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EYE RESEARCH LOOKING GOOD
By Sean Henahan, Access Excellence
IOWA CITY, Iowa (2/6/97) Genetic discoveries, retina cell transplantation and bionic eye chips are three signs that the prospects for eye research are looking good. One the genetic front, University of Iowa reearchers report that mutations in a gene located on chromosome 1 are responsible for primary open angle glaucoma, the second leading cause of legal blindness in the United States. "These study results point to the potential availability of a blood test that can identify people at risk for primary open angle glaucoma," says Dr. Thomas Weingeist, UI professor and head of ophthalmology. "Therefore, more attention can be paid to following these patients and perhaps treating them earlier. It also means that it may be possible to identify drugs that can be more effective in treating this disease."  Glaucoma is a condition in which loss of vision is characterized by the degeneration of the optic nerve, usually associated with increased pressure within the eye. This increased pressure can be caused by problems in the eye's trabecular meshwork, which filters the watery fluid, known as the aqueous humor, that bathes the inside chamber of the eye behind the cornea and in front of the lens. "In 1993 we studied a large family with a history of glaucoma and identified a region of chromosome 1 to be associated with juvenile open angle glaucoma, which is a subset of primary open angle glaucoma and occurs at an earlier age," Stone says. "Studies by other researchers subsequently identified additional families that mapped to this region. For this study, we used genetic linkage analysis and shared genetic information among the families to further narrow that area on chromosome 1." Several genes that mapped to this region of chromosome 1 were considered by the UI researchers as the glaucoma-causing gene. The researchers determined that one gene in particular -- known as the TIGR gene -- existed within this region and was known to be expressed in the trabecular meshwork and ciliary body of the eye, both of which are involved in the maintenance of the intraocular pressure. Studies of eight families suspected of having gene mutations at this specific area on chromosome 1, allowed UI researchers to identify chromosome 1 three years earlier. Of the eight families, the UI team found three different TIGR gene mutations among four of those families. "At that point we could say that this gene causes a type of inherited glaucoma in families in which the disease gene can be shown to map to chromosome 1," Sheffield says. "But perhaps this is only one-tenth of one percent of all glaucoma. So, the next step was to determine if the gene was also mutated in some unrelated patients with glaucoma." The UI team studied 227 unrelated glaucoma patients with a family history of the disease and 103 unselected, or "walk-in," primary open angle glaucoma patients seen at a single clinic. Of the 227 patients with a family history of the condition, 10 (nearly 4.5 percent) had one of the three TIGR gene mutations. Of the 103 unselected patients, 3 (about 3 percent) harbored one the three TIGR gene mutations. "These findings suggest that this gene plays a role in a portion of all primary open angle glaucoma," Stone says. "When you consider that glaucoma affects between 2.5 and 5 million people in the United States, the 3 percent we found in the unselected patients group suggests that mutations in the TIGR gene cause glaucoma in more than 100,000 people." The identification of the gene for primary open angle glaucoma will help researchers better understand the disease, which could lead to better treatments. "The TIGR gene is believed to cause increased pressure in the eye by obstructing the outflow of the aqueous humor, the fluid that bathes the inside of the eye," Alward says. "Now it's possible to examine whether the mutations we describe in this study play a role in this." Identifying the TIGR gene also increases the possibility of developing accurate testing for genetic predisposition to glaucoma before symptoms arise. "Open angle glaucoma can be treated with existing drugs or surgery in most cases," Alward says. "Discovering specific glaucoma-causing mutations will make it possible to identify patients at risk for this disease before significant visual loss has occurred." Engineering New Eyes On the high-tech engineering front, scientists at North Carolina State University, University of North Carolina-Chapel Hill and Johns Hopkins University are developing a microchip that could restore sight to people with retinal pigmentosa, a genetic disorder charaterized by deteriorating vision and eventual blindness. The artificial retina component chip (ARCC) is just two millimeters square, the wafer-thin silicon microchip is imbedded with photosensor cells and electrodes. At that size, the ARCC can be implanted in the blind person's eye near the vision center of the retina. Powered by an exterior laser aimed at a photovoltaic cell, the photosensor cells in the microchip receive light and images through the pupil. The photosensor cells convert the light and images into electrical impulses that stimulate the nerve ganglia behind the retina. By stimulating the retina with a pattern of electrodes, the device partially recreates the visual information. The prototype chip is being polished to less than .02 millimeter thickness that will enable light and images to pass through the chip to the photosensors located at the back of the chip. While the current design of the ARCC will not restore clear vision, it can produce vision compatible with limited mobility such as the ability to see forms or direction of movement. The idea for the project began in 1988 when researchers stimulated with electrical current the nerve ganglia behind the retina of a blind person and demonstrated that he could be made to see points of light. The demonstration proved that the nerves behind the retina still functioned even though the retina had degenerated. Scientists reasoned that if the retina could be replaced with a device that could translate images to electrical impulses, then vision could conceivably be restored. An implantable artificial retina device could enable many of the 10 million people afflicted with retinal diseases, such as retinal pigmentosa and age-related macular degeneration, to regain a sense of sight. In these diseases, the rods and cones that work much like pixels on a video screen become inoperative, but the ganglion cells lining the retina remain intact. The researchers have designed the chip to be as noninvasive to the eye tissues as possible. By using an external laser to power the chip, they have eliminated the need for future surgeries to replace the power source and eliminated the problem of how to keep a battery viable in the wet, salty environment of the eye. The laser and photovoltaic cell is the power source of choice because the laser beam can pass through the cornea without damaging the corneal tissue. Also, the laser, powered by a small battery pack, can be fitted to a pair of regular eyeglasses and aimed at the ARCC's photovoltaic cell without clumsy or obtrusive headgear. To further protect the eye tissues, the ARCC is designed so that the electrodes do not pass current to stimulate the ganglia. Instead the electrodes charge a plate that stimulates the ganglia. By charging a plate instead of passing current, the ARCC protects the retinal tissues from damage from the electrical current. The prototype chips are expected to be ready for shipment to Johns Hopkins for testing in the spring of 1997. FIRST RETINAL TRANSPLANTS Meanwhile, in Chicago, surgeons for the first time recently transplanted retinal cells from an aborted fetus into the eye of an 80-year-old woman with macular degeneration, the leading cause of blindness among older Americans. Age-related macular degeneration (ARMD) is characterized by gradual loss retinal cells accompanied by loss of vision and eventually blindness. At present there is no treatment or cure. The hope is that the new cells will replace cells lost to the disease, with a corresponding improvement in vision. It will be several months before any results will be known. The glaucoma gene research was published in the Jan. 31 issue of the journal Science
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