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

Abstract

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 ( fB 32R), Gln ( fB32Q), or Trp ( fB32W) at position 32 in the Ba domain. The fB 32Q 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 fB 32Q had decreased activity compared with fB 32R. Biacore comparison revealed markedly different proenzyme formation; fB 32R bound C3b with 4-fold higher affinity, and formation of activated convertase was enhanced. Binding and functional differences were confirmed with recombinant fB 32R and fB 32Q; an intermediate affinity was revealed for fB32W. To confirm contribution of Ba to binding, affinity of Ba for C3b was determined. Ba-fB 32R had 3-fold higher affinity compared with Ba-fB32Q. We demonstrate that the disease-protective effect of fB 32Q 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.alternative pathway ͉ AMD

Complement in age-related macular degeneration: a focus on function

“…This polymorphic residue is in the Ba fragment of the CFB protein, which does not serve any known function after it is split from CFB by CFD. 23 The polymorphisms are adjacent to the site of enzymatic cleavage and occur in a binding site for C3b during the formation of the proenzyme C3bB. Montes et al, using both human and then recombinant CFB, showed that the 32R variant of CFB binds to C3b with four times stronger affinity than the 32Q variant and formed proenzyme and enzyme more efficiently than the 32Q variant.…”

“…The 32W variant formed enzyme at a rate intermediate between the 32R and 32Q variants. 23 The protection against AMD associated with the 32Q variant of CFB is due to less efficient formation of the C3 convertase, a less efficient complement system being advantageous in this context. The findings of the Montes et al study would suggest that the 32W variant should also be protective, though with a less strong effect than 32Q.…”

“…91 The risks associated with some variants of complement genes, however, are high, particularly in combination. Montes et al 23 suggested the term 'complotype' to describe the combination of complement variants that may alter systemic complement activation and affect disease specificity. This is an idea that deserves further exploration, as it may be that the cumulative risk of combinations of risk variants is more complex than a simple combination of probabilities would suggest.…”

“…The 32Q variant of CFB, which decreases risk of AMD, increases susceptibility to some infections and to the complications of infections, for example, cardiomyopathy in Chagas disease. 23 The CFH Y402H polymorphism associated with high AMD risk has recently been found to be protective against systemic inflammatory response syndrome in children. 92 Deletion of CFHR3 and CFHR1 is protective against AMD, 17 but homozygous deletion predisposes to the very serious systemic disease, aHUS, because of the formation of auto-antibodies that cross-react with CFH protein.…”

“…67 The role of CFB in the alternative pathway and subsequent evidence of the functional effect of R32Q support this idea. 23 CFI is a cofactor with CFH for the inactivation of C3b. Non-coding polymorphisms adjacent to the gene and in an intron of CFI are associated with altered risk of AMD.…”