Factor H mutations in hemolytic uremic syndrome cluster in exons 18-20, a domain important for host cell recognition

Access to the complement factor B scissile bond is facilitated by association of factor B with C3b protein

Factor B is a zymogen that carries the catalytic site of the complement alternative pathway C3 convertase. During convertase assembly, factor B associates with C3b and Mg(2+) forming a pro-convertase C3bB(Mg(2+)) that is cleaved at a single factor B site by factor D. In free factor B, a pair of salt bridges binds the Arg(234) side chain to Glu(446) and to Glu(207), forming a double latch structure that sequesters the scissile bond (between Arg(234) and Lys(235)) and minimizes its unproductive cleavage. It is unknown how the double latch is released in the pro-convertase. Here, we introduce single amino acid substitutions into factor B that preclude one or both of the Arg(234) salt bridges, and we examine their impact on several different pro-convertase complexes. Our results indicate that loss of the Arg(234)-Glu(446) salt bridge partially stabilizes C3bB(Mg(2+)). Loss of the Arg(234)-Glu(207) salt bridge has lesser effects. We propose that when factor B first associates with C3b, it bears two intact Arg(234) salt bridges. The complex rapidly dissociates unless the Arg(234)-Glu(446) salt bridge is released whereupon conformational changes occur that activate the metal ion-dependent adhesion site and partially stabilize the complex. The remaining salt bridge is then released, exposing the scissile bond and permitting factor D cleavage.

Crystallographic determination of the disease-associated T1184R variant of complement regulator factor H

The soluble 155 kDa glycoprotein factor H (FH) protects host tissue from damage by the human complement system. It accelerates decay of the alternative-pathway C3 convertase, C3bBb, and is a cofactor for factor I-mediated cleavage of the opsonin C3b. Numerous mutations and single-nucleotide polymorphisms (SNPs) occur in the gene encoding FH and the resulting missense mutations and truncation products result in altered functionality that predisposes to the development of the serious renal condition atypical haemolytic uraemic syndrome (aHUS). Other polymorphisms are linked to membranoproliferative glomerulonephritis and macular degeneration. The two C-terminal modules of FH (FH19-20) harbour numerous aHUS-associated mutations that disrupt the ability of factor H to protect host cells from complement-mediated damage. In this work, the crystal structure of an aHUS-associated T1184R variant of FH19-20 at a resolution of 1.52 Å is described. It is shown that this mutation has negligible structural effects but causes a significant change in the electrostatic surface of these two domains. Mechanisms are discussed by which this mutation may alter FH-ligand interactions, particularly with regard to the extension of a region of this molecule within module 20 that has been associated with the binding of glycosaminoglycans (GAGs) or sialic acid residues.

Do complement factor H 402Y and C7 M allotypes predispose to (typical) haemolytic uraemic syndrome?

Typical haemolytic uraemic syndrome (HUS) is mainly caused by infections with enterohaemorrhagic Escherichia coli, whereas in atypical, nonbacteria-associated HUS, complement plays a dominant role. Recently, complement has also been shown to be involved in typical HUS. In this study, mostly weakly significant associations with homozygosities of complement allotype C7 M and inversely with factor H 402H were found, suggesting that 402Y and C7 M allotypes predispose to (typical) haemolytic uraemic syndrome.

Successful simultaneous liver-kidney transplant in an adult with atypical hemolytic uremic syndrome associated with a mutation in complement factor H

Atypical hemolytic uremic syndrome was diagnosed in a 62-year-old man. Sequencing of the CFH gene, which encodes complement factor H, revealed a heterozygous adenine to guanine mutation at nucleotide 3550 of the complementary DNA, leading to a predicted substitution of alanine for threonine at amino acid position 1184 in the protein (c.3550A>G, p.Thr1184Ala). Three years later, he received a simultaneous liver-kidney transplant with plasmapheresis and intratransplant plasma infusion. The postoperative course was complicated by an anastomotic biliary stricture that was treated successfully using endoscopic stenting. One year later, he has excellent function of both transplants, emphasizing that simultaneous liver-kidney transplant is a valuable treatment option in the management of adult patients with atypical hemolytic uremic syndrome.

The effect of electrostatics on factor H function and related pathologies

Factor H (FH) contributes to the regulation of the complement system by binding to polyanionic surfaces and the proteins C3b/C3c/C3d. This implicates charge and electrostatic interactions in recognition and binding of FH. Despite the large amount of experimental and pathology data the exact mechanism at molecular level is not yet known. We have implemented a computational framework for comparative analysis of the charge and electrostatic diversity of FH modules and C3b domains to identify electrostatic hotspots and predict potential binding sites. Our electrostatic potential clustering analysis shows that charge distributions and electrostatic potential distributions are more useful in understanding C3b-FH interactions than net charges alone. We present a model of non-specific electrostatic interactions of FH with polyanion-rich surfaces and specific interactions with C3b, using our computational data and existing experimental data. We discuss the electrostatic contributions to the formation of the C3b-FH complex and the competition between FH and Factor Bb (Bb) for binding to C3b. We also discuss the significance of mutations of charged amino acids in the pathobiology of FH-mediated disease, such as age-related macular degeneration, atypical hemolytic uremic syndrome, and dense deposit disease. Our data can be used to guide future experimental studies.

Efficacy of eculizumab in a patient with factor-H-associated atypical hemolytic uremic syndrome

Atypical hemolytic uremic syndrome (aHUS) is a rare, chronic, life-threatening disease due to complement dysregulation. The use of early-onset plasma therapy is recommended, but optimal long-term treatment regimen is not well defined. Eculizumab, a monoclonal humanized anti-C5 antibody, has shown success in patients with aHUS. We report a 7-year-old girl with aHUS associated with factor H mutations successfully treated with eculizumab. Weekly plasma infusion (PI) of 25-30 ml/kg with short-term intensified PI during aHUS exacerbations was effective for 4.3 years. Progressive mild renal failure (stage 2) was attributed to chronic glomerular lesions. Subsequently, she exhibited aHUS exacerbation unresponsive to intensified PI. Eculizumab was initiated at 600 mg, resulting in immediate and complete inhibition of terminal complement activation. During the week following treatment, we observed a complete reversal of aHUS activity. She has been receiving 600 mg eculizumab every 2 weeks for the last 12 months. She had no aHUS exacerbation, and serum creatinine level returned to normal. In this patient, eculizumab led to control of PI-resistant aHUS exacerbation and chronic microangiopathic hemolytic activity. Clinical trials are ongoing to assess the safety and efficacy of this drug in the management of aHUS.

Production of biologically active complement factor H in therapeutically useful quantities

Human complement factor H (FH), an abundant 155-kDa plasma glycoprotein with 40 disulphide bonds, regulates the alternative-pathway complement cascade. Mutations and single nucleotide polymorphisms in the FH gene predispose to development of age-related macular degeneration, atypical haemolytic uraemic syndrome and dense deposit disease. Supplementation with FH variants protective against disease is an enticing therapeutic prospect. Current sources of therapeutic FH are restricted to human blood plasma highlighting a need for recombinant material. Previously FH expression in cultured plant, mammalian or insect cells yielded protein amounts inadequate for full characterisation, and orders of magnitude below therapeutic usefulness. Here, the V62,Y402 variant of FH has been produced recombinantly (rFH) in Pichia pastoris cells. Codon-optimisation proved essential whilst exploitation of the yeast mating α-factor peptide ensured secretion. We thereby produced multiple 10s-of-milligram of rFH. Following endoglycosidase H digestion of N-linked glycans, rFH (with eight residual N-acetylglucosamine moieties) was purified on heparin-affinity resin and anion-exchange chromatography. Full-length rFH was verified by mass spectrometry and Western blot using monoclonal antibodies to the C-terminus. Recombinant FH is a single non-aggregated species (by dynamic light scattering) and fully functional in biochemical and biological assays. An additional version of rFH was produced in which eight N-glycosylation sequons were ablated by Asn-Gln substitutions resulting in a glycan-devoid product. Successful production of rFH in this potentially very highly expressing system makes production of therapeutically useful quantities economically viable. Furthermore, ease of genetic manipulation in P. pastoris would allow production of engineered FH versions with enhanced pharmacokinetic and pharmacodynamic properties.

[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).

Complement factor H deficiency and endocapillary glomerulonephritis due to paternal isodisomy and a novel factor H mutation

Complement factor H (CFH) is a regulator of the alternative complement activation pathway. Mutations in the CFH gene are associated with atypical hemolytic uremic syndrome, membranoproliferative glomerulonephritis type II and C3 glomerulonephritis. Here, we report a 6-month-old CFH-deficient child presenting with endocapillary glomerulonephritis rather than membranoproliferative glomerulonephritis (MPGN) or C3 glomerulonephritis. Sequence analyses showed homozygosity for a novel CFH missense mutation (Pro139Ser) associated with severely decreased CFH plasma concentration (<6%) but normal mRNA splicing and expression. The father was heterozygous carrier of the mutation, but the mother was a non-carrier. Thus, a large deletion in the maternal CFH locus or uniparental isodisomy was suspected. Polymorphic markers across chromosome 1 showed homozygosity for the paternal allele in all markers and a lack of the maternal allele in six informative markers. This combined with a comparative genomic hybridization assay demonstrated paternal isodisomy. Uniparental isodisomy increases the risk of homozygous variations in other genes on the affected chromosome. Therefore, we analyzed other susceptibility genes on chromosome 1 and found no sequence variation in membrane cofactor protein, but homozygosity for the common deletion of CFH-related proteins 1 and 3, which may contribute to the early onset of disease.

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.

Age-related macular degeneration: genetic and clinical findings

Age-related macular degeneration (AMD) is a sight threatening eye disease that affects millions of humans over the age of 65 years. It is considered to be the major cause of irreversible blindness in the elderly population in the developed world. The disease is prevalent in Europe and the United States, which has a large number of individuals of European descent. AMD is characterized by a progressive loss of central vision attributable to degenerative and neovascular changes that occur in the interface between the neural retina and the underlying choroid. This location contains the retinal photoreceptors, the retinal pigmented epithelium, a basement membrane complex known as Bruch's membrane and a network of choroidal capillaries. AMD is increasingly recognized as a complex genetic disorder where one or more genes contribute to an individual's susceptibility to development of the condition, while the prevailing view is that the disease stems from the interaction of multiple genetic and environmental factors. Although it has been proposed that a threshold event occurs during normal aging, the sequelae of biochemical, cellular, and molecular events leading to AMD are not fully understood. Here, we review the clinical aspects of AMD and summarize the genes which have been reported to have a positive association with the disease.

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.

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.

DEAP-HUS: deficiency of CFHR plasma proteins and autoantibody-positive form of hemolytic uremic syndrome

DEAP-HUS [Deficiency of CFHR (complement factor H-related) plasma proteins and Autoantibody Positive form of Hemolytic Uremic Syndrome] represents a novel subtype of hemolytic uremic syndrome (HUS) with unique characteristics. It affects children and requires special clinical attention in terms of diagnosis and therapy. DEAP-HUS and other atypical forms of HUS share common features, such as microangiopathic hemolytic anemia, acute renal failure, and thrombocytopenia. However, DEAP-HUS has the unique combination of an acquired factor in the form of autoantibodies to the complement inhibitor Factor H and a genetic factor which, in most cases, is the chromosomal deletion of a 84-kbp fragment within human chromosome 1 that results in the absence of the CFHR1 and CFHR3 proteins in plasma. Special attention is required to diagnose and treat DEAP-HUS patients. Most patients show a favorable response to the reduction of autoantibody titers by either plasma therapy, steroid treatment, and/or immunosuppression. In addition, in those DEAP-HUS patients with end-stage renal disease, the reduction of autoantibody titers prior to transplantation is expected to prevent post-transplant disease recurrence by aiming for full complement control at the endothelial cell surface in order to minimize adverse complement and immune reactions.

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.

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.

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.

Role of genetic polymorphisms in factor H and MBL genes in Tunisian patients with immunoglobulin A nephropathy

The molecular mechanisms of IgA nephropathy (IgAN) remain poorly understood. Several different polymorphic genes have been investigated in order to demonstrate their possible association with this disease. It is evident that mainly alternative and lectin pathways complement activation and play an important role in renal injury of IgAN. This study was conducted to determine eventual deficiencies of factor H in the SCR20 gene region and to look for a possible association between the polymorphism (+54) exon 1 of the MBL gene and the predisposition in Tunisian patients with IgAN. We then evaluated the effects of these FH mutations and/or this MBL polymorphism on nephropathy susceptibility and progression. Polymorphism A/B (+54) in the exon1 of the MBL gene and analysis within the C-terminal domain of the protein SCR20 in the exon 22 of the factor H (FH) gene were conducted in 36 sporadic IgAN Tunisian patients and 117 age and gender matched healthy subjects recruited from blood donors, by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and direct sequencing respectively. The analysis of the Gly54Asp (+54) mutation of the MBL gene according to the criteria of gravity of the IgAN reveals that the patients with genotype AB present more frequently with end-stage renal disease (ESRD) compared with those of genotype AA [OR: 8, CI (1.74-54.49), P = 0.019]. Moreover, the variant allele B was statistically more frequent than the allele A in patients with an association with initial arterial high blood pressure, ESRD and class V of the Haas classification compared to those without this association (P = 0.009). The direct sequencing of exon 22 (SCR 20) of FH gene did not reveal any abnormal mutational deficiency for this factor in all patients and controls. The data did not support the hypothesis that FH is a susceptibility factor for the IgAN. However the data did show there was an association between AB (+54) exon1 MBL genotype and severe sporadic forms of this disease in Tunisian patients. Because of the small number of subjects studied, a much larger cohort of IgAN patients with varying severity of the disease and its progression would seem necessary to confirm these findings.

Epidemiological approach to identifying genetic predispositions for atypical hemolytic uremic syndrome

Atypical hemolytic uremic syndrome (aHUS) is caused by several susceptibility genes. A registry including analyses of susceptibility genes, familial occurrence and genotype-phenotype correlation should provide classification insights. Registry data of 187 unrelated index patients included age at onset, gender, family history, relapse of aHUS and potentially triggering conditions. Mutation analyses were performed in the genes CFH, CD46 and CFI and in the six potential susceptibility genes, FHR1 to FHR5 and C4BP. Germline mutations were identified in 17% of the index cases; 12% in CFH, 3% in CD46 and 2% in CFI. Twenty-nine patients had heterozygous mutations and one each had a homozygous and compound heterozygous mutation. Mutations were not found in the genes FHR1-5 and C4BP. In 40% of the patients with familial HUS a mutation was found. Penetrance by age 45 was 50% among carriers of any mutation including results of relatives of mutation-positive index cases. The only risk factor for a mutation was family history of HUS (p = 0.02). Penetrance of aHUS in carriers of mutations is not complete. Occurrence of homo- and heterozygous mutations in the same gene suggests that the number of necessary DNA variants remains unclear. Among clinical information only familial occurrence predicts a mutation.

Severe atypical HUS caused by CFH S1191L--case presentation and review of treatment options

Atypical hemolytic uremic syndrome (aHUS) has been associated with defective regulation of the alternative complement pathway. Although the use of plasma therapy is recommended, there is little consensus on the optimal treatment regimen. The outcome in many cases remains poor despite an improvement in our understanding of the pathology of aHUS. We have followed a female patient with aHUS associated with heterozygous complement Factor H (CFH) mutation (S1191L) over a period of 15 years. She has been plasma dependent since infancy and has subsequently progressed to end stage kidney disease (ESKD) requiring dialysis treatment. Despite ESKD she still depends on regular plasma infusions to prevent thrombocytopenia. The long-term treatment plan for this patient is challenging. Renal transplantation in patients with the S1191L mutation of the CFH gene carries a high risk of failure due to recurrence of aHUS in the renal graft. Thus, the only available curative treatment seems to be combined liver-kidney transplantation, covered by intensive plasma therapy, which comes with a high risk of morbidity and mortality. Antibodies against key activating components of the complement cascade may provide a promising alternative therapeutic strategy in the future. Eculizumab, a monoclonal humanized anti-C5 antibody, has recently been shown to be effective and well-tolerated in patients with paroxysmal nocturnal hemoglobinuria by preventing complement-mediated lysis of affected erythrocytes. Treatment of our patient with eculizumab is supported by recent reports on its successful use in two (pediatric and adult) patients with complement-based aHUS.

The amplification loop of the complement pathways

The C3 amplification loop lies at the core of all the complement pathways, rather than the alternative pathway alone. It is, in evolutionary terms, the oldest part of the complement system and its antecedents can be seen in insects and in echinoderms. The amplification loop is the balance between two competing cycles both acting on C3b: the C3 feedback cycle which enhances amplification and the C3 breakdown cycle which downregulates it. It is solely the balance between their rates of reaction on which amplification depends. The C3 breakdown cycle generates iC3b as its primary reaction product. iC3b, through its reaction with the leukocyte integrins (and complement receptors) CR3 (CD11b/CD18) and CR4 (CD11c/CD18), is the most important mechanism by which complement mediates inflammation. A variety of genetic polymorphisms in components of the amplification loop have been shown to predispose to two kidney diseases-dense deposit disease and atypical haemolytic uraemic syndrome-and to age-related macular degeneration. All predisposing alleles enhance amplification, whereas protective alleles downregulate amplification. This leads to the conclusion that there is a "hyperinflammatory complement phenotype" determined by these polymorphisms. This hyperinflammatory phenotype protects against bacterial infections in early life but in later life is associated with immunopathology. Besides the diseases already mentioned, there is evidence that this hyperinflammatory complement phenotype may predispose to accelerated atherosclerosis and also shows an association with Alzheimer's disease. Downregulation of the amplification loop therefore constitutes an important therapeutic target.

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.

Maintenance of kidney function following treatment with eculizumab and discontinuation of plasma exchange after a third kidney transplant for atypical hemolytic uremic syndrome associated with a CFH mutation

Kidney transplant in patients with atypical hemolytic uremic syndrome (aHUS) is associated with a poor outcome because of recurrent disease, especially in patients known to have a factor H mutation. Long-term prophylactic plasma exchange and combined liver-kidney transplant have prevented graft loss caused by recurrence. However, the mortality associated with liver transplant is not negligible, and prophylactic plasma exchange requires permanent vascular access and regular hospitalization and exposes the patient to potential allergic reactions to plasma. Eculizumab is a high-affinity humanized monoclonal antibody that binds to C5 and thus prevents generation of C5a and the membrane attack complex. We report the case of a 17-year-old girl with aHUS associated with a mutation in the gene for complement factor H (CFH; c.3572C>T, Ser1191Leu) who was highly dependent on plasma exchange. Because of severe allergic reactions to plasma after the third renal graft, eculizumab was introduced in place of plasma exchange without problems. This and other reports suggest that the promise of complement inhibitors in the management of aHUS is going to be fulfilled.

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.

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.

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.

Lack of association between polymorphisms in C4b-binding protein and atypical haemolytic uraemic syndrome in the Spanish population

Dysregulation of the alternative pathway of complement activation, caused by mutations or polymorphisms in the genes encoding factor H, membrane co-factor protein, factor I or factor B, is associated strongly with predisposition to atypical haemolytic uraemic syndrome (aHUS). C4b-binding protein (C4BP), a major regulator of the classical pathway of complement activation, also has capacity to regulate the alternative pathway. Interestingly, the C4BP polymorphism p.Arg240His has been associated recently with predisposition to aHUS and the risk allele His240 showed decreased capacity to regulate the alternative pathway. Identification of novel aHUS predisposition factors has important implications for diagnosis and treatment in a significant number of aHUS patients; thus, we sought to replicate these association studies in an independent cohort of aHUS patients. In this study we show that the C4BP His240 allele corresponds to the C4BP*2 allele identified previously by isoelectric focusing in heterozygosis in 1.9-3.7% of unrelated Caucasians. Crucially, we found no differences between 102 unrelated Spanish aHUS patients and 128 healthy age-matched Spanish controls for the frequency of carriers of the His240 C4BP allele. This did not support an association between the p.Arg240His C4BP polymorphism and predisposition to aHUS in the Spanish population. In a similar study, we also failed to sustain an association between C4BP polymorphisms and predisposition to age-related macular degeneration, another disorder which is associated strongly with polymorphisms in factor H, and is thought to involve alternative pathway dysregulation.

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.

A new map of glycosaminoglycan and C3b binding sites on factor H

Human complement factor H, consisting of 20 complement control protein (CCP) modules, is an abundant plasma glycoprotein. It prevents C3b amplification on self surfaces bearing certain polyanionic carbohydrates, while complement activation progresses on most other, mainly foreign, surfaces. Herein, locations of binding sites for polyanions and C3b are reexamined rigorously by overexpressing factor H segments, structural validation, and binding assays. As anticipated, constructs corresponding to CCPs 7-8 and 19-20 bind well in heparin-affinity chromatography. However, CCPs 8-9, previously reported to bind glycosaminoglycans, bind neither to heparin resin nor to heparin fragments in gel-mobility shift assays. Introduction of nonnative residues N-terminal to a construct containing CCPs 8-9, identical to those in proteins used in the previous report, converted this module pair to an artificially heparin-binding one. The module pair CCPs 12-13 does not bind heparin appreciably, notwithstanding previous suggestions to the contrary. We further checked CCPs 10-12, 11-14, 13-15, 10-15, and 8-15 for ability to bind heparin but found very low affinity or none. As expected, constructs corresponding to CCPs 1-4 and 19-20 bind C3b amine coupled to a CM5 chip (K(d)s of 14 and 3.5 microM, respectively) or a C1 chip (K(d)s of 10 and 4.5 microM, respectively). Constructs CCPs 7-8 and 6-8 exhibit measurable affinities for C3b according to surface plasmon resonance, although they are weak compared with CCPs 19-20. Contrary to expectations, none of several constructs encompassing modules from CCP 9 to 15 exhibited significant C3b binding in this assay. Thus, we propose a new functional map of factor H.

Mutations in complement C3 predispose to development of atypical hemolytic uremic syndrome

Atypical hemolytic uremic syndrome (aHUS) is a disease of complement dysregulation. In approximately 50% of patients, mutations have been described in the genes encoding the complement regulators factor H, MCP, and factor I or the activator factor B. We report here mutations in the central component of the complement cascade, C3, in association with aHUS. We describe 9 novel C3 mutations in 14 aHUS patients with a persistently low serum C3 level. We have demonstrated that 5 of these mutations are gain-of-function and 2 are inactivating. This establishes C3 as a susceptibility factor for aHUS.

What's new in haemolytic uraemic syndrome?

Recent advances in understanding the aetiology of the disorders that make up the haemolytic uraemic syndrome (HUS) permit a revised classification of the syndrome. With appropriate laboratory support, an aetiologically-based subgroup diagnosis can be made in all but a few cases. HUS caused by enterohaemorrhagic Escherichia coli remains by far the most prevalent subgroup, and new insights into this zoonosis are discussed. The most rapidly expanding area of interest is the subgroup of inherited and acquired abnormalities of complement regulation. Details of the pathogenesis are incomplete but it is reasonable to conclude that local activation of the alternative pathway of complement in the glomerulus is a central event. There is no evidence-based treatment for this diagnostic subgroup. However, in circumstances where there is a mutated plasma factor such as complement factor H, strategies to replace the abnormal protein by plasmapheresis or more radically by liver transplantation are logical, and anecdotal successes are reported. In summary, the clinical presentation of HUS gives a strong indication as to the underlying cause. Patients without evidence of EHEC infection should be fully investigated to determine the aetiology. Where complement abnormalities are suspected there is a strong argument for empirical and early plasma exchange, although rapid advances in this field may provide more specific treatments in the near future.

Efficacy of plasma therapy in atypical hemolytic uremic syndrome with complement factor H mutations

Atypical hemolytic uremic syndrome (aHUS) frequently results in end-stage renal failure and can be lethal. Several studies have established an association between quantitative or qualitative abnormalities in complement factor H and aHUS. Although plasma infusion and exchange are often advocated, guidelines have yet to be established. Long-term outcome for patients under treatment is still unknown. We describe a patient who, at 7 months of age, presented with aHUS associated with combined de novo complement factor H mutations (S1191L and V1197A) on the same allele. Laboratory investigations showed normal levels of complements C4, C3 and factor H. Plasma exchanges and large-dose infusion therapy resulted in a resolution of hemolysis and recovery of renal function. Three recurrences were successfully treated by intensification of the plasma infusion treatment to intervals of 2 or 3 days. This patient showed good response to large doses of plasma infusions and her condition remained stable for 30 months with weekly plasma infusions (30 ml/kg). Long-term tolerance and efficacy of such intensive plasma therapy are still unknown. Reported secondary failure of plasma therapy in factor H deficiency warrants the search for alternative therapeutic approaches.

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.

Factor H dysfunction in patients with atypical hemolytic uremic syndrome contributes to complement deposition on platelets and their activation

Atypical hemolytic uremic syndrome (aHUS) may be associated with mutations in the C-terminal of factor H (FH). FH binds to platelets via the C-terminal as previously shown using a construct consisting of short consensus repeats (SCRs) 15 to 20. A total of 4 FH mutations, in SCR15 (C870R) and SCR20 (V1168E, E1198K, and E1198Stop) in patients with aHUS, were studied regarding their ability to allow complement activation on platelet surfaces. Purified FH-E1198Stop mutant exhibited reduced binding to normal washed platelets compared with normal FH, detected by flow cytometry. Washed platelets taken from the 4 patients with aHUS during remission exhibited C3 and C9 deposition, as well as CD40-ligand (CD40L) expression indicating platelet activation. Combining patient serum/plasma with normal washed platelets led to C3 and C9 deposition, CD40L and CD62P expression, aggregate formation, and generation of tissue factor-expressing microparticles. Complement deposition and platelet activation were reduced when normal FH was preincubated with platelets and were minimal when using normal serum. The purified FH-E1198Stop mutant added to FH-deficient plasma (complemented with C3) allowed considerable C3 deposition on washed platelets, in comparison to normal FH. In summary, mutated FH enables complement activation on the surface of platelets and their activation, which may contribute to the development of thrombocytopenia in aHUS.

Update on evaluating complement in hemolytic uremic syndrome

PURPOSE OF REVIEW

The last few years have seen the decoding of the genetic basis for atypical hemolytic uremic syndrome.

RECENT FINDINGS

Mutations in complement factor H were the first to be associated with atypical hemolytic uremic syndrome. These mutations cluster in the C-terminus of complement factor H. This year has seen the publication of a transgenic mouse model lacking the C-terminus of complement factor H, which spontaneously develops atypical hemolytic uremic syndrome. This mouse model regulated C3 activation in plasma but failed to bind to endothelial cells in an analogous manner to the mutations seen in atypical hemolytic uremic syndrome patients. This year has also seen the emergence of genotype-phenotype correlations in atypical hemolytic uremic syndrome. Patients with membrane cofactor protein mutations have a good prognosis and in those who do develop endstage renal disease, recurrence after transplantation is rare. By contrast, the outcome for patients with complement factor H and complement factor I mutations is poor and the rate of recurrence after transplantation is high. New complement genes associated with atypical hemolytic uremic syndrome have also been described in the past year including factor B, C3, C4b-binding protein, FHR1 and FHR3.

SUMMARY

Genetic screening is now providing prognostically significant information in predicting survival, renal recovery and transplant outcome. It paves the way for the use of complement inhibitors in the future.

Translational mini-review series on complement factor H: renal diseases associated with complement factor H: novel insights from humans and animals

Factor H is the major regulatory protein of the alternative pathway of complement activation. Abnormalities in factor H have been associated with renal disease, namely glomerulonephritis with C3 deposition including membranoproliferative glomerulonephritis (MPGN) and the atypical haemolytic uraemic syndrome (aHUS). Furthermore, a common factor H polymorphism has been identified as a risk factor for the development of age-related macular degeneration. These associations suggest that alternative pathway dysregulation is a common feature in the pathogenesis of these conditions. However, with respect to factor H-associated renal disease, it is now clear that distinct molecular defects in the protein underlie the pathogenesis of glomerulonephritis and HUS. In this paper we review the associations between human factor H dysfunction and renal disease and explore how observations in both spontaneous and engineered animal models of factor H dysfunction have contributed to our understanding of the pathogenesis of factor H-related renal disease.

Basal laminar drusen caused by compound heterozygous variants in the CFH gene

Age-related macular degeneration (AMD) is a multifactorial disease that is strongly associated with the Tyr402His variant in the complement factor H (CFH) gene. Drusen are hallmark lesions of AMD and consist of focal-inflammatory and/or immune-mediated depositions of extracellular material at the interface of the retinal pigment epithelium (RPE) and the Bruch membrane. We evaluated the role of CFH in 30 probands with early-onset drusen and identified heterozygous nonsense, missense, and splice variants in five families. The affected individuals all carried the Tyr402His AMD risk variant on the other allele. This supports an autosomal-recessive disease model in which individuals who carry a CFH mutation on one allele and the Tyr402His variant on the other allele develop drusen. Our findings strongly suggest that monogenic inheritance of CFH variants can result in basal laminar drusen in young adults, and this can progress to maculopathy and severe vision loss later in life.

The complement factor H R1210C mutation is associated with atypical hemolytic uremic syndrome

Mutations in the gene encoding complement factor H (CFH) that alter the C3b/polyanions-binding site in the C-terminal region impair the capacity of factor H to protect host cells. These mutations are also strongly associated with atypical hemolytic uremic syndrome (aHUS). Although most of the aHUS-associated CFH mutations seem "unique" to an individual patient or family, the R1210C mutation has been reported in several unrelated aHUS patients from distinct geographic origins. Five aHUS pedigrees and 7 individual aHUS patients were analyzed to identify potential correlations between the R1210C mutation and clinical phenotype and to characterize the origins of this mutation. The clinical phenotype of aHUS patients carrying the R1210C mutation was heterogeneous. Interestingly, 12 of the 13 affected patients carried at least one additional known genetic risk factor for aHUS. These data are in accord with the 30% penetrance of aHUS in R1210C mutation carriers, as it seems that the presence of other genetic or environmental risk factors significantly contribute to the manifestation and severity of aHUS in these subjects. Genotype analysis of CFH and CFHR3 polymorphisms in the 12 unrelated carriers suggested that the R1210C mutation has a single origin. In conclusion, the R1210C mutation of complement factor H is a prototypical aHUS mutation that is present as a rare polymorphism in geographically separated human populations.