Functional basis of protection against age-related macular degeneration conferred by a common polymorphism in complement factor B

Mutations and polymorphisms in complement genes have been linked with numerous rare and prevalent disorders, implicating dysregulation of complement in pathogenesis. The 3 common alleles of factor B (fB) encode Arg (fB₃₂R), Gln (fB₃₂Q), or Trp (fB₃₂W) at position 32 in the Ba domain. The fB₃₂Q allele is protective for age-related macular degeneration, the commonest cause of blindness in developed countries. Factor B variants were purified from plasma of homozygous individuals and were tested in hemolysis assays. The protective variant fB₃₂Q had decreased activity compared with fB₃₂R. Biacore comparison revealed markedly different proenzyme formation; fB₃₂R bound C3b with 4-fold higher affinity, and formation of activated convertase was enhanced. Binding and functional differences were confirmed with recombinant fB₃₂R and fB₃₂Q; an intermediate affinity was revealed for fB₃₂W. To confirm contribution of Ba to binding, affinity of Ba for C3b was determined. Ba-fB₃₂R had 3-fold higher affinity compared with Ba-fB₃₂Q. We demonstrate that the disease-protective effect of fB₃₂Q is consequent on decreased potential to form convertase and amplify complement activation. Knowledge of the functional consequences of polymorphisms in complement activators and regulators will aid disease prediction and inform targeting of diagnostics and therapeutics.