Clustering of missense mutations in the C-terminal region of factor H in atypical hemolytic uremic syndrome

Complement factor H and hemicentin-1 in age-related macular degeneration and renal phenotypes

In this study, we investigated the associations of complement factor H (CFH) and hemicentin-1 (HMCN1) with age-related macular degeneration (AMD) and renal function. Three scales, measuring the course of AMD and drusen development, were examined in two samples: the Family Age-Related Macular degeneration Study (FARMS), consisting of families ascertained through a single individual with severe AMD, and an unascertained population-based family cohort, the Beaver Dam Eye Study (BDES), which was also used to assess longitudinal changes in AMD and associations with renal function. Associations were performed by a regression accounting for known risk factors as well as familial and sibling effects. Strong evidence of the association of rs1061170 (Y402H) variation with AMD was confirmed (P = 9.15 x 10(-5) in BDES, P = 0.016 in FARMS). This association was observed in multiple AMD scales, suggesting that its role is not phenotype-specific. Polymorphisms in both CFH and HMCN1 appeared to influence the longitudinal rate of change of AMD. The rs1061170 polymorphism was also associated with a reduction in estimated glomerular filtration rate (eGFR) (P = 0.046). Another CFH polymorphism, rs800292, was similarly associated with eGFR [beta = -0.90 (P = 0.022)]. Associations between rs743137 (P = 0.05) and rs680638 (P = 0.022) in HMCN1 with calculated creatinine clearance progression were also observed. Both genes appear to play a role in both AMD and renal pathophysiology. These findings support evidence for common pathways influencing ocular and renal function and suggest that further work is required on their common determinants.

Complete factor H deficiency-associated atypical hemolytic uremic syndrome in a neonate

Recent advances have shown that atypical hemolytic uremic syndrome (aHUS) is a disease of complement dysregulation. Almost 50% of cases are associated with mutations in the three complement regulatory genes, factor H (HF1), membrane co-factor protein (MCP) and factor I (IF). The corresponding gene products act in concert and affect the same enzyme, alternative pathway convertase C3bBb, which initiates the alternative pathway and amplification of the complement system. Factor H (FH) deficiency-associated aHUS usually occurs in infants to middle-aged adults and only rarely in neonates. Moreover, the vast majority of patients are heterozygous for the HF1 gene mutations. We report on a case of neonatal-onset aHUS associated with complete FH deficiency due to novel compound heterozygous mutations in the HF1 gene. A 22-day-old baby girl developed acute renal failure and a remarkably low serum complement C3 level, which was rapidly followed by the development of micro-angiopathic hemolytic anemia. Western blot analysis revealed nearly zero plasma FH levels, and an HF1 gene study showed compound heterozygous mutations, C1077W/Q1139X. Renal pathology findings were compatible with glomerular involvement in HUS. The baby recovered completely after the repetitive infusion of fresh frozen plasma. During follow-up (until she was 20 months old) after the initial plasma therapy, the disease recurred three times; twice after the tapering off of plasma therapy, and once during a weekly plasma infusion. All recurrence episodes were preceded by an upper respiratory tract infection, and were successfully managed by restarting or increasing the frequency of plasma therapy.

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

Factor H (FH) is an abundant serum glycoprotein that regulates the alternative pathway of complement-preventing uncontrolled plasma C3 activation and nonspecific damage to host tissues. Age-related macular degeneration (AMD), atypical hemolytic uremic syndrome (aHUS), and membranoproliferative glomerulonephritis type II (MPGN2) are associated with polymorphisms or mutations in the FH gene (Cfh), suggesting the existence of a genotype–phenotype relationship. Although AMD and MPGN2 share pathological similarities with the accumulation of complement-containing debris within the eye and kidney, respectively, aHUS is characterized by renal endothelial injury. This pathological distinction was reflected in our Cfh association analysis, which demonstrated that although AMD and MPGN2 share a Cfh at-risk haplotype, the haplotype for aHUS was unique. FH-deficient mice have uncontrolled plasma C3 activation and spontaneously develop MPGN2 but not aHUS. We show that these mice, transgenically expressing a mouse FH protein functionally equivalent to aHUS-associated human FH mutants, regulate C3 activation in plasma and spontaneously develop aHUS but not MPGN2. These animals represent the first model of aHUS and provide in vivo evidence that effective plasma C3 regulation and the defective control of complement activation on renal endothelium are the critical events in the molecular pathogenesis of FH-associated aHUS.

The decay accelerating factor mutation I197V found in hemolytic uraemic syndrome does not impair complement regulation

Hemolytic uremic syndrome is the clinical triad of thrombocytopenia, microangiopathic hemolytic anaemia and acute renal failure. Cases not associated with a preceding Shiga-like toxin producing Escherichia coli are described as atypical HUS (aHUS). Approximately 50% of patients with aHUS have mutations in one of three complement regulatory proteins, Factor H (CFH), membrane cofactor protein (MCP;CD46) or factor I (IF). A common feature of these three proteins is that they regulate complement by cofactor activity. Decay accelerating factor (DAF; CD55) regulates the complement system by disassociating the alternative and classical pathway convertases. Like CFH and MCP, the gene for DAF lies within the regulators of complement activation (RCA) gene cluster at 1q32. In 1998, we described linkage to this region in families with aHUS which led to the discovery of mutations in CFH and MCP. We therefore genotyped DAF in a panel of 46 aHUS patients including families with linkage to the RCA cluster. A mutation, I197V, was identified in one patient with familial HUS which was not found in 100 healthy controls. Molecular modelling of this mutation shows that the I197V mutation does not reside in an area which would be predicted to be important in decay accelerating activity. The expression of I197V on EBV-transformed B lymphocytes was equivalent to that of wild type controls. There was no significant decrease in decay acceleration activity of the recombinantly produced I197V mutant compared with wild type, as measured by a complement-mediated lytic assay. In conclusion, this study, identifies only one mutation in DAF in 46 patients with aHUS. This mutation, I197V, does not impair complement regulation and cannot be implicated in the pathogenesis of aHUS in this patient. This suggests that the complement regulatory abnormality in aHUS is principally one of deficient cofactor activity rather than of decay acceleration activity.

The interactive Factor H-atypical hemolytic uremic syndrome mutation database and website: update and integration of membrane cofactor protein and Factor I mutations with structural models

Atypical hemolytic uremic syndrome (aHUS) is a disease of hemolytic anemia, thrombocytopenia, and renal failure associated with defective alternative pathway (AP) complement control. Previously, we presented a database (www.FH-HUS.org) focusing on aHUS mutations in the Factor H gene (CFH). Here, new aHUS mutations are reported for the complement regulatory proteins Factor H (FH), Factor I (FI), and membrane cofactor protein (MCP). Additional mutations or polymorphisms within CFH have been associated with membranoproliferative glomerulonephritis (MPGN) and age-related macular degeneration (AMD). Accordingly, the database now includes substitutions that predispose to aHUS, MPGN, and AMD. For this, structural models for the domains in MCP and FI were developed using homology modeling. With this new database, patients with mutations in more than one gene can be displayed and interpreted in a coherent manner. The database also includes SNP polymorphisms in CFH, MCP, and IF. There are now a total of 167 genetic alterations, including 100 in CFH, 43 in MCP, and 24 in IF. The mutations characterize clinical outcomes that vary from several AMD-associated polymorphisms to those associated with aHUS, MPGN, or FI deficiency. A consensus short complement regulator (SCR) domain structure facilitated the interpretations of aHUS mutations. Specific locations within this consensus domain often correlate with the occurrence of clinical phenotypes. The AMD Tyr402His polymorphism is structurally located at a hotspot for several aHUS mutations. The database emphasizes the causative role of the alternative pathway of complement in disease and provides a repository of knowledge to assist future diagnosis and novel therapeutic approaches.

Genetic analysis of the complement factor H related 5 gene in haemolytic uraemic syndrome

Several mutations in the CFH gene have been described in non-Shiga-toxin-associated haemolytic uraemic syndrome (non-Stx-HUS), a rare syndrome characterized by haemolytic anaemia, thrombocytopenia and acute renal failure. Mutations in genes encoding other complement regulatory proteins, membrane cofactor protein (CD46) and complement factor I (CFI), were also involved in the pathogenesis of the disease. Anyway, mutations in the three genes account for no more than 50% of cases of non-Stx-HUS. Human complement factor H related 5 (CFHR5) is a recently characterised member of the human complement factor H (CFH) family that has been found as a component of immune deposits in human kidney with sclerotic lesions from different causes. CFHR5 possesses cofactor activity and has been proposed to play a role in complement regulation in the glomerulus. We screened CFHR5 gene for variations potentially involved in the aetiology of HUS. Forty-five patients with HUS and 80 controls were analysed. Altogether, 5 genetic variants in CFHR5 were found in overall 9/45 HUS patients and in 4/80 controls. Statistical analysis showed that allelic variants in CFHR5 were prefentially associated with HUS. Based on these data, we conclude that, though not causative, CFHR5 genetic alterations may play a secondary role in the pathogenesis of HUS.

The C-terminus of complement factor H is essential for host cell protection

Complement is a powerful self-amplifying system of innate immune defense with the capacity to eliminate microbes directly. Factor H is a central regulator in plasma which protects host tissue from complement mediated damage. Here we characterize the relevance of surface attached factor H, and study the regulatory activity of factor H on endothelial cells. Although these cells expressed membrane bound regulators, cell bound factor H contributed substantially to complement regulatory activity at the cell surface. Blockade of the C-terminus of factor H with monoclonal antibodies inhibited cell binding of this soluble regulator and resulted in enhanced complement activation on the cells. In the absence of factor H, increased deposition and slower inactivation of C3b resulted in higher amount of membrane attack complexes on the cell surface. When the membrane regulators CD55 and CD59 were removed by enzymatic treatment, complement mediated cell lysis was enhanced in the absence of factor H. Importantly, inhibition of the C-terminus did not compromise the regulatory function of factor H in fluid phase. Altogether these data point to a highly relevant, yet so far underestimated role of factor H for complement control at cellular surfaces, and reveal a decisive role of the factor H C-terminus in host cell recognition and protection.

Hemolytic Uremic Syndrome: A Factor H Mutation (E1172Stop) Causes Defective Complement Control at the Surface of Endothelial Cells

Defective complement regulation results in hemolytic uremic syndrome (HUS), a disease that is characterized by microangiopathy, thrombocytopenia, and acute renal failure and that causes endothelial cell damage. For characterization of how defective complement regulation relates to the pathophysiology, the role of the complement regulator factor H and also of a mutant factor H protein was studied on the surface of human umbilical vein endothelial cells. The mutant 145-kD factor H protein was purified to homogeneity, from plasma of a patient with HUS, who is heterozygous for a factor H gene mutation G3587T, which introduces a stop codon at position 1172. Functional analyses show that the lack of the most C-terminal domain short consensus repeats 20 severely affected recognition functions (i.e., binding to heparin, C3b, C3d, and the surface of endothelial cells). Wild-type factor H as well as the mutant protein formed dimers in solution as shown by cross-linking studies and mass spectroscopy. When assayed in fluid phase, the complement regulatory activity of the mutant protein was normal and comparable to wild-type factor H. However, on the surface of endothelial cells, the mutant factor H protein showed severely reduced regulatory activities and lacked protective functions. Similarly, with the use of sheep erythrocytes, the mutant protein lacked the protective activity and caused increased hemolysis when it was added to factor H-depleted plasma. This study shows how a mutation that affects the C-terminal region of the factor H protein leads to defective complement control on cell surfaces and damage to endothelial cells in patients with HUS. These effects explain how mutant factor H causes defective complement control and in HUS-particularly under condition of inflammation and complement activation-causes endothelial cell damage.

Inherited complement regulatory protein deficiency predisposes to human disease in acute injury and chronic inflammatory statesthe examples of vascular damage in atypical hemolytic uremic syndrome and debris accumulation in age-related macular degeneration

In this chapter, we examine the role of complement regulatory activity in atypical hemolytic uremic syndrome (aHUS) and age-related macular degeneration (AMD). These diseases are representative of two distinct types of complement-mediated injury, one being acute and self-limited, the other reflecting accumulation of chronic damage. Neither condition was previously thought to have a pathologic relationship to the immune system. However, alterations in complement regulatory protein genes have now been identified as major predisposing factors for the development of both diseases. In aHUS, heterozygous mutations leading to haploinsufficiency and function-altering polymorphisms in complement regulators have been identified, while in AMD, polymorphic haplotypes in complement genes are associated with development of disease. The basic premise is that a loss of function in a plasma or membrane inhibitor of the alternative complement pathway allows for excessive activation of complement on the endothelium of the kidney in aHUS and on retinal debris in AMD. These associations have much to teach us about the host's innate immune response to acute injury and to chronic debris deposition. We all experience cellular injury and, if we live long enough, will deposit debris in blood vessel walls (atherosclerosis leading to heart attacks and strokes), the brain (amyloid proteins leading to Alzheimer's disease), and retina (lipofuscin pigments leading to AMD). These are three common causes of morbidity and mortality in the developed world. The clinical, genetic, and immunopathologic understandings derived from the two examples of aHUS and AMD may illustrate what to anticipate in related conditions. They highlight how a powerful recognition and effector system, the alternative complement pathway, reacts to altered self. A response to acute injury or chronic debris accumulation must be appropriately balanced. In either case, too much activation or too little regulation promotes undesirable tissue damage and human disease.

Complement dysfunction in hemolytic uremic syndrome

PURPOSE OF REVIEW

Hemolytic uremic syndrome is a rare disease of microangiopathic hemolytic anemia, low platelet count and is associated with renal impairment. The atypical form, which occurs in adult patients, is associated with defective complement control.

RECENT FINDINGS

Recent data show that atypical hemolytic uremic syndrome is a genetic disease and gene mutations have been reported for factor H, membrane cofactor protein/CD46 and factor I. All corresponding gene products act in concert and control the activity of the complement convertase C3bBb. This enzyme initiates the alternative pathway as well as amplification of the complement system. Similar to genetic defects, autoantibodies which bind to factor H have been linked to the disease. Defective complement control resulting in hemolytic uremic syndrome explains the disease mechanism and allows improved diagnosis and therapy.

SUMMARY

The atypical form of hemolytic uremic syndrome is associated with defective complement control and inappropriate protein function and may influence disease progression and provide new ways for treatment. Positive effects were reported upon substitution of a defective protein by plasma exchange or plasmaphoreses. The disease recurrence rate for renal transplants depends on the type of gene mutated; patients with mutations in the membrane cofactor protein gene have a better prognosis than patients with mutations in other genes.

An interactive web database of factor H-associated hemolytic uremic syndrome mutations: insights into the structural consequences of disease-associated mutations

Factor H (FH) is a central complement regulator comprised of 20 short complement repeat (SCR) domains. Nucleotide changes within this gene (CFH) have been observed in patients with hemolytic uremic syndrome (HUS), and also membranoproliferative glomerulonephritis and age-related macular degeneration. All parts of FH are affected, but many mutations are clustered in the C-terminal part of FH. Up to now, structural analyses of HUS have been based on SCR-20, a domain that is involved in FH interactions with C3b, heparin, and endothelial cells. In order to identify the structural and functional consequence of HUS mutations, further disease-associated mutations were analyzed in terms of homology and nuclear magnetic resonance (NMR) models for factor H SCR domains. An interactive web database of 54 human HUS-associated mutations and others was created from the literature (www.FH-HUS.org). This has comprehensive search and analysis tools, integrating phenotypic and genetic data with structural analysis. Each mutation can be highlighted on the SCR structure together with the patient FH and C3 levels where available. Two new insights were obtained from our collection of data. First, phenotypic data on FH clarify our previously-proposed classification of Type I and Type II disorders that both lead to HUS, where Type I affects FH secretion and folding, and Type II leads to expressed protein in plasma that is functionally defective. Second, the new mutations show more clearly that SCR domains from SCR-16 to SCR-19 are important for the ligand binding activities of FH as well as SCR-20. This FH web database will facilitate the interpretation of new mutations and polymorphisms when these are identified in patients, and it will clarify the functional role of FH.

Gain-of-function mutations in complement factor B are associated with atypical hemolytic uremic syndrome

Hemolytic uremic syndrome (HUS) is an important cause of acute renal failure in children. Mutations in one or more genes encoding complement-regulatory proteins have been reported in approximately one-third of nondiarrheal, atypical HUS (aHUS) patients, suggesting a defect in the protection of cell surfaces against complement activation in susceptible individuals. Here, we identified a subgroup of aHUS patients showing persistent activation of the complement alternative pathway and found within this subgroup two families with mutations in the gene encoding factor B (BF), a zymogen that carries the catalytic site of the complement alternative pathway convertase (C3bBb). Functional analyses demonstrated that F286L and K323E aHUS-associated BF mutations are gain-of-function mutations that result in enhanced formation of the C3bBb convertase or increased resistance to inactivation by complement regulators. These data expand our understanding of the genetic factors conferring predisposition to aHUS, demonstrate the critical role of the alternative complement pathway in the pathogenesis of aHUS, and provide support for the use of complement-inhibition therapies to prevent or reduce tissue damage caused by dysregulated complement activation.

Determination of complement factor H functional polymorphisms (V62I, Y402H, and E936D) using sequence-specific primer PCR and restriction fragment length polymorphisms

BACKGROUND

Complement factor H (CFH; HF) is an essential regulatory protein that plays a critical role in the homeostasis of the complement system in plasma. Several polymorphisms and mutations in the complement factor H gene (CFH; HF1) have been identified. These have revealed interesting associations with hemolytic-uremic syndrome and age-related macular degeneration.

METHODS AND RESULTS

The aim of this study was to develop a rapid and reliable assay for determining genotypic variants of the CFH gene. Sequence-specific primer PCR and restriction fragment length polymorphism techniques were chosen for the analysis of CFH polymorphisms. The assays detected the following published single nucleotide polymorphisms of CFH in our Caucasian population (n = 271): rs800292, 257G-->A (V62I); rs1061170, 1277T-->C (Y402H); and rs1065489, 2881G-->T (E936D). The allele frequencies (257G = 0.850, 1277T = 0.574, and 2881G = 0.839) that we obtained from a healthy Hungarian population were consistent with previously published results.

CONCLUSION

These analytical methods are simple, reliable, and rapid to perform, and are amenable to automation. Therefore, they could facilitate large-scale genotypic analyses of the CFH gene in various diseases, such as hemolytic-uremic syndrome, age-related macular degeneration, and cardiovascular diseases.

Critical role of the C-terminal domains of factor H in regulating complement activation at cell surfaces

The plasma protein factor H primarily controls the activation of the alternative pathway of complement. The C-terminal of factor H is known to be involved in protection of host cells from complement attack. In the present study, we show that domains 19-20 alone are capable of discriminating between host-like and complement-activating cells. Furthermore, although factor H possesses three binding sites for C3b, binding to cell-bound C3b can be almost completely inhibited by the single site located in domains 19-20. All of the regulatory activities of factor H are expressed by the N-terminal four domains, but these activities toward cell-bound C3b are inhibited by isolated recombinant domains 19-20 (rH 19-20). Direct competition with the N-terminal site is unlikely to explain this because regulation of fluid phase C3b is unaffected by domains 19-20. Finally, we show that addition of isolated rH 19-20 to normal human serum leads to aggressive complement-mediated lysis of normally nonactivating sheep erythrocytes and moderate lysis of human erythrocytes, which possess membrane-bound regulators of complement. Taken together, the results highlight the importance of the cell surface protective functions exhibited by factor H compared with other complement regulatory proteins. The results may also explain why atypical hemolytic uremic syndrome patients with mutations affecting domains 19-20 can maintain complement homeostasis in plasma while their complement system attacks erythrocytes, platelets, endothelial cells, and kidney tissue.

Modeling how CD46 deficiency predisposes to atypical hemolytic uremic syndrome

Mutations in complement regulatory proteins predispose to the development of aHUS. Approximately 50% of patients bear a mutation in one of three complement control proteins, factor H, factor I, or membrane cofactor protein (MCP; CD46). Another membrane regulator that is closely related to MCP, decay accelerating factor (DAF; CD55) thus far has shown no association with aHUS and continues to be investigated. The goal of this study was to compare the regulatory profile of MCP and DAF and to assess how alterations in MCP predispose to complement dysregulation. We employed a model system of complement activation on Chinese hamster ovary (CHO) cell transfectants. The four regularly expressed isoforms of MCP and DAF inhibited C3b deposition by the alternative pathway. DAF, but not MCP, inhibited the classical pathway. Most patients with MCP-aHUS are heterozygous and express only 25-50% of the wild-type protein. We, therefore, analyzed the effect of reduced levels of wild-type MCP and found that cells with lowered expression levels were less efficient in inhibiting alternative pathway activation. Further, a dysfunctional MCP mutant, expressed at normal levels and identified in five patients with aHUS (S206P), failed to protect against C3b amplification on CHO cells, even if expression levels were increased 10-fold. Our results add new information relative to the necessity for appropriate expression levels of MCP and further implicate the alternative pathway in disease processes such as aHUS.

Primary glomerulonephritis with isolated C3 deposits: a new entity which shares common genetic risk factors with haemolytic uraemic syndrome

INTRODUCTION

Abnormal control of the complement alternative pathway (CAP) (factor H, factor I and membrane cofactor protein (MCP) deficiencies) is a well established risk factor for the occurrence of haemolytic uraemic syndrome (HUS). In some instances, HUS may be associated with an unusual glomerulonephritis with isolated C3 deposits (glomerulonephritis C3). We determined whether HUS and glomerulonephritis C3 share common genetic susceptibility factors.

METHODS

We identified 19 patients with glomerulonephritis C3. We measured levels of circulating complement components, performed assays for the detection of C3 nephritic factor (C3NeF) and screened factor H, factor I and MCP coding genes for the presence of mutations.

RESULTS

Patients were divided in two groups based on renal pathology findings: group I (n = 13) had typical features of type I membranoproliferative glomerulonephritis (glomerulonephritis C3 with membranoproliferative glomerulonephritis (MPGN)) and group II (n = 6) was characterised by mesangial and epimembranous C3 deposits in the absence of mesangial proliferation (glomerulonephritis C3 without MPGN). Mutations in complement regulatory genes were detected in 4/6 patients with glomerulonephritis C3 without MPGN (heterozygous mutations in factor H gene (two patients) with low factor H antigenic level in one case, heterozygous mutations in factor I gene (two patients)) and in only 2/13 patients with glomerulonephritis C3 with MPGN (heterozygous mutations in factor H gene (one patient) and double heterozygous mutation in CD 46 gene (one patient)). In contrast, C3NeF was present in 5/13 patients with glomerulonephritis C3 with MPGN and in 2/6 patients with glomerulonephritis C3 without MPGN, one of whom had a factor H mutation.

CONCLUSION

HUS and glomerulonephritis C3 without MPGN share common genetic risk factors. Constitutional or acquired dysregulation of the CAP is probably associated with a wide spectrum of diseases, ranging from HUS to glomerulonephritis C3 with MPGN.

Atypical haemolytic uraemic syndrome associated with a hybrid complement gene

BACKGROUND

Sequence analysis of the regulators of complement activation (RCA) cluster of genes at chromosome position 1q32 shows evidence of several large genomic duplications. These duplications have resulted in a high degree of sequence identity between the gene for factor H (CFH) and the genes for the five factor H-related proteins (CFHL1-5; aliases CFHR1-5). CFH mutations have been described in association with atypical haemolytic uraemic syndrome (aHUS). The majority of the mutations are missense changes that cluster in the C-terminal region and impair the ability of factor H to regulate surface-bound C3b. Some have arisen as a result of gene conversion between CFH and CFHL1. In this study we tested the hypothesis that nonallelic homologous recombination between low-copy repeats in the RCA cluster could result in the formation of a hybrid CFH/CFHL1 gene that predisposes to the development of aHUS.

METHODS AND FINDINGS

In a family with many cases of aHUS that segregate with the RCA cluster we used cDNA analysis, gene sequencing, and Southern blotting to show that affected individuals carry a heterozygous CFH/CFHL1 hybrid gene in which exons 1-21 are derived from CFH and exons 22/23 from CFHL1. This hybrid encodes a protein product identical to a functionally significant CFH mutant (c.3572C>T, S1191L and c.3590T>C, V1197A) that has been previously described in association with aHUS.

CONCLUSIONS

CFH mutation screening is recommended in all aHUS patients prior to renal transplantation because of the high risk of disease recurrence post-transplant in those known to have a CFH mutation. Because of our finding it will be necessary to implement additional screening strategies that will detect a hybrid CFH/CFHL1 gene.

Genetics of HUS: the impact of MCP, CFH, and IF mutations on clinical presentation, response to treatment, and outcome

Hemolytic uremic syndrome (HUS) is a thrombotic microangiopathy with manifestations of hemolytic anemia, thrombocytopenia, and renal impairment. Genetic studies have shown that mutations in complement regulatory proteins predispose to non-Shiga toxin-associated HUS (non-Stx-HUS). We undertook genetic analysis on membrane cofactor protein (MCP), complement factor H (CFH), and factor I (IF) in 156 patients with non-Stx-HUS. Fourteen, 11, and 5 new mutational events were found in MCP, CFH, and IF, respectively. Mutation frequencies were 12.8%, 30.1%, and 4.5% for MCP, CFH, and IF, respectively. MCP mutations resulted in either reduced protein expression or impaired C3b binding capability. MCP-mutated patients had a better prognosis than CFH-mutated and nonmutated patients. In MCP-mutated patients, plasma treatment did not impact the outcome significantly: remission was achieved in around 90% of both plasma-treated and plasma-untreated acute episodes. Kidney transplantation outcome was favorable in patients with MCP mutations, whereas the outcome was poor in patients with CFH and IF mutations due to disease recurrence. This study documents that the presentation, the response to therapy, and the outcome of the disease are influenced by the genotype. Hopefully this will translate into improved management and therapy of patients and will provide the way to design tailored treatments.

Implications of the initial mutations in membrane cofactor protein (MCP; CD46) leading to atypical hemolytic uremic syndrome

The hemolytic uremic syndrome is characterized by the triad of microangiopathic hemolytic anemia, thrombocytopenia and acute renal failure. There are two general types. One occurs in epidemic form and is diarrheal associated (D+HUS). It has a good prognosis. The second is a rare form known as atypical (aHUS), which may be familial or sporadic, and has a poor prognosis. aHUS is increasingly recognized to be a disease of defective complement regulation, particularly cofactor activity. Mutations in membrane cofactor protein (MCP; CD46) that predispose to the development of aHUS were first identified in 2003. MCP is a membrane-bound complement regulator that acts as a cofactor for the factor I-mediated cleavage of C3b and C4b deposited on host cells. More than 20 different mutations in MCP have now been identified in patients with aHUS. Many of these mutants have been functionally characterized and have helped to define the pathogenic mechanisms leading to aHUS development. Over 75% of the reported mutations cause a reduction in MCP expression, due to homozygous, compound heterozygous or heterozygous mutations. This deficiency of MCP leads to inadequate control of complement activation on endothelial cells after an initiating injury. The remaining MCP mutants are expressed, but demonstrate reduced ligand (C3b/C4b) binding capacity and cofactor activity of MCP. MCP mutations in aHUS demonstrate incomplete penetrance, indicating that additional genetic and environmental factors are required to manifest disease. MCP mutants as a cause of aHUS have a favorable clinical outcome in comparison to patients with factor H (CFH) or factor I (IF) mutations. In 90% of the renal transplants performed in patients with MCP-HUS, there has been no recurrence of the primary disease, whilst >50% of factor I or factor H deficient patients have had a prompt recurrence. This highlights the importance of defining and characterizing the underlying genetic defects in patients with aHUS.

Favorable long-term outcome after liver-kidney transplant for recurrent hemolytic uremic syndrome associated with a factor H mutation

A male child initially presented with atypical hemolytic uremic syndrome (HUS) at the age of 4 months and progressed within weeks to end stage renal disease (ESRD). At the age of 2 years he received a live-related kidney transplant from his mother, which, despite initial good function, was lost to recurrent disease after 2 weeks. Complement factor H analysis showed low serum levels and the presence of two mutations on different alleles (c.2918G > A, Cys973Tyr and c.3590T > C, Val1197Ala). His survival on dialysis was at risk because of access failure and recurrent bacteremic episodes. Therefore, at the age of 5 years he received a combined liver-kidney transplant with pre-operative plasma exchange. Initial function of both grafts was excellent and this has been maintained for over 2 years. This report suggests that despite setbacks in previous experience, combined liver-kidney transplantation offers the prospect of a favorable long-term outcome for patients with HUS associated with complement factor H mutations.

The C-terminus of complement regulator Factor H mediates target recognition: evidence for a compact conformation of the native protein

The complement inhibitor Factor H has three distinct binding sites for C3b and for heparin, but in solution uses specifically the most C-terminal domain, i.e. short consensus repeats (SCR) 20 for ligand interaction. Two novel monoclonal antibodies (mABs C14 and C18) that bind to the most C-terminal domain SCR 20 completely blocked interaction of Factor H with the ligands C3b, C3d, heparin and binding to endothelial cells. In contrast, several mAbs that bind to the N-terminus and to the middle regions of the molecule showed no or minor inhibitory effects when assayed by enzyme-linked immunosorbent assay (ELISA) and ligand interaction assays. This paradox between a single functional binding site identified for native Factor H versus multiple interaction sites reported for deletion constructs is explained by a compact conformation of the fluid phase protein with one accessible binding site. On zymosan particles mAbs C14 and C18 blocked alternative pathway activation completely. Thus demonstrating that native Factor H makes the first and initial contact with the C terminus, which is followed by N terminally mediated complement regulation. These results are explained by a conformational hypothetical model: the native Factor H protein has a compact structure and only one binding site accessible. Upon the first contact the protein unfolds and exposes the additional binding sites. This model does explain how Factor H mediates recognition functions during complement control and the clustering of disease associated mutations in patients with haemolytic uraemic syndrome that have been reported in the C-terminal recognition domain of Factor H.

Genetic and functional analyses of membrane cofactor protein (CD46) mutations in atypical hemolytic uremic syndrome

Hemolytic uremic syndrome (HUS) is characterized by the triad of thrombocytopenia, microangiopathic hemolytic anemia, and acute renal failure. The non-Shiga toxin-associated HUS (atypical HUS [aHUS]) has been shown to be a disease of complement dysregulation. Mutations in the plasma complement regulators factor H and factor I and the widely expressed membrane cofactor protein (MCP; CD46) have been described recently. This study looked for MCP mutations in a panel of 120 patients with aHUS. In this cohort, approximately 10% of patients with aHUS (11 patients; nine pedigrees) have mutations in MCP. The onset typically was in early childhood. Unlike patients with factor I or factor H mutations, most of the patients do not develop end-stage renal failure after aHUS. The majority of patients have a mutation that causes reduced MCP surface expression. A small proportion expressed normal levels of a dysfunctional protein. As in other studies, incomplete penetrance is shown, suggesting that MCP is a predisposing factor rather than a direct causal factor. The low level of recurrence of aHUS in transplantation in patients with MCP mutations is confirmed, and the first MCP null individuals are described. This study confirms the association between MCP deficiency and aHUS and further establishes that a deficiency in complement regulation, specifically cofactor activity, predisposes to severe thrombotic microangiopathy in the renal vasculature.

No association between complement factor H gene polymorphism and exudative age-related macular degeneration in Japanese

Age-related macular degeneration (ARMD) is the leading cause of blindness in the elderly population not only Western but also Asian industrial countries. In Caucasian, a polymorphism of the complement factor H gene (CFH), the C allele of rs1061170 (Y402H), was established as the first strong genetic factor for excursively exudative type of ARMD. In this study, we performed an extensive sequencing of the 22 exons in the CFH gene by recruiting 146 exudative ARMD patients and 105 normal controls of Japanese origin and identified 61 polymorphisms. We found that the frequency of the C allele of rs1061170 (Y402H) is much lower (0.04) in Japanese controls than in Caucasians (0.45). No case disease susceptibility to exudative ARMD was noted for rs1061170 (Y402H) (chi (2) = 3.19, P (corr) = 0.423), or other 12 single nucleotide polymorphisms (SNPs) whose frequency is greater than 0.05. When haplotypes were inferred for 13 SNPs (these 12 SNPs with a frequency greater than 0.05 and rs1061170), three haplotypes whose pattern was similar to those in Caucasians were identified but with substantial difference in frequency. Again we failed to identify genetic association between Japanese exudative ARMD and any of the haplotypes including the J1 haplotype which was shown to be susceptible to ARMD in Caucasians (chi (2 )=( )3.92, P (corr) = 0.157). CFH does not appear to be a primary hereditary contributor to ARMD in Japanese. The absence of CFH contribution to ARMD in Japanese may correlate with the findings in ethnic differences of ARMD phenotypes.

Genetics of HUS: the impact of MCP, CFH, and IF mutations on clinical presentation, response to treatment, and outcome

Hemolytic uremic syndrome (HUS) is a thrombotic microangiopathy with manifestations of hemolytic anemia, thrombocytopenia, and renal impairment. Genetic studies have shown that mutations in complement regulatory proteins predispose to non-Shiga toxin-associated HUS (non-Stx-HUS). We undertook genetic analysis on membrane cofactor protein (MCP), complement factor H (CFH), and factor I (IF) in 156 patients with non-Stx-HUS. Fourteen, 11, and 5 new mutational events were found in MCP, CFH, and IF, respectively. Mutation frequencies were 12.8%, 30.1%, and 4.5% for MCP, CFH, and IF, respectively. MCP mutations resulted in either reduced protein expression or impaired C3b binding capability. MCP-mutated patients had a better prognosis than CFH-mutated and nonmutated patients. In MCP-mutated patients, plasma treatment did not impact the outcome significantly: remission was achieved in around 90% of both plasma-treated and plasma-untreated acute episodes. Kidney transplantation outcome was favorable in patients with MCP mutations, whereas the outcome was poor in patients with CFH and IF mutations due to disease recurrence. This study documents that the presentation, the response to therapy, and the outcome of the disease are influenced by the genotype. Hopefully this will translate into improved management and therapy of patients and will provide the way to design tailored treatments.

Structure of complement factor H carboxyl-terminus reveals molecular basis of atypical haemolytic uremic syndrome

Factor H (FH) is the key regulator of the alternative pathway of complement. The carboxyl-terminal domains 19-20 of FH interact with the major opsonin C3b, glycosaminoglycans, and endothelial cells. Mutations within this area are associated with atypical haemolytic uremic syndrome (aHUS), a disease characterized by damage to endothelial cells, erythrocytes, and kidney glomeruli. The structure of recombinant FH19-20, solved at 1.8 A by X-ray crystallography, reveals that the short consensus repeat domain 20 contains, unusually, a short alpha-helix, and a patch of basic residues at its base. Most aHUS-associated mutations either destabilize the structure or cluster in a unique region on the surface of FH20. This region is close to, but distinct from, the primary heparin-binding patch of basic residues. By mutating five residues in this region, we show that it is involved, not in heparin, but in C3b binding. Therefore, the majority of the aHUS-associated mutations on the surface of FH19-20 interfere with the interaction between FH and C3b. This obviously leads to impaired control of complement attack on plasma-exposed cell surfaces in aHUS.

The central role of the alternative complement pathway in human disease

The complement system is increasingly recognized as important in the pathogenesis of tissue injury in vivo following immune, ischemic, or infectious insults. Within the complement system, three pathways are capable of initiating the processes that result in C3 activation: classical, alternative, and lectin. Although the roles that proinflammatory peptides and complexes generated during complement activation play in mediating disease processes have been studied extensively, the relative contributions of the three activating pathways is less well understood. Herein we examine recent evidence that the alternative complement pathway plays a key and, in most instances, obligate role in generating proinflammatory complement activation products in vivo. In addition, we discuss new concepts regarding the mechanisms by which the alternative pathway is activated in vivo, as recent clinical findings and experimental results have provided evidence that continuous active control of this pathway is necessary to prevent unintended targeting and injury to self tissues.

Complement factor H-associated atypical hemolytic uremic syndrome in monozygotic twins: concordant presentation, discordant response to treatment

Hemolytic uremic syndrome not associated with diarrhea (diarrhea negative, atypical) is less common than the diarrhea-positive typical form, but frequently results in end-stage renal failure. Although there are anecdotal cases of successful treatment with fresh frozen plasma alone, the value of this treatment compared with plasma exchange (PE) is difficult to assess. We describe monozygotic female twins who presented at 5 years of age with factor H-related (c.3572 > T; Ser1191Leu) atypical hemolytic uremic syndrome within months of each other. In the first twin to present, 10 sessions of PE with fresh frozen plasma replacement (40 mL/kg) resulted in resolution of hemolysis and improvement in plasma creatinine level (1.9 to 1.5 mg/dL [166 to 137 micromol/L]). Subsequently, 17 infusions of fresh frozen plasma were administered during a 4-month period for recurrent thrombocytopenia. However, within 4 months, plasma creatinine level increased to 5.1 mg/dL (450 micromol/L), necessitating peritoneal dialysis. When the second twin presented with the same disease, an extended PE regimen was instituted. After 10 daily sessions, PE was continued once every 2 weeks. Two recurrences were treated successfully with daily PE for 7 days. After 44 months of follow-up, kidney function is normal (plasma creatinine, 0.6 mg/dL [53 micromol/L]; creatinine clearance, 119 mL/min/1.73 m2 [1.98 mL/s/1.73 m2]) on maintenance PE therapy. In conclusion, the response to treatment of these monozygotic twins suggests that long-term PE may have benefits over plasma infusion alone.

Disease-associated sequence variations congregate in a polyanion recognition patch on human factor H revealed in three-dimensional structure

Mutations and polymorphisms in the regulator of complement activation, factor H, have been linked to atypical hemolytic uremic syndrome (aHUS), membranoproliferative glomerulonephritis, and age-related macular degeneration. Many aHUS patients carry mutations in the two C-terminal modules of factor H, which normally confer upon this abundant 155-kDa plasma glycoprotein its ability to selectively bind self-surfaces and prevent them from inappropriately triggering the complement cascade via the alternative pathway. In the current study, the three-dimensional solution structure of the C-terminal module pair of factor H has been determined. A binding site for a fully sulfated heparin-derived tetrasaccharide has been delineated using chemical shift mapping and the C3d/C3b-binding site inferred from sequence comparisons and computational docking. The resultant information allows assessment of the likely consequences of aHUS-associated amino acid substitutions in this critical region of factor H. It is striking that, excepting those likely to perturb the three-dimensional structure, aHUS-associated missense mutations congregate in the polyanion-binding site delineated in this study, thus potentially disrupting a vital mechanism for control of complement on self-surfaces in the microvasculature of the kidney. It is intriguing that a single nucleotide polymorphism predisposing to age-related macular degeneration occupies another region of factor H that harbors a polyanion-binding site.

Does complement factor B have a role in the pathogenesis of atypical HUS?

Atypical haemolytic uraemic syndrome (aHUS) is a disorder of complement dysregulation. Because complement factor B (fB) carries the catalytic site of the alternative pathway convertase we examined it as both a potential candidate gene and modifier in the pathogenesis of aHUS. No factor B gene (BF) mutations were found in 20 patients with aHUS. There was no statistical difference between controls and aHUS patients in either BF allele or haplotype frequency. In conclusion, in this small series of aHUS patients we found no evidence that fB has a major role in the pathogenesis of aHUS.

Phenotypic expression of factor H mutations in patients with atypical hemolytic uremic syndrome

We investigated the phenotypic expression of factor H mutations in two patients with atypical hemolytic uremic syndrome (HUS). Factor H in serum was assayed by rocket immunoelectrophoresis, immunoblotting, and double immunodiffusion and in tissue by immunohistochemistry. Functional activity was analyzed by hemolysis of sheep erythrocytes and binding to endothelial cells. A homozygous mutation in complement control protein (CCP) domain 10 of factor H was identified in an adult man who first developed membranoproliferative glomerulonephritis and later HUS. C3 levels were very low. The patient had undetectable factor H levels in serum and a weak factor H 150 kDa band. Double immunodiffusion showed partial antigenic identity with factor H in normal serum owing to the presence of factor H-like protein 1. Strong specific labeling for factor H was detected in glomerular endothelium, mesangium and in glomerular and tubular epithelium as well as in bone marrow cells. A heterozygous mutation in CCP 20 of factor H was found in a girl with HUS. C3 levels were moderately decreased at onset. Factor H levels were normal and a normal 150 kDa band was present. Double immunodiffusion showed antigenic identity with normal factor H. Factor H labeling was minimal in the renal cortex. Factor H dysfunction was demonstrated by increased sheep erythrocyte hemolysis and decreased binding to endothelial cells. In summary, two different factor H mutations associated with HUS were examined: in one, factor H accumulated in cells, and in the other, membrane binding was reduced.

Complement and diseases: Defective alternative pathway control results in kidney and eye diseases

The complement system is a central part of innate immunity and in its normal setting aimed to recognize and eliminate microbes. For elimination toxic activation products are generated locally and are reported directly of the surface of the invading microbe. A deregulation of the alternative pathway results in defective recognition and toxic activation products can be formed on the surface of host tissues and structures. Recent studies have shown that mutated or defective regulators of the alternative pathway of complement are associated with auto immune diseases of the kidney, including the atypical form of hemolytic uremic syndrome (HUS), membranoproliferative glomerulonephritis (MPGN) and also of the eye, such as age-related macular degeneration (ARMD). Current research provides clues how mutations occurring in genes coding for single complement components or the inactivation of single regulators lead to defective alternative pathway amplification, via the convertase C3bBb. These scenarios explain how defects of a single regulator lead to local, organ specific damage.

Extended haplotypes in the complement factor H (CFH) and CFH-related (CFHR) family of genes protect against age-related macular degeneration: characterization, ethnic distribution and evolutionary implications

BACKGROUND

Variants in the complement factor H gene (CFH) are associated with age-related macular degeneration (AMD). CFH and five CFH-related genes (CFHR1-5) lie within the regulators of complement activation (RCA) locus on chromosome 1q32. Aims and Methods. In this study, the structural and evolutionary relationships between these genes and AMD was refined using a combined genetic, molecular and immunohistochemical approach.

RESULTS

We identify and characterize a large, common deletion that encompasses both the CFHR1 and CFHR3 genes. CFHR1, an abundant serum protein, is absent in subjects homozygous for the deletion. Genotyping analyses of AMD cases and controls from two cohorts demonstrates that deletion homozygotes comprise 1.1% of cases and 5.7% of the controls (chi-square=32.8; P= 1.6 E-09). CFHR1 and CFHR3 transcripts are abundant in liver, but undetectable in the ocular retinal pigmented epithelium/choroid complex. AMD-associated CFH/CFHR1/CFHR3 haplotypes are widespread in human populations.

CONCLUSION

The absence of CFHR1 and/or CFHR3 may account for the protective effects conferred by some CFH haplotypes. Moreover, the high frequencies of the 402H allele and the delCFHR1/CFHR3 alleles in African populations suggest an ancient origin for these alleles. The considerable diversity accumulated at this locus may be due to selection, which is consistent with an important role for the CFHR genes in innate immunity.

Extended haplotypes in the complement factor H (CFH) and CFH-related (CFHR) family of genes protect against age-related macular degeneration: Characterization, ethnic distribution and evolutionary implications

BACKGROUND

Variants in the complement factor H gene (CFH) are associated with age-related macular degeneration (AMD). CFH and five CFH-related genes (CFHR1-5) lie within the regulators of complement activation (RCA) locus on chromosome 1q32.

AIMS AND METHODS

In this study, the structural and evolutionary relationships between these genes and AMD was refined using a combined genetic, molecular and immunohistochemical approach.

RESULTS

We identify and characterize a large, common deletion that encompasses both the CFHR1 and CFHR3 genes. CFHR1, an abundant serum protein, is absent in subjects homozygous for the deletion. Genotyping analyses of AMD cases and controls from two cohorts demonstrates that deletion homozygotes comprise 1.1% of cases and 5.7% of the controls (chi-square = 32.8; P = 1.6 E-09). CFHR1 and CFHR3 transcripts are abundant in liver, but undetectable in the ocular retinal pigmented epithelium/choroid complex. AMD-associated CFH/CFHR1/CFHR3 haplotypes are widespread in human populations.

CONCLUSION

The absence of CFHR1 and/or CFHR3 may account for the protective effects conferred by some CFH haplotypes. Moreover, the high frequencies of the 402H allele and the delCFHR1/CFHR3 alleles in African populations suggest an ancient origin for these alleles. The considerable diversity accumulated at this locus may be due to selection, which is consistent with an important role for the CFHR genes in innate immunity.

Factor H and atypical hemolytic uremic syndrome: mutations in the C-terminus cause structural changes and defective recognition functions

Atypical hemolytic uremic syndrome is a disease that is characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. Mutations in the complement regulator factor H are associated with the inherited form of the disease, and >60% of the mutations are located within the C terminus of factor H. The C-terminus of factor H, represented by short consensus repeat 19 (SCR19) and SCR20, harbors multiple functions; consequently, this study aimed to examine the functional effects of clinically reported mutations in these SCR. Mutant factor H proteins (W1157R, W1183L, V1197A, R1210C, R1215G, and P1226S) were recombinantly expressed and functionally characterized. All six mutant proteins showed severely reduced heparin, C3b, C3d, and endothelial cell binding. By peptide spot analyses, four linear regions that are involved in heparin, C3b, and C3d binding were localized in SCR19 and SCR20. A three-dimensional homology model of the two domains suggests that these four regions form a common binding site across both domains. In addition, this structural model identifies two types of residues: Type A residues are positioned on the SCR surface and are represented by mutants W1157R, W1183L, R1210C, and R1215G; and type B residues are buried within the SCR structure and affect mutations V1197A and P1226S. Mutations of both types of residue result in the same functional defects, namely the reduced binding of factor H to surface-attached C3b molecules and reduced complement regulatory activity at the cell surfaces. The buried type B mutations seem to affect ligand interaction of factor H more severely than the surface-exposed mutations.

Outcome of renal transplantation in patients with non-Shiga toxin-associated hemolytic uremic syndrome: prognostic significance of genetic background

More than 50% of patients with non-Shiga toxin-associated hemolytic uremic syndrome (non-Stx-HUS) progress to ESRD. Kidney transplant failure for disease recurrence is common; hence, whether renal transplantation is appropriate in this clinical setting remains a debated issue. The aim of this study was to identify possible prognostic factors for renal transplant outcome by focusing on specific genetic abnormalities associated with the disease. All articles in literature that describe renal transplant outcome in patients with ESRD secondary to non-Stx-HUS, genotyped for CFH, MCP, and IF mutations, were reviewed, and data of patients who were referred to the International Registry of Recurrent and Familial HUS/TTP and data from the Newcastle cohort were examined. This study confirmed that the overall outcome of kidney transplantation in patients with non-Stx-HUS is poor, with disease recurring in 60% of patients, 91.6% of whom developed graft failure. No clinical prognostic factor that could identify patients who were at high risk for graft failure was found. The presence of a factor H (CFH) mutation was associated with a high incidence of graft failure (77.8 versus 54.9% in patients without CFH mutation). Similar results were seen in patients with a factor I (IF) mutation. In contrast, graft outcome was favorable in all patients who carried a membrane co-factor protein (MCP) mutation. Patients with non-Stx-HUS should undergo genotyping before renal transplantation to help predict the risk for graft failure. It is debatable whether a kidney transplant should be recommended for patients with CFH or IF mutation. Reasonably, patients with an MCP mutation can undergo a kidney transplant without risk for recurrence.

The hemolytic uremic syndromes

PURPOSE OF REVIEW

Recent studies have provided a better understanding of the molecular mechanisms responsible for hemolytic uremic syndromes. In this review, we summarize biochemical and genetic data that may lead to new clinical approaches.

RECENT FINDINGS

The structures and modes of action of Shiga toxins have been deciphered. Patients with non-Shiga-like toxin hemolytic uremic syndrome have been found to carry mutations in three genes that encode for regulators of the complement system (factor H, membrane cofactor protein, and factor I).

SUMMARY

Shiga-like toxin-1 and Shiga-like toxin-2 regulate genes that encode for chemokines, cytokines, cell adhesion molecules, and transcription factors involved in immune response and apoptosis. Mutations in factor H, membrane cofactor protein and factor I have recently been identified. Reduced expression of compliment regulators might prevent restriction of complement deposition on glomerular endothelial cells, leading to microvascular cell damage and tissue injury. Shiga-like toxin hemolytic uremic syndrome in children has a favorable prognosis in 90% of cases; kidney transplantation shows a good graft survival rate (80%) in children who progress to end stage renal disease. As for non-Shiga-like toxin hemolytic uremic syndrome, treatment with plasma infusion or exchange has been used with controversial effects. Kidney transplantation is not recommended in those patients with mutations in factor H and factor I; however, a kidney transplant corrects membrane cofactor protein dysfunction. These findings vividly underscore the clinical heterogeneity of outcomes depending upon the nature of the underlying cause of the disease.

Atypical haemolytic uraemic syndrome and mutations in complement regulator genes

Haemolytic uraemic syndrome (HUS) is a thrombotic microangiopathy (TMA) disorder characterised by the association of haemolytic anaemia, thrombocytopenia and acute renal failure. Atypical forms (non-related to shigatoxin) may be familial or sporadic, with frequent recurrences and most of them lead to end stage renal failure. During the last years, different groups have demonstrated genetic predisposition of atypical HUS involving complement components factor H (FH), CD46 [or membrane co-factor protein (MCP)] and factor I. These three proteins are involved in the regulation of the alternative pathway of the complement system. Several series have reported mutations in the FH gene (called HF1) in between 10 and 22% of atypical HUS patients. At this time, four pedigrees corresponding to 13 cases have been reported with an MCP mutation and four cases with a sporadic disease presented factor I mutation. Whereas FH mutations were reported in both familial and sporadic forms of HUS, CD46 mutations were restricted to familial HUS, and factor I mutations were only observed in cases of sporadic HUS. We speculate that the penetrance of the disease may be variable regarding the identified susceptibility factors. Recently, the analysis of single nucleotide polymorphisms in both HF1 and MCP in three large cohorts of HUS patients identified significant association between atypical HUS and HF1 and MCP particular alleles. All these results, together with the finding of anti-FH antibodies in some atypical HUS patients, strongly suggest that an abnormality in the regulation of the alternative pathway participates in the patho-physiological mechanisms of atypical HUS. The recent progress made in the determination of susceptibility factors for atypical HUS has permitted the development of new diagnostic tests and may eventually lead to new specific treatments to block the pathological process.

Successful plasma therapy for atypical hemolytic uremic syndrome caused by factor H deficiency owing to a novel mutation in the complement cofactor protein domain 15

Quantitative or functional deficiency of complement factor H results in uncontrolled complement activation. This leads to thrombotic microangiopathy and finally causes renal failure (atypical hemolytic uremic syndrome [aHUS]). By regular analysis of factor H in patients with aHUS, the authors found a complete factor H deficiency in an infant in whom aHUS developed at 8 months of age. Factor H was quantified by enzyme-linked immunosorbent assay and further analyzed by Western blot using a factor H-specific antibody. Complement activation was determined by measuring total hemolytic activity of the classical (CH50) and alternative (APH50) pathways, C3 and C3d. The sequence of factor H gene was determined. Serial factor H measurements after fresh frozen plasma infusion allowed calculation of a factor H half-life. Factor H was absent in plasma (<1 mug/mL), and the complement system was highly activated (CH50, APH50, C3 decreased; C3d increased). Genetic analysis identified a novel homozygous factor H mutation (T2770A; Y899Stop) in CCP domain 15, most likely causing defective protein secretion. Time course measurements of factor H after plasma infusion established a factor H half-life of about 6 days. By repetitive plasma infusions (20 mL/kg over about 2 to 3 hours) the authors were able to interrupt the vicious circle of thrombotic microangiopathy in a factor H-deficient patient with aHUS. Based on the measured factor H half-life of about 6 days, regular plasma infusions in 2-week intervals were given, which prevented further aHUS episodes and stopped the decline of kidney function.

Comparison of surface recognition and C3b binding properties of mouse and human complement factor H

Factor H (FH) is a central complement regulator both in plasma and on certain cellular and acellular surfaces that are in contact with plasma. Although FH deficiency has been shown to lead to similar diseases in man and mice (membranoproliferative glomerulonephritis or dense deposit disease) little is known about the similarity between the human and murine FH functions. We here characterize the interactions of murine FH (mFH) with C3b, glycosaminoglycans, and endothelial cells and compare these interactions with those of human FH (hFH). To achieve this we purified mFH and murine C3 from plasma, prepared murine C3b, and expressed recombinant mFH constructs containing domains 1-5 and 18-20 (mFH1-5 and mFH18-20). For comparisons, hFH, human C3b, and recombinant hFH1-5 and hFH18-20 were used. We demonstrate that mFH and mFH1-5 do act as cofactors for factor I-mediated cleavage of human C3b. Surface plasmon resonance analysis showed binding of mFH18-20 to murine C3b and weak binding to human C3b. The mFH18-20 construct bound to heparin in a manner comparable to hFH18-20. It was demonstrated by flow cytometry that mFH and mFH18-20 bind to human endothelial cells in a similar manner to hFH and hFH18-20. Taken together, locations of the key functions of mFH, i.e. complement regulation and surface recognition, are comparable to hFH. Recently, mutations in the carboxy-terminal end of hFH have been found to be associated with atypical hemolytic uremic syndrome (aHUS). Based on the results in this report it is conceptually attractive to establish a murine model for aHUS.

Mutations in complement factor I predispose to development of atypical hemolytic uremic syndrome

Mutations in the plasma complement regulator factor H (CFH) and the transmembrane complement regulator membrane co-factor protein (MCP) have been shown to predispose to atypical hemolytic uremic syndrome (HUS). Both of these proteins act as co-factors for complement factor I (IF). IF is a highly specific serine protease that cleaves the alpha-chains of C3b and C4b and thus downregulates activation of both the classical and the alternative complement pathways. This study looked for IF mutations in a panel of 76 patients with HUS. Mutations were detected in two patients, both of whom had reduced serum IF levels. A heterozygous bp change, c.463 G>A, which results in a premature stop codon (W127X), was found in one, and in the other, a heterozygous single base pair deletion in exon 7 (del 922C) was detected. Both patients had a history of recurrent HUS after transplantation. This is in accordance with the high rate of recurrence in patients with CFH mutations. Patients who are reported to have mutations in MCP, by contrast, do not have recurrence after transplantation. As with CFH- and MCP-associated HUS, there was incomplete penetrance in the family of one of the affected individuals. This study provides further evidence that atypical HUS is a disease of complement dysregulation.

Complement factor H mutation in familial thrombotic thrombocytopenic purpura with ADAMTS13 deficiency and renal involvement

Thrombotic thrombocytopenic purpura is a rare disorder of small vessels that is associated with deficiency of the von Willebrand factor-cleaving protease ADAMTS13, which favors platelet adhesion and aggregation in the microcirculation. The disease manifests mainly with central nervous system symptoms, but cases of renal insufficiency have been reported. Presented are findings of the genetic basis of phenotype heterogeneity in thrombotic thrombocytopenic purpura in two sisters within one family. The patients had ADAMTS13 deficiency as a result of two heterozygous mutations (causing V88M and G1239V changes). In addition, a heterozygous mutation (causing an S890I change) in factor H of complement was found in the patient who developed chronic renal failure but not in her sister, who presented with exclusive neurologic symptoms.

Predisposition to atypical hemolytic uremic syndrome involves the concurrence of different susceptibility alleles in the regulators of complement activation gene cluster in 1q32

The efficiency of the complement system as an innate immune defense mechanism depends on a fine control that restricts its action to pathogens and prevents non-specific damage to host tissues. Genetic and functional analyses have shown that this critical control of complement activation may be impaired in atypical hemolytic uremic syndrome (aHUS) patients. Mutations in HF1, MCP or FI have been found in aHUS patients, but incomplete penetrance of the disease in individuals carrying these mutations is relatively frequent and no genetic defect has yet been found in a majority of aHUS patients. We report here the identification of a specific SNP haplotype block, spanning the MCP gene in the regulators of complement activation gene cluster, which is over-represented in aHUS patients and strongly associates with the severity of the disease. Linkage disequilibrium analyses suggest that this SNP haplotype also includes the CR1, DAF and C4BP genes. Initial studies identified two SNPs in the haplotype that influence the transcription activity of the MCP promoter in transient transfection experiments. Notably, the SNP haplotype block was found to be particularly frequent among patients who carry mutations in HF1, MCP or FI. These findings and the identification of aHUS patients carrying mutations in two complement regulatory genes provide an important insight into the etiology of aHUS. Together, they suggest that complement regulatory molecules act as a protein network and that multiple hits, involving plasma- and membrane-associated complement regulatory proteins, are necessary to impair protection to host tissues and to confer significant predisposition to aHUS.

Anti-Factor H autoantibodies associated with atypical hemolytic uremic syndrome

Several studies have demonstrated genetic predisposition in non-shigatoxin-associated hemolytic uremic syndrome (HUS), involving regulatory proteins of the complement alternative pathway: Factor H (FH) and membrane co-factor protein (CD46). Regarding the observations of thrombotic thrombocytopenic purpura patients, in whom a von Willebrand factor protease (ADAMST-13) deficiency may be inherited or acquired secondary to IgG antibodies, it was speculated that HUS might occur in a context of an autoimmune disease with the development of anti-FH antibodies leading to an acquired FH deficiency. The presence of FH autoantibodies was investigated by an ELISA method using coated purified human FH in a series of 48 children who presented with atypical HUS and were recruited from French university hospitals. Anti-FH IgG antibodies were detected in the plasma of three children who presented with recurrent HUS. The anti-FH specificity was conserved by the Fab'2 fraction. The plasma FH activity was found to be decreased, whereas plasma FH antigenic levels and FH gene analysis were normal, indicating that the presence of anti-FH antibodies led to an acquired functional FH deficiency. This report supports for the first time that HUS may occur in a context of an autoimmune disease with the development of anti-FH-specific antibody leading to an acquired FH deficiency. This new mechanism of functional FH deficiency may lead to the design of new approaches of diagnosis and treatment with a particular interest in plasma exchanges or immunosuppressive therapies.

Mutations in CD46, a complement regulatory protein, predispose to atypical HUS

Membrane cofactor protein (MCP, CD46) is a widely expressed transmembrane complement regulator. As does the soluble regulator factor H, it inhibits complement activation by inactivating the C3b that is deposited on target membranes. Factor H mutations have been described in 15-30% of patients with atypical haemolytic uraemic syndrome (HUS). Recent studies have identified mutations in the MCP gene in four families. In one, a heterozygous deletion resulted in the intracellular retention of the mutant protein. In another, a different heterozygous deletion led to a premature stop codon and the loss of the C-terminus. In the other two, a substitution (S206P) resulted in cell-surface expression but inefficient inactivation of surface-bound C3b. These findings provide further evidence that complement dysregulation predisposes to the development of HUS.