FAMILIAL HYPOPARATHYROIDISM - IDENTIFICATION OF A NOVEL GAIN OF FUNCTION MUTATION IN TRANSMEMBRANE DOMAIN-5 OF THE CALCIUM-SENSING RECEPTOR

Activating mutations of the extracellular calcium (Ca(2+)e)-sensing re ceptor (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) le vels 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 seizur es and tetany, respectively, likewise had hypoparathyroidism. A hetero zygous missense mutation substituting a cysteine for the phenylalanine normally present at codon 788 (F788C) was identified in the CaR's fif th 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 tr ansiently transfected HEK293 cells loaded with fura-a by assessing Ca( 2+)e-evoked increases in the cytosolic calcium concentration (Ca(2+)i) . There was a leftward shift in the concentration-response curve for t he mutant receptor [EC50 (effective concentration of Ca(2+)e producing half of the maximal Ca(2+)i response, 2.7 +/- 0.1 us. 4.1 +/- 0.1 mmo l/L for the wild-type receptor]. HEK293 cells cotransfected with both the wild-type and mutant CaRs (to mimic the heterozygous state in affe cted family members) showed an EC50 (3.0 +/- 0.1 mmol/L) similar to th at of the mutant CaR alone. Thus, we confirm that 1) a gain of functio n mutation in the fifth transmembrane domain of the CaR causes severe familial hypoparathyroidism by rendering the receptor more sensitive t han normal to activation by Ca(2+)e; 2) some patients in the family do not experience seizures despite their severe hypocalcemia; and 3) thi s condition needs to be differentiated from other causes of hypoparath yroidism.