BES2003 Oral Communications Thyroid and Calcium (8 abstracts)
Idiopathic (Normocalcaemic) hypercalciuric nephrolithiasis due to an activating calcium sensing receptor mutation
PT Christie 1 , AJ Curley 1 , B Harding 1 , MR Bowl 1 , JJO Turner 1 , FP Cappuccio 2 , CB Langman 3 , AK Saggar 4 , T Taylor 5 & RV Thakker 1
1Nuffield Department of Clinical Medicine, University of Oxford, Oxford, UK; 2Community Health Sciences, St George's Hospital Medical School, London, UK; 3Northwestern University Medical School, Chicago, USA; 4Clinical Genetics, St George's Hospital Medical School, London, UK; 5Department of Paediatrics, St Richard's Hospital, Chichester, UK.
Calcium sensing receptor (CaSR) mutations may result in either hypocalciuric hypercalcaemia or hypocalcaemic hypercalciuria due to a loss or gain of function, respectively. It has also been postulated that some gain of function CaSR mutations may result in idiopathic (i.e. normocalcaemic) hypercalciuria (IH). We reasoned that such CaSR mutations would lead to an early onset of IH and have sought for them in 12 unrelated children who were normocalcaemic and developed IH nephrolithiasis before the age of 4 years and who had a family history for IH. Leukocyte DNA samples were used to search for CaSR mutations. This revealed, in one proband, a missense mutation at codon 994 which involved the substitution of the wild type (WT) histidine residue for a tyrosine in the cytoplasmic tail of the CaSR. This His994Tyr mutation was demonstrated to co-segregate with IH in the proband's family and to be absent in 250 alleles from 125 unrelated normals. Expression of the mutant and WT CaSRs in HEK293 cells together with an assessment of their responses, as measured by changes in intracellular calcium concentration [Ca]i to alterations in extracellular [Ca]o, revealed the Tyr994 mutant CaSR to have a left-ward shift in the dose-response curve (Tyr994 EC50 = 1.43 mM plus/minus 0.22 versus WT (His994) EC50 = 2.74 mM plus/minus 0.22, p<0.02), thereby demonstrating a gain of function. An examination of the mutant sequence indicated that it resulted in a tyrosine kinase phosphorylation consensus sequence and Western blot analysis using transiently-transfected HEK293 cellular extracts and the p-Tyr (PY99) antibody, which specifically detects phosphorylated tyrosine groups, demonstrated a significantly increased (p<0.02) phosphorylation of the mutant CaSR. Thus our results, which report the first CaSR mutation in IH nephrolithiasis, reveal a novel cytoplasmic mutation that increases phosphorylation and activity of the CaSR.
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Endocrine Abstracts
ISSN 1470-3947 (print) | ISSN 1479-6848 (online)
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