Calcium-sensing receptor (CaSR) mutations and disorders of calcium, electrolyte and water metabolism

Persistent hypercalcaemia associated with two pathogenic variants in the CYP24A1 gene and a parathyroid adenoma-a case report and review

Introduction

24-Hydroxylase, encoded by the CYP24A1 gene, is a crucial enzyme involved in the catabolism of vitamin D. Loss-of-function mutations in CYP24A1 result in PTH-independent hypercalcaemia with high levels of 1,25(OH)2D3. The variety of clinical manifestations depends on age, and underlying genetic predisposition mutations can lead to fatal infantile hypercalcaemia among neonates, whereas adult symptoms are usually mild.

Aim of the study

We report a rare case of an adult with primary hyperparathyroidism and loss-of-function mutations in the CYP24A1 gene and a review of similar cases.

Case presentation

We report the case of a 58-year-old woman diagnosed initially with primary hyperparathyroidism. Preoperatively, the suspected mass adjoining the upper pole of the left lobe of the thyroid gland was found via ultrasonography and confirmed by 99mTc scintigraphy and biopsy as the parathyroid gland. The patient underwent parathyroidectomy (a histopathology report revealed parathyroid adenoma), which led to normocalcaemia. After 10 months, vitamin D supplementation was introduced due to deficiency, and the calcium level remained within the reference range. Two years later, biochemical tests showed recurrence of hypercalcaemia with suppressed parathyroid hormone levels and elevated 1,25(OH)2D3 concentrations. Further investigation excluded the most common causes of PTH-independent hypercalcaemia, such as granulomatous disease, malignancy, and vitamin D intoxication. Subsequently, vitamin D metabolites were measured using LC-MS/MS, which revealed high levels of 25(OH)D3, low levels of 24,25(OH)2D3 and elevated 25(OH)2D3/24,25(OH)2D3 ratios, suggesting a defect in vitamin D catabolism. Molecular analysis of the CYP24A1 gene using the NGS technique revealed two pathogenic variants: p.(Arg396Trp) and p.(Glu143del) (rs114368325 and rs777676129, respectively).

Conclusions

The diagnostic process for hypercalcaemia becomes complicated when multiple causes of hypercalcaemia coexist. The measurement of vitamin D metabolites using LC-MS/MS may help to identify carriers of CYP24A1 mutations. Subsequent molecular testing may contribute to establishing the exact frequency of pathogenic variants of the CYP24A1 gene and introducing personalized treatment.

Small vs. Large Library Docking for Positive Allosteric Modulators of the Calcium Sensing Receptor

Drugs acting as positive allosteric modulators (PAMs) to enhance the activation of the calcium sensing receptor (CaSR) and to suppress parathyroid hormone (PTH) secretion can treat hyperparathyroidism but suffer from side effects including hypocalcemia and arrhythmias. Seeking new CaSR modulators, we docked libraries of 2.7 million and 1.2 billion molecules against transforming pockets in the active-state receptor dimer structure. Consistent with simulations suggesting that docking improves with library size, billion-molecule docking found new PAMs with a hit rate that was 2.7-fold higher than the million-molecule library and with hits up to 37-fold more potent. Structure-based optimization of ligands from both campaigns led to nanomolar leads, one of which was advanced to animal testing. This PAM displays 100-fold the potency of the standard of care, cinacalcet, in ex vivo organ assays, and reduces serum PTH levels in mice by up to 80% without the hypocalcemia typical of CaSR drugs. Cryo-EM structures with the new PAMs show that they induce residue rearrangements in the binding pockets and promote CaSR dimer conformations that are closer to the G-protein coupled state compared to established drugs. These findings highlight the promise of large library docking for therapeutic leads, especially when combined with experimental structure determination and mechanism.

Genetics of hereditary forms of primary hyperparathyroidism

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

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

Context

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

Objective

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

Design

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

Results

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

Conclusion

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

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.

Low expression of SLC34A1 is associated with poor prognosis in clear cell renal cell carcinoma

OBJECTIVE

Clear cell renal cell carcinoma (ccRCC) is a malignant renal tumor that is highly prone to metastasis and recurrence. The exact pathogenesis of this cancer is still not well understood. This study aimed to identify novel hub genes in renal clear cell carcinoma and determine their diagnostic and prognostic value.

METHODS

Intersection genes were obtained from multiple databases, and protein-protein interaction analysis and functional enrichment analysis were performed to identify key pathways related to the intersection genes. Hub genes were identified using the cytoHubba plugin in Cytoscape. GEPIA and UALCAN were utilized to observe differences in mRNA and protein expression of hub genes between KIRC and adjacent normal tissues. The Wilcoxon rank sum test was used to analyze hub gene levels between paired KIRC and matched non-cancer samples. IHC results were obtained from the HPA online database, and according to the median gene expression level, they were divided into a high-expression group and a low-expression group. The correlation of these groups with the prognosis of KIRC patients was analyzed. Logistic regression and the Wilcoxon rank sum test were used to test the relationship between SLC34A1 level and clinicopathological features. The diagnostic value of SLC34A1 was evaluated by drawing the receiver operating characteristic (ROC) curve and calculating the area under the curve (AUC). Cox regression analysis was used to analyze the relationship between clinicopathological features, SLC34A1 expression, and KIRC survival rate. LinkedOmics was used to obtain the genes most related to SLC34A1 and their functional enrichment. Genetic mutations and methylation levels of SLC34A1 in KIRC were obtained from the cBioPortal website and the MethSurv website, respectively.

RESULTS

Fifty-eight ccRCC differential genes were identified from six datasets, and they were mainly enriched in 10 functional items and 4 pathways. A total of 5 hub genes were identified. According to the GEPIA database analysis, low expression of SLC34A1, CASR, and ALDOB in tumors led to poor prognosis. Low expression of SLC34A1 mRNA was found to be related to clinicopathological features of patients. SLC34A1 expression in normal tissues could accurately identify tumors (AUC 0.776). SLC34A1 was also found to be an independent predictor of ccRCC in univariate and multivariate Cox analyses. The mutation rate of the SLC34A1 gene was 13%. Eight of the 10 DNA methylated CpG sites were associated with the prognosis of ccRCC. SLC34A1 expression in ccRCC was positively correlated with B cells, eosinophils, neutrophils, T cells, TFH, and Th17 cells, and negatively correlated with Tem, Tgd, and Th2 cells.

CONCLUSION

The expression level of SLC34A1 in KIRC samples was found to be decreased, which predicted a decreased survival rate of KIRC. SLC34A1 may serve as a molecular prognostic marker and therapeutic target for KIRC patients.

Hyperaminoacidemia induces pancreatic α cell proliferation via synergism between the mTORC1 and CaSR-Gq signaling pathways

Glucagon has emerged as a key regulator of extracellular amino acid (AA) homeostasis. Insufficient glucagon signaling results in hyperaminoacidemia, which drives adaptive proliferation of glucagon-producing α cells. Aside from mammalian target of rapamycin complex 1 (mTORC1), the role of other AA sensors in α cell proliferation has not been described. Here, using both genders of mouse islets and glucagon receptor (gcgr)-deficient zebrafish (Danio rerio), we show α cell proliferation requires activation of the extracellular signal-regulated protein kinase (ERK1/2) by the AA-sensitive calcium sensing receptor (CaSR). Inactivation of CaSR dampened α cell proliferation, which was rescued by re-expression of CaSR or activation of Gq, but not Gi, signaling in α cells. CaSR was also unexpectedly necessary for mTORC1 activation in α cells. Furthermore, coactivation of Gq and mTORC1 induced α cell proliferation independent of hyperaminoacidemia. These results reveal another AA-sensitive mediator and identify pathways necessary and sufficient for hyperaminoacidemia-induced α cell proliferation.

The calcium-sensing receptor in inflammation: Recent updates

The Calcium-Sensing Receptor (CaSR) is a member of the class C of G-proteins coupled receptors (GPCRs), it plays a pivotal role in calcium homeostasis by directly controlling calcium excretion in the kidneys and indirectly by regulating parathyroid hormone (PTH) release from the parathyroid glands. The CaSR is found to be ubiquitously expressed in the body, playing a plethora of additional functions spanning from fluid secretion, insulin release, neuronal development, vessel tone to cell proliferation and apoptosis, to name but a few. The present review aims to elucidate and clarify the emerging regulatory effects that the CaSR plays in inflammation in several tissues, where it mostly promotes pro-inflammatory responses, with the exception of the large intestine, where contradictory roles have been recently reported. The CaSR has been found to be expressed even in immune cells, where it stimulates immune response and chemokinesis. On the other hand, CaSR expression seems to be boosted under inflammatory stimulus, in particular, by pro-inflammatory cytokines. Because of this, the CaSR has been addressed as a key factor responsible for hypocalcemia and low levels of PTH that are commonly found in critically ill patients under sepsis or after burn injury. Moreover, the CaSR has been found to be implicated in autoimmune-hypoparathyroidism, recently found also in patients treated with immune-checkpoint inhibitors. Given the tight bound between the CaSR, calcium and vitamin D metabolism, we also speculate about their roles in the pathogenesis of severe acute respiratory syndrome coronavirus-19 (SARS-COVID-19) infection and their impact on patients' prognosis. We will further explore the therapeutic potential of pharmacological targeting of the CaSR for the treatment and management of aberrant inflammatory responses.

Molecular dynamics study of tropical calcific pancreatitis (TCP) associated calcium-sensing receptor single nucleotide variation

Tropical Calcific Pancreatitis (TCP) is a chronic non-alcoholic pancreatitis characterised by extensive calcification. The disease usually appears at a younger age and is more common in tropical regions. This disease’s progression can lead to pancreatic diabetes, which can subsequently lead to pancreatic cancer. The CASR gene encodes a calcium-sensing receptor (CaSR), which is a GPCR protein of class C. It is expressed in the islets of Langerhans, the parathyroid gland, and other tissues. It primarily detects small gradients in circulating calcium concentrations and couples this information to intracellular signalling, which helps to regulate PTH (parathyroid hormone) secretion and mineral ion homeostasis. From co-leading insulin release, CaSR modulates ductal HCO₃₋ secretion, Ca²⁺ concentration, cell-cell communication, β-cell proliferation, and intracellular Ca²⁺ release. In pancreatic cancer, the CaSR limits cell proliferation. TCP-related four novel missense mutations P163R, I427S, D433H and V477A, found in CaSR extracellular domain (ECD) protein, which were reported in the mutTCPdb Database (https://lms.snu.edu.in/mutTCPDB/index.php). P163R mutation occurs in ligand-binding domain 1 (LBD-1) of the CaSR ECD. To investigate the influence of these variations on protein function and structural activity multiple in-silico prediction techniques such as SIFT, PolyPhen, CADD scores, and other methods have been utilized. A 500 ns molecular dynamic simulation was performed on the CaSR ECD crystal structure and the corresponding mutated models. Furthermore, Principal Component Analysis (PCA) and Essential Dynamics analysis were used to forecast collective motions, thermodynamic stabilities, and the critical subspace crucial to CaSR functions. The results of molecular dynamic simulations showed that the mutations P163R, I427S, D433H, and V477A caused conformational changes and decreased the stability of protein structures. This study also demonstrates the significance of TCP associated mutations. As a result of our findings, we hypothesised that the investigated mutations may have an effect on the protein’s structure and ability to interact with other molecules, which may be related to the protein’s functional impairment.

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.

The genetics of kidney stone disease and nephrocalcinosis

Kidney stones (also known as urinary stones or nephrolithiasis) are highly prevalent, affecting approximately 10% of adults worldwide, and the incidence of stone disease is increasing. Kidney stone formation results from an imbalance of inhibitors and promoters of crystallization, and calcium-containing calculi account for over 80% of stones. In most patients, the underlying aetiology is thought to be multifactorial, with environmental, dietary, hormonal and genetic components. The advent of high-throughput sequencing techniques has enabled a monogenic cause of kidney stones to be identified in up to 30% of children and 10% of adults who form stones, with ~35 different genes implicated. In addition, genome-wide association studies have implicated a series of genes involved in renal tubular handling of lithogenic substrates and of inhibitors of crystallization in stone disease in the general population. Such findings will likely lead to the identification of additional treatment targets involving underlying enzymatic or protein defects, including but not limited to those that alter urinary biochemistry.

Is routine 24-hour urine calcium measurement useful during the evaluation of primary hyperparathyroidism?

BACKGROUND

Primary hyperparathyroidism and familial hypocalciuric hypercalcemia have similar biochemical profiles, and calcium-to-creatinine-clearance ratio helps distinguish the two. Additionally, 24-hour urine calcium >400 mg/day indicates surgery and guidelines recommend obtaining 24-hour urine calcium preoperatively. Our aim was to assess how 24-hour urine calcium altered care in the evaluation of suspected primary hyperparathyroidism.

METHODS

Consecutive patients assessed for primary hyperparathyroidism from 2018 to 2020 were reviewed. Primary hyperparathyroidism was diagnosed by 2016 American Association of Endocrine Surgeons Parathyroidectomy Guidelines criteria. 24-hour urine calcium-directed change in care was defined as familial hypocalciuric hypercalcemia diagnosis, surgical deferment for additional testing, or 24-hour urine calcium >400 mg/day as the sole surgical indication.

RESULTS

Of 613 patients, 565 (92%) completed 24-hour urine calcium and 477 (84%) had concurrent biochemical testing to calculate calcium-to-creatinine-clearance ratio. 24-hour urine calcium was <100 mg/day in 9% (49/565) and calcium-to-creatinine-clearance ratio was <0.01 in 17% (82/477). No patient had confirmed familial hypocalciuric hypercalcemia, although 1 had a CASR variant of undetermined significance. When calcium-to-creatinine-clearance ratio was <0.01, familial hypocalciuric hypercalcemia was excluded by 24-hour urine calcium >100 mg/day (56%), prior normal calcium (16%), renal insufficiency (11%), absence of familial hypercalcemia (3%), normal repeat 24-hour urine calcium (10%), or interfering diuretic (1%). 24-hour urine calcium-directed change in care occurred in 25 (4%), including 4 (1%) who had genetic testing. Four-gland hyperplasia was more common with calcium-to-creatinine-clearance ratio <0.01 (17% vs calcium-to-creatinine-clearance ratio ≥ 0.01, 4%, P < .001), but surgical failure rates were equivalent (P = .24).

CONCLUSION

24-hour urine calcium compliance was high, and results affected management in 4%, including productive identification of hypercalciuria as the sole surgical indication in 2 patients. When calcium-to-creatinine-clearance ratio <0.01, clinical assessment was sufficient to exclude familial hypocalciuric hypercalcemia and only 1% required genetic testing. 24-hour urine calcium should be ordered judiciously during primary hyperparathyroidism assessment.

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.

Parathyroid Disease in Pregnancy and Lactation: A Narrative Review of the Literature

Pregnancy and lactation are characterized by sophisticated adaptations of calcium homeostasis, aiming to meet fetal, neonatal, and maternal calcium requirements. Pregnancy is primarily characterized by an enhancement of intestinal calcium absorption, whereas during lactation additional calcium is obtained through resorption from the maternal skeleton, a process which leads to bone loss but is reversible following weaning. These maternal adaptations during pregnancy and lactation may influence or confound the presentation, diagnosis, and management of parathyroid disorders such as primary hyperparathyroidism or hypoparathyroidism. Parathyroid diseases are uncommon in these settings but can be severe when they occur and may affect both maternal and fetal health. This review aims to delineate the changes in calcium physiology that occur with pregnancy and lactation, describe the disorders of calcium and parathyroid physiology that can occur, and outline treatment strategies for these diseases in the above settings.

Hypercalcemia during pregnancy: management and outcomes for mother and child

Diagnosing and treating hypercalcemia during pregnancy can be challenging due to both the physiological changes in calcium homeostasis and the underlying cause for the hypercalcemia. During pregnancy and lactation there is increased mobilization of calcium in the mother to meet the fetus' calcium requirements. Here we discuss the diagnostic challenges, management, and patient perspective of hypercalcemia during pregnancy in two particular cases and in other rare conditions causing hypercalcemia.

Role of the calcium-sensing receptor (CaSR) in cancer metastasis to bone: Identifying a potential therapeutic target

Cancer is a major cause of morbidity and mortality worldwide due to its ability to evade immune surveillance and metastasize from its origin to a secondary point of contact. Though several treatment techniques have been developed to suppress or manage cancer spread, a strategy for total control over the disease continues to evade researchers. In considering ways to control or prevent cancer from metastasizing to the bone, we analyze the impact of the calcium-sensing receptor (CaSR), whose primary role is to maintain calcium (Ca²⁺) homeostasis in cellular and systemic physiological processes. CaSR is a pleiotropic receptor capable of enhancing the proliferation of some cancers such as breast, lung, prostate and kidney cancers at its primary site(s) and stimulating bone metastasis, while exerting a suppressive effect in others such as colon cancer. The activity of CaSR not only increases cancer cell proliferation, migration and suppression of apoptosis in the organs indicated, but also increases the secretion of parathyroid hormone-related protein (PTHrP) and epiregulin, which induce osteolytic activity and osteoblastic suppression. In addition, released cytokines and Ca²⁺ from bone resorption are critical factors that further promote cancer proliferation. In this review, we seek to highlight previous viewpoints on CaSR, discuss its role in a new context, and consider its potential clinical application in cancer treatment.

Calcium Transport in the Kidney and Disease Processes

Calcium is a key ion involved in cardiac and skeletal muscle contractility, nerve function, and skeletal structure. Global calcium balance is affected by parathyroid hormone and vitamin D, and calcium is shuttled between the extracellular space and the bone matrix compartment dynamically. The kidney plays an important role in whole-body calcium balance. Abnormalities in the kidney transport proteins alter the renal excretion of calcium. Various hormonal and regulatory pathways have evolved that regulate the renal handling of calcium to maintain the serum calcium within defined limits despite dynamic changes in dietary calcium intake. Dysregulation of renal calcium transport can occur pharmacologically, hormonally, and via genetic mutations in key proteins in various nephron segments resulting in several disease processes. This review focuses on the regulation transport of calcium in the nephron. Genetic diseases affecting the renal handling of calcium that can potentially lead to changes in the serum calcium concentration are reviewed.

Biased signaling in naturally occurring mutations of G protein-coupled receptors associated with diverse human diseases

G protein-coupled receptors (GPCRs) play critical roles in transmitting a variety of extracellular signals into the cells and regulate diverse physiological functions. Naturally occurring mutations that result in dysfunctions of GPCRs have been known as the causes of numerous diseases. Significant progresses have been made in elucidating the pathophysiology of diseases caused by mutations. The multiple intracellular signaling pathways, such as G protein-dependent and β-arrestin-dependent signaling, in conjunction with recent advances on biased agonism, have broadened the view on the molecular mechanism of disease pathogenesis. This review aims to briefly discuss biased agonism of GPCRs (biased ligands and biased receptors), summarize the naturally occurring GPCR mutations that cause biased signaling, and propose the potential pathophysiological relevance of biased mutant GPCRs associated with various endocrine diseases.

Identification of genes involved in inbreeding depression of reproduction in Langshan chickens

Objective

Inbreeding depression of reproduction is a major concern in the conservation of native chicken genetic resources. Here, based on the successful development of strongly inbred (Sinb) and weakly inbred (Winb) Langshan chickens, we aimed to evaluate inbreeding effects on reproductive traits and identify candidate genes involved in inbreeding depression of reproduction in Langshan chickens.

Methods

A two-sample t-test was performed to estimate the differences in phenotypic values of reproductive traits between Sinb and Winb chicken groups. Three healthy chickens with reproductive trait values around the group mean values were selected from each of the groups. Differences in ovarian and hypothalamus transcriptomes between the two groups of chickens were analyzed by RNA sequencing (RNA-Seq).

Results

The Sinb chicken group showed an obvious inbreeding depression in reproduction, especially for traits of age at the first egg and egg number at 300 days (p<0.01). Furthermore, 68 and 618 differentially expressed genes (DEGs) were obtained in the hypothalamus and ovary between the two chicken groups, respectively. In the hypothalamus, DEGs were mainly enriched in the pathways related to vitamin metabolism, signal transduction and development of the reproductive system, such as the riboflavin metabolism, Wnt signaling pathway, extracellular matrix-receptor interaction and focal adhesion pathways, including stimulated by retinoic acid 6, serpin family F member 1, secreted frizzled related protein 2, Wnt family member 6, and frizzled class receptor 4 genes. In the ovary, DEGs were significantly enriched in pathways associated with basic metabolism, including amino acid metabolism, oxidative phosphorylation, and glycosaminoglycan degradation. A series of key DEGs involved in folate biosynthesis (gamma-glutamyl hydrolase, guanosine triphosphate cyclohydrolase 1), oocyte meiosis and ovarian function (cytoplasmic polyadenylation element binding protein 1, structural maintenance of chromosomes 1B, and speedy/RINGO cell cycle regulator family member A), spermatogenesis and male fertility (prostaglandin D2 synthase 21 kDa), Mov10 RISC complex RNA helicase like 1, and deuterosome assembly protein 1) were identified, and these may play important roles in inbreeding depression in reproduction.

Conclusion

The results improve our understanding of the regulatory mechanisms underlying inbreeding depression in chicken reproduction and provide a theoretical basis for the conservation of species resources.

[Hereditary syndromal and nonsyndromal forms of primary hyperparathyroidism]

Primary hyperparathyroidism is a common disorder of mineral homeostasis, characterized by overproduction of parathyroid hormone and upper normal or elevated calcium levels due to hyperplasia or a tumor of parathyroid gland. 90&minus;95% of cases of primary hyperparathyroidism are sporadic, while hereditary genetic forms occur in 5&ndash;10% of all cases. Primary hyperparathyroidism as the component of hereditary syndromes can present in various clinical forms (asymptomatic, symptomatic), can be associated with other endocrine or non-endocrine diseases, and require special approaches to treatment. Given that primary hyperparathyroidism is one of the most common components of these syndromes, it can be used as an important diagnostic tool in identifying affected families. This review is devoted to modern ideas about the clinical course and genetic characteristics of hereditary variants of primary hyperparathyroidism and the diagnostic and treatment algorithms recommended today. The review considers primary hyperparathyroidism as a component of hereditary syndromes including multiple endocrine neoplasias types 1, 2A and 4 and syndrome of hyperparathyroidism with a jaw tumor. Also non-syndromic hereditary forms are descripted, such as familial isolated hyperparathyroidism, familial hypocalciuric hypercalcemia, and severe neonatal primary hyperparathyroidism.

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

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

Autosomal dominant hypocalcaemia: identification of two novel variants of CASR gene

Autosomal dominant hypocalcaemia is a rare aetiology of hypocalcaemia, caused by gain-of-function mutations of the calcium-sensing receptor (CASR) gene. We present two cases of two asymptomatic women (50-year-old-case 1 and 25-year-old-case 2), referred to our endocrinology department for investigation of hypocalcaemia, hyperphosphatemia and inappropriately low parathormone. Both patients had relatives with the same laboratorial findings. At diagnosis, both patients presented basal ganglia calcifications. Genetic analysis was performed, identifying two novel heterozygous CASR variants: c.2269G>A (p.Glu757Lys) and c.2086C>G (p.Leu696Val), respectively, for case 1 and case 2. Affected individuals started oral calcium and vitamin D analogues, aiming to a low-normal calcium level. They remain under observation and are asymptomatic.

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

The calcium-sensing receptor (CaSR) regulates serum calcium concentrations. CASR loss- or gain-of-function mutations cause familial hypocalciuric hypercalcemia type 1 (FHH1) or autosomal-dominant hypocalcemia type 1 (ADH1), respectively, but the population prevalence of FHH1 or ADH1 is unknown. Rare CASR variants were identified in whole-exome sequences from 51,289 de-identified individuals in the DiscovEHR cohort derived from a single US healthcare system. We integrated bioinformatics pathogenicity triage, mean serum Ca concentrations, and mode of inheritance to identify potential FHH1 or ADH1 variants, and we used a Sequence Kernel Association Test (SKAT) to identify rare variant-associated diseases. We identified predicted heterozygous loss-of-function CASR variants (6 different nonsense/frameshift variants and 12 different missense variants) in 38 unrelated individuals, 21 of whom were hypercalcemic. Missense CASR variants were identified in two unrelated hypocalcemic individuals. Functional studies showed that all hypercalcemia-associated missense variants impaired heterologous expression, plasma membrane targeting, and/or signaling, whereas hypocalcemia-associated missense variants increased expression, plasma membrane targeting, and/or signaling. Thus, 38 individuals with a genetic diagnosis of FHH1 and two individuals with a genetic diagnosis of ADH1 were identified in the 51,289 cohort, giving a prevalence in this population of 74.1 per 100,000 for FHH1 and 3.9 per 100,000 for ADH1. SKAT combining all nonsense, frameshift, and missense loss-of-function variants revealed associations with cardiovascular, neurological, and other diseases. In conclusion, FHH1 is a common cause of hypercalcemia, with prevalence similar to that of primary hyperparathyroidism, and is associated with altered disease risks, whereas ADH1 is a major cause of non-surgical hypoparathyroidism.

In Vitro Control of Genes Critical for Parathyroid Embryogenesis by Extracellular Calcium

Background

The expression of the parathyroid transcription factors, encoded by the genes GATA3, GCM2, and MAFB, persists after parathyroid morphogenesis. This suggests a role of these genes in the regulatory program that governs parathyroid function in the adult. Indeed, these 3 genes form a transcriptional cascade able to activate PTH gene expression.

Materials and Methods

Adult adenoma parathyroid tissues were put in primary cell culture to evaluate the messenger ribonucleic acid (mRNA) expression of the PTH gene, of the genes involved in the calcium regulatory signaling pathway (CaSR, GNA11, and AP2S1), and of the 3 genes (GATA3, GCM2, and MAFB) involved in the parathyroid morphogenesis in the presence of different extracellular calcium concentrations from 0.1 mM to 3.0 mM.

Aim

The aim of the study was to investigate whether different extracellular calcium conditions could control the expression of transcription factors critical for parathyroid embryogenesis.

Results

The results of the experiments showed that the mRNA expression of GATA3, GCM2, and MAFB genes follows the same response as the PTH gene to extracellular calcium concentrations, with the highest expression at low calcium (0.1 mM) and the lowest at high calcium (3.0 mM). Conversely, the genes involved in the calcium signaling in the parathyroid cells showed a variable response to the extracellular calcium concentrations, with the CaSR and GNA11 genes exhibiting a sensitivity to low calcium concentrations.

Conclusions

These findings indicate that transcription factors recognized for their role in parathyroid embryogenesis show a response to extracellular calcium later in adulthood that parallels the behavior of the PTH gene.

Familial Hypocalciuric Hypercalcemia in Pregnancy: Diagnostic Pitfalls

Familial hypocalciuric hypercalcemia (FHH) is a group of autosomal dominant disorders caused by dysfunction of the calcium sensing receptor (CaSR) and its downstream signaling proteins, leading to generally asymptomatic hypercalcemia. During pregnancy, distinguishing FHH from primary hyperparathyroidism (PHPT) is important, as the latter is associated with adverse outcomes and can be treated surgically during pregnancy, whereas the former is benign. This case report highlights the difficulties in diagnosing FHH during pregnancy. A 32‐year‐old woman was found to have asymptomatic hypercalcemia at 14‐weeks’ gestation. Investigations showed a corrected calcium (cCa) of 2.61 mmol/L (2.10 to 2.60), ionized Ca (iCa) of 1.40 mmol/L (1.15 to 1.28), 25OHD of 33 nmol/L (75 to 250), and PTH of 9.5 pmol/L (1.5 to 7.0). The patient was treated with 2000 IU cholecalciferol daily with normalization of 25OHD. The urine calcium / creatinine clearance ratio (CCCR) was 0.0071, and neck US did not visualize a parathyroid adenoma. Upon a retrospective review of the patient's biochemistry from 2 years prior, hypercalcemia was found that was not investigated. The patient was monitored with serial iCa levels and obstetric US. She delivered a healthy boy at 38‐weeks’ gestation. Postnatal iCa was 1.48 mmol/L and remained elevated. Her son had elevated iCa at birth of 1.46 mmol/L (1.15 to 1.33), which rose to 1.81 mmol/L by 2 weeks. He was otherwise well. Given the familial hypercalcemia, a likely diagnosis of FHH was made. Genetic testing of the son revealed a missense mutation, NM_000388.3(CASR):c.2446A > G, in exon 7 of the CaSR, consistent with FHH type 1. To our knowledge, there are only three existing reports of FHH in pregnancy. When differentiating between FHH and PHPT in pregnancy, interpretation of biochemistry requires an understanding of changes in Ca physiology, and urine CCCR may be unreliable. If the decision is made to observe, clinical symptoms, calcium levels, and fetal US should be monitored, with biochemistry and urine CCCR performed postpartum, once lactation is completed © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Control of PTH secretion by the TRPC1 ion channel

Familial hypocalciuric hypercalcemia (FHH) is a genetic condition associated with hypocalciuria, hypercalcemia, and, in some cases, inappropriately high levels of circulating parathyroid hormone (PTH). FHH is associated with inactivating mutations in the gene encoding the Ca2+-sensing receptor (CaSR), a GPCR, and GNA11 encoding G protein subunit α 11 (Gα11), implicating defective GPCR signaling as the root pathophysiology for FHH. However, the downstream mechanism by which CaSR activation inhibits PTH production/secretion is incompletely understood. Here, we show that mice lacking the transient receptor potential canonical channel 1 (TRPC1) develop chronic hypercalcemia, hypocalciuria, and elevated PTH levels, mimicking human FHH. Ex vivo and in vitro studies revealed that TRPC1 serves a necessary and sufficient mediator to suppress PTH secretion from parathyroid glands (PTGs) downstream of CaSR in response to high extracellular Ca2+ concentration. Gα11 physically interacted with both the N- and C-termini of TRPC1 and enhanced CaSR-induced TRPC1 activity in transfected cells. These data identify TRPC1-mediated Ca2+ signaling as an essential component of the cellular apparatus controlling PTH secretion in the PTG downstream of CaSR.

A clinical perspective of parathyroid hormone related hypercalcaemia

There are many causes of hypercalcaemia including hyperparathyroidism, drugs, granulomatous disorders and malignancy. Parathyroid hormone (PTH) related hypercalcaemia is most commonly caused by primary hyperparathyroidism (PHPT) and more rarely by familial hypocalciuric hypercalcaemia (FHH). Algorithms for diagnosis of PTH related hypercalcaemia require assessment of a 24-h urinary calcium and creatinine excretion to calculate calcium/creatinine clearance ratio and radiological investigations including ultrasound scan and ^99mTc-sestamibi-SPECT/CT. To illustrate investigations and management of parathyroid-related hypercalcaemia, we present a selection of distinct cases of PHPT due to eutopic and ectopic parathyroid adenomas, as well as a case with a syndromic form of PHPT (multiple endocrine neoplasia type 1), and a case with FHH type 1 due to a CASR inactivating mutation. Additional cases with normocalcaemic hyperparathyroidism and secondary hyperparathyroidism are included for completeness of differential diagnosis. The common eutopic parathyroid adenomas are easily treated with parathyroidectomy while the less common ectopic parathyroid adenomas require more complex investigations and operative procedures such as video-assisted thoracoscopic surgery. On the other hand, the much less common FHH does not require treatment. Assessment of kin with FHH is important to identify members with this inherited condition in order to prevent unnecessary interventions.

Activating Mutations of the G-protein Subunit α 11 Interdomain Interface Cause Autosomal Dominant Hypocalcemia Type 2

CONTEXT

Autosomal dominant hypocalcemia types 1 and 2 (ADH1 and ADH2) are caused by germline gain-of-function mutations of the calcium-sensing receptor (CaSR) and its signaling partner, the G-protein subunit α 11 (Gα 11), respectively. More than 70 different gain-of-function CaSR mutations, but only 6 different gain-of-function Gα 11 mutations are reported to date.

METHODS

We ascertained 2 additional ADH families and investigated them for CaSR and Gα 11 mutations. The effects of identified variants on CaSR signaling were evaluated by transiently transfecting wild-type (WT) and variant expression constructs into HEK293 cells stably expressing CaSR (HEK-CaSR), and measuring intracellular calcium (Ca2+i) and MAPK responses following stimulation with extracellular calcium (Ca2+e).

RESULTS

CaSR variants were not found, but 2 novel heterozygous germline Gα 11 variants, p.Gly66Ser and p.Arg149His, were identified. Homology modeling of these revealed that the Gly66 and Arg149 residues are located at the interface between the Gα 11 helical and GTPase domains, which is involved in guanine nucleotide binding, and this is the site of 3 other reported ADH2 mutations. The Ca2+i and MAPK responses of cells expressing the variant Ser66 or His149 Gα 11 proteins were similar to WT cells at low Ca2+e, but significantly increased in a dose-dependent manner following Ca2+e stimulation, thereby indicating that the p.Gly66Ser and p.Arg149His variants represent pathogenic gain-of-function Gα 11 mutations. Treatment of Ser66- and His149-Gα 11 expressing cells with the CaSR negative allosteric modulator NPS 2143 normalized Ca2+i and MAPK responses.

CONCLUSION

Two novel ADH2-causing mutations that highlight the Gα 11 interdomain interface as a hotspot for gain-of-function Gα 11 mutations have been identified.

A NOVEL MUTATION IN CALCIUM-SENSING RECEPTOR PRESENTING AS FAMILIAL HYPOCALCIURIC HYPERCALCEMIA IN A YOUNG MAN

Objective

Familial hypocalciuric hypercalcemia (FHH) is considered a relatively benign condition characterized by elevated serum calcium with relatively low urinary calcium excretion. It typically results from an altered set point in calcium homeostasis originating from mutations in the calcium-sensing receptor (CASR), AP2S1, or GNA11 genes, which encode for the calcium-sensing receptor (CaSR), adaptor-related protein complex 2, and G-protein alpha-11 subunit, respectively. Despite numerous reports of novel variants in these genes associated with FHH, new variants continue to be discovered.

Methods

We describe a 20-year-old man with a family history of hypercalcemia who had clinical findings compatible with FHH and no evidence of multiple endocrine neoplasia who underwent CASR gene sequencing for evaluation of hypercalcemia. Parathyroid gland single-photon emission computerized tomography scan was normal.

Results

CASR gene sequencing revealed a previously unreported heterozygous intronic variant at position 1608+3A>G (chromosome 3: 121994892) resulting in a 77-residue deletion. His mother has a history of bipolar disorder and hyperparathyroidism with an adenoma found on imaging, yet our patient had no evidence of adenoma and therefore no surgical intervention was recommended. Given that CaSR plays a role in parathyroid growth, some variants in CASR may ultimately lead to parathyroid hypertrophy and be mistaken for primary hyperparathyroidism.

Conclusion

Long-term clinical follow up will be helpful in understanding the ultimate effects of specific CASR mutations on parathyroid growth or progression to significant hypercalcemia.

Nephrocalcinosis: A Review of Monogenic Causes and Insights They Provide into This Heterogeneous Condition

The abnormal deposition of calcium within renal parenchyma, termed nephrocalcinosis, frequently occurs as a result of impaired renal calcium handling. It is closely associated with renal stone formation (nephrolithiasis) as elevated urinary calcium levels (hypercalciuria) are a key common pathological feature underlying these clinical presentations. Although monogenic causes of nephrocalcinosis and nephrolithiasis are rare, they account for a significant disease burden with many patients developing chronic or end-stage renal disease. Identifying underlying genetic mutations in hereditary cases of nephrocalcinosis has provided valuable insights into renal tubulopathies that include hypercalciuria within their varied phenotypes. Genotypes affecting other enzyme pathways, including vitamin D metabolism and hepatic glyoxylate metabolism, are also associated with nephrocalcinosis. As the availability of genetic testing becomes widespread, we cannot be imprecise in our approach to nephrocalcinosis. Monogenic causes of nephrocalcinosis account for a broad range of phenotypes. In cases such as Dent disease, supportive therapies are limited, and early renal replacement therapies are necessitated. In cases such as renal tubular acidosis, a good renal prognosis can be expected providing effective treatment is implemented. It is imperative we adopt a precision-medicine approach to ensure patients and their families receive prompt diagnosis, effective, tailored treatment and accurate prognostic information.

Parathyroid hormone-dependent familial hypercalcemia with low measured PTH levels and a presumptive novel pathogenic mutation in CaSR

Familial hypocalciuric hypercalcemia (FHH) is a benign autosomal dominant condition characterized by lifelong asymptomatic hypercalcemia. FHH is typically caused by a heterozygous inactivating mutation of the calcium-sensing receptor (CaSR) and characterized by moderate hypercalcemia, inappropriately normal or elevated serum parathyroid hormone (PTH), and relative hypocalciuria (Fe_Ca < 2%) with histologically normal parathyroid glands. FHH should be distinguished from primary hyperparathyroidism so that unnecessary parathyroid surgery is avoided. We report a case that presented with asymptomatic, familial hypercalcemia but low PTH and normal (non-low) urinary calcium excretion found to be secondary to a novel pathogenic inactivating mutation of the CaSR gene. We present an asymptomatic 54-year-old Malaysian woman with incidentally discovered hypercalcemia, intermittent hypophosphatemia, and Fe_Ca > 2%. PTH levels were repeatedly below the mean of the reference range (on two separate assays) and sometimes even below the lower reference limit. Two siblings, one niece, and her son had hypercalcemia without nephrolithiasis. Cinacalcet, used as a PTH-suppression test, normalized serum total and ionized calcium after 7 days of cinacalcet 30 mg BID, confirming her hypercalcemia was PTH-mediated. Given her family history, genetic testing was pursued and discovered a novel pathogenic mutation of the CaSR gene confirming the diagnosis of FHH type 1. Our case represents an atypical presentation of FHH1 with low PTH and Fe_Ca > 2%. This contributes to the expanding clinical and biochemical spectrum of CaSR inactivating mutations and presents an innovative approach to evaluating biochemically uncertain familial hypercalcemia with cinacalcet before pursuing expensive genetic analysis.

The roles of calcium-sensing receptor (CaSR) in heavy metals-induced nephrotoxicity

The kidney is a vital organ responsible for regulating water, electrolyte and acid-base balance as well as eliminating toxic substances from the blood in the body. Exposure of humans to heavy metals in their natural and occupational environments, foods, water, and drugs has serious implications on the kidney's health. The accumulation of heavy metals in the kidney has been linked to acute or chronic renal injury, kidney stones or even renal cancer, at the expense of expensive treatment options. Therefore, unearthing novel biomarkers and potential therapeutic agents or targets against kidney injury for efficient treatment are imperative. The calcium-sensing receptor (CaSR), a G-protein-coupled receptor (GPCR) is typically expressed in the parathyroid glands and renal tubules. It modulates parathyroid hormone secretion according to the serum calcium (Ca²⁺) concentration. In the kidney, it modulates electrolyte and water excretion by regulating the function of diverse tubular segments. Notably, CaSR lowers passive and active Ca²⁺ reabsorption in distal tubules, which facilitates phosphate reabsorption in proximal tubules and stimulates proton and water excretion in collecting ducts. Moreover, at the cellular level, modulation of the CaSR regulates cytosolic Ca²⁺ levels, reactive oxygen species (ROS) generation and the mitogen-activated protein kinase (MAPK) signaling cascades as well as autophagy and the suppression of apoptosis, an effect predominantly triggered by heavy metals. In this regard, we present a review on the CaSR at the cellular level and its potential as a therapeutic target for the development of new and efficient drugs against heavy metals-induced nephrotoxicity.

Clinical genetic testing in endocrinology: Current concepts and contemporary challenges

Recent advances in DNA sequencing technology have led to an unprecedented period of disease-gene discovery offering many new opportunities for genetic testing in the clinical setting. Endocrinology has seen a rapid expansion in the taxonomy of monogenic disorders, which can be detected by an expanding portfolio of genetic tests in both diagnostic and predictive settings. Successful testing relies on many factors including the ability to identify those at increased risk of genetic disease in the busy clinic as well as a working knowledge of the various testing platforms and their limitations. The clinical utility of a given test is dependent upon many factors, which include the reliability of the genetic testing platform, the accuracy of the test result interpretation and knowledge of disease penetrance and expression. The increasing adoption of "high-content" genetic testing based on next-generation sequencing (NGS) to diagnose hereditary endocrine disorders brings a number of challenges including the potential for uncertain test results and/or genetic findings unrelated to the indication for testing. Therefore, it is increasingly important that the clinician is aware of the current evolution in genetic testing, and understands the different settings in which it may be employed. This review provides an overview of the genetic testing workflow, focusing on each of the major components required for successful testing in adult and paediatric endocrine settings. In addition, the challenges of variant interpretation are highlighted, as are issues related to informed consent, prenatal diagnosis and predictive testing. Finally, the future directions of genetic testing relevant to endocrinology are discussed.

A Novel Mutation of the Calcium-Sensing Receptor Gene Causing Familial Hypocalciuric Hypercalcemia Complicates Medical Followup after Roux-en-Y Gastric Bypass: A Case Report and a Summary of Mutations Found in the Same Hospital Laboratory

Heterozygous inactivating mutations in the calcium-sensing receptor (CaSR) gene are known to cause familial hypocalciuric hypercalcemia (FHH), usually a benign form of hypercalcemia without symptoms of a disrupted calcium homeostasis. FHH can be mistaken for the more common primary hyperparathyroidism (PHPT), for which surgical treatment may be needed. We describe a case of a 36-year-old woman with hypercalcemia and elevated PTH, initially suspected of having PHPT. Sequencing of the CaSR-gene revealed a mutation in nucleotide 437, changing the amino acid in position 146 from Glycine to Aspartate. The mutation was previously undescribed in the literature, but a very low calcium:creatinine clearance ratio supported the diagnosis FHH. A few years later, the patient's two daughters were tested and the association between mutation and hypercalcemia could be confirmed. The patient was gastric bypass-operated and therefore, due to malabsorption and increased risk of fracture, was in need of adequate calcium supplementation. The chronically elevated calcium levels challenged medical followup, as calcium sufficiency could not be monitored in a traditional manner. Eventually the patient developed elevated alkaline phosphatase, a further increased PTH and a decreased DXA T-score indicating calcium deficiency and bone resorption. As a supplement, all CaSR-mutations found at our hospital, 2005-2018.

Large-scale exome datasets reveal a new class of adaptor-related protein complex 2 sigma subunit (AP2σ) mutations, located at the interface with the AP2 alpha subunit, that impair calcium-sensing receptor signalling

Mutations of the sigma subunit of the heterotetrameric adaptor-related protein complex 2 (AP2σ) impair signalling of the calcium-sensing receptor (CaSR), and cause familial hypocalciuric hypercalcaemia type 3 (FHH3). To date, FHH3-associated AP2σ mutations have only been identified at one residue, Arg15. We hypothesized that additional rare AP2σ variants may also be associated with altered CaSR function and hypercalcaemia, and sought for these by analysing >111 995 exomes (>60 706 from ExAc and dbSNP, and 51 289 from the Geisinger Health System-Regeneron DiscovEHR dataset, which also contains clinical data). This identified 11 individuals to have 9 non-synonymous AP2σ variants (Arg3His, Arg15His (x3), Ala44Thr, Phe52Tyr, Arg61His, Thr112Met, Met117Ile, Glu122Gly and Glu142Lys) with 3 of the 4 individuals who had Arg15His and Met117Ile AP2σ variants having mild hypercalcaemia, thereby indicating a prevalence of FHH3-associated AP2σ mutations of ∼7.8 per 100 000 individuals. Structural modelling of the novel eight AP2σ variants (Arg3His, Ala44Thr, Phe52Tyr, Arg61His, Thr112Met, Met117Ile, Glu122Gly and Glu142Lys) predicted that the Arg3His, Thr112Met, Glu122Gly and Glu142Lys AP2σ variants would disrupt polar contacts within the AP2σ subunit or affect the interface between the AP2σ and AP2α subunits. Functional analyses of all eight AP2σ variants in CaSR-expressing cells demonstrated that the Thr112Met, Met117Ile and Glu142Lys variants, located in the AP2σ α4-α5 helical region that forms an interface with AP2α, impaired CaSR-mediated intracellular calcium (Cai2+) signalling, consistent with a loss of function, and this was rectified by treatment with the CaSR positive allosteric modulator cinacalcet. Thus, our studies demonstrate another potential class of FHH3-causing AP2σ mutations located at the AP2σ-AP2α interface.

Immunosuppressive therapy of autoimmune hypoparathyroidism in a patient with activating autoantibodies against the calcium-sensing receptor

CONTEXT

Activating antibodies directed at the extracellular calcium-sensing receptor (CaSR) have been described in autoimmune hypoparathyroidism in the setting of isolated hypoparathyroidism or autoimmune polyglandular syndrome type 1.

MATERIALS AND METHODS

A 34-year-old female presented with hypocalcaemia (6.0 mg/dL) and hypomagnesaemia (1.1 mg/dL) accompanied by low serum PTH (2.4 pg/mL) as well as urinary calcium and magnesium wasting. She was diagnosed with hypoparathyroidism, which was refractory to standard therapy. She was started on 60 mg prednisone and 150 mg azathioprine treatment daily on suspicion of an autoimmune aetiology. The patient was tested for CaSR antibodies.

RESULTS

The patient was positive for CaSR antibodies of the IgG1 subtype, which stimulated phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) and inositol phosphate (IP) accumulation. Post-treatment with prednisone and azathioprine, her serum calcium and magnesium normalized, as did her CaSR antibody titre and antibody-mediated stimulation of ERK1/2 phosphorylation and IP accumulation.

CONCLUSION

This is the first demonstration of CaSR antibody-mediated hypoparathyroidism responsive to immunosuppressive therapy, adding to the evidence that autoimmune hypoparathyroidism can be, in some cases, reversible and not the result of autoimmune parathyroid destruction.

The calcium-sensing receptor in physiology and in calcitropic and noncalcitropic diseases

The Ca²⁺-sensing receptor (CaSR) is a dimeric family C G protein-coupled receptor that is expressed in calcitropic tissues such as the parathyroid glands and the kidneys and signals via G proteins and β-arrestin. The CaSR has a pivotal role in bone and mineral metabolism, as it regulates parathyroid hormone secretion, urinary Ca²⁺ excretion, skeletal development and lactation. The importance of the CaSR for these calcitropic processes is highlighted by loss-of-function and gain-of-function CaSR mutations that cause familial hypocalciuric hypercalcaemia and autosomal dominant hypocalcaemia, respectively, and also by the fact that alterations in parathyroid CaSR expression contribute to the pathogenesis of primary and secondary hyperparathyroidism. Moreover, the CaSR is an established therapeutic target for hyperparathyroid disorders. The CaSR is also expressed in organs not involved in Ca²⁺ homeostasis: it has noncalcitropic roles in lung and neuronal development, vascular tone, gastrointestinal nutrient sensing, wound healing and secretion of insulin and enteroendocrine hormones. Furthermore, the abnormal expression or function of the CaSR is implicated in cardiovascular and neurological diseases, as well as in asthma, and the CaSR is reported to protect against colorectal cancer and neuroblastoma but increase the malignant potential of prostate and breast cancers.

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.

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

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

LINKED ARTICLES

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

Insights into calcium-sensing receptor trafficking and biased signalling by studies of calcium homeostasis

The calcium-sensing receptor (CASR) is a class C G-protein-coupled receptor (GPCR) that detects extracellular calcium concentrations, and modulates parathyroid hormone secretion and urinary calcium excretion to maintain calcium homeostasis. The CASR utilises multiple heterotrimeric G-proteins to mediate signalling effects including activation of intracellular calcium release; mitogen-activated protein kinase (MAPK) pathways; membrane ruffling; and inhibition of cAMP production. By studying germline mutations in the CASR and proteins within its signalling pathway that cause hyper- and hypocalcaemic disorders, novel mechanisms governing GPCR signalling and trafficking have been elucidated. This review focusses on two recently described pathways that provide novel insights into CASR signalling and trafficking mechanisms. The first, identified by studying a CASR gain-of-function mutation that causes autosomal dominant hypocalcaemia (ADH), demonstrated a structural motif located between the third transmembrane domain and the second extracellular loop of the CASR that mediates biased signalling by activating a novel β-arrestin-mediated G-protein-independent pathway. The second, in which the mechanism by which adaptor protein-2 σ-subunit (AP2σ) mutations cause familial hypocalciuric hypercalcaemia (FHH) was investigated, demonstrated that AP2σ mutations impair CASR internalisation and reduce multiple CASR-mediated signalling pathways. Furthermore, these studies showed that the CASR can signal from the cell surface using multiple G-protein pathways, whilst sustained signalling is mediated only by the G_q/11 pathway. Thus, studies of FHH- and ADH-associated mutations have revealed novel steps by which CASR mediates signalling and compartmental bias, and these pathways could provide new targets for therapies for patients with calcaemic disorders.

Discovery and Development of Calcimimetic and Calcilytic Compounds

The extracellular calcium receptor (CaR) is a G protein-coupled receptor (GPCR) and the pivotal molecule regulating systemic Ca²⁺ homeostasis. The CaR was a challenging target for drug discovery because its physiological ligand is an inorganic ion (Ca²⁺) rather than a molecule so there was no structural template to guide medicinal chemistry. Nonetheless, small molecules targeting this receptor were discovered. Calcimimetics are agonists or positive allosteric modulators of the CaR, while calcilytics are antagonists and all to date are negative allosteric modulators. The calcimimetic cinacalcet was the first allosteric modulator of a GPCR to achieve regulatory approval and is a first-in-class treatment for secondary hyperparathyroidism in patients on dialysis, and for hypercalcemia in some forms of primary hyperparathyroidism. It is also useful in treating some rare genetic diseases that cause hypercalcemia. Two other calcimimetics are now on the market (etelcalcetide) or under regulatory review (evocalcet). Calcilytics stimulate the secretion of parathyroid hormone and were initially developed as treatments for osteoporosis. Three different calcilytics of two different chemotypes failed in clinical trials due to lack of efficacy. Calcilytics are now being repurposed and might be useful in treating hypoparathyroidism and several rare genetic diseases causing hypocalcemia. The challenges ahead for medicinal chemists are to design compounds that select conformations of the CaR that preferentially target a particular signalling pathway and/or that affect the CaR in a tissue-selective manner.

Gain-of-function mutations in G-protein-coupled receptor genes associated with human endocrine disorders

The human genome encodes more than 700 G-protein-coupled receptors (GPCRs), many of which are involved in hormone secretion. To date, more than 100 gain-of-function (activating) mutations in at least ten genes for GPCRs, in addition to several loss-of-function mutations, have been implicated in human endocrine disorders. Previously reported gain-of-function GPCR mutations comprise various missense substitutions, frameshift mutations, intragenic inframe deletions and copy-number gains. Such mutations appear in both germline and somatic tumour cells, and lead to various hormonal abnormalities reflecting excessive receptor activity. Phenotypic consequences of these mutations include distinctive endocrine syndromes, as well as relatively common hormonal abnormalities. Such mutations encode hyperfunctioning receptors with increased constitutive activity, broadened ligand specificity, increased ligand sensitivity and/or delayed receptor desensitization. Furthermore, recent studies proposed a paradoxical gain-of-function mechanism caused by inactive GPCR mutants. Molecular diagnosis of GPCR activating mutations serves to improve the clinical management of mutation-positive patients. This review aims to introduce new aspects regarding gain-of-function mutations in GPCR genes associated with endocrine disorders.

The importance of the thick ascending limb of Henle's loop in renal physiology and pathophysiology

The thick ascending limb (TAL) of Henle’s loop is a crucial segment for many tasks of the nephron. Indeed, the TAL is not only a mainstay for reabsorption of sodium (Na⁺), potassium (K⁺), and divalent cations such as calcium (Ca²⁺) and magnesium (Mg²⁺) from the luminal fluid, but also has an important role in urine concentration, overall acid–base homeostasis, and ammonia cycle. Transcellular Na⁺ transport along the TAL is a prerequisite for Na⁺, K⁺, Ca²⁺, Mg²⁺ homeostasis, and water reabsorption, the latter through its contribution in the generation of the cortico-medullar osmotic gradient. The role of this nephron site in acid–base balance, via bicarbonate reabsorption and acid secretion, is sometimes misunderstood by clinicians. This review describes in detail these functions, reporting in addition to the well-known molecular mechanisms, some novel findings from the current literature; moreover, the pathophysiology and the clinical relevance of primary or acquired conditions caused by TAL dysfunction are discussed. Knowing the physiology of the TAL is fundamental for clinicians, for a better understanding and management of rare and common conditions, such as tubulopathies, hypertension, and loop diuretics abuse.

Cinacalcet Rectifies Hypercalcemia in a Patient With Familial Hypocalciuric Hypercalcemia Type 2 (FHH2) Caused by a Germline Loss-of-Function Gα11 Mutation

G-protein subunit α-11 (Gα₁₁ ) couples the calcium-sensing receptor (CaSR) to phospholipase C (PLC)-mediated intracellular calcium (Ca²⁺_i ) and mitogen-activated protein kinase (MAPK) signaling, which in the parathyroid glands and kidneys regulates parathyroid hormone release and urinary calcium excretion, respectively. Heterozygous germline loss-of-function Gα₁₁ mutations cause familial hypocalciuric hypercalcemia type 2 (FHH2), for which effective therapies are currently not available. Here, we report a novel heterozygous Gα₁₁ germline mutation, Phe220Ser, which was associated with hypercalcemia in a family with FHH2. Homology modeling showed the wild-type (WT) Phe220 nonpolar residue to form part of a cluster of hydrophobic residues within a highly conserved cleft region of Gα₁₁ , which binds to and activates PLC; and predicted that substitution of Phe220 with the mutant Ser220 polar hydrophilic residue would disrupt PLC-mediated signaling. In vitro studies involving transient transfection of WT and mutant Gα₁₁ proteins into HEK293 cells, which express the CaSR, showed the mutant Ser220 Gα₁₁ protein to impair CaSR-mediated Ca²⁺_i and extracellular signal-regulated kinase 1/2 (ERK) MAPK signaling, consistent with diminished activation of PLC. Furthermore, engineered mutagenesis studies demonstrated that loss of hydrophobicity within the Gα₁₁ cleft region also impaired signaling by PLC. The loss-of-function associated with the Ser220 Gα₁₁ mutant was rectified by treatment of cells with cinacalcet, which is a CaSR-positive allosteric modulator. Furthermore, in vivo administration of cinacalcet to the proband harboring the Phe220Ser Gα₁₁ mutation, normalized serum ionized calcium concentrations. Thus, our studies, which report a novel Gα₁₁ germline mutation (Phe220Ser) in a family with FHH2, reveal the importance of the Gα₁₁ hydrophobic cleft region for CaSR-mediated activation of PLC, and show that allosteric CaSR modulation can rectify the loss-of-function Phe220Ser mutation and ameliorate the hypercalcemia associated with FHH2. © 2017 The Authors. Journal of Bone and Mineral Research Published by Wiley Periodicals Inc.

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.