A prevalent C3 mutation in aHUS patients causes a direct C3 convertase gain of function

Abstract

Atypical hemolytic uremic syndrome (aHUS) is a rare renal thrombotic microangiopathy commonly associated with rare genetic variants in complement system genes, unique to each patient/family. Here, we report 14 sporadic aHUS patients carrying the same mutation, R139W, in the complement C3 gene. The clinical presentation was with a rapid progression to end-stage renal disease (6 of 14) and an unusually high frequency of cardiac (8 of 14) and/or neurologic (5 of 14) events. Although resting glomerular endothelial cells (GEnCs) remained unaffected by R139W-C3 sera, the incubation of those sera with GEnC preactivated with proinflammatory stimuli led to increased C3 deposition, C5a release, and procoagulant tissue-factor expression. This functional consequence of R139W-C3 resulted from the formation of a hyperactive C3 convertase. Mutant C3 showed an increased affinity for factor B and a reduced binding to membrane cofactor protein (MCP; CD46), but a normal regulation by factor H (FH). In addition, the frequency of at-risk FH and MCP haplotypes was significantly higher in the R139W-aHUS patients, compared with normal donors or to healthy carriers. These genetic background differences could explain the R139W-aHUS incomplete penetrance. These results demonstrate that this C3 mutation, especially when associated with an at-risk FH and/or MCP haplotypes, becomes pathogenic following an inflammatory endothelium-damaging event. (Blood. 2012;119(18):4182-4191)

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

“…19,29 The pathogenic role of C5 convertase overactivation also explains the therapeutic efficacy of Eculizumab in aHUS. Eculizumab leaves the overactive C3 convertase untouched but blocks the C5 cleavage.…”

“…19,29 Therefore, apoptonecrotic HUVECs were used as a model surface to study the activity of the alternative pathway C5 convertase in the presence of FB mutations. Because the Ba fragment does not participate in the C5 convertase activity, only the mutations within the Bb part as well as I217L were tested ( Figure 5D).…”

An Engineered Construct Combining Complement Regulatory and Surface-Recognition Domains Represents a Minimal-Size Functional Factor H

“…purified FH was able to control the increased complement activation on the endothelial cell surface in the presence of sera of aHUS patients with a C3 mutation (24). Nevertheless, attention should be paid because certain mutations in factor B or C3 are resistant to regulation by FH, and C3 nephritic factors may also render the C3 convertase resistant to regulation (61)(62)(63)(64)(65).…”

“…C3 deposition on HUVECs in the presence of FH-depleted serum was performed as described previously (24). Briefly, primary HUVECs at passage 3 were activated with 10 ng/ml TNF-a, 10 3 U/ml IFN-g, and 10 ng/ml IL-1 overnight, washed, and incubated with 30% FH-depleted serum diluted in M199 medium containing 5 mM Ca 2+ and Mg 2+ and different doses of FH or mini-FH.…”

“…Purified full-length FH showed efficacy in controlling the C3 convertase in a mouse model of FH deficiency (22) and in in vitro C3-deposition and cell-protection assays in the presence of aHUS-associated anti-FH autoantibodies or mutations in FH or C3 (23)(24)(25). Domains 19-20, which naturally evolved to dock FH on host cells, could be exploited in drug design to direct the regulatory activity of FH, or other complement inhibitor, to host surfaces (26).…”