Several log increase in therapeutic transgene delivery by distinct adeno-associated viral serotype vectors

Adeno-associated virus vectorology, manufacturing, and clinical applications

Adeno-associated virus (AAV) has emerged as an attractive vector for gene therapy. The benefits of using AAV for gene therapy include long-term gene expression, the inability to autonomously replicate without a helper virus, transduction of dividing and nondividing cells, and the lack of pathogenicity from wild-type infections. A number of Phase I and Phase II clinical trials utilizing AAV have been carried out worldwide (Aucoin et al., 2008; Mueller and Flotte, 2008). A number of challenges have been identified based upon data generated from these clinical trials. These challenges include (1) large scale manufacturing technologies in accordance with current Good Manufacturing Practices (cGMP), (2) tissue specific tropism of AAV vectors, (3) high-quality/high potency recombinant AAV vectors (rAAV), and (4) immune response to AAV capsids and transgene. In this chapter, we will provide an overview of AAV biology, AAV vectorology, rAAV manufacturing, and the current status on the latest rAAV clinical trials.

Quantitative 3D tracing of gene-delivery viral vectors in human cells and animal tissues

Trafficking through a variety of cellular structures and organelles is essential for the interaction between gene-delivery vectors (i.e., adeno-associated virus (AAV) and liposomes) and host cells/tissues. Here, we present a method of computer-assisted quantitative 3D biodistribution microscopy that samples the whole population of fluorescently-labeled vectors and document their trafficking routes. Using AAV as a working model, we first experimentally defined numerical parameters for the singularity of Cy5-labeled particles by combining confocal microscopy and atomic force microscopy (AFM). We then developed a robust approach that integrates single-particle fluorescence imaging with 3D deconvolution and isosurface rendering to quantitate viral distribution and trafficking in human cells as well as animal tissues at the single-particle level. Using this quantitative method, we uncovered an as yet uncharacterized rate-limiting step during viral cell entry, while delineating nuclear accumulation of virions during the first 8 hours postinfection. Further, our studies revealed for the first time that following intramuscular injection, AAV spread progressively across muscle tissues through endomysium between myofibers instead of traversing through target cells. Such 3D resolution and quantitative dissection of vector-host interactions at the subcellular level should significantly improve our ability to resolve trafficking mechanisms of gene-delivery particles and facilitate the development of enhanced viral vectors.

Examining the cross-reactivity and neutralization mechanisms of a panel of mAbs against adeno-associated virus serotypes 1 and 5

Neutralizing antibodies play a central role in the prevention and clearance of viral infections, but can be detrimental to the use of viral capsids for gene delivery. Antibodies present a major hurdle for ongoing clinical trials using adeno-associated viruses (AAVs); however, relatively little is known about the antigenic epitopes of most AAV serotypes or the mechanism(s) of antibody-mediated neutralization. We developed panels of AAV mAbs by repeatedly immunizing mice with AAV serotype 1 (AAV1) capsids, or by sequentially immunizing with AAV1 followed by AAV5 capsids, in order to examine the efficiency and mechanisms of antibody-mediated neutralization. The antibodies were not cross-reactive between heterologous AAV serotypes except for a low level of recognition of AAV1 capsids by the AAV5 antibodies, probably due to the initial immunization with AAV1. The neutralization efficiency of different IgGs varied and Fab fragments derived from these antibodies were generally poorly neutralizing. The antibodies appeared to display various alternative mechanisms of neutralization, which included inhibition of receptor-binding and interference with a post-attachment step.

Muscle-directed gene therapy for hemophilia B with more efficient and less immunogenic AAV vectors

BACKGROUND

Adeno-associated viral vector (AAV)-mediated and muscle-directed gene therapy is a safe and non-invasive approach to treatment of hemophilia B and other genetic diseases. However, low efficiency of transduction, inhibitor formation and high prevalence of pre-existing immunity to the AAV capsid in humans remain as main challenges for AAV2-based vectors using this strategy. Vectors packaged with AAV7, 8 and 9 serotypes have improved gene transfer efficiencies and may provide potential alternatives to overcome these problems.

OBJECTIVE

To compare the long-term expression of canine factor IX (cFIX) levels and anti-cFIX antibody responses following intramuscular injection of vectors packaged with AAV1, 2, 5, 7, 8 and 9 capsid in immunocompetent hemophilia B mice.

RESULTS

Highest expression was detected in mice injected with AAV2/8 vector (28% of normal), followed by AAV2/9 (15%) and AAV2/7 (10%). cFIX expression by AAV2/1 only ranged from 0 to 5% of normal levels. High incidences of anti-cFIX inhibitor (IgG) were detected in mice injected with AAV2 and 2/5 vectors, followed by AAV2/1. None of the mice treated with AAV2/7, 2/8 and 2/9 developed inhibitors or capsid T cells.

CONCLUSIONS

AAV7, 8 and 9 are more efficient and safer vectors for muscle-directed gene therapy with high levels of transgene expression and absence of inhibitor formation. The absence of antibody response to transgene by AAV7, 8 and 9 is independent of vector dose but may be due to the fact that these three serotypes are associated with high level distribution to, and transduction of, hepatocytes following i.m. injection.

Phase 2 clinical trial of a recombinant adeno-associated viral vector expressing α1-antitrypsin: interim results

Recombinant adeno-associated virus (rAAV) vectors offer promise for the gene therapy of α(1)-antitrypsin (AAT) deficiency. In our prior trial, an rAAV vector expressing human AAT (rAAV1-CB-hAAT) provided sustained, vector-derived AAT expression for >1 year. In the current phase 2 clinical trial, this same vector, produced by a herpes simplex virus complementation method, was administered to nine AAT-deficient individuals by intramuscular injection at doses of 6.0×10(11), 1.9×10(12), and 6.0×10(12) vector genomes/kg (n=3 subjects/dose). Vector-derived expression of normal (M-type) AAT in serum was dose dependent, peaked on day 30, and persisted for at least 90 days. Vector administration was well tolerated, with only mild injection site reactions and no serious adverse events. Serum creatine kinase was transiently elevated on day 30 in five of six subjects in the two higher dose groups and normalized by day 45. As expected, all subjects developed anti-AAV antibodies and interferon-γ enzyme-linked immunospot responses to AAV peptides, and no subjects developed antibodies to AAT. One subject in the mid-dose group developed T cell responses to a single AAT peptide unassociated with any clinical effects. Muscle biopsies obtained on day 90 showed strong immunostaining for AAT and moderate to marked inflammatory cell infiltrates composed primarily of CD3-reactive T lymphocytes that were primarily of the CD8(+) subtype. These results support the feasibility and safety of AAV gene therapy for AAT deficiency, and indicate that serum levels of vector-derived normal human AAT >20 μg/ml can be achieved. However, further improvements in the design or delivery of rAAV-AAT vectors will be required to achieve therapeutic target serum AAT concentrations.

Worsening of cardiomyopathy using deflazacort in an animal model rescued by gene therapy

We have previously demonstrated that gene therapy can rescue the phenotype and extend lifespan in the delta-sarcoglycan deficient cardiomyopathic hamster. In patients with similar genetic defects, steroids have been largely used to slow down disease progression. Aim of our study was to evaluate the combined effects of steroid treatment and gene therapy on cardiac function. We injected the human delta-sarcoglycan cDNA by adeno-associated virus (AAV) 2/8 by a single intraperitoneal injection into BIO14.6 Syrian hamsters at ten days of age to rescue the phenotype. We then treated the hamsters with deflazacort. Treatment was administered to half of the hamsters that had received the AAV and the other hamsters without AAV, as well as to normal hamsters. Both horizontal and vertical activities were greatly enhanced by deflazacort in all groups. As in previous experiments, the AAV treatment alone was able to preserve the ejection fraction (70±7% EF). However, the EF value declined (52±14%) with a combination of AAV and deflazacort. This was similar with all the other groups of affected animals. We confirm that gene therapy improves cardiac function in the BIO14.6 hamsters. Our results suggest that deflazacort is ineffective and may also have a negative impact on the cardiomyopathy rescue, possibly by boosting motor activity. This is unexpected and may have significance in terms of the lifestyle recommendations for patients.

Viral-mediated overexpression of mutant huntingtin to model HD in various species

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by an expansion of CAG repeats in the huntingtin (Htt) gene. Despite intensive efforts devoted to investigating the mechanisms of its pathogenesis, effective treatments for this devastating disease remain unavailable. The lack of suitable models recapitulating the entire spectrum of the degenerative process has severely hindered the identification and validation of therapeutic strategies. The discovery that the degeneration in HD is caused by a mutation in a single gene has offered new opportunities to develop experimental models of HD, ranging from in vitro models to transgenic primates. However, recent advances in viral-vector technology provide promising alternatives based on the direct transfer of genes to selected sub-regions of the brain. Rodent studies have shown that overexpression of mutant human Htt in the striatum using adeno-associated virus or lentivirus vectors induces progressive neurodegeneration, which resembles that seen in HD. This article highlights progress made in modeling HD using viral vector gene transfer. We describe data obtained with of this highly flexible approach for the targeted overexpression of a disease-causing gene. The ability to deliver mutant Htt to specific tissues has opened pathological processes to experimental analysis and allowed targeted therapeutic development in rodent and primate pre-clinical models.

Therapeutic in vivo gene transfer for genetic disease using AAV: progress and challenges

In vivo gene replacement for the treatment of inherited disease is one of the most compelling concepts in modern medicine. Adeno-associated virus (AAV) vectors have been extensively used for this purpose and have shown therapeutic efficacy in a range of animal models. Successful translation to the clinic was initially slow, but long-term expression of donated genes at therapeutic levels has now been achieved in patients with inherited retinal disorders and haemophilia B. Recent exciting results have raised hopes for the treatment of many other diseases. As we discuss here, the prospects and challenges for AAV gene therapy are to a large extent dependent on the target tissue and the specific disease.

Enhancers of adeno-associated virus AAV2 transduction via high throughput siRNA screening

Intracellular barriers to adeno-associated virus (AAV) transduction may limit gene delivery. We screened a short interfering RNA (siRNA) library targeting 5,520 genes to help identify pathways that modulate AAV transduction of human endothelium. In replicate screening, 50 pools (three siRNAs per gene) resulted in greater than eightfold reporter gene expression enhancement. Single siRNA confirmation tests demonstrated that at least one siRNA from each of the top 10 pools provided greater than twofold enhancement. Several siRNAs when used together resulted in additive effects and two of the most potent siRNA sequences were enhancers in cultured airway epithelium. However, enhanced transduction was not correlated with mRNA knockdown by quantitative real time PCR, indicating an off-target mechanism. In fact, four of the five most potent siRNAs contained a consensus hexamer region 5'-UGUUUC-3' at positions 2-7 of the antisense strand. The point mutation U4A within this region (but not mutations at positions 1 or 14) disrupted transduction enhancement, indicating a microRNA (miRNA)-like mechanism. Transcription profiling indicated that the hexamer also resulted in perturbation of the interferon pathway via reduced interferon-induced protein 44-like (IFI44L), interferon-inducible myxovirus resistance 1 (MX1), and interferon-induced protein with tetratricopeptide repeats (IFIT5) mRNAs. Direct interferon (α, β, and ω) receptor 2 (IFNAR2) knockdown resulted in greater than twofold transduction enhancement. In addition to providing insight into AAV biology and enhanced transduction, the results demonstrate certain beneficial siRNA off-target effects.

Adipose tissue-derived mesenchymal stem cell-based liver gene delivery

BACKGROUND & AIMS

The adipose tissue represents an accessible, abundant, and replenishable source of adult stem cells for potential applications in regenerative medicine. Adipose tissue-derived mesenchymal stem cells (AT-MSCs) resemble bone marrow-derived mesenchymal stem cells (BM-MSCs) with respect to morphology, immune-phenotype, and multiple differentiation capability. In the present study, we investigated the feasibility of AT-MSC-based liver gene delivery for the treatment of alpha 1-antitrypsin deficiency.

METHODS

Mouse AT-MSCs were transduced by rAAV vectors and transplanted into the mouse liver.

RESULTS

We showed that AT-MSCs can be transduced by recombinant adeno-associated viral vector serotype 1 (rAAV1-CB-hAAT). After transplanting to the mouse liver, ex vivo transduced AT-MSCs expressed the transgene product, human alpha 1-antitrypsin (hAAT). Importantly, serum levels of hAAT were sustained and no anti-hAAT antibody was detected in any recipients.

CONCLUSIONS

These results demonstrated that AT-MSCs can be transduced by rAAV vectors, engrafted into recipient livers, contribute to liver regeneration, and serve as a platform for transgene expression without eliciting an immune response. AT-MSC-based gene therapy presents a novel approach for the treatment of liver diseases, such as AAT deficiency.

Adeno-associated virus serotypes 7 and 8 outperform serotype 9 in expressing atheroprotective human apoE3 from mouse skeletal muscle

Intramuscular injection of adeno-associated viral (AAV) vectors is potentially a safe, minimally invasive procedure for the long-term gene expression of circulating antiatherogenic proteins. Here, we compare secretion and atheroprotective effects of human apoE3 after injection of 3 pseudotyped AAV vectors (AAV2/7, AAV2/8, or AAV2/9), driven by the CMV enhancer/chicken β-actin (CAG) promoter, into skeletal muscle of hyperlipidemic apolipoprotein E-deficient (apoE⁻/⁻) mice. Vector viabilities were verified by transducing cultured C2C12 mouse myotubes and assessing secretion of human apoE3 protein. Both hind limb tibialis anterior muscles of female C57BL/6 apoE⁻/⁻ mice, 2 months old and fed a high-fat diet, were each injected with 1 x 10¹⁰ vector genomes of AAV vector. Identical noninjected mice served as controls; and blood was collected at weeks 0, 1, 2, 4, and 13. At termination (13 weeks), the brachiocephalic artery was excised; and after staining sections, plaque morphometry and fractional lipid content were quantified by computerized image analysis. Intramuscular injection of AAV2/7 and AAV2/8 vectors produced up to 2 μg human apoE3 per milliliter plasma, just below the threshold to reverse dyslipoproteinemia. AAV2/9 was notably less effective, mice having a 3-fold lower level of plasma apoE3 at 13 weeks and a 50% greater burden of atherosclerotic plaque lipid in their brachiocephalic arteries. We conclude that although vector refinement is needed to exploit fully apoE3 atheroprotective functions, AAV2/7 and AAV2/8 are promising gene transfer vectors for muscle-based expression of antiatherogenic circulating proteins.

Successful transfection of genes using AAV-2/9 vector in swine coronary and peripheral arteries

BACKGROUND

Gene therapy has attracted attention for its potential to treat several cardiovascular diseases. The use of adeno-associated viral (AAV) vectors to facilitate therapeutic gene transfer to suppress intimal hyperplasia is a promising concept. The objective of this study was to analyze the in vivo transduction of a novel recombinant AAV-2/9 vector with SM22α promoter, containing β-galactosidase gene (LacZ) or green fluorescent protein (GFP) as reporter genes, to the medial layer smooth muscle cells (SMCs) of swine coronary and peripheral arteries.

METHODS

The AAV-2/9 vector containing SM22α (1 × 10(13) pfu) were administered into carotid/femoral/coronary arteries of domestic swine using irrigating balloon catheter-based gene delivery. Following gene transfer, cryosections of arteries were processed for X-Gal and GFP analysis. Fluorescence microscopy and Western blotting were done to analyze the GFP expression in the SMCs.

RESULTS

LacZ mRNA expression was visualized in the medial layer 7 d after vector administration. The GFP expression was detected at day 7 and lasted for at least 2 mo showing the longer-lasting expression of the AAV-2/9 vector. Control arteries did not show any expression of GFP or LacZ. There was no significant effect of AAV-2/9 viral transduction on serum amylase, fibrinogen, and serum CRP levels.

CONCLUSION

These finding support the use of AAV-2/9 as a vector to effectively transduce a gene in SMCs of coronary and peripheral arteries without causing inflammation.

Gene therapy strategies for hemophilia: benefits versus risks

Hemophilia is an inherited bleeding disorder caused by a deficiency of functional clotting factors VIII or IX in the blood plasma. The drawbacks of the classical protein substitution therapy fueled interest in alternative treatments by gene therapy. Hemophilia has been recognized as an ideal target disease for gene therapy because a relatively modest increase in clotting factor levels can result in a significant therapeutic benefit. Consequently, introducing a functional FVIII or FIX gene copy into the appropriate target cells could ultimately provide a cure for hemophilic patients. Several cell types have been explored for hemophilia gene therapy, including hepatocytes, muscle, endothelial and hematopoietic cells. Both nonviral and viral vectors have been considered for the development of hemophilia gene therapy, including transposons, γ-retroviral, lentiviral, adenoviral and adeno-associated viral vectors. Several of these strategies have resulted in stable correction of the bleeding diathesis in hemophilia A and B murine as well as canine models, paving the way towards clinical trials. Although clotting factor expression has been detected in hemophilic patients treated by gene therapy, the challenge now lies in obtaining prolonged therapeutic FVIII or FIX levels in these patients. This review highlights the benefits and potential risks of the different gene therapy strategies for hemophilia that have been developed.

An adeno-associated virus vector efficiently and specifically transduces mouse skeletal muscle

Expression of a therapeutic gene in the skeletal muscle is a practical strategy to compensate a patients' insufficient circulating factor. Its clinical application requires a muscle-targeting vector capable of inducing a continuous high-level transgene expression. We modified an adeno-associated virus serotype 2 (AAV2) vector expressing luciferase from the mouse muscle creatine kinase gene promoter-enhancer (Ckm). First, AAVS1 insulator was inserted into the vector genome for transcriptional enhancement. This increased transduction of mouse quadriceps muscle by 11-fold at 4 weeks after intramuscular injection. Second, two capsid modifications were combined (21F capsid): incorporation of a segment of AAV1 capsid to produce a hybrid capsid and substitution of a tyrosine with a phenylalanine. Use of 21F capsid increased muscle transduction further by 18-fold, resulting in 200-fold higher efficacy than that of the unmodified vector. Compared with a vector having human elongation factor 1α promoter which showed similar efficacy in the muscle, this vector having Ckm transduced non-muscle organs less efficiently after intravenous administration. The AAV2 vector composed of the modified genome and capsid provides a backbone to develop a clinical vector expressing a therapeutic gene in the muscle.

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.

Introduction to viral vectors

Viral vector is the most effective means of gene transfer to modify specific cell type or tissue and can be manipulated to express therapeutic genes. Several virus types are currently being investigated for use to deliver genes to cells to provide either transient or permanent transgene expression. These include adenoviruses (Ads), retroviruses (γ-retroviruses and lentiviruses), poxviruses, adeno-associated viruses, baculoviruses, and herpes simplex viruses. The choice of virus for routine clinical use will depend on the efficiency of transgene expression, ease of production, safety, toxicity, and stability. This chapter provides an introductory overview of the general characteristics of viral vectors commonly used in gene transfer and their advantages and disadvantages for gene therapy use.

AAV's anatomy: roadmap for optimizing vectors for translational success

Adeno-Associated Virus based vectors (rAAV) are advantageous for human gene therapy due to low inflammatory responses, lack of toxicity, natural persistence, and ability to transencapsidate the genome allowing large variations in vector biology and tropism. Over sixty clinical trials have been conducted using rAAV serotype 2 for gene delivery with a number demonstrating success in immunoprivileged sites, including the retina and the CNS. Furthermore, an increasing number of trials have been initiated utilizing other serotypes of AAV to exploit vector tropism, trafficking, and expression efficiency. While these trials have demonstrated success in safety with emerging success in clinical outcomes, one benefit has been identification of issues associated with vector administration in humans (e.g. the role of pre-existing antibody responses, loss of transgene expression in non-immunoprivileged sites, and low transgene expression levels). For these reasons, several strategies are being used to optimize rAAV vectors, ranging from addition of exogenous agents for immune evasion to optimization of the transgene cassette for enhanced therapeutic output. By far, the vast majority of approaches have focused on genetic manipulation of the viral capsid. These methods include rational mutagenesis, engineering of targeting peptides, generation of chimeric particles, library and directed evolution approaches, as well as immune evasion modifications. Overall, these modifications have created a new repertoire of AAV vectors with improved targeting, transgene expression, and immune evasion. Continued work in these areas should synergize strategies to improve capsids and transgene cassettes that will eventually lead to optimized vectors ideally suited for translational success.

Gene therapy with helper-dependent adenoviral vectors: current advances and future perspectives

Recombinant Adenoviral vectors represent one of the best gene transfer platforms due to their ability to efficiently transduce a wide range of quiescent and proliferating cell types from various tissues and species. The activation of an adaptive immune response against the transduced cells is one of the major drawbacks of first generation Adenovirus vectors and has been overcome by the latest generation of recombinant Adenovirus, the Helper-Dependent Adenoviral (HDAd) vectors. HDAds have innovative features including the complete absence of viral coding sequences and the ability to mediate high level transgene expression with negligible chronic toxicity. This review summarizes the many aspects of HDAd biology and structure with a major focus on in vivo gene therapy application and with an emphasis on the unsolved issues that these vectors still presents toward clinical application.

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.

Ex vivo transduction and transplantation of bone marrow cells for liver gene delivery of alpha1-antitrypsin

Adult stem cell-based gene therapy holds several unique advantages including avoidance of germline or other undesirable cell transductions. We have previously shown that liver progenitor (oval) cells can be used as a platform for liver gene delivery of human alpha1-antitrypsin (hAAT). However, this cell source cannot be used in humans for autologous transplantation. In the present study, we tested the feasibility of bone marrow (BM) cell-based liver gene delivery of hAAT. In vitro studies showed that BM cells can be transduced by lentiviral vector (Lenti-CB-hAAT) and recombinant adeno-associated viral vectors (rAAV1-CB-hAAT, and rAAV8-CB-hAAT). Transplantation studies showed that transplanted BM cells homed into liver, differentiated into hepatocytes and expressed hAAT in the liver. Importantly, we showed that transplantation of rAAV8-CB-hAAT vector-transduced BM cells resulted in sustained levels of hAAT in the systemic circulation of recipient mice. These results demonstrated that rAAV vector-mediated BM cell-based liver gene therapy is feasible for the treatment of AAT deficiency and implies a novel therapy for the treatment of liver diseases.

Gene therapy for dyslipidemia: a review of gene replacement and gene inhibition strategies

Despite numerous technological and pharmacological advances and more detailed knowledge of molecular etiologies, cardiovascular diseases remain the leading cause of morbidity and mortality worldwide claiming over 17 million lives a year. Abnormalities in the synthesis, processing and catabolism of lipoprotein particles can result in severe hypercholesterolemia, hypertriglyceridemia or low HDL-C. Although a plethora of antidyslipidemic pharmacological agents are available, these drugs are relatively ineffective in many patients with Mendelian lipid disorders, indicating the need for new and more effective interventions. In vivo somatic gene therapy is one such intervention. This article summarizes current strategies being pursued for the development of clinical gene therapy for dyslipidemias that cannot effectively be treated with existing drugs.

Transient demyelination increases the efficiency of retrograde AAV transduction

Adeno-associated virus (AAV) is capable of mediating retrograde viral transduction of central and peripheral neurons. This occurs at a relatively low efficiency, which we previously found to be dependent upon capsid serotype. We sought to augment retrograde transduction by providing increased axonal access to peripherally delivered AAV. Others have described utilizing full transection of peripheral nerves to mediate retrograde viral transduction of motor neurons. Here, we examined the ability of a transient demyelinating event to modulate levels of retrograde AAV transduction. Transient demyelination does not cause lasting functional deficits. Ethidium bromide (EtBr)-induced transient demyelination of the sciatic nerve resulted in significant elevation of retrograde transduction of both motor and sensory neurons. Retrograde transduction levels of motor neurons and heavily myelinated, large-diameter sensory neurons increased at least sixfold following peripheral delivery of self-complementary AAV serotype 1 (scAAV1) and serotype 2 (scAAV2), when preceded by demyelination. These findings identify a means of significantly enhancing retrograde vector transport for use in experimental paradigms requiring either retrograde neuronal identification and gene expression, or translational treatment paradigms.

Comparison of adeno-associated virus pseudotype 1, 2, and 8 vectors administered by intramuscular injection in the treatment of murine phenylketonuria

Phenylketonuria (PKU) is caused by hepatic phenylalanine hydroxylase (PAH) deficiency and is associated with systemic accumulation of phenylalanine (Phe). Previously we demonstrated correction of murine PKU after intravenous injection of a recombinant type 2 adeno-associated viral vector pseudotyped with type 8 capsid (rAAV2/8), which successfully directed hepatic transduction and Pah gene expression. Here, we report that liver PAH activity and phenylalanine clearance were also restored in PAH-deficient mice after simple intramuscular injection of either AAV2 pseudotype 1 (rAAV2/1) or rAAV2/8 vectors. Serotype 2 AAV vector (rAAV2/2) was also investigated, but long-term phenylalanine clearance has been observed only for pseudotypes 1 and 8. Therapeutic correction was shown in both male and female mice, albeit more effectively in males, in which correction lasted for the entire period of the experiment (>1 year). Although phenylalanine levels began to rise in female mice at about 8-10 months after rAAV2/8 injection they remained only mildly hyperphenylalaninemic thereafter and subsequent supplementation with synthetic tetrahydrobiopterin resulted in a transient decrease in blood phenylalanine. Alternatively, subsequent administration of a second vector with a different AAV pseudotype to avoid immunity against the previously administrated vector was also successful for long-term treatment of female PKU mice. Overall, this relatively less invasive gene transfer approach completes our previous studies and allows comparison of complementary strategies in the development of efficient PKU gene therapy protocols.

Long-term protection against human papillomavirus e7-positive tumor by a single vaccination of adeno-associated virus vectors encoding a fusion protein of inactivated e7 of human papillomavirus 16/18 and heat shock protein 70

We investigated a gene vaccine strategy against human papillomavirus (HPV)-induced cancer and premalignant diseases, using adeno-associated virus (AAV) vector encoding the viral E7 oncoproteins as the tumor antigens from HPV serotypes 16 (HPV16) and 18 (HPV18). Genetically inactivated E7 proteins were fused with a heat shock protein 70 (hsp70) to minimize the risk of cell transformation and enhance immune responses. The fusion protein gene was packaged in AAV serotype 1 or 2 (AAV1 or 2) for efficient in vivo gene expression. Our results showed that after a single intramuscular injection, the AAV1 vector elicited stronger HPV-specific cytotoxic T lymphocyte (CTL) responses and interferon-gamma secretion when compared with the AAV2 vector. Prophylactic immunization with AAV1 protected 100% of the mice from tumor growth for more than 1 year, whereas all the control mice immunized with either a LacZ vector or saline grew large tumors and died within 6 weeks after inoculation of E7-positive tumor cell line TC-1. In addition, this single-dose AAV1 vaccination completely protected the mice against second and third challenges with higher numbers of TC-1 cells. Despite lower CTL responses against the E7 antigens, AAV2 vector prophylactic immunization was also sufficient to protect 100% of the mice against the initial and second tumor challenges and 70% of the mice against the third challenge. In addition, therapeutic immunization with AAV1 after palpable tumor formation inhibited tumor growth and caused tumor regression in some mice. Thus, our studies support the potential of AAV vectors as a genetic vaccine for the prevention and treatment of HPV-induced malignancies.

Comparison of AAV serotypes for gene delivery to dorsal root ganglion neurons

For many experiments in the study of the peripheral nervous system, it would be useful to genetically manipulate primary sensory neurons. We have compared vectors based on adeno-associated virus (AAV) serotypes 1, 2, 3, 4, 5, 6, and 8, and lentivirus (LV), all expressing green fluorescent protein (GFP), for efficiency of transduction of sensory neurons, expression level, cellular tropism, and persistence of transgene expression following direct injection into the dorsal root ganglia (DRG), using histological quantification and qPCR. Two weeks after injection, AAV1, AAV5, and AAV6 had transduced the most neurons. The time course of GFP expression from these three vectors was studied from 1 to 12 weeks after injection. AAV5 was the most effective serotype overall, followed by AAV1. Both these serotypes showed increasing neuronal transduction rates at later time points, with some injections of AAV5 yielding over 90% of DRG neurons GFP(+) at 12 weeks. AAV6 performed well initially, but transduction rates declined dramatically between 4 and 12 weeks. AAV1 and AAV5 both transduced large-diameter neurons, IB4(+) neurons, and CGRP(+) neurons. In conclusion, AAV5 is a highly effective gene therapy vector for primary sensory neurons following direct injection into the DRG.

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.

A simplified baculovirus-AAV expression vector system coupled with one-step affinity purification yields high-titer rAAV stocks from insect cells

Scalable methods of recombinant adeno-associated virus (rAAV) production have gained much recent interest as the field of rAAV-mediated gene therapy approaches the clinic. In particular, the production of rAAV vectors in insect cells via the use of recombinant baculovirus technology has proven to be an efficient and scalable means of rAAV production. Here, we describe a method for the production of rAAV serotypes 1 and 2 in insect cells using a simplified baculovirus-AAV expression vector system coupled with particle purification via affinity chromatography. The number of separate baculovirus constructs required for rAAV production was reduced by genetically modifying the AAV rep gene to allow expression of the AAV-encoded replication enzymes, Rep78 and Rep52, from a single mRNA species and combining the modified rep gene with an AAV cap gene expression cassette in a single baculovirus construct. Additionally, we describe lysis, binding, and elution conditions compatible with a commercially available affinity medium (AVB Sepharose High Performance) used to purify rAAV particles to near homogeneity in a single chromatography step. Using the described method, we obtained an average yield of 7 x 10(4) purified rAAV particles per cell (range: 3.7 x 10(4) to 9.6 x 10(4)) from suspension cultures of recombinant baculovirus-infected insect cells.

Adeno-associated virus capsid structure drives CD4-dependent CD8+ T cell response to vector encoded proteins

The immunological sequelae of adeno-associated virus (AAV)-mediated gene transfer in vivo is quite complex. In murine models, most AAV capsids are associated with minimal or dysfunctional T cell responses to antigenic transgene products. In this study we compared T cell activation against AAV2/8 and AAV2/rh32.33 vectors expressing nuclear-targeted LacZ (nLacZ), GFP, or firefly luciferase in murine skeletal muscle. We show that, unlike AAV8, AAVrh32.33 yields qualitatively and quantitatively robust T cell responses to both the capsid and transgene product. AAV2/rh32.33.CB.nLacZ, but not AAV2/8, drives a high degree of cellular infiltration and a loss of detectable transgene expression in C57BL/6 mice. However, cellular immunity to AAVrh32.33 is ablated in the absence of CD4, CD40L, or CD28, permitting stable beta-galactosidase expression. Treatment of CD40L(-/-) mice with the CD40 agonist, FGK45, failed to restore the CD8 response to AAV2/rh32.33.nLacZ, suggesting that additional factors are involved. Our results suggest that specific domains within the AAVrh32.33 capsid augment the adaptive response to both capsid and transgene Ags in a CD4-dependent pathway involving CD40L signaling and CD28 costimulation. Structural comparison of the AAV8 and rh32.33 capsids has identified key differences that may drive differential immunity by affecting tropism, Ag presentation or the activation of innate immunity. This murine model of AAV-mediated cytotoxicity allows us to delineate the mechanism of viral immune activation, which is relevant to the translation of AAV technology in higher order species.

Cytotoxic-T-lymphocyte-mediated elimination of target cells transduced with engineered adeno-associated virus type 2 vector in vivo

A recent clinical trial in patients with hemophilia B has suggested that adeno-associated virus (AAV) capsid-specific cytotoxic T lymphocytes (CTLs) eliminated AAV-transduced hepatocytes and resulted in therapeutic failure. AAV capsids elicit a CTL response in animal models; however, these capsid-specific CTLs fail to kill AAV-transduced target cells in mice. To better model the human clinical trial data in mice, we introduced an immunodominant epitope derived from ovalbumin (OVA; SIINFEKL) into the AAV capsid and tested CTL-mediated killing of AAV2-transduced target tissues in vivo. Initially, in vitro experiments demonstrated both classical class I and cross-presentation of the OVA antigen, following endogenous expression or AAV2-OVA vector transduction, respectively. Furthermore, an OVA-specific CTL response was elicited after muscular or systemic injection of the AAV2-OVA vector. Finally, CTL reactivity was enhanced in mice with established SIINFEKL-specific immunity after AAV2-OVA/alpha1 anti-trypsin (AAT) administration. Most importantly, these OVA-specific CTLs decreased AAT expression in mice treated with AAV2-OVA/AAT vector that followed a time course mimicking uncoating kinetics of AAV2 transduction in OVA-immunized mice. These results demonstrate that AAV capsid-derived antigens elicit CD8(+) CTL reactivity, and these CTLs eliminated AAV-transduced target cells in mice. Notably, this model system can be exploited to study the kinetics of capsid presentation from different serotypes of AAV and permit the design of novel strategies to block CTL-mediated killing of AAV-transduced cells.

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.

A myocardium tropic adeno-associated virus (AAV) evolved by DNA shuffling and in vivo selection

To engineer gene vectors that target striated muscles after systemic delivery, we constructed a random library of adeno-associated virus (AAV) by shuffling the capsid genes of AAV serotypes 1 to 9, and screened for muscle-targeting capsids by direct in vivo panning after tail vein injection in mice. After 2 rounds of in vivo selection, a capsid gene named M41 was retrieved mainly based on its high frequency in the muscle and low frequency in the liver. Structural analyses revealed that the AAVM41 capsid is a recombinant of AAV1, 6, 7, and 8 with a mosaic capsid surface and a conserved capsid interior. AAVM41 was then subjected to a side-by-side comparison to AAV9, the most robust AAV for systemic heart and muscle gene delivery; to AAV6, a parental AAV with strong muscle tropism. After i.v. delivery of reporter genes, AAVM41 was found more efficient than AAV6 in the heart and muscle, and was similar to AAV9 in the heart but weaker in the muscle. In fact, the myocardium showed the highest gene expression among all tissues tested in mice and hamsters after systemic AAVM41 delivery. However, gene transfer in non-muscle tissues, mainly the liver, was dramatically reduced. AAVM41 was further tested in a genetic cardiomyopathy hamster model and achieved efficient long-term delta-sarcoglycan gene expression and rescue of cardiac functions. Thus, direct in vivo panning of capsid libraries is a simple tool for the de-targeting and retargeting of viral vector tissue tropisms facilitated by acquisition of desirable sequences and properties.

Induction of immune tolerance to FIX following muscular AAV gene transfer is AAV-dose/FIX-level dependent

Direct intramuscular injection (IM) of adeno-associated virus (AAV) has been proven a safe and potentially efficient procedure for gene therapy of many genetic diseases including hemophilia B. It is, however, contentious whether high antigen level induces tolerance or immunity to coagulation factor IX (FIX) following IM of AAV. We recently reported induction of FIX-specific immune tolerance by IM of AAV serotype one (AAV1) vector in mice. We hypothesize that the expression of high levels of FIX is critical to induction of FIX tolerance. In this study, we investigated the correlation among AAV dose, FIX expression, and tolerance induction. We observed that induction of immune tolerance or immunity to FIX was dependent on the dose of AAV1-human FIX (hFIX) given and the level of FIX antigen expressed in both normal and hemophilia mice. We then defined the minimum AAV1-hFIX dose and the lowest level of FIX needed for FIX tolerance. Different from hepatic AAV-hFIX gene transfer, we found that FIX tolerance induced by IM of AAV1 was not driven by regulatory T cells. These results provided further insight into the mechanism(s) of FIX tolerance, contributing to development of hemophilia gene therapy, and optimization of FIX tolerance induction protocols.

Gene therapy for cystic fibrosis

Cystic Fibrosis (CF) is an autosomal recessive disorder due to mutations in the CF transmembrane conductance regulator (CFTR) gene that lead to defective ion transport in the conducting pulmonary airways and exocrine glands. Through a process that is not fully understood, CFTR defects predispose affected patients to chronic endobronchial infections with organisms such as Pseudomonas aeruginosa and Staphylococcus aureus. Following the discovery of the CFTR gene in 1989, CF became one of the primary targets for gene therapy research. Early enthusiasm surrounded the new field of gene therapy during most of the 1990s and it led academics and clinicians on a big effort to apply gene therapy for cystic fibrosis. Clinical studies have been pursued using recombinant adenovirus, recombinant adeno-associated virus, cationic liposomes, and cationic polymer vectors. Although to this date no dramatic therapeutic benefits have been observed, a lot of information has been gained from the pre-clinical and clinical studies that were performed. This learning curve has led to the optimization of vector technology and an appreciation of immune and mechanical barriers that have to be overcome for successful delivery.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Recombinant adeno-associated virus transduction and integration

Recombinant adeno-associated virus (rAAV) holds promise as a gene therapy vector for a multitude of genetic disorders such as hemophilia, cystic fibrosis, and the muscular dystrophies. Given the variety of applications and tissue types toward which these vectors may be targeted, an understanding of rAAV transduction is crucial for the effective application of therapy. rAAV transduction mechanisms have been the subject of much study, resulting in a body of knowledge relating to events from virus-cell attachment through to vector genome conformation in the target cell nucleus. Instead of utilizing one mechanism in each phase of vector transduction, rAAV appears to employ multiple possible pathways toward transgene expression, in part dependent on rAAV serotype, dose, and target cell type. Once inside the nucleus, the rAAV genome exists in a predominantly episomal form; therefore, nondividing cells tend to be most stably transduced. However, rAAV has a low frequency of integration into the host cell genome, often in or near genes, and can be associated with host genome mutations. This review describes the current understanding of the mechanisms and rate-limiting steps involved in rAAV transduction.

Clinical gene therapy using recombinant adeno-associated virus vectors

Recombinant adeno-associated virus (rAAV) vectors possess a number of properties that may make them suitable for clinical gene therapy, including being based upon a virus for which there is no known pathology and a natural propensity to persist in human cells. Wild-type adeno-associated viruses (AAVs) are now known to be very diverse and ubiquitous in humans and nonhuman primates, which adds to the degree of confidence one may place in the natural history of AAV, namely that it has never been associated with any human tumors or other acute pathology, other than sporadic reports of having been isolated from spontaneously aborted fetuses. On the basis of this understanding of AAV biology and a wide range of preclinical studies in mice, rabbits, dogs and nonhuman primates, a growing number of clinical trials have been undertaken with this class of vectors. Altogether, over 40 clinical trials have now been approved. Although all previous trials were undertaken using AAV serotype 2 vectors, at least two current trials utilize AAV2 vector genomes cross-packaged or pseudotyped into AAV1 capsids, which appear to mediate more efficient gene delivery to muscle. The explosion of capsid isolates available for use as vectors to over 120 has now provided the potential to broaden the application of AAV-based gene therapy to other cell types.

Analysis of AAV serotypes 1-9 mediated gene expression and tropism in mice after systemic injection

This study examines transgene expression and biodistribution of adeno-associated virus (AAV) pseudotyped 1-9 after tail vein (TV) injection in male mice. Using a cytomegalovirus (CMV)-luciferase transgene, the time-course of expression in each animal was tracked throughout the experiment. The animals were imaged at 7, 14, 29, 56, and 100 days after the TV injection. The total number of photons emitted from each animal was recorded, allowing examination of expression level and kinetics for each pseudotyped virus. The bioluminescence imaging revealed three expression levels (i) low-expression group, AAV2, 3, 4, and 5; (ii) moderate-expression group, AAV1, 6, and 8; and (iii) high-expression group, AAV7 and 9. In addition, imaging revealed two classes of kinetics (i) rapid-onset, for AAV1, 6, 7, 8, and 9; and (ii) slow-onset, for AAV2, 3, 4, and 5. We next evaluated protein expression and viral genome copy numbers in dissected tissues. AAV9 had the best viral genome distribution and highest protein levels. The AAV7 protein and genome copy numbers were comparable to those of AAV9 in the liver. Most surprisingly, AAV4 showed the greatest number of genome copies in lung and kidney, and a high copy number in the heart. AAV6 expression was observed in the heart, liver, and skeletal muscle, and the genome distribution corroborated these observations.

Correction of murine PKU following AAV-mediated intramuscular expression of a complete phenylalanine hydroxylating system

Phenylketonuria (PKU) caused by phenylalanine hydroxylase (PAH) deficiency leads to toxic accumulation of phenylalanine (Phe). PAH is predominantly expressed in liver and its activity requires a supply of tetrahydrobiopterin (BH(4)) cofactor, but we propose that expression of a complete Phe hydroxylating system (PAH plus BH(4) synthetic enzymes) in skeletal muscle will lead to therapeutic reduction of blood Phe levels in Pah(enu2) mice, a model of human PKU. In order to test this hypothesis, we first developed transgenic Pah(enu2) mice that lack liver PAH activity but coexpress, in their skeletal muscle, PAH and guanosine triphosphate cyclohydrolase I (GTPCH). The latter is responsible for the committing enzymatic step in BH(4) biosynthesis. Despite sufficient muscle enzyme expression, these mice remained hyperphenylalaninemic, thereby suggesting that expression of additional BH(4) synthetic enzymes would be necessary. A recombinant triple-cistronic adeno-associated virus-2 (AAV2) pseudotype 1 vector expressing PAH along with GTPCH and 6-pyruvoyltetrahydrobiopterin synthase (PTPS), the next step in BH(4) synthesis, was generated. Injection of this vector into the gastrocnemius muscles of Pah(enu2) mice led to stable and long-term reduction of blood Phe and reversal of PKU-associated coat hypopigmentation. We propose that muscle-directed gene therapy will be a viable alternative treatment approach to PKU and other inborn errors of metabolism.

Intranasal vaccination of recombinant adeno-associated virus encoding receptor-binding domain of severe acute respiratory syndrome coronavirus (SARS-CoV) spike protein induces strong mucosal immune responses and provides long-term protection against SARS-CoV infection

We have previously reported that a subunit protein vaccine based on the receptor-binding domain (RBD) of severe acute respiratory syndrome coronavirus (SARS-CoV) spike protein and a recombinant adeno-associated virus (rAAV)-based RBD (RBD-rAAV) vaccine could induce highly potent neutralizing Ab responses in immunized animals. In this study, systemic, mucosal, and cellular immune responses and long-term protective immunity induced by RBD-rAAV were further characterized in a BALB/c mouse model, with comparison of the i.m. and intranasal (i.n.) routes of administration. Our results demonstrated that: 1) the i.n. vaccination induced a systemic humoral immune response of comparable strength and shorter duration than the i.m. vaccination, but the local humoral immune response was much stronger; 2) the i.n. vaccination elicited stronger systemic and local specific cytotoxic T cell responses than the i.m. vaccination, as evidenced by higher prevalence of IL-2 and/or IFN-gamma-producing CD3+/CD8+ T cells in both lungs and spleen; 3) the i.n. vaccination induced similar protection as the i.m. vaccination against SARS-CoV challenge in mice; 4) higher titers of mucosal IgA and serum-neutralizing Ab were associated with lower viral load and less pulmonary pathological damage, while no Ab-mediated disease enhancement effect was observed; and 5) the vaccination could provide long-term protection against SARS-CoV infection. Taken together, our findings suggest that RBD-rAAV can be further developed into a vaccine candidate for prevention of SARS and that i.n. vaccination may be the preferred route of administration due to its ability to induce SARS-CoV-specific systemic and mucosal immune responses and its better safety profile.

[Gene therapy using AAV]

AAV (adeno-associated virus) vectors are considered to be promising gene-delivery vehicles for gene therapy, because they are derived from non-pathogenic virus, efficiently transduce non-dividing cells, and cause long-term gene expression. Appropriate AAV serotypes are utilized depending on the type of target cells. Among various neurological disorders, Parkinson's disease (PD) is one of the most promising candidates of gene therapy. PD is a progressive neurodegenerative disorder that predominantly affects dopaminergic neurons in the substantia nigra. One of the major approaches to gene therapy of PD is the intrastriatal expression of dopamine (DA)-synthesizing enzyme genes. As for the initial step of clinical application, AAV vector-mediated AADC (aromatic L-amino acid decarboxylase; the enzyme converting L-DOPA to DA) gene transfer in combination with oral administration of L-DOPA would be appropriate, since DA production can be regulated by adjusting the dose of L-DOPA. Second, intramuscular injection of AAV vectors is appropriate to protein-supplement gene therapy. Monogenic diseases such as hemophilia and Fabry disease are suitable candidates. Regarding cancer gene therapy, AAV vectors may be utilized to inhibit tumor angiogenesis, metastasis, and invasion. When long-term transgene expression in stem cells is needed, a therapeutic gene should be introduced with a minimal risk of insertional mutagenesis. To this end, site-specific integration into the AAVS1 locus on the chromosome 19 (19q13.4) by using the integration machinery of AAV would be particularly valuable.

Gene therapy: the first two decades and the current state-of-the-art

The concept of gene therapy was envisioned soon after the emergence of restriction endonucleases and subcloning of mammalian genes in phage and plasmids. Over the ensuing decades, vectors were developed, including nonviral methods, integrating virus vectors (gammaretrovirus and lentivirus), and non-integrating virus vectors (adenovirus, adeno-associated virus, and herpes simplex virus vectors). Preclinical data demonstrated potential efficacy in a broad range of animal models of human diseases, but clinical efficacy in humans remained elusive in most cases, even after decades of experience in over 1000 trials. Adverse effects from gene therapy have been observed in some cases, often because of viral vectors retaining some of the pathogenic potential of the viruses upon which they are based. Later generation vectors have been developed in which the safety and/or the efficiency of gene transfer has been improved. Most recently this work has involved alterations of vector envelope or capsid proteins either by insertion of ligands to target specific receptors or by directed evolution. The disease targets for gene therapy are multiple, but the most promising data have come from monogenic disorders. As the number of potential targets for gene therapy continues to increase, and a substantial number of trials continue with both the standard and the later generation vector systems, it is hoped that a therapeutic niche for gene therapy will emerge in the coming decades.

Characterizing clearance of helper adenovirus by a clinical rAAV1 manufacturing process

Recombinant adeno-associated viral vectors (rAAV) are being developed as gene therapy delivery vehicles and as genetic vaccines, and some of the most scaleable manufacturing methods for rAAV use live adenovirus to induce production. One aspect of establishing safety of rAAV products is therefore demonstrating adequate and reliable clearance of this helper virus by the vector purification process. The ICH Q5A regulatory guidance on viral safety provides recommendations for process design and characterization of viral clearance for recombinant proteins, and these principles were adapted to a rAAV serotype 1 purification process for clinical vectors. Specific objectives were to achieve overall adenovirus clearance factors significantly greater than input levels by using orthogonal separation and inactivation methods, and to segregate adenovirus from downstream operations by positioning a robust clearance step early in the process. Analytical tools for process development and characterization addressed problematic in-process samples, and a viral clearance validation study was performed using adenovirus and two non-specific model viruses. Overall clearance factors determined were >23 LRV for adenovirus, 11 LRV for BVDV, and >23 LRV for AMuLV.

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.

Muscle as a target for supplementary factor IX gene transfer

Immune responses to the factor IX (F.IX) transgene product are a concern in gene therapy for the X-linked bleeding disorder hemophilia B. The risk for such responses is determined by several factors, including the vector, target tissue, and others. Previously, we have demonstrated that hepatic gene transfer with adeno-associated viral (AAV) vectors can induce F.IX-specific immune tolerance. Muscle-derived F.IX expression, however, is limited by a local immune response. Here, skeletal muscle was investigated as a target for supplemental gene transfer. Given the low invasiveness of intramuscular injections, this route would be ideal for secondary gene transfer, thereby boosting levels of transgene expression. However, this is feasible only if immune tolerance established by compartmentalization of expression to the liver extends to other sites. Immune tolerance to human F.IX established by prior hepatic AAV-2 gene transfer was maintained after subsequent injection of AAV-1 or adenoviral vector into skeletal muscle, and tolerized mice failed to form antibodies or an interferon (IFN)-gamma(+) T cell response to human F.IX. A sustained increase in systemic transgene expression was obtained for AAV-1, whereas an increase after adenoviral gene transfer was transient. A CD8(+) T cell response specifically against adenovirus-transduced fibers was observed, suggesting that cytotoxic T cell responses against viral antigens were sufficient to eliminate expression in muscle. In summary, the data demonstrate that supplemental F.IX gene transfer to skeletal muscle does not break tolerance achieved by liver-derived expression. The approach is efficacious, if the vector for muscle gene transfer does not express immunogenic viral proteins.

Adeno-associated virus type 12 (AAV12): a novel AAV serotype with sialic acid- and heparan sulfate proteoglycan-independent transduction activity

Recombinant adeno-associated virus (rAAV) is a promising vector for gene therapy. Recent isolations of novel AAV serotypes have led to significant advances by broadening the tropism and increasing the efficiency of gene transfer to the desired target cell. However, a major concern that remains is the strong preexisting immune responses to several vectors. In this paper, we describe the isolation and characterization of AAV12, an AAV serotype with unique biological and immunological properties. In contrast to those of all other reported AAVs, AAV12 cell attachment and transduction do not require cell surface sialic acids or heparan sulfate proteoglycans. Furthermore, rAAV12 is resistant to neutralization by circulating antibodies from human serum. The feasibility of rAAV12 as a vector was demonstrated in a mouse model in which muscle and salivary glands were transduced. These characteristics make rAAV12 an interesting candidate for gene transfer applications.