ABSTRACT
Atypical hemolytic uremic syndrome (aHUS); hemolysis, elevated liver function tests, and low platelets syndrome; and transplant-associated thrombotic microangiopathy are related conditions, in that many patients harbor germline heterozygous mutations in genes that regulate the alternative pathway of complement (APC). Penetrance is variable because development of clinically significant disease appears to require supervention of a process such as inflammation. Complement activation on the endothelial surfaces leads to endothelial damage, platelet consumption, microthrombi, and a mechanical hemolytic anemia with schistocytes. Paroxysmal nocturnal hemoglobinuria (PNH) is a clonal hematopoietic disease caused by expansion of a stem cell that harbors a somatic mutation in PIGA PIGA mutant blood cells are deficient in the complement regulator proteins CD55 and CD59, making them susceptible to intravascular hemolysis due to a failure to regulate the APC on erythrocytes. Eculizumab is a monoclonal antibody that binds to C5 and inhibits terminal complement by interfering with the cleavage of C5 by the C5 convertases. The drug is approved by the US Food and Drug Administration for the treatment of aHUS and PNH; however, a new generation of complement inhibitors that block C5 and other components of the complement cascade is showing promise in preclinical and clinical trials.
Subject(s)
Antibodies, Monoclonal, Humanized/therapeutic use , Atypical Hemolytic Uremic Syndrome , Hemoglobinuria, Paroxysmal , Mutation , Atypical Hemolytic Uremic Syndrome/blood , Atypical Hemolytic Uremic Syndrome/drug therapy , Atypical Hemolytic Uremic Syndrome/genetics , CD55 Antigens/blood , CD55 Antigens/genetics , CD59 Antigens/blood , CD59 Antigens/genetics , Complement C5/antagonists & inhibitors , Complement C5/genetics , Complement C5/metabolism , Complement C5 Convertase, Alternative Pathway/antagonists & inhibitors , Complement C5 Convertase, Alternative Pathway/genetics , Complement C5 Convertase, Alternative Pathway/metabolism , Complement Pathway, Alternative/drug effects , Complement Pathway, Alternative/genetics , Hemoglobinuria, Paroxysmal/blood , Hemoglobinuria, Paroxysmal/drug therapy , Hemoglobinuria, Paroxysmal/genetics , Hemolysis/drug effects , Hemolysis/genetics , Humans , Membrane Proteins/blood , Membrane Proteins/genetics , PenetranceABSTRACT
Atypical hemolytic uremic syndrome (aHUS) is a genetic ultrarare renal disease associated with overactivation of the alternative pathway of complement. Four gain-of-function mutations that form a hyperactive or deregulated C3 convertase have been identified in Factor B (FB) ligand binding sites. Here, we studied the functional consequences of 10 FB genetic changes recently identified from different aHUS cohorts. Using several tests for alternative C3 and C5 convertase formation and regulation, we identified two gain-of-function and potentially disease-relevant mutations that formed either an overactive convertase (M433I) or a convertase resistant to decay by FH (K298Q). One mutation (R178Q) produced a partially cleaved protein with no ligand binding or functional activity. Seven genetic changes led to near-normal or only slightly reduced ligand binding and functional activity compared with the most common polymorphism at position 7, R7. Notably, none of the algorithms used to predict the disease relevance of FB mutations agreed completely with the experimental data, suggesting that in silico approaches should be undertaken with caution. These data, combined with previously published results, suggest that 9 of 15 FB genetic changes identified in patients with aHUS are unrelated to disease pathogenesis. This study highlights that functional assessment of identified nucleotide changes in FB is mandatory to confirm disease association.