Mapping interactions between complement C3 and regulators using mutations in atypical hemolytic uremic syndrome

Abstract

Key Points• C3 mutations in aHUS commonly result in impaired complement regulation, C3 consumption, and a poor renal outcome.• C3 mutations tend to cluster at the protein surface and facilitate mapping of putative binding sites for the regulatory proteins.The pathogenesis of atypical hemolytic uremic syndrome (aHUS) is strongly linked to dysregulation of the alternative pathway of the complement system. Mutations in complement genes have been identified in about two-thirds of cases, with 5% to 15% being in C3. In this study, 23 aHUS-associated genetic changes in C3 were characterized relative to their interaction with the control proteins factor H (FH), membrane cofactor protein (MCP; CD46), and complement receptor 1 (CR1; CD35). In surface plasmon resonance experiments, 17 mutant recombinant proteins demonstrated a defect in binding to FH and/or MCP, whereas 2 demonstrated reduced binding to CR1. In the majority of cases, decreased binding affinity translated to a decrease in proteolytic inactivation (known as cofactor activity) of C3b via FH and MCP. These results were used to map the putative binding regions of C3b involved in the interaction with MCP and CR1 and interrogated relative to known FH binding sites. Seventy-six percent of patients with C3 mutations had low C3 levels that correlated with disease severity. This study expands our knowledge of the functional consequences of aHUS-associated C3 mutations relative to the interaction of C3 with complement regulatory proteins mediating cofactor activity. (Blood. 2015;125(15):2359-2369

Functional Characterization of Autoantibodies against Complement Component C3 in Patients with Lupus Nephritis

“…If we assume the first scenario, we can hypothesize that the anti-C3 IgG binds to an area of the C3b molecule carrying the FH and CR1 binding sites and will compete for the binding with these two regulators. The location of the FH binding sites has been well described (45)(46)(47), and data are available for the approximate location of the CR1 binding site because it overlaps with the binding site of the first two domains of FH (46,48,49). This part of the C3b molecule is located in the C3c fragment and is available in the immobilized C3, C3b, C3c, and iC3b but not in C3d.…”

The role of the alternative pathway of complement activation in glomerular diseases

“…In general, mutations in the C3 gene are uncommon in C3G, and they are more likely to be associated with DDD rather than C3 glomerulonephritis (C3GN) phenotype [61,62]. In contrast, genetic abnormalities in C3 gene occur more often in aHUS [104].…”

“…Mutations located in the N-terminal end (SCR 1-4) or autoantibodies against this part of CFH influence the C3b and C3-convertase binding in plasma and most of them are responsible for the development of C3G (Fig. 3) [10,25,104]. However, in the recent studies, some N-terminal genetic abnormalities in CFH, as well as anti-CFH antibodies to SCRs1-4, have also been reported in aHUS.…”

Mutations in complement C3 from aHUS patients

“…1 T he underlying molecular mechanism is shown to be decreased cofactor activity of complement regulators. The resulting increased formation and stability of the C3 degradation product C3b are translated into increased C3 deposition onto endothelial cells, and the majority of aHUS patients carrying mutations in C3 were observed to exhibit low levels of plasma C3.…”