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Gene Tied to Age-Related Vision Loss

In many cases, a single mutation may be to blame

THURSDAY, March 10, 2005 (HealthDay News) -- Researchers say they have identified a genetic flaw involved in a substantial number of cases of age-related macular degeneration (AMD), the leading cause of vision loss and blindness in the elderly.

Three teams with members at 10 medical institutions and biomedical companies, using different approaches, report separately in the March 10 issue of Science that they've zeroed in on a gene expressing a protein called complement factor H, a molecule involved in inflammation.

While the end goal of this type of research is a treatment or cure for AMD, scientists caution that that could still be years away.

The finding comes after years of work on the genetics of the sight-robbing disease, said Dr. Albert O. Edwards, who participated in the work at the University of Texas Southwestern Medical Center in Dallas and has just become president of the Institute for Retinal Research at Presbyterian Hospital, also in Dallas.

Earlier work pointed to a region of chromosome 1, one of the 23 pairs that make up the human genome, Edwards said. "We published on this locus in 1998," he noted. "Once we found the location, we found the specific gene."

One curious aspect of the discovery is that it strengthens a long-suspected link between AMD and cardiovascular disease, Edwards said. Inflammation has emerged as a major factor in heart attack, stroke and other cardiovascular conditions in recent years, and it now appears to be important in development of AMD.

This condition affects an estimated 15 million people in the United States. It causes progressive impairment of central vision, with the size of a visual "dead spot" in the macula (the central part of the retina) increasing over time.

There are two forms of AMD, wet and dry. Ninety percent of cases are the dry form, which causes vision loss over months or years. In the other 10 percent of "wet" cases, vision loss occurs much faster, as abnormal blood vessels under the macula leak blood and fluid that damage the retina.

Vitamin supplements can sometimes slow the progression of dry AMD, while laser surgery and injection of a drug in the eye can help slow vision loss in wet AMD. However, at this point, there's still no available treatment to stop the disease from progressing.

The team to which Edwards belonged identified the gene in two studies: the first one involved genetic analysis of 224 people with AMD and 134 persons with no visual loss; while the second included 176 cases and 68 healthy "control" individuals.

Similarly, a team of researchers at Duke University and Vanderbilt University reached the same conclusion through genetic studies of members of 182 families affected by AMD and 495 other individuals with the condition.

The results indicate that the replacement of a single amino acid in the long chain that makes up the complement factor H molecule prevents that protein from protecting the eye against damage caused by inflammation. They believe the genetic defect could be implicated in up to 43 percent of cases of AMD.

A third study, this time by researchers at Yale University and the National Eye Institute, came to much the same conclusion.

Edwards stressed that more work needs to be done to determine the exact mechanism connecting this gene variant with age-related vision loss. However, he said "we now know that a variant of complement factor H is a major factor in some cases of AMD."

Putting that knowledge to immediate medical use may not be easy, however. Screening people for the variant form of the gene has been proposed, "But I have to ask you -- if we screen people, what would we do with that information?" he said. "Knowing that you have that gene would not change what we now advise."

The advice now given to slow progression of AMD is the same given to improve cardiovascular health, Edwards said -- avoid tobacco, eat a diet rich in fish and low in fat. "There is no need for a genetic test to tell you to do these things," he said.

Stephen P. Daiger, who heads the Human Genetics Center at the University of Texas Health Sciences Center in Dallas and wrote an accompanying editorial, called the finding "a major advance in the understanding of the genetics of AMD and of other diseases as well."

The AMD work is the latest example of research made possible by the Human Genome Project, Daiger said, and "many, many major laboratories around the world will be working on this very soon."

But immediate medical applications of this achievement in basic science won't come quickly, he said. "The full implications will take two to three years to become evident," Daiger said.

More information

Get the full AMD story at the National Eye Institute.

SOURCES: Albert O. Edwards, M.D., Ph.D, president, Institute for Retinal Research, Dallas; Stephen P. Daiger, director, Human Genetics Center, University of Texas Health Sciences Center, Dallas; March 10, 2005, Science
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