Improved transduction of primary murine hepatocytes by recombinant adeno-associated virus 2 vectors in vivo

Host Cell Restriction Factors Blocking Efficient Vector Transduction: Challenges in Lentiviral and Adeno-Associated Vector Based Gene Therapies

Gene therapy relies on the delivery of genetic material to the patient's cells in order to provide a therapeutic treatment. Two of the currently most used and efficient delivery systems are the lentiviral (LV) and adeno-associated virus (AAV) vectors. Gene therapy vectors must successfully attach, enter uncoated, and escape host restriction factors (RFs), before reaching the nucleus and effectively deliver the therapeutic genetic instructions to the cell. Some of these RFs are ubiquitously expressed in mammalian cells, while others are cell-specific, and others still are expressed only upon induction by danger signals as type I interferons. Cell restriction factors have evolved to protect the organism against infectious diseases and tissue damage. These restriction factors can be intrinsic, directly acting on the vector, or related with the innate immune response system, acting indirectly through the induction of interferons, but both are intertwined. The innate immunity is the first line of defense against pathogens and, as such cells derived from myeloid progenitors (but not only), are well equipped with RFs to detect pathogen-associated molecular patterns (PAMPs). In addition, some non-professional cells, such as epithelial cells, endothelial cells, and fibroblasts, play major roles in pathogen recognition. Unsurprisingly, foreign DNA and RNA molecules are among the most detected PAMPs. Here, we review and discuss identified RFs that block LV and AAV vector transduction, hindering their therapeutic efficacy.

Evaluation of engineered AAV capsids for hepatic factor IX gene transfer in murine and canine models

Background

Adeno-associated virus (AAV) gene therapy vectors have shown the best outcomes in human clinical studies for the treatment of genetic diseases such as hemophilia. However, these pivotal investigations have also identified several challenges. For example, high vector doses are often used for hepatic gene transfer, and cytotoxic T lymphocyte responses against viral capsid may occur. Therefore, achieving therapy at reduced vector doses and other strategies to reduce capsid antigen presentation are desirable.

Methods

We tested several engineered AAV capsids for factor IX (FIX) expression for the treatment of hemophilia B by hepatic gene transfer. These capsids lack potential phosphorylation or ubiquitination sites, or had been generated through molecular evolution.

Results

AAV2 capsids lacking either a single lysine residue or 3 tyrosine residues directed substantially higher coagulation FIX expression in mice compared to wild-type sequence or other mutations. In hemophilia B dogs, however, expression from the tyrosine-mutant vector was merely comparable to historical data on AAV2. Evolved AAV2-LiC capsid was highly efficient in hemophilia B mice but lacked efficacy in a hemophilia B dog.

Conclusions

Several alternative strategies for capsid modification improve the in vivo performance of AAV vectors in hepatic gene transfer for correction of hemophilia. However, capsid optimization solely in mouse liver may not predict efficacy in other species and thus is of limited translational utility.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-017-1200-1) contains supplementary material, which is available to authorized users.

Improved transduction efficiencies of adeno-associated virus vectors by synthetic cell-permeable peptides

Various serotypes of adeno-associated virus (AAV) vectors have been used for gene therapy and as research tools. Among these serotypes, the AAV type 2 vector has been used successfully in human gene therapies. However, the transduction efficiency of AAV2 depends on the cell type, and this poses a problem in the efficacy of gene therapy. To improve the transduction efficiency of AAV2, we designed a small peptide consisting of epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor peptide and the HIV-Tat sequence Tat-Y1068. Pre- or co-treatment of CYNOM-K1 cells from cynomolgus monkey embryo skin with Tat-Y1068 increased the transduction efficiencies in a dose-dependent manner and caused p38 phosphorylation. The transduction efficiency of AAV2 into the rat fibroblast cell line RAT-1 highly expressing EGFR was less than the transduction efficiency of AAV2 into CYNOM-K1 cells. Tat-Y1068 increased the transduction efficiency in RAT-1 cells in the same manner as in CYNOM-K1 cells. In conclusion, cell-permeable peptides possessing the EGFR tyrosine kinase inhibitor function might serve as a useful ingredient of AAV2 vector solution for increasing the transduction efficiency of gene therapies.

Progresses towards safe and efficient gene therapy vectors

The emergence of genetic engineering at the beginning of the 1970′s opened the era of biomedical technologies, which aims to improve human health using genetic manipulation techniques in a clinical context. Gene therapy represents an innovating and appealing strategy for treatment of human diseases, which utilizes vehicles or vectors for delivering therapeutic genes into the patients' body. However, a few past unsuccessful events that negatively marked the beginning of gene therapy resulted in the need for further studies regarding the design and biology of gene therapy vectors, so that this innovating treatment approach can successfully move from bench to bedside. In this paper, we review the major gene delivery vectors and recent improvements made in their design meant to overcome the issues that commonly arise with the use of gene therapy vectors. At the end of the manuscript, we summarized the main advantages and disadvantages of common gene therapy vectors and we discuss possible future directions for potential therapeutic vectors.

Development of Novel Recombinant AAV Vectors and Strategies for the Potential Gene Therapy of Hemophilia

Recombinant vectors based on a non-pathogenic human parvovirus, the adeno-associated virus (AAV), have gained attention as a potentially safe and useful alternative to the more commonly used retroviral and adenoviral vectors. AAV vectors are currently in use in Phase I/II clinical trials for gene therapy of a number of diseases such as cystic fibrosis, α-1 antitrypsin deficiency, muscular dystrophy, Batten's disease, and Parkinson's disease, and have shown efficacy in patients with Leber's congenital amaurosis, and hemophilia B. For patients with hemophilia B, however, relatively large vector doses are needed to achieve therapeutic benefits. Large vector doses also trigger an immune response as significant fraction of the vectors fails to traffic efficiently to the nucleus, and is targeted for degradation by the host cell proteasome machinery. With a better understanding of the various steps in the life cycle of AAV vectors, strategies leading to the development of novel AAV vectors that are capable of high-efficiency transduction at lower doses are needed. In this review, we summarize our strategies to develop novel AAV vectors for the potential gene therapy of both hemophilia B and hemophilia A, based on our recent studies on the basic molecular biology of AAV. These strategies, including the development of novel AAV vectors by site-directed mutagenesis of critical surface-exposed tyrosine residues on AAV2 capsids to circumvent the ubiquitination step and the use of different AAV serotypes and self-complementary (sc) AAV2 vectors, and their use as helper vectors to circumvent the obstacles of second-strand DNA synthesis of single-stranded (ss) AAV, should dramatically accelerate the progress towards the potential gene therapy of both hemophilia A and hemophilia B.

Tetradecanoylphorbol-13-acetate (TPA) significantly increases AAV2/5 transduction of human neuronal cells in vitro

Recombinant adeno-associated virus type 2 (AAV2) vectors have shown great promise in current ophthalmology clinical trials targeting gene delivery to the retinal pigment epithelium (RPE). To treat the majority of retinal diseases, however, gene delivery would need to be targeted to photoreceptor neurons of the outer retina. AAV2 pseudotyped with the AAV5 capsid (AAV2/5) has shown far greater transduction efficiency in photoreceptors compared to standard AAV2 vectors. For clinical trial applications using gene therapy, it is helpful to generate pre-clinical data in human cells wherever possible. There is however very little data, indeed some controversy, as to whether AAV2/5 can be used effectively in differentiated neurons in culture. In this study we show that transduction of the human neuroblastoma cell line SH-SY5Y with recombinant AAV2/5 expressing GFP is well tolerated. Furthermore, we explore the mechanism whereby exposure to retinoic acid (RA) and the phorbol ester 12-O-Tetradecanoylphorbol-13- acetate (TPA) can induce this cell line to differentiate into a stable population of human neurons, with significantly increased levels of AAV2/5 transduction. These observations may be helpful for assessing AAV2/5 vectors in vitro, particularly where it is necessary to generate pre-clinical data for clinical trials of gene therapy to the human central nervous system.

A simple method to increase the transduction efficiency of single-stranded adeno-associated virus vectors in vitro and in vivo

We have recently shown that co-administration of conventional single-stranded adeno-associated virus 2 (ssAAV2) vectors with self-complementary (sc) AAV2-protein phosphatase 5 (PP5) vectors leads to a significant increase in the transduction efficiency of ssAAV2 vectors in human cells in vitro as well as in murine hepatocytes in vivo. In the present study, this strategy has been further optimized by generating a mixed population of ssAAV2-EGFP and scAAV2-PP5 vectors at a 10:1 ratio to achieve enhanced green fluorescent protein (EGFP) transgene expression at approximately 5- to 10-fold higher efficiency, both in vitro and in vivo. This simple coproduction method should be adaptable to any ssAAV serotype vector containing transgene cassettes that are too large to be encapsidated in scAAV vectors.

Development of optimized AAV3 serotype vectors: mechanism of high-efficiency transduction of human liver cancer cells

Our recent studies have revealed that among the 10 different commonly used adeno-associated virus (AAV) serotypes, AAV3 vectors transduce human liver cancer cells extremely efficiently because these cells express high levels of human hepatocyte growth factor receptor (hHGFR), and AAV3 utilizes hHGFR as a cellular co-receptor for viral entry. In this report, we provide further evidence that both extracellular as well as intracellular kinase domains of hHGFR are involved in AAV3 vector entry and AAV3-mediated transgene expression. We also document that AAV3 vectors are targeted for degradation by the host cell proteasome machinery, and that site-directed mutagenesis of surface-exposed tyrosine (Y) to phenylalanine (F) residues on AAV3 capsids significantly improves the transduction efficiency of Y701F, Y705F and Y731F mutant AAV3 vectors. The transduction efficiency of the Y705+731F double-mutant vector is significantly higher than each of the single mutants in liver cancer cells in vitro. In immunodeficient mouse xenograft models, direct intratumoral injection of AAV3 vectors also led to high-efficiency transduction of human liver tumor cells in vivo. We also document here that the optimized tyrosine-mutant AAV3 vectors lead to increased transduction efficiency following both intratumoral and tail-vein injections in vivo. The optimized tyrosine-mutant AAV3 serotype vectors containing proapoptotic genes should prove useful for the potential gene therapy of human liver cancers.

Autologous transplantation of endothelial progenitor cells genetically modified by adeno-associated viral vector delivering insulin-like growth factor-1 gene after myocardial infarction

The regenerative potential of bone marrow-derived endothelial progenitor cells (EPCs) has been adapted for the treatment of myocardial and limb ischemia via ex vivo expansion. We sought to enhance EPC function by the efficient genetic modification of EPCs in a rat model of myocardial infarction. Peripheral blood EPCs were expanded and transduced, using adeno-associated virus (AAV). AAV-mediated EPC transduction efficacy was 23 ± 1.2%, which was improved by 4.0- to 7.2-fold after pretreatment with the tyrosine kinase inhibitor genistein. Adult rats (n = 7 in each group) underwent myocardial infarction by left anterior descending coronary artery occlusion, and received autologous EPCs transduced by AAV-IGF-1 or AAV-lacZ into the periinfarct area. Echocardiography demonstrated that cardiac function in the IGF-1-EPC group was significantly improved compared with the lacZ-EPC control group 12 weeks after myocardial infarction. In addition, IGF-1-expressing EPCs led to reduced cardiac apoptosis, increased cardiomyocyte proliferation, and increased numbers of capillaries in the periinfarct area. AAV expression was limited to the targeted heart region only. Pretreatment with genistein markedly improved AAV transduction of EPCs. IGF-1-expressing EPCs exhibit favorable cell-protective effects with tissue-limited expression in rat heart postinfarction.

High-efficiency transduction and correction of murine hemophilia B using AAV2 vectors devoid of multiple surface-exposed tyrosines

Elimination of specific surface-exposed single tyrosine (Y) residues substantially improves hepatic gene transfer with adeno-associated virus type 2 (AAV2) vectors. Here, combinations of mutations in the seven potentially relevant Y residues were evaluated for further augmentation of transduction efficiency. These mutant capsids packaged viral genomes to similar titers and retained infectivity. A triple-mutant (Y444+500+730F) vector consistently had the highest level of in vivo gene transfer to murine hepatocytes, approximately threefold more efficient than the best single-mutants, and ~30-80-fold higher compared with the wild-type (WT) AAV2 capsids. Improvement of gene transfer was similar for both single-stranded AAV (ssAAV) and self-complementary AAV (scAAV) vectors, indicating that these effects are independent of viral second-strand DNA synthesis. Furthermore, Y730F and triple-mutant vectors provided a long-term therapeutic and tolerogenic expression of human factor IX (hF.IX) in hemophilia B (HB) mice after administration of a vector dose that only results in subtherapeutic and transient expression with WT AAV2 encapsidated vectors. In summary, introduction of multiple tyrosine-mutations into the AAV2 capsid results in vectors that yield at least 30-fold improvement of transgene expression, thereby lowering the required therapeutic dose and potentially vector-related immunogenicity. Such vectors should be attractive for treatment of hemophilia and other genetic diseases.

Optimized adeno-associated virus (AAV)-protein phosphatase-5 helper viruses for efficient liver transduction by single-stranded AAV vectors: therapeutic expression of factor IX at reduced vector doses

Abstract Our studies have shown that coinjection of conventional single-stranded adeno-associated virus 2 (ssAAV2) vectors carrying the enhanced green fluorescent protein (EGFP) gene with self-complementary (sc) AAV2-T cell protein tyrosine phosphatase (TC-PTP) and scAAV2-protein phosphatase-5 (PP5) vectors resulted in an approximately 16-fold increase in EGFP expression in primary murine hepatocytes in vivo [Jayandharan, G.R., Zhong, L., Li, B., Kachniarz, B., and Srivastava, A. (2008). Gene Ther. 15, 1287-1293]. In the present studies, this strategy was further optimized to achieve transgene expression at reduced vector/helper virus doses. These included the use of scAAV helper viruses containing (1) hepatocyte-specific promoters, (2) tyrosine-mutant AAV2 capsids, and (3) additional AAV serotype vectors known to efficiently transduce hepatocytes. The hepatocyte-specific transthyretin (TTR) promoter was approximately 6- to 7-fold more efficient than the Rous sarcoma virus (RSV) promoter; tyrosine-mutant AAV2 capsids were approximately 6- to 11-fold more efficient than the wild-type AAV2 capsids; and the AAV8 serotype helper virus was approximately 16-fold more efficient than AAV2 serotype helper virus. With these modifications, the vector dose of the helper virus could be further reduced by approximately 50-fold. Last, coadministration of scAAV8-PP5 helper virus increased coagulation factor IX expression from an ssAAV2 vector by approximately 7- to 10-fold, thereby achieving therapeutic levels at lower vector doses. No adverse effect on hepatocytes was observed under any of these experimental conditions. The strategy presented here should be adaptable to any ssAAV transgene cassette and, specifically, liver-directed applications of ssAAV2 vectors containing larger genes that cannot be encapsidated in scAAV vectors.

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 approximately 68% and approximately 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.

Next generation of adeno-associated virus 2 vectors: point mutations in tyrosines lead to high-efficiency transduction at lower doses

Recombinant adeno-associated virus 2 (AAV2) vectors are in use in several Phase I/II clinical trials, but relatively large vector doses are needed to achieve therapeutic benefits. Large vector doses also trigger an immune response as a significant fraction of the vectors fails to traffic efficiently to the nucleus and is targeted for degradation by the host cell proteasome machinery. We have reported that epidermal growth factor receptor protein tyrosine kinase (EGFR-PTK) signaling negatively affects transduction by AAV2 vectors by impairing nuclear transport of the vectors. We have also observed that EGFR-PTK can phosphorylate AAV2 capsids at tyrosine residues. Tyrosine-phosphorylated AAV2 vectors enter cells efficiently but fail to transduce effectively, in part because of ubiquitination of AAV capsids followed by proteasome-mediated degradation. We reasoned that mutations of the surface-exposed tyrosine residues might allow the vectors to evade phosphorylation and subsequent ubiquitination and, thus, prevent proteasome-mediated degradation. Here, we document that site-directed mutagenesis of surface-exposed tyrosine residues leads to production of vectors that transduce HeLa cells approximately 10-fold more efficiently in vitro and murine hepatocytes nearly 30-fold more efficiently in vivo at a log lower vector dose. Therapeutic levels of human Factor IX (F.IX) are also produced at an approximately 10-fold reduced vector dose. The increased transduction efficiency of tyrosine-mutant vectors is due to lack of capsid ubiquitination and improved intracellular trafficking to the nucleus. These studies have led to the development of AAV vectors that are capable of high-efficiency transduction at lower doses, which has important implications in their use in human gene therapy.

Single-polarity recombinant adeno-associated virus 2 vector-mediated transgene expression in vitro and in vivo: mechanism of transduction

Recombinant adeno-associated virus 2 (AAV) vectors encapsidate single-stranded genomes of either polarity equally frequently in separate mature virions. Because viral genomes of either polarity are transcriptionally inactive, both the failure to undergo viral second-strand DNA synthesis and the failure to undergo DNA strand annealing have been proposed as possible reasons to account for the observed low efficiency of transgene expression. We compared the transduction efficiencies of conventional AAV vectors containing both [-] and [+] polarity genomes with those containing either the [-] or the [+] polarity genomes, in vitro as well as in vivo. We document that the transduction efficiency of single-polarity AAV vectors is significantly enhanced by (i) co-infection with adenovirus; (ii) small interfering RNA (siRNA)-mediated down-modulation of a cellular protein, FKBP52, tyrosine-phosphorylated forms of which inhibit AAV second-strand DNA synthesis; (iii) over-expression of a cellular protein tyrosine phosphatase, T cell protein tyrosine phosphatase (TC-PTP), which catalyzes tyrosine-dephosphorylation of FKBP52; and (iv) deliberate over-expression of TC-PTP, or the absence of FKBP52, respectively, in TC-PTP-transgenic mice and in FKBP52-knockout mice. These data confirm that viral second-strand DNA synthesis, rather than DNA strand annealing, is the rate-limiting step in efficient transduction by AAV vectors. This finding has implications in the use of these vectors in human gene therapy.

Quiescent subpopulations of human CD34-positive hematopoietic stem cells are preferred targets for stable recombinant adeno-associated virus type 2 transduction

We have previously demonstrated recombinant adeno-associated viral (rAAV) transduction of human CD34(+) hematopoietic stem cells (HSCs) capable of serial engraftment in vivo. Here we evaluated the capacity of rAAV2 to mediate gene transfer into nondividing, quiescent, primitive CD34(+) cells subdivided on the basis of metabolic, mitotic, and phenotypic properties. Results revealed that CD34(+)CD38() marrow cells are the most quiescent, exist primarily in G(0) at isolation and are the only population to remain nondividing during the entire exposure to free rAAV. Despite significant differences in the extended clonogenic capacities of CD34(+) subsets in stromal cultures, the frequency of rAAV marking of colonies derived from primitive progenitors was similar in all three populations, suggesting that both primitive and more differentiated progenitors were initially transduced at equal levels. After transduction, episomal and integrated rAAV genomes were detected in all CD34(+) subsets. However, the more quiescent cells displayed higher levels of integrated rAAV than did rapidly dividing cells. Importantly, stable long-term integration was observed only in the most primitive, quiescent CD34(+)CD38(-) subset, indicating that this HSC compartment comprises the preferred substrate for stable rAAV2 transduction. Previously described rate limitations to transgene expression were observed in transduced CD34(+) cells and could be overcome by tyrphostin pretreatment, which resulted in augmented second-strand synthesis. These results represent the first demonstration of rAAV-mediated gene transfer to primitive, quiescent human CD34(+)CD38(-) stem cells and reveal that nondividing CD34(+)CD38(-) HSCs are the optimal CD34(+) targets for rAAV transduction.

Local expression of secondary lymphoid tissue chemokine delivered by adeno-associated virus within the tumor bed stimulates strong anti-liver tumor immunity

Development of an effective antitumor immune response depends on the appropriate interaction of effector and target cells. Thus, the expression of chemokines within the tumor may induce a more potent antitumor immune response. Secondary lymphoid tissue chemokine (SLC) is known to play a critical role in establishing a functional microenvironment in secondary lymphoid tissues. Its capacity to attract dendritic cells (DCs) and colocalize them with T cells makes it a good therapeutic candidate against cancer. In this study, we used SLC as a treatment for tumors established from a murine hepatocellular carcinoma model. SLC was encoded by recombinant adeno-associated virus (rAAV), a system chosen for the low host immunity and high efficiency of transduction, enabling long-term expression of the gene of interest. As a result, rAAV-SLC induced a significant delay of tumor progression, which was paralleled by a profound infiltration of DCs and activated CD4(+) T cells and CD8(+) T cells (CD3(+) CD69(+) cells) into the tumor site. In addition, rAAV-SLC treatment was also found to reduce tumor growth in nude mice, most likely due to inhibition of neoangiogenesis. In conclusion, local expression of SLC by rAAV represents a promising approach to induce immune-mediated regression of malignant tumors.

A dual role of EGFR protein tyrosine kinase signaling in ubiquitination of AAV2 capsids and viral second-strand DNA synthesis

A 52 kd cellular protein, FK506-binding protein (FKBP52), phosphorylated at tyrosine residues by epidermal growth factor receptor protein tyrosine kinase (EGFR-PTK), inhibits adeno-associated virus 2 (AAV2) second-strand DNA synthesis and transgene expression. FKBP52 is dephosphorylated at tyrosine residues by T-cell protein tyrosine phosphatase (TC-PTP), and TC-PTP over-expression leads to improved viral second-strand DNA synthesis and improved transgene expression. In these studies, we observed that perturbation of EGFR-PTK signaling by a specific inhibitor, Tyrphostin 23 (Tyr23), augmented the transduction efficiency of the single-stranded AAV (ssAAV) vector as well as the self-complementary AAV (scAAV) vector. Similarly, tyrosine-dephosphorylation of FKBP52 by TC-PTP resulted in increased transduction by both vectors. These data suggested that EGFR-PTK signaling also affects aspects of AAV transduction other than viral second-strand DNA synthesis. We document that inhibition of EGFR-PTK signaling leads to decreased ubiquitination of AAV2 capsids which, in turn, facilitates nuclear transport by limiting proteasome-mediated degradation of AAV vectors. We also document that Tyr23-mediated increase in AAV2 transduction efficiency is not further enhanced by a specific proteasome inhibitor, MG132. Thus, EGFR-PTK signaling modulates ubiquitin (Ub)/proteasome pathway-mediated intracellular trafficking as well as FKBP52-mediated second-strand DNA synthesis of AAV2 vectors. This has implications in the optimal use of AAV vectors in gene therapy.

A dual role of EGFR protein tyrosine kinase signaling in ubiquitination of AAV2 capsids and viral second-strand DNA synthesis

A 52 kd cellular protein, FK506-binding protein (FKBP52), phosphorylated at tyrosine residues by epidermal growth factor receptor protein tyrosine kinase (EGFR-PTK), inhibits adeno-associated virus 2 (AAV2) second-strand DNA synthesis and transgene expression. FKBP52 is dephosphorylated at tyrosine residues by T-cell protein tyrosine phosphatase (TC-PTP), and TC-PTP over-expression leads to improved viral second-strand DNA synthesis and improved transgene expression. In these studies, we observed that perturbation of EGFR-PTK signaling by a specific inhibitor, Tyrphostin 23 (Tyr23), augmented the transduction efficiency of the single-stranded AAV (ssAAV) vector as well as the self-complementary AAV (scAAV) vector. Similarly, tyrosine-dephosphorylation of FKBP52 by TC-PTP resulted in increased transduction by both vectors. These data suggested that EGFR-PTK signaling also affects aspects of AAV transduction other than viral second-strand DNA synthesis. We document that inhibition of EGFR-PTK signaling leads to decreased ubiquitination of AAV2 capsids which, in turn, facilitates nuclear transport by limiting proteasome-mediated degradation of AAV vectors. We also document that Tyr23-mediated increase in AAV2 transduction efficiency is not further enhanced by a specific proteasome inhibitor, MG132. Thus, EGFR-PTK signaling modulates ubiquitin (Ub)/proteasome pathway-mediated intracellular trafficking as well as FKBP52-mediated second-strand DNA synthesis of AAV2 vectors. This has implications in the optimal use of AAV vectors in gene therapy.

Adeno-associated virus 2-mediated gene transfer: role of a cellular serine/threonine protein phosphatase in augmenting transduction efficiency

We have documented that a cellular chaperone protein, FKBP52, when phosphorylated at tyrosine and/or serine/threonine (Ser/Thr) residues, interacts with the D-sequence in the inverted terminal repeats of the adeno-associated virus 2 (AAV) genome, inhibits the viral second-strand DNA synthesis, and leads to inefficient transgene expression from recombinant AAV vectors in certain cell types. We have also demonstrated that FKBP52 is dephosphorylated at tyrosine residues by T-cell protein tyrosine phosphatase (TC-PTP), and that deliberate overexpression of TC-PTP leads to more efficient viral second-strand DNA synthesis, and increased transgene expression. However, the identity of the putative Ser/Thr protein phosphatase that dephosphorylates FKBP52 at Ser/Thr residues has remained elusive. Using known inhibitors of Ser/Thr phosphatases, we have now identified protein phosphatase 5 (PP5) to be a candidate enzyme. Deliberate overexpression of PP5 in 293 cells, which does not influence cellular growth, leads to approximately 5-fold increase in the transduction efficiency of conventional single-stranded AAV vectors, but no significant enhancement in the transduction efficiency of self-complementary AAV vectors, suggesting that PP5 plays a role in AAV second-strand DNA synthesis. Electrophoretic mobility-shift assays show that in cells overexpressing PP5, the extent of the complex formation between FKBP52 and the AAV D-sequence is significantly reduced. These studies suggest that PP5-mediated dephosphorylation of FKBP52 at Ser/Thr residues augments viral second-strand DNA synthesis and enhances AAV transduction efficiency, which has implications in the optimal use of these vectors in human gene therapy.

Intrahepatic injection of recombinant adeno-associated virus serotype 2 overcomes gender-related differences in liver transduction

The liver is an attractive organ for gene therapy because of its important role in many inherited and acquired diseases. Recombinant adeno-associated viruses (rAAVs) have been shown to be good candidates for liver gene delivery, leading to long-term gene expression. We evaluated the influence of the route of administration on rAAV-mediated liver transduction by comparing levels of luciferase expression in the livers of male and female mice after injection of rAAV serotype 2, using three different routes of administration: intravenous (IV), intraportal (IP), or direct intrahepatic (IH) injection. To determine transgene expression we used a noninvasive optical bioluminescence imaging system that allowed long-term in vivo analysis. After IV injection dramatic differences in liver transgene expression were observed, depending on gender. When IP injection was used the differences were reduced although they were still significant. Interestingly, direct intrahepatic injection of rAAV vectors was associated with the fastest and strongest onset of luciferase expression. Moreover, no gender differences in liver transduction were observed and luciferase expression was confined to the site of injection. Thus, direct intrahepatic injection of rAAV offers specific advantages, which support the potential of this route of administration for future clinical applications.

Long-term efficacy of adeno-associated virus serotypes 8 and 9 in hemophilia a dogs and mice

We reported total correction of blood coagulation plasma factor VIII (FVIII) activity, using adeno-associated virus serotype 8 (AAV8) vectors for liver-specific gene transfer in hemophilia A mice. We now show, irrespective of immunosuppression or route of administration, total long-term correction of hemophilia A mice with pseudotyped AAV8 and AAV9 vectors. We delivered two FVIII vectors, one expressing canine heavy chain and the other expressing canine light chain. Interestingly, when these vectors were given by hepatic portal vein to hemophilia A dogs, only modest FVIII levels were seen despite the species-specific transgene. No dogs treated developed FVIII inhibitors. However, of three dogs treated with AAV8 vector, the single male, given 1.25 x 10(13) genome copies per vector per kilogram (GC/vector/kg), maintained a level of >4.5% for more than 2 years. In contrast, the two female dogs expressed only 2% FVIII activity despite receiving higher doses of 1.52 x 10(13) and 3 x 10(13) GC/vector/kg, respectively. On the other hand, a male dog treated with AAV9 vector at a low dose (6 x 10(12) GC/vector/kg) maintained FVIII levels of 2-2.5% of normal without bleeding for 200 days (observation ongoing). Although hemophilia A mice were not predictive of vector efficacy in dogs, the two treated male dogs became symptom-free for long periods. Even so, translation of these robust vectors either in appropriate large animals or human beings remains challenging.

Role of cellular FKBP52 protein in intracellular trafficking of recombinant adeno-associated virus 2 vectors

We have reported that tyrosine-phosphorylated forms of a cellular protein, FKBP52, inhibit the second-strand DNA synthesis of adeno-associated virus 2 (AAV), leading to inefficient transgene expression from recombinant AAV vectors. To further explore the role of FKBP52 in AAV-mediated transduction, we established murine embryo fibroblasts (MEFs) cultures from FKBP52 wild-type (WT), heterozygous (HE), and knockout (KO) mice. Conventional AAV vectors failed to transduce WT MEFs efficiently, and the transduction efficiency was not significantly increased in HE or KO MEFs. AAV vectors failed to traffic efficiently to the nucleus in these cells. Treatment with hydroxyurea (HU) increased the transduction efficiency of conventional AAV vectors by approximately 25-fold in WT MEFs, but only by approximately 4-fold in KO MEFs. The use of self-complementary AAV (scAAV) vectors, which bypass the requirement of viral second-strand DNA synthesis, revealed that HU treatment increased the transduction efficiency approximately 23-fold in WT MEFs, but only approximately 4-fold in KO MEFs, indicating that the lack of HU treatment-mediated increase in KO MEFs was not due to failure of AAV to undergo viral second-strand DNA synthesis. Following HU treatment, approximately 59% of AAV genomes were present in the nuclear fraction from WT MEFs, but only approximately 28% in KO MEFs, indicating that the pathway by which HU treatment mediates nuclear transport of AAV was impaired in KO MEFs. When KO MEFs were stably transfected with an FKBP52 expression plasmid, HU treatment-mediated increase in the transduction efficiency was restored in these cells, which correlated directly with improved intracellular trafficking. Intact AAV particles were also shown to interact with FKBP52 as well as with dynein, a known cellular protein involved in AAV trafficking. These studies suggest that FKBP52, being a cellular chaperone protein, facilitates intracellular trafficking of AAV, which has implications in the optimal use of recombinant AAV vectors in human gene therapy.

Evaluation of primitive murine hematopoietic stem and progenitor cell transduction in vitro and in vivo by recombinant adeno-associated virus vector serotypes 1 through 5

Conflicting data exist on hematopoietic cell transduction by AAV serotype 2 (AAV2) vectors, and additional AAV serotype vectors have not been evaluated for their efficacy in hematopoietic stem/progenitor cell transduction. We evaluated the efficacy of conventional, single-stranded AAV serotype vectors 1 through 5 in primitive murine hematopoietic stem/progenitor cells in vitro as well as in vivo. In progenitor cell assays using Sca1+ c-kit+ Lin- hematopoietic cells, 9% of the colonies in cultures infected with AAV1 expressed the transgene. Coinfection of AAV1 with self-complementary AAV vectors carrying the gene for T cell protein tyrosine phosphatase (scAAV-TC-PTP) increased the transduction efficiency to 24%, indicating that viral secondstrand DNA synthesis is a rate-limiting step. This was further corroborated by the use of scAAV vectors, which bypass this requirement. In bone marrow transplantation studies involving lethally irradiated syngeneic mice, Sca1+ c-kit+ Lin- cells coinfected with AAV1 +/- scAAV-TC-PTP vectors led to transgene expression in 2 and 7.5% of peripheral blood (PB) cells, respectively, 6 months posttransplantation. In secondary transplantation experiments, 7% of PB cells and 3% of bone marrow (BM) cells expressed the transgene 6 months posttransplantation. Approximately 21% of BM-derived colonies harbored the proviral DNA sequences in integrated forms. These results document that AAV1 is thus far the most efficient vector in transducing primitive murine hematopoietic stem/progenitor cells. Further studies involving scAAV genomes and hematopoietic cell-specific promoters should further augment the transduction efficiency of AAV1 vectors, which should have implications in the optimal use of these vectors in hematopoietic stem cell gene therapy.

Physiological Role for the Cochaperone FKBP52 in Androgen Receptor Signaling

Molecular chaperones mediate multiple aspects of steroid receptor function, but the physiological importance of most receptor-associated cochaperones has not been determined. To help fill this gap, we targeted for disruption the mouse gene for the 52-kDa FK506 binding protein, FKBP52, a 90-kDa heat shock protein (Hsp90)-binding immunophilin found in steroid receptor complexes. A mouse line lacking FKBP52 (52KO) was generated and characterized. Male 52KO mice have several defects in reproductive tissues consistent with androgen insensitivity; among these defects are ambiguous external genitalia and dysgenic prostate. FKBP52 and androgen receptor (AR) are coexpressed in prostate epithelial cells of wild-type mice. However, FKBP52 and AR are similarly coexpressed in testis even though testis morphology and spermatogenesis in 52KO males are usually normal. Molecular studies confirm that FKBP52 is a component of AR complexes, and cellular studies in yeast and human cell models demonstrate that FKBP52 can enhance AR-meditated transactivation. AR enhancement requires FKBP52 peptidylprolyl isomerase activity as well as Hsp90-binding ability, and enhancement probably relates to an affect of FKBP52 on AR-folding pathways. In the presence of FKBP52, but not other cochaperones, the function of a minimally active AR point mutant can be dramatically restored. We conclude that FKBP52 is an AR folding factor that has critically important physiological roles in some male reproductive tissues.

Adeno-associated virus: from defective virus to effective vector

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

Impaired nuclear transport and uncoating limit recombinant adeno-associated virus 2 vector-mediated transduction of primary murine hematopoietic cells

Controversies abound concerning hematopoietic stem cell transduction by recombinant adeno-associated virus 2 (AAV) vectors. For human hematopoietic cells, we have shown that this problem is related to the extent of expression of the cellular receptor for AAV. At least a small subset of murine hematopoietic cells, on the other hand, does express both the AAV receptor and the coreceptor, yet is transduced poorly. In the present study, we have found that approximately 85% of AAV genomes were present in the cytoplasmic fraction of primary murine c-Kit(+)Lin- hematopoietic cells. However, when mice were injected intraperitoneally with hydroxyurea before isolation of these cells, the extent to which AAV genomes were detected in the cytoplasmic fraction was reduced to approximately 40%, with a corresponding increase to approximately 60% in the nuclear fraction, indicating that hydroxyurea facilitated nuclear transport of AAV. It was apparent, nonetheless, that a significant fraction of the AAV genomes present in the nuclear fraction from cells obtained from hydroxyurea-treated mice was single stranded. We next tested whether the single-stranded AAV genomes were derived from virions that failed to undergo uncoating in the nucleus. A substantial fraction of the signal in the nuclear fraction of hematopoietic cells obtained from hydroxyurea-treated mice was also resistant to DNase I. That AAV particles were intact and biologically active was determined by successful transduction of 293 cells by virions recovered from murine hematopoietic cells 48 hr postinfection. Although hydroxyurea facilitated nuclear transport of AAV, most of the virions failed to undergo uncoating, thereby leading to only a partial improvement in viral second- strand DNA synthesis and transgene expression. A better understanding of the underlying mechanism of viral uncoating has implications in the optimal use of recombinant AAV vectors in hematopoietic stem cell gene therapy.

Self-complementary adeno-associated virus 2 (AAV)-T cell protein tyrosine phosphatase vectors as helper viruses to improve transduction efficiency of conventional single-stranded AAV vectors in vitro and in vivo

Recombinant vectors based on adeno-associated virus type 2 (AAV) target the liver efficiently, but the transgene expression is limited to approximately 5% of hepatocytes. The lack of efficient transduction is due, in part, to the presence of a cellular protein, FKBP52, phosphorylated forms of which inhibit the viral second-strand DNA synthesis. We have documented that dephosphorylation of FKBP52 at tyrosine residues by the cellular T cell protein tyrosine phosphatase (TC-PTP) enhances AAV-mediated transduction in primary murine hematopoietic cells from TC-PTP-transgenic mice. We have also documented that AAV-mediated transduction is significantly enhanced in hepatocytes in TC-PTP-transgenic as well as in FKBP52-deficient mice because of efficient viral second-strand DNA synthesis. In this study, we evaluated whether co-infection of conventional single-stranded AAV vectors with self-complementary AAV-TC-PTP vectors leads to increased transduction efficiency of conventional AAV vectors in established human cell lines in vitro and in primary murine hepatocytes in vivo. We demonstrate here that scAAV-TC-PTP vectors serve as a helper virus in augmenting the transduction efficiency of conventional AAV vectors in vitro as well as in vivo which correlates directly with the extent of second-strand DNA synthesis of conventional single-stranded AAV vectors. Toxicological studies following tail-vein injections of scAAV-TC-PTP vectors in experimental mice show no evidence of any adverse effect in any of the organs in any of the mice for up to 13 weeks. Thus, this novel co-infection strategy should be useful in circumventing one of the major obstacles in the optimal use of recombinant AAV vectors in human gene therapy.

Unrestricted hepatocyte transduction with adeno-associated virus serotype 8 vectors in mice

Recombinant adeno-associated virus (rAAV) vectors can mediate long-term stable transduction in various target tissues. However, with rAAV serotype 2 (rAAV2) vectors, liver transduction is confined to only a small portion of hepatocytes even after administration of extremely high vector doses. In order to investigate whether rAAV vectors of other serotypes exhibit similar restricted liver transduction, we performed a dose-response study by injecting mice with beta-galactosidase-expressing rAAV1 and rAAV8 vectors via the portal vein. The rAAV1 vector showed a blunted dose-response similar to that of rAAV2 at high doses, while the rAAV8 vector dose-response remained unchanged at any dose and ultimately could transduce all the hepatocytes at a dose of 7.2 x 10(12) vector genomes/mouse without toxicity. This indicates that all hepatocytes have the ability to process incoming single-stranded vector genomes into duplex DNA. A single tail vein injection of the rAAV8 vector was as efficient as portal vein injection at any dose. In addition, intravascular administration of the rAAV8 vector at a high dose transduced all the skeletal muscles throughout the body, including the diaphragm, the entire cardiac muscle, and substantial numbers of cells in the pancreas, smooth muscles, and brain. Thus, rAAV8 is a robust vector for gene transfer to the liver and provides a promising research tool for delivering genes to various target organs. In addition, the rAAV8 vector may offer a potential therapeutic agent for various diseases affecting nonhepatic tissues, but great caution is required for vector spillover and tight control of tissue-specific gene expression.

Intracellular viral processing, not single-stranded DNA accumulation, is crucial for recombinant adeno-associated virus transduction

Adeno-associated virus (AAV) is a unique gene transfer vector which takes approximately 4 to 6 weeks to reach its expression plateau. The mechanism for this slow-rise expression profile was proposed to be inefficient second-strand DNA synthesis from the input single-stranded (ss) DNA viral genome. In order to clarify the status of ss AAV genomes, we generated AAV vectors labeled with bromodeoxyuridine (BrdU), a nucleotide analog that can be incorporated into the AAV genome and packaged into infectious virions. Since BrdU-DNA can be detected only by an anti-BrdU antibody when DNA is in an ss form, not in a double-stranded (ds) form, ss AAV genomes with BrdU can be readily tracked in situ. Although ss AAV DNA was abundant by Southern blot analysis, free ss AAV genomes were not detectable after AAV transduction by this new detection method. Further Southern blot analysis of viral DNA and virions revealed that ss AAV DNA was protected within virions. Extracted cellular fractions demonstrated that viral particles in host cells remained infectious. In addition, a significant amount of AAV genomes was degraded after AAV transduction. Therefore, we conclude that the amount of free ss DNA is not abundant during AAV transduction. AAV transduction is limited by the steps that affect AAV ss DNA release (i.e., uncoating) before second-strand DNA synthesis can occur. AAV ss DNA released from viral uncoating is either converted into ds DNA efficiently or degraded by cellular DNA repair mechanisms as damaged DNA. This study elucidates a mechanism that can be exploited to develop new strategies to improve AAV vector transduction efficiency.