Plasmid DNA gene therapy by electroporation: principles and recent advances

Simple plasmid DNA injection is a safe and feasible gene transfer method, but it confers low transfection efficiency and transgene expression. This non-viral gene transfer method is enhanced by physical delivery methods, such as electroporation and the use of a gene gun. In vivo electroporation has been rapidly developed over the last two decades to deliver DNA to various tissues or organs. It is generally considered that membrane permeabilization and DNA electrophoresis play important roles in electro-gene transfer. Skeletal muscle is a well characterized target tissue for electroporation, because it is accessible and allows for long-lasting gene expression ( > one year). Skin is also a target tissue because of its accessibility and immunogenicity. Numerous studies have been performed using in vivo electroporation in animal models of disease. Clinical trials of DNA vaccines and immunotherapy for cancer treatment using in vivo electroporation have been initiated in patients with melanoma and prostate cancer. Furthermore, electroporation has been applied to DNA vaccines for infectious diseases to enhance immunogenicity, and the relevant clinical trials have been initiated. The gene gun approach is also being applied for the delivery of DNA vaccines against infectious diseases to the skin. Here, we review recent advances in the mechanism of in vivo electroporation, and summarize the findings of recent preclinical and clinical studies using this technology.     

Gene therapy approaches for cardiovascular diseases

Cardiovascular diseases are one of the main causes of mortality in Western countries. Gene therapy is emerging as a potential strategy for the treatment of cardiovascular diseases, such as peripheral arterial disease, ischemic heart disease, restenosis after angioplasty, vascular bypass graft occlusion and transplant-associated coronary artery disease. Since the initial experiments more than one decade ago, remarkable progress has been made in the field of gene transfer and human clinical trials are underway. In here we give an overview of available gene transfer strategies describing several delivery routes and currently used vectors in animal studies and clinical trials. Hereby we want to focus on new approaches including the potential combination of gene therapy with cell therapy and tissue engineering, gene silencing and recently developed techniques for targeting genes to the vascular wall and the myocardium.     

Biomaterials for gene delivery: atelocollagen-mediated controlled release of molecular medicines

Over the last decade, increasing attention has been paid to the development of systems to deliver drugs for long periods at controlled rates. Some of these systems can deliver drugs continuously for over one year. However, little effort has been given to developing systems for the controlled release of nucleic acids. Recently, a novel gene transfer method which allows prolonged release and expression of plasmid DNA in vivo in normal adult animals was established. In this system, a biocompatible natural polymer such as collagen or its derivatives acts as the carrier for the delivery of DNA vectors. The biomaterial carrying the plasmid DNA was administered into animals and, once introduced, gradually released plasmid DNA in vivo. A single injection of plasmid DNA/biomaterial produced physiologically significant levels of gene-encoding proteins in the local/systemic circulation of animals and resulted in prolonged biological effects. These results suggest that the biomaterials carrying plasmid DNA may enhance the clinical potency of plasmid-based gene transfer, facilitating a more effective and long-term use of naked plasmid vectors for gene therapy. Furthermore, the biomaterials can be removed surgically, minimizing the effect of gene products if some unexpected side effects should be observed after application. The application of these systems to expand the bioavailability of molecular medicine, including antisense oligonucleotides and adenovirus vectors, and to aid in stem cell transplantation in the context of DNA-based tissue engineering will be discussed.     

Improvement of nonviral gene therapy by Epstein-Barr virus (EBV)-based plasmid vectors

The nonviral gene transfer technologies include naked DNA administration, electrical or particle-mediated transfer of naked DNA, and administration of DNA-synthetic macromolecule complex vectors. Each method has its advantage, such as low immunogenicity, inexpensiveness, ease in handling, etc., but the common disadvantage is that the transfection efficiency has been relatively poor as far as conventional plasmid vectors are involved. To improve the nonviral gene transfer systems, Epstein-Barr virus (EBV)-based plasmid vectors (also referred to EBV-based episomal vectors) have been employed. These vectors contain the EBNA1 gene and oriP element that enable high transfer efficiency, strong transgene expression and long term maintenance of the expression. In the current article, I review recent preclinical gene therapy studies with the EBV plasmid vectors conducted against various diseases. For gene therapy against malignancies, drastic tumor suppression was achieved by gancyclovir administrations following an intratumoral injection with an EBV plasmid vector encoding the HSV1-TK suicide gene. Equiping the plasmid with carcinoembryonic antigen (CEA) promoter sequences enabled targeted killing of CEA-positive tumor cells, which was not accomplished by conventional plasmid vectors without the EBV genetic elements. Transfection with an apoptosis-inducing gene was also effective in inhibiting tumors. Interleukin (IL)-12 and IL-18 gene transfer, either local or systemic, induced therapeutic antitumoral immune responses including augmentation of the cytotoxic T lymphocyte (CTL) and natural killer (NK) activities, while an autologous tumor vaccine engineered to secrete Th1 cytokines via the EBV system also induced growth retardation of tumors. Non-EBV conventional plasmids were much less effective in eliciting these therapeutic outcomes. Intracardiomuscular transfer of the beta-adrenergic receptor gene induced a significant elevation in cardiac output in cardiomyopathic animals, suggesting the usefulness of the EBV system in treating heart failure. The EBV-based nonviral delivery also worked as genetic vaccine that triggered prophylactic cellular and humoral immunity against acute lethal viral infection. All the nonviral delivery vehicles so far tested showed an improved transfection rate when combined with the EBV-plasmids. Collectively, the EBV-based plasmid vectors may greatly contribute to nonviral gene therapy against a variety of disorders, including malignant, congenital, chronic and infectious diseases.     

Does gene therapy become pharmacotherapy?

Recent progress in molecular and cellular biology has led to the development of numerous effective cardiovascular drugs. However, there are still a number of diseases for which no known effective therapy exists, such as peripheral arterial disease, ischaemic heart disease, restenosis after angioplasty, and vascular bypass graft occlusion. Currently, gene therapy is emerging as a potential strategy for the treatment of cardiovascular disease despite its limitations. The first human trial in gene therapy for cardiovascular disease was started at 1994 to treat peripheral vascular disease using vascular endothelial growth factor (VEGF). Then, many different potent angiogenic growth factors were tested in clinical trials to treat peripheral arterial disease and ischaemic heart disease. Improvement of clinical symptoms in peripheral arterial disease and ischaemic heart disease has been reported. This review focuses on the future potential of gene therapy for the treatment of cardiovascular disease. In the future, gene therapy might become a real pharmacotherapy to treat cardiovascular disease.     

Muscular gene transfer using nonviral vectors

Skeletal muscle is a target tissue of choice for the gene therapy of both muscle and non-muscle disorders. Investigations of gene transfer into muscle have progressed considerably from the expression of plasmid reporter genes to the production of therapeutic proteins such as trophic factors, hormones, antigens, ion channels or cytoskeletal proteins. Viral vectors are intrinsically the most efficient vehicles to deliver genes into skeletal muscles. But, because viruses are associated with a variety of problems (such as immune and inflammatory responses, toxicity, limited large scale production yields, limitations in the size of the carried therapeutic genes), nonviral vectors remain a viable alternative. In addition, as nonviral vectors allow to transfer genetic structures of various sizes (including large plasmid DNA carrying full-length coding sequences of the gene of interest), they can be used in various gene therapy approaches. However, given the lack of efficiency of nonviral vectors in experimental studies and in the clinical settings, the overall outcome clearly indicates that improved synthetic vectors and/or delivery techniques are required for successful clinical gene therapy. Today, most of the potential muscle-targeted clinical applications seem geared toward peripheral ischemia (mainly through local injections) and cancer and infectious vaccines, and one locoregional administration of naked DNA in Duchenne muscular dystrophy. This review updates the developments in clinical applications of the various plasmid-based non-viral methods under investigation for the delivery of genes to muscles.     

The HVJ-envelope as an innovative vector system for cardiovascular disease

Recently promising results of gene therapy clinical trials have been reported for treatment of peripheral vascular and cardiovascular diseases using various angiogenic growth factors and other therapeutic genes. Viral vector and non-viral vector systems were employed in preclinical studies and clinical trials. Adenoviral vector and naked plasmid have been used most in the clinical studies. HVJ (hemagglutinating virus of Japan or Sendai virus)-liposome vector, a hybrid non-viral vector system with fusion of inactivated HVJ virus particle and liposome, has developed and demonstrated high transfection efficiency in preclinical studies of many different disease models, including a wide range of cardiovascular disease models. However, some limitations exist in the HVJ-liposome technology, especially in the scalability of its production. Recently an innovative vector technology, HVJ envelope (HVJ-E) has been developed as a non-viral vector, consisting of HVJ envelope without its viral genome, which is eliminated by a combination of inactivation and purification steps. HVJ-E is able to enclose various molecule entities, including DNA, oligonucleotides, proteins, as single or multiple therapeutic remedies. The therapeutic molecule-included HVJ-E vector can transfect various cell types in animals and humans with high efficiency. In this review, vector technology for cardiovascular disease and the biology of HVJ-E vector technology is discussed.     

Electro-Gene-Transfer: A New Approach for Muscle Gene Delivery

Gene transfer into skeletal muscle cells by direct injection of naked plasmid DNA results in sustained gene expression. Intramuscular injection of plasmid DNA might thus be used to correct myopathies, to secrete locally or systematic therapeutic proteins and to elicit an immune response against specific antigens. However, the potential utility of this technique for gene application in humans is limited by the poor transduction efficiency and the low and highly variable level of gene expression. Different methods are thus being developed to increase the efficiency of gene transfer in muscles. It has been recently reported that a dramatic improvement of DNA transfer is achieved by applying an electric field to the muscle fibers subsequent to local DNA injection. Electro-gene-transfer increases gene expression by several orders of magnitude and strongly reduces interindividual variability. Electroinjection of genes encoding for secreted proteins resulted in sustained expression and disease correction in animal models of gene therapy. Moreover, the immunogenicity of DNA vaccines is dramatically increased when antigen-encoding plasmids are delivered by this technique. This technique may thus have broad and important applications in human gene therapy. This review provides a brief overview of the theory of electro-gene-transfer and describes parameters governing its efficiency in muscle. We also summarize the results obtained with electro-gene-transfer in animal models to date and the technical issues that must be solved before its use for human therapy can be considered.     

Current Status of Gene Therapy for Rheumatoid Arthritis

Despite the high prevalence of the disease, at present little effective pharmacological treatment of rheumatoid arthritis is available. Novel approaches utilising biological agents have resulted in the development of new antiarthritic and antiinflammatory agents, such as tumour necrosis factor-alpha (TNFalpha)-specific antibodies and interleukin-1 receptor antagonist (IL-1ra). Local gene therapy not only allows the pharmaceutical use of these biologicals, but also allows for continuous drug supply, which is necessary for chronic diseases like rheumatoid arthritis. We discuss the basics of rheumatoid arthritis therapy, candidate genes and possible gene transfer methods. A current clinical gene therapy trial is focusing on the IL-1 system using IL-1ra as a transgene. The transfer system, clinical protocol and preliminary results are described. After treatment of 11 patients we feel that gene therapy will offer potential as a new avenue to treat rheumatoid arthritis.     

Antiphospholipid antibodies as non-traditional risk factors in atherosclerosis based cardiovascular diseases without overt autoimmunity. A critical updated review

The role of antiphospholipid antibodies (aPL) associated with cardiovascular diseases has been extensively studied in autoimmune patients, however it was largely unknown whether and how aPL associate with coronary artery disease (CAD), ishemic stroke (IS) and peripheral artery disease (PAD) in non-autoimmune patients. The current review attempts to prioritize for the first time clinical studies based on cause-outcome and strengths relationships in reference to aPL and CAD/PAD, in addition to supplementing Brey's comprehensive review on IS with other, additional studies. Our overview indicates that all case-control and cross-sectional studies found an aPL association with CAD, PAD and IS, while cohort and nested case-control studies reported a prevailing negative risk association between aPL and IS (confirming Brey), with an unclear/unresolved risk association between aPL and CAD. The only cohort, follow-up study found in PAD reported on positive risk association between aPL and disease. The most frequently associated aPL in all studies reported, irrespective of disease, was aCL, with a less frequent association reported for LA, aβ2GPI and other aPL.     

The significance of plasmid DNA preparations contaminated with bacterial genomic DNA on inflammatory responses following delivery of lipoplexes to the murine lung

Non-viral gene transfer using plasmid DNA (pDNA) is generally acknowledged as safe and non-immunogenic compared with the use of viral vectors. However, pre-clinical and clinical studies have shown that non-viral (lipoplex) gene transfer to the lung can provoke a mild, acute inflammatory response, which is thought to be, partly, due to unmethylated CG dinucleotides (CpGs) present in the pDNA sequence. Using a murine model of lung gene transfer, bronchoalveolar lavage fluid was collected following plasmid delivery and a range of inflammatory markers was analysed. The results showed that a Th1-related inflammatory cytokine response was present that was substantially reduced, though not abolished, by using CpG-free pDNA. The remaining minor level of inflammation was dependent on the quality of the pDNA preparation, specifically the quantity of contaminating bacterial genomic DNA, also a source of CpGs. Successful modification of a scalable plasmid manufacturing process, suitable for the production of clinical grade pDNA, produced highly pure plasmid preparations with reduced genomic DNA contamination. These studies help define the acceptable limit of genomic DNA contamination that will impact FDA/EMEA regulatory guidelines defining clinical grade purity of plasmid DNA for human use in gene therapy and vaccination studies.     

The safety and efficacy of gene therapy treatment for monogenic retinal and optic nerve diseases: A systematic review

PurposeThis study aimed to systematically review and summarize gene therapy treatment for monogenic retinal and optic nerve diseases.MethodsThis review was prospectively registered (CRD42021229812). A comprehensive literature search was performed in Ovid MEDLINE, Ovid Embase, Cochrane Central, and clinical trial registries (February 2021). Clinical studies describing DNA-based gene therapy treatments for monogenic posterior ocular diseases were eligible for inclusion. Risk of bias evaluation was performed. Data synthesis was undertaken applying Synthesis Without Meta-analysis guidelines.ResultsThis study identified 47 full-text publications, 50 conference abstracts, and 54 clinical trial registry entries describing DNA-based ocular gene therapy treatments for 16 different genetic variants. Study summaries and visual representations of safety and efficacy outcomes are presented for 20 unique full-text publications in RPE65-mediated retinal dystrophies, choroideremia, Leber hereditary optic neuropathy, rod-cone dystrophy, achromatopsia, and X-linked retinoschisis. The most common adverse events were related to lid/ocular surface/cornea abnormalities in subretinal gene therapy trials and anterior uveitis in intravitreal gene therapy trials.ConclusionThere is a high degree of variability in ocular monogenic gene therapy trials with respect to study design, statistical methodology, and reporting of safety and efficacy outcomes. This review improves the accessibility and transparency in interpreting gene therapy trials to date.     

The pathology of cystic fibrosis

Cystic fibrosis (CF) is one of the commonest lethal inherited conditions among Caucasians. It affects multiple organ systems and exhibits a range of clinical problems of varying severity. Life expectancy has improved in recent years as treatment regimes have become more intensive, but current treatments are expensive, often time consuming and may affect quality of life. New treatments for the pulmonary disease are under clinical trial and include antiproteases, amiloride, a sodium channel blocker, DNase and gene therapy. The gene for cystic fibrosis was identified in 1989 and this together with the emerging technology of gene therapy heralded a new dawn for the treatment of genetic disease. The lung is considered an ideal organ to target due to ease of access, but subsequent research has shown that the airway surface provides an efficient barrier to topically applied gene transfer agents. A number of Phase I clinical safety trials were carried out through the 1990s and provided proof of concept evidence that delivery of DNA by either viral or non-viral means was safe though not clinically efficacious. Current research is now focusing more on the barriers faced by delivery agents, with the aim that more efficient gene delivery will lead to a gene therapy for cystic fibrosis. The histopathologist is rarely called upon to make the initial diagnosis as cystic fibrosis is usually diagnosed clinically, being characterized by chronic bronchopulmonary infection, malabsorption due to pancreatic insufficiency and a high sweat-sodium concentration on sweat testing. Most information concerning both macroscopic and microscopic findings in cystic fibrosis has come from autopsy studies, so the pathological features are often extreme. However, with increasing survival of patients with cystic fibrosis, we are seeing more subtle changes in other organs and in addition, more aggressive drug therapy, gene therapy and lung transplantation are bringing with them new disease entities and complications.     

Therapeutic approach to familial hypercholesterolemia by HVJ-liposomes in LDL receptor knockout mouse

Familial hypercholesterolemia (FH) is an inherited disease in humans, which we have used as a model to develop a new strategy of gene therapy. This disease, which is due to mutation in the low density lipoprotein (LDL) receptor gene and results in deficiency of the LDL receptor, is associated with hypercholesterolemia and premature development of coronary heart disease. This disease has been identified as one of the target diseases for gene therapy, because a 50% reduction of cholesterol level would be beneficial in such patients. In this study, we examined the feasibility of gene therapy by the delivery of the human LDL receptor plasmid into the liver via the portal vein. For gene transfer we utilized HVJ-liposome method with which many successful gene transfers have been reported. Administration of the human LDL receptor plasmid by the HVJ-liposome method into the liver resulted in a decrease of total cholesterol level. Moreover, second administration of this gene two weeks after the first administration resulted in sustained lowering of total cholesterol level. Although single administration of plasmid by the HVJ-liposome method induced antibodies against HVJ, this antibody production did not affect gene expression following second administration. These results suggest the possibility of a novel repetitive gene therapy for FH, using human LDL receptor plasmid transfer directly into the liver by the HVJ-liposome method.     

Enhanced expressions and histological characteristics of intravenously administered plasmid DNA in rat lung

Cationic liposome-mediated gene transfection is a promising method for gene therapy. In this study, the transfection efficiency and histological patterns were evaluated in rat lung after intravenous administration via femoral vein of naked plasmid DNA, naked plasmid DNA with pretreatment of DOTAP, and DOTAP-cholesterol-plasmid DNA complex. Plasmid DNA encoding bacterial LacZ gene was used. For quantification of LacZ gene expression, beta-galactosidase assay was performed. For histologic examination, X-gal staining and immunohistochemical staining for transfected gene products were performed. Pretreatment of DOTAP prior to the infusion of naked plasmid DNA increased transfection efficiency up to a level comparable to DOTAP-cholesterol-plasmid DNA complex injection. Transfected genes were mainly expressed in type II pneumocytes and alveolar macrophages in all animals. We conclude that the high transfection efficiency is achievable by intravenous administration of naked plasmid DNA with pretreatment of DOTAP, to a level comparable to DOTAP-cholesterol-plasmid DNA complex. In this regard, naked plasmid DNA administration with pretreatment of DOTAP could be a more feasible option for intravenous gene transfer than DOTAP-cholesterol-plasmid DNA complex, in that the former is technically easier and more cost-effective than the latter with a comparable efficacy, in terms of intravenous gene delivery to the lung.     

Intracellular barriers to non-viral gene transfer

Non-viral vector mediated gene transfer, compared to viral vector mediated one, is a promising tool for the safe delivery of therapeutic DNA in genetic and acquired human diseases. Although the lack of specific immune response favor the clinical application of non-viral vectors, comprising of an expression cassette complexed to cationic liposome or cationic polymer, the limited efficacy and short duration of transgene expression impose major hurdles in the widespread application of non-viral gene therapy. The trafficking of transgene, complexed with chemical vectors, has been the subject of intensive investigations to improve our understanding of cellular and extracellular barriers impeding gene delivery. Here, we review those physical and metabolic impediments that account, at least in part, for the inefficient translocation of transgene into the nucleus of target cells. Following the internalization of the DNA-polycation complex by endocytosis, a large fraction is targeted to the lysosomal compartment by default. Since the cytosolic release of heterelogous DNA is a prerequisite for nuclear translocation, entrapment and degradation of plasmid DNA in endo-lysosomes constitute a major impediment to efficient gene transfer. Only a small fraction of internalized plasmid DNA penetrates the cytoplasm. Plasmid DNA encounters the diffusional and metabolic barriers of the cytoplasm, further decreasing the number of intact plasmid molecules reaching the nuclear pore complex (NPC), the gateway of nucleosol. Nuclear translocation of DNA requires either the disassembly of the nuclear envelope or active nuclear transport via the NPC. Comparison of viral and plasmid DNA cellular trafficking should reveal strategies that viruses have developed to overcome those cellular barriers that impede non-viral DNA delivery in gene therapy attempts.     

Artificial cells for replacement of metabolic organ functions

Artificial cells are being actively investigated for use in the replacement of cell and organ functions, especially related to metabolic functions. The earliest routine clinical use of artificial cells is in the form of coated activated charcoal for hemoperfusion. Implantation of encapsulated cells are being studied for the treatment of diabetes, liver failure, kidney failure and the use of encapsulated genetically engineered cells for gene therapy. Blood substitutes based on modified hemoglobin are already in Phase III clinical trials in patients with as much as 20 units infused into each patient during trauma surgery. Artificial cells containing enzymes are being developed for clinical trial in hereditary enzyme deficiency diseases and other diseases. Artificial cell is also being investigated for drug delivery and for other uses in biotechnology, chemical engineering and medicine.     

In vivo electroporation of the central nervous system: a non-viral approach for targeted gene delivery

Electroporation is a widely used technique for enhancing the efficiency of DNA delivery into cells. Application of electric pulses after local injection of DNA temporarily opens cell membranes and facilitates DNA uptake. Delivery of plasmid DNA by electroporation to alter gene expression in tissue has also been explored in vivo. This approach may constitute an alternative to viral gene transfer, or to transgenic or knock-out animals. Among the most frequently electroporated target tissues are skin, muscle, eye, and tumors. Moreover, different regions in the central nervous system (CNS), including the developing neural tube and the spinal cord, as well as prenatal and postnatal brain have been successfully electroporated. Here, we present a comprehensive review of the literature describing electroporation of the CNS with a focus on the adult brain. In addition, the mechanism of electroporation, different ways of delivering the electric pulses, and the risk of damaging the target tissue are highlighted. Electroporation has been successfully used in humans to enhance gene transfer in vaccination or cancer therapy with several clinical trials currently ongoing. Improving the knowledge about in vivo electroporation will pave the way for electroporation-enhanced gene therapy to treat brain carcinomas, as well as CNS disorders such as Alzheimer's disease, Parkinson's disease, and depression.