Inter-α-trypsin inhibitor heavy chain 4 (ITIH4) as a compensatory protease inhibitor in hereditary angioedema

Abstract

BackgroundHereditary angioedema (HAE) is a genetic disorder that manifests as recurrent angioedema attacks, most frequently due to absent or reduced C1 inhibitor (C1-INH) activity. C1-INH is a crucial regulator of enzymatic cascades in the complement, fibrinolytic, and contact systems. Inter-alpha-trypsin inhibitor heavy chain 4 (ITIH4) is an abundant plasma protease inhibitor that can inhibit enzymes in the proteolytic pathways associated with HAE. Nothing is known about its role in HAE. ObjectiveInvestigate ITIH4 activation in HAE, establish it as a potential biomarker, and explore its involvement in HAE-associated proteolytic pathways. MethodsSpecific immunoassays for non-cleaved ITIH4 (Intact ITIH4) and an assay detecting both intact and cleaved ITIH4 (Total ITIH4) were developed. We initially tested serum samples from HAE patients (n=20), ACEI-induced edema patients (ACEI) (n=20), and unknown HAE patients (U-HAE) (n=20). Validation involved an extended cohort of 80 HAE patients (60 type I, 20 type II), including samples taken at attack and quiescent disease periods, as well as 100 healthy controls. ResultsIn 63% of HAE patients, the Intact ITIH4 assay showed lower signals than the Total ITIH4 assay. This difference was not observed in ACEI and U-HAE patients. Western blotting confirmed cleaved ITIH4 in low-Intact ITIH4 samples. In serum samples lacking intact endogenous ITIH4, we observed immediate cleavage of added recombinant ITIH4 suggesting continuous enzymatic activity in the serum. Confirmatory HAE cohort analysis revealed significantly lower intact ITIH4 levels in both HAE type 1 and type 2 patients compared to controls, with consistently low Intact/Total ITIH4 ratios during clinical HAE attacks. ConclusionThe disease-specific low intact ITIH4 levels highlight its unique nature in HAE. The results suggest that ITIH4 may exhibit compensatory mechanisms in HAE, suggesting its utility as a diagnostic and prognostic biomarker. The variations during quiescent and active disease periods raise intriguing questions about the dynamics of proteolytic pathways in HAE.