Integrating adenovirus-adeno-associated virus hybrid vectors devoid of all viral genes

Best of most possible worlds: Hybrid gene therapy vectors based on parvoviruses and heterologous viruses

Parvoviruses and especially the adeno-associated virus (AAV) species provide an exciting and versatile platform for the rational design or molecular evolution of human gene-therapy vectors, documented by literature from over half a century, hundreds of clinical trials, and the recent commercialization of multiple AAV gene therapeutics. For the last three decades, the power of these vectors has been further potentiated through various types of hybrid vectors created by intra- or inter-genus juxtaposition of viral DNA and protein cis elements or by synergistic complementation of parvoviral features with those of heterologous, prokaryotic, or eukaryotic viruses. Here, we provide an overview of the history and promise of this rapidly expanding field of hybrid parvoviral gene-therapy vectors, starting with early generations of chimeric particles composed of a recombinant AAV genome encapsidated in shells of synthetic AAVs or of adeno-, herpes-, baculo-, or protoparvoviruses. We then dedicate our attention to two newer, highly promising types of hybrid vectors created via (1) pseudotyping of AAV genomes with bocaviral serotypes and capsid mutants or (2) packaging of AAV DNA into, or tethering of entire vector particles to, bacteriophages. Finally, we conclude with an outlook summarizing critical requirements and improvements toward clinical translation of these original concepts.

Gene Therapy Applications of Viral Vectors

Viral vectors have frequently been applied in gene therapy with the final goal of treating various diseases in the areas of neurology, neurodegeneration, metabolic disease, and cancer. Vectors have been engineered based on AAV, adenoviruses, alphaviruses, herpes simplex viruses, lentiviruses, and retroviruses. Some vectors are suitable for short-term episomal transgene expression, whereas others are integrated into the host cell genome to provide long-term expression. Additionally, hybrid vectors with favorable features from different viruses have been developed. Therapeutic genes of choice have typically been toxic genes such as thymidine kinase, pro-apoptotic genes like Bax, and immunostimulatory genes (for instance, interleukin-12). A large number of animal studies have demonstrated proof of concept of viral gene therapy. Many types of viral vectors have been employed in more than 700 clinical trials that have been carried out or are currently in progress.

Development of a recombinant adenovirus vector production system free of replication-competent adenovirus by utilizing a packaging size limit of the viral genome

In a conventional adenovirus (Ad) vector production method using 293 cells, homologous recombination between Ad vector DNA and 293 cell-derived Ad E1 DNA occurs with low efficiency, resulting in the generation of replication-competent adenovirus (RCA). RCA can induce the spread of replication-incompetent Ad vectors, leading to unexpected tissue damage. In order to overcome this problem, we developed an Ad vector production system free of RCA generation by utilizing the Ad packaging size limit of the viral genome. It is well known that up to approximately 105% (37.7 kb) of the wild-type genome (35.9 kb) can be packaged in the Ad virion. We designed the Ad vector genome by insertion of a transgene expression cassette into the E3 region, such that homologous recombination between the Ad vector DNA and 293 cell-derived Ad E1 DNA would produce an Ad vector genome that exceeds in the size of the packaging limit. In accord with our strategy, no RCA generation was observed during the passages when we used the E1 (3.2kb)-deleted Ad vectors containing a more than 3.0-kb transgene expression cassette in the E3 region. In contrast, the E1 (3.2kb)-deleted Ad vectors, which retain 37.7 kb of the viral genome and have an insertion of a 2.1-kb transgene expression cassette in the E3 region, generated RCA, although RCA derived from this Ad vector exceeded the packaging size limit (105.0%). These results suggest that RCA generation can be avoided when the genome size of RCA is more than 108.3% (38.9 kb) of the wild-type Ad genome. This Ad vector production system generates safe, easy, and efficient Ad vector stock for both basic study as well as clinical research.

In vivo stable transduction of humanized liver tissue in chimeric mice via high-capacity adenovirus-lentivirus hybrid vector

We developed hybrid vectors employing high-capacity adenovirus as a first-stage carrier encoding all the components required for in situ production of a second-stage lentivirus, thereby achieving stable transgene expression in secondary target cells. Such vectors have never previously been tested in normal tissues, because of the scarcity of suitable in vivo systems permissive for second-stage lentivirus assembly. Here we employed a novel murine model in which endogenous liver tissue is extensively reconstituted with engrafted human hepatocytes, and successfully achieved stable transduction by the second-stage lentivirus produced in situ from first-stage adenovirus. This represents the first demonstration of the functionality of adenoviral-lentiviral hybrid vectors in a normal parenchymal organ in vivo.

Molecular and traditional chemotherapy: a united front against prostate cancer

Castrate resistant prostate cancer (CRPC) is essentially incurable. Recently though, chemotherapy demonstrated a survival benefit ( approximately 2months) in the treatment of CRPC. While this was a landmark finding, suboptimal efficacy and systemic toxicities at the therapeutic doses warranted further development. Smart combination therapies, acting through multiple mechanisms to target the heterogeneous cell populations of PC and with potential for reduction in individual dosing, need to be developed. In that, targeted molecular chemotherapy has generated significant interest with the potential for localized treatment to generate systemic efficacy. This can be further enhanced through the use of oncolytic conditionally replicative adenoviruses (CRAds) to deliver molecular chemotherapy. The prospects of chemotherapy and molecular-chemotherapy as single and as components of combination therapies are discussed.

Viral hybrid vectors for somatic integration - are they the better solution?

The turbulent history of clinical trials in viral gene therapy has taught us important lessons about vector design and safety issues. Much effort was spent on analyzing genotoxicity after somatic integration of therapeutic DNA into the host genome. Based on these findings major improvements in vector design including the development of viral hybrid vectors for somatic integration have been achieved. This review provides a state-of-the-art overview of available hybrid vectors utilizing viruses for high transduction efficiencies in concert with various integration machineries for random and targeted integration patterns. It discusses advantages but also limitations of each vector system.

Viral vectors for neurotrophic factor delivery: a gene therapy approach for neurodegenerative diseases of the CNS

The clinical manifestation of most diseases of the central nervous system results from neuronal dysfunction or loss. Diseases such as stroke, epilepsy and neurodegeneration (e.g. Alzheimer's disease and Parkinson's disease) share common cellular and molecular mechanisms (e.g. oxidative stress, endoplasmic reticulum stress, mitochondrial dysfunction) that contribute to the loss of neuronal function. Neurotrophic factors (NTFs) are secreted proteins that regulate multiple aspects of neuronal development including neuronal maintenance, survival, axonal growth and synaptic plasticity. These properties of NTFs make them likely candidates for preventing neurodegeneration and promoting neuroregeneration. One approach to delivering NTFs to diseased cells is through viral vector-mediated gene delivery. Viral vectors are now routinely used as tools for studying gene function as well as developing gene-based therapies for a variety of diseases. Currently, many clinical trials using viral vectors in the nervous system are underway or completed, and seven of these trials involve NTFs for neurodegeneration. In this review, we discuss viral vector-mediated gene transfer of NTFs to treat neurodegenerative diseases of the central nervous system.

A rapid protocol for construction and production of high-capacity adenoviral vectors

High-capacity adenoviral vectors (HC-AdVs) lacking all viral coding sequences were shown to result in long-term transgene expression and phenotypic correction in small and large animal models. It has been established that HC-AdVs show significantly reduced toxicity profiles compared with early-generation adenoviral vectors. Furthermore, with capsid-modified HC-AdV becoming available, we are just starting to understand the full potential of this vector system. However, for many researchers, the wide-scale use of HC-AdV is hampered by labor-intensive and complex production procedures. Herein, we provide a feasible and detailed protocol for efficient generation of HC-AdV. We introduce an efficient cloning strategy for the generation of recombinant HC-AdV vector genomes. Infection and amplification of the HC-AdV are performed in a spinner culture system. For purification, we routinely apply cesium chloride gradients. Finally, we describe various methods for establishing vector titers. Generation of high-titer HC-AdV can be achieved in 3 weeks.

The development of gene therapy: from monogenic recessive disorders to complex diseases such as cancer

During the last 4 decades, gene therapy has moved from preclinical to clinical studies for many diseases ranging from monogenic recessive disorders such as hemophilia to more complex diseases such as cancer, cardiovascular disorders, and human immunodeficiency virus (HIV). To date, more than 1,340 gene therapy clinical trials have been completed, are ongoing, or have been approved in 28 countries, using more than 100 genes. Most of those clinical trials (66.5%) were aimed at the treatment of cancer. Early hype, failures, and tragic events have now largely been replaced by the necessary stepwise progress needed to realize clinical benefits. We now understand better the strengths and weaknesses of various gene transfer vectors; this facilitates the choice of appropriate vectors for individual diseases. Continuous advances in our understanding of tumor biology have allowed the development of elegant, more efficient, and less toxic treatment strategies. In this introductory chapter, we review the history of gene therapy since the early 1960s and present in detail two major recurring themes in gene therapy: (1) the development of vector and delivery systems and (2) the design of strategies to fight or cure particular diseases. The field of cancer gene therapy experienced an "awkward adolescence." Although this field has certainly not yet reached maturity, it still holds the potential of alleviating the suffering of many individuals with cancer.

Gene therapy via inducible nitric oxide synthase: a tool for the treatment of a diverse range of pathological conditions

Nitric oxide (NO(.)) is a reactive nitrogen radical produced by the NO synthase (NOS) enzymes; it affects a plethora of downstream physiological and pathological processes. The past two decades have seen an explosion in the understanding of the role of NO(.) biology, highlighting various protective and damaging modes of action. Much of the controversy surrounding the role of NO(.) relates to the differing concentrations generated by the three isoforms of NOS. Both calcium-dependent isoforms of the enzyme (endothelial and neuronal NOS) generate low-nanomolar/picomolar concentrations of NO(.). By contrast, the calcium-independent isoform (inducible NOS (iNOS)) generates high concentrations of NO(.), 2-3 orders of magnitude greater. This review summarizes the current literature in relation to iNOS gene therapy for the therapeutic benefit of various pathological conditions, including various states of vascular disease, wound healing, erectile dysfunction, renal dysfunction and oncology. The available data provide convincing evidence that manipulation of endogenous NO(.) using iNOS gene therapy can provide the basis for future clinical trials.

Somatic integration from an adenoviral hybrid vector into a hot spot in mouse liver results in persistent transgene expression levels in vivo

We have developed a hybrid vector that combines the high transduction efficiency of a gene-deleted adenoviral vector and the integration machinery of the bacteriophage-derived integrase PhiC31 for stable transduction and limited integration sites. We based our system on a two-vector system in which the transgene expression cassette is circularized from a helper-dependent vector by Flp-mediated recombination, followed by PhiC31-mediated integration. Integration of the transgene expression cassette from the adenoviral vector resulted in 5-fold higher transgene expression levels in the active PhiC31 group compared to the control group, which received a mutated and inactive version of PhiC31. We confirmed transgene integration into the previously described mpsL1 hot spot of integration by polymerase chain reaction analyses of DNA isolated from mouse livers. In addition, we cloned 40 integration sites. The hot spot mpsL1 was detected only once, and 44% of all integration events were found to be present in gene regions. With further optimization, this system represents a new tool for gene therapy protocols that may offer an alternative to gene therapy approaches based on random integrating viral vectors.

Gene therapy to the kidney using viral vectors

Many pediatric diseases have reached a therapeutic plateau using currently available surgical and pharmacological approaches. Gene therapy has emerged as an exciting new technology to manipulate cells in the mammalian system, and in some cases, this method has achieved amazing therapeutic benefits. Compared to other organs, such as the brain, liver and lung, methods to genetically modify renal cells have received relatively little attention. The current review will discuss the challenges and important developments regarding gene therapy to the kidney, and relate the recent successes and failures to the future potential of gene therapy as a treatment modality in the context of pediatric disease.

Successful compensation for dystrophin deficiency by a helper-dependent adenovirus expressing full-length utrophin

Helper-dependent adenovirus vector (AdV)-mediated full-length dystrophin expression leads to significant mitigation of the dystrophic phenotype of the mdx mouse. However, dystrophin, as a neoantigen, elicits antibody formation. As an alternative approach, we evaluated gene transfer of full-length murine utrophin, a functional homologue of dystrophin that is normally present only at the neuromuscular junction. A single injection in the tibialis anterior (TA) muscle of the helper-dependent adenovirus vector encoding utrophin provided very good transduction, with 58% of fibers demonstrating sarcolemmal utrophin expression in the neonates, and 35% utrophin-positive (Utr(+)) fibers in adults. The presence of utrophin prevented extensive necrosis in the neonates, halted further necrosis in the adults, and led to restoration of sarcolemmal expression of dystrophin-associated proteins up to 1 year after injection. Marked physiological improvement was observed in both neonates and adults. Neither increased humoral responses nor cellular immune responses were evident. However, there was a time-related decline of the initial high utrophin expression. Although viral DNA persisted in animals that were injected in the neonatal stage, viral DNA levels decreased in muscles of adult mice. These results demonstrate that although utrophin gene transfer leads to amelioration of the dystrophic phenotype, the effects are not sustained upon loss of utrophin expression.

Protection of mice from lethal Escherichia coli infection by chimeric human bactericidal/permeability-increasing protein and immunoglobulin G1 Fc gene delivery

To evaluate the potentiality of applying gene therapy to bacterial infections, especially for preventing infection in high-risk patients, we investigated protection of mice from challenge with lethal Escherichia coli infection by adeno-associated virus serotype 2 (AAV2)-mediated gene transfer of a chimeric BPI23-Fcgamma1 gene, which consisted of human bactericidal/permeability-increasing protein (BPI) gene encoding the functional N terminus (amino acid residues 1 to 199) of human BPI and an Fcgamma1 gene encoding the Fc segment of human immunoglobulin G1. Here we show that the target protein that was expressed and secreted into the serum of the gene-transferred mice demonstrated the activity of a neutralizing endotoxin, killing E. coli and mediating opsonization. After lethal E. coli infection, the count of bacteria and the levels of endotoxin and proinflammatory cytokines in the gene-transferred mice were decreased. The survival rate of BPI23-Fcgamma1 gene-transferred mice markedly increased, especially in conjunction with antibiotics. Our data suggest that AAV2-mediated chimeric BPI23-Fcgamma1 gene delivery could potentially be used clinically for the protection and treatment of infection with gram-negative bacteria in high-risk individuals.

Islet cell differentiation in liver by combinatorial expression of transcription factors neurogenin-3, BETA2, and RIPE3b1

Transcription factors, such as PDX-1, that normally mediate pancreatic development are capable of inducing hepatic progenitor cells to differentiate into cells with pancreatic islet characteristics. We hypothesized that simultaneous expression of multiple transcription factors involved in islet development might enhance the differentiation of hepatic progenitor cells. Bi- or tri-cistronic constructs were generated in hybrid adenovirus/adeno-associated virus (Ad/AAV) vectors containing neurogenin 3 (NGN3), BETA2 (NeuroD), and RIPE3b1 (MafA), each of which plays a role in islet cell differentiation. These vectors efficiently express multiple transcription factors and stimulate insulin promoter activity in a combinatorial manner. When these multi-cistronic constructs were administered in vivo, they induce hepatic expression of islet-specific markers, including PDX-1, insulin, glucagon, somatostatin, and islet-amyloid peptide. Administration of the Ad/AAV hybrid vectors to streptozotocin-induced diabetic mice reversed hyperglycemia, consistent the differentiation of functional hepatic insulin-secreting cells. These results indicate that Ad/AAV hybrid vectors can be used to administer combinations of factors that induce islet cell differentiation in hepatic progenitor cells.

A hybrid vector for ligand-directed tumor targeting and molecular imaging

Merging tumor targeting and molecular-genetic imaging into an integrated platform is limited by lack of strategies to enable systemic yet ligand-directed delivery and imaging of specific transgenes. Many eukaryotic viruses serve for transgene delivery but require elimination of native tropism for mammalian cells; in contrast, prokaryotic viruses can be adapted to bind to mammalian receptors but are otherwise poor vehicles. Here we introduce a system containing cis-elements from adeno-associated virus (AAV) and single-stranded bacteriophage. Our AAV/phage (AAVP) prototype targets an integrin. We show that AAVP provides superior tumor transduction over phage and that incorporation of inverted terminal repeats is associated with improved fate of the delivered transgene. Moreover, we show that the temporal dynamics and spatial heterogeneity of gene expression mediated by targeted AAVP can be monitored by positron emission tomography. This new class of targeted hybrid viral particles will enable a wide range of applications in biology and medicine.

Adenoviral vectors--how to use them in cancer gene therapy?

Gene therapy is most often described as a technique for introducing the foreign genetic material into cells with a correction of a dysfunctional gene as its final goal. Today, it is well known that cancer is one of the leading causes of mortality in the world. Besides classical methods for cancer treatment new strategies against cancer are needed. Although originally being designed as a treatment for monogenetic illness, soon after, gene therapy appeared as a potential new strategy in cancer therapy. One of the widely used vectors for cancer gene therapy is adenovirus. In this review we have described molecular biology of adenoviruses and basis for construction of adenoviral vectors. We have also described concepts for cancer gene therapy including their in vitro and in vivo application. Special attention is drawn toward retargeting of adenovirus as a new approach in vector design for cancer gene therapy, in order to restrict transgene expression in tumor tissue. This approach uses biophysical as well as genetic characteristics of tumor itself and its supporting tissue, allowing new "bypass" in cancer gene therapy.

Mutagenesis and oncogenesis by chromosomal insertion of gene transfer vectors

Increasing evidence reveals that random insertion of gene transfer vectors into the genome of repopulating hematopoietic cells may alter their fate in vivo. Although most insertional mutations are expected to have few if any consequences for cellular survival, clonal dominance caused by retroviral vector insertions in (or in the vicinity of) proto-oncogenes or other signaling genes has been described for both normal and malignant hematopoiesis. Important insights into these side effects were initially obtained in murine models. Results from ongoing clinical studies have revealed that similar adverse events may also occur in human gene therapy. However, it remains unknown to what extent the outcome of insertional mutagenesis induced by gene vectors is related to (1) the architecture and type of vector used, (2) intrinsic properties of the target cell, and (3) extrinsic and potentially disease-specific factors influencing clonal competition in vivo. This review discusses reports addressing these questions, underlining the need for models that demonstrate and quantify the functional consequences of insertional mutagenesis. Improving vector design appears to be the most straightforward approach to increase safety, provided all relevant cofactors are considered.

A capsid-modified helper-dependent adenovirus vector containing the beta-globin locus control region displays a nonrandom integration pattern and allows stable, erythroid-specific gene expression

Gene therapy for hemoglobinopathies requires efficient gene transfer into hematopoietic stem cells and high-level erythroid-specific gene expression. Toward this goal, we constructed a helper-dependent adenovirus vector carrying the beta-globin locus control region (LCR) to drive green fluorescent protein (GFP) expression, whereby the LCR-GFP cassette is flanked by adeno-associated virus (AAV) inverted terminal repeats (Ad.LCR-beta-GFP). This vector possesses the adenovirus type 35 fiber knob that allows efficient infection of hematopoietic cells. Transduction and vector integration studies were performed in MO7e cells, a growth factor-dependent CD34(+) erythroleukemic cell line, and in cord blood-derived human CD34(+) cells. Stable transduction of MO7e cells with Ad.LCR-beta-GFP was more efficient and less subject to position effects and silencing than transduction with a vector that did not contain the beta-globin LCR. Analysis of integration sites indicated that Ad.LCR-beta-GFP integration in MO7e cells was not random but tethered to chromosome 11, specifically to the globin LCR. More than 10% of analyzed integration sites were within the chromosomal beta-globin LCR. None of the Ad.LCR-beta-GFP integrations occurred in exons. The integration pattern of a helper-dependent vector that contained X-chromosomal stuffer DNA was different from that of the beta-globin LCR-containing vector. Infection of primary CD34(+) cells with Ad.LCR-beta-GFP did not affect the clonogenic capacity of CD34(+) cells. Transduction of CD34(+) cells with Ad.LCR-beta-GFP resulted in vector integration and erythroid lineage-specific GFP expression.

Adeno-associated virus: from defective virus to effective vector

The initial discovery of adeno-associated virus (AAV) mixed with adenovirus particles was not a fortuitous one but rather an expression of AAV biology. Indeed, as it came to be known, in addition to the unavoidable host cell, AAV typically needs a so-called helper virus such as adenovirus to replicate. Since the AAV life cycle revolves around another unrelated virus it was dubbed a satellite virus. However, the structural simplicity plus the defective and non-pathogenic character of this satellite virus caused recombinant forms to acquire centre-stage prominence in the current constellation of vectors for human gene therapy. In the present review, issues related to the development of recombinant AAV (rAAV) vectors, from the general principle to production methods, tropism modifications and other emerging technologies are discussed. In addition, the accumulating knowledge regarding the mechanisms of rAAV genome transduction and persistence is reviewed. The topics on rAAV vectorology are supplemented with information on the parental virus biology with an emphasis on aspects that directly impact on vector design and performance such as genome replication, genetic structure, and host cell entry.

Delivery systems for pulmonary gene therapy

Delivery of therapeutic genes to the lungs is an attractive strategy to correct a variety of pulmonary dysfunctions such as cystic fibrosis, alpha-1 antitrypsin deficiency, pulmonary hypertension, asthma, and lung cancer. Different delivery routes such as intratracheal instillation, aerosol and intravenous injection have been utilized with varying degrees of efficiency. Both viral and non-viral vectors, with their respective strengths and weaknesses, have achieved significant levels of transgene expression in the lungs. However, the application of gene therapy for the treatment of pulmonary disease has been handicapped by various barriers to the delivery vectors such as serum proteins during intravenous delivery, and surfactant proteins and mucus in the airway lumen during topical application of therapeutic genes. Immune and cytokine responses against the delivery vehicle are also major problems encountered in pulmonary gene therapy. Despite these shortcomings much progress has been made to enhance the efficiency, as well as lower the toxicity of gene therapy vehicles in the treatment of pulmonary disorders such as cystic fibrosis, lung cancer and asthma.

Approaches to improving the kinetics of adenovirus-delivered genes and gene products

Adenovirus (Ad) vectors have been expected to play a great role in gene therapy because of their extremely high transduction efficiency and wide tropism. However, due to the intrinsic deficiency of their immunogenic toxicities, Ad vectors are rapidly cleared from the host, transgene expression is transient, and readministration of the same serotype Ad vectors is problematic. As a result, Ad vectors are continually undergoing refinement to realize their potential for gene therapy application. Even after 1999, when a patient fatally succumbed to the toxicity associated with Ad vector administration at a University of Pennsylvania (U.S.) experimental clinic, enthusiasm of gene therapists for Ad vectors has not waned. With great efforts from various research groups, significant advances have been achieved through comprehensive approaches to improving the kinetics of Ad vector-delivered genes and gene products.

Prospects and problems of gene therapy: an update

The gene therapy approach can vary from delivering extra copies of a gene, through modifications of a genome using the properties of ribozymes or chimeraplasts, to injection of modified cells. For the treatment of genetic deficits the ultimate goal would be the repair of the mutated gene in the target tissue(s). The techniques required for such an approach are emerging, albeit slowly. Therefore, delivery of an extra copy of a normal gene in a specific vector remains the predominant approach. Moreover, this method finds wider applications in gene therapy relating to disorders other than heritable defects, e.g., malignancies, cardiovascular diseases and infections. The major and most intensive areas of research are: i) vectors and delivery methods, ii) regulation of transgene expression and iii) stability of expression. Targeting of the therapeutic gene is being accomplished by using viral vectors or non-viral delivery systems, either ex vivo or in vivo. The choice of vectors and delivery routes depends on the nature of the target cells and the required levels and stability of expression. Although there have been the first positive clinical results and significant technical achievements over the past 2 years, there are still obstacles to the development of effective clinical products and these remain largely unchanged. The most important barriers are the low levels and stability of expression and immune responses to vectors and/or gene products. The safety aspects of gene therapy have become painfully evident with the first death conclusively linked to gene therapy. The progress in AAV and lentiviral vectors, improved regulation of transgene expression and advances in stem cell technology are among the recent most exciting developments.

Targeting site-specific chromosome integration

The concept of gene therapy was introduced with great promise and high expectations. However, what appeared simple in theory has not translated into practice. Despite some success in clinical trials, the research community is still facing an old problem: namely, the need for a vector that can deliver a gene to target cells without adverse events while maintaining a long-term therapeutic effect. Some of these challenges are being addressed by the development of hybrid vectors which meld two different viral systems to incorporate efficient gene delivery and large cloning capacity with site-specific integration. The two known systems that integrate genes into specific sites in mammalian genomes are the adeno-associated virus and phage integrases. Recent experiments with hybrid vectors incorporating both of these systems are encouraging. However, extensive research should be directed towards the safety and efficacy of this approach before it will be available for gene therapy.

Genome size and structure determine efficiency of postinternalization steps and gene transfer of capsid-modified adenovirus vectors in a cell-type-specific manner

Adenovirus serotype 5 (Ad5) vectors containing Ad B-group fibers have become increasingly popular as gene transfer vectors because they efficiently transduce human cell types that are relatively refractory to Ad5 infection. So far, most B-group fiber-containing vectors have been first-generation vectors, deleted of E1 and/or E3 genes. Transduction with these vectors, however, results in viral gene expression and is associated with cytotoxicity and immune responses against transduced cells. To circumvent these problems, we developed fiber-chimeric Ad vectors devoid of all viral genes that were produced either by the homologous recombination of first-generation vectors or by using the Cre/lox-based helper virus system. In this study we compared early steps of infection between first-generation (35-kb genome) and Ad vectors devoid of all viral genes with genome sizes of 28 kb and 12.6 kb. All vectors possessed an Ad35-derived fiber knob domain, which uses CD46 as a primary attachment receptor. Using immortalized human hematopoietic cell lines and primary human CD34-positive hematopoietic cells, we found that the Ad genome size did not affect the efficiency of virus attachment to and internalization into cells. Furthermore, independently of the genome length and structure, all vectors migrated to the nucleus through late endosomal and lysosomal cellular compartments. However, the vector containing the short 12.6-kb genome was unable to efficiently escape from endosomes and deliver its DNA into the nucleus. Moreover, compared to other vectors, these Ad particles were less stable and had an abnormal capsid protein composition, including a lack of capsid-stabilizing protein IX. Our data indicate that the size and structure of the packaged viral genomes can affect the integrity of Ad particles, which in turn results in lower infectivity of Ad vectors.

Sleeping beauty transposition: biology and applications for molecular therapy

Transposable elements can be considered as natural, nonviral gene-delivery vehicles and are valuable and widely used tools for germ-line transgenesis and insertional mutagenesis in invertebrate systems such as flies and worms. Such tools were not available for genome manipulations in vertebrates until recently, when an active element was resurrected from transposon fossils found in fish genomes. This element, the Sleeping Beauty transposon, shows efficient transposition in cells of a wide range of vertebrates, including humans. Sleeping Beauty transposition is a cut-and-paste process, during which the element "jumps" from one DNA molecule to another. Transposon integration into chromosomes provides the basis for long-term, or possibly permanent, transgene expression in transgenic cells and organisms. Thus, the reconstruction of the Sleeping Beauty element generated considerable interest in developing efficient and safe vectors for vertebrate transgenesis as well as for human gene therapy. In this review we summarize our current knowledge of Sleeping Beauty biology and describe the strengths and current limitations of transposon technology for gene therapeutic applications.

Gene therapy with viral vectors

A key factor in the success of gene therapy is the development of gene delivery systems that are capable of efficient gene transfer in a broad variety of tissues, without causing any pathogenic effect. Currently, viral vectors based on many different viruses have been developed, and their performance and pathogenicity has been evaluated in animal models. The results of these studies form the basis for the first clinical trials for correcting genetic disorders using retroviral, adenoviral, and adeno-associated viral vectors. Even though the results of these trials are encouraging, vector development is still required to improve and refine future treatment of hereditary disorders.

Delivering antisense telomerase RNA by a hybrid adenovirus/ adeno-associated virus significantly suppresses the malignant phenotype and enhances cell apoptosis of human breast cancer cells

Activated telomerase is frequently detected in cancer cells and is able to maintain and stabilize the integrity of telomeres; it also contributes to unlimited divisions in cancer cells. Recently, a new generation of selective anticancer strategies is under development targeting the blockage of telomerase activity either at the protein level or telomerase RNA. Here, we report suppression of the malignant phenotype by the expression of the full-length antisense human telomerase RNA (hTR) delivered by a novel hybrid vector recombining adenovirus and adeno-associated virus (vAd-AAV). The hybrid vector vAd-AAV retained the unique traits from two parental viruses, such as high efficiency of gene transfer in mammalian cells and the ability to integrate into the genomic DNA of host cells. The stable expression of antisense hTR in MCF-7 cells significantly suppressed telomerase activity and progressively shortened telomere length for 30 population doublings (PD30). Expression of antisense hTR leads to a telomere-based growth arrest and the induction of spontaneous apoptosis. Antisense hTR decreased soft agar colony formation and reduced the cell proliferation, leading to exit from the cell cycle at G1 at PD15. The expression of antisense hTR also sensitized MCF-7 cells to apoptosis induced by sodium butyrate or serum starvation. Our study demonstrates that delivering antisense hTR by the hybrid Ad/AAV vector is an effective antineoplastic gene therapeutic strategy, which significantly suppresses the malignant phenotype and enhances apoptosis of human breast cancer cells.

Packaging of an AAV vector encoding human acid alpha-glucosidase for gene therapy in glycogen storage disease type II with a modified hybrid adenovirus-AAV vector

We have developed an improved method for packaging adeno-associated virus (AAV) vectors with a replication-defective adenovirus-AAV (Ad-AAV) hybrid virus. The AAV vector encoding human acid alpha-glucosidase (hGAA) was cloned into an E1, polymerase/preterminal protein-deleted adenovirus, such that it is packaged as an Ad vector. Importantly, the Ad-AAV hybrid cannot replicate during AAV vector packaging in 293 cells, because of deletion of polymerase/preterminal protein. The residual Ad-AAV in the AAV vector stock was reduced to <1 infectious particle per 10(10) AAV vector particles. These modifications resulted in approximately 30-fold increased packaging of the AAV vector for the hybrid Ad-AAV vector method as compared with standard transfection-only methods. Similarly improved packaging was demonstrated for pseudotyping the AAV vector as AAV6, and for AAV vector packaging with a second Ad-AAV vector encoding canine glucose-6-phosphatase. Liver-targeted delivery of either the Ad-AAV hybrid or AAV vector particles in acid alpha-glucosidase-knockout (GAA-KO) mice revealed secretion of hGAA with the Ad-AAV vector, and sustained secretion of hGAA with an AAV vector in hGAA-tolerant GAA-KO mice. Further development of hybrid Ad-AAV vectors could offer distinct advantages for gene therapy in glycogen storage diseases.

Latest development in viral vectors for gene therapy

Gene therapy includes the application of various viral vectors, which represent most types and families of viruses, suitable for infection of mammalian host cells. Both hereditary diseases and acquired illnesses, such as cancer, can be targeted. Because of the various properties of each viral vector, the definition of their application range depends on factors such as packaging capacity, host range, cell- or tissue-specific targeting, replication competency, genome integration and duration of transgene expression. Recent engineering of modified viral vectors has contributed to improved gene delivery efficacy.

In vivo transduction of photoreceptors or ciliary body by intravitreal injection of pseudotyped adenoviral vectors

Strategies for retargeting adenoviral (Ad) vectors have been developed, but their in vivo efficacy remains to be demonstrated. Gene delivery to specific ocular cell types represents an approach to treating many diseases that cause irreversible blindness. One of these cell types, the photoreceptor (PR), is not infected by standard Ad5-based vectors. We evaluated gene delivery after intraocular injection of Ads pseudotyped with three different fiber proteins and found three distinct patterns of infection. An intravitreally injected Ad5 vector readily infected the iris, corneal endothelium, and ciliary body, while few cells in the retina expressed transgene product. In contrast, an Ad3-pseudotyped virus selectively transduced ciliary body, of interest for treating diseases such as glaucoma. A vector pseudotyped with the fiber protein of Ad37 transduced PRs as well as ciliary body. This finding has potential application to the treatment of retinal degenerative or neovascular diseases. These studies demonstrate cell type-selective gene delivery in vivo with retargeted Ads, provide information about the cellular tropisms of several Ad serotypes, and should lead to improved strategies for preserving vision.

Transposition from a gutless adeno-transposon vector stabilizes transgene expression in vivo

A major limitation of adenovirus-mediated gene therapy for inherited diseases is the instability of transgene expression in vivo, which originates at least in part from the loss of the linear, extrachromosomal vector genomes. Herein we describe the production of a gene-deleted adenovirus-transposon vector that stably maintains virus-encoded transgenes in vivo through integration into host cell chromosomes. This system utilizes a donor transposon vector that undergoes Flp-mediated recombination and excision of its therapeutic payload in the presence of the Flp and Sleeping Beauty recombinases. Systemic in vivo delivery of this system resulted in efficient generation of transposon circles and stable transposase-mediated integration in mouse liver. Somatic integration was sufficient to maintain therapeutic levels of human coagulation Factor IX for more than six months in mice undergoing extensive liver proliferation. These vectors combine the versatility of adenoviral vectors with the integration capabilities of a eukaryotic DNA transposon and should prove useful in the treatment of genetic diseases.

Gene transfer to salivary glands

This article provides a review of the application of gene transfer technology to studies of salivary glands. Salivary glands provide an uncommon target site for gene transfer but offer many experimental situations likely of interest to the cell biologist. The reader is provided with a concise overview of salivary biology, along with a general discussion of the strategies available for gene transfer to any tissue. In particular, adenoviral vectors have been useful for proof of concept studies with salivary glands. Several examples are given, using adenoviral-mediated gene transfer, for addressing both biological and clinical questions. Additionally, benefits and shortcomings affecting the utility of this technology are discussed.

Restoration of a functional open reading frame by homologous recombination between two adenoviral vectors

In this study, we examined the ability of adenoviral (Ad) vectors to undergo homologous recombination. The lacZ gene was divided between two parental, first-generation vectors such that neither encoded a functional product but both shared 494 bp in common. The open reading frame could only be restored by homologous recombination. We observed beta-galactosidase activity only upon co-infection of both parental vectors and after the onset of viral DNA replication, creating a delay in expression of 24-36 hours in HeLa cells. At peak efficiency, this recombination vector system resulted in beta-galactosidase activity levels 100x above background and just 18x less than a conventional, first-generation vector in HeLa cells. After recombination, the resultant progeny vector genomes containing reconstituted expression cassettes were devoid of all viral genes and contained two packaging signals. These progeny genomes were efficiently packaged, could be separated from their parental vectors based on their lighter buoyant densities in CsCl gradients, and were subsequently used as functional gene transfer vectors. This novel recombination vector system should be useful for transferring large transgenes (because the carrying capacity of two Ad vectors can be exploited) or expressing any cytotoxic or Ad replication inhibitory protein (because the parental vectors exhibit no background expression).

An adenoviral expression system for AAV rep78 using homologous recombination

The construction and amplification of adenoviral (Ad) vectors expressing biologically active transgenes that are cytotoxic or inhibit Ad replication can be extremely difficult, if not impossible. In this study, we harnessed the ability of Ad genomes to undergo efficient homologous recombination to reconstitute the adeno-associated virus (AAV) rep78 gene, a cytotoxic gene that strongly inhibits Ad replication, which was divided between two parental, first-generation Ad vectors. A functional open reading frame was generated by recombination only upon co-infection of both parental vectors and after the onset of viral DNA replication. We were able to amplify both parental rep78 vectors to normal titers without any signs of inhibition or toxicity and could use them to generate progeny vectors containing a functional rep78 gene without any Ad genes. Using this vector recombination system in AAV rescue assays demonstrated that no Ad protein was essential for Rep78 mediated rescue of AAV ITR flanked DNA from plasmid or Ad backbones; the amount of rescue product generated was substantially greater in the presence of Ad infection; neither cellular nor viral DNA replication was necessary for rescue to occur; and progeny vector genomes were efficiently co-replicated along with conventional, first-generation Ad vectors.

A novel, helper-dependent, adenovirus-retrovirus hybrid vector: stable transduction by a two-stage mechanism

We have developed a novel vector system that uses a helper-dependent adenoviral vector as a carrier to deliver a fully functional retrovirus vector. The helper-dependent adenovirus (HDAd) can accommodate large inserts, provide high titers, and infect nondividing as well as dividing cells. However, adenoviral DNA is rarely integrated into the host cell genome, and its episomal expression is transient. Therefore we inserted a replication-competent, ecotropic retrovirus vector containing the green fluorescent protein (GFP) reporter gene as a second-stage component. The well-characterized host species tropism of each vector component provided a stringent biological assay system that demonstrates the two-stage transduction mechanism of the hybrid vector, because the adenovirus stage can efficiently transduce human cells but cannot replicate in murine cells, and conversely, the ecotropic retrovirus stage cannot enter human cells but can efficiently proliferate in murine cells, resulting in permanent integration and progressive spread of reporter gene expression.

Hematopoietic stem cell gene therapy

Gene therapy applications that target hematopoietic stem cells (HSCs) offer great potential for the treatment of hematologic disease. Despite this promise, clinical success has been limited by poor rates of gene transfer, poor engraftment of modified cells, and poor levels of gene expression. We describe here the basic approach used for HSC gene therapy, briefly review some of the seminal clinical trials in the field, and describe several recent advances directed toward overcoming these limitations.

Critical issues in gene therapy for neurologic disease

Gene therapy for the nervous system is a newly emerging field with special issues related to modes of delivery, potential toxicity, and realistic expectations for treatment of this vital and highly complex tissue. This review focuses on the potential for gene delivery to the brain, as well as possible risks and benefits of these procedures. This includes discussion of appropriate vectors, such as adeno-associated virus, lentivirus, gutless adenovirus, and herpes simplex virus hybrid amplicons, and cell vehicles, such as neuroprogenitor cells. Routes of delivery for focal and global diseases are enumerated, including use of migratory cells, facilitation of vascular delivery across the blood-brain barrier, cerebrospinal fluid delivery, and convection injection. Attention is given to examples of diseases falling into different etiologic types: metabolic deficiency states, including Canavan disease and lysosomal storage disorders; and degenerative conditions, including Parkinson's disease and other neurodegenerative conditions.

A high-capacity, capsid-modified hybrid adenovirus/adeno-associated virus vector for stable transduction of human hematopoietic cells

To achieve stable gene transfer into human hematopoietic cells, we constructed a new vector, DeltaAd5/35.AAV. This vector has a chimeric capsid containing adenovirus type 35 fibers, which conferred efficient infection of human hematopoietic cells. The DeltaAd5/35.AAV vector genome is deleted for all viral genes, allowing for infection without virus-associated toxicity. To generate high-capacity DeltaAd5/35.AAV vectors, we employed a new technique based on recombination between two first-generation adenovirus vectors. The resultant vector genome contained an 11.6-kb expression cassette including the human gamma-globin gene and the HS2 and HS3 elements of the beta-globin locus control region. The expression cassette was flanked by adeno-associated virus (AAV) inverted terminal repeats (ITRs). Infection with DeltaAd5/35.AAV allowed for stable transgene expression in a hematopoietic cell line after integration into the host genome through the AAV ITR(s). This new vector exhibits advantages over existing integrating vectors, including an increased insert capacity and tropism for hematopoietic cells. It has the potential for stable ex vivo transduction of hematopoietic stem cells in order to treat sickle cell disease.

Highly efficient targeted transduction of undifferentiated human hematopoietic cells by adenoviral vectors displaying fiber knobs of subgroup B

Human hematopoietic stem cells (HSCs) are poorly transduced by vectors based on adenovirus serotype 5 (Ad5). This is primarily due to the paucity of the coxsackievirus-Ad receptor on these cells. In an attempt to change the tropism of Ad5, we constructed a series of chimeric E1-deleted Ad5 vectors in which the shaft and knob of the capsid fibers were exchanged with those of other Ad serotypes. In all these vectors, the Ad E1 region was replaced by an expression cassette containing the cytomegalovirus immediate-early promoter and the gene for enhanced green fluorescent protein (GFP). Experiments performed in vitro showed an efficient transduction of umbilical cord blood (UCB) monocytes, granulocytes, and their precursors as well as the undifferentiated CD34(+) CD33(-) CD38(-) CD71(-) cells by Ad5 vectors carrying Ad subgroup B-specific fiber chimeras (Ad5FBs). In the latter subpopulation, which comprises less than 1% of the CD34(+) cells and is highly enriched with cells repopulating immunodeficient mice, more than 90% of the cells were GFP(+). Transduction by Ad5FBs of the less primitive fraction within UCB CD34(+) cells was significant lower. Actually, the transduction frequency and GFP level declined gradually with increased expression of the CD33, CD38, and CD71 antigens. Flow cytometric analysis of transduced UCB CD34(+) cells that were cultured for 5 days on an allogeneic human bone marrow stroma layer showed maintenance of the phenotypically defined HSCs at levels similar to those of control cultures. The latter finding indicates that neither the transduction procedure nor the high levels of GFP were toxic for these cells.

Generation of a high-capacity hybrid vector: packaging of recombinant adenoassociated virus replicative intermediates in adenovirus capsids overcomes the limited cloning capacity of adenoassociated virus vectors

Gene therapy aims to complement or, ideally, correct defective genes. The broad clinical application of this emerging technology requires the development of safe high-capacity gene delivery vehicles that combine efficient transduction of dividing as well as quiescent cells with sustained transgene expression. Here we present a new hybrid vector system that unites favorable attributes of adenoassociated virus (AAV) and adenovirus (Ad) vectors in a single particle. This was achieved by inclusion of Ad packaging elements in different sized recombinant AAV genomes. In the presence of AAV replicative functions and a recombinant helper Ad, AAV/Ad hybrid particles were generated via encapsidation of AAV-dependent replicative intermediates into Ad capsids. In stringent in vitro models based on transduction of proliferating cells we show that AAV/Ad hybrid vectors are superior to Ad vectors in establishing prolonged transgene expression and can be used to deliver DNA fragments of at least 27 kb.