Genes for macular degeneration.

Variations in two genes could account for three-quarters of all cases of age-related macular degeneration, a new study reports.

The disease is the leading cause of blindness in people over 60 and affects more than 50 million people worldwide. It occurs when light-sensing cells malfunction in a part of the retina called the macula and block the central field of vision.

Last year, a team led by Columbia University researcher Rando Allikmets reported that certain forms of a gene for a protein called factor H can increase a person's risk of age-related macular degeneration. Other versions of this gene seem to protect a person from the disease. Factor H shuts off inflammation throughout the body once an infection is eliminated and the immune reaction is no longer needed.

However, of the 1,300 elderly people examined in that study, 29 percent had a risky version of the factor H gene but didn't have macular degeneration. That led Allikmets' team to suspect that other genes affecting the inflammation response that factor H regulates also play a part in the disease.

After reexamining blood and tissue samples from the previous study, the researchers honed in on a gene for a protein called factor B, which turns on the same pathway that factor H shuts off. Like the factor H gene, the factor B gene appears to have some variants that increase the risk of macular degeneration and others that provide a protective effect. About 74 percent of the study subjects with the disease had risky gene variants for factor H, factor B, or both, the researchers report in an upcoming Nature Genetics. --C.B.