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Novel Inactivating Mutations of the Calcium-Sensing Receptor: The Calcimimetic NPS R-568 Improves Signal Transduction of Mutant Receptors

Division of Neuroendocrinology (R.R., B.M., C.S.), Department of Neurosurgery, Friedrich-Alexander University Erlangen-Nuremberg, 91054 Erlangen, Germany; Department of Endocrinology, Diabetes and Nutrition (R.R., C.B.-V., V.B., M.M., C.S.), Charité-University Medicine Berlin, Campus Benjamin Franklin, 12200 Berlin, Germany; and Endocrine Practice (C.H., E.S., K.F.-R., F.R.), 69120 Heidelberg, Germany

Address all correspondence and requests for reprints to: Christof Schöfl, M.D., Division of Neuroendocrinology, Department of Neurosurgery, Friedrich-Alexander University Erlangen-Nuremberg, Schwabachanlage 6, 91054 Erlangen, Germany. E-mail: christof.schoefl{at}uk-erlangen.de.

Context and Objective: Inactivating mutations in the calcium-sensing receptor (CaSR) gene cause neonatal severe hyperparathyroidism and familial hypocalciuric hypercalcemia (FHH). The aims of the present study were the functional characterization of novel mutations of the CaSR found in FHH patients, the comparison of in vitro receptor function with clinical parameters, and the effect of the allosteric calcimimetic NPS R-568 on the signaling of mutant receptors.

Methods: Wild-type and mutant CaSRs (W530G, C568Y, W718X, M734R, L849P, Q926R, and D1005N) were expressed in human embryonic kidney 293 cells. Receptor signaling was studied by measuring intracellular free calcium in response to different concentrations of extracellular calcium ([Ca²⁺]_o).

Results: Four CaSR mutations (C568Y, W718X, M734R, and L849P) demonstrated a complete lack of a [Ca²⁺]_o-induced cytosolic Ca²⁺ response up to 30 mM [Ca²⁺]_o, whereas the CaSR mutants W530G, Q926R, and D1005N retained some sensitivity to [Ca²⁺]_o. There was no significant relation between the in vitro calcium sensitivity, serum calcium, and intact PTH levels in the patients. Patients with C-terminal CaSR mutations had a calcium to creatine ratio above the established diagnostic threshold of 0.01 for FHH. The calcimimetic NPS R-568 enhanced the responsiveness to [Ca²⁺]_o in CaSR mutants of the extracellular domain (W530G and C568Y) as well as the intracellular C-terminal domain (Q926R and D1005N).

Conclusion: Therefore, calcimimetics might offer medical treatment for symptomatic FHH patients, and more important, for patients with neonatal severe hyperparathyroidism that harbor calcimimetic-sensitive CaSR mutants.