An acceptor splice site mutation in the calcium-sensing receptor (CASR) gene in familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism

Glycolate oxidase-1 gene variants influence the risk of hyperoxaluria and renal stone development

PURPOSE

Oxalate is an excellent calcium ion attractor with great abundance in the human body, and the liver is the major source of oxalate. The Glycolate oxidase-1 (GOX1) gene is solely responsible for the glycolate and glyoxylate metabolism and produces oxalate. This study has been designed to comprehend the association of genetic variants of the GOX1 gene with the risk of hyperoxaluria and renal stone disease in the Indian population.

METHOD

The present study is a candidate gene approach prospective case-control study carried out on 300 participants (150 cases and 150 controls) at Muljibhai Patel Urological Hospital, Gujarat, India. Biochemical parameters, including serum levels of calcium, creatinine, parathyroid hormone, and 24-h urine metabolites, were performed. The genotyping of GOX1 gene variants rs6086287, rs2235250, rs2255183, and rs2294303 was performed using a customized TaqMan assay probe by RT-PCR.

RESULT

Parathyroid hormone, serum creatinine, and urine metabolites were significantly elevated in nephrolithiasis compared to healthy individuals. All mutated homozygous genotypes GG (rs6086287), TT (rs2235250), GG (rs2255183), and CC (rs2294303) were significantly associated with a high risk of renal stone disease. Individuals diagnosed with hyperoxaluria and carrying TG (rs6086287), AG (rs2255183), and TT (rs2294303) genotypes have a significantly high risk of renal stone disease. Moreover, haplotype analysis and correlation analysis also confirmed the strong association between genetic variants and nephrolithiasis.

CONCLUSION

Genetic variants of the GOX1 genes were associated with renal stone disease. In the presence of risk genotype and hyperoxaluria, the susceptibility to develop renal stone disease risk gets modulated.

Haplotype of CaSR gene is associated with risk of renal stone disease in West Indian population

Calcium is the most abundant metabolite involved in the stone matrix. The CaSR gene controls calcium homeostasis, and genetic variation in the CaSR gene could lead to the development of renal stone disease. Therefore, the current study has been designed to assess the association of genetic variants of CaSR gene polymorphisms with renal stone disease. A single-centric prospective study has been carried out on a total of 300 participants (150 cases and 150 controls). Serum levels of calcium, creatinine, parathyroid hormone, and 24 h urine metabolites were measured. Two polymorphisms, rs1801725 and rs1042636, of the CaSR gene, have been genotyped for each participant. T test, binary logistic regression, and Chi-square analysis were used for statistical analysis. Renal stone patients had significantly higher levels of serum parathyroid hormone, creatinine, and 24-h urine metabolites in comparison to the controls. CaSR gene variants, rs1801725 (GG) and rs1042636 (AA), both have shown significant association with renal stone disease. In addition, individuals having specific genotypes along with metabolic abnormalities such as hypercalcemia and hyperparathyroidism are found to be at a higher significant risk of developing the renal stone disease. In the present study, the haplotype of the CaSR gene has shown an association with renal stone disease. Individuals with hyperparathyroidism and hypercalcemia and risk genotype have a higher susceptibility to developing renal stone disease.

Sequencing the CaSR locus in Pakistani stone formers reveals a novel loss-of-function variant atypically associated with nephrolithiasis

BACKGROUND

Nephrolithiasis (NL) affects 1 in 11 individuals worldwide and causes significant morbidity and cost. Common variants in the calcium sensing receptor gene (CaSR) have been associated with NL. Rare inactivating CaSR variants classically cause hyperparathyroidism, hypercalcemia and hypocalciuria. However, NL and familial hypercalciuria have been paradoxically associated with select inactivating CaSR variants in three kindreds from Europe and Australia.

METHODS

To discover novel NL-associated CaSR variants from a geographically distinct cohort, 57 Pakistani families presenting with pediatric onset NL were recruited. The CaSR locus was analyzed by directed or exome sequencing.

RESULTS

We detected a heterozygous and likely pathogenic splice variant (GRCh37 Chr3:122000958A>G; GRCh38 Chr3:12228211A>G; NM_000388:c.1609-2A>G) in CaSR in one family with recurrent calcium oxalate stones. This variant would be predicted to cause exon skipping and premature termination (p.Val537Metfs*49). Moreover, a splice variant of unknown significance in an alternative CaSR transcript (GRCh37 Chr3:122000929G>C; GRCh38 Chr3:122282082G >C NM_000388:c.1609-31G >C NM_001178065:c.1609-1G >C) was identified in two additional families.

CONCLUSIONS

Sequencing of the CaSR locus in Pakistani stone formers reveals a novel loss-of-function variant, expanding the connection between the CaSR locus and nephrolithiasis.

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.

Homozygous Calcium-Sensing Receptor Polymorphism R544Q Presents as Hypocalcemic Hypoparathyroidism

CONTEXT

Autosomal dominant hypocalcemia type 1 (ADH1) is caused by heterozygous activating mutations in the calcium-sensing receptor gene (CASR). Whether polymorphisms that are benign in the heterozygous state pathologically alter receptor function in the homozygous state is unknown.

OBJECTIVE

To identify the genetic defect in an adolescent female with a history of surgery for bilateral cataracts and seizures. The patient has hypocalcemia, hyperphosphatemia, and low serum PTH level. The parents of the proband are healthy.

METHODS

Mutation testing of PTH, GNA11, GCM2, and CASR was done on leukocyte DNA of the proband. Functional analysis in transfected cells was conducted on the gene variant identified. Public single nucleotide polymorphism (SNP) databases were searched for the presence of the variant allele.

RESULTS

No mutations were identified in PTH, GNA11, and GCM2 in the proband. However, a germline homozygous variant (c.1631G>A; p.R544Q) in exon 6 of the CASR was identified. Both parents are heterozygous for the variant. The variant allele frequency was near 0.1% in SNP databases. By in vitro functional analysis, the variant was significantly more potent in stimulating both the Ca2+i and MAPK signaling pathways than wild type when transfected alone (P < 0.05) but not when transfected together with wild type. The overactivity of the mutant CaSR is due to loss of a critical structural cation-π interaction.

CONCLUSIONS

The patient's hypoparathyroidism is due to homozygosity of a variant in the CASR that normally has weak or no phenotypic expression in heterozygosity. Although rare, this has important implications for genetic counseling and clinical management.

Associations between single nucleotide polymorphisms in the calcium sensing receptor and chronic kidney disease-mineral and bone disorder in cats

Feline chronic kidney disease (CKD) is associated with high variability in severity of CKD-mineral and bone disorder (CKD-MBD). The calcium sensing receptor (CaSR) regulates circulating parathyroid hormone (PTH) and calcium concentrations. Single nucleotide polymorphisms (SNPs) in the CaSR are associated with severity of secondary renal hyperparathyroidism and total calcium concentrations in human patients receiving haemodialysis. The objective of this study was to explore associations between polymorphisms in the feline CaSR (fCaSR) and biochemical changes observed in CKD-MBD. Client owned cats (≥9years) were retrospectively included. SNP discovery was performed in 20 cats with azotaemic CKD and normal or dysregulated calcium concentrations. Non-pedigree cats (n=192) (125 with azotaemic CKD and 66 healthy), Persians (n=40) and Burmese (n=25) were genotyped for all identified SNPs using KASP. Biochemical parameters from the date of CKD diagnosis or from first visit to the clinic (healthy cats) were used. Associations between genotype and ionized calcium, total calcium, phosphate, PTH and FGF-23 were performed for non-pedigree cats using logistic regression. Sequence alignment against the fCaSR sequence revealed eight novel exonic SNPs. KASP genotyping had high accuracy (99.6%) and a low failure rate (<6%) for all SNPs. Allele frequencies varied between breeds. In non-pedigree cats, one synonymous SNP CaSR:c.1269G>A was associated with logPTH concentration (adjusted for plasma creatinine concentration), with a recessive model having the best fit (G/G vs A/A-G/A, P=0.031). Genetic variation in the fCaSR is unlikely to explain the majority of the variability in presence and severity of CKD-MBD in cats.

Monocrotaline Induces Endothelial Injury and Pulmonary Hypertension by Targeting the Extracellular Calcium-Sensing Receptor

BACKGROUND

Monocrotaline has been widely used to establish an animal model of pulmonary hypertension. The molecular target underlying monocrotaline-induced pulmonary artery endothelial injury and pulmonary hypertension remains unknown. The extracellular calcium-sensing receptor (CaSR) and particularly its extracellular domain hold the potential structural basis for monocrotaline to bind. This study aimed to reveal whether monocrotaline induces pulmonary hypertension by targeting the CaSR.

METHODS AND RESULTS

Nuclear magnetic resonance screening through WaterLOGSY (water ligand-observed gradient spectroscopy) and saturation transfer difference on protein preparation demonstrated the binding of monocrotaline to the CaSR. Immunocytochemical staining showed colocalization of monocrotaline with the CaSR in cultured pulmonary artery endothelial cells. Cellular thermal shift assay further verified the binding of monocrotaline to the CaSR in pulmonary arteries from monocrotaline-injected rats. Monocrotaline enhanced the assembly of CaSR, triggered the mobilization of calcium signaling, and damaged pulmonary artery endothelial cells in a CaSR-dependent manner. Finally, monocrotaline-induced pulmonary hypertension in rats was significantly attenuated or abolished by the inhibitor, the general or lung knockdown or knockout of CaSR.

CONCLUSIONS

Monocrotaline aggregates on and activates the CaSR of pulmonary artery endothelial cells to trigger endothelial damage and, ultimately, induces pulmonary hypertension.

Calcium-Sensing Receptor Gene: Regulation of Expression

The human calcium-sensing receptor gene (CASR) has 8 exons, and localizes to chromosome 3q. Exons 1A and 1B encode alternative 5′-untranslated regions (UTRs) that splice to exon 2 encoding the AUG initiation codon. Exons 2–7 encode the CaSR protein of 1078 amino acids. Promoter P1 has TATA and CCAAT boxes upstream of exon 1A, and promoter P2 has Sp1/3 motifs at the start site of exon 1B. Exon 1A transcripts from the P1 promoter are reduced in parathyroid tumors and colon carcinomas. Studies of colon carcinomas and neuroblastomas have emphasized the importance of epigenetic changes—promoter methylation of the GC-rich P2 promoter, histone acetylation—as well as involvement of microRNAs in bringing about CASR gene silencing and reduced CaSR expression. Functional cis-elements in the CASR promoters responsive to 1,25-dihydroxyvitamin D [1,25(OH)₂D], proinflammatory cytokines, and the transcription factor glial cells missing-2 (GCM2) have been characterized. Reduced levels of CaSR and reduced responsiveness to active vitamin D in parathyroid neoplasia and colon carcinoma may blunt the “tumor suppressor” activity of the CaSR. The hypocalcemia of critically ill patients with burn injury or sepsis is associated with CASR gene upregulation by TNF-alpha and IL-1beta via kappaB elements, and by IL-6 via Stat1/3 and Sp1/3 elements in the CASR gene promoters, respectively. The CASR is transactivated by GCM2—the expression of which is essential for parathyroid gland development. Hyperactive forms of GCM2 may contribute to later parathyroid hyperactivity or tumorigenesis. The expression of the CaSR—the calciostat—is regulated physiologically and pathophysiologically at the gene level.

Novel calcium-sensing receptor cytoplasmic tail deletion mutation causing autosomal dominant hypocalcemia: molecular and clinical study

OBJECTIVE

Autosomal dominant hypocalcemia (ADH) is a rare disorder caused by activating mutations of the calcium-sensing receptor (CASR). The treatment of ADH patients with 1α-hydroxylated vitamin D derivatives can cause hypercalciuria leading to nephrocalcinosis.

DESIGN AND METHODS

We studied a girl who presented with hypoparathyroidism and asymptomatic hypocalcemia at age 2.5 years. Mutations of CASR were investigated by DNA sequencing. Functional analyses of mutant and WT CASRs were done in transiently transfected human embryonic kidney (HEK293) cells.

RESULTS

The proband and her father are heterozygous for an eight-nucleotide deletion c.2703_2710delCCTTGGAG in the CASR encoding the intracellular domain of the protein. Transient expression of CASR constructs in kidney cells in vitro suggested greater cell surface expression of the mutant receptor with a left-shifted extracellular calcium dose-response curve relative to that of the WT receptor consistent with gain of function. Initial treatment of the patient with calcitriol led to increased urinary calcium excretion. Evaluation for mosaicism in the paternal grandparents of the proband was negative.

CONCLUSIONS

We describe a novel naturally occurring deletion mutation within the CASR that apparently arose de novo in the father of the ADH proband. Functional analysis suggests that the cytoplasmic tail of the CASR contains determinants that regulate the attenuation of signal transduction. Early molecular analysis of the CASR gene in patients with isolated idiopathic hypoparathyroidism is recommended because of its relevance to clinical outcome and treatment choice. In ADH patients, calcium supplementation and low-dose cholecalciferol avoids hypocalcemic symptoms without compromising renal function.

NKCC2 activity is inhibited by the Bartter's syndrome type 5 gain-of-function CaR-A843E mutant in renal cells

BACKGROUND INFORMATION

The gain-of-function A843E mutation of the calcium sensing receptor (CaR) causes Bartter syndrome type 5. Patients carrying this CaR variant show a remarkably reduced renal NaCl reabsorption in the thick ascending limb (TAL) of Henle's loop resulting in renal loss of NaCl in the absence of mutations in renal Na(+) and Cl(-) ion transporters. The molecular mechanisms underlying this clinical phenotype are incompletely understood. We investigated, in human embryonic kidney 293 (HEK 293) cells and porcine kidney epithelial (LLC-PK1) cells, the functional cross-talk of CaR-A843E with the Na(+):K(+):2Cl(-) co-transporter, NKCC2, which provides NaCl reabsorption in the TAL.

RESULTS

The expression of the CaR mutant did not alter the apical localisation of NKCC2 in LLC-PK1 cells. However, the steady-state NKCC2 phosphorylation and activity were decreased in cells transfected with CaR-A843E compared with the control wild-type CaR (CaR WT)-transfected cells. Of note, low-Cl(-)-dependent NKCC2 activation was also strongly inhibited upon the expression of CaR-A843E mutant. The use of either P450 ω-hydroxylase (CYP4)- or phospholipase A2 (PLA2)-blockers suggests that this effect is likely mediated by arachidonic acid (AA) metabolites.

CONCLUSIONS

The data suggested that the activated CaR affects intracellular pathways modulating NKCC2 activity rather than NKCC2 intracellular trafficking in renal cells, and throw further light on the pathological role played by active CaR mutants in Bartter syndrome type 5.

Parathyroid: The Pathology of Hyperparathyroidism

This review focuses on the pathologic entities associated with hyperparathyroidism in humans. A discussion of the lesions, their embryology, and pathologic features is included. Immunohistology, cytopathology, and a brief overview of molecular aspects of the lesion are included.

Engendering biased signalling from the calcium-sensing receptor for the pharmacotherapy of diverse disorders

The human calcium-sensing receptor (CaSR) is widely expressed in the body, where its activity is regulated by multiple orthosteric and endogenous allosteric ligands. Each ligand stabilizes a unique subset of conformational states, which enables the CaSR to couple to distinct intracellular signalling pathways depending on the extracellular milieu in which it is bathed. Differential signalling arising from distinct receptor conformations favoured by each ligand is referred to as biased signalling. The outcome of CaSR activation also depends on the cell type in which it is expressed. Thus, the same ligand may activate diverse pathways in distinct cell types. Given that the CaSR is implicated in numerous physiological and pathophysiological processes, it is an ideal target for biased ligands that could be rationally designed to selectively regulate desired signalling pathways in preferred cell types.

A comprehensive survey of non-canonical splice sites in the human transcriptome

We uncovered the diversity of non-canonical splice sites at the human transcriptome using deep transcriptome profiling. We mapped a total of 3.7 billion human RNA-seq reads and developed a set of stringent filters to avoid false non-canonical splice site detections. We identified 184 splice sites with non-canonical dinucleotides and U2/U12-like consensus sequences. We selected 10 of the herein identified U2/U12-like non-canonical splice site events and successfully validated 9 of them via reverse transcriptase-polymerase chain reaction and Sanger sequencing. Analyses of the 184 U2/U12-like non-canonical splice sites indicate that 51% of them are not annotated in GENCODE. In addition, 28% of them are conserved in mouse and 76% are involved in alternative splicing events, some of them with tissue-specific alternative splicing patterns. Interestingly, our analysis identified some U2/U12-like non-canonical splice sites that are converted into canonical splice sites by RNA A-to-I editing. Moreover, the U2/U12-like non-canonical splice sites have a differential distribution of splicing regulatory sequences, which may contribute to their recognition and regulation. Our analysis provides a high-confidence group of U2/U12-like non-canonical splice sites, which exhibit distinctive features among the total human splice sites.

Mutations in pseudohypoparathyroidism 1a and pseudopseudohypoparathyroidism in ethnic Chinese

An inactivating mutation in the GNAS gene causes either pseudohypoparathyroidism 1a (PHP1A) when it is maternally inherited or pseudopseudohypoparathyroidism (PPHP) when it is paternally inherited. We investigated clinical manifestations and mutations of the GNAS gene in ethnic Chinese patients with PHP1A or PPHP. Seven patients from 5 families including 4 girls and 2 boys with PHP1A and 1 girl with PPHP were studied. All PHP1A patients had mental retardation. They were treated with calcitriol and CaCO3 with regular monitoring of serum Ca levels, urinary Ca/Cr ratios, and renal sonography. Among them, 5 patients also had primary hypothyroidism suggesting TSH resistance. One female patient had a renal stone which was treated with extracorporeal shockwave lithotripsy. She had an increased urinary Ca/Cr ratio of 0.481 mg/mg when the stone was detected. We detected mutations using PCR and sequencing as well as analysed a splice acceptor site mutation using RT-PCR, sequencing, and minigene construct. We detected 5 mutations: c.85C>T (Q29*), c.103C>T (Q35*), c.840-2A>G (R280Sfs*21), c.1027_1028delGA (D343*), and c.1174G>A (E392K). Mutations c.840-2A>G and c.1027_1028delGA were novel. The c.840-2A>G mutation at the splice acceptor site of intron 10 caused retention of intron 10 in the minigene construct but skipping of exon 11 in the peripheral blood cells. The latter was the most probable mechanism which caused a frameshift, changing Arg to Ser at residue 280 and invoking a premature termination of translation at codon 300 (R280Sfs*21). Five GNAS mutations in ethnic Chinese with PHP1A and PPHP were reported. Two of them were novel. Mutation c.840-2A>G destroyed a spice acceptor site and caused exon skipping. Regular monitoring and adjustment in therapy are mandatory to achieve optimal therapeutic effects and avoid nephrolithiasis in patients with PHP1A.

The CASR gene: alternative splicing and transcriptional control, and calcium-sensing receptor (CaSR) protein: structure and ligand binding sites

The calcium-sensing receptor (CaSR) is a G protein-coupled receptor encoded by a single copy gene. The human CASR gene spans ~103-kb and has eight exons. Promoters P1 and P2 drive transcription of exons 1A and 1B, respectively, encoding alternative 5'-UTRs that splice to exon 2 encoding the common part of the 5'-UTR. Exons 2-7 encode the CaSR protein of 1078 amino acids. Functional elements responsive to 1,25-dihydroxyvitamin D, proinflammatory cytokines, and glial cells missing-2 are present in the CASR promoters. Evolutionarily, the exon structure, first seen in aquatic vertebrates, is well-conserved with a single linkage disequilibrium haplotype block for protein coding exons 2-7. Structural features of the human CaSR protein are: an N-terminal signal peptide (19 amino acids (aa)); an extracellular domain (~600 aa) having a bi-lobed Venus Flytrap (VFT) domain with several Ca(2+)-binding sites; and a nine-cysteines domain that transduces the activation signal to the 7-transmembrane domain (250 aa) and the C-terminal tail (216 aa).

Minireview: the intimate link between calcium sensing receptor trafficking and signaling: implications for disorders of calcium homeostasis

The calcium-sensing receptor (CaSR) regulates organismal Ca(2+) homeostasis. Dysregulation of CaSR expression or mutations in the CASR gene cause disorders of Ca(2+) homeostasis and contribute to the progression or severity of cancers and cardiovascular disease. This brief review highlights recent findings that define the CaSR life cycle, which controls the cellular abundance of CaSR and CaSR signaling. A novel mechanism, termed agonist-driven insertional signaling (ADIS), contributes to the unique hallmarks of CaSR signaling, including the high degree of cooperativity and the lack of functional desensitization. Agonist-mediated activation of plasma membrane-localized CaSR increases the rate of insertion of CaSR at the plasma membrane without altering the constitutive endocytosis rate, thereby acutely increasing the maximum signaling response. Prolonged CaSR signaling requires a large intracellular ADIS-mobilizable pool of CaSR, which is maintained by signaling-mediated increases in biosynthesis. This model provides a rational framework for characterizing the defects caused by CaSR mutations and the altered functional expression of wild-type CaSR in disease states. Mechanistic dissection of ADIS of CaSR should lead to optimized pharmacological approaches to normalize CaSR signaling in disorders of Ca(2+) homeostasis.

CYP17A1 intron mutation causing cryptic splicing in 17α-hydroxylase deficiency

17α-hydroxylase/17, 20-lyase deficiency (17OHD) is an autosomal recessive disease causing congenital adrenal hyperplasia and a rare cause of hypertension with hypokalemia. The CYP17A1 gene mutation leads to 17OHD and its clinical features. We described an 18 y/o female with clinical features of 17α-hydroxylase/17, 20-lyase deficiency and characterized the functional consequences of an intronic CYP17A1 mutation. The coding regions and flanking intronic bases of the CYP17A1 gene were amplified by PCR and sequenced. The patient is a compound heterozygote for the previously described p.R358X and IVS1 +2T>C mutations. A first intron splice donor site mutation was re-created in minigene and full-length expression vectors. Pre-mRNA splicing of the variant CYP17A1 intron was studied in transfected cells and in a transformed lymphoblastoid cell line. When the full-length CYP17A1 gene and minigene containing the intronic mutation was expressed in transfected cells, the majority (>90%) of mRNA transcripts were incorrectly spliced. Only the p.R358X transcript was detected in the EBV-transformed lymphoblastoid cell line. The IVS1 +2T>C mutation abolished most 17α-hydroxylase/17, 20-lyase enzyme activity by aberrant mRNA splicing to an intronic pseudo-exon, causing a frame shift and early termination.

Neonatal severe hyperparathyroidism: further clinical and molecular delineation

We report a newborn female from a consanguineous Sri Lankan family with clinical and biochemical features of neonatal severe hyperparathyroidism (NSHPT). Mutation screening of the calcium-sensing receptor (CASR) gene in genomic DNA revealed a homozygous truncating mutation (c.679C>T, predicting p.R227X), confirming the clinical diagnosis. Other mutations at the R227 position are reported to cause varying degrees of hypercalcemia and hyperparathyroidism, but this nonsense variant is novel and expected to induce unremitting hyperparathyroidism from birth onward. In our patient with NSHPT, early bisphosphonate therapy was crucial in counteracting the marked hypercalcemia and allowed for safe surgical intervention ("total" parathyroidectomy, "thymectomy and hemithyroidectomy") at 3 months of age.

CONCLUSION

This report highlights the continuing challenges in diagnosis and management of this life-threatening condition.

Calcium-sensing receptor (CASR) mutations in hypercalcemic states: studies from a single endocrine clinic over three years

CONTEXT

Inactivating mutations of the calcium-sensing receptor (CASR) are implicated in different hypercalcemic syndromes, including familial hypocalciuric hypercalcemia (FHH), primary hyperparathyroidism (PHPT), and familial isolated hyperparathyroidism (FIHP). However, molecular diagnostics applied to large nonselected hypercalcemic cohorts from a single center have not been reported.

OBJECTIVE

Our objective was to describe the prevalence, type, and potential pathogenicity of CASR mutations in a series of cases with FHH (n = 17), PHPT (n = 165), and FIHP (n = 3) and controls (n = 198) presenting at a single endocrine clinic.

SUBJECTS

All were prospectively evaluated at the "Casa Sollievo della Sofferenza" Hospital in southern Italy over a 3-yr period.

METHODS

CASR screening was conducted by denaturing HPLC. The variant CASRs were functionally characterized by transient transfection studies in kidney cells in vitro.

RESULTS

A single novel missense variant was identified in one PHPT case. However, in FHH probands, mutations were found in eight of 17 (47%). With a hypercalcemic family member, mutation detection rate in FHH rose to seven of eight (87%), whereas only one of nine sporadic cases was positive, and none of the three FIHP cases had detectable CASR mutations. Five missense variant CASRs, identified in control subjects, performed as wild type in functional assays, whereas the missense mutant CASRs identified in the FHH patients, and in the one PHPT case, exhibited significant impairment. A novel intronic mutation (IVS4-19a-->c) found in one FHH family, created an abnormally spliced product in an in vitro minigene assay.

CONCLUSION

CASR testing, with functional analysis, provides critical confirmatory evidence in the differential diagnosis of hypercalcemic states.

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.

Anorganic bovine bone and a silicate-based synthetic bone activate different microRNAs

Bio-Oss (BO), composed of anorganic bovine bone, is widely used in several bone regeneration procedures in oral surgery. PerioGlas (PG) is an alloplastic material that has been used for grafting of periodontal osseous defects since the 1990s. However, how these biomaterials alter osteoblast activity to promote bone formation is poorly understood. We attempted to address this question by using microRNA microarray techniques to investigate differences in translational regulation in osteoblasts exposed to BO and PG. By using miRNA microarrays containing 329 probes designed from human miRNA sequences, we investigated miRNAs whose expression was significantly modified in an osteoblast-like cell line (MG-63) cultured with BO vs PG. Three up-regulated miRNAs (mir-337, mir-200b, mir-377) and 4 down-regulated miRNAs (mir-130a, mir-214, mir-27a, mir-93) were identified. Our results indicated that BO and PG act on different miRNAs. Globally, PG causes activation of bone-forming signaling, whereas BO also activates cartilage-related pathways.

THE CALCIUM-SENSING RECEPTOR IN NORMAL PHYSIOLOGY AND PATHOPHYSIOLOGY: A Review

The discovery of a G protein-coupled, calcium-sensing receptor (CaR) a decade ago and of diseases caused by CaR mutations provided unquestionable evidence of the CaR's critical role in the maintenance of systemic calcium homeostasis. On the cell membrane of the chief cells of the parathyroid glands, the CaR "senses" the extracellular calcium concentration and, subsequently, alters the release of parathyroid hormone (PTH). The CaR is likewise functionally expressed in bone, kidney, and gut--the three major calcium-translocating organs involved in calcium homeostasis. Intracellular signal pathways to which the CaR couples via its associated G proteins include phospholipase C (PLC), protein kinase B (AKT); and mitogen-activated protein kinases (MAPKs). The receptor is widely expressed in various tissues and regulates important cellular functions in addition to its role in maintaining systemic calcium homeostasis, i.e., protection against apoptosis, cellular proliferation, and membrane voltage. Functionally significant mutations in the receptor have been shown to induce diseases of calcium homeostasis owing to changes in the set point for calcium-regulated PTH release as well as alterations in the renal handling of calcium. Gain-of-function mutations cause hypocalcemia, whereas loss-of-function mutations produce hypercalcemia. Recent studies have shown that the latter clinical presentation can also be caused by inactivating autoantibodies directed against the CaR Newly discovered type II allosteric activators of the CaR have been found to be effective as a medical treatment for renal secondary hyperparathyroidism.

Gene discovery and the genetic basis of calcium consumption

This review makes the case that gene discovery is a worthwhile approach to the study of ingestive behavior in general and to calcium appetite in particular. A description of the methods used to discover genes is provided for non-geneticists. Areas covered include the characterization of an appropriate phenotype, the choice of suitable mouse strains, the generation of a hybrid cross, interval mapping, congenic strain production, and candidate gene analysis. The approach is illustrated with an example involving mice of the C57BL/6J and PWK/PhJ strains, which differ in avidity for calcium solutions. The variation between the strains can be attributed to at least seven quantitative trait loci (QTLs). One of these QTLs is most likely accounted for by Tas1r3, which is a gene involved in the detection of sweet and umami tastes. The discovery of a novel function for a gene with no previously known role in calcium consumption illustrates the power of gene discovery methods to uncover novel mechanisms.

The proinflammatory cytokine, interleukin-6, up-regulates calcium-sensing receptor gene transcription via Stat1/3 and Sp1/3

Increased expression of the calcium-sensing receptor (CASR), which controls blood calcium homeostasis, leads to a decrease in the extracellular calcium set-point, thereby reducing parathyroid hormone secretion and renal calcium reabsorption and increasing calcitonin secretion resulting in reduced circulating calcium levels. Critically ill patients with elevated proinflammatory cytokine levels commonly have hypocalcemia, although the mechanism is not known. After intraperitoneal injection of interleukin (IL)-6 in the rat, circulating levels of parathyroid hormone, 1,25-dihydroxyvitamin D, and calcium fell within hours and remained low at 24 h. Expression of CASR (mRNA and protein) increased within hours in parathyroid, thyroid, and kidney and remained elevated at 24 h. The CASR gene has two promoters (P1 and P2) yielding transcripts having alternative 5'-untranslated regions but encoding the same receptor protein. Activities of P1 and P2 promoter/luciferase reporter constructs were stimulated approximately 2-3-fold by IL-6 in proximal tubule HKC cells and TT thyroid C-cells. Studies with P1 deleted and mutated promoter-reporter and Stat1 and/or Stat3 dominant-negative constructs showed that a Stat1/3 element downstream of the P1 start site accounted for the IL-6 induction. There are no Stat elements in the P2 promoter, but Sp1/3 elements are clustered at the transcription start site. A series of transfection P2 promoter-reporter analyses showed that Sp1 together with Stat1/3 was critical for IL-6 responsiveness of P2. By oligonucleotide precipitation assay, IL-6 rapidly promoted a complex containing both Sp1/3 and Stat1/3 on the Sp1/3 elements. In conclusion, Stat1/3 directly controls promoter P1, and the Stats indirectly regulate promoter P2 via Sp1/3 in response to IL-6. By this mechanism, the cytokine likely contributes to altered extracellular calcium homeostasis.

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.

Molecular genetic analysis of the calcium sensing receptor gene in patients clinically suspected to have familial hypocalciuric hypercalcemia: phenotypic variation and mutation spectrum in a Danish population

CONTEXT

The autosomal dominantly inherited condition familial hypocalciuric hypercalcemia (FHH) is characterized by elevated plasma calcium levels, relative or absolute hypocalciuria, and normal to moderately elevated plasma PTH. The condition is difficult to distinguish clinically from primary hyperparathyroidism and is caused by inactivating mutations in the calcium sensing receptor (CASR) gene.

OBJECTIVE

We sought to define the mutation spectrum of the CASR gene in a Danish FHH population and to establish genotype-phenotype relationships regarding the different mutations.

DESIGN AND PARTICIPANTS

A total of 213 subjects clinically suspected to have FHH, and 121 subjects enrolled as part of a family-screening program were studied. Genotype-phenotype relationships were established in 66 mutation-positive index patients and family members.

MAIN OUTCOME MEASURES

We determined CASR gene mutations, and correlating levels of plasma calcium (albumin corrected), ionized calcium (pH 7.4), and PTH were measured.

RESULTS

We identified 22 different mutations in 39 FHH families. We evaluated data on circulating calcium and PTH for 11 different mutations, representing a spectrum of clinical phenotypes, ranging from calcium concentrations moderately above the upper reference limit, to calcium levels more than 20% above the upper reference limit. Furthermore, the mean plasma PTH concentration was within the normal range in eight of 11 studied mutations, but mild to moderately elevated in families with the mutations p.C582Y, p.C582F, and p.G553R.

CONCLUSIONS

The present data add 19 novel mutations to the catalog of inactivating CASR mutations and illustrate a variety of biochemical phenotypes in patients with the molecular genetic diagnosis FHH.

Identification and functional characterization of a novel mutation in the calcium-sensing receptor gene in familial hypocalciuric hypercalcemia: modulation of clinical severity by vitamin D status

CONTEXT

Familial hypocalciuric hypercalcemia (FHH) is a benign condition associated with heterogeneous inactivating mutations in the calcium-sensing receptor (CASR) gene.

OBJECTIVE

The objective of the study was to identify and characterize a CASR mutation in a moderately hypercalcemic, hyperparathyroid individual and his family and assess the influence of vitamin D status on the clinical expression of the defect.

SUBJECTS

We studied a kindred with FHH, in which the proband (a 34-yr-old male) was initially diagnosed with primary hyperparathyroidism due to frankly elevated serum PTH levels.

METHODS

CASR gene mutation analysis was performed on genomic DNA of the proband and family members. The mutant CASR was functionally characterized by transient transfection studies in kidney cells in vitro.

RESULTS

A novel heterozygous mutation (F180C, TTC>TGC) in exon 4 of the CASR gene was identified. Although the mutant receptor was expressed normally at the cell surface, it was unresponsive with respect to intracellular signaling (MAPK activation) to increases in extracellular calcium concentrations. The baby daughter of the proband presented with neonatal hyperparathyroidism with markedly elevated PTH. Vitamin D supplementation of both the proband and the baby resulted in reduction of serum PTH levels to the normal range. The serum calcium level remained at a constant and moderately elevated level.

CONCLUSION

The identification of a novel CASR gene mutation established the basis of the hypercalcemia in the kindred. Concomitant vitamin D deficiency modulates the severity of the presentation of FHH.

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.

Inactivating mutations of G protein-coupled receptors and diseases: Structure-function insights and therapeutic implications

Since the discovery of the first rhodopsin mutation that causes retinitis pigmentosa in 1990, significant progresses have been made in elucidating the pathophysiology of diseases caused by inactivating mutations of G protein-coupled receptors (GPCRs). This review aims to compile the compelling evidence accumulated during the past 15 years demonstrating the etiologies of more than a dozen diseases caused by inactivating GPCR mutations. A generalized classification scheme, based on the life cycle of GPCRs, is proposed. Insights gained through detailed studies of these naturally occurring mutations into the structure-function relationship of these receptors are reviewed. Therapeutic approaches directed against the different classes of mutants are being developed. Since intracellular retention emerges as the most common defect, recent progresses aimed at correcting this defect through membrane permeable pharmacological chaperones are highlighted.

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.

Calcium-sensing receptor gene transcription is up-regulated by the proinflammatory cytokine, interleukin-1beta. Role of the NF-kappaB PATHWAY and kappaB elements

The calcium-sensing receptor (CASR) in parathyroid, thyroid, and kidney is essential for calcium homeostasis. Hypocalcemia is common in critically ill patients having increased circulating proinflammatory cytokines, although the causes are unknown. We hypothesized that the cytokines increase CASR expression and reduce the set point for parathyroid hormone suppression by extracellular calcium, leading to hypocalcemia and hypoparathyroidism. Here, we show in vivo in the rat that parathyroid, thyroid, and kidney CASR mRNA and protein increased after injection of interleukin-1beta. This was associated with decreased circulating parathyroid hormone, calcium, and 1,25-dihydroxyvitamin D levels. Interleukin-1beta stimulated endogenous CASR gene transcripts and transfected promoter reporter activity in human thyroid C-cells (TT cells) and kidney proximal tubule (HKC) cells. Cotransfection of NF-kappaB proteins enhanced activity of the reporter constructs, whereas cotransfection with inhibitor-kappaB or application of an NF-kappaB nuclear localization sequence peptide abrogated responsiveness to cytokine or NF-kappaB proteins. Mutagenesis of some, but not all, of the potential kappaB elements in the 5' part of the CASR gene led to loss of responsiveness to cytokine. These elements conferred cytokine responsiveness to a heterologous promoter, and in electrophoretic mobility shift assays, NF-kappaB complexes formed on the same three kappaB elements. In summary, the CASR gene has several functional kappaB elements that mediate its upregulation by proinflammatory cytokines and probably contribute to altered extracellular calcium homeostasis in the critically ill.

Mutant G-protein-coupled receptors as a cause of human diseases

G-protein-coupled receptors (GPCR) are involved in directly and indirectly controlling an extraordinary variety of physiological functions. Their key roles in cellular communication have made them the target for more than 60% of all currently prescribed drugs. Mutations in GPCR can cause acquired and inherited diseases such as retinitis pigmentosa (RP), hypo- and hyperthyroidism, nephrogenic diabetes insipidus, several fertility disorders, and even carcinomas. To date, over 600 inactivating and almost 100 activating mutations in GPCR have been identified which are responsible for more than 30 different human diseases. The number of human disorders is expected to increase given the fact that over 160 GPCR have been targeted in mice. Herein, we summarize the current knowledge relevant to understanding the molecular basis of GPCR function, with primary emphasis on the mechanisms underlying GPCR malfunction responsible for different human diseases.

Recurrent familial hypocalcemia due to germline mosaicism for an activating mutation of the calcium-sensing receptor gene

De novo activating mutations in the calcium-sensing receptor (CASR) gene are a common cause of sporadic isolated hypoparathyroidism. Here, we describe a family in which two affected siblings were found to be heterozygous for a novel F788L mutation in the fifth transmembrane domain encoded by exon 7 of the CASR. Both parents and the third sibling were clinically unaffected and genotypically normal by direct sequencing of their leukocyte exon 7 PCR amplicons. However, the mother was revealed to be a mosaic for the mutation by sequence analysis of multiple subclones as well as denaturing HPLC of the CASR exon 7 leukocyte PCR product. A functional analysis of the mutation was performed by transiently transfecting wild-type and mutant CASRs tagged with a c-Myc epitope in human embryonic kidney (HEK293) cells. The mutant CASR was expressed at a similar level as the wild type. The F788L mutant produced a significant shift to the left relative to the wild-type CASR in the MAPK response to increasing extracellular calcium concentrations. This is the first report of mosaicism for an activating CASR mutation and suggests that care should be exercised in counseling for risks of recurrence in a situation where a de novo mutation appears likely.

Human calcium-sensing receptor gene. Vitamin D response elements in promoters P1 and P2 confer transcriptional responsiveness to 1,25-dihydroxyvitamin D

The calcium-sensing receptor (CASR), expressed in parathyroid chief cells, thyroid C-cells, and cells of the kidney tubule, is essential for maintenance of calcium homeostasis. Here we show parathyroid, thyroid, and kidney CASR mRNA levels increased 2-fold at 15 h after intraperitoneal injection of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in rats. Human thyroid C-cell (TT) and kidney proximal tubule cell (HKC) CASR gene transcription increased approximately 2-fold at 8 and 12 h after 1,25(OH)2D3 treatment. The human CASR gene has two promoters yielding alternative transcripts containing either exon 1A or exon 1B 5'-untranslated region sequences that splice to exon 2 some 242 bp before the ATG translation start site. Transcriptional start sites were identified in parathyroid gland and TT cells; that for promoter P1 lies 27 bp downstream of a TATA box, whereas that for promoter P2, which lacks a TATA box, lies in a GC-rich region. In HKC cells, transcriptional activity of a P1 reporter gene construct was 11-fold and of P2 was 33-fold above basal levels. 10(-8) m 1,25(OH)2D3 stimulated P1 activity 2-fold and P2 activity 2.5-fold. Vitamin D response elements (VDREs), in which half-sites (6 bp) are separated by three nucleotides, were identified in both promoters and shown to confer 1,25(OH)2D3 responsiveness to a heterologous promoter. This responsiveness was lost when the VDREs were mutated. In electrophoretic mobility shift assays with either in vitro transcribed/translated vitamin D receptor and retinoid X receptor-alpha, or HKC nuclear extract, specific protein-DNA complexes were formed in the presence of 1,25(OH)2D3 on oligonucleotides representing the P1 and P2 VDREs. In summary, functional VDREs have been identified in the CASR gene and provide the mechanism whereby 1,25(OH)2D up-regulates parathyroid, thyroid C-cell, and kidney CASR expression.