Endothelial targeting of the Sleeping Beauty transposon within lung

Wide Awake and Ready to Move: 20 Years of Non-Viral Therapeutic Genome Engineering with the Sleeping Beauty Transposon System

Gene therapies will only become a widespread tool in the clinical treatment of human diseases with the advent of gene transfer vectors that integrate genetic information stably, safely, effectively, and economically. Two decades after the discovery of the Sleeping Beauty (SB) transposon, it has been transformed into a vector system that is fulfilling these requirements. SB may well overcome some of the limitations associated with viral gene transfer vectors and transient non-viral gene delivery approaches that are being used in the majority of ongoing clinical trials. The SB system has achieved a high level of stable gene transfer and sustained transgene expression in multiple primary human somatic cell types, representing crucial steps that may permit its clinical use in the near future. This article reviews the most important aspects of SB as a tool for gene therapy, including aspects of its vectorization and genomic integration. As an illustration, the clinical development of the SB system toward gene therapy of age-related macular degeneration and cancer immunotherapy is highlighted.

Gene Therapy with the Sleeping Beauty Transposon System

The widespread clinical implementation of gene therapy requires the ability to stably integrate genetic information through gene transfer vectors in a safe, effective, and economical manner. The latest generation of Sleeping Beauty (SB) transposon vectors fulfills these requirements, and may overcome limitations associated with viral gene transfer vectors and transient nonviral gene delivery approaches that are prevalent in ongoing clinical trials. The SB system enables high-level stable gene transfer and sustained transgene expression in multiple primary human somatic cell types, thereby representing a highly attractive gene transfer strategy for clinical use. Here, we review the most important aspects of using SB for gene therapy, including vectorization as well as genomic integration features. We also illustrate the path to successful clinical implementation by highlighting the application of chimeric antigen receptor (CAR)-modified T cells in cancer immunotherapy.

Going non-viral: the Sleeping Beauty transposon system breaks on through to the clinical side

Molecular medicine has entered a high-tech age that provides curative treatments of complex genetic diseases through genetically engineered cellular medicinal products. Their clinical implementation requires the ability to stably integrate genetic information through gene transfer vectors in a safe, effective and economically viable manner. The latest generation of Sleeping Beauty (SB) transposon vectors fulfills these requirements, and may overcome limitations associated with viral gene transfer vectors and transient non-viral gene delivery approaches that are prevalent in ongoing pre-clinical and translational research. The SB system enables high-level stable gene transfer and sustained transgene expression in multiple primary human somatic cell types, thereby representing a highly attractive gene transfer strategy for clinical use. Here we review several recent refinements of the system, including the development of optimized transposons and hyperactive SB variants, the vectorization of transposase and transposon as mRNA and DNA minicircles (MCs) to enhance performance and facilitate vector production, as well as a detailed understanding of SB's genomic integration and biosafety features. This review also provides a perspective on the regulatory framework for clinical trials of gene delivery with SB, and illustrates the path to successful clinical implementation by using, as examples, gene therapy for age-related macular degeneration (AMD) and the engineering of chimeric antigen receptor (CAR)-modified T cells in cancer immunotherapy.

Genomic analysis of Sleeping Beauty transposon integration in human somatic cells

The Sleeping Beauty (SB) transposon is a non-viral integrating vector system with proven efficacy for gene transfer and functional genomics. However, integration efficiency is negatively affected by the length of the transposon. To optimize the SB transposon machinery, the inverted repeats and the transposase gene underwent several modifications, resulting in the generation of the hyperactive SB100X transposase and of the high-capacity “sandwich” (SA) transposon. In this study, we report a side-by-side comparison of the SA and the widely used T2 arrangement of transposon vectors carrying increasing DNA cargoes, up to 18 kb. Clonal analysis of SA integrants in human epithelial cells and in immortalized keratinocytes demonstrates stability and integrity of the transposon independently from the cargo size and copy number-dependent expression of the cargo cassette. A genome-wide analysis of unambiguously mapped SA integrations in keratinocytes showed an almost random distribution, with an overrepresentation in repetitive elements (satellite, LINE and small RNAs) compared to a library representing insertions of the first-generation transposon vector and to gammaretroviral and lentiviral libraries. The SA transposon/SB100X integrating system therefore shows important features as a system for delivering large gene constructs for gene therapy applications.

Endothelial indoleamine 2,3-dioxygenase protects against development of pulmonary hypertension

RATIONALE

A proliferative and apoptosis-resistant phenotype in pulmonary arterial smooth muscle cells (PASMCs) is key to pathologic vascular remodeling in pulmonary hypertension (PH). Expression of indoleamine-2,3-dioxygenase (IDO) by vascular endothelium is a newly identified vasomotor-regulatory mechanism also involved in molecular signaling cascades governing vascular smooth muscle cell (vSMC) plasticity.

OBJECTIVES

To investigate the therapeutic potential of enhanced endothelial IDO in development of PH and its associated vascular remodeling.

METHODS

We used loss and gain of function in vivo studies to establish the role and determine the therapeutic effect of endothelial IDO in hypoxia-induced PH in mice and monocrotaline-induced PH in rats. We also studied PASMC phenotype in an IDO-high in vivo and in vitro tissue microenvironment.

MEASUREMENTS AND MAIN RESULTS

The endothelium was the primary site for endogenous IDO production within mouse lung, and the mice lacking this gene had exaggerated hypoxia-induced PH. Conversely, augmented pulmonary endothelial IDO expression, through a human IDO-encoding Sleeping Beauty (SB)-based nonviral gene-integrating approach, halted and attenuated the development of PH, right ventricular hypertrophy, and vascular remodeling in both preclinical models of PH. IDO derived from endothelial cells promoted apoptosis in PH-PASMCs through depolarization of mitochondrial transmembrane potential and down-regulated PH-PASMC proliferative/synthetic capacity through enhanced binding of myocardin to CArG box DNA sequences present within the promoters of vSMC differentiation-specific genes.

CONCLUSIONS

Enhanced endothelial IDO ameliorates PH and its associated vascular structural remodeling through paracrine phenotypic modulation of PH-PASMCs toward a proapoptotic and less proliferative/synthetic state.

DNA transposon-based gene vehicles - scenes from an evolutionary drive

DNA transposons are primitive genetic elements which have colonized living organisms from plants to bacteria and mammals. Through evolution such parasitic elements have shaped their host genomes by replicating and relocating between chromosomal loci in processes catalyzed by the transposase proteins encoded by the elements themselves. DNA transposable elements are constantly adapting to life in the genome, and self-suppressive regulation as well as defensive host mechanisms may assist in buffering ‘cut-and-paste’ DNA mobilization until accumulating mutations will eventually restrict events of transposition. With the reconstructed Sleeping Beauty DNA transposon as a powerful engine, a growing list of transposable elements with activity in human cells have moved into biomedical experimentation and preclinical therapy as versatile vehicles for delivery and genomic insertion of transgenes. In this review, we aim to link the mechanisms that drive transposon evolution with the realities and potential challenges we are facing when adapting DNA transposons for gene transfer. We argue that DNA transposon-derived vectors may carry inherent, and potentially limiting, traits of their mother elements. By understanding in detail the evolutionary journey of transposons, from host colonization to element multiplication and inactivation, we may better exploit the potential of distinct transposable elements. Hence, parallel efforts to investigate and develop distinct, but potent, transposon-based vector systems will benefit the broad applications of gene transfer. Insight and clever optimization have shaped new DNA transposon vectors, which recently debuted in the first DNA transposon-based clinical trial. Learning from an evolutionary drive may help us create gene vehicles that are safer, more efficient, and less prone for suppression and inactivation.

Efficient sleeping beauty DNA transposition from DNA minicircles

DNA transposon-based vectors have emerged as new potential delivery tools in therapeutic gene transfer. Such vectors are now showing promise in hematopoietic stem cells and primary human T cells, and clinical trials with transposon-engineered cells are on the way. However, the use of plasmid DNA as a carrier of the vector raises safety concerns due to the undesirable administration of bacterial sequences. To optimize vectors based on the Sleeping Beauty (SB) DNA transposon for clinical use, we examine here SB transposition from DNA minicircles (MCs) devoid of the bacterial plasmid backbone. Potent DNA transposition, directed by the hyperactive SB100X transposase, is demonstrated from MC donors, and the stable transfection rate is significantly enhanced by expressing the SB100X transposase from MCs. The stable transfection rate is inversely related to the size of circular donor, suggesting that a MC-based SB transposition system benefits primarily from an increased cellular uptake and/or enhanced expression which can be observed with DNA MCs. DNA transposon and transposase MCs are easily produced, are favorable in size, do not carry irrelevant DNA, and are robust substrates for DNA transposition. In accordance, DNA MCs should become a standard source of DNA transposons not only in therapeutic settings but also in the daily use of the SB system.

Recent developments in transposon-mediated gene therapy

INTRODUCTION

The continuous improvement of gene transfer technologies has broad implications for stem cell biology, gene discovery, and gene therapy. Although viral vectors are efficient gene delivery vehicles, their safety, immunogenicity and manufacturing challenges hamper clinical progress. In contrast, non-viral gene delivery systems are less immunogenic and easier to manufacture.

AREAS COVERED

In this review, we explore the emerging potential of transposons in gene and cell therapy. The safety, efficiency, and biology of novel hyperactive Sleeping Beauty (SB) and piggyBac (PB) transposon systems will be highlighted for ex vivo gene therapy in clinically relevant adult stem/progenitor cells, particularly hematopoietic stem cells (HSCs), mesenchymal stem cells (MSCs), myoblasts, and induced pluripotent stem (iPS) cells. Moreover, efforts toward in vivo transposon-based gene therapy will be discussed.

EXPERT OPINION

The latest generation SB and PB transposons currently represent some of the most attractive systems for stable non-viral genetic modification of primary cells, particularly adult stem cells. This paves the way toward the use of transposons as a non-viral gene therapy approach to correct hereditary disorders including those that affect the hematopoietic system. The development of targeted integration into "safe harbor" genetic loci may further improve their safety profile.

Gene targeting in ischemic heart disease and failure: translational and clinical studies

Alternative and innovative targeted strategies hold relevance in improving the current treatments for ischemic heart disease (IHD). One potential treatment modality, gene targeting, may provide a unique alternative to current IHD therapies. The principal function of gene targeting in IHD is to augment the expression of an endogenous gene through amplification of an exogenous gene, delivered by a plasmid or a viral vector to enhance myocardial perfusion, and limit the long-term sequelae. The initial clinical studies of gene targeting in IHD were focused upon induction of angiogenic factors and the outcomes were equivocal. Nevertheless, significant advancements have been made in viral vectors, mode of delivery, and potentially relevant targets for IHD. Several of these advancements, particularly with a focus on translational large animal studies, are the focus of this review. The development of novel vectors with prolonged transduction efficiency and minimal inflammation, coupled with hybrid perfusion-mapping delivery devices, and improving the safety of vector use and efficacy of gene systems are but a few of the exciting progresses that are likely to proceed to clinical studies in the near future.

miR-29 inhibits bleomycin-induced pulmonary fibrosis in mice

Loss of microRNA-29 (miR-29) is known to be a mechanism of transforming growth factor-β (TGF-β)-mediated pulmonary fibrosis, but the therapeutic implication of miR-29 for pulmonary fibrosis remains unexplored. The present study investigated whether miR-29 had therapeutic potential for lung disease induced by bleomycin in mice. In addition, the signaling mechanisms that regulated miR-29 expression were investigated in vivo and in vitro. We found that miR-29 was a downstream target gene of Smad3 and negatively regulated by TGF-β/Smad signaling in fibrosis. This was evidenced by the findings that mice or pulmonary fibroblasts null for Smad3 were protected against bleomycin or TGF-β1-induced loss of miR-29 along with fibrosis in vivo and in vitro. Interestingly, overexpression of miR-29 could in turn negatively regulated TGF-β and connective tissue growth factor (CTGF) expression and Smad3 signaling. Therefore, Sleeping Beauty (SB)-mediated miR-29 gene transfer into normal and diseased lung tissues was capable of preventing and treating pulmonary fibrosis including inflammatory macrophage infiltration induced by bleomycin in mice. In conclusion, miR-29 is negatively regulated by TGF-β/Smad3 and has a therapeutic potential for pulmonary fibrosis. SB-mediated miR-29 gene therapy is a non-invasive therapeutic strategy for lung disease associated with fibrosis.

Nonviral gene delivery with the sleeping beauty transposon system

Effective gene therapy requires robust delivery of therapeutic genes into relevant target cells, long-term gene expression, and minimal risks of secondary effects. Nonviral gene transfer approaches typically result in only short-lived transgene expression in primary cells, because of the lack of nuclear maintenance of the vector over several rounds of cell division. The development of efficient and safe nonviral vectors armed with an integrating feature would thus greatly facilitate clinical gene therapy studies. The latest generation transposon technology based on the Sleeping Beauty (SB) transposon may potentially overcome some of these limitations. SB was shown to provide efficient stable gene transfer and sustained transgene expression in primary cell types, including human hematopoietic progenitors, mesenchymal stem cells, muscle stem/progenitor cells (myoblasts), induced pluripotent stem cells, and T cells. These cells are relevant targets for stem cell biology, regenerative medicine, and gene- and cell-based therapies of complex genetic diseases. Moreover, the first-in-human clinical trial has been launched to use redirected T cells engineered with SB for gene therapy of B cell lymphoma. We discuss aspects of cellular delivery of the SB transposon system, transgene expression provided by integrated transposon vectors, target site selection of the transposon vectors, and potential risks associated with random genomic insertion.

The Sleeping Beauty transposon system for clinical applications

Extensive efforts have been made to establish efficient and safe gene delivery protocols that could meet demanding expectations of a successful gene therapy. The Sleeping Beauty (SB) transposon system combines simplicity and inexpensive manufacture offered by plasmid-based vector formulation with integrative features exhibited by some viral vectors. Activated after over ten million years of silent genomic existence, the SB transposable element entered the 21st century as a potent technology for a broad range of applications in genome engineering, including gene therapy. Beneficially for gene therapy purposes, the SB system has been demonstrated to enable persistent expression of therapeutic genes followed by restoration of homeostasis in a variety of disease models. Importantly, this non-viral gene delivery vehicle is postulated to constitute a relatively safe vector system, because it lacks a preference for inserting into transcription units and their upstream regulatory regions, thereby minimizing genotoxic risks that might be associated with vector integration. Further evolution and wide, comprehensive preclinical testing of the SB transposon system in the context of several disease models is expected to further refine this valuable technology matched by enhanced biosafety towards disease treatment.

Efficient non-viral integration and stable gene expression in multipotent adult progenitor cells

Non-viral integrating systems, PhiC31 phage integrase (ϕC31), and Sleeping Beauty transposase (SB), provide an effective method for ex vivo gene delivery into cells. Here, we used a plasmid-encoding GFP and neomycin phosphotransferase along with recognition sequences for both ϕC31 and SB integrating systems to demonstrate that both systems effectively mediated integration in cultured human fibroblasts and in rat multipotent adult progenitor cells (rMAPC). Southern blot analysis of G418-resistant rMAPC clones showed a 2-fold higher number of SB-mediated insertions per clone compared to ϕC31. Sequence identification of chromosomal junction sites indicated a random profile for SB-mediated integrants and a more restricted profile for ϕC31 integrants. Transgenic rMAPC generated with both systems maintained their ability to differentiate into liver and endothelium albeit with marked attenuation of GFP expression. We conclude that both SB and ϕC31 are effective non-viral integrating systems for genetic engineering of MAPC in basic studies of stem cell biology.

Lifelong reporter gene imaging in the lungs of mice following polyethyleneimine-mediated sleeping-beauty transposon delivery

Polyethyleneimine (PEI) is a cationic polymer that is effective in gene delivery in vivo. Plasmid DNA incorporating the Sleeping-Beauty (SB) transposon has been shown to induce long-term transgene expression in mouse lungs after PEI-mediated delivery. In the current report, we followed the reporter gene expression mediated by PEI/SB delivery in lungs of mice using the non-invasive bioluminescent imaging (BLI) technology. After delivery, the reporter gene signal showed a rapid decay in the first two weeks to a nearly undetectable level, but then the signal augmented gradually in the following weeks and finally reached a stable level that maintained until the natural death of animals. The stabilization of transgene expression is associated with the multiplication of a small number of PEI/SB-labeled alveolar cells, which proliferated both under normal conditions and in response to acute local injury for epithelia repair, and may play a role in long-term homeostatic maintenance in alveoli. The data presented here suggests that systemic delivery of PEI/SB induces stable transfection specifically in a small population of alveolar progenitor cells. The technique provides a promising platform for future research in distal lung biology and tissue regenerative therapy.

The expanding universe of transposon technologies for gene and cell engineering

Transposable elements can be viewed as natural DNA transfer vehicles that, similar to integrating viruses, are capable of efficient genomic insertion. The mobility of class II transposable elements (DNA transposons) can be controlled by conditionally providing the transposase component of the transposition reaction. Thus, a DNA of interest (be it a fluorescent marker, a small hairpin (sh)RNA expression cassette, a mutagenic gene trap or a therapeutic gene construct) cloned between the inverted repeat sequences of a transposon-based vector can be used for stable genomic insertion in a regulated and highly efficient manner. This methodological paradigm opened up a number of avenues for genome manipulations in vertebrates, including transgenesis for the generation of transgenic cells in tissue culture, the production of germline transgenic animals for basic and applied research, forward genetic screens for functional gene annotation in model species, and therapy of genetic disorders in humans. Sleeping Beauty (SB) was the first transposon shown to be capable of gene transfer in vertebrate cells, and recent results confirm that SB supports a full spectrum of genetic engineering including transgenesis, insertional mutagenesis, and therapeutic somatic gene transfer both ex vivo and in vivo. The first clinical application of the SB system will help to validate both the safety and efficacy of this approach. In this review, we describe the major transposon systems currently available (with special emphasis on SB), discuss the various parameters and considerations pertinent to their experimental use, and highlight the state of the art in transposon technology in diverse genetic applications.

Translating Sleeping Beauty transposition into cellular therapies: victories and challenges

Recent results confirm that long-term expression of therapeutic transgenes can be achieved by using a transposon-based system in primary stem cells and in vivo. Transposable elements are natural DNA transfer vehicles that are capable of efficient genomic insertion. The latest generation, Sleeping Beauty transposon-based hyperactive vector (SB100X), is able to address the basic problem of non-viral approaches - that is, low efficiency of stable gene transfer. The combination of transposon-based non-viral gene transfer with the latest improvements of non-viral delivery techniques could provide a long-term therapeutic effect without compromising biosafety. The new challenges of pre-clinical research will focus on further refinement of the technology in large animal models and improving the safety profile of SB vectors by target-selected transgene integration into genomic "safe harbors." The first clinical application of the SB system will help to validate the safety of this approach.

Herpes simplex virus/Sleeping Beauty vector-based embryonic gene transfer using the HSB5 mutant: loss of apparent transposition hyperactivity in vivo

The Sleeping Beauty (SB) transposon system has been successfully used as a gene delivery tool in nonviral and viral vector platforms. Since its initial reconstruction, a series of hyperactive mutants of SB have been generated. Questions remain as to whether the enhanced in vitro activities of these SB transposase mutants translate to the in vivo setting, and whether such increased integration efficiencies will ultimately compromise the safety profile of the transposon platform by raising the risk of genomic insertional mutagenesis. Herein, we compared the in vivo impact of a herpes simplex virus (HSV) amplicon-vectored "wild-type" SB transposase (SB10) and a "hyperactive" SB mutant (HSB5), codelivered in utero with the HSVT-βgeo transposable reporter amplicon vector to embryonic day 14.5 C57BL/6 mice. The SB10 and HSB5 transposases do not disparately affect the viability and development of injected mouse embryos. Quantitation of brain-resident βgeo expression on postnatal day 21 revealed that mice receiving HSB5 exhibited only a trending increase in transgene expression compared with the SB10-infused group, an outcome that did not mirror the marked enhancement of HSB5-mediated transposition observed in vitro. These findings indicate that in vivo application of hyperactive SB mutants, although not differentially genotoxic to the developing mouse embryo, does not necessarily provide a significant therapeutic advantage over the employment of a lesser active SB when delivered in the context of the HSV/SB amplicon platform.

Physiological and tissue-specific vectors for treatment of inherited diseases

After more than 1500 gene therapy clinical trials in the past two decades, the overall conclusion is that for gene therapy (GT) to be successful, the vector systems must still be improved in terms of delivery, expression and safety. The recent development of more efficient and stable vector systems has created great expectations for the future of GT. Impressive results were obtained in three primary immunodeficiencies and other inherited diseases such as congenital blindness, adrenoleukodystrophy or junctional epidermolysis bullosa. However, the development of leukemia in five children included in the GT clinical trials for X-linked severe combined immunodeficiency and the silencing of the therapeutic gene in the chronic granulomatous disease clearly showed the importance of improving safety and efficiency. In this review, we focus on the main strategies available to achieve physiological or tissue-specific expression of therapeutic transgenes and discuss the importance of controlling transgene expression to improve safety. We propose that tissue-specific and/or physiological viral vectors offer the best balance between efficiency and safety and will be the tools of choice for future clinical trials in GT of inherited diseases.

Administration of nonviral gene vector encoding rat beta-defensin-2 ameliorates chronic Pseudomonas aeruginosa lung infection in rats

BACKGROUND

Beta-defensin-2 (BD-2) plays an important role in host defense against pathogenic microbe challenge by its direct antimicrobial activity and immunomodulatory functions. The present study aimed to determine whether genetic up-regulation of rat BD-2 (rBD-2) could ameliorate chronic Pseudomonas aeruginosa lung infection in rats.

METHODS

Plasmid-encoding rBD-2 was delivered to lungs in vivo using linear polyethylenimine at 48 h before challenging with seaweed alginate beads containing P. aeruginosa. Macroscopic and histopathological changes of the lungs, bacterial loads, inflammatory infiltration, and the levels of cytokines/chemokines [interleukin (IL)-1beta, tumor necrosis factor (TNF)-alpha, kertinocyte-derived chemokine (KC), macrophage inflammatory protein-2 (MIP-2)] were measured at 3 and 7 days post-infection (p.i.).

RESULTS

The overexpression of rBD-2 resulted in a significant increase in animal survival rate (at 3 days p.i.), a significant decrease in bacterial loads in the lungs (at 3 and 7 days p.i.), and significantly milder lung pathology. In addition, the overexpression of rBD-2 led to increased infiltration of polymorphonuclear neutrophils (PMN), and elevated protein expression of cytokines/chemokines (IL-1beta, TNF-alpha, KC and MIP-2) at the early stage of infection (at 3 days p.i.), at the same time as being dramatically decreased at the later stage of infection (at 7 days p.i.).

CONCLUSIONS

Genetic up-regulation of rBD-2 increased animal survival rate, and reduced bacterial loads in lungs after bacterial infection. The overexpression of rBD-2 also modulated the production of several cytokines/chemokines and increased PMN recruitment at the early stage of infection. Our findings indicate that the enhancement of BD-2 may be an efficacious intervention for chronic P. aeruginosa lung infection.

Overexpression of Bcl-2 in vascular endothelium inhibits the microvascular lesions of diabetic retinopathy

Recent studies on the pathogenesis of diabetic retinopathy have focused on correcting adverse biochemical alterations, but there have been fewer efforts to enhance prosurvival pathways. Bcl-2 is the archetypal member of a group of antiapoptotic proteins. In this study, we investigated the ability of overexpressing Bcl-2 in vascular endothelium to protect against early stages of diabetic retinopathy. Transgenic mice overexpressing Bcl-2 regulated by the pre-proendothelin promoter were generated, resulting in increased endothelial Bcl-2. Diabetes was induced with streptozotocin, and mice were sacrificed at 2 months of study to measure superoxide generation, leukostasis, and immunohistochemistry, and at 7 months to assess retinal histopathology. Diabetes of 2 months duration caused a significant decrease in expression of Bcl-2 in retina, upregulation of Bax in whole retina and isolated retinal microvessels, and increased generation of retinal superoxide and leukostasis. Seven months of diabetes caused a significant increase in the number of degenerate (acellular) capillaries in diabetic animals. Furthermore, overexpression of Bcl-2 in the vascular endothelium inhibited the diabetes-induced degeneration of retinal capillaries and aberrant superoxide generation, but had no effect on Bax expression or leukostasis. Therefore, overexpression of Bcl-2 in endothelial cells inhibits the capillary degeneration that is characteristic of the early stages of diabetic retinopathy, and this effect seems likely to involve inhibition of oxidative stress.

Gene expression in lung and liver after intravenous infusion of polyethylenimine complexes of Sleeping Beauty transposons

Two methods of systemic gene delivery have been extensively explored, using the mouse as a model system: hydrodynamic delivery, wherein a DNA solution equivalent in volume to 10% of the mouse weight is injected intravenously in less than 10 sec, and condensation of DNA with polyethylenimine (PEI) for standard intravenous infusion. Our goal in this study was to evaluate quantitatively the kinetics of gene expression, using these two methods for delivery of Sleeping Beauty transposons. Transposons carrying a luciferase expression cassette were injected into mice either hydrodynamically or after condensation with PEI at a PEI nitrogen-to-DNA phosphate ratio of 7. Gene expression in the lungs and liver after hydrodynamic delivery resulted in exponential decay with a half-life of about 35-40 hr between days 1 and 14 postinjection. The decay kinetics of gene expression after PEI-mediated gene delivery were more complex; an initial decay rate of 6 hr was followed by a more gradual loss of activity. Consequently, the liver became the primary site of gene expression about 4 days after injection of PEI-DNA, and by 14 days expression in the liver was 10-fold higher than in the lung. Overall levels of gene expression 2 weeks postinjection were 100- to 1000-fold lower after PEI-mediated delivery compared with hydrodynamic injection. These results provide insight into the relative effectiveness and organ specificity of these two methods of nonviral gene delivery when coupled with the Sleeping Beauty transposon system.

Unique functions of repetitive transcriptomes

Repetitive sequences occupy a huge fraction of essentially every eukaryotic genome. Repetitive sequences cover more than 50% of mammalian genomic DNAs, whereas gene exons and protein-coding sequences occupy only ~3% and 1%, respectively. Numerous genomic repeats include genes themselves. They generally encode "selfish" proteins necessary for the proliferation of transposable elements (TEs) in the host genome. The major part of evolutionary "older" TEs accumulated mutations over time and fails to encode functional proteins. However, repeats have important functions also on the RNA level. Repetitive transcripts may serve as multifunctional RNAs by participating in the antisense regulation of gene activity and by competing with the host-encoded transcripts for cellular factors. In addition, genomic repeats include regulatory sequences like promoters, enhancers, splice sites, polyadenylation signals, and insulators, which actively reshape cellular transcriptomes. TE expression is tightly controlled by the host cells, and some mechanisms of this regulation were recently decoded. Finally, capacity of TEs to proliferate in the host genome led to the development of multiple biotechnological applications.

PiggyBac transposon-based inducible gene expression in vivo after somatic cell gene transfer

Somatic cell gene transfer has permitted inducible gene expression in vivo through coinfection of multiple viruses. We hypothesized that the highly efficient plasmid-based piggyBac transposon system would enable long-term inducible gene expression in mice in vivo. We used a multiple-transposon delivery strategy to create a tetracycline-inducible expression system in vitro in human cells by delivering the two genes on separate transposons for inducible reporter gene expression along with a separate selectable transposon marker. Evaluation of stable cell lines revealed 100% of selected clones exhibited inducible expression via stable expression from three separate transposons simultaneously. We next tested and found that piggyBac-mediated gene transfer to liver or lung could achieve stable reporter gene expression in mice in vivo in either immunocompetent or immune deficient animals. A single injection of piggyBac transposons could achieve long-term inducible gene expression in the livers of mice in vivo, confirming our multiple-transposon strategy used in cultured cells. The plasmid-based piggyBac transposon system enables constitutive or inducible gene expression in vivo for potential therapeutic and biological applications without using viral vectors.

Indoleamine 2,3-dioxygenase attenuates inhibitor development in gene-therapy-treated hemophilia A mice

A serious impediment to gene and protein replacement therapy in hemophilia A is the development of inhibitors. Mechanisms responsible for inhibitor development include T-cell-dependent adaptive immune responses and the CD28-B7 signaling pathway that eventually leads to the formation of antibodies directed against factor VIII (FVIII). Indoleamine 2,3-dioxygenase (IDO) is a potent immunosuppressive enzyme that can inhibit T-cell responses and induce T-cell apoptosis by regulation of tryptophan metabolism. Kynurenine, one of the metabolites of tryptophan, has been implicated as an immune modulator. Here we hypothesize that co-delivery of the genes for FVIII and IDO can attenuate inhibitor formation. Using transposon-based gene delivery, we observed long-term therapeutic FVIII expression and significantly reduced inhibitor titers when the genes were co-delivered. Co-expression of FVIII and IDO in the liver was associated with increased plasma kynurenine levels, an inhibition of T-cell infiltration and increased apoptosis of T cells within the liver. These experiments suggest that modulation of tryptophan catabolism through IDO expression provides a novel strategy to reduce inhibitor development in hemophilia gene/protein therapy.

Hybrid lentivirus-transposon vectors with a random integration profile in human cells

Gene delivery by human immunodeficiency virus type 1 (HIV-1)-based lentiviral vectors (LVs) is efficient, but genomic integration of the viral DNA is strongly biased toward transcriptionally active loci resulting in an increased risk of insertional mutagenesis in gene therapy protocols. Nonviral Sleeping Beauty (SB) transposon vectors have a significantly safer insertion profile, but efficient delivery into relevant cell/tissue types is a limitation. In an attempt to combine the favorable features of the two vector systems we established a novel hybrid vector technology based on SB transposase-mediated insertion of lentiviral DNA circles generated during transduction of target cells with integrase (IN)-defective LVs (IDLVs). By construction of a lentivirus-transposon hybrid vector allowing transposition exclusively from circular viral DNA substrates, we demonstrate that SB transposase added in trans directs efficient transposon mobilization from DNA circles in vector-transduced cells. Both transfected plasmid DNA and transduced IDLVs can serve as the source of active transposase. Most important, we demonstrate that the SB transposase overrides the natural lentiviral integration pathway and directs vector integration less frequently toward transcriptional units, resulting in a random genomic integration profile. The novel hybrid vector system combines the attractive features of efficient gene delivery by viral transduction and a safer genomic integration profile by DNA transposition.

Shielding of sleeping beauty DNA transposon-delivered transgene cassettes by heterologous insulators in early embryonal cells

The Sleeping Beauty (SB) transposon system represents an important alternative to viral integrating vector systems but may, as its viral counterparts, be subject to transcriptional silencing. To investigate shielding of SB-delivered transgene cassettes against transcriptional repression, we establish silencing assays in which SB vector-containing F9 murine teratocarcinoma cell clones are identified by strategies that include or exclude selection for transgene expression. Among clones carrying one or more SB transposon vectors, more than one-third are immediately silenced, and most of the remaining clones move toward silencing during prolonged passage. In line with the lack of an intrinsic ability of SB to resist silencing, we show that the stable transfection rate of SB vectors in F9 cells is significantly improved by flanking the transgene with heterologous 5'-HS4 chicken beta-globin (cHS4) insulators. In approaches based on drug selection and subsequent flow-cytometric detection of transgene expression, clones containing cHS4-insulated vectors are to a much higher degree protected against transcriptional silencing, resulting in long-term expression of the fluorescent marker. Our findings demonstrate that SB vectors, prone for transcriptional silencing by positional effects in F9 cells, are protected by insulators. We believe that insulated SB-derived vectors will become useful tools in transposon-based transgenesis and therapeutic gene transfer.

Liver-directed gene therapy using the sleeping beauty transposon system

Sleeping Beauty (SB) is a transposon system genetically reconstructed from teleost fish that mediates chromosomal integration of DNA sequences by a cut-and-paste mechanism. SB has been shown to mediate transposition in a variety of cells and tissues, has been used for the generation of transgenic animals and has been tested as a vector for gene therapy in several animal models of human disease. Here, we describe methods that we have developed for testing SB-mediated transposition, first in cultured mammalian cells, and then in vivo using a combination of rapid, high-volume tail vein injection for DNA delivery to the liver along with in vivo bioluminescence imaging to monitor sustained luciferase gene expression in individual animals.

Lung-directed gene therapy in mice using the nonviral Sleeping Beauty transposon system

The Sleeping Beauty (SB) transposon is an integrative nonviral plasmid system. Here, we describe a protocol for SB-mediated transgene delivery using DNA/polyethyleneimine (PEI) complexes for long-term expression in mouse lungs. This protocol can be used for delivery of any plasmid-based vector system to mouse lungs, although long-term transgene expression will be obtained only when using the SB transposon or other integrating vector systems. The stages of this protocol are preparation of DNA-PEI complexes and injection of the complexes into the lateral tail vein of mice. We also provide protocols for assessing transgene expression using in vivo bioluminescence imaging and enzymatic assay of lung homogenates. The procedure can be completed within 24 h, starting from preparation of DNA-PEI complexes to analysis of transient transgene expression.

Cis-acting gene regulatory activities in the terminal regions of sleeping beauty DNA transposon-based vectors

Sleeping Beauty (SB) DNA transposon-based vectors belong to a growing family of nonviral integrating vectors that represent attractive alternatives to conventional virus-based integrating gene vehicles. Because of concerns related to mutagenesis and/or activation of cellular genes by integrating vectors, much attention has been paid to integration site preferences and the ability of vectors to influence expression of neighboring genes. Here, we test the hypothesis that terminal repeats of transposons carry cis-acting regulatory sequences. In transient gene expression studies, we demonstrate that the inverted repeats of SB direct gene expression in HeLa cells to levels that are 3-fold higher than in promoter-deficient controls. Inverted repeats pointing toward the transposon center consistently facilitate the highest levels of activity in a number of cell lines. We show that transposon sequences flanking the inverted repeats of SB are required for positive effects on gene expression and, moreover, that these regions contain both stimulatory and inhibitory cis-acting elements. In the context of an integrated SB vector the regulatory activities of the transposon termini are sufficient to drive expression of selectable marker genes carried by the transposon, indicating that opposing transcriptional activities originating from the transposon termini may influence expression of its genetic cargo. Finally, detection of regulatory properties of the terminal repeats of the active Tc3 element from Caenorhabditis elegans leads to the suggestion that transcriptional activities of the inverted repeats are conserved among Tc1/mariner transposons in nature. Our data suggest that SB-based gene vectors may carry ancient properties of self-regulation with potential relevance for SB-directed therapeutic gene transfer.

Development and validation of an approach to produce large-scale quantities of CpG-methylated plasmid DNA

The prokaryotic CpG‐specific DNA methylase from Spiroplasma, SssI methylase, has been extensively used to methylate plasmid DNA in vitro to investigate the effects of methylation in vertebrate systems. Currently available methods to produce CpG‐methylated plasmid DNA have certain limitations and cannot generate large quantities of methylated DNA without cost or problems of purity. Here we describe an approach in which the SssI methylase gene has been introduced into the Escherichia coli bacterial genome under the control of an inducible promoter. Plasmid DNA propagated in this bacterium under conditions which induce the methylase gene result in significant (> 90%) CpG methylation. Methylated DNA produced by this approach behaves similarly to methylated DNA produced in vitro using the purified methylase. The approach is scalable allowing for the production of milligram quantities of methylated plasmid DNA.

Pigs taking wing with transposons and recombinases

Swine production has been an important part of our lives since the late Mesolithic or early Neolithic periods, and ranks number one in world meat production. Pig production also contributes to high-value-added medical markets in the form of pharmaceuticals, heart valves, and surgical materials. Genetic engineering, including the addition of exogenous genetic material or manipulation of the endogenous genome, holds great promise for changing pig phenotypes for agricultural and medical applications. Although the first transgenic pigs were described in 1985, poor survival of manipulated embryos; inefficiencies in the integration, transmission, and expression of transgenes; and expensive husbandry costs have impeded the widespread application of pig genetic engineering. Sequencing of the pig genome and advances in reproductive technologies have rejuvenated efforts to apply transgenesis to swine. Pigs provide a compelling new resource for the directed production of pharmaceutical proteins and the provision of cells, vascular grafts, and organs for xenotransplantation. Additionally, given remarkable similarities in the physiology and size of people and pigs, swine will increasingly provide large animal models of human disease where rodent models are insufficient. We review the challenges facing pig transgenesis and discuss the utility of transposases and recombinases for enhancing the success and sophistication of pig genetic engineering. 'The paradise of my fancy is one where pigs have wings.' (GK Chesterton).

InXy and SeXy, compact heterologous reporter proteins for mammalian cells

Mammalian reporter proteins are essential for gene-function analysis, drugscreening initiatives and as model product proteins for biopharmaceutical manufacturing. Bacillus subtilis can maintain its metabolism by secreting Xylanase A (XynA), which converts xylan into shorter xylose oligosaccharides. XynA is a family 11 xylanase monospecific for D-xylose containing substrates. Mammalian cells transgenic for constitutive expression of wild-type xynA showed substantial secretion of this prokaryotic enzyme. Deletion analysis confirmed that a prokaryotic signal sequence encoded within the first 81 nucleotides was compatible with the secretory pathway of mammalian cells. Codon optimization combined with elimination of the prokaryotic signal sequence resulted in an exclusively intracellular mammalian Xylanase A variant (InXy) while replacement by an immunoglobulin-derived secretion signal created an optimal secreted Xylanase A derivative (SeXy). A variety of chromogenic and fluorescence-based assays adapted for use with mammalian cells detected InXy and SeXy with high sensitivity and showed that both reporter proteins resisted repeated freeze/thaw cycles, remained active over wide temperature and pH ranges, were extremely stable in human serum stored at room temperature and could independently be quantified in samples also containing other prominent reporter proteins such as the human placental alkaline phosphatase (SEAP) and the Bacillus stearothermophilus-derived secreted alpha-amylase (SAMY). Glycoprofiling revealed that SeXy produced in mammalian cells was N- glycosylated at four different sites, mutation of which resulted in impaired secretion. SeXy was successfully expressed in a variety of mammalian cell lines and primary cells following transient transfection and transduction with adeno-associated virus particles (AAV) engineered for constitutive SeXy expression. Intramuscular injection of transgenic AAVs into mice showed significant SeXy levels in the bloodstream. InXy and SeXy are highly sensitive, compact and robust reporter proteins, fully compatible with pre-existing marker genes and can be assayed in high-throughput formats using very small sample volumes.

Nonviral gene delivery with indoleamine 2,3-dioxygenase targeting pulmonary endothelium protects against ischemia-reperfusion injury

Pulmonary endothelial dysfunction induced by inflammation and inflammation-associated reactive oxygen species is a central component in the pathophysiology of lung transplant ischemia-reperfusion (IR) injury. Indoleamine-2,3-dioxygenase (IDO) is a unique cytosolic enzyme possessing both immune modulating and antioxidant properties. This study investigated whether enhanced pulmonary endothelial IDO activity by a targeted nonviral gene transfer approach ameliorates lung IR injury. Orthotopic syngeneic lung transplants were performed in Lewis rats. A human IDO (hIDO)-expressing plasmid driven by an endothelial cell-specific endothelin-1 promoter was generated and intravenously delivered to donor lung using cationic polymer polyethylenimine. This nonviral gene transfer approach augmented hIDO expression specifically in endothelial cells within lung grafts. Importantly, enhanced IDO activity induced by the hIDO transgene prevented endothelial cell apoptosis, reduced vascular permeability and leukocyte extravasation, and consequently improved graft function and histologic appearance. Furthermore, our in vitro studies showed that increased IDO activity in endothelial cells protected its mitochondrial function and ultrastructure from oxidative stress through stabilization of intracellular redox status. The approach used in these experiments has properties that could eliminate the inherent side effects associated with viral vectors and/or antibody-directed targeted therapy, and thus may represent a potential therapeutic strategy against lung IR injury in patients.

Endothelin-1 induces alveolar epithelial-mesenchymal transition through endothelin type A receptor-mediated production of TGF-beta1

Endothelin-1 (ET-1) is implicated in the pathogenesis of idiopathic pulmonary fibrosis (IPF), but the cellular mechanisms underlying the role it plays in this disease are not well characterized. Epithelial-mesenchymal transition (EMT), which was recently demonstrated in alveolar epithelial cells (AEC), may play an important role in the pathogenesis of IPF and other forms of pulmonary fibrosis. Whether ET-1 contributes to the induction of EMT in AEC is unknown. The aims of this study were to evaluate AEC production of ET-1 and to determine if ET-1 induces EMT in AEC. We demonstrate that ET-1 is produced at physiologically relevant levels by primary AEC and is secreted preferentially toward the basolateral surface. We also demonstrate that AEC express high levels of endothelin type A receptors (ET-A) and, to a lesser extent, type B receptors (ET-B), suggesting autocrine or paracrine function for alveolar ET-1. In addition, ET-1 induces EMT through ET-A activation. Furthermore, TGF-beta1 synthesis is increased by ET-1, ET-1 induces Smad3 phosphorylation, and ET-1-induced EMT is attenuated by a TGF-beta1-neutralizing antibody. Thus, ET-1 is an important mediator of EMT in AEC, acting through ET-A-mediated TGF-beta1 production. These findings increase our basic understanding of the role of ET-1 in pulmonary fibrosis and suggest potential roles for AEC-derived ET-1 in the pathogenesis of other alveolar epithelial-mediated lung diseases.

Sleeping beauty transposon-mediated nonviral gene therapy

Safe and effective delivery of genetic material to mammalian tissues would significantly expand the therapeutic possibilities for a large number of medical conditions. Unfortunately, the promise of gene therapy has been hampered by technical challenges, the induction of immune responses, and inadequate expression over time. Despite these setbacks, progress continues to be made and the anticipated benefits may come to fruition for certain disorders. In terms of delivery, nonviral vector systems are particularly attractive as they are simple to produce, can be stored for long periods of time, and induce no specific immune responses. A significant drawback to nonviral systems has been the lack of persistent expression, as plasmids are lost or degraded when delivered to living tissues. The recent application of integrating transposons to nonviral gene delivery has significantly helped to overcome this obstacle, because it allows for genomic integration and long-term expression. Recent advances in transposon-based vector systems hold promise as new technologies that may unlock the potential of gene therapy; however, technical and safety issues still need refinement.

Stable gene transfer to human CD34(+) hematopoietic cells using the Sleeping Beauty transposon

OBJECTIVE

Methods of gene transfer to hematopoietic stem cells that result in stable integration may provide treatments for many inherited and acquired blood diseases. It has been demonstrated previously that a gene delivery system based on the Sleeping Beauty (SB) transposon can be derived where a plasmid transiently expressing the SB transposase can mediate the stable chromosomal integration of a codelivered second plasmid containing a gene expression unit flanked by the inverted repeats derived from the transposon.

METHODS

Plasmid DNA containing the elements required for SB transposition was delivered to hematopoietic cells via electroporation. Integrated transgene (enhanced green fluorescent protein [eGFP]) expression was assessed in vitro and in vivo.

RESULTS

In the K562 human hematopoietic cell line, we observed stable expression of eGFP in >60% of cells for over 2 months after electroporation of the two plasmids; in contrast, in control cells either not treated with transposase or exposed to a defective mutant transposase, the level of gene expression had fallen to near background (<0.1%) by 2 weeks. In purified human cord blood CD34(+) progenitor cells, the transposase led to stable gene transfer at levels up to 6% for over 4 weeks, but gene transfer to more primitive nonobese diabetic/severe combined immunodeficient repopulating cells or CD34(+)/CD38(-) in long-term culture was low and electroporation of the cells with plasmid DNA caused significant cell death.

CONCLUSION

The long-term stable expression highlights the potential of this transposase-based gene delivery method for ameliorating diseases affecting the hematopoietic system, although further improvements in gene transfer efficacy are needed.

Sleeping Beauty-mediated eNOS gene therapy attenuates monocrotaline-induced pulmonary hypertension in rats

Pulmonary hypertension (PH) is a life-threatening disorder with high mortality rates and limited treatment options. Gene therapy is an alternative treatment strategy, yet viral vectors have inherent disadvantages including immune activation. The Sleeping Beauty (SB) transposon is a nonviral method of gene delivery that overcomes some of these drawbacks. A SB-based transposon harboring a constitutively active endothelial nitric oxide synthase (eNOS) gene was administered to Sprague-Dawley rats via tail vein injection using the carrier polyethylenimine. Two days after transposon delivery, monocrotaline (MCT) was administered to induce PH. Hemodynamic, histological, and molecular measurements were performed four weeks later. Animals coinjected with transposase showed a significant reduction in pulmonary arterial pressure (PABP, 31.67+/-6.03 mmHg, P<0.01), an attenuation of right ventricle (RV) to whole heart (WH) wt ratios (0.227+/-0.0252, P<0.05) and a decrease in the pulmonary vessel wall thickness index (36.87%, P<0.001), compared with those animals receiving the eNOS transposon and a nonfunctional transposase (PABP 44.33+/-4.04 mmHg; RV/WH ratio 0.280+/-0.01; wall thickness index 62.14%) or control animals receiving MCT injection alone (PABP 49.67+/-3.22 mmHg; RV/WH ratio 0.290+/-0.0265; wall thickness index 71.99%). The physiological improvements correlated with therapeutic gene expression, suggesting that transposon-based genetic approaches have utility in the treatment of PH.

Sleeping Beauty-based gene therapy with indoleamine 2,3-dioxygenase inhibits lung allograft fibrosis

Sleeping Beauty (SB) transposon is a natural nonviral gene transfer system that can mediate long-term transgene expression. Its potential utility in treating organ transplantation-associated long-term complications has not yet been explored. In the present study we generated an improved SB transposon encoding the human gene indoleamine-2,3-dioxygenase (hIDO), an enzyme that possesses both T cell-suppressive and antioxidant properties and selectively delivered the SB transposon in combination with a hyperactive transposase plasmid to donor lung using the cationic polymer polyethylenimine (PEI) as transfection reagent. This nonviral gene therapeutic approach led to persistent and uniform transgene expression in the rat lung tissue without noticeable toxicity and inflammation. Importantly, IDO activity produced by hIDO transgene showed a remarkable therapeutic response, as evident by near normal pulmonary function (peak airway pressure and oxygenation), histological appearance, and reduced collagen content in lung allografts. In addition, we established a hIDO-overexpressing type II cell line using the SB-based gene transfer system and found that hIDO-overexpressing lung cells effectively inhibited transforming growth factor-beta-stimulated fibroblast proliferation in vitro. In summary, the SB-based gene therapy with hIDO represents a new strategy for treating lung transplantation-associated chronic complications, e.g., obliterative bronchiolitis.

Novel Action of Indoleamine 2,3-Dioxygenase Attenuating Acute Lung Allograft Injury

RATIONALE

Lung allografts are prone to reperfusion injury and acute rejection, which, in addition to infiltrating lymphocytes, are accompanied by neutrophil infiltration and neutrophil-associated oxidative stress. Indoleamine 2,3-dioxygenase (IDO) is a unique cytosolic enzyme that possesses T-cell-suppressive and antioxidant properties.

OBJECTIVES

The purpose of this study was to determine if genetic up-regulation of IDO could ameliorate acute lung allograft injury.

METHODS

Lung orthotopic transplants were performed using Lewis donors and Sprague-Dawley rat recipients (allografts) or the same strain (isografts). Plasmid-encoding human IDO was delivered to donor lungs in vivo using a nonviral gene-transfer vector, polyethylenimine. Transplanted lungs were evaluated at 6 d post-transplantation based on pulmonary function, histology, inflammatory responses, and their associated oxidative stress. Basic biology of the IDO-overexpressing lung cells was evaluated in vitro in response to external oxidant.

MEASUREMENTS AND MAIN RESULTS

This gene delivery method led to uniform transgene expression in lung tissue distributed in airway, alveolar epithelial, and endothelial cells. IDO overexpression in lung allografts resulted in a significant protective effect with improvement in functional properties (peak airway pressure and oxygenation) and histologic appearance. Although IDO was able to block local T-cell responses, it failed to abrogate neutrophilic infiltration and the inflammation-associated oxidative stress. IDO-enhanced lung cells were resistance to oxidant-induced necrosis and apoptosis by limiting intracellular reactive oxygen species formation.

CONCLUSIONS

These results demonstrate that IDO prevents acute lung allograft injury through augmenting the local antioxidant defense system and inhibiting alloreactive T-cell responses.

Sustained FVIII expression and phenotypic correction of hemophilia A in neonatal mice using an endothelial-targeted sleeping beauty transposon

Hemophilia A, deficiency of coagulation factor VIII (FVIII), is an attractive candidate for gene therapy as expression of modest amounts of FVIII can provide therapeutic benefit. Most gene transfer approaches for hemophilia have focused on the liver, as this is the major source of endogenous FVIII; however, increasing evidence suggests that endothelial cells are capable of synthesis and release of FVIII. Here the Sleeping Beauty (SB) transposon is employed to target long-term expression of the human B-domain-depleted FVIII gene (approved gene symbol F8) within lung endothelia of hemophilic mice. As the formation of inhibitory antibodies to FVIII has been a significant impediment toward achieving therapeutic plasma levels after gene or protein therapy, we chose to perform gene transfer in neonatal mice, which are more likely to be immune tolerant. Using this approach, low therapeutic levels of FVIII ( approximately 10%), as well as phenotypic correction of the bleeding disorder, were achieved in all animals that received the FVIII transposon and functional transposase throughout the duration of the study (24 weeks). Rechallenge of these animals with additional gene transfer did not result in significant increases in FVIII levels, due mainly to increases in inhibitory antibodies. These studies demonstrate the feasibility of using endothelial-targeted SB transposons for the treatment of hemophilia A.

Retroviral insertional mutagenesis: past, present and future

Retroviral insertion mutagenesis screens in mice are powerful tools for efficient identification of oncogenic mutations in an in vivo setting. Many oncogenes identified in these screens have also been shown to play a causal role in the development of human cancers. Sequencing and annotation of the mouse genome, along with recent improvements in insertion site cloning has greatly facilitated identification of oncogenic events in retrovirus-induced tumours. In this review, we discuss the features of retroviral insertion mutagenesis screens, covering the mechanisms by which retroviral insertions mutate cellular genes, the practical aspects of insertion site cloning, the identification and analysis of common insertion sites, and finally we address the potential for use of somatic insertional mutagens in the study of nonhaematopoietic and nonmammary tumour types.

Hyperactive transposase mutants of the Sleeping Beauty transposon

Transposable elements have enormous potential to overcome one of the major hurdles in nonviral gene delivery, namely the lack of long-term gene expression. The Sleeping Beauty (SB) transposon is a promising vector system for nonviral gene therapy as it has the highest transposition activity of all known DNA transposons within mammalian cells. In an effort to generate a more efficient delivery vehicle, we conducted a systematic evaluation of several novel and previously identified SB transposase mutants. The results indicate that certain combinations of mutants do not enhance transposition, whereas others give a synergistic response. The most active mutant, designated HSB17, shows nearly 17-fold higher transposition activity compared to the original transposase SB10 when tested within the same expression cassette. In addition, synergistic activity is observed when this hyperactive mutant is combined with an improved transposon. Animal studies utilizing the hyperactive transposase show enhanced long-term reporter gene expression. These modifications further expand the utility of this transposon-based gene transfer system.

Retargeting mobile group II introns to repair mutant genes

Retroposable elements such as retroviral and lentiviral vectors have been employed for many gene therapy applications. Unfortunately, such gene transfer vectors integrate genes into many different DNA sequences and unintended integration of the vector near a growth-promoting gene can engender pathological consequences. For example, retroviral vector-mediated gene transfer induced leukemia in 2 of 11 children treated for severe combined immunodeficiency, raising significant safety issues for gene transfer strategies that cannot be targeted to specific sequences. Here, we examine the use of a mobile retroposable genetic element that can be targeted to introduce therapeutic sequences site specifically into mutant genes. The data demonstrate that the mobile group II intron from Lactococcus lactis can be targeted to insert into and repair mutant lacZ (approved gene symbol GLB1) and beta-globin (approved gene symbol HBB) genes with high efficiency and fidelity in model systems in bacteria. These results suggest that these mobile genetic elements represent a novel class of agents for performing targeted genetic repair.

High-resolution genome-wide mapping of transposon integration in mammals

The Sleeping Beauty (SB) transposon is an emerging tool for transgenesis, gene discovery, and therapeutic gene delivery in mammals. Here we studied 1,336 SB insertions in primary and cultured mammalian cells in order to better understand its target site preferences. We report that, although widely distributed, SB integration recurrently targets certain genomic regions and shows a small but significant bias toward genes and their upstream regulatory sequences. Compared to those of most integrating viruses, however, the regional preferences associated with SB-mediated integration were much less pronounced and were not significantly influenced by transcriptional activity. Insertions were also distinctly nonrandom with respect to intergenic sequences, including a strong bias toward microsatellite repeats, which are predominantly enriched in noncoding DNA. Although we detected a consensus sequence consistent with a twofold dyad symmetry at the target site, the most widely used sites did not match this consensus. In conjunction with an observed SB integration preference for bent DNA, these results suggest that physical properties may be the major determining factor in SB target site selection. These findings provide basic insights into the transposition process and reveal important distinctions between transposon- and virus-based integrating vectors.