The Use of Calcimimetics for the Treatment of Secondary Hyperparathyroidism: A 10 Year Evidence Review

Until the discovery of calcimimetics, the management of secondary hyperparathyroidism (SHPT) relied exclusively on treatment with phosphate binders, vitamin D derivatives or surgical parathyroidectomy with limited success. The therapeutic use of calcimimetic agents, together with a better understanding of the pivotal role of the calcium‐sensing receptor (CaSR) in the physiological regulation of parathyroid gland function, substantially advanced the management of hyperparathyroidism in dialysis practice. Calcimimetics bind selectively to the CaSR receptor in parathyroid tissue and enhance the inhibitory effect of extracellular calcium ions on parathyroid hormone (PTH) secretion, thereby reducing PTH levels even when serum calcium concentrations are normal or low. The availability of calcimimetic agents for clinical use has opened a new era in the management of patients with SHPT. Indeed, calcimimetic compounds have been shown to reduce PTH levels and to lower serum calcium concentrations in all forms of hyperparathyroidism, including primary hyperparathyroidism (PHPT) and parathyroid carcinoma. Such findings underscore the critical importance of the CaSR as a therapeutic target in this family of clinical disorders. New calcimimetic agents are being developed that have the potential to offer improved efficacy and safety compared with currently available calcimimetic compounds.     

Calcilytic Ameliorates Abnormalities of Mutant Calcium‐Sensing Receptor (CaSR) Knock‐In Mice Mimicking Autosomal Dominant Hypocalcemia (ADH)

Activating mutations of calcium‐sensing receptor (CaSR) cause autosomal dominant hypocalcemia (ADH). ADH patients develop hypocalcemia, hyperphosphatemia, and hypercalciuria, similar to the clinical features of hypoparathyroidism. The current treatment of ADH is similar to the other forms of hypoparathyroidism, using active vitamin D₃ or parathyroid hormone (PTH). However, these treatments aggravate hypercalciuria and renal calcification. Thus, new therapeutic strategies for ADH are needed. Calcilytics are allosteric antagonists of CaSR, and may be effective for the treatment of ADH caused by activating mutations of CaSR. In order to examine the effect of calcilytic JTT‐305/MK‐5442 on CaSR harboring activating mutations in the extracellular and transmembrane domains in vitro, we first transfected a mutated CaSR gene into HEK cells. JTT‐305/MK‐5442 suppressed the hypersensitivity to extracellular Ca²⁺ of HEK cells transfected with the CaSR gene with activating mutations in the extracellular and transmembrane domains. We then selected two activating mutations locating in the extracellular (C129S) and transmembrane (A843E) domains, and generated two strains of CaSR knock‐in mice to build an ADH mouse model. Both mutant mice mimicked almost all the clinical features of human ADH. JTT‐305/MK‐5442 treatment in vivo increased urinary cAMP excretion, improved serum and urinary calcium and phosphate levels by stimulating endogenous PTH secretion, and prevented renal calcification. In contrast, PTH(1‐34) treatment normalized serum calcium and phosphate but could not reduce hypercalciuria or renal calcification. CaSR knock‐in mice exhibited low bone turnover due to the deficiency of PTH, and JTT‐305/MK‐5442 as well as PTH(1‐34) increased bone turnover and bone mineral density (BMD) in these mice. These results demonstrate that calcilytics can reverse almost all the phenotypes of ADH including hypercalciuria and renal calcification, and suggest that calcilytics can become a novel therapeutic agent for ADH. © 2015 American Society for Bone and Mineral Research.     

A semimechanistic model of the time‐course of release of PTH into plasma following administration of the calcilytic JTT‐305/MK‐5442 in humans

JTT‐305/MK‐5442 is a calcium‐sensing receptor (CaSR) allosteric antagonist being investigated for the treatment of osteoporosis. JTT‐305/MK‐5442 binds to CaSRs, thus preventing receptor activation by Ca²⁺. In the parathyroid gland, this results in the release of parathyroid hormone (PTH). Sharp spikes in PTH secretion followed by rapid returns to baseline are associated with bone formation, whereas sustained elevation in PTH is associated with bone resorption. We have developed a semimechanistic, nonpopulation model of the time‐course relationship between JTT‐305/MK‐5442 and whole plasma PTH concentrations to describe both the secretion of PTH and the kinetics of its return to baseline levels. We obtained mean concentration data for JTT‐305/MK‐5442 and whole PTH from a multiple dose study in U.S. postmenopausal women at doses of 5, 10, 15, and 20 mg. We hypothesized that PTH is released from two separate sources: a reservoir that is released rapidly (within minutes) in response to reduction in Ca²⁺ binding, and a second source released more slowly following hours of reduced Ca²⁺ binding. We modeled the release rates of these reservoirs as maximum pharmacologic effect (E_max) functions of JTT‐305/MK‐5442 concentration. Our model describes both the dose‐dependence of PTH time of occurrence for maximum drug concentration (T_max) and maximum concentration of drug (C_max), and the extent and duration of the observed nonmonotonic return of PTH to baseline levels following JTT‐305/MK‐5442 administration.     

Familial Hypocalciuric Hypercalcemia as an Atypical Form of Primary Hyperparathyroidism

Familial hypocalciuric hypercalcemia (FHH) causes lifelong hypercalcemia with features that overlap with typical primary hyperparathyroidism (PHPT). The incompleteness of this overlap has led to divergent nomenclatures for FHH. I compare two nomenclatures. One sets FHH as an entity distinct from PHPT. The other groups FHH with PHPT but conditions FHH as atypical PHPT. I analyzed selected articles about calcium‐sensing receptors, FHH, PHPT, CASR, GNA11, and AP2S1. FHH usually results from a heterozygous germline inactivating mutation of the CASR, and less frequently from mutation of GNA11 or AP2S1. The CASR encodes the calcium‐sensing receptors. These are highly expressed on parathyroid cells, where they sense serum calcium concentration and regulate suppression of PTH secretion by serum calcium. Their mutated expression in the kidney in FHH causes increased renal tubular reabsorption of calcium (hypocalciuria). Many FHH features are shared with PHPT and thus support FHH as a form of PHPT. These include a driver mutation expressed mainly in the parathyroid cells. The mutation causes a parathyroid cell insensitivity to extracellular calcium in vivo and in vitro, a right‐shift of the set point for suppression of PTH secretion by calcium. Serum PTH is normal or mildly elevated; ie, it is not appropriately suppressed by hypercalcemia. Total parathyroidectomy causes hypoparathyroidism and durable remission of hypercalcemia. Some other features are not shared with PHPT and could support FHH as a distinct entity. These include onset of hypercalcemia in the first week of life, frequent persistence of hypercalcemia after subtotal parathyroidectomy, and hypocalciuria. The features supporting FHH as a form of PHPT are stronger than those favoring FHH as a distinct entity. Classifying FHH as an atypical form of PHPT represents compact nomenclature and supports current concepts of pathophysiology of FHH and PHPT. Published 2017. This article is a U.S. Government work and is in the public domain in the USA.     

Calcimimetic agents and secondary hyperparathyroidism: rationale for use and results from clinical trials

Calcimimetic agents are small organic molecules that act as allosteric activators of the calcium sensing receptor (CaSR). In parathyroid cells, they lower the threshold for receptor activation by extracellular calcium ions and diminish parathyroid hormone (PTH) secretion. Calcimimetic compounds thus represent a novel way of controlling excess PTH secretion in clinical disorders such as secondary hyperparathyroidism (SHPT) due to chronic renal failure. Clinical trials have documented that the calcimimetic agent cinacalcet hydrochloride effectively lowers plasma PTH levels without increasing serum calcium or phosphorus concentrations in adult hemodialysis patients with SHPT. Serum phosphorus levels and values for the calcium-phosphorus ion product in serum often decline as plasma PTH levels fall during treatment. Experimental evidence suggests that calcimimetic agents may also impede the development of parathyroid gland hyperplasia, an integral component of SHPT due to chronic renal failure. Calcimimetic agents have considerable potential, therefore, as part of new therapeutic strategies for SHPT.     

45例甲状旁腺肿瘤临床分析

目的:探讨甲状旁腺肿瘤的诊断、术前定位、外科治疗及预后.方法:对2001年-2012年间收治的45例甲状旁腺肿瘤病例的临床资料进行回顾性分析.结果:全组45例甲状旁腺肿瘤术前均通过B超和或CT、99mTc-MIBI检查得以定位,阳性率分别为86.7％,93.8％,100％.45例均行手术治疗,均为单发肿瘤,其中左上7例,左下23例,右上4例,右下11例.术后病理证实39例为腺瘤,1例腺癌和5例囊肿.囊肿均为非功能性,腺瘤、腺癌均伴原发性甲状旁腺功能亢进(PHPT).术前PHPT患者均有不同程度的血钙升高和血磷降低,其中24例甲状旁腺激素(PTH)升高;术后血钙明显下降,血磷明显上升(均P＜0.05),1周至3个月恢复正常,PTH明显降低(P＜0.05),22例术后1～5 d即正常,2例1年后正常.术中PTH(IOTPH)判断腺瘤成功切除率100％.40例获随访3个月至10年,均无复发和病灶遗漏.结论:甲状旁腺肿瘤起病缓慢,临床表现复杂多样.血钙、血磷以及血PTH的检测有助于PHPT的诊断;B超可作为甲状旁腺肿瘤的术前定位的首选检查,联合CT和99mTc-MIBI核素扫描能提高定位率;手术治疗为甲状旁腺肿瘤的首选治疗方法,效果良好.     

钙敏感受体在甲状旁腺功能亢进中的作用

钙敏感受体(CaSR)主要表达于甲状旁腺细胞和肾小管七皮细胞,它可通过细胞内信号传导通路改变甲状旁腺激素(PTH)的分泌和肾小管对电解质和水的处理,调节体内钙平衡,同时CaSR也参与了骨重塑的过程.其遗传性的基因异常可引起各种不同的疾病,包括CaSR失活突变引起的良性家族性低尿钙高钙血症和新生儿期重度甲状旁腺功能亢进,激活突变可引起常染色体显性遗传性低钙血症,与原发性甲状旁腺功能亢进的发病也存在一定的联系.这也为CaSR靶向治疗提供了依据.     

钙敏感受体在甲状旁腺功能亢进中的作用

钙敏感受体(CaSR)主要表达于甲状旁腺细胞和肾小管七皮细胞,它可通过细胞内信号传导通路改变甲状旁腺激素(PTH)的分泌和肾小管对电解质和水的处理,调节体内钙平衡,同时CaSR也参与了骨重塑的过程.其遗传性的基因异常可引起各种不同的疾病,包括CaSR失活突变引起的良性家族性低尿钙高钙血症和新生儿期重度甲状旁腺功能亢进,...     

The calcium‐sensing receptor and 25‐hydroxyvitamin D–1α‐hydroxylase interact to modulate skeletal growth and bone turnover

We examined parathyroid and skeletal function in 3‐month‐old mice expressing the null mutation for 25‐hydroxyvitamin D–1α‐hydroxylase [1α(OH)ase^?/?] and in mice expressing the null mutation for both the 1α(OH)ase and the calcium‐sensing receptor [Casr^?/?1α(OH)ase^?/?] genes. On a normal diet, all mice were hypocalcemic, with markedly increased parathyroid hormone (PTH), increased trabecular bone volume, increased osteoblast activity, poorly mineralized bone, enlarged and distorted cartilaginous growth plates, and marked growth retardation, especially in the compound mutants. Osteoclast numbers were reduced in the Casr^?/?1α(OH)ase^?/? mice. On a high‐lactose, high‐calcium, high‐phosphorus “rescue” diet, serum calcium and PTH were normal in the 1α(OH)ase^?/? mice but increased in the Casr^?/?1α(OH)ase^?/? mice with reduced serum phosphorus. Growth plate architecture and mineralization were improved in both mutants, but linear growth of the double mutants remained abnormal. Mineralization of bone improved in all mice, but osteoblast activity and trabecular bone volume remained elevated in the Casr^?/?1α(OH)ase^?/? mice. These studies support a role for calcium‐stimulated maturation of the cartilaginous growth plate and mineralization of the growth plate and bone and calcium‐stimulated CaSR‐mediated effects on bone resorption. PTH‐mediated bone resorption may require calcium‐stimulated CaSR‐mediated enhancement of osteoclastic activity. © 2010 American Society for Bone and Mineral Research. © 2010 American Society for Bone and Mineral Research     

Development and progression of secondary hyperparathyroidism in chronic kidney disease: lessons from molecular genetics

The identification of the calcium-sensing receptor (CaSR) and the clarification of its role as the major regulator of parathyroid gland function have important implications for understanding the pathogenesis and evolution of secondary hyperthyroidism in chronic kidney disease (CKD). Signaling through the CaSR has direct effects on three discrete components of parathyroid gland function, which include parathyroid hormone (PTH) secretion, PTH synthesis, and parathyroid gland hyperplasia. Disturbances in calcium and vitamin D metabolism that arise owing to CKD diminish the level of activation of the CaSR, leading to increases in PTH secretion, PTH synthesis, and parathyroid gland hyperplasia. Each represents a physiological adaptive response by the parathyroid glands to maintain plasma calcium homeostasis. Studies of genetically modified mice indicate that signal transduction via the CaSR is a key determinant of parathyroid cell proliferation and parathyroid gland hyperplasia. Because enlargement of the parathyroid glands has important implications for disease progression and disease severity, it is possible that clinical management strategies that maintain adequate calcium-dependent signaling through the CaSR will ultimately prove useful in diminishing parathyroid gland hyperplasia and in modifying disease progression.     

Mapping of human autoantibody binding sites on the calcium‐sensing receptor

Previously, we have demonstrated the presence of anti‐calcium‐sensing receptor (CaSR) antibodies in patients with autoimmune polyglandular syndrome type 1 (APS1), a disease that is characterized in part by hypoparathyroidism involving hypocalcemia, hyperphosphatemia, and low serum levels of parathyroid hormone. The aim of this study was to define the binding domains on the CaSR of anti‐CaSR antibodies found in APS1 patients and in one patient suspected of having autoimmune hypocalciuric hypercalcemia (AHH). A phage‐display library of CaSR peptides was constructed and used in biopanning experiments with patient sera. Selectively enriched IgG‐binding peptides were identified by DNA sequencing, and subsequently, immunoreactivity to these peptides was confirmed in ELISA. Anti‐CaSR antibody binding sites were mapped to amino acid residues 41–69, 114–126, and 171–195 at the N‐terminal of the extracellular domain of the receptor. The major autoepitope was localized in the 41–69 amino acid sequence of the CaSR with antibody reactivity demonstrated in 12 of 12 (100%) APS1 patients with anti‐CaSR antibodies and in 1 AHH patient with anti‐CaSR antibodies. Minor epitopes were located in the 114–126 and 171–195 amino acid domains, with antibody reactivity shown in 5 of 12 (42%) and 4 of 12 (33%) APS1 patients, respectively. The results indicate that epitopes for anti‐CaSR antibodies in the AHH patient and in the APS1 patients who were studied are localized in the N‐terminal of the extracellular domain of the receptor. The present work has demonstrated the successful use of phage‐display technology in the discovery of CaSR‐specific epitopes targeted by human anti‐CaSR antibodies. © 2010 American Society for Bone and Mineral Research     

Direct in vitro evidence of the suppressive effect of cinacalcet HCl on parathyroid hormone secretion in human parathyroid cells with pathologically reduced calcium-sensing receptor levels

Clinical studies have been performed to determine the effect of cinacalcet HCl (cinacalcet), an allosteric modulator of the calcium-sensing receptor (CaR), on primary hyperparathyroidism (PHPT) and secondary hyperparathyroidism of uremia (SHPT). However, no in vitro studies on human parathyroid cells have been reported to date. In this study, the inhibitory effect of cinacalcet on PTH secretion was analyzed in primary cultured parathyroid cells obtained from patients. The investigation involved three PHPT and three SHPT patients subjected to therapeutic parathyroidectomy. Notably, all SHPT patients were resistant to intravenous vitamin D analogue therapy. Removed parathyroid tumors were used for immunohistochemistry and parathyroid cell primary culture. Immunohistochemical analyses revealed diminished expression of CaR and vitamin D receptor (VDR) in all parathyroid tumors. PTH secretion from cultured parathyroid cells of PHPT and SHPT patients was suppressed by extracellular Ca²⁺ and cinacalcet in a dose-dependent manner. Rates of suppression of PTH secretion in PHPT and SHPT by cinacalcet (1000 nmol/l) were 61% ± 21% and 61% ± 19%, respectively. Cinacalcet demonstrates significant potency in the suppression of PTH secretion in primary cultured human parathyroid cells in vitro, despite reduced levels of the target protein, CaR. Data from this in vitro analysis support the clinical application of cinacalcet in PHPT and SHPT therapy.     

Diphtheria Toxin‐ and GFP‐Based Mouse Models of Acquired Hypoparathyroidism and Treatment With a Long‐Acting Parathyroid Hormone Analog

Hypoparathyroidism (HP) arises most commonly from parathyroid (PT) gland damage associated with neck surgery, and is typically treated with oral calcium and active vitamin D. Such treatment effectively increases levels of serum calcium (sCa), but also brings risk of hypercalciuria and renal damage. There is thus considerable interest in using PTH or PTH analogs to treat HP. To facilitate study of this disease and the assessment of new treatment options, we developed two mouse models of acquired HP, and used them to assess efficacy of PTH(1–34) as well as a long‐acting PTH analog (LA‐PTH) in regulating blood calcium levels. In one model, we used PTHcre‐iDTR mice in which the diphtheria toxin (DT) receptor (DTR) is selectively expressed in PT glands, such that systemic DT administration selectively ablates parathyroid cells. For the second model, we generated GFP‐PT mice in which green fluorescent protein (GFP) is selectively expressed in PT cells, such that parathyroidectomy (PTX) is facilitated by green fluorescence of the PT glands. In the PTHcre‐iDTR mice, DT injection (2 × 5 μg/kg, i.p.) resulted in moderate yet consistent reductions in serum PTH and sCa levels. The more severe hypoparathyroid phenotype was observed in GFP‐PT mice following GFP‐guided PTX surgery. In each model, a single subcutaneous injection of LA‐PTH increased sCa levels more effectively and for a longer duration (>24 hours) than did a 10‐fold higher dose of PTH(1–34), without causing excessive urinary calcium excretion. These new mouse models thus faithfully replicate two degrees of acquired HP, moderate and severe, and may be useful for assessing potential new modes of therapy. © 2015 American Society for Bone and Mineral Research.     

Cinacalcet Rectifies Hypercalcemia in a Patient With Familial Hypocalciuric Hypercalcemia Type 2 (FHH2) Caused by a Germline Loss‐of‐Function Gα11 Mutation

G‐protein subunit α‐11 (Gα₁₁) couples the calcium‐sensing receptor (CaSR) to phospholipase C (PLC)‐mediated intracellular calcium (Ca²⁺_i) and mitogen‐activated protein kinase (MAPK) signaling, which in the parathyroid glands and kidneys regulates parathyroid hormone release and urinary calcium excretion, respectively. Heterozygous germline loss‐of‐function Gα₁₁ mutations cause familial hypocalciuric hypercalcemia type 2 (FHH2), for which effective therapies are currently not available. Here, we report a novel heterozygous Gα₁₁ germline mutation, Phe220Ser, which was associated with hypercalcemia in a family with FHH2. Homology modeling showed the wild‐type (WT) Phe220 nonpolar residue to form part of a cluster of hydrophobic residues within a highly conserved cleft region of Gα₁₁, which binds to and activates PLC; and predicted that substitution of Phe220 with the mutant Ser220 polar hydrophilic residue would disrupt PLC‐mediated signaling. In vitro studies involving transient transfection of WT and mutant Gα₁₁ proteins into HEK293 cells, which express the CaSR, showed the mutant Ser220 Gα₁₁ protein to impair CaSR‐mediated Ca²⁺_i and extracellular signal‐regulated kinase 1/2 (ERK) MAPK signaling, consistent with diminished activation of PLC. Furthermore, engineered mutagenesis studies demonstrated that loss of hydrophobicity within the Gα₁₁ cleft region also impaired signaling by PLC. The loss‐of‐function associated with the Ser220 Gα₁₁ mutant was rectified by treatment of cells with cinacalcet, which is a CaSR‐positive allosteric modulator. Furthermore, in vivo administration of cinacalcet to the proband harboring the Phe220Ser Gα₁₁ mutation, normalized serum ionized calcium concentrations. Thus, our studies, which report a novel Gα₁₁ germline mutation (Phe220Ser) in a family with FHH2, reveal the importance of the Gα₁₁ hydrophobic cleft region for CaSR‐mediated activation of PLC, and show that allosteric CaSR modulation can rectify the loss‐of‐function Phe220Ser mutation and ameliorate the hypercalcemia associated with FHH2. © 2017 The Authors. Journal of Bone and Mineral Research Published by Wiley Periodicals Inc.     

Parathyroid hormone signaling via Gαs is selectively inhibited by an NH2‐terminally truncated Gαs: Implications for pseudohypoparathyroidism

Pseudohypoparathyroid patients have resistance predominantly to parathyroid hormone (PTH), and here we have examined the ability of an alternative Gαs‐related protein to inhibit Gαs activity in a hormone‐selective manner. We tested whether the GNAS exon A/B‐derived NH₂‐terminally truncated (Tr) αs protein alters stimulation of adenylate cyclase by the PTH receptor (PTHR1), the thyroid‐stimulating hormone (TSH) receptor (TSHR), the β₂‐adrenergic receptor (β₂AR), or the AVP receptor (V2R). HEK293 cells cotransfected with receptor and full‐length (FL) Gαs ± Tr αs protein expression vectors were stimulated with agonists (PTH [10^?7 to 10^?9 M], TSH [1 to 100 mU], isoproterenol [10^?6 to 10^?8 M], or AVP [10^?6 to 10^?8 M]). Following PTH stimulation, HEK293 cells cotransfected with PTHR1 + FL Gαs + Tr αs had a significantly lower cAMP response than those transfected with only PTHR1 + FL Gαs. Tr αs also exerted an inhibitory effect on the cAMP levels stimulated by TSH via the TSHR but had little or no effect on isoproterenol or AVP acting via β₂AR or V2R, respectively. These differences mimic the spectrum of hormone resistance in pseudohypoparathyroidism type 1a (PHP‐1a) and type 1b (PHP‐1b) patients. In opossum kidney (OK) cells, endogenously expressing the PTHR1 and β₂AR, the exogenous expression of Tr αs at a level similar to endogenous FL Gαs resulted in blunting of the cAMP response to PTH, whereas that to isoproterenol was unaltered. A pseudopseudohypoparathyroid patient with Albright hereditary osteodystrophy harbored a de novo paternally inherited M1I Gαs mutation. Similar maternally inherited mutations at the initiation codon have been identified previously in PHP‐1a patients. The M1I αs mutant (lacking the first 59 amino acids of Gαs) blunted the increase in cAMP levels stimulated via the PTHR1 in both HEK293 and OK cells similar to the Tr αs protein. Thus NH₂‐terminally truncated forms of Gαs may contribute to the pathogenesis of pseudohypoparathyroidism by inhibiting the activity of Gαs itself in a GPCR selective manner. © 2011 American Society for Bone and Mineral Research     

A G‐protein Subunit‐α11 Loss‐of‐Function Mutation,Thr54Met,Causes Familial Hypocalciuric Hypercalcemia Type 2 (FHH2)

Familial hypocalciuric hypercalcemia (FHH) is a genetically heterogeneous disorder with three variants, FHH1 to FHH3. FHH1 is caused by loss‐of‐function mutations of the calcium‐sensing receptor (CaSR), a G‐protein coupled receptor that predominantly signals via G‐protein subunit alpha‐11 (Gα₁₁) to regulate calcium homeostasis. FHH2 is the result of loss‐of‐function mutations in Gα₁₁, encoded by GNA11, and to date only two FHH2‐associated Gα₁₁ missense mutations (Leu135Gln and Ile200del) have been reported. FHH3 is the result of loss‐of‐function mutations of the adaptor protein‐2 σ‐subunit (AP2σ), which plays a pivotal role in clathrin‐mediated endocytosis. We describe a 65‐year‐old woman who had hypercalcemia with normal circulating parathyroid hormone concentrations and hypocalciuria, features consistent with FHH, but she did not have CaSR and AP2σ mutations. Mutational analysis of the GNA11 gene was therefore undertaken, using leucocyte DNA, and this identified a novel heterozygous GNA11 mutation (c.161C>T; p.Thr54Met). The effect of the Gα₁₁ variant was assessed by homology modeling of the related Gα_q protein and by measuring the CaSR‐mediated intracellular calcium (Ca²⁺_i) responses of HEK293 cells, stably expressing CaSR, to alterations in extracellular calcium (Ca²⁺_o) using flow cytometry. Three‐dimensional modeling revealed the Thr54Met mutation to be located at the interface between the Gα₁₁ helical and GTPase domains, and to likely impair GDP binding and interdomain interactions. Expression of wild‐type and the mutant Gα₁₁ in HEK293 cells stably expressing CaSR demonstrate that the Ca²⁺_i responses after stimulation with Ca²⁺_o of the mutant Met54 Gα₁₁ led to a rightward shift of the concentration‐response curve with a significantly (p < 0.01) increased mean half‐maximal concentration (EC₅₀) value of 3.88 mM (95% confidence interval [CI] 3.76–4.01 mM), when compared with the wild‐type EC₅₀ of 2.94 mM (95% CI 2.81–3.07 mM) consistent with a loss‐of‐function. Thus, our studies have identified a third Gα₁₁ mutation (Thr54Met) causing FHH2 and reveal a critical role for the Gα₁₁ interdomain interface in CaSR signaling and Ca²⁺_o homeostasis. © 2016 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research (ASBMR).     

Primary hyperparathyroidism in a patient with familial hypocalciuric hypercalcaemia due to a novel mutation in the calcium-sensing receptor gene

We describe the clinical and genetic findings in pedigree with a novel mutation in the calcium sensing receptor (CaSR) gene and the unusual coexistence of primary hyperparathyroidism (HPT) and familial hypocalciuric hypercalcaemia (FHH) and its clinical management. The occurrence of both FHH and primary HPT in the same patient has been described rarely. Our pedigree has a novel mutation in the CaSR gene. Parathyroidectomy led to a reduction, but not normalization of the calcium levels in the patient identified as having HPT. The coexistence of HPT and FHH was considered in this patient as her calcium and PTH levels were rising with time. Surgical resection of her parathyroid adenoma resulted in reduction of her calcium levels to above normal and significant reduction in her symptoms of fatigue and low mood.     

Cinacalcet HCl Suppresses <Emphasis Type="Italic">Cyclin D1</Emphasis> Oncogene-Derived Parathyroid Cell Proliferation in a Murine Model for Primary Hyperparathyroidism

Cinacalcet HCl (cinacalcet) is a calcimimetic compound, which suppresses parathyroid (PTH) hormone secretion from parathyroid glands in both primary hyperparathyroidism (PHPT) and secondary hyperparathyroidism (SHPT). We previously reported the suppressive effect of cinacalcet on PTH secretion in vivo in a PHPT model mouse, in which parathyroid-targeted overexpression of the cyclin D1 oncogene caused chronic biochemical hyperparathyroidism and parathyroid cell hyperplasia. Although cinacalcet suppressed parathyroid cell proliferation in SHPT in 5/6-nephrectomized uremic rats, its effect on PHPT has not yet been determined. In this study, the effect of cinacalcet on parathyroid cell proliferation was analyzed in PHPT mice. Cinacalcet (1 mg/g) was mixed into the rodent diet and orally administrated to 80-week-old PHPT mice for 10 days before death. 5-Bromo-2′-deoxyuridine (BrdU, 6 mg/day) was infused by an osmotic pump for 5 days before death, followed by immunostaining of the thyroid–parathyroid complex using an anti-BrdU antibody to estimate parathyroid cell proliferation. Compared to untreated PHPT mice, cinacalcet significantly suppressed both serum calcium and PTH. The proportion of BrdU-positive cells to the total cell number in the parathyroid glands increased considerably in untreated PHPT mice (9.5 ± 3.1%) compared to wild-type mice (0.7 ± 0.1%) and was significantly suppressed by cinacalcet (1.2 ± 0.2%). Cinacalcet did not affect apoptosis in the parathyroid cells of PHPT mice. These data suggest that cinacalcet suppressed both serum PTH levels and parathyroid cell proliferation in vivo in PHPT.     

Calcium-sensing receptors

It is now known that variations in extracellular calcium concentration exert diverse physiologic effects in a variety of tissues that are mediated by a calcium-sensing receptor (CaSRs). In parathyroid tissue, the CaSR represents the molecular mechanism by which parathyroid cells detect changes in blood ionized calcium concentration, modulate parathyroid hormone (PTH) secretion accordingly, and thus maintain serum calcium levels within a narrow physiologic range. In the kidney, the CaSR regulates renal calcium excretion and influences the transepithelial movement of water and other electrolytes. More generally, activation of the CaSR represents an important signal transduction pathway in intestine, placenta, brain, and perhaps bone. Some of these actions involve cell cycle regulation, changes that may be relevant to understanding the pathogenesis of parathyroid gland hyperplasia in secondary hyperparathyroidism caused by chronic kidney disease. The CaSR represents an appealing target for therapeutic agents designed to modify parathyroid gland function in vivo, offering the prospect of novel therapies for selected disorders of bone and mineral metabolism. Other receptors capable of responding to extracellular calcium ions also have been identified, but the functional importance of these interactions remains to be determined.     

Phosphocalcic metabolism: regulation and explorations

Calcium and phosphate play a key role in bone mineralization but have also many other physiological functions. The control of serum phosphate concentration is mandatory to avoid the occurrence of severe metabolic disorders, but is less tightly regulated than serum ionized calcium concentration, which is maintained in a very limited range thanks to parathyroid hormone (PTH) and the active vitamin D metabolite calcitriol. Any change in serum ionized calcium concentration is detected by the calcium sensing receptor (CaSR), a membranous protein located principally in the parathyroid glands and the kidney. A decrease in ionized calcium level inactivates the CaSR, thus stimulating PTH secretion. PTH in turn stimulates the release of calcium and phosphate from bone, renal calcium reabsorption and calcium and phosphate intestinal absorption by inducing renal calcitriol production. Moreover, PTH inhibits phosphate reabsorption in proximal tubular cells, thus contributing towards phosphate homeostasis. Fibroblast growth factor 23 (FGF23) is a circulating factor that decreases serum levels of inorganic phosphate by inhibiting renal phosphate reabsorption and calcitriol production and may have a great physiological role in phosphate homeostasis. Recently, vitamin D actions independent of calcium and phosphate homeostasis were discovered. Basal exploration of phosphocalcic metabolism abnormalities consists in measurement of serum calcium (ionized calcium if possible), phosphate, 25-hydroxy vitamine D and PTH and of 24 hours urinary calcium excretion as well as renal function. Hence, the understanding of physiopathological mechanisms has been improved by newly identified genetic disorders responsible for phophocalcic homeostasis disturbances.