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

Area of Science:

• Immunology
• Molecular Biology
• Genetics

Background:

• Atypical hemolytic uremic syndrome (aHUS) pathogenesis involves complement system dysregulation.
• Mutations in complement genes, particularly C3, are found in a significant portion of aHUS cases.

Purpose of the Study:

• To characterize aHUS-associated genetic changes in C3.
• To investigate the functional consequences of these C3 mutations on interactions with complement regulatory proteins: factor H (FH), membrane cofactor protein (MCP), and complement receptor 1 (CR1).

Main Methods:

• Surface plasmon resonance was used to assess binding affinities between mutant C3 proteins and regulatory proteins.
• Cofactor activity assays were performed to measure the proteolytic inactivation of C3b.
• Binding sites were mapped and compared to known FH binding sites.

Main Results:

• 17 of 23 aHUS-associated C3 mutants showed impaired binding to FH and/or MCP.
• 2 mutants exhibited reduced binding to CR1.
• Decreased binding affinity correlated with reduced cofactor activity, impacting C3b inactivation.
• Low C3 levels in 76% of patients with C3 mutations correlated with disease severity.

Conclusions:

• Atypical hemolytic uremic syndrome-associated C3 mutations functionally impair interactions with complement regulatory proteins.
• These functional defects contribute to aHUS pathogenesis by disrupting complement regulation.
• The study provides insights into C3 binding regions and their role in aHUS, correlating C3 levels with disease severity.