Pteridine dependent hydroxylases as autoantigens in autoimmune polyendocrine syndrome type 1

Abstract: Autoimmune polyendocrine syndrome type I (APS) is a monogenous, recessively inherited disease characterised by endocrine and non-endocrine autoimmune manifestations. One fifth of APS I patients suffer from periodic intestinal dysfunction with varying degrees of malabsorbtion, steatorrhea and constipation. Alopecia areata is found in one third of APS I patients. By immunoscreening human cDNA libraries derived from normal human duodenum and scalp with APS I sera, we identified tryptophan hydroxylase (TPH) as an intestinal autoantigen and tyrosine hydroxylase (TH) as a dermal autoantigen. Forty-eight percent (38/80) of the APS I patients had TPH antibodies (Ab) and 44% (41/94) showed TH immunoreactivity. No reactivity against TPH or TH was seen in healthy controls. TPH-Abs showed a statistically significant correlation with gastrointestinal dysfunction (p<0.0001) and TH-Abs were significantly correlated to alopecia (p=0.02). TPH-Ab positive APS I sera specifically immunostained TPH containing enterochromaffin cells in normal duodenal mucosa. In affected mucosa a depletion of the TPH containing EC cells was seen. In enzyme inhibition experiments TPH and TH activity in vitro was reduced by adding APS I sera. TPH and TH together with phenylalanine hydroxylase (PAH) constitute the group of pteridine dependent hydroxylases. These are highly homologous enzymes involved in the biosynthesis of neurotransmitters. Immunoprecipitation of PAH expressed in vitro showed that 27% (25/94) of APS I patients had antibodies reacting with PAH, but no associations with clinical manifestations was observed. An immunocompetition assay showed that the PAH reactivity reflects a cross-reactivity with TPH.In conclusion, we have identified TPH and TH as intestinal and dermal autoantigens in APS I, coupled to gastrointestinal dysfunction and alopecia. We have also demonstrated immunoreactivity against PAH in APS I patient sera reflecting a cross-reactivity with TPH.