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Autoimmune polyendocrine syndrome type I (APS I) is characterized by autoantibodies, often directed towards tissue-specific enzymes in the affected organs. We have earlier reported the identification of tryptophan hydroxylase (TPH) and tyrosine hydroxylase (TH) as autoantigens in APS I associated with intestinal dysfunction and alopecia, respectively. These two enzymes, together with phenylalanine hydroxylase (PAH), constitute the group of biopterin-dependent hydroxylases, which all are involved in the biosynthesis of neurotransmitters. A clone encoding PAH was used for in vitro transcription/translation, followed by immunoprecipitation with sera from 94 APS I patients and 70 healthy controls. Of the APS I patients, 25% had PAH antibodies, and no reactivity was detected in the controls. No association with the main clinical components of APS I was found with PAH antibodies. Altogether, 59 sera from the 94 APS I patients reacted with at least one of TPH, TH, or PAH, whereas 35 showed no reactivity. Nineteen of the sera contained antibodies towards all enzymes, 12 to TPH only and 12 to TH only. No sera showed antibodies that reacted to only PAH. An immunocompetition assay demonstrated that the reactivity against PAH represents a cross-reactivity with TPH, whereas antibodies against TPH and TH are directed towards epitopes unique for the two enzymes.

Background: Autoimmune polyendocrine syndrome type 1 (APS1) is an autosomal recessive disorder with both endocrine and non-endocrine features. Periodic gastrointestinal dysfunction occurs in 25-30% of APS1 patients. We aimed to identify an intestinal autoantigen., Methods: A human duodenal cDNA library was immunoscreened with serum samples from APS1 patients. A positive clone was identified and used for in-vitro transcription and translation, followed by immunoprecipitation with serum samples from 80 APS1 patients from Norway, Finland, and Sweden. Sections of normal and APS1-affected small intestine were immunostained with serum from APS1 patients and specific antibodies. An enzyme-inhibition assay was used to characterise the autoantibodies., Findings: We isolated a cDNA clone coding for tryptophan hydroxylase. 48% (38/80) of APS1 patients had antibodies to tryptophan hydroxylase, whereas no reactivity to this antigen was detected in patients with other autoimmune diseases (n=372) or healthy blood donors (n=70). 89% (17/19) of APS1 patients with gastrointestinal dysfunction were positive for antibodies to tryptophan hydroxylase, compared with 34% (21/61) of patients with no gastrointestinal dysfunction (p<0.0001). Serum from antibody-positive APS1 patients specifically immunostained tryptophan-hydroxylase-containing enterochromaffin cells in normal duodenal mucosa. No serotonin-containing cells were seen in duodenal biopsy samples from APS1 patients. Serum from antibody-positive APS1 patients almost completely inhibited activity of tryptophan hydroxylase., Interpretation: Tryptophan hydroxylase is an endogenous intestinal autoantigen in APS1, and there is an association between antibodies to the antigen and gastrointestinal dysfunction. Analysis of antibodies to tryptophan hydroxylase may be a valuable diagnostic tool to predict and monitor gastrointestinal dysfunction in APS1.