Gene Delivery in Gene Therapy Viral-Based and Non-Viral Systems - Term Paper Example

Gene delivery in gene therapy – viral-based and non-viral systems Author Course Tutor Date Abstract Gene therapy has grown tremendously from clinical trials to commercial products. There has also been an increase in the understanding of the gene delivery systems. This has led to the development of both viral and non-viral vectors, and new techniques that promote gene therapy. The review intends to highlight issues related to gene therapy and gene delivery systems. It shall also discuss the relevance of both viral and non-viral vectors in the administration of gene therapy in human. The paper will examine the properties of the vectors respectively, and the associated advantages and disadvantages of using both viral and non-viral vectors in gene therapy. Viral vectors like adenovirus, herpes simplex virus, poxvirus, and adeno-associate virus will be highlighted with much focus on their individual properties. The paper will then develop a summary of issues discussed in the review. Table of Contents 1.0 Introduction.................................................................................................................4 1.1 Gene therapy...............................................................................................................4 1.2 Understanding gene delivery systems..........................................................................4 1.3 Gene delivery systems and gene therapy...............………….......................................4 2.0 Viral vector gene delivery systems…..........................................................................5 2.1 Properties of viral vectors for gene therapy.................................................................4 2.2 Advantages of viral vector delivery systems…............................................................7 2.3 Disadvantages of viral vector delivery systems...........................................................8 2.4 Other uses of viral vectors.....…………………………………………………………...9 2.5 Scientific thinking on gene delivery methods....………………...................................10 3.0 Non-viral vector delivery systems.......…....................................................................10 3.1 Properties of non-viral vector delivery systems..........................................................10 3.2 Advantages of non-viral vector delivery systems........................................................11 3.3 Disadvantages of non-viral vector delivery systems....................................................11 3.4 Effectiveness of non-viral vector delivery system in gene therapy….…….....……….11 3.5 The future of viral and non-viral vector delivery systems...........................................12 4.0 Conclusion…………………........................................................................................12 References.....................................................................................................…….....14 1.0 Introduction 1.1 Gene therapy Gene therapy has become a significant topic of discussion in science-related news. Gene therapy is used to introduce a gene into the body which has the capacity to prevent the spread of disease. It introduces a normal gene into a cell in the body in which that particular gene is not functional (Niidome and Huang, 2002). When cells, tissues or individuals go through the process of gene therapy, they are considered to have been genetically modified. Gene therapy is performed in order to prevent cardiovascular diseases, eradicate cancerous cells, eliminate infectious pathogens, and to prevent neurological disorders (Cevher, Sezer and Çağlar, 2012). Gene therapy is classified into two types; reproductive gene therapy and somatic cell gene therapy. 1.2 Understanding gene delivery systems Different methods are used in the gene delivery system to enable the uptake of the selected genes to target the cell (Conwell et al, 2005). In order to design an effective gene delivery system, it is necessary to understand the relationship between the delivery system and the target cell. There is also the need to understand the targeting mechanism and the intercellular traffic. Cell targeting is the transfer of the therapeutic agent to a particular organelle of the cell. It is used a mechanism used in endocytosis gene therapy, especially in the non-viral delivery systems (Prokop and Davidson, 2007). A gene delivery procedure that is successful minimizes possible inhibitory inflammatory response, and at the same time overcomes barriers at all steps in the procedure (Conwell et al, 2005). 1.3 Gene delivery systems on genetic therapy The background of most of the genetic disorders was known, and the gene therapy introduced by the end of 1970s. However, the first trial on human gene therapy was performed in the early 1970s. It was found that naturally existing RNA and DNA tumor viruses provided genetic information to the mammal cells' genomes (Escors and Brecpot, 2010). Conwell et al, 2005 argue that the introduction of the science of genetics and gene delivery systems in the twentieth century necessitated the understanding of genetic-based diseases like hemophilia, diabetes, retinoblastoma, and colon cancer. 2.0 Viral Vector Gene Delivery System Methods used to deliver DNA in gene therapy ought to be appropriate, since the targeted tissues must receive the right genes. Gene therapy can be done by delivering naked DNA molecules directly into the cells. The procedure of delivering these molecules into the target cells was discovered to have low efficiency rate (Witlox et al, 2007). In order to facilitate the uptake of the DNA, special vectors were introduced for gene transfer. Vectors are plasmids used to facilitate the movement of DNA molecules from cell to cell. A retro-virus is a class of RNA that has the ability to insert the nucleic acid it produces into host cells (Kresina, 2001). Other vectors used in gene therapy are retrotransposons, Adenovirus, and liposomes. The vectors are able to integrate and transfer genes from one cell to another. Retroviruses used in gene therapy are designed to remove the harmful genes in the body. The genes are replaced with the corrective genes, and then modified retrovirus is put into the patient (Cevher, Sezer and Çağlar, 2012). 2.1 Properties of viral vectors for gene therapy When performing gene therapy trials, the gene delivery systems were based on retrovirus, adenovirus (Ad), herpes simplex virus (HSV), poxvirus, and adeno-associated virus (AAV). These vectors were used in most of the clinical trials to date. In fact, these viruses led to the development of the gene delivery systems since they are easy to manipulate in vitro, and were studied in detail (Stone, 2010). The viral vectors have various characteristics that make them suitable to be used in different therapeutic applications (Warnock et al, 2011). The adenovirus vectors have been used in various pre-clinic applications. Such applications include cancer gene therapies, and vaccination that does not require long -term transfer of particular type of cell or organs. The Ad has no integration machinery and appropriate host immune responses, making it suitable for only short-time therapies (Stone, 2010). Its high level of toxicity and human exposure make it unsuitable for some therapies. HSV is among the largest human viruses and most complex as far as pathogenesis and replication cycle are concerned. However, it is a vital vector that is applied in neurological therapies because of its natural neurotropism. The vector is reliably used in the treatment of neurodegenerative disorders. Like the Ad, HSV vector has inherent toxicity that limits its use in other applications (Cevher, Sezer and Çağlar, 2012). Retrovirus vectors were used in marking genes and performing clinical trials in human beings. They are based on the murine leukemia oncoretrovirus abbreviated as MLV The vectors have the capacity to infect dividing cells and integrate the genome to allow long-term gene expression. This makes the vectors suitable for use in different therapies, not excluding hematopoietic gene therapy. However, during clinical trials, the MLV sequences integrate to form tumorgenesis, which are related to oncogenes. This has been observed to be a limiting factor of the vectors (Stone, 2010). Poxviruses or vaccinia virus are commonly used as gene therapy vectors. They act as agents for vaccination. The vector has been found to safe for use in humans, especially in the vaccination against small pox. Due to it safety, the poxviruses vector is used in other genetic therapies. Poxviruses are currently being used as oncolytic vectors because of their ability to kill tumor cells in a selective manner (Warnock et al, 2011). AAV is known to be the most safe viral vector system. This is because it based on a human virus which is non-pathogenic, and it only replicates when a helper virus co-infection is available. The viral vector is preferred in many clinical trials, especially in those which take long time to enhance a genetic defect. Though the vector is safe to use and able to enhance long-term gene expression, it can only accommodate less therapeutic genes due to its small genome (Stone, 2010). The vector is also unable to infect particular cell types like hematopoietic stem cells that are targeted by genetic therapies. 2.2 Advantages of viral vector delivery system Different vectors have been designed for use in gene therapy applications. The viral vectors have own advantages which are related to their methods of delivery, and how they express their genomes (Kamimura et al, 2011). The Ad vectors are not oncogenic and are usually manufactured at high titers. They are also preferred because their ability to stabilize the recombinant vectors. These vectors are commonly used in clinical gene therapy applications because they are able to infect the quiescent and dividing cells. On the other hand, AAV vectors are preferred in gene therapy applications because they have a wide range of cell type tropism. They are nonpathogenic and have the ability to transduce the dividing and other cells (Warnock et al, 2011). Another advantage for the AAV is the ability to withstand extreme levels of gene expression for a considerably long time in vivo. The advantage with retrovirus vector is the potential to reverse their RNA genome transcription. This allows the creation of dsDNA used to replicate after the host cells have been infected. HSV vectors remain latent after the host cells have been infected. This allows the vector to infect the neural cells, and thus useful in the treatment of neural diseases. The poxviruses are preferred in vaccination because they are incapable of producing infective viruses in the tissues of human body. They are also replication-defective in nature. (Warnock et al, 2011) 2.3 Disadvantages of viral vector delivery system The disadvantages of viral vector can also be identified by looking at the performance of individual vectors in the gene therapy applications. The viral vectors have own disadvantages. According to Thomas, Ehrhardt and Kay (2003), the Ad vector does not support a long-term correlation. When used in high doses, the vector causes cellular immune and humoral response. The AVV vectors have also their disadvantages like slow gene expression. They are small in size and this reduces the amount of inserted foreign genes. Other vectors like retroviruses are not able to infect the non-dividing cells. However, according to Witlox et al (2007), the use of vital systems has some limitations which include immunogenicity, toxicity, use of the viruses in production, and failure to optimize when producing on a large scale. This is illustrated in the figure 1 below. Fig 1: Gene therapy systems (suicide gene therapy, mutation compensation, and immune-protection (Witlox et al, 2007). Source:http://cdn.intechopen.com/pdfs/40265/InTechGene_delivery_systems_recent_progress_in_viral_and_non_viral_therapy.pdf 2.4 Other uses of viral vectors Apart from gene therapy vector viruses are used in cellular and molecular biological studies. They are used to manipulate and investigate how cells functions. Viral vectors are also used in bacteriophage therapy. In this application, they are used to infect and eliminate pathogenic bacteria. Viral vectors can be modified by use of genetic engineering methods so as the genomes are carried into animals and plants. Viruses are also used to control pests in agriculture, and the agents significant to human health (Mandal, 2013). 2.5 Scientific thinking on gene delivery methods The major goal of different scientific applications in the gene delivery methods is to have a safer and efficient foreign DNA transfer into cells. This has led to the advancement in the stability and transport of the DNA. Due to the weaknesses of the viral gene delivery system, scientists saw the need to develop stronger carriers of the viral vehicles (Khosravi-Darani et al, 2010). Scientists logically think that in order to overcome the barriers to gene delivery, drastic changes have to be made. Calcium phosphate nanoparticles were found to be effective in transferring gene. 3.0 Viral Vector delivery system The non-viral vectors were developed after the weaknesses encountered in using the viral vectors. Non-viral vectors like polymers, cationic lipids, peptides, and dendrimers are used to deliver DNA into the cells. Non-viral vectors exhibit reduced efficiency in transfection since they are prevented by intracellular obstacles (Mintzer and Simanek, 2009). There are three classes of non-viral vector delivery systems namely; physical methods, inorganic particles, and natural biodegradable particles. 3.1 Properties of non-viral vector delivery system According to Gascón, Pozo-Rodríguez and Solinís (2013), inorganic nanoparticles are different in shape, size, and porosity, which are engineered to protect molecular payload from denaturation. They are also characterized by good storage stability, and are less prone to microbial attack. Gold nanorods have strong absorption bands which allow them to absorb light. Most of the synthetic biodegradable particles are safe and non-toxic when experimented both on human beings and animals. They are also biocompatible to the human body. Chitosan, in particular is able to open its intercellular tight junctions, which enhances its movement into the cells. 3.2 Advantages of non-viral vector delivery systems Non-viral vectors are safe to use, and less costly. They are more producible and they can accommodate different sizes of DNA. Availability of non-viral vectors has led to the increase in the number of products involved in clinical trials. This is because they are safe to use (Gascón, Pozo-Rodríguez and Solinís, 2013). Cationic lipid, lipid-polymer hybrid, and cationic polymer systems are highly effective when used in vitro, and are easy to prepare. Needle injection is easy to use while delivery systems like sonoporation, laser assisted, magnetofection, and hydrodynamic delivery have high efficiency and are site specific. 3.3 Disadvantages of non-viral vector delivery systems The main disadvantage of non-viral vector delivery is low transfection efficiency (Gascón, Pozo-Rodríguez and Solinís, 2013). Although cationic lipid, cationic polymer, and lipid-polymer hybrid system are effective in vitro, they perform poorly in vivo, and may stimulate severe immune response. Gene gun and electroporation delivery systems limit gene transfer to the targeted areas, and require surgical procedures to deliver the genes into internal organs. Some non-viral delivery systems like sonoporation and magnetofection damage the tissue of cells while hydrodynamic delivery system requires catheter insertion when used in large animals (Kamimura et al, 2011). 3.4 Effectiveness of non-viral vector delivery system in gene therapy The most significant and challenging factor in gene therapy is the delivery. The non-viral delivery systems have been found to be effective and efficient in gene therapy applications. These vectors have successfully overcome various extracellular barriers like guarding the acid from being degraded, and targeting specific tissues in the cells. The non-viral vectors delivery system have driven gene therapy to higher levels because of being effective, safe, long lasting, and specific (Gascón, Pozo-Rodríguez and Solinís, 2013). 3.5 The future of viral and non-viral vector delivery systems A lot of research and advances have been made in the area of gene transfer. Despite the advancement, absolute efficiency has not been attained. In this respect, viral and non-viral vector delivery systems have to be improved in the future. Calcium phosphate, electroporation, and lipofection are commonly used in the analysis of gene expression because of their stability. Before new developments emerge, it is likely the viral and non-viral systems shall continue to be functional (Mitrović, 2003). There is high expectation in the delivery of gene therapy services, and this can only be achieved by use of viral and non-viral gene delivery systems. The success of both viral and non-viral gene delivery in the future depends on having systems that can reduce toxicity in place. Mitrović (2003) further states that strategies which link the advantages of viral and non-viral vectors have been reported, and this has provided opportunity for growth in gene therapy. 4.0 Conclusion In conclusion, gene therapy is used to eliminate diseases in the body of human and animals. However, it could be used in other applications like in agriculture and bacteriophage therapy. The viral vectors include the adenovirus, HSV, retrovirus, poxvirus, and AAV. The vectors have unique properties that make them suitable in gene therapy. The paper also indicates that viral-vector gene therapy is efficient compared to non-viral gene therapy. The non-viral vectors include polymers, cationic lipids, peptides, and dendrimers. The development of the non-viral vectors facilitated the process of gene therapy. In order to attain maximum efficiency in the gene delivery systems, more improvements have to be made in the future. References Cevher, E Sezer, A. D & Çağlar, E. Ş 2012 "Gene Delivery Systems: Recent Progress in Viral and Non-Viral Therapy". Conwell CC, Huang L, In K. Taira, K. Kataoka, T. Niidome (ed) 2005 " Recent Progress in Non-viral Gene Delivery", Non-viral Gene TherapyGene Design and Delivery. Springer-Verlag Tokyo. Japan; pp: 3-11. Escors D, & Brecpot K 2010 "Lentiviral vectors in gene therapy: their current status and future potential. Archivum Immunologiae et Therapia Experimentalis; 58; pp:107-119. Gascón, A. R del Pozo-Rodríguez, A & Solinís, M. Á 2013 "Non-Viral Delivery Systems in Gene Therapy". Kamimura, K., Suda, T Zhang, G & Liu, D 2011 "Advances in gene delivery systems", Pharmaceutical medicine, 25(5), pp:293-306. Khosravi-Darani, K. Mozafari, M. R Rashidi, L & Mohammadi, M 2010 "Calcium based non-viral gene delivery: an overview of methodology and applications,. 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