Atypical hemolytic uremic syndrome associated with a factor B genetic variant and fluid-phase complement activation: an exception to the rule?

Gain-of-function variants in CFB encoding factor B (FB), a component of the alternative pathway C3 convertase, have been reported in a minority of patients with aHUS and result in massive C3 activation. Zhang et al. describe the functional characterization of a novel FB variant (p.Ser367Arg) that they identified in 2 unrelated aHUS pedigrees who had undetectable C3 levels. The mutant FB caused strong C3 cleavage in fluid-phase but also C3 deposition on cell surface. This commentary addresses the implications of these findings for understanding the complexity of complement-related genetic renal diseases.