Mapping interactions between complement C3 and regulators using mutations in atypical hemolytic uremic syndrome

• Published in: Blood Vol. 125; no. 15; pp. 2359 - 2369
• Main Authors: Schramm, Elizabeth C., Roumenina, Lubka T., Rybkine, Tania, Chauvet, Sophie, Vieira-Martins, Paula, Hue, Christophe, Maga, Tara, Valoti, Elisabetta, Wilson, Valerie, Jokiranta, Sakari, Smith, Richard J.H., Noris, Marina, Goodship, Tim, Atkinson, John P., Fremeaux-Bacchi, Veronique
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 09.04.2015; American Society of Hematology
• Subjects: Atypical Hemolytic Uremic Syndrome - genetics; Atypical Hemolytic Uremic Syndrome - metabolism; Atypical Hemolytic Uremic Syndrome - pathology; Binding Sites; Cohort Studies; Complement C3 - analysis; Complement C3 - genetics; Complement C3 - metabolism; Female; Humans; Immunobiology; Male; Models, Molecular; Mutation; Protein Interaction Maps
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood-2014-10-609073

The pathogenesis of atypical hemolytic uremic syndrome (aHUS) is strongly linked to dysregulation of the alternative pathway of the complement system. Mutations in complement genes have been identified in about two-thirds of cases, with 5% to 15% being in C3. In this study, 23 aHUS-associated genetic changes in C3 were characterized relative to their interaction with the control proteins factor H (FH), membrane cofactor protein (MCP; CD46), and complement receptor 1 (CR1; CD35). In surface plasmon resonance experiments, 17 mutant recombinant proteins demonstrated a defect in binding to FH and/or MCP, whereas 2 demonstrated reduced binding to CR1. In the majority of cases, decreased binding affinity translated to a decrease in proteolytic inactivation (known as cofactor activity) of C3b via FH and MCP. These results were used to map the putative binding regions of C3b involved in the interaction with MCP and CR1 and interrogated relative to known FH binding sites. Seventy-six percent of patients with C3 mutations had low C3 levels that correlated with disease severity. This study expands our knowledge of the functional consequences of aHUS-associated C3 mutations relative to the interaction of C3 with complement regulatory proteins mediating cofactor activity. •C3 mutations in aHUS commonly result in impaired complement regulation, C3 consumption, and a poor renal outcome.•C3 mutations tend to cluster at the protein surface and facilitate mapping of putative binding sites for the regulatory proteins.