Interpretive Handbook

The extracellular G-protein-coupled calcium sensing receptor (CASR) is an essential component of calcium homeostasis. CASR is expressed at particularly high levels in the parathyroid glands and kidneys. It forms stable homodimeric cell-membrane complexes, which signal upon binding of extracellular calcium-ions (Ca[++]). In the parathyroid glands, this results in downregulation of gene expression of the main short-term regulator of calcium homeostasis, parathyroid hormone (PTH), as well as diminished secretion of already synthesized PTH. At the same time, renal calcium excretion is upregulated and sodium-chloride excretion is downregulated. Ca(++) binding to CASR is highly cooperative within the physiological Ca(++) concentration range, leading to a steep dose-response curve, which results in tight control of serum calcium levels.

To date over 100 different alterations in the CASR gene have been described. Many of these cause diseases of abnormal serum calcium regulation. Inactivating mutations result in undersensing of Ca(++) concentrations and consequent PTH overproduction and secretion. This leads to either familial hypocalciuric hypercalcemia (FHH) or neonatal severe primary hyperparathyroidism (NSPHT), depending on the severity of the functional impairment.

Except for a very small percentage of cases with no apparent CASR mutations, FHH is due to heterozygous inactivating CASR mutations. Serum calcium levels are mildly-to-moderately elevated. PTH is within the reference range or modestly elevated, phosphate is normal or slightly low, and urinary calcium excretion is low for the degree of hypercalcemia. Unlike patients with primary hyperparathyroidism (PHT), which can be difficult to distinguish from FHH, the majority of FHH patients do not seem to suffer any adverse long-term effects from hypercalcemia and elevated PTH levels. They should therefore generally not undergo parathyroidectomy.

NSPHT is usually due to homozygous or compound heterozygous inactivating CASR mutations, but can occasionally be caused by dominant-negative heterozygous mutations. The condition presents at birth, or shortly thereafter, with severe hypercalcemia requiring urgent parathyroidectomy.

Activating mutations lead to oversensing of Ca(++), resulting in suppression of PTH secretion and consequently hypoparathyroidism. All activating mutations described are functionally dominant and disease inheritance is therefore autosomal dominant. However, sporadic cases also occur. Autosomal dominant hypoparathyroidism may account for many cases of idiopathic hypoparathyroidism. Disease severity depends on the degree of gain of function, spanning the spectrum from mild hypoparathyroidism, which is diagnosed incidentally, to severe and early onset disease. In addition, while the majority of patients suffer only from hypoparathyroidism, a small subgroup with extreme gain of function mutations suffer from concomitant inhibition of renal sodium-chloride transport. These individuals may present with additional symptoms of hypokalemic metabolic alkalosis, hyperreninemia, hyperaldosteronism, and hypomagnesemia, consistent with type V Bartter’s syndrome.

Confirming or ruling out a suspected diagnosis of familial hypocalciuric hypercalcemia

As part of the workup of some patients with parathyroid hormone

Confirming or ruling out a suspected diagnosis of neonatal severe primary hyperparathyroidism

Confirming or ruling out a suspected diagnosis of autosomal dominant hypoparathyroidism

As part of the workup of idiopathic hypoparathyroidism

As part of the workup of patients with Bartter's syndrome

Patients with apparent mild-to-moderate hyperparathyroidism, who have a ratio of calcium clearance to creatinine clearance that is <0.01, are likely to suffer from familial hypocalciuric hypercalcemia (FHH). Identification of a heterozygous inactivating CASR mutation confirms this diagnosis, while identification of novel alteration(s) increases the likelihood of a FHH diagnosis, but does not confirm it until family studies or functional studies support its pathogenicity. Absence of any mutations or the presence of polymorphism(s) that are known to be functionally neutral makes the diagnosis very unlikely (see Cautions for exceptions).

Approximately 20% of FHH patients are reported to have calcium to creatinine clearance ratios of >0.01. These patients can be difficult or impossible to distinguish from individuals with primary hyperparathyroidism (PTH). A diagnosis of FHH should therefore be considered in all PHT patients who do not have markedly elevated serum calcium, low phosphate, high urinary calcium excretion, and demonstrable parathyroid tumor, indicative of typical PHT. CASR mutation screening, as described above, can assist in reaching a definite diagnosis.

Severe hypercalcemia and hyperparathyroidism in neonates and small infants is highly suspicious of neonatal severe primary hyperparathyroidism (NSPHT), but can be caused by other diseases. While treatment must not be delayed until a final diagnosis is reached, CASR mutation screening allows confirmation or exclusion of NSPHT. Identification of homozygous or compound heterozygous inactivating CASR mutations or a known dominant negative CASR mutation confirms the diagnosis, while identification of novel alterations(s) increases the likelihood of a NSPHT diagnosis, but does not confirm it until family studies or functional studies support its pathogenicity. Absence of any mutations or the presence of polymorphism(s) that are known to be functionally neutral makes the diagnosis very unlikely (see Cautions for exceptions).

Children with isolated hypoparathyroidism and no other syndrome-related abnormalities are likely to suffer from autosomal dominant hypoparathyroidism (ADH). Identification of a heterozygous activating CASR mutation confirms this diagnosis, while identification of novel alteration(s) increases the likelihood of an ADH diagnosis, but does not confirm it until family studies or functional studies support its pathogenicity. Absence of any mutations or the presence of polymorphism(s) that are known to be functionally neutral makes the diagnosis unlikely (see Cautions for exceptions).

Since ADH cases may have mild disease that does not present in childhood, all cases of apparent idiopathic hypoparathyroidism should be considered for CASR mutation screening. The correct diagnosis is important, as ADH patients are more susceptible to the side effects of 1,25-dihydroxy vitamin D therapy than other patients with hypoparathyroidism and because of genetic counseling considerations. CASR mutation screening, as described above, can assist in reaching a definite diagnosis.

Patients with Bartter's syndrome who have hypocalcemia and inappropriately low serum PTH levels that do not change after repletion of magnesium stores, might be suffering from Bartter's syndrome type V, which is caused by extreme gain of function mutations of the CASR. Identification of the known Bartter's type V associated L125P mutation confirms the diagnosis, while identification of other ADH-causing mutations or novel alteration(s) increase the likelihood of CASR mutation-related Bartter's syndrome, but do not confirm it without additional family or functional studies. Absence of any mutations or the presence of polymorphism(s) that are known to be functionally neutral makes the diagnosis very unlikely (see Cautions for exceptions).

See Parathyroid Disease and the Calcium-Sensing Receptor Gene (November 2005 Communique') in publications for additional information.

A small percentage of individuals who are carriers or have a diagnosis of familial hypocalciuric hypercalcemia (FHH) or neonatal severe primary hyperparathyroidism (NSPHT) may have a mutation that is not identified by this method (eg, large genomic deletions, promoter mutations). The absence of a mutation(s), therefore, does not eliminate the possibility of positive carrier status or the diagnosis of FHH or NSPHT. For carrier testing, it is important to first document the presence of a CASR gene mutation in an affected family member.

In some cases, DNA alterations of undetermined significance may be identified.

Rare polymorphisms exist that could lead to false-negative or false-positive results. If results obtained do not match the clinical findings, additional testing should be considered.

A previous bone marrow transplant from an allogenic donor will interfere with testing. Call Mayo Medical Laboratories for instructions for testing patients who have received a bone marrow transplant.   

Test results should be interpreted in the context of clinical findings, family history, and other laboratory data. Errors in our interpretation of results may occur if information given is inaccurate or incomplete.

Very rarely, patients with typical biochemical findings of FHH, with or without a supporting family history, will have no CASR mutations. In 2 such families, linkage to chromosome 19 has been established, suggesting that a small percentage of FHH cases are caused by mutations in other genes, possibly related to CASR downstream signaling.

Up to 20% of patients with clinically typical ADH may also lack demonstrable CASR mutations.

An interpretive report will be provided.

1. Hendy GN, D'Souza-Li L, Yang B, et al: Mutations of the calcium-sensing receptor (CASR) in familial hypocalciuric hypercalcemia, neonatal severe hypocalciuric hyperparathyroidism, and autosomal dominant hypocalcemia. Hum Mutat 2000 Oct;16(4):281-296. The authors maintain a CASR polymorphism/mutation database: http://www.casrdb.mcgill.ca/

2. Lienhardt A, Bai M, Lgarde JP, et al: Activating mutations of the calcium-sensing receptor: management of hypocalcemia. J Clin Endocrinol Metab 2001 Nov;86(1):5313-5323 

3. Hu J, Spiegel AM: Naturally occurring mutations of the extracellular Ca2+ -sensing receptor: implications for its structure and function. Trends Endocrinol Metab 2003 Aug;14(6):282-288

4. Naesens M, Steels P, Verberckmoes R, et al: Bartter's and Gitelman's syndromes: from gene to clinic. Nephron Physiol 2004;96(3):65-78

5. Egbuna OI, Brown EM: Hypercalcaemic and hypocalcaemic conditions due to calcium-sensing receptor mutations. Best Pract Res Clin Rheumatol 2008; 22:129-148