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

Absence of calcium-sensing receptor basal activity due to inter-subunit disulfide bridges

G protein-coupled receptors naturally oscillate between inactive and active states, often resulting in receptor constitutive activity with important physiological consequences. Among the class C G protein-coupled receptors that typically sense amino-acids and their derivatives, the calcium sensing receptor (CaSR) tightly controls blood calcium levels. Its constitutive activity has not yet been studied. Here, we demonstrate the importance of the inter-subunit disulfide bridges in maintaining the inactive state of CaSR, resulting in undetectable constitutive activity, unlike the other class C receptors. Deletion of these disulfide bridges results in strong constitutive activity that is abolished by mutations preventing amino acid binding. It shows that this inter-subunit disulfide link is necessary to limit the agonist effect of amino acids on CaSR. Furthermore, human genetic mutations deleting these bridges and associated with hypocalcemia result in elevated CaSR constitutive activity. These results highlight the physiological importance of fine tuning the constitutive activity of G protein-coupled receptors.

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.

Genetics of monogenic disorders of calcium and bone metabolism

Disorders of calcium homeostasis are the most frequent metabolic bone and mineral disease encountered by endocrinologists. These disorders usually manifest as primary hyperparathyroidism (PHPT) or hypoparathyroidism (HP), which have a monogenic aetiology in 5%-10% of cases, and may occur as an isolated endocrinopathy, or as part of a complex syndrome. The recognition and diagnosis of these disorders is important to facilitate the most appropriate management of the patient, with regard to both the calcium-related phenotype and any associated clinical features, and also to allow the identification of other family members who may be at risk of disease. Genetic testing forms an important tool in the investigation of PHPT and HP patients and is usually reserved for those deemed to be an increased risk of a monogenic disorder. However, identifying those suitable for testing requires a thorough clinical evaluation of the patient, as well as an understanding of the diversity of relevant phenotypes and their genetic basis. This review aims to provide an overview of the genetic basis of monogenic metabolic bone and mineral disorders, primarily focusing on those associated with abnormal calcium homeostasis, and aims to provide a practical guide to the implementation of genetic testing in the clinic.

[Syndrome of hypocalсiuric hypercalcemia. Is it rare? Two clinical cases in an outpatient clinic]

Hypocalciuric hypercalcemia syndrome (familial hypocalciuric hypercalcemia, FHH) is an inherited condition based on dysfunction of the calcium receptor or its associated partner proteins. Recent evidence suggests that the prevalence of this condition may be comparable to that of primary hyperparathyroidism. Clinical manifestations of FHH are usually absent; however the classic symptoms of hypercalcemia may be present in some cases. Timely differential diagnosis of FHH avoids unnecessary and expensive instrumental examination, as well as ineffective treatment. The clinical cases presented in this publication demonstrate the unjustified difficulties in this issue and the necessity to raise the awareness of physicians about the familial hypocalciuric hypercalcemia.

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.

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.

Pathophysiology of Hypercalcemia

Extracellular calcium is normally tightly regulated by parathyroid hormone (PTH), 1,25-dihydroxyvitamin D, as well as by calcium ion (Ca⁺⁺) itself. Dysregulated PTH production leading to hypercalcemia occurs most commonly in sporadic primary hyperparathryoidism (PHPT) but may also result from select genetic mutations in familial disorders. Parathyroid hormone-related protein shares molecular mechanisms of action with PTH and is the most common cause of hypercalcemia of malignancy. Other cytokines and mediators may also cause resorptive hypercalcemia once bone metastases have occurred. Less commonly, extrarenal production of calcitriol can occur in malignancies and in infectious and noninfectious inflammatory conditions and can cause hypercalcemia.

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.

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.

A Case of a Heterozygous Inactivating CASR Variant with Adult-Onset Symptomatic Hypercalcemia Requiring Extensive Surgery

We describe the case of an adult female patient with symptomatic familial hypocalciuric hypercalcemia requiring a step-wise therapeutic approach and the eventual need for a total parathyroidectomy and thyroidectomy to cure symptoms. Genetic analysis demonstrated a heterozygous R227L inactivating CASR gene variant, previously only described in neonatal severe hyperparathyroidism. Post-operative histology showed diffuse hyperplasia of all four parathyroid glands along with the presence of intrathyroidal parathyroid tissue. With regard to clinical management this case suggests that familial hypocalciuric hypercalcemia should be classified as an atypical form of primary hyperparathyroidism rather than a distinct entity.

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.

Overview of Genetically Determined Diseases/Multiple Endocrine Neoplasia Syndromes Predisposing to Endocrine Tumors

In this chapter, we present an overview of multiple endocrine neoplasia syndromes including their most important clinical and molecular features. Multiple endocrine neoplasia type 1 and 2 syndromes (MEN1 and MEN2) are discussed in detail. Syndromes that are presented in other chapters are only briefly mentioned. We discuss the relevance of germline gene alterations in apparently sporadic endocrine tumors, e.g., medullary thyroid cancer, primary hyperparathyroidism, and neuroendocrine tumors. McCune-Albright syndrome that only exists in non-hereditary, sporadic forms is also discussed in detail, as tumors of several endocrine organs can develop in the same individual.

Evolution of Our Understanding of the Hyperparathyroid Syndromes: A Historical Perspective

We review advancing and overlapping stages for our understanding of the expressions of six hyperparathyroid (HPT) syndromes: multiple endocrine neoplasia type 1 (MEN1) or type 4, multiple endocrine neoplasia type 2A (MEN2A), hyperparathyroidism-jaw tumor syndrome, familial hypocalciuric hypercalcemia, neonatal severe primary hyperparathyroidism, and familial isolated hyperparathyroidism. During stage 1 (1903 to 1967), the introduction of robust measurement of serum calcium was a milestone that uncovered hypercalcemia as the first sign of dysfunction in many HPT subjects, and inheritability was reported in each syndrome. The earliest reports of HPT syndromes were biased toward severe or striking manifestations. During stage 2 (1959 to 1985), the early formulations of a syndrome were improved. Radioimmunoassays (parathyroid hormone [PTH], gastrin, insulin, prolactin, calcitonin) were breakthroughs. They could identify a syndrome carrier, indicate an emerging tumor, characterize a tumor, or monitor a tumor. During stage 3 (1981 to 2006), the assembly of many cases enabled recognition of further details. For example, hormone non-secreting skin lesions were discovered in MEN1 and MEN2A. During stage 4 (1985 to the present), new genomic tools were a revolution for gene identification. Four principal genes ("principal" implies mutated or deleted in 50% or more probands for its syndrome) (MEN1, RET, CASR, CDC73) were identified for five syndromes. During stage 5 (1993 to the present), seven syndromal genes other than a principal gene were identified (CDKN1B, CDKN2B, CDKN2C, CDKN1A, GNA11, AP2S1, GCM2). Identification of AP2S1 and GCM2 became possible because of whole-exome sequencing. During stages 4 and 5, the newly identified genes enabled many studies, including robust assignment of the carriers and non-carriers of a mutation. Furthermore, molecular pathways of RET and the calcium-sensing receptor were elaborated, thereby facilitating developments in pharmacotherapy. Current findings hold the promise that more genes for HPT syndromes will be identified and studied in the near future. © 2018 American Society for Bone and Mineral Research.

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.

Familial Hypocalciuric Hypercalcemia as an Atypical Form of Primary Hyperparathyroidism

Familial hypocalciuric hypercalcemia (FHH) causes lifelong hypercalcemia with features that overlap with typical primary hyperparathyroidism (PHPT). The incompleteness of this overlap has led to divergent nomenclatures for FHH. I compare two nomenclatures. One sets FHH as an entity distinct from PHPT. The other groups FHH with PHPT but conditions FHH as atypical PHPT. I analyzed selected articles about calcium-sensing receptors, FHH, PHPT, CASR, GNA11, and AP2S1. FHH usually results from a heterozygous germline inactivating mutation of the CASR, and less frequently from mutation of GNA11 or AP2S1. The CASR encodes the calcium-sensing receptors. These are highly expressed on parathyroid cells, where they sense serum calcium concentration and regulate suppression of PTH secretion by serum calcium. Their mutated expression in the kidney in FHH causes increased renal tubular reabsorption of calcium (hypocalciuria). Many FHH features are shared with PHPT and thus support FHH as a form of PHPT. These include a driver mutation expressed mainly in the parathyroid cells. The mutation causes a parathyroid cell insensitivity to extracellular calcium in vivo and in vitro, a right-shift of the set point for suppression of PTH secretion by calcium. Serum PTH is normal or mildly elevated; ie, it is not appropriately suppressed by hypercalcemia. Total parathyroidectomy causes hypoparathyroidism and durable remission of hypercalcemia. Some other features are not shared with PHPT and could support FHH as a distinct entity. These include onset of hypercalcemia in the first week of life, frequent persistence of hypercalcemia after subtotal parathyroidectomy, and hypocalciuria. The features supporting FHH as a form of PHPT are stronger than those favoring FHH as a distinct entity. Classifying FHH as an atypical form of PHPT represents compact nomenclature and supports current concepts of pathophysiology of FHH and PHPT. Published 2017. This article is a U.S. Government work and is in the public domain in the USA.

Calcimimetic Use in Familial Hypocalciuric Hypercalcemia-A Perspective in Endocrinology

CONTEXT

Familial hypocalciuric hypercalcemia (FHH) causes lifelong hypercalcemia that even persists after subtotal parathyroidectomy. Symptoms are usually mild. Past recommendations have often been for monitoring and against surgical or pharmacologic treatments.

METHODS

Review of publications about FHH, calcium-sensing receptors (CaSRs), and calcimimetics.

RESULTS

FHH reflects heterozygous germline mutation of CASR, GNA11, or AP2S1. These mutations inactivate the CaSRs in the parathyroid cell. Thereby, they shift the serum calcium set point to higher values and cause hypercalcemia. Calcimimetic drugs enhance the effects of calcium on the CaSRs and thereby inhibit the parathyroid cell. Calcimimetic drugs are indicated in adults with primary hyperparathyroidism without a good surgical option. Calcimimetic safety and efficacy are not established in children younger than age 18 years. Recent case reports have described treatment of FHH with calcimimetics. Success was classified as combinations of subjective improvements and decreases of serum calcium levels, but not necessarily into the normal range. Treatment was successful in 14 of 16 cases (88%).

CONCLUSION

Deductions based on these case reports have limitations. For example, failures of therapy may not have been reported. Cost of the drug might be rate limiting. Calcimimetics can be offered to adults with FHH and those in whom the serum calcium level is >0.25 mM (1 mg/dL) beyond the upper limit of normal or with possible symptoms of hypercalcemia. Calcimimetics can now be offered to more adults with FHH.

Hypercalcemic Disorders in Children

Hypercalcemia is defined as a serum calcium concentration that is greater than two standard deviations above the normal mean, which in children may vary with age and sex, reflecting changes in the normal physiology at each developmental stage. Hypercalcemic disorders in children may present with hypotonia, poor feeding, vomiting, constipation, abdominal pain, lethargy, polyuria, dehydration, failure to thrive, and seizures. In severe cases renal failure, pancreatitis and reduced consciousness may also occur and older children and adolescents may present with psychiatric symptoms. The causes of hypercalcemia in children can be classified as parathyroid hormone (PTH)-dependent or PTH-independent, and may be congenital or acquired. PTH-independent hypercalcemia, ie, hypercalcemia associated with a suppressed PTH, is commoner in children than PTH-dependent hypercalcemia. Acquired causes of PTH-independent hypercalcemia in children include hypervitaminosis; granulomatous disorders, and endocrinopathies. Congenital syndromes associated with PTH-independent hypercalcemia include idiopathic infantile hypercalcemia (IIH), William's syndrome, and inborn errors of metabolism. PTH-dependent hypercalcemia is usually caused by parathyroid tumors, which may give rise to primary hyperparathyroidism (PHPT) or tertiary hyperparathyroidism, which usually arises in association with chronic renal failure and in the treatment of hypophosphatemic rickets. Acquired causes of PTH-dependent hypercalcemia in neonates include maternal hypocalcemia and extracorporeal membrane oxygenation. PHPT usually occurs as an isolated nonsyndromic and nonhereditary endocrinopathy, but may also occur as a hereditary hypercalcemic disorder such as familial hypocalciuric hypercalcemia, neonatal severe primary hyperparathyroidism, and familial isolated primary hyperparathyroidism, and less commonly, as part of inherited complex syndromic disorders such as multiple endocrine neoplasia (MEN). Advances in identifying the genetic causes have resulted in increased understanding of the underlying biological pathways and improvements in diagnosis. The management of symptomatic hypercalcemia includes interventions such as fluids, antiresorptive medications, and parathyroid surgery. This article presents a clinical, biochemical, and genetic approach to investigating the causes of pediatric hypercalcemia. © 2017 American Society for Bone and Mineral Research.

Genetics of Magnesium Disorders

Background

Magnesium (Mg²⁺), the second most abundant cation in the cell, is woven into a multitude of cellular functions. Dysmagnesemia is associated with multiple diseases and, when severe, can be life-threatening.

Summary

This review discusses Mg²⁺ homeostasis and function with specific focus on renal Mg²⁺ handling. Intrarenal channels and transporters related to Mg²⁺ absorption are discussed. Unraveling the rare genetic diseases with manifestations of dysmagnesemia has greatly increased our understanding of the complex and intricate regulatory network in the kidney, specifically, functions of tight junction proteins including claudin-14, -16, -19, and -10; apical ion channels including: TRPM6, K_v1.1, and ROMK; small regulatory proteins including AC3 and ANK3; and basolateral proteins including EGF receptor, γ-subunit (FXYD2) of Na-K-ATPase, K_ir4.1, CaSR, CNNM2, and SLC41A. Although our understanding of Mg²⁺ handling of the kidney has expanded considerably in the last two decades, many questions remain. Future studies are needed to elucidate a multitude of unknown aspects of Mg²⁺ handling in the kidney.

Key Message

Understanding rare and genetic diseases of Mg²⁺ dysregulation has expanded our knowledge and furthers the development of strategies for preventing and managing dysmagnesemia.

Diseases associated with calcium-sensing receptor

The calcium-sensing receptor (CaSR) plays a pivotal role in systemic calcium metabolism by regulating parathyroid hormone secretion and urinary calcium excretion. The diseases caused by an abnormality of the CaSR are genetically determined or are more rarely acquired. The genetic diseases consist of hyper- or hypocalcemia disorders. Hypercalcaemia disorders are related to inactivating mutations of the CASR gene either heterozygous (autosomal dominant familial benign hypercalcaemia, still named hypocalciuric hypercalcaemia syndrome type 1) or homozygous (severe neonatal hyperparathyroidism). The A986S, R990G and Q1011E variants of the CASR gene are associated with higher serum calcium levels than in the general population, hypercalciuria being also associated with the R990G variant. The differential diagnosis consists in the hypocalciuric hypercalcaemia syndrome, types 2 (involving GNA11 gene) and 3 (involving AP2S1 gene); hyperparathyroidism; abnormalities of vitamin D metabolism, involving CYP24A1 and SLC34A1 genes; and reduced GFR. Hypocalcemia disorders, which are more rare, are related to heterozygous activating mutations of the CASR gene (type 1), consisting of autosomal dominant hypocalcemia disorders, sometimes with a presentation of pseudo-Bartter’s syndrome. The differential diagnosis consists of the hypercalciuric hypocalcaemia syndrome type 2, involving GNA11 gene and other hypoparathyroidism aetiologies. The acquired diseases are related to the presence of anti-CaSR antibodies, which can cause hyper- or especially hypocalcemia disorders (for instance in APECED syndromes), determined by their functionality. Finally, the role of CaSR in digestive, respiratory, cardiovascular and neoplastic diseases is gradually coming to light, providing new therapeutic possibilities. Two types of CaSR modulators are known: CaSR agonists (or activators, still named calcimimetics) and calcilytic antagonists (or inhibitors of the CasR). CaSR agonists, such as cinacalcet, are indicated in secondary and primary hyperparathyroidism. Calcilytics have no efficacy in osteoporosis, but could be useful in the treatment of hypercalciuric hypocalcaemia syndromes.