Neonatal hyperparathyroidism with a heterozygous calcium-sensing receptor (CASR) R185Q mutation: clinical benefit from cinacalcet

Rare genetic disorders that impair parathyroid hormone synthesis, secretion, or bioactivity provide insights into the diagnostic utility of different parathyroid hormone assays

PURPOSE OF REVIEW

Parathyroid hormone (PTH) is the major peptide hormone regulator of blood calcium homeostasis. Abnormal PTH levels can be observed in patients with various congenital and acquired disorders, including chronic kidney disease (CKD). This review will focus on rare human diseases caused by PTH mutations that have provided insights into the regulation of PTH synthesis and secretion as well as the diagnostic utility of different PTH assays.

RECENT FINDINGS

Over the past years, numerous diseases affecting calcium and phosphate homeostasis have been defined at the molecular level that are responsible for reduced or increased serum PTH levels. The underlying genetic mutations impair parathyroid gland development, involve the PTH gene itself, or alter function of the calcium-sensing receptor (CaSR) or its downstream signaling partners that contribute to regulation of PTH synthesis or secretion. Mutations in the pre sequence of the mature PTH peptide can, for instance, impair hormone synthesis or intracellular processing, while amino acid substitutions affecting the secreted PTH(1-84) impair PTH receptor (PTH1R) activation, or cause defective cleavage of the pro-sequence and thus secretion of a pro- PTH with much reduced biological activity. Mutations affecting the secreted hormone can alter detection by different PTH assays, thus requiring detailed knowledge of the utilized diagnostic test.

SUMMARY

Rare diseases affecting PTH synthesis and secretion have offered helpful insights into parathyroid biology and the diagnostic utility of commonly used PTH assays, which may have implications for the interpretation of PTH measurements in more common disorders such as CKD.

Genetics of hereditary forms of primary hyperparathyroidism

Primary hyperparathyroidism (PHPT), a relatively common disorder characterized by hypercalcemia with raised or inappropriately normal serum parathyroid hormone (PTH) concentrations, may occur as part of a hereditary syndromic disorder or as a non-syndromic disease. The associated syndromic disorders include multiple endocrine neoplasia types 1-5 (MEN1-5) and hyperparathyroidism with jaw tumor (HPT-JT) syndromes, and the non-syndromic forms include familial hypocalciuric hypercalcemia types 1-3 (FHH1-3), familial isolated hyperparathyroidism (FIHP), and neonatal severe hyperparathyroidism (NS-HPT). Such hereditary forms may occur in > 10% of patients with PHPT, and their recognition is important for implementation of gene-specific screening protocols and investigations for other associated tumors. Syndromic PHPT tends to be multifocal and multiglandular with most patients requiring parathyroidectomy with the aim of limiting end-organ damage associated with hypercalcemia, particularly osteoporosis, nephrolithiasis, and renal failure. Some patients with non-syndromic PHPT may have mutations of the MEN1 gene or the calcium-sensing receptor (CASR), whose loss of function mutations usually cause FHH1, a disorder associated with mild hypercalcemia and may follow a benign clinical course. Measurement of the urinary calcium-to-creatinine ratio clearance (UCCR) may help to distinguish patients with FHH from those with PHPT, as the majority of FHH patients have low urinary calcium excretion (UCCR < 0.01). Once genetic testing confirms a hereditary cause of PHPT, further genetic testing can be offered to the patients' relatives and subsequent screening can be carried out in these affected family members, which prevents inappropriate testing in normal individuals.

Identification and characterization of a novel CASR mutation causing familial hypocalciuric hypercalcemia

Context

Although a monoallelic mutation in the calcium-sensing receptor (CASR) gene causes familial hypocalciuric hypercalcemia (FHH), the functional characterization of the identified CASR mutation linked to the clinical response to calcimimetics therapy is still limited.

Objective

A 45-year-old male presenting with moderate hypercalcemia, hypocalciuria, and inappropriately high parathyroid hormone (PTH) had a good response to cinacalcet (total serum calcium (Ca2+) from 12.5 to 10.1 mg/dl). We identified the genetic mutation and characterized the functional and pathophysiological mechanisms, and then linked the mutation to calcimimetics treatment in vitro.

Design

Sanger sequencing of the CASR, GNA11, and AP2S1 genes was performed in his family. The simulation model was used to predict the function of the identified mutant. In vitro studies, including immunoblotting, immunofluorescence, a cycloheximide chase study, Calbryte™ 520 Ca2+ detection, and half-maximal effective concentration (EC50), were examined.

Results

This proband was found to carry a de novo heterozygous missense I554N in the cysteine-rich domain of CASR, which was pathogenic based on the different software prediction models and ACGME criteria. The simulation model showed that CASR I554N mutation decreased its binding energy with Ca2+. Human CASR I554N mutation attenuated the stability of CASR protein, reduced the expression of p-ERK 1/2, and blunted the intracellular Ca2+ response to gradient extracellular Ca2+ (eCa2+) concentration. The EC50 study also demonstrated the correctable effect of calcimimetics on the function of the CASR I554N mutation.

Conclusion

This novel CASR I554N mutation causing FHH attenuates CASR stability, its binding affinity with Ca2+, and the response to eCa2+ corrected by therapeutic calcimimetics.

Subtotal Parathyroidectomy Successfully Controls Calcium Levels of Patients with Neonatal Severe Hyperparathyroidism Carrying a Novel CASR Mutation

INTRODUCTION

Inactivating mutations of the calcium-sensing receptor (CASR) gene result in neonatal severe hyperparathyroidism (NSHPT). Total parathyroidectomy is an effective way to control life-threatening hypercalcemia in NSHPT but leads to permanent hypoparathyroidism. An alternative surgical option is subtotal parathyroidectomy. However, few cases were reported in the literature. Here, we report two unrelated NSHPT patients, one with a novel homozygous mutation (c.1817T>C; p.Leu606Pro) in CASRand the other with heterozygous for the same mutation who also carried two rare intronic variants in CASR. The outcomes of subtotal parathyroidectomy in these patients are also described.

CASE PRESENTATION

Two infants presented with an alteration of consciousness, respiratory distress, and bradycardia. Severe hypercalcemia, hypophosphatemia, and markedly elevated parathyroid hormone levels were identified, suggesting NSHPT. Cinacalcet was unable to control calcium (Ca) levels of both patients. A novel heterozygous and homozygous missense mutation c.1817T>C; p.Leu606Pro was identified in patients 1 and 2, respectively. Based on the model prediction, proline substitution at Leu606 is likely to disrupt conversion between the active and inactive conformations at the extracellular to transmembrane domain interface of CASR. In addition, two extremely rare intronic variants in CASR (chr3:g.122180314A>G and chr3:g.122251601G>A, based on GRCh38) were identified in patient 1 and his mother. These variants might have contributed to the clinical manifestations of patient 1 who was heterozygous for the c.1817T>C; p.Leu606Pro variant. Subtotal parathyroidectomy was performed by removing three and a half parathyroid glands. So far, patient 1 has been in normocalcemia for 5 years. Patient 2 was in normocalcemia for 16 months after surgery and subsequently developed mild hypoparathyroidism which required only low-dose calcitriol treatment.

CONCLUSION

We report a novel heterozygous and homozygous missense variant (c.1817T>C; p.Leu606Pro) in CASR in two NSHPT patients. The mutation likely disrupts conformational changes of CASR and results in cinacalcet unresponsiveness. Intronic variants in CASR identified in the patient with heterozygous variant might have contributed to the clinical manifestations of the patient. Although total parathyroidectomy is widely accepted as a standard treatment for NSHPT, we demonstrate that subtotal parathyroidectomy is also an effective procedure to normalize Ca levels and allow these patients to be in normocalcemia or mild hypoparathyroidism, which is simply controlled by low-dose calcitriol treatment. Subtotal parathyroidectomy appeared to be an effective treatment for NSHPT regardless of the molecular etiologies.

Familial Hyperparathyroidism: A Diagnostic and Treatment Challenge in Saudi Arabia

Familial hyperparathyroidism is a rare, inherited endocrine disorder characterized by abnormally elevated serum calcium due to increased parathyroid hormone levels.

In this case report, we present a two-day-old male newborn who was admitted with severe respiratory distress, hyperparathyroidism, and hypercalcemia with a family history of hyperparathyroidism in his two siblings, both diagnosed in childhood and treated with parathyroidectomy. He was diagnosed with familial hyperparathyroidism without other endocrinopathies. His left parathyroid glands were surgically removed, and post-operatively, his parathyroid hormones and calcium levels normalized. Pathological examination of the removed parathyroid glands confirmed parathyroid hyperplasia.

This is a successfully managed case of familial hyperparathyroidism in the neonatal period. Therefore, as the patient grows up, a close follow-up is recommended for early detection and managing multiple endocrine neoplasia type 1 that may be present later in life.

Disorders of the Calcium Sensing Signaling Pathway: From Familial Hypocalciuric Hypercalcemia (FHH) to Life Threatening Conditions in Infancy

Familial hypocalciuric hypercalcemia (FHH) is a mostly benign condition of elevated calcium and PTH levels based on a hyposensitive calcium sensing receptor (CaSR) in FHH 1 or its downstream regulatory pathway in FHH2 and FHH3. In children, adolescents and young adults with FHH the main challenge is to distinguish the condition from primary hyperparathyroidism and thereby to avoid unnecessary treatments including parathyroidectomy. However, inheritance of FHH may result in neonatal hyperparathyroidism (NHPT) or neonatal severe hyperparathyroidism (NSHPT), conditions with high morbidity, and in the latter even high mortality. This review focuses on the genetic and pathophysiological framework that leads to the severe neonatal form, gives recommendations for counselling and summarizes treatment options.

Successful use of cinacalcet monotherapy in the management of siblings with homozygous calcium-sensing receptor mutation

OBJECTIVES

Neonatal severe hyperparathyroidism (NSHPT) due to pathogenic mutations in the calcium-sensing receptor (CASR) is a serious medical condition that can lead to symptomatic hypercalcaemia and has detrimental effects on a child's growth and development. What is new: This report adds to evidence that homozygous CASR mutations can be managed with cinacalcet monotherapy as an alternative to parathyroidectomy. And, early use of cinacalcet in NSHPT can result in improvements in symptoms, growth and developmental milestones.

CASE PRESENTATION

We present two siblings with NSHPT due to homozygous mutation in the CASR gene with moderate hypercalcaemia. Both were treated with cinacalcet monotherapy and showed significant improvement in growth parameters including head circumference, developmental milestones and hypercalcaemic symptoms, once their calcium and parathyroid hormone levels normalised.

CONCLUSIONS

This report highlights the role of cinacalcet in managing elevated serum calcium levels in a select group of infants with NSHPT due to homozygous CASR mutations, resulting in improvement in hypercalcaemic symptoms, growth and neurodevelopmental outcomes.

PRENATAL BONE ABNORMALITIES IN THREE CASES OF FAMILIAL HYPOCALCIURIC HYPERCALCEMIA

INTRODUCTION

Prenatal diagnosis of bone and mineralization anomalies is associated with a wide range of etiologies and prognoses. The improvement of antenatal ultrasound combined with the development of molecular diagnosis in genetics has transformed antenatal medicine into a challenging discipline. Of the various known causes of bone abnormalities and hypomineralization, calcium and phosphate metabolism disorders are exceptional. An accurate diagnosis is crucial for providing appropriate genetic counseling and medical follow-up after birth.

CASE

We report on three siblings with severe bone abnormalities diagnosed during the second trimester ultrasound of pregnancy. Postnatal follow-up showed transitory hyperparathyroidism, with hypercalcemia and hypocalciuria.

METHODS

Sanger sequencing performed after birth in the three newborns revealed a monoallelic pathogenic variant in the CASR gene, encoding the calcium sensing receptor, confirming the diagnosis of familial hypocalciuric hypercalcemia, paternally inherited. Postnatal evolution was favorable after treatment with a calcimimetic agent.

CONCLUSIONS

Previously, prenatal bone abnormalities caused by familial hypocalciuric hypercalcemia had only been described in one patient. This entity should be considered as differential diagnosis of bones abnormalities. Knowing about this unusual etiology is important to guide the diagnosis, the prenatal counselling and to improve medical management. This article is protected by copyright. All rights reserved.

Case Report: Unusual Presentations of Loss-of-Function Mutations of the Calcium-Sensing Receptor

Background

In recent years, heterozygous loss-of-function mutations of the Calcium Sensing Receptor gene (CaSR) were implicated in different hypercalcemic syndromes besides familial hypocalciuric hypercalcemia (FHH), including neonatal severe primary hyperparathyroidism (NSHPT) and primary hyperparathyroidism (PHPT).

Cases presentation

Here we describe two unusual presentations of heterozygous inactivating CaSR mutations. Case 1: a case of NSHPT due to a de novo, p.(ArgR185Gln) CaSR mutation and successfully treated with cinacalcet monotherapy for 8 years until definitive surgical resolution. Case 2: a 37 years-old woman with PHPT complicated with hypercalcemia and nephrocalcinosis with a novel heterozygous p.(Pro393Arg) CaSR mutation and cured with parathyroidectomy.

Conclusions

These cases reinforce the fact that the clinical spectrum of inactivating mutations of the CaSR has widened and, although carrying a mutation suggestive of FHH, some patients may have different clinical phenotypes and complications requiring individualized therapies.

Genetic causes of neonatal and infantile hypercalcaemia

The causes of hypercalcaemia in the neonate and infant are varied, and often distinct from those in older children and adults. Hypercalcaemia presents clinically with a range of symptoms including failure to thrive, poor feeding, constipation, polyuria, irritability, lethargy, seizures and hypotonia. When hypercalcaemia is suspected, an accurate diagnosis will require an evaluation of potential causes (e.g. family history) and assessment for physical features (such as dysmorphology, or subcutaneous fat deposits), as well as biochemical measurements, including total and ionised serum calcium, serum phosphate, creatinine and albumin, intact parathyroid hormone (PTH), vitamin D metabolites and urinary calcium, phosphate and creatinine. The causes of neonatal hypercalcaemia can be classified into high or low PTH disorders. Disorders associated with high serum PTH include neonatal severe hyperparathyroidism, familial hypocalciuric hypercalcaemia and Jansen's metaphyseal chondrodysplasia. Conditions associated with low serum PTH include idiopathic infantile hypercalcaemia, Williams-Beuren syndrome and inborn errors of metabolism, including hypophosphatasia. Maternal hypocalcaemia and dietary factors and several rare endocrine disorders can also influence neonatal serum calcium levels. This review will focus on the common causes of hypercalcaemia in neonates and young infants, considering maternal, dietary, and genetic causes of calcium dysregulation. The clinical presentation and treatment of patients with these disorders will be discussed.

Off-label use of cinacalcet in pediatric primary hyperparathyroidism: A French multicenter experience

BACKGROUND

Cinacalcet is a calcimimetic approved in adults with primary hyperparathyroidism (PHPT). Few cases reports described its use in pediatric HPT, with challenges related to the risk of hypocalcemia, increased QT interval and drug interactions. In this study, we report the French experience in this setting.

METHODS

We retrospectively analyzed data from 18 pediatric patients from 7 tertiary centers who received cinacalcet for PHPT. The results are presented as median (interquartile range).

RESULTS

At a median age of 10.8 (2.0-14.4) years, 18 patients received cinacalcet for primary HPT (N = 13 inactive CASR mutation, N = 1 CDC73 mutation, N = 1 multiple endocrine neoplasia type 1, N=3 unknown etiology). Cinacalcet was introduced at an estimated glomerular filtration rate (eGFR) of 120 (111-130) mL/min/1.73 m², plasma calcium of 3.04 (2.96-3.14) mmol/L, plasma phosphate of 1.1 (1.0-1.3) mmol/L, age-standardized (z score) phosphate of -3.0 (-3.5;-1.9), total ALP of 212 (164-245) UI/L, 25-OHD of 37 (20-46) ng/L, age-standardized (z score) ALP of -2.4 (-3.7;-1.4), PTH of 75 (59-123) ng/L corresponding to 1.2 (1.0-2.3)-time the upper limit for normal (ULN). The starting daily dose of cinacalcet was 0.7 (0.6-1.0) mg/kg, with a maximum dose of 1.0 (0.9-1.4) mg/kg per day. With a follow-up of 2.2 (1.3-4.3) years on cinacalcet therapy, PTH and calcium significantly decreased to 37 (34-54) ng/L, corresponding to 0.8 (0.5-0.8) ULN (p = 0.01), and 2.66 (2.55-2.90) mmol/L (p = 0.002), respectively. In contrast, eGFR, 25-OHD, ALP and phosphate and urinary calcium levels remained stable. Nephrocalcinosis was not reported but one patient displayed nephrolithiasis. Cinacalcet was progressively withdrawn in three patients; no side effects were reported.

CONCLUSIONS

Cinacalcet in pediatric HPT can control hypercalcemia and PTH without significant side effects.

Challenges in diagnosis and management of neonatal hyperparathyroidism in a resource-limited country: a case series from a Sudanese family

Neonatal hyperparathyroidism is a rare disease caused by a homozygous inactivating mutation in the calcium sensing receptor gene. It presents early in life with life threatening manifestations of hypercalcemia, if left untreated the condition may be lethal. This is the first case series reported from Sudan. Three Sudanese siblings presented with severe symptoms of hypercalcemia in the form of polyuria, failure to thrive and multiple bone fractures. Serum calcium and parathyroid hormone levels were very high with low phosphate and normal alkaline phosphatase levels. Ultrasonography and sestamibi scan were normal and did not assist in diagnosing their condition. Medical management was a great challenge due to unavailability of medications such as parentral bisphosphonates and calcimimetics. Parathyroidectomy was inevitable. Tissue biopsies revealed parathyroid hyperplasia and no adenoma. Gene sequencing revealed a homozygous missense mutation: c 2038 C T p (Arg680Cys) in two siblings, both parents were heterozygous for the same missense mutation. Our report reflects the challenges in diagnosis and management of neonatal hyperparathyroidism in resource limited countries. We also highlight the importance of genetic testing in the diagnosis and management of such cases in countries with high rates of consanguineous marriage.

Cinacalcet as a First-Line Treatment in Neonatal Severe Hyperparathyroidism Secondary to Calcium Sensing Receptor (CaSR) Mutation

INTRODUCTION

Neonatal severe hyperparathyroidism (NSHPT) is a rare cause of neonatal hypercalcemia caused by a loss of function mutation in the calcium-sensing receptor (CaSR). Hypercalcemia in NSHPT can be life-threatening. Maintenance of serum calcium within a safe range is the primary goal of treatment through hydration, forced diuresis, and bisphosphonate treatment, nevertheless most cases require parathyroidectomy. We report a case with NSHPT diagnosed on the first day of life (DoL) and successfully treated with cinacalcet as the first-line treatment from the 2nd DoL up to the age of 18 months.

CASE REPORT

A full-term baby evaluated for weight loss at postnatal 14th hour and found to have hypercalcemia (14.4 mg/dL, reference range [RR]: 8.0-11.3). Despite hydration and diuresis, hypercalcemia persisted. Further evaluation revealed a parathyroid hormone (PTH) level of 1,493 pg/mL (RR: 15-65) and urine Ca/Cr of 0.09 mg/mg (RR: 0.03-0.81). Cinacalcet treatment was initiated on the 2nd DoL with the diagnosis of NSHPT due to hypocalciuric hypercalcemia and elevated PTH level. Ca levels decreased to normal levels on the 7th DoL. She was discharged from hospital at postnatal day 15 on cinacalcet treatment and still continued at 18 months of age. Sequencing of CaSR revealed a novel homozygous c.1836G>A (p.G613E) mutation in the patient, for which the parents and sister were heterozygous.

CONCLUSION

This case represents the youngest age at cinacalcet initiation and the longest duration without parathyroidectomy in a homozygous NSHPT and demonstrates that cinacalcet is an effective first-line treatment in patients who are responsive to this treatment modality and allows avoiding/delay in surgical intervention in NSHPT.

Prenatal features and neonatal management of severe hyperparathyroidism caused by the heterozygous inactivating calcium-sensing receptor variant, Arg185Gln: A case report and review of the literature

Background

Loss-of-function variants in the calcium-sensing receptor (CASR) gene are known to be involved in a clinical spectrum ranging from asymptomatic familial hypocalciuric hypercalcemia (FHH) to neonatal severe hyperparathyroidism (NSHPT). Homozygous or compound heterozygous variants are usually responsible for severe neonatal forms, whereas heterozygous variants cause benign forms. One recurrent pathogenic variant, p.Arg185Gln, has been reported in both forms, in a heterozygous state. This variant can be a de novo occurrence or can be inherited from a father with FHH.

NSHPT leads to global hypotonia, failure to thrive, typical X-ray anomalies (diffuse demineralization, fractures, metaphyseal irregularities), and acute respiratory distress which can be fatal. Phosphocalcic markers show severe hypercalcemia, abnormal urinary calcium resorption, and hyperparathyroidism as major signs.

Classical treatment involves calcium restriction, hyperhydration, and bisphosphonates. Unfortunately, the disease often leads to parathyroidectomy. Recently, calcimimetics have been used with variable efficacy. Efficacy in NSHPT seems to be particularly dependent on CASR genotype.

Case presentation

We describe the antenatal presentation of a male with short ribs, initially suspected having skeletal ciliopathy. At birth, he presented with NSHPT linked to the pathogenic heterozygous CASR variant, Arg185Gln, inherited from his father who had FHH. Postnatal therapy with cinacalcet was successful.

Discussion

An exhaustive literature review permits a comparison with all reported cases of Arg185Gln and to hypothesize that cinacalcet efficacy depends on CASR genotype. This confirms the importance of pedigree and parental history in antenatal short rib presentation and questions the feasibility of phosphocalcic exploration during pregnancy or prenatal CASR gene sequencing in the presence of specific clinical signs. It could in fact enable early calcimimetic treatment which might be effective in the CASR variant Arg185Gln.

The role of calcium-sensing receptor signaling in regulating transepithelial calcium transport

The calcium-sensing receptor (CaSR) plays a critical role in sensing extracellular calcium (Ca²⁺) and signaling to maintain Ca²⁺ homeostasis. In the parathyroid, the CaSR regulates secretion of parathyroid hormone, which functions to increase extracellular Ca²⁺ levels. The CaSR is also located in other organs imperative to Ca²⁺ homeostasis including the kidney and intestine, where it modulates Ca²⁺ reabsorption and absorption, respectively. In this review, we describe CaSR expression and its function in transepithelial Ca²⁺ transport in the kidney and intestine. Activation of the CaSR leads to G protein dependent and independent signaling cascades. The known CaSR signal transduction pathways involved in modulating paracellular and transcellular epithelial Ca²⁺ transport are discussed. Mutations in the CaSR cause a range of diseases that manifest in altered serum Ca²⁺ levels. Gain-of-function mutations in the CaSR result in autosomal dominant hypocalcemia type 1, while loss-of-function mutations cause familial hypocalciuric hypercalcemia. Additionally, the putative serine protease, FAM111A, is discussed as a potential regulator of the CaSR because mutations in FAM111A cause Kenny Caffey syndrome type 2, gracile bone dysplasia, and osteocraniostenosis, diseases that are characterized by hypocalcemia, hypoparathyroidism, and bony abnormalities, i.e. share phenotypic features of autosomal dominant hypocalcemia. Recent work has helped to elucidate the effect of CaSR signaling cascades on downstream proteins involved in Ca²⁺ transport across renal and intestinal epithelia; however, much remains to be discovered.

A novel case of neonatal severe hyperparathyroidism successfully treated with a type II calcimimetic drug

We report a boy with hypercalcemia due to neonatal severe hyperparathyroidism (NSHPT) caused by a compound heterozygous mutation in the calcium sensing receptor (CaSR) managed successfully on a type II calcimimetic drug. The hypercalcemia was temporarily treated by hyperhydration, bisphosphonate and calcium depleted milk. At 29 days of age cinacalcet was introduced. The starting dose was 0.5 mg/kg/day and was subsequently titrated to the point of efficacy (5.2 mg/kg/day) when a persuasive reduction in parathyroid hormone and calcium concentrations was observed. We propose a trial of type II calcimimetics in newborns with NSHPT irrespective of the genetic mutation and advocate that residual functionality of the CaSR predict the drug efficacy.

Rare diseases caused by abnormal calcium sensing and signalling

The calcium-sensing receptor (CaSR) provides the major mechanism for the detection of extracellular calcium concentration in several cell types, via the induction of G-protein-coupled signalling. Accordingly, CaSR plays a pivotal role in calcium homeostasis, and the CaSR gene defects are related to diseases characterized by serum calcium level changes. Activating mutations of the CaSR gene cause enhanced sensitivity to extracellular calcium concentration resulting in autosomal dominant hypocalcemia or Bartter-syndrome type V. Inactivating CaSR gene mutations lead to resistance to extracellular calcium. In these cases, familial hypocalciuric hypercalcaemia (FHH1) or neonatal severe hyperparathyroidism (NSHPT) can develop. FHH2 and FHH3 are associated with mutations of genes of partner proteins of calcium signal transduction. The common polymorphisms of the CaSR gene have been reported not to affect the calcium homeostasis itself; however, they may be associated with the increased risk of malignancies.

Case Report: Severe Neonatal Course in Paternally Derived Familial Hypocalciuric Hypercalcemia

Familial hypocalciuric hypercalcemia (FHH, [OMIM #145980]) is recognized as a benign endocrine condition affecting PTH and calcium levels due to heterozygous inactivating mutations in the calcium sensing receptor (CaSR). The condition is often un- or misdiagnosed but may have a prevalence as high as 74 in 100.000. Here, the neonatal courses of two brothers with paternally inherited FHH (CaSR c.554G>A; p.(Arg185Gln)) are described. The older brother was born preterm at 25 weeks gestation with hypercalcemia and hyperparathyroidism. The younger brother, born full-term, had severe hyperparathyroidism, muscular hypotonia, thrombocytopenia, failure to thrive and multiple metaphyseal fractures. Treatment with cinacalcet was initiated, which resulted in subsequent reduction of PTH levels and prompt clinical improvement. While it is known that homozygous mutations in CaSR may lead to life-threatening forms of neonatal severe hyperparathyroidism (NSHPT), few reports have described a severe clinical course in neonates with FHH due to heterozygous mutations. However, based on the pathophysiological framework, in de novo or paternally transmitted FHH the differing calcium needs of mother and fetus can be expected to induce fetal hyperparathyroidism and may result in severe perinatal complications as described in this report. In summary, FHH is a mostly benign condition, but transient neonatal hyperparathyroidism may occur in affected neonates if the mutation is paternally inherited. If severe, the condition can be treated successfully with cinacalcet. Patients with FHH should be informed about the risk of neonatal disease manifestation in order to monitor pregnancies and neonates.

Hereditary Parathyroid Disease: Sometimes Pathologists Do Not Know What They Are Missing

Parathyroid gland excision specimens are common and sometimes underestimated cases that many surgical pathologists encounter regularly. In the vast majority of cases, these will be spot diagnoses of sporadic primary parathyroid adenomas or, perhaps, hyperplasias commonly in the setting of renal failure. However, a small but significant number of parathyroid gland excisions may be due to heritable disease. In most cases, hereditary disease is suspected by the referring clinicians. Nevertheless, a subset of these are undetected which is significant, particularly in the setting of the multiple endocrine neoplasia (MEN), and the hyperparathyroidism jaw tumour (HPT-JT) syndromes. There have been recent advances in recognition of the morphological and immunohistochemical characteristics of these tumours and hyperplasias. While hereditary kindreds are over-represented at specialist referral centres, with awareness of the characteristic clinical and morphological features, the general surgical pathologist is frequently able to suggest the possibility of hereditary parathyroid disease. We therefore provide a succinct guide for pathologists to increase the recognition of hereditary parathyroid disease.

International Union of Basic and Clinical Pharmacology. CVIII. Calcium-Sensing Receptor Nomenclature, Pharmacology, and Function

The calcium-sensing receptor (CaSR) is a class C G protein-coupled receptor that responds to multiple endogenous agonists and allosteric modulators, including divalent and trivalent cations, L-amino acids, γ-glutamyl peptides, polyamines, polycationic peptides, and protons. The CaSR plays a critical role in extracellular calcium (Ca2+ o) homeostasis, as demonstrated by the many naturally occurring mutations in the CaSR or its signaling partners that cause Ca2+ o homeostasis disorders. However, CaSR tissue expression in mammals is broad and includes tissues unrelated to Ca2+ o homeostasis, in which it, for example, regulates the secretion of digestive hormones, airway constriction, cardiovascular effects, cellular differentiation, and proliferation. Thus, although the CaSR is targeted clinically by the positive allosteric modulators (PAMs) cinacalcet, evocalcet, and etelcalcetide in hyperparathyroidism, it is also a putative therapeutic target in diabetes, asthma, cardiovascular disease, and cancer. The CaSR is somewhat unique in possessing multiple ligand binding sites, including at least five putative sites for the "orthosteric" agonist Ca2+ o, an allosteric site for endogenous L-amino acids, two further allosteric sites for small molecules and the peptide PAM, etelcalcetide, and additional sites for other cations and anions. The CaSR is promiscuous in its G protein-coupling preferences, and signals via Gq/11, Gi/o, potentially G12/13, and even Gs in some cell types. Not surprisingly, the CaSR is subject to biased agonism, in which distinct ligands preferentially stimulate a subset of the CaSR's possible signaling responses, to the exclusion of others. The CaSR thus serves as a model receptor to study natural bias and allostery. SIGNIFICANCE STATEMENT: The calcium-sensing receptor (CaSR) is a complex G protein-coupled receptor that possesses multiple orthosteric and allosteric binding sites, is subject to biased signaling via several different G proteins, and has numerous (patho)physiological roles. Understanding the complexities of CaSR structure, function, and biology will aid future drug discovery efforts seeking to target this receptor for a diversity of diseases. This review summarizes what is known to date regarding key structural, pharmacological, and physiological features of the CaSR.

Neonatal Severe Hyperparathyroidism: Novel Insights From Calcium, PTH, and the CASR Gene

CONTEXT

Neonatal severe hyperparathyroidism (NSHPT) is rare and potentially lethal. It is usually from homozygous or heterozygous germline-inactivating CASR variant(s). NSHPT shows a puzzling range of serum calcium and parathyroid hormone (PTH) levels. Optimal therapy is unclear.

EVIDENCE ACQUISITION

We categorized genotype/phenotype pairings related to CASRs. For the 2 pairings in NSHPT, each of 57 cases of neonatal severe hyperparathyroidism required calcium, PTH, upper normal PTH, and dosage of a germline pathogenic CASR variant.

EVIDENCE SYNTHESIS

Homozygous and heterozygous NSHPT are 2 among a spectrum of 9 genotype/phenotype pairings relating to CASRs and NSHPT. For the 2 NSHPT pairings, expressions differ in CASR allelic dosage, CASR variant severity, and sufficiency of maternofetal calcium fluxes. Homozygous dosage of CASR variants was generally more aggressive than heterozygous. Among heterozygotes, high-grade CASR variants in vitro were more pathogenic in vivo than low-grade variants. Fetal calcium insufficiency as from maternal hypoparathyroidism caused fetal secondary hyperparathyroidism, which persisted and was reversible in neonates. Among NSHPT pairings, calcium and PTH were higher in CASR homozygotes than in heterozygotes. Extreme hypercalcemia (above 4.5 mM; normal 2.2-2.6 mM) is a robust biomarker, occurring only in homozygotes (83% of that pairing). It could occur during the first week.

CONCLUSIONS

In NSHPT pairings, the homozygotes for pathogenic CASR variants show higher calcium and PTH levels than heterozygotes. Calcium levels above 4.5 mM among NSHPT are frequent and unique only to most homozygotes. This cutoff supports early and robust diagnosis of CASR dosage. Thereby, it promotes definitive total parathyroidectomy in most homozygotes.

Cinacalcet therapy in an infant with an R185Q calcium-sensing receptor mutation causing hyperparathyroidism: a case report and review of the literature

Background Neonatal severe hyperparathyroidism (NSHPT) is commonly treated with either parathyroidectomy or pharmacologic agents with varying efficacy and numerous side effects. Reports of using cinacalcet for NSHPT have increased, however, the effective dose for pediatric patients from the onset of symptoms through infancy has not been established. Case presentation We describe the clinical course of a newborn with a de novo R185Q mutation in the calcium-sensing receptor (CASR) gene, causing NSHPT. The infant received cinacalcet from the first days of life until 1 year of age. Conclusions Cinacalcet therapy effectively controlled the patient's serum calcium, phosphorus, and parathyroid hormone (PTH) levels without side effects.

Novel homozygous inactivating mutation of the calcium-sensing receptor gene in neonatal severe hyperparathyroidism responding to cinacalcet therapy: A case report and literature review

RATIONALE

Calcium-sensing receptor (CaSR) mutations can cause life-threatening neonatal severe hyperparathyroidism (NSHPT). The medical management of NSHPT is often challenging and complex. Here, we present a case of NSHPT caused by a novel homozygous CaSR mutation.

PATIENT CONCERNS

A Chinese female infant presented with poor feeding, constipation, severe hypotonia, and periodic bradycardia. Biochemistry tests revealed markedly elevated serum levels of Ca and parathyroid hormone (PTH).

DIAGNOSES

Genetic sequencing revealed a previously undescribed CaSR mutation in exon 3 (c.242T>A; p.I81K). A diagnosis of NSHPT secondary to homozygously inherited familial hypocalciuric hypercalcemia syndrome was established.

INTERVENTIONS

Cinacalcet was administered after the common treatments (low-calcium intake, hydration, and furosemide), calcitonin, and pamidronate therapy all failed.

OUTCOMES

Serum Ca decreased and stabilized with cinacalcet therapy. During a 10-month follow-up, total calcium was maintained within the high-normal range and PTH was normalized.

LESSONS

A trial of cinacalcet therapy might be undertaken in cases of NSHPT while definitive results of the genetic analysis are awaited.

Genetic polymorphism (rs6776158) in CaSR gene is associated with risk of nephrolithiasis in Chinese population

The objective of this study is to find about the association between calcium-sensing receptor (CaSR) genetic variants and susceptibility to nephrolithiasis in the Chinese Han population.This hospital-based case-control study included 319 nephrolithiasis cases and 378 healthy controls subjects. Two SNPs in CaSR were genotyped using the TaqMan assay.We found that subjects carrying the G allele of rs6776158 (AG and GG) had significantly higher risk of nephrolithiasis compared to the AA genotype (P = .015 and .009, respectively).Our results indicate that rs6776158 polymorphism that might elevate the risk of nephrolithiasis in the Chinese population.

Calcimimetic and calcilytic therapies for inherited disorders of the calcium-sensing receptor signalling pathway

The calcium-sensing receptor (CaS receptor) plays a pivotal role in extracellular calcium homeostasis, and germline loss-of-function and gain-of-function mutations cause familial hypocalciuric hypercalcaemia (FHH) and autosomal dominant hypocalcaemia (ADH), respectively. CaS receptor signal transduction in the parathyroid glands is probably regulated by G-protein subunit α11 (Gα11 ) and adaptor-related protein complex-2 σ-subunit (AP2σ), and recent studies have identified germline mutations of these proteins as a cause of FHH and/or ADH. Calcimimetics and calcilytics are positive and negative allosteric modulators of the CaS receptor that have potential efficacy for symptomatic forms of FHH and ADH. Cellular studies have demonstrated that these compounds correct signalling and/or trafficking defects caused by mutant CaS receptor, Gα11 or AP2σ proteins. Moreover, mouse model studies indicate that calcilytics can rectify the hypocalcaemia and hypercalciuria associated with ADH, and patient-based studies reveal calcimimetics to ameliorate symptomatic hypercalcaemia caused by FHH. Thus, calcimimetics and calcilytics represent targeted therapies for inherited disorders of the CaS receptor signalling pathway.

LINKED ARTICLES

This article is part of a themed section on Molecular Pharmacology of GPCRs. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.21/issuetoc.

Case report: acute clinical presentation and neonatal management of primary hyperparathyroidism due to a novel CaSR mutation

Background

Neonatal severe primary hyperparathyroidism (NSHPT) is a rare autosomal recessive disorder of calcium homeostasis, characterized by striking hyperparathyroidism, marked hypercalcemia and hyperparathyroid bone disease. We report the case of a newborn with a novel homozygous mutation of the CaSR, treated by successful subtotal parathyroidectomy, who had an acute presentation of the disease, i.e. out-of hospital cardiorespiratory arrest. .

Case presentation

A 8-day-old female newborn was admitted to the NICU of University of Bari “Aldo Moro” (Italy) after a cardiorespiratory arrest occurred at home. Severe hypercalcemia was found and different drug therapies were employed (Furosemide, Cinacalcet and bisphosphonate), as well as hyperhydration, until subtotal parathyroidectomy, was performed at day 32. Our patient’s mutation was never described before so that a strict and individualized long-term follow-up was started.

Conclusions

This case of NSHPT suggests that a near-miss event, labelled as a possible case of SIDS, could also be due to severe hypercalcemia and evidentiates the difficulties of the neonatal management of NSHPT. Furthermore, the identification of the specific CaSR mutation provides the substrate for prenatal diagnosis.

Neonatal severe hyperparathyroidism secondary to a novel homozygous CASR gene mutation

Neonatal severe hyperparathyroidism (NSHPT) is a rare autosomal recessive disease. Children present within the first 6 months of life and more commonly in the first few weeks. Common presentation is poor feeding, polyuria, dehydration, lethargy, failure to thrive, hypotonia, gastrointestinal dysmotility, osteopenia and symptoms of respiratory distress due to a poorly developed chest cage. We present a case of a 2-month old girl with severe hypercalcemia and hyperparathyroidism. She was found to have a novel homozygous mutation in the acceptor splicing site of intron 4 (c.1378 -2A>G) of the calcium sensing receptor gene (CASR). This mutation causes frame shift deletion of exon 5 and insensitivity of CASR to calcium. The patient was treated with intravenous fluids, fruosemide, calcitonin, intravenous pamidronate and oral cinacalcet. She did not respond to medical treatment. Parathyroid gland imaging including ultrasound, MRI and sestamibi nuclear scan were not helpful in localizing the glands. Her symptoms resolved following total parathyroidectomy. She is being treated with alfacalcidiol and calcium supplements to maintain normal serum calcium and phosphate. She achieved her normal developmental milestones.

A novel SLC12A1 gene mutation associated with hyperparathyroidism, hypercalcemia, nephrogenic diabetes insipidus, and nephrocalcinosis in four patients

Solute Carrier Family 12 member 1 (SLC12A1) gene encodes the sodium-potassium-chloride co-transporter (NKCC2) at the apical membrane of the thick ascending loop of Henle (TAL). Bartter's syndrome (BS) type I is a rare, autosomal recessive, renal tubular disorder associated with mutation of the SLC12A1 gene. Presenting features include: hypokalemic metabolic alkalosis, hypercalciuria and nephrocalcinosis. The many allelic variants reported present with a spectrum of phenotypes, biochemical abnormalities and clinical severities. However, to date, only two reports have described hyperparathyroidism and hypercalcemia in patients with SLC12A1 gene mutations. We describe 4 patients with 4 novel mutation variants in the SLC12A1 gene (c.735C>G, c.1137del, c.2498-2499del, and c.1833delT) presenting with variable degrees of hyperparathyroidism, hypercalcemia, hypokalemic metabolic alkalosis, nephrocalcinosis, and nephrogenic diabetes insipidus. The link between calcium and parathyroid hormone abnormalities in patients with SLC12A1 mutations is unclear; the cases described suggest an association between primary hyperparathyroidism and loss of function mutation of SLC12A1, which may result in an aberrant threshold of the calcium sensing receptor at the level of the kidney.

Heterozygous inactivating CaSR mutations causing neonatal hyperparathyroidism: function, inheritance and phenotype

BACKGROUND

Homozygous inactivating mutations of the calcium-sensing receptor (CaSR) lead to neonatal severe hyperparathyroidism (NSHPT), whereas heterozygous inactivating mutations result in familial hypocalciuric hypercalcemia (FHH). It is unknown why in some cases heterozygous CaSR mutations cause neonatal hyperparathyroidism (NHPT) clinically similar to NSHPT but with only moderately elevated serum calcium.

METHODS

A literature survey was conducted to identify patients with heterozygous CaSR mutations and NHPT. The common NHPT CaSR mutants R185Q and R227L were compared with 15 mutants causing only FHH in the heterozygous state. We studied in vitro calcium signaling including the functional consequences of co-expression of mutant and wild-type (wt) CaSR, patients' phenotype, age of disease manifestation and mode of inheritance.

RESULTS

All inactivating CaSR mutants impaired calcium signaling of wt-CaSR regardless of the patients' clinical phenotype. The absolute intracellular calcium signaling response to physiologic extracellular calcium concentrations in vitro showed a high correlation with patients' serum calcium concentrations in vivo, which is similar in NHPT and FHH patients with the same genotype. Pedigrees of FHH families revealed that paternal inheritance per se does not necessarily lead to NHPT but may only cause FHH.

CONCLUSIONS

There is a significant correlation between in vitro functional impairment of the CaSR at physiologic calcium concentrations and the severity of alterations in calcium homeostasis in patients. Whether a particular genotype leads to NHPT or FHH appears to depend on additional predisposing genetic or environmental factors. An individual therapeutic approach appears to be warranted for NHPT patients.

Associations of the calcium-sensing receptor gene CASR rs7652589 SNP with nephrolithiasis and secondary hyperparathyroidism in haemodialysis patients

Nephrolithiasis, secondary hyperparathyroidism (sHPT), and cardiovascular complications are associated with disturbances in Ca handling and contribute to morbidity/mortality during haemodialysis (HD). Calcimimetics, activators of the calcium-sensing receptor (CaSR), provide an effective means of reducing parathyroid hormone (PTH) secretion in sHPT. Polymorphism in CaSR gene (CASR) influences Ca-related parameters, however it was not shown in HD patients for CASR rs7652589. The minor allele at this polymorphism modifies the binding sites of transcription factors and CaSR expression. We hypothesized that CASR rs7652589 variants may also influence CaSR in end stage renal disease (ESRD). We aimed to determine the associations of rs7652589 with nephrolithiasis-related ESRD, Ca, P, ALP, PTH, response to treatment with cinacalcet, prevalence of coronary artery disease, and all-cause/cardiovascular mortality in HD patients (n = 1162). Healthy individuals (n = 918) were controls. This study shows that the A allele of rs7652589 is a risk allele for nephrolithiasis-related ESRD. The AA genotype is associated with more severe sHPT (higher Ca and PTH concentrations). The A allele is associated with reduced CaSR transcript level in peripheral blood mononuclear cells. According to computational analysis, potential binding sites for GLI3, AHR and TP53 are removed by the A allele, whereas binding sites for SOX18 and TP63 are created.

GENETICS IN ENDOCRINOLOGY: Gain and loss of function mutations of the calcium-sensing receptor and associated proteins: current treatment concepts

The calcium-sensing receptor (CASR) is the main calcium sensor in the maintenance of calcium metabolism. Mutations of the CASR, the G protein alpha 11 (GNA11) and the adaptor-related protein complex 2 sigma 1 subunit (AP2S1) genes can shift the set point for calcium sensing causing hyper- or hypo-calcemic disorders. Therapeutic concepts for these rare diseases range from general therapies of hyper- and hypo-calcemic conditions to more pathophysiology oriented approaches such as parathyroid hormone (PTH) substitution and allosteric CASR modulators. Cinacalcet is a calcimimetic that enhances receptor function and has gained approval for the treatment of hyperparathyroidism. Calcilytics in turn attenuate CASR activity and are currently under investigation for the treatment of various diseases. We conducted a literature search for reports about treatment of patients harboring inactivating or activating CASR, GNA11 or AP2S1 mutants and about in vitro effects of allosteric CASR modulators on mutated CASR. The therapeutic concepts for patients with familial hypocalciuric hypercalcemia (FHH), neonatal hyperparathyroidism (NHPT), neonatal severe hyperparathyroidism (NSHPT) and autosomal dominant hypocalcemia (ADH) are reviewed. FHH is usually benign, but symptomatic patients benefit from cinacalcet. In NSHPT patients pamidronate effectively lowers serum calcium, but most patients require parathyroidectomy. In some patients cinacalcet can obviate the need for surgery, particularly in heterozygous NHPT. Symptomatic ADH patients respond to vitamin D and calcium supplementation but this may increase calciuria and renal complications. PTH treatment can reduce relative hypercalciuria. None of the currently available therapies for ADH, however, prevent tissue calcifications and complications, which may become possible with calcilytics that correct the underlying pathophysiologic defect.

Neonatal severe hyperparathyroidism caused by homozygous mutation in CASR: A rare cause of life-threatening hypercalcemia

Neonatal severe hyperparathyroidism (NSHPT) is a rare, life-threatening condition that presents with severe hypercalcemia, hyperparathyroidism, and osteopenia in the newborn period. Treatment of NSHPT traditionally includes hydration and bisphosphonates; however newer calcimimetic agents, such as cinacalcet, are now being utilized to prevent or delay parathyroidectomy which is technically difficult in the newborn. Medical treatment success is related to calcium sensing receptor (CaSR) genotype. We report a 4-day-old infant who presented with hyperbilirubinemia, poor feeding, weight loss, severe hypotonia and was ultimately diagnosed with NSHPT. The patient's total serum calcium level of 36.8 mg/dL (reference range: 8.5-10.4 mg/dL) is, to our knowledge, the highest ever documented in this setting. Exome data previously obtained on the infant's parents was re-analyzed demonstrating bi-parental heterozygosity for a mutation of the CASR gene: c.206G > A, and Sanger sequencing data confirmed the patient was a homozygote for the same mutation. Though a patient with the same CaSR gene mutation described here has responded to cinacalcet, our patient did not respond and required parathyroidectomy. Though this case has previously been published as a surgical case report, a full report of the medical management and underlying genetic etiology is warranted; this case underscores the importance of disclosing bi-parental heterozygosity for a gene causing severe neonatal disease particularly when treatment is available and illustrates the need for further in vitro studies of this CaSR mutation.

The calcium-sensing receptor: A promising target for prevention of colorectal cancer

The inverse correlation between dietary calcium intake and the risk of colorectal cancer (CRC) is well known, but poorly understood. Expression of the calcium-sensing receptor (CaSR), a calcium-binding G protein-coupled receptor is downregulated in CRC leading us to hypothesize that the CaSR has tumor suppressive roles in the colon. The aim of this study was to understand whether restoration of CaSR expression could reduce the malignant phenotype in CRC. In human colorectal tumors, expression of the CaSR negatively correlated with proliferation markers whereas loss of CaSR correlated with poor tumor differentiation and reduced apoptotic potential. In vivo, dearth of CaSR significantly increased expression of proliferation markers and decreased levels of differentiation and apoptotic markers in the colons of CaSR/PTH double knock-out mice confirming the tumor suppressive functions of CaSR. In vitro CRC cells stably overexpressing wild-type CaSR showed significant reduction in proliferation, as well as increased differentiation and apoptotic potential. The positive allosteric modulator of CaSR, NPS R-568 further enhanced these effects, whereas treatment with the negative allosteric modulator, NPS 2143 inhibited these functions. Interestingly, the dominant-negative mutant (R185Q) was able to abrogate these effects. Our results demonstrate a critical tumor suppressive role of CaSR in the colon. Restoration of CaSR expression and function is linked to regulation of the balance between proliferation, differentiation, and apoptosis and provides a rationale for novel strategies in CRC therapy.

Successful treatment of neonatal severe hyperparathyroidism with cinacalcet in two patients

Neonatal severe hyperparathyroidism (NSHPT) is a rare disorder caused by inactivating calcium-sensing receptor (CASR) mutations that result in life-threatening hypercalcemia and metabolic bone disease. Until recently, therapy has been surgical parathyroidectomy. Three previous case reports have shown successful medical management of NSHPT with cinacalcet. Here we present the detailed description of two unrelated patients with NSHPT due to heterozygous R185Q CASR mutations. Patient 1 was diagnosed at 11 months of age and had developmental delays, dysphagia, bell-shaped chest, and periosteal bone reactions. Patient 2 was diagnosed at 1 month of age and had failure to thrive, osteopenia, and multiple rib fractures. Cinacalcet was initiated at 13 months of age in patient 1, and at 4 months of age in patient 2. We have successfully normalized their parathyroid hormone and alkaline phosphatase levels. Despite the continuance of mild hypercalcemia (11–12 mg/dl), both patients showed no hypercalcemic symptoms. Importantly, patient 1 had improved neurodevelopment and patient 2 never experienced any developmental delays after starting cinacalcet. Neither experienced fractures after starting cinacalcet. Both have been successfully managed long-term without any significant adverse events. These cases expand the current literature of cinacalcet use in NSHPT to five successful reported cases. We propose that cinacalcet may be considered as an option for treating the severe hypercalcemia and metabolic bone disease found in infants and children with inactivating CASR disorders.

Familial hypocalciuric hypercalcemia with a de novo heterozygous mutation of calcium-sensing receptor

A de novo heterozygous inactivating mutation of calcium-sensing receptor (CASR) gene typically causes neonatal hyperparathyroidism (NHPT) with moderate hypercalcemia and hyperparathyroid bone disease. We present a case of asymptomatic hypocalciuric hypercalcemia with a de novo heterozygous mutation in CASR, S591C, which is primarily reported to be responsible for NHPT. A 54-year-old female was referred for investigation of asymptomatic hypercalcemia that was initially found in the 1980s but without a history of bone disease during the perinatal period. She had moderate hypercalcemia (12.4 mg/dl) and relative hypocalciuria (fractional extraction of calcium 1.07%) but normal intact parathyroid hormone and serum 1,25-dihydroxyvitamin D3. Pedigree analysis revealed that she carried a de novo heterozygous mutation of S591C, which she transmitted to an affected child with moderate hypercalcemia but not to other children, who had normal serum calcium levels. A de novo heterozygous CASR mutation that is responsible for NHPT may also present in individuals with asymptomatic hypocalciuric hypercalcemia. Caution is required when predicting course and outcome in a pedigree with CASR mutation, as well as incidental hypercalcemia, because of its variable phenotypes.

LEARNING POINTS

The phenotype and severity of CASR mutations are thought to be dependent on genotypes.We report an asymptomatic case of the de novo heterozygous S591C mutation in CASR, which has previously been reported as a responsible mutation of NHPT with bone diseases.Variable phenotypes of CASR raise a cautionary note about predicting outcome by genotyping in a pedigree with CASR mutation.

The calcium-sensing receptor in bone--mechanistic and therapeutic insights

The extracellular calcium-sensing receptor, CaSR, is a member of the G protein-coupled receptor superfamily and has a critical role in modulating Ca(2+) homeostasis via its role in the parathyroid glands and kidneys. New evidence suggests that CaSR expression in cartilage and bone also directly regulates skeletal homeostasis. This Review discusses the role of CaSR in chondrocytes, through which CaSR contributes to the development of the cartilaginous growth plate, as well as in osteoblasts and osteoclasts, through which CaSR has effects on skeletal development and bone turnover in young and mature animals. The interaction of skeletal CaSR activation with parathyroid hormone (PTH), which is secreted by the parathyroid gland, can lead to net bone formation in trabecular bone or net bone resorption in cortical bone. Allosteric modulators of CaSR are beneficial in some clinical conditions, with effects that are mediated by the ability of these agents to alter levels of PTH and improve Ca(2+) homeostasis. However, further insights into the action of CaSR in bone cells might lead to CaSR-based drugs that maximize not only the effects of the receptor on the parathyroid glands and kidneys but also on bone.

Towards tissue-specific pharmacology: insights from the calcium-sensing receptor as a paradigm for GPCR (patho)physiological bias

The calcium-sensing receptor (CaSR) is a widely expressed G protein-coupled receptor (GPCR) that mediates numerous tissue-specific functions. Its multiple ligands and diverse roles attest to the need for exquisite control over the signaling pathways that mediate its effects. 'Biased signaling' is the phenomenon by which distinct ligands stabilize preferred receptor signaling states. The CaSR is subject to biased signaling in response to its endogenous ligands. Interestingly, the 'natural' bias of the CaSR is altered in disease states, and small molecule drugs engender biased allosteric modulation of downstream signaling pathways. Thus, biased signaling from the CaSR also has important implications pathophysiologically and therapeutically. As outlined in this review, this novel paradigm extends to other GPCRs, making the CaSR a model for studies of ligand-biased signaling and for understanding how it may be used to foster selective drug activity in different tissues.

Calcium-sensing receptor sequencing in 21 patients with idiopathic or familial parathyroid disorder: pitfalls and characterization of a novel I32 V loss-of-function mutation

The calcium-sensing receptor (CaSR) is a G-protein-coupled receptor with a crucial role in calcium homeostasis. Mutations in the CaSR gene may lead to specific parathyroid disorders due to either gain-of-function (autosomal dominant hypercalciuric hypocalcemia; ADHH) or loss-of-function (familial hypocalciuric hypercalcemia; FHH). Our aim was to evaluate CaSR mutations as a cause of disease in selected patients. We identified and recruited patients with phenotypes suggestive of CaSR-related parathyroid disorders. DNA was extracted, and CaSR gene was sequenced. Live-ratiometric measurements of intracellular [Ca(2+)] and Western blot assays for evaluation of MAPK phosphorylation in response to changes in extracellular [Ca(2+)] were performed in transiently transfected HEK-293T cells to functionally characterize mutants. A total of 21 patients were evaluated, seven of them with idiopathic hypoparathyroidism (suspected ADHH) and 14 with hyperparathyroidism (suspected FHH). In the latter group two patients were found to harbor missense mutations: a novel heterozygous I32 V mutation in a female index case and a sporadic known R185Q mutation in a 1-year-old girl. In-vitro functional studies showed that I32 V is an inactivating mutation. In our study, most patients had normal CaSR sequencing. This suggests that phenotypic pitfalls may occur at time of patients' selection for CaSR sequencing. In one patient with strong positive pre-test probability based on both familial history and appropriate phenotype, a novel I32 V mutation leading to FHH was identified and characterized. In cases of familial parathyroid disorders, CaSR sequencing should be performed, but if negative, one should consider involvement of alternative genes or mechanisms.

Engendering biased signalling from the calcium-sensing receptor for the pharmacotherapy of diverse disorders

The human calcium-sensing receptor (CaSR) is widely expressed in the body, where its activity is regulated by multiple orthosteric and endogenous allosteric ligands. Each ligand stabilizes a unique subset of conformational states, which enables the CaSR to couple to distinct intracellular signalling pathways depending on the extracellular milieu in which it is bathed. Differential signalling arising from distinct receptor conformations favoured by each ligand is referred to as biased signalling. The outcome of CaSR activation also depends on the cell type in which it is expressed. Thus, the same ligand may activate diverse pathways in distinct cell types. Given that the CaSR is implicated in numerous physiological and pathophysiological processes, it is an ideal target for biased ligands that could be rationally designed to selectively regulate desired signalling pathways in preferred cell types.

Diagnosis and management of primary hyperparathyroidism--a scientific statement from the Department of Bone Metabolism, the Brazilian Society for Endocrinology and Metabolism

OBJECTIVE

To conduct a literature review on the diagnosis and management of primary hyperparathyroidism including the classical hipercalcemic form as well as the normocalcemic variant.

MATERIALS AND METHODS

This scientific statement was generated by a request from the Brazilian Medical Association (AMB) to the Brazilian Society for Endocrinology as part of its Clinical Practice Guidelines program. Articles were identified by searching in PubMed and Cochrane databases as well as abstracts presented at the Endocrine Society, Brazilian Society for Endocrinology Annual Meetings and the American Society for Bone and Mineral Research Annual Meeting during the last 5 years. Grading quality of evidence and strength of recommendation were adapted from the first report of the Oxford Centre for Evidence-based Medicine. All grades of recommendation, including "D", are based on scientific evidence. The differences between A, B, C and D, are due exclusively to the methods employed in generating evidence.

CONCLUSION

We present a scientific statement on primary hyperparathyroidism providing the level of evidence and the degree of recommendation regarding causes, clinical presentation as well as surgical and medical treatment.

Novel homozygous inactivating mutation of the calcium-sensing receptor gene (CASR) in neonatal severe hyperparathyroidism-lack of effect of cinacalcet

BACKGROUND

NSHPT is a life-threatening disorder caused by homozygous inactivating calcium-sensing receptor (CASR) mutations. In some cases, the CaSR allosteric activator, cinacalcet, may reduce serum PTH and calcium levels, but surgery is the treatment of choice.

OBJECTIVE

To describe a case of NSHPT unresponsive to cinacalcet.

PATIENT AND RESULTS

A 23-day-old girl was admitted with hypercalcemia, hypotonia, bell-shaped chest and respiratory distress. The parents were first-degree cousins once removed. Serum Ca was 4.75 mmol/l (N: 2.10-2.62), P: 0.83 mmol/l (1.55-2.64), PTH: 1096 pg/ml (9-52) and urinary Ca/Cr ratio: 0.5mg/mg. First, calcitonin was given (10 IU/kg × 4/day), and then 2 days later, pamidronate (0.5mg/kg) for 2 days. Doses of cinacalcet were given daily from day 28 of life starting at 30 mg/m2 and increasing to 90 mg/m2 on day 43. On day 33, 6 days after pamidronate, serum Ca levels had fallen to 2.5 mmol/l but, thereafter, rose to 5 mmol/l despite the cinacalcet. Total parathyroidectomy was performed at day 45. Hungry bone disease after surgery required daily Ca replacement and calcitriol for 18 days. At 3 months, the girl was mildly hypercalcemic, with no supplementation, and at 6 months, she developed hypocalcemia and has since been maintained on Ca and calcitriol. By CASR mutation analysis, the infant was homozygous and both parents heterozygous for a deletion-frameshift mutation.

CONCLUSION

The predicted nonfunctional CaSR is consistent with lack of response to cinacalcet, but total parathyroidectomy was successful. An empiric trial of the drug and/or prompt mutation testing should help minimize the period of unnecessary pharmacotherapy.

Cinacalcet monotherapy in neonatal severe hyperparathyroidism: a case study and review

CONTEXT

Neonatal severe hyperparathyroidism (NSHPT) is a severe form of familial hypocalciuric hypercalcemia characterized by severe hypercalcemia and skeletal demineralization. In most cases, NSHPT is due to biallelic loss-of-function mutations in the CASR gene encoding the calcium-sensing receptor (CaSR), but some patients have heterozygous mutations. Conventional treatment consists of iv saline, bisphosphonates, and parathyroidectomy.

OBJECTIVE

The aim of this project was to characterize the molecular basis for NSHPT in an affected newborn and to describe the response to monotherapy with cinacalcet.

METHODS

Clinical and biochemical features were monitored as cinacalcet therapy was initiated and maintained. Genomic DNA was obtained from the proband and parents. The CASR gene was amplified by PCR and sequenced directly.

RESULTS

The patient was a full-term male who developed hypotonia and respiratory failure soon after birth. He was found to have multiple fractures and diffuse bone demineralization, with a marked elevation in serum ionized calcium (1.99 mmol/L) and elevated serum levels of intact PTH (1154 pg/mL); serum 25-hydroxyvitamin D was low, and fractional excretion of calcium was reduced. The serum calcium level was not reduced by iv saline infusion. Based on an extensive family history of autosomal dominant hypercalcemia, a diagnosis of NSHPT was made, and cinacalcet therapy was initiated with a robust and durable effect. Molecular studies revealed a heterozygous R185Q missense mutation in the CASR in the patient and his father, whereas normal sequences for the CASR gene were present in the patient's mother.

CONCLUSIONS

We describe the first use of cinacalcet as monotherapy for severe hypercalcemia in a newborn with NSHPT. The rapid and durable response to cinacalcet suggests that a trial of calcimimetic therapy should be considered early in the course of NSHPT.

Pharmacoperones and the calcium sensing receptor: exogenous and endogenous regulators

Calcium sensing receptor (CaSR) mutations or altered expression cause disorders of calcium handling. Recent studies suggest that reduced targeting to the plasma membrane is a feature common to many CaSR loss-of-function mutations. Allosteric agonists (calcimimetics) can rescue signaling of a subset of CaSR mutants. This review evaluates our current understanding of the subcellular site(s) for allosteric modulator rescue of CaSR mutants. Studies to date make a strong case for calcimimetic potentiation of signaling not only at plasma membrane-localized CaSR, but at the endoplasmic reticulum, acting as pharmacoperones to assist in navigation of multiple quality control checkpoints. The possible role of endogenous pharmacoperones, calcium and glutathione, in folding and stabilization of the CaSR extracellular and transmembrane domains are considered. Finally, the possibility that dihydropyridines act as unintended pharmacoperones of CaSR is proposed. While our understanding of pharmacoperone rescue of CaSR requires refinement, promising results to date argue that this may be a fruitful avenue for drug discovery.

Is calcium signaling relevant for long bone growth?

BACKGROUND

Neonatal severe hyperparathyroidism (NSHPT) is a rare autosomal recessive disorder of calcium homeostasis, more often induced by homozygous inactivating mutations of the calcium-sensing receptor gene. This rare syndrome can be lethal if total parathyroidectomy is not performed within the first weeks of life.

CLINICAL REPORT

We report the clinical case of a male patient, son of consanguineous hypercalcemic parents, with clinical and biochemical features of NSHPT, followed until the age of 21 years. The patient underwent total parathyroidectomy, and then, due to the low compliance to calcium and calcitriol supplementation, an attempt was made with recombinant human parathyroid hormone [rhPTH (1-84)]. The patient did not reach the predicted height with an increased ratio of the upper and lower segments.

CONCLUSIONS

While this case is unique for the length of follow-up, the continuous and detailed description of NSHPT after total parathyroidectomy in its adult phenotype, and the treatment of hypoparathyroidism with rhPTH (1-84). Following this first description of a statural defect due to shortening of long bones in NSHPT, future investigations will attempt to uncover the role of calcium signaling in growth plate cartilage in humans.

Calcimimetic and calcilytic drugs for treating bone and mineral-related disorders

The calcium-sensing receptor (CaSR) plays a pivotal role in regulating systemic Ca(2+) homeostasis and is a target for drugs designed to treat certain disorders of bone and mineral metabolism. Calcimimetics are agonists or positive allosteric modulators of the CaSR; they inhibit parathyroid hormone (PTH) secretion and stimulate renal Ca(2+) excretion. The first calcimimetic drug is cinacalcet, a positive allosteric modulator of the CaSR that is approved for treating secondary hyperparathyroidism (HPT) in patients on renal replacement therapy and for some forms of primary HPT characterized by clinically significant hypercalcemia. Cinacalcet is also being investigated as a therapy for other hypercalcemic conditions and certain hypophosphatemic disorders. Calcilytics are CaSR inhibitors that stimulate the secretion of PTH and decrease renal excretion of Ca(2+). Although calcilytics have failed thus far as anabolic therapies for osteoporosis, they are currently being evaluated as novel therapies for new indications involving hypocalcemia and/or hypercalciuria.