Context: Autoimmune polyendocrine syndrome type 1 (APS1) is characterised by multiple autoimmune endocrinopathies and results from mutations in the autoimmune regulator (AIRE) gene. Approximately 80% of patients present with hypoparathyroidism which is suggested to result from autoimmune responses against the parathyroid glands. The calcium-sensing receptor (CaSR), which plays a pivotal role in maintaining calcium homeostasis by sensing blood calcium levels and regulating release of parathyroid hormone, has been identified as a parathyroid autoantibody target in APS1.
Objective: The aim of the study was to characterise APS1 patient anti-CaSR autoantibodies in relation to their epitopes, specificity, IgG subclass, and effects upon CaSR function.
Methods: Phage-display; ELISA; and bioassays.
Results: Anti-CaSR autoantibody binding sites (epitopes) were identified between amino acids 4169, 114126, 171195, and 260340 in the extracellular domain of the receptor. Absorption experiments confirmed the specificity of anti-CaSR autoantibodies in recognising their respective epitope. Anti-CaSR autoantibodies were analysed for their ability to increase both Ca2+-dependent ERK phosphorylation and inositol phosphate accumulation in HEK293 cells expressing the CaSR. The results indicated that two APS1 patients had anti-CaSR-activating autoantibodies, suggesting that although the majority of APS1 patients do not have anti-CaSR-stimulating autoantibodies, there may be a small minority of patients in whom the hypoparathyroid state is the result of functional suppression of the parathyroid glands. Autoantibodies against CaSR epitopes 4169, 171195, and 260340 were exclusively of the IgG1 subclass. Autoantibody responses against CaSR epitope 114126 were predominantly of the IgG1 with a minority of the IgG3 subclass.
Conclusion: The study provides a detailed analysis of the characteristics of anti-CaSR autoantibodies in APS1 patients, although further investigations are required to determine the exact role played by the autoimmune response against the CaSR in the pathogenesis of APS1-associated hypoparathyroidism.