Design of a nonviral vector for site-selective, efficient integration into the human genome

Synthetically Engineered Adeno-Associated Virus for Efficient, Safe, and Versatile Gene Therapy Applications

Gene therapy directly targets mutations causing disease, allowing for a specific treatment at a molecular level. Adeno-associated virus (AAV) has been of increasing interest as a gene delivery vehicle, as AAV vectors are safe, effective, and capable of eliciting a relatively contained immune response. With the recent FDA approval of two AAV drugs for treating rare genetic diseases, AAV vectors are now on the market and are being further explored for other therapies. While showing promise in immune privileged tissue, the use of AAV for systemic delivery is still limited due to the high prevalence of neutralizing antibodies (nAbs). To avoid nAb-mediated inactivation, engineered AAV vectors with modified protein capsids, materials tethered to the capsid surface, or fully encapsulated in a second, larger carrier have been explored. Many of these engineered AAVs have added benefits, including avoided immune response, overcoming the genome size limit, targeted and stimuli-responsive delivery, and multimodal therapy of two or more therapeutic modalities in one platform. Native and engineered AAV vectors have been tested to treat a broad range of diseases, including spinal muscular atrophy, retinal diseases, cancers, and tissue damage. This review will cover the benefits of AAV as a promising gene vector by itself, the progress and advantages of engineered AAV vectors, particularly synthetically engineered ones, and the current state of their clinical translation in therapy.

A self-assembling amphiphilic peptide nanoparticle for the efficient entrapment of DNA cargoes up to 100 nucleotides in length

To overcome the low efficiency and cytotoxicity associated with most non-viral DNA delivery systems we developed a purely peptidic self-assembling system that is able to entrap single- and double-stranded DNA of up to 100 nucleotides in length. (HR)3gT peptide design consists of a hydrophilic domain prone to undergo electrostatic interactions with DNA cargo, and a hydrophobic domain at a ratio that promotes the self-assembly into multi-compartment micellar nanoparticles (MCM-NPs). Self-assembled (HR)3gT MCM-NPs range between 100 to 180 nm which is conducive to a rapid and efficient uptake by cells. (HR)3gT MCM-NPs had no adverse effects on HeLa cell viability. In addition, they exhibit long-term structural stability at 4 °C but at 37 °C, the multi-micellar organization disassembles overtime which demonstrates their thermo-responsiveness. The comparison of (HR)3gT to a shorter, less charged H3gT peptide indicates that the additional arginine residues result in the incorporation of longer DNA segments, an improved DNA entrapment efficiency and an increase cellular uptake. Our unique non-viral system for DNA delivery sets the stage for developing amphiphilic peptide nanoparticles as candidates for future systemic gene delivery.

CAR-T cell therapy in ovarian cancer: from the bench to the bedside

Ovarian cancer (OC) is the most lethal gynecological malignancy and is responsible for most gynecological cancer deaths. Apart from conventional surgery, chemotherapy, and radiotherapy, chimeric antigen receptor-modified T (CAR-T) cells as a representative of adoptive cellular immunotherapy have received considerable attention in the research field of cancer treatment. CARs combine antigen specificity and T-cell-activating properties in a single fusion molecule. Several preclinical experiments and clinical trials have confirmed that adoptive cell immunotherapy using typical CAR-engineered T cells for OC is a promising treatment approach with striking clinical efficacy; moreover, the emerging CAR-Ts targeting various antigens also exert great potential. However, such therapies have side effects and toxicities, such as cytokine-associated and “on-target, off-tumor” toxicities. In this review, we systematically detail and highlight the present knowledge of CAR-Ts including the constructions, vectors, clinical applications, development challenges, and solutions of CAR-T-cell therapy for OC. We hope to provide new insight into OC treatment for the future.

Progresses towards safe and efficient gene therapy vectors

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

Chimeric piggyBac transposases for genomic targeting in human cells

Integrating vectors such as viruses and transposons insert transgenes semi-randomly and can potentially disrupt or deregulate genes. For these techniques to be of therapeutic value, a method for controlling the precise location of insertion is required. The piggyBac (PB) transposase is an efficient gene transfer vector active in a variety of cell types and proven to be amenable to modification. Here we present the design and validation of chimeric PB proteins fused to the Gal4 DNA binding domain with the ability to target transgenes to pre-determined sites. Upstream activating sequence (UAS) Gal4 recognition sites harbored on recipient plasmids were preferentially targeted by the chimeric Gal4-PB transposase in human cells. To analyze the ability of these PB fusion proteins to target chromosomal locations, UAS sites were randomly integrated throughout the genome using the Sleeping Beauty transposon. Both N- and C-terminal Gal4-PB fusion proteins but not native PB were capable of targeting transposition nearby these introduced sites. A genome-wide integration analysis revealed the ability of our fusion constructs to bias 24% of integrations near endogenous Gal4 recognition sequences. This work provides a powerful approach to enhance the properties of the PB system for applications such as genetic engineering and gene therapy.

Size matters: versatile use of PiggyBac transposons as a genetic manipulation tool

Transposons have been promising elements for gene integration, and the Sleeping Beauty (SB) system has been the major one for many years, although there have been several other transposon systems available, for example, Tol2. However, recently another system known as PiggyBac (PB) has been introduced and developed for fulfilling the same purposes, for example, mutagenesis, transgenesis and gene therapy and in some cases with improved transposition efficiency and advantages over the Sleeping Beauty transposon system, although improved hyperactive transposase has highly increased the transposition efficacy for SB. The PB systems have been used in many different scientific research fields; therefore, the purpose of this review is to describe some of these versatile uses of the PiggyBac system to give readers an overview on the usage of PiggyBac system.

Using chimeric piggyBac transposase to achieve directed interplasmid transposition in silkworm Bombyx mori and fruit fly Drosophila cells

The piggyBac transposon has been long used to integrate foreign DNA into insect genomes. However, undesirable transgene expression can result from random insertions into the genome. In this study, the efficiency of chimeric Gal4-piggyBac transposase in directing integration onto a DNA target plasmid was evaluated in cultured silkworm Bombyx mori Bm-12 and fruit fly Drosophila Schneider 2 (S2) cells. The Gal4-piggyBac transposase has a Gal4 DNA-binding domain (DBD), and the target plasmid has upstream activating sequences (UAS) to which the Gal4 DBD can bind with high affinity. The results indicate that, in the Bm-12 and S2 cells, transpositional activity of Gal4-piggyBac transposase was increased by 4.0 and 7.5 times, respectively, compared to controls, where Gal4-UAS interaction was absent. Moreover, the Gal4-piggyBac transposase had the ability of directing piggyBac element integration to certain sites of the target plasmid, although the target-directing specificity was not as high as expected. The chimeric piggyBac transposase has the potential for use in site-directed transgenesis and gene function research in B. mori.

A site-specific genomic integration strategy for sustained expression of glucagon-like peptide-1 in mouse muscle for controlling energy homeostasis

The incretin hormone glucagon-like peptide-1 (GLP-1) exerts important functions in controlling glucose and energy homeostasis. Endogenous GLP-1 has a very short half-life due to DPP-IV-mediated degradation and renal clearance, which limits the therapeutic use of native GLP-1. We have shown previously that immunoglobulin fragment-fused GLP-1 (GLP-1/Fc) is a structurally stable GLP-1 analog. Here, we report a non-viral GLP-1/Fc gene therapy strategy utilizing a REP78-in-trans and REB-in-cis element system to achieve a site-specific genomic integration. For this purpose, the GLP-1/Fc expression cassette, which is fused with the RBE element, was co-injected with the Rep78 plasmid into the muscles of transgenic mice carrying the AAVS1 locus of human chromosome 19. The Rep protein-mediated site-specific integration was demonstrated by nested PCR, dot-blot, and Southern blotting. We found that this approach reduced weight gain and improved lipid profiles in the AAVS1-mice on high-fat diet challenge. Our observations reveal a new GLP-1 therapeutic strategy with an apparent absence of side effects, which may find applications in diabetes treatment and obesity prevention.

Genome-wide mapping of PiggyBac transposon integrations in primary human T cells

The piggyBac transposon system represents a promising nonviral tool for gene delivery and discovery, and may also be of value for clinical gene therapy. PiggyBac is a highly efficient integrating vector that stably transfects (approximately 40%) of primary human T cells for potential adoptive immunotherapy applications. To evaluate the potential genotoxicity of piggyBac, we compared 228 integration sites in primary human T cells to integrations in 2 other human-derived cell lines (HEK293 and HeLa) and randomly simulated integrations into the human genome. Our results revealed distinct differences between cell types. PiggyBac had a nonrandom integration profile and a preference for transcriptional units (approximately 50% into RefSeq genes in all cell types), CpG islands (18% in T cells and 8% in other human cells), and transcriptional start sites (<5 kb, 16% to 20% in all cell types). PiggyBac also preferred TTAA but not AT-rich regions of the human genome. We evaluated the expression of mapped genes into which piggyBac integrated, and found selection of more active genes in primary human T cells compared with other human cell types, possibly due to concomitant T-cell activation during transposition. Importantly, we found that in comparison to what has been reported for gammaretroviral and human lenitviral vectors, piggyBac had decreased integration frequency into or within 50 kb of the transcriptional start sites of known proto-oncogenes. Hence the piggyBac nonviral gene delivery system seems to represent a promising gene transfer system for clinical applications using human T lymphocytes.

Helper-independent piggyBac plasmids for gene delivery approaches: strategies for avoiding potential genotoxic effects

Efficient integration of functional genes is an essential prerequisite for successful gene delivery such as cell transfection, animal transgenesis, and gene therapy. Gene delivery strategies based on viral vectors are currently the most efficient. However, limited cargo capacity, host immune response, and the risk of insertional mutagenesis are limiting factors and of concern. Recently, several groups have used transposon-based approaches to deliver genes to a variety of cells. The piggyBac (pB) transposase in particular has been shown to be well suited for cell transfection and gene therapy approaches because of its flexibility for molecular modification, large cargo capacity, and high transposition activity. However, safety considerations regarding transposase gene insertions into host genomes have rarely been addressed. Here we report our results on engineering helper-independent pB plasmids. The single-plasmid gene delivery system carries both the piggyBac transposase (pBt) expression cassette as well as the transposon cargo flanked by terminal repeat element sequences. Improvements to the helper-independent structure were achieved by developing new plasmids in which the pBt gene is rendered inactive after excision of the transposon from the plasmid. As a consequence, potentially negative effects that may develop by the persistence of an active pBt gene posttransposition are eliminated. The results presented herein demonstrate that our helper-independent plasmids represent an important step in the development of safe and efficient gene delivery methods that should prove valuable in gene therapy and transgenic approaches.

Interaction of DNA/nuclear protein/polycation and the terplexes for gene delivery

Nuclear transport of exogenous DNA is a major barrier to nonviral gene delivery that needs to be addressed in the design of new vectors. In this study, we prepared pDNA/HMGB1/PEG-PEI terplexes to promote nuclear import. HMGB1 in the terplexes was used to assist the transportation of pDNA into the nucleus of cells, since it contained nuclear localization signal (NLS); PEG chains were introduced to stabilize pDNA/vector terplexes and reduce the cytotoxicity. HMGB1/PEG-PEI combined vectors have been investigated specifically for their structure interaction by atomic force microscopy and circular dichroic spectroscopy. The results demonstrated that the HMGB1 molecule could bind with the pDNA chains, but not condense pDNA well. The PEG-PEI further compacted pDNA/HMGB1 complexes into nanosized spherical terplexes. The pDNA delivered by HMGB1/PEG-PEI combined vectors was significantly accumulated in the nucleus of cells, as observed by confocal laser scanning microscopy. The percentage of GFP-transfected cells and VEGF protein expression level induced by HMGB1/PEG-PEI were 2.6-4.9-fold and 1.4-2.8-fold higher, respectively, than that of a common cationic polymer PEI 25 kDa. Therefore, the HMGB1/PEG-PEI combined vector could be used as a versatile vector for promoting exogenous DNA nuclear localization, thereby enhancing its expression.

Steps toward targeted insertional mutagenesis with class II transposable elements

Insertional mutagenesis can be achieved by a variety of approaches, including both random and targeted methods. In contrast to chemical mutagenesis, insertional mutagens provide a molecular tag, thereby allowing rapid identification of the mutated genomic region. Integration into defined genomic locations has great utility for both gene insertion and mutagenesis. Our laboratories have explored targeted integration through the use of transposases coupled to defined DNA-binding domains. This technology holds great promise for targeted insertional mutagenesis by biasing integration events to regions recognized by the chosen DNA-binding domain. Herein, we provide a brief background on targeted transposon integration and detailed protocols for testing chimeric transposases in both mammalian cell culture and insect embryos.

Pompe disease: current state of treatment modalities and animal models

Pompe disease is a rare autosomal recessive lysosomal storage disease caused by deficiency of acid-alpha-glucosidase (GAA). This deficiency results in glycogen accumulation in the lysosomes, leading to lysosomal swelling, cellular damage and organ dysfunction. In early-onset patients (the classical infantile form and juvenile form) this glycogen accumulation leads to death. The only therapy clinically available is enzyme replacement therapy, which compensates for the missing enzyme by i.v. administration of recombinant produced enzyme. The development of clinically relevant animal models gained more insight in the disease and allowed evaluation of recombinant enzyme therapy. Several therapies are currently under investigation for Pompe disease, including gene therapy. This review gives an overview of the available knockout mouse models, of the in vitro and in vivo studies performed using recombinant produced enzyme. Furthermore, it describes current therapeutic approaches for Pompe disease as well as experimental therapies like gene correction therapy.

Site-specific transfer of an intact β-globin gene cluster through a new targeting vector

The ideal gene-therapy vector for treating genetic disorders should deliver intact therapeutic genes and their essential regulatory elements into the specific "safe genomic site" and realize long-term, self-regulatory expression. For beta-thalassemia gene therapy, viral vectors have been broadly used, but the accompanying insertional mutation and immunogenicity remain problematic. Hence, we aimed to develop new non-viral vectors that are efficient and safe in treating diseases. As previous studies have demonstrated that physiological expression of beta-globin genes requires both a 5' locus control region and 3' specific elements, we constructed a new human chromosome-derived targeting vector to transfer the intact beta-globin gene cluster into K562 cells. The whole beta-globin gene cluster was precisely integrated into the target site and expressed in a self-regulatory pattern. The results proved that the human chromosome-derived vector was specifically targeted to the human genome and this could provide a novel platform for further gene therapy research.

Active integration: new strategies for transgenesis

This paper presents novel methods for producing transgenic animals, with a further emphasis on how these techniques may someday be applied in gene therapy. There are several passive methods for transgenesis, such as pronuclear microinjection (PNI) and Intracytoplasmic Sperm Injection-Mediated Transgenesis (ICSI-Tr), which rely on the repair mechanisms of the host for transgene (tg) insertion. ICSI-Tr has been shown to be an effective means of creating transgenic animals with a transfection efficiency of approximately 45% of animals born. Furthermore, because this involves the injection of the transgene into the cytoplasm of oocytes during fertilization, limited mosaicism has traditionally occurred using this technique. Current active transgenesis techniques involve the use of viruses, such as disarmed retroviruses which can insert genes into the host genome. However, these methods are limited by the size of the sequence that can be inserted, high embryo mortality, and randomness of insertion. A novel active method has been developed which combines ICSI-Tr with recombinases or transposases to increase transfection efficiency. This technique has been termed "Active Transgenesis" to imply that the tg is inserted into the host genome by enzymes supplied into the oocyte during tg introduction. DNA based methods alleviate many of the costs and time associated with purifying enzyme. Further studies have shown that RNA can be used for the transposase source. Using RNA may prevent problems with continued transposase activity that can occur if a DNA transposase is integrated into the host genome. At present piggyBac is the most effective transposon for stable integration in mammalian systems and as further studies are done to elucidate modifications which improve piggyBac's specificity and efficacy, efficiency in creating transgenic animals should improve further. Subsequently, these methods may someday be used for gene therapy in humans.

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.

piggyBac is a flexible and highly active transposon as compared to sleeping beauty, Tol2, and Mos1 in mammalian cells

A nonviral vector for highly efficient site-specific integration would be desirable for many applications in transgenesis, including gene therapy. In this study we directly compared the genomic integration efficiencies of piggyBac, hyperactive Sleeping Beauty (SB11), Tol2, and Mos1 in four mammalian cell lines. piggyBac demonstrated significantly higher transposition activity in all cell lines whereas Mos1 had no activity. Furthermore, piggyBac transposase coupled to the GAL4 DNA-binding domain retains transposition activity whereas similarly manipulated gene products of Tol2 and SB11 were inactive. The high transposition activity of piggyBac and the flexibility for molecular modification of its transposase suggest the possibility of using it routinely for mammalian transgenesis.

Chimeric Mos1 and piggyBac transposases result in site-directed integration

Genetic transformation systems based on Mos1 and piggyBac transposable elements are used to achieve stable chromosomal integration. However, integration sites are randomly distributed in the genome and transgene expression can be influenced by position effects. We developed a novel technology that utilizes chimeric transposases to direct integration into specific sites on a target DNA molecule. The Gal4 DNA binding domain was fused to the NH(2) terminus of the Mos1 and piggyBac transposases and a target plasmid was created that contained upstream activating sequences (UAS), to which the Gal4 DBD binds with high affinity. The transpositional activity of the Gal4-Mos1 transposase was 12.7-fold higher compared to controls where the Gal4-UAS interaction was absent and 96% of the recovered transposition products were identical, with integration occurring at the same TA site. In a parallel experiment, a Gal4-piggyBac transposase resulted in an 11.6-fold increase in transpositional activity compared to controls, with 67% of the integrations occurring at a single TTAA site. This technology has the potential to minimize nonspecific integration events that may result in insertional mutagenesis and reduced fitness. Site-directed integration will be advantageous to the manipulation of genomes, study of gene function, and for the development of gene therapy techniques.

Functional zinc finger/sleeping beauty transposase chimeras exhibit attenuated overproduction inhibition

The sleeping beauty (SB) transposon system has potential utility in gene transfer applications but lacks specificity for genomic integration and exhibits overproduction inhibition which limits in vivo activity. Targeting transposition may be possible by coupling a specific DNA binding domain to the SB transposase, but it is not known if this strategy will preserve or disrupt activity of the system. We engineered and tested chimeric SB transposases with two different human zinc finger DNA binding domain elements, Sp1 and zinc finger 202 (ZNF202). Addition of Sp1 to the C-terminus abolished transposase activity whereas N-terminal addition of either Sp1 or ZNF202 did not. Transposition activity exhibited by N-terminal chimeras was increased to levels similar to native SB through the use of a hyperactive transposase (SB12) and activating transposon mutations. Importantly, addition of DNA binding domains to the transposase N-terminus resulted in attenuation of overproduction inhibition, a major limitation of this system. These findings suggest that SB transposase chimeras may have specific advantages over the native enzyme.

Site-directed genome modification: nucleic acid and protein modules for targeted integration and gene correction

A variety of technological advances in recent years have made permanent genetic manipulation of an organism a technical possibility. As the details of natural biological processes for genome modification are elucidated, the enzymes catalyzing these events (transposases, recombinases, integrases and DNA repair enzymes) are being harnessed or modified for the purpose of intentional gene modification. Targeted integration and gene repair can be mediated by the DNA-targeting specificity inherent to a particular enzyme, or rely on user-designed specificities. Integration sites can be defined by using DNA base-pairing or protein-DNA interaction as a means of targeting. This review will describe recent progress in the development of 'user-targetable' systems, particularly highlighting the application of custom DNA-binding proteins or nucleic acid homology to confer specificity.

Synthetic, self-assembly ABCD nanoparticles; a structural paradigm for viable synthetic non-viral vectors

Gene therapy research is still in trouble owing to a paucity of acceptable vector systems to deliver nucleic acids to patients for therapy. Viral vectors are efficient but may be too dangerous. Synthetic non-viral vectors are inherently safer but are currently not efficient enough to be clinically viable. The solution for gene therapy lies with improved synthetic non-viral vectors systems. This review is focused on synthetic cationic liposome/micelle-based non-viral vector systems and is a critical review written to illustrate the increasing importance of chemistry in gene therapy research. This review should be of primary interest to synthetic chemists and biomedical researchers keen to appreciate emerging technologies, but also to biological scientists who remain to be convinced about the relevance of chemistry to biology.

Site-directed genome modification: derivatives of DNA-modifying enzymes as targeting tools

The modification of mammalian genomes is an important goal in gene therapy and animal transgenesis. To generate stable genetic and biochemical changes, the therapeutic genes or transgenes need to be incorporated into the host genome. Ideally, the integration of the foreign gene should occur at sites that ensure their continual expression in the absence of any unwanted side effects on cellular metabolism. In this article, we discuss the opportunities provided by natural DNA-modifying enzymes, such as transposases, recombinases and integrases, to mediate the stable integration of foreign genes into host genomes. In addition, we discuss the approaches that have been taken to improve the efficiency and to modify the site-specificity of these enzymes.

Gene therapy progress and prospects: targeted gene repair

The capacity to correct a mutant gene within the context of the chromosome holds great promise as a therapy for inherited disorders but fulfilling this promise has proven to be challenging. However, steady progress is being made and the development of gene repair as a viable and robust approach is underway. Here, we present some of the recent advances that are helping to shape our thinking about the feasibility and the limitations of this technique. For the most part, these advances center on understanding the regulation of the reaction and validating its application in animal models.

Sleeping Beauty Transposon‐Mediated Gene Therapy for Prolonged Expression

The Sleeping Beauty (SB) transposon system represents a new vector for non-viral gene transfer that melds advantages of viruses and other forms of naked DNA transfer. The transposon itself is comprised of two inverted terminal repeats of about 340 base pairs each. The SB system directs precise transfer of specific constructs from a donor plasmid into a mammalian chromosome. The excision of the transposon from a donor plasmid and integration into a chromosomal site is mediated by Sleeping Beauty transposase, which can be delivered to cells vita its gene or its mRNA. As a result of its integration in chromosomes, and its lack of viral sequences that are often detected by poorly understood cellular defense mechanisms, a gene in a chromosomally integrated transposon can be expressed over the lifetime of a cell. SB transposons integrate nearly randomly into chromosomes at TA-dinucleotide base pairs although the sequences flanking the TAs can influence the probability of integration at a given site. Although random integration of vectors into human genomes is often thought to raise significant safety issues, evidence to date does not indicate that random insertions of SB transposons represent risks that are equal to those of viral vectors. Here we review the activities of the SB system in mice used as a model for human gene therapy, methods of delivery of the SB system, and its efficacy in ameliorating disorders that model human disease.

Transfer and Stable Transgene Expression of a Mammalian Artificial Chromosome into Bone Marrow-Derived Human Mesenchymal Stem Cells

Mammalian artificial chromosomes (ACEs) transferred to autologous adult stem cells (SCs) provide a novel strategy for the ex vivo gene therapy of a variety of clinical indications. Unlike retroviral vectors, ACEs are stably maintained, autonomous, and nonintegrating. In this report we assessed the delivery efficiency of ACEs and evaluated the subsequent differentiation potential of ACE-transfected bone marrow-derived human mesenchymal stem cells (hMSCs). For this, an ACE carrying multiple copies of the red fluorescent protein (RFP) reporter gene was transferred under optimized conditions into hMSCs using standard cationic transfection reagents. RFP expression was detectable in 11% of the cells 4-5 days post-transfection. The RFP-expressing hMSCs were enriched by high-speed flow cytometry and maintained their potential to differentiate along adipogenic or osteogenic lineages. Fluorescent in situ hybridization and fluorescent microscopy demonstrated that the ACEs were stably maintained as single chromosomes and expressed the RFP transgenes in both differentiated cultures. These findings demonstrate the potential utility of ACEs for human adult SC ex vivo gene therapy.

Transgene manipulation in zebrafish by using recombinases

Although much remains to be done, our results to date suggest that efficient and precise genome engineering in zebrafish will be possible in the future by using Cre recombinase and SB transposase in combination with their respective target sites. In this study, we provide the first evidence that Cre recombinase can mediate effective site-specific deletion of transgenes in zebrafish. We found that the efficiency of target site utilization could approach 100%, independent of whether the target site was provided transiently by injection or stably within an integrated transgene. Microinjection of Cre mRNA appeared to be slightly more effective for this purpose than microinjection of Cre-expressing plasmid DNA. Our work has not yet progressed to the point where SB-mediated mobilization of our transgene constructs would be observed. However, a recent report has demonstrated that SB can enhance transgenesis rates sixfold over conventional methods by efficiently mediating multiple single-copy insertion of transgenes into the zebrafish genome (Davidson et al., 2003). Therefore, it seems likely that a combined system should eventually allow both SB-mediated transgene mobilization and Cre-mediated transgene modification. Our goal is to validate methods for the precise reengineering of the zebrafish genome by using a combination of Cre-loxP and SB transposon systems. These methods can be used to delete, replace, or mobilize large pieces of DNA or to modify the genome only when and where required by the investigator. For example, it should be possible to deliver particular RNAi genes to well-expressed chromosomal loci and then exchange them easily with alternative RNAi genes for the specific suppression of alternative targets. As a nonviral vector for gene therapy, the transposon component allows for the possibility of highly efficient integration, whereas the Cre-loxP component can target the integration and/or exchange of foreign DNA into specific sites within the genome. The specificity and efficiency of this system also make it ideal for applications in which precise genome modifications are required (e.g., stock improvement). Future work should establish whether alternative recombination systems (e.g., phiC31 integrase) can improve the utility of this system. After the fish system is fully established, it would be interesting to explore its application to genome engineering in other organisms.

Immunotherapy and prostate cancer

Prostate cancer is the second leading cause of cancer death in the US, largely because of the limitations of our current therapeutic options, especially once the cancer has metastasized. Investigators have long sought new therapeutic modalities such as angiogenesis inhibitors, vaccines, and gene therapy, among others. It appears that a combination approach will be required to cure the majority of malignancies. Immunotherapy for prostate cancer appears feasible and a likely therapeutic modality in the armamentarium. Unfortunately, further research in basic immunology and the interaction of the immune system with other forms of therapy is needed. Many obstacles exist in immunotherapy, including vector design, tumouricidal specificity, and tumor evasion, which will have to be overcome in order to realize the maximum therapeutic benefit from this treatment modality.

Highly efficient, large volume flow electroporation

Electroporation is widely used to transfect and load cells with various molecules. Traditional electroporation using a static mode is typically restricted to volumes less than 1 mL, which limits its use in clinical and industrial bioprocessing applications. Here we report efficient, large volume transfection results by using a scalable-volume electroporation system. Suspended (Jurkat) and adherent cells (10T1/2 and Huh-7) were tested. A large macromolecule, FITC-conjugated dextran (MW=500 kD) was used to measure cell uptake, while a plasmid carrying the gene coding for enhanced green fluorescence protein (eGFP) was used to quantitate the flow electrotransfection efficiency as determined by flow cytometry. The flow electroloading efficiency of FITC-dextran was >90%, while the cell viability was highly maintained (>90%). High flow electrotransfection efficiency (up to 75%) and cell viability (up to 90%) were obtained with processing volumes ranging from 1.5 to 50 mL. No significant difference of electrotransfection efficiency was observed between flow and static electrotransfection. When 50 mL of cell volume was processed and samples collected at different time points during electroporation, the transgene expression and cell viability results were identical. We also demonstrated that DNA plasmid containing EBNA1-OriP elements from Epstein-Barr virus were more efficient in transgene expression than standard plasmid without the elements (at least 500 too 1000-fold increase in expression level). Finally, to examine the feasibility of utilizing flow electrotransfected cells as a gene delivery vehicle, 10T1/2 cells were transfected with a DNA plasmid containing the gene coding for mIL12. mIL12 transfected cells were injected subcutaneously into mice, and produced functional mIL12, as demonstrated by anti-angiogenic activity. This is the first demonstration of efficient, large volume, flow electroporation and the in vivo efficacy of flow electrotransfected cells. This technology may be useful for clinical gene therapy and large-scale bioprocesses.