Identification and functional characterization of loss-of-function mutations of the calcium-sensing receptor in four Italian kindreds with familial hypocalciuric hypercalcemia

Abstract

Objective: Identification and characterization of calcium-sensing receptor (CASR) mutations in four unrelated Italian kindreds with familial hypocalciuric hypercalcemia. Design: Clinical evaluation and genetic analysis of CASR gene. Functional characterization of mutated CASRs. Methods: Direct sequencing of CASR gene in genomic DNA. Studies of CASR-mediated increases in cytosolic calcium concentration [Ca 2C ] i in CASR-transfected COS-7 cells in vitro. Results: Four unreported heterozygous CASR mutations were identified, including three missense (H595Y, P748H, and C765W) and one splice site (IVS2C1GOC) mutation. The H595Y, P748H, and C765W mutant receptors, although expressed at normal levels on the cell surface, showed a reduced response in [Ca 2C ] i relative to the wildtype (WT) CASR to increasing extracellular calcium concentrations. Cotransfection experiments showed that the H595Y and P748H mutants did not affect the apparent affinity of the WT CASR for calcium, suggesting that they do not exert a dominantnegative effect. On the other hand, the co-transfected C765W mutant decreased the maximum response of the WT CASR to calcium, suggesting that it may reduce the effective concentration of the normal CASR on the cell surface or impair its maximal signaling capacity. Conclusions: Four CASR mutations were identified. The reduced functional responses to extracellular calcium and normal expression of the mutant receptors suggest that conformational changes account for altered CASR activity. Moreover, a reduced complement of normal CASRs in these heterozygous patients, perhaps combined with a mutant receptor-induced decrease in maximal activity of the WT receptor, may contribute to defective calcium-sensing in vivo.

Mutations of calcium-sensing receptor gene: two novel mutations and overview of impact on calcium homeostasis

“…On the other hand, activating mutations have been described in the context of ADH and type V Bartter syndrome (see http://www.casrdb.mcgill.ca). Taking into account the last 6 years, about 36 CASR gene mutations have been described (25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38), with 34 being inactivating and two activating. Among them, most have been found in exons 4 and 7.…”