Targeted adeno-associated virus vector transduction of nonpermissive cells mediated by a bispecific F(ab'gamma)2 antibody

AAV vectors: The Rubik's cube of human gene therapy

Defective genes account for ∼80% of the total of more than 7,000 diseases known to date. Gene therapy brings the promise of a one-time treatment option that will fix the errors in patient genetic coding. Recombinant viruses are highly efficient vehicles for in vivo gene delivery. Adeno-associated virus (AAV) vectors offer unique advantages, such as tissue tropism, specificity in transduction, eliciting of a relatively low immune responses, no incorporation into the host chromosome, and long-lasting delivered gene expression, making them the most popular viral gene delivery system in clinical trials, with three AAV-based gene therapy drugs already approved by the US Food and Drug Administration (FDA) or European Medicines Agency (EMA). Despite the success of AAV vectors, their usage in particular scenarios is still limited due to remaining challenges, such as poor transduction efficiency in certain tissues, low organ specificity, pre-existing humoral immunity to AAV capsids, and vector dose-dependent toxicity in patients. In the present review, we address the different approaches to improve AAV vectors for gene therapy with a focus on AAV capsid selection and engineering, strategies to overcome anti-AAV immune response, and vector genome design, ending with a glimpse at vector production methods and the current state of recombinant AAV (rAAV) at the clinical level.

Adeno-Associated Virus Vector Design-Moving the Adeno-Associated Virus to a Bioengineered Therapeutic Nanoparticle

Although the number of market-approved gene therapies is still low, this new class of therapeutics has become an integral part of modern medicine. The success and safety of gene therapy depend on the vectors used to deliver the therapeutic material. Adeno-associated virus (AAV) vectors have emerged as the most frequently used delivery system for in vivo gene therapy. This success was achieved with first-generation vectors, using capsids derived from natural AAV serotypes. Their broad tropism, the high seroprevalence for many of the AAV serotypes in the human population, and the high vector doses needed to transduce a sufficient number of therapy-relevant target cells are challenges that are addressed by engineering the capsid and the vector genome, improving the efficacy of these biological nanoparticles.

Fantastic AAV Gene Therapy Vectors and How to Find Them—Random Diversification, Rational Design and Machine Learning

Parvoviruses are a diverse family of small, non-enveloped DNA viruses that infect a wide variety of species, tissues and cell types. For over half a century, their intriguing biology and pathophysiology has fueled intensive research aimed at dissecting the underlying viral and cellular mechanisms. Concurrently, their broad host specificity (tropism) has motivated efforts to develop parvoviruses as gene delivery vectors for human cancer or gene therapy applications. While the sum of preclinical and clinical data consistently demonstrates the great potential of these vectors, these findings also illustrate the importance of enhancing and restricting in vivo transgene expression in desired cell types. To this end, major progress has been made especially with vectors based on Adeno-associated virus (AAV), whose capsid is highly amenable to bioengineering, repurposing and expansion of its natural tropism. Here, we provide an overview of the state-of-the-art approaches to create new AAV variants with higher specificity and efficiency of gene transfer in on-target cells. We first review traditional and novel directed evolution approaches, including high-throughput screening of AAV capsid libraries. Next, we discuss programmable receptor-mediated targeting with a focus on two recent technologies that utilize high-affinity binders. Finally, we highlight one of the latest stratagems for rational AAV vector characterization and optimization, namely, machine learning, which promises to facilitate and accelerate the identification of next-generation, safe and precise gene delivery vehicles.

Parvovirus Vectors: The Future of Gene Therapy

The unique diversity of parvoviral vectors with innate antioncogenic properties, autonomous replication, ease of recombinant vector production and stable transgene expression in target cells makes them an attractive choice as viral vectors for gene therapy protocols. Amongst various parvoviruses that have been identified so far, recombinant vectors originating from adeno-associated virus, minute virus of mice (MVM), LuIII and parvovirus H1 have shown promising results in many preclinical models of human diseases including cancer. The adeno-associated virus (AAV), a non-pathogenic human parvovirus, has gained attention as a potentially useful vector. The improved understanding of the metabolism of vector genomes and the mechanism of transduction by AAV vectors is leading to advancement in the development of more sophisticated AAV vectors. The in-depth studies of AAV vector biology is opening avenues for more robust design of AAV vectors that have potentially increased transduction efficiency, increased specificity in cellular targeting, and an increased payload capacity. This chapter gives an overview of the application of autonomous parvoviral vectors and AAV vectors, based on our current understanding of viral biology and the state of the platform.

Deep Parallel Characterization of AAV Tropism and AAV-Mediated Transcriptional Changes via Single-Cell RNA Sequencing

Engineered variants of recombinant adeno-associated viruses (rAAVs) are being developed rapidly to meet the need for gene-therapy delivery vehicles with particular cell-type and tissue tropisms. While high-throughput AAV engineering and selection methods have generated numerous variants, subsequent tropism and response characterization have remained low throughput and lack resolution across the many relevant cell and tissue types. To fully leverage the output of these large screening paradigms across multiple targets, we have developed an experimental and computational single-cell RNA sequencing (scRNA-seq) pipeline for in vivo characterization of barcoded rAAV pools at high resolution. Using this platform, we have both corroborated previously reported viral tropisms and discovered unidentified AAV capsid targeting biases. As expected, we observed that the tropism profile of AAV.CAP-B10 in mice was shifted toward neurons and away from astrocytes when compared with AAV-PHP.eB. Transcriptomic analysis revealed that this neuronal bias is due mainly to increased targeting efficiency for glutamatergic neurons, which we confirmed by RNA fluorescence in situ hybridization. We further uncovered cell subtype tropisms of AAV variants in vascular and glial cells, such as low transduction of pericytes and Myoc+ astrocytes. Additionally, we have observed cell-type-specific transitory responses to systemic AAV-PHP.eB administration, such as upregulation of genes involved in p53 signaling in endothelial cells three days post-injection, which return to control levels by day twenty-five. The presented experimental and computational approaches for parallel characterization of AAV tropism will facilitate the advancement of safe and precise gene delivery vehicles, and showcase the power of understanding responses to gene therapies at the single-cell level.

Capsid-Engineering for Central Nervous System-Directed Gene Therapy with Adeno-Associated Virus Vectors

Closing the gap in knowledge on the cause of neurodegenerative disorders is paving the way toward innovative treatment strategies, among which gene therapy has emerged as a top candidate. Both conventional gene therapy and genome editing approaches are being developed, and a great number of human clinical trials are ongoing. Already 2 years ago, the first gene therapy for a neurodegenerative disease, spinal muscular atrophy type 1 (SMA1), obtained market approval. To realize such innovative strategies, gene therapy delivery tools are key assets. Here, we focus on recombinant adeno-associated virus (AAV) vectors and report on strategies to improve first-generation vectors. Current efforts focus on the viral capsid to modify the host-vector interaction aiming at increasing the efficacy of target cell transduction, at simplifying vector administration, and at reducing the risk of vector dose-related side effects.

Development of a Bispecific Antibody-Based Platform for Retargeting of Capsid Modified AAV Vectors

A major limiting factor for systemically delivered gene therapies is the lack of novel tissue specific AAV (Adeno-associated virus) derived vectors. Bispecific antibodies can be used to redirect AAVs to specific target receptors. Here, we demonstrate that the insertion of a short linear epitope “2E3” derived from human proprotein-convertase subtilisin/kexin type 9 (PCSK9) into different surface loops of the VP capsid proteins can be used for AAV de-targeting from its natural receptor(s), combined with a bispecific antibody-mediated retargeting. We chose to target a set of distinct disease relevant membrane proteins—fibroblast activation protein (FAP), which is upregulated on activated fibroblasts within the tumor stroma and in fibrotic tissues, as well as programmed death-ligand 1 (PD-L1), which is strongly upregulated in many cancers. Upon incubation with a bispecific antibody recognizing the 2E3 epitope and FAP or PD-L1, the bispecific antibody/rAAV complex was able to selectively transduce receptor positive cells. In summary, we developed a novel, rationally designed vector retargeting platform that can target AAVs to a new set of cellular receptors in a modular fashion. This versatile platform may serve as a valuable tool to investigate the role of disease relevant cell types and basis for novel gene therapy approaches.

AAV Targeting of Glial Cell Types in the Central and Peripheral Nervous System and Relevance to Human Gene Therapy

Different glial cell types are found throughout the central (CNS) and peripheral nervous system (PNS), where they have important functions. These cell types are also involved in nervous system pathology, playing roles in neurodegenerative disease and following trauma in the brain and spinal cord (astrocytes, microglia, oligodendrocytes), nerve degeneration and development of pain in peripheral nerves (Schwann cells, satellite cells), retinal diseases (Müller glia) and gut dysbiosis (enteric glia). These cell type have all been proposed as potential targets for treating these conditions. One approach to target these cell types is the use of gene therapy to modify gene expression. Adeno-associated virus (AAV) vectors have been shown to be safe and effective in targeting cells in the nervous system and have been used in a number of clinical trials. To date, a number of studies have tested the use of different AAV serotypes and cell-specific promoters to increase glial cell tropism and expression. However, true glial-cell specific targeting for a particular glial cell type remains elusive. This review provides an overview of research into developing glial specific gene therapy and discusses some of the issues that still need to be addressed to make glial cell gene therapy a clinical reality.

Towards Clinical Implementation of Adeno-Associated Virus (AAV) Vectors for Cancer Gene Therapy: Current Status and Future Perspectives

Adeno-associated virus (AAV) vectors have gained tremendous attention as in vivo delivery systems in gene therapy for inherited monogenetic diseases. First market approvals, excellent safety data, availability of large-scale production protocols, and the possibility to tailor the vector towards optimized and cell-type specific gene transfer offers to move from (ultra) rare to common diseases. Cancer, a major health burden for which novel therapeutic options are urgently needed, represents such a target. We here provide an up-to-date overview of the strategies which are currently developed for the use of AAV vectors in cancer gene therapy and discuss the perspectives for the future translation of these pre-clinical approaches into the clinic.

Multiplexed Cre-dependent selection yields systemic AAVs for targeting distinct brain cell types

Recombinant adeno-associated viruses (rAAVs) are efficient gene delivery vectors via intravenous delivery; however, natural serotypes display a finite set of tropisms. To expand their utility, we evolved AAV capsids to efficiently transduce specific cell types in adult mouse brains. Building upon our Cre-recombination-based AAV targeted evolution (CREATE) platform, we developed Multiplexed-CREATE (M-CREATE) to identify variants of interest in a given selection landscape through multiple positive and negative selection criteria. M-CREATE incorporates next-generation sequencing, synthetic library generation and a dedicated analysis pipeline. We have identified capsid variants that can transduce the central nervous system broadly, exhibit bias toward vascular cells and astrocytes, target neurons with greater specificity or cross the blood-brain barrier across diverse murine strains. Collectively, the M-CREATE methodology accelerates the discovery of capsids for use in neuroscience and gene-therapy applications.

Site-Specific Post-translational Surface Modification of Adeno-Associated Virus Vectors Using Leucine Zippers

Adeno-associated virus (AAV) is widely favored as a gene therapy vector, tested in over 200 clinical trials internationally. To improve targeted delivery a variety of genetic capsid modifications, such as insertion of targeting proteins/peptides into the capsid shell, have been explored with some success but larger insertions often have unpredictable deleterious impacts on capsid formation and gene delivery. Here, we demonstrate a modular platform for the integration of exogenous peptides and proteins onto the AAV capsid post-translationally while preserving vector functionality. We decorated the AAV capsid with leucine-zipper coiled-coil binding motifs that exhibit specific noncovalent heterodimerization. AAV capsids successfully display hexahistidine tagged-peptides using this approach, as demonstrated through nickel column affinity. This protein display platform may facilitate the incorporation of biological moieties on the AAV surface, expanding possibilities for vector enhancement and engineering.

rAAV-Mediated Cochlear Gene Therapy: Prospects and Challenges for Clinical Application

Over the last decade, pioneering molecular gene therapy for inner-ear disorders have achieved experimental hearing improvements after a single local or systemic injection of adeno-associated, virus-derived vectors (rAAV for recombinant AAV) encoding an extra copy of a normal gene, or ribozymes used to modify a genome. These results hold promise for treating congenital or later-onset hearing loss resulting from monogenic disorders with gene therapy approaches in patients. In this review, we summarize the current state of rAAV-mediated inner-ear gene therapies including the choice of vectors and delivery routes, and discuss the prospects and obstacles for the future development of efficient clinical rAAV-mediated cochlear gene medicine therapy.

Efficient and Highly Specific Gene Transfer Using Mutated Lentiviral Vectors Redirected with Bispecific Antibodies

The goal of gene therapy is specific delivery and expression of therapeutic genes to target cells and tissues. Common lentivirus (LV) vectors are efficient gene delivery vehicles but offer little specificity. Here, we report an effective and versatile strategy to redirect LV to target cells using bispecific antibodies (bsAbs) that bind both cell receptors and LV envelope domains. Importantly, we ablated the native receptor binding of LV to minimize off-target transduction. Coupling bsAb specificity and ablated native LV tropism synergistically enhanced the selectivity of our targeted gene delivery system. The modular nature of our bsAb-based redirection enables facile targeting of the same LV to diverse tissues/cells. By abrogating the native broad tropism of LV, our bsAb-LV redirection strategy may enable lentivirus-based gene delivery in vivo, expanding the current use of LV beyond ex vivo applications.

Despite their exceptional potencies, the broad tropism of most commonly used lentivirus (LV) vectors limits their use for targeted gene delivery in vivo. We hypothesized that we could improve the specificity of LV targeting by coupling (i) reduction of their binding to off-target cells with (ii) redirection of the vectors with a bispecific antibody (bsAb) that binds both LV and receptors on target cells. As a proof of concept, we pseudotyped nonreplicating LV using a mutated Sindbis envelope (mSindbis) with ablated binding to native receptors, while retaining the capacity to facilitate efficient fusion and endosomal escape. We then evaluated the transduction potencies of the mSindbis LV for HER2-positive (HER2⁺) (SKBR3) breast and HER2-negative (HER2⁻) (A2780) cells when redirected with different bsAbs. mSindbis LV alone failed to induce appreciable green fluorescent protein (GFP) expression in either cell. When mixed with HER2-targeting bsAb, mSindbis LV was exceptionally potent, transducing 12% to 16% of the SKBR3 cells at a multiplicity of infection (MOI [ratio of viral genome copies to target cells]) of 3. Transduction was highly specific, resulting in ∼50-fold-greater selectivity toward SKBR3 cells versus A2780 cells. Redirecting mSindbis LV led to a 10-fold improvement in cell-specific targeting compared to redirecting wild-type Sindbis LV with the same bsAb, underscoring the importance of ablating native virus tropism in order to maximize targeting specificity. The redirection of mutated LV using bsAb represents a potent and highly versatile platform for targeted gene therapy.

Nanobody-Enhanced Targeting of AAV Gene Therapy Vectors

A limiting factor for the use of adeno-associated viruses (AAVs) as vectors in gene therapy is the broad tropism of AAV serotypes, i.e., the parallel infection of several cell types. Nanobodies are single immunoglobulin variable domains from heavy chain antibodies that naturally occur in camelids. Their small size and high solubility allow easy reformatting into fusion proteins. Herein we show that a membrane protein-specific nanobody can be inserted into a surface loop of the VP1 capsid protein of AAV2. Using three structurally distinct membrane proteins—a multispan ion channel, a single-span transmembrane protein, and a glycosylphosphatidylinositol (GPI)-anchored ectoenzyme—we show that this strategy can dramatically enhance the transduction of specific target cells by recombinant AAV2. Moreover, we show that the nanobody-VP1 fusion of AAV2 can be incorporated into the capsids of AAV1, AAV8, and AAV9 and thereby effectively redirect the target specificity of other AAV serotypes. Nanobody-mediated targeting provides a highly efficient AAV targeting strategy that is likely to open up new avenues for genetic engineering of cells.

A CD33 Antigen-Targeted AAV6 Vector Expressing an Inducible Caspase-9 Suicide Gene Is Therapeutic in a Xenotransplantation Model of Acute Myeloid Leukemia

Current chemotherapeutic regimens for acute myeloid leukemia (AML) have been modestly effective in patients and are associated with poor long-term survival (<30% at 5 years). Viral vector-based suicide gene therapy is an attractive option, if these vectors can target the AML cells with high specificity and efficiency. In this study, we have developed a receptor-specific adeno-associated virus (AAV) based vector to target the CD33 antigen which is overexpressed in leukemic cells. A targeting peptide was rationally designed from the antigen-binding regions of a CD33 monoclonal antibody. This peptide was further expressed on the capsid of the AAV6 vector, since this serotype was most efficient among AAV1-rh10 vectors to infect the pro-monocytic, human myeloid leukemia cells (U937). AAV6-CD33 vectors expressing a suicide gene, the inducible caspase 9 (iCasp9), and its prodrug AP20187 significantly reduced (∼59%) the viability of U937 cells. To further test its efficacy and specificity in vivo, AAV6-CD33 vectors were administered into a xenotransplantation model of AML in zebrafish through systemic delivery. We observed a significant antileukemic effect with AAV6-CD33 vectors, with a markedly higher survival (100% for AAV6-CD33 vectors vs 15% for mock-treated) and a higher number of TUNEL positive apoptotic cells after systemic vector delivery. Taken together, our work demonstrates the efficacy and translational potential of CD33-targeted AAV6 vectors for cytotoxic gene therapy in AML.

Polarized AAVR expression determines infectivity by AAV gene therapy vectors

Adeno-associated virus (AAV) has been investigated to transfer the cystic fibrosis transmembrane conductance regulator (CFTR) to airways. Inhaled AAV2-CFTR in people with cystic fibrosis (CF) is safe, but inefficient. In vitro, AAV2 transduction of human airway epithelia on the apical (luminal) side is inefficient, but efficient basolaterally. We previously selected AAV2.5T, a novel capsid that apically transduces CF human airway epithelia and efficiently restores CFTR function. We hypothesize the AAV receptor (AAVR) is basolaterally localized, and that AAV2.5T utilizes an alternative apical receptor. We found AAVR in human airway epithelia by western blot and RNA-Seq analyses. Using immunocytochemistry we did not find endogenous AAVR at membranes but overexpression localized AAVR to the basolateral membrane, where it preferentially increased transduction. Anti-AAVR antibodies blocked transduction by AAV2 from the basolateral side but not AAV2.5T from the apical side, suggesting a unique apical receptor. Finally, we found infection by AAV2 but not AAV2.5T was blocked by CRISPR knockout of AAVR in cell lines. Our data suggest the absence of apical AAVR is rate limiting for AAV2, and efficient transduction by AAV2.5T is accomplished using an AAVR independent pathway. Our findings inform the development of gene therapy for CF, and AAV vectors in general.

Codon-Optimization of Wild-Type Adeno-Associated Virus Capsid Sequences Enhances DNA Family Shuffling while Conserving Functionality

Adeno-associated virus (AAV) vectors have become one of the most widely used gene transfer tools in human gene therapy. Considerable effort is currently being focused on AAV capsid engineering strategies with the aim of developing novel variants with enhanced tropism for specific human cell types, decreased human seroreactivity, and increased manufacturability. Selection strategies based on directed evolution rely on the generation of highly variable AAV capsid libraries using methods such as DNA-family shuffling, a technique reliant on stretches of high DNA sequence identity between input parental capsid sequences. This identity dependence for reassembly of shuffled capsids is inherently limiting and results in decreased shuffling efficiency as the phylogenetic distance between parental AAV capsids increases. To overcome this limitation, we have developed a novel codon-optimization algorithm that exploits evolutionarily defined codon usage at each amino acid residue in the parental sequences. This method increases average sequence identity between capsids, while enhancing the probability of retaining capsid functionality, and facilitates incorporation of phylogenetically distant serotypes into the DNA-shuffled libraries. This technology will help accelerate the discovery of an increasingly powerful repertoire of AAV capsid variants for cell-type and disease-specific applications.

Adeno-Associated Virus Vectors and Stem Cells: Friends or Foes?

The infusion of healthy stem cells into a patient-termed "stem-cell therapy"-has shown great promise for the treatment of genetic and non-genetic diseases, including mucopolysaccharidosis type 1, Parkinson's disease, multiple sclerosis, numerous immunodeficiency disorders, and aplastic anemia. Stem cells for cell therapy can be collected from the patient (autologous) or collected from another "healthy" individual (allogeneic). The use of allogenic stem cells is accompanied with the potentially fatal risk that the transplanted donor T cells will reject the patient's cells-a process termed "graft-versus-host disease." Therefore, the use of autologous stem cells is preferred, at least from the immunological perspective. However, an obvious drawback is that inherently as "self," they contain the disease mutation. As such, autologous cells for use in cell therapies often require genetic "correction" (i.e., gene addition or editing) prior to cell infusion and therefore the requirement for some form of nucleic acid delivery, which sets the stage for the AAV controversy discussed herein. Despite being the most clinically applied gene delivery context to date, unlike other more concerning integrating and non-integrating vectors such as retroviruses and adenovirus, those based on adeno-associated virus (AAV) have not been employed in the clinic. Furthermore, published data regarding AAV vector transduction of stem cells are inconsistent in regards to vector transduction efficiency, while the pendulum swings far in the other direction with demonstrations of AAV vector-induced toxicity in undifferentiated cells. The variation present in the literature examining the transduction efficiency of AAV vectors in stem cells may be due to numerous factors, including inconsistencies in stem-cell collection, cell culture, vector preparation, and/or transduction conditions. This review summarizes the controversy surrounding AAV vector transduction of stem cells, hopefully setting the stage for future elucidation and eventual therapeutic applications.

Enhanced Targeted Gene Transduction: AAV2 Vectors Conjugated to Multiple Aptamers via Reducible Disulfide Linkages

Enhanced targeted gene transduction by AAV2 vectors is achieved by linking the vector to multiple sgc8 aptamers, which are selective for cell membrane protein PTK7. Aptamer molecules are conjugated to multiple sites on a DNA dendrimer (G-sgc8), which is then linked to AAV2 via a dithiobis(succinimidyl propionate) cross-linker containing a disulfide group, which can facilitate the release of AAV2 vectors by reaction with the reduced form of intracellular glutathione. The G-sgc8-AAV2 vectors showed a 21-fold enhancement in binding affinity and an enhanced ability to protect sgc8 aptamers against nuclease degradation to cells expressing PTK7 compared to single aptamer-AAV2 conjugates. The transduction efficiency was tested by loading AAV2 with the gene for green fluorescent protein. Therefore, this modified recombinant vector is an attractive and promising tool for targeted biomedical applications.

A directed evolution approach to select for novel Adeno-associated virus capsids on an HIV-1 producer T cell line

A directed evolution approach was used to select for Adeno-associated virus (AAV) capsids that would exhibit more tropism toward an HIV-1 producer T cell line with the long-term goal of developing improved gene transfer vectors. A library of AAV variants was used to infect H9 T cells previously infected or uninfected by HIV-1 followed by AAV amplification with wild-type adenovirus. Six rounds of biological selection were performed, including negative selection and diversification after round three. The H9 T cells were successfully infected with all three wild-type viruses (AAV, adenovirus, and HIV-1). Four AAV cap mutants best representing the small number of variants emerging after six rounds of selection were chosen for further study. These mutant capsids were used to package an AAV vector and subsequently used to infect H9 cells that were previously infected or uninfected by HIV-1. A quantitative polymerase chain reaction assay was performed to measure cell-associated AAV genomes. Two of the four cap mutants showed a significant increase in the amount of cell-associated genomes as compared to wild-type AAV2. This study shows that directed evolution can be performed successfully to select for mutants with improved tropism for a T cell line in the presence of HIV-1.

Covalent coupling of high-affinity ligands to the surface of viral vector particles by protein trans-splicing mediates cell type-specific gene transfer

We have established a novel approach for the covalent coupling of large polypeptides to the surface of fully assembled adeno-associated viral gene transfer vector (AAV) particles via split-intein mediated protein-trans-splicing (PTS). This way, we achieved selective gene transfer to distinct cell types. Single-chain variable fragments (scFvs) or designed ankyrin repeat proteins (DARPins), exhibiting high-affinity binding to cell surface receptors selectively expressed on the surface of target cells, were coupled to AAV particles harboring mutations in the capsid proteins which ablate natural receptor usage. Both, the AAV capsid protein VP2 and multiple separately produced targeting ligands recognizing Her2/neu, EpCAM, CD133 or CD30 were genetically fused with complementary split-intein domains. Optimized coupling conditions led to an effective conjugation of each targeting ligand to the universal AAV capsid and translated into specific gene transfer into target receptor-positive cell types in vitro and in vivo. Interestingly, PTS-based AAVs exhibited significantly less gene transfer into target receptor-negative cells than AAVs displaying the same targeting ligand but coupled genetically. Another important consequence of the PTS technology is the possibility to now display scFvs or other antibody-derived domain formats harboring disulfide-bonds in a functionally active form on the surface of AAV particles. Hence, the custom combination of a universal AAV vector particle and targeting ligands of various formats allows for an unprecedented flexibility in the generation of gene transfer vectors targeted to distinct cell types.

Seek and destroy: targeted adeno-associated viruses for gene delivery to hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the most common form of primary liver cancer with high incidence globally. Increasing mortality and morbidity rates combined with limited treatment options available for advanced HCC press for novel and effective treatment modalities. Gene therapy represents one of the most promising therapeutic options. With the recent approval of herpes simplex virus for advanced melanoma, the field of gene therapy has received a major boost. Adeno-associated virus (AAV) is among the most widely used and effective viral vectors today with safety and efficacy demonstrated in a number of human clinical trials. This review identifies the obstacles for effective AAV based gene delivery to HCC which primarily include host immune responses and off-target effects. These drawbacks could be more pronounced for HCC because of the underlying liver dysfunction in most of the patients. We discuss approaches that could be adopted to tackle these shortcomings and manufacture HCC-targeted vectors. The combination of transductional targeting by modifying the vector capsid and transcriptional targeting using HCC-specific promoters has the potential to produce vectors which can specifically seek HCC and deliver therapeutic gene without significant side effects. Finally, the identification of novel HCC-specific ligands and promoters should facilitate and expedite this process.

Gene therapy approaches in the non-human primate model of Parkinson's disease

The field of gene therapy has recently witnessed a number of major conceptual changes. Besides the traditional thinking that comprises the use of viral vectors for the delivery of a given therapeutic gene, a number of original approaches have been recently envisaged, focused on using vectors carrying genes to further modify basal ganglia circuits of interest. It is expected that these approaches will ultimately induce a therapeutic potential being sustained by gene-induced changes in brain circuits. Among others, at present, it is technically feasible to use viral vectors to (1) achieve a controlled release of neurotrophic factors, (2) conduct either a transient or permanent silencing of any given basal ganglia circuit of interest, (3) perform an in vivo cellular reprogramming by promoting the conversion of resident cells into dopaminergic-like neurons, and (4) improving levodopa efficacy over time by targeting aromatic L-amino acid decarboxylase. Furthermore, extensive research efforts based on viral vectors are currently ongoing in an attempt to better replicate the dopaminergic neurodegeneration phenomena inherent to the progressive intraneuronal aggregation of alpha-synuclein. Finally, a number of incoming strategies will soon emerge over the horizon, these being sustained by the underlying goal of promoting alpha-synuclein clearance, such as, for instance, gene therapy initiatives based on increasing the activity of glucocerebrosidase. To provide adequate proof-of-concept on safety and efficacy and to push forward true translational initiatives based on these different types of gene therapies before entering into clinical trials, the use of non-human primate models undoubtedly plays an instrumental role.

A brain microvasculature endothelial cell-specific viral vector with the potential to treat neurovascular and neurological diseases

Gene therapy critically relies on vectors that combine high transduction efficiency with a high degree of target specificity and that can be administered through a safe intravenous route. The lack of suitable vectors, especially for gene therapy of brain disorders, represents a major obstacle. Therefore, we applied an in vivo screening system of random ligand libraries displayed on adeno‐associated viral capsids to select brain‐targeted vectors for the treatment of neurovascular diseases. We identified a capsid variant showing an unprecedented degree of specificity and long‐lasting transduction efficiency for brain microvasculature endothelial cells as the primary target of selection. A therapeutic vector based on this selected viral capsid was used to markedly attenuate the severe cerebrovascular pathology of mice with incontinentia pigmenti after a single intravenous injection. Furthermore, the versatility of this selection system will make it possible to select ligands for additional in vivo targets without requiring previous identification of potential target‐specific receptors.

Cre-dependent selection yields AAV variants for widespread gene transfer to the adult brain

Recombinant adeno-associated viruses (rAAVs) are commonly used vehicles for in vivo gene transfer. However, the tropism repertoire of naturally occurring AAVs is limited, prompting a search for novel AAV capsids with desired characteristics. Here we describe a capsid selection method, called Cre recombination-based AAV targeted evolution (CREATE), that enables the development of AAV capsids that more efficiently transduce defined Cre-expressing cell populations in vivo. We use CREATE to generate AAV variants that efficiently and widely transduce the adult mouse central nervous system (CNS) after intravenous injection. One variant, AAV-PHP.B, transfers genes throughout the CNS with an efficiency that is at least 40-fold greater than that of the current standard, AAV9 (refs. 14,15,16,17), and transduces the majority of astrocytes and neurons across multiple CNS regions. In vitro, it transduces human neurons and astrocytes more efficiently than does AAV9, demonstrating the potential of CREATE to produce customized AAV vectors for biomedical applications.

Controlled release strategies for rAAV-mediated gene delivery

The development of efficient and safe gene transfer vectors capable of achieving appropriate levels of therapeutic gene expression in a target is one of the most challenging issues in clinical gene therapy. Diverse nonviral and viral gene vehicles have been developed to modify human cells and tissues that may be affected in a variety of diseases, among which the nonpathogenic, effective, and relatively safe recombinant adeno-associated viral (rAAV) vectors that make them a preferred gene delivery system to treat human disorders. Yet, their adapted clinical application is still limited by several hurdles including the presence of immune responses in the host organism and the existence of rate-limiting steps associated with physiological barriers. The use of controlled release strategies to deliver gene vectors such as rAAV may provide powerful tools to enhance the temporal and spatial presentation of therapeutic agents in a defined target and to overcome such obstacles in vivo. The goal of this review is to provide an overview of the most recent advances in gene therapy with a focus on rAAV vectors for clinical translation based on the controlled release from adapted biomaterials as a means to improve the performance of the gene transfer procedure. We also discuss the challenges that remain to be addressed for a safe and efficient adaptation and use of such approaches in the patient.

STATEMENT OF SIGNIFICANCE

The development of effective gene vectors to achieve suitable levels of a therapeutic agent in a target is a critical issue in clinical gene therapy and regenerative medicine. Diverse vehicles are currently available among which the nonpathogenic recombinant adeno-associated virus (rAAV) vectors, a preferred system to effectively treat human disorders. Yet, the clinical use of rAAV is impaired by the host immune responses and by rate-limiting steps of transgene expression. Controlled rAAV delivery systems may provide workable approaches to overcome such obstacles. Here, we give an overview of the most recent advances on the controlled release of vectors with a focus on rAAV using adapted biomaterials and discuss the key challenges for a safe translation in patients.

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.

Adeno-associated virus serotypes for gene therapeutics

Gene transfer vectors based on adeno-associated virus (AAV) are showing exciting therapeutic promise in early phase clinical trials. The ability to cross-package the prototypic AAV2 vector genome into different capsids is a powerful way of conferring novel tropism and biology, with evolving capsid engineering technologies and directed evolution approaches further enhancing the utility and flexibility of these vectors. Novel properties of specific capsids show unpredictable species and cell-type specificity. Therefore, full realisation of the therapeutic potential of AAV vectors requires the development of more therapeutically predictive preclinical methods for evaluating capsid performance. This will strongly complement an iterative approach to the evaluation of capsid variants in the clinic and, should wherever possible, include the determination of gene transfer efficiencies.

Recombinant adeno-associated virus vectors in the treatment of rare diseases

INTRODUCTION

An estimated 25 million Americans are living with rare diseases. Adeno-associated virus (AAV)-mediated gene therapy is an emerging therapeutic option for the more than 7,000 identified rare diseases. This paper highlights the benefits of AAV therapy compared to conventional small molecules, discusses current pre-clinical and clinical applications of AAV-mediated gene therapy, and offers insights into cutting edge research that will shape the future of AAV for broad therapeutic use.

AREAS COVERED

In this review the biology of AAV and our ability to generate disease-specific variants is summarized. Limitations of current therapy are reviewed, with an emphasis on immune detection of virus, viral tropism and tissue targeting, and limitations of gene expression. Information for this review was found using PubMed and clinicaltrials.gov.

EXPERT OPINION

Currently the scope of clinical trials of AAV gene therapy is concentrated in an array of phase I/II safety trials with less than two dozen rare diseases featured. Pre-clinical, translational studies are expanding in number as developments within the last decade have made generation of improved AAV vectors available to more researchers. Further, one bottleneck that is being overcome is the availability of disease models, which will allow for improved preclinical testing and advancement of AAV to more clinical applications.

A viral vector expressing hypoxia-inducible factor 1 alpha inhibits hippocampal neuronal apoptosis

Hypoxia-inducible factor 1 (HIF-1) attenuates amyloid-beta protein neurotoxicity and decreases apoptosis induced by oxidative stress or hypoxia in cortical neurons. In this study, we constructed a recombinant adeno-associated virus (rAAV) vector expressing the human HIF-1α gene (rAAV-HIF-1α), and tested the assumption that rAAV-HIF-1α represses hippocampal neuronal apoptosis induced by amyloid-beta protein. Our results confirmed that rAAV-HIF-1α significantly reduces apoptosis induced by amyloid-beta protein in primary cultured hippocampal neurons. Direct intracerebral rAAV-HIF-1α administration also induced robust and prolonged HIF-1α production in rat hippocampus. Single rAAV-HIF-1α administration resulted in decreased apoptosis of hippocampal neurons in an Alzheimer's disease rat model established by intracerebroventricular injection of aggregated amyloid-beta protein (25-35). Our in vitro and in vivo findings demonstrate that HIF-1 has potential for attenuating hippocampal neuronal apoptosis induced by amyloid-beta protein, and provides experimental support for treatment of neurodegenerative diseases using gene therapy.

Enhancing gene delivery of adeno-associated viruses by cell-permeable peptides

Adeno-associated virus type 2 (AAV2) is considered a promising gene delivery vector and has been extensively applied in several disease models; however, inefficient transduction in various cells and tissues has limited its widespread application in many areas of gene therapy. In this study, we have developed a general, but efficient, strategy to enhance viral transduction, both in vitro and in vivo, by incubating viral particles with cell-permeable peptides (CPPs). We show that CPPs increase internalization of viral particles into cells by facilitating both energy-independent and energy-dependent endocytosis. Moreover, CPPs can significantly enhance the endosomal escape process of viral particles, thus enhancing viral transduction to those cells that have exhibited very low permissiveness to AAV2 infection as a result of impaired intracellular viral processing. We also demonstrated that this approach could be applicable to other AAV serotypes. Thus, the membrane-penetrating ability of CPPs enables us to generate an efficient method for enhanced gene delivery of AAV vectors, potentially facilitating its applicability to human gene therapy.

Ultrasound‑targeted microbubble destruction enhances gene transduction of adeno-associated virus in a less-permissive cell type, NIH/3T3

Adeno‑associated virus (AAV) is a common vector utilized in gene therapy. The NIH/3T3 cell line, which is a potential induced pluripotent stem (iPS) cell type, was identified to be a less-permissive cell type to AAV due to its defective endosomal processing. Ultrasound‑targeted microbubble destruction (UTMD) enhanced the gene transduction of AAV in permissive cells. However, there are no data concerning UTMD enhancement in less-permissive cells, and the exact mechanism of UTMD enhancement in cellular uptake is unclear. Greater knowledge concerning the rate-limiting steps in NIH/3T3 cells would aid in the elucidation of the mechanism of UTMD enhancement in the gene transduction of AAV. In the present study, UTMD enhanced the gene transduction of AAV in NIH/3T3 cells, suggesting that UTMD‑enhanced AAV‑mediated gene transduction may be beneficial for gene therapy in iPS cells. The dose dependence of UTMD enhancement indicated that mechanisms other than sonoporation were involved in the cellular uptake of AAV. However, UTMD did not greatly increase the gene transduction of AAV in NIH/3T3 cells. Additionally, the similar degree of enhancement in the two cell types resulted in no correlation between UTMD and endosomal processing. Future studies on UTMD‑mediated AAV transduction in other non- or less‑permissive cell types may aid in elucidating the exact mechanism of UTMD enhancement in cellular uptake.

Site-specific modification of adeno-associated viruses via a genetically engineered aldehyde tag

As a consequence of their well-defined nanostructure and intrinsic bioactive functionality, virus-based nanoparticles have shown promise for mediating gene delivery. Adeno-associated virus (AAV) nanoparticles, which possess an excellent safety profile and therapeutic potential, hold potential for use in human gene therapy. However, because of their native tropisms, the applicability of AAV nanoparticles is often limited to restricted ranges of cells or tissues. Thus, retargeting AAV particles to the desired cell populations has continued to be a major research focus in many gene therapy applications. In this study, a general strategy is reported for nanoparticle targeting. This involves the site-specific modification of AAV type 2 (AAV2) by genetically incorporating a short peptide, in this case an aldehyde tag, in the viral capsid. Such a tag can be exploited for site-specific attachment of targeting molecules and allows for further introduction of targeting antibodies or ligands. It is shown that this modification neither affects the level of infectious viral titer nor intracellular trafficking properties. Furthermore, the site-specific conjugation of targeting antibodies could significantly enhance viral transduction to those target cells that have otherwise exhibited very low permissiveness to AAV2 infection. This method also allows the functional incorporation of RGD peptides onto AAV2 for enhanced delivery with implications for cancer gene therapy.

Random Insertion of mCherry Into VP3 Domain of Adeno-associated Virus Yields Fluorescent Capsids With no Loss of Infectivity

Adeno-associated virus (AAV)-derived vectors are promising gene delivery systems, and a number of design strategies have been pursued to improve their performance. For example, genetic insertion of proteins into the capsid may be used to achieve vector retargeting, reduced immunogenicity, or to track vector transport. Unfortunately, rational approaches to genetic insertion have experienced limited success due to the unpredictable context-dependent nature of protein folding and the complexity of the capsid's macroassembly. We report the construction and use of a frame-enriched DNase-based random insertion library based on AAV2 cap, called pAAV2_RaPID (Random Peptide Insertion by DNase). The fluorescent mCherry protein was inserted randomly throughout the AAV2 capsid and the library was selected for fluorescent and infectious variants. A capsid site was identified in VP3 that can tolerate the large protein insertion. In contrast to previous efforts to incorporate fluorescent proteins into the AAV2 capsid, the isolated mCherry mutant maintains native infectivity while displaying robust fluorescence. Collectively, these results demonstrate that the pAAV2_RaPID platform library can be used to create fully infectious AAV vectors carrying large functional protein domains on the capsid.Molecular Therapy - Nucleic Acids (2012) 1, e54; doi:10.1038/mtna.2012.46; published online 13 November 2012.

Development and validation of novel AAV2 random libraries displaying peptides of diverse lengths and at diverse capsid positions

Libraries based on the insertion of random peptide ligands into the capsid of adeno-associated virus type 2 (AAV2) have been widely used to improve the efficiency and selectivity of the AAV vector system. However, so far only libraries of 7-mer peptide ligands have been inserted at one well-characterized capsid position. Here, we expanded the combinatorial AAV2 display system to a panel of novel AAV libraries, displaying peptides of 5, 7, 12, 19, or 26 amino acids in length at capsid position 588 or displaying 7-mer peptides at position 453, the most prominently exposed region of the viral capsid. Library selections on two unrelated cell types-human coronary artery endothelial cells and rat cardiomyoblasts-revealed the isolation of cell type-characteristic peptides of different lengths mediating strongly improved target-cell transduction, except for the 26-mer peptide ligands. Characterization of vector selectivity by transduction of nontarget cells and comparative gene-transduction analysis using a panel of 44 human tumor cell lines revealed that insertion of different-length peptides allows targeting of distinct cellular receptors for cell entry with similar efficiency, but with different selectivity. The application of such novel AAV2 libraries broadens the spectrum of targetable receptors by capsid-modified AAV vectors and provides the opportunity to choose the best suited targeting ligand for a certain application from a number of different candidates.

Retargeting of viruses to generate oncolytic agents

Oncolytic virus therapy is based on the ability of viruses to effectively infect and kill tumor cells without destroying the normal tissues. While some viruses seem to have a natural preference for tumor cells, most viruses require the modification of their tropism to specifically enter and replicate in such cells. This review aims to describe the transductional targeting strategies currently employed to specifically redirect viruses towards surface receptors on tumor cells. Three major strategies can be distinguished; they involve (i) the incorporation of new targeting specificity into a viral surface protein, (ii) the incorporation of a scaffold into a viral surface protein to allow the attachment of targeting moieties, and (iii) the use of bispecific adapters to mediate targeting of a virus to a specified moiety on a tumor cell. Of each strategy key features, advantages and limitations are discussed and examples are given. Because of their potential to cause sustained, multiround infection—a desirable characteristic for eradicating tumors—particular attention is given to viruses engineered to become self-targeted by the genomic expression of a bispecific adapter protein.

AAV2-mediated in vivo immune gene therapy of solid tumours

Background

Many strategies have been adopted to unleash the potential of gene therapy for cancer, involving a wide range of therapeutic genes delivered by various methods. Immune therapy has become one of the major strategies adopted for cancer gene therapy and seeks to stimulate the immune system to target tumour antigens. In this study, the feasibility of AAV2 mediated immunotherapy of growing tumours was examined, in isolation and combined with anti-angiogenic therapy.

Methods

Immune-competent Balb/C or C57 mice bearing subcutaneous JBS fibrosarcoma or Lewis Lung Carcinoma (LLC) tumour xenografts respectively were treated by intra-tumoural administration of AAV2 vector encoding the immune up-regulating cytokine granulocyte macrophage-colony stimulating factor (GM-CSF) and the co-stimulatory molecule B7-1 to subcutaneous tumours, either alone or in combination with intra-muscular (IM) delivery of AAV2 vector encoding Nk4 14 days prior to tumour induction. Tumour growth and survival was monitored for all animals. Cured animals were re-challenged with tumourigenic doses of the original tumour type. In vivo cytotoxicity assays were used to investigate establishment of cell-mediated responses in treated animals.

Results

AAV2-mediated GM-CSF, B7-1 treatment resulted in a significant reduction in tumour growth and an increase in survival in both tumour models. Cured animals were resistant to re-challenge, and induction of T cell mediated anti-tumour responses were demonstrated. Adoptive transfer of splenocytes to naïve animals prevented tumour establishment. Systemic production of Nk4 induced by intra-muscular (IM) delivery of Nk4 significantly reduced subcutaneous tumour growth. However, combination of Nk4 treatment with GM-CSF, B7-1 therapy reduced the efficacy of the immune therapy.

Conclusions

Overall, this study demonstrates the potential for in vivo AAV2 mediated immune gene therapy, and provides data on the inter-relationship between tumour vasculature and immune cell recruitment.

Novel cytotoxic vectors based on adeno-associated virus

Vectors based on adeno-associated virus (AAV) are promising tools for gene therapy. The production of strongly toxic vectors, for example for cancer-directed gene transfer, is often unfeasible due to uncontrolled expression of toxic genes in vector-producing cells. Using an approach based on transcriptional repression, we have created novel AAV vectors carrying the genes coding for diphtheria toxin A (DTA) and the pro-apoptotic PUMA protein. The DTA vector had a significant toxic effect on a panel of tumor cell lines, and abrogation of protein synthesis could be shown. The PUMA vector had a toxic effect on HeLa and RPMI 8226 cells, and sensitized transduced cells to doxorubicin. To permit targeted gene transfer, we incorporated the DTA gene into a genetically modified AAV-2 capsid previously developed by our group that mediates enhanced transduction of murine breast cancer cells in vitro. This vector had a stronger cytotoxic effect on breast cancer cells than DTA vectors with wildtype AAV capsid or vectors with a random capsid modification. The vector production and application system presented here allows for easy exchange of promotors, transgenes and capsid specificity for certain target cells. It will therefore be of great possible value in a broad range of applications in cytotoxic gene therapy and significantly broadens the spectrum of available tools for AAV-based gene therapy.

High-efficiency transduction of fibroblasts and mesenchymal stem cells by tyrosine-mutant AAV2 vectors for their potential use in cellular therapy

Adeno-associated virus 2 (AAV2) vectors transduce fibroblasts and mesenchymal stem cells (MSCs) inefficiently, which limits their potential widespread applicability in combinatorial gene and cell therapy. We have reported that AAV2 vectors fail to traffic efficiently to the nucleus in murine fibroblasts. We have also reported that site-directed mutagenesis of surface-exposed tyrosine residues on viral capsids leads to improved intracellular trafficking of the mutant vectors, and the transduction efficiency of the single tyrosine-mutant vectors is ∼10-fold higher in human cells. In the current studies, we evaluated the transduction efficiency of single as well as multiple tyrosine-mutant AAV2 vectors in murine fibroblasts. Our results indicate that the Y444F mutant vectors transduce these cells most efficiently among the seven single-mutant vectors, with >30-fold increase in transgene expression compared with the wild-type vectors. When the Y444F mutation is combined with additional mutations (Y500F and Y730F), the transduction efficiency of the triple-mutant vectors is increased by ∼130-fold and the viral intracellular trafficking is also significant improved. Similarly, the triple-mutant vectors are capable of transducing up to 80-90% of bone marrow-derived primary murine as well as human MSCs. Thus, high-efficiency transduction of fibroblasts with reprogramming genes to generate induced pluripotent stem cells, and the MSCs for delivering therapeutic genes, should now be feasible with the tyrosine-mutant AAV vectors.

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 viral vectors and their redirection to cell-type specific receptors

Efficient and specific delivery of genes to the cell type of interest is a crucial issue in gene therapy. Adeno-associated virus (AAV) has gained particular interest as gene vector recently and is therefore the focus of this chapter. Its low frequency of random integration into the genome and the moderate immune response make AAV an attractive platform for vector design. Like in most other vector systems, the tropism of AAV vectors limits their utility for certain tissues especially upon systemic application. This may in part be circumvented by using AAV serotypes with an in vivo gene transduction pattern most closely fitting the needs of the application. Also, the tropism of AAV capsids may be changed by combining parts of the natural serotype diversity. In addition, peptides mediating binding to the cell type of interest can be identified by random phage display library screening and subsequently be introduced into an AAV capsid region critical for receptor binding. Such peptide insertions can abrogate the natural tropism of AAV capsids and result in detargeting from the liver in vivo. In a novel approach, cell type-directed vector capsids can be selected from random peptide libraries displayed on viral capsids or serotype-shuffling libraries in vitro and in vivo for optimized transduction of the cell type or tissue of interest.

Using viral vectors as gene transfer tools (Cell Biology and Toxicology Special Issue: ETCS-UK 1 day meeting on genetic manipulation of cells)

In recent years, the development of powerful viral gene transfer techniques has greatly facilitated the study of gene function. This review summarises some of the viral delivery systems routinely used to mediate gene transfer into cell lines, primary cell cultures and in whole animal models. The systems described were originally discussed at a 1-day European Tissue Culture Society (ETCS-UK) workshop that was held at University College London on 1st April 2009. Recombinant-deficient viral vectors (viruses that are no longer able to replicate) are used to transduce dividing and post-mitotic cells, and they have been optimised to mediate regulatable, powerful, long-term and cell-specific expression. Hence, viral systems have become very widely used, especially in the field of neurobiology. This review introduces the main categories of viral vectors, focusing on their initial development and highlighting modifications and improvements made since their introduction. In particular, the use of specific promoters to restrict expression, translational enhancers and regulatory elements to boost expression from a single virion and the development of regulatable systems is described.

Adeno-associated virus infection of murine fibroblasts with help provided by mouse adenovirus

Adeno-associated virus (AAV-2) replicates to high titers when host cells are coinfected with a helper virus. Here we analyzed the coinfection of AAV-2 and mouse adenovirus (MAV-1) in murine fibroblasts. We observed that AAV-2/MAV-1 coinfected NIH 3T3 cells produced approximately 10-40-fold less AAV-2 DNAse resistant particles than Hela cells. Levels of AAV-2 DNA replication were approximately 30-fold less in 3T3 cells as compared to Hela cells coinfected with human adenovirus (Ad-5). A study of these lower levels of infection in 3T3 cells compared to Hela cells revealed that receptor binding and internalization of AAV-2 in 3T3 and Hela cells was comparable. However, AAV-2 did not enter into the nucleus of mouse cells as efficiently as it does in human cells. Furthermore, viral DNA replication levels of AAV-2 DNA were found to be lower in mouse cells than human cells, indicating limitations in the murine nucleus for viral replication.

Gene therapy and intracytoplasmatic sperm injection (ICSI) - a review

Human gene therapy (HGT), the treatment or prevention of disease by gene transfer is, regarded by many, as a potential revolution in medicine, because gene therapies target the causes of disease, whereas most current drugs treat the symptoms. Micro-assisted fertilization in the form of intracytoplasmatic sperm injection (ICSI) has truly revolutionized the treatment options for couples with impaired semen quality, and those with both obstructive and non-obstructive azoospermia. ICSI involves the injection of a single sperm directly into the cytoplasm of a mature egg (oocyte) using a glass needle (pipette). Fertilization with this technique occurs in 50%-80% of injected oocytes, but may damage a small percentage of them. With gene therapy, there are new and varied strategies for gene transfer and genome sequence manipulation with improved methodologies that use the technique of microinjection such as the intracytoplasmatic sperm injection-mediated transgenesis (ICSI-Tr), active transgenesis or the pronuclear microinjection technique. This review will look at these methods as well as their potential applications and limitations.

Estrogen plays a critical role in AAV2-mediated gene transfer in ovarian cancer

AIM

The aim of our study was to develop an effective gene delivery system for ovarian cancer gene therapy.

METHODS

The expression of heparin sulfate proteoglycan (HSPG) and integrins alpha(upsilon)beta(3) and alpha(upsilon)beta(5) were analyzed with flow cytometry on 2 human ovarian cancer cell lines (OVCAR-3 and SKOV-3ip). The gene transduction efficiencies were evaluated with recombinant adeno-associated viral vector (rAAV)2-green fluorescent protein or rAAV2-lactase Z followed by flow cytometry or cytohistochemistry staining. The effect of 17beta-estradiol on ovarian cancer cell proliferation, HSPG, the expressions of integrins alpha(upsilon)beta(3) and alpha(upsilon)beta(5), and adeno-associated viral vector (AAV)2-mediated gene transduction were determined.

RESULTS

In the present study, we found: (1) a variation in HSPG and the expressions of integrins alpha(upsilon)beta(3) and alpha(upsilon)beta(5) between OVCAR-3 and SKOV-3ip; (2) that 17beta-estradiol was shown to significantly stimulate cell proliferation and integrin beta(5) expression in certain ovarian cancer cell lines; and (3) integrintargeted A520/N584RGD-rAAV2, which has alternative interactivity with integrins and abrogates the binding capacity HSPG, showed much higher gene transduction efficiency in ovarian cancer cells than rAAV2 in the presence/absence of 17beta-estradiol. Moreover, this RGD-modified rAAV2 exerted more efficient transduction in ovarian cancer cells in response to 17beta-estradiol.

CONCLUSION

Our findings implied that A520/N584RGD-rAAV2 may offer great potential for ovarian cancer treatment in vivo.