ADVANCES IN GENETIC MEDICINE

Not long ago, Ian MacDonald MD, professor and chair of ophthalmology and visual sciences at the University of Alberta, Edmonton, Canada, examined a young boy with presumed Leber’s congenital amaurosis (LCA). Based on the boy’s phenotype, Prof MacDonald, who is also a clinical geneticist, suspected a mutation in the LRAT gene. One of at least 17 identified genetic causes of LCA, the gene is named for its role in synthesising lecithin retinol acryltransferase.

LRAT defects, along with those of the gene RPE65, promote retinal degeneration by producing an abnormal protein that disrupts recycling of photopigments. Gene sequencing confirmed the diagnosis of LRAT-involved LCA, also known as LCA14, in the boy. so Prof MacDonald referred him to a colleague conducting a clinical trial of an oral synthetic retinoid designed to bypass this biochemical blockade. “His vision improved,” recalled Dr MacDonald, who has studied and described the molecular basis of choroideraemia and is taking part in a gene therapy study for treating it beginning this fall. “I can count on one hand the patients i have treated for genetic eye diseases who have improved.” The case illustrates an important point about genetic medicine and its application in paediatric ophthalmology. Gene therapies designed to cure outright by replacing defective genes may grab more headlines. But genetic testing coupled with conventional drug and surgical treatments, as well as new compounds targeting specific metabolic pathways, are advancing much more quickly – and may be much more clinically useful, at least in the short to intermediate term. indeed, in a phase 1b trial involving 17 retinitis pigmentosa patients due to mutations in either LRAT or RPE65 receiving an oral synthetic retinoid oral treatment for seven days, statistically significant increases in the area of visual fields at day seven and day 14 respectively were observed, hendrik PN scholl MD, professor of ophthalmology at Johns hopkins University, Baltimore, Us, told the American Academy of Ophthalmology annual meeting last November.

Two-thirds of all patients saw best-corrected vision improve in at least one eye at seven and 30 days. in addition, two patients tested with dark-adapted perimetry and full field sensitivity showed “significant improvement” of up to 36 dB, or 4,000-fold, in light sensitivity after prolonged dark adaptation within days of treatment, he added. Genetic diagnosis also results in better understanding of disease mechanisms, making it possible to repurpose existing compounds, Dr MacDonald noted. For example, dorzolamide, developed for glaucoma, has improved visual function and reduced macular oedema in some patients with x-linked juvenile retinoschisis. “We know what the gene is, we know what the gene product is and so we can address it.” And in the case of retinoblastoma, genetic testing has completely changed the treatment dynamic, enabling positive identification of those at risk. “We can truly differentiate between a heritable risk and an initially occurring mutation,” Dr MacDonald said. This allows early monitoring and surgical treatment of infants with inherited disease while sparing those without risk a lifetime of intrusive and expensive screening for tumours that will never develop. Nonetheless, genetic technology for confirming diagnoses and guiding conventional treatment is technically complex, challenging clinicians to keep up with increasingly rapid developments, said David B Granet MD, professor of ophthalmology and paediatrics at the University of California san Diego, Us. Moreover, it is fraught with ethical and philosophical questions, especially when applied to children who cannot give informed consent. Uncovering genetic markers for incurable diseases is a particular challenge, Dr Granet noted. It may present opportunities for early treatment in cases where the gene is strongly associated with a disease phenotype, as with retinoblastoma. But more often, heritable diseases involve multiple genes and even non-genetic factors, complicating the risk calculation and any treatment or management decision. For diseases with multiple genes involved, such as age-related macular degeneration and glaucoma, the presence of some genes suggest a higher risk, but it is currently not possible to predict who will manifest the diseases when and to what degree. Other diseases, such as retinitis pigmentosa, result from defects in single gene pairs, but more than 50 different gene causes have been identified, and even these do not account for all cases.

About one in five individuals carries one copy of an RP-causing gene, though only about one in 5,000 develop the disease, which requires inheriting two copies of the same defective gene segment. Even diseases caused by defects in just one gene and that are fairly Genetic testing, coupled with conventional drug and surgical treatments and new compounds targeting metabolic pathways, is advancing. We can truly differentiate between a heritable risk and an initially occurring mutation “ Ian MacDonald MD You can imagine a world in which the impact of genetic medicine could be unbelievable. But the conversation quickly gets into should we test people, and what do we do with the information? “ David B Granet MD “ ...two patients tested with dark-adapted perimetry and full field sensitivity showed ‘significant improvement’ of up to 36 dB, or 4,000- fold, in light sensitivity after prolonged dark adaptation within days of treatment” Hendrik PN Scholl MD common are relatively low incidence. For example, one in 25 Caucasian people carry one gene for cystic fibrosis, but there is no risk of their children developing the disease unless they mate with another carrier. “You can imagine a world in which the impact of genetic medicine could be unbelievable. But the conversation quickly gets into should we test people, and what do we do with the information? If you are a prospective parent, do you avoid having children, or test embryos for selective implantation or terminate a pregnancy if a foetus has a genetic disease or is a carrier? Who gets to decide? Who pays? These are the sort of questions we face with what we do now,” Dr Granet said. Effectively and ethically harnessing the power of genetic medicine require that ophthalmologists understand what tests are available, what their limits are and how they relate to clinical observation, Dr Granet said.

Above all, physicians or expert genetic counsellors must be able to work through these questions with patients in a way that both informs and helps them determine their own reasons for seeking and using genetic information. Early treatment: promise and peril Technical difficulties aside, the potential advantages of genetic medicine in paediatric ophthalmology are obvious. Identifying and treating heritable eye diseases before they manifest or advance presents the prospect of reducing or completely preventing vision loss. But at this early stage of development, the risks are just as obvious. “It is generally easier to justify the risks of experimental gene therapy in adults than in children. Adults are better able to evaluate the risks for themselves, and are relatively protected against the impact of side effects because they typically have more advanced sight loss.” said James W Bainbridge PhD, FRCOphth of Moorfields Eye hospital and the UCL institute of Ophthalmology, London, UK. “it is obviously critical to ensure that the gene defect is responsible for a particular phenotype,” noted Dr Bainbridge, whose pioneering work targeting RPE65-related Leber’s congenital amaurosis established the short-term safety and efficacy of viral vectors for delivering gene therapy to the retina. These risks are why it will take longer for gene therapy – of which only a handful exist and none yet past human clinical trials – to enter clinical practice in paediatric ophthalmology, Dr Granet said. “This technology is in a translational stage. When you are looking at kids, you have to be careful.” still, early treatment is the goal, Dr Bainbridge said. Many experimental studies of gene therapy for retinal degenerations demonstrate that earlier intervention improves outcomes, and the same is anticipated in clinical application. however, potential therapy targets and agents are highly specific, and further development is required before gene replacement is reliable and robust, Dr Bainbridge said. “There are many questions to be answered about the magnitude and durability of the response, and whether the impact can be sufficient to protect against on-going degeneration. These are all relevant to the optimal timing of intervention in children.” Indeed, patients treated with experimental genetic replacement therapy for RPE65-related LCA, also known as LCA2, have generally continued to lose vision over time, as have patients receiving new genes for choroideraemia, Dr MacDonald noted. similar issues crop up in other gene therapy fields, he added. For example, transplanted pancreatic islet cells seemed to cure Type 1 diabetes, but most patients eventually again required insulin or other supportive treatment. Dr MacDonald suggests that treating inherited eye disease may require multiple interventions along the lines of chemotherapy for cancer or long-term management of blood pressure and diabetes. “These aren’t infectious diseases, these are degenerative diseases. Chronic management may require many additional changes in lifestyle, preventive measures and perhaps neuroprotective agents supplementing gene therapy. What they might be i don’t know.” such approaches also may be amenable to gene insertion therapy, Dr Bainbridge said. he envisions adding genes for suppressing toxic proteins created by single-gene dominant mutations as well as genes that express anti-VEGF or neuroprotective factors for treating a broader range of genetic disease.

Dr Bainbridge also remains confident that early diagnosis and treatment will eventually become common. “The increasing power of genetic screening will enable us to identify children at an age when they are most likely to benefit from intervention.” In the popular mind, genetic testing is conceived as a mechanical process in which identification of specific genes or gene combinations infallibly predicts disease. It’s just a matter of time before all genetic disease is catalogued, screened for and prevented. The reality, of course, is much messier. While more genetic causes of heritable disease are continuously discovered, many remain unknown; for example in 30 per cent or so of LCA cases, none of the 17 known gene defects are present. Technical issues also can interfere, leading to false positives and false negatives. In addition, many diseases are influenced by multiple genes, and every human genome includes many, many mutations that increase risk for specific diseases to varying degrees. Many more mutations are harmless. And, of course, many genetic diseases are as-yet incurable.

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