Three novel activating mutations in the calcium-sensing receptor responsible for autosomal dominant hypocalcemia

We report three novel activating mutations in the calcium-sensing receptor (CASR) that are responsible for autosomal dominant hypocalcemia (ADH) in three unrelated families. Each mutation involves a missense substitution resulting in a nonconservative amino acid alteration, P221L, E228Q, and Q245R. These mutations were observed in affected family members, but not in unaffected family members or in unrelated control samples. All three mutations are clustered in the extracellular domain of the CASR in a region dominated by negatively charged amino acids. Each mutant and wild-type receptor was expressed in Cos-1 cells. A luciferase reporter gene assay was utilized to detect the level of receptor activity by utilizing a protein kinase C-activated promoter to drive the production of luciferin, the reporter gene product. All three mutant receptors exhibited an increased sensitivity to calcium at all concentrations tested when compared to the wild-type receptor, supporting the hypothesis that these are activating mutations and are responsible for the ADH phenotype in these families. The data presented in this study suggest the importance of this highly negatively charged region of the extracellular domain in normal CASR function.