Structural and functional characterization of mutations in C1-ihn responsible for hereditary angioedema

Hereditary angioedema (HAE) is due to mutations in C1 inhibitor (C1inh) gene causing its deficiency. C1inh is a serine protease inhibitor (serpin) that controls activation of complement and contact systems. The variable HAE phenotype remain fairly undisclosed. Hence, identifying mutations and their molecular effect is relevant for genotype/phenotypes as well as for structure/function correlates. Out of our case list of 471 HAE patients characterized for clinical phenotype, at present we have screened 132 unrelated patients: 112 type I, 20 type II. We found 96 different mutations: 35 missense, 22 frameshift, 12 large deletions/duplications, 3 insertion/deletion of 1â 2codons, 11 nonsense, 1 sinonimus, 5 splicing defects, 3 possible splicing defects, 2 in the signal peptide, 1 in the promoter and 1 gene deletion. In order to define structure-function correlates in C1inh, we selected mutations affecting residues primarily involved in the unique suicide substrate-like inhibitory mechanism of serpins (delT280, T308N, R378C, L427P/R) and expressed the mutated proteins in Pichia pastoris expression system. The family presenting the R378C variant has a peculiar clinical and biochemical profile with just atypical: recurrent abdominal attacks, no cutaneous angioedema, and C1-inh levels ranging between very low and normal values both during and outside Danazol treatment. The structural and functional data on the recombinant protein are in accordance with our working hypothesis that the R378C variant could undergo different conformations (native, latent, polymer). These conformations may result in different degrees of impaired secretion and/or function depending on specific environmental conditions. This variability could explain the inconstant levels of C1-inh function and antigen detected in the patient plasma.