A functional variant in the CFI gene confers a high risk of age-related macular degeneration

Abstract

Purpose: Genome-wide association studies have identified common variants in or near genes within the complement cascade, high-density lipoprotein cholesterol pathway, and extracellular matrix that are associated with age-related macular degeneration (AMD). Combined, these common variants explain up to half of the heritability of AMD. To explore the role of rare, highly penetrant variants in AMD we focused on the complement factor I (CFI) gene.

Methods: All CFI coding exons were sequenced from the genomic DNA of 84 unrelated cases with AMD. The recurrent p.Gly119Arg variant was subsequently screened in 3,483 AMD cases and 3,746 controls from Nijmegen, Cologne, Baltimore and Rotterdam. The concentration of Factor I and the degradation of C3b in plasma and serum samples was measured. Expression and secretion of recombinant wild-type and mutant Factor I proteins were assessed in HEK293 cells. The effect of overexpression and morpholino knockdown of cfi on vascular architecture was assessed in zebrafish fli:EGFP embryos.

Results: We identified a rare missense mutation, p.Gly119Arg, in 20 of 3,567 cases versus only one of 3,937 controls, consistent with p.Gly119Arg conferring a high risk for developing AMD (P= 3.79 x 10-6, odds ratio = 22.20; 95% confidence interval 2.98-164.49). The p.Gly119Arg mutation affects a highly conserved glycine residue in the CD5 domain of Factor I and is predicted to alter the packing and stability of this domain. The cases carrying the p.Gly119Arg variant had lower plasma Factor I levels compared to both controls and cases without a CFI variant. Plasma and serum samples of cases carrying the p.Gly119Arg substitution mediated the degradation of C3b both in the fluid phase and on the cell surface at a lower level compared to controls. Recombinant protein studies revealed that the p.Gly119Arg mutant protein is both expressed and secreted at lower levels than wildtype Factor I protein. Consistent with these findings, 119Arg-encoding human mRNA had reduced activity compared to 119Gly in regulating vessel thickness and branching in the zebrafish retina.

Conclusions: This study provides the first direct evidence that Factor I deficiency confers a high risk of developing AMD. Taken together, these findings demonstrate that rare, highly penetrant mutations contribute to the genetic variance of AMD, with important implications for predictive testing and personalized treatment.

Keywords: 412 age-related macular degeneration • 539 genetics • 660 proteins encoded by disease genes