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

Genetic and epigenetic dysregulation of innate immune mechanisms in autoinflammatory diseases

Dysregulation and hyperactivation of innate immune responses can lead to the onset of systemic autoinflammatory diseases. Monogenic autoinflammatory diseases are caused by inborn genetic errors and based on molecular mechanisms at play, can be divided into inflammasomopathies, interferonopathies, relopathies, protein misfolding, and endogenous antagonist deficiencies. On the other hand, more common autoinflammatory diseases are multifactorial, with both genetic and non-genetic factors playing an important role. During the last decade, long-term memory characteristics of innate immune responses have been described (also called trained immunity) that in physiological conditions provide enhanced host protection from pathogenic re-infection. However, if dysregulated, induction of trained immunity can become maladaptive, perpetuating chronic inflammatory activation. Here, we describe the mechanisms of genetic and epigenetic dysregulation of the innate immune system and maladaptive trained immunity that leads to the onset and perpetuation of the most common and recently described systemic autoinflammatory diseases.

Genetic investigation of Nordic patients with complement-mediated kidney diseases

Background

Complement activation in atypical hemolytic uremic syndrome (aHUS), C3 glomerulonephropathy (C3G) and immune complex-mediated membranoproliferative glomerulonephritis (IC-MPGN) may be associated with rare genetic variants. Here we describe gene variants in the Swedish and Norwegian populations.

Methods

Patients with these diagnoses (N=141) were referred for genetic screening. Sanger or next-generation sequencing were performed to identify genetic variants in 16 genes associated with these conditions. Nonsynonymous genetic variants are described when they have a minor allele frequency of <1% or were previously reported as being disease-associated.

Results

In patients with aHUS (n=94, one also had IC-MPGN) 68 different genetic variants or deletions were identified in 60 patients, of which 18 were novel. Thirty-two patients had more than one genetic variant. In patients with C3G (n=40) 29 genetic variants, deletions or duplications were identified in 15 patients, of which 9 were novel. Eight patients had more than one variant. In patients with IC-MPGN (n=7) five genetic variants were identified in five patients. Factor H variants were the most frequent in aHUS and C3 variants in C3G. Seventeen variants occurred in more than one condition.

Conclusion

Genetic screening of patients with aHUS, C3G and IC-MPGN is of paramount importance for diagnostics and treatment. In this study, we describe genetic assessment of Nordic patients in which 26 novel variants were found.

Proteomic characteristics of plasma and blood cells in natural aging rhesus monkeys

Aging has become a serious social issue that places a heavy burden on society. However, the underlying mechanisms of aging remain unclear. This study sought to understand the aging process as it may be affected by proteins in the blood, the most important functional system for material transportation in the body. We analyzed and compared the protein expression spectrums in the blood of old and young rhesus monkeys and found 257 proteins expressed differentially in plasma and 1183 proteins expressed differentially in blood cells. Through bioinformatics analysis, we found that the differentially-expressed proteins in plasma were involved in signal pathways related to complement and coagulation cascades, pertussis, malaria, phagosome, and cholesterol metabolism, while the differentially-expressed proteins in blood cells were involved in endocytosis, proteasome, ribosome, protein processing in the endoplasmic reticulum, and Parkinson's disease. We confirmed that the protein levels of complement C2 in plasma and actin-related protein 2/3 complex subunit 2 (ARPC2) in blood cells obviously decreased, whereas the complement C3 and complement component 4 binding protein beta (C4BPB) significantly increased in plasma of old rhesus monkeys and C57BL/6 mice. Our results suggest that C2, C3, C4BPB, and ARPC2 can be used as target proteins for anti-aging research.

Distinct genetic profile with recurrent population-specific missense variants in Korean adult atypical hemolytic uremic syndrome

INTRODUCTION

Atypical hemolytic uremic syndrome (aHUS) is a rare thrombotic microangiopathy (TMA), characterized by micro-angiopathic hemolytic anemia, thrombocytopenia, and renal failure. In more than half of cases, genetic defects leading to overactivation of the alternative complement system have been identified. In this study, we investigated genetic defects in Korean adult patients with aHUS.

MATERIALS AND METHODS

Sixty-six Korean adult patients with aHUS were ascertained from the Korean TMA Registry. Genetic variants of 15 aHUS-related genes (eight core genes [CFH, CFB, CFI, CD46, C3, THBD, PLG, and DGKE] and seven candidate genes [CFP, C4BPA, and CHFR1-5]) were analyzed from exome sequencing data. Multiplex ligation-dependent probe amplification of CFH and related genes was performed to detect hybrid genes or large deletions.

RESULTS

Thirty patients (45%) had at least one aHUS-related variant (s) in eight core genes (total 40 variant alleles). The most frequently affected gene was CFH (13/40, 32%), followed by THBD (8/40, 20%) and CD46 (7/40, 18%). The two most common variants were Asp486Tyr of THBD (N = 7) and Tyr1058His-Val1060Leu of CFH (N = 5, linked on the same allele), accounting for 30% (12/40). In seven candidate genes, 19 variants were detected. When combined, 40 patients (61%) had at least one variant in 15 core or candidate genes. No patients had anti-CFH Ab or hybrid gene/CFHR1 homozygous deletions.

CONCLUSIONS

The genetic profile of Korean adult aHUS was unique with recurrent missense variants, demonstrating ethnicity- and age-dependent differences in the genetic background of aHUS.

Genetic Analysis of 400 Patients Refines Understanding and Implicates a New Gene in Atypical Hemolytic Uremic Syndrome

BACKGROUND

Genetic variation in complement genes is a predisposing factor for atypical hemolytic uremic syndrome (aHUS), a life-threatening thrombotic microangiopathy, however interpreting the effects of genetic variants is challenging and often ambiguous.

METHODS

We analyzed 93 complement and coagulation genes in 400 patients with aHUS, using as controls 600 healthy individuals from Iowa and 63,345 non-Finnish European individuals from the Genome Aggregation Database. After adjusting for population stratification, we then applied the Fisher exact, modified Poisson exact, and optimal unified sequence kernel association tests to assess gene-based variant burden. We also applied a sliding-window analysis to define the frequency range over which variant burden was significant.

RESULTS

We found that patients with aHUS are enriched for ultrarare coding variants in the CFH, C3, CD46, CFI, DGKE, and VTN genes. The majority of the significance is contributed by variants with a minor allele frequency of <0.1%. Disease-related variants tend to occur in specific complement protein domains of FH, CD46, and C3. We observed no enrichment for multiple rare coding variants in gene-gene combinations.

CONCLUSIONS

In known aHUS-associated genes, variants with a minor allele frequency >0.1% should not be considered pathogenic unless valid enrichment and/or functional evidence are available. VTN, which encodes vitronectin, an inhibitor of the terminal complement pathway, is implicated as a novel aHUS-associated gene. Patients with aHUS are not enriched for multiple rare variants in complement genes. In aggregate, these data may help in directing clinical management of aHUS.

High protein S activity due to C4b-binding protein deficiency in a 34-year-old Surinamese female with ischemic retinopathy

In this study, we present the first case of a 34‐year‐old Surinamese female with ischemic retinopathy and increased free protein S due to C4BP deficiency. Possibly, the low PS/C4BP complex level has increased the risk of arterial thrombosis in our patient.

Complement in cancer: untangling an intricate relationship

In tumour immunology, complement has traditionally been considered as an adjunctive component that enhances the cytolytic effects of antibody-based immunotherapies, such as rituximab. Remarkably, research in the past decade has uncovered novel molecular mechanisms linking imbalanced complement activation in the tumour microenvironment with inflammation and suppression of antitumour immune responses. These findings have prompted new interest in manipulating the complement system for cancer therapy. This Review summarizes our current understanding of complement-mediated effector functions in the tumour microenvironment, focusing on how complement activation can act as a negative or positive regulator of tumorigenesis. It also offers insight into clinical aspects, including the feasibility of using complement biomarkers for cancer diagnosis and the use of complement inhibitors during cancer treatment.

Complementopathies

The complement system is an essential part of the innate immune system that requires careful regulation to ensure responses are appropriately directed against harmful pathogens, while preventing collateral damage to normal host cells and tissues. While deficiency in some components of the complement pathway is associated with increased susceptibility to certain infections, it has also become clear that inappropriate activation of complement is an important contributor to human disease. A number of hematologic disorders are driven by complement, and these disorders may be termed "complementopathies". This includes paroxysmal nocturnal hemoglobinuria (PNH), atypical hemolytic uremic syndrome (aHUS), cold agglutinin disease (CAD) and other related disorders, which will be the focus of this review. A better understanding of the central role of the complement system in the pathophysiology of these disorders may allow for application of therapies directed at blocking the complement cascade.

Complement Dysregulation and Disease: Insights from Contemporary Genetics

The vertebrate complement system consists of sequentially interacting proteins that provide for a rapid and powerful host defense. Nearly 60 proteins comprise three activation pathways (classical, alternative, and lectin) and a terminal cytolytic pathway common to all. Attesting to its potency, nearly half of the system's components are engaged in its regulation. An emerging theme over the past decade is that variations in these inhibitors predispose to two scourges of modern humans. One, occurring most often in childhood, is a rare but deadly thrombomicroangiopathy called atypical hemolytic uremic syndrome. The other, age-related macular degeneration, is the most common form of blindness in the elderly. Their seemingly unrelated clinical presentations and pathologies share the common theme of overactivity of the complement system's alternative pathway. This review summarizes insights gained from contemporary genetics for understanding how dysregulation of this powerful innate immune system leads to these human diseases.

C4b-binding Protein Protects β-Cells from Islet Amyloid Polypeptide-induced Cytotoxicity

C4BP (C4b-binding protein) is a polymer of seven identical α chains and one unique β chain synthesized in liver and pancreas. We showed previously that C4BP enhances islet amyloid polypeptide (IAPP) fibril formation in vitro Now we report that polymeric C4BP strongly inhibited lysis of human erythrocytes incubated with monomeric IAPP, whereas no lysis was observed after incubation with preformed IAPP fibrils. In contrast, incubation with the monomeric α-chain of C4BP was less effective. These data indicate that polymeric C4BP with multiple binding sites for IAPP neutralizes lytic activity of IAPP. Furthermore, addition of monomeric IAPP to a rat insulinoma cell line (INS-1) resulted in decreased cell viability, which was restored in the presence of physiological concentrations of C4BP. Treatment of INS-1 cells and primary rat islets with IAPP also diminished their ability to secrete insulin upon stimulation with glucose, which was reversed in the presence of C4BP. Further, C4BP was internalized together with IAPP into INS-1 cells. Pathway analyses of mRNA expression microarray data indicated that cells exposed to C4BP and IAPP in comparison with IAPP alone increased expression of genes involved in cholesterol synthesis. Depletion of cholesterol through methyl-β-cyclodextrin or cholesterol oxidase abolished the protective effect of C4BP on IAPP cytotoxicity of INS-1 cells. Also, inhibition of phosphoinositide 3-kinase but not NF-κB had a similar effect. Taken together, C4BP protects β-cells from IAPP cytotoxicity by modulating IAPP fibril formation extracellularly and also, after uptake by the cells, by enhancing cholesterol synthesis.

C4b-binding protein: The good, the bad and the deadly. Novel functions of an old friend

C4b-binding protein (C4BP) is best known as a potent soluble inhibitor of the classical and lectin pathways of the complement system. This large 500 kDa multimeric plasma glycoprotein is expressed mainly in the liver but also in lung and pancreas. It consists of several identical 75 kDa α-chains and often also one 40 kDa β-chain, both of which are mainly composed of complement control protein (CCP) domains. Structure-function studies revealed that one crucial binding site responsible for inhibition of complement is located to CCP1-3 of the α-chain. Binding of anticoagulant protein S to the CCP1 of the β-chain provides C4BP with the ability to strongly bind apoptotic and necrotic cells in order to prevent inflammation arising from activation of complement by these cells. Further, C4BP interacts strongly with various types of amyloid and enhances fibrillation of islet amyloid polypeptide secreted from pancreatic beta cells, which may attenuate pro-inflammatory and cytotoxic effects of this amyloid. Full deficiency of C4BP has not been identified but non-synonymous alterations in its sequence have been found in haemolytic uremic syndrome and recurrent pregnancy loss. Furthermore, C4BP is bound by several bacterial pathogens, notably Streptococcus pyogenes, which due to inhibition of complement and enhancement of bacterial adhesion to endothelial cells provides these bacteria with a survival advantage in the host. Thus, depending on the context, C4BP has a protective or detrimental role in the organism.

Regulation of complement and modulation of its activity in monoclonal antibody therapy of cancer

The complement system is a powerful tool of the innate immune system to eradicate pathogens. Both in vitro and in vivo evidence indicates that therapeutic anti-tumor monoclonal antibodies (mAbs) can activate the complement system by the classical pathway. However, the contribution of complement to the efficacy of mAbs is still debated, mainly due to the lack of convincing data in patients. A beneficial role for complement during mAb therapy is supported by the fact that cancer cells often upregulate complement-regulatory proteins (CRPs). Polymorphisms in various CRPs were previously associated with complement-mediated disorders.

In this review the role of complement in anti-tumor mAb therapy will be discussed with special emphasis on strategies aiming at modifying complement activity. In the future, clinical efficacy of mAbs with enhanced effector functions together with comprehensive analysis of polymorphisms in CRPs in mAb-treated patients will further clarify the role of complement in mAb therapy.

Atypical hemolytic uremic syndrome

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

Where genotype is not predictive of phenotype: towards an understanding of the molecular basis of reduced penetrance in human inherited disease

Some individuals with a particular disease-causing mutation or genotype fail to express most if not all features of the disease in question, a phenomenon that is known as 'reduced (or incomplete) penetrance'. Reduced penetrance is not uncommon; indeed, there are many known examples of 'disease-causing mutations' that fail to cause disease in at least a proportion of the individuals who carry them. Reduced penetrance may therefore explain not only why genetic diseases are occasionally transmitted through unaffected parents, but also why healthy individuals can harbour quite large numbers of potentially disadvantageous variants in their genomes without suffering any obvious ill effects. Reduced penetrance can be a function of the specific mutation(s) involved or of allele dosage. It may also result from differential allelic expression, copy number variation or the modulating influence of additional genetic variants in cis or in trans. The penetrance of some pathogenic genotypes is known to be age- and/or sex-dependent. Variable penetrance may also reflect the action of unlinked modifier genes, epigenetic changes or environmental factors. At least in some cases, complete penetrance appears to require the presence of one or more genetic variants at other loci. In this review, we summarize the evidence for reduced penetrance being a widespread phenomenon in human genetics and explore some of the molecular mechanisms that may help to explain this enigmatic characteristic of human inherited disease.

Complement therapy in atypical haemolytic uraemic syndrome (aHUS)

Central to the pathogenesis of atypical haemolytic uraemic syndrome (aHUS) is over-activation of the alternative pathway of complement. Inherited defects in complement genes and autoantibodies against complement regulatory proteins have been described. The use of plasma exchange to replace non-functioning complement regulators and hyper-functional complement components in addition to the removal of CFH-autoantibodies made this the ‘gold-standard’ for management of aHUS. In the last 4 years the introduction of the complement inhibitor Eculizumab has revolutionised the management of aHUS. In this review we shall discuss the available literature on treatment strategies to date.

Analysis of genes coding for CD46, CD55, and C4b-binding protein in patients with idiopathic, recurrent, spontaneous pregnancy loss

Since a tightly regulated complement system is needed for a successful pregnancy, we hypothesized that alterations in complement inhibitors may be associated with idiopathic, recurrent miscarriage. We sequenced all exons coding for three complement inhibitors: C4b-binding protein (C4BP), CD46, and CD55 in 384 childless women with at least two miscarriages that could not be explained by known risk factors. Several alterations were found in C4BPA, of which the R120H, I126T, and the G423T mutations affected the expression level and/or the ability of recombinant C4BP to serve as cofactor for factor I. The only variant in C4BPB was located in the C-terminal part, and did not impair the polymerization of the molecule. Our results identify for the first time alterations in C4BP in women experiencing recurrent miscarriages. We also found four CD46 alterations in individual patients that were not found in healthy controls. One of the rare variants, P324L, showed decreased expression, whereas N213I resulted in deficient protein processing as well as an impaired cofactor activity in the degradation of both C4b and C3b. The identified alterations may result in in vivo consequences and contribute to the disorder but the degree of association must be evaluated in larger cohorts.

Treatment of atypical hemolytic uremic syndrome and thrombotic microangiopathies: a focus on eculizumab

Uncontrolled complement activation is central to the occurrence of atypical hemolytic uremic syndrome (aHUS) and can result in thrombotic microangiopathies (TMAs). These terms encompass a group of heterogenic inherited or acquired diseases that recent research suggests may be triggered by the complement cascade. Pathogenetic triggers of complement activation include immunologic disorders, genetics, infections, systemic diseases, pregnancy, drug administration, metabolic diseases, transplantation, or triggers of mixed cause. Hallmarks of aHUS and other TMAs include increased vascular endothelium thromboresistance, leukocyte adhesion to damaged endothelium, complement consumption, coagulation abnormalities, and vascular shear stress, whereas common end points of these mechanisms include hemolytic anemia, thrombocytopenia with microvascular infarction, and predisposition for decreased kidney function and other organ involvement. The central role of the complement cascade as a disease trigger suggests a possible therapeutic target. Eculizumab, a first-in-class humanized monoclonal anti-C5 antibody that has been successful in the treatment of paroxysmal nocturnal hemoglobinuria, a disorder of complement-induced hemolytic anemia, received approval for the treatment of aHUS in the United States and Europe in late 2011. We review the treatment of aHUS and other TMAs, focusing on the role of eculizumab, including its pharmacology, mechanism of action, and approved dosing recommendations and health economic considerations. Finally, the potential for future indications for eculizumab use in other complement-driven diseases is discussed.

Thrombotic microangiopathy and associated renal disorders

Thrombotic microangiopathy (TMA) is a pathological process involving thrombocytopenia, microangiopathic haemolytic anaemia and microvascular occlusion. TMA is common to haemolytic uraemic syndrome (HUS) associated with shiga toxin or invasive pneumococcal infection, atypical HUS (aHUS), thrombotic thrombocytopenic purpura (TTP) and other disorders including malignant hypertension. HUS complicating infection with shiga toxin-producing Escherichia coli (STEC) is a significant cause of acute renal failure in children worldwide, occurring sporadically or in epidemics. Studies in aHUS have revealed genetic and acquired factors leading to dysregulation of the alternative complement pathway. TTP has been linked to reduced activity of the ADAMTS13 cleaving protease (typically with an autoantibody to ADAMTS13) with consequent disruption of von Willebrand factor multimer processing. However, the convergence of pathogenic pathways and clinical overlap create diagnostic uncertainty, especially at initial presentation. Furthermore, recent developments are challenging established management protocols. This review addresses the current understanding of molecular mechanisms underlying TMA, relating these to clinical presentation with an emphasis on renal manifestations. A diagnostic and therapeutic approach is presented, based on international guidelines, disease registries and published trials. Early treatment remains largely empirical, consisting of plasma replacement/exchange with the exception of childhood STEC-HUS or pneumococcal sepsis. Emerging therapies such as the complement C5 inhibitor eculizumab for aHUS and rituximab for TTP are discussed, as is renal transplantation for those patients who become dialysis-dependent as a result of aHUS.

Polymorphisms of innate immunity receptors in infection by parasites

The innate immune system is the first line of defence against infection by pathogenic bacteria, virus and parasites and is also responsible for initiating an adaptive immune response. In contrast to the receptors of adaptive immunity (TCRs and antibodies) which are generated by gene recombination, receptors of the innate immune system are encoded in the germline and are thus inherited from generation to generation. Although evolutionarily selected, the genes encoding the innate recognition receptors show variations among individuals, and these polymorphisms may have an impact on the ability of an individual to deal with an infection. In recent years, several polymorphisms have been identified in innate recognition receptors, and efforts are being made to determine whether these polymorphisms are associated with a higher or lower susceptibility to infectious diseases. These studies will allow a better understanding of the role of innate receptors in specific diseases and are valuable in the design of preventive or therapeutic interventions to fight the disease. In this review, we summarize studies aimed at determining the influence of polymorphisms in innate recognition receptors on the susceptibility to diseases caused by parasites.

Complement-mediated injury and protection of endothelium: lessons from atypical haemolytic uraemic syndrome

The complement system provides a vital defence against invading pathogens. As an intrinsic system it is always 'on', in a state of constant, low level activation. This activation is principally mediated through the deposition of C3b on to pathogenic surfaces and host tissues. C3b is generated by spontaneous 'tick over' and formal activation of the alternative pathway, and by activation of the classical and lectin pathways. If the deposited C3b is not appropriately regulated, there is progression to terminal pathway complement activation via the C5 convertases, generating the potent anaphylotoxin C5a and the membrane attack complex C5b-9. Unsurprisingly, these highly active components have the potential to cause injury to bystander host tissue, including the vascular endothelium. As such, complement activation on endothelium is normally tightly controlled by a large number of fluid-phase and membrane bound inhibitors, in an attempt to ensure that propagation of complement activation is appropriately restricted to invading pathogens and altered 'self', e.g. apoptotic and necrotic cells. The kidney is increasingly recognised as a site at particular risk from complement-mediated endothelial injury. Both genetic and acquired defects which impact on complement regulation predispose to this susceptibility. The thrombotic microangiopathy, haemolytic uraemic syndrome (HUS), will be used to illustrate the mechanisms by which the endothelial cell injury occurs. Finally, the underlying rationale for current and future potential therapeutic interventions in HUS and also the opportunities for enhancing endothelial defence to prevent relapsing disease through increased complement cytoprotective strategies will be summarised.

Atypical hemolytic uremic syndrome

Hemolytic uremic syndrome (HUS) is defined by the triad of mechanical hemolytic anemia, thrombocytopenia and renal impairment. Atypical HUS (aHUS) defines non Shiga-toxin-HUS and even if some authors include secondary aHUS due to Streptococcus pneumoniae or other causes, aHUS designates a primary disease due to a disorder in complement alternative pathway regulation. Atypical HUS represents 5 -10% of HUS in children, but the majority of HUS in adults. The incidence of complement-aHUS is not known precisely. However, more than 1000 aHUS patients investigated for complement abnormalities have been reported. Onset is from the neonatal period to the adult age. Most patients present with hemolytic anemia, thrombocytopenia and renal failure and 20% have extra renal manifestations. Two to 10% die and one third progress to end-stage renal failure at first episode. Half of patients have relapses. Mutations in the genes encoding complement regulatory proteins factor H, membrane cofactor protein (MCP), factor I or thrombomodulin have been demonstrated in 20-30%, 5-15%, 4-10% and 3-5% of patients respectively, and mutations in the genes of C3 convertase proteins, C3 and factor B, in 2-10% and 1-4%. In addition, 6-10% of patients have anti-factor H antibodies. Diagnosis of aHUS relies on 1) No associated disease 2) No criteria for Shigatoxin-HUS (stool culture and PCR for Shiga-toxins; serology for anti-lipopolysaccharides antibodies) 3) No criteria for thrombotic thrombocytopenic purpura (serum ADAMTS 13 activity > 10%). Investigation of the complement system is required (C3, C4, factor H and factor I plasma concentration, MCP expression on leukocytes and anti-factor H antibodies; genetic screening to identify risk factors). The disease is familial in approximately 20% of pedigrees, with an autosomal recessive or dominant mode of transmission. As penetrance of the disease is 50%, genetic counseling is difficult. Plasmatherapy has been first line treatment until presently, without unquestionable demonstration of efficiency. There is a high risk of post-transplant recurrence, except in MCP-HUS. Case reports and two phase II trials show an impressive efficacy of the complement C5 blocker eculizumab, suggesting it will be the next standard of care. Except for patients treated by intensive plasmatherapy or eculizumab, the worst prognosis is in factor H-HUS, as mortality can reach 20% and 50% of survivors do not recover renal function. Half of factor I-HUS progress to end-stage renal failure. Conversely, most patients with MCP-HUS have preserved renal function. Anti-factor H antibodies-HUS has favourable outcome if treated early.

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.

Alternative complement pathway assessment in patients with atypical HUS

The atypical Hemolytic Uremic Syndrome (aHUS) is a rare thrombotic microangiopathy leading to end stage renal disease in approximately 60% of patients. Over the last decade, a clear link has been demonstrated between this disease and defective complement regulation. The hallmark of the aHUS is the association with mutations in complement alternative pathway genes. Endothelial damage is related to complement dysregulation, but the exact mechanism is just starting to be elucidated. Screening for and characterization of mutations in the components of the C3 convertase (C3 and FB) or its regulators (FH, FI, MCP, and Thrombomodulin) or anti-FH antibodies has become an indispensable part of the disease's diagnostic. This review will initially summarize current knowledge on the understanding of complement activation and regulation, followed by a description on the genetic analysis as well as the methods used for complement protein quantification. Another part of this review will focus on the mechanisms of action of aHUS-associated mutations. We will emphasize on when and why some mutations lead to protein deficiency, while others result in - to dysfunctional but normally expressed proteins. Finally, we will discuss how the therapy of aHUS patients can be modified according to the functional consequences of each particular genetic defect.

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.

Thrombotic microangiopathies: new insights and new challenges

PURPOSE OF REVIEW

Thrombotic microangiopathies (TMAs) manifest as a spectrum of related disorders in the form of thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS). New data on both diseases support more and more the relatedness of the disorders and reveal related pathomechanisms, which, however, manifest in different organs. TTP develops primarily at neurological sites, and also in the kidney, and HUS is a kidney disease. In TTP thrombi formation occurs subsequently to the release of multimers of von Willebrand factor (vWF), and in HUS endothelial cell damage is considered the reason for complement and platelet activation leading to thrombus formation.

RECENT FINDINGS

Genetic mutations are associated with both disorders: in TTP the ADAMTS13 gene, the vWF cleaving protease, is affected, and in HUS several complement genes are mutated. In addition autoimmune forms, with acquired, de-novo generated inhibitors in the form of autoantibodies exist for both disorders, affecting ADAMTS13 in TTP or the central complement inhibitor factor H in HUS. In HUS autoantibodies can develop in the context of a specific mostly homozygous chromosomal deletion that represents a new subform of the disease, which is termed DEAP-HUS (deficient for CFHR proteins and autoantibody positive HUS).

SUMMARY

As the underlying disease mechanisms of TMA are now being better understood new options for a more precise diagnosis, improved therapy and prognosis for kidney transplantation become available for the benefit of patients. Here we summarize the recent developments in this rapidly progressing field.

Updating the physiology, exploration and disease relevance of complement factor H

The factor H (FH) protein (also known as beta1H globulin) is the main regulator of the complement alternative pathway. It exhibits multivalent binding sites to the complement component C3b, and polyanions and one binding site to sialic acid and cell surfaces. These multiple binding sites confer to FH a decay-accelerating factor activity in the fluid phase as well as at the cell surface. A defect in FH activity or a FH protein deficiency triggers chronic inflammation and tissue injury, leading to various disorders impacting the kidney or the eye. In contrast, some pathogens, as well as cancer cells, develop various strategies to bind FH and thereby subvert a complement attack. We focus on the functions of FH, and review the main pathological conditions in which FH is involved. Since the pathogenesis is elusive, appropriate FH dosage in biological fluids and FH gene analysis may help in improving understanding of such diseases.

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.

Mutations in complement factor I as found in atypical hemolytic uremic syndrome lead to either altered secretion or altered function of factor I

The complement system is regulated by inhibitors such as factor I (FI), a serine protease that degrades activated complement factors C4b and C3b in the presence of specific cofactors. Mutations and polymorphisms in FI and its cofactors are associated with atypical hemolytic uremic syndrome (aHUS). All 14 complement factor I mutations associated with aHUS analyzed in this study were heterozygous and generated premature stop codons (six) or amino acid substitutions (eight). Almost all of the mutants were expressed by human embryonic kidney 293 cells but only six mutants were secreted into the medium, three of which were at lower levels than WT. The remaining eight mutants were not secreted but sensitive to deglycosylation with endoglycosidase H, indicating that they were retained early in the secretory pathway. Six secreted mutants were purified and five of them were functionally altered in degradation of C4b/C3b in the fluid-phase in the presence of various cofactors and on endothelial cells. Three mutants cleaved surface-bound C3b less efficiently than WT. The D501N mutant was severely impaired both in solution and on surface irrespective of the cofactor used. In conclusion, mutations in complement factor I affect both secretion and function of FI, which leads to impaired regulation of the complement system in aHUS.

Mutations in components of complement influence the outcome of Factor I-associated atypical hemolytic uremic syndrome

Genetic studies have shown that mutations of complement inhibitors such as membrane cofactor protein, Factors H, I, or B and C3 predispose patients to atypical hemolytic uremic syndrome (aHUS). Factor I is a circulating serine protease that inhibits complement by degrading C3b and up to now only a few mutations in the CFI gene have been characterized. In a large cohort of 202 patients with aHUS, we identified 23 patients carrying exonic mutations in CFI. Their overall clinical outcome was unfavorable, as half died or developed end-stage renal disease after their first syndrome episode. Eight patients with CFI mutations carried at least one additional known genetic risk factor for aHUS, such as a mutation in MCP, CFH, C3 or CFB; a compound heterozygous second mutation in CFI; or mutations in both the MCP and CFH genes. Five patients exhibited homozygous deletion of the Factor H-related protein 1 (CFHR-1) gene. Ten patients with aHUS had one mutation in their CFI gene (Factor I-aHUS), resulting in a quantitative or functional Factor I deficiency. Patients with a complete deletion of the CFHR-1 gene had a significantly higher risk of a bad prognosis compared with those with one Factor I mutation as their unique vulnerability feature. Our results emphasize the necessity of genetic screening for all susceptibility factors in patients with aHUS.

Thrombomodulin mutations in atypical hemolytic-uremic syndrome

BACKGROUND

The hemolytic-uremic syndrome consists of the triad of microangiopathic hemolytic anemia, thrombocytopenia, and renal failure. The common form of the syndrome is triggered by infection with Shiga toxin-producing bacteria and has a favorable outcome. The less common form of the syndrome, called atypical hemolytic-uremic syndrome, accounts for about 10% of cases, and patients with this form of the syndrome have a poor prognosis. Approximately half of the patients with atypical hemolytic-uremic syndrome have mutations in genes that regulate the complement system. Genetic factors in the remaining cases are unknown. We studied the role of thrombomodulin, an endothelial glycoprotein with anticoagulant, antiinflammatory, and cytoprotective properties, in atypical hemolytic-uremic syndrome.

METHODS

We sequenced the entire thrombomodulin gene (THBD) in 152 patients with atypical hemolytic-uremic syndrome and in 380 controls. Using purified proteins and cell-expression systems, we investigated whether thrombomodulin regulates the complement system, and we characterized the mechanisms. We evaluated the effects of thrombomodulin missense mutations associated with atypical hemolytic-uremic syndrome on complement activation by expressing thrombomodulin variants in cultured cells.

RESULTS

Of 152 patients with atypical hemolytic-uremic syndrome, 7 unrelated patients had six different heterozygous missense THBD mutations. In vitro, thrombomodulin binds to C3b and factor H (CFH) and negatively regulates complement by accelerating factor I-mediated inactivation of C3b in the presence of cofactors, CFH or C4b binding protein. By promoting activation of the plasma procarboxypeptidase B, thrombomodulin also accelerates the inactivation of anaphylatoxins C3a and C5a. Cultured cells expressing thrombomodulin variants associated with atypical hemolytic-uremic syndrome had diminished capacity to inactivate C3b and to activate procarboxypeptidase B and were thus less protected from activated complement.

CONCLUSIONS

Mutations that impair the function of thrombomodulin occur in about 5% of patients with atypical hemolytic-uremic syndrome.

Complement inhibitor eculizumab in atypical hemolytic uremic syndrome

BACKGROUND AND OBJECTIVES

Atypical hemolytic uremic syndrome (aHUS) is associated with a congenital or acquired dysregulation of the complement alternative pathway that leads to continuous complement activation on host cells causing inflammation and damage. Eculizumab, a humanized mAb against complement protein C5, inhibits activation of the terminal complement pathway.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We report an adolescent with relapsing unclassified aHUS. On admission, a high plasma creatinine level indicated a poor prognosis, and hemodialysis had to be started. Plasma exchanges were initially effective against the microangiopathic hemolytic activity and allowed a temporary improvement of renal function with termination of hemodialysis after 7 wk. Subsequently, plasma exchanges (three times per week) failed to prevent ongoing aHUS activity and progressive renal failure. After 12 wk, aHUS treatment was switched to eculizumab.

RESULTS

Eculizumab was effective in terminating the microangiopathic hemolytic process in two aHUS relapses; however, after normalization of complement activity, aHUS recurred and ultimately led to anuric end-stage renal failure.

CONCLUSIONS

In this patient, complement inhibition by eculizumab temporarily terminated the microangiopathic hemolytic activity. Nevertheless, renal damage as a result of preceding and subsequent aHUS activity resulted in end-stage renal failure; therefore, therapeutic success may depend on early administration of eculizumab. The optimal duration of treatment may be variable and remains to be determined.

Complement evasion strategies of pathogens-acquisition of inhibitors and beyond

Activation of the complement system and resulting opsonisation with C3b are key events of the innate immune defense against infections. However, a wide variety of bacterial pathogens subvert complement attack by binding host complement inhibitors such as C4b-binding protein, factor H and vitronectin, which results in diminished opsonophagocytosis and killing of bacteria by lysis. Another widely used strategy is production of proteases, which can effectively degrade crucial complement components. Furthermore, bacterial pathogens such as Moraxella catarrhalis and Staphylococcus aureus capture and incapacitate the key complement component C3. The current review describes examples of these three strategies. Targeting binding sites for complement inhibitors on bacterial surfaces and complement-degrading proteases with vaccine-induced antibodies may be used to enhance a common vaccine design strategy that depends on the generation of complement-dependent bactericidal and opsonophagocytic antibody activities.

Contribution of interactions between complement inhibitor C4b-binding protein and pathogens to their ability to establish infection with particular emphasis on Neisseria gonorrhoeae

Complement activation and resulting opsonisation with C3b form key arms of the innate immune defense against infections. However, a wide variety of pathogens subvert complement attack by binding host complement inhibitors, which results in diminished opsonophagocytosis and killing of bacteria by lysis. Human C4b-binding protein (C4BP) binds Neisseria gonorrhoeae and Streptococcus pyogenes, both uniquely human pathogens. This binding specificity is circumvented by other bacterial species, which bind C4BP from numerous mammalian hosts that they infect. Binding of C4BP to Neisseria is mediated by outer membrane porin proteins and appears to be one of the main factors mediating serum resistance. Targeting C4BP binding sites on bacterial surfaces with vaccine-induced antibodies may block binding of C4BP and enhance a common vaccine design strategy that depends on the generation of complement-dependent bactericidal and opsonophagocytic antibody activities.

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.