A novel loss-of-function mutation, Gln459Arg, of the calcium-sensing receptor gene associated with apparent autosomal recessive inheritance of familial hypocalciuric hypercalcemia

Severe Hyperparathyroidism in a 3-year-old girl Due to Homozygous Inactivating Variant of the Calcium Sensing Receptor Gene

Homozygous inactivating pathogenic variants of calcium-sensing receptors cause severe hyperparathyroidism (HPT), typically presenting in early infancy. In the pediatric age group, HPT is uncommon and can be due to parathyroid adenoma or hyperplasia. We describe the case of a 3-year, 10-month-old girl who presented with severe HPT with symptomatic hypercalcemia, skeletal demineralization, and bone pains resulting from a homozygous missense variant p.Gly670Arg in the CaSR gene that was previously reported in association with familial hypocalciuric hypercalcemia in its heterozygous state. Parathyroidectomy with autotransplant of one-fourth of the gland in the forearm did not result in a cure even after the removal of autotransplant. In addition to controlling preoperative hypercalcemia, pamidronate helped alleviate bone pains and improve skeletal remineralization when used postoperatively for a brief period.

Adolescent primary hyperparathyroidism

Adolescent primary hyperparathyroidism (PHPT) is a rare endocrine disorder bearing distinctions from the adult form. This review examines its unique aspects, focusing on clinical presentation, genetic etiologies, genotype-phenotype correlations, and therapeutic management. Adolescent PHPT often has a genetic basis, whether familial, syndromic, or apparently sporadic, and identifying the underlying genetic cause is important for patient care. The clinical presentation is predominantly symptomatic worldwide. Unique manifestations in this age group include rickets, short stature, and slipped capital femoral epiphysis. Genotype-specific differences are evident in the adolescent PHPT characteristics. Diagnostic evaluation requires careful interpretation of biochemical and dual-energy X-ray absorptiometry findings using age and gender-specific reference ranges, with targeted screening for syndrome-associated neoplasms. Surgery remains the cornerstone of management. Current knowledge gaps in their management include treatment protocols for multiple endocrine neoplasia type 1-associated PHPT, the efficacy and safety of nonsurgical options, and long-term post-surgical outcomes.

Generation of CRISPR/Cas9 modified human iPSC line with correction of heterozygous mutation in exon 6 of the CaSR gene

The calcium-sensing receptor (CaSR) gene encodes a cell membrane G protein-coupled receptor (GPCR) which has a key role in maintaining the extracellular Ca2+ homeostasis. We aimed at correcting the compound heterozygous mutation in the 6th [c.1656delA, p.I554SfsX73] and 7th [c.2217 T > A, p.C739X] exons of the CASR gene which the original patient-derived iPSC line had. The mutation is associated with neonatal severe primary hyperparathyroidism of the patient. We generated and characterized a CRISP/Cas9-edited hiPSC line with the restored sequence in the sixth exon of the CASR gene, bearing only heterozygous mutation in the 7th exon. The results showed that the new genetically modified cell line has karyotype without abnormalities, typical hiPSCs morphology, characteristic expression of pluripotency markers, and ability to develop into three germ layers, and differentiates in chondrogenic, adipogenic, osteogenic directions. This new cell line will complement the existing pool of CaSR-mutated cell lines, a valuable resource for in-depth understanding of neonatal severe primary hyperparathyroidism. This will allow further exploration of the application of pharmacological drugs in the context of personalized medicine to correct Ca-homeostasis disorders.

Novel Calcium-Sensing Receptor (CASR) Mutation in a Family with Autosomal Dominant Hypocalcemia Type 1 (ADH1): Genetic Study over Three Generations and Clinical Characteristics

INTRODUCTION

Activating mutation of the calcium-sensing receptor gene (CASR) reduces parathyroid hormone secretion and renal tubular reabsorption of calcium, defined as autosomal dominant hypocalcemia type 1 (ADH1). Patients with ADH1 may present with hypocalcemia-induced seizures. Calcitriol and calcium supplementation in symptomatic patients may exacerbate hypercalciuria, leading to nephrocalcinosis, nephrolithiasis, and compromised renal function.

METHODS

We report on a family with seven members over three generations with ADH1 due to a novel heterozygous mutation in exon 4 of CASR: c.416T>C.

RESULTS

This mutation leads to substitution of isoleucine with threonine in the ligand-binding domain of CASR. HEK293T cells transfected with wild type or mutant cDNAs demonstrated that p.Ile139Thr substitution led to increased sensitivity of the CASR to activation by extracellular calcium relative to the wild-type CASR (EC50 of 0.88 ± 0.02 mM vs. 1.1 ± 0.23 mM, respectively, p < 0.005). Clinical characteristics included seizures (2 patients), nephrocalcinosis and nephrolithiasis (3 patients), and early lens opacity (2 patients). In 3 of the patients, serum calcium and urinary calcium-to-creatinine ratio levels obtained simultaneously over 49 patient-years were highly correlated. Using the age-specific maximal-normal levels of calcium-to-creatinine ratio in the correlation equation, we obtained age-adjusted serum calcium levels that are high enough to reduce hypocalcemia-induced seizures and low enough to reduce hypercalciuria.

CONCLUSION

We report on a novel CASR mutation in a three-generation kindred. Comprehensive clinical data enabled us to suggest age-specific upper limit of serum calcium levels, considering the association between serum calcium and renal calcium excretion.

Genotype-Phenotype Correlations in Asian Indian Children and Adolescents with Primary Hyperparathyroidism

Childhood and adolescent primary hyperparathyroidism (PHPT) is a very rare disease. Data on its molecular genetics are scarce. We performed a retrospective analysis (January 2000-January 2021) to determine the deleterious germline variants and genotype-phenotype correlations in children and adolescents < 20 years diagnosed with PHPT from a single referral center. Clinical features, biochemistry, imaging, management, and genetics (clinical exome analyzed for 11 PHPT and 7 pancreatitis-associated genes, MLPA for CDC73) were recorded. Thirty-six patients (20 males; median age 17 years) were classified into those with familial and/or syndromic (F/S) or apparently sporadic (AS) presentation. Sixteen (44.4%) harbored pathogenic/likely pathogenic germline variants in PHPT-associated genes. The genetic yield in F/S group was 90% (MEN1:8/10; CDC73:1/10), and AS group was 26.9% (CDC73:4/26; CASR:3/26). F/S group had frequent asymptomatic presentation (60% vs none; P < 0.001), lower serum PTH (237.5 vs 1369.1 pg/mL; P = 0.001), and maximum parathyroid dimension (0.9 vs 2.2 cm; P = 0.01) than AS group. Among the AS group, renal involvement was higher in those with molecular diagnoses (71.4% vs 10.5%; P = 0.01). All those with novel CASR variants (including one homozygous) had hypercalciuria and histology-proven parathyroid adenoma/carcinoma. A missense CTRC VUS occurred in one patient with chronic pancreatitis. In summary, Asian Indian children and adolescents with PHPT have high genetic yield, even with apparently sporadic presentation. The phenotypic spectrum of CASR variants is expanded to include childhood/adolescent PHPT with hypercalciuria and single gland neoplasia. The proposed roles for renal involvement to predict molecular diagnosis among those with apparently sporadic presentation require further elucidation.

A novel case of neonatal severe hyperparathyroidism successfully treated with a type II calcimimetic drug

We report a boy with hypercalcemia due to neonatal severe hyperparathyroidism (NSHPT) caused by a compound heterozygous mutation in the calcium sensing receptor (CaSR) managed successfully on a type II calcimimetic drug. The hypercalcemia was temporarily treated by hyperhydration, bisphosphonate and calcium depleted milk. At 29 days of age cinacalcet was introduced. The starting dose was 0.5 mg/kg/day and was subsequently titrated to the point of efficacy (5.2 mg/kg/day) when a persuasive reduction in parathyroid hormone and calcium concentrations was observed. We propose a trial of type II calcimimetics in newborns with NSHPT irrespective of the genetic mutation and advocate that residual functionality of the CaSR predict the drug efficacy.

[A series of clinical cases of familial hypocalciuric hypercalcemia syndrome]

Familial hypocalciuric hypercalcemia (FHH) - rare disease with predominantly autosomal dominant inheritance. FHH typically develops due to a heterozygous inactivating mutation in the calcium-sensitive receptor gene (CASR), less commonly due to heterozygous mutations in GNA11 and AP2S1. CASR mutations lead to an increase in the threshold for calcium sensitivity, which requires a higher concentration in serum to reduce the release of PTH. These changes are accompanied by an increase of calcium and magnesium reabsorption in the proximal tubules, which leads to hypercalcemia and hypocalciuria. Basically, FHH may be asymptomatic or accompanied by mild hypercalcemia. FHH doesn't require surgical treatment, unlike primary hyperparathyroidism (PHPT), therefore, differential diagnosis of these two conditions is extremely important. In addition, immediate relatives of a proband with FHH also require the exclusion of disease inheritance. We analyzed a series of clinical cases with a genetically confirmed diagnosis of FHH. Our clinical cases indicate a variety of clinical manifestations and the difficulties of differential diagnosis with PHPT.

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

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

Neonatal Severe Hyperparathyroidism: Novel Insights From Calcium, PTH, and the CASR Gene

CONTEXT

Neonatal severe hyperparathyroidism (NSHPT) is rare and potentially lethal. It is usually from homozygous or heterozygous germline-inactivating CASR variant(s). NSHPT shows a puzzling range of serum calcium and parathyroid hormone (PTH) levels. Optimal therapy is unclear.

EVIDENCE ACQUISITION

We categorized genotype/phenotype pairings related to CASRs. For the 2 pairings in NSHPT, each of 57 cases of neonatal severe hyperparathyroidism required calcium, PTH, upper normal PTH, and dosage of a germline pathogenic CASR variant.

EVIDENCE SYNTHESIS

Homozygous and heterozygous NSHPT are 2 among a spectrum of 9 genotype/phenotype pairings relating to CASRs and NSHPT. For the 2 NSHPT pairings, expressions differ in CASR allelic dosage, CASR variant severity, and sufficiency of maternofetal calcium fluxes. Homozygous dosage of CASR variants was generally more aggressive than heterozygous. Among heterozygotes, high-grade CASR variants in vitro were more pathogenic in vivo than low-grade variants. Fetal calcium insufficiency as from maternal hypoparathyroidism caused fetal secondary hyperparathyroidism, which persisted and was reversible in neonates. Among NSHPT pairings, calcium and PTH were higher in CASR homozygotes than in heterozygotes. Extreme hypercalcemia (above 4.5 mM; normal 2.2-2.6 mM) is a robust biomarker, occurring only in homozygotes (83% of that pairing). It could occur during the first week.

CONCLUSIONS

In NSHPT pairings, the homozygotes for pathogenic CASR variants show higher calcium and PTH levels than heterozygotes. Calcium levels above 4.5 mM among NSHPT are frequent and unique only to most homozygotes. This cutoff supports early and robust diagnosis of CASR dosage. Thereby, it promotes definitive total parathyroidectomy in most homozygotes.

Heterozygous Mutation (Q459R) in the Calcium-Sensing Receptor Gene Causes Familial Hypocalciuric Hypercalcemia 1 (FHH1)

CONTEXT

Several heterozygous loss-of-function mutations in the calcium-sensing receptor gene (CASR) leading to elevated ionized serum calcium and familial hypocalciuric hypercalcemia 1 (FHH1) have been characterized. Few mutations are not pathogenic, and previous studies suggested that the Q459R mutation does not result in an FHH1 phenotype.

OBJECTIVE

We identified a family with a heterozygous CASR Q459R mutation and characterized their calcium homeostasis and the pathophysiological mechanisms of a homozygous and heterozygous Q459R mutation in vitro.

DESIGN

The index patient and her family had clinical, biochemical, and genetic analyses performed. In vitro functional characterization of homozygous and heterozygous (Q459R) mutations was conducted by determining CaSR cell-surface expression and inositol monophosphate (IP1) signaling in transiently transfected human embryonic kidney 293A (HEK293A) cells.

RESULTS

All 3 heterozygous carriers had mild asymptomatic hypercalcemia, hypocalciuria, and 2 had elevated serum parathyroid hormone (PTH). In vitro characterization in HEK293A cells revealed that CASR Q459R is a loss-of-function mutation with no impact on cell-surface expression. Cells with the homozygous Q459R genotype had significantly reduced calcium potency of IP1 signaling compared to wild type, whereas the heterozygous Q459R also had lower calcium potency albeit not significantly different from wild type.

CONCLUSION

A loss-of-function Q459R mutation in CASR in a family caused FHH1 characterized by elevated ionized calcium and PTH and low calcium excretion. The marked presence of CaSR at the membrane and inhibition of IP1 signaling in vitro suggest that calcimimetics may be functional in patients with this mutation, which seems to be a mild loss-of-function mutation associated with autosomal dominant transmission of FHH1.

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.

CALCIUM CREATININE CLEARANCE RATIO IS NOT HELPFUL IN DIFFERENTIATING PRIMARY HYPERPARATHYROIDISM FROM FAMILIAL HERPERCALCEMIC HYPOCALCIURIA: A STUDY OF 1000 PATIENTS

OBJECTIVE

With increasing recognition of more subtle presentations of primary hyperparathyroidism (pHPT), laboratory values are frequently seen in a range that would be expected for patients who have familial hypercalcemic hypocalciuria (FHH). Calcium creatinine clearance ratio (CCCR) has been advocated as a diagnostic tool to differentiate between these two disorders. However, it is limited by an indeterminate range (0.01-0.02). The aim of this study is to assess the relevance of CCCR in a modern series of patients with surgically managed pHPT.

METHODS

We performed a retrospective cohort study of 1000 patients who underwent parathyroid surgery for pHPT over eleven years. CCCR was evaluated by degree of biochemical derangement, single versus multiple gland disease and interfering medications.

RESULTS

Patient demographics and resected histopathology were typical for a current series of patients with pHPT. In retrospect, none of the patients were suspected to have FHH post operatively. CCCR was less than 0.01 for 19.0%, between 0.01-0.02 for 43.7% and greater than 0.02 in 37.3%. Distribution of CCCR for patients free from interfering medications and different histological subtypes were the same. One third of the cohort had mild calcium elevations, more typical for FHH. Of these, almost two thirds had a CCCR in a range suspect for FHH (<0.02).

CONCLUSION

To our knowledge this is the largest series to evaluate the validity of CCCR for patients with surgically confirmed pPHT. The utility of CCCR in screening for FHH is limited, as 63% of modern patients with confirmed pHPT have low values.

Pro-FHH: A Risk Equation to Facilitate the Diagnosis of Parathyroid-Related Hypercalcemia

CONTEXT

Parathyroid-related hypercalcemia is due to primary hyperparathyroidism (PHPT) or to familial hypocalciuric hypercalcemia (FHH). PHPT can lead to complications that necessitate parathyroidectomy. FHH is a rare genetic disease resembling PHPT; surgery is ineffective. A reliable method for distinguishing FHH from PHPT is needed.

OBJECTIVE

To develop an easy-to-use tool to predict if a patient has PHPT.

DESIGN

Retrospective analysis of two prospective cohorts. Development of an unsupervised risk equation (Pro-FHH).

SETTING

University hospitals in Paris, France, and Aarhus, Denmark.

PARTICIPANTS

Patients (Paris: 65 with FHH, 85 with PHPT; Aarhus: 38 with FHH, 55 with PHPT) were adults with hypercalcemia and PTH concentration within normal range.

MAIN OUTCOME MEASURES

Performance of Pro-FHH to predict PHPT.

RESULTS

Pro-FHH takes into account plasma calcium, PTH, and serum osteocalcin concentrations, and calcium-to-creatinine clearance ratio calculated from 24-hour urine collection (24h-CCCR). In the Paris cohort, area under the receiver operating characteristic curve (AUROC) of Pro-FHH was 0.961, higher than that of 24h-CCCR. With a cutoff value of 0.928, Pro-FHH had 100% specificity and 100% positive predictive value for the diagnosis of PHPT; it correctly categorized 51 of 85 patients with PHPT; the remaining 34 were recommended to undergo genetic testing. No patients with FHH were wrongly categorized. In an independent cohort from Aarhus, AUROC of Pro-FHH was 0.951, higher than that of 24h-CCCR.

CONCLUSION

Pro-FHH effectively predicted whether a patient has PHPT. A prospective trial is necessary to assess its usefulness in a larger population and in patients with elevated PTH concentration.

TRPV6 Variants Interfere with Maternal-Fetal Calcium Transport through the Placenta and Cause Transient Neonatal Hyperparathyroidism

Transient neonatal hyperparathyroidism (TNHP) is etiologically a heterogeneous condition. One of the etiologies is an insufficient maternal-fetal calcium transport through the placenta. We report six subjects with homozygous and/or compound-heterozygous mutations in the gene encoding the transient receptor potential cation channel, subfamily V, member 6 (TRPV6), an epithelial Ca²⁺-selective channel associated with this condition. Exome sequencing on two neonates with skeletal findings consistent with neonatal hyperparathyroidism identified homozygous frameshift mutations before the first transmembrane domain in a subject born to first-cousins parents of Pakistani descent as well as compound-heterozygous mutations (a combination of a frameshift mutation and an intronic mutation that alters mRNA splicing) in an individual born to a non-consanguineous couple of African descent. Subsequently, targeted mutation analysis of TRPV6 performed on four other individuals (born to non-consanguineous Japanese parents) with similar X-rays findings identified compound-heterozygous mutations. The skeletal findings improved or resolved in most subjects during the first few months of life. We identified three missense variants (at the outer edges of the second and third transmembrane domains) that alter the localization of the TRPV6: one recurrent variant at the S2-S3 loop and two recurrent variants (in the fourth ankyrin repeat domain) that impair TRPV6 stability. Compound heterozygous loss-of-function mutations for the pathogenic frameshift allele and the allele with an intronic c.607+5G>A mutation resulted in the most severe phenotype. These results suggest that TNHP is an autosomal-recessive disease caused by TRPV6 mutations that affect maternal-fetal calcium transport.

Diseases associated with calcium-sensing receptor

The calcium-sensing receptor (CaSR) plays a pivotal role in systemic calcium metabolism by regulating parathyroid hormone secretion and urinary calcium excretion. The diseases caused by an abnormality of the CaSR are genetically determined or are more rarely acquired. The genetic diseases consist of hyper- or hypocalcemia disorders. Hypercalcaemia disorders are related to inactivating mutations of the CASR gene either heterozygous (autosomal dominant familial benign hypercalcaemia, still named hypocalciuric hypercalcaemia syndrome type 1) or homozygous (severe neonatal hyperparathyroidism). The A986S, R990G and Q1011E variants of the CASR gene are associated with higher serum calcium levels than in the general population, hypercalciuria being also associated with the R990G variant. The differential diagnosis consists in the hypocalciuric hypercalcaemia syndrome, types 2 (involving GNA11 gene) and 3 (involving AP2S1 gene); hyperparathyroidism; abnormalities of vitamin D metabolism, involving CYP24A1 and SLC34A1 genes; and reduced GFR. Hypocalcemia disorders, which are more rare, are related to heterozygous activating mutations of the CASR gene (type 1), consisting of autosomal dominant hypocalcemia disorders, sometimes with a presentation of pseudo-Bartter’s syndrome. The differential diagnosis consists of the hypercalciuric hypocalcaemia syndrome type 2, involving GNA11 gene and other hypoparathyroidism aetiologies. The acquired diseases are related to the presence of anti-CaSR antibodies, which can cause hyper- or especially hypocalcemia disorders (for instance in APECED syndromes), determined by their functionality. Finally, the role of CaSR in digestive, respiratory, cardiovascular and neoplastic diseases is gradually coming to light, providing new therapeutic possibilities. Two types of CaSR modulators are known: CaSR agonists (or activators, still named calcimimetics) and calcilytic antagonists (or inhibitors of the CasR). CaSR agonists, such as cinacalcet, are indicated in secondary and primary hyperparathyroidism. Calcilytics have no efficacy in osteoporosis, but could be useful in the treatment of hypercalciuric hypocalcaemia syndromes.

Heterozygous inactivating CaSR mutations causing neonatal hyperparathyroidism: function, inheritance and phenotype

BACKGROUND

Homozygous inactivating mutations of the calcium-sensing receptor (CaSR) lead to neonatal severe hyperparathyroidism (NSHPT), whereas heterozygous inactivating mutations result in familial hypocalciuric hypercalcemia (FHH). It is unknown why in some cases heterozygous CaSR mutations cause neonatal hyperparathyroidism (NHPT) clinically similar to NSHPT but with only moderately elevated serum calcium.

METHODS

A literature survey was conducted to identify patients with heterozygous CaSR mutations and NHPT. The common NHPT CaSR mutants R185Q and R227L were compared with 15 mutants causing only FHH in the heterozygous state. We studied in vitro calcium signaling including the functional consequences of co-expression of mutant and wild-type (wt) CaSR, patients' phenotype, age of disease manifestation and mode of inheritance.

RESULTS

All inactivating CaSR mutants impaired calcium signaling of wt-CaSR regardless of the patients' clinical phenotype. The absolute intracellular calcium signaling response to physiologic extracellular calcium concentrations in vitro showed a high correlation with patients' serum calcium concentrations in vivo, which is similar in NHPT and FHH patients with the same genotype. Pedigrees of FHH families revealed that paternal inheritance per se does not necessarily lead to NHPT but may only cause FHH.

CONCLUSIONS

There is a significant correlation between in vitro functional impairment of the CaSR at physiologic calcium concentrations and the severity of alterations in calcium homeostasis in patients. Whether a particular genotype leads to NHPT or FHH appears to depend on additional predisposing genetic or environmental factors. An individual therapeutic approach appears to be warranted for NHPT patients.

GENETICS IN ENDOCRINOLOGY: Gain and loss of function mutations of the calcium-sensing receptor and associated proteins: current treatment concepts

The calcium-sensing receptor (CASR) is the main calcium sensor in the maintenance of calcium metabolism. Mutations of the CASR, the G protein alpha 11 (GNA11) and the adaptor-related protein complex 2 sigma 1 subunit (AP2S1) genes can shift the set point for calcium sensing causing hyper- or hypo-calcemic disorders. Therapeutic concepts for these rare diseases range from general therapies of hyper- and hypo-calcemic conditions to more pathophysiology oriented approaches such as parathyroid hormone (PTH) substitution and allosteric CASR modulators. Cinacalcet is a calcimimetic that enhances receptor function and has gained approval for the treatment of hyperparathyroidism. Calcilytics in turn attenuate CASR activity and are currently under investigation for the treatment of various diseases. We conducted a literature search for reports about treatment of patients harboring inactivating or activating CASR, GNA11 or AP2S1 mutants and about in vitro effects of allosteric CASR modulators on mutated CASR. The therapeutic concepts for patients with familial hypocalciuric hypercalcemia (FHH), neonatal hyperparathyroidism (NHPT), neonatal severe hyperparathyroidism (NSHPT) and autosomal dominant hypocalcemia (ADH) are reviewed. FHH is usually benign, but symptomatic patients benefit from cinacalcet. In NSHPT patients pamidronate effectively lowers serum calcium, but most patients require parathyroidectomy. In some patients cinacalcet can obviate the need for surgery, particularly in heterozygous NHPT. Symptomatic ADH patients respond to vitamin D and calcium supplementation but this may increase calciuria and renal complications. PTH treatment can reduce relative hypercalciuria. None of the currently available therapies for ADH, however, prevent tissue calcifications and complications, which may become possible with calcilytics that correct the underlying pathophysiologic defect.

Association of Mutations in SLC12A1 Encoding the NKCC2 Cotransporter With Neonatal Primary Hyperparathyroidism

CONTEXT

Primary hyperparathyroidism with hypercalciuria has not been described in the newborn period.

OBJECTIVE

Our objectives are to identify the genetic basis for neonatal primary hyperparathyroidism in a family with 2 affected children.

SUBJECTS

An African American boy presenting with mild neonatal primary hyperparathyroidism and hypercalciuria was evaluated at The Children's Hospital of Philadelphia. His older brother with neonatal primary hyperparathyroidism had died in infancy of multiple organ failure.

METHODS

We collected clinical and biochemical data and performed exome sequencing analysis on DNA from the patient and his unaffected mother after negative genetic testing for known causes of primary hyperparathyroidism.

RESULTS

Exome sequencing followed by Sanger sequencing disclosed 2 heterozygous mutations, c.1883C>A, p.(A628D) and c.2786_2787insC, p.(T931fsX10), in the SLC12A1 gene, which was previously implicated in antenatal type 1 Bartter syndrome. Sanger sequencing confirmed the 2 mutations in the proband and his deceased brother; both parents were heterozygous for different mutations and an unaffected sister was homozygous for wild-type alleles.

CONCLUSIONS

These results demonstrate a previously unrecognized association between neonatal primary hyperparathyroidism and mutation of SLC12A1, the cause of antenatal Bartter syndrome type 1, and suggest that the loss of sodium-potassium-chloride cotransporter-2 cotransporter activity influences parathyroid gland function.

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.

Hereditary hyperparathyroidism--a consensus report of the European Society of Endocrine Surgeons (ESES)

BACKGROUND

Hereditary hyperparathyroidism has been reported to occur in 5-10 % of cases of primary hyperparathyroidism in the context of multiple endocrine neoplasia (MEN) types 1, 2A and 4; hyperparathyroidism-jaw tumour (HPT-JT); familial isolated hyperparathyroidism (FIHPT); familial hypocalciuric hypercalcaemia (FHH); neonatal severe hyperparathyroidism (NSHPT) and autosomal dominant moderate hyperparathyroidism (ADMH). This paper aims to review the controversies in the main genetic, clinical and pathological features and surgical management of hereditary hyperparathyroidism.

METHODS

A peer review literature analysis on hereditary hyperparathyroidism was carried out and analyzed in an evidence-based perspective. Results were discussed at the 2015 Workshop of the European Society of Endocrine Surgeons devoted to hyperparathyroidism due to multiple gland disease.

RESULTS

Literature reports scarcity of prospective randomized studies; thus, a low level of evidence may be achieved.

CONCLUSIONS

Hereditary hyperparathyroidism typically presents at an earlier age than the sporadic variants. Gene penetrance and expressivity varies. Parathyroid multiple gland involvement is common, but in some variants, it may occur metachronously often with long disease-free intervals, simulating a single-gland involvement. Bilateral neck exploration with subtotal parathyroidectomy or total parathyroidectomy + autotransplantation should be performed, especially in MEN 1, in order to decrease the persistent and recurrent hyperparathyroidism rates; in some variants (MEN 2A, HPT-JT), limited parathyroidectomy can achieve long-term normocalcemia. In FHH, surgery is contraindicated; in NSHPT, urgent total parathyroidectomy is required. In FIHPT, MEN 4 and ADMH, a tailored case-specific approach is recommended.

Familial hypocalciuric hypercalcemia with a de novo heterozygous mutation of calcium-sensing receptor

A de novo heterozygous inactivating mutation of calcium-sensing receptor (CASR) gene typically causes neonatal hyperparathyroidism (NHPT) with moderate hypercalcemia and hyperparathyroid bone disease. We present a case of asymptomatic hypocalciuric hypercalcemia with a de novo heterozygous mutation in CASR, S591C, which is primarily reported to be responsible for NHPT. A 54-year-old female was referred for investigation of asymptomatic hypercalcemia that was initially found in the 1980s but without a history of bone disease during the perinatal period. She had moderate hypercalcemia (12.4 mg/dl) and relative hypocalciuria (fractional extraction of calcium 1.07%) but normal intact parathyroid hormone and serum 1,25-dihydroxyvitamin D3. Pedigree analysis revealed that she carried a de novo heterozygous mutation of S591C, which she transmitted to an affected child with moderate hypercalcemia but not to other children, who had normal serum calcium levels. A de novo heterozygous CASR mutation that is responsible for NHPT may also present in individuals with asymptomatic hypocalciuric hypercalcemia. Caution is required when predicting course and outcome in a pedigree with CASR mutation, as well as incidental hypercalcemia, because of its variable phenotypes.

LEARNING POINTS

The phenotype and severity of CASR mutations are thought to be dependent on genotypes.We report an asymptomatic case of the de novo heterozygous S591C mutation in CASR, which has previously been reported as a responsible mutation of NHPT with bone diseases.Variable phenotypes of CASR raise a cautionary note about predicting outcome by genotyping in a pedigree with CASR mutation.

Bones, stones, moans and groans: hypercalcaemia revisited

Disorders of calcium homeostasis are uncommon but important because of the broad spectrum of potential underlying causes that lie on a spectrum from the benign to the life-threatening. Paediatricians may find them challenging because they do not arise often enough for the investigative approach to be second nature. We report a 4-year-old with acute onset profound hypercalcaemia. We focus on an approach to the clinical problem that is based on the potential organ systems affected, namely the gut, bone and kidney. Key biochemical parameters that may help the paediatric team to reach a diagnosis are discussed, as well as important components of acute management.

A novel CASR mutation associated with neonatal severe hyperparathyroidism transmitted as an autosomal recessive disorder

BACKGROUND

Neonatal severe primary hyperparathyroidism (NSHPT, MIM 239200) is most often an isolated disorder that is due to biallelic inactivating mutations in the CASR, the gene encoding the calcium sensing receptor; NSHPT is inherited from parents with familial hypocalciuric hypercalcemia, each of whom has one mutated CASR allele.

OBJECTIVES

To report clinical and genetic findings in a brother and sister with NSHPT due to a novel mutation in the CASR transmitted as an autosomal recessive trait and to examine the functional effect of the mutation.

SUBJECTS AND METHODS

A brother and sister with marked hypercalcemia due to NSHPT were identified; the boy also had craniosynostosis requiring surgical repair. The genotyping of the CASR in both children and their parents who were eucalcemic and normophosphatemic was undertaken. In order to examine the significance of the variant CASR identified, the CASR variant was expressed in vitro and examined by three computer computational programs [PolyPhen2, MutationTaster, Sorting Intolerant From Tolerant (SIFT)] designed to evaluate the effect of a nucleotide variant on the structure and likely functional consequence upon the protein product.

RESULTS

A sequence variant in the CASR was identified [G>T point mutation at nucleotide c.2303 in exon 7 (c.2303G>T) resulting in the replacement of glycine by valine at codon 768 (p.Gly768Val)]. Two copies of this CASR variant were present in the genome of the siblings while a single copy of the CASR variant was present in both of the clinically and biochemically normal parents, a pattern of transmission consistent with autosomal recessive inheritance of NSHPT in this family. When expressed in HEK293 cells in vitro, the novel Gly768Val variant did not interfere with protein generation or migration to the cell membrane in vitro. The analysis of the functional effect of the Gly768Val CASR variant by the PolyPhen2, MutationTaster, and Sorting Intolerant From Tolerant computer programs revealed that this mutation was very likely to be deleterious.

CONCLUSION

The NSHPT associated with biallelic Gly768Val mutations of the CASR in two siblings with severe hypercalcemia and hyperparathyroidism and their clinically and biochemically normal heterozygous parents was transmitted as an autosomal recessive disorder in this family.

A homozygous CaSR mutation causing a FHH phenotype completely masked by vitamin D deficiency presenting as rickets

CONTEXT

Heterozygous inactivating calcium-sensing receptor (CaSR) mutations lead to familial hypocalciuric hypercalcemia (FHH), whereas homozygous mutations usually cause neonatal severe hyperparathyroidism.

OBJECTIVE

The objective of the study was to investigate the pathophysiological mechanisms of a homozygous inactivating CaSR mutation identified in a 16-year-old female.

DESIGN

Clinical, biochemical, and genetic analyses of the index patient and her family were performed. Functional capacity of CaSRQ459R and CaSR mutants causing FHH (Q27R, P39A, S417C) or neonatal severe hyperparathyroidism (W718X) was assessed. Activation of the cytosolic calcium pathway and inhibition of PTH-induced cAMP signaling were measured.

RESULTS

A 16-year-old girl presented with adolescent rickets, vitamin D deficiency, and secondary hyperparathyroidism. Vitamin D treatment unmasked features resembling FHH, and genetic testing revealed a homozygous CaSRQ459R mutation. Two apparently healthy siblings were homozygous for CaSRQ459R and had asymptomatic hypercalcemia and hypocalciuria. The CaSRQ459R mutation leads to mild functional inactivation in vitro, which explains the FHH-like phenotype in homozygous family members and the grossly exaggerated PTH response to vitamin D deficiency in the index case. The patient's parents and two other siblings were heterozygous, had normal serum calcium and PTH, but had marked hypocalciuria, which appeared to be associated with impaired in vitro activation of the calcium signaling pathway by CaSRQ459R. The Q459R mutation responded well to calcimimetic treatment in vitro.

CONCLUSION

CaSR mutations causing mild functional impairment can lead to FHH, even in homozygous patients. The skeletal deformities in the index case were mainly due to severe vitamin D deficiency, and the CaSR mutation did not appear to have played a major independent role in the skeletal phenotype.

Cinacalcet monotherapy in neonatal severe hyperparathyroidism: a case study and review

CONTEXT

Neonatal severe hyperparathyroidism (NSHPT) is a severe form of familial hypocalciuric hypercalcemia characterized by severe hypercalcemia and skeletal demineralization. In most cases, NSHPT is due to biallelic loss-of-function mutations in the CASR gene encoding the calcium-sensing receptor (CaSR), but some patients have heterozygous mutations. Conventional treatment consists of iv saline, bisphosphonates, and parathyroidectomy.

OBJECTIVE

The aim of this project was to characterize the molecular basis for NSHPT in an affected newborn and to describe the response to monotherapy with cinacalcet.

METHODS

Clinical and biochemical features were monitored as cinacalcet therapy was initiated and maintained. Genomic DNA was obtained from the proband and parents. The CASR gene was amplified by PCR and sequenced directly.

RESULTS

The patient was a full-term male who developed hypotonia and respiratory failure soon after birth. He was found to have multiple fractures and diffuse bone demineralization, with a marked elevation in serum ionized calcium (1.99 mmol/L) and elevated serum levels of intact PTH (1154 pg/mL); serum 25-hydroxyvitamin D was low, and fractional excretion of calcium was reduced. The serum calcium level was not reduced by iv saline infusion. Based on an extensive family history of autosomal dominant hypercalcemia, a diagnosis of NSHPT was made, and cinacalcet therapy was initiated with a robust and durable effect. Molecular studies revealed a heterozygous R185Q missense mutation in the CASR in the patient and his father, whereas normal sequences for the CASR gene were present in the patient's mother.

CONCLUSIONS

We describe the first use of cinacalcet as monotherapy for severe hypercalcemia in a newborn with NSHPT. The rapid and durable response to cinacalcet suggests that a trial of calcimimetic therapy should be considered early in the course of NSHPT.

Loss-of-function and gain-of-function mutations of calcium-sensing receptor: functional analysis and the effect of allosteric modulators NPS R-568 and NPS 2143

OBJECTIVE

Activating mutations in the calcium-sensing receptor (CASR) gene cause autosomal dominant hypoparathyroidism, and heterozygous inactivating CASR mutations cause familial hypocalciuric hypercalcemia. Recently, there has been a focus on the use of allosteric modulators to restore the functional activity of mutant CASRs. In this study, the effect of allosteric modulators NPS R-568 and NPS 2143 on CASR mutants was studied in vitro.

METHODS

DNA sequence analysis of the CASR gene was undertaken in autosomal dominant hypoparathyroidism and familial hypocalciuric hypercalcemia Japanese patients, and the functional consequences for the Gi-MAPK pathway and cell surface expression of CASR were determined. Furthermore, we studied the effect of NPS R-568 and NPS 2143 on the signal transduction activity and cell surface expression of each mutant CASR.

RESULTS

We identified 3 activating mutations (S122C, P569H, and I839T) and 2 inactivating mutations (A110T and R172G) in patients. The activating and inactivating mutations caused leftward and rightward shifts, respectively, in the dose-response curves of the signaling pathway. NPS R-568 rescued the signal transduction capacity of 2 inactivating mutants without increasing cell surface expression levels. NPS 2143 suppressed the enhanced activity of the activating mutants without altering cell surface expression levels, although A843E, which is a constitutively active mutant, was suppressed to a lesser degree.

CONCLUSIONS

We have identified 4 novel mutations of CASR. Moreover, our results indicate that allosteric modulators can restore the activity of the loss- and gain-of-function mutant CASRs, identified in this study.

Primary hyperparathyroidism in children and adolescents

Primary hyperparathyroidism (PHPT) is a common endocrine disorder in adults in whom the typical presentation is incidentally discovered as asymptomatic hypercalcemia. PHPT is much less common in children and adolescents, but has greater morbidity in this age group, as most young patients with PHPT will have symptomatic hypercalcemia or complications such as kidney stones, abdominal pain, and skeletal fragility. An important feature of PHPT in younger patients is the relatively high prevalence of germline inactivating mutations of the CASR gene, which encodes the calcium-sensing receptor. Biallelic CASR mutations cause neonatal severe hyperparathyroidism, a life-threatening condition that presents within days of life with marked hypercalcemia, respiratory distress, failure to thrive, and skeletal demineralization. By contrast, more common heterozygous CASR mutations are generally associated with a benign variant of PHPT termed familial hypocalciuric hypercalcemia. Appropriate management of PHPT in children and adolescents requires distinction between familial hypocalciuric hypercalcemia, which generally requires no specific treatment, and other forms of PHPT that are best treated by parathyroidectomy.

Mutations of calcium-sensing receptor gene: two novel mutations and overview of impact on calcium homeostasis

OBJECTIVE

Genetic disorders of calcium metabolism arise in a familial or sporadic setting. The calcium-sensing receptor (CASR) plays a key role in maintaining calcium homeostasis and study of the CASR gene can be clinically useful in determining etiology and appropriate therapeutic approaches. We report two cases of novel CASR gene mutations that illustrate the varying clinical presentations and discuss these in terms of the current understanding of CASR function.

PATIENTS AND METHODS

A 16-year-old patient had mild hypercalcemia associated with low-normal urinary calcium excretion and normal-to-high parathyroid hormone (PTH) levels. Because of negative family history, familial hypocalciuric hypercalcemia was originally excluded. The second patient was a 54-year-old man with symptomatic hypocalcemia, hyperphosphatemia, low PTH, and mild hypercalciuria. Familial investigation revealed the same phenotype in the patient's sister. The coding region of the CASR gene was sequenced in both probands and their available first-degree relatives.

RESULTS

The first patient had a novel heterozygous inactivating CASR mutation in exon 4, which predicted a p.A423K change; genetic analysis was negative in the parents. The second patient had a novel heterozygous activating CASR mutation in exon 6, which predicted a p.E556K change; the affected sister of the proband was also positive.

CONCLUSIONS

We reported two novel heterozygous mutations of the CASR gene, an inactivating mutation in exon 4 and the first activating mutation reported to date in exon 6. These cases illustrate the importance of genetic testing of CASR gene to aid correct diagnosis and to assist in clinical management.

[Usefulness of genetic tests in familial hypocalciuric hypercalcemia with atypical clinical presentation]

OBJECTIVES

Biochemical tests related to calcium and phosphorus metabolism have traditionally been considered as a reliable tool to differentiate familial hypocalciuric hypercalcemia (FHH) from primary hyperparathyroidism (PHPT). However, diagnosis may sometimes be difficult even for experienced clinicians. Our objective was to assess the accuracy of diagnostic tests in FHH and the circumstances in which genetic studies are required.

PATIENTS AND METHODS

A descriptive study was conducted of two families with hypercalcemia and suspected atypical FHH. Urinary calcium excretion was measured in 24-hour urine using different tests (calcium excretion (CE), urinary calcium/creatinine clearance ratio (UCCR)), and serum PTH and 25-hydroxyvitamin D levels were tested. Index cases underwent genetic study.

RESULTS

One patient from the first family showed overt, persistent hypercalciuria with values more consistent with PHPT than with FHH if we consider, as proposed by guidelines, a UCCR lower than 0.01 as diagnostic of FHH and a value higher than 0.02 as diagnostic of PHPT. The index case of the second family underwent surgery for a parathyroid adenoma. Both cases had a mutation c. 164C>T (Pro55Leu) in exon 2 in heterozygosis.

CONCLUSIONS

According to current clinical guidelines, definitive diagnosis of FHH requires genetic confirmation, which allowed in our case for detection of two families with FHH and atypical clinical presentations. We think that rational use of genetic tests may avoid unnecessary surgery and excess monitoring costs.

Clinical utilization of cinacalcet in hypercalcemic conditions

INTRODUCTION

Cinacalcet has recently been introduced as a treatment for secondary hyperparathyroidism in dialysis patients and for parathyroid carcinoma. However, there has been an increasing interest in finding out whether cinacalcet can be used as a treatment for other parathyroid hormone (PTH)-dependent hypercalcemic conditions also.

AREAS COVERED

The article reports the most relevant recent contributions dealing with calcium sensing receptor (CaSR) physiology as well as cinacalcet pharmacokinetics and pharmacodynamics. It also looks at the different hypercalcemic conditions where the use of cinacalcet has been proposed. This article was researched using clinical trials, case reports and outstanding basic research published in the last 3 years (MEDLINE database up to 31 November 2010). It provides the reader with an insight into the many unaddressed issues regarding cinacalcet that need to be resolved before it can be used in newly proposed fields.

EXPERT OPINION

Since cinacalcet may not only have an effect on parathyroid CaSR but also on CaSR expressed at bone and renal levels, it can currently only be considered a good alternative to parathyroidectomy in PTH-dependent hypercalcemic conditions when surgical intervention is burdened by a high failure rate or when it can be considered a risky procedure. At present, cinacalcet cannot be considered the first choice treatment in asymptomatic primary hyperparathyroidism or in mild-to-moderate forms of familial hypocalciuric hypocalcemia.

Hypercalcemia in children and adolescents

PURPOSE OF REVIEW

In this review, we define hypercalcemia levels, common causes for hypercalcemia in children, and treatment in order to aid the practicing pediatrician.

RECENT FINDINGS

One rare cause of hypercalcemia in the child is familial hypocalciuric hypercalcemia (also termed familial benign hypercalcemia). Mutations that inactivate the Ca-sensing receptor gene FHH have been described as an autosomal dominant disorder, but recently milder mutations in the CASR have been shown to cause hypercalcemia when homozygous.

SUMMARY

Normal serum levels of calcium are maintained through the interplay of parathyroid, renal, and skeletal factors. In this review, we have distinguished the neonate and infant from the older child and adolescent because the causes and clinical features of hypercalcemia can differ in these two age groups. However, the initial approach to the medical treatment of severe or symptomatic hypercalcemia is to increase the urinary excretion of calcium in both groups. In most cases, hypercalcemia is due to osteoclastic bone resorption, and agents that inhibit or destroy osteoclasts are, therefore, effective treatments. Parathyroid surgery, the conventional treatment for adults with symptomatic primary hyperparathyroidism, is recommended for all children with primary hyperparathyroidism.

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.