AMD often affects older people

Age-related macular degeneration (AMD) causes blindness in millions of older people across the globe.

A team led by New York's Columbia University hope their work could help aid the development of new treatments for the condition.

Details are published in the journal Nature Genetics.

I am not aware of any other complex disorder where nearly 75% of genetic causality has been identified Dr Rando Allikmets

AMD is marked by a progressive loss of central vision due to degeneration of the macula - a region of the retina responsible for fine, central vision.

Previous work had shown that several variants of a gene called Factor H significantly increase the risk of AMD.

Factor H controls production of a protein that helps shut down the body's immune response to infection once it has been successfully fought off.

People with these inherited variants of Factor H are less able to control inflammation caused by infectious triggers, which may spark AMD in later life.

However, the previous research found that about one in three people with a risky variant of Factor H had not been diagnosed with AMD.

Complementary roles

The latest research focused not only on Factor H, but on other genes that play a role in the same immune response pathway.

A genetic analysis of 1,300 people quickly identified a second gene, Factor B, as playing a significant role.

While Factor H is an inhibitor of the immune response to infection, Factor B is an activator.

Because of the complementary roles of the these two genes, a protective Factor B variation can protect against AMD, even if one carries a risk-increasing variant of Factor H, and vice versa.

The researchers found 74% of the people with AMD had either the Factor H or Factor B risk factor or both - but no protective variants of either gene.

Lead researcher Dr Rando Allikmets said "I am not aware of any other complex disorder where nearly 75% of genetic causality has been identified.

"These findings are significant because they absolutely confirm the roles of these two genes and, consequently, the central role of a specific immune response pathway, in the development of AMD.

"In just a few short years, we've gone from knowing very little about what causes AMD to knowing quite a lot. We now have clear targets for early therapeutic intervention."

The researchers are now searching for the specific triggers that set off the immune response, and subsequent inflammation.

A spokesperson for the UK Macular Disease Society described the research as "interesting".

"It is a very useful part of the jigsaw, but only one small piece. We estimate that it will take seven to 10 years before see any cure for AMD."