Endocrine Abstracts

The calcium-sensing receptor (CaSR) plays a central role in regulating parathyroid hormone (PTH) secretion in response to changes in extracellular calcium. The CaSR is a G-protein-coupled receptor and ligand binding results in stimulation of phospholipase C (PLC) activity, causing accumulation of inositol 1,4,5-triphosphate (IP₃) and the rapid release of calcium ions from intracellular stores. Given the pivotal role of the CaSR in calcium homeostasis, we decided to investigate a 51 year old woman, who was the daughter of normocalcaemic consanguineous parents and suffered from severe hypercalcaemia, for CaSR mutations. The study was approved by the local ethical committee. She presented with nephrolithiasis and osteoporosis in association with hypercalcaemia, an elevated serum PTH of 65 pg/ml (0–30 pg/ml), and a normal calcium: creatinine clearance ratio of 0.024. At surgery two parathyroid adenomas were removed but she remained hypercalcaemic. Mutational analysis of the CaSR gene revealed a novel homozygous germline CaSR mutation, Pro339Thr. Functional characterisation of the mutant (Thr339) and wild-type (Pro339) CaSRs was performed by transient transfection into HEK293 cells, and this demonstrated the Thr339 mutant CaSR to have a rightward shift in the dose-response curve (EC₅₀=3.18 mM (±0.19)) versus the wild-type (EC₅₀=2.16 mM (±0.1), P=0.0007), consistent with a loss-of-function mutation. Co-expression of the wild-type and Thr339 mutant CaSRs also revealed a rightward shift in the dose-response curve (wild-type/Thr339 EC₅₀=2.88 mM (±0.19) versus wild-type, EC₅₀=2.16 mM (±0.1), P=0.006), demonstrating the mutant CaSR to exert a dominant negative effect on the wild-type receptor. An analysis of the three-dimensional structure of the CaSR, based on the crystal structure of the metabotropic glutamate receptor type 1, predicted the Thr339 mutant residue to interfere with ligand binding. Thus, our findings demonstrate that a homozygous inactivating CaSR mutation may cause adult onset primary hyperparathyroidism.