A mechanism for hereditary angioedema caused by a methionine-379–to–lysine substitution in kininogens

Hereditary angioedema (HAE) is associated with episodic kinin-induced swelling of the skin and mucosal membranes. Most patients with HAE have low plasma C1-inhibitor activity, leading to increased generation of the protease plasma kallikrein (PKa) and excessive release of the nanopeptide bradykinin from high-molecular-weight kininogen (HK). However, disease-causing mutations in at least 10% of patients with HAE appear to involve genes for proteins other than C1-inhibitor. A point mutation in the Kng1 gene encoding HK and low–molecular weight kininogen (LK) was identified recently in a family with HAE. The mutation changes a methionine (Met³⁷⁹) to lysine (Lys³⁷⁹) in both proteins. Met³⁷⁹ is adjacent to the Lys³⁸⁰-Arg³⁸¹ cleavage site at the N-terminus of the bradykinin peptide. Recombinant wild-type (Met³⁷⁹) and variant (Lys³⁷⁹) versions of HK and LK were expressed in HEK293 cells. PKa-catalyzed kinin release from HK and LK was not affected by the Lys³⁷⁹ substitutions. However, kinin release from HK-Lys³⁷⁹ and LK-Lys³⁷⁹ catalyzed by the fibrinolytic protease plasmin was substantially greater than from wild-type HK-Met³⁷⁹ and LK-Met³⁷⁹. Increased kinin release was evident when fibrinolysis was induced in plasma containing HK-Lys³⁷⁹ or LK-Lys³⁷⁹ compared with plasma containing wild-type HK or LK. Mass spectrometry revealed that the kinin released from wild-type and variant kininogens by PKa is bradykinin. Plasmin also released bradykinin from wild-type kininogens but cleaved HK-Lys³⁷⁹ and LK-Lys³⁷⁹ after Lys³⁷⁹ rather than Lys³⁸⁰, releasing the decapeptide Lys-bradykinin (kallidin). The Met³⁷⁹Lys substitutions make HK and LK better plasmin substrates, reinforcing the relationship between fibrinolysis and kinin generation.