C3 glomerulopathies (C3G) are a group of severe renal diseases with distinct patterns of glomerular inflammation and C3 deposition caused by complement dysregulation. Here we report the identification of a familial C3G-associated genomic mutation in the gene complement factor H-related 1 (CFHR1), which encodes FHR1. The mutation resulted in the duplication of the N-terminal short consensus repeats (SCRs) that are conserved in FHR2 and FHR5. We determined that native FHR1, FHR2, and FHR5 circulate in plasma as homo-and hetero-oligomeric complexes, the formation of which is likely mediated by the conserved N-terminal domain. In mutant FHR1, duplication of the N-terminal domain resulted in the formation of unusually large multimeric FHR complexes that exhibited increased avidity for the FHR1 ligands C3b, iC3b, and C3dg and enhanced competition with complement factor H (FH) in surface plasmon resonance (SPR) studies and hemolytic assays. These data revealed that FHR1, FHR2, and FHR5 organize a combinatorial repertoire of oligomeric complexes and demonstrated that changes in FHR oligomerization influence the regulation of complement activation. In summary, our identification and characterization of a unique CFHR1 mutation provides insights into the biology of the FHRs and contributes to our understanding of the pathogenic mechanisms underlying C3G.

Dense deposit disease (DDD) is a severe renal disease characterized by accumulation of electron-dense material in the mesangium and glomerular basement membrane. Previously, DDD has been associated with deficiency of factor H (fH), a plasma regulator of the alternative pathway (AP) of complement activation, and studies in animal models have linked pathogenesis to the massive complement factor 3 (C3) activation caused by this deficiency. Here, we identified a unique DDD pedigree that associates disease with a mutation in the C3 gene. Mutant C3 923ΔDG , which lacks 2 amino acids, could not be cleaved to C3b by the AP C3-convertase and was therefore the predominant circulating C3 protein in the patients. However, upon activation to C3b by proteases, or to C3(H 2 O) by spontaneous thioester hydrolysis, C3 923ΔDG generated an active AP C3-convertase that was regulated normally by decay accelerating factor (DAF) but was resistant to decay by fH. Moreover, activated C3b 923ΔDG and C3(H 2 O) 923ΔDG were resistant to proteolysis by factor I (fI) in the presence of fH, but were efficiently inactivated in the presence of membrane cofactor protein (MCP). These characteristics cause a fluid phase-restricted AP dysregulation in the patients that continuously activated and consumed C3 produced by the normal C3 allele. These findings expose structural requirements in C3 that are critical for recognition of the substrate C3 by the AP C3-convertase and for the regulatory activities of fH, DAF, and MCP, all of which have implications for therapeutic developments. IntroductionComplement is a major component of innate immunity, with crucial roles in microbial killing, apoptotic cell clearance, immune complex handling, and modulation of adaptive immune responses. Complement is activated by 3 independent activation pathways: the classical pathway (CP), the lectin pathway (LP), and the alternative pathway (AP). The critical steps in complement activation are the formation of unstable protease complexes, named complement factor 3-convertases (C3-convertases; specifically, C3bBb for AP and C4b2a for CP and LP), and the cleavage of C3 by the convertases to generate C3b. Convertase-generated C3b can form more AP C3-convertase, providing exponential amplification to the initial activation. Binding of C3b to the C3-convertases generates the C5-convertases with the capacity to bind and cleave C5, initiating formation of the lytic membrane attack complex (MAC). In contrast to the CP and the LP, whose activation is triggered by immune complexes and bacterial mannose groups, respectively, the AP is intrinsically activated. Spontaneous activation of C3 in plasma occurs through the tick-over mecha-

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