Familial Hypoparathyroidism: Identification of a Novel Gain of Function Mutation in Transmembrane Domain 5 of the Calcium-Sensing Receptor1

Activating mutations of the extracellular calcium (Ca2+e)-sensing receptor (CaR) gene, mostly in its extracellular domain, can cause both familial and sporadic hypoparathyroidism. We report a Japanese family with severe hypoparathyroidism with pretreatment serum calcium (Ca) levels of 4.9–5.9 mg/dL. The proband presented with a seizure at 6 days of age. Her older brother and mother, who had also experienced seizures and tetany, respectively, likewise had hypoparathyroidism. A heterozygous missense mutation substituting a cysteine for the phenylalanine normally present at codon 788 (F788C) was identified in the CaR’s fifth transmembrane domain and was shown to cosegregate with the disease. The mutation was absent in DNA from 50 control subjects. Analysis of the functional properties of the mutant receptor was carried out in transiently transfected HEK293 cells loaded with fura-2 by assessing Ca2+e-evoked increases in the cytosolic calcium concentration (Ca2+i). There was a leftward shift in the concentrati...