No evidence for mutations in the calcium-sensing receptor gene in sporadic parathyroid adenomas

Molecular and Clinical Spectrum of Primary Hyperparathyroidism

Recent data suggest an increase in the overall incidence of parathyroid disorders, with primary hyperparathyroidism (PHPT) being the most prevalent parathyroid disorder. PHPT is associated with morbidities (fractures, kidney stones, chronic kidney disease) and increased risk of death. The symptoms of PHPT can be nonspecific, potentially delaying the diagnosis. Approximately 15% of patients with PHPT have an underlying heritable form of PHPT that may be associated with extraparathyroidal manifestations, requiring active surveillance for these manifestations as seen in multiple endocrine neoplasia type 1 and 2A. Genetic testing for heritable forms should be offered to patients with multiglandular disease, recurrent PHPT, young onset PHPT (age ≤40 years), and those with a family history of parathyroid tumors. However, the underlying genetic cause for the majority of patients with heritable forms of PHPT remains unknown. Distinction between sporadic and heritable forms of PHPT is useful in surgical planning for parathyroidectomy and has implications for the family. The genes currently known to be associated with heritable forms of PHPT account for approximately half of sporadic parathyroid tumors. But the genetic cause in approximately half of the sporadic parathyroid tumors remains unknown. Furthermore, there is no systemic therapy for parathyroid carcinoma, a rare but potentially fatal cause of PHPT. Improved understanding of the molecular characteristics of parathyroid tumors will allow us to identify biomarkers for diagnosis and novel targets for therapy.

Novel PRUNE2 Germline Mutations in Aggressive and Benign Parathyroid Neoplasms

Parathyroid tumors are mostly sporadic but can also occur in familial forms, including different kinds of genetic syndromes with varying phenotypes and penetrance. Recently, somatic mutations of the tumor suppressor gene PRUNE2 were found to be frequent in parathyroid cancer (PC). The germline mutation status of PRUNE2 was investigated in a large cohort of patients with parathyroid tumors from the genetically homogenous Finnish population, 15 of which had PC, 16 atypical parathyroid tumors (APT), and 6 benign parathyroid adenomas (PA). Mutations in previously established hyperparathyroidism-related genes were screened with a targeted gene panel analysis. Nine PRUNE2 germline mutations with a minor allele frequency (MAF) of <0.05 were found in our cohort. Five of these were predicted to be potentially damaging and were identified in two patients with PC, two with APT, and three with PA. The mutational status was not associated with the tumor group nor related to the clinical picture or severity of the disease. Still, the frequent finding of rare germline mutations of PRUNE2 may point to the gene playing a role in the pathogenesis of parathyroid neoplasms.

Epidemiology, Pathophysiology, and Genetics of Primary Hyperparathyroidism

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

Integrated Whole-Exome and Transcriptome Sequencing of Sporadic Parathyroid Adenoma

PURPOSE

Hyperparathyroidism is the third most common endocrine disease. Parathyroid adenoma (PA) accounts for approximately 85% of cases of primary hyperparathyroidism, but the molecular mechanism is not fully understood. Herein, we aimed to investigate the genetic and transcriptomic profiles of sporadic PA.

METHODS

Whole-exome sequencing (WES) and transcriptome sequencing (RNA-seq) of 41 patients with PA and RNA-seq of 5 normal parathyroid tissues were performed. Gene mutations and characterized expression changes were identified. To elucidate the molecular mechanism underlying PA, unsupervised consensus clustering of RNA-seq data was performed. The correlations between the sequencing data and clinicopathological features of these patients were analyzed.

RESULTS

Previously reported PA driver gene mutations, such as MEN1 (9/41), mTOR (4/41), ZFX (3/41), CASR (3/41), EZH2 (2/41) and FAT1 (2/41), were also identified in our cohort. Furthermore, somatic mutation of EZH1, which had not been reported in PA, was found in 4 samples. RNA-seq showed that the expression levels of 84 genes were upregulated and 646 were downregulated in PA samples compared with normal samples. Unsupervised clustering analysis of RNA-seq data clustered these patients into 10 subgroups related to mutation or abnormal expression of a group of potential pathogenic genes.

CONCLUSION

MEN1, EZH2, CASR, EZH1, ZFX, mTOR and FAT1 mutations in PA were revealed. According to the RNA-seq data clustering analysis, cyclin D1, β-catenin, VDR, CASR and GCM2 may be important factors contributing to the PA gene expression profile.

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 characteristics of familial hypocalciuric hypercalcaemia type 1: A multicentre study of 77 adult patients

OBJECTIVE

Familial hypocalciuric hypercalcaemia type 1 (FHH1), related to heterozygous loss-of-function mutations of the calcium-sensing receptor gene, is the main differential diagnosis for primary hyperparathyroidism. The aim of our study was to describe clinical characteristics of adult patients living in France with a genetically confirmed FHH1.

DESIGN AND PATIENTS

This observational, retrospective, multicentre study included 77 adults, followed up in 32 clinical departments in France, with a genetic FHH1 diagnosis between 2001 and 2012.

RESULTS

Hypercalcaemia was diagnosed at a median age of 53 years [IQR: 38-61]. The diagnosis was made after clinical manifestations, routine analysis or familial screening in 56, 34 and 10% of cases, respectively, (n = 58; data not available for 19 patients). Chondrocalcinosis was present in 11/51 patients (22%), bone fractures in 8/56 (14%) and renal colic in 6/55 (11%). The median serum calcium was 2.74 mmol/L [IQR: 2.63-2.86 mmol/L], the median plasma parathyroid hormone level was 4.9 pmol/L [3.1-7.1], and the median 24-hour urinary calcium excretion was 2.8 mmol/24 hours [IQR: 1.9-4.0]. Osteoporosis (dual X-ray absorptiometry) or kidney stones (renal ultrasonography) were found in 6/38 patients (16%) and 9/32 patients (28%), respectively. Fourteen patients (18%) underwent parathyroid surgery; parathyroid adenoma was found in three patients (21%) and parathyroid hyperplasia in nine patients (64%). No correlation between genotype and phenotype was established.

CONCLUSION

This large cohort study demonstrates that FHH1 clinical characteristics can be atypical in 33 patients (43%). Clinicians should be aware of this rare differential diagnosis in order to adopt an appropriate treatment strategy.

Reduced Calcium Sensing Receptor (CaSR) Expression Is Epigenetically Deregulated in Parathyroid Adenomas

AIM

Reduced calcium sensing receptor (CaSR) expression has been implicated in parathyroid tumorigenesis, but the underlying mechanism remains elusive. Accordingly, we aimed to explore the epigenetic changes (DNA methylation and histone modifications) involved in CaSR regulation in sporadic parathyroid adenomas and correlate epigenetic state with disease indices.

EXPERIMENTAL DESIGN

Forty sporadic parathyroid adenomas and 10 control parathyroid tissues were studied. Real-time quantitative PCR (qPCR) for mRNA and immunohistochemistry for protein expression of CaSR were performed. The methylation status of the CaSR promoter 2 was determined by bisulphite sequencing analysis of sodium bisulphite-converted DNA. To determine the role of histone modifications in the CaSR regulation, chromatin immunoprecipitation-qPCR assay was performed.

RESULTS

Real-time qPCR revealed reduced CaSR mRNA expression with a fold reduction of 0.12 (P < 0.0001) in parathyroid adenomas. Immunohistochemistry revealed reduced protein expression of CaSR in 90% (36/40) of adenomas. The promoter 2 region of CaSR displayed significant hypermethylation in 45% (18/40) of the adenomas compared with the controls (6.7%; 1 of 10) (P < 0.002). Bisulphite sequencing analysis revealed maximum methylated CpG at glial cell missing 2 binding site on the CaSR promoter 2 compared to other CpG sites. The methylation status of CaSR correlated directly with plasma intact parathyroid hormone levels in patients with parathyroid adenoma. With chromatin immunoprecipitation-qPCR analysis, H3K9me3 levels showed increased enrichment by 10-fold in adenomas and correlated with CaSR-mRNA expression (r = 0.61; P < 0.003). Treatment with 5-aza-2'deoxycytidine restored the expression of CaSR in a parathyroid cell line.

CONCLUSION

Our data suggest that hypermethylation and increased H3K9me3 of the CaSR promoter 2 are involved in silencing CaSR expression in sporadic parathyroid adenoma.

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.

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

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

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.

Genetics of parathyroids disorders: Overview

Several familial forms of primary hyperparathyroidism (PHTP) have been discovered over the past 25 years, and molecular test for their risk assessment has been widely increasing. These syndromic and non-syndromic forms have received benefits from the identification of the responsible genes whose mutations account for the genetic susceptibility to develop parathyroid tumours as also other endocrine and nonendocrine tumours. In recent years, care options have been made available to patients and families with hereditary PHPT, and the process of systematically assessing the genetic risk has been becoming increasingly important. The aim of this review is to help health providers not frequently dealing with genetic testing use, introducing general concepts with regard to genetic diagnosis issues. The role and the practical usefulness of DNA-based diagnosis in patients affected by different forms of "congenital" PHPT is described, closely looking on why, when and how genetic testing should be performed in these subjects and their relatives. Moreover, this review will provide some practical suggestions and recommendations concerning on how to deal with a suspected or known case of familial PHPT.

Calcium-sensing receptor in colorectal inflammation and cancer: Current insights and future perspectives

The extracellular calcium-sensing receptor (CaSR) is best known for its action in the parathyroid gland and kidneys where it controls body calcium homeostasis. However, the CaSR has different roles in the gastrointestinal tract, where it is ubiquitously expressed. In the colon, the CaSR is involved in controlling multiple mechanisms, including fluid transport, inflammation, cell proliferation and differentiation. Although the expression pattern and functions of the CaSR in the colonic microenvironment are far from being completely understood, evidence has been accumulating that the CaSR might play a protective role against both colonic inflammation and colorectal cancer. For example, CaSR agonists such as dipeptides have been suggested to reduce colonic inflammation, while dietary calcium was shown to reduce the risk of colorectal cancer. CaSR expression is lost in colonic malignancies, indicating that the CaSR is a biomarker for colonic cancer progression. This dual anti-inflammatory and anti-tumourigenic role of the CaSR makes it especially interesting in colitis-associated colorectal cancer. In this review, we describe the clinical and experimental evidence for the role of the CaSR in colonic inflammation and colorectal cancer, the intracellular signalling pathways which are putatively involved in these actions, and the possibilities to exploit these actions of the CaSR for future therapies of colonic inflammation and cancer.

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.

Primary Hyperparathyroidism: Effects on Bone Health

Primary hyperparathyroidism (PHPT) is the most common cause of chronic hypercalcemia. With the advent of routine calcium screening, the classic presentation of renal and osseous symptoms has been largely replaced with mild, asymptomatic disease. In hypercalcemia caused by PHPT, serum parathyroid hormone levels are either high, or inappropriately normal. A single-gland adenoma is responsible for 80% of PHPT cases. Less frequent causes include 4-gland hyperplasia and parathyroid carcinoma. Diminished bone mineral density and nephrolithiasis are the major current clinical sequelae. Parathyroidectomy is the only definitive treatment for PHPT, and in experienced hands, cure rates approach 98%.

Hyperplasia in glands with hormone excess

Five syndromes share predominantly hyperplastic glands with a primary excess of hormones: neonatal severe primary hyperparathyroidism, from homozygous mutated CASR, begins severely in utero; congenital non-autoimmune thyrotoxicosis, from mutated TSHR, varies from severe with fetal onset to mild with adult onset; familial male-limited precocious puberty, from mutated LHR, expresses testosterone oversecretion in young boys; hereditary ovarian hyperstimulation syndrome, from mutated FSHR, expresses symptomatic systemic vascular permeabilities during pregnancy; and familial hyperaldosteronism type IIIA, from mutated KCNJ5, presents in young children with hypertension and hypokalemia. The grouping of these five syndromes highlights predominant hyperplasia as a stable tissue endpoint and as their tissue stage for all of the hormone excess. Comparisons were made among this and two other groups of syndromes, forming a continuum of gland staging: predominant oversecretions express little or no hyperplasia; predominant hyperplasias express little or no neoplasia; and predominant neoplasias express nodules, adenomas, or cancers. Hyperplasias may progress (5 of 5) to neoplastic stages while predominant oversecretions rarely do (1 of 6; frequencies differ P<0.02). Hyperplasias do not show tumor multiplicity (0 of 5) unlike neoplasias that do (13 of 19; P<0.02). Hyperplasias express mutation of a plasma membrane-bound sensor (5 of 5), while neoplasias rarely do (3 of 14; P<0.002). In conclusion, the multiple distinguishing themes within the hyperplasias establish a robust pathophysiology. It has the shared and novel feature of mutant sensors in the plasma membrane, suggesting that these are major contributors to hyperplasia.

Polyclonality of Parathyroid Tumors in Neonatal Severe Hyperparathyroidism

Neonatal severe hyperparathyroidism (NSHPT) is a rare disorder characterized by major hypercalcemia, elevated parathyroid hormone levels, and marked enlargement of multiple parathyroid glands, usually associated with germline mutations in the calcium receptor gene CASR. However, little is known about the outgrowth of parathyroid tumors in NSHPT, including whether they represent monoclonal or polyclonal expansions. We sought to examine the clonality of parathyroid tissues resected from a patient with NSHPT and biallelic CASR mutations. DNA from two distinct parathyroid tumors resected from a girl with NSHPT, plus polyclonal/monoclonal control samples, were subjected to analyses of clonality by two independent methods, X-chromosome inactivation analysis at the androgen receptor locus (HUMARA) and BAC array comparative genomic hybridization (CGH). Both parathyroid tumor samples revealed polyclonal patterns by X-inactivation analysis, with polyclonal and monoclonal controls yielding the expected patterns. Similarly, by BAC array CGH, neither parathyroid sample contained monoclonal copy number changes and both appeared identical to the patient-matched polyclonal controls. Our observations provide direct experimental evidence that the markedly enlarged parathyroid tumors in the setting of NSHPT constitute polyclonal, generalized hyperplastic growths rather than monoclonal neoplasms.

Clinicopathological correlates of hyperparathyroidism

Hyperparathyroidism is a common endocrine disorder with potential complications on the skeletal, renal, neurocognitive and cardiovascular systems. While most cases (95%) occur sporadically, about 5% are associated with a hereditary syndrome: multiple endocrine neoplasia syndromes (MEN-1, MEN-2A, MEN-4), hyperparathyroidism-jaw tumour syndrome (HPT-JT), familial hypocalciuric hypercalcaemia (FHH-1, FHH-2, FHH-3), familial hypercalciuric hypercalcaemia, neonatal severe hyperparathyroidism and isolated familial hyperparathyroidism. Recently, molecular mechanisms underlying possible tumour suppressor genes (MEN1, CDC73/HRPT2, CDKIs, APC, SFRPs, GSK3β, RASSF1A, HIC1, RIZ1, WT1, CaSR, GNA11, AP2S1) and proto-oncogenes (CCND1/PRAD1, RET, ZFX, CTNNB1, EZH2) have been uncovered in the pathogenesis of hyperparathyroidism. While bi-allelic inactivation of CDC73/HRPT2 seems unique to parathyroid malignancy, aberrant activation of cyclin D1 and Wnt/β-catenin signalling has been reported in benign and malignant parathyroid tumours. Clinicopathological correlates of primary hyperparathyroidism include parathyroid adenoma (80-85%), hyperplasia (10-15%) and carcinoma (<1-5%). Secondary hyperparathyroidism generally presents with diffuse parathyroid hyperplasia, whereas tertiary hyperparathyroidism reflects the emergence of autonomous parathyroid hormone (PTH)-producing neoplasm(s) from secondary parathyroid hyperplasia. Surgical resection of abnormal parathyroid tissue remains the only curative treatment in primary hyperparathyroidism, and parathyroidectomy specimens are frequently encountered in this setting. Clinical and biochemical features, including intraoperative PTH levels, number, weight and size of the affected parathyroid gland(s), are crucial parameters to consider when rendering an accurate diagnosis of parathyroid proliferations. This review provides an update on the expanding knowledge of hyperparathyroidism and highlights the clinicopathological correlations of this prevalent disease.

Parathyroid adenoma in a patient with familial hypocalciuric hypercalcaemia

A 57-year-old man with symptoms of fatigue, joint pains and insomnia was found to have hypercalcaemia secondary to hyperparathyroidism with a corrected calcium of 2.61 mmol/L (2.2-2.6 mmol/L) and a serum parathyroid hormone (PTH) of 86 pg/mL (10-65 pg/mL). Preoperative workup demonstrated a parathyroid adenoma in the right upper position and he proceeded to surgery. Postoperatively, however, his symptoms remained unchanged and the corrected calcium remained elevated at 2.87 mmol/L with a PTH of 59 pg/mL. He had no family history of hypercalcaemia. Further investigations revealed low 24 h urinary calcium level and a low urine calcium to creatinine ratio. Genetic testing revealed a mutation in exon 4 of the calcium sensing receptor (CaSR) confirming a diagnosis of familial hypocalciuric hyercalcaemia (FHH). The case is an example of a rare phenomenon when a parathyroid adenoma develops in patients with FHH.

Changes in parathyroid proteome in patients with primary hyperparathyroidism due to sporadic parathyroid adenomas

PURPOSE

The pathogenesis of parathyroid tumours is only partially understood. A direct approach using proteomics could be a promising tool to increase our understanding of parathyroid tumorigenesis. The aim of the study was to investigate differentially expressed proteins to explore the underlying molecular basis of the disease and identify potential target proteins responsible for the genesis of adenoma.

METHODS

Proteins were extracted from adenomatous and normal parathyroid tissues. Differentially expressed proteins were separated by two-dimensional gel electrophoresis (2-D) and identified by matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry. Statistical analysis was performed using spss 10.01 software.

RESULTS

Comparative analysis of the 2-D profiles of proteins isolated from adenomatous and normal parathyroid tissues showed 15 differentially expressed proteins, of which 11 were overexpressed. The characterized proteins were associated with diverse cellular functions including regulation of cell organization, programmed cell death, transcription and signal transduction.

CONCLUSION

The differentially expressed proteins in parathyroid adenomas may potentially serve as new targets to investigate the mechanisms of parathyroid adenoma transformation.

Genetic and epigenetic changes in sporadic endocrine tumors: parathyroid tumors

Parathyroid neoplasia is most commonly due to benign parathyroid adenoma but rarely can be caused by malignant parathyroid carcinoma. Evidence suggests that parathyroid carcinomas rarely, if ever, evolve through an identifiable benign intermediate, with the notable exception of carcinomas associated with the familial hyperparathyroidism-jaw tumor syndrome. Several genes have been directly implicated in the pathogenesis of typical sporadic parathyroid adenoma; somatic mutations in the MEN1 tumor suppressor gene are the most frequent finding, and alterations in the cyclin D1/PRAD1 oncogene are also firmly established molecular drivers of sporadic adenomas. In addition, good evidence supports mutation in the CDKN1B/p27 cyclin-dependent kinase inhibitor (CDKI) gene, and in other CDKI genes as contributing to disease pathogenesis in this context. Somatic defects in additional genes, including β-catenin, POT1 and EZH2 may contribute to parathyroid adenoma formation but, for most, their ability to drive parathyroid tumorigenesis remains to be demonstrated experimentally. Further, genetic predisposition to sporadic presentations of parathyroid adenoma appears be conferred by rare, and probably low-penetrance, germline variants in CDKI genes and, perhaps, in other genes such as CASR and AIP. The HRPT2 tumor suppressor gene is commonly mutated in parathyroid carcinoma.

Aryl hydrocarbon receptor interacting protein (AIP) mutations occur rarely in sporadic parathyroid adenomas

CONTEXT

The molecular pathogenesis of primary hyperparathyroidism is still largely unknown. The aryl hydrocarbon receptor interacting protein (AIP) gene has a major role in the pathogenesis of familial isolated pituitary adenoma.

OBJECTIVE

We evaluated the involvement of the AIP gene in sporadic parathyroid adenomas.

PATIENTS AND DESIGN

We performed direct sequencing and multiplex ligation-dependent probe amplification analyses of the AIP gene in a large series of sporadic parathyroid adenomas. Loss of heterozygosity (LOH) at the AIP locus was studied, and aryl hydrocarbon receptor interacting protein immunostaining was also performed. In addition, alterations in the MEN1 gene were studied.

RESULTS

A somatic AIP mutation, substitution of arginine with glutamine at codon 304 (R304Q), was identified in 2 of 132 tumors. The mutation was germline in both cases despite the nonfamilial presentation. Heterozygous AIP large deletions were detected in 29 cases including 1 of the 2 mutated tumors, confirming a biallelic inactivation of the AIP gene. The AIP-mutated tumor with LOH showed decreased AIP immunostaining compared with normal parathyroid. LOH at the MEN1 locus, which often shared LOH at the AIP locus, was found in one third of tumors. Somatic MEN1 mutations were found in the 1 of the 2 AIP-mutated tumors and in 22 parathyroid adenomas. In addition, multiplex ligation-dependent probe amplification analysis revealed 1 large deletion of the MEN1 gene in 1 patient.

CONCLUSIONS

The AIP gene is rarely involved in parathyroid adenomas, but the germline nature of the mutations suggests that it might predispose to primary hyperparathyroidism. MEN1 gene alterations occur in a substantial proportion of sporadic parathyroid adenomas.

Increased protein kinase A type Iα regulatory subunit expression in parathyroid gland adenomas of patients with primary hyperparathyroidism

Protein kinase A (PKA) regulatory subunit type Iα (RIα) is a major regulatory subunit that functions as an inhibitor of PKA kinase activity. We have previously demonstrated that elevated RIα expression is associated with diffuse-to-nodular transformation of hyperplasia in parathyroid glands of renal hyperparathyroidism. The aim of the current study was to determine whether or not RIα expression is increased in adenomas of primary hyperparathyroidism (PHPT), because monoclonal proliferation has been demonstrated in both adenomas and nodular hyperplasia. Surgical specimens comprising 22 adenomas and 11 normal glands, obtained from 22 patients with PHPT, were analyzed. Western blot and immunohistochemical analyses were employed to evaluate RIα expression. PKA activities were determined in several adenomas highly expressing RIα. RIα expression was also separately evaluated in chief and oxyphilic cells using the "Allred score" system. Expression of proliferating cell nuclear antigen (PCNA), a proliferation marker, was also immunohistochemically examined. Western blot analysis revealed that 5 out of 8 adenomas highly expressed RIα, compared with normal glands. PKA activity in adenomas was significantly less than in normal glands. Immunohistochemical analysis further demonstrated high expression of RIα in 20 out of 22 adenomas. In adenomas, the greater RIα expression and more PCNA positive cells were observed in both chief and oxyphilic cells. The present study suggested that high RIα expression could contribute to monoclonal proliferation of parathyroid cells by impairing the cAMP/PKA signaling pathway.

Calcium sensing receptor signalling in physiology and cancer

The calcium sensing receptor (CaSR) is a class C G-protein-coupled receptor that is crucial for the feedback regulation of extracellular free ionised calcium homeostasis. While extracellular calcium (Ca(2+)o) is considered the primary physiological ligand, the CaSR is activated physiologically by a plethora of molecules including polyamines and l-amino acids. Activation of the CaSR by different ligands has the ability to stabilise unique conformations of the receptor, which may lead to preferential coupling of different G proteins; a phenomenon termed 'ligand-biased signalling'. While mutations of the CaSR are currently not linked with any malignancies, altered CaSR expression and function are associated with cancer progression. Interestingly, the CaSR appears to act both as a tumour suppressor and an oncogene, depending on the pathophysiology involved. Reduced expression of the CaSR occurs in both parathyroid and colon cancers, leading to loss of the growth suppressing effect of high Ca(2+)o. On the other hand, activation of the CaSR might facilitate metastasis to bone in breast and prostate cancer. A deeper understanding of the mechanisms driving CaSR signalling in different tissues, aided by a systems biology approach, will be instrumental in developing novel drugs that target the CaSR or its ligands in cancer. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

The role of the calcium-sensing receptor in human disease

Following the discovery of the calcium-sensing receptor (CaSR) in 1993, its pivotal role in disorders of calcium homeostasis such as Familial Hypocalciuric Hypercalcemia (FHH) was quickly demonstrated. Since then, it has become clear that the CaSR has immense functional versatility largely through its ability to activate many different signaling pathways in a ligand- and tissue-specific manner. This allows the receptor to play diverse and crucial roles in human physiology and pathophysiology, both in calcium homeostasis and in tissues and biological processes unrelated to calcium balance. This review covers current knowledge of the role of the CaSR in disorders of calcium homeostasis (FHH, neonatal severe hyperparathyroidism, autosomal dominant hypocalcemia, primary and secondary hyperparathyroidism, hypercalcemia of malignancy) as well as unrelated diseases such as breast and colorectal cancer (where the receptor appears to play a tumor suppressor role), Alzheimer's disease, pancreatitis, diabetes mellitus, hypertension and bone and gastrointestinal disorders. In addition, it examines the use or potential use of CaSR agonists or antagonists (calcimimetics and calcilytics) and other drugs mediated through the CaSR, in the management of disorders as diverse as hyperparathyroidism, osteoporosis and gastrointestinal disease.

Molecular diagnosis of parathyroid carcinoma: a reality in the near future

INTRODUCTION

Parathyroid carcinoma (PC) is a rare endocrine malignancy that still suffers from a problem of delayed clinical diagnosis. Consequently, it usually is not recognized preoperatively and often is not conclusively identified during the operation either.

AREAS COVERED

The role played by parafibromin in the development of PC, representing an important advance in understanding the pathogenesis of this malignancy, is discussed. Through a careful search of the international literature, using "parathyroid carcinoma", "molecular genetics of parathyroid carcinoma" and "parathyroid tumorigenesis" as key words, other less mentioned molecular mechanisms are reappraised as potential molecular markers of PC; we also discuss their potential role in 20 parathyroid outgrowths. Finally, both the major efforts and the limitations of reported molecular diagnostic techniques and diagnostic markers are considered.

EXPERT OPINION

Currently, several critical issues still need to be addressed, such as the lack of: i) common criteria for the histopathological diagnosis of parathyroid malignancy and ii) timely appropriated preoperative diagnosis of PC. The latter issue would be of fundamental importance to assist the surgeon in performing a complete resection of all carcinomatous tissue at the time of the initial surgery, allowing for the greatest likelihood of a cure.

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.

Residue 826 in the calcium-sensing receptor is implicated in the response to calcium and to R-568 calcimimetic compound

Within the extracellular loops of the seven-transmembrane domain of the calcium-sensing receptor (CaR) there is a region (I819-E837) relevant for calcimimetic activity. As the naturally occurring variant Ala826Thr is within this important region, it may be postulated that this change may influence the CaR response to calcium and R-568. Human embryonic kidney (HEK-293) cells transiently transfected with three different human CaRs (wild-type [A826], variant allele [T826], and artificial mutant [W826]) were used to test the ability of calcium alone or in combination with the calcimimetic R-568 to modulate CaR activity. CaR activation was detected by flow cytometry using a fluorescent probe. Intracellular calcium changes were measured in response to changes in extracellular calcium alone or with different R-568 concentrations. The change of the alanine in the 826 position (A826) for threonine (T826) worsened calcium sensitivity, increasing the EC(50) value from 2.34 +/- 0.48 mM (A826, wild-type) to 2.96 +/- 0.75 mM (T826) (P < 0.05). The T826 receptor reached a similar response with 1 muM R-568 compared with the wild-type receptor. On the contrary, the artificial introduction of a tryptophan in the same position (W826) did not affect calcium sensitivity (EC(50) = 2.64 +/- 0.81 mM) but reduced the ability of the receptor to respond to R-568. The results demonstrate the importance of the 826 residue in the CaR response to calcium and calcimimetics. Since the A826T change was described as a natural variant, the differences in the calcium and calcimimetic responses observed between the alleles could have potential clinical impact.

Calcium-sensing receptor in cancer: good cop or bad cop?

The extracellular calcium-sensing receptor (CaR) is a versatile 'sensor' for di- and polycationic molecules in the body. CaR plays a key role in the defense against hypercalcemia by "sensing" extracellular calcium levels in the parathyroid and kidney, the key organs maintaining systemic calcium homeostasis. Although mutation of CaR gene has so far not been associated with any malignancy, aberrant functions of CaR have implications in malignant progression. One situation is loss of CaR expression, resulting in loss of growth suppressing effects of elevated extracellular Ca(2+) by CaR, reported in parathyroid adenoma and in colon carcinoma. Another situation is activation of CaR, resulting in increased production of parathyroid hormone-related peptide (PTHrP), a primary causal factor in hypercalcemia of malignancy and a contributor to metastatic processes involving bone. CaR signaling and effects have been studied in several cancers including ovarian cancers, gastrinomas, and gliomas in addition to comparatively detailed studies in breast, prostate, and colon cancers. Studies on H-500 rat Leydig cells, a xenotransplantable model of humoral hypercalcemia of malignancy has shed much light on the mechanisms of CaR-induced cancer cell growth and survival. Pharmacological agonists and antagonists of CaR hold therapeutic promise depending on whether activation of CaR is required such as in case of colon cancer or inactivating the receptor is required as in the case of breast- and prostate tumors.

The role of the calcium-sensing receptor in the development and progression of cancer

The calcium-sensing receptor (CaR) is responsive to changes in the extracellular Ca(2+) (Ca(2+)(o)) concentration. It is a member of the largest family of cell surface receptors, the G protein-coupled receptors, and it has been shown to be involved in Ca(2+)(o) homeostasis. Apart from its primary role in Ca(2+)(o) homeostasis, the CaR may be involved in phenomena that allow for the development of many types of benign or malignant tumors, from parathyroid adenomas to breast, prostate, and colon cancers. For example, whereas the CaR is expressed in both normal and malignant breast tissue, increased CaR levels have been reported in highly metastatic primary breast cancer cells and breast cancer cell lines, possibly contributing to their malignancy and associated alterations in their biological properties. In these settings the CaR exhibits oncogenic properties. Enhanced CaR expression and altered proliferation of prostate cancer cells in response to increased Ca(2+)(o) have also been described. In contrast, colon and parathyroid cancers often present with reduced or absent CaR expression, and activation of this receptor decreases cell proliferation, suggesting a role for the CaR as a tumor suppressor gene. Thus, the CaR may play an important role in the development of many types of neoplasia. Herein, we review the role of the CaR in various benign and malignant tumors in further detail, describing its contribution to parathyroid tumors, breast, prostate, and colon cancers, and we evaluate how pharmacological manipulations of this receptor may be of interest for the treatment of certain cancers in the future.

Identification and functional characterization of loss-of-function mutations of the calcium-sensing receptor in four Italian kindreds with familial hypocalciuric hypercalcemia

OBJECTIVE

Identification and characterization of calcium-sensing receptor (CASR) mutations in four unrelated Italian kindreds with familial hypocalciuric hypercalcemia.

DESIGN

Clinical evaluation and genetic analysis of CASR gene. Functional characterization of mutated CASRs.

METHODS

Direct sequencing of CASR gene in genomic DNA. Studies of CASR-mediated increases in cytosolic calcium concentration [Ca(2)(+)](i) in CASR-transfected COS-7 cells in vitro.

RESULTS

Four unreported heterozygous CASR mutations were identified, including three missense (H595Y, P748H, and C765W) and one splice site (IVS2+1G>C) mutation. The H595Y, P748H, and C765W mutant receptors, although expressed at normal levels on the cell surface, showed a reduced response in [Ca(2)(+)](i) relative to the wildtype (WT) CASR to increasing extracellular calcium concentrations. Cotransfection experiments showed that the H595Y and P748H mutants did not affect the apparent affinity of the WT CASR for calcium, suggesting that they do not exert a dominant-negative effect. On the other hand, the co-transfected C765W mutant decreased the maximum response of the WT CASR to calcium, suggesting that it may reduce the effective concentration of the normal CASR on the cell surface or impair its maximal signaling capacity.

CONCLUSIONS

Four CASR mutations were identified. The reduced functional responses to extracellular calcium and normal expression of the mutant receptors suggest that conformational changes account for altered CASR activity. Moreover, a reduced complement of normal CASRs in these heterozygous patients, perhaps combined with a mutant receptor-induced decrease in maximal activity of the WT receptor, may contribute to defective calcium-sensing in vivo.

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.

Genetic variation at the calcium-sensing receptor (CASR) locus: implications for clinical molecular diagnostics

OBJECTIVES

The calcium-sensing receptor (CASR) is critical for maintenance of blood calcium in a narrow physiologic range. Naturally occurring mutations in the calcium-sensing receptor gene (CASR) cause hypocalcaemia or hypercalcaemia, and molecular diagnosis of these mutations is clinically important. Knowledge of SNP frequency and haplotype structure is essential in understanding molecular test results.

DESIGN AND METHODS

Genotyping and haplotype analysis of 26 CASR SNPs (and a tetranucleotide insertion/deletion polymorphism) in control cohorts of Caucasian, Asian and African-American origin (n=1136, 88 and 104 chromosomes, respectively).

RESULTS

The three SNPs in exon 7 (A986S, R990G, Q1011E) are the only common exonic variants in our cohorts, and synonymous exonic SNPs are uncommon. Linkage disequilibrium analysis of the Caucasian cohort (Haploview) showed that the CASR locus is divided into three haplotype blocks, coincident with 5' regulatory, coding, and 3' regulatory domains.

CONCLUSIONS

These analyses provide an important framework for appropriate interpretation of CASR mutation screening now offered by a number of laboratories for the diagnosis of calcium disorders. They will assist in the study of CASR polymorphisms as predictors of complex disease states.

The diagnosis and management of asymptomatic primary hyperparathyroidism

Classical primary hyperparathyroidism--a disease of 'bones, stones, and psychic groans'--is a rarity in the US, although it is still seen in some parts of the world. Today, most patients with primary hyperparathyroidism are asymptomatic. This change in clinical profile is not merely because of improved detection techniques, and patients with primary hyperparathyroidism who lack symptoms, if left untreated, rarely develop the 'classical' features that were common previously. Indeed, currently available data suggest that there is, today, a disorder with different, yet characteristic, effects on the end-organs of the hyperparathyroid process. In this review, the features of the skeletal, renal, neuropsychologic, cardiovascular, and gastrointestinal manifestations of 'asymptomatic' primary hyperparathyroidism will be discussed. The considerable body of data that has emerged since the recognition of the new clinical profile of primary hyperparathyroidism has allowed a reconsideration of standards of care for this disease. These data were reviewed and incorporated into new clinical guidelines at the 2002 NIH workshop on asymptomatic primary hyperparathyroidism. These recommendations highlight both what is known about the disease, and important areas that need investigation. Continued interest in the skeletal, cardiovascular, and neuropsychiatric manifestations of the disease, and therapeutic trials of medical approaches for its management, will further refine our current understanding, and could lead to additional modifications in the approach to patients with this common endocrine disorder.

The calcium-sensing receptor and related diseases

The calcium-sensing receptor (CASR) adjusts the extracellular calcium set point regulating PTH secretion and renal calcium excretion. The receptor is expressed in several tissues and is also involved in other cellular functions such as proliferation, differentiation and other hormonal secretion. High extracellular calcium levels activate the receptor resulting in modulation of several signaling pathways depending on the target tissues. Mutations in the CASR gene can result in gain or loss of receptor function. Gain of function mutations are associated to Autossomal dominant hypocalcemia and Bartter syndrome type V, while loss of function mutations are associated to Familial hypocalciuric hypercalcemia and Neonatal severe hyperparathyroidism. More than one hundred mutations were described in this gene. In addition to calcium, the receptor also interacts with several ions and polyamines. The CASR is a potential therapeutic target to treatment of diseases including hyperparathyroidism and osteoporosis, since its interaction with pharmacological compounds results in modulation of PTH secretion.

Le récepteur sensible au calcium : physiologie et pathologie

Calcium is a major ion in human metabolism and its level is highly controlled. This regulation is performed via the Calcium Sensing Receptor, a discovery which ten years ago led to the explanation of a number of clinical disorders. The syndromes caused by CaSR abnormalities are characterized by hypercalcemia or hypocalcemia, associated with inappropriate calciuria. An underlying genetic or auto-immune cause may be demonstrated. High blood calcium levels linked to mutations of the CaSR gene lead to familial hypocalciuric hypercalcemia and the neonatal and non neonatal forms with severe hypercalcemic. Hypocalcemia determined by mutations in the CaSR gene include autosomal dominant hypocalcemia and its sporadic form. Another clinical presentation similar to Bartter syndrome has been reported. Auto-antibodies directed against CaSRs, seen in auto-immune diseases, can lead to similar clinical presentations. Finally, CaSR polymorphisms modulate the range of blood calcium levels. With diagnosis of these diseases deleterious therapeutics can be avoided. The discovery of this receptor has led to new therapeutic prospects such as calcimimetics for hyperthyroidism.

Relationship between parathyroid calcium-sensing receptor expression and potency of the calcimimetic, cinacalcet, in suppressing parathyroid hormone secretion in an in vivo murine model of primary hyperparathyroidism

Cinacalcet HCl, an allosteric modulator of the calcium-sensing receptor (CaR), has recently been approved for the treatment of secondary hyperparathyroidism in patients with chronic kidney disease on dialysis, due to its suppressive effect on parathyroid hormone (PTH) secretion. Although cinacalcet's effects in patients with primary and secondary hyperparathyroidism have been reported, the crucial relationship between the effect of calcimimetics and CaR expression on the parathyroid glands requires better understanding. To investigate its suppressive effect on PTH secretion in primary hyperparathyroidism, in which hypercalcemia may already have stimulated considerable CaR activity, we investigated the effect of cinacalcet HCl on PTH-cyclin D1 transgenic mice (PC2 mice), a model of primary hyperparathyroidism with hypo-expression of CaR on their parathyroid glands. A single administration of 30 mg/kg body weight (BW) of cinacalcet HCl significantly suppressed serum calcium (Ca) levels 2 h after administration in 65- to 85-week-old PC2 mice with chronic biochemical hyperparathyroidism. The percentage reduction in serum PTH was significantly correlated with CaR hypo-expression in the parathyroid glands. In older PC2 mice (93-99 weeks old) with advanced hyperparathyroidism, serum Ca and PTH levels were not suppressed by 30 mg cinacalcet HCl/kg. However, serum Ca and PTH levels were significantly suppressed by 100 mg/kg of cinacalcet HCl, suggesting that higher doses of this compound could overcome severe hyperparathyroidism. To conclude, cinacalcet HCl demonstrated potency in a murine model of primary hyperparathyroidism in spite of any presumed endogenous CaR activation by hypercalcemia and hypo-expression of CaR in the parathyroid glands.

The effect of vitamin D status on the severity of bone disease and on the other features of primary hyperparathyroidism (pHPT) in a vitamin D deficient region

A clear relationship between vitamin D status and the clinical indices of primary hyperparathyroidism (pHPT) severity has not been convincingly established. We proposed that such a relationship might exist, in so far as vitamin D deficiency could contribute to the severity of metabolic bone disease and promote the growth of the parathyroid tumor. Accordingly, we undertook a retrospective study and analyzed the clinical, biochemical, radiological and histopathological findings in a group of 49 patients who underwent parathyroidectomy at our center. Patients who had skeletal X-rays were grouped, according to their X-ray findings, in group A (19 patients; 45%) if they had severe bone changes, or group B (23 patients; 55%) if they had mild or no bone changes. Patients were also stratified according to their 25-hydroxyvitamin D (25-OHD) levels in tertiles. The 2 groups were compared using Fisher's exact test or analysis of variance as appropriate. Group A patients were younger (p=0.001), had more musculoskeletal symptoms (p=0.0003), and complained more frequently of fatigue (p=0.02). They had higher alkaline phosphatase (AP; p=0.0002), PTH index (p=0.0007), and serum Ca level (p=0.006). There were more patients from the lower and middle vitamin D tertiles and fewer patients from the upper vitamin D tertile in group A (p=0.02). Post-operative severe hypo-calcemia was more prevalent in group A patients (p<0.0001). Resected parathyroid tumors were larger in size in group A patients (p=0.01), and weighed more (p=0.01). There was a positive correlation between the weight of the parathyroid tumor and the PTH index (p=0.002), and AP level (p=0.0007). We concluded that vitamin D deficiency is a contributing factor to both the severity of bone disease and the high activity of parathyroid tumors seen in many patients with pHPT in vitamin D deficient regions.

Asymptomatic primary hyperparathyroidism: a medical perspective

The clinical phenotype of primary hyperparathyroidism in the United States has changed markedly over the past century, with the majority of patients having minimal overt symptomatology. It has become clear, however, that "asymptomatic" primary hyperparathyroidism is a disease with distinct physiologic characteristics. Data accumulated over recent years suggest that asymptomatic patients meeting specific criteria can be safely followed without surgery. These guidelines are elucidated, and new options in the medical therapeutics of primary hyperparathyroidism are discussed.

The role of the calcium-sensing receptor in cancer

The extracellular calcium-sensing receptor (CaR) is a versatile sensor of small, polycationic molecules ranging from Ca2+ and Mg2+ through polyarginine, spermine, and neomycin. The sensitivity of the CaR to changes in extracellular Ca2+ over the range of 0.05-5 mM positions the CaR as a key mediator of cellular responses to physiologically relevant changes in extracellular Ca2+. For many cell types, including intestinal epithelial cells, breast epithelial cells, keratinocytes, and ovarian surface epithelial cells, changes in extracellular Ca2+ concentration over this range can switch the cellular behaviour from proliferation to terminal differentiation or quiescence. As cancer is predominantly a disease of disordered balance between proliferation, differentiation, and apoptosis, disruptions in the function of the CaR could contribute to the progression of neoplastic disease. Loss of the growth suppressing effects of elevated extracellular Ca2+ have been demonstrated in parathyroid hyperplasias and in colon carcinoma, and have been correlated with changes in the level of CaR expression. Activation of the CaR has also been linked to increased expression and secretion of PTHrP (parathyroid hormone-related peptide), a primary causal factor in hypercalcemia of malignancy and a contributor to metastatic processes involving bone. Although mutation of the CaR does not appear to be an early event in carcinogenesis, loss or upregulation of normal CaR function can contribute to several aspects of neoplastic progression, so that therapeutic strategies directed at the CaR could potentially serve a supportive function in cancer management.

Familial hypocalciuric hypercalcemia in a woman with metastatic breast cancer: a case report of mistaken identity

We describe a 45-yr-old woman with metastatic breast cancer and hypercalcemia previously diagnosed as hypercalcemia of malignancy and treated with bisphosphonates without changes of serum calcium (s-Ca). At the time of our evaluation, biochemical data [s-Ca, 10.8 mg/dl (2.70 mmol/liter); PTH, 24.4 pg/ml (2.6 pmol/liter); 24-h urinary calcium, 160 mg (4.0 mmol); calcium/creatinine clearance, 0.007] suggested the diagnosis of familial hypocalciuric hypercalcemia. Three of five relatives had mild hypercalcemia [s-Ca, 10.7-11.2 mg/dl (2.67-2.80 mmol/liter)] and detectable serum PTH [24.5-29.0 pg/ml (2.6-3.1 pmol/liter)]. A novel heterozygous I212T missense mutation in exon 4 of the calcium-sensing receptor (CaR) gene was found in the proband and affected relatives but not in unaffected relatives. Expression of the mutant I212T CaR in COS-7 cells resulted in no response of inositol phosphates to any calcium concentration. The calcium dose-response curve of the coexpressed receptors [wild-type/I212T] suggested that the mutant receptor interferes with the function of the wild-type receptor. In conclusion, we describe a case of familial hypocalciuric hypercalcemia due to a novel CaR mutation, in a woman with breast cancer in whom hypercalcemia was initially attributed to hypercalcemia of malignancy.

Update on genetic and clinical aspects of primary hyperparathyroidism

Primary hyperparathyroidism (pHPT) is a common endocrine disorder that predominantly affects postmenopausal women. It is mostly caused by solitary tumours within the parathyroid glands. Although the pathophysiology of pHPT is still incompletely understood, recent studies provide new clues on the development and cellular growth of tumours within the parathyroids associated with hypersecretion of parathyroid hormone and hypercalcaemia. The natural course of pHPT is rather benign. Nowadays, it has become an oligo- or asymptomatic disease often only detected by routine blood tests. These facts raise the question whether to perform parathyroidectomy on oligo- and asymptomatic patients with pHPT or whether it is possible to monitor these patients without surgery. The aim of this article is to review the literature as regards (i) the pathophysiological mechanisms that underlie parathyroid neoplasia and (ii) the defective calcium-sensing in patients with pHPT (iii) environmental and/or genetic risk factors that predispose to or promote parathyroid neoplasia, as well as (iv) alternative approaches to treat oligo- and asymptomatic patients with pHPT medically.

Sodium bicarbonate infusion test: a new method for evaluating parathyroid function

We have developed a new test for estimating the secretory capacity of parathyroid hormone (PTH) from the parathyroid gland. Sodium bicarbonate solution [8.4% (w/v); 35 ml/m(2) body surface area] was infused for 2 min, and blood samples for the determination of plasma ionized calcium, plasma PTH (intact, midregion, carboxy-terminus) and related parameters were serially obtained. In 8 healthy volunteers, the mean (+/-SE) plasma ionized calcium fell promptly and significantly (from 1.21 +/- 0.01 to 1.11 +/- 0.01 mmol/L) after the sodium bicarbonate infusion. The mean (+/-SE) plasma intact PTH increased promptly and significantly, by more than four fold (42.3 +/- 4.2 to 182.4 +/- 34.7 pg/ml), and then gradually returned to basal levels. In patients with partial hypoparathyroidism who have detectable basal plasma levels of PTH, the absolute increment in PTH levels was much less, and in the plasma obtained from patients with complete hypoparathyroidism, absolutely no response was observed. Plasma obtained from patients diagnosed with primary hyperparathyroidism (parathyroid adenoma or hyperplasia) has high basal PTH levels. The response to the sodium bicarbonate infusion in these patients was markedly blunted (less than a two-fold increase in all cases examined). No significant adverse effects were observed during the procedure. Therefore, the sodium bicarbonate infusion test is a simple and sensitive method to stimulate PTH release, and is clinically useful for evaluating parathyroid gland function.

Use of calcimimetics in uremic patients with secondary hyperparathyroidism: review

Recognition of the role of the extracellular calcium sensing receptor (CaR) in mineral metabolism has greatly improved our understanding of calcium homeostasis. The activation of this receptor by small changes in extracellular ionized calcium (ec(Ca2+)) regulates PTH, calcitonin secretion, urinary calcium excretion, and ultimately, bone turnover. The cloning of this CaR and the discovery of mutations making the receptor less or more sensitive to calcium allowed a better understanding of several hereditary disorders characterized either by hyperparathyroidism or hypoparathyroidism. This CaR became an ideal target for the development of compounds, the calcimimetics, able to amplify the sensitivity of the CaR to ec(Ca2+) suppressing PTH levels with a resultant fall in blood Ca2+. The first clinical studies with first-generation calcimimetic agents have demonstrated their efficacy lowering plasma intact PTH concentration in uremic patients with secondary hyperparathyroidism. However, the low bioavailability of these first calcimimetics predicts a difficult clinical utilization. The second-generation calcimimetic AMG-073, with a better pharmacokinetic profile, appears to be effective and safe for the treatment of secondary hyperparathyroidism, producing suppression of PTH levels with a simultaneous reduction in serum phosphorus levels and the calcium X phosphorus product. The advantage of controlling PTH secretion without the complications related to hypercalcemia, hyperphosphatemia, and increased calcium X phosphorus product is very promising.

Two Italian kindreds with familial hypocalciuric hypercalcaemia caused by loss-of-function mutations in the calcium-sensing receptor (CaR) gene: functional characterization of a novel CaR missense mutation

OBJECTIVES

Description of two unrelated Italian kindreds with familial hypocalciuric hypercalcaemia (FHH), an autosomal dominant disease mostly caused by heterozygous inactivating mutations of the Ca2+ sensing receptor (CaR).

PATIENTS AND DESIGN

We studied 11 members of the two families. Genomic DNA was isolated from peripheral blood leucocytes in all family members and in 50 unrelated Italian controls. Total serum and ionized calcium, PTH, creatinine, phosphate, magnesium, and urinary calcium clearance to creatinine clearance ratio were measured. Direct sequencing of the entire coding region of the CaR was performed in the probands. Functional studies were performed in COS-7 cells transiently expressing the mutated CaR.

RESULTS

In the proband of family A direct sequencing revealed a novel heterozygous Y218C missense mutation in exon 4. The same mutation was identified in the affected but not in the unaffected family members or in any of the 50 unrelated Italian controls. Transient expression of the Y218C CaR in COS-7 cells revealed a blunted Ca2+-evoked accumulation of inositol trisphosphates, indicating that the Y218C is a loss-of-function mutation. Cotransfection experiments showed that the mutant receptor had no impact on the function of the wild-type receptor, suggesting that a reduced expression of the normal CaR, rather than a dominant-negative effect, accounted for the functional impairment. In the proband of family B an already described heterozygous P55L missense mutation in exon 2 of the CaR gene was found. The same mutation was identified in the affected family members.

CONCLUSIONS

We described two familial hypocalciuric hypercalcaemia kindreds with loss-of-function mutations of the Ca2+ receptor gene and identified a novel heterozygous mutation (Y218C) characterized by a blunted response to Ca2+ stimulation compared to the wild-type receptor and no interference with the function of the wild-type Ca2+ receptor.