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.

The factor H-related protein family (CFHR) is a group of minor plasma proteins genetically and structurally related to complement factor H (fH). Notably, deficiency of CFHR1/CFHR3 associates with protection against age-related macular degeneration and with the presence of anti-fH autoantibodies in atypical hemolytic uremic syndrome (aHUS). We have developed a proteomics strategy to analyze the CFHR proteins in plasma samples from controls, patients with aHUS, and patients with type II membranoproliferative glomerulonephritis. Here, we report on the identification of persons carrying novel deficiencies of CFHR1, CFHR3, and CFHR1/CFHR4A, resulting from point mutations in CFHR1 and CFHR3 or from a rearrangement involving CFHR1 and CFHR4. Remarkably, patients with aHUS lacking CFHR1, but not those lacking CFHR3, present anti-fH autoantibodies, suggesting that generation of these antibodies is specifically related to CFHR1 deficiency. We also report the characterization of a novel CFHR1 polymorphism, resulting from a gene conversion event between CFH and CFHR1, which strongly associates with aHUS. The risk allotype CFHR1*B, with greater sequence similarity to fH, may compete with fH, decreasing protection of cellular surfaces against complement damage. In summary, our comprehensive analyses of the CFHR proteins have improved our understanding of these proteins and provided further insights into aHUS pathogenesis. (Blood. 2009;114:4261-4271)

Hemolytic-uremic syndrome (HUS) is a microvasculature disorder leading to microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. Most cases of HUS are associated with epidemics of diarrhea caused by verocytotoxin-producing bacteria, but atypical cases of HUS not associated with diarrhea (aHUS) also occur. Early studies describing the association of aHUS with deficiencies of factor H suggested a role for this complement regulator in aHUS. Molecular evidence of factor H involvement in aHUS was first provided by Warwicker et al., who demonstrated that aHUS segregated with the chromosome 1q region containing the factor H gene (HF1) and who identified a mutation in HF1 in a case of familial aHUS with normal levels of factor H. We have performed the mutational screening of the HF1 gene in a novel series of 13 Spanish patients with aHUS who present normal complement profiles and whose plasma levels of factor H are, with one exception, within the normal range. These studies have resulted in the identification of five novel HF1 mutations in four of the patients. Allele HF1 Delta exon2, a genomic deletion of exon 2, produces a null HF1 allele and results in plasma levels of factor H that are 50% of normal. T956M, W1183L, L1189R, and V1197A are missense mutations that alter amino acid residues in the C-terminal portion of factor H, within a region--SCR16-SCR20--that is involved in the binding to solid-phase C3b and to negatively charged cellular structures. This remarkable clustering of mutations in HF1 suggests that a specific dysfunction in the protection of cellular surfaces by factor H is a major pathogenic condition underlying aHUS.