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

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.

Attempted treatment of factor H deficiency by liver transplantation

Complement factor H (FH) deficiency is one of the causes of atypical hemolytic uremic syndrome (HUS). Most patients with FH deficiency associated HUS progress to end-stage renal disease despite plasma therapy. Moreover, the disease invariably recurs in the graft kidney and causes graft failure. We confirmed FH deficiency in a 30-month-old boy with recurrent HUS of 2 years duration, and attempted an auxiliary partial orthotopic liver transplantation (APOLT) to overcome the sustained intractable dependency on plasma therapy. APOLT restored the plasma FH level, without HUS recurrence, for 7 months. However, thereafter he suffered from serious infectious complications associated with immunosuppression and finally died 11 months after APOLT. In conclusion, although APOLT showed clinical and laboratory improvement for some period in this patient, the final fatal outcome suggests that liver transplantation should be cautiously applied to patients with HUS associated with FH deficiency.

The Japanese experience with thrombotic thrombocytopenic purpura-hemolytic uremic syndrome

A total of 290 Japanese patients with thrombotic thrombocytopenic purpura-hemolytic uremic syndrome (TTP-HUS) were analyzed with respect to ADAMTS-13 activity and its inhibitor. Twenty-one patients (17 families) had Upshaw-Schulman syndrome, and 12 patients (six families) had familial HUS of undetermined etiology. The number of patients with acquired HUS and TTP was 44 and 213, respectively. In acquired TTP, patients with severe deficiency of ADAMTS-13 activity secondary to the presence of an inhibitor were high responders to plasma exchange, but others were low responders to plasma exchange. The former patients were associated with "idiopathic" TTP, drugs, and pregnancy, and the latter patients with malignancy and stem cell transplantation. Patients with autoimmune disease-associated TTP fit into both groups.

Familial haemolytic uraemic syndrome and an MCP mutation

BACKGROUND

Mutations in factor H (HF1) have been reported in a consistent number of diarrhoea-negative, non-Shiga toxin-associated cases of haemolytic uraemic syndrome (D-HUS). However, most patients with D-HUS have no HF1 mutations, despite decreased serum concentrations of C3. Our aim, therefore, was to assess whether genetic abnormalities in other complement regulatory proteins are involved.

METHODS

We screened genes that encode the complement regulatory proteins-ie, factor H related 5, complement receptor 1, and membrane cofactor protein (MCP)-by PCR-single-strand conformation polymorphism (PCR-SSCP) and by direct sequencing, in 25 consecutive patients with D-HUS, an abnormal complement profile, and no HF1 mutation, from our International Registry of Recurrent and Familial HUS/TTP (HUS/thrombotic thrombocytopenic purpura).

FINDINGS

We identified a heterozygous mutation in MCP, a surface-bound complement regulator, in two patients with a familial history of HUS. The mutation causes a change in three aminoacids at position 233-35 and insertion of a premature stop-codon, which results in loss of the transmembrane domain of the protein and severely reduced cell-surface expression of MCP.

INTERPRETATION

Results of previous studies on HF1 indicate an association between HF1 deficiency and D-HUS. Our findings of an MCP mutation in two related patients suggest that impaired regulation of complement activation might be a factor in the pathogenesis of genetic forms of HUS. MCP could be a second putative candidate gene for D-HUS. The protein is highly expressed in the kidney and plays a major part in regulation of glomerular C3 activation. We propose, therefore, that reduced expression of MCP in response to complement-activating stimuli could prevent restriction of complement deposition on glomerular endothelial cells, leading to microvascular cell damage and tissue injury.

Genetic screening in haemolytic uraemic syndrome

PURPOSE OF REVIEW

Haemolytic uraemic syndrome (HUS) is a disease of diverse origin. The last year has witnessed the identification of a novel genetic marker of this disease, the description of the frequency of the factor H associated form of HUS in a registry of over 100 patients and a better understanding of the pathophysiology of the disease.

RECENT FINDINGS

In patients with atypical HUS, heterozygous mutations in the gene coding for the soluble complement regulator factor H are reported and most of the mutations cluster in the C-terminal recognition domain of the protein. A novel genetic marker for HUS has also been identified. Mutations occurring in the gene of the von Willebrand factor cleaving protease, ADAMTS13, which were previously linked to thrombotic thrombocytopenic purpura have now been identified in HUS patients. The frequency of factor H-associated HUS was established as 14% in a registry of German speaking countries and also 16 novel disease associated mutations were reported. The pathophysiology of factor H-associated HUS was analysed. Three analysed mutant proteins show normal complement regulatory activities but display defective recognition functions: reduced binding to surface attached C3b, to heparin/polyanions and to endothelial cells.

SUMMARY

The identification of effector molecules of the complement as well as the coagulation cascade as disease associated molecules indicate a regulatory protein network, which maintains integrity of endothelial cells during stress or infection. Defining the individual components and how their functional interaction causes microangiopathies will identify additional disease markers and will allow the design of proper diagnostic and therapeutic approaches.

Mutations in human complement regulator, membrane cofactor protein (CD46), predispose to development of familial hemolytic uremic syndrome

Membrane cofactor protein (MCP; CD46) is a widely expressed transmembrane complement regulator. Like factor H it inhibits complement activation by regulating C3b deposition on targets. Factor H mutations occur in 10-20% of patients with hemolytic uremic syndrome (HUS). We hypothesized that MCP mutations could predispose to HUS, and we sequenced MCP coding exons in affected individuals from 30 families. MCP mutations were detected in affected individuals of three families: a deletion of two amino acids (D237/S238) in family 1 (heterozygous) and a substitution, S206P, in families 2 (heterozygous) and 3 (homozygous). We evaluated protein expression and function in peripheral blood mononuclear cells from these individuals. An individual with the D237/S238 deletion had reduced MCP levels and approximately 50% C3b binding compared with normal controls. Individuals with the S206P change expressed normal quantities of protein, but demonstrated approximately 50% reduction in C3b binding in heterozygotes and complete lack of C3b binding in homozygotes. MCP expression and function was evaluated in transfectants reproducing these mutations. The deletion mutant was retained intracellularly. S206P protein was expressed on the cell surface but had a reduced ability to prevent complement activation, consistent with its reduced C3b binding and cofactor activity. This study presents further evidence that complement dysregulation predisposes to development of thrombotic microangiopathy and that screening patients for such defects could provide informed treatment strategies.

Complement factor H mutations and gene polymorphisms in haemolytic uraemic syndrome: the C-257T, the A2089G and the G2881T polymorphisms are strongly associated with the disease

Mutations in complement factor H (HF1) gene have been reported in non-Shiga toxin-associated and diarrhoea-negative haemolytic uraemic syndrome (D-HUS). We analysed the complete HF1 in 101 patients with HUS, in 32 with thrombotic thrombocytopenic purpura (TTP) and in 106 controls to evaluate the frequency of HF1 mutations, the clinical outcome in mutation and non-mutation carriers and the role of HF1 polymorphisms in the predisposition to HUS. We found 17 HF1 mutations (16 heterozygous, one homozygous) in 33 HUS patients. Thirteen mutations were located in exons XXII and XXIII. No TTP patient carried HF1 mutations. The disease manifested earlier and the mortality rate was higher in mutation carriers than in non-carriers. Kidney transplants invariably failed for disease recurrences in patients with HF1 mutations, while in non-mutated patients half of the grafts were functioning after 1 year. Three HF1 polymorphic variants were strongly associated with D-HUS: -257T (promoter region), 2089G (exonXIV, silent) and 2881T (963Asp, SCR16). The association was stronger in patients without HF1 mutations. Two or three disease-associated variants led to a higher risk of HUS than a single one. Analysis of available relatives of mutated patients revealed a penetrance of 50%. In 5/9 families the proband inherited the mutation from one parent and two disease-associated variants from the other, while unaffected carriers inherited the protective variants. In conclusion HF1 mutations are frequent in patients with D-HUS (24%). Common polymorphisms of HF1 may contribute to D-HUS manifestation in subjects with and without HF1 mutations.

Haemolytic uraemic syndrome and mutations of the factor H gene: a registry-based study of German speaking countries

BACKGROUND

The aetiology of atypical haemolytic uraemic syndrome (aHUS) is, in contrast to classical, Shiga-like toxin induced HUS in children, largely unknown. Deficiency of human complement factor H and familial occurrence led to identification of the factor H gene (FH1) as the susceptibility gene, but the frequency and relevance of FH1 mutations are unknown.

METHODS

We established a German registry for aHUS and analysed in all patients and 100 controls the complete FH1 gene by single strand confirmational polymorphism and DNA sequencing. In addition, complement C3 and factor H serum levels were assayed. Demographic data at onset of aHUS and follow up were compared for the mutation positive and negative groups.

RESULTS

Of 111 patients with aHUS (68 female, 43 male, mean age 33 years) 14% had FH1 germline mutations, including two of eight patients with familial aHUS. For each of these eight patients, both parents were tested, and we were able to trace the mutation for five cases. In the other three cases (one with the mutation 3749 C/T, one with 3200 T/C, and one with 3566+1 G/A), we could not detect the mutation in either parent, although paternity was proven by genetic fingerprinting, suggesting that these subjects have new mutations. C3 was decreased in five mutation carriers but also in two non-carriers, and factor H was decreased in none of the carriers, but elevated in six carriers and 15 non-carriers. Clinical parameters including associated medications and diseases, and outcome of aHUS and of post-aHUS kidney transplantation were similar in the mutation positive and negative groups.

CONCLUSION

FH1 germline mutations occur with considerable frequency in patients with aHUS. Hypocomplementaemia is not regularly associated with a germline mutation, and factor H serum levels can even be elevated. Screening for FH1 mutations contributes to the classification of aHUS.

Successful (?) therapy of hemolytic-uremic syndrome with factor H abnormality

We report a patient with continuously recurring hemolytic-uremic syndrome due to factor H deficiency. First at the age of 3 months he showed signs of hemolytic anemia, thrombocytopenia and renal insufficiency, often recurring concomitantly with respiratory tract infections, despite weekly to twice weekly plasma substitution (20 ml/kg body weight). Now at the age of 3.5 years glomerular filtration rate is approximately 50 ml/min/1.73 m(2) and psychomotoric development is normal. Since factor H is mainly synthesized in the liver, hepatic transplantation has been proposed as curative treatment. Before justification of liver transplantation as the ultimate treatment for these patients, an international registry should be developed to optimize and standardize therapeutic alternatives.

Research on complement: old issues revisited and a novel sphere of influence

Immunology in recent years has taken a somewhat surprising turn, expressed by a renewed interest in innate immunity. Especially intriguing is the regulatory role exerted by the innate components on the adaptive response, with Toll receptors and complement components being the most investigated. This function has been firmly established for complement protein CR2 (CD21) as part of the BCR co-receptor CD19/CD21/CD81. New findings are now providing a broader picture of complement and its tuning of the immune response; for example, complement proteins have been implicated in the control of T-cell-mediated responses. We will review some of these data here and summarize new discoveries in areas of research on more traditional topics within the complement literature, such as complement and renal diseases, and the therapeutic use of C1-Inhibitor. We cover papers selected from studies presented at the XIX International Complement Workshop, held in Palermo in September 2002, and published within the past six months.

Mutations in factor H reduce binding affinity to C3b and heparin and surface attachment to endothelial cells in hemolytic uremic syndrome

Hemolytic uremic syndrome (HUS) is a disease characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. Recent studies have identified a factor H-associated form of HUS, caused by gene mutations that cluster in the C-terminal region of the complement regulator factor H. Here we report how three mutations (E1172Stop, R1210C, and R1215G; each of the latter two identified in three independent cases from different, unrelated families) affect protein function. All three mutations cause reduced binding to the central complement component C3b/C3d to heparin, as well as to endothelial cells. These defective features of the mutant factor H proteins explain progression of endothelial cell and microvascular damage in factor H-associated genetic HUS and indicate a protective role of factor H for tissue integrity during thrombus formation.

Complement C3b/C3d and cell surface polyanions are recognized by overlapping binding sites on the most carboxyl-terminal domain of complement factor H

Factor H (FH) is a potent suppressor of the alternative pathway of C in plasma and when bound to sialic acid- or glycosaminoglycan-rich surfaces. Of the three interaction sites on FH for C3b, one interacts with the C3d part of C3b. In this study, we generated recombinant constructs of FH and FH-related proteins (FHR) to define the sites required for binding to C3d. In FH, the C3d-binding site was localized by surface plasmon resonance analysis to the most C-terminal short consensus repeat domain (SCR) 20. To identify amino acids of FH involved in binding to C3d and heparin, we compared the sequences of FH and FHRs and constructed a homology-based molecular model of SCR19-20 of FH. Subsequently, we created an SCR15-20 mutant with substitutions in five amino acids that were predicted to be involved in the binding interactions. These mutations reduced binding of the SCR15-20 construct to both C3b/C3d and heparin. Binding of the wild-type SCR15-20, but not the residual binding of the mutated SCR15-20, to C3d was inhibited by heparin. This indicates that the heparin- and C3d-binding sites are overlapping. Our results suggest that a region in the most C-terminal domain of FH is involved in target recognition by binding to C3b and surface polyanions. Mutations in this region, as recently reported in patients with familial hemolytic uremic syndrome, may lead to indiscriminatory C attack against self cells.

The molecular basis for hereditary porcine membranoproliferative glomerulonephritis type II: point mutations in the factor H coding sequence block protein secretion

Porcine membranoproliferative glomerulonephritis type II in piglets of the Norwegian Yorkshire breed is considered the first animal model of human dense deposit disease. Porcine dense deposit disease is caused by the absence of the complement regulator factor H in plasma. Here we report the molecular basis for this absence. Single nucleotide exchanges at position C1590G and T3610G in the coding region of the factor H gene result in amino acid exchanges at nonframework residues L493V and I1166R that are located within SCR 9 and SCR 20, respectively. Apparently the L493V mutation represents a polymorphism whereas the I1166R causes the physiological consequences a block in protein secretion. Expression analysis shows comparable mRNA levels for factor H in liver tissue derived from both affected and healthy animals. In affected piglets, factor H protein is detected in increased amounts in liver cells. Factor H accumulates inside the hepatocytes and is not released as shown by Western blot analysis and immunohistochemistry. These data demonstrate that single amino acid exchanges of two nonframework amino acids either alone or in combination block protein secretion of factor H. This observation is also of interest for other human diseases in which factor H is involved, such as human factor H-associated form of hemolytic uremic syndrome.

Recurrence of hemolytic uremic syndrome after live related renal transplantation associated with subsequent de novo disease in the donor

There is a significant risk of disease recurrence in patients with nondiarrheal (D-) hemolytic uremic syndrome (HUS) undergoing renal transplantation. Recent studies have found that approximately 20% of sporadic cases of HUS have mutations in the gene for the complement regulatory protein factor H. The authors report on 2 families, in each of which a family member initially presented with sporadic HUS and subsequently received a live-related renal transplant, one from a sibling and the other from the father. Subsequently, both recipients suffered recurrent HUS in the allograft, and both donors had HUS within a year of the transplant. Neither family has a factor H mutation. This report underlines the risk of disease recurrence in recipients associated with live-related renal transplantation in HUS and also suggests that the donors may be at risk.

Structural and functional characterization of factor H mutations associated with atypical hemolytic uremic syndrome

Genetic studies have demonstrated the involvement of the complement regulator factor H in nondiarrheal, nonverocytotoxin (i.e., atypical) cases of hemolytic uremic syndrome. Different factor H mutations have been identified in 10%-30% of patients with atypical hemolytic uremic syndrome (aHUS), and most of these mutations alter single amino acids in the C-terminal region of factor H. Although these mutations are considered to be responsible for the disease, the precise role that factor H plays in the pathogenesis of aHUS is unknown. We report here the structural and functional characterization of three different factor H proteins purified from the plasma of patients with aHUS who carry the factor H mutations W1183L, V1197A, or R1210C. Structural anomalies in factor H were found only in R1210C carriers; these individuals show, in their plasma, a characteristic high-molecular-weight factor H protein that results from the covalent interaction between factor H and human serum albumin. Most important, all three aHUS-associated factor H proteins have a normal cofactor activity in the proteolysis of fluid-phase C3b by factor I but show very low binding to surface-bound C3b. This functional impairment was also demonstrated in recombinant mutant factor H proteins expressed in COS7 cells. These data support the hypothesis that patients with aHUS carry a specific dysfunction in the protection of cellular surfaces from complement activation, offering new possibilities to improve diagnosis and develop appropriate therapies.

Cutting edge: localization of the host recognition functions of complement factor H at the carboxyl-terminal: implications for hemolytic uremic syndrome

Incidents of hemolytic uremic syndrome (HUS) include a subset of patients that exhibit mutations in C factor H. These mutations cluster in the C-terminal domains of factor H where previous reports have identified polyanion and C3b-binding sites. In this study, we show that recombinant human factor H with deletions at the C-terminal end of the protein loses the ability to control the spontaneous activation of the alternative C pathway on host-like surfaces. For the pathology of HUS, the findings imply that mutations that disrupt the normal functions of the C-terminal domains prevent host polyanion recognition. The resulting uncontrolled activation of complement on susceptible host tissues appears to be the initiating event behind the acute renal failure of familial HUS patients.

The genetics and pathogenesis of haemolytic uraemic syndrome and thrombotic thrombocytopenic purpura

PURPOSE OF REVIEW

In recent years there has been a substantial increase in the understanding of the genetics and pathogenesis of haemolytic uraemic syndrome and thrombotic thrombocytopenic purpura.

RECENT FINDINGS

In diarrhoeal associated haemolytic uraemic syndrome it has been established that the virulence of Escherichia coli O157 is related to intimin adhesion and the transport of verocytotoxin on polymorphonuclear cells. It has been shown that early changes in the coagulation pathway predate the onset of diarrhoeal haemolytic uraemic syndrome. Mutations in factor H, a fluid-phase regulator of the alternative complement pathway, have been identified in 10-20% of patients with both familial and sporadic (non-diarrhoeal-associated) haemolytic uraemic syndrome. The mutations mainly cluster in the C terminal part of factor H, a region that is important for both binding to C3b and also polyanionic structures on cell surfaces. The identification of antibodies against a plasma metalloproteinase responsible for cleaving ultralarge von Willebrand factor multimers in thrombotic thrombocytopenic purpura has been followed by the elucidation of the identity of the proteinase. It has been shown to be a member of the ADAMTS family, and mutations have been identified in the gene in families with inherited thrombotic thrombocytopenic purpura.

SUMMARY

The molecular pathogenesis of haemolytic uraemic syndrome and thrombotic thrombocytopenic purpura is an exciting and rapidly evolving field. These recent advances will lead to logical, targetted changes in the management of these conditions.

Molecular modelling of the C-terminal domains of factor H of human complement: a correlation between haemolytic uraemic syndrome and a predicted heparin binding site

Factor H (FH) of the complement system acts as a regulatory cofactor for the factor I-mediated cleavage of C3b and binds to polyanionic substrates. FH is composed of 20 short consensus/complement repeat (SCR) domains. A set of 12 missense mutations in the C-terminal domains between SCR-16 to SCR-20 is associated with haemolytic uraemic syndrome. Recent structural models for intact FH permit the molecular interpretation of these amino acid substitutions. As all nine SCR-20 substitutions correspond to normal amounts of FH in plasma, and were localised in mostly surface-exposed positions, these are inferred to lead to a functional defect in FH. The nine substitutions occur in the same spatial region of SCR-20. As this surface coincides with conserved basic residues in the C-terminal SCR-20 domain, the substitutions provide direct evidence for a polyanionic binding surface. The positions of these conserved basic residues coincide with those of heparin-binding residues in the crystal structure of the acidic fibroblast growth factor-heparin complex. A tenth substitution and another conserved basic residue in SCR-19 are proximate to this binding site. As the remaining FH substitutions could also be correlated with their proximity to conserved basic residues, haemolytic uraemic syndrome may result from a failure of FH to interact with polyanions at cell surfaces in the kidney.

Thrombotic microangiopathy, hemolytic uremic syndrome, and thrombotic thrombocytopenic purpura

The term thrombotic microangiopathy (TMA) defines a lesion of vessel wall thickening (mainly arterioles or capillaries), intraluminal platelet thrombosis, and partial or complete obstruction of the vessel lumina. Depending on whether renal or brain lesions prevail, two pathologically indistinguishable but somehow clinically different entities have been described: the hemolytic uremic syndrome (HUS) and the thrombotic thrombocytopenic purpura (TTP). Injury to the endothelial cell is the central and likely inciting factor in the sequence of events leading to TMA. Loss of physiological thromboresistance, leukocyte adhesion to damaged endothelium, complement consumption, abnormal von Willebrand factor release and fragmentation, and increased vascular shear stress may then sustain and amplify the microangiopathic process. Intrinsic abnormalities of the complement system and of the von Willebrand factor pathway may account for a genetic predisposition to the disease that may play a paramount role in particular in familial and recurrent forms. Outcome is usually good in childhood, Shiga toxin-associated HUS, whereas renal and neurological sequelae are more frequently reported in adult, atypical, and familial forms of HUS and in TTP. Plasma infusion or exchange is the only treatment of proven efficacy. Bilateral nephrectomy and splenectomy may serve as rescue therapies in very selected cases of plasma resistant HUS or recurrent TTP, respectively.

Hemolytic uremic syndrome: how do factor H mutants mediate endothelial damage?

Hemolytic uremic syndrome is characterized by microangiopathic hemolytic anemia, thrombocytopenia and acute renal failure. Data from recent genetic analyses reveal a clear association between the complement regulator factor H and the atypical form of this fatal human disease. The clustering of the identified mutations in the C-terminus of factor H identifies a "hot spot" that is central to the pathogenesis of the disease. What are the possible biological and functional consequences of the identified mutations for the disease process and mechanisms of disease progression?