Novel activating mutations of the calcium-sensing receptor: the calcilytic NPS-2143 mitigates excessive signal transduction of mutant receptors.

CONTEXT AND OBJECTIVE

Activating mutations in the calcium-sensing receptor (CaSR) gene cause autosomal dominant hypocalcemia (ADH). The aims of the present study were the functional characterization of novel mutations of the CaSR found in patients, the comparison of in vitro receptor function with clinical parameters, and the effect of the allosteric calcilytic NPS-2143 on the signaling of mutant receptors as a potential new treatment for ADH patients.

METHODS

Wild-type and mutant CaSR (T151R, P221L, E767Q, G830S, and A844T) were expressed in human embryonic kidney cells (HEK 293T). Receptor signaling was studied by measuring intracellular free calcium in response to different concentrations of extracellular calcium (Ca(2+)) in the presence or absence of NPS-2143.

RESULTS

All ADH patients had lowered serum calcium ranging from 1.7 to 2.0 mm and inadequate intact PTH and urinary calcium excretion. In vitro testing of CaSR mutations from these patients revealed exaggerated Ca(2+)-induced cytosolic Ca(2+) responses with EC(50) values for Ca(2+) ranging from 1.56 to 3.15 mM, which was lower than for the wild-type receptor (4.27 mM). The calcilytic NPS-2143 diminished the responsiveness to Ca(2+) in the CaSR mutants T151R, E767Q, G830S, and A844T. The mutant P221L, however, was only responsive when coexpressed with the wild-type CaSR.

CONCLUSION

Calcilytics might offer medical treatment for patients with autosomal dominant hypocalcemia caused by calcilytic-sensitive CaSR mutants.