Spontaneous hemolytic uremic syndrome triggered by complement factor H lacking surface recognition domains

Association of genetic variants in complement factor H and factor H-related genes with systemic lupus erythematosus susceptibility

Systemic lupus erythematosus (SLE), a complex polygenic autoimmune disease, is associated with increased complement activation. Variants of genes encoding complement regulator factor H (CFH) and five CFH-related proteins (CFHR1-CFHR5) within the chromosome 1q32 locus linked to SLE, have been associated with multiple human diseases and may contribute to dysregulated complement activation predisposing to SLE. We assessed 60 SNPs covering the CFH-CFHRs region for association with SLE in 15,864 case-control subjects derived from four ethnic groups. Significant allelic associations with SLE were detected in European Americans (EA) and African Americans (AA), which could be attributed to an intronic CFH SNP (rs6677604, in intron 11, P_meta = 6.6×10⁻⁸, OR = 1.18) and an intergenic SNP between CFHR1 and CFHR4 (rs16840639, P_meta = 2.9×10⁻⁷, OR = 1.17) rather than to previously identified disease-associated CFH exonic SNPs, including I62V, Y402H, A474A, and D936E. In addition, allelic association of rs6677604 with SLE was subsequently confirmed in Asians (AS). Haplotype analysis revealed that the underlying causal variant, tagged by rs6677604 and rs16840639, was localized to a ∼146 kb block extending from intron 9 of CFH to downstream of CFHR1. Within this block, the deletion of CFHR3 and CFHR1 (CFHR3-1Δ), a likely causal variant measured using multiplex ligation-dependent probe amplification, was tagged by rs6677604 in EA and AS and rs16840639 in AA, respectively. Deduced from genotypic associations of tag SNPs in EA, AA, and AS, homozygous deletion of CFHR3-1Δ (P_meta = 3.2×10⁻⁷, OR = 1.47) conferred a higher risk of SLE than heterozygous deletion (P_meta = 3.5×10⁻⁴, OR = 1.14). These results suggested that the CFHR3-1Δ deletion within the SLE-associated block, but not the previously described exonic SNPs of CFH, might contribute to the development of SLE in EA, AA, and AS, providing new insights into the role of complement regulators in the pathogenesis of SLE.

Systemic lupus erythematosus (SLE) is a complex autoimmune disease, associated with increased complement activation. Previous studies have provided evidence for the presence of SLE susceptibility gene(s) in the chromosome 1q31-32 locus. Within 1q32, genes encoding complement regulator factor H (CFH) and five CFH-related proteins (CFHR1-CFHR5) may contribute to the development of SLE, because genetic variants of these genes impair complement regulation and predispose to various human diseases. In this study, we tested association of genetic variants in the region containing CFH and CFHRs with SLE. We identified genetic variants predisposing to SLE in European American, African American, and Asian populations, which might be attributed to the deletion of CFHR3 and CFHR1 genes but not previously identified disease-associated exonic variants of CFH. This study provides the first evidence for consistent association between CFH/CFHRs and SLE across multi-ancestral SLE datasets, providing new insights into the role of complement regulators in the pathogenesis of SLE.

Common polymorphisms in C3, factor B, and factor H collaborate to determine systemic complement activity and disease risk

Common polymorphisms in complement alternative pathway (AP) proteins C3 (C3(R102G)), factor B (fB(R32Q)), and factor H (fH(V62I)) are associated with age-related macular degeneration (AMD) and other pathologies. Our published work showed that fB(R32Q) influences C3 convertase formation, whereas fH(V62I) affects factor I cofactor activity. Here we show how C3(R102G) (C3S/F) influences AP activity. In hemolysis assays, C3(102G) activated AP more efficiently (EC(50) C3(102G): 157 nM; C3(102R): 191 nM; P < 0.0001). fB binding kinetics and convertase stability were identical, but native and recombinant fH bound more strongly to C3b(102R) (K(D) C3b(102R): 1.0 μM; C3b(102G): 1.4 μM; P < 0.0001). Accelerated decay was unaltered, but fH cofactor activity was reduced for C3b(102G), favoring AP amplification. Combining disease "risk" variants (C3(102G), fB(32R), and fH(62V)) in add-back assays yielded sixfold higher hemolytic activity compared with "protective" variants (C3(102R), fB(32Q), and fH(62I); P < 0.0001). These data introduce the concept of a functional complotype (combination of polymorphisms) defining complement activity in an individual, thereby influencing susceptibility to AP-driven disease.

Binding of factor H to tubular epithelial cells limits interstitial complement activation in ischemic injury

Factor H is a regulator of the alternative pathway of complement, and genetic studies have shown that patients with mutations in factor H are at increased risk for several types of renal disease. Pathogenic activation of the alternative pathway in acquired diseases, such as ischemic acute kidney injury, suggests that native factor H has a limited capacity to control the alternative pathway in the kidney. Here we found that an absolute deficiency of factor H produced by gene deletion prevented complement activation on tubulointerstitial cells after ischemia/reperfusion (I/R) injury, likely because alternative pathway proteins were consumed in the fluid phase. In contrast, when fluid-phase regulation by factor H was maintained while the interaction of factor H with cell surfaces was blocked by a recombinant inhibitor protein, complement activation after renal I/R increased. Finally, a recombinant form of factor H, specifically targeted to sites of C3 deposition, reduced complement activation in the tubulointerstitium after ischemic injury. Thus, although factor H does not fully prevent activation of the alternative pathway of complement on ischemic tubules, its interaction with the tubule epithelial cell surface is critical for limiting complement activation and attenuating renal injury after ischemia.

Advances in our understanding of the pathogenesis of glomerular thrombotic microangiopathy

Glomerular thrombotic microangiopathy is a hallmark feature of haemolytic uraemic syndrome, the leading cause of acute renal failure in childhood. This paper is a review of the different mechanistic pathways that lead to this histological picture in the kidney. It will focus on atypical HUS and complement dysregulation, but will also highlight some other recent advances in our understanding of this condition, including the potential role of the molecule vascular endothelial growth factor-A (VEGF-A).

[Atypical hemolytic-uremic syndrome related to abnormalities within the complement system]

Hemolytic uremic syndrome (HUS) is a thrombotic microangiopathy (TMA) disorder characterised by the association of haemolytic anaemia, thrombocytopenia and acute renal failure. Atypical forms (non-shigatoxin related forms) may be familial or sporadic, frequently with relapses and most of them lead to end stage renal failure. During the last years, different groups have demonstrated genetic predisposition to atypical HUS (aHUS) involving five genes encoding for complement components which play a role in the activation or control of the alternative pathway: encoding factor H (CFH), accounting for 30% of aHUS; CD46 (encoding membrane cofactor protein [MCP]) accounting for approximately 10% of aHUS; CFI (encoding factor I) accounting for an estimated 5-15% of patients; C3 (encoding C3) accounting for approximately 10% of aHUS; and rarely CFB (encoding factor B). Predisposition to aHUS is inherited with incomplete penetrance. It is admitted that mutations confer a predisposition to develop aHUS rather than directly causing the disease and that a second event (genetic or environmental) is required for disease manifestation. HUS onset follows a triggering event in most cases (frequently banal seasonal infection and pregnancy). Uncontrolled C3 convertase leads to increased deposition of C3b on vascular endothelium and participates to the prothrombotic state. The phenotype of aHUS is variable ranging from mild forms, with complete recovery of renal function to severe forms with end stage renal disease within the first year after the onset. Overall, the outcome is severe with a mortality rate of 10% and with more than 60% of patients on dialysis. The most severe prognosis was in the CFH mutation group. There is a high risk of recurrence of the disease after renal transplantation in patients with mutations in CFH, CFI, CFB and C3. Plasma therapy may allow complete haematological remission but frequently with persistent renal damage. Some patients are plasma resistant and some are plasma dependent. The recent progress in the determination of the susceptibility factors for aHUS, have allowed to propose new diagnostic tests including a molecular genetic testing and may permit to consider some new specific treatments in this disease (human plasma-derived CFH or complement inhibitors).

Review: Complement and its regulatory proteins in kidney diseases

Complement is a part of the body's innate immune system that helps defend the host from microbial infection. It is tightly controlled by a number of cell surface and fluid-phase proteins so that under normal circumstances injury to autologous tissues is avoided. In many pathological settings, such as when the complement regulatory mechanisms are dysfunctional or overwhelmed, complement attack of autologous tissues can occur with severe, sometimes life-threatening consequences. The kidney appears to be particularly vulnerable to complement-mediated inflammatory injury and many kidney pathologies have been linked to abnormal complement activation. Clinical and experimental studies have shown that complement attack can be a primary cause in rare, genetically predisposed kidney diseases or a significant contributor to kidney injury caused by other etiological factors. Here we provide a brief review of recent advances on the activation and regulation of the complement system in kidney disease, with a particular emphasis on the relevance of complement regulatory proteins.

Disease-associated N-terminal complement factor H mutations perturb cofactor and decay-accelerating activities

Many mutations associated with atypical hemolytic uremic syndrome (aHUS) lie within complement control protein modules 19-20 at the C terminus of the complement regulator factor H (FH). This region mediates preferential action of FH on self, as opposed to foreign, membranes and surfaces. Hence, speculation on disease mechanisms has focused on deficiencies in regulation of complement activation on glomerular capillary beds. Here, we investigate the consequences of aHUS-linked mutations (R53H and R78G) within the FH N-terminal complement control protein module that also carries the I62V variation linked to dense-deposit disease and age-related macular degeneration. This module contributes to a four-module C3b-binding site (FH1-4) needed for complement regulation and sufficient for fluid-phase regulatory activity. Recombinant FH1-4(V62) and FH1-4(I62) bind immobilized C3b with similar affinities (K(D) = 10-14 μM), whereas FH1-4(I62) is slightly more effective than FH1-4(V62) as cofactor for factor I-mediated cleavage of C3b. The mutant (R53H)FH1-4(V62) binds to C3b with comparable affinity (K(D) ∼12 μM) yet has decreased cofactor activities both in fluid phase and on surface-bound C3b, and exhibits only weak decay-accelerating activity for C3 convertase (C3bBb). The other mutant, (R78G)FH1-4(V62), binds poorly to immobilized C3b (K(D) >35 μM) and is severely functionally compromised, having decreased cofactor and decay-accelerating activities. Our data support causal links between these mutations and disease; they demonstrate that mutations affecting the N-terminal activities of FH, not just those in the C terminus, can predispose to aHUS. These observations reinforce the notion that deficiency in any one of several FH functional properties can contribute to the pathogenesis of this disease.

Atypical hemolytic uremic syndrome, genetic basis, and clinical manifestations

Atypical hemolytic uremic syndrome (aHUS) is now well recognized to be a disease characterized by excessive complement activation in the microvasculature. In both the familial and sporadic forms, inherited and acquired abnormalities affecting components of the alternative complement pathway are found in ~ 60% of patients. These include mutations in the genes encoding both complement regulators (factor H, factor I, membrane cofactor protein, and thrombomodulin) and activators (factors B and C3) and autoantibodies against factor H. Multiple hits are necessary for the disease to manifest, including a trigger, mutations, and at-risk haplotypes in complement genes. The prognosis for aHUS is poor, with most patients developing end-stage renal failure. Renal transplantation in most patients also has a poor prognosis, with frequent loss of the allograft to recurrent disease. However, improving results with combined liver-kidney transplantation and the advent of complement inhibitors such as eculizumab offer hope that the prognosis for aHUS will improve in future years.

The development of atypical hemolytic uremic syndrome depends on complement C5

Gene variants in the alternative pathway of the complement system strongly associate with atypical hemolytic uremic syndrome (aHUS), presumably by predisposing to increased complement activation within the kidney. Complement factor H (CFH) is the major regulator of complement activation through the alternative pathway. Factor H-deficient mice transgenically expressing a mutant CFH protein (Cfh(-/-).FHΔ16-20) that functionally mimics the CFH mutations reported in aHUS patients spontaneously develop thrombotic microangiopathy. To investigate the role of complement C5 activation in this aHUS model, we generated C5-deficient Cfh(-/-).FHΔ16-20 mice. Both C5-sufficient and C5-deficient Cfh(-/-).FHΔ16-20 mice had abnormal C3 deposition within the kidney, but spontaneous aHUS did not develop in any of the C5-deficient mice. Furthermore, although Cfh(-/-).FHΔ16-20 animals demonstrated marked hypersensitivity to experimentally triggered renal injury, animals with concomitant C5 deficiency did not. These data demonstrate a critical role for C5 activation in both spontaneous aHUS and experimentally triggered renal injury in animals with defective complement factor H function. This study provides a rationale to investigate therapeutic inhibition of C5 in human aHUS.

Escherichia coli Shiga Toxin Mechanisms of Action in Renal Disease

Shiga toxin-producing Escherichia coli is a contaminant of food and water that in humans causes a diarrheal prodrome followed by more severe disease of the kidneys and an array of symptoms of the central nervous system. The systemic disease is a complex referred to as diarrhea-associated hemolytic uremic syndrome (D⁺HUS). D⁺HUS is characterized by thrombocytopenia, microangiopathic hemolytic anemia, and acute renal failure. This review focuses on the renal aspects of D⁺HUS. Current knowledge of this renal disease is derived from a combination of human samples, animal models of D⁺HUS, and interaction of Shiga toxin with isolated renal cell types. Shiga toxin is a multi-subunit protein complex that binds to a glycosphingolipid receptor, Gb3, on select eukaryotic cell types. Location of Gb3 in the kidney is predictive of the sites of action of Shiga toxin. However, the toxin is cytotoxic to some, but not all cell types that express Gb3. It also can cause apoptosis or generate an inflammatory response in some cells. Together, this myriad of results is responsible for D⁺HUS disease.

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.

Lessons from functional and structural analyses of disease-associated genetic variants in the complement alternative pathway

Complement is an essential component of innate immunity and a major trigger of inflammatory responses. A critical step in complement activation is the formation of the C3 convertase of the alternative pathway (AP), a labile bimolecular complex formed by activated fragments of the C3 and factor B components that is fundamental to provide exponential amplification of the initial complement trigger. Regulation of the AP C3 convertase is essential to maintain complement homeostasis in plasma and to protect host cells and tissues from damage by complement. During the last decade, several studies have associated genetic variations in components and regulators of the AP C3 convertase with a number of chronic inflammatory diseases and susceptibility to infection. The functional characterization of these protein variants has helped to decipher the critical pathogenic mechanisms involved in some of these complement related disorders. In addition, these functional data together with recent 3D structures of the AP C3 convertase have provided fundamental insights into the assembly, activation and regulation of the AP C3 convertase.

Identification of a mutation in complement factor H-related protein 5 in patients of Cypriot origin with glomerulonephritis

BACKGROUND

Complement is a key component of the innate immune system, and variation in genes that regulate its activation is associated with renal and other disease. We aimed to establish the genetic basis for a familial disorder of complement regulation associated with persistent microscopic haematuria, recurrent macroscopic haematuria, glomerulonephritis, and progressive renal failure.

METHODS

We sought patients from the West London Renal and Transplant Centre (London, UK) with unusual renal disease and affected family members as a method of identification of new genetic causes of kidney disease. Two families of Cypriot origin were identified in which renal disease was consistent with autosomal dominant transmission and renal biopsy of at least one individual showed C3 glomerulonephritis. A mutation was identified via a genome-wide linkage study and candidate gene analysis. A PCR-based diagnostic test was then developed and used to screen for the mutation in population-based samples and in individuals and families with renal disease.

FINDINGS

Occurrence of familial renal disease cosegregated with the same mutation in the complement factor H-related protein 5 gene (CFHR5). In a cohort of 84 Cypriots with unexplained renal disease, four had mutation in CFHR5. Overall, we identified 26 individuals with the mutation and evidence of renal disease from 11 ostensibly unrelated kindreds, including the original two families. A mutant CFHR5 protein present in patient serum had reduced affinity for surface-bound complement. We term this renal disease CFHR5 nephropathy.

INTERPRETATION

CFHR5 nephropathy accounts for a substantial burden of renal disease in patients of Cypriot origin and can be diagnosed with a specific molecular test. The high risk of progressive renal disease in carriers of the CFHR5 mutation implies that isolated microscopic haematuria or recurrent macroscopic haematuria should not be regarded as a benign finding in individuals of Cypriot descent.

FUNDING

UK Medical Research Council and Wellcome Trust.

Complement control protein factor H: the good, the bad, and the inadequate

The complement system is an essential component of the innate immune system that participates in elimination of pathogens and altered host cells and comprises an essential link between the innate and adaptive immune system. Soluble and membrane-bound complement regulators protect cells and tissues from unintended complement-mediated injury. Complement factor H is a soluble complement regulator essential for controlling the alternative pathway in blood and on cell surfaces. Normal recognition of self-cell markers (i.e. polyanions) and C3b/C3d fragments is necessary for factor H function. Inadequate recognition of host cell surfaces by factor H due to mutations and polymorphisms have been associated with complement-mediated tissue damage and disease. On the other hand, unwanted recognition of pathogens and altered self-cells (i.e. cancer) by factor H is used as an immune evasion strategy. This review will focus on the current knowledge related to these versatile recognition properties of factor H.

The complement inhibitors Crry and factor H are critical for preventing autologous complement activation on renal tubular epithelial cells

Congenital and acquired deficiencies of complement regulatory proteins are associated with pathologic complement activation in several renal diseases. To elucidate the mechanisms by which renal tubular epithelial cells (TECs) control the complement system, we examined the expression of complement regulatory proteins by the cells. We found that Crry is the only membrane-bound complement regulator expressed by murine TECs, and its expression is concentrated on the basolateral surface. Consistent with the polarized localization of Crry, less complement activation was observed when the basolateral surface of TECs was exposed to serum than when the apical surface was exposed. Furthermore, greater complement activation occurred when the basolateral surface of TECs from Crry(-/-)fB(-/-) mice was exposed to normal serum compared with TECs from wild-type mice. Complement activation on the apical and basolateral surfaces was also greater when factor H, an alternative pathway regulatory protein found in serum, was blocked from interacting with the cells. Finally, we injected Crry(-/-)fB(-/-) and Crry(+/+)fB(-/-) mice with purified factor B (an essential protein of the alternative pathway). Spontaneous complement activation was seen on the tubules of Crry(-/-)fB(-/-) mice after injection with factor B, and the mice developed acute tubular injury. These studies indicate that factor H and Crry regulate complement activation on the basolateral surface of TECs and that factor H regulates complement activation on the apical surface. However, congenital deficiency of Crry or reduced expression of the protein on the basolateral surface of injured cells permits spontaneous complement activation and tubular injury.

Impaired binding of the age-related macular degeneration-associated complement factor H 402H allotype to Bruch's membrane in human retina

Age-related macular degeneration (AMD) is the predominant cause of blindness in the industrialized world where destruction of the macula, i.e. the central region of the retina, results in loss of vision. AMD is preceded by the formation of deposits in the macula, which accumulate between the Bruch's membrane and the retinal pigment epithelium (RPE). These deposits are associated with complement-mediated inflammation and perturb retinal function. Recent genetic association studies have demonstrated that a common allele (402H) of the complement factor H (CFH) gene is a major risk factor for the development of AMD; CFH suppresses complement activation on host tissues where it is believed to bind via its interaction with polyanionic structures. We have shown previously that this coding change (Y402H; from a tyrosine to histidine residue) alters the binding of the CFH protein to sulfated polysaccharides. Here we demonstrate that the AMD-associated polymorphism profoundly affects CFH binding to sites within human macula. Notably, the AMD-associated 402H variant binds less well to heparan sulfate and dermatan sulfate glycosaminoglycans within Bruch's membrane when compared with the 402Y form; both allotypes exhibit a similar level of binding to the RPE. We propose that the impaired binding of the 402H variant to Bruch's membrane results in an overactivation of the complement pathway leading to local chronic inflammation and thus contributes directly to the development and/or progression of AMD. These studies therefore provide a putative disease mechanism and add weight to the genetic association studies that implicate the 402H allele as an important risk factor in AMD.

Advances in understanding the aetiology of atypical Haemolytic Uraemic Syndrome

Atypical Haemolytic Uraemic Syndrome (aHUS) is a thrombotic microangiopathy that often provokes irreversible renal damage and post-transplantation recurrence. Studies performed during the last decade have shown that 50-60% of aHUS patients present genetic or acquired defects in the complement system that enhance the initial endothelial damage and favour disease development. This review analyses the complement proteins and processes that are disturbed in aHUS patients, and outlines the relevance of a prompt genetic/molecular diagnosis for improving clinical management and prognosis.

Atypical hemolytic uremic syndrome and mutation analysis of factor H gene in two Tunisian families

We carried out a protein and genetic investigation of the factor H gene mutations within two families presenting with a diagnostic suspicion of atypical hemolytic uremic syndrome (aHUS). The results within the patients of the first family revealed a factor H-deficiency. Direct sequencing allowed the detection of a 4-nucleotide deletion in the factor H gene. This deletion was found as the homozygote form in the proband and as the heterozygote form in the parents. Protein and functional analyses of the complement system were normal in all members of the second family. However, the molecular investigation for the father showed the presence of an amino acid substitution in the FH gene. Unfortunately, his two affected children died without being investigated for mutations. The functional consequences of these abnormal proteins are still to be demonstrated.

Both domain 19 and domain 20 of factor H are involved in binding to complement C3b and C3d

Factor H (FH) regulates the alternative pathway of complement in plasma and mediates discrimination of cellular surfaces to alternative pathway activators and non-activators. The carboxyl-terminal domains 19 and 20 of FH are essential in target discrimination and are known to contain binding sites for the C3d part of C3b, heparin, and endothelial cells. Mutations in FH19-20 are frequently found in patients with atypical haemolytic uremic syndrome (aHUS). Most aHUS-associated and some other mutations have been shown to lead to impaired binding to C3d and C3b by the recombinant FH19-20 fragment. Most of these mutated residues, such as R1203, are located close to each other in domain 20 but some, such as Q1139, are located in domain 19. We generated mutant proteins Q1139A and R1203A of FH19-20 and showed that their binding to C3d and C3b was clearly impaired. To show that the effects on C3d/C3b binding are due to direct interactions rather than structural changes, we solved the X-ray crystal structures of the R1203A and Q1139A mutant proteins at 1.65 and 2.0A, respectively. Neither of the mutations caused any overall structural changes in FH19-20. It is thus evident that Q1139 in domain 19 and R1203 in domain 20 are directly involved in binding to the C3d part of C3b and therefore both the domains are involved in the interaction with C3d and C3b. This explains why several aHUS-associated FH mutations are found within domain 19 in addition to domain 20.

Pregnancy-associated hemolytic uremic syndrome revisited in the era of complement gene mutations

In contrast to pregnancy-associated thrombotic thrombocytopenic purpura, the pathogenesis and presentation of pregnancy-associated atypical hemolytic uremic syndrome (P-aHUS) remain ill-defined. We conducted a retrospective study to assess the presentation and outcomes of patients presenting with P-aHUS and the prevalence of alternative C3 convertase dysregulation. P-aHUS occurred in 21 of the 100 adult female patients with atypical HUS, with 79% presenting postpartum. We detected complement abnormalities in 18 of the 21 patients. The outcomes were poor: 62% reached ESRD by 1 month and 76% by last follow-up. The risk for P-aHUS was highest during a second pregnancy. Thirty-five women, 26 (74%) of whom had complement abnormalities, had at least one pregnancy before the onset of a non-pregnancy-related aHUS. Outcomes did not differ between patients with pregnancy-related and non-pregnancy-related aHUS. Mutations in the SCR19-20 domains of factor H were less frequent in P-aHUS patients compared with non-pregnancy-related aHUS. Pregnancies in female patients with complement abnormalities (n = 44) were complicated by fetal loss and preeclampsia in 4.8% and 7.7%, respectively. Better understanding of complement dysregulation in pregnancy complications is essential, especially to guide development of pharmacologic agents to modulate this system.

Association of factor H autoantibodies with deletions of CFHR1, CFHR3, CFHR4, and with mutations in CFH, CFI, CD46, and C3 in patients with atypical hemolytic uremic syndrome

Factor H autoantibodies have been reported in approximately 10% of patients with atypical hemolytic uremic syndrome (aHUS) and are associated with deficiency of factor H-related proteins 1 and 3. In this study we examined the prevalence of factor H autoantibodies in the Newcastle cohort of aHUS patients, determined whether the presence of such autoantibodies is always associated with deficiency of factor H-related proteins 1 and 3, and examined whether such patients have additional susceptibility factors and/or mutations in the genes encoding complement regulator/activators. We screened 142 patients with aHUS and found factor H autoantibodies in 13 individuals (age 1-11 years). The presence of the autoantibodies was confirmed by Western blotting. By using multiplex ligation-dependent probe amplification we measured complement factor H-related (CFHR)1 and CFHR3 copy number. In 10 of the 13 patients there were 0 copies of CFHR1, and in 3 patients there were 2. In 3 of the patients with 0 copies of CFHR1 there was 1 copy of CFHR3, and these individuals exhibited a novel deletion incorporating CFHR1 and CFHR4. In 5 patients mutations were identified: 1 in CFH, 1 in CFI, 1 in CD46, and 2 in C3. The latter observation emphasizes that multiple concurrent factors may be necessary in individual patients for disease manifestation.

Characterization of complement factor H-related (CFHR) proteins in plasma reveals novel genetic variations of CFHR1 associated with atypical hemolytic uremic syndrome

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.

Functional analyses indicate a pathogenic role of factor H autoantibodies in atypical haemolytic uraemic syndrome

BACKGROUND

Atypical haemolytic uraemic syndrome (aHUS) is associated with defective complement regulation. Recently, an autoimmune aHUS form has been described that is associated with complement factor H (CFH) autoantibodies. The aim of this study was to address the pathologic relevance of CFH autoantibodies in aHUS.

METHODS

CFH autoantibodies were identified and antibody levels were analysed in three aHUS patients during the disease course by the ELISA method. Epitope mapping was performed using recombinant factor H fragments and domain-mapped monoclonal antibodies. The effect of the antibodies on cell-protective activity of CFH was measured by haemolytic assays. CFH:autoantibody complexes were analysed by ELISA.

RESULTS

All three autoantibodies bound to the C-terminal domain of CFH, which is essential for CFH binding to cell surfaces. In patient 1, plasma exchanges and immune adsorption temporarily reduced the autoantibody titre and led to temporary clinical improvement. In patient 2, plasma exchanges and long-term immunosuppression strongly reduced the CFH autoantibody level, and induced a stable remission of aHUS. Patient 3 had lower autoantibody levels that decreased during the follow-up and is in good clinical condition. The patients' plasma samples caused enhanced lysis of sheep erythrocytes, and the degree of lysis correlated with the CFH autoantibody titre and the amount of CFH:autoantibody complexes. An addition of purified CFH to aHUS plasma or removal of IgG inhibited the haemolytic activity.

CONCLUSION

These results support a direct role of the autoantibodies in aHUS pathology by inhibiting the regulatory function of CFH at cell surfaces and suggest that reduction of the autoantibody titre is beneficial for the patients.

A large family with a gain-of-function mutation of complement C3 predisposing to atypical hemolytic uremic syndrome, microhematuria, hypertension and chronic renal failure

BACKGROUND AND OBJECTIVES

Atypical hemolytic uremic syndrome (aHUS) is associated with mutations in genes encoding complement-regulatory proteins factor H, I and B and membrane cofactor protein. Recently, heterozygous gain-of-function mutations in the complement C3 gene have been found in patients with aHUS.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

A large family with a C3 R570Q mutation is described. Clinical and laboratory findings of carriers of the mutation and unaffected family members are reported.

RESULTS

The index patient suffered from recurrent aHUS at age 22 and developed end-stage renal failure. Of 24 family members, nine harbored the C3 R570Q mutation. Carriers showed reduced or borderline C3 levels. Arterial hypertension was found in six family members, microhematuria in five and chronic kidney disease stage 3 in two elderly carrier patients. Despite marked consumption of C3, serum terminal complement complex levels were not elevated in carriers compared with other family members.

CONCLUSIONS

The penetrance of the C3 R570Q mutation to induce aHUS is incomplete and lower compared with mutations in other genes predisposing to the disease. The mutation is possibly also associated with hypertension, hematuria and chronic kidney disease, all of which may represent consequences of long-term complement activation in the renal vasculature.

The disease-protective complement factor H allotypic variant Ile62 shows increased binding affinity for C3b and enhanced cofactor activity

Mutations and polymorphisms in the gene encoding factor H (CFH) have been associated with atypical haemolytic uraemic syndrome, dense deposit disease and age-related macular degeneration. The disease-predisposing CFH variants show a differential association with pathology that has been very useful to unravel critical events in the pathogenesis of one or other disease. In contrast, the factor H (fH)-Ile(62) polymorphism confers strong protection to all three diseases. Using ELISA-based methods and surface plasmon resonance analyses, we show here that the protective fH-Ile(62) variant binds more efficiently to C3b than fH-Val(62) and competes better with factor B in proconvertase formation. Functional analyses demonstrate an increased cofactor activity for fH-Ile(62) in the factor I-mediated cleavage of fluid phase and surface-bound C3b; however, the two fH variants show no differences in decay accelerating activity. From these data, we conclude that the protective effect of the fH-Ile(62) variant is due to its better capacity to bind C3b, inhibit proconvertase formation and catalyze inactivation of fluid-phase and surface-bound C3b. This demonstration of the functional consequences of the fH-Ile(62) polymorphism provides relevant insights into the complement regulatory activities of fH that will be useful in disease prediction and future development of effective therapeutics for disorders caused by complement dysregulation.

The binding of factor H to a complex of physiological polyanions and C3b on cells is impaired in atypical hemolytic uremic syndrome

Factor H (fH) is essential for complement homeostasis in fluid-phase and on surfaces. Its two C-terminal domains (CCP 19-20) anchor fH to self-surfaces where it prevents C3b amplification in a process requiring its N-terminal four domains. In atypical hemolytic uremic syndrome (aHUS), mutations clustering toward the C terminus of fH may disrupt interactions with surface-associated C3b or polyanions and thereby diminish the ability of fH to regulate complement. To test this, we compared a recombinant protein encompassing CCP 19-20 with 16 mutants. The mutations had only very limited and localized effects on protein structure. Although we found four aHUS-linked fH mutations that decreased binding to C3b and/or to heparin (a model compound for cell surface polyanionic carbohydrates), we identified five aHUS-associated mutants with increased affinity for either or both ligands. Strikingly, these variable affinities for the individual ligands did not correlate with the extent to which all the aHUS-associated mutants were found to be impaired in a more physiological assay that measured their ability to inhibit cell surface complement functions of full-length fH. Taken together, our data suggest that disruption of a complex fH-self-surface recognition process, involving a balance of affinities for protein and physiological carbohydrate ligands, predisposes to aHUS.

Successful renal transplantation in a patient with atypical hemolytic uremic syndrome carrying mutations in both factor I and MCP

Kidney transplantation in patients with atypical hemolytic uremic syndrome (aHUS) carrying mutations in the soluble complement regulators factor H (CFH) or factor I (CFI) is associated with elevated risk of disease recurrence and almost certain graft loss. In contrast, recurrence is unusual in patients with mutations in the membrane-associated complement regulator membrane cofactor protein (MCP) (CD46). Therefore, a panel of experts recently recommended the combined liver-kidney transplantation to minimize aHUS recurrence in patients with mutations in CFH or CFI. There was, however, very limited information regarding transplantation in patients carrying mutations in both soluble and membrane-associated complement regulators to support a recommendation. Here, we report the case of an aHUS patient with a heterozygous mutation in both CFI and MCP who received an isolated kidney transplant expressing normal MCP levels. Critically, the patient suffered from a severe antibody-mediated rejection that was successfully treated with plasmapheresis and IvIgG. Most important, despite the complement activation in the allograft, there was no evidence of thrombotic microangiopathy, suggesting that the normal MCP levels in the grafted kidney were sufficient to prevent the aHUS recurrence. Our results suggest that isolated kidney transplantation may be a good first option for care in aHUS patients carrying CFI/MCP combined heterozygous mutations.

Complement in human diseases: Lessons from complement deficiencies

Complement deficient cases reported in the second half of the last century have been of great help in defining the role of complement in host defence. Surveys of the deficient individuals have been instrumental in the recognition of the clinical consequences of the deficiencies. This review focuses on the analysis of the diseases associated with the deficiencies of the various components and regulators of the complement system and their therapeutic implications. The diagnostic approach leading to the identification of the deficiency is discussed here as a multistep process that starts with the screening assays and proceeds in specialized laboratories with the characterization of the defect at the molecular level. The organization of a registry of complement deficiencies is presented as a means to collect the cases identified in and outside Europe with the aim to promote joint projects on treatment and prevention of diseases associated with defective complement function.

A novel mutation in the complement regulator clusterin in recurrent hemolytic uremic syndrome

A novel heterozygous mutation in the clusterin gene, nucleotide position A1298C (glutamine>proline Q433P), was detected in exon 7 of a child with recurrent hemolytic uremic syndrome (HUS). The same mutation was found in the child's two siblings and mother but not in 120 controls. In addition, a previously described heterozygous mutation was detected in the gene encoding membrane cofactor protein (MCP) causing a 6 base-pair deletion 811-816delGACAGT in exon 6. It was found in the patient, both siblings and the father. One sibling had recovered from post-streptococcal glomerulonephritis. Clusterin levels in the patient, siblings and parents were normal as was the migration pattern in a gel. Patient serum induced C3 and C9 deposition on normal washed platelets, and platelet activation, as detected by flow cytometry. The same phenomenon was found in serum taken from the siblings and the mother but not in the sample from the father and controls. Addition of clusterin to patient serum did not inhibit complement activation on platelets. The Q433P mutant, in isolated form, was further studied by binding to the components of the terminal complement complex. The mutant did not bind to C5b-7 that was immobilized onto a BIAcore chip, whereas wild-type clusterin did, indicating that the mutation could lead to defective inhibition of formation of the membrane attack complex under these conditions. Hemolysis of rabbit erythrocytes was inhibited by wild-type clusterin but not by the mutant. Mutated clusterin could thus not prevent assembly of the membrane attack complex on platelets and erythrocytes.

Factor H facilitates the clearance of GBM bound iC3b by controlling C3 activation in fluid phase

Dense deposit disease (DDD) is strongly associated with the uncontrolled activation of the complement alternative pathway. Factor H (CFH)-deficient (Cfh^−/−) mice spontaneously develop C3 deposition along the glomerular basement membrane (GBM) with subsequent development of glomerulonephritis with features of DDD, a lesion dependent on C3 activation. In order to understand the role of CFH in preventing renal damage associated with the dysregulation of the alternative pathway we administered purified mouse CFH (mCFH) to Cfh^−/− mice. 24 h following the administration of mCFH we observed an increase in plasma C3 levels with presence of intact C3 in circulation showing that mCFH restored control of C3 activation in fluid phase. mCFH resulted in the reduction of iC3b deposition along the GBM. The exogenous mCFH was readily detectable in plasma but critically not in association with C3 along the GBM. Thus, the reduction in GBM C3 was dependent on the ability of mCFH to regulate C3 activation in plasma. Western blot analysis of glomeruli from Cfh^−/− mice demonstrated the presence of iC3b. Our data show that the C3 along the GBM in Cfh^−/− mice is the C3 fragment iC3b and that this is derived from plasma C3 activation. The implication is that successful therapy of DDD is likely to be achieved by therapies that inhibit C3 turnover in plasma.

Mutations of factor H impair regulation of surface-bound C3b by three mechanisms in atypical hemolytic uremic syndrome

Atypical hemolytic uremic syndrome (aHUS) is a thrombotic microangiopathy associated with mutations in complement proteins, most frequently in the main plasma alternative pathway regulator factor H (FH). The hotspot for the FH mutations is in domains 19-20 (FH19-20) that are indispensable for FH activity on C3b bound covalently to host cells. In aHUS, down-regulation of cell-bound C3b by FH is impaired, but it is not clear whether this is due to an altered FH binding to surface-bound C3b or to cell surface structures. To explore the molecular pathogenesis of aHUS we tested binding of 14 FH19-20 point mutants to C3b and its C3d fragment, mouse glomerular endothelial cells (mGEnC-1), and heparin. The cell binding correlated well, but not fully, with heparin binding and the cell binding site was overlapping but distinct from the C3b/C3d binding site that was shown to extend to domain 19. Our results show that aHUS-associated FH19-20 mutants have different combinations of three primary defects: impaired binding to C3b/C3d, impaired binding to the mGEnC-1 cells/heparin, and, as a novel observation, an enhanced mGEnC-1 cell or heparin binding. We propose a model of the molecular pathogenesis of aHUS where all three mechanisms lead eventually to impaired control of C3b on the endothelial cell surfaces. Based on the results with the aHUS patient mutants and the overlap in FH19-20 binding sites for mGEnC-1/heparin and C3b/C3d we conclude that binding of FH19-20 to C3b/C3d is essential for target discrimination by the alternative pathway.

Polyanion-induced self-association of complement factor H

Factor H is the primary soluble regulator of activation of the alternative pathway of complement. It prevents activation of complement on host cells and tissues upon association with C3b and surface polyanions such as sialic acids, heparin, and other glycosaminoglycans. Here we show that interaction with polyanions causes self-association forming tetramers of the 155,000 Da glycosylated protein. Monomeric human factor H is an extended flexible protein that exhibits an apparent size of 330,000 Da, relative to globular standards, during gel filtration chromatography in the absence of polyanions. In the presence of dextran sulfate (5000 Da) or heparin an intermediate species of apparent m.w. 700,000 and a limit species of m.w. 1,400,000 were observed by gel filtration. Sedimentation equilibrium analysis by analytical ultracentrifugation indicated a monomer Mr of 163,000 in the absence of polyanions and a Mr of 607,000, corresponding to a tetramer, in the presence of less than a 2-fold molar excess of dextran sulfate. Increasing concentrations of dextran sulfate increased binding of factor H to zymosan-C3b 4.5-fold. This result was accompanied by an increase in both the decay accelerating and cofactor activity of factor H on these cells. An expressed fragment encompassing the C-terminal polyanion binding site (complement control protein domains 18-20) also exhibited polyanion-induced self-association, suggesting that the C-terminal ends of factor H mediate self-association. The results suggest that recognition of polyanionic markers on host cells and tissues by factor H, and the resulting regulation of complement activation, may involve formation of dimers and tetramers of factor H.

Initiation of the alternative pathway of murine complement by immune complexes is dependent on N-glycans in IgG antibodies

OBJECTIVE

Collagen antibody-induced arthritis in mice exhibits a requirement for amplification by the alternative pathway of complement. Although the alternative pathway is activated by spontaneous hydrolysis, it is not known whether this pathway can also be initiated directly by IgG antibodies in immune complexes (ICs). IgG lacking terminal sialic acid and galactose (G0 IgG) can activate the lectin pathway of complement, but it is not known if G0 IgG can also activate the classical or alternative pathway. The purpose of this study was to examine the mechanism of initiation of the alternative pathway of complement by ICs.

METHODS

We used adherent ICs containing bovine type II collagen (CII) and 4 monoclonal antibodies (mAb) to CII (adCII-IC). C3 activation was measured in the presence of sera from wild-type C57BL/6 mice or from mice deficient in informative complement components. The mAb were used intact or after enzyme digestion to create G0 IgG or to completely remove the N-glycan.

RESULTS

Both the classical and alternative pathways, but not the lectin pathway, mediated C3 activation induced by the adCII-IC. Mannose inhibited the alternative pathway-mediated C3 activation but had no effect on the classical pathway, and N-glycans in IgG were required by the alternative pathway but not the classical pathway. Both the classical and alternative pathways mediated C3 activation induced by G0 IgG. Mannose-binding lectin bound avidly to G0 IgG, but lectin pathway-mediated C3 activation was only slightly increased by G0 IgG.

CONCLUSION

The alternative pathway of complement is capable of initiating C3 activation induced by adCII-IC and requires the presence of N-glycans on the IgG. G0 IgG activates both the classical and alternative pathways more strongly than the lectin pathway.

Factor H family proteins and human diseases

Complement is a major defense system of innate immunity and aimed to destroy microbes. One of the central complement regulators is factor H, which belongs to a protein family that includes CFHL1 and five factor H-related (CFHR) proteins. Recent evidence shows that factor H family proteins (factor H and CFHRs) are associated with diverse and severe human diseases and are also used by human pathogenic microbes for complement evasion. Therefore, dissecting the exact functions of the individual CFHR proteins will provide insights into the pathophysiology of such inflammatory and infectious diseases and will define the therapeutic potential of these proteins.

A novel non-synonymous polymorphism (p.Arg240His) in C4b-binding protein is associated with atypical hemolytic uremic syndrome and leads to impaired alternative pathway cofactor activity

Atypical hemolytic uremic syndrome (aHUS) is a disorder characterized by hemolytic anemia, thrombocytopenia, and acute renal failure. Mutations, polymorphisms, and copy number variation in complement factors and inhibitors are associated with aHUS. In this study, we report the first functional non-synonymous polymorphism in the complement inhibitor C4b-binding protein (C4BP) alpha-chain (c.719G>A; p.Arg240His), which is associated with aHUS. This heterozygous change was found in 6/166 aHUS patients compared with 5/542 normal (chi2 = 6.021; p = 0.014), which was replicated in a second cohort of aHUS patients in which we found 5/170 carriers. The polymorphism does not decrease expression efficiency of C4BP. p.Arg240His is equally efficient as the wild type in binding and supporting degradation of C4BP but its ability to bind C3b and act as cofactor to its degradation both in fluid phase and on surfaces is impaired. This observation supports the hypothesis that dysregulation of the alternative pathway of complement is pivotal for aHUS. Three of the patients carry also mutations in membrane cofactor protein and factor H strengthening the hypothesis that individuals may carry multiple susceptibility factors with an additive effect on the risk of developing aHUS.

Complement regulatory genes and hemolytic uremic syndromes

Hemolytic uremic syndrome is a triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. It is one of a group of conditions termed the thrombotic microangiopathies, which are characterized by prominent endothelial cell injury. It may be diarrheal-associated or atypical (aHUS). Evidence for a pathogenic role of the alternative pathway of complement was first suggested in 1974. Mutations in the complement regulatory proteins factor H, membrane cofactor protein (CD46), and factor I predispose to aHUS development. Mutations of the activating components factor B and complement C3 have also been reported. Penetrance is approximately 50%, suggesting other genetic and environmental modifiers are needed for disease expression. Identification of mutations is important owing to differences in mortality, renal survival, and outcome of renal transplantation. Current treatment is plasma infusion/exchange, but complement inhibitor therapy provides hope for the future.

Mutations in proteins of the alternative pathway of complement and the pathogenesis of atypical hemolytic uremic syndrome

Atypical hemolytic uremic syndrome is associated with mutations in the complement proteins factor H, factor I, factor B, C3, or membrane cofactor protein in about 50% of patients. The evolution and prognosis of the disease in patients carrying mutations in factor H is particularly poor, and renal transplantation most often fails because of recurrence of the disease in the graft. The risk of rapid loss of renal function in patients with functional mutations in factor H requires that effective treatment be initiated as soon as possible, but identification of these patients relies on genetic studies that are time consuming. We describe a case in which an in vitro hemolytic assay proved useful for rapidly assessing factor H dysfunction and for testing whether this dysfunction could be corrected with fresh frozen plasma. In the context of this case, we summarize recent advances in understanding the molecular mechanisms contributing to atypical hemolytic uremic syndrome, including descriptions of DNA- and protein-based analysis. We conclude that functional analysis of factor H should help rationalize the plasma treatment of patients with atypical hemolytic uremic syndrome.

Association of a factor H mutation with hemolytic uremic syndrome following a diarrheal illness

Hemolytic uremic syndrome (HUS) takes 2 forms: diarrheal HUS and nondiarrheal HUS. As its name suggests, diarrheal HUS classically follows an enteric infection. The classic infective organism is the Escherichia coli O157 serotype, although other bacteria, including Shigella species, can produce the verocytotoxin required to cause HUS. The usual clinical course is an episode of bloody diarrhea followed by thrombotic microangiopathy and acute renal failure. Supportive treatment sees recovery of renal function in the vast majority of patients. Most cases occur in children, but all age groups can be affected. Conversely, nondiarrheal HUS may have one of a number of predisposing factors, including drugs, irradiation, and hypertension. It also is well established that mutations in the genes encoding the complement regulator proteins factor H, factor I, and membrane cofactor protein predispose to nondiarrheal HUS. In patients with nondiarrheal HUS, recovery of renal function is much less common. Here, we present a case of HUS after a diarrheal illness in which the patient did not recover renal function in the long term. A novel mutation in exon 23 of the factor H gene was discovered. This is clinically important. If this patient underwent transplantation, he would be expected to have an 80% risk of graft loss at 2 years because of recurrent HUS. We recommend consideration of complement gene mutations in any patient with HUS after a diarrheal episode in which there are unusual features.

Complement factor H polymorphisms, renal phenotypes and age-related macular degeneration: the Blue Mountains Eye Study

Complement factor H (CFH) is a key regulator of the alternative pathway of complement and its mutations have been associated with membranoproliferative glomerulonephritis type II, atypical hemolytic uremic syndrome and age-related macular degeneration (AMD), suggesting that alternative pathway dysregulation is a common pathogenetic feature of these ocular and renal conditions. In this study we tested the hypothesis that common CFH variants have a global role in renal function in the Australian population-based Blue Mountains Eye Study (BMES). We replicated the association of I62V with estimated glomerular filtration rate (GFR; P=0.017) and creatinine clearance (CRCL; P=0.015). The minor allele of I62V (G) was deleterious: adding one copy of the G allele decreased GFR/CRCL by approximately 0.98 ml min(-1) per 1.73 m(2) (95% confidence interval (CI): 0.97, 0.99). We also replicated the association of Y402H with AMD and provided an unbiased estimate of population attributable risk (PAR). The minor allele of Y402H (C) was deleterious: the odds ratio estimate of CC genotype compared to TT was 1.87 (95% CI: 1.44, 2.45). The PAR of the C allele was estimated as 0.22 (95% CI: 0.15, 0.28). In summary, in the BMES population we confirmed the association between I62V and renal function, as measured by the estimated GFR, plus the association of Y402H with both early- and late-stage AMD.

Translational mini-review series on complement factor H: structural and functional correlations for factor H

The 155-kDa glycoprotein, complement factor H (CFH), is a regulator of complement activation that is abundant in human plasma. Three-dimensional structures of over half the 20 complement control protein (CCP) modules in CFH have been solved in the context of single-, double- and triple-module segments. Proven binding sites for C3b occupy the N and C termini of this elongated molecule and may be brought together by a bend in CFH mediated by its central CCP modules. The C-terminal CCP 20 is key to the ability of the molecule to adhere to polyanionic markers on self-surfaces where CFH acts to regulate amplification of the alternative pathway of complement. The surface patch on CCP 20 that binds to model glycosaminoglycans has been mapped using nuclear magnetic resonance (NMR), as has a second glycosaminoglycan-binding patch on CCP 7. These patches include many of the residue positions at which sequence variations have been linked to three complement-mediated disorders: dense deposit disease, age-related macular degeneration and atypical haemolytic uraemic syndrome. In one plausible model, CCP 20 anchors CFH to self-surfaces via a C3b/polyanion composite binding site, CCP 7 acts as a 'proof-reader' to help discriminate self- from non-self patterns of sulphation, and CCPs 1-4 disrupt C3/C5 convertase formation and stability.