Factor H-related protein 1 neutralizes anti-factor H autoantibodies in autoimmune hemolytic uremic syndrome.

The autoimmune form of atypical hemolytic uremic syndrome (HUS) is characterized by circulating autoantibodies against the complement regulator factor H, and is often associated with deficiency of the factor H-related proteins CFHR1 and CFHR3. Here we studied whether anti-factor H autoantibodies crossreact with CFHR1, and determined functional consequences of this. In ELISA, anti-factor H immunoglobulin G (IgG) autoantibodies from 24 atypical HUS patients bound to the short consensus repeat 20 domain of factor H, 21 antibodies also recognized CFHR1, but none CFHR3. Three patients also had anti-factor H IgA autoantibodies crossreacting with CFHR1. Analysis of the IgG fractions in CFHR1-deficient patients found that CFHR1-IgG complexes were formed during plasma exchange treatment, indicating that autoantibodies recognize CFHR1 in vivo. Recombinant CFHR1 prevented hemolysis of sheep erythrocytes caused by patient plasma containing anti-factor H IgG, but it did not inhibit red cell lysis caused by a factor H mutation (W1183 L) in the short consensus repeat 20 domain. Thus, exogenous CFHR1 provided during plasma exchange therapy may neutralize anti-factor H autoantibodies and help in the treatment of autoimmune atypical HUS

The major autoantibody epitope on factor H in atypical hemolytic uremic syndrome is structurally different from its homologous site in factor H-related protein 1, supporting a novel model for induction of autoimmunity in this disease

Atypical hemolytic uremic syndrome (aHUS) is characterized by complement attack against host cells due to mutations in complement proteins or autoantibodies against complement factor H (CFH). It is unknown why nearly all patients with autoimmune aHUS lack CFHR1 (CFH-related protein-1). These patients have autoantibodies against CFH domains 19 and 20 (CFH19-20), which are nearly identical to CFHR1 domains 4 and 5 (CFHR14-5). Here, binding site mapping of autoantibodies from 17 patients using mutant CFH19-20 constructs revealed an autoantibody epitope cluster within a loop on domain 20, next to the two buried residues that are different in CFH19-20 and CFHR14-5. The crystal structure of CFHR14-5 revealed a difference in conformation of the autoantigenic loop in the C-terminal domains of CFH and CFHR1, explaining the variation in binding of autoantibodies from some aHUS patients to CFH19-20 and CFHR14-5. The autoantigenic loop on CFH seems to be generally flexible, as its conformation in previously published structures of CFH19-20 bound to the microbial protein OspE and a sialic acid glycan is somewhat altered. Cumulatively, our data suggest that association of CFHR1 deficiency with autoimmune aHUS could be due to the structural difference between CFHR1 and the autoantigenic CFH epitope, suggesting a novel explanation for CFHR1 deficiency in the pathogenesis of autoimmune aHUS

C3 glomerulopathy-associated CFHR1 mutation alters FHR oligomerization and complement regulation

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

Genetics and complement in atypical HUS

Central to the pathogenesis of atypical hemolytic uremic syndrome (aHUS) is over-activation of the alternative pathway of complement. Following the initial discovery of mutations in the complement regulatory protein, factor H, mutations have been described in factor I, membrane cofactor protein and thrombomodulin, which also result in decreased complement regulation. Autoantibodies to factor H have also been reported to impair complement regulation in aHUS. More recently, gain of function mutations in the complement components C3 and Factor B have been seen. This review focuses on the genetic causes of aHUS, their functional consequences, and clinical effect

Actualización en síndrome hemolítico urémico atípico: diagnóstico y tratamiento. Documento de consenso. Revisión

Podeu consultar la versió en castellà del document a: http://dx.doi.org/10.1016/j.nefro.2015.07.005Haemolytic uraemic syndrome (HUS) is a clinical entity defined as the triad of nonimmune haemolytic anaemia, thrombocytopenia, and acute renal failure, in which the underlying lesions are mediated by systemic thrombotic microangiopathy (TMA). Different causes can induce the TMA process that characterises HUS. In this document we consider atypical HUS (aHUS) a sub-type of HUS in which the TMA phenomena are the consequence of the endotelial damage in the microvasculature of the kidneys and other organs due to a disregulation of the activity of the complement system. In recent years, a variety of aHUs-related mutations have been identified in genes of the complement system, which can explain approximately 60% of the aHUS cases, and a number of mutations and polymorphisms have been functionally characterised. These findings have stablished that aHUS is a consequence of the insufficient regulation of the activation of the complement on cell surfaces, leading to endotelial damage mediated by C5 and the complement terminal pathway. Eculizumab is a monoclonal antibody that inhibits the activation of C5 and blocks the generation of the pro-inflammatory molecule C5a and the formation of the cell membrane attack complex. In prospective studies in patients with aHUS, the use of Eculizumab has shown a fast and sustained interruption of the TMA process and it has been associated with significative long-term improvements in renal function, the interruption of plasma therapy and important reductions in the need of dialysis. According to the existing literature and the accumulated clinical experience, the Spanish aHUS Group published a consensus document with recommendations for the treatment of aHUs (Nefrologia 2013;33[1]:27-45). In the current online version of this document, we update the aetiological classification of TMAs, the pathophysiology of aHUS, its differential diagnosis and its therapeutic management

Atypical Hemolytic Uremic Syndrome

SummaryHemolytic uremic syndrome (HUS) is a triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. The atypical form of HUS is a disease characterized by complement overactivation. Inherited defects in complement genes and acquired autoantibodies against complement regulatory proteins have been described. Incomplete penetrance of mutations in all predisposing genes is reported, suggesting that a precipitating event or trigger is required to unmask the complement regulatory deficiency. The underlying genetic defect predicts the prognosis both in native kidneys and after renal transplantation. The successful trials of the complement inhibitor eculizumab in the treatment of atypical HUS will revolutionize disease management

aHUS caused by complement dysregulation: new therapies on the horizon

Atypical hemolytic uremic syndrome (aHUS) is a heterogeneous disease that is caused by defective complement regulation in over 50% of cases. Mutations have been identified in genes encoding both complement regulators [complement factor H (CFH), complement factor I (CFI), complement factor H-related proteins (CFHR), and membrane cofactor protein (MCP)], as well as complement activators [complement factor B (CFB) and C3]. More recently, mutations have also been identified in thrombomodulin (THBD), an anticoagulant glycoprotein that plays a role in the inactivation of C3a and C5a. Inhibitory autoantibodies to CFH account for an additional 5–10% of cases and can occur in isolation or in association with mutations in CFH, CFI, CFHR 1, 3, 4, and MCP. Plasma therapies are considered the mainstay of therapy in aHUS secondary to defective complement regulation and may be administered as plasma infusions or plasma exchange. However, in certain cases, despite initiation of plasma therapy, renal function continues to deteriorate with progression to end-stage renal disease and renal transplantation. Recently, eculizumab, a humanized monoclonal antibody against C5, has been described as an effective therapeutic strategy in the management of refractory aHUS that has failed to respond to plasma therapy. Clinical trials are now underway to further evaluate the efficacy of eculizumab in the management of both plasma-sensitive and plasma-resistant aHUS

Factor H-related proteins determine complement-activating surfaces.

Complement factor H-related proteins (FHRs) are strongly associated with different diseases involving complement dysregulation, which suggests a major role for these proteins regulating complement activation. Because FHRs are evolutionarily and structurally related to complement inhibitor factor H (FH), the initial assumption was that the FHRs are also negative complement regulators. Whereas weak complement inhibiting activities were originally reported for these molecules, recent developments indicate that FHRs may enhance complement activation, with important implications for the role of these proteins in health and disease. We review these findings here, and propose that FHRs represent a complex set of surface recognition molecules that, by competing with FH, provide improved discrimination of self and non-self surfaces and play a central role in determining appropriate activation of the complement pathway

Genetic insights into age-related macular degeneration: Controversies addressing risk, causality, and therapeutics

Age-related macular degeneration (AMD) is a common condition among the elderly population that leads to the progressive central vision loss and serious compromise of quality of life for its sufferers. It is also one of the few disorders for whom the investigation of its genetics has yielded rich insights into its diversity and causality and holds the promise of enabling clinicians to provide better risk assessments for individuals as well as to develop and selectively deploy new therapeutics to either prevent or slow the development of disease and lessen the threat of vision loss. The genetics of AMD began initially with the appreciation of familial aggregation and increase risk and expanded with the initial association of APOE variants with the disease. The first major breakthroughs came with family-based linkage studies of affected (and discordant) sibs, which identified a number of genetic loci and led to the targeted search of the 1q31 and 10q26 loci for associated variants. Three of the initial four reports for the CFH variant, Y402H, were based on regional candidate searches, as were the two initial reports of the ARMS2/HTRA1 locus variants. Case-control association studies initially also played a role in discovering the major genetic variants for AMD, and the success of those early studies have been used to fuel enthusiasm for the methodology for a number of diseases. Until 2010, all of the subsequent genetic variants associated with AMD came from candidate gene testing based on the complement factor pathway. In 2010, several large-scale genome-wide association studies (GWAS) identified genes that had not been previously identified. Much of this historical information is available in a number of recent reviews.(Chen et al., 2010b; Deangelis et al., 2011; Fafowora and Gorin, 2012b; Francis and Klein, 2011; Kokotas et al., 2011) Large meta analysis of AMD GWAS has added new loci and variants to this collection.(Chen et al., 2010a; Kopplin et al., 2010; Yu et al., 2011) This paper will focus on the ongoing controversies that are confronting AMD genetics at this time, rather than attempting to summarize this field, which has exploded in the past 5 years