Molecular and Clinical Analysis of a Neonatal Severe Hyperparathyroidism Case Caused by a Stop Mutation in the Calcium-Sensing Receptor Extracellular Domain Representing in Effect a Human "Knockout".

ObjectiveLoss-of-function calcium-sensing receptor (CaR) mutations cause elevated parathyroid hormone (PTH) secretion and hypercalcaemia. Though full CaR deletion is possible in mice, most human CaR mutations result from single amino acid substitutions which maintain partial function. However, here we report a case of neonatal severe hyperparathyroidism (NSHPT) in which the truncated CaR lacks any transmembrane domain (CaRR392X), in effect a full "CaR-knockout".Case ReportThe infant (daughter of distant cousins) presented with hypercalcaemia (5.5-6 mmol/l [2.15-2.65]) and elevated PTH concentrations (650-950 pmol/l [12-81]) together with skeletal demineralisation. NSHPT was confirmed by CaR gene sequencing (homozygous c.1174C-to-T mutation) requiring total parathyroidectomy during which only two glands were located and removed, resulting in normalisation of her serum PTH/calcium levels.Design and MethodsThe R392X stop codon was inserted into human CaR and the resulting mutant (CaRR392X) expressed transiently in HEK-293 cells.ResultsCaRR392X expressed as a 54kDa dimeric glycoprotein that was undetectable in conditioned medium or in the patient's urine. The membrane localisation observed for wild-type CaR in parathyroid gland and transfected HEK-293 cells was absent from the proband's parathyroid gland and from CaRR392X-transfected cells. Expression of the mutant was localised to endoplasmic reticulum consistent with its lack of functional activity.ConclusionsIntriguingly, the patient remained normocalcaemic throughout childhood (2.5mM Ca2+, 11pg/ml PTH [10-71], age-8) but exhibited mild asymptomatic hypocalcaemia at age-10, now treated with 1-hydroxycholecalciferol and Ca2+ supplementation. Despite representing a virtual CaR-knockout, the patient displays no obvious pathologies beyond her calcium homeostatic dysfunction