Activating autoantibodies against the calcium-sensing receptor detected in two patients with autoimmune polyendocrine syndrome type 1

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).

Etiology and Pathophysiology of Hypoparathyroidism: A Narrative Review

The approach utilized a systematic review of the medical literature executed with specifically designed criteria that focused on the etiologies and pathogenesis of hypoparathyroidism. Enhanced attention by endocrine surgeons to new knowledge about parathyroid gland viability are reviewed along with the role of intraoperative parathyroid hormone (ioPTH) monitoring during and after neck surgery. Nonsurgical etiologies account for a significant proportion of cases of hypoparathyroidism (~25%), and among them, genetic etiologies are key. Given the pervasive nature of PTH deficiency across multiple organ systems, a detailed review of the skeletal, renal, neuromuscular, and ocular complications is provided. The burden of illness on affected patients and their caregivers contributes to reduced quality of life and social costs for this chronic endocrinopathy. © 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).

Novel Gene Mutations Regulating Immune Responses in Autoimmune Polyglandular Syndrome With an Atypical Course

Context

Autoimmune polyglandular syndrome (APS) is a cluster of endocrine disorders arising from immune dysregulation, often combined with damage to nonendocrine organs. There are 2 types of APS: type 1 and type 2 (APS-1 and APS-2, respectively). In clinical practice, an atypical course of APS is often observed.

Objective

This work aims to find a novel genetic predictor of APS.

Methods

We performed exome sequencing in 2 patients with an atypical clinical APS picture and members of their families. Patient A presented with a manifestation of APS-2 in early childhood and patient B with a late manifestation of the main components of APS-1.

Results

In patient B, we identified inherited compound mutations as a novel combination of the c.769C > T and c.821delG alleles of AIRE and genetic variation in the CIITA gene. No homozygous or compound mutations in AIRE were found in patient A, but we did reveal mutations in genes encoding regulatory proteins of innate and acquired immunity in this patient.

Conclusion

Our data revealed novel combination of mutations in the AIRE gene in atypical APS and imply that mutations in immune-related genes may modify the clinical manifestation of APS in AIRE-mutation carriers and contribute to the development of autoimmune pathology in non-AIRE carriers with atypical APS.

Calcium-sensing receptor autoantibody-mediated hypoparathyroidism associated with immune checkpoint inhibitor therapy: diagnosis and long-term follow-up

Background

Immune checkpoint inhibitors (ICIs) have produced significant survival benefit across many tumor types. However, immune-related adverse events are common including autoimmune responses against different endocrine organs. Here, a case of ICI-mediated hypoparathyroidism focusing on long-term follow-up and insights into its etiology is presented.

Case and methods

A 73-year-old man developed severe symptomatic hypocalcemia after the initiation of ipilimumab and nivolumab for the treatment of metastatic melanoma. Hypoparathyroidism was diagnosed with undetectable intact parathyroid hormone (PTH). Immunoprecipitation assays, ELISAs, and cell-based functional assays were used to test the patient for antibodies against the calcium-sensing receptor (CaSR). NACHT leucine-rich repeat protein 5 (NALP5) and cytokine antibodies were measured in radioligand binding assays and ELISAs, respectively.

Results

The patient’s symptoms improved with aggressive calcium and vitamin D supplementation. At 3 years and 3 months since the diagnosis of hypoparathyroidism, PTH was still inappropriately low at 7.6 pg/mL, and attempted discontinuation of calcium and calcitriol resulted in recurrent symptomatic hypocalcemia. Analysis for an autoimmune etiology of the patient’s hypoparathyroidism indicated that CaSR antibodies were negative before treatment and detected at multiple time points afterwards, and corresponded to the patient’s clinical course of hypoparathyroidism. CaSR antibodies purified from the patient’s serum activated the human CaSR. The patient was seronegative for NALP5 and cytokine antibodies, indicating that their hypoparathyroidism was not a manifestation of autoimmune polyendocrine syndrome type 1.

Conclusion

The etiology of hypocalcemia is likely autoimmune hypoparathyroidism caused by the development of CaSR-activating antibodies that might prevent PTH release from the parathyroid.

Activating Antibodies to The Calcium-sensing Receptor in Immunotherapy-induced Hypoparathyroidism

CONTEXT

Immune checkpoint inhibitors (ICIs), such as programmed cell death protein-1 (PD-1), programmed cell death protein-ligand 1 (PD-L1), and cytotoxic T lymphocyte antigen-4 (CTLA-4) monoclonal antibodies, are approved for the treatment of some types of advanced cancer. Their main treatment-related side-effects are immune-related adverse events (irAEs), especially thyroid dysfunction and hypophysitis. Hypoparathyroidism, on the contrary, is an extremely rare irAE.

OBJECTIVES

The aim of the study was to investigate the etiology of autoimmune hypoparathyroidism in a lung cancer patient treated with pembrolizumab, an anti-PD-1.

METHODS

Calcium-sensing receptor (CaSR) autoantibodies, their functional activity, immunoglobulin (Ig) subclasses and epitopes involved in the pathogenesis of autoimmune hypoparathyroidism were tested.

RESULTS

The patient developed hypocalcemia after 15 cycles of pembrolizumab. Calcium levels normalized with oral calcium carbonate and calcitriol and no remission of hypocalcemia was demonstrated during a 9-month follow-up. The patient was found to be positive for CaSR-stimulating antibodies, of IgG1 and IgG3 subclasses, that were able to recognize functional epitopes on the receptor, thus causing hypocalcemia.

CONCLUSION

The finding confirms that ICI therapy can trigger, among other endocrinopathies, hypoparathyroidism, which can be caused by pathogenic autoantibodies.

Inflammation-induced hypoparathyroidism triggered by combination immune checkpoint blockade for melanoma

BACKGROUND

Treatment with a combination of PD-1 and CTLA-4 targeted checkpoint inhibition has improved outcome of melanoma patients and led to durable remissions but is also associated with significant toxicities. Endocrinopathies such as thyroiditis and hypophysitis are often seen, but other, rarer disturbances have also been described. Endocrinopathies affecting the parathyroid gland are rarely reported and no clear pathomechanism has been proposed.

CASE PRESENTATION

Here, we report a case of severe hypocalcemia due to an antibody-mediated hypoparathyroidism as an immune-related adverse event (irAE) in a patient who was treated with the anti-PD-1 antibody nivolumab and anti-CTLA-4 antibody ipilimumab. Hypocalcemia was rapidly corrected by substitution, but the endogenous serum parathyroid hormone (PTH) remained low. The patient demonstrated a rapid and profound tumor response to the combination immune checkpoint blockade, but developed a severe colitis that required high-dose intravenous corticosteroid and anti-TNFα therapy. During this strong immunosuppression the PTH level normalized and the calcium levels were stable without substitution. However, during tapering of immunosuppressants, the PTH and calcium levels decreased again to a level requiring calcium substitution.

CONCLUSION

Our report demonstrates a rare endocrinopathy as a complication of combined PD-1 and CTLA-4 blockade. In addition, it provides evidence from the course of the disease that inflammation within the parathyroid gland is involved in the mechanism.

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.

Calcium-Sensing Receptor Autoantibodies in Patients with Autoimmune Polyendocrine Syndrome Type 1: Epitopes, Specificity, Functional Affinity, IgG Subclass, and Effects on Receptor Activity

A major manifestation of autoimmune polyendocrine syndrome type 1 (APS1) is hypoparathyroidism, which is suggested to result from aberrant immune responses against the parathyroid glands. The calcium-sensing receptor (CaSR), which plays a pivotal role in maintaining calcium homeostasis by sensing blood calcium levels and regulating release of parathyroid hormone (PTH), is an autoantibody target in APS1. In this study, the aim was to characterize the binding sites, specificity, functional affinity, IgG subclass, and functional effects of CaSR autoantibodies using phage-display technology, ELISA, and bioassays. The results indicated that CaSR autoantibody binding sites were at aa 41-69, 114-126, 171-195, and 260-340 in the extracellular domain of the receptor. Autoantibodies against CaSR epitopes 41-69, 171-195, and 260-340 were exclusively of the IgG1 subclass. Autoantibody responses against CaSR epitope 114-126 were predominantly of the IgG1 with a minority of the IgG3 subclass. Only autoantibodies recognizing CaSR epitopes 114-126 and 171-195 affected receptor activity; inositol-phosphate accumulation was increased significantly in HEK293-CaSR cells, and PTH secretion from PTH-C1 cells was reduced significantly when either were incubated with purified Ab and Ca2+ compared with Ca2+ alone. In conclusion, although the majority of APS1 patients do not have CaSR-stimulating autoantibodies, the hypoparathyroid state in a small minority of patients is the result of functional suppression of the parathyroid glands.

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.

Glucocorticoid-responsive lymphocytic parathyroiditis and hypocalciuric hypercalcemia due to autoantibodies against the calcium-sensing receptor: a case report and literature review

OBJECTIVE

Autoimmune lymphocytic parathyroiditis and acquired hypocalciuric hypercalcemia associated with autoantibodies against the calcium-sensing receptor (anti-CaSR) are rare and poorly understood conditions. Here, we describe a patient with acquired parathyroid hormone (PTH)-dependent hypercalcemia with associated hypocalciuria, found to have true lymphocytic parathyroiditis on histopathology, and circulating anti-CaSR antibodies in serum.

DESIGN AND METHODS

A 64-year-old woman was referred to our clinic for persistent hypercalcemia after a subtotal parathyroidectomy. She was normocalcemic until the age of 63 years when she was diagnosed with primary hyperparathyroidism. She underwent subtotal parathyroidectomy with appropriate intraoperative PTH decline. Two weeks post-parathyroidectomy, she presented with persistent hypercalcemia and hyperparathyroidism. Urine studies revealed an inappropriately low 24-h urine calcium (Ca)/creatinine clearance ratio. Surgical pathology was consistent with true lymphocytic parathyroiditis with lymphoid follicles. The presence of circulating anti-CaSR antibodies was detected by immunoprecipitation of CaSR by the patient's serum. After a 4-week course of prednisone, serum Ca and PTH normalized, and her anti-CaSR titers declined. She remains normocalcemic 10 months after the discontinuation of glucocorticoid therapy. We present this patient in the context of the relevant published literature on lymphocytic parathyroiditis and acquired hypocalciuric hypercalcemia related to anti-CaSR antibodies.

CONCLUSIONS

Autoimmune lymphocytic parathyroiditis and acquired hypocalciuric hypercalcemia associated with anti-CaSR antibodies is a very rare yet important condition to be considered in a patient with acquired PTH-dependent hypercalcemia with inappropriate hypocalciuria. Although subtotal parathyroidectomy is unlikely to correct the hypercalcemia, this entity may respond to a short course of prednisone therapy.

Pathogenic and Protective Autoantibodies in Autoimmune Polyendocrinopathy-Candidiasis-Ectodermal Dystrophy (APECED)

Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) is a rare disorder caused by mutations in the autoimmune regulator (AIRE) gene, leading to defects in T cell selection. The disease manifestations include both autoimmune tissue destruction and immunodeficiency, with specific susceptibility to chronic mucocutaneous candidiasis. Studies have demonstrated a wide repertoire of high affinity tissue- and cytokine-specific antibodies in patients with APECED. Here, we review the antigenic targets and function of these disease-causing and disease-ameliorating antibodies.

Molecular Basis of the Extracellular Ligands Mediated Signaling by the Calcium Sensing Receptor

Ca²⁺-sensing receptors (CaSRs) play a central role in regulating extracellular calcium concentration ([Ca²⁺]_o) homeostasis and many (patho)physiological processes in multiple organs. This regulation is orchestrated by a cooperative response to extracellular stimuli such as small changes in Ca²⁺, Mg²⁺, amino acids, and other ligands. In addition, CaSR is a pleiotropic receptor regulating several intracellular signaling pathways, including calcium mobilization and intracellular calcium oscillation. Nearly 200 mutations and polymorphisms have been found in CaSR in relation to a variety of human disorders associated with abnormal Ca²⁺ homeostasis. In this review, we summarize efforts directed at identifying binding sites for calcium and amino acids. Both homotropic cooperativity among multiple calcium binding sites and heterotropic cooperativity between calcium and amino acid were revealed using computational modeling, predictions, and site-directed mutagenesis coupled with functional assays. The hinge region of the bilobed Venus flytrap (VFT) domain of CaSR plays a pivotal role in coordinating multiple extracellular stimuli, leading to cooperative responses from the receptor. We further highlight the extensive number of disease-associated mutations that have also been shown to affect CaSR's cooperative action via several types of mechanisms. These results provide insights into the molecular bases of the structure and functional cooperativity of this receptor and other members of family C of the G protein-coupled receptors (cGPCRs) in health and disease states, and may assist in the prospective development of novel receptor-based therapeutics.

Early-onset hypoparathyroidism and chronic keratitis revealing APECED

Early diagnosis of potentially life-threatening autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) is crucial, but is often delayed due to the clinical heterogeneity of the disorder. Even in the absence of the classic disease triad of chronic mucocutaneous candidiasis, hypoparathyroidism, and adrenocortical insufficiency, a diagnosis of APECED should be considered in children who have hypoparathyroidism and chronic keratitis, with a past medical history showing a mild and transient Candida infection.

Hypoparathyroidism and central diabetes insipidus: in search of the link

Two siblings (a 15-year-old boy and an 11-year-old girl) who presented with hypocalcemic seizure at the age of 2 years and 2 months (boy) and 2 years and 4 months (girl) were diagnosed with hypoparathyroidism. At the age of 3 years, the girl developed central diabetes insipidus with good response to desmopressin acetate treatment. The family history was unremarkable, and there was no consanguinity between the parents. The father is of Iraqi/Egyptian Jewish origin and the mother is of Iranian/Romanian Jewish origin. Sequence analysis of the candidate genes for isolated hypoparathyroidism encoding calcium-sensing receptor, parathyroid hormone, and glial cells missing homolog B did not reveal any mutations. Whole-exome sequencing identified a homozygous mutation in the autoimmune regulatory gene (AIRE), c.374A>G;p.Y85C, characteristic for Jewish Iranians with autoimmune polyendocrine syndrome type 1 (APS1), which was confirmed by the Sanger sequencing. Antibodies against the adrenal, pancreatic islet cell, ovary, thyroid, pituitary, celiac, and parietal cell were negative in both siblings, while anti-diuretic hormone antibodies were positive only in the girl. No other symptoms or signs of APS1 developed during all the years of follow-up.

CONCLUSION

APS1 should be part of the differential diagnosis in children presenting with isolated hypoparathyroidism or hypoparathyroidism with central diabetes insipidus (CDI). These cases show that the AIRE mutation characteristic of Iranian Jews can also be found in non-Iranian Jews.

Calcium-sensing receptor 20 years later

The calcium-sensing receptor (CaSR) has played an important role as a target in the treatment of a variety of disease states over the past 20 plus years. In this review, we give an overview of the receptor at the cellular level and then provide details as to how this receptor has been targeted to modulate cellular ion transport mechanisms. As a member of the G protein-coupled receptor (GPCR) family, it has a high degree of homology with a variety of other members in this class, which could explain why this receptor has been identified in so many different tissues throughout the body. This diversity of locations sets it apart from other members of the family and may explain how the receptor interacts with so many different organ systems in the body to modulate the physiology and pathophysiology. The receptor is unique in that it has two large exofacial lobes that sit in the extracellular environment and sense changes in a wide variety of environmental cues including salinity, pH, amino acid concentration, and polyamines to name just a few. It is for this reason that there has been a great deal of research associated with normal receptor physiology over the past 20 years. With the ongoing research, in more recent years a focus on the pathophysiology has emerged and the effects of receptor mutations on cellular and organ physiology have been identified. We hope that this review will enhance and update the knowledge about the importance of this receptor and stimulate future potential investigations focused around this receptor in cellular, organ, and systemic physiology and pathophysiology.

Prevalence and clinical associations of calcium-sensing receptor and NALP5 autoantibodies in Finnish APECED patients

CONTEXT

Previous studies have identified the calcium-sensing receptor (CaSR) and NALP5 as parathyroid autoantibody targets in patients with autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED). However, although NALP5 antibodies have been associated with the occurrence of hypoparathyroidism (HP) in APECED, it is unclear whether CaSR antibodies are a specific or sensitive marker for APECED-associated HP.

OBJECTIVE

The objective of the study was to identify associations between the presence of CaSR and NALP5 antibodies and the disease manifestations and demographic characteristics of Finnish APECED patients.

DESIGN, SUBJECTS, AND METHODS

This was a case-control study including 44 APECED patients and 38 age- and sex-matched healthy controls. Antibodies against the CaSR and NALP5 were detected using immunoprecipitation assays and radioligand binding assays, respectively.

RESULTS

CaSR and NALP5 antibodies were detected in 16 of 44 (36%) and 13 of 44 (30%) patients, respectively. No statistically significant associations were found between the presence of CaSR or NALP5 antibodies and the disease manifestations of APECED including HP (P > .05). For the diagnosis of HP, CaSR and NALP5 antibodies had specificities of 83% and 50%, respectively, and sensitivities of 39% and 26%, respectively. A significant association between both a shorter APECED and HP duration (<10 y) and positivity for CaSR antibodies was noted (P = .019 and P = .0061, respectively).

CONCLUSION

Neither CaSR nor NALP5 antibodies were found to be specific or sensitive markers for HP in APECED. Further investigations are required to determine the exact role of the autoimmune response against the CaSR and NALP5 in the pathogenesis of this autoimmune syndrome.

Diagnosis and classification of autoimmune parathyroid disease

Hypoparathyroidism (HP) is clinically characterized by the presence of hypocalcemia, usually associated with specific signs and symptoms that depend on how severe and chronic the disease becomes. HP is usually caused by surgical removal of all four parathyroids, while other forms are rarer. Autoimmune HP can occur as an isolated disease or as part of an autoimmune polyendocrine syndrome. Here we review what is known about parathyroid gland autoimmunity, focusing on recently-proposed parathyroid autoantibody markers, and particularly those directed against NACHT leucine-rich-repeat protein 5 and calcium-sensing receptor. We also describe the clinical characteristics of HP and design a diagnostic algorithm for autoimmune HP.

Calcium-sensing receptor autoantibodies and idiopathic hypoparathyroidism

CONTEXT

Data on calcium-sensing receptor autoantibodies (CaSRAbs) in hypoparathyroidism are variable.

OBJECTIVE

We assessed the prevalence and significance of CaSRAbs in idiopathic hypoparathyroidism.

DESIGN

This was a case-control study.

SUBJECTS

One hundred forty-seven patients with idiopathic hypoparathyroidism treated during 1998-2011 in a tertiary care setting and 348 controls [healthy, n = 199; type 1 diabetes mellitus (T1DM), n = 99; and chronic lymphocytic thyroiditis (CLT), n = 50] participated in the study.

METHODS

CaSRAb assays included Western blot with CaSR protein expressed in Escherichia coli or human embryonic kidney (HEK)-293 cells, immunoprecipitation (IP) using in vitro-transcribed/translated protein, and indirect immunofluorescence on HEK293-CaSR. Functional significance was assessed by ERK1/2 phosphorylation. PTH and CaSR genes were sequenced for mutations.

RESULTS

E coli-Western blot assay revealed 16.3% CaSRAb positivity in idiopathic hypoparathyroidism, which was comparable with healthy subjects and CLT but significantly less than the T1DM controls. The prevalence of CaSRAbs on HEK293-Western blot (24.5%) against 150 kDa and/or 168 kDa protein in hypoparathyroidism was significantly higher than the healthy subjects, T1DM, and CLT. IP assay showed CaSRAbs in 12.9% of the hypoparathyroid patients but not in controls. The sensitivity and specificity of CaSRAbs in E coli and HEK-293-CaSR Western blot and IP assays were 16.3% and 83.1%, 24.5% and 88.9%, and 12.9% and 100%, respectively, and 42.1% of the cases detected were common in the IP assay and HEK293-Western blot. Duration of illness and coexistent autoimmunity were similar in patients with and without CaSRAbs. The CaSRAb-positive sera showed no immunofluorescence and phosphorylated ERK1/2 activity. The CaSR gene sequence was normal in all patients. One of the patients showed a novel p.Met1_Asp6del mutation in the signal peptide region of the PTH gene.

CONCLUSION

IP performed the best in detecting CaSRAbs in 12.9% of hypoparathyroid patients. Although CaSRAbs were functionally inert, its clinical relevance remains due to 100% specificity. Limited prevalence of CaSRAb suggests heterogeneity in the etiology of idiopathic hypoparathyroidism or the presence of CaSR epitopes other than those measured in the current study.

Acquired hypocalciuric hypercalcemia in a patient with CKD

We present a case of an 82-year-old woman with elevated parathyroid hormone (PTH) levels, hypocalciuria, hypercalcemia, and stage 3 chronic kidney disease. Hypocalciuria initially was attributed to chronic kidney disease, and hypercalcemia was attributed to primary hyperparathyroidism. Subsequent laboratory studies showed autoantibodies in the patient's serum directed against the calcium-sensing receptor (CaSR). Functional testing in a CaSR-transfected human embryonic kidney-293 cell line showed that the patient's antibodies inhibited CaSR-mediated intracellular signaling that ordinarily would have been stimulated by extracellular calcium ions. Her serum calcium and PTH levels were normalized by treatment with the calcimimetic cinacalcet. We advise consideration of the presence of inhibitory autoantibodies directed at the CaSR in patients with hypercalcemic hyperparathyroidism and unexplained hypocalciuria or with confounding conditions affecting interpretation of urinary calcium measurement. A calcimimetic is an effective treatment for the hypercalcemia and elevated PTH levels in acquired hypocalciuric hypercalcemia caused by inhibitory anti-CaSR autoantibodies.

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

The extracellular calcium-sensing receptor (CaSR) is a family C G-protein-coupled receptor (GPCR) that is expressed at multiple sites, including the parathyroids and kidneys. The human CASR gene, located on chromosome 3q21.1, encodes a 1078 amino acid protein. More than 230 different disease-causing mutations of the CaSR have been reported. Loss-of-function mutations lead to three hypercalcemic disorders, which are familial hypocalciuric hypercalcemia (FHH), neonatal severe hyperparathyroidism and primary hyperparathyroidism. Gain-of-function mutations, on the other hand, result in the hypocalcemic disorders of autosomal dominant hypocalcemia and Bartter syndrome type V. Moreover, autoantibodies directed against the extracellular domain of the CaSR have been found to be associated with FHH in some patients, and also in some patients with hypoparathyroidism that may be part of autoimmune polyglandular syndrome type 1. Studies of disease-causing CASR mutations have provided insights into structure-function relationships and highlighted intra-molecular domains that are critical for ligand binding, intracellular signaling, and receptor trafficking.

Graves' Disease, Hypoparathyroidism, Systemic Lupus Erythematosus, Alopecia, and Angioedema: Autoimmune Polyglandular Syndrome Variant or Coincidence?

Data on coexisting Graves' disease (GD), hypoparathyroidism, and systemic lupus erythematosus (SLE) are limited. The thyroid and parathyroid glands may be extra sensitive to irradiation damage in an underlying autoimmune condition. A 34-year-old black woman presented with tetanic-like cramps, easy skin bruising, fatigue, weight gain, nocturia and back pain. She was previously diagnosed with GD in 2001 and underwent radioiodine therapy (RAI) in 9/01 using 6 mCi. PostRAI (November 2001) she developed hypocalcemia and hypothyroidism (2/02). In 2007, SLE was diagnosed. In October 2009, s-calcium and PTH were still low at 7.1 mg/dl and 9 pg/mL, respectively, although the patient denied symptoms on vitamin D and calcium supplementation. To identify possible autoimmune damage of the parathyroids, we evaluated the presence of activating antibodies to the CaSR and also analyzed the DNA sequence of all 6 translated exons and flanking intronic sequences of her CaSR gene for a functionally significant CaSR mutation but neither was positive. The initial autoimmune damage to her thyroid and possibly parathyroid glands followed by irradiation of them seems to have contributed to her developing both hypoparathyroidism (11/01) and hypothyroidism (2002). The patient could potentially have had parathyroid autoantibodies in 2001 that disappeared by 2009 when she was tested for them. We consider that the multiple autoimmune conditions developed over the past decade of her life with the concurrent irradiation contributing to her brittle hypoparathyroidism. Select patients with GD and perhaps parathyroid autoantibodies with a slowly developing destructive impact on the parathyroid glands may then develop overt hyoparathyroidism with rather low dose RAI ablation. This patient adds to the evolving spectrum of polyglandular syndrome variants.

Calcium-sensing receptor: Role in health and disease

The calcium-sensing receptor (CaSR) is a 1,078 amino acid G protein-coupled receptor (GPCR), which is predominantly expressed in the parathyroids and kidney. The CaSR allows regulation of parathyroid hormone (PTH) secretion and renal tubular calcium re-absorption in response to alterations in extracellular calcium concentrations. Loss-of-function CaSR mutations have been reported in the hypercalcemic disorders of familial benign (hypocalciuric) hypercalcemia (FBH or FHH), neonatal severe primary hyperparathyroidism (NSHPT), and adult primary hyperparathyroidism. However, some individuals with loss-of-function CaSR mutations remain normocalcemic. Gain-of-function CaSR mutations have been shown to result in autosomal-dominant hypocalcemia with hypercalciuria (ADHH) and Bartter's syndrome type V. CaSR auto-antibodies have been found in FHH patients who did not have loss-of-function CaSR mutations and in patients with an acquired form (i.e. autoimmune) of hypoparathyroidism. Thus, abnormalities of the CaSR are associated with 4 hypercalcemic and 3 hypocalcemic disorders.

APECED: is this a model for failure of T cell and B cell tolerance?

In APECED, the key abnormality is in the T cell defect that may lead to tissue destruction chiefly in endocrine organs. Besides, APECED is characterized by high-titer antibodies against a wide variety of cytokines that could partly be responsible for the clinical symptoms during APECED, mainly chronic mucocutaneous candidiasis, and linked to antibodies against Th17 cells effector molecules, IL-17 and IL-22. On the other hand, the same antibodies, together with antibodies against type I interferons may prevent the patients from other immunological diseases, such as psoriasis and systemic lupus erythematous. The same effector Th17 cells, present in the lymphocytic infiltrate of target organs of APECED, could be responsible for the tissue destruction. Here again, the antibodies against the corresponding effector molecules, anti-IL-17 and anti-IL-22 could be protective. The occurrence of several effector mechanisms (CD4(+) Th17 cell and CD8(+) CTL and the effector cytokines IL-17 and IL-22), and simultaneous existence of regulatory mechanisms (CD4(+) Treg and antibodies neutralizing the effect of the effector cytokines) may explain the polymorphism of APECED. Almost all the patients develop the characteristic manifestations of the complex, but temporal course and severity of the symptoms vary considerably, even among siblings. The autoantibody profile does not correlate with the clinical picture. One could speculate that a secondary homeostatic balance between the harmful effector mechanisms, and the favorable regulatory mechanisms, finally define both the extent and severity of the clinical condition in the AIRE defective individuals. The proposed hypothesis that in APECED, in addition to strong tissue destructive mechanisms, a controlling regulatory mechanism does exist, allow us to conclude that APECED could be treated, and even cured, with immunological manipulation.

New concepts in calcium-sensing receptor pharmacology and signalling

The calcium-sensing receptor (CaR) is the key controller of extracellular calcium (Ca(2+)(o)) homeostasis via its regulation of parathyroid hormone (PTH) secretion and renal Ca(2+) reabsorption. The CaR-selective calcimimetic drug Cinacalcet stimulates the CaR to suppress PTH secretion in chronic kidney disease and represents the world's first clinically available receptor positive allosteric modulator (PAM). Negative CaR allosteric modulators (NAMs), known as calcilytics, can increase PTH secretion and are being investigated as possible bone anabolic treatments against age-related osteoporosis. Here we address the current state of development and clinical use of a series of positive and negative CaR modulators. In addition, clinical CaR mutations and transgenic mice carrying tissue-specific CaR deletions have provided a novel understanding of the relative functional importance of CaR in both calciotropic tissues and those elsewhere in the body. The development of CaR-selective modulators and signalling reagents have provided us with a more detailed appreciation of how the CaR signals in vivo. Thus, both of these areas of CaR research will be reviewed.

Hypoparathyroidism in the adult: epidemiology, diagnosis, pathophysiology, target-organ involvement, treatment, and challenges for future research

Recent advances in understanding the epidemiology, genetics, diagnosis, clinical presentations, skeletal involvement, and therapeutic approaches to hypoparathyroidism led to the First International Workshop on Hypoparathyroidism that was held in 2009. At this conference, a group of experts convened to discuss these issues with a view towards a future research agenda for this disease. This review, which focuses primarily on hypoparathyroidism in the adult, provides a comprehensive summary of the latest information on this disease.

Hypoparathyroidism and autoimmunity in the 22q11.2 deletion syndrome

OBJECTIVE

To characterize the endocrine and autoimmune disturbances with emphasis on parathyroid dysfunction in patients with 22q11.2 deletion syndrome (22q11.2 DS). Design In this nationwide survey; 59 patients (age 1-54 years) out of 86 invited with a 22q11.2 DS were recruited through all the genetic institutes in Norway.

METHODS

Data was collected from blood tests, medical records, a physical examination and a semi-structured interview. We registered autoimmune diseases and measured autoantibodies, hormone levels and HLA types.

RESULTS

Twenty-eight (47%) patients had hypoparathyroidism or a history of neonatal or transient hypocalcemia. Fifteen patients had neonatal hypocalcemia. Fourteen patients had permanent hypoparathyroidism including seven (54%) of those above age 15 years. A history of neonatal hypocalcemia did not predict later occurring hypoparathyroidism. Parathyroid hormone levels were generally low indicating a low reserve capacity. Twenty-eight patients were positive for autoantibodies. Six (10%) persons had developed an autoimmune disease, and all were females (P<0.02). Hypoparathyroidism correlated with autoimmune diseases (P<0.05), however, no antibodies were detected against the parathyroid glands.

CONCLUSIONS

Hypoparathyroidism and autoimmunity occur frequently in the 22q11.2 DS. Neonatal hypocalcemia is not associated with later development of permanent hypoparathyroidism. Hypoparathyroidism may present at any age, also in adults, and warrants regular measurement of calcium levels. Hypoparathyroidism and autoimmunity occur frequently together. Our findings of autoimmune diseases in 10% of the patients highlight the importance of stringent screening and follow-up routines.

Autoimmune hypocalciuric hypercalcemia unresponsive to glucocorticoid therapy in a patient with blocking autoantibodies against the calcium-sensing receptor

CONTEXT

Autoantibodies directed against the calcium-sensing receptor (CaSR) have been reported in several individuals with various autoimmune disorders and PTH-mediated hypercalcemia. Previously, glucocorticoid treatment has been shown to decrease the CaSR autoantibody titers and normalize the hypercalcemia in a patient with autoimmune hypocalciuric hypercalcemia (AHH).

OBJECTIVE

The objective of the study was to evaluate a patient with AHH for the presence of blocking autoantibodies against the CaSR and to monitor her biochemical and serological responses to a trial of glucocorticoid therapy.

RESULTS

Glucocorticoid treatment had no effect on serum total or ionized calcium concentration or serum PTH levels, all of which remained at higher than normal levels. In contrast, on prednisone, urinary calcium excretion increased from overtly hypocalciuric levels to normal values. Anti-CaSR autoantibodies were detected at similar levels in the patient's serum before, during, and after glucocorticoid treatment. Functional testing of these antibodies showed that they inhibited the stimulatory effect of extracellular Ca(2+) on ERK1/2 but did not suppress the calcium-induced accumulation of inositol-1-phosphate.

CONCLUSIONS

We report a patient with AHH with frankly elevated PTH levels who was found to have autoantibodies against the CaSR. The hypercalcemia and CaSR autoantibody titers failed to respond to glucocorticoid therapy, unlike a previously reported patient with similar clinical and biochemical features. The anti-CaSR antibody-mediated inhibition of CaSR-stimulated ERK1/2 activity, but not of inositol-1-phosphate accumulation, suggests that ERK1/2 may mediate, at least in part, the regulation of PTH secretion and urinary calcium excretion by the CaSR.

The complexity of their activation mechanism opens new possibilities for the modulation of mGlu and GABAB class C G protein-coupled receptors

In the human genome, 22 genes are coding for the class C G protein-coupled receptors that are receptors for the two main neurotransmitters glutamate and γ-aminobutyric acid, for Ca(2+) and for sweet and amino acid taste compounds. In addition to the GPCR heptahelical transmembrane domain responsible for G-protein activation, class C receptors possess a large extracellular domain that is responsible for ligand recognition. Recent studies had revealed that class C receptors are homo- or heterodimers with unique mechanism of activation. In the present review, we present an up-to-date view of the structures and activation mechanism of these receptors in particular the metabotropic glutamate and GABA(B) receptors. We show how the complexity of functioning of these transmembrane proteins can be used for the development of therapeutics to modulate their activity. We emphasize on the new approaches and drugs that could potentially become important in the future pharmacology of these receptors.

Physiology and pathophysiology of the calcium-sensing receptor in the kidney

The extracellular calcium-sensing receptor (CaSR) plays a major role in the maintenance of a physiological serum ionized calcium (Ca2+) concentration by regulating the circulating levels of parathyroid hormone. It was molecularly identified in 1993 by Brown et al. in the laboratory of Dr. Steven Hebert with an expression cloning strategy. Subsequent studies have demonstrated that the CaSR is highly expressed in the kidney, where it is capable of integrating signals deriving from the tubular fluid and/or the interstitial plasma. Additional studies elucidating inherited and acquired mutations in the CaSR gene, the existence of activating and inactivating autoantibodies, and genetic polymorphisms of the CaSR have greatly enhanced our understanding of the role of the CaSR in mineral ion metabolism. Allosteric modulators of the CaSR are the first drugs in their class to become available for clinical use and have been shown to treat successfully hyperparathyroidism secondary to advanced renal failure. In addition, preclinical and clinical studies suggest the possibility of using such compounds in various forms of hypercalcemic hyperparathyroidism, such as primary and lithium-induced hyperparathyroidism and that occurring after renal transplantation. This review addresses the role of the CaSR in kidney physiology and pathophysiology as well as current and in-the-pipeline treatments utilizing CaSR-based therapeutics.