Functional Characterization of a Calcium-Sensing Receptor Mutation in Severe Autosomal Dominant Hypocalcemia with a Bartter-Like Syndrome

Abstract

Abstract. The extracellular Ca 2ϩ -sensing receptor (CaSR) plays an essential role in extracellular Ca 2ϩ homeostasis by regulating the rate of parathyroid hormone (PTH) secretion and the rate of calcium reabsorption by the kidney. Activation of the renal CaSR is thought to inhibit paracellular divalent cation reabsorption in the cortical ascending limb (cTAL) both directly and indirectly via a decrease in NaCl transport. However, in patients with autosomal dominant hypocalcemia (ADH), caused by CaSR gain-of-function mutations, a defect in tubular NaCl reabsorption with renal loss of NaCl has not been described so far. This article describes a patient with ADH due to a gain-of-function mutation in the CaSR, L125P, associated with a Bartter-like syndrome that is characterized by a decrease in distal tubular fractional chloride reabsorption rate and negative NaCl balance with secondary hyperaldosteronism and hypokalemia. The kinetics of activation of the L125P mutant receptor expressed in HEK-293 cells, assessed by measuring CaSR-stimulated changes in intracellular Ca 2ϩ and ERK activity, showed a dramatic reduction in the EC 50 for extracellular Ca 2ϩ compared with the wild-type and a loss-offunction mutant CaSR (I40F). This study describes the first case of ADH associated with a Bartter-like syndrome. It is herein proposed that the L125P mutation of the CaSR, which represents the most potent gain-of-function mutation reported so far, may reduce NaCl reabsorption in the cTAL sufficiently to result in renal loss of NaCl with secondary hyperaldosteronism and hypokalemia.

Epigenetic Regulation of MicroRNAs Controlling CLDN14 Expression as a Mechanism for Renal Calcium Handling

“…49 These second messengers (inositol 1,4,5-trisphosphate and intracellular Ca ++ ) have been broadly used to assess the genetic mutations of CaSR in Xenopus oocytes and HEK293 cells. [15][16][17][18] Our study has revealed a novel second messenger based on miRs that is particularly useful for assessing the activating mutations of CaSR found in ADH disease. The common ADH mutations invariably repress the transcription of miR-9 and miR-374 genes, which can be relieved by HDAC inhibitors.…”

“…Knowing that miRs are an important second messenger of CaSR signaling, 9 we asked whether CaSR mutations deregulated miR abundance as a potential causal mechanism and whether HDAC inhibitors corrected the CaSR mutational effects as a novel therapeutic approach for ADH disease. We generated three common ADH mutations in the human CaSR gene (E127A, F128L, and T151M previously described by Pollak et al 15 and Pearce et al 16 ), transfected the wild-type CaSR gene and its mutants into a well established cell model for studying CaSR signaling (HEK293 cells that expressed no endogenous CaSR), 17 and quantified the transcriptional levels of four miR genes-miR-9-1, miR-9-2, miR-9-3 (all three loci transcribing mature miR-9 hairpin sequence), and miR-374. Among them, the miR-9-1 and miR-374 genes were transcriptionally suppressed by the CaSR mutants.…”

“…15,16 A severe form of ADH has been characterized in patients with hypocalcemia, hypomagnesemia, and urinary loss of Ca ++ and Mg ++ , and it is accompanied by additional Bartter-like phenotypes. 17,18 Despite the rapid expanding repertoire of known CaSR genetic mutations, it remains elusive how these mutations affect CaSR downstream signaling targets. Here, we studied three common CaSR mutations found in patients with ADH-E127A, F128L, and T151M-with quantitative RNA analyses.…”

Treatment of Secondary Hyperparathyroidism in CKD Patients with Cinacalcet and/or Vitamin D Derivatives

“…In contrast, activating CaR mutations result in autosomal dominant hypocalcemia (10,11). The latter may be sometimes associated with hypercalciuria and a Bartter-like syndrome (12,13), explained by the fact that the CaR is also expressed by epithelial cells of Henle's loop. Corresponding phenotypes have been reproduced in genetically modified animal models.…”

Osteomalacia in a Case of Adult-Onset Bartter Syndrome

“…Gain-of-function mutations in the CASR gene have been reported in some patients with Bartter syndrome associated with hypocalcaemia and hypercalciuria. Studies showed that these mutations result in receptor activation that is more severe than other gain-of-function mutations [3] . We describe a case of adultonset Bartter syndrome with hypocalcaemia severe enough to cause osteomalacia.…”