Atypical hemolytic uremic syndrome: genetically-based insights into pathogenesis through an analysis of the complement regulator CD46

The complement system is a critical innate immune defense mechanism that also facilitates antigen recognition as well as antibody production through the adaptive immune response. Overall, complement activation contributes to the immune system’s recognition and response to foreign pathogens and altered self. Regulating complement activation, particularly its powerful alternative pathway (AP) amplification loop, plays a key role in modulating tissue damage at sites of injury. Besides a predisposition to infections and autoimmunity (particularly systemic lupus erythematosus) in individuals deficient in activating components, variants in complement regulators are associated with multiple diseases including atypical hemolytic uremic syndrome (aHUS), paroxysmal nocturnal hemoglobinuria (PNH), and age-related macular degeneration (AMD). In particular, the pathogenesis of aHUS is commonly related to a rare heterozygous loss-of-function (LOF) mutation in the gene for complement factor H (CFH), CD46 [membrane cofactor protein (MCP)] or factor I (CFI) or a gain-of-function (GOF) secondary to a variant in factor B (CFB) or C3. The variants associated with complement regulators are the most prevalent and clearly demonstrate that cofactor activity (CA) is essential to control complement activation and thereby avoid collateral damage to normal tissues. Importantly, multiple studies have now established the therapeutic efficacy of blocking the membrane attack complex (MAC) with a humanized monoclonal antibody that targets the fifth component, C5. In this review, we primarily focus on insights derived from the assessment of rare variants in a membrane complement inhibitor CD46 in aHUS. We also discuss the putative pathological mechanisms relative to these variants of the complement system.