Factor H–Related Protein 1 Drives Disease Susceptibility and Prognosis in C3 Glomerulopathy

Significance Statement Mutations in factor H–related protein 1 (FHR-1) that result in duplication of its dimerization domain associate with the chronic renal disease C3 glomerulopathy (C3G), which is characterized by complement dysregulation. The molecular basis for this association is only partially understood. The authors show that these FHR-1 mutations enhance FHR-1’s binding to C3-activated fragments on opsonized surfaces and promote an excessive complement activation that overcomes FH regulation. They also show that elevated levels of FHR-1 associate with poor renal prognosis for patients with C3G, whereas a genetic deficiency of FHR-1 offers protection against C3G development. These findings advance our understanding of C3G pathogenesis and suggest that inhibition of FHR-1 may have therapeutic potential in C3G. Background C3 glomerulopathy (C3G) is a heterogeneous group of chronic renal diseases characterized predominantly by glomerular C3 deposition and complement dysregulation. Mutations in factor H–related (FHR) proteins resulting in duplicated dimerization domains are prototypical of C3G, although the underlying pathogenic mechanism is unclear. Methods Using in vitro and in vivo assays, we performed extensive characterization of an FHR-1 mutant with a duplicated dimerization domain. To assess the FHR-1 mutant’s association with disease susceptibility and renal prognosis, we also analyzed CFHR1 copy number variations and FHR-1 plasma levels in two Spanish C3G cohorts and in a control population. Results Duplication of the dimerization domain conferred FHR-1 with an increased capacity to interact with C3-opsonized surfaces, which resulted in an excessive activation of the alternative pathway. This activation does not involve C3b binding competition with factor H. These findings support a scenario in which mutant FHR-1 binds to C3-activated fragments and recruits native C3 and C3b; this leads to formation of alternative pathway C3 convertases, which increases deposition of C3b molecules, overcoming FH regulation. This suggests that a balanced FHR-1/FH ratio is crucial to control complement amplification on opsonized surfaces. Consistent with this conceptual framework, we show that the genetic deficiency of FHR-1 or decreased FHR-1 in plasma confers protection against developing C3G and associates with better renal outcome. Conclusions Our findings explain how FHR-1 mutants with duplicated dimerization domains result in predisposition to C3G. They also provide a pathogenic mechanism that may be shared by other diseases, such as IgA nephropathy or age-related macular degeneration, and identify FHR-1 as a potential novel therapeutic target in C3G.