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’

• Published in: European journal of endocrinology Vol. 169; no. 1; pp. K1 - K7
• Main Authors: Ward, D T, Mughal, M Z, Ranieri, M, Dvorak-Ewell, M M, Valenti, G, Riccardi, D
• Format: Journal Article
• Language: English
• Published: Bristol BioScientifica 01.07.2013
• Subjects: Amino Acid Substitution; Arginine; Biological and medical sciences; Calcium - blood; Case Report; Child; Endocrinopathies; Endoplasmic Reticulum - genetics; Endoplasmic Reticulum - metabolism; Female; Fluorescent Antibody Technique; Fundamental and applied biological sciences. Psychology; HEK293 Cells; Humans; Hypercalcemia - blood; Hypercalcemia - etiology; Hyperparathyroidism - blood; Hyperparathyroidism - congenital; Hyperparathyroidism - diagnosis; Hyperparathyroidism - genetics; Immunoblotting; Infant; Infant, Newborn; Medical sciences; Mutagenesis, Insertional; Non tumoral diseases. Target tissue resistance. Benign neoplasms; Parathyroid Hormone - blood; Parathyroid Hormone - genetics; Parathyroidectomy - methods; Parathyroids. Parafollicular cells. Cholecalciferol. Phosphocalcic homeostasis (diseases); Receptors, Calcium-Sensing - genetics; Receptors, Calcium-Sensing - metabolism; Sequence Analysis, DNA - methods; Severity of Illness Index; Transfection; Treatment Outcome; Vertebrates: endocrinology; Endocrinopathy; Human; Neonatal; Newborn; Parathyroid diseases; Mutation; Calcium-sensing receptor; Molecular biology; Severe; Extracellular; Endocrinology; Hyperparathyroidism
• ISSN: 0804-4643; 1479-683X
• DOI: 10.1530/EJE-13-0094

ObjectiveLoss-of-function calcium-sensing receptor (CAR) mutations cause elevated parathyroid hormone (PTH) secretion and hypercalcaemia. Although full Car deletion is possible in mice, most human CAR mutations result from a single amino acid substitution that maintains 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 corrected calcium (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 54 kDa 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 (2.5 mM corrected calcium, 11 pg/ml PTH (10–71), age 8 years) but exhibited mild asymptomatic hypocalcaemia at age 10 years, 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.