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

Reshaping the Treatment Landscape of a Galactose Metabolism Disorder

The Leloir pathway was elucidated decades ago, unraveling how galactose is metabolized in the body. Different inborn errors of metabolism in this pathway are known, the most frequent and well-studied being Classic Galactosemia (CG) (OMIM 230400) due to pathogenic variants in the GALT gene. Substrate reduction using dietary restriction of galactose is currently the only available treatment option. Although this burdensome diet resolves the life-threatening clinical picture in neonates, patients still face long-term complications, including cognitive and neurological deficits as well as primary ovarian insufficiency. Emerging therapies aim to address these challenges on multiple fronts: (1) restoration of GALT activity with nucleic acid therapies, pharmacological chaperones, or enzyme replacement; (2) influencing the pathological cascade of events to prevent accumulation of metabolites (Galactokinase 1 (GALK1) inhibitors, aldose reductase inhibitors), address myo-inositol deficiency, or alleviate cellular stress responses; (3) substrate reduction with synthetic biotics or galactose uptake inhibitors to eliminate the need for lifelong diet; and (4) novel approaches to mitigate existing symptoms, such as non-invasive brain stimulation and reproductive innovations. Early, personalized intervention remains critical for optimizing patient outcomes. We review the advances in the development of different treatment modalities for CG and reflect on the factors that need to be considered and addressed to reshape the landscape of treatment.

Gene Editing by Ferrying of CRISPR/Cas Ribonucleoprotein Complexes in Enveloped Virus-Derived Particles

The invention of next-generation CRISPR/Cas gene editing tools, like base and prime editing, for correction of gene variants causing disease, has created hope for in vivo use in patients leading to wider clinical translation. To realize this potential, delivery vehicles that can ferry gene editing tool kits safely and effectively into specific cell populations or tissues are in great demand. In this review, we describe the development of enveloped retrovirus-derived particles as carriers of "ready-to-work" ribonucleoprotein complexes consisting of Cas9-derived editor proteins and single guide RNAs. We present arguments for adapting viruses for cell-targeted protein delivery and describe the status after a decade-long development period, which has already shown effective editing in primary cells, including T cells and hematopoietic stem cells, and in tissues targeted in vivo, including mouse retina, liver, and brain. Emerging evidence has demonstrated that engineered virus-derived nanoparticles can accommodate both base and prime editors and seems to fertilize a sprouting hope that such particles can be further developed and produced in large scale for therapeutic applications.

Construction and Quantitative Evaluation of a Tissue-Specific Sleeping Beauty by EDL2-Specific Transposase Expression in Esophageal Squamous Carcinoma Cell Line KYSE-30

Gene delivery to esophageal tissue could provide novel treatments for diseases, such as cancer. The Sleeping Beauty (SB) transposon system, as a natural and non-viral tool, is efficient at transferring transgene into the human genome for human cell genetic engineering. The plasmid-based SB transposon can insert into chromosomes through an accurate recombinase-mediated mechanism, providing long-term expression of transgene integrated into the target cells. In this study, we aimed to investigate the activity of ED-L2 tissue-specific promoter that was engineered from the Epstein-Barr Virus (EBV) and combined with the hyperactive SB100X transposase to achieve the stable expression of T2-Onc3 transposon in esophageal squamous epithelial cells. Here we constructed an SB transposon-based plasmid system to obtain the stable expression of transposon upon introduction of a hyperactive SB transposase under the control of tissue-specific ED-L2 promoter via the lipid-based delivery method in the cultured esophageal squamous cell carcinoma cells. Among established human and mouse cell lines, the (ED-L2)-SB100X transposase was active only in human esophageal stratified squamous epithelial and differentiated keratinocytes derived from skin (KYSE-30 and HaCaT cell lines), where it revealed high promoter activity. Data offered that the 782 bp sequence of ED-L2 promoter has a key role in its activity in vitro. The (ED-L2)-SB100X transposase mediated stable integration of T2-Onc3 in KYSE-30 cells, thereby providing further evidence of the tissue specificity of ED-L2 promoter. The KYSE-30 cells modified with the SB system integrate on average 187 copies of the T2-Onc3 transposon in its genome. In aggregate, the (ED-L2)-SB100X transposase can be efficiently applied for the tissue-specific stable expression of a transgene in human KYSE-30 cells using SB transposon.

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.

Delivering genes with human immunodeficiency virus-derived vehicles: still state-of-the-art after 25 years

Viruses are naturally endowed with the capacity to transfer genetic material between cells. Following early skepticism, engineered viruses have been used to transfer genetic information into thousands of patients, and genetic therapies are currently attracting large investments. Despite challenges and severe adverse effects along the way, optimized technologies and improved manufacturing processes are driving gene therapy toward clinical translation. Fueled by the outbreak of AIDS in the 1980s and the accompanying focus on human immunodeficiency virus (HIV), lentiviral vectors derived from HIV have grown to become one of the most successful and widely used vector technologies. In 2022, this vector technology has been around for more than 25 years. Here, we celebrate the anniversary by portraying the vector system and its intriguing properties. We dive into the technology itself and recapitulate the use of lentiviral vectors for ex vivo gene transfer to hematopoietic stem cells and for production of CAR T-cells. Furthermore, we describe the adaptation of lentiviral vectors for in vivo gene delivery and cover the important contribution of lentiviral vectors to basic molecular research including their role as carriers of CRISPR genome editing technologies. Last, we dwell on the emerging capacity of lentiviral particles to package and transfer foreign proteins.

Progenitor cells derived from gene-engineered human induced pluripotent stem cells as synthetic cancer cell alternatives for in vitro pharmacology

Limitations in genetic stability and recapitulating accurate physiological disease properties challenge the utility of patient-derived (PD) cancer models for reproducible and translational research. We have genetically engineered a portfolio of isogenic human induced pluripotent stem cells (hiPSCs) with different pan-cancer relevant oncoprotein signatures followed by differentiation into lineage-committed progenitor cells. Characterization on molecular and biological level validated successful stable genetic alterations in pluripotency state as well as upon differentiation to prove the functionality of our approach Meanwhile proposing core molecular networks possibly involved in early dysregulation of stem cell homeostasis, the application of our cell systems in comparative substance testing indicates the potential for cancer research such as identification of augmented therapy resistance of stem cells in response to activation of distinct oncogenic signatures. This article is protected by copyright. All rights reserved.

Non-Integrating Lentiviral Vectors in Clinical Applications: A Glance Through

Lentiviral vectors (LVs) play an important role in gene therapy and have proven successful in clinical trials. LVs are capable of integrating specific genetic materials into the target cells and allow for long-term expression of the cDNA of interest. The use of non-integrating LVs (NILVs) reduces insertional mutagenesis and the risk of malignant cell transformation over integrating lentiviral vectors. NILVs enable transient expression or sustained episomal expression, especially in non-dividing cells. Important modifications have been made to the basic human immunodeficiency virus (HIV) structures to improve the safety and efficacy of LVs. NILV-aided transient expression has led to more pre-clinical studies on primary immunodeficiencies, cytotoxic cancer therapies, and hemoglobinopathies. Recently, the third generation of self-inactivating LVs was applied in clinical trials for recombinant protein production, vaccines, gene therapy, cell imaging, and induced pluripotent stem cell (iPSC) generation. This review discusses the basic lentiviral biology and the four systems used for generating NILV designs. Mutations or modifications in LVs and their safety are addressed with reference to pre-clinical studies. The detailed application of NILVs in promising pre-clinical studies is also discussed.

Context-aware synthetic biology by controller design: Engineering the mammalian cell

The rise of systems biology has ushered a new paradigm: the view of the cell as a system that processes environmental inputs to drive phenotypic outputs. Synthetic biology provides a complementary approach, allowing us to program cell behavior through the addition of synthetic genetic devices into the cellular processor. These devices, and the complex genetic circuits they compose, are engineered using a design-prototype-test cycle, allowing for predictable device performance to be achieved in a context-dependent manner. Within mammalian cells, context effects impact device performance at multiple scales, including the genetic, cellular, and extracellular levels. In order for synthetic genetic devices to achieve predictable behaviors, approaches to overcome context dependence are necessary. Here, we describe control systems approaches for achieving context-aware devices that are robust to context effects. We then consider cell fate programing as a case study to explore the potential impact of context-aware devices for regenerative medicine applications.

Lentiviral delivery of co-packaged Cas9 mRNA and a Vegfa-targeting guide RNA prevents wet age-related macular degeneration in mice

Therapeutic genome editing requires effective and targeted delivery methods. The delivery of Cas9 mRNA using adeno-associated viruses has led to potent in vivo therapeutic efficacy, but can cause sustained Cas9 expression, anti-Cas9 immune responses and off-target edits. Lentiviral vectors have been engineered to deliver nucleases that are expressed transiently, but in vivo evidence of their biomedical efficacy is lacking. Here, we show that the lentiviral codelivery of Streptococcus pyogenes Cas9 mRNA and expression cassettes that encode a guide RNA that targets vascular endothelial growth factor A (Vegfa) is efficacious in a mouse model of wet age-related macular degeneration induced by Vegfa. A single subretinal injection of engineered lentiviruses knocked out 44% of Vegfa in retinal pigment epithelium and reduced the area of choroidal neovascularization by 63% without inducing off-target edits or anti-Cas9 immune responses. Engineered lentiviruses for the transient expression of nucleases may form the basis of new treatments for retinal neovascular diseases.

Integrase-Defective Lentiviral Vectors for Delivery of Monoclonal Antibodies against Influenza

Delivering rapid protection against infectious agents to non-immune populations is a formidable public health challenge. Although passive immunotherapy is a fast and effective method of protection, large-scale production and administration of monoclonal antibodies (mAbs) is expensive and unpractical. Viral vector-mediated delivery of mAbs offers an attractive alternative to their direct injection. Integrase-defective lentiviral vectors (IDLV) are advantageous for this purpose due to the absence of pre-existing anti-vector immunity and the safety features of non-integration and non-replication. We engineered IDLV to produce the humanized mAb VN04-2 (IDLV-VN04-2), which is broadly neutralizing against H5 influenza A virus (IAV), and tested the vectors’ ability to produce antibodies and protect from IAV in vivo. We found that IDLV-transduced cells produced functional VN04-2 mAbs in a time- and dose-dependent fashion. These mAbs specifically bind the hemagglutinin (HA), but not the nucleoprotein (NP) of IAV. VN04-2 mAbs were detected in the serum of mice at different times after intranasal (i.n.) or intramuscular (i.m.) administration of IDLV-VN04-2. Administration of IDLV-VN04-2 by the i.n. route provided rapid protection against lethal IAV challenge, although the protection did not persist at later time points. Our data suggest that administration of mAb-expressing IDLV may represent an effective strategy for rapid protection against infectious diseases.

Engineering of Primary Human B cells with CRISPR/Cas9 Targeted Nuclease

B cells offer unique opportunities for gene therapy because of their ability to secrete large amounts of protein in the form of antibody and persist for the life of the organism as plasma cells. Here, we report optimized CRISPR/Cas9 based genome engineering of primary human B cells. Our procedure involves enrichment of CD19+ B cells from PBMCs followed by activation, expansion, and electroporation of CRISPR/Cas9 reagents. We are able expand total B cells in culture 10-fold and outgrow the IgD+ IgM+ CD27- naïve subset from 35% to over 80% of the culture. B cells are receptive to nucleic acid delivery via electroporation 3 days after stimulation, peaking at Day 7 post stimulation. We tested chemically modified sgRNAs and Alt-R gRNAs targeting CD19 with Cas9 mRNA or Cas9 protein. Using this system, we achieved genetic and protein knockout of CD19 at rates over 70%. Finally, we tested sgRNAs targeting the AAVS1 safe harbor site using Cas9 protein in combination with AAV6 to deliver donor template encoding a splice acceptor-EGFP cassette, which yielded site-specific integration frequencies up to 25%. The development of methods for genetically engineered B cells opens the door to a myriad of applications in basic research, antibody production, and cellular therapeutics.

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.

Viral delivery of genome-modifying proteins for cellular reprogramming

Following the successful development of virus-based gene vehicles for genetic therapies, exploitation of viruses as carriers of genetic tools for cellular reprogramming and genome editing should be right up the street. However, whereas persistent, potentially life-long gene expression is the main goal of conventional genetic therapies, tools and bits for genome engineering should ideally be short-lived and active only for a limited time. Although viral vector systems have already been adapted for potent genome editing both in vitro and in vivo, regulatable gene expression systems or self-limiting expression circuits need to be implemented limiting exposure of chromatin to genome-modifying enzymes. As an alternative approach, emerging virus-based protein delivery technologies support direct protein delivery, providing a short, robust boost of enzymatic activity in transduced cells. Is this potentially the perfect way of shipping loads of cargo to many recipients and still maintain short-term activity?

Programming CAR-T cells to kill cancer

T cells engineered to express chimeric antigen receptors (CARs) that are specific for tumour antigens have led to high complete response rates in patients with haematologic malignancies. Despite this early success, major challenges to the broad application of CAR-T cells as cancer therapies remain, including treatment-associated toxicities and cancer relapse with antigen-negative tumours. Targeting solid tumours with CAR-T cells poses additional obstacles because of the paucity of tumour-specific antigens and the immunosuppressive effects of the tumour microenvironment. To overcome these challenges, T cells can be programmed with genetic modules that increase their therapeutic potency and specificity. In this Review Article, we survey major advances in the engineering of next-generation CAR-T therapies for haematologic cancers and solid cancers, with particular emphasis on strategies for the control of CAR specificity and activity and on approaches for improving CAR-T-cell persistence and overcoming immunosuppression. We also lay out a roadmap for the development of off-the-shelf CAR-T cells.

Paralleled comparison of vectors for the generation of CAR-T cells

T-lymphocytes genetically engineered with the chimeric antigen receptor (CAR-T) have shown great therapeutic potential in cancer treatment. A variety of preclinical researches and clinical trials of CAR-T therapy have been carried out to lay the foundation for future clinical application. In these researches, several gene-transfer methods were used to deliver CARs or other genes into T-lymphocytes, equipping CAR-modified T cells with a property of recognizing and attacking antigen-expressing tumor cells in a major histocompatibility complex-independent manner. Here, we summarize the gene-transfer vectors commonly used in the generation of CAR-T cell, including retrovirus vectors, lentivirus vectors, the transposon/transposase system, the plasmid-based system, and the messenger RNA electroporation system. The following aspects were compared in parallel: efficiency of gene transfer, the integration methods in the modified T cells, foreground of scale-up production, and application and development in clinical trials. These aspects should be taken into account to generate the optimal CAR-gene vector that may be suitable for future clinical application.

Preclinical and clinical advances in transposon-based gene therapy

Transposons derived from Sleeping Beauty (SB), piggyBac (PB), or Tol2 typically require cotransfection of transposon DNA with a transposase either as an expression plasmid or mRNA. Consequently, this results in genomic integration of the potentially therapeutic gene into chromosomes of the desired target cells, and thus conferring stable expression. Non-viral transfection methods are typically preferred to deliver the transposon components into the target cells. However, these methods do not match the efficacy typically attained with viral vectors and are sometimes associated with cellular toxicity evoked by the DNA itself. In recent years, the overall transposition efficacy has gradually increased by codon optimization of the transposase, generation of hyperactive transposases, and/or introduction of specific mutations in the transposon terminal repeats. Their versatility enabled the stable genetic engineering in many different primary cell types, including stem/progenitor cells and differentiated cell types. This prompted numerous preclinical proof-of-concept studies in disease models that demonstrated the potential of DNA transposons for ex vivo and in vivo gene therapy. One of the merits of transposon systems relates to their ability to deliver relatively large therapeutic transgenes that cannot readily be accommodated in viral vectors such as full-length dystrophin cDNA. These emerging insights paved the way toward the first transposon-based phase I/II clinical trials to treat hematologic cancer and other diseases. Though encouraging results were obtained, controlled pivotal clinical trials are needed to corroborate the efficacy and safety of transposon-based therapies.

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.

Towards a Safer, More Randomized Lentiviral Vector Integration Profile Exploring Artificial LEDGF Chimeras

The capacity to integrate transgenes into the host cell genome makes retroviral vectors an interesting tool for gene therapy. Although stable insertion resulted in successful correction of several monogenic disorders, it also accounts for insertional mutagenesis, a major setback in otherwise successful clinical gene therapy trials due to leukemia development in a subset of treated patients. Despite improvements in vector design, their use is still not risk-free. Lentiviral vector (LV) integration is directed into active transcription units by LEDGF/p75, a host-cell protein co-opted by the viral integrase. We engineered LEDGF/p75-based hybrid tethers in an effort to elicit a more random integration pattern to increase biosafety, and potentially reduce proto-oncogene activation. We therefore truncated LEDGF/p75 by deleting the N-terminal chromatin-reading PWWP-domain, and replaced this domain with alternative pan-chromatin binding peptides. Expression of these LEDGF-hybrids in LEDGF-depleted cells efficiently rescued LV transduction and resulted in LV integrations that distributed more randomly throughout the host-cell genome. In addition, when considering safe harbor criteria, LV integration sites for these LEDGF-hybrids distributed more safely compared to LEDGF/p75-mediated integration in wild-type cells. This approach should be broadly applicable to introduce therapeutic or suicide genes for cell therapy, such as patient-specific iPS cells.

Sleeping Beauty transposition: from biology to applications

Sleeping Beauty (SB) is the first synthetic DNA transposon that was shown to be active in a wide variety of species. Here, we review studies from the last two decades addressing both basic biology and applications of this transposon. We discuss how host-transposon interaction modulates transposition at different steps of the transposition reaction. We also discuss how the transposon was translated for gene delivery and gene discovery purposes. We critically review the system in clinical, pre-clinical and non-clinical settings as a non-viral gene delivery tool in comparison with viral technologies. We also discuss emerging SB-based hybrid vectors aimed at combining the attractive safety features of the transposon with effective viral delivery. The success of the SB-based technology can be fundamentally attributed to being able to insert fairly randomly into genomic regions that allow stable long-term expression of the delivered transgene cassette. SB has emerged as an efficient and economical toolkit for safe and efficient gene delivery for medical applications.

Human adipose derived mesenchymal stromal cells transduced with GFP lentiviral vectors: assessment of immunophenotype and differentiation capacity in vitro

Adipose derived mesenchymal stromal/stem cells (ASCs) are a heterogeneous population characterized by (a) their ability to adhere to plastic; (b) immunophenotypic expression of certain cell surface markers, while lacking others; and (c) the capacity to differentiate into lineages of mesodermal origin including osteocytes, chondrocytes and adipocytes. The long-term goal is to utilize these cells for clinical translation into cell-based therapies. However, preclinical safety and efficacy need to be demonstrated in animal models. ASCs can also be utilized as biological vehicles for vector-based gene delivery systems, since they are believed to home to sites of inflammation and infection in vivo. These factors motivated the development of a labelling system for ASCs using lentiviral vector-based green fluorescent protein (GFP) transduction. Human ASCs were transduced with GFP-expressing lentiviral vectors. A titration study determined the viral titer required to transduce the maximum number of ASCs. The effect of the transduced GFP lentiviral vector on ASC immunophenotypic expression of surface markers as well as their ability to differentiate into osteocytes and adipocytes were assessed in vitro. A transduction efficiency in ASC cultures of approximately 80 % was observed with an MOI of ~118. No significant immunophenotypic differences were observed between transduced and non-transduced cells and both cell types successfully differentiated into adipocytes and osteocytes in vitro. We obtained >80 % transduction of ASCs using GFP lentiviral vectors. Transduced ASCs maintained plastic adherence, demonstrated ASC immunophenotype and the ability to differentiate into cells of the mesodermal lineage. This GFP-ASC transduction technique offers a potential tracking system for future pre-clinical studies.

A High-Capacity Adenoviral Hybrid Vector System Utilizing the Hyperactive Sleeping Beauty Transposase SB100X for Enhanced Integration

For efficient delivery of required genetic elements we utilized high-capacity adenoviral vectors in the past allowing high transgene capacities of up to 36 kb. Previously we explored the hyperactive Sleeping Beauty (SB) transposase (HSB5) for somatic integration from the high-capacity adenoviral vectors genome. To further improve this hybrid vector system we hypothesized that the previously described hyperactive SB transposase SB100X will result in significantly improved efficacies after transduction of target cells. Plasmid based delivery of the SB100X system revealed significantly increased integration efficiencies compared with the previously published hyperactive SB transposase HSB5. After optimizing experimental setups for high-capacity adenoviral vectors-based delivery of the SB100X system we observed up to eightfold and 100-fold increased integration efficiencies compared with the previously published hyperactive SB transposase HSB5 and the inactive transposase mSB, respectively. Furthermore, transposon copy numbers per cell were doubled with SB100X compared with HSB5 when using the identical multiplicity of infection. We believe that this improved hybrid vector system represents a valuable tool for achieving stabilized transgene expression in cycling cells and for treatment of numerous genetic disorders. Especially for in vivo approaches this improved adenoviral hybrid vector system will be advantageous because it may potentially allow reduction of the applied viral dose.

Targeted, homology-driven gene insertion in stem cells by ZFN-loaded 'all-in-one' lentiviral vectors

Biased integration remains a key challenge for gene therapy based on lentiviral vector technologies. Engineering of next-generation lentiviral vectors targeting safe genomic harbors for insertion is therefore of high relevance. In a previous paper (Cai et al., 2014a), we showed the use of integrase-defective lentiviral vectors (IDLVs) as carriers of complete gene repair kits consisting of zinc-finger nuclease (ZFN) proteins and repair sequences, allowing gene correction by homologous recombination (HR). Here, we follow this strategy to engineer ZFN-loaded IDLVs that insert transgenes by a homology-driven mechanism into safe loci. This insertion mechanism is driven by time-restricted exposure of treated cells to ZFNs. We show targeted gene integration in human stem cells, including CD34(+) hematopoietic progenitors and induced pluripotent stem cells (iPSCs). Notably, targeted insertions are identified in 89% of transduced iPSCs. Our findings demonstrate the applicability of nuclease-loaded 'all-in-one' IDLVs for site-directed gene insertion in stem cell-based gene therapies.

Immortalization of pig fibroblast cells by transposon-mediated ectopic expression of porcine telomerase reverse transcriptase

Pigs are the most economically important livestock, but pig cell lines useful for physiological studies and/or vaccine development are limited. Although several pig cell lines have been generated by oncogene transformation or human telomerase reverse transcriptase (TERT) immortalization, these cell lines contain viral sequences and/or antibiotic resistance genes. In this study, we established a new method for generating pig cell lines using the Sleeping Beauty (SB) transposon-mediated ectopic expression of porcine telomerase reverse transcriptase (pTERT). The performance of the new method was confirmed by generating a pig fibroblast cell (PFC) line. After transfection of primary PFCs with the SB transposon system, one cell clone containing the pTERT expression cassette was selected by dilution cloning and passed for different generations. After passage for more than 40 generations, the cell line retained stable expression of ectopic pTERT and continuous growth potential. Further characterization showed that the cell line kept the fibroblast morphology, growth curve, population doubling time, cloning efficiency, marker gene expression pattern, cell cycle distribution and anchorage-dependent growth property of the primary cells. These data suggest that the new method established is useful for generating pig cell lines without viral sequence and antibiotic resistant gene.

Hybrid nonviral/viral vector systems for improved piggyBac DNA transposon in vivo delivery

The DNA transposon piggyBac is a potential therapeutic agent for multiple genetic diseases such as cystic fibrosis (CF). Recombinant piggyBac transposon and transposase are typically codelivered by plasmid transfection; however, plasmid delivery is inefficient in somatic cells in vivo and is a barrier to the therapeutic application of transposon-based vector systems. Here, we investigate the potential for hybrid piggyBac/viral vectors to transduce cells and support transposase-mediated genomic integration of the transposon. We tested both adenovirus (Ad) and adeno-associated virus (AAV) as transposon delivery vehicles. An Ad vector expressing hyperactive insect piggyBac transposase (iPB7) was codelivered. We show transposase-dependent transposition activity and mapped integrations in mammalian cells in vitro and in vivo from each viral vector platform. We also demonstrate efficient and persistent transgene expression following nasal delivery of piggyBac/viral vectors to mice. Furthermore, using piggyBac/Ad expressing Cystic Fibrosis transmembrane Conductance Regulator (CFTR), we show persistent correction of chloride current in well-differentiated primary cultures of human airway epithelial cells derived from CF patients. Combining the emerging technologies of DNA transposon-based vectors with well-studied adenoviral and AAV delivery provides new tools for in vivo gene transfer and presents an exciting opportunity to increase the delivery efficiency for therapeutic genes such as CFTR.

Gene therapy for hemophilia

Hemophilia is an X-linked inherited bleeding disorder consisting of two classifications, hemophilia A and hemophilia B, depending on the underlying mutation. Although the disease is currently treatable with intravenous delivery of replacement recombinant clotting factor, this approach represents a significant cost both monetarily and in terms of quality of life. Gene therapy is an attractive alternative approach to the treatment of hemophilia that would ideally provide life-long correction of clotting activity with a single injection. In this review, we will discuss the multitude of approaches that have been explored for the treatment of both hemophilia A and B, including both in vivo and ex vivo approaches with viral and nonviral delivery vectors.

Sleeping Beauty-baculovirus hybrid vectors for long-term gene expression in the eye

BACKGROUND

A baculovirus vector is capable of efficiently transducing many nondiving and diving cell types. However, the potential of baculovirus is restricted for many gene delivery applications as a result of the transient gene expression that it mediates. The plasmid-based Sleeping Beauty (SB) transposon system integrates transgenes into target cell genome efficiently with a genomic integration pattern that is generally considered safer than the integration of many other integrating vectors; yet efficient delivery of therapeutic genes into cells of target tissues in vivo is a major challenge for nonviral gene therapy. In the present study, SB was introduced into baculovirus to obtain novel hybrid vectors that would combine the best features of the two vector systems (i.e. effective gene delivery and efficient integration into the genome), thus circumventing the major limitations of these vectors.

METHODS

We constructed and optimized SB-baculovirus hybrid vectors that bear either SB100x transposase or SB transposon in the forward or reverse orientations with respect to the viral backbone The functionality of the novel hybrid vectors was investigated in cell cultures and in a proof-of-concept study in the mouse eye.

RESULTS

The hybrid vectors showed high and sustained transgene expression that remained stable and demonstrated no signs of decline during the 2 months follow-up in vitro. These results were verified in the mouse eye where persistent transgene expression was detected two months after intravitreal injection.

CONCLUSIONS

Our results confirm that (i) SB-baculovirus hybrid vectors mediate long-term gene expression in vitro and in vivo, and (ii) the hybrid vectors are potential new tools for the treatment of ocular diseases.

Optimizing delivery and expression of designer nucleases for genome engineering

Genome engineering can be accomplished by designer nucleases. They are specifically designed to cleave double-stranded DNA at the desired target locus. This double-strand break subsequently engages the DNA repair pathway through nonhomologous end-joining (NHEJ), resulting in either gene disruption or gene repair. Alternatively, the presence of homologous donor DNA allows for targeted integration of this exogenous donor DNA in this target locus through homology-directed DNA repair. The key bottleneck in genome engineering relates to the delivery and expression of the designer nucleases. One of the most attractive vector platforms for genome engineering is based on integration-defective lentiviral vectors (IDLVs). The intrinsic episomal nature of IDLVs is well suited to ensure transient expression of designer nucleases and minimize potential risks associated with their sustained expression. Unfortunately, their expression is compromised because of epigenetic silencing that interferes with the transcriptional competence of IDLVs. In this issue, Pelascini and colleagues now showed that this bottleneck could be overcome by interfering with chromatin remodeling using histone deacetylase (HDAC) inhibitors. HDAC inhibition restored expression of designer nucleases from IDLVs and rescued their ability to achieve efficient targeted gene disruption by NHEJ comparable with that achieved with bona fide integrating lentiviral vectors. This study has implications for the ex vivo use of IDLVs for gene repair and gene targeting.

Hybrid lentivirus-phiC31-int-NLS vector allows site-specific recombination in murine and human cells but induces DNA damage

Gene transfer allows transient or permanent genetic modifications of cells for experimental or therapeutic purposes. Gene delivery by HIV-derived lentiviral vector (LV) is highly effective but the risk of insertional mutagenesis is important and the random/uncontrollable integration of the DNA vector can deregulate the cell transcriptional activity. Non Integrative Lentiviral Vectors (NILVs) solve this issue in non-dividing cells, but they do not allow long term expression in dividing cells. In this context, obtaining stable expression while avoiding the problems inherent to unpredictable DNA vector integration requires the ability to control the integration site. One possibility is to use the integrase of phage phiC31 (phiC31-int) which catalyzes efficient site-specific recombination between the attP site in the phage genome and the chromosomal attB site of its Streptomyces host. Previous studies showed that phiC31-int is active in many eukaryotic cells, such as murine or human cells, and directs the integration of a DNA substrate into pseudo attP sites (pattP) which are homologous to the native attP site. In this study, we combined the efficiency of NILV for gene delivery and the specificity of phiC31-int for DNA substrate integration to engineer a hybrid tool for gene transfer with the aim of allowing long term expression in dividing and non-dividing cells preventing genotoxicity. We demonstrated the feasibility to target NILV integration in human and murine pattP sites with a dual NILV vectors system: one which delivers phiC31-int, the other which constitute the substrate containing an attB site in its DNA sequence. These promising results are however alleviated by the occurrence of significant DNA damages. Further improvements are thus required to prevent chromosomal rearrangements for a therapeutic use of the system. However, its use as a tool for experimental applications such as transgenesis is already applicable.

Driving DNA transposition by lentiviral protein transduction

Gene vectors derived from DNA transposable elements have become powerful molecular tools in biomedical research and are slowly moving into the clinic as carriers of therapeutic genes. Conventional uses of DNA transposon-based gene vehicles rely on the intracellular production of the transposase protein from transfected nucleic acids. The transposase mediates mobilization of the DNA transposon, which is typically provided in the context of plasmid DNA. In recent work, we established lentiviral protein transduction from Gag precursors as a new strategy for direct delivery of the transposase protein. Inspired by the natural properties of infecting viruses to carry their own enzymes, we loaded lentivirus-derived particles not only with vector genomes carrying the DNA transposon vector but also with hundreds of transposase subunits. Such particles were found to drive efficient transposition of the piggyBac transposable element in a range of different cell types, including primary cells, and offer a new transposase delivery approach that guarantees short-term activity and limits potential cytotoxicity. DNA transposon vectors, originally developed and launched as a non-viral alternative to viral integrating vectors, have truly become viral. Here, we briefly review our findings and speculate on the perspectives and potential advantages of transposase delivery by lentiviral protein transduction.

Gene Therapy Used in Cancer Treatment

Cancer has been, from the beginning, a target of intense research for gene therapy approaches. Currently, more than 60% of all on-going clinical gene therapy trials worldwide are targeting cancer. Indeed, there is a clear unmet medical need for novel therapies. This is further urged by the fact that current conventional cancer therapies are frequently troubled by their toxicities. Different gene therapy strategies have been employed for cancer, such as pro-drug activating suicide gene therapy, anti-angiogenic gene therapy, oncolytic virotherapy, gene therapy-based immune modulation, correction/compensation of gene defects, genetic manipulation of apoptotic and tumor invasion pathways, antisense, and RNAi strategies. Cancer types, which have been targeted with gene therapy, include brain, lung, breast, pancreatic, liver, colorectal, prostate, bladder, head and neck, skin, ovarian, and renal cancer. Currently, two cancer gene therapy products have received market approval, both of which are in China. In addition, the stimulation of the host’s immune system, using gene therapeutic approaches, has gained vast interest. The intention of this review is to point out the most commonly viral and non-viral vectors and methods used in cancer gene therapy, as well as highlight some key results achieved in clinical trials.

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.

DNA transposition by protein transduction of the piggyBac transposase from lentiviral Gag precursors

DNA transposon-based vectors have emerged as gene vehicles with a wide biomedical and therapeutic potential. So far, genomic insertion of such vectors has relied on the co-delivery of genetic material encoding the gene-inserting transposase protein, raising concerns related to persistent expression, insertional mutagenesis and cytotoxicity. This report describes potent DNA transposition achieved by direct delivery of transposase protein. By adapting integrase-deficient lentiviral particles (LPs) as carriers of the hyperactive piggyBac transposase protein (hyPBase), we demonstrate rates of DNA transposition that are comparable with the efficiency of a conventional plasmid-based strategy. Embedded in the Gag polypeptide, hyPBase is robustly incorporated into LPs and liberated from the viral proteins by the viral protease during particle maturation. We demonstrate lentiviral co-delivery of the transposase protein and vector RNA carrying the transposon sequence, allowing robust DNA transposition in a variety of cell types. Importantly, this novel delivery method facilitates a balanced cellular uptake of hyPBase, as shown by confocal microscopy, and allows high-efficiency production of clones harboring a single transposon insertion. Our findings establish engineered LPs as a new tool for transposase delivery. We believe that protein transduction methods will increase applicability and safety of DNA transposon-based vector technologies.

Histone deacetylase inhibition rescues gene knockout levels achieved with integrase-defective lentiviral vectors encoding zinc-finger nucleases

Zinc-finger nucleases (ZFNs) work as dimers to induce double-stranded DNA breaks (DSBs) at predefined chromosomal positions. In doing so, they constitute powerful triggers to edit and to interrogate the function of genomic sequences in higher eukaryotes. A preferred route to introduce ZFNs into somatic cells relies on their cotransduction with two integrase-defective lentiviral vectors (IDLVs) each encoding a monomer of a functional heterodimeric pair. The episomal nature of IDLVs diminishes the risk of genotoxicity and ensures the strict transient expression profile necessary to minimize deleterious effects associated with long-term ZFN activity. However, by deploying IDLVs and conventional lentiviral vectors encoding HPRT1- or eGFP-specific ZFNs, we report that DSB formation at target alleles is limited after IDLV-mediated ZFN transfer. This IDLV-specific underperformance stems, to a great extent, from the activity of chromatin-remodeling histone deacetylases (HDACs). Importantly, the prototypic and U.S. Food and Drug Administration-approved inhibitors of metal-dependent HDACs, trichostatin A and vorinostat, respectively, did not hinder illegitimate recombination-mediated repair of targeted chromosomal DSBs. This allowed rescuing IDLV-mediated site-directed mutagenesis to levels approaching those achieved by using their isogenic chromosomally integrating counterparts. Hence, HDAC inhibition constitutes an efficacious expedient to incorporate in genome-editing strategies based on transient IDLV-mediated ZFN expression. Finally, we compared two of the most commonly used readout systems to measure targeted gene knockout activities based on restriction and mismatch-sensitive endonucleases. These experiments indicate that these enzymatic assays display a similar performance.

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.

Integration profile and safety of an adenovirus hybrid-vector utilizing hyperactive sleeping beauty transposase for somatic integration

We recently developed adenovirus/transposase hybrid-vectors utilizing the previously described hyperactive Sleeping Beauty (SB) transposase HSB5 for somatic integration and we could show stabilized transgene expression in mice and a canine model for hemophilia B. However, the safety profile of these hybrid-vectors with respect to vector dose and genotoxicity remains to be investigated. Herein, we evaluated this hybrid-vector system in C57Bl/6 mice with escalating vector dose settings. We found that in all mice which received the hyperactive SB transposase, transgene expression levels were stabilized in a dose-dependent manner and that the highest vector dose was accompanied by fatalities in mice. To analyze potential genotoxic side-effects due to somatic integration into host chromosomes, we performed a genome-wide integration site analysis using linker-mediated PCR (LM-PCR) and linear amplification-mediated PCR (LAM-PCR). Analysis of genomic DNA samples obtained from HSB5 treated female and male mice revealed a total of 1327 unique transposition events. Overall the chromosomal distribution pattern was close-to-random and we observed a random integration profile with respect to integration into gene and non-gene areas. Notably, when using the LM-PCR protocol, 27 extra-chromosomal integration events were identified, most likely caused by transposon excision and subsequent transposition into the delivered adenoviral vector genome. In total, this study provides a careful evaluation of the safety profile of adenovirus/Sleeping Beauty transposase hybrid-vectors. The obtained information will be useful when designing future preclinical studies utilizing hybrid-vectors in small and large animal models.

New developments in lentiviral vector design, production and purification

INTRODUCTION

Lentiviruses are a very potent class of viral vectors for which there is presently a rapidly growing interest for a number of gene therapy. However, their construction, production and purification need to be performed according to state-of-the-art techniques in order to obtain sufficient quantities of high purity material of any usefulness and safety.

AREAS COVERED

The recent advances in the field of recombinant lentivirus vector design, production and purification will be reviewed with an eye toward its utilization for gene therapy. Such a review should be helpful for the potential user of this technology.

EXPERT OPINION

The principal hurdles toward the use of recombinant lentivirus as a gene therapy vector are the low titer at which it is produced as well as the difficulty to purify it at an acceptable level without degrading it. The recent advances in the bioproduction of this vector suggest these issues are about to be resolved, making the retrovirus gene therapy a mature technology.

Histone deacetylase inhibition activates transgene expression from integration-defective lentiviral vectors in dividing and non-dividing cells

Integration-defective lentiviral vectors (IDLVs) are being increasingly deployed in both basic and preclinical gene transfer settings. Often, however, the IDLV transgene expression profile is muted when compared to that of their integration-proficient counterparts. We hypothesized that the episomal nature of IDLVs turns them into preferential targets for epigenetic silencing involving chromatin-remodeling histone deacetylation. Therefore, vectors carrying an array of cis-acting elements and transcriptional unit components were assembled with the aid of packaging constructs encoding either the wild-type or the class I mutant D116N integrase moieties. The transduction levels and transgene-product yields provided by each vector class were assessed in the presence and absence of the histone deacetylase (HDAC) inhibitors sodium butyrate and trichostatin A. To investigate the role of the target cell replication status, we performed experiments in growth-arrested human mesenchymal stem cells and in post-mitotic syncytial myotubes. We found that IDLVs are acutely affected by HDACs regardless of their genetic makeup or target cell replication rate. Interestingly, the magnitude of IDLV transgene expression rescue due to HDAC inhibition varied in a vector backbone- and cell type-dependent manner. Finally, investigation of histone modifications by chromatin immunoprecipitation followed by quantitative PCR (ChIP-qPCR) revealed a paucity of euchromatin marks distributed along IDLV genomes when compared to those measured on isogenic integration-competent vector templates. These findings support the view that IDLVs constitute preferential targets for epigenetic silencing involving histone deacetylation, which contributes to dampening their full transcriptional potential. Our data provide leads on how to most optimally titrate and deploy these promising episomal gene delivery vehicles.

Understanding lentiviral vector chromatin targeting: working to reduce insertional mutagenic potential for gene therapy

Replication-deficient retroviruses have been successfully utilized as vectors, offering an efficient, stable method of therapeutic gene delivery. Many examples exist proving this mode of integrative gene transfer is both effective and safe in cultured systems and clinical trials. Along with their success, severe side effects have occurred with early retroviral vectors causing a shift in the approach to vector design before further clinical testing. Several alternative delivery methods are available but lentiviral vectors (LV) are among the most favorable as they are already well understood. LV offer safer integration site selection profiles and a lower degree of genotoxicity, compared with γ-retroviral vectors. Following their introduction, development of the self-inactivating vector configuration was a huge step to this mode of therapy but did not confer full protection against insertional mutagenesis. As a result integration, modeling must be improved to eventually avoid this possibility. The cellular factor LEDGF/p75 seems to play an essential role in the process of LV site selection and its interactions with chromatin are being quickly resolved. LEDGF/p75 is at the center of one example directed integration effort where recombinant products bias the integration event, a step toward fully directed integration into pre-determined benign loci. A more accurate picture of the details of LEDGF/p75 in the natural integration process is emerging, including new binding specificities, chromatin interaction kinetics and additional cellular factors. Together with next-generation sequencing technology and bio-informatics to analyze integration patterns, these advancements will lead to highly focused directed integration, accelerating wide-spread acceptance of LV for gene therapy.

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.

Zinc finger protein designed to target 2-long terminal repeat junctions interferes with human immunodeficiency virus integration

Integration of the human immunodeficiency virus type 1 (HIV-1) genome into the host chromosome is a vital step in the HIV life cycle. The highly conserved cytosine-adenine (CA) dinucleotide sequence immediately upstream of the cleavage site is crucial for integrase (IN) activity. As this viral enzyme has an important role early in the HIV-1 replication cycle, interference with the IN substrate has become an attractive strategy for therapeutic intervention. We demonstrated that a designed zinc finger protein (ZFP) fused to green fluorescent protein (GFP) targets the 2-long terminal repeat (2-LTR) circle junctions of HIV-1 DNA with nanomolar affinity. We report now that 2LTRZFP-GFP stably transduced into 293T cells interfered with the expression of vesicular stomatitis virus glycoprotein (VSV-G)-pseudotyped lentiviral red fluorescent protein (RFP), as shown by the suppression of RFP expression. We also used a third-generation lentiviral vector and pCEP4 expression vector to deliver the 2LTRZFP-GFP transgene into human T-lymphocytic cells, and a stable cell line for long-term expression studies was selected for HIV-1 challenge. HIV-1 integration and replication were inhibited as measured by Alu-gag real-time PCR and p24 antigen assay. In addition, the molecular activity of 2LTRZFP-GFP was evaluated in peripheral blood mononuclear cells. The results were confirmed by Alu-gag real-time PCR for integration interference. We suggest that the expression of 2LTRZFP-GFP limited viral integration on intracellular immunization, and that it has potential for use in HIV gene therapy in the future.

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.