Building a better vector: the manipulation of AAV virions

A Novel Retrograde AAV Variant for Functional Manipulation of Cortical Projection Neurons in Mice and Monkeys

Retrograde adeno-associated viruses (AAVs) are capable of infecting the axons of projection neurons and serve as a powerful tool for the anatomical and functional characterization of neural networks. However, few retrograde AAV capsids have been shown to offer access to cortical projection neurons across different species and enable the manipulation of neural function in non-human primates (NHPs). Here, we report the development of a novel retrograde AAV capsid, AAV-DJ8R, which efficiently labeled cortical projection neurons after local administration into the striatum of mice and macaques. In addition, intrastriatally injected AAV-DJ8R mediated opsin expression in the mouse motor cortex and induced robust behavioral alterations. Moreover, AAV-DJ8R markedly increased motor cortical neuron firing upon optogenetic light stimulation after viral delivery into the macaque putamen. These data demonstrate the usefulness of AAV-DJ8R as an efficient retrograde tracer for cortical projection neurons in rodents and NHPs and indicate its suitability for use in conducting functional interrogations.

An ultra-compact promoter drives widespread neuronal expression in mouse and monkey brains

Promoters are essential tools for basic and translational neuroscience research. An ideal promoter should possess the shortest possible DNA sequence with cell-type selectivity. However, whether ultra-compact promoters can offer neuron-specific expression is unclear. Here, we report the development of an extremely short promoter that enables selective gene expression in neurons, but not glial cells, in the brain. The promoter sequence originates from the human CALM1 gene and is only 120 bp in size. The CALM1 promoter (pCALM1) embedded in an adeno-associated virus (AAV) genome directed broad reporter expression in excitatory and inhibitory neurons in mouse and monkey brains. Moreover, pCALM1, when inserted into an all-in-one AAV vector expressing SpCas9 and sgRNA, drives constitutive and conditional in vivo gene editing in neurons and elicits functional alterations. These data demonstrate the ability of pCALM1 to conduct restricted neuronal gene expression, illustrating the feasibility of ultra-miniature promoters for targeting brain-cell subtypes.

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

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

Transduction of Adeno-Associated Virus Vectors Targeting Hair Cells and Supporting Cells in the Neonatal Mouse Cochlea

Adeno-associated virus (AAV) is the preferred vector for gene therapy of hereditary deafness, and different viral serotypes, promoters and transduction pathways can influence the targeting of AAV to different types of cells and the expression levels of numerous exogenous genes. To determine the transduction and expression patterns of AAV with different serotypes or promoters in hair cells and supporting cells in the neonatal mouse cochlea, we examined the expression of enhanced green fluorescent protein (eGFP) for five different types of AAV vectors [serotypes 2, 9, and Anc80L65 with promoter cytomegalovirus (CMV)-beta-Globin and serotypes 2 and 9 with promoter chicken beta-actin (CBA)] in in vitro cochlear explant cultures and we tested the transduction of AAV2/2-CBA, AAV2/9-CBA, and AAV2/Anc80L65-CMV by in vivo microinjection into the scala media of the cochlea. We found that each AAV vector had its own transduction and expression characteristics in hair cells and supporting cells in different regions of the cochlea. There was a tonotopic gradient for the in vitro transduction of AAV2/2-CBA, AAV2/9-CBA, AAV2/2-CMV, and AAV2/9-CMV in outer hair cells (OHCs), with more OHCs expressing eGFP at the base of the cochlea than at the apex. AAV2/2-CBA in vitro and AAV2/Anc80L65-CMV in vivo induced more supporting cells expressing eGFP at the apex than in the base. We found that AAV vectors with different promoters had different expression efficacies in hair cells and supporting cells of the auditory epithelium. The CMV-beta-Globin promoter could drive the expression of the delivered construct more efficiently in hair cells, while the CBA promoter was more efficient in supporting cells. The in vitro and in vivo experiments both demonstrated that AAV2/Anc80L65-CMV was a very promising vector for gene therapy of deafness because of its high transduction rates in hair cells. These results might be useful for selecting the appropriate vectors for gene delivery into different types of inner ear cells and thus improving the effectiveness of gene therapy.

Diagnosis, Intervention, and Prevention of Genetic Hearing Loss

It is estimated that at least 50% of congenital or childhood hearing loss is attributable to genetic causes. In non-syndromic hearing loss, which accounts for 70% of genetic hearing loss, approximately 80% of cases are autosomal recessive, 15% autosomal dominant, and 1-2% mitochondrial or X-linked. In addition, 30% of genetic hearing loss is syndromic. The genetic causes of hearing loss are highly heterogeneous. So far, more than 140 deafness-related genes have been discovered. Studies on those genes tremendously increased our understanding of the inner ear functions at the molecular level. It also offers important information for the patients and allows personalized and accurate genetic counseling. In many cases, genetic diagnosis of hearing loss can help to avoid unnecessary and costly clinical testing, offer prognostic information, and guide future medical management. On the other hand, a variety of gene therapeutic approaches have been developed aiming to relieve or converse the hearing loss due to genetic causes. Prevention of genetic hearing loss is feasible through prepregnancy and prenatal genetic diagnosis and counseling.

Emerging Gene Therapies for Genetic Hearing Loss

Gene therapy, or the treatment of human disease using genetic material, for inner ear dysfunction is coming of age. Recent progress in developing gene therapy treatments for genetic hearing loss has demonstrated tantalizing proof-of-principle in animal models. While successful translation of this progress into treatments for humans awaits, there is growing interest from patients, scientists, clinicians, and industry. Nonetheless, it is clear that a number of hurdles remain, and expectations for total restoration of auditory function should remain tempered until these challenges have been overcome. Here, we review progress, prospects, and challenges for gene therapy in the inner ear. We focus on technical aspects, including routes of gene delivery to the inner ear, choice of vectors, promoters, inner ear targets, therapeutic strategies, preliminary success stories, and points to consider for translating of these successes to the clinic.

Controlling AAV Tropism in the Nervous System with Natural and Engineered Capsids

More than one hundred naturally occurring variants of adeno-associated virus (AAV) have been identified, and this library has been further expanded by an array of techniques for modification of the viral capsid. AAV capsid variants possess unique antigenic profiles and demonstrate distinct cellular tropisms driven by differences in receptor binding. AAV capsids can be chemically modified to alter tropism, can be produced as hybrid vectors that combine the properties of multiple serotypes, and can carry peptide insertions that introduce novel receptor-binding activity. Furthermore, directed evolution of shuffled genome libraries can identify engineered variants with unique properties, and rational modification of the viral capsid can alter tropism, reduce blockage by neutralizing antibodies, or enhance transduction efficiency. This large number of AAV variants and engineered capsids provides a varied toolkit for gene delivery to the CNS and retina, with specialized vectors available for many applications, but selecting a capsid variant from the array of available vectors can be difficult. This chapter describes the unique properties of a range of AAV variants and engineered capsids, and provides a guide for selecting the appropriate vector for specific applications in the CNS and retina.

Evaluation of the expression pattern of rAAV2/1, 2/5, 2/7, 2/8, and 2/9 serotypes with different promoters in the mouse visual cortex

This study compared the expression pattern, laminar distribution, and cell specificity of several rAAV serotypes (2/1, 2/5, 2/7, 2/8, and 2/9) injected in the primary visual cortex (V1) of adult C57Bl/6J mice. In order to obtain specific expression in certain neuron subtypes, different promoter sequences were evaluated for excitatory cell specificity: a universal cytomegalovirus (CMV) promoter, and two versions of the excitatory neuron-specific Ca(2+) /calmodulin-dependent kinase subunit α (CaMKIIα) promoter, CaMKIIα 0.4 and CaMKIIα 1.3. The spatial distribution as well as the cell type specificity was immunohistochemically verified. Depending on the rAAV serotype used, the transduced volume expressing reporter protein differed substantially (rAAV2/5 ≫ 2/7 ≈ 2/9 ≈ 2/8 ≫ 2/1). Excitatory neuron-specific targeting was promoter-dependent, with a surprising difference between the 1.3 kb and 0.4 kb CaMKIIα promoters. While CaMKIIα 1.3 and CMV carrying vectors were comparable, with 78% of the transduced neurons being excitatory for CMV and 82% for CaMKIIα 1.3, the shorter CaMKIIα 0.4 version resulted in 95% excitatory specificity. This study therefore puts forward the CaMKIIα 0.4 promoter as the best choice to target excitatory neurons with rAAVs. Together, these results can be used as an aid to select the most optimal vector system to deliver transgenes into specific rodent neocortical circuits, allowing further elucidation of their functions.

Adeno-associated virus-mediated microRNA delivery and therapeutics

MicroRNAs (miRNAs) are 20 to 24 nt long, single-stranded RNAs that repress gene expression. Dysregulation of miRNA expression is associated with many human diseases. Modulating the level of endogenous miRNA alters gene profiling and can achieve therapeutic benefits. Here the authors review currently used methods of altering miRNA activity in vivo. They focus on the delivery of miRNAs and miRNA inhibitors using recombinant adeno-associated virus (rAAV). In general, rAAV-mediated miRNA inhibition or overexpression provides a simple, efficient, and informative way to study miRNA function in mammals. This method also provides the opportunity to explore potential miRNA therapeutics for many diseases.

Viral vector-based models of Parkinson's disease

In order to study the molecular pathways of Parkinson's disease (PD) and to develop novel therapeutic strategies, scientific investigators rely on animal models. The identification of PD-associated genes has led to the development of genetic PD models as an alternative to toxin-based models. Viral vector-mediated loco-regional gene delivery provides an attractive way to express transgenes in the central nervous system. Several vector systems based on various viruses have been developed. In this chapter, we give an overview of the different viral vector systems used for targeting the CNS. Further, we describe the different viral vector-based PD models currently available based on overexpression strategies for autosomal dominant genes such as α-synuclein and LRRK2, and knockout or knockdown strategies for autosomal recessive genes, such as parkin, DJ-1, and PINK1. Models based on overexpression of α-synuclein are the most prevalent and extensively studied, and therefore the main focus of this chapter. Many efforts have been made to increase the expression levels of α-synuclein in the dopaminergic neurons. The best α-synuclein models currently available have been developed from a combined approach using newer AAV serotypes and optimized vector constructs, production, and purification methods. These third-generation α-synuclein models show improved face and predictive validity, and therefore offer the possibility to reliably test novel therapeutics.

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.

Convection-enhanced delivery and systemic mannitol increase gene product distribution of AAV vectors 5, 8, and 9 and increase gene product in the adult mouse brain

The use of recombinant adeno-associated viral (rAAV) vectors as a means of gene delivery to the central nervous system has emerged as a potentially viable method for the treatment of several types of degenerative brain diseases. However, a limitation of typical intracranial injections into the adult brain parenchyma is the relatively restricted distribution of the delivered gene to large brain regions such as the cortex, presumably due to confined dispersion of the injected particles. Optimizing the administration techniques to maximize gene distribution and gene expression is an important step in developing gene therapy studies. Here, we have found additive increases in distribution when 3 methods to increase brain distribution of rAAV were combined. The convection enhanced delivery (CED) method with the step-design cannula was used to deliver rAAV vector serotypes 5, 8 and 9 encoding GFP into the hippocampus of the mouse brain. While the CED method improved distribution of all 3 serotypes, the combination of rAAV9 and CED was particularly effective. Systemic mannitol administration, which reduces intracranial pressure, also further expanded distribution of GFP expression, in particular, increased expression on the contralateral hippocampi. These data suggest that combining advanced injection techniques with newer rAAV serotypes greatly improves viral vector distribution, which could have significant benefits for implementation of gene therapy strategies.

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.

Adeno-associated virus serotype 2 induces cell-mediated immune responses directed against multiple epitopes of the capsid protein VP1

Adeno-associated virus serotype 2 (AAV-2) has been developed as a gene therapy vector. Antibody and cell-mediated immune responses to AAV-2 or AAV-2-transfected cells may confound the therapeutic use of such vectors in clinical practice. In one of the most detailed examinations of AAV-2 immunity in humans to date, cell-mediated and humoral immune responses to AAV-2 were characterized from a panel of healthy blood donors. The extent of AAV-2-specific antibody in humans was determined by examination of circulating AAV-2-specific total IgG levels in plasma from 45 normal donors. Forty-one donors were seropositive and responses were dominated by IgG1 and IgG2 subclasses. Conversely, AAV-2-specific IgG3 levels were consistently low in all donors. Cell-mediated immune recall responses were detectable in nearly half the population studied. In vitro restimulation with AAV-2 of peripheral blood mononuclear cell cultures from 16 donors elicited gamma interferon (IFN-γ) (ten donors), interleukin-10 (IL-10) (eight donors) and interleukin-13 (IL-13) (four donors) responses. Using a series of overlapping peptides derived from the sequence of the VP1 viral capsid protein, a total of 59 candidate T-cell epitopes were identified. Human leukocyte antigen characterization of donors revealed that the population studied included diverse haplotypes, but that at least 17 epitopes were recognized by multiple donors and could be regarded as immunodominant. These data indicate that robust immunological memory to AAV-2 is established. The diversity of sequences recognized suggests that attempts to modify the AAV-2 capsid, as a strategy to avoid confounding immunity, will not be feasible.

Gene therapy for retinitis pigmentosa and Leber congenital amaurosis caused by defects in AIPL1: effective rescue of mouse models of partial and complete Aipl1 deficiency using AAV2/2 and AAV2/8 vectors

Defects in the photoreceptor-specific gene encoding aryl hydrocarbon receptor-interacting protein-like 1 (AIPL1) are clinically heterogeneous and present as Leber Congenital Amaurosis, the severest form of early-onset retinal dystrophy and milder forms of retinal dystrophies such as juvenile retinitis pigmentosa and dominant cone-rod dystrophy. [Perrault, I., Rozet, J.M., Gerber, S., Ghazi, I., Leowski, C., Ducroq, D., Souied, E., Dufier, J.L., Munnich, A. and Kaplan, J. (1999) Leber congenital amaurosis. Mol. Genet. Metab., 68, 200-208.] Although not yet fully elucidated, AIPL1 is likely to function as a specialized chaperone for rod phosphodiesterase (PDE). We evaluate whether AAV-mediated gene replacement therapy is able to improve photoreceptor function and survival in retinal degeneration associated with AIPL1 defects. We used two mouse models of AIPL1 deficiency simulating three different rates of photoreceptor degeneration. The Aipl1 hypomorphic (h/h) mouse has reduced Aipl1 levels and a relatively slow degeneration. Under light acceleration, the rate of degeneration in the Aipl1 h/h mouse is increased by 2-3-fold. The Aipl1-/- mouse has no functional Aipl1 and has a very rapid retinal degeneration. To treat the different rates of degeneration, two pseudotypes of recombinant adeno-associated virus (AAV) exhibiting different transduction kinetics are used for gene transfer. We demonstrate restoration of cellular function and preservation of photoreceptor cells and retinal function in Aipl1 h/h mice following gene replacement therapy using an AAV2/2 vector and in the light accelerated Aipl1 h/h model and Aipl1-/- mice using an AAV2/8 vector. We have thus established the potential of gene replacement therapy in varying rates of degeneration that reflect the clinical spectrum of disease. This is the first gene replacement study to report long-term rescue of a photoreceptor-specific defect and to demonstrate effective rescue of a rapid photoreceptor degeneration.

Gene therapy for retinitis pigmentosa and Leber congenital amaurosis caused by defects in AIPL1: effective rescue of mouse models of partial and complete Aipl1 deficiency using AAV2/2 and AAV2/8 vectors

Defects in the photoreceptor-specific gene encoding aryl hydrocarbon receptor-interacting protein-like 1 (AIPL1) are clinically heterogeneous and present as Leber Congenital Amaurosis, the severest form of early-onset retinal dystrophy and milder forms of retinal dystrophies such as juvenile retinitis pigmentosa and dominant cone-rod dystrophy. [Perrault, I., Rozet, J.M., Gerber, S., Ghazi, I., Leowski, C., Ducroq, D., Souied, E., Dufier, J.L., Munnich, A. and Kaplan, J. (1999) Leber congenital amaurosis. Mol. Genet. Metab., 68, 200-208.] Although not yet fully elucidated, AIPL1 is likely to function as a specialized chaperone for rod phosphodiesterase (PDE). We evaluate whether AAV-mediated gene replacement therapy is able to improve photoreceptor function and survival in retinal degeneration associated with AIPL1 defects. We used two mouse models of AIPL1 deficiency simulating three different rates of photoreceptor degeneration. The Aipl1 hypomorphic (h/h) mouse has reduced Aipl1 levels and a relatively slow degeneration. Under light acceleration, the rate of degeneration in the Aipl1 h/h mouse is increased by 2-3-fold. The Aipl1-/- mouse has no functional Aipl1 and has a very rapid retinal degeneration. To treat the different rates of degeneration, two pseudotypes of recombinant adeno-associated virus (AAV) exhibiting different transduction kinetics are used for gene transfer. We demonstrate restoration of cellular function and preservation of photoreceptor cells and retinal function in Aipl1 h/h mice following gene replacement therapy using an AAV2/2 vector and in the light accelerated Aipl1 h/h model and Aipl1-/- mice using an AAV2/8 vector. We have thus established the potential of gene replacement therapy in varying rates of degeneration that reflect the clinical spectrum of disease. This is the first gene replacement study to report long-term rescue of a photoreceptor-specific defect and to demonstrate effective rescue of a rapid photoreceptor degeneration.

Striatal readministration of rAAV vectors reveals an immune response against AAV2 capsids that can be circumvented

Recombinant adeno-associated virus (rAAV) expresses no viral genes after transduction. In addition, because the brain is relatively immunoprivileged, intracranial rAAV transduction may be immunologically benign due to a lack of antigen presentation. However, preexposure to AAV allows neutralizing antibodies (nAbs) to block brain transduction and rAAV readministration in the brain leads to an inflammatory response in the second-injection site. In this study, we replicate our striatal rAAV2/2-GDNF readministration results and extend this effect to a second transgene, green fluorescent protein (GFP). Unlike rAAV2/2-GDNF readministration, striatal rAAV2/2-GFP readministration leads to a loss of transgene in the second site in the absence of detectable circulating nAbs. In order to determine whether the transgene or the AAV2 capsid is the antigenic stimulus in brain for the immune response in the second site, we readministered rAAV2/2-GFP using two different rAAV serotypes (rAAV2/2 followed by rAAV2/5). In this case, there was no striatal inflammation or transgene loss detected in the second-injection site. In addition, striatal readministration of rAAV2/5-GFP also resulted in no detectable immune response. Furthermore, delaying rAAV2/2 striatal readministration to a 11-week interval abrogated the immune response in the second-injection site. Finally, while striatal readministration of rAAV2/2 leads to significant loss of transgene in the second-injection site, this effect is not due to loss of vector genomes as determined by quantitative real-time PCR. We conclude that intracellular processing of AAV capsids after transduction is the immunogenic antigen and capsid serotypes that are processed more quickly than rAAV2/2 are less immunogenic.

Immune-directed gene therapeutic development for Alzheimer's, prion, and Parkinson's diseases

The development of novel immune-based therapeutics for neurodegenerative diseases is an area of intense focus. Neurodegenerative diseases represent a particular challenge since in many cases the onset of symptoms occurs after considerable degeneration has ensued. Based on human genetic and histopathological evidence from patients with neurodegenerative diseases, animal models that recapitulate specific pathologic features have been developed. Utilizing these animal models in combination with viral vector-based gene therapeutics, specific epochs of disease can be targeted. One common feature of several neurodegenerative diseases is misfolded proteins. The mechanism by which these altered protein conformers lead to neurodegeneration is not completely understood but much effort has been put forward to either degrade aberrant protein or prevent the formation of misfolded conformers. In this review, we will summarize work that employs viral vector gene therapeutics to modulate the brain's response to misfolded proteins with a specific focus on neurodegeneration.

Recent developments in adeno-associated virus vector technology

Adeno-associated virus (AAV), a single-stranded DNA parvovirus, is emerging as one of the leading gene therapy vectors owing to its nonpathogenicity and low immunogenicity, stability and the potential to integrate site-specifically without known side-effects. A portfolio of recombinant AAV vector types has been developed with the aim of optimizing efficiency, specificity and thereby also the safety of in vitro and in vivo gene transfer. More and more information is now becoming available about the mechanism of AAV/host cell interaction improving the efficacy of recombinant AAV vector (rAAV) mediated gene delivery. This review summarizes the current knowledge of the infectious biology of AAV, provides an overview of the latest developments in the field of AAV vector technology and discusses remaining challenges.

Potent antitumor effect of TRAIL mediated by a novel adeno-associated viral vector targeting to telomerase activity for human hepatocellular carcinoma

BACKGROUND

Adeno-associated virus (AAV) has rapidly become a promising gene delivery vehicle for its excellent advantages of low pathogenicity and long-term gene expression. However, lack of tissue specificity caused low efficiency of AAV transfer to target cells. The promoter of human telomerase reverse transcriptase (hTERT) has been implicated in mediating gene expression in cancer cells as hTERT is transcriptionally upregulated in most cancer cells. Thereby, the hTERT promoter becomes a good candidate to enhance the targeting efficiency of AAV in cancer cells. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) functions as a soluble cytokine to selectively kill various cancer cells without toxicity to most normal cells. It remains to be determined whether the hTERT promoter can efficiently mediate TRAIL gene therapy in cancer cells using AAV vector.

METHODS

A novel AAV vector containing the TRAIL gene under the control of the hTERT promoter (AAV-hTERT-TRAIL) was generated. The specific expression of hTERT-controlled genes was evaluated in cell lines. The antitumor efficacy of AAV-hTERT-TRAIL was assessed in tumor cell lines and human hepatocellular carcinoma xenograft mouse model.

RESULTS

TRAIL expression was observed in tumor cells infected with AAV-hTERT-TRAIL at both the protein and mRNA level. AAV-hTERT-TRAIL displayed cancer-specific cytotoxicity and induced tumor cell apoptosis. Moreover, in animal experiments, intratumoral administration of AAV-hTERT-TRAIL significantly suppressed the growth of xenograft tumors and resulted in tumor cell death.

CONCLUSIONS

AAVs in combination with hTERT-mediated therapeutic gene expression provide a promising targeting approach for developing effective therapy for human cancers. These data suggest that AAV-hTERT-TRAIL is a potent therapeutic agent for cancer therapy.

Coating of adeno-associated virus with reactive polymers can ablate virus tropism, enable retargeting and provide resistance to neutralising antisera

BACKGROUND

Copolymers based on poly-[N-(2-hydroxypropyl) methacrylamide] (HPMA) have been used previously to enable targeted delivery of adenovirus. Here we demonstrate polymer-coating techniques can also be used to modify and retarget adeno-associated virus (AAV) types 5 and 8.

METHODS

Three strategies for modifying transductional targeting of AAV were employed. The first involved direct reaction of AAV5 or AAV8 with amino-reactive HPMA copolymer. The second approach used carbodiimide (EDC) chemistry to increase the number of surface amino groups on the AAV5 capsid, thereby improving coating efficiency. In the third approach, the AAV5 genome was isolated from capsid proteins and delivered in a synthetic polyplex consisting of polyethylenimine (PEI) and HPMA.

RESULTS

Efficient covalent attachment of HPMA copolymer to AAV5 could only be achieved following modification of the virus with EDC. Coating inhibited sialic acid dependent infection and provided a platform for retargeting via new ligands, including basic fibroblast growth factor. Retargeted infection was shown to be partially resistant to neutralising antisera. Delivery of AAV5 genomes using PEI and HPMA was efficient and provided absolute control of tropism and protection from antisera. In contrast AAV8 could be reacted directly with HPMA copolymer and allowed specific retargeting via the epidermal growth factor receptor, but gave no protection against neutralising antisera.

CONCLUSIONS

Reactive HPMA polymers can be used to ablate the natural tropism of both AAV8 and EDC-modified AAV5 and enable receptor-specific infection by incorporation of targeting ligands. These data show transductional targeting strategies can be used to improve the versatility of AAV vectors.

Strategies to enhance capillary formation inside biomaterials: a computational study

Control over angiogenesis (formation of new capillaries from preexisting vessels) is often a crucial requirement for implantable porous biomaterials serving as scaffolds for tissue regeneration. Angiogenesis is influenced by the transport of chemoattractants such as vascular endothelial growth factor (VEGF) through the implant. To investigate this influence, we developed a computational model of capillary formation based on endothelial cell migration by modeling the random motion of sprout tips biased along spatially and temporally evolving concentration gradients of VEGF. The model focuses on the effect of diffusive VEGF transport inside a 2D domain on the directed migration of sprouts to test several chemical and physical strategies to stimulate and control angiogenesis. We considered a 2D porous membrane that is located between the primary vessel and a line source of VEGF. We assess the vascular network formed in 2 cases of a high and zero VEGF degradation rates applying 3 strategies of VEGF production: (1) only a line source; (2) a line source plus controlled release from a small number of VEGF sources that are randomly dispersed on the pore boundaries; and (3) a line source plus controlled release of VEGF from the pore boundaries themselves. Results show that in the limiting cases where VEGF degradation rate is relatively high, strategies 2 and 3 lead to a substantial increase in the number of vessels. This increase depends on the relative rates at which the line source and embedded sources or solid boundaries produce VEGF. Using strategy 2 results in a newly formed capillary network that is highly localized around the embedded sources. However, using strategy 3 leads to a more uniformly distributed vessel network and a higher degree of vessel ingrowth inside the porous membrane. In addition, the duration at which we engineer the embedded sources or pore boundaries to release VEGF determines the morphology of the capillary network. Although a higher release duration leads to a dense network of newly formed vessels near the primary vessel, it hinders further vessel penetration inside the porous membrane. Therefore, in applying both strategies 2 and 3, there is an optimum release duration that leads to a deeper penetration of vessels inside the membrane. It is hoped that insights from this study will aid in the design of materials with optimal structural and chemical properties to facilitate controlled angiogenesis.

Effect of sense or anti-sense GCRG213 fragment and GCRG213 RNA interference on transplanted gastric carcinoma in vivo

AIM: To investigate the influence of sense or anti-sense GCRG213 fragment and GCRG213 RNA interference (RNAi) on transplanted gastric cancer in vivo.

METHODS: Gastric carcinoma cell line MKN45 at logarithmic growth phase was subcutaneously inoculated in 20 athymic mice. After tumor formation, the adeno-associated virus (AAV) containing sense or anti-sense GCRG213 fragment and GCRG213 RNA interference fragment was injected into tubercles. Meanwhile, empty and normal saline control groups were designed. All the mice were killed 2 weeks after injection. Tumors were weighted and the expression of GCRG213 mRNA was detected by semi-quantitative reverse transcription-polymerase chain reaction.

RESULTS: Three weeks after inoculation of MKN45 cells, small tumor tubercles came into formation. The weights of tumor tissues were 5.12 ± 1.02, 1.22 ± 0.46, 0.81 ± 0.37, 3.13 ± 0.69 and 3.45 ± 0.87 g in sense, anti-sense, RNAi, empty and normal saline group, and the mRNA expression of GCRG213 were 0.406 ± 0.013, 0.211 ± 0.021, 0.087 ± 0.015, 0.312 ± 0.050 and 0.283 ± 0.061 in the above groups, respectively.

CONCLUSION: Target genes including sense or anti-sense GCRG213 and RNAi fragment can be effectively expressed in vivo through AAV vector. Increased or decreased GCRG213 expression can promote or inhibit growth of gastric cancer.

Construction of recombined adeno-associated virus vector expressing GCRG213 short interference RNA and preparation of high-titer virus

AIM: To construct the recombinant adeno-associated virus vector expressing gastric cancer related gene GCRG213 short interference RNA (rAAV-GCRG213-siRNA) and use it for the preparation of high-titer virus.

METHODS: IMG800-GCRG213-siRNA plasmid, which was constructed with GCRG213 siRNA, was digested with BamHⅠ and BglⅡ. A 410-bp fragment, which contained U6 promoter and GCRG213 siRNA, was obtained and inserted into the adeno-associated virus vector plasmid pSNAV2.0 that was digested with BamHⅠ. The positive vectors were analyzed through enzyme digestion and DNA sequencing. The recombinant plasmid was transfected into BHK cells using LipofecLamine^TM2000. The G418-resistant cells were obtained and infected with HSVl-rc/ΔUL2, which had the function of packaging rAAV. After purification, the target vector and virus were collected.

RESULTS: The target vector, rAAV-GCRG213-siRNA, was successfully constructed, and the rAAV with a titer of 5 × 10¹² vg/L (vg: vector genome) was obtained.

CONCLUSION: Recombitant rAAV vector may be used for further investigation of GCRG213 function in vivo and gene therapy.

Induction of robust immune responses against human immunodeficiency virus is supported by the inherent tropism of adeno-associated virus type 5 for dendritic cells

The ability of adeno-associated virus serotype 1 to 8 (AAV1 to AAV8) vectors expressing the human immunodeficiency virus type 1 (HIV-1) Env gp160 (AAV-HIV) to induce an immune response was evaluated in BALB/c mice. The AAV5 vector showed a higher tropism for both mouse and human dendritic cells (DCs) than did the AAV2 vector, whereas other AAV serotype vectors transduced DCs only poorly. AAV1, AAV5, AAV7, and AAV8 were more highly expressed in muscle cells than AAV2. An immunogenicity study of AAV serotypes indicates that AAV1, AAV5, AAV7, and AAV8 vectors expressing the Env gp160 gene induced higher HIV-specific humoral and cell-mediated immune responses than the AAV2 vector did, with the AAV5 vector producing the best responses. Furthermore, mice injected with DCs that had been transduced ex vivo with an AAV5 vector expressing the gp160 gene elicited higher HIV-specific cell-mediated immune responses than did DCs transduced with AAV1 and AAV2 vectors. We also found that AAV vectors produced by HEK293 cells and insect cells elicit similar levels of antigen-specific immune responses. These results demonstrate that the immunogenicity of AAV vectors depends on their tropism for both antigen-presenting cells (such as DCs) and non-antigen-presenting cells (such as muscular cells) and that AAV5 is a better vector than other AAV serotypes. These results may aid in the development of AAV-based vaccine and gene therapy.

Complete Prevention of Atherosclerosis in ApoE-Deficient Mice by Hepatic Human ApoE Gene Transfer With Adeno-Associated Virus Serotypes 7 and 8

OBJECTIVE

Using intravenous injection of adeno-associated viral (AAV) vectors based on novel serotypes 7 and 8, we examined whether liver-specific expression of human apolipoprotein E (apoE) in apoE-deficient mice would completely prevent atherosclerosis after 1 year of sustained expression.

METHODS AND RESULTS

Chow-fed apoE-/- mice were injected via the tail vein with vectors based on AAV2 or novel serotypes AAV7 and AAV8 encoding human apoE3 driven by a liver-specific promoter. In contrast to the first-generation AAV2 vector, apoE levels of mice injected with chimeric AAV2/7 and AAV2/8 vectors reached approximately 2-fold greater than normal human plasma levels by week 4 and maintained therapeutic levels up to 1 year. Cholesterol levels of AAV2/7-apoE and AAV2/8-apoE-treated mice were reduced to normal murine wild-type levels and were maintained for 1 year. At termination after 1 year, extensive atherosclerosis was present in the thoracic aortas and aortic roots of control AAV2/8-lacZ and AAV2-apoE-injected mice, but was completely prevented in both the AAV2/7 and AAV2/8-apoE-treated mice.

CONCLUSIONS

We demonstrate that intravenous administration of AAV2/7- and AAV2/8-apoE vectors effectively mediated robust and sustained hepatic-specific expression of apoE and completely prevented atherosclerosis at 1 year.

Adeno-associated virus vectors: potential applications for cancer gene therapy

Augmenting cancer treatment by protein and gene delivery continues to gain momentum based on success in animal models. The primary hurdle of fully exploiting the arsenal of molecular targets and therapeutic transgenes continues to be efficient delivery. Vectors based on adeno-associated virus (AAV) are of particular interest as they are capable of inducing transgene expression in a broad range of tissues for a relatively long time without stimulation of a cell-mediated immune response. Perhaps the most important attribute of AAV vectors is their safety profile in phase I clinical trials ranging from CF to Parkinson's disease. The utility of AAV vectors as a gene delivery agent in cancer therapy is showing promise in preclinical studies. In this review, we will focus on the basic biology of AAV as well as recent progress in the use of this vector in cancer gene therapy.

Genetic medicines: treatment strategies for hereditary disorders

The treatment of the more than 1,800 known monogenic hereditary disorders will depend on the development of 'genetic medicines' - therapies that use the transfer of DNA and/or RNA to modify gene expression to correct or compensate for an abnormal phenotype. Strategies include the use of somatic stem cells, gene transfer, RNA modification and, in the future, embryonic stem cells. Despite the efficacy of these technologies in treating experimental models of hereditary disorders, applying them successfully in the clinic is a great challenge, which will only be overcome by expending considerable intellectual and economic resources, and by solving societal concerns about modifications of the human genetic repertoire.

Biological effects of rAAV-caAlk2 coating on structural allograft healing

Structural bone allografts often fracture due to their lack of osteogenic and remodeling potential. To overcome these limitations, we utilized allografts coated with recombinant adeno-associated virus (rAAV) that mediate in vivo gene transfer. Using beta-galactosidase as a reporter gene, we show that 4-mm murine femoral allografts coated with rAAV-LacZ are capable of transducing adjacent inflammatory cells and osteoblasts in the fracture callus following transplantation. While this LacZ vector had no effect on allograft healing, bone morphogenetic protein signals delivered via rAAV-caAlk2 coating induced endochondral bone formation directly on the cortical surface of the allograft by day 14. By day 28 there was evidence of remodeling of the new woven bone and massive osteoclastic resorption of the cortical surface of the rAAV-caAlk2-coated allografts only. Micro-CT analysis of rAAV-LacZ- vs rAAV-caAlk2-coated allografts after 42 days of healing demonstrated a significant increase in new bone formation (0.67 +/- 0.21 vs 2.49 +/- 0.40 mm(3); P < 0.005). Furthermore, the 3D micro-CT images of femurs grafted with rAAV-Alk2-coated allografts provided the first evidence that complete bridging of bone around a cortical allograft is possible. These results indicate that cell-free, rAAV-coated allografts have the potential to revitalize in vivo following transplantation.

Altering AAV tropism with mosaic viral capsids

Over the past decade, AAV-based vectors have emerged as promising candidates for gene therapeutic applications. Despite the broad tropism of the first eight serotypes identified, certain cell types are refractory to transduction with AAV-based vectors. Furthermore, for certain applications the targeting of specific cell types is desirable. To improve on present methods to alter AAV2 tropism, we take advantage of AAV2 mosaics. Here, we show that AAV2 mosaics have improved infectivity compared with all-mutant virions. Using an AAV2 mutant that contains the immunoglobulin-binding Z34C fragment of protein A, we demonstrate the utility of AAV2 mosaics to alter AAV2 tropism. This system allows us to transduce selectively and efficiently MO7e and Jurkat cells. The use of AAV2 mosaics with a protein A fragment inserted into their capsid, together with targeting antibodies, is a versatile method that allows the specific transduction of a wide array of cell types.

Adeno-associated virus-mediated gene transfer to hair cells and support cells of the murine cochlea

More than 28 million Americans suffer from various forms of hearing loss. The lack of effective treatments for many forms of hearing disorders has prompted interest in the potential application of gene delivery techniques to treat both inherited and pathological hearing disorders. However, to develop a gene therapy strategy that will successfully treat hearing disorders, appropriate vectors that are capable of transducing cochlear hair cells and support cells must be identified. In the present study, we examined the efficiency with which AAV vectors (serotypes 1, 2, and 5) transduce hair cells and support cells in cochlear explants from P0 and E13 mice. We further examined the ability of the CBA and GFAP promoters to drive expression of a GFP marker gene in hair cells and support cells. Robust GFP expression was observed in hair cells and support cells following transduction of primary murine cochlear explants with AAV serotypes 1 and 2, but not serotype 5. The CBA promoter predominantly drove GFP expression in hair cells. In contrast, strong expression from the GFAP promoter was observed primarily in support cells. Thus, using AAV vectors and specific promoters, cell-type-specific expression of transgenes can be established within the cochlea.

Custom adeno-associated virus capsids: the next generation of recombinant vectors with novel tropism

Recombinant gene delivery vehicles based on adeno-associated virus (rAAV) have emerged as promising vectors for the correction of genetic and acquired human disease states. These vectors possess many characteristics, including low pathogenicity and immunogenicity, and long-term gene expression after a single administered dose, that make them leading candidates for clinical gene therapy applications. Yet, the broad tissue tropism of the available AAV serotypes remains a disadvantage for the safest, most effective in vivo delivery of transgenes to target tissues. In addition, clinically relevant cell types exist that are poorly transduced by current rAAV vectors. As a result, increased efforts are now being made to tailor the tropism of rAAV to improve their transduction and selectivity profiles. Flexible, diverse methodologies have emerged that allow more control over the cell surface receptors rAAV employs for cell entry. These novel rAAV production strategies have resulted in unique vectors characterized by unique capsid protein sequences that employ alternative receptors, and have provided a better understanding of many basic aspects of the rAAV life cycle. This review aims to summarize the genetic methods currently being employed to customize rAAV capsids.

One year transgene expression with adeno-associated virus cardiac gene transfer

BACKGROUND

Adeno-associated virus (AAV) has shown promise as a vector for cardiac gene transfer given its ability to stably integrate into the host genome and its lack of immune reactivity. This study examined the feasibility of AAV-mediated myocardial gene transfer in mice, the animal which, because of transgenic technology, has become the disease model of choice for cardiovascular research.

METHODS

AAV encoding the cytomegalovirus promoter driven LacZ reporter gene (10(7) LacZ-forming units per animal) or vehicle control was injected into the hearts of young adult C57Bl/6 mice by a transdiaphragmatic approach. At one, two, three, six, and twelve months post-injection, cardiac function was assessed by transthoracic echocardiography and hearts were assayed by X-gal histochemical staining.

RESULTS

Echocardiography revealed normal left ventricular function in both AAV and control groups at all time points. X-gal staining of cryostat sections of hearts revealed uniform LacZ expression at all time points. There were minimal signs of immunologic infiltration by hematoxylin and eosin staining.

CONCLUSIONS

AAV-mediated myocardial gene transfer by transdiaphragmatic injection can be conducted safely and results in long-term expression of the LacZ gene for at least one year without causing significant inflammatory response or adversely affecting LV systolic function.

A molecular and genetic arsenal for systems neuroscience

Neural circuits are composed of a meshwork of numerous neuron types, each with their own distinctive morphological and intrinsic physiological properties, connectivity and biochemistry. How do distinct neural subcircuits, composed of different classes of neuron, contribute to brain function? Approaching this question requires methods that can target specific neurons types. This can be achieved by harnessing the same machinery that builds sophistication into the brain and using it to make novel tools for investigating and manipulating the brain: molecular and genetic technology. These tools can be used to target gene expression to specific neuron types within complicated neuronal circuits, and the transgenes that are expressed can be used to elucidate and manipulate these circuits with unprecedented precision and control. These methods are likely to become the archetype for future studies linking perception, cognition and behavior to specific components of the brain.

Adeno-associated virus: from defective virus to effective vector

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

Remodeling of cortical bone allografts mediated by adherent rAAV-RANKL and VEGF gene therapy

Structural allograft healing is limited because of a lack of vascularization and remodeling. To study this we developed a mouse model that recapitulates the clinical aspects of live autograft and processed allograft healing. Gene expression analyses showed that there is a substantial decrease in the genes encoding RANKL and VEGF during allograft healing. Loss-of-function studies showed that both factors are required for autograft healing. To determine whether addition of these signals could stimulate allograft vascularization and remodeling, we developed a new approach in which rAAV can be freeze-dried onto the cortical surface without losing infectivity. We show that combination rAAV-RANKL- and rAAV-VEGF-coated allografts show marked remodeling and vascularization, which leads to a new bone collar around the graft. In conclusion, we find that RANKL and VEGF are necessary and sufficient for efficient autograft remodeling and can be transferred using rAAV to revitalize structural allografts.

Adeno-associated virus serotypes 1 to 5 mediated tumor cell directed gene transfer and improvement of transduction efficiency

BACKGROUND

Gene therapy is an attractive new approach for the treatment of cancer. Therefore, the development of efficient vector systems is of crucial importance in this field. Different adeno-associated virus (AAV) serotypes have been characterized so far, which show considerable differences in tissue tropism. Consequently, we aimed to characterize the most efficient serotype for this application.

METHODS

To exclude all influences other than those provided by the capsid, all serotypes contained the same transgene cassette flanked by the AAV2 inverted terminal repeats. We systematically compared these vectors for efficiency in human cancer cell directed gene transfer. In order to identify limiting steps, the influence of second-strand synthesis and proteasomal degradation of AAV in a poorly transducible cell line were examined.

RESULTS

AAV2 was the most efficient serotype in all solid tumor cells and primary melanoma cells with transduction rates up to 98 +/- 0.3%. Transduction above 70% could be reached with serotypes 1 (in cervical and prostate carcinoma) and 3 (in cervical, breast, prostate and colon carcinoma) using 1000 genomic particles per cell. In the colon carcinoma cell line HT-29 proteasomal degradation limited AAV1-AAV4-mediated gene transfer. Moreover, inefficient second-strand synthesis prevents AAV2-mediated transgene expression in this cell line.

CONCLUSIONS

Recent advances in AAV-vector technology suggest that AAV-based vectors can be used for cancer gene therapy. Our comparative analysis revealed that, although AAV2 is the most promising candidate for such an application, serotypes 1 and 3 are valid alternatives. Furthermore, the use of self-complementary AAV vectors and proteasome inhibitors significantly improves cancer cell transduction.

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.

Adeno-associated virus-mediated gene transfer of a secreted decoy human macrophage scavenger receptor reduces atherosclerotic lesion formation in LDL receptor knockout mice

Macrophage scavenger receptors (MSR) promote atherosclerotic lesion formation, and modulation of MSR activity has been shown to influence atherosclerosis. Soluble receptors are effective in inhibiting receptor-mediated functions in various diseases. We have generated a secreted macrophage scavenger receptor (sMSR) that consists of the bovine growth hormone signal sequence and the human MSR A I extracellular domains. sMSR reduces degradation of atherogenic modified low-density lipoproteins and monocyte/macrophage adhesion on endothelial cells in vitro. To test long-term effects of sMSR, atherosclerosis-susceptible LDLR knockout mice were transduced via the tail vein with an adeno-associated virus (AAV) expressing sMSR or control enhanced green fluorescent protein (EGFP), and a Western-type diet was started. Gene transfer caused a temporary elevation in alkaline phosphatase and aspartate amino transferase values without a change in C-reactive protein. sMSR protein was detected in the plasma of the transduced mice by a specific ELISA 6 months after the gene transfer. AAV-mediated sMSR gene transfer reduced atherosclerotic lesion area in the aorta by 21% (P < 0.05) compared to EGFP-transduced control mice. Even though eradication of established disease was not possible, atherosclerotic lesion formation could be modified using AAV-mediated gene transfer of the decoy sMSR.

In utero recombinant adeno-associated virus gene transfer in mice, rats, and primates

Background

Gene transfer into the amniotic fluid using recombinant adenovirus vectors was shown previously to result in high efficiency transfer of transgenes into the lungs and intestines. Adenovirus mediated in utero gene therapy, however, resulted in expression of the transgene for less than 30 days. Recombinant adenovirus associated viruses (rAAV) have the advantage of maintaining the viral genome in daughter cells thus providing for long-term expression of transgenes.

Methods

Recombinant AAV2 carrying green fluorescent protein (GFP) was introduced into the amniotic sac of fetal rodents and nonhuman primates. Transgene maintenance and expression was monitor.

Results

Gene transfer resulted in rapid uptake and long-term gene expression in mice, rats, and non-human primates. Expression and secretion of the reporter gene, GFP, was readily demonstrated within 72 hours post-therapy. In long-term studies in rats and nonhuman primates, maintenance of GFP DNA, protein expression, and reporter gene secretion was documented for over one year.

Conclusions

Because only multipotential stem cells are present at the time of therapy, these data demonstrated that in utero gene transfer with AAV2 into stem cells resulted in long-term systemic expression of active transgene roducts. Thus, in utero gene transfer via the amniotic fluid may be useful in treatment of gene disorders.

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.

Gene therapy for neurological disease

Significant advances have been made in the last 20 years in understanding the basic biology of the normal nervous system and in elucidating molecular and cellular mechanisms underlying neurological disease. This progress has generated, for the first time, a realistic possibility of treating what have historically been common and tragically untreatable diseases of the nervous system. In particular, therapeutic delivery of genes to the degenerating, injured or developmentally-deficient nervous system offers the potential to prevent cell death, induce new growth and restore function. Clinical trials of gene therapy are beginning to move forward in several neurological disorders. We have thereby begun the transition to molecular-based medicine which has the potential to alter the landscape and prognosis of neurological disease.

P53-dependent cell-killing by selective repression of thymidine kinase and reduced prodrug activation

Selective killing of tumor cells is an important goal for cancer therapeutics. The tumor suppressor transcription factor p53 is absent or mutated in more than 50% of human tumors. Thus, determining approaches that use p53 status to regulate therapy may be an important strategy for attaining cancer selectivity. We have shown previously that a designed transcriptional repressor, K2-5F, strongly and selectively reduces the expression of its target gene MDR1. In this study, we exploited p53 status and the strong repressor activity of K2-5F to establish a system for preferential killing of p53-negative cells. In this system, the expression of K2-5F is induced by p53 in normal cells, and the K2-5F repressor then inhibits the expression of herpes simplex virus thymidine kinase (HSV-TK) driven by an MDR1 minipromoter. In p53-deficient cells, little K2-5F is expressed, and thus HSV-TK is expressed, allowing the cells to be killed by ganciclovir (GCV). K2-5F induced by exogenous p53 dramatically reduced the expression of HSV-TK in human embryonic kidney 293 cells, and it subsequently increased cell survival in response to GCV. To further evaluate this approach in a uniform genetic background, we developed Saos-2 cells stably expressing physiological levels of p53 and paired them with wild-type p53-negative Saos-2 cells. Stable expression of moderate levels of p53 in Saos-2 cells was able to induce the expression of K2-5F and reduce HSV-TK expression and resulted in a modest but distinct protection from GCV toxicity. Thus, this system may be suitable for further development as an approach to selective cancer therapy.

Receptor targeting of adeno-associated virus vectors

Adeno-associated virus (AAV) is a promising vector for human somatic gene therapy. However, its broad host range is a disadvantage for in vivo gene therapy, because it does not allow the selective tissue- or organ-restricted transduction required to enhance the safety and efficiency of the gene transfer. Therefore, increasing efforts are being made to target AAV-2-based vectors to specific receptors. The studies summarized in this review show that it is possible to target AAV-2 to a specific cell. So far, the most promising approach is the genetic modification of the viral capsid. However, the currently available AAV-2 targeting vectors need to be improved with regard to the elimination of the wild-type AAV-2 tropism and the improvement of infectious titers. The creation of highly efficient AAV-2 targeting vectors will also require a better understanding of the transmembrane and intracellular processing of this virus.