Background
The calcium-sensing receptor (CaSR), a G-protein coupled receptor (GPCR) family member, is ubiquitously expressed, but mostly in the parathyroid gland and the renal tubule. It enables CaSR-expressing cells to sense alterations in the level of blood calcium and to normalize its concentration, by regulating parathyroid hormone (PTH) secretion and urinary calcium excretion. The CaSR is able to bind numerous ligands, to interact with multiple G-proteins, and to regulate highly divergent downstream signalling pathways and cell fate, through epigenetics and miRNA [
1]. Besides Ca
2+, the ligands include other divalent cations such as Mg
2+, Ba
2+, Mn
2+, Ni
2+, Sr
2+ and trivalent cations La
3+ and Gd
3+, basic peptides (such as poly-arginine, protamine, and poly-lysine), glutathione and its γ-glutamyl peptides, agonists (such as AMG 416), antagonists and drugs [
2]. The human
CASR gene localizes on chromosome 3q and has 8 exons, the first (1A and 1B) encoding alternative 5′-untranslatesd regions splicing. The CASR promoters are responsive to 1,25-dihydroxyvitamin D, proinflammatory cytokines (TNF-alpha, IL-1beta and IL-6) and the transcription factor glial cells missing-2 (GCM2) [
1]. Abnormal CaSR function affects the development of both calciotropic disorders, and non-calciotropic disorders, such as cardiovascular disease and cancer [
3].
Several disorders of calcium sensing arise from inherited or acquired abnormalities that ‘reset’ the serum calcium concentration upwards or downwards. They are expressed through a hyper- or hypocalcaemic syndrome [
3] (Table
1).
Table 1
Main diseases related to CaSR anomalies
Hypocalciuric hypercalcaemia syndromes: |
- Genetic via inactivating mutations of the CASR gene
o heterozygous (familial benign hypercalcaemia), NB: Serum calcium levels of the variants A986S, R990G and Q1011E slightly higher than in the general population
o homozygous, compound heterozygous (severe neonatal hyperparathyroidism) - Acquired via anti-CaSR blocking antibodies (rare) |
Hypercalciuric hypocalcaemia syndrome, more rare, - Genetic via heterozygous activating mutations of the CASR gene
o autosomal dominant
o sometimes with presentation of pseudo-Bartter’s syndrome - Acquired via anti-CaSR stimulating antibodies |
Other disorders - Hypercalciuria - lithiases R990G variant of CaSR - Cancers: tumor suppressor or oncogenic (colon, breast, prostate, neuroblastoma, etc.) - Metabolic syndrome - Hypergastrinaemia - Inflammatory digestive and respiratory diseases - Taste (kokumi) |
Familial hypocalciuric hypercalcaemia syndromes are schematically related to inactivating mutations of the
CASR gene:
Hypocalcaemia, which is more rare, is related to heterozygous activating mutations of CASR, corresponding to autosomal dominant hypocalcaemia, sometimes with a presentation of pseudo-Bartter’s syndrome. Acquired diseases, which are much more rare, are associated with the presence of CaSR-stimulating or CaSR-blocking antibodies.
Finally, the role of CaSR in many diseases that do not cause calcium and phosphate disorders is gradually coming to light, thus providing new therapeutic possibilities.
The aim of this review is 1) to make a point on the different disorders of calcium metabolism associated with CaSR anomalies, their main differential diagnoses, and their treatment, 2) to unveil the less known fields in which the CaSR could be involved. To do so, we performed a literature review with the keywords calcium-sensing receptor, hypocalciuric hypercalcaemia, hypocalcemia and hyperparathyroidism.
Genetic causes
Hypercalcaemia via inactivating CASR mutations
Heterozygous inactivating mutations
Homozygous inactivating mutations
Hypocalcaemia via activating mutations of CASR
Hypercalciuric hypocalcaemia syndrome type 1
Hypercalciuric hypocalcaemia syndrome is an isolated form of autosomal dominant, congenital hypoparathyroidism that is the mirror of the presentation of FHH (Table
1). There is therefore hypocalcemia with normal or low PTH but that is maladjusted in all cases. The urine calcium is usually normal, consistent with hypercalciuria relative to the serum calcium levels. There is a tendancy for hypomagnesemia. This syndrome is linked to gain-of-function, or activating, mutations of the
CASR gene [
40‐
42].
-
50% of patients present with moderate and asymptomatic hypocalcemia that is found fortuitously,
-
50% present with paresthesias, tetany, epilepsy, severe hypocalcaemia, sometimes with Bartter syndrome [
43],
-
10% present with hypercalciuria with nephrocalcinosis or lithiasis,
-
Over 35% present with ectopic and/or basal ganglia calcifications.
The A843E, C131W, F788C mutations are generally associated with hypomagnesemia with PTH at the lower limit of normal. The P221L, K47N and finally E481K mutations are associated with normal serum magnesium levels, and an increased PTH in response to the hypocalcemia [
44,
45].
Differential diagnosis
The differential diagnosis of these hypercalciuric hypocalcaemia syndromes consists of the hypercalciuric hypocalcaemia syndrome type 2 linked to a gain-in-function activating mutation of the
GNA11 gene (Table
2). The phenotype is identical to the hypercalciuric hypocalcaemia syndrome type 1 linked to an activating mutation of the
CASR gene, apart from the hypercalciuria and hypomagnesemia that are not present in the type 2 form, [
41,
46]. A gain-of-function mutation has still not been described for the
AP2S1 gene at this time.
The other differential diagnoses consist of hypoparathyroidism aetiologies.
This presentation may be reproduced by the presence of anti-CaSR stimulant antibodies, for which an investigation will therefore need to be done.
Treatments
The emergency treatment for these types of hypocalcaemia is based on parenteral calcium administration and the standard of care treatment would be vitamin and calcium supplementation. Vitamin D stimulates the expression of CASR but causes an increase in urine calcium with the risk of nephrocalcinosis in 57% of treated subjects.
For this reason, it is recommended that treatment should only be given to patients with hypocalcaemia below 76 mg/L and/or who are symptomatic, using the smallest dose of 1 alpha-hydroxycholecalciferol (1 to 2 μg/day), while monitoring the 24-h urine calcium every 3 to 6 months. The combination with thiazides, to decrease the urine calcium, worsens the hypokalemic tendency [
26]. Recombinant PTH may normalise the serum and urine calcium levels. Calcilytic compounds (allosteric inhibitors of CaSR) could be beneficial in the future, through stimulation of PTH secretion and reduction of urine calcium and renal calcifications [
47,
48]. They are however usually ineffective in osteoporosis. Otherwise, certain drugs such as proton pump inhibitors may worsen hypomagnesemia and hypocalcemia and should be used with caution to avoid cardiac arrhythmias and seizures.
Acquired causes
Autoimmune hypocalciuric hypercalcaemia
The clinical presentation of autoimmune hypocalciuric hypercalcaemia is identical to the presentation of the genetically determined forms, with moderate hyperparathyroidism, relative hypocalciuria, and inappropriate PTH. An association with other autoimmune diseases is possible in the case of autoimmune polyendocrinopathy syndrome type 1 (APECED) or type 2 (particularly thyroid diseases and coeliac disease). The case studied histologically did not show lymphocytic infiltration of the parathyroids. Anti-CaSR antibodies, blocking in vitro
, directed against the extracellular portion of the protein have been identified [
49‐
51]. They inhibit the activation of CaSR by the extracellular calcium, resulting in PTH stimulation.
Acquired autoimmune hypoparathyroidism
These types of hypoparathyroidism may occur in isolation or in association with autoimmune polyendocrinopathy type 1 or more rarely with type 2. They are characterised by the presence of antibodies directed against the extracellular portion of CaSR. The prevalence of these antibodies in isolated autoimmune hypoparathyroidism is about 49%. The presence of this type of antibody does not imply that they are necessarily functional.
Anti-
NALP5 antibodies have also been identified, particularly in cases of hypoparathyroidism related to the APECED syndrome. Their specificity is 50% and their sensitivity is 26%, which makes them a poorer indicator than the anti-CaSR antibodies,which have a specificity of 83% and a sensitivity of 50%. The presence of these antibodies in this type of syndrome is more frequent when they are assayed early relative to the date of occurrence of the hypoparathyroidism [
50].
Calcium-sensing receptor and other diseases
Hypercalciuria and lithiasis
The serum calcium level is genetically determined for 50 to 70% of its variability. The A986S variant, as well as the R990G and Q1011E variants, are associated with higher serum calcium levels [
27,
28] than in the general population. Hypercalciuria is associated with the R990G variant of
CASR [
18‐
20]. The minor allele rs6776158 predisposes to kidney stones by decreasing the transcriptional activity of the CASR gene promoter 1 and its renal tubular expression. Moreover, Claudin 14, a protein which regulates the transport of ions ans solutes at epithelial tight junctions, is expressed at a lower level in the rs6776158 GG homozygous subjects. In these cases, nephrolithiasis may occur by another mechanism than hypercalciuria [
1,
19].
Calcium-sensing receptor and the digestive tract
Hypercalcaemia is associated with increased acid discharge, since the secretion of gastrin and calcitonin is stimulated by the hypercalcaemia via the intermediary of the CaSR present in the gastrin-secreting cells. Patients presenting with hypercalcaemia therefore have a frequent tendency for hypergastrinaemia, which may explain the increased frequency of digestive disorders. CaSR reportedly has anti-inflammatory, anti-secretory, pro-absorbent and inhibitory properties on intestinal motility. Its activation could provide a new therapeutic approach for diarrhoea [
52,
53].
Calcium-sensing receptor and the respiratory tract
The activation of CaSR also reportedly has anti-inflammatory and anti-allergic properties, which could provide a therapeutic benefit [
54].
Calcium-sensing receptor and cancer
A correlation was observed between certain
CASR rs17251221 type polymorphisms and coronary artery disease, type 2 diabetes, cancer and mortality. In cancer, CaSR appears to have paradoxical roles, and depending on the tissue involved, it is able to prevent or promote tumour growth. This effect would be mediated through genetic or epigenetic mechanisms such as methylation of the
CASR P2 promoter especially in colon cancer and neuroblastic tumors [
1]. In tissues like the parathyroid or colon, CaSR inhibits proliferation and induces terminal differentiation of the cells. Therefore, loss of the receptor, as seen in colorectal or parathyroid tumours, confers malignant potential, suggestive of a tumour suppressor role. In contrast, in prostate and breast tumours, expression of CaSR is increased, and it seems that it favours metastasis to the bone, acting as an oncogene [
55,
56]. Normal and neoplastic mammary epithelial cells express CaSR. During lactation, CaSR activation in the mammary cells causes negative feedback of parathyroid hormone-related protein (PTHrP) in the milk and blood and increases calcium transport in the milk. Conversely, in breast cancer, CaSR stimulates the expression of PTHrP. A switch in the function of the G-proteins underlies the opposite effects of CaSR on the expression of PTHrP in normal and neoplastic mammary epithelial cells.
Calcium-sensing receptor and cardiovascular risk
CaSR is present on the β cells and the enteral endocrine cells, as well as on the adipocytes and the myocytes. This may explain why alterations in cardiac function and metabolic regulation are associated with genetically determined signalling abnormalities of CaSR, particularly with regard to insulin secretion, postprandial blood glucose regulation, lipolysis and inhibition of myocardial cell proliferation [
57‐
63].
Calcium-sensing receptor and taste regulation
CaSR could participate in taste control (particularly the taste of kokumi) and the regulation of enterogastric hormones, such as ghrelin, which are orexigenic and participate in glucose regulation in response to the amino acid intake in the intestine [
64].
Knowledge of the calcium-sensing receptor enables the development of various approaches, especially those of an imaging agent (Calhex-231) in parathyroid diseases and perhaps in the medullary thyroid cancer.
Calcium-sensing receptor modulators
There are two known types of CaSR modulators: CaSR agonists (or activators, still named calcimimetics) and calcilytic antagonists (or inhibitors of the calcium-sensing receptor) [
48,
60,
65‐
67].
CaSR agonists or calcimimetics
These drugs are capable of making CaSR more sensitive to serum calcium levels, thereby decreasing the parathyroid hormone and the serum calcium in hyperparathyroidism, whether they are primary, secondary or neoplastic. Two types have been described: type I, which is a direct agonist, and type II, which is a positive allosteric activator.
Cinacalcet is a type II calcimimetic and thus results in a risk of dose-dependent hypocalcaemia by reducing the parathyroid hormone. In patients on dialysis, cinacalcet reduces calcium, phosphorus and FGF23 levels, hyperplasia of the parathyroids and bone remodelling, with a bone gain of around 3% in the femur; it may diminish skeletal fracture rates and the need for parathyroidectomy. It is also indicated in primary hyperparathyroidism, parathyroid carcinomatosis, parathyromatosis, and refusal or contraindications of parathyroid surgery.
The second generation type II calcimimetic, Velcalcetide (AMG416) is currently under regulatory review.
Their benefits have also been discussed in patients with arterial hypertension and vascular calcifications. The
R-568 or
AMG641 calcimimetics are in fact capable of increasing CaSR expression and decreasing mineralisation of the vascular smooth muscle cells, which could have a beneficial effect on vascular calcifications [
68]. Finally, the combination of cinacalcet with small doses of vitamin D has been shown to be effective in slowing down the progression of vascular calcifications compared with treatments using larger doses of vitamin D alone.
Calcilytic antagonists or inhibitors of the calcium-sensing receptor
Calcilytics are CaSR antagonists that stimulate the secretion of PTH and reduce renal excretion of calcium. They have not been shown to be effective in osteoporosis, especially post-menopausal osteoprosis. They could be indicated in cases of idiopathic hypocalcaemia and hypercalciuria.
Future outlook and Conclusions
There are many CaSR diseases, but familial hypocalciuric hypercalcaemia syndrome is the most common, even if it may sometimes be associated with, depending on the variant, a hypercalciuric tendency, thus posing problems for the differential diagnosis with hyperparathyroidism.
The genetic causes identified in adults usually require no treatment. Some cases of parathyroid adenomas, however, are associated with CASR gene mutations, thus justifying additional morphological investigations in case of severe hypercalcaemia, since excision of these adenomas can improve serum calcium levels.
The CASR gene might also be involved in tumourigenesis, particularly in the colon, breasts and the prostate, as well as in cardiovascular and inflammatory diseases, including both digestive and respiratory.
It is important to know how to make the diagnosis in complex situations, such as mammary neoplasm or chronic renal failure, since these disorders, which remain asymptomatic for a long time, can suggest neoplastic hypercalcaemia or that related to tertiary hyperparathyroidism, while it is in fact a genetically determined disorder.
Acknowledgements
We thank Mr Pierre-Olivier Pryen for his secretarial assistance and Mrs Janet Ratziu for the English editing
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