A CA2-SENSING RECEPTOR MUTATION CAUSES HYPOPARATHYROIDISM BY INCREASING RECEPTOR SENSITIVITY TO CA2+ AND MAXIMAL SIGNAL-TRANSDUCTION()

Activating mutations of the Ca2+-sensing receptor (caR) gene cause aut osomal dominant hypoparathyroidism. Functional expression studies have been reported for several mutations, but have produced conflicting re sults. Thus, the mechanism by which these mutations activate the recep tor is unclear. We describe here a new family with autosomal dominant hypoparathyroidism. The mother and three daughters experienced muscle spasms and/or seizures from early childhood. They were treated with or al calcium and vitamin D analogs, and all four patients developed hype rcalciuria, nephrocalcinosis, and renal insufficiency. In this family, we identified a heterozygous missense mutation (F612S) involving the extracellular region of the CaR. The mutation cosegregated with diseas e. It was not present in 50 normal control individuals, We used site-d irected mutagenesis to introduce this mutation into the caR cDNA, and then expressed the mutant receptor in human embryonic kidney (HEK)-293 cells. In these cells, the accumulation of inositol phosphates was me asured as a function of extracellular Ca2+ concentration. compared wit h the wild-type receptor, the mutant receptor showed a left-shift in t he concentration-response curve and an increase in the maximal respons e to high Ca2+ concentrations. These effects did not appear to be medi ated by changes in levels of receptor expression, as judged by ELISA, or by changes in receptor glycosylation, as judged by Western analysis . We conclude that this CaR mutation causes hypoparathyroidism by a du al increase in receptor sensitivity to extracellular Ca2+ and maximal signal transduction capacity.