THURSDAY, Oct. 19 (HealthDay News) -- Two international research teams say they've found a genetic variation that greatly increases the risk of age-related macular degeneration, the leading cause of vision loss in older people.

But several vision experts aren't convinced that the studies prove their point.

The studies, from the United States and Hong Kong, link what is called a single nucleotide polymorphism (SNP) in a gene designated HTRA1 with age-related macular degeneration (AMD). The Hong Kong study compared the incidence of SNP in 96 people with AMD and 130 people with normal vision. The U.S. study included 581 people with AMD and 309 with normal vision.

"This gene confers about 50 percent of the risk of AMD," said Dr. Kang Zhang, associate professor of ophthalmology and visual sciences at the University of Utah School of Medicine, and lead author of the U.S. report. The presence of SNP, together with a previously identified gene for a protein called complement factor H, can explain up to 80 percent of AMD cases, Zhang added.

There are two forms of the eye disease. "Wet" AMD is caused by an overgrowth of blood vessels that creates a dead spot in the macula, the center of the retina. A different version, "dry" AMD, which accounts for 90 percent of all AMD cases, causes less sight loss.

The discovery of SNP could make possible screening programs to detect persons at high risk of AMD and might lead to treatments directed at the overproduction of a protein called serine protease that causes the damage of wet AMD, Zhang said.

The study findings are published in the Oct. 20 issue of the journal Science.

Several experts expressed doubt about the findings, however.

"They claim that they have found the real functional gene," said Rando Allikmets, associate professor of ophthalmology, pathology and cell biology at Columbia University in New York City. "Well, they may be correct but they don't prove it in this paper."

The region of chromosome 10 on which the studies focused was described a year ago by American and German researchers as being associated with AMD, Allikmets said. There are three closely related genes at that location, and it's unclear whether the specific SNP in the specific gene the new studies describe is responsible for AMD, he said.

"They might have found a causal link, but they don't prove it here," he said.

Similar caution was expressed by Anand Swaroop, professor of ophthalmology and visual sciences at the University of Michigan Kellogg Eye Center.

"Whether it is causal, that needs to be verified," he said of SNP. "There are many variations in this region that could have the same effect."

Previous reports have identified other SNPs in the region of chromosome 10 as associated with AMD, Swaroop said. "It is hard to say which is the causal one. The genetic data is fine, but it is not convincing and needs more work. Causality requires further verification."

Josephine Hoh, associate professor of epidemiology and public health at Yale University, and co-author of the Hong Kong paper, said the studies provide "preliminary proof. It would take several years to provide complete proof. The preliminary data already shows very interesting results, a connection between the function of this particular SNP and wet AMD."

Zhang added: "There is no question in my mind that this is a major gene for macular degeneration. The work from now on is to look at why this gene is responsible for macular degeneration. We can also go on to real drugs for AMD."

More information

For more on AMD, visit the U.S.National Eye Institute.

SOURCES: Kang Zhang, M.D., professor, ophthalmology and visual sciences, University of Utah School of Medicine, Salt Lake City; Rando Allikmets, Ph.D, professor, ophthalmology, pathology and cell biology, Columbia University, New York City; Anand Swaroop, Ph.D, professor, ophthalmology and visual sciences, University of Michigan Kellogg Eye Center, Ann Arbor; Josephine Hoh, Ph.D, associate professor, epidemiology and public health, Yale University, New Haven, Conn.; Oct. 20, 2006, Science

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