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Original Investigation
January 2017

The Functional Effect of Rare Variants in Complement Genes on C3b Degradation in Patients With Age-Related Macular Degeneration

Author Affiliations
  • 1Department of Ophthalmology, Donders Institute for Brain, Cognition, and Behaviour, Radboud University Medical Center, Nijmegen, the Netherlands
  • 2Division of Medical Protein Chemistry, Department of Translational Medicine, Lund University, Malmö, Sweden
  • 3Department of Ophthalmology, University Hospital of Cologne, Cologne, Germany
  • 4Department of Human Genetics, Donders Institute for Brain, Cognition, and Behaviour, Radboud University Medical Center, Nijmegen, the Netherlands
JAMA Ophthalmol. 2017;135(1):39-46. doi:10.1001/jamaophthalmol.2016.4604
Key Points

Question  Is there a functional effect of rare genetic variants in the complement system on complement levels and activity in serum?

Findings  In this study, carriers of complement factor I gene variants had decreased factor I levels; carriers of the complement 9 Pro167Ser gene had increased C9 levels, whereas C3 and factor H levels were not altered. Carriers of complement factor H and complement factor I gene variants had a reduced ability to degrade C3b, which for the complement factor I gene was associated with reduced serum factor I levels.

Meaning  This study suggests that carriers of rare variants in the complement factor I and complement factor H genes are less able to inhibit complement activation and may benefit more from complement-inhibiting therapy than patients with age-related macular degeneration in general.

Abstract

Importance  In age-related macular degeneration (AMD), rare variants in the complement system have been described, but their functional consequences remain largely unexplored.

Objectives  To identify new rare variants in complement genes and determine the functional effect of identified variants on complement levels and complement regulation in serum samples from carriers and noncarriers.

Design, Setting, and Participants  This study evaluated affected (n = 114) and unaffected (n = 60) members of 22 families with AMD and a case-control cohort consisting of 1831 unrelated patients with AMD and 1367 control individuals from the European Genetic Database from March 29, 2006, to April 26, 2013, in Nijmegen, the Netherlands, and Cologne, Germany. Exome sequencing data of families were filtered for rare variants in the complement factor H (CFH), complement factor I (CFI), complement C9 (C9), and complement C3 (C3) genes. The case-control cohort was genotyped with allele-specific assays. Serum samples were obtained from carriers of identified variants (n = 177) and age-matched noncarriers (n = 157). Serum concentrations of factor H (FH), factor I (FI), C9, and C3 were measured, and C3b degradation ability was determined.

Main Outcomes and Measures  Association of rare variants in the CFH, CFI, C9, and C3 genes with AMD, serum levels of corresponding proteins, and C3b degradation ability of CFH and CFI variant carriers.

Results  The 1831 unrelated patients with AMD had a mean (SD) age of 75.0 (9.4) years, and 60.5% were female. The 1367 unrelated control participants had a mean (SD) age of 70.4 (7.0), and 58.7% were female. All individuals were of European descent. Rare variants in CFH, CFI, C9, and C3 contributed to an increased risk of developing AMD (odds ratio, 2.04; 95% CI, 1.47-2.82; P < .001). CFI carriers had decreased median FI serum levels (18.2 μg/mL in Gly119Arg carriers and 16.2 μg/mL in Leu131Arg carriers vs 27.2 and 30.4 μg/mL in noncarrier cases and controls, respectively; both P < .001). Elevated C9 levels were observed in Pro167Ser carriers (10.7 µg/mL vs 6.6 and 6.1 µg/mL in noncarrier cases and controls, respectively; P < .001). The median FH serum levels were 299.4 µg/mL for CFH Arg175Gln and 266.3 µg/mL for CFH Ser193Leu carriers vs 302.4 and 283.0 µg/mL for noncarrier cases and controls, respectively. The median C3 serum levels were 943.2 µg/mL for C3 Arg161Trp and 946.7 µg/mL for C3 Lys155Gln carriers vs 874.0 and 946.7 µg/mL for noncarrier cases and controls, respectively. The FH and FI levels correlated with C3b degradation in noncarriers (R2 = 0.35 and R2 = 0.31, respectively; both P < .001).

Conclusions and Relevance  Reduced serum levels were associated with C3b degradation in carriers of CFI but not CFH variants, suggesting that CFH variants affect functional activity of FH rather than serum levels. Carriers of CFH (Arg175Gln and Ser193Leu) and CFI (Gly119Arg and Leu131Arg) variants have an impaired ability to regulate complement activation and may benefit more from complement-inhibiting therapy than patients with AMD in general.

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