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

Department of Pediatrics, Chiba University School of Medicine (T.W., K.M., M.M., H.N., T.Y.), Chiba 260-8670, Japan; Endocrine-Hypertension Division, Department of Medicine (M.B., C.R.L., E.M.B.), Brigham and Women’s Hospital and Harvard Medical School, Boston, Massachusetts 02115; and Saitama Yorii Children’s Hospital (S.M.), Yorii, Saitama 369–1200, Japan

Address all correspondence and requests for reprints to: Toshiyuki Yasuda, M.D., Department of Pediatrics, Chiba University School of Medicine, 1–8-1 Inohana, Chuo-ku, Chiba 260-8670, Japan. E-mail: toshi{at}med.m.chiba-u.ac.jp

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 concentration-response curve for the mutant receptor [EC₅₀ (effective concentration of Ca²⁺e producing half of the maximal Ca²⁺i response, 2.7 ± 0.1 vs. 4.1 ± 0.1 mmol/L for the wild-type receptor]. HEK293 cells cotransfected with both the wild-type and mutant CaRs (to mimic the heterozygous state in affected family members) showed an EC₅₀ (3.0 ± 0.1 mmol/L) similar to that of the mutant CaR alone. Thus, we confirm that 1) a gain of function mutation in the fifth transmembrane domain of the CaR causes severe familial hypoparathyroidism by rendering the receptor more sensitive than normal to activation by Ca²⁺e; 2) some patients in the family do not experience seizures despite their severe hypocalcemia; and 3) this condition needs to be differentiated from other causes of hypoparathyroidism.

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