Adeno-associated virus (AAV) vectors for gene transfer

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.

Genome DNA leakage of Adeno-Associated virus under freeze-thaw stress

Adeno-associated virus (AAV) has become an emerging tool for human gene therapies. Currently, AAV gene therapies are subjected to multiple freeze-thaw cycles during manufacturing, storage, transportation, and administration. While studies have shown that multiple freeze-thaw cycles led to a decrease in transduction efficiency, the AAV degradation mechanism during freeze-thaw is not well understood. Here, we have characterized the impact of freeze-thaw on AAV8 by employing a variety of assays, which revealed significant increases in the amount of free single-stranded DNA (ssDNA) in AAV8 formulations after multiple freeze-thaw cycles. Subsequent analysis using Next Generation Sequencing (NGS) revealed that the ssDNA primarily consisted of genome DNA, indicating that the increased ssDNA leaked out from AAV8. Experiments performed using different serotypes of AAV confirmed the pervasiveness of such behavior amongst AAVs. In addition, formulation screening studies were performed to understand the impact on genome DNA leakage from AAV. The formulation screening results showed that the addition of 10% sucrose and 0.1% poloxamer 188 to Dulbecco's phosphate-buffered saline (DPBS) reduced the leakage of ssDNA in AAV samples after freeze-thaw cycles compared to the base formulation of DPBS alone. These findings shed new light on the degradation mechanism of AAVs and stabilization of the AAV-based gene therapies.

Bacteriosomes as a Promising Tool in Biomedical Applications: Immunotherapy and Drug Delivery

Bacteriosomes are a member of cell-derived vesicles that are proposed as promising tools in diagnosis, therapy, and drug delivery. These vesicles could be derived from a virus, bacterial cells, and animal cells. Biotechnology techniques were used in bioengineering of cell-derived vesicles in vitro, and in vivo. Bacterial vesicles such as bacterial cells, bacterial ghost, or bacteriosomes are vesicular structures derived from bacteria produced by manipulation of bacterial cells by chemical agents or gene-mediated lysis. Subsequently, bacterial vesicles (bacteriosomes) are non-living, non-denatured bacterial cell envelopes free of the cytoplasm and genetic materials. Gram-negative and Gram-positive bacteria are exploited in the production of bacteriosomes. Bacteriosomes have instinct organs, tissues, cells, as well as subcellular tropism. Moreover, bacteriosomes might be used as immunotherapy and/or drug delivery shuttles. They could act as cargoes for the delivery of small drugs, large therapeutics, and nanoparticles to the specific location. Furthermore, bacteriosomes have nature endosomal escaping ability, hence they could traffic different bio-membranes by endocytosis mechanisms. Therefore, bacterial-derived vesicles could be used in therapy and development of an innovative drug delivery systems. Consequently, utilizing bacteriosomes as drug cargoes enhances the delivery and efficacy of administered therapeutic agents. This review highlighted bacteriosomes in terms of source, engineering, characterization, applications, and limitations.

Co-vaccination with adeno-associated virus vectors encoding human papillomavirus 16 L1 proteins and adenovirus encoding murine GM-CSF can elicit strong and prolonged neutralizing antibody

Non-infectious human papillomavirus-like particles (VLPs), encoded by the major capsid gene L1, have been shown to be effective as vaccines to prevent cervical cancer. We have developed the genetic immunization of the L1 gene to induce a neutralizing antibody. We constructed and generated a recombinant adeno-associated virus encoding human papillomavirus (HPV) 16 L1 protein that could form virus-like particles in transduced cells. Previous reports have demonstrated that the formation of VLP is necessary to induce high titers of neutralizing antibodies to protect an animal from viral challenge. Therefore, we carried out a single intramuscular (i.m.) injection with recombinant adeno-associated virus encoding HPV-16 L1 protein (rAAV-16L1) in BALB/c mice, which ultimately produced stronger and more prolonged neutralizing L1 antibodies, when compared to the DNA vaccine. Immunohistochemistry showed that the accumulation of antigen presenting cells, such as macrophages and dendritic cells, in rAAV-16L1 and L1 DNA-injected muscle fibers may be due to the L1 protein expression, but not to AAV infection. When compared to the L1 VLP vaccine, however, the titers of neutralizing L1 antibodies induced by VLP were higher than those induced by rAAV-16L1. Co-vaccinating with rAAV-16L1 and adenovirus encoding murine GM-CSF (rAAV-16L1/rAd-mGM-CSF) induced comparable higher levels of neutralizing L1 antibodies with those of VLP. This implies that a single i.m. co-injection with rAAV-16L1/rAd-mGM-CSF can achieve the same vaccine effect as a VLP vaccine requiring 3 booster injections.

Feasibility of generating adeno-associated virus packaging cell lines containing inducible adenovirus helper genes

The adeno-associated virus (AAV) vector system is based on nonpathogenic and helper-virus-dependent parvoviruses. The vector system offers safe, efficient, and long-term in vivo gene transfer in numerous tissues. Clinical trials using AAV vectors have demonstrated vector safety as well as efficiency. The increasing interest in the use of AAV for clinical studies demands large quantities of vectors and hence a need for improvement in vector production. The commonly used transient-transfection method, although versatile and free of adenovirus (Ad), is not cost-effective for large-scale production. While the wild-type-Ad-dependent AAV producer cell lines seem to be cost-effective, this method faces the problem of wild-type Ad contamination. To overcome these shortcomings, we have explored the feasibility of creating inducible AAV packaging cell lines that require neither transfection nor helper virus infection. As a first step toward that goal, we have created a cell line containing highly inducible Ad E1A and E1B genes, which are essential for AAV production. Subsequently, the AAV Rep and Cap genes and an AAV vector containing a green fluorescent protein (GFP) reporter gene were stably introduced into the E1A-E1B cell line, generating inducible AAV-GFP packaging cell lines. Upon induction of E1A and E1B genes and infection with replication-defective Ad with E1A, E1B, and E3 deleted, the packaging cells yielded high-titer AAV-GFP vectors. Finally, the E2, E4, and VA genes of Ad, under the control of their endogenous promoters, were also introduced into these cells. A few producer cell lines were obtained, which could produce AAV-GFP vectors upon simple drug induction. Although future improvement is necessary to increase the stability and vector yield of the cells, our study has nonetheless demonstrated the feasibility of generating helper-virus-free inducible AAV producer cell lines.

Gene regulation in the magnocellular hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

Recombinant adeno-associated virus expressing human papillomavirus type 16 E7 peptide DNA fused with heat shock protein DNA as a potential vaccine for cervical cancer

In this study, we explore a potential vaccine for human papillomavirus (HPV)-induced tumors, using heat shock protein as an adjuvant, a peptide vaccine for safety, and adeno-associated virus (AAV) as a gene delivery vector. The tumor vaccine was devised by constructing a chimeric gene which contained HPV type 16 E7 cytotoxic T-lymphocyte (CTL) epitope DNA (M. C. Feltkamp, H. L. Smits, M. P. Vierboom, R. P. Minnaar, B. M. de Jongh, J. W. Drijfhout, J. ter Schegget, C. J. Melief, and W. M. Kast, Eur. J. Immunol. 23:2242-2249, 1993) fused with the heat shock protein gene as a tumor vaccine delivered via AAV. Our results demonstrate that this vaccine can eliminate tumor cells in syngeneic animals and induce CD4- and CD8-dependent CTL activity in vitro. Moreover, studies with knockout mice with distinct T-cell deficiencies confirm that CTL-induced tumor protection is CD4 and CD8 dependent. Taken together, the evidence indicates that this chimeric gene delivered by AAV has potential as a cervical cancer vaccine.

Cell-specific gene expression in oxytocin and vasopressin magnocellular neurons

The oxytocin (OT) and vasopressin (VP) expressing magnocellular neurons in the hypothalamic-neurohypophysial system (HNS) have been the most studied of all the neuroendocrine cell-types. Despite this, our understanding of the mechanisms that underly the cell-specific expression of the peptide genes in these neurons has remained obscure. Part of the reason for this may be related to the close apposition of the OT and VP genes in the chromosomal locus, the genes being separated by as little as 3.5 kb in the mouse, and their interactions which are critical for cell-specific expression of the genes. Recent studies using intact rat OT and VP constructs in transgenic mice, and rat and mouse VP genes with CAT inserts in exon III as reporters in transgenic rats and mice, respectively, have suggested the presence of cell-specific enhancer elements in the 3' downstream (intergenic region, IGR) region of the VP gene. Evidence in favor of this view is presented from transgenic mouse studies on the expression of mouse OT- and VP-CAT gene constructs. Oxytocin and vasopressin phenotypes in the magnocellular neuronal population have traditionally been assessed by either immunocytochemical or in situ hybridization histochemical methods leading to the view that these genes are never coexpressed. However, more sensitive methods show that most OT cells also express some VP mRNA, and most VP cells contain some OT mRNA. A third phenotype containing equivalent levels of both OT and VP mRNA can also be found under some conditions, thereby complicating our analysis of cell-specificity. A continuing problem hindering studies of the regulation of OT and VP gene expression in neurons, is the absence of an appropriate cell line to examine these issues. We have found that stationary slice-explant cultures allow for excellent preservation of highly differentiated magnocellular neurons in long-term culture, and that these cultures can be used for physiological and pharmacological studies and analysis of gene expression.

Transient immunosuppression allows transgene expression following readministration of adeno-associated viral vectors

Adeno-associated viral (AAV) vectors have much promise in gene therapy. Among the many properties that make AAV an ideal vector for gene therapy are its ability to infect both dividing and nondividing cells and the longevity of expression in tissues such as brain, skeletal muscle, and liver. However, like other viral vectors, readministration of vector is limited because of the host's immune response to viral components of the vector. Using class I, class II, and CD40 ligand (CD40L)-deficient mice, we demonstrate that neutralizing antibodies to the viral capsid proteins prevent transgene expression following readministration of rAAV vectors. Transient immunosuppression of mice by treatment with antibody to CD4 at the time of primary infection allowed transgene expression after readministration of rAAV vectors to animals. Transient immunosuppression with antibody to CD40L had only a modest effect on the efficacy of readministration. The ability to readminister virus was inversely correlated with both AAV capsid enzyme-linked immunosorbent assay titers and AAV neutralizing antibody titers. These studies demonstrate that readministration of rAAV can be accomplished by down regulating the anti-AAV immune response and suggest the use of repeated administration of rAAV as a viable form of therapy for the treatment of chronic diseases.

Genetic immunization with adeno-associated virus vectors expressing herpes simplex virus type 2 glycoproteins B and D

Intramuscular injection of mice with an adeno-associated virus (AAV) vector expressing herpes simplex virus type 2 glycoprotein B led to the generation of both gB-specific major histocompatibility complex class I-restricted cytotoxic T lymphocytes and anti-gB antibody. AAV-mediated immunization was more potent than plasmid DNA or protein in generating antibody responses.

Adeno-associated virus as a gene delivery system

Adeno-associated virus (AAV) has several characteristics which make it extremely attractive as a gene transfer vector: (1) no known pathogenicity; (2) high efficiency and the ability to remain latent; (3) a minimal number of antigens ensuring minimal immunogenicity; (4) the ability to transduce post-mitotic cells; (5) possible advantages of site-specific integration; and (6) a broad host and cell range. The human isolate, AAV-2, is the best studied and has been the focus for gene delivery experiments. This review will discuss recent in vivo experiments demonstrating the utility of AAV in animal models of neurodegenerative disease.

Role for highly regulated rep gene expression in adeno-associated virus vector production

Recent success achieving long-term in vivo gene transfer without a significant immune response by using adeno-associated virus (AAV) vectors (X. Xiao, J. Li, and R. J. Samulski, J. Virol. 70:8098-8108, 1996) has encouraged further development of this vector for human gene therapy. Currently, studies focus on the generation of high-titer vectors by using the two-plasmid helper-vector system in adenovirus (Ad)-infected cells. To examine the effects of the AAV replication (rep) genes on recombinant AAV (rAAV) vector production, we have constructed a series of AAV helper plasmids that contain strong heterologous promoters in place of the endogenous p5 promoter. Although high-level rep gene expression was achieved, rAAV DNA failed to replicate in the absence of Ad infection. Moreover, unregulated overexpression of Rep78/68 led to substantially lower rAAV yields in the presence of Ad (10(4-5) versus 10(7-8)). In contrast, under similar conditions, reduced Rep78/68 expression resulted in much higher rAAV yields (10(9)). Molecular characterization showed that overexpression of the rep gene decreased rAAV DNA replication and severely inhibited capsid (cap) gene expression. Interestingly, a reduced rep level enhanced cap gene expression and supported normal rAAV DNA replication. These studies suggest a critical role for regulated rep gene expression in rAAV production and have facilitated the development of a new AAV helper plasmid that increases vector production eightfold over currently used constructs.

Potent inhibition of human immunodeficiency virus type 1 in primary T cells and alveolar macrophages by a combination anti-Rev strategy delivered in an adeno-associated virus vector

The rate of viral replication appears to play a pivotal role in human immunodeficiency virus type 1 (HIV-1) pathogenesis and disease progression as it outstrips the capacity of the immune system to respond. Important cellular sites for HIV-1 production include T lymphocytes and tissue macrophages. Antiviral strategies, including newer treatment modalities such as gene therapy of HIV-1-susceptible cell populations, must be capable of engendering durable inhibitory effects to HIV-1 replication in both of these primary cell types in order to be effective. Among the potential genetic targets for intervention in the HIV-1 life cycle, the Rev regulatory system, consisting of Rev and its binding site, the Rev-responsive element (RRE), stands out as particularly attractive. Rev is essential for maintaining the stability of the viral genomic RNA as well as viral mRNAs encoding key structural and regulatory proteins. Moreover, it exhibits favorable threshold kinetics, in that Rev concentrations must rise above a critical level to exert their effect. To disable Rev function, primary T cells or macrophages were transduced with anti-Rev single-chain immunoglobulin (SFv) or RRE decoy genes either singly or in combination by employing adeno-associated virus vectors and then challenged with HIV-1. By directing both a protein and a nucleic acid against the normal interaction between Rev and the RRE, this genetic antiviral strategy effectively inhibited infection by either clinical or laboratory virus isolates. These results provide a framework for novel interventions to reduce virus production in the infected host.

Binding sites for adeno-associated virus Rep proteins within the human genome

The Rep proteins of adeno-associated virus type 2 (AAV) are known to bind to Rep recognition sequences (RRSs) in the AAV inverted terminal repeats (ITRs), the AAV p5 promoter, and the preferred AAV integration site in human chromosome 19, called AAVS1. Integration of the AAV genome into AAVS1 appears to be mediated by an interaction between the Rep proteins of AAV and Rep binding sites within the viral genome and the integration locus. In an attempt to identify potential alternate integration sites, we looked for recognition sites for AAV Rep proteins in the human genome by performing a BLASTN computerized homology search. We used the 16-mer core sequences of the RRSs in the AAV ITRs and AAVS1 separately as query sequences and identified 18 new RRSs in or flanking the genes coding for the following: tyrosine kinase activator protein 1 (TKA-1); colony stimulating factor-1; insulin-like growth factor binding protein 2 (IGFBP-2); histone H2B.1; basement membrane heparan sulfate proteoglycan, also known as perlecan; the AF-9 gene product, which is involved in the chromosomal translocation t (9:11)(p22:q23); the betaB subunit of the hormone known as inhibin; interleukin-2 enhancer binding factor; an endoplasmic reticulum-Golgi intermediate compartment resident protein called p63; a global transcription activator (hSNF2L); the beta-actin repair domain; a retinoic acid-inducible factor, also known as midkine; a breast tumor autoantigen; a growth-arrest- and DNA-damage-inducible protein called gadd45; the cyclin-dependent kinase inhibitor called KIP2, which inhibits several G1 cyclin-cyclin-dependent kinase complexes; and the hereditary breast and ovarian cancer gene (BRCA1). RRSs were also identified in a newly discovered open reading frame on chromosome 10 and in the ERCC1 locus on human chromosome 19. The ability of a maltose binding protein-Rep68 fusion protein to bind to these sequences was confirmed by electrophoretic mobility shift assays. These sites may serve as alternate integration sites for AAV or play a role in Rep-mediated effects on human cells.

Gene transfer by adeno-associated virus vectors into the central nervous system

Adeno-associated virus (AAV) vectors are derived from a nonpathogenic and defective human parvovirus. Although currently unable to display the integration specificity featured by its wild-type parent, the recombinant AAV (rAAV) system has continued to attract enormous interest primarily due to its unique features such as safety, high titers, broad host range, transduction of quiescent cells, and vector integration. Recently, rAAV-mediated in vivo gene transfers have demonstrated efficient long-term transduction (from 3 months to more than 1.5 years) and lack of cytotoxicity and cellular immune responses in the target tissues, especially in the CNS. Alternative approaches using rAAV plasmid DNA in nonviral gene delivery systems also generated promising results. Propelled by various efforts to improve the system, rAAV vectors will provide numerous opportunities to explore the potential therapeutic applications in humans.

Lack of site-specific integration of the recombinant adeno-associated virus 2 genomes in human cells

The adeno-associated virus 2 (AAV)-based vector system has been suggested for its potential use in human gene therapy because the wild-type (wt) AAV genome appears to integrate into the human chromosomal DNA in a site-specific manner. We systematically investigated the integration patterns of the recombinant AAV genomes lacking one or both the viral coding sequences. Four recombinant AAV genomes were constructed containing the genes for resistance to tetracycline (TcR) and the herpesvirus thymidine kinase (TK) promoter-driven gene for resistance to neomycin (neoR; vTc.Neo), the genes for resistance to ampicillin (ApR) and TK-neoR (vAp.Neo), the genes for AAV replication (rep) genes and TK-neoR (vRep.Neo), and the AAV capsid (cap) genes and TK-neoR (vCap.Neo). The integration pattern of each of the recombinant AAV genomes in individual clonal isolates of the human nasopharyngeal carcinoma cell line (KB) analyzed on Southern blots using a neo-specific DNA probe was distinctly different. In addition, in none of the clones examined was the proviral genome covalently linked to the previously described AAV right-junction (Rt.Jn.) human chromosomal DNA fragment, the putative specific-site of integration for the wt AAV genome. Furthermore, whereas a 276-bp DNA fragment could be readily amplified from each of these clones, using a neo-specific primer-pair by polymerase chain reaction (PCR), no amplified DNA product was obtained using the neo- and the Rt.Jn. primer-pair under identical conditions. Fluorescence in situ hybridization (FISH) analyses further revealed the lack of integration of the recombinant AAV into human chromosome 19, even in the presence of a functional rep gene as determined by rescue of the recombinant AAV genome in the presence of adenovirus. These data suggest that the recombinant AAV genomes integrate at sites that are different from that characterized for the wt AAV genome. These studies may have implications in the development of the AAV-based vector system for its potential use in human gene therapy.

Expression of the human multidrug resistance and glucocerebrosidase cDNAs from adeno-associated vectors: efficient promoter activity of AAV sequences and in vivo delivery via liposomes

Recombinant adeno-associated viruses (rAAV) are attractive tools for gene therapy. We designed plasmids in which the human multidrug resistance gene (hMDR1) cDNA was placed downstream from portions of the 5' end of AAV including either a 234-bp cassette or the entire AAV p5 promoter. The drug-resistant phenotype conferred by the P-glycoprotein (Pgp) efflux pump encoded by the hMDR1 cDNA was used to select NIH-3T3 cells transfected with these plasmids. The 234-bp region alone showed promoter activity similar in strength to that of the entire p5 promoter or the retroviral Harvey murine sarcoma virus long terminal repeat (LTR); this result demonstrates that the 234-bp cassette might be used as a small and efficient promoter in rAAV designed to express large genes approaching the packaging limit of AAV particles. After transfection of AAV-MDR1 vectors, the integration of MDR1 sequences into the host cell genome was demonstrated by fluorescent in situ hybridization (FISH). In addition, Southern analysis of low-molecular-weight DNA extracted from drug-resistant cells grown under continuous selection pressure indicated the persistence of nonintegrated AAV-MDR1 plasmids. Coordinate expression of Pgp and human glucocerebrosidase (hGC) was observed in drug-selected NIH-3T3 cells transfected with a bicistronic vector in which MDR1 cDNA was linked to hGC cDNA via the encephalomyocarditis internal ribosome entry site sequence. Moreover, following a single intravenous injection of the bicistronic vector complexed to cationic liposomes into recipient mice, delivery of MDR1 and GC cDNAs was achieved in all the organs we tested. Our results demonstrate that the efficiency of liposomes as vehicles for in vitro and in vivo gene delivery, the advantages of AAV-vectors, and the use of MDR1 as a selectable marker might be successfully combined in gene therapy protocols.

Recombinant junctions formed by site-specific integration of adeno-associated virus into an episome

A model system using an episomal Epstein-Barr virus shuttle vector was recently developed to study the adeno-associated virus (AAV) site-specific integration event in chromosome 19q13.3-qter (C. Giraud, E. Winocour, and K.I. Berns, Proc. Natl. Acad. Sci. USA 91:10039-10043, 1994). In this study, we analyze the recombinant junctions generated after integration of the AAV genome into an Epstein-Barr virus shuttle vector carrying 8.2, 1.6, or 0.51 kb of the chromosome 19 preintegration sequence (AAVS1 locus). In most of the recombinants, one end of the viral genome was joined to a portion of the AAVS1 DNA previously shown to be a minimum target for AAV integration. Within this AAVS1 segment, the AAV insertion points were strikingly clustered around a binding site for the AAV regulatory protein. In all cases, the second junction with AAV occurred with vector DNA outside of the AAVS1 segment. With respect to the viral genome, one junction with the shuttle vector DNA occurred either within the AAV inverted terminal repeat (itr), or near the P5 promoter, approximately 100 nucleotides distal to a modified itr. The modified itr in 5 of 11 recombinants involved a head-to-tail organization. In one such instance, the AAV insert contained slightly more than one genome equivalent arranged in a head-to-tail manner with a junction close to the P5 promoter; the AAV insert in this recombinant episome could be rescued by adenovirus infection and replicated to virus particles. The significance of the head-to-tail organization is discussed in terms of the possible circularization of AAV DNA before or during integration.