Elsevier

The Lancet

Volume 362, Issue 9395, 8 November 2003, Pages 1542-1547
The Lancet

Fast track — Mechanisms of Disease
Familial haemolytic uraemic syndrome and an MCP mutation

https://doi.org/10.1016/S0140-6736(03)14742-3Get rights and content

Summary

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 (DHUS). 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 PCRsingle-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.

Introduction

Haemolytic uraemic syndrome (HUS) is a rare disease of microangiopathic haemolysis, thrombocytopenia, and renal failure.1, 2 The most common form of HUS in children, with predominant renal failure, is associated with infection by Escherichia coli, which produce a powerful Shiga-like toxin.1 This form of the disease (D+HUS) usually presents with a diarrhoea prodrome and has an excellent prognosis in most cases.3 By contrast, non-Shiga toxin-associated and diarrhoea-negative forms of HUS (D-HUS) have a much poorer outcome (often end-stage renal failure or death4, 5, 6), with patients prone to relapse. There is sometimes a clustering of affected individuals within families, suggesting a genetic predisposition to the disease. Both autosomal dominant and autosomal recessive forms of inheritance have been noted, with precipitating events such as pregnancy, virus-like disease, or sepsis reported in some instances.7, 8, 9, 10, 11 Low serum concentrations of the third component of complement (C3) have been identified in patients with D-HUS.12, 13 Among such patients, a subgroup—between 13% and 30%—carry mutations in the gene encoding for factor H (HF1), a plasma protein that inhibits the activation of the alternative pathway of complement.14, 15, 16, 17, 18, 19 However, two thirds of patients with D-HUS have no HF1 mutations, despite decreased C3 concentrations,1, 18, 19 indicating a role for genetic abnormalities in other complement regulatory proteins.

Section snippets

Participants

Between, 1996, and May, 2003, we enrolled consecutive patients with familial, recurrent, or sporadic D-HUS with no HF1 mutations but an abnormal serum complement profile (defined as C3 serum concentrations < 0·83 g/L12 or a plasma C3d/serum C3 ratio >0·015) through the International Registry of Recurrent and Familial HUS/TTP (HUS/thrombotic thrombocytopenic purpura), a network of 60 Haematology and Nephrology Units established under the coordination of the Clinical Research Centre for Rare

Results

We enrolled 25 patients with familial (n=12), recurrent (n=6), or sporadic (n=7) D-HUS, all of whom were white, had no HF1 mutations, and had an abnormal serum complement profile. We also enrolled 100 healthy blood donors and, for protein expression studies in PBMC, six healthy female controls and three uraemic female controls.

Analysis of FHR5, indicated a heterozygous 343C→T polymorphism leading to a L66F change in short consensus repeat (SCR) 1 in two patients and in one healthy control, a

Discussion

Our results of genetic screening in 25 consecutive D-HUS patients without mutations in HF1 but presenting with abnormalities in the ALTERNATIVE but not in the CLASSICAL PATHWAY OF COMPLEMENT, led us to identify a candidate gene for D-HUS in addition to HF1. Based on the fact that these patients had signs of activation of the complement system in their blood, we focused our search for a genetic cause of the disease on genes involved in complement regulation. We identified a heterozygous mutation

GLOSSARY

classical and alternative complement pathways
Complement is part of the innate immune system and underlies the main effector mechanism of antibody-mediated immunity. The classical pathway is initiated by the binding of C1 complex to antibodies bound to an antigen on the surface of a bacterial cell. The alternative pathway is initiated by the covalent binding of a small amount of C3b to hydroxyl groups on cell-surface carbohydrates and proteins, and is activated by the low-grade cleavage of C3 in

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