Whe exact age-related macular degeneration, the most common cause of old age blindness in the western world, is still largely unknown. However, the so-called complement system, an ancient branch of the immune system, is under urgent suspicion. However, it has so far been unclear which of the numerous members of this family of immune proteins add to the retina in such a way that blindness can occur.
Researchers led by Simon Clark from the Center for Ophthalmic Research at the University of Tübingen could now have succeeded in exposing one of the ringleaders of severe retinal disease. We are talking about a protein from the ranks of the complement system with the bulky name “Factor-H-Related Protein-4”, or FHR4 for short. As Clark says on request, he originally had no intention of investigating the causes of macular degeneration. His actual research area, which he has been dealing with for many years, is the complementary system. In connection with this, he wanted to clarify one day exactly where FHR4 is located in the body. “To do this, we developed an antibody against FHR4 and used it to search for the immune protein in more than 50 human tissues.
We found it in the eyes, among others, but only in some people – and only those with age-related macular degeneration, ”explains the biochemist. In further investigations, they would have discovered that FHR4 is only produced in the liver. “Since it must have got into our eyes from there, we had the idea of searching for FHR4 in the blood.”
The scientists have achieved this goal in the current study, the results of which have now been published in the journal “Nature Communications”. A comparison of the blood values of around 500 patients with age-related macular degeneration and roughly the same number of people without eye problems then confirmed their suspicion. As Valentina Cipriani of Queen Mary University in London, Simon Clark and the other study authors report, the blood of the eye-sick men and women contained much more FHR4 than that of the healthy controls.
In-depth genetic analyzes then revealed another surprise: People with a large amount of FHR4 in the blood were conspicuously often carriers of genetic variants that are associated with a high risk of age-related macular degeneration. “As our observations show, these genetic varieties influence the blood content of FHR4,” Clark explains, adding: “However, they are not in the FHR4 gene, as you would expect, but in that of another complement protein, namely Factor H (FH). We were very surprised. “
Leaky blood vessels give off FHR4 proteins
But why does the immune protein FHR4 accumulate in the eye and is causing trouble here? The British biochemist most likely considers the following scenario: In advanced age, when the small blood vessels become increasingly leaky, FHR4 flows more and more from the blood into the back of the eye, causing inflammatory reactions. The greater its content in the blood, the more likely it is to damage the retina. “People with very high levels of FHR4 in the blood all had macular degeneration,” Clark explains. The immune protein does not infiltrate the back of the eye to combat pathogens there. The main task of the complement system is to ward off such intruders. With age-related macular degeneration, there is no evidence of infection.
Frank Holz from the University Eye Clinic in Bonn describes the findings of Clark and his colleagues as new and interesting. “So far, however, there are no therapy studies that build on this,” admits the ophthalmologist. Other complement inhibitors are already further in this regard. However, one of them, an antibody against factor D, was not effective.
However, an inhibitor against another complement protein (C3) has given promising results in a small study and is currently being tested in a large study. “If it proves itself, many patients with the” dry “late form of age-related macular degeneration could be offered effective therapy for the first time. So far, this has only been possible for people with the “wet” late form, “says Holz. As far as terminology is concerned, the second variant is usually the consequence of the first: For reasons unknown so far, some people develop small vessels in the back of the eyes that have been damaged, which grow into the retina. They transform the previously “dry” defect into a “wet” one. Since the new blood streams are very permeable and bleed easily, they can permanently impair vision. Such a fate can often be averted with drugs that suppress pathological vascular growth.
However, no effective remedy has yet been found against the various “dry” types of age-related macular degeneration, which make up around 80 to 90 percent of cases of illness. Contrary to the common term age-related macular degeneration, this diagnosis hides very different ailments. The results of the new study also show this. For example, only about 30 of the eye-sick people had elevated levels of FHR4 in the blood. For the rest, the demise of the retinal cells is likely to have other causes.