Molecular mechanism for the pathogenicity of the human C1-inhibitor Arg378Cys variant

C1-inhibitor (C1-inh) is a member of serine protease inhibitor (serpin) family which contains a common tertiary structure composed of three -sheets and several α-helices. Genetic variants of C1-inh lead to plasma deficiency, causing Hereditary Angioedema (HAE). HAE is a life-threatening autosomal dominant disorder characterized by episodic local subcutaneous edema and submucosal edema involving the upper respiratory and gastrointestinal tracts. Despite in HAE patients C1-inh plasma levels are low without significant variations, frequency and severity of symptoms are highly variable among patients and even in the same patient from time to time. Nothing is known on the variables responsible for such different clinical expressions. Moreover, the detailed molecular mechanism for the pathogenicity of most variant C1-inh molecules is not known. We examined the structural basis of a specific deficient C1-inh variant, with a peculiar clinical phenotype of food related abdominal pain which extraordinarely improved upon danazol treatment. This patient carryied the Arg378Cys mutation, that was expressed in Pichia pastoris. The expression yield of this variant protein was ten fold lower than the wild-type molecule, but once correctly purified, the stability and inhibitory activity were comparable, as revealed by functional and structural studies. Based on the known structure of other serpins together with molecular modeling, we expect that Arg378 in the native protein forms a salt bridge with Glu429. In alpha(1)-antitrypsin Z variant, it is predicted that loss of this salt bridge would have an effect on the rate of folding. Analogously, we speculate that the low plasma level of this variant could be due to retarded protein folding, that may promote protein aggregation by accumulation of aggregation-prone folding intermediates.