The Molecular and Structural Bases for the Association of Complement C3 Mutations With Atypical Hemolytic Uremic Syndrome

Mol Immunol. 2015 Aug;66(2):263-73. doi: 10.1016/j.molimm.2015.03.248. Epub 2015 Apr 11.

Abstract

Atypical hemolytic uremic syndrome (aHUS) associates with complement dysregulation caused by mutations and polymorphisms in complement activators and regulators. However, the reasons why some mutations in complement proteins predispose to aHUS are poorly understood. Here, we have investigated the functional consequences of three aHUS-associated mutations in C3, R592W, R161W and I1157T. First, we provide evidence that penetrance and disease severity for these mutations is modulated by inheritance of documented "risk" haplotypes as has been observed with mutations in other complement genes. Next, we show that all three mutations markedly reduce the efficiency of factor I-mediated C3b cleavage when catalyzed by membrane cofactor protein (MCP), but not when catalyzed by factor H. Biacore analysis showed that each mutant C3b bound sMCP (recombinant soluble MCP; CD46) at reduced affinity, providing a molecular basis for its reduced cofactor activity. Lastly, we show by electron microscopy structural analysis a displacement of the TED domain from the MG ring in C3b in two of the C3 mutants that explains these defects in regulation. As a whole our data suggest that aHUS-associated mutations in C3 selectively affect regulation of complement on surfaces and provide a structural framework to predict the functional consequences of the C3 genetic variants found in patients.

Keywords: Atypical hemolytic uremic syndrome; C3 mutation; Complement C3; Factor H; MCP; TED.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Atypical Hemolytic Uremic Syndrome / genetics*
  • Atypical Hemolytic Uremic Syndrome / immunology
  • Atypical Hemolytic Uremic Syndrome / pathology
  • Base Sequence
  • Binding Sites
  • Child
  • Complement C3 / chemistry
  • Complement C3 / genetics*
  • Complement C3 / immunology
  • Complement Factor H / genetics
  • Complement Factor H / immunology
  • Female
  • Fibrinogen / genetics
  • Fibrinogen / immunology
  • Gene Expression
  • Genetic Predisposition to Disease*
  • Haplotypes
  • Humans
  • Male
  • Membrane Cofactor Protein / genetics*
  • Membrane Cofactor Protein / immunology
  • Models, Molecular
  • Molecular Sequence Data
  • Mutation*
  • Protein Binding
  • Protein Structure, Tertiary
  • Proteolysis
  • Recombinant Proteins / genetics
  • Recombinant Proteins / immunology
  • Severity of Illness Index

Substances

  • CD46 protein, human
  • Complement C3
  • Membrane Cofactor Protein
  • Recombinant Proteins
  • Complement Factor H
  • Fibrinogen