Familial Hypocalciuric Hypercalcemia Type 1 and Autosomal-Dominant Hypocalcemia Type 1: Prevalence in a Large Healthcare Population

G protein-coupled receptor (GPCR) pharmacogenomics

The field of pharmacogenetics, the investigation of the influence of one or more sequence variants on drug response phenotypes, is a special case of pharmacogenomics, a discipline that takes a genome-wide approach. Massively parallel, next generation sequencing (NGS), has allowed pharmacogenetics to be subsumed by pharmacogenomics with respect to the identification of variants associated with responders and non-responders, optimal drug response, and adverse drug reactions. A plethora of rare and common naturally-occurring GPCR variants must be considered in the context of signals from across the genome. Many fundamentals of pharmacogenetics were established for G protein-coupled receptor (GPCR) genes because they are primary targets for a large number of therapeutic drugs. Functional studies, demonstrating likely-pathogenic and pathogenic GPCR variants, have been integral to establishing models used for in silico analysis. Variants in GPCR genes include both coding and non-coding single nucleotide variants and insertion or deletions (indels) that affect cell surface expression (trafficking, dimerization, and desensitization/downregulation), ligand binding and G protein coupling, and variants that result in alternate splicing encoding isoforms/variable expression. As the breadth of data on the GPCR genome increases, we may expect an increase in the use of drug labels that note variants that significantly impact the clinical use of GPCR-targeting agents. We discuss the implications of GPCR pharmacogenomic data derived from the genomes available from individuals who have been well-phenotyped for receptor structure and function and receptor-ligand interactions, and the potential benefits to patients of optimized drug selection. Examples discussed include the renin-angiotensin system in SARS-CoV-2 (COVID-19) infection, the probable role of chemokine receptors in the cytokine storm, and potential protease activating receptor (PAR) interventions. Resources dedicated to GPCRs, including publicly available computational tools, are also discussed.

G protein-coupled receptor (GPCR) gene variants and human genetic disease

Genetic variations in the genes encoding G protein-coupled receptors (GPCRs) can disrupt receptor structure and function, which can result in human genetic diseases. Disease-causing mutations have been reported in at least 55 GPCRs for more than 66 monogenic diseases in humans. The spectrum of pathogenic and likely pathogenic variants includes loss of function variants that decrease receptor signaling on one extreme and gain of function that may result in biased signaling or constitutive activity, originally modeled on prototypical rhodopsin GPCR variants identified in retinitis pigmentosa, on the other. GPCR variants disrupt ligand binding, G protein coupling, accessory protein function, receptor desensitization and receptor recycling. Next generation sequencing has made it possible to identify variants of uncertain significance (VUS). We discuss variants in receptors known to result in disease and in silico strategies for disambiguation of VUS such as sorting intolerant from tolerant and polymorphism phenotyping. Modeling of variants has contributed to drug development and precision medicine, including drugs that target the melanocortin receptor in obesity and interventions that reverse loss of gonadotropin-releasing hormone receptor from the cell surface in idiopathic hypogonadotropic hypogonadism. Activating and inactivating variants of the calcium sensing receptor (CaSR) gene that are pathogenic in familial hypocalciuric hypercalcemia and autosomal dominant hypocalcemia have enabled the development of calcimimetics and calcilytics. Next generation sequencing has continued to identify variants in GPCR genes, including orphan receptors, that contribute to human phenotypes and may have therapeutic potential. Variants of the CaSR gene, some encoding an arginine-rich region that promotes receptor phosphorylation and intracellular retention, have been linked to an idiopathic epilepsy syndrome. Agnostic strategies have identified variants of the pyroglutamylated RF amide peptide receptor gene in intellectual disability and G protein-coupled receptor 39 identified in psoriatic arthropathy. Coding variants of the G protein-coupled receptor L1 (GPR37L1) orphan receptor gene have been identified in a rare familial progressive myoclonus epilepsy. The study of the role of GPCR variants in monogenic, Mendelian phenotypes has provided the basis of modeling the significance of more common variants of pharmacogenetic significance.

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.

The Role of Demographic and Clinical Factors in Germline Mutation Testing for Patients with Primary Hyperparathyroidism

INTRODUCTION

Guidelines recommending genetic counseling in primary hyperparathyroidism (PHPT) vary. To further delineate current recommendations, this study examined genetic counseling referral patterns and rates of mutations in surgical patients with PHPT.

PATIENTS AND METHODS

A single-institution review was performed of adult patients who underwent parathyroidectomy for presumed sporadic PHPT. Genetic testing indications of hypercalcemia onset ≤ 40 years, multigland disease (MGD), family history (FHx) of PHPT, or other clinical indications suspicious for a PHPT-related endocrinopathy were examined by demographics and mutation detection rates.

RESULTS

Genetic counseling was performed in 237 (37.9%) of 625 patients. Counseling was discussed but not performed in 121 (19.4%) patients. No evidence was noted of genetic referral discussion in the remaining 267 (42.7%). Of these groups, patients who received genetic counseling were youngest, p < 0.001 [median age 55.3 (IQR 43.2, 66.7) years]. The majority of patients with indications of age ≤ 40 years (65.7%), FHx (78.0%), and other clinical indications (70.7%) underwent genetic counseling, while most with MGD (57.0%) did not. Eight mutations were detected in 227 patients (3.5%). Mutations included: MEN1 (n = 2), CDC-73 (n = 4), and CASR (n = 2). Detection was most common in patients with FHx (4/71, 5.6%), then age ≤ 40 years (3/66, 4.5%), and other clinical indications (3/80, 3.8%). No mutations were identified in 48 patients tested solely for MGD.

CONCLUSIONS

Most patients with onset of hypercalcemia age ≤ 40 years, positive FHx, or other clinical concerns underwent genetic counseling, while most with MGD did not. As no germline mutations were identified in patients with MGD alone, further investigation of MGD as a sole indication for genetic counseling may be warranted.

Familial states of primary hyperparathyroidism: an update

BACKGROUND

Familial primary hyperparathyroidism (PHPT) includes syndromic and non-syndromic disorders. The former are characterized by the occurrence of PHPT in association with extra-parathyroid manifestations and includes multiple endocrine neoplasia (MEN) types 1, 2, and 4 syndromes, and hyperparathyroidism-jaw tumor (HPT-JT). The latter consists of familial hypocalciuric hypercalcemia (FHH) types 1, 2 and 3, neonatal severe primary hyperparathyroidism (NSHPT), and familial isolated primary hyperparathyroidism (FIHP). The familial forms of PHPT show different levels of PHPT penetrance, developing earlier and with multiglandular involvement compared to sporadic counterpart. All these diseases exhibit Mendelian inheritance patterns, and for most of them, the genes responsible have been identified. DNA testing for predisposing mutations is helpful in index cases or in individuals with a high suspicion of the disease. Early recognition of hereditary disorders of PHPT is of great importance for the best clinical and surgical approach. Genetic testing is useful in routine clinical practice because it will also involve appropriate screening for extra-parathyroidal manifestations related to the syndrome as well as the identification of asymptomatic carriers of the mutation.

PURPOSE

The aim of the review is to discuss the current knowledge on the clinical and genetic profile of these disorders along with the importance of genetic testing in clinical practice.

Clinical and outcome comparison of genetically positive vs. negative patients in a large cohort of suspected familial hypocalciuric hypercalcemia

OBJECTIVE

Biochemical suspicion of familial hypocalciuric hypercalcemia (FHH) might provide with a negative (FHH-negative) or positive (FHH-positive) genetic result. Understanding the differences between both groups may refine the identification of those with a positive genetic evaluation, aid management decisions and prospective surveillance. We aimed to compare FHH-positive and FHH-negative patients, and to identify predictive variables for FHH-positive cases.

DESIGN

Retrospective, national multi-centre study of patients with suspected FHH and genetic testing of the CASR, AP2S1 and GNA11 genes.

METHODS

Clinical, biochemical, radiological and treatment data were collected. We established a prediction model for the identification of FHH-positive cases by logistic regression analysis and area under the ROC curve (AUROC) was estimated.

RESULTS

We included 66 index cases, of which 30 (45.5%) had a pathogenic variant. FHH-positive cases were younger (p = 0.029), reported more frequently a positive family history (p < 0.001), presented higher magnesium (p < 0.001) and lower parathormone levels (p < 0.001) and were less often treated for hypercalcemia (p = 0.017) in comparison to FHH-negative cases. Magnesium levels showed the highest AUROC (0.825, 95%CI: 0.709-0.941). The multivariate analysis revealed that family history and magnesium levels were independent predictors of a positive genetic result. The predictive model showed an AUROC of 0.909 (95%CI: 0.826-0.991).

CONCLUSIONS

The combination of magnesium and a positive family history offered a good diagnostic accuracy to predict a positive genetic result. Therefore, the inclusion of magnesium measurement in the routine evaluation of patients with suspected FHH might provide insight into the identification of a positive genetic result of any of the CaSR-related genes.

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.

Cinacalcet Reverses Short QT Interval in Familial Hypocalciuric Hypercalcemia Type 1

CONTEXT

Familial hypocalciuric hypercalcemia type 1 (FHH-1) defines an autosomal dominant disease, related to mutations in the CASR gene, with mild hypercalcemia in most cases. Cases of FHH-1 with a short QT interval have not been reported to date.

OBJECTIVE

Three family members presented with FHH-1 and short QT interval (<360 ms), a condition that could lead to cardiac arrhythmias, and the effects of cinacalcet, an allosteric modulator of the CaSR, in rectifying the abnormal sensitivity of the mutant CaSR and in correcting the short QT interval were determined.

METHODS

CASR mutational analysis was performed by next-generation sequencing and functional consequences of the identified CaSR variant (p.Ile555Thr), and effects of cinacalcet were assessed in HEK293 cells expressing wild-type and variant CaSRs. A cinacalcet test consisting of administration of 30 mg cinacalcet (8 Am) followed by hourly measurement of serum calcium, phosphate, and parathyroid hormone during 8 hours and an electrocardiogram was performed.

RESULTS

The CaSR variant (p.Ile555Thr) was confirmed in all 3 FHH-1 patients and was shown to be associated with a loss of function that was ameliorated by cinacalcet. Cinacalcet decreased parathyroid hormone by >50% within two hours, and decreases in serum calcium and increases in serum phosphate occurred within 8 hours, with rectification of the QT interval, which remained normal after 3 months of cinacalcet treatment.

CONCLUSION

Our results indicate that FHH-1 patients should be assessed for a short QT interval and a cinacalcet test used to select patients who are likely to benefit from this treatment.

Wolf-Hirschhorn Syndrome with Hyperparathyroidism: A Case Report and a Narrative Review of the Literature

Wolf-Hirschhorn syndrome (WHS) is a contiguous gene deletion condition. The WHS core phenotype includes developmental delays, intellectual disabilities, seizures, and distinctive facial features. Various other comorbidities have also been reported, such as hearing loss, heart defects, as well as eye problems and kidney problems. In this report, we present a case of WHS accompanied by hyperparathyroidism and hypercalcemia, which has not been previously reported. A girl was born at 37 weeks of gestation by vaginal delivery. She was small for the gestational age (2,045 g) and admitted to neonatal intensive care unit. She had typical WHS facial features and was found to have bilateral small kidneys associated with transient metabolic acidosis and renal insufficiency. She had right-sided sensorineural hearing loss, a small atrial septal defect, and colpocephaly and hypoplasia of corpus callosum. She had a single seizure which was well controlled with an oral antiepileptic medication. Cytogenetic studies demonstrated a large terminal chromosome 4p deletion (21.4 Mb) and 4p duplication (2.1 Mb) adjacent to the deletion. A unique finding in this patient is her consistently elevated levels of parathyroid hormone and serum calcium, suggesting hyperparathyroidism. We present this rare case along with a review of the literature and hope to draw an attention to a potential relationship between WHS and hyperparathyroidism.

Case Report: Multiple prolactinomas in a young man with Kallmann syndrome and familial hypocalciuric hypercalcemia

Introduction

The occurrence of prolactinomas in sex hormone treated patients with central hypogonadism is extremely rare.

Case presentation

We present a Caucasian male patient who was diagnosed with Kallmann syndrome (KS) at age 15 years. Testosterone treatment was started. At age 26 the patient presented with mild headache. MRI revealed two separate pituitary adenomas along with the absence of the olfactory bulbs. Given the presence of marked hyperprolactinemia (17x upper limit of the reference range) the diagnosis prolactinoma was made and treatment with cabergoline was started which resulted in a complete biochemical response and in marked reduction of both adenomas in size. Hypogonadism persisted and testosterone replacement therapy was continued. Genetic testing of genes associated with pituitary tumors, Kallmann syndrome and idiopathic hypogonadotropic hypogonadism was negative. Mild concomitant hypercalcemia in accordance with familial hypocalciuric hypercalcemia (FHH) prompted mutation analysis of the calcium receptor (CASR) gene which yielded a pathogenic inactivating variant.

Discussion/conclusion

The presence of two separate prolactinomas in a patient with KS has not yet been reported in the literature. The effect of sex hormone treatment of KS patients on the possible development of prolactinoma is unknown at present. The occurance of multiple prolactinomas in our patient suggests increased susceptibility. Although CaSR is expressed in GnRH neurons in mouse brain and CaSR deficient mice have a reduced hypothalamic GnRH neuronal population, the relevance of the CASR gene variant in our patient for the KS phenotype is unclear at present.

A pediatric case of autosomal dominant hypocalcemia type 2

OBJECTIVES

Autosomal dominant hypocalcemia (ADH) is characterized by hypocalcemia and hyperphosphatemia secondary to hypoparathyroidism. It is classified as type 1, caused by gain-of-function mutations of the calcium-sensing receptor (CASR), and type 2, caused by activating mutations in GNA11, which is a crucial mediator of CASR signaling. What is new? We report a rare pediatric case of ADH type 2.

CASE PRESENTATION

The patient was a 15-year-old girl with short stature. Blood tests demonstrated hypocalcemia and hyperphosphatemia without elevated parathyroid hormone levels. Brain computed tomography revealed calcification in the bilateral basal ganglia. Genetic testing revealed the rare GNA11 mutation, c.1023C>G (p.Phe341Leu). The patient was diagnosed with ADH type 2. She had experienced numbness and tetany in her hands for several years, which improved with alfacalcidol therapy.

CONCLUSIONS

Our patient is the third female and first pediatric reported case of a variant mutation in the GNA11 gene (ADH type 2), c.1023C>G (p.Phe341Leu).

Stable Incidence and Increasing Prevalence of Primary Hyperparathyroidism in a Population-based Study in Scotland

CONTEXT

Previous studies, including our own, have demonstrated a highly variable incidence of primary hyperparathyroidism (PHPT) from year to year.

OBJECTIVE

We planned to provide a current estimate of the incidence and prevalence of PHPT in a community-based study.

METHODS

A population-based retrospective follow-up study was conducted in Tayside (Scotland) from 2007 to 2018. Record-linkage technology (demography, biochemistry, prescribing, hospital admissions, radiology, and mortality data) was used to identify all patients. Cases of PHPT were defined as those with at least 2 raised serum corrected calcium concentration CCA (> 2.55 mmol/L) and/or hospital admissions with PHPT diagnoses and/or surgery records with parathyroidectomy during the follow-up period. The number of prevalent and incident cases of PHPT per calendar year by age and sex were estimated.

RESULTS

A total of 2118 people (72.3% female, mean age 65 years) were identified with an incident case of PHPT. The overall prevalence of PHPT over the 12 years of the study was 0.84% (95% CI, 0.68%-1.02%), steadily increasing from 0.71% in 2007 to 1.02% in 2018. From 2008, the incidence of PHPT was relatively stable from 4 to 6 cases per 10 000 person-years, declining from 11.5 per 10 000 person-years in 2007. The incidence varied from 0.59 per 10 000 person-years (95% CI, 0.40%-0.77%) for those aged 20 to 29 years, to 12.4 per 10 000 person-years (95% CI, 11.2%-13.3%) in those aged 70 to 79 years. Incidence of PHPT was 2.5 times higher in women than in men.

CONCLUSION

This study is the first showing a relatively steady annual incidence of PHPT at 4 to 6 per 10 000 person-years. This population-based study reports a PHPT prevalence of 0.84%.

Case Report: Calcium sensing receptor gene gain of function mutations: a case series and report of 2 novel mutations

Autosomal dominant hypocalcemia (ADH1) is a genetic disorder characterized by low serum calcium and low or inappropriately normal levels of parathyroid hormone. The disease is caused by a heterozygous activating mutation of the calcium-sensing receptor (CaSR) gene, encoding a G-Protein-coupled cell membrane sensor of extracellular calcium concentration mainly expressed by parathyroid glands, renal tubules, and the brain. ADH1 has been linked to 113 unique germline mutations, of which nearly 96% are missense mutations. There is often a lack of a clear genotype/phenotype correlation in the reported literature. Here, we described a case series of 6 unrelated ADH1 probands, each one bearing a gain-of-function CaSR mutation, and two children of one of these cases, matching our identified mutations to the same ones previously reported in the literature, and comparing the clinical and biochemical characteristics, as well as the complication profile. As a result of these genetic and clinical comparisons, we propose that a genotype/phenotype correlation may exist because our cases showed similar presentation, characteristics, and severity, with respect to published cases with the same or similar mutations. We also contend that the severity of the presentation is highly influenced by the specific CaSR variant. These findings, however, require further evaluation and assessment with a systematic review.

Calcium requests in a primary care; An observational audit of biochemical requests and frequency of abnormal results

BACKGROUND/AIMS

This aim of this audit was to assess the extent of serum calcium testing and the frequency of hypercalcaemia in the primary care setting. We also assessed the appropriateness of subsequent investigations with repeat serum calcium and PTH testing if hypercalcaemia was identified.

METHODS

All laboratory requests for adjusted calcium and PTH samples sent from primary care in Glasgow were analysed over a 12 month period. This covered approximately 125 GP practices and a patient population of over 590,000.

RESULTS

There were 78,845 requests for adjusted calcium and 2053 PTH requests from 62,745 patients aged 16-105 years (median age 57, IQ range 30 years). Of these requests 1423 (2.3%) of patients had biochemical evidence of hypercalcaemia (adjusted calcium ≥ 2.61 mmol/L). Of the 1423 patients with hypercalcaemia, 368 patients (45.8%) had a single raised calcium level that was within the normal range on repeat testing. Of the 400 patients with persistent hypercalcaemia on 2 or more samples, 210 (52.5%) had a PTH measured. Eight patients had a PTH < 2.0 pmol/L, whilst 202 (96.1%) had a PTH ≥ 2.0 pmol/L (range 2.1-106.1 pmol/L).

CONCLUSIONS

Serum calcium was checked in 10.6% of the population per year within primary care. In the 2.4% with a raised calcium on initial testing, approximately half (45.8%) will normalise on repeat testing. Of those who remained persistently hypercalcaemic, only half (52.5%) had a PTH measured and the majority (96.1%) were in keeping with primary hyperparathyroidism being the most common cause of hypercalcaemia.

Airway smooth muscle in contractility and remodeling of asthma: potential drug target mechanisms

INTRODUCTION

Asthma is characterized by enhanced airway contractility and remodeling where airway smooth muscle (ASM) plays a key role, modulated by inflammation. Understanding the mechanisms by which ASM contributes to these features of asthma is essential for the development of novel asthma therapies.

AREAS COVERED

Inflammation in asthma contributes to a multitude of changes within ASM including enhanced airway contractility, proliferation, and fibrosis. Altered intracellular calcium ([Ca²⁺]_i) regulation or Ca²⁺ sensitization contributes to airway hyperreactivity. Increased airway wall thickness from ASM proliferation and fibrosis contributes to structural changes seen with asthma.

EXPERT OPINION

ASM plays a significant role in multiple features of asthma. Increased ASM contractility contributes to hyperresponsiveness, while altered ASM proliferation and extracellular matrix production promote airway remodeling both influenced by inflammation of asthma and conversely even influencing the local inflammatory milieu. While standard therapies such as corticosteroids or biologics target inflammation, cytokines, or their receptors to alleviate asthma symptoms, these approaches do not address the underlying contribution of ASM to hyperresponsiveness and particularly remodeling. Therefore, novel therapies for asthma need to target abnormal contractility mechanisms in ASM and/or the contribution of ASM to remodeling, particularly in asthmatics resistant to current therapies.

Hypoparathyroidism: Genetics and Diagnosis

This narrative report summarizes diagnostic criteria for hypoparathyroidism and describes the clinical presentation and underlying genetic causes of the nonsurgical forms. We conducted a comprehensive literature search from January 2000 to January 2021 and included landmark articles before 2000, presenting a comprehensive update of these topics and suggesting a research agenda to improve diagnosis and, eventually, the prognosis of the disease. Hypoparathyroidism, which is characterized by insufficient secretion of parathyroid hormone (PTH) leading to hypocalcemia, is diagnosed on biochemical grounds. Low albumin-adjusted calcium or ionized calcium with concurrent inappropriately low serum PTH concentration are the hallmarks of the disease. In this review, we discuss the characteristics and pitfalls in measuring calcium and PTH. We also undertook a systematic review addressing the utility of measuring calcium and PTH within 24 hours after total thyroidectomy to predict long-term hypoparathyroidism. A summary of the findings is presented here; results of the detailed systematic review are published separately in this issue of JBMR. Several genetic disorders can present with hypoparathyroidism, either as an isolated disease or as part of a syndrome. A positive family history and, in the case of complex diseases, characteristic comorbidities raise the clinical suspicion of a genetic disorder. In addition to these disorders' phenotypic characteristics, which include autoimmune diseases, we discuss approaches for the genetic diagnosis. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Epidemiology, Pathophysiology, and Genetics of Primary Hyperparathyroidism

In this narrative review, we present data gathered over four decades (1980-2020) on the epidemiology, pathophysiology and genetics of primary hyperparathyroidism (PHPT). PHPT is typically a disease of postmenopausal women, but its prevalence and incidence vary globally and depend on a number of factors, the most important being the availability to measure serum calcium and parathyroid hormone levels for screening. In the Western world, the change in presentation to asymptomatic PHPT is likely to occur, over time also, in Eastern regions. The selection of the population to be screened will, of course, affect the epidemiological data (ie, general practice as opposed to tertiary center). Parathyroid hormone has a pivotal role in regulating calcium homeostasis; small changes in extracellular Ca++ concentrations are detected by parathyroid cells, which express calcium-sensing receptors (CaSRs). Clonally dysregulated overgrowth of one or more parathyroid glands together with reduced expression of CaSRs is the most important pathophysiologic basis of PHPT. The spectrum of skeletal disease reflects different degrees of dysregulated bone remodeling. Intestinal calcium hyperabsorption together with increased bone resorption lead to increased filtered load of calcium that, in addition to other metabolic factors, predispose to the appearance of calcium-containing kidney stones. A genetic basis of PHPT can be identified in about 10% of all cases. These may occur as a part of multiple endocrine neoplasia syndromes (MEN1-MEN4), or the hyperparathyroidism jaw-tumor syndrome, or it may be caused by nonsyndromic isolated endocrinopathy, such as familial isolated PHPT and neonatal severe hyperparathyroidism. DNA testing may have value in: confirming the clinical diagnosis in a proband; eg, by distinguishing PHPT from familial hypocalciuric hypercalcemia (FHH). Mutation-specific carrier testing can be performed on a proband's relatives and identify where the proband is a mutation carrier, ruling out phenocopies that may confound the diagnosis; and potentially prevention via prenatal/preimplantation diagnosis. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

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.

Autosomal Dominant Hypocalcemia Type 1: A Systematic Review

Autosomal dominant hypocalcemia type 1 (ADH1) is a rare form of hypoparathyroidism due to activating variants of the calcium-sensing receptor gene (CASR). Inherited or de novo activating variants of the CASR alter the set point for extracellular calcium, resulting in inadequate parathyroid hormone (PTH) secretion and inappropriate renal calcium excretion leading to hypocalcemia and hypercalciuria. Conventional therapy includes calcium and activated vitamin D, which can worsen hypercalciuria, resulting in renal complications. A systematic literature review, using published reports from 1994 to 2021, was conducted to catalog CASR variants, to define the ADH1 clinical spectrum, and to determine the effect of treatment on patients with ADH1. There were 113 unique CASR variants reported, with a general lack of genotype/phenotype correlation. Clinical data were available in 191 patients; 27% lacked symptoms, 32% had mild/moderate symptoms, and 41% had severe symptoms. Seizures, the most frequent clinical presentation, occurred in 39% of patients. In patients with blood and urine chemistries available at the time of diagnosis (n = 91), hypocalcemia (99%), hyperphosphatemia (59%), low PTH levels (57%), and hypercalciuria (34%) were observed. Blood calcium levels were significantly lower in patients with severe symptoms compared with asymptomatic patients (6.8 ± 0.7 versus 7.6 ± 0.7 mg/dL [mean ± SD]; p < 0.0001), and the age of presentation was significantly lower in severely symptomatic patients (9.1 ± 15.0 versus 19.3 ± 19.4 years; p < 0.01). Assessments for complications including nephrocalcinosis, nephrolithiasis, renal impairment, and brain calcifications in 57 patients on conventional therapy showed that 75% had at least one complication. Hypercalciuria was associated with nephrocalcinosis, nephrolithiasis, renal impairment, or brain calcifications (odds ratio [OR] = 9.3; 95% confidence interval [CI] 2.4-37.2; p < 0.01). In 27 patients with urine calcium measures before and after starting conventional therapy, the incidence of hypercalciuria increased by 91% (p < 0.05) after therapy initiation. ADH1 is a condition often associated with severe symptomatology at presentation with an increase in the risk of renal complications after initiation of conventional therapy. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Frequency of truncating FLCN variants and Birt-Hogg-Dubé-associated phenotypes in a health care system population

PURPOSE

Penetrance estimates of Birt-Hogg-Dubé syndrome (BHD)-associated cutaneous, pulmonary, and kidney manifestations are based on clinically ascertained families. In a health care system population, we used a genetics-first approach to estimate the prevalence of pathogenic/likely pathogenic (P/LP) truncating variants in FLCN, which cause BHD, and the penetrance of BHD-related phenotypes.

METHODS

Exomes from 135,990 patient-participants in Geisinger's MyCode cohort were assessed for P/LP truncating FLCN variants. BHD-related phenotypes were evaluated from electronic health records. Association between P/LP FLCN variants and BHD-related phenotypes was assessed using Firth's logistic regression.

RESULTS

P/LP truncating FLCN variants were identified in 35 individuals (1 in 3234 unrelated individuals), 68.6% of whom had BHD-related phenotype(s), including cystic lung disease (65.7%), pneumothoraces (17.1%), cutaneous manifestations (8.6%), and kidney cancer (2.9%). A total of 4 (11.4%) individuals had prior clinical BHD diagnoses.

CONCLUSION

In this health care population, the frequency of P/LP truncating FLCN variants is 60 times higher than the previously reported prevalence. Although most variant-positive individuals had BHD-related phenotypes, a minority were previously clinically diagnosed, likely because cutaneous manifestations, pneumothoraces, and kidney cancer were observed at lower frequencies than in clinical cohorts. Improved clinical recognition of cystic lung disease and education concerning its association with FLCN variants could prompt evaluation for BHD.

[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.

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.

Autosomal dominant hypocalcemia type 1 (ADH1) associated with myoclonus and intracerebral calcifications

Autosomal dominant hypocalcemia type 1 (ADH1) is a disorder of extracellular calcium homeostasis caused by germline gain-of-function mutations of the calcium-sensing receptor (CaSR). Over 35% of ADH1 patients have intracerebral calcifications predominantly affecting the basal ganglia. The clinical consequences of such calcifications remain to be fully characterized, although the majority of patients with these calcifications are considered to be asymptomatic. We report a 20-year-old female proband with a severe form of ADH1 associated with recurrent hypocalcemic and hypercalcemic episodes, persistent childhood hyperphosphatemia, and a low calcium/phosphate ratio. From the age of 18 years, she had experienced recurrent myoclonic jerks affecting the upper limbs that were not associated with epileptic seizures, extra-pyramidal features, cognitive impairment, or alterations in serum calcium concentrations. Computerised tomography (CT) scans revealed calcifications of the globus pallidus regions of the basal ganglia bilaterally, and also the frontal lobes at the grey-white matter junction, and posterior horn choroid plexuses. The patient’s myoclonus resolved following treatment with levetiracetam. CASR mutational analysis identified a reported germline gain-of-function heterozygous missense mutation, c.2363T&gt;G; p.(Phe788Cys), which affects an evolutionarily conserved phenylalanine residue located in transmembrane domain helix 5 of the CaSR protein. Analysis of the cryo-electron microscopy CaSR structure predicted the wild-type Phe788 residue to form interactions with neighbouring phenylalanine residues, which likely maintain the CaSR in an inactive state. The p.(Phe788Cys) mutation was predicted to disrupt these interactions, thereby leading to CaSR activation. These findings reveal myoclonus as a novel finding in an ADH1 patient with intracerebral calcifications.

Functional Assessment of Calcium-Sensing Receptor Variants Confirms Familial Hypocalciuric Hypercalcaemia

Context

In the clinic it is important to differentiate primary hyperparathyroidism (PHPT) from the more benign, inherited disorder, familial hypocalciuric hypercalcemia (FHH). Since the conditions may sometimes overlap biochemically, identification of calcium-sensing receptor (CASR) gene variants causative of FHH (but not PHPT) is the most decisive diagnostic aid. When novel variants are identified, bioinformatics and functional assessment are required to establish pathogenicity.

Objective

We identified 3 novel CASR transmembrane domain missense variants, Thr699Asn, Arg701Gly, and Thr808Pro, in 3 probands provisionally diagnosed with FHH and examined the variants using bioinformatics and functional analysis.

Methods

Bioinformatics assessment utilized wANNOVAR software. For functional characterization, each variant was cloned into a mammalian expression vector; wild-type and variant receptors were transfected into HEK293 cells, and their expression and cellular localization were assessed by Western blotting and confocal immunofluorescence, respectively. Receptor activation in HEK293 cells was determined using an IP-One ELISA assay following stimulation with Ca⁺⁺ ions.

Results

Bioinformatics analysis of the variants was unable to definitively assign pathogenicity. Compared with wild-type receptor, all variants demonstrated impaired expression of mature receptor reaching the cell surface and diminished activation at physiologically relevant Ca⁺⁺ concentrations.

Conclusion

Three CASR missense variants identified in probands provisionally diagnosed with FHH result in receptor inactivation and are therefore likely causative of FHH. Inactivation may be due to inadequate processing/trafficking of mature receptor and/or conformational changes induced by the variants affecting receptor signaling. This study demonstrates the value of functional studies in assessing genetic variants identified in hypercalcemic patients.

European Expert Consensus on Practical Management of Specific Aspects of Parathyroid Disorders in Adults and in Pregnancy: Recommendations of the ESE Educational Program of Parathyroid Disorders

This European expert consensus statement provides recommendations for the diagnosis and management of primary hyperparathyroidism (PHPT), chronic hypoparathyroidism in adults (HypoPT), and parathyroid disorders in relation to pregnancy and lactation. Specified areas of interest and unmet needs identified by experts at the second ESE Educational Program of Parathyroid Disorders (PARAT) in 2019, were discussed during two virtual workshops in 2021, and subsequently developed by working groups with interest in the specified areas. PHPT is a common endocrine disease. However, its differential diagnosing to familial hypocalciuric hypercalcemia (FHH), the definition and clinical course of normocalcemic PHPT, and the optimal management of its recurrence after surgery represent areas of uncertainty requiring clarifications. HypoPT is an orphan disease characterized by low calcium concentrations due to insufficient PTH secretion, most often secondary to neck surgery. Prevention and prediction of surgical injury to the parathyroid glands are essential to limit the disease-related burden. Long-term treatment modalities including the place for PTH replacement therapy and the optimal biochemical monitoring and imaging surveillance for complications to treatment in chronic HypoPT, need to be refined. The physiological changes in calcium metabolism occurring during pregnancy and lactation modify the clinical presentation and management of parathyroid disorders in these periods of life. Modern interdisciplinary approaches to PHPT and HypoPT in pregnant and lactating women and their newborns children are proposed. The recommendations on clinical management presented here will serve as background for further educational material aimed for a broader clinical audience, and were developed with focus on endocrinologists in training.

Topical therapy with negative allosteric modulators of the calcium-sensing receptor (calcilytics) for the management of asthma: the beginning of a new era?

In this review article we present the evidence to date supporting the role of the calcium-sensing receptor (CaSR) as a key, pluripotential molecular trigger for asthma and speculate on the likely benefits of topical therapy of asthma with negative allosteric modulators of the CaSR: calcilytics.

A contemporary clinical approach to genetic testing for heritable hyperparathyroidism syndromes

PURPOSE

The improved access and affordability of next generation sequencing has facilitated the clinical use of gene panel testing to test concurrently patients for multiple heritable hyperparathyroidism syndromes. However, there is little guidance as to which patients should be selected for gene panel testing and which genes should be included in such panels. In this review, we provide a practical approach to considering, interpreting and managing genetic testing for familial primary hyperparathyroidism (PHPT) syndromes and familial hypocalciuric hypercalcaemia (FHH) in patients with PTH-dependent hypercalcaemia. We discuss known genes implicated in PHPT and FHH, testing criteria and yields, pre-test counselling, laboratory considerations, and post-test management.

METHODS

In addition to reviewing the literature, we conducted audits of local genetic testing data to examine the real-world yield of genetic testing in patients with PTH-dependent hypercalcaemia.

RESULTS

Our local audits revealed a positive genetic testing rate of 15-26% in patients with suspected hyperparathyroidism syndromes.

CONCLUSION

Based on the particular testing criteria met, affected patients should be tested for variants in the genes currently implicated in PHPT (MEN1, CDC73, RET, CDKN1B, GCM2, CASR) and/or FHH (CASR, GNA11, AP2S1). Patients should be provided with pre- and post-test counselling, including consideration of potential implications for family members.

Compendium of causative genes and their encoded proteins for common monogenic disorders

A compendium is presented of inherited monogenic disorders that have a prevalence of >1:20,000 in the human population, along with their causative genes and encoded proteins. "Simple" monogenic diseases are those for which the clinical features are caused by mutations impacting a single gene, usually in a manner that alters the sequence of the encoded protein. Of course, for a given "monogenic disorder", there is sometimes more than one potential disease gene, mutations in any one of which is sufficient to cause phenotypes of that disorder. Disease-causing mutations for monogenic disorders are usually passed on from generation to generation in a Mendelian fashion, and originate from spontaneous (de novo) germline founder mutations. In the past monogenic disorders have often been written off as targets for drug discovery because they sometimes are assumed to be rare disorders, for which the meager projected financial payoff of drug discovery and development has discouraged investment. However, not all monogenic diseases are rare. Here, we report that that currently available data identifies 72 disorders with a prevalence of at least 1 in 20,000 humans. For each, we tabulate the gene(s) for which mutations cause the spectrum of phenotypes associated with that disorder. We also identify the gene and protein that most commonly causes each disease. 34 of these disorders are caused exclusively by mutations in only a single gene and encoded protein.

Hereditary Primary Hyperparathyroidism

Primary hyperparathyroidism (PHPT) is a commonly encountered clinical problem and occurs as part of an inherited disorder in ∼10% of patients. Several features may alert the clinician to the possibility of a hereditary PHPT disorder (eg, young age of disease onset) whilst establishing any relevant family history is essential to the clinical evaluation and will help inform the diagnosis. Genetic testing should be offered to patients at risk of a hereditary PHPT disorder, as this may improve management and allow the identification and investigation of other family members who may also be at risk of disease.

Structural basis for activation and allosteric modulation of full-length calcium-sensing receptor

Calcium-sensing receptor (CaSR) is a class C G protein-coupled receptor (GPCR) that plays an important role in calcium homeostasis and parathyroid hormone secretion. Here, we present multiple cryo-electron microscopy structures of full-length CaSR in distinct ligand-bound states. Ligands (Ca²⁺ and l-tryptophan) bind to the extracellular domain of CaSR and induce large-scale conformational changes, leading to the closure of two heptahelical transmembrane domains (7TMDs) for activation. The positive modulator (evocalcet) and the negative allosteric modulator (NPS-2143) occupy the similar binding pocket in 7TMD. The binding of NPS-2143 causes a considerable rearrangement of two 7TMDs, forming an inactivated TM6/TM6 interface. Moreover, a total of 305 disease-causing missense mutations of CaSR have been mapped to the structure in the active state, creating hotspot maps of five clinical endocrine disorders. Our results provide a structural framework for understanding the activation, allosteric modulation mechanism, and disease therapy for class C GPCRs.

The Complexities of Organ Crosstalk in Phosphate Homeostasis: Time to Put Phosphate Sensing Back in the Limelight

Phosphate homeostasis is essential for health and is achieved via interaction between the bone, kidney, small intestine, and parathyroid glands and via intricate processes involving phosphate transporters, phosphate sensors, and circulating hormones. Numerous genetic and acquired disorders are associated with disruption in these processes and can lead to significant morbidity and mortality. The role of the kidney in phosphate homeostasis is well known, although it is recognized that the cellular mechanisms in murine models and humans are different. Intestinal phosphate transport also appears to differ in humans and rodents, with recent studies demonstrating a dominant role for the paracellular pathway. The existence of phosphate sensing has been acknowledged for decades; however, the underlying molecular mechanisms are poorly understood. At least three phosphate sensors have emerged. PiT2 and FGFR1c both act as phosphate sensors controlling Fibroblast Growth Factor 23 secretion in bone, whereas the calcium-sensing receptor controls parathyroid hormone secretion in response to extracellular phosphate. All three of the proposed sensors are expressed in the kidney and intestine but their exact function in these organs is unknown. Understanding organ interactions and the mechanisms involved in phosphate sensing requires significant research to develop novel approaches for the treatment of phosphate homeostasis disorders.

The Molecular Basis of Calcium and Phosphorus Inherited Metabolic Disorders

Calcium (Ca) and Phosphorus (P) hold a leading part in many skeletal and extra-skeletal biological processes. Their tight normal range in serum mirrors their critical role in human well-being. The signalling “voyage” starts at Calcium Sensing Receptor (CaSR) localized on the surface of the parathyroid glands, which captures the “oscillations” of extracellular ionized Ca and transfers the signal downstream. Parathyroid hormone (PTH), Vitamin D, Fibroblast Growth Factor (FGF23) and other receptors or ion-transporters, work synergistically and establish a highly regulated signalling circuit between the bone, kidneys, and intestine to ensure the maintenance of Ca and P homeostasis. Any deviation from this well-orchestrated scheme may result in mild or severe pathologies expressed by biochemical and/or clinical features. Inherited disorders of Ca and P metabolism are rare. However, delayed diagnosis or misdiagnosis may cost patient’s quality of life or even life expectancy. Unravelling the thread of the molecular pathways involving Ca and P signaling, we can better understand the link between genetic alterations and biochemical and/or clinical phenotypes and help in diagnosis and early therapeutic intervention.

Therapeutic Opportunities of Targeting Allosteric Binding Sites on the Calcium-Sensing Receptor

The CaSR is a class C G protein-coupled receptor (GPCR) that acts as a multimodal chemosensor to maintain diverse homeostatic functions. The CaSR is a clinical therapeutic target in hyperparathyroidism and has emerged as a putative target in several other diseases. These include hyper- and hypocalcaemia caused either by mutations in the CASR gene or in genes that regulate CaSR signaling and expression, and more recently in asthma. The development of CaSR-targeting drugs is complicated by the fact that the CaSR possesses many different binding sites for endogenous and exogenous agonists and allosteric modulators. Binding sites for endogenous and exogenous ligands are located throughout the large CaSR protein and are interconnected in ways that we do not yet fully understand. This review summarizes our current understanding of CaSR physiology, signaling, and structure and how the many different binding sites of the CaSR may be targeted to treat disease.

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.

Calcium-sensing receptor signaling - How human disease informs biology

The calcium-sensing receptor (CaSR) is a class C G-protein-coupled receptor (GPCR) that plays a fundamental role in extracellular calcium homeostasis by regulating parathyroid hormone (PTH) release. Although the CaSR was identified over 25 years ago, new mechanistic details of how the CaSR controls PTH secretion have recently been uncovered demonstrating heteromerization and phosphate binding affect CaSR-mediated suppression of PTH release. In addition, understanding of how the CaSR performs diverse functions in different cellular contexts is just beginning to be elucidated, with new evidence of tissue-specific regulation, and endo-somal signaling. Insights into CaSR activation mechanisms and signaling bias have arisen from studies of CaSR mutations, which cause disorders of calcium homeostasis. Functional assessment of these mutations demonstrated the importance of the homodimer interface and transmembrane domain in biased signaling and showed CaSR mutations can facilitate G-protein-independent signaling. Population genetics studies have allowed a greater understanding of the prevalence of calcemic disorders and revealed new pathophysiological roles.

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.

Calcilytic NPSP795 Increases Plasma Calcium and PTH in an Autosomal Dominant Hypocalcemia Type 1 Mouse Model

Calcilytics are calcium‐sensing receptor (CaSR) antagonists that reduce the sensitivity of the CaSR to extracellular calcium. Calcilytics have the potential to treat autosomal dominant hypocalcemia type 1 (ADH1), which is caused by germline gain‐of‐function CaSR mutations and leads to symptomatic hypocalcemia, inappropriately low PTH concentrations, and hypercalciuria. To date, only one calcilytic compound, NPSP795, has been evaluated in patients with ADH1: Doses of up to 30 mg per patient have been shown to increase PTH concentrations, but did not significantly alter ionized blood calcium concentrations. The aim of this study was to further investigate NPSP795 for the treatment of ADH1 by undertaking in vitro and in vivo studies involving Nuf mice, which have hypocalcemia in association with a gain‐of‐function CaSR mutation, Leu723Gln. Treatment of HEK293 cells stably expressing the mutant Nuf (Gln723) CaSR with 20nM NPSP795 decreased extracellular Ca²⁺‐mediated intracellular calcium and phosphorylated ERK responses. An in vivo dose‐ranging study was undertaken by administering a s.c. bolus of NPSP795 at doses ranging from 0 to 30 mg/kg to heterozygous (Casr^+/Nuf) and to homozygous (Casr^Nuf/Nuf) mice, and measuring plasma PTH responses at 30 min postdose. NPSP795 significantly increased plasma PTH concentrations in a dose‐dependent manner with the 30 mg/kg dose causing a maximal (≥10‐fold) rise in PTH. To determine whether NPSP795 can rectify the hypocalcemia of Casr^+/Nuf and Casr^Nuf/Nuf mice, a submaximal dose (25 mg/kg) was administered, and plasma adjusted‐calcium concentrations measured over a 6‐hour period. NPSP795 significantly increased plasma adjusted‐calcium in Casr^+/Nuf mice from 1.87 ± 0.03 mmol/L to 2.16 ± 0.06 mmol/L, and in Casr^Nuf/Nuf mice from 1.70 ± 0.03 mmol/L to 1.89 ± 0.05 mmol/L. Our findings show that NPSP795 elicits dose‐dependent increases in PTH and ameliorates the hypocalcemia in an ADH1 mouse model. Thus, calcilytics such as NPSP795 represent a potential targeted therapy for ADH1. © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.