Autosomal dominant hypocalcaemia: identification of two novel variants of CASR gene

Evolutionary history of calcium-sensing receptors unveils hyper/hypocalcemia-causing mutations

Despite advancements in understanding the structure and functions of the Calcium Sensing Receptor (CaSR), gaps persist in our knowledge of the specific functions of its residues. In this study, we used phylogeny-based techniques to identify functionally equivalent orthologs of CaSR, predict residue significance, and compute specificity-determining position (SDP) scores to understand its evolutionary basis. The analysis revealed exceptional conservation of the CaSR subfamily, emphasizing the critical role of residues with high SDP scores in receptor activation and pathogenicity. To further enhance the findings, gradient-boosting trees were applied to differentiate between gain- and loss-of-function mutations responsible for hypocalcemia and hypercalcemia. Lastly, we investigated the importance of these mutations in the context of receptor activation dynamics. In summary, through comprehensive exploration of the evolutionary history of the CaSR subfamily, coupled with innovative phylogenetic methodologies, we identified activating and inactivating residues, providing valuable insights into the regulation of calcium homeostasis and its connections to associated disorders.

Functional evaluation of a novel nonsense variant of the calcium-sensing receptor gene leading to hypocalcemia

OBJECTIVE

The calcium-sensing receptor (CASR) gene encodes a G protein-coupled receptor crucial for calcium homeostasis. Gain-of-function CASR variants result in hypocalcemia, while loss-of-function variants lead to hypercalcemia. This study aims to assess the functional consequences of the novel nonsense CASR variant [c.2897_2898insCTGA, p.(Gln967*) (Q967*)] identified in adolescent patient with chronic hypocalcemia, a phenotype expected for a gain-of-function variants.

DESIGN AND METHODS

To functionally characterize the Q967* mutant receptor, both wild-type (WT) and mutant CASR were transiently transfected into HEK293T cells and calcium-sensing receptor (CaSR) protein expression and functions were comparatively evaluated using multiple read-outs.

RESULTS

Western blot analysis revealed that the CaSR mutant protein displayed a lower molecular weight compared with the WT, consistent with the loss of the last 122 amino acids in the intracellular domain. Mitogen-activated protein kinase activation and serum responsive element luciferase assays demonstrated that the mutant receptor had higher baseline activity than the WT. Extracellular-signal-regulated kinase/c-Jun N-terminal kinase phosphorylation, however, remained consistently high in the mutant, without significant modulations following exposure to increasing extracellular calcium (Ca2+o) levels, suggesting that the mutant receptor is more sensitive to Ca2+o compared with the WT.

CONCLUSIONS

This study provides functional validation of the pathogenicity of a novel nonsense CASR variant, resulting in an abnormally hyperfunctioning protein consistent with the patient's phenotype. Functional analyses indicate that mutant receptor is constitutively active and poorly sensitive to increasing concentrations of extracellular calcium, suggesting that the cytoplasmic tail may contain elements regulating signal transduction.

Identification of a Novel Homozygous Missense Mutation in the CLDN16 Gene to Decipher the Ambiguous Clinical Presentation Associated with Autosomal Dominant Hypocalcaemia and Familial Hypomagnesemia with Hypercalciuria and Nephrocalcinosis in an Indian Family

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHNNC) is a rare autosomal recessive renal tubulopathy disorder characterized by excessive urinary loss of calcium and magnesium, polyuria, polydipsia, bilateral nephrocalcinosis, progressive chronic kidney disease, and renal failure. Also, sometimes amelogenesis imperfecta and severe ocular abnormalities are involved. The CLDN-16 and CLDN-19 genes encode the tight junction proteins claudin-16 and claudin-19, respectively, in the thick ascending loop of Henle in the kidney, epithelial cells of the retina, dental enamel, etc. Loss of function of the CLDN-16 and/or CLDN-19 genes leads to FHHNC. We present a case of FHHNC type 1, which was first confused with autosomal dominant hypocalcaemia (ADH) due to the presence of a very low serum parathyroid hormone (PTH) concentration and other similar clinical features before the genetic investigations. After the exome sequencing, FHHNC type 1 was confirmed by uncovering a novel homozygous missense mutation in the CLDN-16 gene (Exon 2, c.374 T > C) which causes, altered protein structure with F55S. Associated clinical, biochemical, and imaging findings also corroborate final diagnosis. Our findings expand the spectrum of the CLDN-16 mutation, which will further help in the genetic diagnosis and management of FHNNC.

Autosomal Dominant Hypocalcemia Type 1 and Neonatal Focal Seizures

Autosomal dominant hypocalcemia type 1 (ADH1) is a rare form of hypoparathyroidism that is characterized by gain-of-function mutations in the CASR gene, which provides instructions for producing the protein called calcium-sensing receptor (CaSR). Hypocalcemia in the neonatal period has a wide differential diagnosis. We present the case of a female newborn with genetic hypoparathyroidism (L125P mutation of CASR gene), hypocalcemia, and neonatal seizures due to the potential correlation between refractory neonatal seizures and ADH1. Neonatal seizures were previously described in patients with ADH1 but not in association with the L125P mutation of the CASR gene. Prompt diagnosis and management by a multidisciplinary and an appropriate therapeutic approach can prevent neurological and renal complications.

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.

Autosomal dominant hypocalcemia type 1 (ADH1) associated with myoclonus and intracerebral calcifications

Autosomal dominant hypocalcemia type 1 (ADH1) is a disorder of extracellular calcium homeostasis caused by germline gain-of-function mutations of the calcium-sensing receptor (CaSR). Over 35% of ADH1 patients have intracerebral calcifications predominantly affecting the basal ganglia. The clinical consequences of such calcifications remain to be fully characterized, although the majority of patients with these calcifications are considered to be asymptomatic. We report a 20-year-old female proband with a severe form of ADH1 associated with recurrent hypocalcemic and hypercalcemic episodes, persistent childhood hyperphosphatemia, and a low calcium/phosphate ratio. From the age of 18 years, she had experienced recurrent myoclonic jerks affecting the upper limbs that were not associated with epileptic seizures, extra-pyramidal features, cognitive impairment, or alterations in serum calcium concentrations. Computerised tomography (CT) scans revealed calcifications of the globus pallidus regions of the basal ganglia bilaterally, and also the frontal lobes at the grey-white matter junction, and posterior horn choroid plexuses. The patient’s myoclonus resolved following treatment with levetiracetam. CASR mutational analysis identified a reported germline gain-of-function heterozygous missense mutation, c.2363T>G; p.(Phe788Cys), which affects an evolutionarily conserved phenylalanine residue located in transmembrane domain helix 5 of the CaSR protein. Analysis of the cryo-electron microscopy CaSR structure predicted the wild-type Phe788 residue to form interactions with neighbouring phenylalanine residues, which likely maintain the CaSR in an inactive state. The p.(Phe788Cys) mutation was predicted to disrupt these interactions, thereby leading to CaSR activation. These findings reveal myoclonus as a novel finding in an ADH1 patient with intracerebral calcifications.