Generation of a high-capacity hybrid vector: packaging of recombinant adenoassociated virus replicative intermediates in adenovirus capsids overcomes the limited cloning capacity of adenoassociated virus vectors

Gene therapy aims to complement or, ideally, correct defective genes. The broad clinical application of this emerging technology requires the development of safe high-capacity gene delivery vehicles that combine efficient transduction of dividing as well as quiescent cells with sustained transgene expression. Here we present a new hybrid vector system that unites favorable attributes of adenoassociated virus (AAV) and adenovirus (Ad) vectors in a single particle. This was achieved by inclusion of Ad packaging elements in different sized recombinant AAV genomes. In the presence of AAV replicative functions and a recombinant helper Ad, AAV/Ad hybrid particles were generated via encapsidation of AAV-dependent replicative intermediates into Ad capsids. In stringent in vitro models based on transduction of proliferating cells we show that AAV/Ad hybrid vectors are superior to Ad vectors in establishing prolonged transgene expression and can be used to deliver DNA fragments of at least 27 kb.     

Chimeric herpes simplex virus/adeno-associated virus amplicon vectors

Chimeric or hybrid herpes simplex virus type 1/adeno-associated virus amplicon vectors combine the large transgene capacity of HSV-1 with the potential for site-specific genomic integration and stable transgene expression of AAV. These chimeric vectors have been demonstrated to support transgene expression for significantly longer periods than standard HSV-1 amplicons. Moreover, HSV/AAV hybrid vectors can mediate integration at the AAVS1 pre-integration site on human chromosome 19 at a relatively high rate, although random integration has also been observed. One major remaining hurdle of HSV/AAV hybrid vectors is the low packaging efficiency and titers when AAV rep sequences are included in the amplicon vector. In the conditions prevalent during the replication/packaging of HSV/AAV hybrid amplicons into HSV-1 virions, in particular the presence of HSV-1 replication factors and AAV Rep protein, at least three different viral origins of DNA replication are active: the HSV-1 ori, the AAV inverted terminal repeats (ITRs), and the p5 promoter/ori driving expression of the AAV rep gene. A detailed understanding of the properties of these origins of DNA replication and the molecular mechanisms of interactions between them, may allow designing novel hybrid vectors that allow the efficient and precise integration of large transgenes in the human genome.     

Emerging adenoviral vectors for stable correction of genetic disorders

Recent drawbacks in treating patients with severe combined immunodeficiency disorders with retroviral vectors underline the importance of generating novel tools for stable transduction of mammalian cells. Substantial progress has been made over the recent years which may offer important steps towards stable and more importantly safer correction of genetic diseases. This article discusses recent advances for stable transduction of target cells based on adenoviral gene transfer. There is accumulating evidence that recombinant adenoviral vectors (AdVs) based on various human serotypes with a broad cellular tropism and adenoviruses (Ads) from different species will play an important role in future gene therapy applications. In combination with recombinant AdVs for somatic integration these gene transfer vectors offer high transduction efficiencies with potentially safer integration patterns. Other approaches for persistent transgene expression include excision of stable episomes from the adenoviral vector genome, but also long-term persistence of the complete adenoviral vector genome as an episomal DNA molecule was demonstrated and exemplified by the treatment of various genetic diseases in small and large animal models. This review displays advantages but also limitations of these Ad based vector systems. This is the perfect time to pursue such approaches because alternative strategies for stable transduction of mammalian cells undergoing many cell divisions are urgently needed. Looking into the future, we believe that a combination of different components from different viral vectors in concert with non-viral vector systems will be successful in designing significantly optimized transfer vehicles for a broad range of different genetic diseases.     

Development of AAV serotype-specific ELISAs using novel monoclonal antibodies

Adeno-associated viruses (AAV) have been developed and evaluated as recombinant vectors for gene therapy. More recently, due to the advantages they offer for gene transfer, several AAV serotypes have gained increasing interest. However, monoclonal antibodies for the characterization and quantitation of vectors derived from different serotypes are at present not available. Serotype-specific monoclonal antibodies (mAbs) against the capsids of the serotypes 1/6, 4 and 5 are described. These antibodies, designated as ADK1a and b, ADK4 or ADK5a and b, reacted specifically with the indicated serotype capsids in cell lysates. ADK 1a and b cross-reacted with its highly related AAV6 serotype, but not with the other serotypes tested. The new antibodies recognized exclusively assembled capsids and neither free nor denatured capsid proteins as shown by fractionation experiments. In immunofluorescence experiments, the mAbs stained only distinct intranuclear foci in cells expressing the capsid protein. The development of capture ELISAs for quantitation of AAV1 and 6, AAV4 or AAV5 capsids illustrates that these novel monoclonal antibodies provide valuable tools for characterization of vector stocks.     

Tyrosine-phosphorylation of AAV2 vectors and its consequences on viral intracellular trafficking and transgene expression

We have documented that epidermal growth factor receptor protein tyrosine kinase (EGFR-PTK) signaling negatively affects intracellular trafficking and transduction efficiency of recombinant adeno-associated virus 2 (AAV2) vectors. Specifically, inhibition of EGFR-PTK signaling leads to decreased ubiquitination of AAV2 capsid proteins, which in turn, facilitates viral nuclear transport by limiting proteasome-mediated degradation of AAV2 vectors. In the present studies, we observed that AAV capsids can indeed be phosphorylated at tyrosine residues by EGFR-PTK in in vitro phosphorylation assays and that phosphorylated AAV capsids retain their structural integrity. However, although phosphorylated AAV vectors enter cells as efficiently as their unphosphorylated counterparts, their transduction efficiency is significantly reduced. This reduction is not due to impaired viral second-strand DNA synthesis since transduction efficiency of both single-stranded AAV (ssAAV) and self-complementary AAV (scAAV) vectors is decreased by ∼ 68% and ∼ 74%, respectively. We also observed that intracellular trafficking of tyrosine-phosphorylated AAV vectors from cytoplasm to nucleus is significantly decreased, which results from ubiquitination of AAV capsids followed by proteasome-mediated degradation, although downstream consequences of capsid ubiquitination may also be affected by tyrosine-phosphorylation. These studies provide new insights into the role of tyrosine-phosphorylation of AAV capsids in various steps in the virus life cycle, which has implications in the optimal use of recombinant AAV vectors in human gene therapy.     

Gene therapy vectors based on adeno-associated virus: characteristics and applications to acquired and inherited diseases (review)

Adeno-associated virus (AAV), a defective parvovirus, was discovered more than 30 years ago. Interest in this virus for human gene therapy applications focuses on its non-pathogenicity, broad tropism and infectivity, site-specific integration and long-term persistence. The field of rAAV research has considerably advanced: titers of 1014 p/ml have been achieved, plasmid systems devised to produce helper-free viruses, chimaeric vectors combining properties of rAAV ITRs and large sequence capacity from Ad/HS vectors in parallel with the revolutionary intron strategy based on heterodimerisation of the forming concatamers have expanded the vector capacity. Muscle cells and neurons (post-mitotic cells) are amongst the most efficient targets of rAAV delivery and AAV receptors and co-receptors have been identified. This review will describe advances in the field of rAAV technology that overcome certain limitations of the vector as a gene delivery system and overview applications involving these recombinant vectors for the treatment of acquired and inherited diseases.     

Development of adenovirus hybrid vectors for Sleeping Beauty transposition in large mammals

The Sleeping Beauty (SB) transposase system for somatic integration offers great potential for in vivo gene therapeutic applications and genome engineering. Until recently, however, efficacy of SB transposase as a gene transfer vector especially in large animals was lacking. Herein, we report about the newest viral vector development for delivery of the SB transposase system into large mammals. Over the past decade various hyperactive versions of SB transposase and advanced adenovirus vectors enabling efficient and safe delivery of transgenes in vivo were developed. Already several years ago it was demonstrated that adenovirus vectors can be used for delivery of the SB transposase system into murine liver. Our newest study showed for the first time that a hyperactive transposase system delivered by high-capacity adenoviral vectors can result in somatic integration of exogenous DNA in canine liver, facilitating stabilized transgene expression and phenotypic correction for up to three years in a canine model of human disease. In this review we discuss safety issues and further improvements of this adenovirus based hybrid vector system for somatic integration. In the future this approach paves new paths towards the possible cure of human genetic diseases and novel strategies for in vivo genome engineering in large mammals.     

New recombinant serotypes of AAV vectors

AAV based vectors can achieve stable gene transfer with minimal vector related toxicities. AAV serotype 2 (AAV2) is the first AAV that was vectored for gene transfer applications. However, the restricted tissue tropism of AAV and its low transduction efficiency have limited its further development as vector. Recent studies using vectors derived from alternative AAV serotypes such as AAV1, 4, 5 and 6 have shown improved potency and broadened tropism of the AAV vector by packaging the same vector genome with different AAV capsids. In an attempt to search for potent AAV vectors with enhanced performance profiles, molecular techniques were employed for the detection and isolation of endogenous AAVs from a variety of human and non-human primate (NHP) tissues. A family of novel primate AAVs consisting of 110 non-redundant species of proviral sequences was discovered and turned to be prevalent in 18-19% of the tissues evaluated. Phylogenetic and functional analyses revealed that primate AAVs are segregated into clades based on phylogenetic relatedness. The members within a clade share functional and serological properties. Initial evaluation in mouse models of vectors based on these novel AAVs for tissue tropism and gene transfer potency led to the identification of some vector with improved gene transfer to different target tissues. Gene therapy treatment of several mouse and canine models with novel AAV vectors achieved long term phenotypic corrections. Vectors based on new primate AAVs could become the next generation of efficient gene transfer vehicles for various gene therapy applications.     

Inhibition of adenovirus cytotoxicity, replication, and E2a gene expression by adeno-associated virus

Adeno-associated virus (AAV) and the other parvoviruses have long been known to inhibit proliferation of nonpermissive cells. The mechanism of this inhibition is not thoroughly understood. To learn how AAV interacts with host cells, we have begun an investigation into AAV's relationship with adenovirus (Ad), AAV's most efficient helper virus. AAV, but not UV-inactivated AAV, delayed Ad-induced cytotoxicity and inhibited Ad E2a gene expression. AAV, but not UV-inactivated AAV or a recombinant AAV vector, inhibited Ad DNA replication. To determine whether AAV or its replication (Rep) proteins alter Ad early gene expression, we measured steady state E2a mRNA levels in AAV and Ad coinfected cultures and in a cell line (Neo6) that inducibly expresses the Rep proteins. AAV, but not UV-AAV, and Rep expression resulted in diminution of E2a protein and mRNA levels. To determine whether the AAV Rep proteins directly affect the individual Ad early promoters, we constructed luciferase reporter plasmids containing each of the five early promoters. Cotransfection of Ad-luciferase and an AAV rep gene-expressing plasmid in HeLa cells demonstrated that Rep78 repressed the E1a, E2a, and E4 promoters but trans-activated the E1b and E3 promoters. In the presence of a cotransfected E1a-expressing plasmid, Rep78 repressed expression from all five promoters. These results indicate that Rep may have different effects on the Ad early promoters dependent upon the presence of the E1a trans-activating protein.     

Long-term expression of a transferred gene in Epstein-Barr virus transformed human B cells

Delivering a gene into the Epstein-Barr virus (EBV)-transformed B cells is useful in studying effects of the gene on B-cell functions. However, although people have been able to efficiently transfer genes into and get them expressed in B-lympho blastoid cells for a time probably long enough to kill the cells using vectors harbouring oriP, the expression time of the delivered gene is not long enough in order to study the gene function in B cells. To solve this problem, we constructed an adeno-associated virus (AAV) plasmid pAGX(+) based on plasmids pSub201 and pRc/CMV. We developed and packaged recombinant AAV (rAAV) expression vectors containing an antisense or a sense DNA fragment of 6A8 cDNA encoding a human alpha-mannosidase, or an antisense fragment of 5D4 cDNA encoding a human cell membrane protein, or EYFP DNA. EBV-transformed B cell SKW6 and 3D5 were transduced with those rAAV or the mock. Transduction with the rAAV-EYFP showed an infection frequency of 64 +/- 3.5% and 58 +/- 6.2% for SKW6 and 3D5 cell, respectively. Genomic polymerase chain reaction (PCR) for neoR gene indicated an integration of the transferred gene into the host DNA. After being cultured and propagated for over 12 months, the cells were detected for the expression of the transferred gene. The RT-PCR, enzymatic assay and Con A binding test demonstrated an inhibition of 6A8 alpha-mannosidase in both SKW6 and 3D5 cells transduced with the antisense 6A8 DNA. Immunofluorescence staining with monoclonal antibodies (MoAb) 5D4 showed a reduction of the 5D4 protein expression on both the cells transduced with the antisense 5D4 DNA. The DNA fragmentation assay showed a resistance of the cells with 6A8 alpha-mannosidase inhibition to apoptosis induction by anti-Fas antibody. The data indicate that the AAV vector pAGX(+) can efficiently introduce genes into EBV-transformed B cells and the delivered gene can be expressed in the cells for more than 12 months which may be long enough for the study of gene functions in B cells.     

Adeno-associated virus vector gene transfer and sarcolemmal expression of a 144 kDa micro-dystrophin effectively restores the dystrophin-associated protein complex and inhibits myofibre degeneration in nude/mdx mice

Duchenne muscular dystrophy is a severe life-threatening X-linked recessive disorder, caused by mutations in the dystrophin gene, for which currently there is no effective treatment. Because of the large size of the dystrophin cDNA (14 kb) this precluded it from being used in early adenovirus- or retrovirus-based gene therapy vectors. However, some therapeutic success has been achieved in mdx mice using adenovirus- and retrovirus-mediated transfer of a 6.3 kb recombinant mini-dystrophin cDNA. Despite this, problems with immunogenicity and inefficient transduction of mature myofibres make these vectors less than ideal for gene transfer to skeletal muscle. Adeno-associated viral (AAV) vectors overcome many of the problems associated with other vector systems. However, AAV vectors can only accommodate <5 kb of foreign DNA. For this reason we have produced a micro-dystrophin cDNA gene construct that is <3.8 kb. This construct, driven by a CMV promoter, was introduced into the skeletal muscle of 12-day-old nude/mdx mice using an AAV vector, resulting in specific sarcolemmal expression of micro-dystrophin in >50% of myofibres up to 20 weeks of age, and effective restoration of the dystrophin-associated protein (DAP) complex components. Additionally, evaluation of central nucleation indicated a significant inhibition of degenerative dystrophic muscle pathology. We have therefore shown that the current micro-dystrophin gene delivered in vivo using an AAV vector is not only capable of restoring sarcolemmal DAP complexes, but can also ameliorate dystrophic pathology at the cellular level.     

Adeno-associated viral vectors as gene delivery vehicles

Adeno-associated virus (AAV), a non-pathogenic human parvovirus, is gaining attention for its potential use as a human gene therapy vector. One of the most attractive features of recombinant AAV vectors is the ability to be stably maintained in host cells as integrated proviruses. This property is particularly desireable for therapies requiring long-term correction of a genetic defect. This review highlights recent advances made in the AAV field and will discuss some limitations of rAAV vector integration. A novel method for enhancing the integration efficiency of these vectors will be presented.     

From virus evolution to vector revolution: use of naturally occurring serotypes of adeno-associated virus (AAV) as novel vectors for human gene therapy

Gene transfer vectors based on the human adeno-associated virus serotype 2 (AAV-2) have been developed and tested in pre-clinical studies for almost 20 years, and are currently being evaluated in clinical trials. So far, all these studies have provided evidence that AAV-2 vectors possess many properties making them very attractive for therapeutic gene delivery to humans, such as a lack of pathogenicity or toxicity, and the ability to confer long-term gene expression. However, there is concern that two restrictions of AAV-2 vectors might limit their clinical use in humans. First, these vectors are rather inefficient at transducing some cells of therapeutic interest, such as liver and muscle cells. Second, gene transfer might be hampered by neutralizing anti-AAV-2 antibodies, which are highly prevalent in the human population. In efforts to overcome both limitations, an increasing number of researchers are now focusing on the seven other naturally occurring serotypes of AAV (AAV-1 and AAV-3 to -8), which are structurally and functionally different from AAV-2. To this end, several strategies have been devised to cross-package an AAV-2 vector genome into the capsids of the other AAV serotypes, resulting in a new generation of "pseudotyped" AAV vectors. In vitro and in vivo, these novel vectors were shown to have a host range different from AAV-2, and to escape the anti-AAV-2 immune response, thus underscoring the great potential of this approach. Here the biology of the eight AAV serotypes is summarized, existing technology for pseudotyped AAV vector production is described, initial results from pre-clinical evaluation of the vectors are reviewed, and finally, the prospects of these promising novel tools for human gene therapy are discussed.     

Adeno-associated virus (AAV) capsid genes isolated from rat and mouse liver genomic DNA define two new AAV species distantly related to AAV-5

Using polymerase chain reactions and genome walking strategies, adeno-associated virus (AAV)-like capsid genes were isolated from rat and mouse liver genomic DNA, where they are present at <5 copies per cell. These genes define two new species of AAVs since their amino acid sequences are <60% identical to each other or to any other AAV capsid. They are most similar to the AAV-5 and goat AAV capsids. A recombinant vector with the mouse AAV capsid and a lacZ transgene (rAAV-mo.1 lacZ) was able to transduce rodent cell lines in vitro. However, it was not able to transduce eight human cell lines or primary human fibroblasts in vitro. It did not bind heparin and its ability to transduce cells in vitro was not inhibited by heparin, mucin, or sialic acid suggesting it uses a novel entry receptor. rAAV-mo.1 lacZ was 29 times more resistant to in vitro neutralization by pooled, purified human IgG than AAV-2. In vivo, rAAV-mo.1 lacZ efficiently transduced murine ocular cells after a subretinal injection. Intramuscular injection of a rAAV-mo.1 human factor IX (hFIX) vector into mice resulted in no detectable hFIX in plasma, but intravenous injection resulted in high plasma levels of hFIX, equivalent to that obtained from a rAAV-8 hFIX vector. Biodistribution analysis showed that rAAV-mo.1 primarily transduced liver after an intravenous injection. These AAV capsids may be useful for gene transfer in rodents.     

Adeno-associated virus vectors for human gene therapy

Adeno-associated virus(AAV) is a small,non-enveloped virus that contains a single-stranded DNA genome. It was the first gene therapy drug approved in the Western world in November 2012 to treat patients with lipoprotein lipase deficiency. AAV made history and put human gene therapy in the forefront again. More than four decades of research on AAV vector biology and human gene therapy has generated a huge amount of valuable information. Over 100 AAV serotypes and variants have been isolated and at least partially characterized. A number of them have been used for preclinical studies in a variety of animal models. Several AAV vector production platforms,especially the baculovirus-based system have been established for commercial-scale AAV vector production. AAV purification technologies such as density gradient centrifugation,column chromatography,or a combination,have been well developed. More than 117 clinical trials have been conducted with AAV vectors. Although there are still challenges down the road,such as crossspecies variation in vector tissue tropism and gene transfer efficiency,pre-existing humoral immunity to AAV capsids and vector dose-dependent toxicity in patients,the gene therapy community is forging ahead with cautious optimism. In this review I will focus on the properties and applications of commonly used AAV serotypes and variants,and the technologies for AAV vector production and purification. I will also discuss the advancement of several promising gene therapy clinical trials.     

Adeno-associated virus vectors serotyped with AAV8 capsid are more efficient than AAV-1 or -2 serotypes for widespread gene delivery to the neonatal mouse brain

Adeno-associated virus (AAV) vectors have gained a preeminent position in the field of gene delivery to the normal brain through their ability to achieve extensive transduction of neurons and to mediate long-term gene expression with no apparent toxicity. In adult animals direct infusion of AAV vectors into the brain parenchyma results in highly efficient transduction of target structures. However AAV-mediated global delivery to the adult brain has been an elusive goal. In contrast, widespread global gene delivery has been obtained by i.c.v. injection of AAV1 or AAV2 in neonates. Among the novel AAV serotypes cloned and engineered for production of recombinant vectors, AAV8 has shown a tremendous potential for in vivo gene delivery with nearly complete transduction of many tissues in rodents after intravascular infusion. Here we compare the efficiency of an AAV8 serotyped vector with that of AAV1 and AAV2 serotyped vectors for the extent of gene delivery to the brain after neonatal injection into the lateral ventricles. The vectors all encoded green fluorescent protein (GFP) under control of a hybrid CMV enhancer/chicken beta-actin promoter with AAV2 inverted terminal repeats, but differed from each other with respect to the capsid type. A total of 6.8 x 10(10) genome copies were injected into the lateral ventricles of postnatal day 0 mice. Mice were killed at postnatal day 30 and brains analyzed for distribution of GFP-positive cells. AAV8 proved to be more efficient than AAV1 or AAV2 vectors for gene delivery to all of the structures analyzed, including the cerebral cortex, hippocampus, olfactory bulb, and cerebellum. Moreover the intensity of gene expression, assessed using a microarray reader, was considerably higher for AAV8 in all structures analyzed. In conclusion, the enhanced transduction achieved by AAV8 compared with AAV1 and AAV2 indicates that AAV8 is the superior serotype for gene delivery to the CNS.