Familial hypocalciuric hypercalcemia and other disorders with resistance to extracellular calcium

Autosomal dominant hypercalciuric hypocalcaemia: the calcium-sensing receptor in renal calcium homeostasis and the impact of renal transplantation

Calcium-sensing receptors (CaSRs) are G protein-coupled receptors that help maintain Ca2+ concentrations, modulating calciotropic hormone release (parathyroid hormone (PTH), calcitonin and 1,25-dihydroxyvitamin D) by direct actions in the kidneys, gastrointestinal tract and bone. Variability in population calcium levels has been attributed to single nucleotide polymorphisms in CaSR genes, and several conditions affecting calcium and phosphate homeostasis have been attributed to gain- or loss-of-function mutations. An example is autosomal dominant hypercalciuric hypocalcaemia, because of a missense mutation at codon 128 of chromosome 3, as reported in our specific case and her family. As a consequence of treating symptomatic hypocalcaemia as a child, this female subject slowly developed progressive end-stage kidney failure because of nephrocalcinosis and nephrolithiasis. After kidney transplantation, she remains asymptomatic, with decreased vitamin D and elemental calcium requirements, stable fluid and electrolyte homeostasis during intercurrent illnesses and has normalised urinary calcium and phosphate excretion, reducing the likelihood of hypercalciuria-induced graft impairment. We review the actions of the CaSR, its role in regulating renal Ca2+ homeostasis along with the impact of a proven gain-of-function mutation in the CaSR gene resulting in autosomal dominant hypercalciuric hypocalcaemia before and after kidney transplantation.

Familial hypocalciuric hypercalcaemia type 1 caused by a novel heterozygous missense variant in the CaSR gene, p(His41Arg): two case reports

BACKGROUND

Familial hypocalciuric hypercalcaemia (FHH) is a rare, inherited disorder of extracellular calcium sensing. It is clinically characterised by mild to moderate parathyroid hormone dependent hypercalcaemia, an autosomal dominant pattern of inheritance, and a normal to reduced urinary calcium excretion in spite of high serum calcium.

CASE PRESENTATION

We report two cases of FHH in a family caused by a novel pathogenic missense variant in the CaSR gene, p. His41Arg. Case 1, describes a 17 year old female with no significant past medical history, admitted with acute appendicitis requiring laparoscopic appendectomy and reporting a six month history of polydipsia. Routine investigations were significant for hypercalcaemia, corrected calcium 3.19 mmol/L (2.21-2.52mmol/L), elevated parathyroid hormone of 84pg/ml (15-65pg/ml) and a low 24-hour urine calcium of 0.75mmol/24 (2.50-7.50mmol/24). She was initially managed with intravenous fluids and Zolendronic acid with temporary normalisation of calcium though ultimately required commencement of Cinacalcet 30 mg daily for persistent symptomatic hypercalcaemia. Genetic analysis was subsequently positive for the above variant. Case 2, a 50-year-old female, was referred to the endocrine outpatient clinic for the management of type 2 diabetes and reported a longstanding history of asymptomatic hypercalcaemia which had not been investigated previously. Investigation revealed hypercalcaemia; corrected calcium of 2.6 mmol/L (reference range: 2.21-2.52 mmol/L); PTH of 53.7ng/L (reference range: 15-65 ng/L) and an elevated 24-hour urine calcium of 10 mmol/24 (2.50-7.50 mmol/24hr) with positive genetic analysis and is managed conservatively. Despite sharing this novel mutation, these cases have different phenotypes and their natural history is yet to be determined. Two further relatives are currently undergoing investigation for hypercalcaemia and the family have been referred for genetic counselling.

CONCLUSION

Accurate diagnosis of FHH and differentiation from classic primary hyperparathyroidism can be challenging, however it is essential to avoid unnecessary investigations and parathyroid surgery. Genetic analysis may be helpful in establishing a diagnosis of FHH in light of the biochemical heterogeneity in this population and overlap with other causes of hypercalcaemia.

Amelioration of hypercalcemia by cinacalcet treatment in a subject with relapsing acquired hypocalciuric hypercalcemia: A case report

RATIONALE

Hypocalciuric hypercalcemia is classified as acquired hypocalciuric hypercalcemia (AHH) and familial hypocalciuric hypercalcemia (FHH). While FHH is inherited as an autosomal dominant trait, AHH is one of the rare acquired diseases and is usually treated with prednisolone. Here, we report a case with relapsing AHH which was well controlled with cinacalcet therapy.

PATIENT CONCERN

A 68-year-old Japanese man was referred to our institution because of hypercalcemia. Despite such hypercalcemia, he was almost asymptomatic.

DIAGNOSTICS

We diagnosed him as AHH due to the following reason. First, the ratio of calcium (Ca)/creatinine clearance was very low which met the criteria. Second, there was no overt family history of hypercalcemia. Third, his serum Ca level was within the normal range 3 years before. Fourth, despite hypercalcemia, he was almost asymptomatic and had no evidence of primary hyperparathyroidism.

INTERVENTIONS

Although it is known that steroid therapy is useful for AHH, optimal treatment remains unknown and cinacalcet therapy is very much limited for the treatment of AHH. In this subject, we introduced cinacalcet therapy for the treatment of relapsing AHH.

OUTCOMES

Serum Ca and parathyroid hormone levels were normalized after such therapy with cinacalcet.

CONCLUSIONS

We should bear in mind that cinacalcet treatment is effective for the treatment of relapsing AHH.

Familial Hyperparathyroidism

Regulation of the serum calcium level in humans is achieved by the endocrine action of parathyroid glands working in concert with vitamin D and a set of critical target cells and tissues including osteoblasts, osteoclasts, the renal tubules, and the small intestine. The parathyroid glands, small highly vascularized endocrine organs located behind the thyroid gland, secrete parathyroid hormone (PTH) into the systemic circulation as is needed to keep the serum free calcium concentration within a tight physiologic range. Primary hyperparathyroidism (HPT), a disorder of mineral metabolism usually associated with abnormally elevated serum calcium, results from the uncontrolled release of PTH from one or several abnormal parathyroid glands. Although in the vast majority of cases HPT is a sporadic disease, it can also present as a manifestation of a familial syndrome. Many benign and malignant sporadic parathyroid neoplasms are caused by loss-of-function mutations in tumor suppressor genes that were initially identified by the study of genomic DNA from patients who developed HPT as a manifestation of an inherited syndrome. Somatic and inherited mutations in certain proto-oncogenes can also result in the development of parathyroid tumors. The clinical and genetic investigation of familial HPT in kindreds found to lack germline variants in the already known HPT-predisposition genes represents a promising future direction for the discovery of novel genes relevant to parathyroid tumor development.

Clinical and Molecular Genetics of Primary Hyperparathyroidism

Calcium homeostasis is maintained by the actions of the parathyroid glands, which release parathyroid hormone into the systemic circulation as necessary to maintain the serum calcium concentration within a tight physiologic range. Excessive secretion of parathyroid hormone from one or more neoplastic parathyroid glands, however, causes the metabolic disease primary hyperparathyroidism (HPT) typically associated with hypercalcemia. Although the majority of cases of HPT are sporadic, it can present in the context of a familial syndrome. Mutations in the tumor suppressor genes discovered by the study of such families are now recognized to be pathogenic for many sporadic parathyroid tumors. Inherited and somatic mutations of proto-oncogenes causing parathyroid neoplasia are also known. Future investigation of somatic changes in parathyroid tumor DNA and the study of kindreds with HPT yet lacking germline mutation in the set of genes known to predispose to HPT represent two avenues likely to unmask additional novel genes relevant to parathyroid neoplasia.

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.

Hypoparathyroidism and the Kidney

Hypocalcemia and hyperphosphatemia are the pathognomonic biochemical features of hypoparathyroidism, and result directly from lack of parathyroid hormone (PTH) action on the kidney. In the absence of PTH action, the renal mechanisms transporting calcium and phosphate reabsorption deregulate, resulting in hypocalcemia and hyperphosphatemia. Circulating calcium negatively regulates PTH secretion. Hypocalcemia causes neuromuscular disturbances ranging from epilepsy and tetany to mild paresthesia. Circulating phosphate concentration does not directly regulate PTH secretion. Hyperphosphatemia is subclinical, but chronically promotes ectopic mineralization disease. Vitamin D-thiazide treatment leads to ectopic mineralization and renal damage. PTH treatment has the potential for fewer side effects.

Incidence of hypocalcemia and hypercalcemia in hospitalized patients: Is it changing?

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Disorders of calcium metabolism are frequently encountered in clinical practice.

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Hypocalcemia accounted for 27.72% and hypercalcemia for 4.74% of inpatients.

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Incidence of hypo- and hypercalcemia changed over time.

Disorders of calcium metabolism are frequently encountered in routine clinical practice. However limited data are available on the epidemiology of hypocalcemia and hypercalcemia in hospitalized patients. Our aim was to evaluate the frequency of hypocalcemia and hypercalcemia in hospitalized patients.

This is a retrospective study based on the laboratory results of all hospitalized subjects (n = 12,334) whose calcemia was determined between January 1st, 2011 and December 31st, 2014. Measurements of serum calcium were carried out by a single centralized laboratory. Hypocalcemia was defined as serum calcium levels <8.2 mg/dl and hypercalcemia as serum calcium levels >10.4 mg/dl. Albumin correction was applied to adjust serum calcium values.

Overall, hypocalcemia accounted for 27.72% (n = 3420) and hypercalcemia for 4.74% (n = 585) of the 12,334 inpatients. The highest prevalence of hypocalcemia was found in patients over 65 yr. (n = 2097, 61.31%) vs. younger subjects, while the highest prevalence of hypercalcemia was observed in patients aged 0–18 yr. (n = 380, 64.95%). Hypocalcemia was more often encountered in males (n = 1952, 57.07%) while no gender differences were found regarding hypercalcemia. Incidence of hypocalcemia changed over time varying from 35.42% (n = 1061) in 2011 to 21.93% (n = 672) in 2014 (r = −0.98; p = 0.01). Differently, incidence of hypercalcemia did not significantly increase significantly from 3.47% (n = 104) in 2011 to 6.92% (n = 211) in 2014 (r = 0.94; p = 0.052).

Despite increased awareness about electrolytes disturbance, physicians should consider calcium levels because of life-threatening consequences associated to hypo- and hypercalcemia. Patient’s gender and age could be associated to a different risk of calcium disturbance in hospitalized patients.

Genetics of Hyperparathyroidism, Including Parathyroid Cancer

Primary hyperparathyroidism (HPT) is a metabolic disease caused by the excessive secretion of parathyroid hormone from 1 or more neoplastic parathyroid glands. HPT is largely sporadic, but it can be associated with a familial syndrome. The study of such families led to the discovery of tumor suppressor genes whose loss of function is now recognized to underlie the development of many sporadic parathyroid tumors. Heritable and acquired oncogenes causing parathyroid neoplasia are also known. Studies of somatic changes in parathyroid tumor DNA and investigation of kindreds with unexplained familial HPT promise to unmask more genes relevant to parathyroid neoplasia.

An Excess of CYP24A1, Lack of CaSR, and a Novel lncRNA Near the PTH Gene Characterize an Ectopic PTH-Producing Tumor

Thus far, only 23 cases of the ectopic production of parathyroid hormone (PTH) have been reported. We have characterized the genome-wide transcription profile of an ectopic PTH-producing tumor originating from a retroperitoneal histiocytoma. We found that the calcium-sensing receptor (CaSR) was barely expressed in the tumor. Lack of CaSR, a crucial braking apparatus in the presence of both intraparathyroid and, probably, serendipitous PTH expression, might contribute strongly to the establishment and maintenance of the ectopic transcriptional activation of the PTH gene in nonparathyroid cells. Along with candidate drivers with a crucial frameshift mutation or copy number variation at specific chromosomal areas obtained from whole exome sequencing, we identified robust tumor-specific cytochrome P450 family 24 subfamily A member 1 (CYP24A1) overproduction, which was not observed in other non–PTH-expressing retroperitoneal histiocytoma and parathyroid adenoma samples. We then found a 2.5-kb noncoding RNA in the PTH 3′-downstream region that was exclusively present in the parathyroid adenoma and our tumor. Such a co-occurrence might act as another driver of ectopic PTH-producing tumorigenesis; both might release the control of PTH gene expression by shutting down the other branches of the safety system (e.g., CaSR and the vitamin D3–vitamin D receptor axis).

Low urine calcium excretion in African American patients with primary hyperparathyroidism

OBJECTIVE

To evaluate the prevalence of low urine calcium excretion in African American patients with primary hyperparathyroidism (PHPT), a common disorder associated with bone and renal complications, and to assess the distinction between PHPT and familial hypocalciuric hypercalcemia (FHH), a rare benign genetic disease.

METHODS

We conducted a retrospective study on a cohort of 1,297 patients in whom a 24-hour urine study was performed for measurement of urine calcium and creatinine. PHPT was diagnosed if the serum calcium concentration was ≥10.5 mg/dL and intact parathyroid hormone (PTH) was ≥40 pg/mL. Patients receiving medications that affect urine calcium or with glomerular filtration rate ≤30 mL/min were excluded.

RESULTS

Ninety-six patients satisfied the diagnostic criteria for PHPT. The African American (n = 70) and non-African American (n = 26) patients did not differ in their mean age, body mass index, glomerular filtration rate, serum PTH, 25-hydroxyvitamin D levels, and 24-hour urine creatinine values. Median values of urine calcium/creatinine (mg/g) were 122 for African American versus 214 for non-African American patients (P = .006). Thirty-one of 70 African American patients (44%) had a urine calcium/creatinine ratio ≤100 mg/g, whereas only 2 of 26 non-African American patients (8%) had this value (P = .001).

CONCLUSION

The prevalence of low urine calcium excretion among African American patients with PHPT is unexpectedly high. A threshold of 100 mg/g urine calcium/creatinine identified 44% of such patients with PHPT as having FHH in this cohort. Therefore, other clinical criteria and laboratory variables should be used to distinguish PHPT from FHH in African American patients with PTH-dependent hypercalcemia.

Clinical and biochemical outcomes of cinacalcet treatment of familial hypocalciuric hypercalcemia: a case series

Introduction

Familial hypocalciuric hypercalcemia is a rare benign autosomal-dominant genetic disease with high penetrance. In most cases, patients with familial hypocalciuric hypercalcemia experience unspecific physical discomfort or asymptomatic disease. These patients are typically characterized by mild to moderately increased blood ionized calcium and a normal to slightly elevated serum parathyroid hormone.

Case presentation

Four female patients with familial hypocalciuric hypercalcemia with inactivating mutations in the CaSR gene were included in the treatment study. Three patients were related: two were siblings and one was the daughter of one of these. The ages of the related patients were 51 years, 57 years and 35 years. All three patients were carriers of the same mutation. The fourth patient, unrelated to the others, was 53 years old, and a carrier of a novel and previously unknown mutation leading to familial hypocalciuric hypercalcemia. All four patients were Caucasians of Danish nationality. Biochemically, all patients had elevated blood ionized calcium, serum parathyroid hormone, serum magnesium and total serum calcium, except one, whose serum parathyroid hormone was within the normal range prior to treatment. All patients were treated with cinacalcet in a dosage of 30 mg to 60 mg per day.

Conclusion

Three months after the initiation of cinacalcet treatment, all our patients experiencing clinical signs of hypercalcemia had improved in self -reported well-being and in biochemical parameters. None of our patients suffered adverse events to cinacalcet treatment. Biochemical markers of calcium homeostasis were improved and remained stable during the observation period of 12 months (two patients), 24 and 36 months, in both the symptomatic and the asymptomatic patients.

Outpatient management of primary hyperparathyroidism

Primary hyperparathyroidism is the most common cause of hypercalcemia in an outpatient setting. Most patients are asymptomatic with mild hypercalcemia and only require periodic follow-up. Symptomatic patients and those with end-organ damage should undergo surgery according to the National Institutes of Health guidelines. No medical therapy has been approved for treatment of this disorder. Treatment with bisphosphonate, calcimimetic drugs, or alcohol ablation techniques has been used in select patients.

Calcilytics: antagonists of the calcium-sensing receptor for the treatment of osteoporosis

The only bone anabolic agents currently available on the market are based on the parathyroid hormone (PTH). Secretion of endogenous PTH is controlled by a calcium-sensing receptor at the surface of the parathyroid glands. Antagonists of this receptor (calcilytics) induce the release of the hormone. Provided the effect of the calcilytic is of short duration, a bone anabolic effect should also result. Although the first calcilytic series became known approximately 10 years ago, the number of different structural types is still small today. This article outlines the quest from hits to potent development candidates of all relevant calcilytic series currently known. Even after the front-runners unexpectedly failed in the clinic, the approach for an oral alternative to parenteral PTH remains highly attractive.

A novel CASR mutation in a Tunisian FHH/NSHPT family associated with a mental retardation

The calcium-sensing receptor (CASR), a plasma membrane G-protein coupled receptor, is expressed in parathyroid gland and kidney, and controls systemic calcium homeostasis. Inactivating CASR mutations have previously been identified in patients with familial hypocalciuric hypercalcemia (FHH) and neonatal severe hyperparathyroidism (NSHPT). The aim of the present study is to determine the underlying molecular defect of FHH/NSHPT disease in a consanguineous Tunisian family. Mutation screening was carried out using RFLP-PCR and direct sequencing. We found that the proband is homozygous for a novel 15 bp deletion in the exon 7 (c.1952_1966del) confirming the diagnosis of NSHPT. All the FHH members were found to be heterozygous for the novel detected mutation. The mutation, p.S651_L655del, leads to the deletion of 5 codons in the second trans-membrane domain of the CASR which is thought to be involved in the processes of ligand-induced signaling. This alteration was associated with the evidence of mental retardation in the FHH carriers and appears to be a novel inactivating mutation in the CASR gene. Our findings provide additional support for the implication of CASR gene in the FHH/NSHPT pathogenesis.

Different vitamin D substrate-product relationship after oral vitamin D supplementation in familial benign hypercalcemia, primary hyperparathyroidism, and healthy controls

CONTEXT

In healthy subjects and in patients with primary hyperparathyroidism (PH), the administration of a low dose of 25(OH)D (25 μg/day) increases the serum levels of both 25(OH)D and 1,25(OH)(2)D. It is unknown whether this relationship is present in patients affected by familial benign hypocalciuric hypercalcemia (FBH).

OBJECTIVE

To evaluate the different vitamin D substrate-product relationship after oral vitamin D supplementation in familial benign hypercalcemia, PH, and healthy controls.

DESIGN

We evaluated the main physiological regulators of 1α-hydroxylase and the substrate-product relationship of 25(OH)D and 1,25(OH)(2)D in 20 patients with PH, 25 with FBH, and 122 healthy sex- and age-matched controls before and after administration of 25(OH)D for 2 weeks.

RESULTS

25(OH)D increased significantly in all subjects, whereas 1,25(OH)(2)D serum levels increased significantly in PH patients and healthy controls but not in patients with FBH. Therefore, a significant positive substrate-product relationship of 25(OH)D-1,25(OH)(2)D was found in PH and healthy controls, but not in FBH. Monomeric calcitonin (hCT-M) was significantly lower at baseline and after 25(OH)D supplementation in the FBH group compared with the other two groups.

CONCLUSIONS

The lack of 1,25(OH)(2)D increase in FBH may be due to a direct inhibitory effect on 1α-hydroxylase of hypercalcemia per se, increased metabolic clearance of 1,25(OH)(2)D, or a decreased stimulus of 1α-hydroxylase related to persistently low levels of hCT.

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

Neonatal hyperparathyroidism can be caused by a heterozygous inactivating mutation in the calcium-sensing receptor. Calcimimetics, allosteric activators of the calcium-sensing receptor, may provide an effective means of reducing PTH secretion in such patients. OBJECTIVE/PATIENT: The objective of the study was to identify the molecular defect and to monitor the postnatal course of a 1-wk-old infant with elevated blood ionized calcium, serum PTH, and alkaline phosphatase and low calcium excretion. The parents were normocalcemic.

METHODS

CASR gene mutation analysis was performed on genomic DNA of the proband and her parents. The infant was treated initially with pamidronate and then cinacalcet.

RESULTS

A heterozygous mutation (R185Q, CGA > CAA) in exon 4 of the CASR gene was identified in the proband. The CASR gene of both parents was normal. At 1 wk of age, iv fluids and furosemide were initiated, but hypercalcemia, hyperparathyroidism, and low calcium excretion persisted. At 2 wk of age, a single iv dose of pamidronate resulted in hypocalcemia and further increase in PTH levels, but hypercalcemia recurred within 1 wk. At 3 wk of age, a single oral dose of cinacalcet resulted in decreased PTH levels at 2 h; blood-ionized calcium reached a nadir at 10 h. Three days later daily cinacalcet was initiated, resulting in normalization of ionized calcium. The suppression of serum PTH and reduction in total serum calcium was maintained long term.

CONCLUSIONS

In neonatal hyperparathyroidism secondary to presumed de novo heterozygous CASR mutation, treatment with cinacalcet decreases PTH secretion and serum calcium levels and mitigates the need for parathyroidectomy.

Common variants in the calcium-sensing receptor gene are associated with total serum calcium levels

Serum calcium levels are tightly regulated. We performed genome-wide association studies (GWAS) in population-based studies participating in the CHARGE Consortium to uncover common genetic variations associated with total serum calcium levels. GWAS of serum calcium concentrations was performed in 20 611 individuals of European ancestry for ∼2.5 million genotyped and imputed single-nucleotide polymorphisms (SNPs). The SNP with the lowest P-value was rs17251221 (P = 2.4 * 10(-22), minor allele frequency 14%) in the calcium-sensing receptor gene (CASR). This lead SNP was associated with higher serum calcium levels [0.06 mg/dl (0.015 mmol/l) per copy of the minor G allele] and accounted for 0.54% of the variance in serum calcium concentrations. The identification of variation in CASR that influences serum calcium concentration confirms the results of earlier candidate gene studies. The G allele of rs17251221 was also associated with higher serum magnesium levels (P = 1.2 * 10(-3)), lower serum phosphate levels (P = 2.8 * 10(-7)) and lower bone mineral density at the lumbar spine (P = 0.038), but not the femoral neck. No additional genomic loci contained SNPs associated at genome-wide significance (P < 5 * 10(-8)). These associations resemble clinical characteristics of patients with familial hypocalciuric hypercalcemia, an autosomal-dominant disease arising from rare inactivating mutations in the CASR gene. We conclude that common genetic variation in the CASR gene is associated with similar but milder features in the general population.

Clinical and molecular genetics of parathyroid neoplasms

Primary hyperparathyroidism (HPT) results from the excessive secretion of parathyroid hormone from parathyroid tumours. While most HPT is sporadic, it is associated with a familial syndrome in a minority of cases. The study of these syndromes has helped define the pathophysiology of both familial and sporadic parathyroid neoplasms. Investigation of kindred with multiple endocrine neoplasia type 1 (MEN1) and the hyperparathyroidism-jaw tumour syndrome (HPT-JT) led to the discovery of the tumour suppressor genes MEN1 and HRPT2. We now recognise that somatic mutations in MEN1 and HRPT2 tumour suppressor genes are frequent events in sporadic parathyroid adenomas and carcinomas, respectively. Parathyroid tumours in the MEN2A syndrome result from mutational activation of the RET oncogene. The CCND1/PRAD1 oncogene was discovered by analysis of sporadic parathyroid tumours. Studies of familial isolated HPT and analysis of chromosomal loss and gain in parathyroid tumours suggest that other genes relevant to parathyroid neoplasia await identification.

Hypercalcaemic and hypocalcaemic conditions due to calcium-sensing receptor mutations

The extracellular calcium (Ca2+o)-sensing receptor (CaSR) enables the parathyroid glands and other CaSR-expressing cells involved in calcium homeostasis, such as the kidney and bone, to sense alterations in the level of Ca2+o and to respond with changes in function that are directed at normalizing the blood calcium concentration. Several disorders of Ca2+o sensing arise from inherited or acquired abnormalities that 'reset' the serum calcium concentration upwards or downwards. Heterozygous inactivating mutations of the CaSR produce a benign form of hypercalcaemia, termed 'familial hypocalciuric hypercalcaemia', while homozygous mutations produce a much more severe hypercalcaemic disorder resulting from marked hyperparathyroidism, called 'neonatal severe hyperparathyroidism'. Activating mutations cause a hypocalcaemic syndrome of varying severity, termed 'autosomal-dominant hypocalcaemia or hypoparathyroidism' as well as Bartter's syndrome type V. Calcimimetic CaSR activators and calcilytic CaSR antagonists have also been developed with potential for use in the treatment of these disorders.

The calcium-sensing receptor: physiology, pathophysiology and CaR-based therapeutics

The extracellular calcium (Ca(o)2+)-sensing receptor (CaR) enables the parathyroid glands and other CaR-expressing cells to sense alterations in the level of Ca(o)2+ and to respond with changes in function that are directed at normalizing the blood calcium concentration. In addition to the parathyroid gland, the kidney is a key site for Ca(o)2(+)-sensing that enables it to make physiologically relevant alterations in divalent cation and water metabolism. Several disorders of Ca(o)2(+)-sensing arise from inherited or acquired abnormalities that "reset" the serum calcium concentration upward or downward. Inactivating mutations produce a benign form of hypercalcemia when present in the heterozygous state, termed Familial Hypocalciuric Hypercalcemia (FHH), while homozygous mutations produce a much more severe hypercalcemic disorder resulting from marked hyperparathyroidism, called Neonatal Severe Hyperparathyroidism (NSHPT). Activating mutations cause a hypocalcemic syndrome of varying severity, termed autosomal dominant hypocalcemia or hypoparathyroidism. Inactivating or activating antibodies directed at the CaR produce the expected hyper- or hypocalcemic syndromes, respectively. "Calcimimetic" CaR activators and "calcilytic" CaR antagonists have been developed. The calcimimetics are currently in use for controlling severe hyperparathyroidism in patients receiving dialysis treatment for end stage renal disease or with parathyroid cancer. Calcilytics are being evaluated as a means of inducing a "pulse" in the circulating parathyroid hormone (PTH) concentration, which would mimic that resulting from injection of PTH, an established anabolic form of treatment for osteoporosis.

Normalization of lithium-induced hypercalcemia and hyperparathyroidism with cinacalcet hydrochloride

An underrecognized side effect of long-term lithium carbonate therapy is hyperparathyroidism with associated hypercalcemia and hypocalciuria. Because cessation of lithium carbonate therapy usually does not correct the hyperparathyroidism and associated hypercalcemia, parathyroidectomy frequently is necessary. This is the initial report of 2 patients with lithium carbonate-induced hyperparathyroidism treated with cinacalcet hydrochloride (HCl), which normalized serum calcium levels and reduced intact parathyroid hormone (iPTH) secretion. The patients, both with bipolar disease and a 15- to 30-year history of lithium carbonate therapy, were evaluated for stage 3 chronic kidney disease, persistent hypercalcemia, and hyperparathyroidism. A 67-year-old woman was administered cinacalcet HCl, 30 mg/d, for 11 months. Mean serum calcium level decreased from 10.8 +/- 0.4 mg/dL (2.69 +/- 0.10 mmol/L) to 9.9 +/- 0.4 mg/dL (2.47 +/- 0.10 mmol/L; P < 0.001), and iPTH level decreased from 139 +/- 31 pg/mL (139 +/- 31 ng/L) to 114 +/- 39 pg/mL (114 +/- 39 ng/L; P = not significant). A 63-year-old man was administered 30 mg/d of cinacalcet HCl for 8 months, then 60 mg/d for another 2 months. Mean serum calcium and iPTH levels decreased from 11.0 +/- 0.5 mg/dL (2.74 +/- 0.12 mmol/L) to 10.3 +/- 0.4 mg/dL (2.57 +/- 0.10 mmol/L; P < 0.001) and 138 +/- 10 pg/mL (138 +/- 10 ng/L) to 73 +/- 7 pg/mL (73 +/- 7 ng/L; P = 0.03), respectively. Urinary fractional excretion of calcium was low for both patients before (<0.026 and <0.015) and after (0.026 and 0.008) treatment with cinacalcet HCl. These findings suggest that cinacalcet HCl can provide an alternative nonsurgical means to control this disorder in patients with hypercalcemia of variable severity for whom surgical treatment is not a consideration because of perceived mildness of disease or unsuitability of the patient for surgical intervention.

Role of calcium-sensing receptor in mineral ion metabolism and inherited disorders of calcium-sensing

The extracellular calcium-sensing receptor (CaR), a G protein-coupled receptor that resides on the parathyroid cell surface negatively regulates secretion of parathyroid hormone (PTH). The CaR is functionally expressed in bone, kidney, and gut--the three major calcium-translocating organs involved in calcium homeostasis. Further studies are needed to define fully the homeostatic roles of the CaR in tissues that are involved in systemic extracellular calcium [Ca(2+)](o) homeostasis. The role of the CaR in regulating calcium metabolism has been greatly clarified by the identification and studies of genetically determined disorders that either activate or inactivate the receptor. Antibodies to the CaR that either activate or inactivate it produce syndromes resembling the corresponding genetic diseases. Expression of the CaR is significantly reduced in primary and secondary hyperparathyroidism, which could contribute to the defective [Ca(2+)](o)-sensing in these conditions. Calcimimetics act as CaR agonists or allosteric activators and thereby potentiate the effects of [Ca(2+)](o) on parathyroid cell function. This kind of pharmacological manipulation of the CaR is now used for the treatment of hyperparathyroid states, whereby the calcimimetics increase the activation of the CaR at any given level of extracellular calcium. Calcimimetics are also an effective element in the treatment of secondary hyperparathyroidism, particularly in dialysis patients, by virtue of reducing plasma levels of PTH, calcium and phosphate.

Identification of a novel inactivating R465Q mutation of the calcium-sensing receptor

In this study, we describe a 52-year-old woman, who was diagnosed with familial benign hypocalciuric hypercalcemia (FBHH), a condition characterized by hypercalcemia, low urinary calcium excretion, and normal parathyroid hormone PTH levels, resulting from inactivating mutations of the calcium-sensing receptor (CaSR). In order to identify and characterize the underlying mutation in the CASR gene, direct sequence analysis of CASR exons 2-7 was performed, and functional activity was examined by transient transfection of human embryonic kidney (HEK-293) cells with wild-type and mutant CaSRs, followed by intracellular calcium measurement using fluorometry, and Western blot analysis. Sequence analysis demonstrated, in addition to the already described A986S polymorphism, a novel heterozygous G--> A substitution in CASR exon 5 that causes an arginine to glutamine substitution at codon 465 (R465Q). Functional analysis showed a rightward shift of the dose-response curve with a significant increase of the EC50 from 5.4 mM of the CaSR carrying the A986S polymorphism alone to 11.3 mM of the CaSR carrying the R465Q mutation in the presence of the A986S polymorphism. Western blot analysis of membrane protein revealed an even higher expression level of the R465Q mutant protein compared to wild-type CaSR. In conclusion, we identified a novel heterozygous loss-of-function R465Q mutation of the CASR gene, which is characterized by a blunted response to calcium stimulation, thereby causing FBHH.

Parathyroid and autoimmunity

Data in favor of chronic hypoparathyroidism as an autoimmune disease are examined. The article takes into consideration the different clinical forms, genetic patterns, histopathology, animal models, cellular immunity, circulating autoantibodies, target autoantigens, clinical manifestations, laboratory diagnosis and therapy. Furthermore, data on 71 Italian patients with chronic hypoparathyroidism are presented.

Dissimilar PTH, gastrin, and calcitonin responses to oral calcium and peptones in hypocalciuric hypercalcemia, primary hyperparathyroidism, and normal subjects: a useful tool for differential diagnosis

We analyzed gastrin, PTH, and calcitonin responses to oral calcium and peptones in hypocalciuric hypercalcemia, mild primary hyperparathyroidism, and normal controls. We observed diverse hormonal responses that may help in the differential diagnosis of these conditions.

INTRODUCTION

Hypocalciuric hypercalcemia (HH) is consequent to calcium-sensing receptor (CaSR) genetic mutations or anti-CaSR antibodies. CaSR is expressed in parathyroid tissue, thyroid C cells, and gastrin-secreting cells, where it has been suggested that on calcium and/or amino acid allosteric activation, promotes gastrin secretion.

MATERIALS AND METHODS

We evaluated gastrin, PTH, and calcitonin responses to oral calcium (1 g) and peptones (10 g) in 10 patients with HH (mean age, 58.5 +/- 10.3 years; F/M = 9/1), 15 patients with primary hyperparathyroidism (PH; mean age, 60.4 +/- 8.3 years; F/M = 11/4), and 30 healthy controls (mean age, 60.3 +/- 8.1 years). Statistical analyses for differences during oral loading tests were calculated with ANOVA for repeated measurements and comparisons between two groups were performed with Student's t-test.

RESULTS

PTH response to peptones was markedly increased in patients with PH compared with flat responses in controls and HH patients (p < 0.05). Gastrin increase after oral calcium was absent in HH and PH subjects (p < 0.05 versus controls), and gastrin responses to peptones were blunted in HH and PH subjects compared with controls (p < 0.05). PTH drop and calcitonin increase after calcium load observed in controls were absent in HH and PH subjects (p < 0.05).

CONCLUSIONS

The marked difference in PTH response elicited by peptones observed in PH compared with subjects with HH may help in the differential diagnosis of these conditions without genetic studies. Peptones may stimulate CaSR-controlled hormones as an allosteric regulatory pathway. CaSR abnormalities may help to explain the different calcium- and peptones-induced hormonal responses observed in PH and HH compared with normal subjects.

[Calcimimetics, mechanisms of action and therapeutic applications]

The extracellular calcium-sensing receptor (CaR) on the parathyroid cell surface negatively regulates secretion of parathyroid hormone (PTH). Its activation by small changes in the extracellular concentration of ionized calcium (ec[Ca2+]) decreases PTH secretion and secondarily bone turnover. CaR is an ideal target for compounds that may be developed to modulate its activity - activating calcimimetics and inhibiting calcilytics. Calcimimetics can amplify the sensitivity of the CaR to ec(Ca2+), thereby suppressing PTH levels and in turn reducing blood Ca++. They dose-dependently reduce the secretion of PTH in cultured parathyroid cells, in animal models and in humans. In uremic animals, these compounds prevent parathyroid cell hyperplasia when given at the onset of the disease and stop cell proliferation if they are administered afterwards, when the hyperplasia already exists. They normalize plasma PTH levels and bone remodeling. In uremic patients undergoing hemodialysis, calcimimetics reduce plasma PTH concentrations in the short (12 weeks) and long (2 years) terms. They also reduce serum levels of calcium-phosphorus product. Calcimimetics are therefore an alternative for the treatment of secondary hyperparathyroidism, particularly in dialysis patients, when increased serum levels of calcium-phosphorus product, the attendant risk of cardiovascular calcification, and its lack of efficacy limit use of the standard treatment.

Laboratory screening for hyperparathyroidism

INTRODUCTION

The clinical syndrome produced by excess parathyroid hormone (PTH) is referred to as hyperparathyroidism (HPT). Autonomous growth of PTH producing cells is defined as primary hyperparathyroidism (pHPT). In its classic form pHPT is characterized by painful bones, kidney stones, abdominal groans, psychic moans, fatigue overtones and hypercalcemia. Chronic stimulation of the parathyroid glands secondary to low circulating calcium level results in secondary hyperparathyroidism (sHPT). Tertiary hyperparathyroidism (tHPT) results from prolonged secondary hyperparathyroidism when the glands take on an autonomous function manifested by hypercalcemia and high PTH levels despite resolution of the original stimulus.

REVIEW

The paper reviews the physiologic regulation of PTH secretion and types and forms of HPT. Calcium homeostasis is discussed, emphasizing interactions of PTH, PO4 and vitamin D that can lead to HPT. In addition, the paper reviews the contribution of serum calcium, chloride, phosphorus and PTH levels to the diagnosis of HPT, the role of urinary calcium in the diagnosis of familial benign hypocalciuric hypercalcemia (FBHH), and the role of alkaline phosphatase and bone mass measurements as markers of severity of hyperparathyroid bone disease.

CONCLUSIONS

It is concluded that the diagnosis of hyperparathyroidism can be made with a very high confidence rate by documenting an increased serum PTH level with an increased ionized or total calcium level in pHPT, increased serum PTH level with low or normal calcium level and an underlying renal failure or vitamin D deficiency in sHPT. Early management of HPT is important because many of the nonspecific complains, or classic symptoms, or metabolic conditions often improve after proper control of hyperparathyroidism.

Hypercalcemic hypocalciuria: a critical differential diagnosis for hyperparathyroidism

A differential diagnosis for hyperparathyroidism includes all causes of hypercalcemia; the primary causes of which are primary hyperparathyroidism and humoral hypercalcemia of malignancy. Benign familial hypocalciuric hypercalcemia is a rare condition of primary importance because failure to recognize this entity leads to unnecessary surgery. There are a plethora of other rare causes of hypercalcemia that can be distinguished from primary hyperparathyroidism by history and laboratory profiles. This article discusses the main and less common causes of hypercalcemia and provides guidance for distinguishing among them.

Calcium sensing by endocrine cells

The elucidation of the structure and function of the Ca2+(o)-sensing receptor (CaR) has provided important insights into the normal control of Ca2+(o) homeostasis, particularly the key role of the receptor in kidney and parathyroid. Further studies are needed to define more clearly the homeostatic role of the CaR in additional tissues, both those that are involved and those that are uninvolved in systemic Ca2+(o) homeostasis. The availability of the cloned CaR has also permitted documentation of the molecular basis of inherited disorders of Ca2+(o) sensing, including those in which the receptor is less and or more sensitive than normal to Ca2+(o). Antibodies to the CaR that either activate it or inactivate it produce syndromes resembling the corresponding genetic diseases. Expression of the receptor is abnormally low in 1 degree and 2 degrees hyperparathyroidism, which could contribute to the defective Ca2+(o) sensing in these conditions. The recent discovery of calcimimetics, which sensitize the CaR to Ca2+(o), has provided what will likely be an effective medical therapy for the secondary/tertiary hyperparathyroidism of end stage renal failure as well as for 1 degree hyperparathyroidism.

Osteoclastogenesis, bone resorption, and osteoclast-based therapeutics

Over the past decade, advances in molecular tools, stem cell differentiation, osteoclast and osteoblast signaling mechanisms, and genetically manipulated mice models have resulted in major breakthroughs in understanding osteoclast biology. This review focuses on key advances in our understanding of molecular mechanisms underlying the formation, function, and survival of osteoclasts. These include key signals mediating osteoclast differentiation, including PU.1, RANK, CSF-1/c-fms, and src, and key specializations of the osteoclast including HCl secretion driven by H+-ATPase and the secretion of collagenolytic enzymes including cathepsin K and matrix metalloproteinases (MMPs). These pathways and highly expressed proteins provide targets for specific therapies to modify bone degradation. The main outstanding issues, basic and translational, will be considered in relation to the osteoclast as a target for antiresorptive therapies.

Renal genetic disorders related to K+ and Mg2+

The recent knowledge of the renal epithelial transport systems has exploded with the identification, cloning, and characterization of a large number of membrane transport proteins. The fundamental aspects of these transporters are beginning to emerge at the molecular level and are summarized in the accompanying contributions in this volume of the Annual Review of Physiology. The aim of my review is to integrate this body of knowledge with the understanding of the clinical disorders of human mineral homeostasis that accompany gain, loss, or dysregulation of function of these transport systems. The specific focus is on the best defined human clinical syndromes in which there are derangements in K(+) and Mg(2+) homeostasis.

Hypercalcemia in malignant and inflammatory diseases

Hypercalcemia is one of the most common metabolic abnormalities in human disease. Although there are many causes, most cases are due to neoplasia. Understanding the pathophysiology can lead to correct diagnosis and effective therapy for most patients.

Parathyroid hyperplasia: an unusual cause of neonatal hypercalcemia

Primary hyperparathyroidism (PHPT) in infants is caused by parathyroid chief cell hyperplasia. Patients present with symptoms of chronic hypercalcemia, such as failure to thrive, irritability, abdominal pain, and anorexia. Medical therapy is inadequate, often resulting in chronic hypercalcemia or death. Partial or total surgical removal of the parathyroid gland is the preferred treatment. We describe a case of a 7-month-old infant with PHPT secondary to hyperplasia successfully treated with a subtotal parathyroidectomy.

Familial calcium crystal diseases: what have we learned?

The spectrum of heterotopic calcification or ossification is expanding because of the reports of several kindreds with calcium pyrophosphate deposition disease, apatite deposition disease, and others with less common syndromes associated with extracellular matrix calcification, such as fibrodysplasia ossificans progressiva and related syndromes. Genomic DNA studies in both humans and mice provide a shortcut to understanding the genetic basis of promotion and prevention of ECM calcification. Mutation in the COL2A1 gene has been identified in one family with spondyloepiphyseal dysplasia and calcium pyrophosphate and apatite crystalline deposits. In another kindred with precocious osteoarthritis without spondyloepiphyseal dysplasia, the phenotype was linked to markers of chromosome 8. In four other kindreds, the phenotypes were linked to an area of chromosome 5p. Two genes located in this region, which are expressed in articular cartilage, are being investigated as possible calcium pyrophosphate deposition disease genes. The results of linkage studies in three kindreds with articular/periarticular ADD with the COL2A1 gene were noninformative. Two different mouse mutations, the ank/ank and the ttw/ttw mice, are associated with intra-articular and ligament apatite deposits caused by a decrease in extracellular pyrophosphate concentrations, mimicking human arthritis caused by apatite deposition disease. Mutations in the matrix GLA protein, both in mice and in humans, are also associated with vascular and articular calcification. These mouse mutations provide cutting-edge information in the investigation of the mechanisms of apatite deposition in humans.