Effect of the E4 region on the persistence of transgene expression from adenovirus vectors

Protease-deleted adenovirus as an alternative for replication-competent adenovirus vector

For cancer therapy and vaccination an amplified expression of the therapeutic gene is desired. Previously, we have developed a single-cycle adenovirus vector (SC-AdV) by deleting the adenovirus protease (PS) gene. In order to keep the E1 region intact within the PS-deleted adenoviruses, we examined the insertion of two transgenes under the control of a constitutive or inducible promoters. These were inserted between E4 and the right inverted terminal repeat in a wide variety of backbones with various combinations of PS, E3 and E4 deletion. Our data showed that PS-deleted adenoviruses, expressed transgenes as strongly as replication-competent AdVs in HEK293A and a variant of HeLa cells. In a head-to-head comparison in four human cell lines, we demonstrated that SC-AdV, was comparable for transgene expression efficacy with its replication-competent counterpart. However, the SC-AdV expresses its transgene 10 to 16,000 times higher than its replication-defective counterpart.

Capsid and Genome Modification Strategies to Reduce the Immunogenicity of Adenoviral Vectors

Adenovirus-based gene transfer vectors are the most frequently used vector type in gene therapy clinical trials to date, and they play an important role as genetic vaccine candidates during the ongoing SARS-CoV-2 pandemic. Immediately upon delivery, adenovirus-based vectors exhibit multiple complex vector-host interactions and induce innate and adaptive immune responses. This can severely limit their safety and efficacy, particularly after delivery through the blood stream. In this review article we summarize two strategies to modulate Ad vector-induced immune responses: extensive genomic and chemical capsid modifications. Both strategies have shown beneficial effects in a number of preclinical studies while potential synergistic effects warrant further investigations.

Introducing Genes into Mammalian Cells: Viral Vectors

Over the years, many different viral vector systems have been developed to take advantage of the specific biological properties and tropisms of a large number of mammalian viruses. As a result, researchers wanting to introduce and/or express genes in mammalian cells have many options, as discussed here.

Biological Approaches to Bone Regeneration by Gene Therapy

Safe, effective approaches for bone regeneration are needed to reverse bone loss caused by trauma, disease, and tumor resection. Unfortunately, the science of bone regeneration is still in its infancy, with all current or emerging therapies having serious limitations. Unlike current regenerative therapies that use single regenerative factors, the natural processes of bone formation and repair require the coordinated expression of many molecules, including growth factors, bone morphogenetic proteins, and specific transcription factors. As will be developed in this article, future advances in bone regeneration will likely incorporate therapies that mimic critical aspects of these natural biological processes, using the tools of gene therapy and tissue engineering. This review will summarize current knowledge related to normal bone development and fracture repair, and will describe how gene therapy, in combination with tissue engineering, may mimic critical aspects of these natural processes. Current gene therapy approaches for bone regeneration will then be summarized, including recent work where combinatorial gene therapy was used to express groups of molecules that synergistically interacted to stimulate bone regeneration. Last, proposed future directions for this field will be discussed, where regulated gene expression systems will be combined with cells seeded in precise three-dimensional configurations on synthetic scaffolds to control both temporal and spatial distribution of regenerative factors. It is the premise of this article that such approaches will eventually allow us to achieve the ultimate goal of bone tissue engineering: to reconstruct entire bones with associated joints, ligaments, or sutures. Abbreviations used: BMP, bone morphogenetic protein; FGF, fibroblast growth factor; AER, apical ectodermal ridge; ZPA, zone of polarizing activity; PZ, progress zone; SHH, sonic hedgehog; OSX, osterix transcription factor; FGFR, fibroblast growth factor receptor; PMN, polymorphonuclear neutrophil; PDGF, platelet-derived growth factor; IGF, insulin-like growth factor; TGF-β, tumor-derived growth factor β; CAR, coxsackievirus and adenovirus receptor; MLV, murine leukemia virus; HIV, human immunodeficiency virus; AAV, adeno-associated virus; CAT, computer-aided tomography; CMV, cytomegalovirus; GAM, gene-activated matrix; MSC, marrow stromal cell; MDSC, muscle-derived stem cell; VEGF, vascular endothelial growth factor.

Suppression of leaky expression of adenovirus genes by insertion of microRNA-targeted sequences in the replication-incompetent adenovirus vector genome

Leaky expression of adenovirus (Ad) genes occurs following transduction with a conventional replication-incompetent Ad vector, leading to an induction of cellular immunity against Ad proteins and Ad protein-induced toxicity, especially in the late phase following administration. To suppress the leaky expression of Ad genes, we developed novel Ad vectors by incorporating four tandem copies of sequences with perfect complementarity to miR-122a or miR-142-3p into the 3′-untranslated region (UTR) of the E2A, E4, or pIX gene, which were mainly expressed from the Ad vector genome after transduction. These Ad vectors easily grew to high titers comparable to those of a conventional Ad vector in conventional 293 cells. The leaky expression of these Ad genes in mouse organs was significantly suppressed by 2- to 100-fold, compared with a conventional Ad vector, by insertion of the miRNA-targeted sequences. Notably, the Ad vector carrying the miR-122a–targeted sequences into the 3′-UTR of the E4 gene expressed higher and longer-term transgene expression and more than 20-fold lower levels of all the Ad early and late genes examined in the liver than a conventional Ad vector. miR-122a–mediated suppression of the E4 gene expression in the liver significantly reduced the hepatotoxicity which an Ad vector causes via both adaptive and non-adaptive immune responses.

[Development of a novel adenovirus vector exhibiting microRNA-mediated suppression of the leaky expression of adenovirus genes]

Replication-incompetent adenovirus (Ad) vectors are widely used in gene therapy studies because they are beneficial as a gene delivery vehicle enabling high-titer production and highly efficient gene transfer into a wide spectrum of dividing and non-dividing cells in vitro and in vivo. Theoretically, Ad genes should not be expressed following transduction with a replication-incompetent Ad vector. However, leaky expression of viral genes is known to occur following transduction with a conventional Ad vector, which leads to a cellular immunity against Ad proteins as well as Ad protein-induced toxicity. Such Ad protein-induced cellular immunity and toxicity frequently cause both an elimination of Ad vector-transduced cells and tissue damage, leading to short-lived transgene expression. To date, no detailed analysis of the leaky expression profile of Ad genes has been performed. First, we systematically examined the expression profiles of Ad genes in cells using real-time RT-PCR following transduction with a conventional Ad vector. The results revealed that significant expression was found for E2A, E4, and pIX genes. Next, in order to suppress the leaky expression of Ad genes, complementary sequences for microRNA (miRNA) were inserted into the 3'-untranslated region of the E2A, E4, or pIX genes. miRNAs are an approximately 22-nt length non-coding RNA, and bind to imperfectly complementary sequences in the 3'-untranslated region of target mRNA, leading to suppression of gene expression via post-transcriptional regulation. Incorporation of the miRNA-targeted sequences significantly suppressed the leaky expression of Ad genes in an miRNA-dependent manner.

Inhalation of nebulized perfluorochemical enhances recombinant adenovirus and adeno-associated virus-mediated gene expression in lung epithelium

Use of perfluorochemical liquids during intratracheal vector administration enhances recombinant adenovirus and adeno-associated virus (AAV)-mediated lung epithelial gene expression. We hypothesized that inhalation of nebulized perfluorochemical vapor would also enhance epithelial gene expression after subsequent intratracheal vector administration. Freely breathing adult C57BL/6 mice were exposed for selected times to nebulized perflubron or sterile saline in a sealed Plexiglas chamber. Recombinant adenoviral vector was administered by transtracheal puncture at selected times afterward and mice were killed 3 days after vector administration to assess transgene expression. Mice tolerated the nebulized perflubron without obvious ill effects. Vector administration 6 hr after nebulized perflubron exposure resulted in an average 540% increase in gene expression in airway and alveolar epithelium, compared with that with vector alone or saline plus vector control (p<0.05). However, vector administration 1 hr, 1 day, or 3 days after perflubron exposure was not different from either nebulized saline with vector or vector alone and a 60-min exposure to nebulized perflubron is required. In parallel pilot studies in macaques, inhalation of nebulized perflubron enhanced recombinant AAV2/5 vector expression throughout the lung. Serial chest radiographs, bronchoalveolar lavages, and results of complete blood counts and serum biochemistries demonstrated no obvious adverse effects of nebulized perflubron. Further, one macaque receiving nebulized perflubron only was monitored for 1 year with no obvious adverse effects of exposure. These results demonstrate that inhalation of nebulized perflubron, a simple, clinically more feasible technique than intratracheal administration of liquid perflubron, safely enhances lung gene expression.

The non-essential left end region of the fowl adenovirus 9 genome is suitable for foreign gene insertion/replacement

The goals of this study were to demonstrate that a non-essential region at the left end of the fowl adenovirus 9 (FAdV-9) genome could be used to generate recombinant viruses, examine their in vitro growth characteristics and determine their ability to transduce non-avian cells. Three FAdV-9 vectors (rFAdV-9s) were generated carrying the enhanced-green fluorescent protein (EGFP) gene: FAdV-9inEGFP, FAdV-9 Delta 1-EGFP and FAdV-9 Delta 4-EGFP. FAdV-9inEGFP carried the EGFP cassette inserted into the non-essential region without deletion resulting in an increase of the genome size to 103.7% of the wild-type. FAdV-9 Delta 1-EGFP and FAdV-9 Delta 4-EGFP (rFAdV-9 Delta s) carried the EGFP cassette replacing the non-essential sequences at nucleotides 1194-2342 and 491-2782, respectively. All rFAdV-9s had wild-type growth kinetics and plaque morphology. The rFAdV-9 Delta s replicated in CH-SAH cells with the same titers as the wild-type virus. The FAdV-9inEGFP titers were approximately 1 log lower than those of rFAdV-9 Delta s and wt FAdV-9 at 36 and 48 h post-infection (h.p.i.). EGFP was expressed in avian and mammalian cells infected with rFAdV-9s. EGFP expression, based on spectrofluorometry, was significantly higher in chicken hepatoma cells infected with FAdV-9inEGFP than in those with rFAdV-9 Delta s at 18 and 24h.p.i, suggesting a functional role of some or all non-essential ORFs on foreign gene expression. This study demonstrated the suitability of the non-essential region as an insertion/replacement site for foreign genes to generate FAdV-9-based vectors that can be applied as recombinant vaccines for poultry or gene delivery vehicles for mammalian systems.

Viral vector approaches to modify gene expression in the brain

The use of viral vectors as gene transfer tools for the central nervous system has seen a significant growth in the last decade. Improvements in the safety, efficiency and specificity of vectors for clinical applications have proven to be beneficial also for basic neuroscience research. This review will discuss the viral systems currently available to neuroscientists and some of the recent achievements in the study of synaptic function, memory and drug addiction.

Gene delivery by adenoviruses

Adenoviruses have a number advantages as gene delivery vectors, including ability to transduce a wide variety of non-dividing and dividing cells with high efficiency, relative ease of construction, and ability to be purified as high-titer viral stocks. These characteristics make adenoviruses particularly attractive for over-expressing specific genes in vitro and for evaluating in vivo biological activity in animal models. In this chapter, procedures will be described for adenovirus construction and virus delivery to in vivo sites for measurement of osteogenic activity.

PEGylated adenovirus for targeted gene therapy

Bifunctional polyethylene glycol (PEG) molecules provide a novel approach to retargeting viral vectors without the need to genetically modify the vector. Modification of the surface of adenovirus with heterofunctional PEG allows further modification of the capsid with ligands. In addition, heterofunctional PEG modification ablates the normal tropism of the virus and reduces transduction of non-target tissues in vivo. Moreover, the addition of PEG chains to the surface of the virus shields antigen-binding sites, significantly reducing the susceptibility of the virus to antibody neutralization. Finally, T cell subsets from mice exposed to the PEGylated vector demonstrate a marked decrease in Th1 and Th2 responses, suggesting that PEG modification may help reduce the immune response to the vector.

Viral-mediated gene therapy for the muscular dystrophies: successes, limitations and recent advances

Much progress has been made over the past decade elucidating the molecular basis for a variety of muscular dystrophies (MDs). Accordingly, there are examples of mouse models of MD whose disease progression has been halted in large part with the use of viral vector technology. Even so, we must acknowledge significant limitations of present vector systems that must be overcome prior to successful treatment of humans with such approaches. This review will present a variety of viral-mediated therapeutic strategies aimed at counteracting the muscle-wasting symptoms associated with muscular dystrophy. We include viral vector systems used for muscle gene transfer, with a particular emphasis on adeno-associated virus. Findings of several encouraging studies focusing on repair of the mutant dystrophin gene are also included. Lastly, we present a discussion of muscle compensatory therapeutics being considered that include pathways involved in the up-regulation of utrophin, promotion of cellular adhesion, enhancement of muscle mass, and antagonism of the inflammatory response. Considering the complexity of the muscular dystrophies, it appears likely that a multilayered approach tailored to a patient sub-group may be warranted in order to effectively contest the progression of this devastating disease.

Transcription mapping and characterization of proteins produced from early region 4 of porcine adenovirus type 3

The early region 4 (E4) of porcine adenovirus 3 (PAdV-3) was characterized by Northern blot, rapid amplification of cDNA ends (RACE), RT-PCR and cDNA sequence analysis. Northern blot analysis revealed three different classes of transcripts, which appeared and peaked at different times post-infection. The RT-PCR, RACE and cDNA sequence analysis identified nine major E4 transcripts, all of which shared a 107-bp 5' leader sequence and a 126-bp 3' terminus. These transcripts have one to three introns removed. Interestingly, of the nine major transcripts, there was one fusion transcript of ORFp1 and ORFp7 (ORFp1/7), which codes for a protein of 119 amino acids. All transcripts initiated at nucleotide 33740 of the PAdV-3 genome. To identify proteins, rabbit antiserum was prepared using a bacterial fusion protein encoding p2, p3, p4 or p7 proteins. Serum against p2, p3 and p4 immunoprecipitated proteins of 13.5, 13.6 and 15.3 kDa, respectively, in in-vitro transcribed and translated mRNA and in PAdV-3-infected cells. Serum against p7 immunoprecipitated a protein of 19.8 kDa in in-vitro transcription and translation analysis but recognized two proteins of 19.8 kDa (encoded by ORFp7) and 14 kDa (encoded by the fusion transcript ORF1/7) in PAdV-3-infected cells. The protein encoded by ORFp2 was localized in the nucleus of PAdV-3-infected cells. The proteins encoded by ORFp3 and ORFp7\ORFp1/7 were detected in the cytoplasm of PAdV-3-infected cells. However, the protein encoded by ORFp4 was observed both in the cytoplasm and nucleus of PAdV-3-infected cells.

Gene therapy in cardiac arrhythmias

Gene therapy has progressed from a dream to a bedside reality in quite a few human diseases. From its first application in adenosine deaminase deficiency, through the years, its application has evolved to vascular angiogenesis and cardiac arrhythmias. Gene based biological pacemakers using viral vectors or mesenchymal cells tested in animal models hold much promise. Induction of pacemaker activity within the left bundle branch can provide stable heart rates. Genetic modification of the AV node mimicking beta blockade can be therapeutic in the management of atrial fibrillation. G protein overexpression to modify the AV node also is experimental. Modification and expression of potassium channel genes altering the delayed rectifier potassium currents may permit better management of congenital long QT syndromes. Arrhythmias in a failing heart are due to abnormal calcium cycling. Potential targets for genetic modulation include the sarcoplasmic reticulum calcium pump, calsequestrin and sodium calcium exchanger. Lastly the ethical concerns need to be addressed.

Gutless adenovirus: last-generation adenovirus for gene therapy

Last-generation adenovirus vectors, also called helper-dependent or gutless adenovirus, are very attractive for gene therapy because the associated in vivo immune response is highly reduced compared to first- and second-generation adenovirus vectors, while maintaining high transduction efficiency and tropism. Nowadays, gutless adenovirus is administered in different organs, such as the liver, muscle or the central nervous system achieving high-level and long-term transgene expression in rodents and primates. However, as devoid of all viral coding regions, gutless vectors require viral proteins supplied in trans by a helper virus. To remove contamination by a helper virus from the final preparation, different systems based on the excision of the helper-packaging signal have been generated. Among them, Cre-loxP system is mostly used, although contamination levels still are 0.1-1% too high to be used in clinical trials. Recently developed strategies to avoid/reduce helper contamination were reviewed.

Production and formulation of adenovirus vectors

Adenovirus vectors have attracted considerable interest over the past decade, with ongoing clinical development programs for applications ranging from replacement therapy for protein deficiencies to cancer therapeutics to prophylactic vaccines. Consequently, considerable product, process, analytical, and formulation development has been undertaken to support these programs. For example, "gutless" vectors have been developed in order to improve gene transfer capacity and durability of expression; new cell lines have been developed to minimize recombination events; production conditions have been optimized to improve volumetric productivities; analytical techniques and scaleable purification processes have advanced towards the goal of purified adenovirus becoming a "well-characterized biological"; and liquid formulations have been developed which maintain virus infectivity at 2-8 degrees C for over 18 months. These and other advances in the production of adenovirus vectors are discussed in detail in this review. In addition, the needs for the next decade are highlighted.

Local Delivery of a Viral Vector Mitigates Neutralization by Antiviral Antibodies and Results in Efficient Transduction of Rabbit Liver

Antiviral antibodies within the human population remain a barrier to the effective clinical use of viral gene transfer vectors. We have asked whether local, balloon catheter-mediated delivery of a viral vector to the rabbit liver using a hepatic vein might mitigate the neutralizing effects of antiviral antibodies. We have compared directly the ability of adenovirus (Ad2) encoding nuclear-localized beta-galactosidase to infect the rabbit liver after local and systemic delivery in both the presence and the absence of defined anti-Ad2 antibody titers. In naive rabbits, local delivery resulted in higher beta-galactosidase expression compared to systemic delivery. In the presence of passively administered anti-Ad2 antibodies, local delivery resulted in expression levels that were comparable to those obtained in naive rabbits by systemic delivery. Local delivery also resulted in the majority of expression originating from hepatocytes, even in passively immunized animals, a result that could not be duplicated using the systemic approach. Since systemic delivery of adenovirus in naive animal models results in transgene expression levels often regarded as therapeutic, these results predict that local hepatic vein delivery of a viral vector is a clinically practical approach to mitigate neutralizing antiviral antibodies and generate therapeutic levels of transgene expression.

Gene therapy: twenty-first century medicine

Broadly defined, the concept of gene therapy involves the transfer of genetic material into a cell, tissue, or whole organ, with the goal of curing a disease or at least improving the clinical status of a patient. A key factor in the success of gene therapy is the development of delivery systems that are capable of efficient gene transfer in a variety of tissues, without causing any associated pathogenic effects. Vectors based upon many different viral systems, including retroviruses, lentiviruses, adenoviruses, and adeno-associated viruses, currently offer the best choice for efficient gene delivery. Their performance and pathogenicity has been evaluated in animal models, and encouraging results form the basis for clinical trials to treat genetic disorders and acquired diseases. Despite some initial success in these trials, vector development remains a seminal concern for improved gene therapy technologies.

Novel molecular approaches to cystic fibrosis gene therapy

Gene therapy holds promise for the treatment of a range of inherited diseases, such as cystic fibrosis. However, efficient delivery and expression of the therapeutic transgene at levels sufficient to result in phenotypic correction of cystic fibrosis pulmonary disease has proved elusive. There are many reasons for this lack of progress, both macroscopically in terms of airway defence mechanisms and at the molecular level with regard to effective cDNA delivery. This review of approaches to cystic fibrosis gene therapy covers these areas in detail and highlights recent progress in the field. For gene therapy to be effective in patients with cystic fibrosis, the cDNA encoding the cystic fibrosis transmembrane conductance regulator protein must be delivered effectively to the nucleus of the epithelial cells lining the bronchial tree within the lungs. Expression of the transgene must be maintained at adequate levels for the lifetime of the patient, either by repeat dosage of the vector or by targeting airway stem cells. Clinical trials of gene therapy for cystic fibrosis have demonstrated proof of principle, but gene expression has been limited to 30 days at best. Results suggest that viral vectors such as adenovirus and adeno-associated virus are unsuited to repeat dosing, as the immune response reduces the effectiveness of each subsequent dose. Nonviral approaches, such as cationic liposomes, appear more suited to repeat dosing, but have been less effective. Current work regarding non-viral gene delivery is now focused on understanding the mechanisms involved in cell entry, endosomal escape and nuclear import of the transgene. There is now increasing evidence to suggest that additional ligands that facilitate endosomal escape or contain a nuclear localization signal may enhance liposome-mediated gene delivery. Much progress in this area has been informed by advances in our understanding of the mechanisms by which viruses deliver their genomes to the nuclei of host cells.

Detection of plasmid DNA vectors following gene transfer to the murine airways

Non-viral gene therapy is being considered as a treatment for cystic fibrosis. In clinical studies and in studies using the mouse airways as a model, current formulations result in only transient transgene expression. A number of reasons for this have been proposed including the loss of plasmid DNA from cells. The aim of these studies was to investigate why transgene expression from non-viral vectors is transient in the mouse lung. Plasmid DNA encoding the luciferase reporter gene was complexed with the cationic lipid GL67 and delivered to the mouse airways. The persistence of plasmid DNA in the mouse lungs was investigated using quantitative PCR and Southern hybridization. Results showed that intact plasmid DNA persisted in the mouse lung in the absence of any detectable luciferase activity. The de novo methylation of plasmid DNA in vivo was investigated as a potential cause of this transient gene expression but results suggested that plasmid DNA does not become de novo methylated in the mouse lung. Therefore processes other than the loss of plasmid DNA from the lung or the de novo methylation of plasmid DNA vectors must be responsible for the transient transgene expression.

Induction and Inhibition of Innate Inflammatory Responses by Adenovirus Early Region Proteins

First-generation adenovirus (Ad) gene therapy vectors deleted for the E1A, E1B, and E3 regions and carrying foreign genes under the control of strong foreign promoters induce high-level innate inflammatory responses within the first 24 hrs after transduction. Both uptake of the capsid and expression of gene products encoded by the vector contribute to the innate inflammatory response. Natural infections by Ad are frequently asymptomatic, suggesting that Ad has potent methods of inhibiting inflammation. The inability of Ad vectors to counter inflammatory responses suggests that the products of the Ad genes deleted in vector construction play critical roles in inhibiting these responses. Genetic analysis of the roles of Ad early region gene functions in vivo demonstrated that a virus made replication-incompetent by deletion of the preterminal protein gene and deleted for the transcriptional activation function of E1A effectively inhibits the innate inflammatory processes induced by Ad vectors. The mechanism(s) by which the Ad early region proteins inhibit inflammation is complex, as certain early region proteins can promote as well as inhibit inflammation, depending on the genetic context of the virus. Understanding of the roles of the Ad gene products in the induction and inhibition of innate inflammatory functions offers potential for the development of non-inflammatory vectors as well as for understanding of the mechanisms by which inflammation is regulated.

Adenoviral vectors: development and application

One of the prerequisites for the successful application of gene vaccination and therapy is the development of efficient gene delivery vectors. The rate-limiting nature of vectors was clearly manifested during the first wave of gene therapy testing, resulting in the demand for more effective and suitable vector systems. Adenoviral (Ad) vectors have recently played a central role in the development of gene-vector technology due to their practical advantages and potential applications. A large number of preclinical and clinical studies both have generated an overwhelming amount of data and literature on this vector system. It is the intention of this article to provide a systematic and broad spectrum review of this system, outlining the principle, potential, and limitations, and evaluating the rational development of this delivery approach. Recombinant adenoviruses (Ad), helper cell lines, and related technologies have been developed and applied to many indications owing to progress in virological research, molecular and cellular biology, eukaryotic protein expression, recombinant vaccines, and gene therapy. The technical depth this article covers should be useful to both the experienced researcher and to beginners in this field.

Immune responses against adenoviral vectors and their transgene products: a review of strategies for evasion

Human adenoviruses have been adopted as attractive vectors for in vivo gene therapy since they have a well-characterized genomic organization, can be grown to high titres and efficiently transduce a wide spectrum of dividing and non-dividing cells. However, the first-generation of adenoviral (Ad) vectors yielded only transient expression of the transgene in most immunocompetent mice. This constituted a major limitation of this early vector type. In contrast, persistent transgene expression can be established in immunodeficient mice. This suggests that the immunogenicity of adenoviral vectors limits the effective period of adenovirus-based gene therapy. Much effort has been put in devising strategies to circumvent the limitations imposed onto gene therapy by the immune system. Improvements in vector design have significantly improved the performance of the adenovirus vectors. Based on these results it is reasonable to anticipate that new modifications of the vectors will overcome some of the immunological barriers and will further expand the applicability of adenovirus-derived vectors.

Prevalence of neutralizing antibodies to adenoviral serotypes 5 and 35 in the adult populations of The Gambia, South Africa, and the United States

One of the major limitations of the use of adenoviruses as gene therapy vectors is the existence of preformed immunity in various populations. Recent studies have linked failure of adenoviral gene therapy trials to the presence of antiadenoviral neutralizing antibodies (NAb). Understanding the distribution and specificity of such antibodies will assist in the design of successful recombinant adenoviral gene therapies and vaccines. To assess the prevalence of NAb to adenovirus serotypes 5 and 35 (Ad5 and Ad35), we analyzed serum samples from adult immunocompetent individuals living in The Gambia, South Africa, and the United States by using a neutralization assay. Serum samples were incubated with A549 lung carcinoma cells and adenoviruses encoding enhanced green or yellow fluorescent proteins; results were analyzed by fluorescence microscopy and flow cytometry. Using this technique, we found a high prevalence of NAb against Ad5 in Gambian, South African, and U.S. subjects at both low and high titers. Conversely, all subjects displayed a low prevalence of NAb to Ad35; when present, anti-Ad35 NAb were seen at low titers. Because of the ability of adenoviruses to elicit systemic and mucosal immune responses, Ad35 with its low NAb prevalence appears to be an attractive candidate vector for gene therapy applications.

Transcription-controlled gene therapy against tumor angiogenesis

A major drawback of current approaches to antiangiogenic gene therapy is the lack of tissue-specific targeting. The aim of this work was to trigger endothelial cell-specific apoptosis, using adenoviral vector-mediated delivery of a chimeric death receptor derived from the modified endothelium-specific pre-proendothelin-1 (PPE-1) promoter. In the present study, we constructed an adenovirus-based vector that targets tumor angiogenesis. Transcriptional control was achieved by use of a modified endothelium-specific promoter. Expression of a chimeric death receptor, composed of Fas and TNF receptor 1, resulted in specific apoptosis of endothelial cells in vitro and sensitization of cells to the proapoptotic effect of TNF-alpha. The antitumoral activity of the vectors was assayed in two mouse models. In the model of B16 melanoma, a single systemic injection of virus to the tail vein caused growth retardation of tumor and reduction of tumor mass with central tumor necrosis. When the Lewis lung carcinoma lung-metastasis model was applied, i.v. injection of vector resulted in reduction of lung-metastasis mass, via an antiangiogenic mechanism. Moreover, by application of the PPE-1-based transcriptional control, a humoral immune response against the transgene was avoided. Collectively, these data provide evidence that transcriptionally controlled, angiogenesis-targeted gene therapy is feasible.

Factors involved in the sensitivity of different hematopoietic cell lines to infection by subgroup C adenovirus: implication for gene therapy of human lymphocytic malignancies

Gene transfer approaches using viruses such human adenovirus (HAdV) may provide an alternative treatment for diseases involving hematopoietic cells. Better understanding of the cellular mechanisms by which the HAdV introduces DNA into these cells should help in vector design. We examined HAdV intracellular delivery in several cell lines including B and T lymphocytes. We demonstrated that HAdV resistance in most B lymphocytes is the result of moderate HAdV uptake. In contrast, high levels of coxsackie and HAdV receptor (hCAR) are expressed on the surface of HSB2 (T cells), allowing efficient binding and uptake but no transgene expression, probably because of deficient endosomolysis and subsequent exocytose. This work demonstrates the existence of hCAR-dependent and -independent endocytic route in hematopoietic cells. Moreover, it precises the intracellular barriers to be overcome by HAdV in such cells to be infectious and gives previous information's to design new vectors for gene transfer.

Delivery of GDNF by an E1,E3/E4 deleted adenoviral vector and driven by a GFAP promoter prevents dopaminergic neuron degeneration in a rat model of Parkinson's disease

A new adenoviral vector (Ad-GFAP-GDNF) (Ad=adenovirus, GFAP=glial fibrillary acidic protein, GDNF=glial cell line-derived neurotrophic factor) was constructed in which (i) the E1,E3/E4 regions of Ad5 were deleted and (ii) the GDNF transgene is driven by the GFAP promoter. We verified, in vitro, that the recombinant GDNF was expressed in primary cultures of astrocytes. In vivo, the Ad-GFAP-GDNF was injected into the striatum of rats 1 week before provoking striatal 6-OHDA lesion. After 1 month, the striatal GDNF levels were 37 pg/microg total protein. This quantity was at least 120-fold higher than in nontransduced striatum or after injection of the empty adenoviral vector. At 3 months after viral injection, GDNF expression decreased, whereas the viral DNA remained unchanged. Furthermore, around 70% of the dopaminergic (DA) neurons were protected from degeneration up to 3 months as compared to about 45% in the control groups. In addition, the amphetamine-induced rotational behavior was decreased. The results obtained in this study on DA neuron protection and rotational behavior are similar to those previously reported using vectors with viral promoters. In addition to these results, we established that a high level of GDNF was present in the striatum and that the period of GDNF expression was prolonged after injection of our adenoviral vector.

Adenovirus vectors: biology, design, and production

The use of adenovirus as a gene transfer vehicle arose from early reports of recombinant viruses carrying heterologous DNA fragments. Adenovirus vectors offer many advantages for gene delivery: they are easy to propagate to high titers, they can infect most cell types regardless of their growth state, and in their most recent embodiments they can accommodate large DNA inserts. In this chapter, the development of adenovirus vectors is reviewed, from the use of so-called first-generation, E1-deleted viruses to the latest generation high-capacity, helper-dependent vectors. Examples of their use in the clinic are described, as are the current areas in which improvements to these vectors are being explored.

Transient increase in lung epithelial tight junction permeability: an additional mechanism for enhancement of lung transgene expression by perfluorochemical liquids

Intratracheal instillation of perfluorochemical (PFC) liquids enhances lung epithelial transgene expression by improved vector propulsion throughout lung airways. We now demonstrate that PFC liquids also facilitate gene transfer by increasing transepithelial permeability. Apical application of PFC liquid to well-differentiated human airway epithelial cells resulted in a transient decrease in transepithelial resistance peaking approximately 2 h after PFC liquid administration and returning to normal approximately 24 h later. The permeability change was sufficient to enhance access of apically applied 100-nm latex beads and adenoviral vectors to the basolateral side of the culture. Adenovirus-mediated gene expression was concurrently enhanced. Following intratracheal instillation of PFC liquid into mouse lungs, tight junction permeability, as assessed by electron microscopic evaluation of lanthanum deposition, was increased with peak effect between 6 h and 1 day after instillation. Importantly, alveolar-capillary permeability remained unchanged with the treatment. Administration of PFC liquid 6 h or 1 day, but not 3 days, prior to instillation of a recombinant adenovirus vector enhanced gene expression comparable to that observed with concurrent administration of PFC liquid and vector. We conclude that transient increase in epithelial permeability after PFC liquid administration contributes to the enhancement of adenovirus vector-mediated gene expression in lung epithelium.

Comparative analysis of the effects of packaging signal, transgene orientation, promoters, polyadenylation signals, and E3 region on growth properties of first-generation adenoviruses

First-generation adenovectors have been developed for gene therapy and vaccine applications. The construction of these adenovectors has entailed the use of numerous types of expression cassettes. It has long been known that first-generation adenovectors can be rescued more easily and to higher titers with some transgenes than with others. This study has systematically shown that there can be marked differences in growth properties of recombinant adenovectors attributable to the use of promoters, the orientation of the transgene within the E1A/E1B-deleted region, and the inclusion of the E3 region. In addition, we had demonstrated the benefit of extending the packaging signal region to include elements V, VI, and VII. The effects of the complete packaging region were studied by plasmid competition studies between original and modified adenovectors. Similar competition studies between E3(+) and E3(-) adenovectors were performed and showed that the E3(+) vector had a growth advantage over its E3(-) counterpart. By making various changes, we have enhanced the growth capacity of our recombinant adenovector by more than 3-fold under serum-free and cell suspension growth conditions. Along with this enhanced growth, our adenovectors have maintained their genetic stability after 21 successive passages in cell culture. This increased robustness will be critical when adapting first-generation recombinant adenovectors to commercial production.

In vivo ligand-inducible regulation of gene expression in a gutless adenoviral vector system

Transcriptional regulation that is rapid, reversible, and repeatedly inducible would greatly enhance the safety and efficacy of many gene therapy strategies. We developed a chimeric ligand-inducible regulation system based on the human estrogen receptor. This system has two components, the responsive promoter driving expression of the transgene of interest, and the ligand-inducible chimeric transcription factor. The transcription factor is composed of a novel DNA binding domain and a modified estrogen receptor ligand-binding domain. A point mutation in the ligand-binding domain significantly reduces estrogen binding while allowing binding of the estrogen antagonist, tamoxifen. We used a gutless adenoviral vector system and incorporated both components into two separate vectors. A single gutless vector encoding both system components was also generated. The tamoxifen-mediated induciblity of transgene expression of the gutless vector system was compared in vitro and in vivo with the analogous components incorporated into early generation, E1/E2a/E3-deficient adenoviral vectors. In normal mice, both the gutless vector and early generation systems displayed inducibility in the presence of tamoxifen. Importantly, the gutless vector system was inducible to extremely high levels, at least four times over a 2-month period. In contrast, the early generation vector system was inducible only once. Furthermore, the early generation system displayed significant toxicity, as evidenced by extremely high liver enzyme levels, abnormal liver pathology, and rapid loss of vector DNA from the liver, while the gutless vector system displayed minimal toxicity. These data directly demonstrate the improved in vivo function of the tamoxifen-inducible transcriptional regulation system in the context of the gutless adenoviral vectors.

Gene therapy and tissue engineering in repair of the musculoskeletal system

Historically, surgeons have sought and used different procedures in order to augment the repair of various skeletal tissues. Now, with the completion of the Human Genome Project, many researchers have turned to gene therapy as a means to aid various ailments. In the orthopedic field, many strides have been made toward using gene therapy and tissue engineering in a clinical setting. In this review, several studies are outlined in different areas that gene therapy has or will influence orthopedic surgery. Gene therapy and tissue engineering can aid in fracture healing and spinal fusions by inducing bone formation, ligamentous repairs by increasing the production of connective tissue fibers, intervertebral disc disease by creating potential replacements, and articular cartilage repairs by providing means to improve cartilage. As we continue to see great contributions, such as the few mentioned here, this field will continue to mature and develop.

Persistent hepatic expression of human apo A-I after transfer with a helper-virus independent adenoviral vector

Gene transfer with 'gutted' vectors is associated with persistent transgene expression and absence of hepatotoxicity, but the requirement of helper viruses hampers efficient production and leads to contamination of viral batches with these helper-viruses. In the present study, gene transfer with a helper-virus independent E(1)/E(3)/E(4)-deleted adenoviral vector induced persistent expression of human apo A-I (200 +/- 16 mg/dl at day 35, 190 +/- 15 mg/dl at 4 months, 170 +/- 16 mg/dl at 6 months) and stable transgene DNA levels (3.5 +/- 0.60 at day 35, 3.3 +/- 0.39 at 4 months, 3.1 +/- 0.47 mg/dl at 6 months) in C57BL/6 mice in the absence of significant toxicity. The vector contained the 1.5 kb human alpha(1)-antitrypsin promoter in front of the genomic human apo A-I sequence and four copies of the human apo E enhancer (hAAT.gA-I.4xapoE) and was deleted in E(1), E(3) and E(4). Reintroduction of E(4) ORF 3 and E(4) ORF 4 in the viral backbone caused a more than four-fold decline of transgene DNA between day 35 and 4 months after transfer both in wild-type and in C57BL/6 SCID and C57BL/6 Rag-1(-/-) mice, indicating that the effect of E(4) ORF 3 and E(4) ORF 4 is independent of a cellular immune response against viral epitopes. Co-injection of an E(1)-deleted vector containing no expression cassette and the E(1)/E(3)/E(4)-deleted vector containing the hAAT.gA-I.4xapoE expression cassette indicated that E(4) gene products destabilize transgene DNA in trans. Gene transfer with an E(1)/E(3)/E(4)-deleted vector containing only E(4) ORF 3 and the hAAT.gA-I.4xapoE expression cassette was associated with transgene DNA decline, but not with hepatotoxicity, indicating that transgene DNA persistence and hepatotoxicity are dissociated processes. After transfer with E(1)/E(3)/E(4)-deleted vectors containing expression cassettes with a different promoter or a different position of the apo E enhancers, transgene DNA levels were less stable than after transfer with the vector containing hAAT.gA-I.4xapoE, indicating that the expression cassette is an important determinant of episomal stability. In conclusion, gene transfer with an E(1)/E(3)/E(4)-deleted vector containing the hAAT.gA-I.4xapoE expression cassette induces persistent expression of human apo A-I in the absence of hepatotoxicity. Transgene DNA turnover is independent of an adaptive cellular immune response against viral epitopes and of hepatotoxicity. E(1)/E(3)/E(4)-deleted vectors containing transgenes under control of the hAAT promoter in combination with four copies of the human apo E enhancer may be suitable for hepatocyte-specific overexpression of transgenes after gene transfer. doi:10.1038/sj.gt.3301824

PEGylation of E1-deleted adenovirus vectors allows significant gene expression on readministration to liver

Systemic administration of adenoviral vectors leads to activation of innate and antigen-specific immunity. In an attempt to diminish T and B cell-specific immune responses to E1-deleted adenoviral vectors, capsid proteins were modified with various activated monomethoxypolyethylene glycols (MPEGs). The impact of this modification was studied in a murine model of liver-directed gene transfer in which an E1-deleted adenovirus expressing the lacZ gene was given intravenously. The efficiency of vector transduction of hepatocytes in vivo was not compromised by any of the polymer chemistries. PEGylation of the virus, however, diminished the activation of cytotoxic T lymphocytes and helper T cells of the type 1 subset (Th1 cells) against native viral antigens; neutralizing antibodies to native virus were also diminished. PEGylation prolonged transgene expression and allowed partial readministration with native virus or with a virus PEGylated with a heterologous chemical moiety. Apparently, modification of the capsid leads to a shift in antigenic epitopes because vector readministration was not possible when the immunizing vector had been modified by the same PEGylation chemistry used to modify the second vector. In light of these results, the concept of improving the performance of adenoviral vectors through modification of the capsid with PEG shows promise.

Improved performance of a fully gutted adenovirus vector containing two full-length dystrophin cDNAs regulated by a strong promoter

Dystrophin gene transfer using gutted or helper-dependent adenoviruses (HDAd), which have most of their genes deleted, is a promising approach to treat Duchenne muscular dystrophy. In an attempt to boost the amount of dystrophin produced after gene transfer, we have constructed a fully deleted HDAd (HDCBDys2x) containing two human dystrophin cDNAs controlled by the powerful hybrid cytomegalovirus enhancer/beta-actin promoter. We demonstrated high dystrophin expression after infection of muscle cultures with HDCBDys2x. Similarly, high (mean=583) and moderate (mean=124) numbers of muscle fibers were transduced in anterior tibialis muscle after intramuscular injection of HDCBDys2x in neonate and adult dystrophindeficient (mdx) mice 10 days postinjection. In fact, in the neonatally injected mdx mice, the transferred dystrophin was five times more abundant than in normal human muscle. However, the high early transduction level was transient in both animal groups, and we observed a humoral response against the human dystrophin. In contrast, we demonstrated sustained dystrophin expression in immunodeficient mouse muscles. Dystrophin expression of HDCBDys2x could be further increased in the presence of an E1/E3-deleted (first-generation) adenovirus, thus demonstrating that the latter vector synthesizes trans-acting enhancing factors. We have achieved abundant dystrophin expression with our new, improved HDAd. It is anticipated that high longterm transgene expression will be possible by employing weaker immunogenic transgenes.

Optimisation of real-time quantitative RT-PCR for the evaluation of non-viral mediated gene transfer to the airways

Naked plasmid DNA and DNA/liposome complexes are currently being considered as gene therapy treatments for cystic fibrosis (CF) pulmonary disease. Current methods of gene delivery to the airways result only in transient correction of the CF ion transport defect, and disease treatment is likely to require repeated administrations of vector. However, it is unclear if repeat administration will be tolerated by CF individuals. Technologies including TaqMan (Applied Biosystems) real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR) can be used to determine the efficacy of gene transfer formulations. TaqMan RT-PCR assays were designed and optimised to detect plasmid vector-derived and endogenous gene expression. Subsequently, these assays were used to quantify vector-derived mRNA after delivery of naked DNA and DNA/liposome formulations expressing human and murine cystic fibrosis transmembrane conductance regulator (CFTR) to the mouse airways. Vector-derived mRNA was detected in samples following the delivery of naked DNA or DNA/liposomes to the mouse airways, and no reduction in vector-derived mRNA was observed upon repeat administration, a finding that is consistent with the murine and human CFTR being tolerated by the mouse. Although it remains to be seen if CF patients can tolerate long-term expression of wild-type CFTR, these data demonstrate that TaqMan RT-PCR is an effective tool to accurately quantify transgene expression in the airways.

Demonstration of feasibility of in vivo gene therapy for Gaucher disease using a chemically induced mouse model

Progress towards developing gene therapy for Gaucher disease has been hindered by the lack of an animal model. Here we describe a mouse model of Gaucher disease which has a chemically induced deficiency of glucocerebrosidase and that accumulates elevated levels of glucosylceramide (GL-1) in the lysosomes of Kupffer cells. Administration of mannose-terminated glucocerebrosidase (Cerezyme) resulted in the reduction of GL-1 levels in the livers of these animals. Gene transduction of hepatocytes with a plasmid DNA vector encoding human glucocerebrosidase (pGZB-GC) generated high-level expression and secretion of the enzyme into systemic circulation with consequent normalization of Kupffer cell GL-1 levels. This suggested that the de novo synthesized and unmodified enzyme produced by hepatocyte transduction was also capable of being delivered to the cells that are primarily affected in Gaucher disease. Immunolocalization studies also revealed that preferential transduction and expression of human glucocerebrosidase in the Kupffer cells with subsequent reduction in the GL-1 levels could be attained with a low dose of a recombinant adenoviral vector encoding the human enzyme (Ad2/CMV-GC). This observation raises the possibility of gene therapy for Gaucher disease that involves directly transducing the affected histiocytes using recombinant adenoviral vectors. Together, these data demonstrate the potential for use of in vivo gene therapy vectors for treating Gaucher disease.

Adenovirus-transduced lung as a portal for delivering alpha-galactosidase A into systemic circulation for Fabry disease

Gene therapy efforts have focused primarily on the use of either the liver or skeletal muscle as depot organs for the production of a variety of therapeutic proteins that act systemically. Here we examined the lung to determine whether it could function as yet another portal for the secretion of proteins into the circulation. Fabry disease is caused by a deficiency of the lysosomal hydrolase alpha-galactosidase A, resulting in the abnormal deposition of the glycosphingolipid globotriaosylceramide (GL-3) in vascular lysosomes. Pulmonary instillation of a recombinant adenoviral vector (Ad2/CMVHI-alpha(gal)) encoding human alpha-galactosidase A into Fabry mice resulted in high-level transduction and expression of the enzyme in the lung. Importantly, enzymatic activity was also detected in the plasma, liver, spleen, heart, and kidneys of the Fabry mice. The detection of enzymatic activity outside of the lung, along with the finding that viral DNA was limited to the lung, indicates that the enzyme crossed the air/blood barrier, entered the systemic circulation, and was internalized by the distal visceral organs. The levels of alpha-galactosidase A attained in these tissues were sufficient to reduce GL-3 to basal levels in the lung, liver, and spleen and to approximately 50% of untreated levels in the heart. Together, these results suggest that the lung may be a viable alternate depot organ for the production and systemic secretion of alpha-galactosidase A for Fabry disease.

Bilamellar cationic liposomes protect adenovectors from preexisting humoral immune responses

Adenoviral vectors have been widely used for gene therapy, but they are limited both by the presence of a humoral immune response that dramatically decreases the level of transduction after reinjection and by their requirement for target cells to express appropriate receptors such as Coxsackie adenovirus receptor (CAR). To overcome both limits, we encapsulated adenovectors using bilamellar DOTAP:chol liposomes. Electron micrography (EM) showed that these liposomes efficiently encapsulated the vectors, allowing CAR-independent adenovector transduction of otherwise resistant cells. DOTAP:chol-encapsulated adenovectors encoding LacZ or alpha(1)-antitrypsin inhibitor (AAT) were also functionally resistant ex vivo and in vivo to the neutralizing effects of human anti-adenoviral antibodies, unlike other liposomal systems. Hence, bilamellar DOTAP:chol liposomes may be useful for applications using adenovectors in which the target cells lack adenoviral receptors or in which the recipient already has or develops a neutralizing antibody response that would otherwise inactivate readministered vector.

Variables affecting in vivo performance of high-capacity adenovirus vectors

In high-capacity adenovirus (HC-Ad) vectors the size and/or composition of the vector genome influences vector stability during production and the expression profile following gene transfer. Typically, an HC-Ad vector will contain both a gene or an expression cassette and stuffer DNA that is required to balance the final vector genome to a size of between 27 and 36 kb. To gain an improved understanding of factors that may influence gene expression from HC-Ad vectors, we have generated a series of vectors that carry different combinations of human alpha-1 antitrypsin (hAAT) expression constructs and stuffer DNAs. Expression in vitro did not predict in vivo performance: all vectors expressed hAAT at similar levels when tested in cell culture. Hepatic expression was evaluated following in vivo gene transfer in C57BL/6J mice. hAAT levels obtained from genomic DNA were significantly higher than levels achieved with small cDNA expression cassettes. Expression was independent of the orientation and only marginally influenced by the location of the expression cassette within the vector genome. The use of lambda stuffer DNA resulted in low-level but stable expression for at least 3 months when higher doses were applied. A potential matrix attachment region element was identified within the hAAT gene and caused a 10-fold increase in expression when introduced in an HC-Ad vector genome carrying a phosphoglycerate kinase (pgk) hAAT cDNA construct. We also illustrate the influence of the promoter on anti-hAAT antibody formation in C57BL/6J mice: a human cytomegalovirus but not a pgk promoter resulted in an anti-hAAT antibody response. Thus, the overall design of HC-Ad vectors may significantly influence amounts and duration of gene expression at different levels.