Mutations of calcium-sensing receptor gene: two novel mutations and overview of impact on calcium homeostasis

Autosomal dominant hypocalcemia with a novel CASR mutation: a case study and literature review

Autosomal dominant hypocalcemia type 1 (ADH1) is a rare inherited disorder characterized by hypocalcemia with low parathyroid hormone (PTH) levels and high urinary calcium. Its clinical presentation varies from mild asymptomatic to severe hypocalcemia. It is caused by gain-of-function mutations in the calcium-sensing receptor gene (CASR) which affect PTH secretion from the parathyroid gland and calcium resorption in the kidney. Here, we describe a case who presented with symptoms of recurrent seizure caused by hypocalcemia with a novel CASR variant. We comprehensively analyzed the phenotypic features of this presentation and reviewed the current literature to better understand clinical manifestations and the genetic spectrum.

Landscape genomics and selection signatures of local adaptation of Eritrean indigenous cattle along environmental gradients

Most parts of Eritrea are classified as arid and semi-arid land (ASALs) which are characterised by dryness, high ambient temperatures and low precipitation. The country has three different environmental landscapes: highland, Western lowland and Eastern lowland agro-ecological zones (AEZs). The geophysical and biophysical characteristics of the AEZs vary. Indigenous cattle which are mainly kept in extensive production system have to cope with the adverse climatic conditions of these AEZs. They have to adapt to the harsh climate, poor feed quality and endemic diseases and parasites and reproduction challenges in the AEZs. The underlying genetic basis of the indigenous cattle adaption to the specific AEZs in Eritrea is limited. In this study, it is hypothesized that the indigenous cattle have evolved and developed unique adaptive characteristics because of being reared in specific ecological landscapes. This was tested using the landscape genomics and analysis of selection signatures to ascertain the genetic basis of their adaptability to the nutritional and environmental constraints that characterise the AEZ conditions. The study was carried out in three different environmental landscapes: highland, Eastern lowland and Western lowland AEZs. Blood samples were collected from a total of 188 indigenous cattle populations sampled from the three AEZs. DNA was extracted and genotyped using the genotype by sequencing (GBS) method. Analysis revealed the existence of medium to high genetic diversity between and within the indigenous cattle populations. Scanning of selection signatures revealed genomic regions under positive selection. The regions harboured genes (IFNAR2, CASR, AHSG, ATP1B3, AIRE, ROBO2, SCHIP1 and PARS2) of importance for facial morphology, locomotion, mineral homeostasis, immune system and skeletal and muscle system development. Findings pointed out the influence of AEZ landscapes on the genetic differentiation of indigenous cattle appearances probably due to selection driven by diverse climatic and biophysical conditions. The differentiation is orientated towards adaptive characteristics.

Calcium-sensing receptor 20 years later

The calcium-sensing receptor (CaSR) has played an important role as a target in the treatment of a variety of disease states over the past 20 plus years. In this review, we give an overview of the receptor at the cellular level and then provide details as to how this receptor has been targeted to modulate cellular ion transport mechanisms. As a member of the G protein-coupled receptor (GPCR) family, it has a high degree of homology with a variety of other members in this class, which could explain why this receptor has been identified in so many different tissues throughout the body. This diversity of locations sets it apart from other members of the family and may explain how the receptor interacts with so many different organ systems in the body to modulate the physiology and pathophysiology. The receptor is unique in that it has two large exofacial lobes that sit in the extracellular environment and sense changes in a wide variety of environmental cues including salinity, pH, amino acid concentration, and polyamines to name just a few. It is for this reason that there has been a great deal of research associated with normal receptor physiology over the past 20 years. With the ongoing research, in more recent years a focus on the pathophysiology has emerged and the effects of receptor mutations on cellular and organ physiology have been identified. We hope that this review will enhance and update the knowledge about the importance of this receptor and stimulate future potential investigations focused around this receptor in cellular, organ, and systemic physiology and pathophysiology.

Two novel mutations of the calcium-sensing receptor gene affecting the same amino acid position lead to opposite phenotypes and reveal the importance of p.N802 on receptor activity

OBJECTIVE

Gain-of-function mutations of the calcium-sensing receptor (CASR) gene have been identified in patients with sporadic or familial autosomal dominant hypocalcemia (ADH). Inactivating mutations of the CASR gene cause familial hypocalciuric hypercalcemia (FHH). Here, we report two novel CASR mutations affecting the same amino acid (p.N802); one causes ADH and the other atypical FHH.

PATIENTS AND METHODS

The first patient, an 11-year-old girl suffering from hypocalcemia, developed nephrocalcinosis when she was only 5 years old. The second patient is a 30-year-old woman who presented with mild hypercalcemia. PCR amplification of CASR coding exons and direct sequencing of PCR products were used to identify mutations. Site-directed mutagenesis was used to generate mutated CASR cDNAs in an expression plasmid. Using the MAPK assay system and transient transfection of Cos-7 cells with wild-type (WT) and mutated CASR, we studied the responses of these mutated receptors to extracellular Ca(2+) and to the negative allosteric CASR modulator, NPS2143.

RESULTS

Two heterozygous missense mutations (p.N802I and p.N802S) affecting a residue in the sixth transmembrane domain of CASR were identified. In functional tests, the response of the p.N802S mutant to calcium was typical of an inactivating mutation. However, the p.N802I mutant had 70% of the maximally stimulated WT receptor activity even in the absence of extracellular calcium. This constitutive activity was only partially inhibited by the inhibitor, NPS2143.

CONCLUSIONS

The asparagine at amino acid position 802 appears to be essential for the activity of the CASR protein and is implicated in the mechanism of CASR signaling.