A Gain-of-Function CASR Mutation Causing Hypocalcemia in a Recessive Manner

Abstract

A new gain-of-function CASR mutation has been described as a cause of hypocalcemia. This mutation, unlike other such mutations, is inherited recessively and has implications in genetic evaluation.

The calcium-sensing receptor (CaSR) signaling is critical for the regulation of blood calcium levels (1). Activation of CaSR suppresses PTH secretion from the parathyroid chief cells and inhibits Ca²⁺ reabsorption in the renal distal tubule, thus leading to a reduction in serum calcium level. The CaSR is a G protein-coupled receptor that functions as a homodimer connected by intermolecular disulfide bonds. Binding of extracellular Ca²⁺ to CaSR results in a conformational change that allows coupling to one of several heterotrimeric G proteins including, but not limited to, G_q/11.

Germline mutations that lead to enhanced CaSR signaling are responsible for autosomal dominant hypocalcemia (ADH), characterized by mild-to-moderate hypocalcemia in the presence of inappropriately low or normal PTH, elevated serum phosphate, and increased urinary calcium excretion relative to the serum calcium level (2, 3). Most patients with ADH (∼70%) carry gain-of-function mutations in the gene encoding CaSR (CASR), located on chromosome 3q. This form of ADH is referred to as type 1. A small percentage of ADH cases carry gain-of-function mutations in the gene encoding the α-subunit of G₁₁ (GNA11), referred to as ADH type 2 (4, 5). More than 70 different gain-of-function mutations of CASR have been described (3). The vast majority of those are missense mutations that render the receptor more sensitive to Ca²⁺. Three domains within CaSR harbor most of these mutations, including the second peptide loop of the extracellular domain, the Venus Flytrap-like domain, and the region encompassing the transmembrane domains 6 and 7.

Cavaco et al. (6) describe a new CASR mutation (Arg544Gln) as a cause of symptomatic hypocalcemia in an adolescent patient. The mutated residue is located in the cysteine-rich domain that bridges Venus Flytrap-like domain to the first transmembrane domain. Previously described mutations in this domain, except for a phenylalanine-to-leucine substitution at residue 589 (7), cause receptor inactivity and are thus found in patients with the opposite phenotype, i.e., hypercalcemia. In contrast, Cavaco et al. (6) have demonstrated, by performing a series of well-designed experiments, that the Arg544Gln mutation promotes signaling by disrupting a hitherto unrecognized intramolecular interaction critical for inactive receptor confirmation. Although this finding is consistent with the hypocalcemia phenotype, it is striking that the proband is homozygous for the Arg544Gln mutation. The parents are heterozygous for the same mutation but are clinically unaffected. It is unknown why this mutation leads to hypocalcemia only in the homozygous state, but the mutant seems to display augmented signaling only in the absence of wild type CaSR in transfected cells (6). It is thus possible that the effect is conditional upon the formation of mutant-mutant homodimers. There is another report of homozygous CASR mutation in ADH type 1 (8). The latter is an in-frame deletion of 181 amino acids resulting in receptor overactivity. However, both homozygotes and heterozygotes with that deletion were reported to be affected, and the disease severity did not appear to depend on zygosity. Thus, the Arg544Gln variant is a unique gain-of-function CASR mutation that is associated with hypocalcemia only in the homozygous state.

These findings are highly important for genetic evaluation of patients with hypocalcemia. A patient with ADH is typically heterozygous for the gain-of-function mutation in CASR or GNA11. Often, one of the parents is also affected and carries the same mutation. Thus, the chance of having an affected offspring in the next pregnancy, assuming full penetrance, is calculated as 50%. If neither parent is a carrier, the mutation in the proband may have occurred de novo, predicting a much lower recurrence risk (<1%). The absence of the mutation in the parents may also reflect germline mosaicism for one of the parents. The likelihood, in that case, of having another affected offspring is higher than 1% but is difficult to estimate. A patient without clinically affected parents may suggest an autosomal recessive disorder causing hypocalcemia, such as familial isolated hypoparathyroidism. The latter results from mutations in either the PTH gene or the gene encoding glial cells missing homolog-2 (GCM2). The mutations are loss-of-function or dominant negative, so that both kinds of hypoparathyroidism can present as a recessive or a dominant condition. If genetic testing identifies a mutation in PTH or GCM2 and confirms the recessive inheritance pattern, the recurrence risk would be 25%. In such a patient, however, one of the dominantly inherited forms of hypocalcemia is also possible, as a de novo mutation may be present in one of the latter genes, as well as in CASR or GNA11. In addition, based on the findings of Cavaco et al. (6), a recessively inherited gain-of-function mutation in CASR could also be responsible for the disease.

Identification of the underlying gene defect is not only important for appropriate genetic counseling but also provides guidance in clinical management of a patient with hypocalcemia. In contrast to patients carrying PTH or GCM2 mutations, patients who carry activating CASR mutations show relative hypercalciuria owing to enhanced CaSR signaling in the renal distal tubule. In the latter patients, treatment with calcium and vitamin D should be commenced with the lowest doses possible and only if there are symptoms. Urinary calcium excretion should be monitored regularly to reduce the risk of nephrocalcinosis, nephrolithiasis, and renal impairment.

Diseases caused by gain-of-function mutations are primarily dominant, resulting from a single copy of the mutant gene. Homozygous activating mutations are rarely found and often cause a severe early-onset form of the disease, such as the homozygous FGFR3 mutations found in thanatophoric dysplasia (MIM# 187600). The Arg544Gln mutation in CASR is different in that regard, causing disease only when present in two copies. It is also quite frequent in the population, with the highest minor allele frequency of 0.6% observed in the Ashkenazi Jewish cohort of the Genome Aggregation Database (rs115230894). No homozygotes are listed, consistent with heterozygotes being clinically unaffected. Similarly, conditional or mild gain-of-function mutations for other disorders may be present in healthy controls at a relatively high frequency.

Glossary

Abbreviations:

ADH

autosomal dominant hypocalcemia

CaSR

calcium-sensing receptor