Genetic mutations account for much of the risk for age-related macular degeneration, the most common cause of irreversible vision loss in older people in the developed world, researchers say.

Age-related macular degeneration, or AMD, is the leading cause of blindness among Canadians over the age of 50, and affects about 15 million people in North America.

Vision is blurred in the macula, the part of the eye that perceives details in central vision.

The most serious form, wet macular degeneration, occurs when abnormal blood vessels grow and leak in the retina. It may prevent people from being able to see well enough to read or drive.

The dry version accounts for 90 per cent of AMD cases and causes less loss of sight.

Dr. Kang Zhang, an ophthalmology professor at the University of Utah, and his team describe the discovery of a tiny gene molecular difference or "single nucleotide polymorphism" (SNP) linked to AMD in Thursday's online issue of the journal Science.

The combination of the newly discovered SNP and a previously identified one could explain about 70 per cent of cases of AMD, the researchers said.

Until now, researchers had implicated a gene on chromosome 10, but haven't identified it precisely, Zhang said. He called the discovery a new clue for thinking about, diagnosing and treating the disease.

Genetic screening

To do so, Zhang's team looked at the genes of 581 Utah residents with AMD and 309 without it. Those with a mutant copy of the HTRA1 gene had a significantly higher risk of developing the disorder during their lifetime.

The discovery could make it possible to screen about five per cent of the population who might be at high risk of AMD. "If anyone in your family has a history of macular degeneration, this test would be advised," Zhang said. 

When a genetic test becomes commercially available, people who carry the mutation could be encouraged to change their risk factors for the disease, such as quitting smoking, eating a proper diet, taking nutrition supplements or perhaps a drug.

New avenue for drug investigators

Since the findings also suggest the gene plays a key role in the formation of a protein that leads to damage in wet AMD, the research opens a new target for treatment. The gene is a protease, a type of enzyme that drug developers have successfully blocked in HIV/AIDS research, Zhang noted.

A second study appearing in the same issue looked at the incidence of the SNP in Hong Kong, among 96 people with AMD and 130 people with normal vision. Populations studies suggest wet AMD is more prevalent in Asians than Caucasians.

People with the mutated type of the SNP were estimated to have a likelihood of developing wet AMD 10 times that of people with the normal type, the researchers said.

Although what's true for populations is a poor predictor for individual patients, finding the same SNP in more than one population makes geneticists more confident about the link.

Ophthalmologists, who were not involved in the research, cautioned Zhang's research suggests a causal link but doesn't prove it. To do so, his team plans to put the human gene in a mouse model to see whether it causes AMD-type symptoms.