If you’ve got a product being built on an assembly line, and one part of that product breaks down, do you have to replace that piece of machinery? Or is there a way to fix it farther downstream so you’ll have a functional product?”
For the past 25 years, geneticists have been trying to answer questions like these, said Richard Alan Lewis, MD, professor of ophthalmology at Baylor College of Medicine and consultant in genetic eye disorders for the Kleberg Genetics Center at Texas Children’s Hospital in Houston. The answers are dependent on finding the gene, reading its “instruction manual,” and deciphering its role in the entire matrix of the visual system. More than 500 genes contributing to eye diseases have been identified over the past 15 years.1
Although treatments aimed specifically at the pathophysiology of genetic eye diseases are in their infancy, genetic testing has already brought other benefits: confirmation of diagnosis at the molecular level, the segregation of one responsible gene from many that might cause the same or similar-appearing disorders, the ability to test siblings and allay a family’s anxiety and the identification of gene carriers, which allows preconceptual counseling.
A few recent developments have pushed the envelope a bit further.
Gene Transfer: First Dogs, Now Humans
Several years ago, a strain of vision-impaired Briard sheepdogs, subjects in a gene transfer therapy trial, became an overnight sensation when they regained partial sight. A team of scientists at the University of Pennsylvania, the University of Florida and Cornell University found that this strain of Briards had a naturally occurring mutation in a gene called Retinal Pigment Epithelium 65 (RPE65)—one of the known causes of Leber’s congenital amaurosis (LCA) in humans.
LCA is a group of rare, inherited diseases characterized by poor vision from birth and a variety of retinal changes, sometimes accompanied by nystagmus, strabismus, photophobia and cataracts, among other problems. RPE65 is one of the 12 LCA genes identified thus far, which, combined, account for about 70 percent of all cases of LCA. The discovery last year of CEP290 identified the responsible gene in a full 20 percent of LCA families.
The proof was in the puppy. Using a highly modified adenoviral vector, the RPE65 researchers delivered several million copies of a normal gene into the eyes of the Briard pups. The pups then regained enough sight to be able to navigate around doors, tables and chairs. “This gave incredible impetus to those of us interested in genetic therapy of hereditary eye disease,” said Dr. Lewis. “The investigators got the gene in, got it turned on, got it working and got it to keep working. It was absolutely incredible. Now we have a large mammal with an eye disease similar, but not identical, to humans’ that would otherwise have been blind.”
The dogs’ legacy is now bearing fruit with the announcement in May that a human trial for 12 patients with LCA has begun at Moorfields Eye Hospital and University College London. Trials will soon be conducted as well at the University of Pennsylvania and Children’s Hospital in Philadelphia.
“This will be the first formal gene therapy for human hereditary eye disease with specifically targeted gene transfer technology,” said Dr. Lewis. “It may open the gate to use similar vectors on other genes in other stages.” But, he added, it must first be proven safe, successful and sustainable.
Transferring the Goods
Gene transfer in the eye has many advantages, not just for the treatment of the retinal pigment epithelium for LCA but, potentially, for the photoreceptors.
Delivery. “When you place it between the retina and the retinal pigment epithelium, it’s easy for the virus to get into the RPE itself,” said Elias I. Traboulsi, MD, director of the Center for Genetic Eye Diseases at the Cleveland Clinic. “With a needle, you lift up the retina and deliver the viral particles right there, under direct observation.”
Isolation. The retina is advantageous for yet another reason: The blood-retina barrier provides an immune-protected environment, which means the vector will not immediately become neutralized. The eye is also largely isolated by its scleral coat.
Concentration. The eye’s small volume allows for a high concentration of the virus at the area of injection, said Dr. Traboulsi. This contrasts with blood, bone marrow, liver, brain or cerebrospinal fluid, he noted, which require a huge volume of virus and which carry a higher risk for dispersal to other organs, where incorporation might be undesirable.
Observation. Using robust methods of observation, such as electroretinography, it is possible to assess the effects of treatment over time in a relatively easy fashion.
Larger Gene Defect Means Challenge for Gene Delivery
Although Drs. Traboulsi and Lewis greet the LCA trials with optimism, they both caution that challenges remain for using gene transfer with many other genetic eye diseases. For example, most of the retinal dystrophies are due to problems in areas other than the pigment epithelium, which means delivery to cells may be more problematic. Also, some eye diseases are caused by defects in larger genes, as is the case with Stargardt’s disease.
The gene that broke the scales. In Stargardt’s, “it’s a huge gene and none of the usual viral vectors will hold it,” said Dr. Lewis. “The shuttle isn’t big enough to get the whole gene in.” In addition, Stargardt’s is not caused by null mutations, which would allow replacement by an entirely new functioning gene; instead, some of the Stargardt genes work but produce an abnormal protein, which then becomes the target of treatment.
Nonetheless, Dr. Lewis predicts an impending story with Stargardt’s. In fact, last October the National Neurovision Research Institute and Oxford BioMedica, a gene therapy developer, announced an agreement to collaborate on the production of “StarGen” gene replacement therapy for the treatment of Stargardt’s and other retinal degenerative diseases.2
A Glance at Other Advances
From preclinical stem cell therapies—showing promise with differentiation into photoreceptor and RPE cells—to the potential use of the glaucoma drug dorzolamide to modify some of the features of X-linked juvenile retinoschisis, translation from the laboratory to the clinical setting is an exciting, albeit laborious and complex, process.
The recent discovery of the association of several genetic polymorphisms with AMD, for example, now leads researchers in a fresh direction, with a focus on the immune system rather than on just genes affecting retinal degeneration, said Dr. Traboulsi.
Risk identification. Dr. Lewis added that understanding the genetic variants that predict who might progress to the advanced form of AMD has a more immediate payoff as a risk profile predictor. It allows for more targeted screening and closer monitoring of those who might benefit most from lifestyle changes, such as from smoking cessation, controlling weight or limiting intake of saturated fats.
Gene identification has made other contributions to prevention. “Knowing the genetic contribution to strabismus allows us to examine siblings earlier,” said Dr. Traboulsi. “We’re going to put them in glasses early and follow them closely,” which can help to prevent progression to amblyopia and binocular vision impairment.
Click Here for the Future
Below are resources that can enhance collaboration on genetic disorders:
Carver Lab. The University of Iowa houses the Carver Nonprofit Genetic Testing Laboratory, the first such entity to provide clinical molecular testing for a significant number of genetic eye diseases. The director of the lab is Edwin M. Stone, MD. (www.carverlab.org)
eyeGene. In the fall of 2006, the National Eye Institute formally started up the National Ophthalmic Disease Genotyping Network (eyeGENE), a cost-effective way to collect DNA samples from patients with inherited eye diseases. This helps ophthalmologists incorporate molecular diagnosis into their clinical practice, eventually directing interested patients into clinical trials. (www.nei.nih.gov/resources/eyegene.asp)
GeneTests. This site includes selected summaries and links to current review articles about a broad range of inherited diseases; lists of research and clinical laboratories that perform molecular testing; and links to patient resources. (www.genetests.org)
OMIM. Online Mendelian Inheritance in Man provides a continuously updated overview of peer-reviewed publications of genes, clinical syndromes and mutations. (www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM)
Diagnosis: Stop, Look and Listen
As more treatments for genetic eye disorders become available, the ophthalmologist’s role as diagnostician becomes ever more critical. Even now, Dr. Traboulsi emphasized, diagnosis in and of itself has a crucial value that should not be underestimated.
It used to be that all retinal dystrophies were dumped into a generic bin of “retinal degeneration,” he said. And the stock response of the ophthalmologist was, “Sorry, we cannot help you.”
Message of hope. “Now we can say, ‘Your son has a CRX mutation that’s causing Type 1 of 12 known types of LCA. We hope in the next few years to have treatment for CRX mutations and to be able to reduce the severity of visual impairment in your son,’” said Dr. Traboulsi.
To speak honestly of treatment, however, a diagnosis must be accurate. Many disorders, Dr. Traboulsi said, are misdiagnosed or mislabled as retinitis pigmentosa, including LCA, Usher’s syndrome and Bardet-Biedl syndrome. Even some rare metabolic conditions like infantile Refsum’s disease have been misconstrued as retinitis pigmentosa.
Consider your entire patient. “The patient with RPE65 disease may not be thought of by the general ophthalmologist as having LCA,” said Dr. Traboulsi, “because the image is of a child who is totally blind.”
Likewise, he said, it’s essential to always ask, “Are we dealing with just an eye problem or an ocular manifestation of a systemic problem?” This particularly comes into play with syndromes such as Bardet-Biedl, which can include a wide array of signs including obesity, mild mental retardation, renal disease, ataxia, polydactyly, asthma and speech problems.
Dr. Lewis cautioned, “Look at your whole patient, not just at their eyes.” Many Bardet-Biedl patients have a speech so distinctive, he said, that it alone is a strong indicator of the diagnosis.
Collaboration and testing. There’s a paradigm shift in the diagnosis, and now treatment, of patients with inherited eye disorders, according to Terri L. Young, MD, professor of ophthalmology and pediatrics at Duke University and director of the pediatric ophthalmic genetics program there.
“The tools of genetic diagnosis are not just for the laboratory scientist,” she said. “The optimal way to do this is as a collaborative effort, not only between the ophthalmologist and the patient, but including the geneticist and the genetic counselor.”
Dr. Traboulsi agreed. “With the DNA technologies currently available, testing for gene mutations has become much easier and more available.” Testing is now commercially available for 24 genetic eye diseases, and that list is growing.3
1 National Eye Institute Web site www.nei.nih.gov/resources/eyegene.asp
2 National Neurovision Research Institute Web site www.nnri.info
3 Arch Ophthalmol