Funktionelle Charakterisierung der mit altersabhängiger Makuladegeneration (AMD) assoziierten Komplementfaktor-H-Variante Y402H

Abstract

Factor H (FH) is the most important regulator of alternative pathway Complement activation in the fluid phase. The FH-polymorphism Y402H which is strongly associated with age-related macular degeneration (AMD) causes a tyrosine-to-histidine-substitution within the SCR7-domain which is involved in heparin- and CRP-binding. Furthermore FH binds C3b and acts as cofactor for its Factor-I-mediated cleavage.This dissertation shows that the Y402H-polymorhism markedly influences some important FH-functions. In comparison to FH-402Y the AMD-risk-variant FH-402H shows a reduced binding capacity to CRP and a reduced cofactor-activity for the degradation of the C3-fragment iC3b into C3dg. The importance of FH as cofactor for this step of C3b-cleavage in the fluid phase and under physiological conditions has been shown in this work for the first time ever. For comparative study of both FH-variants only native and full-length FH-preparations were used. Those were purified from plasmas of genotyped healthy donors by immunoaffinitychromatography. Furthermore generation of monoclonal antibodies which specifically bind either FH-402Y or FH-402H allowed functional characterization of the FH-402-epitope. The own data suggests that amino acid 402 could be directly involved in FH-binding to heparin and in cofactor activity for cleavage of iC3b into C3dg. The 402-epitope seems not to be directly involved in binding CRP. Because of that an indirect effect of the Y402H-polymorphism on CRP-binding is plausible.The reduced CRP-binding and cofactor activity of FH-402H could result in an impaired complement control and accumulation of iC3b which in turn could cause neovascularization and chronic inflammation, e.g. in the eyes of AMD-patients. Analysis of AMD-patient-plasmas for several inflammatory markers in this work indicates systemic chronic alternative pathway complement activation. The same profile of inflammatory markers has been detected in plasmas of healthy people which are genetically characterized by a combination of several AMD-risk-polymorphisms. The Y402H-polymorphims alone does not seem to cause systemic complement activation. Compatible to this observance it has been shown on molecular level that the FH-polymorphisms I62V and Y402H have additive effects on CRP-binding.Taken together the data of this work implies that the combination of several FH-polymorphisms causes chronic complement activation via additional effects on FH-function. It has been shown before that local complement activation plays a role in AMD. The findings presented here enlarge the actual concepts on the disease-pathology.