An autoinflammatory CDC42 variant M45L demonstrates the mechanism of pyrin inflammasome activation

CDC42 is an evolutionarily conserved guanosine triphosphatase (GTPase) regulating critical cellular and immune processes. Variants in CDC42 therefore cause a spectrum of developmental phenotypes, including hematological and immunological symptoms. Here, we identify a previously unrecognized autoinflammatory condition caused by CDC42^{Met45→Leu(M45L)}. We confirm the pathogenicity of CDC42^M45L given that it promotes pyrin inflammasome activation, consistent with highly elevated circulating interleukin-18 (IL-18) in all affected individuals. Coimmunoprecipitation demonstrates that CDC42 interacts with the PYD and B30.2 domains of pyrin. The B30.2 interaction is greatly enhanced by CDC42^M45L. Models show that CDC42 methionine-45 sits in a hydrophobic cleft of the pyrin B30.2 domain, and substitution with a negatively charged residue prevents colocalization between CDC42 and pyrin. This study supports a model whereby CDC42 nucleates pyrin via the B30.2 domain, after which a threshold is reached to liberate the PYD for inflammasome activation. Blocking the inflammasome-associated cytokine IL-1 was therapeutically effective when tested against this autoinflammatory CDC42^M45L endotype.