Insights into hemolytic uremic syndrome: Segregation of three independent predisposition factors in a large, multiple affected pedigree
Introduction
Hemolytic uremic syndrome (HUS) is clinically defined by thrombocytopenia, Coomb's test negative microangiopathic haemolytic anemia and acute renal failure. Most HUS cases occur associated to Escherichia coli infections leading to hemorragic diarrhea (Karmali, 2004). This typical form of HUS usually resolves satisfactorily and complete recovery of the renal function is achieved. However, 5–10% of HUS cases lack a particular relationship with infection and have a poorer prognosis (Moake, 2002). This idiopathic atypical form of HUS (aHUS) is frequently associated with immunosuppressive drugs, cancer therapies, oral contraceptives or develops during pregnancy or postpartum. The molecular mechanisms underlying atypical HUS are not completely understood. The complement system has been implicated in the pathophysiology of this syndrome for many years (Thompson and Winterborn, 1981), but only recently mutations in the genes for the complement proteins factor H (CFH) (Warwicker et al., 1998, Pérez-Caballero et al., 2001, Caprioli et al., 2001, Richards et al., 2001, Sánchez-Corral et al., 2002, Manuelian et al., 2003, reviewed in Rodríguez de Córdoba et al., 2004), membrane cofactor protein (MCP) (Noris et al., 2003, Richards et al., 2003) and factor I (IF) (Fremeaux-Bacchi et al., 2004, Kavanagh et al., 2005) have been shown to predispose to aHUS. Functional characterization of several of these mutations suggest that the disease likely develops as a consequence of a defective protection of cellular surfaces from complement activation due to an improper function of complement regulatory proteins (Sánchez-Corral et al., 2002, Sánchez-Corral et al., 2004, Manuelian et al., 2003, Richards et al., 2003).
Despite these advances in our understanding of the molecular basis of aHUS, incomplete penetrance of the disease in individuals carrying factor H, MCP or factor I mutations is relatively frequent, suggesting the existence of additional genetic factors contributing to aHUS. Recently, different groups have reported that relatively frequent CHF and MCP SNPs are strongly associated with aHUS (Caprioli et al., 2003, Esparza-Gordillo et al., 2005, Fremeaux-Bacchi et al., 2005). Among these SNPs, the MCP haplotype block MCPggaac is particularly interesting because it includes two SNPs, c.-547G/C and c.-261G/C, that influence transcription from the MCP promoter in transient transfection experiments (Esparza-Gordillo et al., 2005). Moreover, the observation that MCPggaac was especially frequent among patients who carry mutations in CFH, MCP or IF suggested that the concurrence of different mutations and polymorphisms in the complement regulatory genes increases predisposition to aHUS (Esparza-Gordillo et al., 2005). Here we provide further support to this conclusion identifying a pedigree in which the affected individuals carry three different genetic susceptibility factors in two different complement regulatory genes (IF and MCP).
Section snippets
Family RCO
This study focuses on the characterization of aHUS genetic predisposition factors in a large, multiple affected Spanish pedigree, referred to as family RCO. This pedigree was initially selected because we found that two patients in the Spanish HUS registry, HUS 68 and HUS 84, were first cousins and the only affected members in their family. HUS 68 has been reported earlier (Esparza-Gordillo et al., 2005). All protocols included in these studies have been approved by national and/or local
Results
Blood samples obtained from patients HUS 68 and HUS 84 showed normal complement activity through the classical (CH50) and the alternative (AP50) pathways. Similarly, plasma levels of C3, C4 and factor H also were within the normal reference range of variation (Table 1). However, both patients presented half-normal plasma levels of factor I, and also showed a 50% reduction in the MCP expression levels in PBLs (Table 1). These observations prompted us to search for mutations in the MCP and IF
Discussion
We report here the identification of three independent aHUS risk factors in a large Spanish pedigree with two members affected. These factors are: a MCP missense mutation (c.-598C > T; Pro165Ser) that decreases MCP expression on the cell surface, a dinucleotide insertion in the coding sequence of factor I (c.-1610insAT) that introduces a premature stop codon in the factor I protein, and the MCPggaac SNP haplotype block that was previously shown to influence the transcription activity from the MCP
Acknowledgements
We are grateful to all members of family RCO and the collaborating clinicians for their participation in this study. We acknowledge Dra. Mª Cruz García-Rodríguez, from the Immunology Unit of the Hospital Universitario La Paz, for her helpful advice in flow cytometry analyses. We also thank the members of the DNA sequencing laboratory at the Centro de Investigaciones Biológicas, for invaluable help.
These studies were performed with funds provided by the Ministerio de Educación y Cultura
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