Mapping of human autoantibody epitopes on aromatic l-amino acid decarboxylase

Context: Aromatic L-amino acid decarboxylase (AADC) is target ofautoantibodies in autoimmune polyendocrine syndrome I (APS I),especially in patients with autoimmune hepatitis. Little informationis currently available on AADC autoantibody epitopes and on theinterrelation between autoantibody-mediated inhibition of enzymaticactivity and epitope specificity.Design: We tested the immunoreactivity of full-length porcine AADCand of eight fragments of the enzyme with human serum from 18patients with APS I, 199 with non-APS I autoimmune Addison’sdisease, 124 with type 1 diabetes mellitus, 36 with Graves’ disease,and 141 healthy control subjects, and we evaluated the autoantibodymediatedenzymatic inhibition.Results: AADC antibodies (Ab) were detected in 12 of 18 (67%) APSI patients and in six of 199 (3%) autoimmune Addison’s disease patients.Four patients with autoimmune hepatitis were all positive forAADCAb. None of the 141 healthy control subjects, 82 patients withnonautoimmune adrenal insufficiency, 124 with type 1 diabetes mellitus,and 36 with Graves’ disease were found positive. Two epitoperegions, corresponding to amino acids 274–299 (E1) and 380–471(E2) were identified. Localization of E1 was confirmed by displacementstudies with synthetic peptides corresponding to peptides ofporcine AADC. All 12 AADCAb-positive APS I sera reacted with E1,and seven of 12 (58%) reacted also with E2. E2-specific, but notE1-specific, autoantibodies were associated with a significant inhibitionof in vitro AADC enzymatic activity.Conclusions: We mapped the human AADCAb epitopes to the middleand COOH-terminal regions of the enzyme. Autoantibodies to the COOH-terminal region induce a significant inhibition of enzymatic activity