Enhancement of muscle gene delivery with pseudotyped adeno-associated virus type 5 correlates with myoblast differentiation

High-throughput 5' UTR engineering for enhanced protein production in non-viral gene therapies

Despite significant clinical progress in cell and gene therapies, maximizing protein expression in order to enhance potency remains a major technical challenge. Here, we develop a high-throughput strategy to design, screen, and optimize 5' UTRs that enhance protein expression from a strong human cytomegalovirus (CMV) promoter. We first identify naturally occurring 5' UTRs with high translation efficiencies and use this information with in silico genetic algorithms to generate synthetic 5' UTRs. A total of ~12,000 5' UTRs are then screened using a recombinase-mediated integration strategy that greatly enhances the sensitivity of high-throughput screens by eliminating copy number and position effects that limit lentiviral approaches. Using this approach, we identify three synthetic 5' UTRs that outperform commonly used non-viral gene therapy plasmids in expressing protein payloads. In summary, we demonstrate that high-throughput screening of 5' UTR libraries with recombinase-mediated integration can identify genetic elements that enhance protein expression, which should have numerous applications for engineered cell and gene therapies.

Rational Engineering and Preclinical Evaluation of Neddylation and SUMOylation Site Modified Adeno-Associated Virus Vectors in Murine Models of Hemophilia B and Leber Congenital Amaurosis

Synthetic engineering of viral vectors such as adeno-associated virus (AAV) is crucial to overcome host transduction barriers observed during clinical gene therapy. We reasoned that exploring the role of cellular ubiquitin-like modifiers (UBLs) such as Neddylation or SUMOylation during AAV transduction could be beneficial. Using a combination of in silico biochemical and molecular engineering strategies, we have studied the impact of these UBLs during AAV2 infection and further developed Neddylation or SUMOylation site–modified AAV vectors and validated them in multiple disease models in vitro and in vivo. Hepatic gene transfer of two novel vectors developed, K105Q (SUMOylation-site mutant) and K665Q (Neddylation-site mutant), demonstrated a significantly improved human coagulation factor (F) IX expression (up to two-fold) in a murine model of hemophilia B. Furthermore, subretinal gene transfer of AAV2-K105Q vector expressing RPE65 gene demonstrated visual correction in a murine model of a retinal degenerative disease (rd12 mice). These vectors did not have any adverse immunogenic events in vivo. Taken together, we demonstrate that gene delivery vectors specifically engineered at UBLs can improve the therapeutic outcome during AAV-mediated ocular or hepatic gene therapy.

Comparative AAV-eGFP Transgene Expression Using Vector Serotypes 1-9, 7m8, and 8b in Human Pluripotent Stem Cells, RPEs, and Human and Rat Cortical Neurons

Recombinant adeno-associated virus (rAAV), produced from a nonpathogenic parvovirus, has become an increasing popular vector for gene therapy applications in human clinical trials. However, transduction and transgene expression of rAAVs can differ across in vitro and ex vivo cellular transduction strategies. This study compared 11 rAAV serotypes, carrying one reporter transgene cassette containing a cytomegalovirus immediate-early enhancer (eCMV) and chicken beta actin (CBA) promoter driving the expression of an enhanced green-fluorescent protein (eGFP) gene, which was transduced into four different cell types: human iPSC, iPSC-derived RPE, iPSC-derived cortical, and dissociated embryonic day 18 rat cortical neurons. Each cell type was exposed to three multiplicity of infections (MOI: 1E4, 1E5, and 1E6 vg/cell). After 24, 48, 72, and 96 h posttransduction, GFP-expressing cells were examined and compared across dosage, time, and cell type. Retinal pigmented epithelium showed highest AAV-eGFP expression and iPSC cortical the lowest. At an MOI of 1E6 vg/cell, all serotypes show measurable levels of AAV-eGFP expression; moreover, AAV7m8 and AAV6 perform best across MOI and cell type. We conclude that serotype tropism is not only capsid dependent but also cell type plays a significant role in transgene expression dynamics.

Myoediting: Toward Prevention of Muscular Dystrophy by Therapeutic Genome Editing

Muscular dystrophies represent a large group of genetic disorders that significantly impair quality of life and often progress to premature death. There is no effective treatment for these debilitating diseases. Most therapies, developed to date, focus on alleviating the symptoms or targeting the secondary effects, while the underlying gene mutation is still present in the human genome. The discovery and application of programmable nucleases for site-specific DNA double-stranded breaks provides a powerful tool for precise genome engineering. In particular, the CRISPR/Cas system has revolutionized the genome editing field and is providing a new path for disease treatment by targeting the disease-causing genetic mutations. In this review, we provide a historical overview of genome-editing technologies, summarize the most recent advances, and discuss potential strategies and challenges for permanently correcting genetic mutations that cause muscular dystrophies.

Myoblast Therapy: From Bench to Bedside

Myoblasts are defined as stem cells containing skeletal muscle cell precursors. A decade of experimental work has revealed many properties of myoblasts, including the stability of resulting hybrid myofibers without immune suppression, the persistence of transgene expression, and the lack of tumorigenicity. Early phase clinical trials also showed that myoblast-based therapy is a promising approach for many intractable clinical conditions, including both muscle-related and non-muscle-related diseases. The potential application of myoblast therapy may be in the treatment of genetic muscle diseases, cardiomyocyte damaged heart diseases, and urinary incontinence. This review will provide an overview of myoblast biology, along with discussion of the potential application in clinical medicine. In addition, problems in current myoblast therapy and possible future improvements will be addressed.

The potential of adeno-associated viral vectors for gene delivery to muscle tissue

INTRODUCTION

Muscle-directed gene therapy is rapidly gaining attention primarily because muscle is an easily accessible target tissue and is also associated with various severe genetic disorders. Localized and systemic delivery of recombinant adeno-associated virus (rAAV) vectors of several serotypes results in very efficient transduction of skeletal and cardiac muscles, which has been achieved in both small and large animals, as well as in humans. Muscle is the target tissue in gene therapy for many muscular dystrophy diseases, and may also be exploited as a biofactory to produce secretory factors for systemic disorders. Current limitations of using rAAVs for muscle gene transfer include vector size restriction, potential safety concerns such as off-target toxicity and the immunological barrier composing of pre-existing neutralizing antibodies and CD8(+) T-cell response against AAV capsid in humans.

AREAS COVERED

In this article, we will discuss basic AAV vector biology and its application in muscle-directed gene delivery, as well as potential strategies to overcome the aforementioned limitations of rAAV for further clinical application.

EXPERT OPINION

Delivering therapeutic genes to large muscle mass in humans is arguably the most urgent unmet demand in treating diseases affecting muscle tissues throughout the whole body. Muscle-directed, rAAV-mediated gene transfer for expressing antibodies is a promising strategy to combat deadly infectious diseases. Developing strategies to circumvent the immune response following rAAV administration in humans will facilitate clinical application.

Structural insights into adeno-associated virus serotype 5

The adeno-associated viruses (AAVs) display differential cell binding, transduction, and antigenic characteristics specified by their capsid viral protein (VP) composition. Toward structure-function annotation, the crystal structure of AAV5, one of the most sequence diverse AAV serotypes, was determined to 3.45-Å resolution. The AAV5 VP and capsid conserve topological features previously described for other AAVs but uniquely differ in the surface-exposed HI loop between βH and βI of the core β-barrel motif and have pronounced conformational differences in two of the AAV surface variable regions (VRs), VR-IV and VR-VII. The HI loop is structurally conserved in other AAVs despite amino acid differences but is smaller in AAV5 due to an amino acid deletion. This HI loop is adjacent to VR-VII, which is largest in AAV5. The VR-IV, which forms the larger outermost finger-like loop contributing to the protrusions surrounding the icosahedral 3-fold axes of the AAVs, is shorter in AAV5, creating a smoother capsid surface topology. The HI loop plays a role in AAV capsid assembly and genome packaging, and VR-IV and VR-VII are associated with transduction and antigenic differences, respectively, between the AAVs. A comparison of interior capsid surface charge and volume of AAV5 to AAV2 and AAV4 showed a higher propensity of acidic residues but similar volumes, consistent with comparable DNA packaging capacities. This structure provided a three-dimensional (3D) template for functional annotation of the AAV5 capsid with respect to regions that confer assembly efficiency, dictate cellular transduction phenotypes, and control antigenicity.

A novel chimeric adenoassociated virus 2/human bocavirus 1 parvovirus vector efficiently transduces human airway epithelia

Human bocavirus virus-1 (HBoV1), a newly discovered autonomous parvovirus with a 5,500 nt genome, efficiently infects human-polarized airway epithelia (HAE) from the apical membrane. We hypothesized that the larger genome and high airway tropism of HBoV1 would be ideal for creating a viral vector for lung gene therapy. To this end, we successfully generated recombinant HBoV1 (rHBoV1) from an open reading frames-disrupted rHBoV1 genome that efficiently transduces HAE from the apical surface. We next evaluated whether HBoV1 capsids could package oversized rAAV2 genomes. These studies created a rAAV2/HBoV1 chimeric virus (5.5 kb genome) capable of apically transducing HAE at 5.6- and 70-fold greater efficiency than rAAV1 or rAAV2 (4.7-kb genomes), respectively. Molecular studies demonstrated that viral uptake from the apical surface was significantly greater for rAAV2/HBoV1 than for rAAV2 or rAAV1, and that polarization of airway epithelial cells was required for HBoV1 capsid-mediated gene transfer. Furthermore, rAAV2/HBoV1-CFTR virus containing the full-length cystic fibrosis transmembrane conductance regulator (CFTR) gene coding sequence and the strong CBA promoter efficiently corrected CFTR-dependent chloride transport in cystic fibrosis (CF) HAE. In summary, using the combined advantages of AAV and HBoV1, we have developed a novel and promising viral vector for CF lung gene therapy and also potentially HBoV1 vaccine development.

Gene and cell-mediated therapies for muscular dystrophy

Duchenne muscular dystrophy (DMD) is a devastating muscle disorder that affects 1 in 3,500 boys. Despite years of research and considerable progress in understanding the molecular mechanism of the disease and advancement of therapeutic approaches, there is no cure for DMD. The current treatment options are limited to physiotherapy and corticosteroids, and although they provide a substantial improvement in affected children, they only slow the course of the disorder. On a more optimistic note, more recent approaches either significantly alleviate or eliminate muscular dystrophy in murine and canine models of DMD and importantly, many of them are being tested in early phase human clinical trials. This review summarizes advancements that have been made in viral and nonviral gene therapy as well as stem cell therapy for DMD with a focus on the replacement and repair of the affected dystrophin gene.

Extracorporeal delivery of rAAV with metabolic exchange and oxygenation

Over the past decade much progress has been made towards the treatment of disease with recombinant adeno-associated viral vectors, ranging from cancer to muscular dystrophies, and autoimmune diseases to cystic fibrosis. Given inherent challenges of vector delivery we developed a system incorporating commercially available dialysis equipment. This concept was evaluated in vitro utilizing rAAV expressing the reporter gene human placental alkaline phosphatase. A number of pre-circulating conditions were assessed. Vector recovery was evaluated by quantitative vector genome analysis and cellular transduction assays. A dialysis circulation time course was established, and results were recorded across varied conditions ranging from approximately 2 to 90% retention of viable vector. This approach is unique in that it focuses on efficient localized, isolated and continual delivery of vector to target tissues, provides for the preservation of tissue integrity with dialysis for metabolic exchange and allows for the transfer of oxygen through a secondary membrane post-dialysis.

Intracellular transport of recombinant adeno-associated virus vectors

Recombinant adeno-associated viral vectors (rAAVs) have been widely used for gene delivery in animal models, and are currently evaluated for human gene therapy after successful clinical trials in the treatment of inherited, degenerative or acquired diseases, such as Leber congenital amaurosis, Parkinson disease or heart failure. However, limitations in vector tropism, such as limited tissue specificity and insufficient transduction efficiencies of particular tissues and cell types, still preclude therapeutic applications in certain tissues. Wild-type adeno-associated viruses (AAVs) are defective viruses that require the presence of a helper virus to complete their life cycle. On the one hand, this unique property makes AAV vectors one of the safest available viral vectors for gene delivery. On the other, it also represents a potential obstacle because rAAV vectors have to overcome several biological barriers in the absence of a helper virus to transduce successfully a cell. Consequently, a better understanding of the cellular roadblocks that limit rAAV gene delivery is crucial and, during the last 15 years, numerous studies resulted in an expanding body of knowledge of the intracellular trafficking pathways of rAAV vectors. This review describes our current understanding of the mechanisms involved in rAAV attachment to target cells, endocytosis, intracellular trafficking, capsid processing, nuclear import and genome release with an emphasis on the most recent discoveries in the field and the emerging strategies used to improve the efficiency of AAV-derived vectors.

Development of Novel Recombinant AAV Vectors and Strategies for the Potential Gene Therapy of Hemophilia

Recombinant vectors based on a non-pathogenic human parvovirus, the adeno-associated virus (AAV), have gained attention as a potentially safe and useful alternative to the more commonly used retroviral and adenoviral vectors. AAV vectors are currently in use in Phase I/II clinical trials for gene therapy of a number of diseases such as cystic fibrosis, α-1 antitrypsin deficiency, muscular dystrophy, Batten's disease, and Parkinson's disease, and have shown efficacy in patients with Leber's congenital amaurosis, and hemophilia B. For patients with hemophilia B, however, relatively large vector doses are needed to achieve therapeutic benefits. Large vector doses also trigger an immune response as significant fraction of the vectors fails to traffic efficiently to the nucleus, and is targeted for degradation by the host cell proteasome machinery. With a better understanding of the various steps in the life cycle of AAV vectors, strategies leading to the development of novel AAV vectors that are capable of high-efficiency transduction at lower doses are needed. In this review, we summarize our strategies to develop novel AAV vectors for the potential gene therapy of both hemophilia B and hemophilia A, based on our recent studies on the basic molecular biology of AAV. These strategies, including the development of novel AAV vectors by site-directed mutagenesis of critical surface-exposed tyrosine residues on AAV2 capsids to circumvent the ubiquitination step and the use of different AAV serotypes and self-complementary (sc) AAV2 vectors, and their use as helper vectors to circumvent the obstacles of second-strand DNA synthesis of single-stranded (ss) AAV, should dramatically accelerate the progress towards the potential gene therapy of both hemophilia A and hemophilia B.

Recombinant adeno-associated viral vector production and purification

Gene delivery vectors based on recombinant adeno-associated virus (AAV) are powerful tools for studying myogenesis in normal and diseased conditions. Strategies have been developed to use AAV to increase, down-regulate, or modify expression of a particular muscle gene in a specific muscle, muscle group(s), or all muscles in the body. AAV-based muscle gene therapy has been shown to cure several inherited muscle diseases in animal models. Early clinical trials have also yielded promising results. In general, AAV vectors lead to robust, long-term in vivo transduction in rodents, dogs, and non-human primates. To meet specific research needs, investigators have developed numerous AAV variants by engineering viral capsid and/or genome. Here we outline a generic AAV production and purification protocol. Techniques described here are applicable to any AAV variant.

Treatment of focal brain ischemia with viral vector-mediated gene transfer

Promoting functional recovery after ischemic brain injury has emerged as a potential approach for the treatment of ischemic stroke. An ideal restorative approach to enhance long-term functional recovery is to promote postischemic angiogenesis and neurogenesis. This chapter describes a system using adeno-associated viral (AAV) vector-mediated vascular endothelial growth factor (VEGF) gene transfer into the ischemic brain. The methods described here for construction, production, and purification of AAV vector expressing VEGF gene can also be applied to producing AAV vectors expressing other genes. This chapter also illustrates the methods to produce mouse middle cerebral artery occlusion (MCAO), injection of viral vector into the mouse brain, and standard assays for determining the success of brain ischemia and gene transfer.

Preclinical studies for gene therapy of Duchenne muscular dystrophy

The muscular dystrophies are a diverse group of genetic disorders without an effective treatment. Because they are caused by mutations in various genes, the most direct way to treat them involves correcting the underlying gene defect (ie, gene therapy). Such a gene therapy approach involves delivering a therapeutic gene cassette to essentially all the muscles of the body in a safe and efficacious manner. The authors describe gene delivery methods using vectors derived from adeno-associated virus that are showing great promise in preclinical studies for treatment of Duchenne muscular dystrophy. It is hoped that variations on these methods might be applicable for most, if not all, of the different types of muscular dystrophy.

Selective tropism of the recombinant adeno-associated virus 9 serotype for rat cardiac tissue

BACKGROUND

Cardiac gene transfer may serve as a novel therapeutic approach for heart disease. Numerous serotypes of recombinant adeno-associated virus (rAAV) have been identified with variable tropisms to cardiac tissue.

METHODS

Both in vitro and in vivo experiments were undertaken to compare cardiac tropisms of rAAV-2, 5, 7, 8 and 9. For the in vitro studies, 10(7) vector genome (vg) of rAAV-2, 5, 7, 8 or 9 were used to transduce both rat neonatal cardiac myocytes (RNCM) and fibroblasts (RNCF). For the in vivo studies, 4 x 10(10) vg of rAAV-2, 5, 7, 8 or 9, and 4 x 10(11) vg of rAAV8 or 9 were administered in 5-day-old rats via a relatively non-invasive intracardiac injection. One and two months post-administration, green fluorescent protein (GFP) expression in tissues was visualized and GFP mRNA was quantified by the real-time polymerase chain reaction.

RESULTS

At 3 days post-viral transduction, rAAV9 and rAAV2 produced the highest transducing efficiency in RNCM. Only rAAV2 elicited any transduction in the RNCF. The results obtained in vivo indicated that the order for transduction efficiency in the heart was: rAAV9 > rAAV8 > rAAV7 > rAAV2 = rAAV5. The transduction efficiency order in the liver was: rAAV2 > rAAV5 > rAAV7 > rAAV8 > rAAV9. Injection of a higher dose (4 x 10(11) vg) of rAAV9 provided more widespread and highly cardiac-selective GFP expression in the heart than rAAV8. Zero to minimal expression of GFP was found in the lung and kidney for both doses of all rAAV serotypes utilized.

CONCLUSIONS

Collectively, the results obtained in the present study suggest that rAAV9 provides the most selective and stable transduction efficiency in cardiac tissue, and this expression was primarily exhibited in cardiac myocytes.

Evidence for the failure of adeno-associated virus serotype 5 to package a viral genome > or = 8.2 kb

Limited packaging capacity hinders adeno-associated virus (AAV) gene therapy. A recent study seems to have provided a solution to this problem. Allocca et al. reported that AAV-5 could package an 8.9 kb vector genome. Here we tested whether this approach can be used to deliver a large genome for Duchenne muscular dystrophy (DMD) gene therapy. We first evaluated AAV-5 packaging of an 8.2 kb genome. This vector carries two independent reporter gene cassettes, one for alkaline phosphatase (AP) and another for LacZ. Viral yield was log-fold lower than that of a regular AAV-5. Nevertheless, both AP and LacZ genes were detected in purified virus. Injection to dystrophic muscle resulted in both AP and LacZ expression. On electron microscopy, virion structure appeared normal. Surprisingly, we did not find the full-length single-stranded viral genome by alkaline gel electrophoresis. Neither did we see the full-length double-stranded replication forms in adenovirus coinfected cells. We suspect that AP and LacZ expression may have come from partially packaged 5' or 3'-half of the genome. Additional studies revealed failure of AAV-5 to package and express an 8.7 kb minidystrophin gene cassette. In summary, our results do not support the extraordinary packaging capacity of AAV-5.

Emerging strategies for cell and gene therapy of the muscular dystrophies

The muscular dystrophies are a heterogeneous group of over 40 disorders that are characterised by muscle weakness and wasting. The most common are Duchenne muscular dystrophy and Becker muscular dystrophy, which result from mutations within the gene encoding dystrophin; myotonic dystrophy type 1, which results from an expanded trinucleotide repeat in the myotonic dystrophy protein kinase gene; and facioscapulohumeral dystrophy, which is associated with contractions in the subtelomeric region of human chromosome 1. Currently the only treatments involve clinical management of symptoms, although several promising experimental strategies are emerging. These include gene therapy using adeno-associated viral, lentiviral and adenoviral vectors and nonviral vectors, such as plasmid DNA. Exon-skipping and cell-based therapies have also shown promise in the effective treatment and regeneration of dystrophic muscle. The availability of numerous animal models for Duchenne muscular dystrophy has enabled extensive testing of a wide range of therapeutic approaches for this type of disorder. Consequently, we focus here on the therapeutic developments for Duchenne muscular dystrophy as a model of the types of approaches being considered for various types of dystrophy. We discuss the advantages and limitations of each therapeutic strategy, as well as prospects and recent successes in the context of future clinical applications.

Disease rescue and increased lifespan in a model of cardiomyopathy and muscular dystrophy by combined AAV treatments

BACKGROUND

The BIO14.6 hamster is an excellent animal model for inherited cardiomyopathy, because of its lethal and well-documented course, due to a spontaneous deletion of delta-sarcoglycan gene promoter and first exon. The muscle disease is progressive and average lifespan is 11 months, because heart slowly dilates towards heart failure.

METHODOLOGY/PRINCIPAL FINDINGS

Based on the ability of adeno-associated viral (AAV) vectors to transduce heart together with skeletal muscle following systemic administration, we delivered human delta-sarcoglycan cDNA into male BIO14.6 hamsters by testing different ages of injection, routes of administration and AAV serotypes. Body-wide restoration of delta-SG expression was associated with functional reconstitution of the sarcoglycan complex and with significant lowering of centralized nuclei and fibrosis in skeletal muscle. Motor ability and cardiac functions were completely rescued. However, BIO14.6 hamsters having less than 70% of fibers recovering sarcoglycan developed cardiomyopathy, even if the total rescued protein was normal. When we used serotype 2/8 in combination with serotype 2/1, lifespan was extended up to 22 months with sustained heart function improvement.

CONCLUSIONS/SIGNIFICANCE

Our data support multiple systemic administrations of AAV as a general therapeutic strategy for clinical trials in cardiomyopathies and muscle disorders.

Sustained baculovirus-mediated expression in myogenic cells

BACKGROUND

Baculovirus has emerged as a promising gene delivery vector due to its low cytotoxicity and nonreplication nature in mammalian cells. However, baculovirus-mediated expression is transient and generally lasts less than 14 days, which could restrict its application in the treatment of diseases requiring stable transgene expression.

METHODS

We transduced myoblast cell lines C2C12, Sol 8 and primary myoblasts with a baculovirus expressing the enhanced green fluorescent protein (EGFP) under the control of cytomegalovirus immediate-early promoter and measured the transduction efficiency by flow cytometry. Myogenic differentiation was induced after transduction and the longevity of EGFP expression was monitored by fluorescence microscopy. The myogenic differentiation was confirmed by reverse transcription-polymerase chain reaction (RT-PCR). The persistence of the egfp DNA and transcripts was monitored by real-time PCR and quantitative real-time RT-PCR.

RESULTS

Baculovirus efficiently transduced C2C12, Sol 8 and the primary myoblasts. The transgene expression persisted for a prolonged period of time (at least 63 days) in the cells differentiating into myotubes, but was transient in HeLa cells (<7 days). The sustained expression paralleled the myogenic differentiation and stemmed from the intracellular persistence of egfp DNA and mRNA.

CONCLUSIONS

The transgene delivered by baculovirus persists in the myotubes and endows sustained expression, which is distinct from its rapid degradation and transient expression in other cell types. These findings justify the future use of baculovirus for muscle-based gene therapy.

Catalase overexpression does not impair extensor digitorum longus muscle function in normal mice

Catalase is a major antioxidant enzyme. Increasing catalase expression represents a promising avenue to improve muscle function in certain physiological conditions and in some muscle diseases. We hypothesized that catalase overexpression should not impair normal muscle contraction. We delivered a hemagglutinin (HA)-tagged human catalase gene to normal mouse muscle by an adeno-associated viral vector (AAV). Western blot and immunostaining revealed efficient expression of HA-tagged catalase. Enzymatic assay demonstrated an approximately threefold increase in catalase activity in AAV-infected muscles. Catalase overexpression impaired neither twitch nor tetanic tension in the extensor digitorum longus (EDL) muscle. Furthermore, EDL fatigue response was not altered. Taken together, we have developed a novel AAV vector to enhance catalase expression. Lack of apparent toxicity in normal muscle strongly supports further exploration of this vector to reduce oxidative stress-induced muscle damage.

The role of the adeno-associated virus capsid in gene transfer

Adeno-associated virus (AAV) is one of the most promising viral gene transfer vectors that has been shown to effect long-term gene expression and disease correction with low toxicity in animal models, and is well tolerated in human clinical trials. The surface of the AAV capsid is an essential component that is involved in cell binding, internalization, and trafficking within the targeted cell. Prior to developing a gene therapy strategy that utilizes AAV, the serotype should be carefully considered since each capsid exhibits a unique tissue tropism and transduction efficiency. Several approaches have been undertaken in an effort to target AAV vectors to specific cell types, including utilizing natural serotypes that target a desired cellular receptor, producing pseudotyped vectors, and engineering chimeric and mosaic AAV capsids. These capsid modifications are being incorporated into vector production and purification methods that provide for the ability to scale-up the manufacturing process to support human clinical trials. Protocols for small-scale and large-scale production of AAV, as well as assays to characterize the final vector product, are presented here. The structures of AAV2, AAV4, and AAV5 have been solved by X-ray crystallography or cryo-electron microscopy (cryo-EM), and provide a basis for rational vector design in developing customized capsids for specific targeting of AAV vectors. The capsid of AAV has been shown to be remarkably stable, which is a desirable characteristic for a gene therapy vector; however, recently it has been shown that the AAV serotypes exhibit differential susceptibility to proteases. The capsid fragmentation pattern when exposed to various proteases, as well as the susceptibility of the serotypes to a series of proteases, provides a unique fingerprint for each serotype that can be used for capsid identity validation. In addition to serotype identification, protease susceptibility can also be utilized to study dynamic structural changes that must occur for the AAV capsid to perform its various functions during the virus life cycle. The use of proteases for structural studies in solution complements the crystal structural studies of the virus. A generic protocol based on proteolysis for AAV serotype identification is provided here.

Biological Differences in rAAV Transduction of Airway Epithelia in Humans and in Old World Non-human Primates

Non-human primates (NHPs) are considered to be among the most relevant animal models for pre-clinical testing of human therapies, on the basis of their close evolutionary relatedness to humans in terms of organ cell biology and physiology. In this study, we sought to investigate whether NHP models accurately reflect the effectiveness of recombinant adeno-associated virus (rAAV)-mediated gene delivery to the airway in humans. In order to do this, we utilized an identical model system of differentiated airway epithelia from Indian Rhesus monkeys and from humans, cultured at an air-liquid interface (ALI). In addition to assessing the biology of rAAV-mediated transduction for three serotypes, we characterized the bioelectric properties as a reference for biological similarities and differences between the cell cultures from the two species. Our results demonstrate that airway epithelia from NHPs and humans have very similar Na(+) and Cl(-) transport properties. In contrast, rAAV transduction of airway epithelia of NHPs demonstrated significant differences to those in humans with regard to the efficiency of apical and/or basal transduction with three rAAV serotypes (AAV1, AAV2, AAV5). These findings suggest that the IndianRhesusmonkey may not be the best model for preclinical testing of rAAV-mediated gene therapy to the airway in humans.

A hybrid vector system expands adeno-associated viral vector packaging capacity in a transgene-independent manner

The trans-splicing (ts) and overlapping (ov) vectors expand the packaging capacity of adeno-associated virus (AAV). But their application depends on the inherent properties of the target gene. The ts vectors require an optimal gene-splitting site and the ov vectors require a highly recombinogenic domain. In order to overcome these limitations, we developed a hybrid dual (hd) vector system. In the hd vectors, we inserted a highly recombinogenic alkaline phosphatase (AP) sequence in the ts vectors to allow for transgene-independent reconstitution through homologous recombination of the AP sequences. We first tested the hybrid system with the LacZ gene. Both in the cell line (in vitro) and in the mouse muscle (in vivo), the hd vectors significantly outperformed the ts and ov vectors. In muscle, the transduction efficiency of the hybrid vectors reached 80% of that from the single intact vector. Southern blot confirmed AP sequence-mediated transgene reconstitution. In order to validate the hybrid system, we split the 6 kilobase (kb) mini-dystrophin gene at the exon 55/56 junction, a predicted poor site for the ts approach. In dystrophic mdx mouse muscle, the hd vectors yielded 5.6-fold higher transduction than the ts vectors did. Taken together, these data suggest that the hybrid system efficiently expresses large therapeutic genes that are poor candidates for the ts and ov approaches.

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.

AAV serotype-1 mediates early onset of gene expression in mouse hearts and results in better therapeutic effect

Adeno-associated viral vectors (AAV) are attractive tool for gene therapy for coronary artery disease. However, gene expression in myocardium mediated by AAV serotype 2 (AAV2) does not peak until 4-6 weeks after gene transfer. This delayed gene expression may reduce its therapeutic potential for acute cardiac infarction. To determine whether earlier gene expression and better therapeutic effect could be achieved using a different serotype, CMV promoter driving the EPO gene (AAV-EPO) was packaged into AAV serotypes 1-5 capsids and injected into mouse myocardium. EPO expression was studied by measuring the hematocrits and EPO mRNA. After we found that AAV1 mediates the highest gene expression after 4 days of gene transduction, AAV-LacZ (CMV promoter driving LacZ gene expression) and MLCVEGF (hypoxia-inducible and cardiac-specific VEGF expression) were packaged into AAV1 and 2 capsids. LacZ expression was detected in AAV1-LacZ but not in AAV2-LacZ-injected hearts 1 day after vector injection. Compared to AAV2-MLCVEGF that mediated no significant VEGF expression, AAV1-MLCVEGF mediated 13.7-fold induction of VEGF expression in ischemic hearts 4 days after gene transduction and resulted in more neovasculatures, better cardiac function and less myocardial fibrosis. Thus, AAV1 mediates earlier and higher transgene expression in myocardium and better therapeutic effects.

Evaluation of primitive murine hematopoietic stem and progenitor cell transduction in vitro and in vivo by recombinant adeno-associated virus vector serotypes 1 through 5

Conflicting data exist on hematopoietic cell transduction by AAV serotype 2 (AAV2) vectors, and additional AAV serotype vectors have not been evaluated for their efficacy in hematopoietic stem/progenitor cell transduction. We evaluated the efficacy of conventional, single-stranded AAV serotype vectors 1 through 5 in primitive murine hematopoietic stem/progenitor cells in vitro as well as in vivo. In progenitor cell assays using Sca1+ c-kit+ Lin- hematopoietic cells, 9% of the colonies in cultures infected with AAV1 expressed the transgene. Coinfection of AAV1 with self-complementary AAV vectors carrying the gene for T cell protein tyrosine phosphatase (scAAV-TC-PTP) increased the transduction efficiency to 24%, indicating that viral secondstrand DNA synthesis is a rate-limiting step. This was further corroborated by the use of scAAV vectors, which bypass this requirement. In bone marrow transplantation studies involving lethally irradiated syngeneic mice, Sca1+ c-kit+ Lin- cells coinfected with AAV1 +/- scAAV-TC-PTP vectors led to transgene expression in 2 and 7.5% of peripheral blood (PB) cells, respectively, 6 months posttransplantation. In secondary transplantation experiments, 7% of PB cells and 3% of bone marrow (BM) cells expressed the transgene 6 months posttransplantation. Approximately 21% of BM-derived colonies harbored the proviral DNA sequences in integrated forms. These results document that AAV1 is thus far the most efficient vector in transducing primitive murine hematopoietic stem/progenitor cells. Further studies involving scAAV genomes and hematopoietic cell-specific promoters should further augment the transduction efficiency of AAV1 vectors, which should have implications in the optimal use of these vectors in hematopoietic stem cell gene therapy.

Viral serotype and the transgene sequence influence overlapping adeno-associated viral (AAV) vector-mediated gene transfer in skeletal muscle

BACKGROUND

The overlapping approach was developed recently to expand the adeno-associated viral (AAV) packaging capacity. In this approach, a gene is split into two partially overlapping fragments and separately packaged into an upstream and a downstream vector, respectively. Transgene expression is achieved in co-infected cells after homologous recombination. Despite the promising proof-of-principle results in the lung, the efficiency has been very disappointing in skeletal muscle. Here we examined two potential rate-limiting factors including AAV serotype and the transgene sequence.

METHODS

To study serotype effect, we delivered AAV-2, -5 and -6 overlapping vectors (5 x 10(8) vg particles of the upstream and the downstream vectors, respectively) and 5 x 10(8) vg particles of the intact gene vector to the tibialis anterior muscles of 7-week-old C57Bl/6 mice, respectively. To determine the effect of transgene sequence, we compared LacZ and alkaline phosphatase (AP) overlapping vectors. Transduction efficiency was quantified 6 weeks later by scoring the percentage of transgene-positive myofibers.

RESULTS

AAV-2 overlapping vectors barely resulted in detectable transduction. Transduction efficiency was significantly improved in AAV-5 and AAV-6. The highest level was achieved in AAV-6 that reached 42% and 96% of that of the intact gene vector for the LacZ gene and the AP gene, respectively. Surprisingly, AAV-6 overlapping vector resulted in higher transduction than did AAV-2 and AAV-5 intact gene vectors.

CONCLUSIONS

Our findings suggest that AAV serotype and the transgene sequence play critical roles in the overlapping approach. AAV-6 holds great promise for overlapping vector-mediated muscle gene therapy.

Therapeutic strategies for the inherited neuropathies

More than 30 genetic causes have been identified for the inherited neuropathies collectively referred to as Charcot-Marie-Tooth (CMT) disease. Previous therapies for CMT were limited to traditional approaches such as rehabilitation medicine, ambulation aids, and pain management. Identification of the genes causing CMT has led to improved genetic counseling and assistance in family planning. Identification of these genes is beginning to delineate common molecular pathways in multiple forms of CMT that can be exploited in future molecular therapies. Scientifically based clinical trials for CMT are currently being implemented. Techniques of gene therapy are advancing to the point that they may become feasible options for patients with CMT and other neurodegenerative diseases.

Intracellular trafficking of adeno-associated viral vectors

Adeno-associated virus (AAV) has attracted considerable interest as a gene therapy vector over the past decade. In all, 85% of the current 2052 PubMed references on AAV (as of December 2004) have been published in the last 10 years. As researchers have moved forward with using this vector system for gene delivery, an increased appreciation for the complexities of AAV biology has emerged. The biology of recombinant AAV (rAAV) transduction has demonstrated considerable diversity in different cell types and target tissues. This review will summarize the current understanding of events that control rAAV transduction following receptor binding and leading to nuclear uptake. These stages are broadly classified as intracellular trafficking and have been found to be a major rate-limiting step in rAAV transduction for many cell types. Advances in understanding this area of rAAV biology will help to improve the efficacy of this vector system for the treatment of inherited and acquired diseases.

Adeno-associated virus-mediated microdystrophin expression protects young mdx muscle from contraction-induced injury

Duchenne muscular dystrophy (DMD) is the most common inherited lethal muscle degenerative disease. Currently there is no cure. Highly abbreviated microdystrophin cDNAs were developed recently for adeno-associated virus (AAV)-mediated DMD gene therapy. Among these, a C-terminal-truncated DeltaR4-R23/DeltaC microgene (DeltaR4/DeltaC) has been considered as a very promising therapeutic candidate gene. In this study, we packaged a CMV.DeltaR4/DeltaC cassette in AAV-5 and evaluated the transduction and muscle contractile profiles in the extensor digitorum longus muscles of young (7-week-old) and adult (9-month-old) mdx mice. At approximately 3 months post-gene transfer, 50-60% of the total myofibers were transduced in young mdx muscle and the percentage of centrally nucleated myofibers was reduced from approximately 70% in untreated mdx muscle to approximately 22% in microdystrophin-treated muscle. Importantly, this level of transduction protected mdx muscle from eccentric contraction-induced damage. In contrast, adult mdx muscle was more resistant to AAV-5 transduction, as only approximately 30% of the myofibers were transduced at 3 months postinfection. This transduction yielded marginal protection against eccentric contraction-induced injury. The extent of central nucleation was also more difficult to reverse in adult mdx muscle (from approximately 83% in untreated to approximately 58% in treated). Finally, we determined that the DeltaR4/DeltaC microdystrophin did not significantly alter the expression pattern of the endogenous full-length dystrophin in normal muscle. Neither did it have any adverse effects on normal muscle morphology or contractility. Taken together, our results suggest that AAV-mediated DeltaR4/DeltaC microdystrophin expression represents a promising approach to rescue muscular dystrophy in young mdx skeletal muscle.

Viral delivery of recombinant short hairpin RNAs

Recent work demonstrates that RNA interference (RNAi) can coordinate protein expression. Inhibitory RNAs are expressed naturally in cells as microRNAs (miRNAs) or introduced into cells as small interfering RNAs (siRNAs). Both types of small RNAs can be used at the bench to silence mRNA expression. For many researchers, transfection of siRNAs synthesized in vitro or purchased from commercial sources is impractical for the cellular system under study. As an alternative to transfection-based methods, we provide a practical approach to accomplish siRNA-mediated gene silencing through the generation and introduction of recombinant viral vectors expressing short hairpin RNAs (shRNAs). shRNAs are subsequently processed to siRNAs in vivo, leading to efficient, and, in some cases, long-term silencing.

Recombinant adeno-associated virus vectors for gene therapy

Recombinant adeno-associated virus (rAAV) vectors are based on a non-pathogenic human parvovirus (AAV) that is unique in its ability to persist in human cells without causing any pathologic effects. Studies of the potential barriers to rAAV-mediated transduction of relatively resistant cells has led to an understanding of the mechanisms of cell attachment and entry, cytoplasmic translocation, nuclear entry, conversion to active double-stranded DNA, activation of transcription and establishment of persistent molecular forms. Each of these areas is individually discussed, as are recent applications in vivo in preclinical models and clinical trials.

Adeno-associated virus vector-mediated interleukin-10 gene transfer inhibits atherosclerosis in apolipoprotein E-deficient mice

Inflammation is a major contributor to atherosclerosis by its effects on arterial wall biology and lipoprotein metabolism. Interleukin-10 (IL-10) is an anti-inflammatory cytokine that may modulate the atherosclerotic disease process. We investigated the effects of adeno-associated virus (AAV) vector-mediated gene transfer of IL-10 on atherogenesis in apolipoprotein E (ApoE)-deficient mice. A murine myoblast cell line, C2C12, transduced with AAV encoding murine IL-10 (AAV2-mIL10) secreted substantial amounts of IL-10 into conditioned medium. The production of monocyte chemoattractant protein-1 (MCP-1) by the murine macrophage cell line, J774, was significantly inhibited by conditioned medium from AAV2-mIL10-transduced C2C12 cells. ApoE-deficient mice were injected with AAV5-mIL10 into their anterior tibial muscle at 8 weeks of age. The expression of MCP-1 in the vascular wall of the ascending aorta and serum MCP-1 concentration were decreased in AAV5-mIL10-transduced mice compared with AAV5-LacZ-transduced mice. Oil red-O staining of the ascending aorta revealed that IL-10 gene transfer resulted in a 31% reduction in plaque surface area. Serum cholesterol concentrations were also significantly reduced in AAV5-mIL10-transduced mice. To understand the cholesterol-lowering mechanism of IL-10, we measured the cellular cholesterol level in HepG2 cells, resulting in its significant decrease by the addition of IL-10 in a dose-dependent manner. Furthermore, IL-10 suppressed HMG-CoA reductase expression in the HepG2 cells. These observations suggest that intramuscular injection of AAV5-mIL10 into ApoE-deficient mice inhibits atherogenesis through anti-inflammatory and cholesterol-lowering effects.

Novel adeno-associated virus vector vaccine restricts replication of simian immunodeficiency virus in macaques

Gene transfer vectors based on recombinant adeno-associated virus (rAAV) are simple, versatile, and safe. While the conventional applications for rAAV vectors have focused on delivery of therapeutic genes, we have developed the system for delivery of vaccine antigens. In particular, we are interested in generating rAAV vectors for use as a prophylactic human immunodeficiency virus type 1 (HIV-1) vaccine. To that end, we constructed vaccine vectors that expressed genes from the simian immunodeficiency virus (SIV) for evaluation in the monkey SIV model. After a single intramuscular dose, rAAV/SIV vaccines elicited SIV-specific T cells and antibodies in macaques. Furthermore, immunized animals were able to significantly restrict replication of a live, virulent SIV challenge. These data suggest that rAAV vaccine vectors induced biologically relevant immune responses, and thus, warrant continued development as a viable HIV-1 vaccine candidate.

Two novel adeno-associated viruses from cynomolgus monkey: pseudotyping characterization of capsid protein

We demonstrated the presence of two adeno-associated viruses (AAVs), designated AAV10 and AAV11, in cynomolgus monkeys by isolating and sequencing the entire viral coding regions from the monkey DNA. AAV10 and AAV11 capsid proteins shared 84% and 65%, respectively, of amino acids with AAV2. A phylogenetic analysis of AAV capsid proteins showed that AAV10 and AAV11 resembled most AAV8 and AAV4, respectively. To characterize the capsid protein, we pseudotyped an AAV2 vector with the monkey AAV capsid proteins and examined the resulting pseudotypes AAV2/10 and AAV2/11, in comparison with the AAV2 vector, for their host ranges in cell lines and tissue tropism in mice. AAV2/10 and AAV2/11 transduced primate cells less efficiently than AAV2. Whereas AAV2 transduced undifferentiated C2C12 mouse myoblasts more efficiently than differentiated ones, AAV2/10 and AAV2/11 transduced the undifferentiated myoblasts less efficiently than differentiated ones. Three weeks after injection to the muscle of the hind legs, AAV2/10 and AAV2 induced transgene expression similarly, but AAV2/11 did not transduce the skeletal muscle. Six weeks after systemic administration, transduced vector DNA was detected by PCR in the liver and spleen of mice inoculated with AAV2, in the liver, heart, muscle, lung, kidney, and uterus of mice with AAV2/10, and the muscle, kidney, spleen, lung, heart, and stomach of mice with AAV2/11. Mouse antisera against capsid protein VP2 of the three AAVs neutralized the respective vector particles in a type-specific manner. The results indicate that AAV10 and AAV11 capsid proteins, which are antigenically distinct from each other and AAV2, are likely to determine their host ranges and tissue tropism that are different from AAV2s, suggesting that cynomolgus AAVs could provide a broader choice of pseudotype AAV vectors for gene therapy.

Recombinant adeno-associated viral (rAAV) vectors as therapeutic tools for Duchenne muscular dystrophy (DMD)

Duchenne muscular dystrophy (DMD) is a lethal genetic muscle disorder caused by recessive mutations in the dystrophin gene. The size of the gene (2.4 Mb) and mRNA (14 kb) in addition to immunogenicity problems and inefficient transduction of mature myofibres by currently available vector systems are formidable obstacles to the development of efficient gene therapy approaches. Adeno-associated viral (AAV) vectors overcome many of the problems associated with other vector systems (nonpathogenicity and minimal immunogenicity, extensive cell and tissue tropism) but accommodate limited transgene capacity (<5 kb). As a result of these observations, a number of laboratories worldwide have engineered a series of microdystrophin cDNAs based on genotype-phenotype relationship in Duchenne (DMD) and Becker (BMD) dystrophic patients, and transgenic studies in mdx mice. Recent progress in characterization of AAV serotypes from various species has demonstrated that alternative AAV serotypes are far more efficient in transducing muscle than the traditionally used AAV2. This article summarizes the current progress in the field of recombinant adeno-associated viral (rAAV) delivery for DMD, including optimization of recombinant AAV-microdystrophin vector systems/cassettes targeting the skeletal and cardiac musculature.

Intrapleural administration of a serotype 5 adeno-associated virus coding for α1-antitrypsin mediates persistent, high lung and serum levels of α1-antitrypsin

alpha1-Antitrypsin (alpha1AT) is a serine proteinase inhibitor that protects the lung from degradation by neutrophil proteases. In alpha1AT deficiency, an autosomal recessive disorder resulting from mutations in the alpha1AT (approved symbol SERPINA1) gene, serum alpha1AT levels of < 570 microg/ml are associated with development of emphysema. Adeno-associated virus (AAV) serotype 2 (AAV2) vectors expressing alpha1AT administered intramuscularly or intravenously mediate sustained serum levels of alpha1AT in experimental animals. Since the lung is only 2% of the body weight, AAV vector delivery to the muscle or liver is inefficient, as most of the alpha1AT does not reach the lung. The present study evaluates AAV2- and AAV5-mediated delivery of human alpha1AT (halpha1AT) to C57BL/6 mice using the intrapleural space as a platform for local production of alpha1AT. Intrapleural administration of either an AAV5-halpha1AT or an AAV2-halpha1AT vector achieves higher lung and serum levels of alpha1AT than intramuscular delivery. AAV5-mediated serum and lung alpha1AT levels were 10-fold higher than those achieved by AAV2 delivery via either route. The diaphragm, lung, and heart are the major sites of transgene expression following intrapleural administration of an AAV5 reporter vector. At 40 weeks postadministration, intrapleural administration of the AAV5-halpha1AT vector mediated serum alpha1AT levels of 900 +/- 50 microg/ml, 1.6-fold higher than the accepted therapeutic level of 570 microg/ml. In the context that the pleura is a safe site for administration, intrapleural administration using AAV5 vectors may represent an attractive gene therapy strategy for alpha1AT deficiency in humans.

Trans-splicing adeno-associated viral vector-mediated gene therapy is limited by the accumulation of spliced mRNA but not by dual vector coinfection efficiency

Therapeutic application of recombinant adeno-associated virus (AAV) has been limited by its small carrying capacity. To overcome this limitation trans-splicing vectors were developed recently. However, the transduction efficiency of trans-splicing vectors is considerably lower than that of a single intact vector in skeletal muscle. To improve trans-splicing vectors for skeletal muscle gene therapy, we examined whether coinfection efficiency is a rate-limiting factor in the mdx mouse, a model for Duchenne muscular dystrophy. Two different AAV viruses were delivered to the mdx muscle. Similar to previous reports in normal muscle, coinfection efficiency reached approximately 90% in the diseased muscle. This result suggests that coinfection is not a hurdle in dystrophic muscle. Another critical step in the trans-splicing method is the transcription and splicing across the inverted terminal repeat (ITR) junction in the reconstituted genome. To test whether this represented a significant obstacle, we systematically evaluated the transcription, pre-mRNA stability and splicing, and translation in a synthetic lacZ construct that mimicked the reconstituted genome. Although inserting an intron in the lacZ gene had no effect on its expression, inclusion of the ITR junction in the intron reduced expression by 50%. In construct containing the ITR junction, the mRNA transcript level was significantly reduced. This mRNA level reduction was associated with decreased pre-mRNA stability. These data suggest that the accumulation of mRNA is a rate-limiting factor in trans-splicing vector-mediated gene therapy.

Long-term transgene expression in cardiac and skeletal muscle following fetal administration of adenoviral or adeno-associated viral vectors in mice

BACKGROUND

In utero gene transfer may provide advantages for the correction of congenital genetic disorders. In the present study we compare the ability of adenovirus (AdCMVLacZ), and two serotypes of adeno-associated virus (AAVCMVLacZ serotypes 2 and 2/5), to target cardiac and skeletal muscle after prenatal systemic or intramuscular injection in mice and assess the immune response to the vectors.

METHODS

Day 14 gestation fetal mice underwent direct intraperitoneal or intramuscular injection of AdCMVLacZ, and AAVCMVLacZ serotypes 2 and 2/5 vectors. Tissues were processed for beta-galactosidase expression in frozen or high-resolution thin plastic sections at early and late time points. Neutralizing antibodies to Ad and AAV were analyzed in separate fetal experimental and neonatal or adult control groups after administration and re-administration of the vectors.

RESULTS

A single injection of each vector in utero resulted in sustained expression of beta-galactosidase transgene in skeletal and cardiac muscle. Transgene expression was detected for the length of the study, i.e. 86, 58, and 31 weeks after birth for AdCMVLacZ, and AAVCMVLacZ serotypes 2 and 2/5, respectively. High-level expression in the myocardium was observed independent of the vector or route of administration. Neutralizing antibody responses to AAV and Ad antigens were reduced and long-term expression in muscle was not ablated on postnatal re-administration of vector.

CONCLUSIONS

Sustained, high-level cardiac and skeletal muscle transgene expression can be obtained after prenatal gene transfer with each of these vectors. The potential for immune response to viral antigens is altered, but not entirely ablated after in utero exposure.

Cross-dressing the virion: the transcapsidation of adeno-associated virus serotypes functionally defines subgroups

For all adeno-associated virus (AAV) serotypes, 60 monomers of the Vp1, Vp2, and Vp3 structural proteins assemble via an unknown mechanism to form an intact capsid. In an effort to better understand the properties of the capsid monomers and their role in viral entry and infection, we evaluated whether monomers from distinct serotypes can be mixed to form infectious particles with unique phenotypes. This transcapsidation approach consisted of the transfection of pairwise combinations of AAV serotype 1 to 5 helper plasmids to produce mosaic capsid recombinant AAV (rAAV). All ratios (19:1, 3:1, 1:1, 1:3, and 1:19) of these mixtures were able to replicate the green fluorescent protein transgene and to produce capsid proteins. A high-titer rAAV was obtained with mixtures that included either serotype 1, 2, or 3, whereas an rAAV of intermediate titer was obtained from serotype 5 mixtures. Only mixtures containing the AAV4 capsid exhibited reduced packaging capacity. The binding profiles of the mixed-virus preparations to either heparin sulfate (HS) or mucin agarose revealed that only AAV3-AAV5 mixtures at the 3:1 ratio exhibited duality in binding. All other mixtures displayed either an abrupt shift or a gradual alteration in the binding profile to the respective ligand upon increase of a capsid component that conferred either HS or mucin binding. The transduction of cell lines was used to further evaluate the phenotypes of these transcapsidated virions. Three transduction profiles were observed: (i) small to no change regardless of ratio, (ii) a gradual increase in transduction consistent with titration of a second capsid component, or (iii) an abrupt increase in transduction (threshold effect) dependent on the specific ratios used. Interestingly, an unexpected synergistic effect in transduction was observed when AAV1 helper constructs were combined with type 2 or type 3 recipient helpers. Further studies determined that at least two components contributed to this observed synergy: (i) heparin-mediated binding from AAV2 and (ii) an unidentified enhancement activity from AAV1 structural proteins. Using this procedure of mixing different AAV helper plasmids to generate "cross-dressed" AAV virions, we propose an additional means of classifying new AAV serotypes into subgroups based on functional approaches to analyze AAV capsid assembly, receptor-mediated binding, and virus trafficking. Exploitation of this approach in generating custom-designed AAV vectors should be of significant value to the field of gene therapy.

Targeted correction of single-base-pair mutations with adeno-associated virus vectors under nonselective conditions

Recombinant adeno-associated virus (rAAV) vectors possess the unique ability to introduce genetic alterations at sites of homology in genomic DNA through a mechanism thought to predominantly involve homologous recombination. We have investigated the efficiency of this approach using a mutant enhanced green fluorescent protein (eGFP) fluorescence recovery assay that facilitates detection of gene correction events in living cells under nonselective conditions. Our data demonstrate that rAAV infection can correct a mutant eGFP transgene at an efficiency of 0.1% in 293 cells, as determined by fluorescence-activated cell-sorting analysis. Gene repair was also confirmed using clonal expansion of GFP-positive cells and sequencing of the eGFP transgene. These results support previous findings demonstrating the efficacy of rAAV for gene targeting. In an effort to improve gene-targeting efficiencies, we evaluated several agents known to increase rAAV transduction (i.e., expression of an expressed gene), including genotoxic stress and proteasome inhibitors, but observed no correlation between the level of gene repair and rAAV transduction. Interestingly, however, our results demonstrated that enrichment of G(1)/S-phase cells in the target population through the addition of thymidine moderately (approximately 2-fold) increased gene correction compared to cells in other cell cycle phases, including G(0)/G1, G(1), and G(2)/M. These results suggest that the S phase of the cell cycle may more efficiently facilitate gene repair by rAAV. Transgenic mice expressing the mutant GFP were used to evaluate rAAV targeting efficiencies in primary fetal fibroblast and tibialis muscles. However, targeting efficiencies in primary mouse fetal fibroblasts were significantly lower (approximately 0.006%) than in 293 cells, and no correction was seen in tibialis muscles following rAAV infection. To evaluate the molecular structures of rAAV genomes that might be responsible for gene repair, single-cell injection studies were performed with purified viral DNA in a mutant eGFP target cell line. However, the failure of direct cytoplasm- or nucleus-injected rAAV DNA to facilitate gene repair suggests that some aspect of intracellular viral processing may be required to prime recombinant viral genomes for gene repair events.

Adeno-associated viral vectors for retinal gene transfer

Vectors derived from adeno-associated viruses (AAV) represent a promising tool for retinal gene transfer in pre-clinical and clinical settings. AAV vectors efficiently transduce dividing and non-dividing cells, escape cellular immunity and result in long-non-term transduction. In addition, they may be targeted to specific retinal cell types by taking advantage of surface proteins from various AAV serotypes thus limiting transfer of therapeutic genes to those cells requiring correction. This review will provide an overview of the properties of AAV vectors followed by a detailed report of their use in retinal gene transfer for mendelian and non-mendelian disorders.

Safety and efficacy of factor IX gene transfer to skeletal muscle in murine and canine hemophilia B models by adeno-associated viral vector serotype 1

Adeno-associated viral (AAV) vectors (serotype 2) efficiently transduce skeletal muscle, and have been used as gene delivery vehicles for hemophilia B and for muscular dystrophies in experimental animals and humans. Recent reports suggest that AAV vectors based on serotypes 1, 5, and 7 transduce murine skeletal muscle much more efficiently than AAV-2, with reported increases in expression ranging from 2-fold to 1000-fold. We sought to determine whether this increased efficacy could be observed in species other than mice. In immunodeficient mice we saw 10- to 20-fold higher levels of human factor IX (hF.IX) expression at a range of doses, and in hemophilic dogs we observed approximately 50-fold higher levels of expression. The increase in transgene expression was due partly to higher gene copy number and a larger number of cells transduced at each injection site. In all immunocompetent animals injected with AAV-1, inhibitory antibodies to F.IX developed, but in immunocompetent mice treated with high doses of vector, inhibitory antibodies eventually disappeared. These studies emphasize that the increased efficacy of AAV-1 vectors carries a risk of inhibitor formation, and that further studies will be required to define doses and treatment regimens that result in tolerance rather than immunity to F.IX.

Serum-free production and column purification of adeno-associated virus type 5

Viral vectors derived from adeno-associated virus (AAV) are rapidly becoming the vehicles of choice for gene therapy applications. AAV-2 is the adeno-associated virus serotype most commonly employed in AAV-mediated gene therapy studies; however, recently developed vectors derived from alternative serotypes of AAV, such as AAV-5, are receiving special attention due to their disparate tissue tropisms and potential for serial administration. In this report, we describe a rapid and efficient method for the serum-free production and column purification of recombinant AAV-5 particles. This method utilizes a combination of anion-exchange chromatography and gel filtration chromatography to purify recombinant AAV particles to near homogeneity. Importantly, viral particles are captured directly from cellular extracts with high efficiency, and vector purification is achieved in less than one working day with a minimal amount of sample manipulation. The method described in this report does not require partial purification by density centrifugation, detergent treatment, or solvent extraction to achieve efficient levels of column binding and vector purification.

Microdystrophin gene therapy of cardiomyopathy restores dystrophin-glycoprotein complex and improves sarcolemma integrity in the mdx mouse heart

BACKGROUND

More than 90% of Duchenne muscular dystrophy (DMD) patients develop cardiomyopathy, and many die of cardiac failure. Despite tremendous progress in skeletal muscle gene therapy, few attempts have been made to treat cardiomyopathy. Microdystrophin genes are shown to correct skeletal muscle pathological lesions in the mdx mouse model for DMD. Here, we tested the therapeutic potential of adeno-associated virus (AAV)-mediated microdystrophin gene therapy in the mdx mouse heart.

METHODS AND RESULTS

AAV was delivered to the newborn mdx mouse cardiac cavity. The procedure was rapid and well tolerated. Efficient expression was achieved in the inner and the outer layers of the myocardium. The ubiquitous cytomegalovirus promoter resulted in substantially higher expression than the muscle-specific CK6 promoter. The therapeutic effects of microdystrophin were evaluated at 10 months after infection. Immunostaining demonstrated extensive microdystrophin expression and successful restoration of the dystrophin-glycoprotein complex. Importantly, AAV-mediated microdystrophin expression improved the sarcolemma integrity in the mdx heart.

CONCLUSIONS

We established a simple gene transfer method for efficient and persistent transduction of the mdx mouse heart. AAV-mediated microdystrophin expression restored the critical dystrophin-glycoprotein complex and improved sarcolemma integrity of the mdx heart. Our results revealed the promise of AAV-microdystrophin gene therapy for cardiomyopathy in DMD.

Adeno-associated virus vector-mediated transduction in the cat brain

Adeno-associated virus (AAV) vectors are capable of delivering a therapeutic gene to the mouse brain that can result in long-term and widespread protein production. However, the human infant brain is more than 1000 times larger than the mouse brain, which will make the treatment of global neurometabolic disorders in children more difficult. In this study, we evaluated the ability of three AAV serotypes (1,2, and 5) to transduce cells in the cat brain as a model of a large mammalian brain. The human lysosomal enzyme beta-glucuronidase (GUSB) was used as a reporter gene, because it can be distinguished from feline GUSB by heat stability. The vectors were injected into the cerebral cortex, caudate nucleus, thalamus, corona radiata, internal capsule, and centrum semiovale of 8-week-old cats. The brains were evaluated for gene expression using in situ hybridization and enzyme histochemistry 10 weeks after surgery. The AAV2 vector was capable of transducing cells in the gray matter, while the AAV1 vector resulted in greater transduction of the gray matter than AAV2 as well as transduction of the white matter. AAV5 did not result in detectable transduction in the cat brain.