In C3G, the underlying pathophysiology is believed to be an overactive AP, mediated by increased C3 convertase activity.
Ba production serves as a proxy for newly formed AP C3 convertase since factor B can only be cleaved by factor D when it is already complexed with the components required to form the active AP C3 convertase.
With respect to pathophysiology of paraproteinemia-associated C3GP and DDD, it has been suspected but not proved that the paraprotein is functioning as an autoantibody that stabilizes C3 convertase (C3 nephritic factor) or possibly inhibits one of the CRPs, such as Factor H or I [9,10].
Complement factor H (CFH), complement factor I (CFI), membrane cofactor protein (MCP or CD46) and thrombomodulin (THBD) which are regulatory proteins of the alternative complement pathway and C3 and complement factor B (CFB) which are C3 convertase proteins are involved in the pathogenesis of the disease (6).
Complement C3 constitutes the active parts of C3 convertase together with CFB.
A prevalent C3 mutation in aHUS patients causes a direct C3 convertase gain of function.
Mutations in the genes encoding complement regulatory proteins factor H, membrane cofactor protein (MCP), factor I or thrombomodulin have been demonstrated in 20-30%, 5-15%, 4-10% and 3-5% of patients respectively, and mutations in the genes of C3 convertase proteins, C3 and factor B, in 2-10% and 1-4%.
When complement proteins are triggered into action by a microbe, the proteins ultimately form a complex enzyme called C3 convertase, which triggers a cascade of immune and inflammatory reactions.
Researchers at the University of Pennsylvania School of Medicine, in collaboration with colleagues at Utrecht University in the Netherlands, have determined the structure of C3 convertase and of the C3b fragment in complex with factor H.