Role of complement in aHUS and localization of the aHUS mutations in C3. (A) Role of complement alternative pathway in the physiopathology of aHUS. The alternative complement pathway is continuously activated by the spontaneous transformation of the biologically inactive central complement component C3 into a biologically active form C3(H₂O) to serve as a first line of defense against pathogens. C3(H₂O) interacts with factor B to form the fluid phase complex C3(H₂O)Bb, which has a C3 convertase activity, ie, it is able to cleave native C3 into biologically active fragments C3a (with proinflammatory activity) and C3b (which binds covalently to the cell surface). On the surface of pathogens, the complement cascade proceeds, but on healthy host cells (including the endothelial cells represented here) deposited C3b is rapidly inactivated by regulatory molecules, including MCP, FH, and FI. In case of FB binding and C3 convertase formation, it is dissociated by FH and DAF. These regulatory proteins protect healthy cells from complement overactivation and thereby prevent excessive host tissue damage. In case of mutations of the complement regulators FH, MCP, or FI, complement regulation may become inefficient. When mutations in the components of the C3 convertase (C3 and FB) are present, they may induce a formation of an overactive C3 convertase or a convertase, which is resistant to regulation. In both cases, the complement cascade is be activated on the glomerular endothelial cell surface, leading to endothelial damage, thrombosis, erythrocyte lysis–aHUS. (B) Mapping of the aHUS mutations on the C3 gene and protein. The individual exons and protein domains are indicated. Genetic changes indicated in bold are functionally characterized in this study. Rare variants are indicated in italics. Mutations marked with * are recurrent, being found in 2 or more unrelated patients from different cohorts. (C) Mapping of the aHUS mutations on the surface of C3b and C3d (in gray) in a complex with FH domains CCP1-4 and CCP19-20 (in cyan). Mutations are colored red and rare polymorphisms are green. The genetic changes characterized here are given in bold. The model is reconstructed using the structures of C3b-FH 1-4⁸  and C3d-FH 19-20.⁶  (D) Side view of the C3b molecule with mapped aHUS mutations. This surface holds the binding site of the substrate molecule C3, which enters into the convertase to be cleaved. Not visible on C and D are 2 polymorphisms (indicated with dashed arrows) that are located on the surface, opposite to the FH binding site. Further, one mutation (S1597R/M) is buried in the interior of the protein, near the surface and opposite the FH binding site (indicated also with a dashed arrow). The numbering of the mutated residues is according to the mature protein sequence, lacking the 22-amino-acid signal peptide. (E) Summary of the genetic changes in C3 found in aHUS and characterized in this study.