Activating Autoantibodies against the Calcium-Sensing Receptor Detected in Two Patients with Autoimmune Polyendocrine Syndrome Type 1

Context: Autoimmune polyendocrine syndrome type 1 (APS1) is an autosomal recessive disorder caused by mutations in the autoimmune regulator (AIRE) gene. Hypoparathyroidism occurs in 80% of patients with APS1 and has been suggested to result from an autoimmune reaction against the calcium-sensing receptor (CaSR) in parathyroid cells. Anti-CaSR binding antibodies have previously been detected in patients with APS1. Objective: The aim of this study was to determine whether anti-CaSR antibodies present in APS1 patients could modulate the response of the CaSR to stimulation by Ca2+. Results: The results indicated that two of the 14 APS1 patients included in the study had anti-CaSR antibodies that stimulated the receptor. These antibodies were detected by their ability to increase both Ca2+-dependent extracellular signal-regulated kinase phosphorylation and inositol phosphate accumulation in human embryonic kidney 293 cells expressing the CaSR. Conclusion: An important implication of the present results is that although the majority of APS1 patients do not have CaSR-stimulating antibodies, there may be a small but substantial minority of patients in whom the hypoparathyroid state is the result of functional suppression of the parathyroid glands rather than their irreversible destruction