The Binding of Factor H to a Complex of Physiological Polyanions and C3b on Cells Is Impaired in Atypical Hemolytic Uremic Syndrome

Abstract

Factor H (fH) is essential for complement homeostasis in fluid-phase and on surfaces. Its two C-terminal domains (CCP 19-20) anchor fH to self surfaces where it prevents C3b amplification in a process requiring its N-terminal four domains. In atypical hemolytic uremic syndrome (aHUS), mutations clustering towards the C-terminus of fH may disrupt interactions with surface-associated C3b or polyanions and thereby diminish the ability of fH to regulate complement. To test this we compared a recombinant protein encompassing CCP 19-20 with sixteen mutants. The mutations had only very limited and localized effects on protein structure. While we found four aHUS-linked fH mutations that decreased binding to C3b and/or to heparin (a model compound for cell-surface polyanionic carbohydrates), we identified five aHUS-associated mutants with increased affinity for either or both ligands. Strikingly, these variable affinities for the individual ligands did not correlate with the extent to which all the aHUS-associated mutants were found to be impaired in a more physiological assay that measured their ability to inhibit cell surface complement functions of full-length fH. Taken together, our data suggest that disruption of a complex fH-self surface recognition process, involving a balance of affinities for protein and physiological carbohydrate ligands, predisposes to aHUS.

Complement Factor B Mutations in Atypical Hemolytic Uremic Syndrome—Disease-Relevant or Benign?

“…Similar results were obtained for 17 mutations in the C-terminal 19 and 20 domains of FH, for which the functional defect was well characterized [31][32][33] (data summarized in ref.…”

“…Similar results were obtained for the mutations in the C terminus of FH, well known for their functional defect and association with aHUS. 2,[31][32][33] In conclusion, although some FB mutations induce complement overactivation and predispose to aHUS, others have a normal functional activity and hence, are benign and unlikely to be related to the disease. In-depth screening of all susceptibility genes as well as for the presence of anti-FH antibodies is necessary in all patients with identified FB mutations to identify other abnormalities affecting the disease occurrence, especially in the presence of low C3 levels.…”

Atypical Hemolytic Uremic Syndrome

“…More than 50% of CFH mutations are in SCR20 [38]. Functional studies to analyse the interaction between CFH and its ligands (C3b, glycosaminoglycans, heparin and endothelial cells) frequently demonstrate alteration of the binding of CFH19-20 mutants [50,[59][60][61]. Some mutations (named type 1 mutations) are associated with a quantitative deficiency in CFH (decreased CFH plasma levels), but many, including the majority of mutations in SCR19 and 20, are associated with normal plasma levels of CFH, the mutant CFH being functionally deficient (type 2 mutations).…”

A novel factor H-FC chimeric immunotherapeutic molecule against Neisseria gonorrhoeae

“…Mutations of FH that diminish FH binding to glycosaminoglycans and/or C3 fragments on host cells are important in the development of aHUS (reviewed in references (48-50)). Ferreira et al introduced some of these mutations into recombinant molecules that comprised FH domains 19 and 20, and examined whether the mutant molecules prevented full-length human FH from protecting anti-CD59-treated human RBCs from complement-mediated hemolysis (37). As expected, the wild-type FH 19-20 out-competed the full-length FH and resulted in hemolysis.…”