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Increasingly, that efficiency is attracting attention from doctors and scientists who want to harness the infectious characteristics of HIV to deliver gene therapy to treat patients with a variety of diseases, including a number of ophthalmic disorders. Among the most likely candidates for HIV-based gene therapy are ischaemic retinal diseases such as diabetic retinopathy, retinopathy of prematurity and age related macular degeneration. More defined genetic conditions – such as retinitis pigmentosa – are also good candidates for such therapy. These disorders and many more may now benefit from the delivery of external gene constructs using a greatly "edited" version of the HIV virus. Like all viruses, HIV is the ultimate parasite. Incapable of existence outside of a cellular environment, HIV invades certain cells in the body’s immune system to deliver its own deadly pathology. The proteins encoded by the HIV genes may then hijack the cell’s infrastructural machinery to produce more infectious viral particles which complete the cycle, eventually disabling the host cell. Making viruses useful Due to the success of HIV’s infectivity, scientists are now investigating the possibility of removing the pathogenic genes from HIV and replacing them with therapeutically useful genes. The objective of gene therapy is to introduce a functional gene into a target cell to restore protein production that may be absent due to a genetic disorder. A massive barrier in the growing field of gene therapy to date has been the issue of "delivery." In other words, how can doctors deliver useful genes to certain target tissues or cells to replace genes that have become mutated and are no longer functioning correctly? The simple rationale behind using viruses as delivery vehicles is to harness the natural "talent" or ability of viruses as gene delivery vehicles. Viruses have fine tuned their delivery skills over the millennia of evolutionary time. The idea of using viruses as Trojan horses, while simple in principle, has turned out to be enormously challenging in practice. However, once successful, gene therapy is set to bring much needed novel medicines into the clinician’s armament. HIV is a retrovirus, which is a virus made of RNA rather than DNA. An HIV virus consists of two strands of RNA contained within a viral protein core surrounded by a viral envelope made up of host cell membrane and viral encoded glycoprotein which facilitate the virus’ entry onto the cell. The process of converting a potentially pathogenic virus into a beneficial delivery vehicle or "vector" for gene therapy involves the use of sophisticated molecular biological techniques. These specialised techniques essentially delete all potentially harmful viral genes and replace them with a therapeutically beneficial gene. For instance, one form of retinal disorder, autosomal recessive retinitis pigmentosa, may be caused by mutations in a gene known as cGMP phosphodiesterase ß subunit (PDE ß). Dr. Masayo Takahashi and Dr. Hiroyuki Miyoshi, researchers at the Salk Institute for Biological Sciences, in La Jolla, California, in the United States, have been able to strip the HIV virus of any harmful genetic sequences and insert in their place the missing PDE ß gene. Once the vector and its beneficial cargo were constructed, the researchers injected approximately 50,000 infective HIV vectors carrying the PDE ß gene into the sub-retinal space of mice. The mice all had a genetic condition similar to that of the human form of autosomal recessive retinitis pigmentosa. The researchers then examined the eyes of these animals 6, 12 and 24 weeks after the injection. When they did so, the researchers found statistically significant rescue of the disease state, albeit with great variation from eye to eye. The differences in response may have been caused by the differences in the technical procedure of injection. Despite the variation, the results overall clearly indicated the promise of such therapeutic approaches in that the researchers were able to demonstrate the healthy functioning of the PDE ß gene 24 weeks after injection. Blocking angiogenesis Subsequently, a Japanese research team found promise for gene therapy with HIV in patients with proliferative retinopathy. The team, led by Dr. Tsutomu Igarashi at the Department of Ophthalmology at Nippon Medical School, Bunkyo-ku in Tokyo, was able to use HIV-based gene therapy to inhibit neovascularisation in mice with proliferative retinopathy. The researchers inhibited the growth of new blood vessels by delivering angiostatin – an anti-angiogenesis peptide – inside a specially constructed HIV vector. Angiogenesis has been shown to be the final common pathway leading to vision loss in age-related macular degeneration, diabetic retinopathy and retinopathy of prematurity, among others. Consequently there has been a clear clinical need for drugs to counter this unwanted blood vessel expansion. Angiostatin is a potent anti-angiogenic factor originally purified from the serum and urine of mice. The mouse angiostatin gene was cloned by the research team at the Nippon Medical School and inserted into a HIV vector from which all harmful pathogenic genes had been removed, similar to the procedure used in the previous studies at the Salk Institute. Using an experimental mouse model of retinal neovascularisation Dr. Igarishi’s research team injected less than 1/1000th of a milliliter of suspension containing millions of HIV vectors carrying the angiostatin gene intravitreally. Analysis of the retinas post-injection revealed that neovascularisation could be reduced by 90%. Although other therapies to treat angiogenesis in the retina are available – such as pan retinal photocoagulation and cryotherapy – such treatments are not without their own complications and side-effects. Consequently, this new gene therapy approach may find significant demand in the market place should the strategy be progressed through the various regulatory hurdles of drug development. Although this study by Dr. Tsutomu Igarashi was not the first attempt to deliver angiostatic agents to the retina, it demonstrated significant advantages over previous studies through its achievement of stable gene transfer and expression in retinal cells. Additionally, the intravitreal injection succeeded in containing the viral vectors within the ocular space. Furthermore, the mouse's neuronal blood brain barrier provided further insurance that expression of angiostatin in non-target tissues was not a risk. Aside from HIV, the medical and scientific community is actively engaged in the testing of many other types of viruses to safely deliver gene therapeutics across a range of disorders. Presently there are nearly 160 commercial enterprises directly involved in driving the development of gene therapy from bench to bedside. Those companies are expected to spend billions of euro over the next few years on research and development of gene therapy. For now, the vast majority of diseases being targeted by gene therapy are cancers. Over 63% of gene therapy clinical trials are for the treatment of various forms of cancer; however, there is increasing interest in the eye as a target for gene therapy due to the relative ease of administering such therapy safely to the eye and containing the effects – and side-effects – of the gene therapy to the organ. Risk of gene therapy unclear Significant controversy still plagues gene therapy, however. In September 1999 an 18 year old patient, Jesse Gelsinger, died in a gene therapy trial at the University of Pennsylvania’s Institute for Human Gene Therapy. Mr. Gelsinger, suffering from an inherited enzyme deficiency that severely disrupts ammonia metabolism, was the first patient in a gene therapy trial to die from the therapy itself. Mr. Gelsinger had been given a massive dose (38 trillion virus particles) of an adenovirus vector carrying an ornithine transcabamylase (OTC) therapeutic gene and subsequently died within a few days from a systemic inflammatory response. Investigations of how the adenovirus caused this patient’s death continue to be an active and controversial topic of discussion. More recently, two out of 11 patients undergoing gene therapy for the fatal severe combined immunodeficiency-X1 disorder (SCID-X1) developed a leukemia-like disease. Such incidents have raised serious concerns about the future of gene therapy as a safe and efficient medical tool. In spite of such setbacks, there have been significant advances in both biological understanding and technical capability leading to a genuinely realistic potential for gene therapy to radically alter the course of medical intervention over the coming decades. If successful, the practice of medicine today will bear little resemblance to what the coming generations will witness. Would you like to read previous "Bio-ophthalmology" columns? Visit the archive here. |
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