Adeno-associated virus: integration at a specific chromosomal locus

Immune responses in the mammalian inner ear and their implications for AAV-mediated inner ear gene therapy

Adeno-associated virus (AAV)-mediated inner ear gene therapy is a promising treatment option for hearing loss and dizziness. Several studies have shown that AAV-mediated inner ear gene therapy can be applied to various mouse models of hereditary hearing loss to improve their auditory function. Despite the increase in AAV-based animal and clinical studies aiming to rescue auditory and vestibular functions, little is currently known about the host immune responses to AAV in the mammalian inner ear. It has been reported that the host immune response plays an important role in the safety and efficacy of viral-mediated gene therapy. Therefore, in order for AAV-mediated gene therapy to be successfully and safely translated into patients with hearing loss and dizziness, a better understanding of the host immune responses to AAV in the inner ear is critical. In this review, we summarize the current knowledge on host immune responses to AAV-mediated gene therapy in the mammalian inner ear and other organ systems. We also outline the areas of research that are critical for ensuring the safety and efficacy of AAV-mediated inner ear gene therapy in future clinical and translational studies.

Various AAV Serotypes and Their Applications in Gene Therapy: An Overview

Despite scientific discoveries in the field of gene and cell therapy, some diseases still have no effective treatment. Advances in genetic engineering methods have enabled the development of effective gene therapy methods for various diseases based on adeno-associated viruses (AAVs). Today, many AAV-based gene therapy medications are being investigated in preclinical and clinical trials, and new ones are appearing on the market. In this article, we present a review of AAV discovery, properties, different serotypes, and tropism, and a following detailed explanation of their uses in gene therapy for disease of different organs and systems.

Impact of Inverted Terminal Repeat Integrity on rAAV8 Production Using the Baculovirus/Sf9 Cells System

Adeno-associated virus (AAV) inverted terminal repeats (ITRs) are key elements of AAV. These guanine-cytosine-rich structures are involved in the replication and encapsidation of the AAV genome, along with its integration in and excision from the host genome. These sequences are the only AAV-derived DNA sequences conserved in recombinant AAV (rAAV), as they allow its replication, encapsidation, and long-term maintenance and expression in target cells. Due to the original vector design, plasmids containing the gene of interest flanked by ITRs and used for rAAV production often present incomplete, truncated, or imperfect ITR sequences. For example, pSUB201 and its derivatives harbor a truncated (14 nt missing on the external part of the ITR), flop-orientated ITR plus 46 bp of non-ITR viral DNA at each end of the rAAV genome. It has been shown that rAAV genomes can be replicated, even with incomplete, truncated, or imperfect ITR sequences, leading to the production of rAAV vectors in transfection experiments. Nonetheless, it was hypothesized that unmodified wild-type (WT) ITR sequences could lead to a higher yield of rAAV, with less non-rAAV encapsidated DNA originating from the production cells and/or baculovirus shuttle vector genomes. This work studied the impact of imperfect ITRs on the level of encapsidated rAAV genomes and baculovirus-derived DNA sequences using the baculovirus/Sf9 cells production system. Replacement of truncated ITRs with WT and additional wtAAV2 sequences has an impact on the two major features of rAAV production: (1) a rise from 10% to 40% of full capsids obtained, and (2) up to a 10-fold reduction in non-rAAV encapsidated DNA. Furthermore, this study considered the impact on these major parameters of additional ITR elements and ITRs coupled with various regulatory elements of different origins. Implementation of the use of complete ITRs in the frame of the baculovirus-based rAAV expression system is one step that will be required to optimize the quality of rAAV-based gene therapy drugs.

Adeno-Associated Virus (AAV) gene therapy for cystic fibrosis: current barriers and recent developments

INTRODUCTION

Since the cystic fibrosis (CF) gene was discovered in 1989, researchers have worked to develop a gene therapy. One of the most promising and enduring vectors is the AAV, which has been shown to be safe. In particular, several clinical trials have been conducted with AAV serotype 2. All of them detected viral genomes, but identification of mRNA transduction was not consistent; clinical outcomes in Phase II studies were also inconsistent. The lack of a positive outcome has been attributed to a less-than-efficient viral infection by AAV2, a weak transgene promoter and the host immune response to the vector. Areas covered: Herein, the authors focus on AAV gene therapy for CF, evaluating past experience with this approach and identifying ways forward, based on the progress that has already been made in identifying and overcoming the limitations of AAV gene therapy. Expert opinion: Such progress makes it clear that this is an opportune time to push forward toward the development of a gene therapy for CF. Drugs to treat the basic defect in CF represent a remarkable advance but cannot treat a significant cohort of patients with rare mutations. Thus, there is a critical need to develop a gene therapy for those individuals.

Complement yourself: Transcomplementation rescues partially folded mutant proteins

Cystic Fibrosis (CF) is an autosomal disease associated with malfunction in fluid and electrolyte transport across several mucosal membranes. The most common mutation in CF is an in-frame three-base pair deletion that removes a phenylalanine at position 508 in the first nucleotide-binding domain of the cystic fibrosis conductance regulator (CFTR) chloride channel. This mutation has been studied extensively and leads to biosynthetic arrest of the protein in the endoplasmic reticulum and severely reduced channel activity. This review discusses a novel method of rescuing ΔF508 with transcomplementation, which occurs when smaller fragments of CFTR containing the wild-type nucleotide binding domain are co-expressed with the ΔF508 deletion mutant. Transcomplementation rescues the processing and channel activity of ΔF508 and reduces its rate of degradation in airway epithelial cells. To apply transcomplementation as a therapy would require that the cDNA encoding the truncated CFTR be delivered to cells. We also discuss a gene therapeutic approach based on delivery of a truncated form of CFTR to airway cells using adeno-associated viral vectors.

Gene delivery to the airway

This unit describes generation of and gene transfer to several commonly used airway models. Isolation and transduction of primary airway epithelial cells are first described. Next, the preparation of polarized airway epithelial monolayers is outlined. Transduction of these polarized cells is also described. Methods are presented for generation of tracheal xenografts, as well as both ex vivo and in vivo gene transfer to these xenografts. Finally, a method for in vivo gene delivery to the lungs of rodents is included. Methods for evaluating transgene expression are given in the support protocols.

Retargeting transposon insertions by the adeno-associated virus Rep protein

The Sleeping Beauty (SB), piggyBac (PB) and Tol2 transposons are promising instruments for genome engineering. Integration site profiling of SB, PB and Tol2 in human cells showed that PB and Tol2 insertions were enriched in genes, whereas SB insertions were randomly distributed. We aimed to introduce a bias into the target site selection properties of the transposon systems by taking advantage of the locus-specific integration system of adeno-associated virus (AAV). The AAV Rep protein binds to Rep recognition sequences (RRSs) in the human genome, and mediates viral integration into nearby sites. A series of fusion constructs consisting of the N-terminal DNA-binding domain of Rep and the transposases or the N57 domain of SB were generated. A plasmid-based transposition assay showed that Rep/SB yielded a 15-fold enrichment of transposition at a particular site near a targeted RRS. Genome-wide insertion site analysis indicated that an approach based on interactions between the SB transposase and Rep/N57 enriched transgene insertions at RRSs. We also provide evidence of biased insertion of the PB and Tol2 transposons. This study provides a comparative insight into target site selection properties of transposons, as well as proof-of-principle for targeted chromosomal transposition by composite protein–protein and protein–DNA interactions.

Empowering adult stem cells for myocardial regeneration

Treatment strategies for heart failure remain a high priority for ongoing research due to the profound unmet need in clinical disease coupled with lack of significant translational progress. The underlying issue is the same whether the cause is acute damage, chronic stress from disease, or aging: progressive loss of functional cardiomyocytes and diminished hemodynamic output. To stave off cardiomyocyte losses, a number of strategic approaches have been embraced in recent years involving both molecular and cellular approaches to augment myocardial structure and performance. Resultant excitement surrounding regenerative medicine in the heart has been tempered by realizations that reparative processes in the heart are insufficient to restore damaged myocardium to normal functional capacity and that cellular cardiomyoplasty is hampered by poor survival, proliferation, engraftment, and differentiation of the donated population. To overcome these limitations, a combination of molecular and cellular approaches must be adopted involving use of genetic engineering to enhance resistance to cell death and increase regenerative capacity. This review highlights biological properties of approached to potentiate stem cell-mediated regeneration to promote enhanced myocardial regeneration, persistence of donated cells, and long-lasting tissue repair. Optimizing cell delivery and harnessing the power of survival signaling cascades for ex vivo genetic modification of stem cells before reintroduction into the patient will be critical to enhance the efficacy of cellular cardiomyoplasty. Once this goal is achieved, then cell-based therapy has great promise for treatment of heart failure to combat the loss of cardiac structure and function associated with acute damage, chronic disease, or aging.

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.

Development of a slow non-viral DNA release system from PDLLA scaffolds fabricated using a supercritical CO2 technique

Polyamidoamine polymers (PAA) comprising methylene-bisacrylamide/dimethylethylene-diamine monomers were synthesized, complexed with DNA and incorporated into porous P(DL)LA scaffolds by using a supercritical CO(2) (scCO(2)) technique. Scaffolds were made in a dry state consequently there was a need to lyophilize the complexes. A statistically significant reduction of the transfection efficiency was observed in the absence of trehalose when compared to the original complex after freeze-drying. Increasing concentrations (0-10% w/v) of trehalose were added to the complex prior to freeze-drying. Structure dependent differences in DNA binding were evaluated by gel electrophoresis and thermal transition analysis. TEM and PCS showed aggregate formation after freeze-drying without trehalose. Scaffolds were characterized by pore sizes of 173 +/- 73 microm and a porosity of 71%. The transfection potential of the released DNA was investigated by seeding scaffolds with A549 cells and following firefly luciferase as a marker gene after 48 h exposure. Low but continuous levels of transfection were observed for PAA complexes during a 60-day study. Complexes made with Lipofectaminetrade mark gave initially higher levels of DNA release but no further expression was seen after 40 days. Uncomplexed DNA showed background levels of transfection. Culturing cells on 3D scaffolds showed a benefit in retention of transfection activity with time compared to 2D controls. Transfection levels could be increased when cells were grown in OptiMEM. This study demonstrated that PAA/DNA complexes incorporated into a P(DL)LA scaffold made by using scCO(2) processing exhibited a slow release and extended gene expression profile.

Alpha-1-Antitrypsin Deficiency: Current Concepts

Since the condition was first described four decades ago, alpha-1-antitrypsin (A1AT) deficiency has served as a model for other disease processes. A1AT is the archetypal serpin designed to ensnare proteases, a process that involves significant conformational change within the molecule. Mutations in the A1AT gene lead to misfolding of the protein and accumulation within the endoplasmic reticulum of hepatocytes resulting in two different pathologic processes. First, the accumulation of mutant A1AT protein has a directly toxic effect on the liver, resulting in hepatitis and cirrhosis. Second, the resultant decrease in circulating A1AT results in protease-antiprotease imbalance at the lung surface and emphysema ensues. A1AT deficiency therefore can be seen as two distinct disease processes: a conformational disease of the liver and a protease-antiprotease imbalance of the lung. This two-stage model of disease in A1AT deficiency is elegant in its simplicity and goes a long way to explaining the clinical manifestations that occur in patients with the condition. However, some aspects of the disease are not readily explained. Recent findings suggest that there is more to the lung damage in A1AT deficiency than simple proteolytic insult and that the presence of the mutant protein itself is proinflammatory and may indeed cause chronic injury to the cells that produce it. This review discusses some of the emerging concepts in alpha-1-antitrypsin research and outlines the implications these new ideas may have for treatment of this condition.

Identification of a cytoplasmic interaction partner of the large regulatory proteins Rep78/Rep68 of adeno-associated virus type 2 (AAV-2)

Through yeast two-hybrid analysis and coimmunoprecipitation studies, we have identified a novel cellular AAV-2 Rep78/Rep68 interaction partner located predominantly in the cytoplasm. In public databases, it has been assigned as KCTD5, because of a region of high similarity to the cytoplasmic tetramerization domain of voltage-gated potassium channels. Whereas Rep/KCTD5 interaction relied on the region surrounding the Rep nuclear localization signal, nuclear accumulation of Rep was not required. Wildtype Rep78/Rep68 proteins induced the translocation of large portions of KCTD5 into the nucleus pointing to functional interactions both in the cytoplasm and the nucleus. In line with an anticipated functional interference in the cytoplasm, KCTD5 overexpression completely abrogated Rep68-mediated posttranscriptional activation of a HIV-LTR driven luciferase reporter gene. Our study expands the panel of already identified nuclear Rep interaction partners to a cytoplasmic protein, which raises the awareness that important steps in the AAV life cycle may be regulated in this compartment.

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

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

Efficient AAV1-AAV2 hybrid vector for gene therapy of hemophilia

Adeno-associated virus (AAV) serotype 1 (AAV1) has been shown to be more effective than the well-studied AAV serotype 2 (AAV2) in muscle gene transfer. Replacement of amino acids 350 to 430 of AAV2 VP1 with the corresponding amino acids from VP1 of AAV1 resulted in a hybrid vector, termed AAV-221-IV, which behaved similarly to AAV1 in vitro and in vivo in muscle. Intramuscular injection of 1x10(11) vector particles per mouse of hybrid vector carrying a human FIX transgene in CD4 knockout mice resulted in an average level of human FIX in the plasma of 450 ng/ml, 4- to 10-fold higher than in mice injected with an AAV2 vector carrying the same transgene, and 80% of the transgene levels in animals treated with the same dose of AAV1. DNA analysis of injected muscle showed a 10-fold higher copy number after gene delivery by the hybrid vector compared with AAV2. A comparison of total DNA versus DNA from intact virus particles suggests a higher stability of hybrid virus particles. These results suggest that changes in the AAV capsid have an effect on virus-cell receptor interaction, and also influence trafficking and processing of the virus particle in the cell. This "hybrid vector" retains the heparin-binding sites of AAV2 and, therefore, can be purified by passage through a heparin-Sepharose column with the same efficiency as AAV2. When tested in vivo, either in CD4 knockout mice or in a hemophilic mouse model, the heparin-purified hybrid vector showed >10-fold higher activity than similarly purified AAV2. This demonstrates the utility of this hybrid vector in the performance of large-scale heparin column purification to generate a vector with a high expression profile for muscle-directed gene delivery. Initiation of clinical studies with this hybrid vector may be facilitated because it differs from AAV2 by only nine amino acids.

Adeno-associated virus-based gene therapy for inherited disorders

Adeno-associated virus vectors are capable of long-term gene transfer without obvious adverse effects in a number of animal models. Over the last two decades, preclinical and early phase clinical trials in cystic fibrosis and alpha-1 antitrypsin deficiency were undertaken to test the feasibility of this approach. The results of those studies have been important since they have indicated that in vivo gene transfer is feasible and relatively safe. In addition, a number of key limitations to the current generation of AAV2 gene therapy vectors have been defined. The information about these limitations has been used to develop newer AAV vector approaches, based on new mutant and alternative serotype capsids and enhanced promoter systems. The evaluation of safety and efficacy of these newer agents is ongoing.

Safety and efficacy of adeno-associated viral vector-mediated insulin gene transfer via portal vein to the livers of streptozotocin-induced diabetic Sprague-Dawley rats

BACKGROUND

Previous studies demonstrating the efficacy of insulin gene therapy have mostly involved use of adenoviral vectors or naked DNA to deliver the insulin gene. However, this procedure may not guarantee long-term insulin production. To improve the performance, we prepared recombinant adeno-associated viral vectors (rAAV) harboring the gene encoding a furin-modified human insulin under the cytomegalovirus (CMV) promoter [rAAV-hPPI(F12)].

METHODS

Streptozotocin (STZ)-induced diabetic Sprague-Dawley rats were used as a diabetic animal model. The levels of blood glucose, insulin, and HbA1c were measured to test the effect. An intraperitoneal glucose tolerance test was performed to test the capability of blood glucose disposal. Immunohistochemical staining and Northern blot analyses were performed to survey the expression pattern of the therapeutic insulin gene.

RESULTS

STZ-induced diabetic Sprague-Dawley rats infused via the portal vein with rAAV-hPPI(F12) produced human insulin and after a 6-h fast were normoglycemic for over 90 days post-treatment, whereas diabetic rats treated with recombinant adenoviral vector harboring the hPPI(F12) gene [rAV-hPPI(F12)] were normoglycemic only for days 3 to 13 post-treatment. Insulin mRNA was detected mainly in the liver of the rAAV-hPPI(F12)-treated diabetic rats. The glucose tolerance capability of the rAAV-hPPI(F12)-treated diabetic rats was comparable to that of non-diabetic rats, even without injection of recombinant insulin. Furthermore, blood HbA1c concentrations in rAAV-hPPI(F12)-treated diabetic rats were reduced to almost the normal level. Importantly, studies of rAV or rAAV vector-dependent side effects on the targeted liver strongly suggested that only rAAV treatment caused no side effects.

CONCLUSIONS

These results demonstrate that our rAAV-mediated in vivo insulin gene therapy provides safer maintenance of the insulin gene expression required for long-term and thus more effective blood glycemic control.

Enhancing rAAV vector expression in the lung

Despite favorable DNA transfer efficiency, gene expression from recombinant adeno-associated virus (rAAV2) vectors in the lung has been variable in the context of cystic fibrosis (CF) gene therapy. This is due, in part, to the large size of the CF transmembrane regulator (CFTR)-coding sequence which necessitates the use of compact endogenous promoter elements versus stronger exogenous promoters. We evaluated the possibility that gene expression from rAAV could be improved by using AAV capsid serotypes with greater tropism for the apical surface of airway cells (i.e. rAAV5 or rAAV1) and/or using strong promoters such as the cytomegalovirus (CMV) enhancer/chicken beta-actin hybrid (Cbeta) promoter. The relative activity of the CMV immediate-early (CMVie) promoter, the Cbeta promoter, and the Cbeta promoter with a downstream woodchuck hepatitis virus post-transcriptional regulatory element (wpre) were assessed in vitro and in vivo in C57\Bl6 mice using human alpha-1 antitrypsin (hAAT) as a secreted reporter. In vivo, the Cbeta-AAT-wpre group achieved maximum serum levels of 1.5 mg/ml of hAAT. AAV capsid serotypes were then compared in vivo utilizing the transcriptionally optimized CB-wpre cassette in rAAV serotype 1, 2 or 5 capsids (rAAV1, rAAV2, and rAAV5), utilizing luciferase as a reporter to compare expression over a wide dynamic range. The pulmonary luciferase levels at 8 weeks were similar in rAAV5 and rAAV1 groups (2.9 x 10(6) relative light units (RLU)/g tissue and 2.7 x 10(6) RLU/g tissue, respectively), both of which were much higher than rAAV2. Although the advantage of rAAV5 over rAAV2 in the lung has already been described, the availability of another serotype (rAAV1) capable of efficient gene transfer in the lung could be useful.

Intracellular viral processing, not single-stranded DNA accumulation, is crucial for recombinant adeno-associated virus transduction

Adeno-associated virus (AAV) is a unique gene transfer vector which takes approximately 4 to 6 weeks to reach its expression plateau. The mechanism for this slow-rise expression profile was proposed to be inefficient second-strand DNA synthesis from the input single-stranded (ss) DNA viral genome. In order to clarify the status of ss AAV genomes, we generated AAV vectors labeled with bromodeoxyuridine (BrdU), a nucleotide analog that can be incorporated into the AAV genome and packaged into infectious virions. Since BrdU-DNA can be detected only by an anti-BrdU antibody when DNA is in an ss form, not in a double-stranded (ds) form, ss AAV genomes with BrdU can be readily tracked in situ. Although ss AAV DNA was abundant by Southern blot analysis, free ss AAV genomes were not detectable after AAV transduction by this new detection method. Further Southern blot analysis of viral DNA and virions revealed that ss AAV DNA was protected within virions. Extracted cellular fractions demonstrated that viral particles in host cells remained infectious. In addition, a significant amount of AAV genomes was degraded after AAV transduction. Therefore, we conclude that the amount of free ss DNA is not abundant during AAV transduction. AAV transduction is limited by the steps that affect AAV ss DNA release (i.e., uncoating) before second-strand DNA synthesis can occur. AAV ss DNA released from viral uncoating is either converted into ds DNA efficiently or degraded by cellular DNA repair mechanisms as damaged DNA. This study elucidates a mechanism that can be exploited to develop new strategies to improve AAV vector transduction efficiency.

SUMO-1 modification regulates the protein stability of the large regulatory protein Rep78 of adeno associated virus type 2 (AAV-2)

The large Rep proteins Rep78 and Rep68 of the helper-dependent adeno associated virus type 2 (AAV-2) are essential for both site-specific integration of AAV DNA in the absence of helpervirus and productive AAV replication in the presence of helpervirus. We have identified UBC9, the E2 conjugating enzyme for the small ubiquitin-related polypeptide SUMO-1, as binding partner of the large Rep proteins in yeast two-hybrid analysis and in GST pulldown assays. Modification of the large Rep proteins with SUMO-1 could be demonstrated in immunoblot analysis and in immunoprecipitations, with the lysine residue at amino acid position 84 serving as the major attachment site. The largely sumolation-deficient Rep78 lysine to arginine point mutant showed a strongly reduced half-life as compared to the wild-type protein. This finding implicates a role for sumolation in the regulation of Rep78 protein stability that is assumed to be critical for the establishment and maintenance of AAV latency.

Advances in AAV-mediated gene transfer for the treatment of inherited disorders

The holy grail of gene therapy is the cure of genetic diseases. To achieve this goal, a vector system is desirable that offers a high level of safety combined with clinical efficacy and versatility in terms of potential applications. Gene therapy vectors based on recombinant adeno-associated viruses (AAVs) meet all of these criteria: They are nonpathogenic, devoid of viral coding sequences, and mediate long-term gene expression in the absence of an immune or inflammatory response. Moreover, with the recent discovery of novel AAV serotypes, there is now one preferred serotype for nearly every organ or tissue to target. Thus, AAV gene therapy vectors are increasingly becoming the vectors of choice for the treatment of inherited disorders.

Phase I trial of intramuscular injection of a recombinant adeno-associated virus alpha 1-antitrypsin (rAAV2-CB-hAAT) gene vector to AAT-deficient adults

A recombinant virus vector constructed from adeno-associated virus (AAV) that has been altered to carry the human alpha1-antitrypsin (hAAT) gene expressed from a hybrid chicken beta-actin promoter with a cytomegalovirus enhancer has been developed. The construct has been shown to initiate the production of hAAT in animal models closely matching the proposed human trial. The proposed clinical trial is an open-label, phase I study administering recombinant adeno-associated virus alpha1-antitrypsin (rAAV2-CB-hAAT) gene vector intramuscularly to AAT-deficient human subjects where gene expression can be measured directly in blood samples to assess safety. Safety parameters will be measurement of changes in serum chemistries and hematology, urinalysis, pulmonary function testing, semen assay for vector genomes, immunologic response to AAT, and AAV, as well as reported subject history of any symptoms.

Identification and characterization of an adeno-associated virus integration site in CV-1 cells from the African green monkey

Adeno-associated virus (AAV) is a classification given to a group of nonpathogenic, single-stranded DNA viruses known to reside latently in primates. During latency in humans, AAV type 2 (AAV2) preferentially integrates at a site on chromosome 19q13.3ter by targeting a sequence composed of an AAV Rep binding element (RBE), a spacer, and a nicking site. Here, we report the DNA sequence of an African green monkey AAV integration site isolated from CV-1 cells. Overall, it has 98% homology to the analogous human site, including identical spacer and nicking sequences. However, the simian RBE is expanded, having five perfect directly repeated GAGC tetramers. We carried out a number of in vitro and in vivo assays to determine the effect of this expanded RBE sequence on the Rep-RBE interaction and AAV targeted integration. Using electromobility shift assays it was demonstrated that AAV4 Rep68 bound the expanded RBE with a sixfold-greater affinity than the human RBE. To determine the basis for the affinity increase, DNase I protection and methylation interference (MI) assays were performed. Comparison of footprints on both the human and simian RBEs revealed nearly identical protection; however, MI analysis suggested greater interaction with the guanine nucleotides of the expanded RBE, thus providing a biochemical basis for the increased binding activity. In vivo, integration targeted to the simian RBE was demonstrated by PCR analysis of latently infected Cos-7 cells. Interestingly, the frequency of site-specific integration was twofold greater in Cos-7 cells than in HeLa cells. Overall, these experiments establish that the simian RBE, identified in CV-1 cells, functions analogously to the human RBE and provide further evidence for a developing model that proposes individual roles for the RBE and the spacer and nicking site elements.

Virus-based gene delivery systems

Within the past decade, gene therapy strategies have come to the forefront of novel therapeutics. Tremendous advances in vector technology along with deeper understandings of vector biology and the molecular mechanisms of disease have significantly advanced the field of human gene therapy. This manuscript will discuss the viral-based subset of current gene transfer vectors. In particular, the most established viral vectors to date, including parvovirus, adenovirus, retrovirus, lentivirus, and herpesvirus-based vectors, are described, as well as the current innovative improvements being made to each. From past experience, it has become evident that in addition to optimising the vectors in terms of transgene expression, minimising vector-related immunology, and vector production, methods of vector delivery resulting in optimum vector transduction of target cells need to be established. This review will also illustrate several current improved physical delivery systems for optimal vector administration.

Replication competent helper functions for recombinant AAV vector generation

Adeno-associated virus (AAV) is a promising gene transfer vector tested in both animal studies and human clinical trials. However, current production methods are generally inefficient and require improvements to meet the increasing clinical need for economical, high titer and high quality rAAV vectors. The inefficiency of the current systems largely arises from the AAV helper function, which contains only the AAV coding region but lacks inverted terminal repeats. The terminal repeats were originally removed to prevent replication competent AAV contamination. Here we designed a novel and highly efficient rAAV helper function containing AAV terminal repeats. The new helper function not only mimics the wild-type AAV growth as it replicates along with the vector plasmid, but also restores the cis regulating function of the AAV terminal repeats. Addition of heterologous introns to the helper genome and use of a mutant AAV terminal repeat defective in packaging effectively controls the contamination of replication competent AAV particles. This new strategy also performs better in AAV producing cell lines than those based on non-replicating AAV rep and cap genome.

Prevention of chemotherapy-related toxic side effects by infection with adeno-associated virus type 2

Drug resistance and toxic side effects are major limiting factors in the clinical use of antineoplastic chemotherapy. Patients with pancreatic cancer generally do not benefit from chemotherapy. The nonpathogenic adeno-associated virus type 2 (AAV-2) has been shown to sensitize human tumor cells to gamma irradiation and chemotherapeutic drugs. In the present study, we characterized the therapeutic role of AAV-2 infection in combination with 5-fluorouracil (5-FU)-based chemotherapy on pancreatic cancer cells in an animal model. In Lewis rats bearing s.c. implants of syngeneic DSL6A pancreatic cancer cells, intratumoral infection with AAV-2 (MOI 10E8 i.u.) in combination with 5-FU (5 or 50 mg/kg body weight) resulted in significantly reduced tumor growth and prolonged survival time compared with 5-FU single therapy. Most surprisingly, AAV-2-infected rats remained in a much better physical condition compared to their noninfected counterparts. While rats treated with 5-FU single therapy lost weight, were sluggish and died within 4 months after tumor implantation, animals infected with AAV showed much better vigilance, with body weight, leukocyte number and hemoglobin levels similar to healthy rats. In particular, 5-FU-related side effects like thrombocytopenia and leukopenia were significantly reduced in animals treated with the combination regimen. By in vitro analysis, human (Capan-1 and DANG) pancreatic cancer cell lines were shown to be sensitized to 5-FU chemotherapy to an extent similar to DSL6A cells. AAV-2 infection enhanced 5-FU-induced apoptosis by a factor of 8 to 14 in both human and rat pancreatic cancer cell lines. The data suggest that infection with the nonpathogenic AAV-2 significantly improves both chemotherapy efficacy and physical appearance and offers a novel strategy in cancer treatment.

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.

Gene therapy for prostate cancer

Prostate cancer is the most common noncutaneous cancer in man. When confined to the prostate it can be cured by radical prostatectomy or irradiation therapy. However, there are no curative therapies for locally advanced, recurrent or metastatic disease. Prostate cancer gene therapy has recently transition from preclinical studies to clinical trials with the goal of developing novel treatments for prostate cancer. The greatest challenge in treating advanced prostate cancer is therapeutic access to and the elimination of metastases. This review details two aspects of prostate cancer gene therapy, the types of delivery systems under development and specific categories of therapeutic genes available with an emphasis on the mechanism of action of specific gene therapy strategies.

Adeno-associated virus (AAV) site-specific recombination does not require a Rep-dependent origin of replication within the AAV terminal repeat

Adeno-associated virus (AAV) is the only known eukaryotic virus capable of targeted integration in human cells. An AAV Rep binding element (RBE) and terminal resolution site (trs) identical to the viral terminal repeats required for AAV DNA replication are located on chromosome (ch) 19. Both ch-19 RBE and trs elements have been shown to be essential for viral targeting to this locus. To characterize the role of the AAV inverted terminal repeat (ITR) cis-acting sequences in targeted integration an AAV trs mutant incapable of supporting viral replication was tested. Wild-type and mutant substrates were assayed for targeted integration after cotransfection in the presence or absence of Rep. Our results demonstrated that, in the presence of Rep78, both ITR substrates targeted to ch-19 with similar frequency. Molecular characterization of the mutant ITR integrants confirmed the presence of the trs mutation in the majority of samples tested. Complementation analysis confirmed that the mutant targeted viral genomes were unable to rescue and replicate. In addition, Rep78 induced extensive rearrangement and amplification of ch-19 sequences independent of wild-type or mutant targeting substrate. These studies demonstrate that Rep-dependent nicking of the viral cis-acting trs sequence is not a prerequisite for site-specific recombination and suggests AAV targeting is mediated by Rep78/68-dependent replication from the ch-19 origin of replication (ori). These studies have significant impact toward the understanding of AAV site-specific recombination and the development of targeting vectors.

Alpha1-antitrypsin deficiency: biological answers to clinical questions

Alpha1-antitrypsin (alpha1AT) deficiency is a common lethal hereditary disorder of white persons of European descent. The condition is characterized by reduced serum levels of alpha1AT, a 52-kDa glycoprotein synthesized chiefly in the liver and, to a lesser extent, by macrophages and neutrophils. Alpha1AT acts as an antiprotease and is the physiological inhibitor of neutrophil serine proteases such as neutrophil elastase cathepsin G and proteinase 3. The clinical manifestations of alpha1AT deficiency occur chiefly in the lung, with a high risk of emphysema occurring by the third or fourth decade of life. Cigarette smoking accelerates the development of emphysema in persons with alpha1AT deficiency. There is also an increased risk of liver disease in alpha1AT deficiency, which occurs mostly in childhood. In this review, we will define further the diagnosis of alpha1AT deficiency and its clinical manifestations and describe the therapeutic strategies that are currently being developed to treat the hepatic and pulmonary disease associated with this condition.

Progress in gene therapy for Duchenne muscular dystrophy

Gene transfer research for Duchenne muscular dystrophy (DMD) has brought the goal of successful treatment of this devastating, inherited disease closer to being a reality. Although gene therapeutic approaches for DMD patients are not yet in clinical use, recent advances using DMD animal models are encouraging. Progress in vector design, such as high-capacity adenoviral vectors, targeted adenoviral vectors, and heterodimerization of DNA delivered by adeno-associated virus (AAV) vectors have advanced the field considerably. The recent studies into the pharmacologic-induced read-through of stop codons, the increased study of utrophin and its upregulation, and the introduction of point mutation correction using chimeric oligonucleotides have expanded the field, providing new avenues of inquiry.

Remission in models of type 1 diabetes by gene therapy using a single-chain insulin analogue

A cure for diabetes has long been sought using several different approaches, including islet transplantation, regeneration of beta cells and insulin gene therapy. However, permanent remission of type 1 diabetes has not yet been satisfactorily achieved. The development of type 1 diabetes results from the almost total destruction of insulin-producing pancreatic beta cells by autoimmune responses specific to beta cells. Standard insulin therapy may not maintain blood glucose concentrations within the relatively narrow range that occurs in the presence of normal pancreatic beta cells. We used a recombinant adeno-associated virus (rAAV) that expresses a single-chain insulin analogue (SIA), which possesses biologically active insulin activity without enzymatic conversion, under the control of hepatocyte-specific L-type pyruvate kinase (LPK) promoter, which regulates SIA expression in response to blood glucose levels. Here we show that SIA produced from the gene construct rAAV-LPK-SIA caused remission of diabetes in streptozotocin-induced diabetic rats and autoimmune diabetic mice for a prolonged time without any apparent side effects. This new SIA gene therapy may have potential therapeutic value for the cure of autoimmune diabetes in humans.

Selective cleavage of AAVS1 substrates by the adeno-associated virus type 2 rep68 protein is dependent on topological and sequence constraints

The adeno-associated virus type 2 (AAV-2) Rep78 and Rep68 proteins are required for replication of the virus as well as its site-specific integration into a unique site, called AAVS1, of human chromosome 19. Rep78 and Rep68 initiate replication by binding to a Rep binding site (RBS) contained in the AAV-2 inverted terminal repeats (ITRs) and then specifically nicking at a nearby site called the terminal resolution site (trs). Similarly, Rep78 and Rep68 are postulated to trigger the integration process by binding and nicking RBS and trs homologues present in AAVS1. However, Rep78 and Rep68 cleave in vitro AAVS1 duplex-linear substrates much less efficiently than hairpinned ITRs. In this study, we show that the AAV-2 Rep68 endonuclease activity is affected by the topology of the substrates in that it efficiently cleaves in vitro in a site- and strand-specific manner the AAVS1 trs only if this sequence is in a supercoiled (SC) conformation. DNA sequence mutagenesis in the context of SC templates allowed us to elucidate for the first time the AAVS1 trs sequence and position requirements for Rep68-mediated cleavage. Interestingly, Rep68 did not cleave SC templates containing RBS from other sites of the human genome. These findings have intriguing implications for AAV-2 site-specific integration in vivo.

Characteristics of the adeno-associated virus preintegration site in human chromosome 19: open chromatin conformation and transcription-competent environment

Adeno-associated virus (AAV) establishes latency in infected cells by integrating into the cellular genome, with a high preference for a unique region, called AAVS1, of the human chromosome 19. The AAV proteins Rep78 and -68 are postulated to initiate the site-specific integration process by binding to a Rep binding site (RBS) in AAVS1. We provide further evidence to corroborate this model by demonstrating that the AAVS1 RBS in human cell lines is located near a DNase I hypersensitive "open" chromatin region and therefore is potentially easily accessible to Rep proteins. This open conformation is maintained in transgenic rats which carry an AAVS1 3. 5-kb DNA fragment and are proficient for Rep-mediated site-specific integration. Interestingly, the core of the DNAse I hypersensitive site in AAVS1 corresponds to a sequence displaying transcriptional enhancer-like properties, suggesting that AAVS1 constitutes a transcription-competent environment. The implications of our findings for AAV physiology and gene therapy are discussed.

Roles of adeno-associated virus Rep protein and human chromosome 19 in site-specific recombination

Adeno-associated virus type 2 (AAV) is the only known eucaryotic virus capable of targeted integration in human cells. AAV integrates preferentially into human chromosome (ch) 19q13.3qter. The nonstructural proteins of AAV-2, Rep78 and Rep68, are essential for targeted integration. Rep78 and Rep68 are multifunctional proteins with diverse biochemical activities, including site-specific binding to AAV and ch-19 target sequences, helicase activity, and strand-specific, site-specific endonuclease activities. Both a Rep DNA binding element (RBE) and a nicking site essential for AAV replication present within the viral terminal repeats are also located on ch-19. Recently, identical RBE sequences have been identified at other locations in the human genome. This fact raises numerous questions concerning AAV targeted integration; specifically, how many RBE sequences are in the human genome? How does Rep discriminate between these and the ch-19 RBE sequence? Does Rep interact with all sites and, if so, how is targeted integration within a fixed time frame facilitated? To better characterize the role of Rep in targeted integration, we established a Rep-dependent filter DNA binding assay using a highly purified Rep-68 fusion protein. Electron microscopy (EM) analysis was also performed to determine the characteristics of the Rep-RBE interaction. Our results determined that the Rep affinity for ch-19 is not distinct compared to other RBEs in the human genome when utilizing naked DNA. In fact, a minimum-binding site (GAGYGAGC) efficiently associated with Rep, suggesting that as many as 2 x 10(5) sites may exist. In addition, such sites also exist frequently in nonprimate mammalian genomes, although AAV integrates site specifically into primate genomes. EM analysis demonstrated that only one Rep-DNA complex was formed on ch-19 target DNA. Surprisingly, identically sized complexes were observed on all substrates containing a RBE sequence, but never on DNA lacking an RBE. Rep-DNA complexes involved a multimeric protein structure that spanned ca. 60 bp. Immunoprecipitation of AAV latently infected cells determined that 1,000 to 4,000 copies of Rep78 and Rep68 protein are expressed per cell. Comparison of the Rep association constant with those of established DNA binding proteins indicates that sufficient molecules of Rep are present to interact with all potential RBE sites. Moreover, Rep expression in the absence of AAV cis-acting substrate resulted in Rep-dependent amplification and rearrangement of the target sequence in ch-19. This result suggests that this locus is a hot spot for Rep-dependent recombination. Finally, we engineered mice to carry a single 2.7-kb human ch-19 insertion containing the AAV ch-19 target locus. Using cells derived from these mice, we demonstrated that this sequence was sufficient for site-specific recombination after infection with transducing vectors expressing Rep. This result indicates that any host factors required for targeting are conserved between human and mouse. Furthermore, the human ch-19 cis sequences and chromatin structure required for site-specific recombination are contained within this fragment. Overall, these results indicate that the specificity of targeted recombination to human ch-19 is not dictated by differential Rep affinities for RBE sites. Instead, specificity is likely dictated by human ch-19 sequences that serve as a Rep protein-mediated origin of replication, thus facilitating viral targeting through Rep-Rep interactions and host enzymes, resulting in site-specific recombination. Control of specificity is clearly dictated by the ch-19 sequences, since transfer of these sequences into the mouse genome are sufficient to achieve Rep-dependent site-specific integration.

The use of adeno-associated virus to circumvent the maturation-dependent viral transduction of muscle fibers

Muscle-based gene therapy using adenovirus, retrovirus, and herpes simplex virus has been hindered by viral cytotoxicity, host immune response, and the maturation-dependent viral transduction of muscle fibers. The development of new mutant vectors has greatly reduced the toxicity and the immune rejection problems, but the inability of viral vectors to penetrate and transduce mature myofibers remains an important issue. Research has been focused on the characterization of barriers to viral transduction in mature myofibers to develop strategies to circumvent the maturation-dependent viral transduction of myofibers. Here, we report that adeno-associated virus (AAV) can be used to overcome the maturation-dependent viral transduction of myofibers. We have investigated by which mechanism AAV can penetrate and efficiently transduce mature muscle fibers, and have shown that this viral vector is not blocked by the basal lamina and that AAV transduction of myofibers is independent of myoblast mediation. Although AAV can efficiently transduce mature myofibers, a differential transduction is still observed among the different types of myofibers that correlates with the expression of the heparan sulfate proteoglycan receptors, the muscle maturity, the number of viral particles used, and the time postinjection. The identification of the mechanisms by which AAV transduces mature myofibers will help in the development of strategies to achieve an efficient muscle-based gene therapy for inherited and acquired diseases.

A chimeric protein containing the N terminus of the adeno-associated virus Rep protein recognizes its target site in an in vivo assay

The Rep78 and Rep68 proteins of adeno-associated virus (AAV) type 2 are involved in DNA replication, regulation of gene expression, and targeting site-specific integration. They bind to a specific Rep recognition sequence (RRS) found in both the viral inverted terminal repeats and the AAVS1 integration locus on human chromosome 19. Previous in vitro studies implied that an N-terminal segment of Rep is involved in DNA recognition, while additional domains might stabilize binding and mediate multimerization. In order to define the minimal requirements for Rep to recognize its target site in the human genome, we developed one-hybrid assays in which DNA-protein interactions are detected in vivo. Chimeric proteins consisting of the N terminus of Rep fused to different oligomerization motifs and a transcriptional activation domain were analyzed for oligomerization, DNA binding, and activation of reporter gene expression. Expression of reporter genes was driven from RRS motifs cloned upstream of minimal promoters and examined in mammalian cells from transfected plasmids and in Saccharomyces cerevisiae from a reporter cassette integrated into the yeast genome. Our results show for the first time that chimeric proteins containing the amino-terminal 244 residues of Rep are able to target the RRS in vitro and in vivo when incorporated into artificial multimers. These studies suggest that chimeric proteins may be used to harness the unique targeting feature of AAV for gene therapy applications.

Conditional site-specific integration into human chromosome 19 by using a ligand-dependent chimeric adeno-associated virus/Rep protein

It is of great interest for gene therapy to develop vectors that drive the insertion of a therapeutic gene into a chosen specific site on the cellular genome. Adeno-associated virus (AAV) is unique among mammalian viruses in that it integrates into a distinct region of human chromosome 19 (integration site AAVS1). The inverted terminal repeats (ITRs) flanking the AAV genome and the AAV-encoded nonstructural proteins Rep78 and/or Rep68 are the only viral elements necessary and sufficient for site-specific integration. However, it is also known that unrestrained Rep activity may cause nonspecific genomic rearrangements at AAVS1 and/or have detrimental effects on cell physiology. In this paper we describe the generation of a ligand-dependent form of Rep, obtained by fusing a C-terminally deleted Rep68 with a truncated form of the hormone binding domain of the human progesterone receptor, which does not bind progesterone but binds only its synthetic antagonist RU486. The activity of this chimeric protein, named Rep1-491/P, is highly dependent on RU486 in various assays: in particular, it triggers site-specific integration at AAVS1 of an ITR-flanked cassette in a ligand-dependent manner, as efficiently as wild-type Rep68 but without generating unwanted genomic rearrangement at AAVS1.

Chromosomal integration of adeno-associated parvovirus DNA limits proliferation and dispersal of human MKr melanoma cells in co-cultures with human fibroblasts

Adeno-associated parvoviruses (AAVs) possess onco-suppressive potential and inhibit proliferation of cells derived from malignant human tumors in culture. Growth inhibition of human melanoma cells can be achieved when the cells are infected with these viruses but continues subsequent to infection when viral DNA is chromosomally integrated. Integration of AAV2 DNA into the genome of the human melanoma cell line MKr alters the cellular phenotype towards that of diploid cells in culture, and leads to density-arrested growth, strong reduction of the ability to form colonies from single cells and to an increased number of terminally differentiating cultures. The present study aimed at the question whether the altered growth properties were retained upon prolonged co-cultivation with fibroblasts, i.e. under conditions that occur in invasive growth and colonization of distant tissues. The results show that despite the known possibility of growth stimulation by fibroblasts the potential of melanoma cells to proliferate and the potential to further scatter in the fibroblast cultures remain limited when AAV DNA is integrated.

Gene therapy in the inner ear. Mechanisms and clinical implications

The application of gene therapy to the inner ear is an emerging field of study. Most studies report the expression of marker genes (e.g., galactosidase) within the tissues of the cochlea. The first biologic response of an inner ear tissue (i.e., auditory neurons) to transduction by a gene therapy vector expressing a therapeutic gene (a herpes amplicon vector containing a BDNF gene) was observed in spiral explants obtained from early postnatal rat cochleae. This study was important because it demonstrated the feasibility of a gene augmentation approach to treat traumatized cochleae. Long-term expression of transduced or transfected genes in cochlear tissues have been obtained with adenovirus, adeno-associated virus, and herpes amplicon vectors. The herpes amplicon vector (i.e., HSVbdnflac) that evoked a biologic response in vitro has also been successfully used to support the survival of auditory neurons in vivo following loss of the auditory hair cells (i.e., loss of trophic factor). Gene therapy has been successfully applied to the cochlea of a laboratory animal, and future studies will define the types of vectors and therapeutic genes that will work best for the treatment of inner ear diseases in the clinic.

Update on the prevalence of serum antibodies (IgG and IgM) to adeno-associated virus (AAV)

In view of presumed non-pathogenicity, tumor suppressive properties, and site-specific integration of the viral genome the human parvovirus, adeno-associated virus (AAV) has gained great interest as a gene transduction vector. Data on the seroprevalence of antibodies to AAV vary between reports, probably due to the different serological methods used. In order to understand better the immune response to AAV during natural infection, sera from different age groups and various geographical regions were compared for AAV antibodies using an ELISA. The data show that the prevalence of antibodies to AAV is similar in Europe (Germany, France, and Switzerland), Brazil, and Japan, indicating worldwide infection. It was confirmed that infection takes place during childhood. However, declining seropositivity thereafter and a second increase of seropositivity after 30 years of age suggests reinfection or reactivation of latent virus in particular as the prevalence of IgM antibodies in adults is relatively high. Furthermore, pregnant women were found to be significantly more frequently seropositive than non-pregnant controls, hinting at a reactivation of persistent AAV (up to 80% of women carry AAV in genital tissue) in specific hormonal conditions, e.g., pregnancy. Cross-reaction of serum antibodies with the different AAV types (defined by complement fixation) was observed by ELISA and neutralization tests confirming earlier results. The results suggest an unstable AAV antibody response allowing lifelong reinfection or reactivation of persisting virus possibly due to partial immunotolerance after an infection in utero, at delivery or during early infancy.

Gene therapy vectors based on adeno-associated virus type 1

The complete sequence of adeno-associated virus type 1 (AAV-1) was defined. Its genome of 4,718 nucleotides demonstrates high homology with those of other AAV serotypes, including AAV-6, which appears to have arisen from homologous recombination between AAV-1 and AAV-2. Analysis of sera from nonhuman and human primates for neutralizing antibodies (NAB) against AAV-1 and AAV-2 revealed the following. (i) NAB to AAV-1 are more common than NAB to AAV-2 in nonhuman primates, while the reverse is true in humans; and (ii) sera from 36% of nonhuman primates neutralized AAV-1 but not AAV-2, while sera from 8% of humans neutralized AAV-2 but not AAV-1. An infectious clone of AAV-1 was isolated from a replicated monomer form, and vectors were created with AAV-2 inverted terminal repeats and AAV-1 Rep and Cap functions. Both AAV-1- and AAV-2-based vectors transduced murine liver and muscle in vivo; AAV-1 was more efficient for muscle, while AAV-2 transduced liver more efficiently. Strong NAB responses were detected for each vector administered to murine skeletal muscle; these responses prevented readministration of the same serotype but did not substantially cross-neutralize the other serotype. Similar results were observed in the context of liver-directed gene transfer, except for a significant, but incomplete, neutralization of AAV-1 from a previous treatment with AAV-2. Vectors based on AAV-1 may be preferred in some applications of human gene therapy.

Sustained gene expression in transplanted skin fibroblasts in rats

Retrovirus-mediated gene transfer into adult skin fibroblasts has provided measurable amounts of therapeutic proteins in animal models. However, the major problem emerging from these experiments was a limited time of vector encoded gene expression once transduced cells were engrafted. We hypothesized that sustained transduced gene expression in quiescent fibroblasts in vivo might be obtained by using a fibronectin (Fn) promoter. Fibronectin plays a key role in cell adhesion, migration and wound healing and is up-regulated in quiescent fibroblasts. Retroviral vectors containing human adenosine deaminase (ADA) cDNA linked to rat fibronectin promoter (LNFnA) or viral LTR promoter (LASN) were compared for their ability to express ADA from transduced primary rat skin fibroblasts in vivo. Skin grafts formed from fibroblasts transduced with LNFnA showed strong human ADA enzyme activity from 1 week to 3 months. In contrast, skin grafts containing LASN-transduced fibroblasts tested positive for human ADA for weeks 1 and 2, were faintly positive at week 3 and showed no human ADA expression at 1, 2 and 3 months. Thus, a fibronectin promoter provided sustained transduced gene expression at high levels for at least 3 months in transplanted rat skin fibroblasts, perhaps permitting the targeting of this tissue for human gene therapy.

Development of animal models for adeno-associated virus site-specific integration

The adeno-associated virus (AAV) is unique in its ability to target viral DNA integration to a defined region of human chromosome 19 (AAVS1). Since AAVS1 sequences are not conserved in a rodent's genome, no animal model is currently available to study AAV-mediated site-specific integration. We describe here the generation of transgenic rats and mice that carry the AAVS1 3.5-kb DNA fragment. To test the response of the transgenic animals to Rep-mediated targeting, primary cultures of mouse fibroblasts, rat hepatocytes, and fibroblasts were infected with wild-type wt AAV. PCR amplification of the inverted terminal repeat (ITR)-AAVS1 junction revealed that the AAV genome integrated into the AAVS1 site in fibroblasts and hepatocytes. Integration in rat fibroblasts was also observed upon transfection of a plasmid containing the rep gene under the control of the p5 and p19 promoters and a dicistronic cassette carrying the green fluorescent protein (GFP) and neomycin (neo) resistance gene between the ITRs of AAV. The localization of the GFP-Neo sequence in the AAVS1 region was determined by Southern blot and FISH analysis. Lastly, AAV genomic DNA integration into the AAVS1 site in vivo was assessed by virus injection into the quadriceps muscle of transgenic rats and mice. Rep-mediated targeting to the AAVS1 site was detected in several injected animals. These results indicate that the transgenic lines are proficient for Rep-mediated targeting. These animals should allow further characterization of the molecular aspects of site-specific integration and testing of the efficacy of targeted integration of AAV recombinant vectors designed for human gene therapy.

Efficient gene transfer into human keratinocytes with recombinant adeno-associated virus vectors

Gene transfer into the skin is a promising approach to treat inherited or acquired dermatological diseases and systemic monogenic deficiencies. For this purpose, the efficient and sustained gene delivery into keratinocytes is of critical importance. Recombinant adeno-associated virus (rAAV) vectors hold the potential to achieve a long-term gene transfer into various human organs. In order to evaluate this potential for skin gene therapy, human keratinocytes were transduced in vitro with rAAV vectors encoding the reporter genes beta-galactosidase (rAAV/LacZ) or green fluorescent protein (rAAV/GFP). Using rAAV/LacZ at a multiplicity of infection (MOI) of five transducing particles per cell, up to 70% of human keratinocytes were transduced within 48 h. This effect was independent of individual skin donors and different body areas serving as the source for keratinocyte isolation. rAAV had no significant influence on cell viability, but induced a growth arrest in transduced keratinocytes. This growth arrest was overcome by replating cells in fresh media. rAAV/GFP-transduced keratinocytes could be passaged several times, expressed GFP for up to 50 days, and passed the transgene to their daughter cells, suggesting that keratinocyto precursor cells were also transduced. Taken together, the results suggest that rAAV is a promising gene transfer vehicle for skin gene therapy.

Nontargeted stable integration of recombinant adeno-associated virus into human leukemia and lymphoma cell lines as evaluated by fluorescence in situ hybridization

A number of studies on human epithelial cells of varying origin have demonstrated integration of recombinant adeno-associated virus (AAV) vectors into a variety of chromosomes compared with the site-specific integration on chromosome 19 predominantly observed for wild-type (wt) AAV. We have constructed a recombinant AAV (rAAV) vector and tested the integration into hematopoietic cells, using the human acute myeloid leukemia cell line AML5 and the human non-Hodgkin's lymphoma cell line OCI-LY18 as targets. The integration sites were visualized by fluorescence in situ hybridization (FISH). Positive signals were observed for chromosomes 1, 2, 3, 8, 14, 15, 19, and Y. The majority of cells demonstrated integration into one specific site. A minority showed simultaneous integration into more than one chromosome. The frequency of observed integrations was not uniformly distributed among chromosomes; for instance, in AML5 chromosome 2 seemed to be favored. Colony-derived AML5 clones bore unique integration patterns indicating successful transduction of clonogenic progenitor cells with high proliferative potential. The integration was stable and observed for more than 12 months after transduction. FISH has been shown to be a powerful tool for detailed analyses of rAAV integration patterns and can be used to evaluate targets and transduction conditions.