Available RPE65 Genetic Test for enrollment in Retinitis Pigmentosa Genetic Therapy.
NEI-funded study points to a novel approach for treating AMD, Stargardt disease
Combinations of Food and Drug Administration-approved drugs protect against the loss of cells required for vision in a mouse model of blinding retinal diseases. The study, published in Science Signaling, was funded by the National Eye Institute, a part of the National Institutes of Health.
The drugs, which are currently used for a range of conditions, from lowering blood pressure to treating prostate disease, may eventually offer an option for preventing vision loss associated with the degeneration of cells in the retina, the light-sensitive tissue at the back of the eye. The loss of retinal function causes blindness in diseases such as age-related macular degeneration and Stargardt disease, the most common form of inherited juvenile macular degeneration. If the therapy’s success is replicated in humans, it would represent an entirely novel approach to preventing visual impairment, said the study’s principal investigator, Krzysztof Palczewski, Ph.D., chair of the Department of Pharmacology at Case Western Reserve University.
The researchers selected the drugs because they act on G protein coupled receptors (GPCRs), a family of signaling proteins in various cell types throughout the body. When working properly, such signaling keeps the cell in homeostasis, a state of balance with multiple biological processes buzzing along in sync.
The researchers studied wildtype and transgenic mice that were bred to have retinas especially susceptible to light-induced degeneration of their photoreceptors, the key cells in the retina that detect light and convert it into a signal for vision. Both types of mice were exposed to a bright light long enough to lead to significant losses of photoreceptors as well as other types of cells in the retina and a thinning of a retinal layer.
In previous studies, Palczewski and his team had found that when mice develop light-induced retinopathy, it could hinder the activity of GPCRs. So they asked, “What if we expose the mice to compounds that activate GPCRs?” They identified more than a dozen drugs with a particular affinity for GPCRs in the retina and that also appeared to be significantly protective against light-induced photoreceptor degeneration.
In the current study, the researchers zeroed in on two combinations of these drugs that each individually take different pathways to the same target: optimizing the activity of GPCRs. Wildtype and transgenic mice that were dosed with the drug combinations prior to light exposure were more likely to preserve their retinal layer and photoreceptor cells compared to those that were not given the prophylactic therapy prior to light exposure.
The investigators then sought to examine the impact of the drug regimens at the molecular level by performing transcriptome analysis. That is, they looked at patterns of gene expression in the retinas of light-exposed and unexposed mice and they found some key differences. “Light-induced injury to the cells of the retina can trigger the upregulation or downregulation of over 100 genes as the cells attempt to respond to the damage. Studying these gene expression patterns is a far more sensitive marker of retinal degeneration than looking at structural changes to the cells under a microscope,” said the study’s co-investigator, Anand Swaroop, Ph.D., chief of NEI’s Neurobiology-Neurodegeneration and Repair Laboratory.
The combinations of drugs used in the study work synergistically to achieve what’s called systems pharmacology, a relatively new concept, Palczewski explained. Traditionally, patients are often treated by means of poly-pharmacology, multiple drugs that act on different targets, which means that each drug needs to be dosed at high enough levels to achieve a response at their respective targets. While effective, such doses of several drugs over time can lead to side effects.
By contrast, systems pharmacology homes in on a single target – in this case, GPCR function. Multiple drugs acting via multiple biological pathways all affect a single target. Working synergistically on the same target requires much lower doses of each drug to be effective, which avoids the risk of inducing drug-related side effects. “Interestingly, the combined lower doses of two drug treatments were able to reverse the gene expression changes induced by light-induced injury, and one of the combinations did not seem to induce the expression of undesirable genes,” Swaroop said.
“It’s reassuring that the drugs used in the study are already being used in clinical practice, so we know their safety profile,” Palczewski said. Future studies will need to be conducted to ensure that there is no additional risk of toxicity when the low doses of the agents are used together.
“Discovering new uses for drugs that are approved by the FDA provides the quickest possible transition from bench to bedside,” said Neeraj Agarwal, Ph.D., who manages the NEI’s translational research program. “These studies are remarkable as they offer a systems pharmacologic approach for treating retinopathies.”
The study team also included researchers at Shanghai University of TCM, the Louis Stokes Cleveland Veterans Affairs Medical Center, and the Scripps Research Institute Florida.
The study was funded in part by NEI grants EY018139, R24 EY024864, U01 EY025451, EY000474 and EY000546.
Photo Caption: Four imaging techniques show significantly more retinal damage following light exposure in the eyes of untreated versus treated mice. Thinning of the layers of the retina is shown in top images. Autofluorescent spots (shown bottom left), indicating light damage, are plentiful in untreated animals and absent in treated ones. Two-photon microscopy (bottom right) shows enlargement of the photoreceptors in untreated mice compared with treated ones. Credit: Krzysztof Palczewski, Ph.D.
Chen, Y et al. “Synergistically acting agonists and antagonists of G protein–coupled receptors prevent photoreceptor cell degeneration,”Sci. Signal. 26 Jul 2016:Vol. 9, Issue 438, pp. ra74
Color fundus photography image from a Stargardt disease patient showing a central macular scar with some pigmentary changes and surrounding perimacular flecks.
Stargardt disease is an inherited disorder of the retina – the tissue at the back of the eye that senses light. The disease typically causes vision loss during childhood or adolescence, although in some forms, vision loss may not be noticed until later in adulthood. It is rare for people with the disease to become completely blind. For most people, vision loss progresses slowly over time to 20/200 or worse. (Normal vision is 20/20).
Stargardt disease is also called Stargardt macular dystrophy, juvenile macular degeneration, or fundus flavimaculatus. The disease causes progressive damage—or degeneration—of the macula, which is a small area in the center of the retina that is responsible for sharp, straight-ahead vision. Stargardt disease is one of several genetic disorders that cause macular degeneration. Experts estimate that 1 in 8-10 thousand people have Stargardt disease.
The most common symptom of Stargardt disease is variable, often slow loss of central vision in both eyes. People with the disease might notice gray, black, or hazy spots in the center of their vision, or that it takes longer than usual for their eyes to adjust when moving from light to dark environments. Their eyes may be more sensitive to bright light. Some people also develop color blindness later in the disease.
The progression of symptoms in Stargardt disease is different for each person. People with an earlier onset of disease tend to have more rapid vision loss. Vision loss may decrease slowly at first, then worsen rapidly until it levels off. Most people with Stargardt disease will end up with 20/200 vision or worse. People with Stargardt disease may also begin to lose some of their peripheral (side) vision as they get older.
Fundus autofluorescence image of a patient with Stargardt disease.
The retina contains light-sensing cells called photoreceptors. There are two types of photoreceptors: rods and cones. Together, rod and cones detect light and convert it into electrical signals, which are then “seen” by the brain. Rods are found in the outer retina and help us see in dim and dark lighting. Cones are found in the macula and help us see fine visual detail and color. Both cones and rods die away in Stargardt disease, but for unclear reasons, cones are more strongly affected in most cases.
You may have heard about the importance of vitamin A-rich foods in maintaining healthy vision. That’s because vitamin A is needed to make key light-sensitive molecules inside photoreceptors. Unfortunately, this manufacturing process can lead to harmful vitamin A byproducts—which turn out to play a key role in Stargardt disease.
Mutations in a gene called ABCA4 are the most common cause of Stargardt disease. This gene makes a protein that normally clears away vitamin A byproducts inside photoreceptors. Cells that lack the ABCA4 protein accumulate clumps of lipofuscin, a fatty substance that forms yellowish flecks. As the clumps of lipofuscin increase in and around the macula, central vision becomes impaired. Eventually, these fatty deposits lead to the death of photoreceptors and vision becomes further impaired.
Mutations in the ABCA4 gene are also associated with other retinal dystrophies including cone dystrophy, cone-rod dystrophy, and retinitis pigmentosa, a severe form of retinal degeneration.
Genes are bundled together on structures called chromosomes. One copy of each chromosome is passed by a parent at conception through egg and sperm cells. The X and Y chromosomes, known as sex chromosomes, determine whether a person is born female (XX) or male (XY) and also carry other non-sex traits.
In autosomal recessive inheritance, it takes two copies of the mutant gene to give rise to the disease. An individual who has one copy of a recessive gene mutation is known as a carrier. When two carriers have a child, there is a:
In autosomal dominant inheritance, it takes just one copy of the mutant gene to bring about the disease. When an affected parent with one dominant gene mutation has a child, there is a 1 in 2 chance that a child will inherit the disease.
Autosomal recessive mutations in the ABCA4 gene account for about 95 percent of Stargardt disease. The other five percent of cases are caused by rarer mutations in different genes that play a role in lipofuscin function. Some of these mutations are autosomal dominant.
An eye care professional can make a positive diagnosis of Stargardt disease by examining the retina. Lipofuscin deposits can be seen as yellowish flecks in the macula. The flecks are irregular in shape and usually extend outward from the macula in a ring-like pattern. The number, size, color, and appearance of these flecks are widely variable.
A standard eye chart and other tests may be used to assess symptoms of vision loss in Stargardt disease, including:
Currently, there is no treatment for Stargardt disease. Some ophthalmologists encourage people with Stargardt disease to wear dark glasses and hats when out in bright light to reduce the buildup of lipofuscin. Cigarette smoking and second hand smoke should be avoided. Animal studies suggest that high-dose vitamin A may increase lipofuscin accumulation and potentially accelerate vision loss. Therefore, supplements containing more than the recommended daily allowance of vitamin A should be avoided, or taken only under a doctor’s supervision. There is no need to worry about getting too much vitamin A through food.
A number of services and devices can help people with Stargardt disease carry out daily activities and maintain their independence. Low-vision aids can be helpful for many daily tasks and range from simple hand-held lenses to electronic devices such as electronic reading machines or closed circuit video magnification systems. Because many people with Stargardt disease will become visually disabled by their 20s, the disease can have a significant emotional impact. Work, socializing, driving and other activities that may have come easily in the past are likely to become challenging. So counseling and occupational therapies often need to be part of the treatment plan.
Over the past several decades, researchers have identified hundreds of genes that contribute to inherited eye diseases, including Stargardt disease. This information has led to better diagnostic tests, and is providing insight into possible treatments.
Scientists are also working to find new mutations in the ABCA4 gene, and in other genes, that might contribute to Stargardt disease. NEI’s National Ophthalmic Disease Genotyping and Phenotyping Network (eyeGENE®) is an important resource for these efforts. The eyeGENE network gives patients access to genetic studies and clinical trials, while giving researchers access to patient DNA samples and clinical information.
Many studies continue to explore the biology and genetics of Stargardt disease, and of macular degeneration more generally. For example, NEI is conducting a natural history study of Stargardt and other ABCA4-related diseases. The study is following 45 individuals with ABCA4 mutations for five years. The main goals are to better understand the natural course of the disease, to make contact with people who may be interested in future clinical trials, and to collect blood, skin, and DNA samples from those people. These samples can be studied in the lab to explore the mechanisms of Stargardt disease.
Such mechanistic studies can lead to new treatment strategies. One strategy currently under study is to reduce the build-up of lipofuscin and other toxic byproducts in the retina. An NEI-funded group based at Columbia University is working with a synthetic form of vitamin A, called ALK-001, that isn’t readily converted into lipofuscin. In mice and larger animal models, an oral form of ALK-001 slows the formation of lipofuscin deposits. ALK-001 is being tested for safety in healthy volunteers before testing begins on people with Stargardt disease.
A group based at Case Western Reserve University and supported by the NEI Translational Research Program on Therapy for Visual Disorders is taking another approach. The group has tested a panel of drugs already deemed safe by the Food and Drug Administration (FDA) in a mouse model of Stargardt. They found that some FDA-approved drugs can reduce retinal damage in the mice. They are now evaluating chemical compounds with a similar structure to FDA-approved drugs, and exploring different modes of drug delivery.
Gene therapy—that is, repairing or replacing the defective ABCA4 gene—also holds promise for treating Stargardt disease. Gene replacement therapy requires a method for delivering the gene of interest into cells, and for some diseases, the solution has been to package the gene inside a small, harmless virus called the adeno-associated virus (AAV). The ABCA4gene is too large to fit within this virus. Researchers funded by NEI therefore modified a larger virus from the lentivirus family, and engineered it to carry ABCA4. Tests in the Stargardt mouse model showed that this approach can reduce lipofuscin accumulation. Oxford Biomedica has refined this technology, and licensed it under the name StarGen. Human safety trials of StarGen began in 2011.
Finally, stem cell-based therapies are showing promise for Stargardt disease in clinical trials. Stem cells are immature cells that can generate many mature cells types in the body, including the photoreceptors that die off in Stargardt disease. Human stem cells can be derived from embryonic or adult tissues. Both kinds of cells are being tested in patients with Stargardt disease and age-related macular degeneration (AMD), which is a leading cause of vision loss in the United States.
A U.S. company called Advanced Cell Technology (ACT) is conducting a trial of retinal pigment epithelium (RPE) cells for AMD and Stargardt disease. These cells provide support and nourishment to the retina. The RPE cells under study in the ACT trial are derived from human embryonic stem cells.
Other clinical and laboratory studies are making use of adult cells that have been reprogrammed into stem cells, called induced pluripotent – or iPS – cells. In 2014, Japanese scientists launched the first clinical trial of iPS cells to treat AMD, and indeed the first trial ever of iPS cells. The goal is to coax the iPS cells into making RPE cells. NEI scientists are planning a similar trial and going a step further – by seeding the RPE cells onto a scaffold so that they form a sheet, similar to how they arrange themselves in the eye. NEI scientists also plan to make iPS cells from the skin cells collected in the Stargardt natural history study. These iPS cells could be used to make photoreceptors and RPE cells – for use in potential cell therapies and as a research tool to study how potential drugs will affect these cell types.
Last Reviewed: April 2015
The National Eye Institute (NEI) is part of the National Institutes of Health (NIH) and is the Federal government’s lead agency for vision research that leads to sight-saving treatments and plays a key role in reducing visual impairment and blindness.
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