Two Italian kindreds with familial hypocalciuric hypercalcaemia caused by loss-of-function mutations in the calcium-sensing receptor (CaR) gene: functional characterization of a novel CaR missense mutation

Serum calcium levels are associated with cognitive function in hypoparathyroidism: a neuropsychological and biochemical study in an Italian cohort of patients with chronic post-surgical hypoparathyroidism

PURPOSE

Hypoparathyroidism (HypoPT) is a rare endocrine disease and conventional therapy is based on calcium and vitamin D analogues. Conventional therapy does not restore calcium homeostasis and patients complain with neuropsychological symptoms, which have been evaluated with nonspecific self-administered questionnaires. This study aims to evaluate cognitive functions of patients with chronic post-surgical (PS)-HypoPT compared to a control population, using a standardized neuropsychological approach and evaluating the relationship with serum calcium (Alb-Ca).

METHODS

Observational, monocentric study on 33 patients with PS-HypoPT and 24 controls, in whom biochemical testing and a standardized neuropsychological assessment by a trained psychologist were performed.

RESULTS

In patients with PS-HypoPT, low Alb-Ca correlated with a worse performance on semantic memory abilities and executive function, as suggested by a significant inverse correlation between Alb-Ca and Trail Making Test A (TMT-A) scores (r = - 0.423; p = 0.014) and by a positive correlation with Semantic Fluency Test scores (SF)(r = 0.510; p = 0.002). PS-HypoPT patients with Alb-Ca ≤ 8.9 mg/dl had a significantly lower test performance compared with PS-HypoPT patients with Alb-Ca > 8.9 mg/dl, both at the TMT-A test (mean score: 34.53-18.55; p < 0.0001) and at SF test (mean score: 41.94-48.68; p = 0.01) and also a significantly lower test performance compared with control patients' group at TMT-A (mean score: 34.53-25.5; p = 0.0057).

CONCLUSIONS

Patients with chronic PS-HypoPT in conventional therapy do not show a severe cognitive impairment; however, cognitive functions namely visuo-spatial attention, executive function and semantic memory appear to be modulated by Alb-Ca and impaired by its low levels.

A case of familial hypocalciuric hypercalcemia type 1 due to CASR p.Pro55Leu mutation

BACKGROUND

Familial hypocalciuric hypercalcemia (FHH) is a rare autosomal dominant disease, which requires differential diagnosis from relatively common primary hyperparathyroidism (PHPT) in order to avoid unnecessary surgery.

CASE PRESENTATION

A 16-year-old female had been followed by the department of psychosomatic medicine at our institution. Throughout the follow-up period, her plasma calcium levels were high, plasma Pi levels were relatively low, and plasma intact PTH was relatively high. She was referred to our department to determine the cause of her hypercalcemia. Her 24 h urinary calcium excretion was as low as 100 mg/day, and calcium creatinine clearance ratio was below 0.01. Moreover, she had a family history of hypercalcemia (proband, her brother, and her father). The genetic testing for her family revealed that she, her brother, and her father were definitively diagnosed with FHH type 1 due to the heterozygous calcium-sensing receptor mutation (NM_00388:4:c.164C > T:p.Pro55Leu).

CONCLUSION

We experienced a 16-year-old female with FHH, in whom genetic testing identified the heterozygous calcium-sensing receptor mutation (NM_00388:4:c.164C > T:p.Pro55Leu) as pathogenic, permitting a definitive diagnosis of FHH type 1. The genetic testing for calcium sensing receptor is beneficial to distinguish asymptomatic primary hyperparathyroidism from FHH.

Hypomagnesuria is Associated With Nephrolithiasis in Patients With Asymptomatic Primary Hyperparathyroidism

CONTEXT

The pathogenesis of nephrolithiasis in primary hyperparathyroidism (PHPT) remains to be elucidated. The latest guidelines suggest parathyroidectomy in patients with asymptomatic PHPT with hypercalciuria (> 400 mg/d) and increased stone risk profile.

OBJECTIVE

The objective of this work is to evaluate the association of urinary stone risk factors and nephrolithiasis in patients with asymptomatic sporadic PHPT and its clinical relevance.

DESIGN

A total of 157 consecutive patients with sporadic asymptomatic PHPT were evaluated by measurement of serum and 24-hour urinary parameters and kidney ultrasound.

RESULTS

Urinary parameters were tested in the univariate analysis as continuous and categorical variables. Only hypercalciuria and hypomagnesuria were significantly associated with nephrolithiasis in the univariate and multivariate analysis adjusted for age, sex, body mass index, estimated glomerular filtration rate, parathyroid hormone, 25-hydroxyvitamin D, serum calcium, and urine volume (odds ratio, OR 2.14 [1.10-4.56]; P = .04; OR 3.06 [1.26-7.43]; P = .013, respectively). Hypomagnesuria remained associated with nephrolithiasis in the multivariate analysis (OR 6.09 [1.57-23.5], P = .009) even when the analysis was limited to patients without concomitant hypercalciuria. The urinary calcium/magnesium (Ca/Mg) ratio was also associated with nephrolithiasis (univariate OR 1.62 [1.27-2.08]; P = .001 and multivariate analysis OR 1.74 [1.25-2.42], P = .001). Hypomagnesuria and urinary Ca/Mg ratio had a better, but rather low, positive predictive value compared with hypercalciuria.

CONCLUSIONS

Hypomagnesuria and urinary Ca/Mg ratio are each associated with silent nephrolithiasis and have potential clinical utility as risk factors, besides hypercalciuria, for kidney stones in asymptomatic PHPT patients. The other urinary indices that have been commonly thought to be associated with kidney stones in PHPT are not supported by our results.

Hypercalciuria: its value as a predictive risk factor for nephrolithiasis in asymptomatic primary hyperparathyroidism?

CONTEXT

The latest guidelines of the 4th International Workshop on Asymptomatic Primary Hyperparathyroidism (aPHPT) reintroduced hypercalciuria (i.e. urinary calcium > 400 mg/day) as criterion for surgery. However, the value of hypercalciuria as a predictor of nephrolithiasis and the correct cut-off values still need to be confirmed.

OBJECTIVE

To evaluate the prevalence of silent kidney stones in a large series of patients with aPHPT and the sensibility, specificity and predictive value of different cut-off values of hypercalciuria in identifying patients with nephrolithiasis.

DESIGN

One hundred seventy-six consecutive patients with aPHPT were evaluated at our Institution by serum and urinary parameters and kidney ultrasound.

RESULTS

Silent nephrolithiasis was found in 38 (21.6%) patients. In the univariate and multivariate model, hypercalciuria was a predictor of nephrolithiasis using the criterion of 400 mg/24 h [(OR 2.30, (1.11-4.82) P = 0.025], 4 mg/kg/bw [OR 2.65, (1.14-6.25) P = 0.023], gender criterion [OR 2.79, (1.15-6.79) P = 0.023] and the cut-off value derived from the ROC analysis [(> 231 mg/24 h) OR 5.02 (1.68-14.97) P = 0.004]. Despite these several predictive criteria, however, hypercalciuria had a low positive predictive value (PPV), ranging from 27.4 to 32.7%.

CONCLUSIONS

Hypercalciuria is a predictor of nephrolithiasis, but its PPV is low.

Clinical profile of juvenile primary hyperparathyroidism: a prospective study

INTRODUCTION

Juvenile primary hyperparathyroidism is uncommon and more symptomatic than the adult counterpart. The aim of this prospective monocentric study, conducted in a tertiary referral center, was to evaluate the clinical, biochemical, and densitometric data, and the outcome of a series of patients with juvenile primary hyperparathyroidism.

MATERIAL AND METHODS

The study group included 154 patients with sporadic and familial juvenile primary hyperparathyroidism, aged ≤40 years. Relative frequency of sporadic and familial forms, comparison of the clinical and biochemical characteristics, rate of cure after parathyroidectomy and the outcome of patients not undergoing surgery were evaluated.

RESULTS

Familial cases (n = 42) were younger, less frequently females, and had milder disease compared to sporadic cases (n = 112). No difference was observed in biochemical and densitometric parameters. Among patients undergoing parathyroidectomy (n = 116), familial cases had a higher rate of multigland disease and a higher persistence/relapse rate compared to sporadic cases (73 vs. 3.6% and 48.1 vs. 5.7%, respectively). Patients who did not undergo parathyroidectomy had stable clinical, biochemical, and densitometric parameters during follow-up (median 27 months). Using the cut-off age of 25 years, there was no difference in clinical, biochemical and densitometric parameters between younger and older patients, with the exception of parathyroid hormone and phosphate, which were significantly lower and higher, respectively, in patients <25 years.

CONCLUSIONS

In conclusion, this prospective study shows that juvenile primary hyperparathyroidism is frequently a sporadic disease, with no difference in the biochemical phenotype between sporadic and familial forms. Patients with familial juvenile primary hyperparathyroidism have a milder clinical phenotype and higher rate of persistence/recurrence after PTx than those with sporadic juvenile primary hyperparathyroidism.

Impact of vitamin D deficiency on the clinical and biochemical phenotype in women with sporadic primary hyperparathyroidism

The purpose of the study was to evaluate the relationship between serum 25(OH)D and the clinical phenotype in 215 consecutive Italian Caucasian women with sporadic primary hyperparathyroidism (PHPT) not taking vitamin D supplements. The study was performed at a single Italian tertiary center. PHPT-related manifestations, serum 25(OH)D, and other parameters of calcium metabolism and bone mineral density (BMD) by DXA were recorded. Serum 25(OH)D was negatively correlated with age (r = -0.18; P = 0.006), BMI (r = -0.20; P = 0.002), PTH (r = -0.21; P = 0.001), bone-specific alkaline phosphatase (BSAP) (r = -0.27; P < 0.001), and eGFR (r = -0.22; P = 0.01), and positively with serum creatinine and 1/3 distal radius BMD (R-BMD; r = 0.17; P = 0.015). In a multivariate regression analysis, serum 25(OH)D remained significantly correlated with age (r = -0.18; P = 0.005), BMI (r = -0.23; P = 0.049), serum PTH (r = -0.01; P = 0.023), BSAP (r = -0.01; P = 0.023) and eGFR (r = -0.09; P = 0.001), but not with R-BMD. Serum 25(OHD) was higher in patients with nephrolithiasis than in those without nephrolithiasis (18.5 ± 8.8 vs. 15.6 ± 8.0 ng/ml; P = 0.029), whereas no difference was found between fractured and unfractured patients (16.8 ± 9.3 vs. 16.0 ± 7.7; P = 0.663). There was a statistically significant inverse correlation between vitamin D status [defined by quartiles of measured values as well as commonly accepted cutoffs of serum 25(OH)D] and severity of the disease, as reflected by higher PTH and BSAP, but not by meeting the latest guidelines for parathyroidectomy. In conclusion, a low vitamin D status is associated with some features reflecting a more severe biochemical and clinical phenotype of PHPT in Italian women not taking vitamin D supplements.

A novel mutation in the calcium-sensing receptor in a French family with familial hypocalciuric hypercalcaemia

OBJECTIVE

The calcium-sensing receptor (CASR) has an important role in calcium homoeostasis by controlling PTH secretion and renal calcium handling. Inactivating mutations in the CASR gene (HGNC ID: 1514) cause familial hypocalciuric hypercalcaemia (FHH). We present a case of FHH patient to describe a novel mutation in the CASR.

SUBJECTS AND METHODS

A 34-year-old patient was referred because of recurrent hypercalcaemia after resection of two hyperplastic parathyroids. Extensive evaluation found elevated PTH and low calcium/creatinine clearance ratio. One of her three children had high serum calcium concentrations. Genetic studies were performed by PCR amplification of CASR coding exons and direct sequencing of PCR products. Transient transfection of the wild-type (WT) CASR and the mutant CASR into COS-7 was performed to assess functional impact of the mutation and the capacity of either protein to mediate increases in cellular levels of inositol phosphates (IPs).

RESULTS

CASR sequencing found a previously undescribed heterozygous base substitution, determining a change of threonine to isoleucine at codon 550 (p.T550I) in the sixth exon. In contrast to those transfected with WT CASR, which showed a five- to eightfold increase in total IPs at high levels of calcium, COS-7 cells transfected with the (p.T550I) mutant showed no increase confirming to the inactivating nature of the mutation. COS-7 cells co-transfected with the WT and the (p.T550I) mutant showed an intermediate response suggesting a possible dominant negative effect.

CONCLUSION

This case report presents a not-yet-described mutation in the cysteine-rich region of the CASR extracellular domain, a mutation with a possible dominant negative effect.

Different vitamin D substrate-product relationship after oral vitamin D supplementation in familial benign hypercalcemia, primary hyperparathyroidism, and healthy controls

CONTEXT

In healthy subjects and in patients with primary hyperparathyroidism (PH), the administration of a low dose of 25(OH)D (25 μg/day) increases the serum levels of both 25(OH)D and 1,25(OH)(2)D. It is unknown whether this relationship is present in patients affected by familial benign hypocalciuric hypercalcemia (FBH).

OBJECTIVE

To evaluate the different vitamin D substrate-product relationship after oral vitamin D supplementation in familial benign hypercalcemia, PH, and healthy controls.

DESIGN

We evaluated the main physiological regulators of 1α-hydroxylase and the substrate-product relationship of 25(OH)D and 1,25(OH)(2)D in 20 patients with PH, 25 with FBH, and 122 healthy sex- and age-matched controls before and after administration of 25(OH)D for 2 weeks.

RESULTS

25(OH)D increased significantly in all subjects, whereas 1,25(OH)(2)D serum levels increased significantly in PH patients and healthy controls but not in patients with FBH. Therefore, a significant positive substrate-product relationship of 25(OH)D-1,25(OH)(2)D was found in PH and healthy controls, but not in FBH. Monomeric calcitonin (hCT-M) was significantly lower at baseline and after 25(OH)D supplementation in the FBH group compared with the other two groups.

CONCLUSIONS

The lack of 1,25(OH)(2)D increase in FBH may be due to a direct inhibitory effect on 1α-hydroxylase of hypercalcemia per se, increased metabolic clearance of 1,25(OH)(2)D, or a decreased stimulus of 1α-hydroxylase related to persistently low levels of hCT.

Familial hypocalciuric hypercalcemia revealed by chondrocalcinosis

BACKGROUND

Calcium pyrophosphate dihydrate crystal deposition disease (CPPD-CDD) has been associated to hypercalcemia. Familial hypocalciuric hypercalcemia (FHH) is a rare but important consideration in the differential diagnosis of hypercalcemia. This autosomal dominantly inherited condition is characterized by elevated plasma calcium levels, relative or absolute hypocalciuria, and normal to moderately elevated plasma PTH level. The disease is caused by inactivating mutations in the calcium-sensing receptor gene.

CASE REPORT

We describe a 77-year-old Italian man with arthritis secondary to CPPD-CDD and hypercalcemia. Clinical and biochemical data (s-Ca: 2.94 mmol/L; PTH: 5.9 pmol/L; 24 h urinary calcium: 69.6 mg; calcium/creatinine clearance: 0.004) suggested the diagnosis of FHH. Mild hypocalciuric hypercalcemia was also found in five of seven relatives confirming the diagnosis, of these one showed chondrocalcinosis.

CONCLUSIONS

It is important to screen for FHH using fractional urinary excretion of calcium in subjects with CPPD-CDD associated to hypercalcemia, this approach may prevent unnecessary parathyroidectomy.

Morphometric vertebral fractures in postmenopausal women with primary hyperparathyroidism

CONTEXT

An increased risk of fracture in patients with primary hyperparathyroidism (PHPT) compared to the general population has been reported, but available data are controversial.

OBJECTIVE

The aim of the study was to evaluate the rate of vertebral fractures (VFs) by dual-energy x-ray absorptiometry in postmenopausal women with sporadic PHPT and compare the results with a control group.

DESIGN AND SETTING

A case-control study was performed at a referral center.

PARTICIPANTS

A total of 150 consecutive patients and 300 healthy women matched for age and menopausal age participated in the study.

RESULTS

VFs were detected in 37 of 150 (24.6%) patients and 12 of 300 (4.0%) controls (P < 0.0001). The majority of VFs were mild. Stepwise multiple logistic regression analysis showed that in PHPT patients lumbar spine bone mineral density was the only variable independently associated with the prevalence of VFs (P = 0.003). The rate of fracture was higher in symptomatic (34.1%) than asymptomatic (21.1%) patients, but this difference was not statistically significant (P = 0.15). Among asymptomatic patients, fracture rate was significantly higher in those who met the criteria for parathyroidectomy (28.1%) than in those who did not (11.1%) (P = 0.03). Compared to controls, the fracture rate was significantly higher in patients with symptomatic and asymptomatic PHPT who met the criteria for surgery (P < 0.0001), but not in those who did not meet the criteria (P = 0.06).

CONCLUSIONS

VF rate is increased in postmenopausal women with PHPT compared to controls, independently of whether they are classified as symptomatic or asymptomatic. The question of whether the finding of mild morphometric VFs in the latter represents an indication for parathyroid surgery remains to be established.

Surgical treatment of low- and intermediate-risk papillary thyroid cancer with minimally invasive video-assisted thyroidectomy

BACKGROUND

Minimally invasive video-assisted thyroidectomy (MIVAT) was introduced in the clinical practice to treat small benign thyroid nodules. This method has recently been demonstrated to produce the same completeness as a conventional thyroidectomy in patients with papillary thyroid cancer (PTC). The low number of treated cases and the limited follow-up of these patients represent the major limitations of these studies.

OBJECTIVE

The aim of the study was to compare the outcome of two groups of PTC patients, one treated with MIVAT and the other with conventional thyroidectomy, after a median follow-up of 5 yr.

STUDY GROUP

A total of 221 PTC patients were enrolled in this study according to the following criteria: 171 were treated with MIVAT (group A), and 50 were treated with conventional thyroidectomy (group B).

RESULTS

The outcome and the cumulative (131)I activity administered to achieve curative status were compared. After a mean follow-up of 3.6 +/- 1.5 yr (range, 1-8 yr; median, 5 yr), no differences were found between group A and group B. A similar rate of permanent hypoparathyroidism and/or nerve cord palsy was found in both groups.

CONCLUSION

We demonstrated that PTC patients operated on with MIVAT had a good outcome after 5 yr. This was similar to the outcome of patients treated with conventional thyroidectomy and the same degree of exposure to (131)I. These results, together with the evidence of a similar degree of completeness and rate of complications between the two surgical techniques, show that MIVAT is a valid option to treat low- and intermediate-risk PTC patients.

Identification and functional characterization of loss-of-function mutations of the calcium-sensing receptor in four Italian kindreds with familial hypocalciuric hypercalcemia

OBJECTIVE

Identification and characterization of calcium-sensing receptor (CASR) mutations in four unrelated Italian kindreds with familial hypocalciuric hypercalcemia.

DESIGN

Clinical evaluation and genetic analysis of CASR gene. Functional characterization of mutated CASRs.

METHODS

Direct sequencing of CASR gene in genomic DNA. Studies of CASR-mediated increases in cytosolic calcium concentration [Ca(2)(+)](i) in CASR-transfected COS-7 cells in vitro.

RESULTS

Four unreported heterozygous CASR mutations were identified, including three missense (H595Y, P748H, and C765W) and one splice site (IVS2+1G>C) mutation. The H595Y, P748H, and C765W mutant receptors, although expressed at normal levels on the cell surface, showed a reduced response in [Ca(2)(+)](i) relative to the wildtype (WT) CASR to increasing extracellular calcium concentrations. Cotransfection experiments showed that the H595Y and P748H mutants did not affect the apparent affinity of the WT CASR for calcium, suggesting that they do not exert a dominant-negative effect. On the other hand, the co-transfected C765W mutant decreased the maximum response of the WT CASR to calcium, suggesting that it may reduce the effective concentration of the normal CASR on the cell surface or impair its maximal signaling capacity.

CONCLUSIONS

Four CASR mutations were identified. The reduced functional responses to extracellular calcium and normal expression of the mutant receptors suggest that conformational changes account for altered CASR activity. Moreover, a reduced complement of normal CASRs in these heterozygous patients, perhaps combined with a mutant receptor-induced decrease in maximal activity of the WT receptor, may contribute to defective calcium-sensing in vivo.

Surgery or surveillance for mild asymptomatic primary hyperparathyroidism: a prospective, randomized clinical trial

CONTEXT

It is unclear whether patients with asymptomatic primary hyperparathyroidism (PHPT) do better with parathyroidectomy (PTx) as compared with conservative medical management.

OBJECTIVE

The aim of the study was to evaluate the beneficial effect of PTx vs. conservative management in patients with mild asymptomatic PHPT.

DESIGN

We conducted a prospective, randomized study.

SETTING

The study took place at a referral center.

PATIENTS

We studied 50 patients who did not meet any guidelines for parathyroid surgery as recommended by the National Institutes of Health Consensus Development Conference on Asymptomatic PHPT.

INTERVENTION

Patients were randomly assigned to PTx or no PTx and were evaluated at 6 months and at 1 yr.

MAIN OUTCOME MEASURES

We compared changes (percentage of basal) of lumbar spine bone mineral density (BMD) between the two groups at 1 yr.

RESULTS

The change in BMD at lumbar spine was greater after PTx (+4.16 +/- 1.13 for PTx vs. -1.12 +/- 0.71 for no PTx; P = 0.0002). The change in BMD at the total hip was also significantly greater in the PTx group (+2.61 +/- 0.71 for PTx vs. -1.88 +/- 0.60 for no PTx; P = 0.0001). There was no difference in BMD after 1 yr between both groups at the one-third radius site. In comparison with those who did not undergo surgery, the PTx subjects, after 1 yr, showed significant differences in four quality of life measures as determined by the 36-item short form health survey scale: bodily pain (P = 0.001), general health (P = 0.008), vitality (P = 0.003), and mental health (P = 0.017).

CONCLUSIONS

In patients with mild asymptomatic PHPT, successful PTx is followed by an improvement in BMD and quality of life. Most patients followed without surgery did not show evidence of progression.

Identification and dissection of Ca(2+)-binding sites in the extracellular domain of Ca(2+)-sensing receptor

Ca(2+)-sensing receptors (CaSRs) represent a class of receptors that respond to changes in the extracellular Ca(2+) concentration ([Ca(2+)](o)) and activate multiple signaling pathways. A major barrier to advancing our understanding of the role of Ca(2+) in regulating CaSRs is the lack of adequate information about their Ca(2+)-binding locations, which is largely hindered by the lack of a solved three-dimensional structure and rapid off rates due to low Ca(2+)-binding affinities. In this paper, we have reported the identification of three potential Ca(2+)-binding sites in a modeled CaSR structure using computational algorithms based on the geometric description and surface electrostatic potentials. Mutation of the predicted ligand residues in the full-length CaSR caused abnormal responses to [Ca(2+)](o), similar to those observed with naturally occurring activating or inactivating mutations of the CaR, supporting the essential role of these predicted Ca(2+)-binding sites in the sensing capability of the CaSR. In addition, to probe the intrinsic Ca(2+)-binding properties of the predicted sequences, we engineered two predicted continuous Ca(2+)-binding sequences individually into a scaffold protein provided by a non-Ca(2+)-binding protein, CD2. We report herein the estimation of the metal-binding affinities of these predicted sites in the CaSR by monitoring aromatic-sensitized Tb(3+) fluorescence energy transfer. Removing the predicted Ca(2+)-binding ligands resulted in the loss of or significantly weakened cation binding. The potential Ca(2+)-binding residues were shown to be involved in Ca(2+)/Ln(3+) binding by high resolution NMR and site-directed mutagenesis, further validating our prediction of Ca(2+)-binding sites within the extracellular domain of the CaSR.

Inactivating mutations of G protein-coupled receptors and diseases: Structure-function insights and therapeutic implications

Since the discovery of the first rhodopsin mutation that causes retinitis pigmentosa in 1990, significant progresses have been made in elucidating the pathophysiology of diseases caused by inactivating mutations of G protein-coupled receptors (GPCRs). This review aims to compile the compelling evidence accumulated during the past 15 years demonstrating the etiologies of more than a dozen diseases caused by inactivating GPCR mutations. A generalized classification scheme, based on the life cycle of GPCRs, is proposed. Insights gained through detailed studies of these naturally occurring mutations into the structure-function relationship of these receptors are reviewed. Therapeutic approaches directed against the different classes of mutants are being developed. Since intracellular retention emerges as the most common defect, recent progresses aimed at correcting this defect through membrane permeable pharmacological chaperones are highlighted.

Delineating a Ca2+ Binding Pocket within the Venus Flytrap Module of the Human Calcium-sensing Receptor

The Ca(2+)-sensing receptor (CaSR) belongs to the class III G-protein-coupled receptors (GPCRs), which include receptors for pheromones, amino acids, sweeteners, and the neurotransmitters glutamate and gamma-aminobutyric acid (GABA). These receptors are characterized by a long extracellular amino-terminal domain called a Venus flytrap module (VFTM) containing the ligand binding pocket. To elucidate the molecular determinants implicated in Ca(2+) recognition by the CaSR VFTM, we developed a homology model of the human CaSR VFTM from the x-ray structure of the metabotropic glutamate receptor type 1 (mGluR1), and a phylogenetic analysis of 14 class III GPCR VFTMs. We identified critical amino acids delineating a Ca(2+) binding pocket predicted to be adjacent to, but distinct from, a cavity reminiscent of the binding site described for amino acids in mGluRs, GABA-B receptor, and GPRC6a. Most interestingly, these Ca(2+)-contacting residues are well conserved within class III GPCR VFTMs. Our model was validated by mutational and functional analysis, including the characterization of activating and inactivating mutations affecting a single amino acid, Glu-297, located within the proposed Ca(2+) binding pocket of the CaSR and associated with autosomal dominant hypocalcemia and familial hypocalciuric hypercalcemia, respectively, genetic diseases characterized by perturbations in Ca(2+) homeostasis. Altogether, these data define a Ca(2+) binding pocket within the CaSR VFTM that may be conserved in several other class III GPCRs, thereby providing a molecular basis for extracellular Ca(2+) sensing by these receptors.

Familial hypocalciuric hypercalcemia in a woman with metastatic breast cancer: a case report of mistaken identity

We describe a 45-yr-old woman with metastatic breast cancer and hypercalcemia previously diagnosed as hypercalcemia of malignancy and treated with bisphosphonates without changes of serum calcium (s-Ca). At the time of our evaluation, biochemical data [s-Ca, 10.8 mg/dl (2.70 mmol/liter); PTH, 24.4 pg/ml (2.6 pmol/liter); 24-h urinary calcium, 160 mg (4.0 mmol); calcium/creatinine clearance, 0.007] suggested the diagnosis of familial hypocalciuric hypercalcemia. Three of five relatives had mild hypercalcemia [s-Ca, 10.7-11.2 mg/dl (2.67-2.80 mmol/liter)] and detectable serum PTH [24.5-29.0 pg/ml (2.6-3.1 pmol/liter)]. A novel heterozygous I212T missense mutation in exon 4 of the calcium-sensing receptor (CaR) gene was found in the proband and affected relatives but not in unaffected relatives. Expression of the mutant I212T CaR in COS-7 cells resulted in no response of inositol phosphates to any calcium concentration. The calcium dose-response curve of the coexpressed receptors [wild-type/I212T] suggested that the mutant receptor interferes with the function of the wild-type receptor. In conclusion, we describe a case of familial hypocalciuric hypercalcemia due to a novel CaR mutation, in a woman with breast cancer in whom hypercalcemia was initially attributed to hypercalcemia of malignancy.

Impaired GH secretion to provocative stimuli in two families with hypocalciuric hypercalcaemia

OBJECTIVE

To determine whether hypercalcemia per se might be responsible for an impairment in GH secretion.

DESIGN

Prospective study.

PATIENTS

Six subjects of two unrelated families with familial hypocalciuric hypercalcaemia (FHH), an autosomal dominant disorder due to inactivating mutations in the calcium receptor gene, leading to an increase in serum calcium levels and inappropriately normal serum PTH concentrations. Forty normal subjects, matched for sex and age served as controls.

MEASUREMENTS

Serum GH concentrations were measured after GHRH-Arginine (GHRH-Arg) stimulation test; serum IGF-I, ACTH, cortisol, FT4, FT3, TSH, PRL, LH, FSH levels were measured under basal conditions.

RESULTS

All subjects (two male, four female, age range 24-74 years) had increased serum ionized calcium levels (range 1.36-1.56 mmol/l) and five of six patients had normal PTH levels (range for all patients was 14-68 ng/l). Basal serum GH concentrations ranged from 0.1 to 7.0 micro g/l. Mean serum GH secretory peak after GHRH-Arg stimulation test was reduced in five subjects (mean 9.3 +/- 3.6 microg/l, P < 0.006 vs. Controls, mean 67.0 +/- 44.0 microg/l, cut-off, 16.0 microg/l) and normal in one subject (38.7 microg/l). However, serum IGF-I levels were reduced only in two patients (29 and 57 microg/l) and normal in four subjects (range 127-208 microg/l). The basal secretion of the other anterior pituitary hormones was within their normal ranges.

CONCLUSIONS

The results of the present study support the concept that elevated serum calcium levels impair GH secretion. However, the clinical relevance of GH deficiency in FHH remains to be elucidated.