ReviewGene therapy and DNA delivery systems
Graphical abstract
Introduction
Gene therapy is a promising therapeutic strategy (Friedmann, 1996) based on using genes as a medicine (Mohsen, 2011). It can be employed effectively to cure a wide range of serious acquired and inherited diseases (Gardlík et al., 2005), such as cancer (Rochlitz, 2001), acquired immunodeficiency syndrome (AIDS) (Yu et al., 1994), cardiovascular diseases (Dishart et al., 2003), infectious diseases (Bunnell and Morgan, 1998), cystic fibrosis (Davies et al., 2001), and X-linked severe combined immune deficiency (X-linked SCID) (Kohn et al., 2003). Theoretically, gene therapy is a simple therapeutic method depending on either replacing a distorted gene by healthy one, or completing a missing gene in order to express the required protein (Zhang et al., 2004b). However, in practice this is a complex operation (Tong et al., 2007), due to several obstacles that must be overcome by the transgene to reach the targeted human cell-nucleus, where it should be expressed correctly. Hence, for ensuring the arrival of a transgene into a cell nucleus without degradation, it is necessary to use gene delivery system that can protect the transgene from degradation and pass through the plasma membrane to the nucleus (Luo and Saltzman, 2000, Gao et al., 2007). At present, a perfect delivering system (carrier) capable of ensuring the success of gene therapy must satisfy the following criteria: (i) it must not interact with vascular endothelial cells and blood components (Schatzlein, 2001); (ii) it must be capable of avoiding uptake by the reticuloendothelial system (Mohsen, 2011); (iii) it must be small enough to pass through the cell-membrane and reach the nucleus (Labhasetwar, 2005). In fact, viruses were the first carriers to be used to deliver and protect the therapeutic gene, benefiting from the virus-life cycle. This type of carrier, known as viral vector, is one of the vectors used most in gene therapy, due to its ability to carry the gene efficiently and ensure long-term expression (Boulaiz et al., 2005). However, the risk of provoking immune response by using viruses as delivering vectors (Lv et al., 2006, El-Aneed, 2004), the high cost and difficulty relating to their preparation (Boulaiz et al., 2005), and the limited size of the genetic materials that can be inserted into human cells (Lv et al., 2006, El-Aneed, 2004), have restricted the use of these vectors in gene therapy, and led to research into safer and cheaper alternatives. Therefore non-viral vectors have appeared. Non-viral approaches for delivering transgenes can be divided into two groups:
- 1-
Physical approaches: these depend on a physical force that weakens the cell membrane to facilitate the penetration of the gene into the nucleus. They include needle injection, electroporation, gene gun, ultrasound, and hydrodynamic delivery.
- 2-
Chemical vectors: these can be prepared by electrostatic interaction between poly cationic derivatives that can be lipids or polymers and the anionic phosphate of DNA to form a particle called polyplexe when the interaction occurs between the polymer and the DNA, and lipoplexe when the DNA interacts with a lipid, or by encapsulation of DNA within biodegradable spherical structures that lead to micro and nanoparticles containing DNA, or by adsorption of DNA.
Section snippets
Protein therapy and gene therapy
Proteins have been used for treating various kinds of diseases for a long time (Goddard, 1991, Talmadge, 1993) in what is known as protein therapy, but using proteins for treating diseases is confronted by many obstacles such as low bioavailability in the body, short life in the blood stream due to high rates of hepatic and renal clearance and in vivo instability as it can degrade in the biological medium. The latter two constraints make it necessary to repeat recombinant protein-injection
The harbingers of gene therapy
Since 1944, the year in which Avery, Mcleod and Mc Carthy, proved that DNA encodes human genetic information (Avery et al., 1944, Dahm, 2010), much valuable genetic information was published until, finally, Watson and Crick published their article on the double helix structure of DNA in 1953 (Watson and Crick, 1953). This led to a genuine genetic revolution that led to understanding the mechanisms of many diseases, and to the development of new treatment methods, such as gene therapy. The major
Classification of gene therapy
The main goal of gene therapy is to insert a functional gene that plays the role of drug into the cell targeted in order to cure a disease or to repair a dysfunction caused by a genetic defect. Gene therapy can be classified into two major categories according to the nature of targeted cell.
Clinical gene therapy trials
The expression gene therapy owes its origin to the term “genetic engineering” which was employed for the first time at the Sixth International Congress of Genetics held at Ithaca in 1932 (Wolff and Lederberg, 1994). Though the idea of gene therapy existed already, concrete development in this field began in late 1960s and early 1970s (Friedmann, 1992) (Roemer and Friedmann, 1992) and gene therapy in humans was practiced in the late 1980s (Anderson, 1992) as a result of developments in the field
Gene therapy: principle and vectors
Gene therapy is a technique employed recently to treat serious diseases (acquired or inherited) by correcting their genetic causes (Müller-Reible, 1994), either by replacing the deformed genes by healthy ones or by completing missing genes (Sandhu et al., 1997). Different types of genetic material are used in gene therapy; such as double-stranded DNA (dsDNA), single-stranded DNA (ssDNA), plasmid DNA (Ferreira et al., 2000) and anti-sense oligonucleotides (ASON) (Knipe et al., 2013). The success
Conclusion
The main purpose of all of these pharmaceutical developments is to increase the desired medical impacts of a drug, and to decrease the side effects related to its use. This is the main objective now in gene therapy, in which when DNA is the drug to be administered. Gene therapy has become a promising strategy for treating many incurable diseases, whether acquired or heritable. However, the main challenge facing gene therapy, which prevents its widespread use in vivo is to find efficient
Acknowledgments
The authors thank and appreciate the research grant from Syrian government. The authors also thank MILADI Karim for his technical help and discussions.
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