Current Gene Therapy - Volume 8, Issue 5, 2008

In the widest sense, gene therapy may be defined as the delivery of nucleic acids to patients for therapeutic purposes. Thus, it involves the deliberate introduction, usually by means of a vector, of nucleic acid sequences into cells of a patient to treat a disease. The DNA sequences introduced into these cells may correspond either to a functional gene encoding a protein of therapeutic interest or to sequences capable of interfering with the functioning of a cellular gene. Gene therapy is actively investigated for the treatment of both genetic and acquired diseases. However, although some positive results have already been reported in the clinical setting, the clinical trials performed to date have highlighted that a crucial requirement for successful gene therapy is the use of an efficient gene delivery system perfectly adapted to the pathological situation. Ideally, these DNA delivery systems should : (i) be safe, non-toxic, non-immunogenic and well tolerated, (ii) protect and compact DNA into small particles ; i.e. below 300 nm in diameter to be compatible with cell internalization, (iii) provide a stealthy behavior in order to improve in vivo bioavailability; i.e. prevent particles removal from the blood by the monophagocytic system allowing to target cells of interest, (iv) penetrate into the target cells and escape from the endosomes, (v) target nucleic acid sequences to the nucleus where the expression of the protein will take place. The future of gene therapy will depend on the ability to provide to DNA delivery systems all the properties listed above. Viruses have been tailored by evolution for transferring their genes from one cell to another. Accordingly, viruses can be viewed as sophisticated nucleic acid containing macromolecular assemblies which are primed for the introduction of nucleic acid sequences into their target cells. However, viral vectors suffer from some inconveniences including immunogenicity, safety issues and practical issues relating to large scale production and quality control. Thus, a major research effort has been dedicated to the development of alternative approaches to recombinant viruses. The main alternatives to viral vectors aims at developing complex “virus-like” systems consisting of direct complexation of naked DNA with chemical nanocarriers capable of bringing the required properties. Indeed, nonviral vectors offer a means to create sophisticated modular self-assembling gene delivery systems incorporating various functional elements to overcome the distinct extracellular and cellular barriers to gene transfection. Thus, this review will focus on current work coming from opposite but complementary sciences; (i) chemistry for the synthesis of nanocarriers, (ii) physicochemistry for the formulation of DNA with nanocarriers, (iii) cellular biology for the extracellular targeting and intracellular trafficking, (iv) molecular biology for the sustained expression of the transgene by its genome integration or using matrix attachment regions, and (v) physiology for the preclinical and clinical evaluation of the transfection efficiency and therapeutic effect. These convergent efforts should lead to the designed of versatile non viral vectors that will be organized at the nanometric scale, which will be enable them to cross efficiently, safely the cell barriers and to be targeted to the site or organ where they will be active, and provide a beneficial effect. Ultimately, this will allow to fill in the existing gap between the huge therapeutic potential of the nucleic acid sequences and their concrete translation to innovative treatments or vaccines.

Over the last several years, various gene delivery systems have been developed for gene therapy applications. Although viral vector-based gene therapy has led to the greatest achievements in animal and human studies, synthetic nonviral vectors have also been developed as they offer several advantages over viral systems, including lower immunogenicity and greater nucleic acid packaging capacity. Nevertheless, the transfection efficiency of the current non-viral gene carriers still needs to be improved, especially as regards direct in vivo transfection. In particular, cationic lipid/nucleic acid complexes (termed lipoplexes) have been the subject of intensive investigation with a view to optimize their performance and to better understand their mechanisms of action, and consequently to design new approaches to overcome the critical barriers of cationic liposome-mediated gene delivery. A possible strategy may rely on considering the membrane constituents and properties of the vast variety of living organisms as a source of inspiration for the design of biocompatible, nontoxic and effective novel artificial liposomal systems. Thus, the present forward-looking review provides an overview of the progress already made during the last years in the field of cationic lipid-mediated gene transfection and also focuses on a series of novel bio-inspired lipids for both in vitro and in vivo gene transfection.

Gene therapy is based on the vectorization of nucleic acids to target cells and their subsequent expression. Cationic lipids and polymers are the most widely used vectors for the delivery of DNA into cultured cells. Nowadays, numerous reagents made of these cationic molecules are commercially available and used by researchers from the academic and industrial field. By contrast their evaluations in preclinical programs have revealed that their use for in vivo applications will be more problematic than their massive use in vitro. This is mostly due to the physicochemical properties of cationic vectors/DNA complexes, which are the result of their mode of interaction. Indeed, these cationic vectors interact through electrostatic forces with negatively charged DNA. This results in the formation of highly organized positively charged supramolecular structures where DNA molecules are condensed. Association of DNA with cationic lipids under a micellar or liposomal form leads to lamellar organization with DNA molecules sandwiched between lipid bilayers. Although the lamellar phase is the common described structure, as evidenced by small-angle X-ray scattering and electron microscopy, some cationic lipid combined with a hexagonal forming lipid could also result with DNA in an inverted hexagonal structure. Despite a lot of effort, the precise mechanism of gene transfer with cationic vector is still ill-defined. Here, our objective was to overview the main relationships between the physico chemical properties of cationic lipid/DNA complexes and their transfection efficiency. An overview of a new class of vectors consisting of amphiphilic block copolymers designed for in vivo delivery is also presented and discussed.

Success of nucleic acid based therapies often depends on target-cell specific delivery of genetic materials such as plasmid DNA, antisense oligonucleotides or small interfering RNA. Such extracellular targeting strategies include the incorporation of hydrophilic shielding domains into nucleic acid carriers which protects them from unspecific interactions with non-target tissues (passive targeting), and the inclusion of targeting moieties which allows recognition of targetspecific cellular receptors (active targeting). Furthermore physical targeting methods such as magnetofection, electroporation or by photochemical means have been used to enhance efficiency of nucleic acid transfer. For optimum efficacy, extracellular targeting concepts are combined with programmed bioresponsive carrier chemistry which confers to the formulation stability during extracellular delivery but controlled disassembly and nucleic acid release after reaching the target cell.

Lipoplexes and polyplexes, electrostatic complexes between a plasmid DNA and cationic lipids or polymers are chemical systems that are developed for gene delivery. Considerable efforts have been done to delineate the exact knowledge of their entry mechanisms and the intracellular routing of the plasmid DNA that are of major importance for the designing of these gene delivery systems. While the uptake of lipoplexes made with several types of cationic lipids proceeds mainly by the clathrin-dependent pathway, it appears that for polyplexes the uptake pathway is more dependent on the polymer and the cell types. So, after an overview of the current knowledge of different endocytic pathways, we present here a selection of current reports related to the entry mechanisms and intracellular routing of plasmid DNA complexed with select cationic polymers. The review includes the role of glycosaminoglycans, cell polarization and cell cycle in the polyplex uptake and their transfection efficiency. We also report current data showing that the insertion of specific κB motifs in the nucleic acid sequence provides an increase of the plasmid import into the nucleus. This has been demonstrated by fluorescence methods suitable to investigate the intracellular trafficking of pDNA. Overall, it appears that polyplex uptake proceeds both by the clathrin-dependent pathway and a clathrin-independent (cholesterol- dependent) pathway. These two entry mechanisms are not exclusive and can occur simultaneously in the same cell. Both of them lead to cell transfection but polyplexes still need improvements for clinical use.

Matrix attachment regions (MARs) are DNA sequences that may be involved in anchoring DNA/chromatin to the nuclear matrix and they have been described in both mammalian and plant species. MARs possess a number of features that facilitate the opening and maintenance of euchromatin. When incorporated into viral or non-viral vectors MARs can increase transgene expression and limit position-effects. They have been used extensively to improve transgene expression and recombinant protein production and promising studies on the potential use of MAR elements for mammalian gene therapy have appeared. These illustrate how MARs may be used to mediate sustained or higher levels of expression of therapeutic genes and/or to reduce the viral vector multiplicity of infection required to achieve consistent expression. More recently, the discovery of potent MAR elements and the development of improved vectors for transgene delivery, notably non-viral episomal vectors, has strengthened interest in their use to mediate expression of therapeutic transgenes. This article will describe the progress made in this field, and it will discuss future directions and issues to be addressed.

Gene delivery technologies have been developed for various biotechnology applications. In gene therapy, they are promising for the treatment of several inherited and acquired human diseases. When therapies require the transfection of a transgene, the vector integration is one of the solutions that is used for maintaining and sustaining expression. On the basis of their origin, vectorisation technologies are currently divided in two fields, gathering on one hand viral vectors and, on the other hand, non-viral approaches. In the case of the non-viral therapies, three main sub-fields are in progress to integrate transgenes. The first uses oligonucleotides to stimulate targeted gene repair by homologous recombination. The second is based on site-specific endonucleases for which the cleavage activity is used to stimulate the host recombination mechanisms in the presence of a DNA vector. The third one is developed from phage and transposon enzymatic systems. The two lasts sub-fields use non-viral enzymes and are the scope of this review. Here, our objective was to overview the main non-viral enzymatic systems able to integrate DNA cassettes. Their molecular and functional characteristics are summarized, and their properties and limits in the current state of the art highlighted. An overview of the safety and quality issues is also presented and discussed, taking into account the solutions that might circumvent problems, intellectual property and economic status for each system. As a conclusion, we propose projections of the future technological developments in the context of the different interests for public and private bodies.

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 fulllength 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.