A novel assay to diagnose hereditary angioedema utilizing inhibition of bradykinin‐forming enzymes

• Published in: Allergy (Copenhagen) Vol. 70; no. 1; pp. 115 - 119
• Main Authors: Joseph, K., Bains, S., Tholanikunnel, B. G., Bygum, A., Aabom, A., Koch, C., Farkas, H., Varga, L., Ghebrehiwet, B., Kaplan, A. P.
• Format: Journal Article
• Language: English
• Published: Denmark Blackwell Publishing Ltd 01.01.2015
• Subjects: Allergies; Angioedemas, Hereditary - diagnosis; Angioedemas, Hereditary - enzymology; Bioassays; bradykinin; Bradykinin - biosynthesis; Case-Control Studies; Complement C1 Inhibitor Protein - metabolism; Enzyme-Linked Immunosorbent Assay; Enzymes; Factor XIIa; Genetic disorders; hereditary angioedema; hereditary angioedema diagnosis; Humans; kallikrein; Plasma; Plasma Kallikrein; Reproducibility of Results; Sensitivity and Specificity; factor XIIa; bradykinin; hereditary angioedema; hereditary angioedema diagnosis; kallikrein
• ISSN: 0105-4538; 1398-9995
• DOI: 10.1111/all.12520

Background Hereditary angioedema types I and II are caused by a functional deficiency of C1 inhibitor (C1‐INH), leading to overproduction of bradykinin. The current functional diagnostic assays employ inhibition of activated C1s; however, an alternative, more physiologic method is desirable. Methods ELISAs were developed using biotinylated activated factor XII (factor XIIa) or biotinylated kallikrein bound to avidin‐coated plates. Incubation with plasma was followed by detection of bound C1‐INH. Results After standard curves were developed for quantification of C1‐INH, serial dilutions of normal plasma were employed to validate the ability to detect known concentration of C1‐INH in the plasma as a percent of normal. Hereditary angioedema (HAE) types I and II were then tested. The level of functional C1‐INH in all HAE types I and II plasma tested was less than 40% of our normal control. This was evident regardless of whether we measured factor XIIa–C1‐INH or kallikrein–C1‐INH complexes, and the two assays were in close agreement. By contrast, testing the same samples utilizing the commercial method (complex ELISA, Quidel Corp.) revealed the levels of C1‐INH between 0 and 57% of normal (mean, 38%), and 42 samples were considered equivocal (four controls and 38 patients). Conclusions Diagnosis of HAE types I and II can be ascertained by inhibition of enzymes of the bradykinin‐forming cascade, namely factor XIIa and kallikrein. Either method yields functional C1‐INH levels in patients with HAE (types I and II) that are clearly abnormal with less variance or uncertainty than the commercial method.