An international team of scientists has identified a protein which is strongly linked to the commonest cause of blindness in developed countries when its levels are raised in the blood.

The discovery is a major step forward in the understanding of age-related macular degeneration, which affects 1.5 million people in the UK alone.

The study, carried out by the team from Universities of Manchester, Cardiff, London and Nijmegen, and Manchester Foundation NHS Trust is published in Nature Communications.

The major funder was the Medical Research Council and the study

The protein, called FHR4, was found by the team to be present at higher levels in the blood of patients with AMD compared to individuals of a similar age without the disease.

The findings were confirmed in 484 patient and 522 control samples from two independent collections across Europe.

Analyses of eyes donated for research after life also revealed the FHR4 protein was present in the AMD-affected parts of the eye

FHR4 was shown by the team to activate part of the immune system -called the complement system; over activation is a major causal factor of AMD.

FHR4 is one of a group of proteins that regulate the complement system and the genes encoding these proteins are tightly clustered on chromosome 1, the largest human chromosome.

When the team investigated a set of genetic variants across the human genome, they found that genetic variants in this region on chromosome 1 determined the levels of FHR4 in the blood. And they found that the same genetic variants were associated with AMD.

Professor Paul Bishop and Professor Simon Clark, from the University of Manchester were part of the leadership team on the study.