Heterogeneous Origins of Calcium Homeostasis Disorders Arising From 5 Heterozygous Calcium-Sensing Receptor Variants

Area of Science:

• Endocrinology
• Molecular Biology
• Pharmacology

Background:

• The calcium-sensing receptor (CaSR) is crucial for calcium homeostasis.
• CASR variants are linked to hypercalcemic and hypocalcemic disorders.
• Familial hypocalciuric hypercalcemia (FHH1) and autosomal dominant hypocalcemia (ADH1) are associated with specific CASR variants.

Purpose of the Study:

• To characterize the pharmacological implications of five heterozygous CASR variants.
• To investigate the impact of variants on receptor expression, cell surface localization, and calcium responsiveness.
• To assess the differential efficacy of positive allosteric modulators (PAMs) on variant CaSR function.

Main Methods:

• Expressed wild-type (WT) and variant CaSRs in HEK293T cells.
• Quantified total and cell surface expression using ELISA.
• Evaluated pharmacological properties and Ca2+ responsiveness in functional assays.
• Tested the effects of 7TMD- and ECD-binding PAMs on CaSR signaling.

Main Results:

• Y63C and I81T variants showed reduced cell surface expression and Ca2+ responsiveness.
• Q459R variant exhibited WT-like expression and function.
• W818stop variant (7TMD truncation) lacked cell surface expression.
• R955stop variant (C-terminal truncation) had increased surface expression and Ca2+ potency.
• PAM effectiveness varied: Y63C/I81T responded to 7TMD PAMs, while Q459R/R955stop responded to all tested PAMs.

Conclusions:

• CaSR variant molecular phenotypes align with clinical presentations.
• CASR variant-associated calcium homeostasis disorders have diverse molecular origins.
• Calcimimetic efficacy may differ based on specific CASR variants, necessitating personalized treatment approaches.