Development of a novel helper-dependent adenovirus-Epstein-Barr virus hybrid system for the stable transformation of mammalian cells

Episomes and Transposases-Utilities to Maintain Transgene Expression from Nonviral Vectors

The efficient delivery and stable transgene expression are critical for applications in gene therapy. While carefully selected and engineered viral vectors allowed for remarkable clinical successes, they still bear significant safety risks. Thus, nonviral vectors are a sound alternative and avoid genotoxicity and adverse immunological reactions. Nonviral vector systems have been extensively studied and refined during the last decades. Emerging knowledge of the epigenetic regulation of replication and spatial chromatin organisation, as well as new technologies, such as Crispr/Cas, were employed to enhance the performance of different nonviral vector systems. Thus, nonviral vectors are in focus and hold some promising perspectives for future applications in gene therapy. This review addresses three prominent nonviral vector systems: the Sleeping Beauty transposase, S/MAR-based episomes, and viral plasmid replicon-based EBV vectors. Exemplarily, we review different utilities, modifications, and new concepts that were pursued to overcome limitations regarding stable transgene expression and mitotic stability. New insights into the nuclear localisation of nonviral vector molecules and the potential consequences thereof are highlighted. Finally, we discuss the remaining limitations and provide an outlook on possible future developments in nonviral vector technology.

Generation of Integration-free Induced Pluripotent Stem Cells from Human Peripheral Blood Mononuclear Cells Using Episomal Vectors

Induced Pluripotent Stem Cells (iPSCs) hold great promise for disease modeling and regenerative therapies. We previously reported the use of Episomal Vectors (EV) to generate integration-free iPSCs from peripheral blood mononuclear cells (PB MNCs). The episomal vectors used are DNA plasmids incorporated with oriP and EBNA1 elements from the Epstein-Barr (EB) virus, which allow for replication and long-term retainment of plasmids in mammalian cells, respectively. With further optimization, thousands of iPSC colonies can be obtained from 1 mL of peripheral blood. Two critical factors for achieving high reprogramming efficiencies are: 1) the use of a 2A "self-cleavage" peptide to link OCT4 and SOX2, thus achieving equimolar expression of the two factors; 2) the use of two vectors to express MYC and KLF4 individually. Here we describe a step-by-step protocol for generating integration-free iPSCs from adult peripheral blood samples. The generated iPSCs are integration-free as residual episomal plasmids are undetectable after five passages. Although the reprogramming efficiency is comparable to that of Sendai Virus (SV) vectors, EV plasmids are considerably more economical than the commercially available SV vectors. This affordable EV reprogramming system holds potential for clinical applications in regenerative medicine and provides an approach for the direct reprogramming of PB MNCs to integration-free mesenchymal stem cells, neural stem cells, etc.

Novel HDAd/EBV Reprogramming Vector and Highly Efficient Ad/CRISPR-Cas Sickle Cell Disease Gene Correction

CRISPR/Cas enhanced correction of the sickle cell disease (SCD) genetic defect in patient-specific induced Pluripotent Stem Cells (iPSCs) provides a potential gene therapy for this debilitating disease. An advantage of this approach is that corrected iPSCs that are free of off-target modifications can be identified before differentiating the cells into hematopoietic progenitors for transplantation. In order for this approach to be practical, iPSC generation must be rapid and efficient. Therefore, we developed a novel helper-dependent adenovirus/Epstein-Barr virus (HDAd/EBV) hybrid reprogramming vector, rCLAE-R6, that delivers six reprogramming factors episomally. HDAd/EBV transduction of keratinocytes from SCD patients resulted in footprint-free iPSCs with high efficiency. Subsequently, the sickle mutation was corrected by delivering CRISPR/Cas9 with adenovirus followed by nucleoporation with a 70 nt single-stranded oligodeoxynucleotide (ssODN) correction template. Correction efficiencies of up to 67.9% (β(A)/[β(S)+β(A)]) were obtained. Whole-genome sequencing (WGS) of corrected iPSC lines demonstrated no CRISPR/Cas modifications in 1467 potential off-target sites and no modifications in tumor suppressor genes or other genes associated with pathologies. These results demonstrate that adenoviral delivery of reprogramming factors and CRISPR/Cas provides a rapid and efficient method of deriving gene-corrected, patient-specific iPSCs for therapeutic applications.

Enhanced Generation of Integration-free iPSCs from Human Adult Peripheral Blood Mononuclear Cells with an Optimal Combination of Episomal Vectors

We previously reported the generation of integration-free induced pluripotent stem cells from adult peripheral blood (PB) with an improved episomal vector (EV) system, which uses the spleen focus-forming virus U3 promoter and an extra factor BCL-XL (B). Here we show an ∼100-fold increase in efficiency by optimizing the vector combination. The two most critical factors are: (1) equimolar expression of OCT4 (O) and SOX2 (S), by using a 2A linker; (2) a higher and gradual increase in the MYC (M) to KLF4 (K) ratio during the course of reprogramming, by using two individual vectors to express M and K instead of one. The combination of EV plasmids (OS + M + K + B) is comparable with Sendai virus in reprogramming efficiency but at a fraction of the cost. The generated iPSCs are indistinguishable from those from our previous approach in pluripotency and phenotype. This improvement lays the foundation for broad applications of episomal vectors in PB reprogramming.

•

Expression of MYC and KLF4 with two episomal vectors is critical for PB reprogramming

•

Optimized episomal vector combination shows an ∼100-fold increase in reprogramming

•

This system is comparable with Sendai virus in generating integration-free iPSCs

Hybrid adeno-associated viral vectors utilizing transposase-mediated somatic integration for stable transgene expression in human cells

Recombinant adeno-associated viral (AAV) vectors have been shown to be one of the most promising vectors for therapeutic gene delivery because they can induce efficient and long-term transduction in non-dividing cells with negligible side-effects. However, as AAV vectors mostly remain episomal, vector genomes and transgene expression are lost in dividing cells. Therefore, to stably transduce cells, we developed a novel AAV/transposase hybrid-vector. To facilitate SB-mediated transposition from the rAAV genome, we established a system in which one AAV vector contains the transposon with the gene of interest and the second vector delivers the hyperactive Sleeping Beauty (SB) transposase SB100X. Human cells were infected with the AAV-transposon vector and the transposase was provided in trans either by transient and stable plasmid transfection or by AAV vector transduction. We found that groups which received the hyperactive transposase SB100X showed significantly increased colony forming numbers indicating enhanced integration efficiencies. Furthermore, we found that transgene copy numbers in transduced cells were dose-dependent and that predominantly SB transposase-mediated transposition contributed to stabilization of the transgene. Based on a plasmid rescue strategy and a linear-amplification mediated PCR (LAM-PCR) protocol we analysed the SB100X-mediated integration profile after transposition from the AAV vector. A total of 1840 integration events were identified which revealed a close to random integration profile. In summary, we show for the first time that AAV vectors can serve as template for SB transposase mediated somatic integration. We developed the first prototype of this hybrid-vector system which with further improvements may be explored for treatment of diseases which originate from rapidly dividing cells.

Cellular reprogramming of human peripheral blood cells

Breakthroughs in cell fate conversion have made it possible to generate large quantities of patient-specific cells for regenerative medicine. Due to multiple advantages of peripheral blood cells over fibroblasts from skin biopsy, the use of blood mononuclear cells (MNCs) instead of skin fibroblasts will expedite reprogramming research and broaden the application of reprogramming technology. This review discusses current progress and challenges of generating induced pluripotent stem cells (iPSCs) from peripheral blood MNCs and of in vitro and in vivo conversion of blood cells into cells of therapeutic value, such as mesenchymal stem cells, neural cells and hepatocytes. An optimized design of lentiviral vectors is necessary to achieve high reprogramming efficiency of peripheral blood cells. More recently, non-integrating vectors such as Sendai virus and episomal vectors have been successfully employed in generating integration-free iPSCs and somatic stem cells.

Enhanced and prolonged baculovirus-mediated expression by incorporating recombinase system and in cis elements: a comparative study

Baculovirus (BV) is a promising gene vector but mediates transient expression. To prolong the expression, we developed a binary system whereby the transgene in the substrate BV was excised by the recombinase (ΦC31o, Cre or FLPo) expressed by a second BV and recombined into smaller minicircle. The recombination efficiency was lower by ΦC31o (≈40–75%), but approached ≈90–95% by Cre and FLPo in various cell lines and stem cells [e.g. human adipose-derived stem cells (hASCs)]. Compared with FLPo, Cre exerted higher expression level and lower negative effects; thus, we incorporated additional cis-acting element [oriP/Epstein–Barr virus nuclear antigen 1 (EBNA1), scaffold/matrix attached region or human origin of replication (ori)] into the Cre-based BV system. In proliferating cells, only oriP/EBNA1 prolonged the transgene expression and maintained the episomal minicircles for 30 days without inadvertent integration, whereas BV genome was degraded in 10 days. When delivering bmp2 or vegf genes, the efficient recombination/minicircle formation prolonged and enhanced the growth factor expression in hASCs. The prolonged bone morphogenetic protein 2 expression ameliorated the osteogenesis of hASCs, a stem cell with poor osteogenesis potential. Altogether, this BV vector exploiting Cre-mediated recombination and oriP/EBNA1 conferred remarkably high recombination efficiency, which prolonged and enhanced the transgene expression in dividing and non-dividing cells, thereby broadening the applications of BV.

A rapid Q-PCR titration protocol for adenovirus and helper-dependent adenovirus vectors that produces biologically relevant results

Adenoviruses are employed in the study of cellular processes and as expression vectors used in gene therapy. The success and reproducibility of these studies is dependent in part on having accurate and meaningful titers of replication competent and helper-dependent adenovirus stocks, which is problematic due to the use of varied and divergent titration protocols. Physical titration methods, which quantify the total number of viral particles, are used by many, but are poor at estimating activity. Biological titration methods, such as plaque assays, are more biologically relevant, but are time consuming and not applicable to helper-dependent gene therapy vectors. To address this, a protocol was developed called "infectious genome titration" in which viral DNA is isolated from the nuclei of cells ~3 h post-infection, and then quantified by Q-PCR. This approach ensures that only biologically active virions are counted as part of the titer determination. This approach is rapid, robust, sensitive, reproducible, and applicable to all forms of adenovirus. Unlike other Q-PCR-based methods, titers determined by this protocol are well correlated with biological activity.

A Novel Adenoviral Hybrid-vector System Carrying a Plasmid Replicon for Safe and Efficient Cell and Gene Therapeutic Applications

In dividing cells, the two aims a gene therapeutic approach should accomplish are efficient nuclear delivery and retention of therapeutic DNA. For stable transgene expression, therapeutic DNA can either be maintained by somatic integration or episomal persistence of which the latter approach would diminish the risk of insertional mutagenesis. As most monosystems fail to fulfill both tasks with equal efficiency, hybrid-vector systems represent promising alternatives. Our hybrid-vector system synergizes high-capacity adenoviral vectors (HCAdV) for efficient delivery and the scaffold/matrix attachment region (S/MAR)–based pEPito plasmid replicon for episomal persistence. After proving that this plasmid replicon can be excised from adenovirus in vitro, colony forming assays were performed. We found an increased number of colonies of up to sevenfold in cells that received the functional plasmid replicon proving that the hybrid-vector system is functional. Transgene expression could be maintained for 6 weeks and the extrachromosomal plasmid replicon was rescued. To show efficacy in vivo, the adenoviral hybrid-vector system was injected into C57Bl/6 mice. We found that the plasmid replicon can be released from adenoviral DNA in murine liver resulting in long-term transgene expression. In conclusion, we demonstrate the efficacy of our novel HCAdV-pEPito hybrid-vector system in vitro and in vivo.

Developing extrachromosomal gene expression vector technologies: an overview

Extrachromosomal, or episomal, vectors offer a number of advantages for therapeutic and scientific applications compared to integrating vectors. Extrachromosomal vectors persist in the nucleus without the requirement to integrate into the host genome, hence avoiding the recent concerns surrounding the genotoxic effects of vector integration. By avoiding integration, episomal vectors avoid vector rearrangement, which can occur at integration, and also avoid any effect of surrounding DNA activity on transgene expression ("position effect"). Extrachromosomal vectors offer a very high transgene capacity, allowing either the incorporation of large promoter and regulatory elements into an expression cassette, or the use of complete genomic loci of up to 100 kb or larger as transgenes. Whole genomic loci transgenes offer an elegant means to express genes under physiological and developmental-stage regulation, to express multiple transcript variants from a single locus, and to express multiple genes from a single tract of genomic DNA. The combined advantages of episomal vectors of prolonged transgene persistence in the absence of vector integration, avoiding silencing by flanking heterochromatin, and high capacity, facilitating delivery and expression of genomic DNA transgenes, will be reviewed here and potential therapeutic and scientific uses outlined.

LDLR-Gene therapy for familial hypercholesterolaemia: problems, progress, and perspectives

Coronary artery diseases (CAD) inflict a heavy economical and social burden on most populations and contribute significantly to their morbidity and mortality rates. Low-density lipoprotein receptor (LDLR) associated familial hypercholesterolemia (FH) is the most frequent Mendelian disorder and is a major risk factor for the development of CAD. To date there is no cure for FH. The primary goal of clinical management is to control hypercholesterolaemia in order to decrease the risk of atherosclerosis and to prevent CAD. Permanent phenotypic correction with single administration of a gene therapeutic vector is a goal still needing to be achieved. The first ex vivo clinical trial of gene therapy in FH was conducted nearly 18 years ago. Patients who had inherited LDLR gene mutations were subjected to an aggressive surgical intervention involving partial hepatectomy to obtain the patient's own hepatocytes for ex vivo gene transfer with a replication deficient LDLR-retroviral vector. After successful re-infusion of transduced cells through a catheter placed in the inferior mesenteric vein at the time of liver resection, only low-level expression of the transferred LDLR gene was observed in the five patients enrolled in the trial. In contrast, full reversal of hypercholesterolaemia was later demonstrated in in vivo preclinical studies using LDLR-adenovirus mediated gene transfer. However, the high efficiency of cell division independent gene transfer by adenovirus vectors is limited by their short-term persistence due to episomal maintenance and the cytotoxicity of these highly immunogenic viruses. Novel long-term persisting vectors derived from adeno-associated viruses and lentiviruses, are now available and investigations are underway to determine their safety and efficiency in preparation for clinical application for a variety of diseases. Several novel non-viral based therapies have also been developed recently to lower LDL-C serum levels in FH patients. This article reviews the progress made in the 18 years since the first clinical trial for gene therapy of FH, with emphasis on the development, design, performance and limitations of viral based gene transfer vectors used in studies to ameliorate the effects of LDLR deficiency.

High-level expression by tissue/cancer-specific promoter with strict specificity using a single-adenoviral vector

Tissue-/cancer-specific promoters for use in adenovirus vectors (AdVs) are valuable for elucidating specific gene functions and for use in gene therapy. However, low activity, non-specific expression and size limitations in the vector are always problems. Here, we developed a 'double-unit' AdV containing the Cre gene under the control of an α-fetoprotein promoter near the right end of its genome and bearing a compact 'excisional-expression' unit consisting of a target cDNA 'upstream' of a potent promoter between two loxPs near the left end of its genome. When Cre was expressed, the expression unit was excised as a circular molecule and strongly expressed. Undesired leak expression of Cre during virus preparation was completely suppressed by a dominant-negative Cre and a short-hairpin RNA against Cre. Using this novel construct, a very strict specificity was maintained while achieving a 40- to 90-fold higher expression level, compared with that attainable using a direct specific promoter. Therefore, the 'double-unit' AdV enabled us to produce a tissue-/cancer-specific promoter in an AdV with a high expression level and strict specificity.

When Cre-mediated recombination in mice does not result in protein loss

Cre/loxP recombination enables cellular specificity and, in the case of inducible systems, temporal control of genomic deletions. Here we used a SM22α tamoxifen-inducible Cre line to inactivate β1 integrin in adult smooth muscle. Interestingly, analysis of two distinct β1 loxP transgenic mice revealed vastly different outcomes after β1 integrin deletion. Lethality occurred 4 weeks postinduction in one Cre/loxP line, while no apparent phenotype was seen in the other line. Genetic analysis revealed appropriate DNA excision in both cases; however, differences were found in the degree of protein loss with absolutely no change in protein levels in the model that lacked a phenotype. Seeking to understand protein persistence despite appropriate recombination, we first validated the flox allele using a constitutive Cre line and demonstrated its ability to mediate effective protein inactivation. We then examined the possibility of heterozygous cell selection, protein turnover, and deletion efficiency with no success for explaining the phenotype. Finally, we documented the presence of the Cre-recombination episomal product, which persisted in tissue samples with no protein loss. The product was only noted in cells with low proliferative capacity. These findings highlight the potential for protein expression from the products of Cre-recombinase excised genes, particularly when deletion occurs in low turnover populations.

Delivery of an EBV episome by a self-circularizing helper-dependent adenovirus: long-term transgene expression in immunocompetent mice

Epstein-Barr virus (EBV) evolved an episomal system for maintaining life-long, latent infection of human B lymphocytes. Circular episomes engineered from EBV components required for this latent form of infection have the capacity to persist in most types of replicating mammalian cells without DNA integration and the pitfalls of insertional mutagenesis. EBV episomes are typically transduced using low-efficiency methods. Here we present a method for efficient delivery of EBV episomes to nuclei of hepatocytes in living mice using a helper-dependent adenoviral vector and Cre-mediated recombination in vivo to generate circular EBV episomes following infection. Cre is transiently expressed from a hepatocyte-specific promoter so that vector generation and transgene expression are tissue specific. We show long-term persistence of the circularized vector DNA and expression of a reporter gene in hepatocytes of immunocompetent mice.

In vivo stable transduction of humanized liver tissue in chimeric mice via high-capacity adenovirus-lentivirus hybrid vector

We developed hybrid vectors employing high-capacity adenovirus as a first-stage carrier encoding all the components required for in situ production of a second-stage lentivirus, thereby achieving stable transgene expression in secondary target cells. Such vectors have never previously been tested in normal tissues, because of the scarcity of suitable in vivo systems permissive for second-stage lentivirus assembly. Here we employed a novel murine model in which endogenous liver tissue is extensively reconstituted with engrafted human hepatocytes, and successfully achieved stable transduction by the second-stage lentivirus produced in situ from first-stage adenovirus. This represents the first demonstration of the functionality of adenoviral-lentiviral hybrid vectors in a normal parenchymal organ in vivo.

Stable transgene expression in primitive human CD34+ hematopoietic stem/progenitor cells, using the Sleeping Beauty transposon system

Sleeping Beauty (SB) transposon-mediated integration has been shown to achieve long-term transgene expression in a wide range of host cells. In this study, we improved the SB transposon-mediated gene transfer system for transduction of human CD34(+) stem/progenitor cells by two approaches: (1) to increase the transposition efficacy, a hyperactive mutant of SB, HSB, was used; (2) to improve the expression of the SB transposase and the transgene cassette carried by the transposon, different viral and cellular promoters were evaluated. SB components were delivered in trans into the target cells by Nucleoporation. The SB transposon-mediated integration efficacy was assessed by integrated transgene (enhanced green fluorescent protein [eGFP]) expression both in vitro and in vivo. In purified human cord blood CD34(+) cells, HSB achieved long-term transgene expression in nearly 7-fold more cells than the original SB transposase. Significantly brighter levels of eGFP expression (5-fold) were achieved with the human elongation factor 1alpha (EF1-alpha) promoter in Jurkat human T cells, compared with that achieved with the modified myeloproliferative sarcoma virus long terminal repeat enhancer-promoter (MNDU3); in contrast, the MNDU3 promoter expressed eGFP at the highest level in K-562 myeloid cells. In human CD34(+) cord blood cells studied under conditions directing myeloid differentiation, the highest transgene integration and expression were achieved using the EF1-alpha promoter to express the SB transposase combined with the MNDU3 promoter to express the eGFP reporter. Stable transgene expression was achieved at levels up to 27% for more than 4 weeks of culture after improved gene transfer to CD34(+) cells (average, 17%; n = 4). In vivo studies evaluating engraftment and differentiation of the SB-modified human CD34(+) cells demonstrated that SB-modified human CD34(+) cells engrafted in NOD/SCID/gamma chain(null) (NSG) mice and differentiated into multilineage cell types with eGFP expression. More importantly, secondary transplantation studies demonstrated that the integrated transgene was stably expressed in more primitive CD34(+) hematopoietic stem cells (HSCs) with long-term repopulating capability. This study demonstrates that an improved HSB gene transfer system can stably integrate genes into primitive human HSCs while maintaining the pluripotency of the stem cells, which shows promise for further advancement of non-virus-based gene therapy using hematopoietic stem cells.

A systematic approach to virus-virus interactions

A virus–virus interaction is a measurable difference in the course of infection of one virus as a result of a concurrent or prior infection by a different species or strain of virus. Many such interactions have been discovered by chance, yet they have rarely been studied systematically. Increasing evidence suggests that virus–virus interactions are common and may be critical to understanding viral pathogenesis in natural hosts. In this review we propose a system for classifying virus–virus interactions by organizing them into three main categories: (1) direct interactions of viral genes or gene products, (2) indirect interactions that result from alterations in the host environment, and (3) immunological interactions. We have so far identified 15 subtypes of interaction and assigned each to one of these categories. It is anticipated that this framework will provide for a more systematic approach to investigating virus–virus interactions, both at the cellular and organismal levels.

Antitumor therapy based on cellular competition

A major obstacle for the efficacy of cancer gene therapy is the need to transduce a high proportion of tumor cells with genes that directly or indirectly cause their death. During the formation of certain organs, cells compete among themselves to colonize the whole tissue. We reasoned that cell competition could be used to increase the proportion of cells that become transfected in a tumor. For this, a transgene that provides a selective advantage to the transfected cells should be used. If the same gene conferred a suicide mechanism the tumor could be eradicated after a period of selection. Bystander effect of transfected cells over neighboring nonmodified cells may eliminate tumors even with incomplete replacement of tumor cells. To test this strategy a competitive advantage was provided to colon cancer cells, using a gene encoding a fusion protein of dihydrofolate reductase (DHFR) and thymidine kinase (TK). DHFR confers resistance to methotrexate (MTX) and TK confers sensitivity to ganciclovir (GCV). Modified cells were also transduced with green fluorescent protein and parental cells with red fluorescent protein. In vitro and in vivo experiments were performed, using various proportions of modified cells and applying positive selection with MTX followed by negative selection with GCV. In vitro, cell competition was evident. Under MTX treatment, tumor cells transfected with the DHFR-TK fusion gene efficiently replaced the parental cells (from 0.1 to 90% in 35 days). After this positive selection period, negative selection with GCV eliminated the transfected cells. In vivo, positive selection was also achieved and resulted in a statistically significant therapeutic effect.

A rapid protocol for construction and production of high-capacity adenoviral vectors

High-capacity adenoviral vectors (HC-AdVs) lacking all viral coding sequences were shown to result in long-term transgene expression and phenotypic correction in small and large animal models. It has been established that HC-AdVs show significantly reduced toxicity profiles compared with early-generation adenoviral vectors. Furthermore, with capsid-modified HC-AdV becoming available, we are just starting to understand the full potential of this vector system. However, for many researchers, the wide-scale use of HC-AdV is hampered by labor-intensive and complex production procedures. Herein, we provide a feasible and detailed protocol for efficient generation of HC-AdV. We introduce an efficient cloning strategy for the generation of recombinant HC-AdV vector genomes. Infection and amplification of the HC-AdV are performed in a spinner culture system. For purification, we routinely apply cesium chloride gradients. Finally, we describe various methods for establishing vector titers. Generation of high-titer HC-AdV can be achieved in 3 weeks.

Development of a hybrid baculoviral vector for sustained transgene expression

Baculovirus is a promising gene delivery vector but its widespread application is impeded as it only mediates transient transgene expression in mammalian cells. To prolong the expression, we developed a dual baculovirus system whereby one baculovirus expressed FLP recombinase while the other harbored an Frt-flanking cassette encompassing the transgene and oriP/EBNA1 derived from Epstein-Barr virus. After cotransduction of cells, the expressed FLP cleaved the Frt-flanking cassette off the baculovirus genome and catalyzed circular episome formation, then oriP/EBNA1 within the cassette enabled the self-replication of episomes. The excision/recombination efficiency was remarkably enhanced by sodium butyrate, reaching 75% in human embryonic kidney-293 (HEK293) cells, 85% in baby-hamster kidney (BHK) cells, 77% in primary chondrocytes, and 48% in mesenchymal stem cells (MSCs). The hybrid baculovirus substantially prolonged the transgene expression to approximately 48 days without selection and >63 days with selection, thanks to the maintenance of replicons and transgene transcription. In contrast to the replicating episomes, the baculovirus genome was rapidly degraded. Furthermore, an osteoinductive growth factor gene was efficiently delivered into MSCs using this system, which not only prolonged the growth factor expression but also potentiated the osteogenesis of MSCs. These data collectively implicate the potential of this hybrid baculovirus system in gene therapy applications necessitating sustained transgene expression.

Robust in vivo transduction of a genetically stable Epstein-Barr virus episome to hepatocytes in mice by a hybrid viral vector

To make a safe, long-lasting gene delivery vehicle, we developed a hybrid vector that leverages the relative strengths of adenovirus and Epstein-Barr virus (EBV). A fully gene-deleted helper-dependent adenovirus (HDAd) is used as the delivery vehicle for its scalability and high transduction efficiency. Upon delivery, a portion of the HDAd vector is recombined to form a circular plasmid. This episome includes two elements from EBV: an EBV nuclear antigen 1 (EBNA1) expression cassette and an EBNA1 binding region. Along with a human replication origin, these elements provide considerable genetic stability to the episome in replicating cells while avoiding insertional mutagenesis. Here, we demonstrate that this hybrid approach is highly efficient at delivering EBV episomes to target cells in vivo. We achieved nearly 100% transduction of hepatocytes after a single intravenous injection in mice. This is a substantial improvement over the transduction efficiency of previously available physical and viral methods. Bioluminescent imaging of vector-transduced mice demonstrated that luciferase transgene expression from the hybrid was robust and compared well to a traditional HDAd vector. Quantitative PCR analysis confirmed that the EBV episome was stable at approximately 30 copies per cell for up to 50 weeks and that it remained circular and extrachromosomal. Approaches for adapting the HDAd-EBV hybrid to a variety of disease targets and the potential benefits of this approach are discussed.

Advances in high-capacity extrachromosomal vector technology: episomal maintenance, vector delivery, and transgene expression

Recent developments in extrachromosomal vector technology have offered new ways of designing safer, physiologically regulated vectors for gene therapy. Extrachromosomal, or episomal, persistence in the nucleus of transduced cells offers a safer alternative to integrating vectors which have become the subject of safety concerns following serious adverse events in recent clinical trials. Extrachromosomal vectors do not cause physical disruption in the host genome, making these vectors safe and suitable tools for several gene therapy targets, including stem cells. Moreover, the high insert capacity of extrachromosomal vectors allows expression of a therapeutic transgene from the context of its genomic DNA sequence, providing an elegant way to express normal splice variants and achieve physiologically regulated levels of expression. Here, we describe past and recent advances in the development of several different extrachromosomal systems, discuss their retention mechanisms, and evaluate their use as expression vectors to deliver and express genomic DNA loci. We also discuss a variety of delivery systems, viral and nonviral, which have been used to deliver episomal vectors to target cells in vitro and in vivo. Finally, we explore the potential for the delivery and expression of extrachromosomal transgenes in stem cells. The long-term persistence of extrachromosomal vectors combined with the potential for stem cell proliferation and differentiation into a wide range of cell types offers an exciting prospect for therapeutic interventions.

Enhanced expression of hypersensitive α4* nAChR in adult mice increases the loss of midbrain dopaminergic neurons

We describe an inducible genetic model for degeneration of midbrain dopaminergic neurons in adults. In previous studies, knock-in mice expressing hypersensitive M2 domain Leu9'Ser (L9'S) alpha4 nicotinic receptors (nAChR) at near-normal levels displayed dominant neonatal lethality and dopaminergic deficits in embryonic midbrain, because the hypersensitive nAChR is excitotoxic. However, heterozygous L9'S mice that retain the neomycin resistance cassette (neo) in a neighboring intron express low levels of the mutant allele (approximately 25% of normal levels), and these neo-intact mice are therefore viable and fertile. The neo cassette is flanked by loxP sites. In adult animals, we locally injected helper-dependent adenovirus (HDA) expressing cre recombinase. Local excision of the neo cassette, via cre-mediated recombination, was verified by genomic analysis. In L9'S HDA-cre injected animals, locomotion was reduced both under baseline conditions and after amphetamine application. There was no effect in L9'S HDA-control treated animals or in wild-type (WT) littermates injected with either virus. Immunocytochemical analyses revealed marked losses (> 70%) of dopaminergic neurons in L9'S HDA-cre injected mice compared to controls. At 20-33 days postinjection in control animals, the coexpressed marker gene, yellow fluorescent protein (YFP), was expressed in many neurons and few glial cells near the injection, emphasizing the neurotropic utility of the HDA. Thus, HDA-mediated gene transfer into adult midbrain induced sufficient functional expression of cre in dopaminergic neurons to allow for postnatal deletion of neo. This produced increased L9'S mutant nAChR expression, which in turn led to nicotinic cholinergic excitotoxicity in dopaminergic neurons.

Versatility of gene therapy vectors through viruses

Several viruses have been engineered for gene therapy applications, and the specific properties of each viral vector have been exploited to target a variety of inherited and acquired diseases. Preclinical and clinical studies demonstrated that viral vectors are highly versatile tools capable of efficient transfer of foreign genetic information into almost all cell types and tissues. Gene therapy applications depend on vector characteristics, such as host range, cell- or tissue-specific targeting, genome integration, efficiency and duration of transgene expression, packaging capacity, and suitability for scale-up production. This review discusses the advances in the development of viral vectors, with particular emphasis on how knowledge of virus biology has been exploited to design a variety of vectors with improved safety characteristics and efficiency, potentially suitable for a large number of gene therapy applications.

PTEN deletion in Bergmann glia leads to premature differentiation and affects laminar organization

Development of the central nervous system is controlled by both intrinsic and extrinsic signals that guide neuronal migration to form laminae. Although defects in neuronal mobility have been well documented as a mechanism for abnormal laminar formation, the role of radial glia, which provide the environmental cues, in modulating neuronal migration is less clear. We provide evidence that loss of PTEN in Bergmann glia leads to premature differentiation of this crucial cell population and subsequently to extensive layering defects. Accordingly, severe granule neuron migration defects and abnormal laminar formation are observed. These results uncover an unexpected role for PTEN in regulating Bergmann glia differentiation, as well as the importance of time-dependent Bergmann glia differentiation during cerebellar development.

Targeting site-specific chromosome integration

The concept of gene therapy was introduced with great promise and high expectations. However, what appeared simple in theory has not translated into practice. Despite some success in clinical trials, the research community is still facing an old problem: namely, the need for a vector that can deliver a gene to target cells without adverse events while maintaining a long-term therapeutic effect. Some of these challenges are being addressed by the development of hybrid vectors which meld two different viral systems to incorporate efficient gene delivery and large cloning capacity with site-specific integration. The two known systems that integrate genes into specific sites in mammalian genomes are the adeno-associated virus and phage integrases. Recent experiments with hybrid vectors incorporating both of these systems are encouraging. However, extensive research should be directed towards the safety and efficacy of this approach before it will be available for gene therapy.

Helper-dependent adenoviral vectors in experimental gene therapy

In the majority of potential applications gene therapy will require an effective transfer of a transgene in vivo resulting in high-level and long-term transgene expression, all in the absence of significant toxicity or inflammatory responses. The most efficient vehicles for delivery of foreign genes to the target tissues are modified adenoviruses. Adenoviral vectors of the first generation, despite the high infection efficacy, have an essential drawback: they induce strong immune response, which leads to short term expression of the transgene, and limits their usefulness in clinical trials. In contrast, helper-dependent adenoviral vectors (HdAd) lacking all viral coding sequences display only minimal immunogenicity and negligible side-effects, allowing for long-term transgene expression. Thus, HdAd vehicles have become the carrier of choice for adenoviral vector-mediated experimental gene therapy, effectively used in animal models for delivery of transgenes into the liver, skeletal muscle, myocardium or brain. Strong and long-lasting expression of therapeutic genes has allowed for successful treatment of dyslipidemias, muscular dystrophy, obesity, hemophilia, and diabetes. Additionally, the large cloning capacity of HdAd, up to 37 kb, facilitates the use of physiologically regulated, endogenous promoters, instead of artificial viral promoter sequences. This enables also generation of the single vectors expressing multiple genes, which can be potentially useful for treatment of polygenic diseases. In this review we characterize the basic features of HdAd vectors and describe some of their experimental and potential clinical applications.