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Hum Mol Genet. 2015 Sep 15;24(18):5079-92. doi: 10.1093/hmg/ddv226. Epub 2015 Jun 16.

Adaptor protein-2 sigma subunit mutations causing familial hypocalciuric hypercalcaemia type 3 (FHH3) demonstrate genotype-phenotype correlations, codon bias and dominant-negative effects.

Author information

1
Academic Endocrine Unit, Radcliffe Department of Medicine, University of Oxford, Oxford, UK.
2
Oxford Molecular Genetics Laboratory, Churchill Hospital, Oxford, UK.
3
Renal Unit, Great Ormond Street Hospital for Children NHS Foundation Trust and UCL Institute of Child Health, London, UK.
4
Division of Endocrinology, Boston Children's Hospital, Boston, MA, USA.
5
School of Medicine, Ninewells Hospital, University of Dundee, Dundee, UK.
6
Clinical Genetics Department, Sheffield Children's Hospital NHS Foundation Trust, Sheffield, UK.
7
Queen Elizabeth II Hospital, Welwyn Garden City, UK.
8
Department of Paediatric Endocrinology, Royal Manchester Children's Hospital, Manchester, UK.
9
Regional Centre for Endocrinology and Diabetes, Royal Victoria Hospital, Belfast, UK.
10
Department of Paediatrics, Evelina London Children's Hospital, St. Thomas' Hospital, London, UK.
11
Department of Clinical Genetics, Guy's Hospital, London, UK.
12
Institute of Cancer and Genetics, University Hospital of Wales, Cardiff, UK.
13
Department of Endocrinology, St. Vincent's University Hospital, Dublin, Ireland.
14
Metabolic Unit, Western General Hospital, NHS Lothian and University of Edinburgh, Edinburgh, UK.
15
Centre for Cancer Research and Cell Biology, Queens University of Belfast, Belfast, UK, Department of Genetic Medicine, Belfast HSC Trust, Belfast, UK.
16
Department of Endocrinology, Cork University Hospital, Cork, Ireland.
17
Institute of Genetic Medicine, Newcastle University, Newcastle upon Tyne, UK.
18
SE Scotland Genetic Service, Western General Hospital, Edinburgh, UK.
19
Department of Endocrinology, Northwick Park Hospital, London, UK.
20
Diabetes and Endocrine Centre, Royal Bournemouth Hospital, Bournemouth, UK.
21
Department of Endocrinology, Chesterfield Royal Hospital NHS Foundation Trust, Derbyshire, UK.
22
Pediatric Gastroenterology, Hepatology and Nutrition Unit, Cliniques Universitaires Saint-Luc, Brussels, Belgium.
23
Department of Endocrinology, Russells Hall Hospital, Dudley, UK.
24
Department of Clinical Biochemistry, Frimley Park Hospital, Surrey, UK and.
25
Center for Metabolic Bone Disease and Molecular Research, Shriners Hospital for Children, St. Louis, Missouri, USA.
26
Academic Endocrine Unit, Radcliffe Department of Medicine, University of Oxford, Oxford, UK, rajesh.thakker@ndm.ox.ac.uk.

Abstract

The adaptor protein-2 sigma subunit (AP2σ2) is pivotal for clathrin-mediated endocytosis of plasma membrane constituents such as the calcium-sensing receptor (CaSR). Mutations of the AP2σ2 Arg15 residue result in familial hypocalciuric hypercalcaemia type 3 (FHH3), a disorder of extracellular calcium (Ca(2+) o) homeostasis. To elucidate the role of AP2σ2 in Ca(2+) o regulation, we investigated 65 FHH probands, without other FHH-associated mutations, for AP2σ2 mutations, characterized their functional consequences and investigated the genetic mechanisms leading to FHH3. AP2σ2 mutations were identified in 17 probands, comprising 5 Arg15Cys, 4 Arg15His and 8 Arg15Leu mutations. A genotype-phenotype correlation was observed with the Arg15Leu mutation leading to marked hypercalcaemia. FHH3 probands harboured additional phenotypes such as cognitive dysfunction. All three FHH3-causing AP2σ2 mutations impaired CaSR signal transduction in a dominant-negative manner. Mutational bias was observed at the AP2σ2 Arg15 residue as other predicted missense substitutions (Arg15Gly, Arg15Pro and Arg15Ser), which also caused CaSR loss-of-function, were not detected in FHH probands, and these mutations were found to reduce the numbers of CaSR-expressing cells. FHH3 probands had significantly greater serum calcium (sCa) and magnesium (sMg) concentrations with reduced urinary calcium to creatinine clearance ratios (CCCR) in comparison with FHH1 probands with CaSR mutations, and a calculated index of sCa × sMg/100 × CCCR, which was ≥ 5.0, had a diagnostic sensitivity and specificity of 83 and 86%, respectively, for FHH3. Thus, our studies demonstrate AP2σ2 mutations to result in a more severe FHH phenotype with genotype-phenotype correlations, and a dominant-negative mechanism of action with mutational bias at the Arg15 residue.

PMID:
26082470
PMCID:
PMC4550820
DOI:
10.1093/hmg/ddv226
[Indexed for MEDLINE]
Free PMC Article

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