in the CNS. Our goal is to provide insight into features of the optimal vector and which viral vectors can provide these features.

Induction of immunological tolerance to adenoviral vectors by using a novel dendritic cell-based strategy

The success of helper-dependent adenoviral (HD-Ad) vector-mediated lung gene therapy is hampered by the host immune response, which limits pulmonary transgene expression following multiple rounds of vector readminstration. Here, we show that HD-Ad-mediated pulmonary gene expression is sustained even upon three rounds of readministration to immunodeficient mice, highlighting the need to suppress the adaptive immune response for sustained gene expression following vector readministration. Therefore, we devised a dendritic cell (DC)-based strategy for induction of immunological tolerance toward HD-Ad vectors. DCs derived in the presence of interleukin-10 (IL-10) are refractory to HD-Ad-induced maturation and instead facilitate generation of IL-10-producing Tr1 regulatory T cells which suppress HD-Ad-induced T cell proliferation. Delivery of HD-Ad-pulsed, IL-10-modified DCs to mice induces long-lasting immunological tolerance to HD-Ad vectors, whereby pulmonary DC maturation, the T cell response, and antibody response to HD-Ad vectors are suppressed even after three rounds of pulmonary HD-Ad readministration. Moreover, sustained transgene expression is also observed in the lungs of mice immunized with HD-Ad-pulsed, IL-10-modified DCs even after three rounds of pulmonary HD-Ad delivery. Taken together, these studies identify the use of DCs generated in the presence of IL-10 as a novel strategy to induce long-lasting immune tolerance to HD-Ad vectors.

Gene therapy: light is finally in the tunnel

After two decades of ups and downs, gene therapy has recently achieved a milestone in treating patients with Leber's congenital amaurosis (LCA). LCA is a group of inherited blinding diseases with retinal degeneration and severe vision loss in early infancy. Mutations in several genes, including RPE65, cause the disease. Using adeno-associated virus as a vector, three independent teams of investigators have recently shown that RPE65 can be delivered to retinal pigment epithelial cells of LCA patients by subretinal injections resulting in clinical benefits without side effects. However, considering the whole field of gene therapy, there are still major obstacles to clinical applications for other diseases. These obstacles include innate and immune barriers to vector delivery, toxicity of vectors and the lack of sustained therapeutic gene expression. Therefore, new strategies are needed to overcome these hurdles for achieving safe and effective gene therapy. In this article, we shall review the major advancements over the past two decades and, using lung gene therapy as an example, discuss the current obstacles and possible solutions to provide a roadmap for future gene therapy research.

Development of a suspension serum-free helper-dependent adenovirus production system and assessment of co-infection conditions

Helper-dependent adenovirus (HDAd), deleted in all viral protein-coding sequences has been designed to reduce immune response and favor long-term expression of therapeutic genes in clinical programs. Its production requires co-infection of E1-complementing cells with helper adenovirus (HAd). Significant progresses have been made in the molecular design of HDAd, but large scale production remains a challenge. In this work, a scalable system for HDAd production is designed and evaluated focusing on the co-infection step. A human embryo kidney 293 (293) derived cell line, the 293SF/FLPe was generated to produce efficiently HDAd while restricting the packaging of HAd. This cell line was adapted to grow in suspension and in serum-free medium. Multiplicity of infection (MOI) of HDAd ranging from 0.1 to 50 was evaluated in presence of HAd at a MOI of 5. Optimal MOIs for HDAd amplification were found in the range of 5-10. HAd contamination was only 1%. These results were validated in a 3 L bioreactor under controlled operating conditions where a higher HDAd yield of 2.6 x 10(9) viral particles (VP)/mL or 3.5 x 10(8) infectious units (IU)/mL of HDAd was obtained.

Preclinical development of gene therapy for Parkinson's disease

Multiple targets and pathways may be amenable to the development of gene therapy approaches for Parkinson's disease. This article discusses some of the cellular and brain circuit pathways relevant to Parkinson's disease that would be clinically amenable to gene therapy. Approaches could be classified according to two main categories, i.e. symptomatic vs. neuroprotective/neurorestorative strategies. Examples of the different possibilities currently in development are given and feature both dopaminergic and non-dopaminergic symptomatic treatments of parkinsonian symptoms and/or L-DOPA-induced side effects, anti-apoptotic neuroprotective strategies and growth-factor delivery for neuroprotection/neurorestoration. While gene therapy has been mostly used so far for enhancing the expression of the target gene, the use of dominant negative or siRNA opens new possibilities. This, combined with the key feature of gene delivery that offers access to intracellular signalling pathways, is likely to further expand the number of proposed targets to be studied.

Viral Vector–mediated and Cell-based Therapies for Treatment of Cystic Fibrosis

Gene and cell-based therapies are considered to be potentially powerful new approaches for the management of cystic fibrosis (CF) lung disease. Despite tremendous efforts that have been made, especially in studies to understand the obstacles to gene delivery, major challenges to the application of these approaches remain to be solved. This article will review the advancements made and challenges remaining in the development of viral vector-mediated and cell-based approaches to treat patients with CF.

Gene transfer therapy for neurodegenerative disorders

Recent advances in gene transfer technology have led to promising new therapies for neurodegenerative disorders. This article will review methods of gene transfer therapy and applications of these techniques to both genetic and sporadic neurodegenerative illnesses. The article will focus on Parkinson's disease, Huntington's disease, and Alzheimer's disease. Several promising gene therapy approaches to these diseases are being pursued both in animal models and in early human trials. Initial safety-tolerability results from these trials appear promising. It is therefore likely that the number of human trials of gene therapy for neurodegenerative disorders will increase over the coming years.

Analysis of adenovirus gene transfer into adult neural stem cells

Adult neural stem cells (aNSCs) represent an attractive source for the production of specific types of neurons in degenerative CNS diseases and for the development of new regenerative gene therapies. However, the use of adult NSCs for transplantation and gene replacement strategies requires efficient gene expression in the cells. Due to the low pathogenicity of adenovirus (Ad) for humans, its large delivery capacity, and long-term transgene expression, Ad vectors are widely used. Here, we tested the potential of the Ad vector system to transduce adult NSCs. Analysis of Ad receptor expression in primary aNSCs revealed a complete lack of the coxsackie-adenovirus receptor and no or low expression of alphanu- and beta5-integrins, respectively, on mRNA and protein level. Consistently, transduction at different multiplicities of infection using an Ad vector expressing the enhanced green fluorescent protein (GFP) showed that adult NSCs are particularly resistant to Ad infection even at highest MOI (1000) in contrast to differentiated types of neural cells.

A complementation method for functional analysis of mammalian genes

Our progress in understanding mammalian gene function has lagged behind that of gene identification. New methods for mammalian gene functional analysis are needed to accelerate the process. In yeast, the powerful genetic shuffle system allows deletion of any chromosomal gene by homologous recombination and episomal expression of a mutant allele in the same cell. Here, we report a method for mammalian cells, which employs a helper-dependent adenoviral (HD-Ad) vector to synthesize small hairpin (sh) RNAs to knock-down the expression of an endogenous gene by targeting untranslated regions (UTRs). The vector simultaneously expresses an exogenous version of the same gene (wild-type or mutant allele) lacking the UTRs for functional analysis. We demonstrated the utility of the method by using PRPF3, which encodes the human RNA splicing factor Hprp3p. Recently, missense mutations in PRPF3 were found to cause autosomal-dominant Retinitis Pigmentosa, a form of genetic eye diseases affecting the retina. We knocked-down endogenous PRPF3 in multiple cell lines and rescued the phenotype (cell death) with exogenous PRPF3 cDNA, thereby creating a genetic complementation method. Because Ad vectors can efficiently transduce a wide variety of cell types, and many tissues in vivo, this method could have a wide application for gene function studies.

Down-regulation of IL-8 expression in human airway epithelial cells through helper-dependent adenoviral-mediated RNA interference

Interleukin (IL)-8 is a potent neutrophil chemotactic factor and a crucial mediator in neutrophil-dependent inflammation. Various cell types produce IL-8, either in response to external stimuli such as cytokines or bacterial infection, or after malignant transformation. Anti-IL-8 strategies have been considered for anti-inflammatory therapy. In this paper we demonstrate that the RNA interference technique can be used to efficiently down-regulate IL-8 protein expression in airway epithelial cells. We used a helper-dependent adenoviral vector to express a small hairpin (sh)RNA targeting human IL-8 in cultured airway epithelial cells (IB3-1, Cftr-/-; C38, Cftr-corrected) stimulated with TNF-alpha, IL-1beta or heat-inactivated Burkholderia cenocepacia. Stimulated IL-8 expression in IB3-1 and C38 cells was significantly reduced by shRNA expression. The shRNA targeting IL-8 had no effect on the activation of NF-kappaB, or on the protein levels of IkappaB or IL-6, suggesting that this anti-IL-8 strategy was highly specific, and therefore may offer potential for the treatment of inflammatory diseases.

Cellular vehicles for cancer gene therapy: current status and future potential

Cancer is a difficult target for any therapeutic strategy; therefore, there is a continuous search for new therapeutic modalities, for application either alone or in combination. In this regard, gene-based therapy is a new approach that offers hope of improved control of tumors. Intensive research to apply gene therapy for cancer treatment has led to identification of the most important technical and theoretical barriers that need to be overcome for clinical success. One of the central unresolved challenges remains the issue of specific and efficient delivery of genes to target cells or tissues, emphasizing the importance of the gene carrier. Along with different viral and non-viral vector systems, mammalian cells have also been considered as vehicles for delivery of anti-cancer therapeutics. The cell-based delivery approach was introduced as the first attempt to apply gene therapy to cancer treatment, and in general, has followed most of the ups and downs of gene therapy applications, progressing alongside new knowledge gained in this field. As a result, significant progress has been made in some aspects of the cell-based approach, while the development of other essential issues is only just gaining speed. It appears that the initial phase of development of cell-based protocols - the achievement of efficient ex vivo cell loading with therapeutics - has largely been fulfilled. However, the desired efficacy of cell-based strategies in general has not yet been reached, and specificity of tumor homing needs to be improved considerably. There is hope that advances in related scientific fields will promote the utilization of cells as powerful and versatile vehicles for cancer gene therapy.

Helper-Dependent Adenoviral Vectors for Gene Therapy

Helper-dependent adenoviral vectors possess a number of characteristics that make them attractive gene therapy vectors. These vectors are completely devoid of viral coding sequences and are able to mediate high-efficiency transduction in vivo to direct sustain high-level transgene expression with negligible chronic toxicity. This review focuses on advances in helper-dependent adenoviral vector technology, selected examples of in vivo studies of particular interest, and the issue of vector-mediated acute toxicity.

Adenoviral Vectors for Gene Replacement Therapy

Adenovirus-based vectors are promising vehicles for gene replacement therapy due to their ability to efficiently transduce a wide variety of proliferating and non-proliferating cells. Over the past decade, different versions of adenoviral vectors (Ads) have been developed. These vectors can be classified into two major categories, based on whether the viral coding sequences are partially (first or second-generation Ads) or completely deleted (helper-dependent or gutted Ads). Both types of Ads have been tested in a variety of gene delivery studies, and major obstacles to their clinical application have been identified. Currently, innate and adaptive host immune responses to Ads remain major challenges, limiting both the initial viral dose and the effectiveness of subsequent administrations. Recent developments in vector design and delivery methods have improved the potential of Ads for successful gene therapy application.

Gene transfer in cardiac myocytes

Congestive heart failure (CHF) represents an enormous clinical problem and remains a leading cause of death despite advances in treatment. New treatments significantly impact mortality and disease course; they do not cure the underlying pathology. Gene transfer, the ability to genetically reprogram the heart in relevant cardiovascular disease models, allows testing the role of specific molecular pathways in disease pathogenesis. Potential therapeutic intervention targets can be then identified and approached with the full spectrum of therapeutic options, including traditional pharmacology, targeted synthesis of small molecule agonists or antagonists, biological agents (cells, antibodies, genetic material), or gene-based therapy. Lessons gleaned from gene transfer experiments on local modulation of cardiac genetic programs will guide attempts to transform early investigations into established therapy.

Helper-dependent adenoviral vectors mediate therapeutic factor VIII expression for several months with minimal accompanying toxicity in a canine model of severe hemophilia A

Two helper-dependent (HD) adenoviral vectors encoding a canine factor VIII B-domain–deleted transgene (cFVIII) were constructed and evaluated in 4 hemophilia A dogs. One vector was regulated by the cytomegalovirus (CMV) promoter (HD-CMV-cFVIII), while the other vector contained a tissue-restricted promoter comprised of the human FVIII proximal promoter with an upstream concatemer of 5 hepatocyte nuclear factor 1 binding sites (HD-HNF-cFVIII). We detected no toxicity at low dose (5 × 1011 vp/kg), but at higher vector doses (> 1 × 1012 vp/kg) transient hepatotoxicity and thrombocytopenia were observed. Low-level increases in FVIII activity were detected in all 3 HD-HNF-cFVIII–treated dogs, which corresponded with decreased whole blood clotting times. None of the animals receiving the HD-HNF-cFVIII vector developed FVIII inhibitors, and in 1 of the 3 animals, FVIII activity was sustained for over 6 months after treatment. One animal, which received the HD-CMV-cFVIII vector, achieved peak levels of FVIII above 19 000 mU/mL, but FVIII activity disappeared within 1 week, coincident with the development of a potent anti–canine FVIII antibody response. This study supports previous demonstrations of improved safety using HD gene transfer and suggests that these vectors can provide transient FVIII expression with minimal, acute toxicity in the absence of inhibitor formation.

Invasive drug delivery

The central nervous system is a very attractive target for new therapeutic strategies since many genes involved in neurological diseases are known and often only local low level gene expression is required. However, as the blood brain barrier on one hand prevents some therapeutic agents given systematically from exerting their activity in the CNS, it also provides an immune privileged environment. Neurosurgical technology meanwhile allows the access of nearly every single centre of the CNS and provides the surgical tool for direct gene delivery via minimal invasive surgical approaches to the brain. Successful therapy of the central nervous system requires new tools for delivery of therapeutics in vitro and in vivo (Fig. 1). The application of therapeutic proteins via pumps into the CSF was shown to be only of limited value since the protein mostly is not sufficiently transported within the tissue and the half life of proteins limits the therapeutic success. Direct gene delivery into the host cell has been a main strategy for years, and in the beginning the direct DNA delivery or encapsulation in liposomes or other artificial encapsulation have been applied with different success. For several years the most promising tools have been vectors based on viruses. Viruses are able to use the host cell machinery for protein synthesis, and some of them are able to stably insert into the host cell genome and provide long term transgene expression as long as the cell is alive. The increasing knowledge of viruses and their live cycle promoted the development of viral vectors that function like a shuttle to the cell, with a single round of infection either integrating or transiently expressing the transgene. Viral vectors have proven to be one of the most efficient and stable transgene shuttle into the cell and have gained increasing importance. The limitations of some viral vectors like the adenoviral vector and adeno-associated viral vector have been improved by new constructs like HIV-1 based lentiviral vectors. The immune response caused by expression of viral proteins, or the inability of some viral vectors like the retroviral vector to infect only dividing cells have been overcome by these new constructs. Lentiviral vectors allow an efficient and stable transgene expression over years in vivo without effecting transgene expression or immune response. In this Chapter we will describe synthetic vectors, give an overview of the most common viral vectors and focus our attention on lentiviral vectors, since we consider them to be the most efficient tool for gene delivery in the CNS.

Green fluorescent protein-adenoviral construct as a model for transient gene therapy for human cultured keratinocytes in an athymic mouse model

BACKGROUND

The goal of gene therapy for cultured keratinocyte grafts is to accelerate growth and wound healing following engraftment without producing long-term complications from the delivered gene. We studied a Green Fluorescent Protein-Adenoviral construct (GFP-ADV) to determine the characteristics of gene expression in human cultured keratinocyte grafts.

METHODS

Twelve GFP-ADV grafts and twelve control grafts were transplanted to the flanks of 24 athymic mice. Mouse flanks were monitored with fluorescence-filtered microscopy and, on Day 21, were sectioned and stained with anti-human MHC Class I with H&E counterstaining. Real-time PCR was performed on graft biopsies for adenoviral DNA.

RESULTS

Fluorescence decreased from Days 3 to 5 resulting in no difference between GFP-ADV and control grafts from days 5 to 10. All grafts were positive for human MHC Class I with an epithelial architecture by H&E. Day 21 GFP-ADV grafts were negative for adenoviral DNA.

CONCLUSION

The delivered gene was transiently expressed without the persistence of viral DNA, demonstrating the potential of adenoviral gene delivery for the improvement of wound healing without long-term adverse effects to the graft.

Production of helper-dependent adenovirus vector relies on helper virus structure and complementing

BACKGROUND

The helper-dependent (HD) adenoviral (Ad) vector relies on a helper virus to provide viral proteins for vector amplification. HD-Ad vectors can significantly increase therapeutic gene expression and improve safety. However, the yield of an HD-Ad vector is generally lower than that of an E1-deleted first-generation vector, likely due to the alterations in viral E3 or packaging regions of a helper virus that attenuate its replication and complementing for an HD-Ad vector.

METHODS

To study this question and improve HD-Ad vector production, we have generated four different helper viruses with a wild-type or deleted E3 region, and with a relocated loxP. We have also constructed a first-generation vector with a wild-type E3 region and without the loxP site. We compared the replication of these viruses in Cre-positive and -negative cells and studied their complementing for HD-Ad vector production.

RESULTS

Viruses with deleted E3 formed smaller plaques and produced lower titer compared with viruses containing the E3 region. The site where a loxP is inserted can also affect virus replication. Higher yield of HD-Ad vector was obtained when a helper virus with wild-type E3 was used. We also showed that deletion of the packaging signal in a helper virus through loxP/Cre interaction decreased the viral DNA complementing ability.

CONCLUSIONS

Although the E3 region is not essential for adenovirus replication in vivo, deletion of this region attenuates virus replication. Production of HD-Ad vector can be further improved by modifications in helper virus structure.

High in vivo production of a model monoclonal antibody on adenoviral gene transfer

The therapeutic potential of monoclonal antibodies (MAbs) for treating a variety of severe or life-threatening diseases is high. Although intravenous infusion appears to be the simplest and most obvious mode of administration, it is not applicable in many long-term treatments. It might, however, be advantageously replaced by gene/cell therapies, rendering treatments cost-effective and eliminating the short- and long-term side effects associated with injection of massive doses of antibodies. Grafting of ex vivo genetically modified cells of various types has already been used for in vivo production and systemic delivery of MAbs in mice. However, although sustained for long periods of time, serum levels of ectopic MAbs were low. We show here that in vivo administration to mice of a first-generation adenoviral vector expressing a model MAb also permits achievement of the same goal, but with 100 to 200 times better efficiency that in any other case of gene transfer described thus far. We also investigated for possible anti-idiotypic response against the ectopic MAb. None was detected in the animals expressing the lowest levels of ectopic MAb production; a response was detected among the highest producers. In the latter case, however, the response was low and could not exert any significant neutralizing activity. In conclusion, our work indicates that high levels of circulating ectopic MAb can be obtained on direct in vivo gene transfer without inducing an anti-idiotypic response sufficiently robust to exert a neutralizing effect. This observation is encouraging in the perspective of clinical applications of this technology.

Cre levels limit packaging signal excision efficiency in the Cre/loxP helper-dependent adenoviral vector system

Helper-dependent (HD) adenovirus vectors devoid of all viral coding sequences have a large cloning capacity and provide long-term transgene expression in vivo with negligible toxicity, making them attractive vectors for gene therapy. Currently, the most efficient means of producing HD vectors involves coinfecting 293 cells expressing Cre with the HD vector and a helper virus bearing a packaging signal flanked by loxP sites. Cre-mediated packaging signal excision renders the helper virus genome unpackageable but still able to replicate and provide helper functions for HD vector propagation. Typically, helper virus contamination is < or =1% pre- and < or =0.1% postpurification by CsCl banding. While these contamination levels are low, further reduction is desirable. However, this objective has not been realized since the Cre/loxP system was first developed. This lack of progress is due, at least in part, to our lack of understanding of the origins of the contaminating helper virus, thus rendering its reduction or elimination difficult to achieve. This study was designed to investigate the possible sources of contaminating helper virus persisting during HD vector amplification. The results revealed that Cre is limiting in helper virus-infected Cre-expressing 293 cells, thereby permitting helper viruses to escape packaging signal excision and propagate. The results of this study should provide a foundation for developing rational strategies to further reduce or possibly eliminate the contaminating helper virus.

Recent advances in liver-directed gene therapy for dyslipidemia

As currently available preventive and therapeutic interventions for hypercholesterolemia are ineffective in a substantial proportion of patients, severe dyslipidemias associated with atherosclerotic vascular disease remain an important target for the development of novel gene therapies. The development of a safe and efficient gene transfer vector has been a major challenge in liver-directed gene therapy, but recently significant progress has been made in this area. Proof-of-principle experiments indicate that the transfer of lipid-modifying genes to the liver is an effective method to restore normal plasma lipids and protect against atherosclerosis. This article summarizes recent developments in liver-directed gene delivery and reviews data on the treatment of dyslipidemias and prevention of atherosclerosis in animals. The evidence presented suggests that some of the approaches taken in animals may be ready for clinical trials in the near future.

Generation of helper-dependent adenoviral vectors by homologous recombination

Helper-dependent adenoviral vectors (HD-Ad) represent a potentially valuable tool for safe and prolonged gene expression in vivo. The current approach for generating these vectors is based on ligation of the expression cassette into large plasmids containing the viral inverted terminal repeats flanking "stuffer" DNA to maintain a final size above the lower limit for efficient packaging into the adenovirus capsid (approximately 28 kb). The ligation to produce the viral plasmid is generally very inefficient. Similar problems in producing first-generation adenoviral (FG-Ad) vectors were circumvented with the development of a system taking advantage of efficient homologous recombination between a shuttle plasmid containing the expression cassette and a FG-Ad vector backbone in the Escherichia coli strain BJ5183. Here we describe a method for fast and efficient generation of HD-Ad vector plasmids that can accommodate expression cassettes of any size up to 35 kb. To validate the system, we generated a HD-Ad vector expressing the fusion protein between beta-galactosidase and neomycin resistance genes under the control of the SR alpha promoter, and one expressing the enhanced green fluorescent protein under the control of the cytomegalovirus promoter. The viruses were rescued and tested in vitro and for in vivo expression in mice. The data collected indicate the possibility for achieving a high level of hepatocyte transduction using HD-Ad vectors derived from plasmids obtained by homologous recombination in E. coli, with no significant alteration of liver enzymes and a less severe, transient thrombocytopenia in comparison with previous reports with similar doses of a FG-Ad vector.

Gene therapy for cardiovascular disorders: is there a future?

Incidence of cardiovascular disease has reached epidemic proportions in spite of recent advances in improving the efficacy of pharmacotherapeutics. This has led many to conclude that drug therapy has reached a plateau in its effectiveness. As a result, our efforts have been diverted to explore the use of gene transfer approaches for long-term control of these pathophysiological conditions. The purpose of this review is to present various approaches that are being undertaken to provide "proof of principle" for gene therapy for cardiovascular diseases. Finally, we will discuss the future of gene therapy and other new technologies that may further advance this field of therapeutics.

Heparan sulfate glycosaminoglycans are receptors sufficient to mediate the initial binding of adenovirus types 2 and 5

Cell infection by adenovirus serotypes 2 and 5 (Ad2/5) initiates with the attachment of Ad fiber to the coxsackievirus and Ad receptor (CAR) followed by alpha(v) integrin-mediated entry. We recently demonstrated that heparan sulfate glycosaminoglycans (HS GAGs) expressed on cell surfaces are involved in the binding and infection of Ad2/5 (M. C. Dechecchi, A. Tamanini, A. Bonizzato, and G. Cabrini, Virology 268:382-390, 2000). The role of HS GAGs was investigated using extracellular soluble domain 1 of CAR (sCAR-D1) and heparin as soluble receptor analogues of CAR and HS GAGs in A549 and recombinant CHO cell lines with differential levels of expression of the two receptors and cultured to various densities. Complete inhibition of binding and infection was obtained by preincubating Ad2/5 with both heparin (10 microg/ml) and sCAR-D1 (200 microg/ml) in A549 cells. Partial inhibition was observed when heparin and sCAR-D1 were preincubated separately with Ad. The level of heparin-sensitive [(3)H]Ad2/5 binding doubled in sparse A549 cells (50 to 70,000 cells/cm(2)) with respect to that of cells grown to confluence (200 to 300,000 cells/cm(2)), in parallel with increased expression of HS GAGs. [(3)H]Ad2 bound to sparse CAR-negative CHO cells expressing HS GAGs (CHO K1). No [(3)H]Ad2 binding was observed in CHO K1 cells upon competitive inhibition with heparin and in HS GAG-defective CHO A745, D677, and E606 clones. HS-sensitive Ad2 infection was obtained in CAR-negative sparse CHO K1 cells but not in CHO A745 cells, which were permissive to infection only upon transfection with CAR. These results demonstrate that HS GAGs are sufficient to mediate the initial binding of Ad2/5.

Gene therapy for Fabry disease

Fabry disease is an X-linked metabolic disorder caused by a deficiency of alpha-galactosidase A (alpha-Gal A). Lack of this lysosomal hydrolase results in the accumulation of galactose-terminal glycosphingolipids in a number of tissues, including vascular endothelial cells. Premature death is predominantly associated with vascular conditions of the heart, kidneys and brain. Historically, treatment has largely been palliative. Alternative treatments for many lysosomal storage diseases have been developed, including allogeneic organ and bone marrow transplantation, enzyme replacement therapy, and gene therapy. Significant clinical risks still exist with allogeneic transplantations. Alpha-Gal A enzyme replacement therapy has been implemented in clinical trials. This approach has been effective but may have limitations for long-term systemic or cost-effective correction. As an alternative, gene therapy approaches, involving a variety of gene delivery systems, have been pursued for the amelioration of Fabry disease. Fabry disease is a compelling disorder for gene therapy, as target cells are readily accessible and relatively low levels of enzyme correction may suffice to reduce storage. Importantly, metabolic cooperativity effects are also manifested in Fabry disease, wherein corrected cells secrete alpha-Gal A that can correct bystander cells. In addition, a broad therapeutic window probably exists, and mouse models of Fabry disease have been generated to assist studies. As an example, in vitro and in vivo studies using alpha-Gal A-transduced haematopoietic cells from Fabry mice have demonstrated enzymatic correction of recipient cells and dissemination of alpha-Gal A upon transplantation, leading to reduced lipid storage in a number of clinically relevant organs. This corrective enzymatic effect has recently been shown to be even further enhanced upon pre-selection of therapeutically transduced cells prior to transplantation. This review will briefly detail current gene delivery methods and summarize results to date in the context of gene therapy for Fabry disease.