Remittent C1-inhibitor deficiency due to Arg378Cys mutation

Background: C1 inhibitor (C1-INH) deficiency causes hereditary angioedema. Arg378→Cys C1-INH variant was found in a patient suffering from abdominal pain and presenting markedly reduced C1-INH plasma levels episodically undergoing spontaneous normalization with concomitant disappearance of symptoms. Purpose: To elucidate the effect of the mutation on protein function and conformation and explain the biochemical and clinical phenotype. Methods: We expressed Arg378→Cys C1-INH variant in Pichia pastoris and performed functional and structural studies on the purified protein. Results: Expression of C1-INH resulted in a ten fold drop in mutant protein secretion compared to wild type. Both proteins formed complexes with target proteases, but the kinetic of inhibition of the mutant was slightly diminished and this reduction increased with temperature. Gel filtration indicated that the mutant protein can form oligomers. Thermal denaturation experiments in conditions reproducing the intracellular molecular crowding demonstrated that, compared to wild-type, the mutant had higher tendency to oligomerize. Conclusion: our findings suggest that the Arg378→Cys C1-INH variant maintains inhibitory functions against target proteases, but its structural conformation is abnormally susceptible to environmental factors that may occasionally promote protein oligomerization and interfere with its function and secretion from cells, accounting for variability in plasma levels. Clinical Implication: We describe a new biochemical and clinical phenotype caused by a mutation in C1-INH gene.