Present the Current Status of Gene Therapy for Retinal Diseases - Research Paper Example

?Current Status of Gene Therapy for Retinal Diseases Using genes thus to prevent or treat a disease is known as gene therapy. This is an experimental technique and is done by inserting genetic material into the patient’s cells. The genes thus introduced exert their benefits by either compensating for the abnormal genes or by producing beneficial protein or by supplementing the abnormal gene. Thus, gene therapy has emerged as one of the novel treatment strategies for several untreatable diseases. One significant area in which enormous research has been done with regard to gene therapy is retinal disease. This articles reviews the current status of gene therapy for retinal diseases. Diseases of the eye which have the potential of being treatable with gene therapy in future are retinitis pigmentosa, proliferative retinopathy, autoimmune uveitis, gyrate atrophy, age-related macular degeneration, retinoblastoma and Leber's congential amaurosis. Thus gene therapy has the potential to treat various inheritable retinal diseases in future. As of now, no product has been approved for clinical use and most of them are in clinical trial stage. Introduction Whenever a particular gene is defective or absent, there is either absence of a particular protein, or there is production of abnormal protein or the amount of protein produced is insufficient. This causes diseased states. Many of these diseases can be either prevented or treated by replacing the abnormal gene, or by introducing the absent gene or by supplementing the defective gene. Using genes thus to prevent or treat a disease is known as gene therapy (HGPI, 2009). This is an experimental technique and is done by inserting genetic material into the patient’s cells. The genes thus introduced exert their benefits by either compensating for the abnormal genes or by producing beneficial protein or by supplementing the abnormal gene (Hecht, 2004). The first gene therapy clinical trial started in 1990 (HGPI, 2009). In this therapy, diseases are either prevented or treated through transfer of genes which manipulate human genetic material. Gene therapy is useful in replacing absent or defective genes, delivering genetic material into cells that speed the destruction of cancer cells, delivering genetic material into cells that cause cancer cells to revert back to normal cells, delivering genes of bacteria or virus as a form of vaccination, elivering genetic material that either impedes or promotes growth of a new tissue and delivering genetic material that stimulates healing of a damaged tissue (Hecht, 2004). Currently, research is going on for gene therapy by various approaches in different parts of the world for various diseases like combined immuno-deficiencies, hemophilia, Parkinson's disease, cancer and even HIV. Thus, gene therapy has emerged as one of the novel treatment strategies for several untreatable diseases (Singh and Tripathi, 2002). One significant area in which enormous research has been done with regard to gene therapy is retinal diseases. In this article, the current status of gene therapy for retinal diseases will be discussed through review of suitable literature. Why is retina suitable for gene therapy? Several genes which are responsible for various inherited retinal disorders have been identified (Dejneka and Bennett, 2001). Eye, especially retina, is one of the best suitable targets for gene therapy because of easy accessibility and allowance for local application of various therapeutic agents, along with decreased risk for systemic effects. Another major advantage is that the effects of therapy can be monitored through various non-invasive tests. Of the parts in the ocular system, retina is the most suitable candidate for gene therapy. This is because; the amount of virus that needs to be injected into the retina is just 1/1000 of that needed for other systemic diseases (Singh and Tripathi, 2002). The blood ocular barrier that is present in the eye separates eye from other parts of the body and protects retina. This barrier prevents escape of the large molecules into the blood stream. This is the main reason why any virus injected into the retina is unlikely to result in any systemic disease, unlike gene therapy of other regions. It is for this reason, that gene therapy is not useful in treating various retinal diseases, but also is studying the pathophysiology of various retinal diseases (Singh and Tripathi, 2002). Several gene transfer experiments have demonstrated the possibility and effectiveness of transferring transgenes in vivo into retina in an efficient and stable manner with minimal toxic effects. As of now, lentivirus, adenovirus and adeno-associated virus have been employed for successful gene therapy wherein genetic material is transferred into the retinal pigment epithelium and also photoreceptor cells (Dejneka and Bennett, 2001). Gene therapy is suitable for autosomal recessive disorders wherein there is absence of normal protein production. However, this is unsuitable for autosomal dominant diseases where one copy produces mutant protein and the other produces normal protein, resulting in dominant negative effect. Thus in autosomal dominant types, silencing of RNA is useful and this can be achieved by ribozyme therapy or interfering RNA therapy (Uthra and Kumaramanickavel, 2009). According to Chung et al (2009), "ongoing human clinical trials support the safety and efficacy of adeno-associated virus-mediated gene therapy for retinal disease. These and other studies will establish the foundation for methodology to treat additional ocular diseases using gene therapy strategies." Barriers for retinal gene therapy include risk of insertional mutagenesis and risk of innate inflammatory responses (Kumar-Singh, 2008). Retinitis pigmentosa Retinitis pigmentosa or RP is a group of retinal degenerative disorders that are hereditary and mainly characterized by progressive loss of vision because of photoreceptor degeneration. These disorders have been studied immensely for gene therapy and are considered as suitable candidates for the same. In the animal models, researchers have been able to investigate various molecular mechanisms involved in the phototransduction cascade, providing valuable information pertaining to pathogenesis of RP. Some researchers have also been able to rescue loss of vision in animal models by harnessing genes that are therapeutic to the viruses. Thus, it has been proposed that gene therapy may be useful in delaying cell death of photoreceptors (Singh and Tripathi, 2002). Currently, researchers are evaluating different approaches for slowing progression . Recently ribozyme therapy has been shown to be effective in slowing the progression of RP in animal models. In this therapy, specialized ribozymes act as molecular scissors for targeting mutant gene products or naturally occurring protein growth factors known as neutropins, which cause inhibition of apoptotic cell death. The ribozyme actually targets the mutant gene and then causes cleavage of its mutant messenger RNA causing slowing of the production of destructive protein. In some studies using rat models with dominant forms of RP, it has been found that ribozyme therapy causes reduction of vision loss in the rats dramatically. It was found that photoreceptor cell function was atleast 93 percent greater in the treated group when compared to the untreated group (Singh and Tripathi, 2002). Another interesting finding in research is that when the AAV inserted ribozyme was injected into the subretinal space, the vector bound ribozyme bound to the mutated mRNA of the photoreceptors that were defective (Singh and Tripathi, 2002). The ribozyme then cut the mRNA and decreased the quantity of production of injury causing protein before it caused damage to the photoreceptor. Investigators have even created a ribozyme that is able to recognize the mRNA for RP that is autosomal dominant, thus being more specific. The ribozyme encodes P23H rhodopsin that is mutant and destroys it much before it makes photoreceptor-degenerating P23H protein. The photoreceptors in these cases survive with only a single normal copy of the rhodopsin gene present in their other allelle (Singh and Tripathi, 2002). Some researchers have also been able to treat RP in animal models by injecting them with neurotrophin carrying vectors. These genes, when introduced into the retina caused increased secretion of neutrophins that slowed the progression of the disease by atleast 30 percent. Cai et al (2010) conducted a study to evaluate and ascertain the therapeutic efficacy and also safety aspects of compacted-DNA nanoparticle-mediated gene delivery into the subretinal space of an animal model with retinitis pigmentosa. The animal models were mice carrying a haploinsufficiency mutation in the retinal degeneration slow (rds+/?) gene. those in the study group were injected with Nanoparticles containing the mouse opsin promoter and wild-type mouse Rds gene and those in the control group were injected with one of these: plain saline or uncompacted naked plasmid DNA carrying the Rds gene. Another set of control group did not receive any treatment. From this study, it was evident that gene expression can be safely driven through gene therapy in both postmitoic and mitotic photo receptors, causing retardation of degeneration. Lau et al evaluated adeno-associated virus-mediated gene transfer of FGF-2 as a therapy for degeneration of photoreceptor in rat models of retinitis pigmentosa. From the results of the study, the researchers concluded that transduction of retinal cells with this form of gene therapy reduced rate of degeneration of retina in animal models, pointing to the potential use of this therapy in patients with RP. Proliferative vitroretinopathy Proliferative vitroretinopathy or PVR is a condition of the retina in which retinal detachment occurs due to traction secondary to formation and contraction of pre-and subretinal membrane formation. PVR is the most common cause of retinal reattachment surgery failure. The most important component of PVR is proliferation of cells. Some researchers have evaluated the efficacy of mutant gene, ribonucleotide-reductase-deficient HSV-I mutant hrR3 in destroying the proliferation of RPE. This acts by prevention of epiretinal membrane formation and also PVR. At the same time, it spares non-dividing cells like neurons. Thus, this mutant strain is useful for selective destruction proliferating RPE, while sparing non-replicating normal cells (Singh and Tripathi, 2002). Experimental autoimmune uveitis Experimental autoimmune uveitis or EAU is a T-cell mediated autoimmune disease that mainly targets the neural retina. It also serves as a model for posterior uveitis among humans. This condition can be induced easily in animal models. It shares many similarities with clinical uveitis. However, it is more self-limited and demands an adjuvant for induction of disease when compared to clinical uveitis. One of the therapeutic strategies for EAU appears to be gene therapy. Infact, investigators have constructed immunoglobulins using major uveitogenic epitope containg retroviral vector and this was used to transduce peripheral B cells which were then infused into syngeneic recipients. Infusion of these transduced cells 10 days before challenge of the uveis protected the animal modes from development of clinical disease. Thus, this form of gene therapy can be a useful protocol for treatment of autoimmune problems (Singh and Tripathi, 2002). Gyrate atrophy Gyrate atrophy is a genetic disorder of the retina in which patients lack the specific gene ornithine amino transferase (OAT). This leads to blinding retinal degeneration. Researchers have developed an adenovirus construct of the missing OAT gene that can be inserted into animal models with gyrate atrophy. Infact, through gene therapy, they have been able to study and demonstrate the molecular genetic defects associated with gyrate atrophy. It is interesting to note that OAT associated chorioretinal degeneration is because of hyperornithemia. Hence one of the suggested potential treatment is enhanced clearing of ornithine. Skin has been sought as the site for such therapies (Singh and Tripathi, 2002). Leber's Congenital Amaurosis Leber's Congenital Amaurosis or LCA is a group of retinal dystrophies that are hereditary characterised by severe loss of visual and retinal functions at a very early stage in life leading to progressive degeneration of the retinal cellular structure. the condition is usually inherited as an autosomal recessive trait and more than 15 different mutations have been noted. One of the potential therapeutic strategy that has been under investigation is RPE65 gene replacement. This gene therapy has been specifically useful for LCA2. The vector that has been used is replication-defective adeno-associated viral vector. Simonelli et al (2009) evaluated the long term efficacy and also safety of subretinal administration of AAV2-hRPE65v2 in animal models with LCA2. The study noted transient rise in neutralizing antibodies to AAV capsid, but no humoral response to RPE65 protein was reported. Functional amelioration also was noticeable and the physiologic effect was persistent suggestive of the possibility of role of gene therapy in improvement in retinal and visual function in LCA patients. In 2007, Dr. Robin Ali and his team at the Moorsfield Eye Hospital and the Institute of Ophthalmology, University College London, treated patients with LCA successfully with gene therapy. This success instilled hope for further clinical trials and research in the treatment of various retinal diseases (Uthra and Kumaramanickavel, 2009). Age-related macular degeneration Age related macular degeneration or AMD occurs due to ocular neovascularization and the key modulator for angiogenesis appears to be Vascular endothelial growth factor (VEGF) which is over expressed. In murine models, inhibition of VEGF through recombinant adeno-associated virus (rAAV) mediated transfer of gene coding for its receptor, known as soluble VEGF receptor 1has caused reduction of retinal neovascularization by 50 percent (Uthra and Kumaramanickavel, 2009). Retinoblastoma Retinoblastoma is the most common childhood primary ocular malignancy with poor prognosis. The primary goal of treatment of this condition is saving the life of the patient and the secondary goal is to save the eyeball and the vision of the patient. Oncolytic adenovirus has been found to be useful in treating retinoblastoma in mice models. Thus, it is suggested that this therapy may be a potential treatment strategy along with chemotherapy and radiotherapy (Song et al, 2009). In the study by Song et al (2009), the researchers found that H101 effectively inhibits growth of retinoblastoma cells in ice and also in vitro and thus may serve as a novel therapy for retinoblastoma in future. Conclusion Thus gene therapy has the potential to treat various inheritable retinal diseases in future. As of now, no product has been approved for clinical use and most of them are in clinical trial stage. References Cai, X., Conley, S.M, Nash, Z, Fliesler, S.J., Cooper, M.J., and Naash, M.I. (2010). Gene delivery to mitotic and postmitotic photoreceptors via compacted DNA nanoparticles results in improved phenotype in a mouse model of retinitis pigmentosa. FASEB J., 24(4), 1178–1191. Chung, D.C., Lee, V., Maguire, A.M. (2009). Recent advances in ocular gene therapy. Curr Opin Ophthalmol., 20(5), 377-81. Dejneka, N.S., and Bennett, J. (2001). Gene therapy and retinitis pigmentosa: advances and future challenges. Bioessays, 23, 662-68. Fibison, W.J. Gene therapy. Nurs Clin North Am., 35, 757-64. Human Genome Project Information or HGPI. (2009). Gene Therapy. Retrieved on 9th May, 2011 from http://www.ornl.gov/sci/techresources/Human_Genome/medicine/genetherapy.shtml Hecht, F. (2004). Gene Therapy - The Future Is Here! MedicineNet.com. Retrieved on 9th May, 2011 from http://www.medicinenet.com/script/main/art.asp?articlekey=12662 Kumar-Singh, R. (2008). Barriers for retinal gene therapy: separating fact from fiction. Vision Res.,48(16), 1671-80. Lau, D., McGee, L.H., Zhou, S., et al. (2000). Retinal Degeneration Is Slowed in Transgenic Rats by AAV-Mediated Delivery of FGF-2. Invest. Ophthalmol. Vis. Sci., 41 (11), 3622-3633 Singh, V.K., and Tripathi, P. (2002). Gene therapy in ocular diseases. Current Ophthalmology, 50(3), 173- 181. Simonelli, F., Maguire, A.M., Testa, F., et al. (2010). Gene Therapy for Leber's Congenital Amaurosis is Safe and Effective Through 1.5 Years After Vector Administration. Mol Ther., 18, (3), 643–650. Song, X., Zhou, Y., Jia, R., et al. (2010). Inhibition of Retinoblastoma In Vitro and In Vivo with Conditionally Replicating Oncolytic Adenovirus H101. Invest. Ophthalmol. Vis. Sci., 51 (5), 2626-2635. Uthra, S., and Kumaramanickavel, G. (2009). Gene therapy in ophthalmology. Oman J Ophthalmol., 2(3), 108–110. Read More