Autoantibodies to the extracellular domain of the calcium sensing receptor in patients with acquired hypoparathyroidism

Abstract

Acquired hypoparathyroidism (AH) has been considered to result from an autoimmune process but the self-antigens have not been identified. We studied 25 patients with AH, of which 17 had type I autoimmune polyglandular syndrome and 8 had AH associated with autoimmune hypothyroidism. Five of 25 (20%) AH sera reacted to a membrane-associated antigen of 120-140 kD in human parathyroid gland extracts using immunoblot analysis. This is the exact size of the calcium sensing receptor (Ca-SR). The AH sera were then tested by immunoblot using a membrane fraction of HEK-293 cells transfected with Ca-SR cDNA. Eight of 25 (32%) AH sera reacted to a 120-140-kD protein, which closely matched that recognized by the anti-Ca-SR IgG raised in rabbits. The Ca-SR cDNA was translated in vitro into two parts in order to identify the antigenic epitopes. By using this technique, 14 of 25 (56%) AH sera were positive to the extracellular domain of the Ca-SR, whereas none of the AH patient sera reacted to the intracellular domain. The reactivity of the positive sera was completely removed after pre-absorption with the Ca-SR containing membranes. Sera from 50 patients with various other autoimmune diseases as well as 22 normal controls were also tested, and none of them was positive. In conclusion, the Ca-SR has been identified as an autoantigen in AH.