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

CONTEXT: The human calcium-sensing receptor (CaSR) plays a key role in calcium homeostasis, and most identified CASR variants are associated with hypercalcemic and hypocalcemic disorders.

OBJECTIVE

Here we characterized the pharmacological implications of 5 heterozygous CASR variants from individuals with familial hypocalciuric hypercalcemia 1 (FHH1: Y63C, I81T, Q459R, W818stop) or autosomal dominant hypocalcemia 1 (ADH1: R955stop).

METHODS

Total and cell surface expression levels of wild-type (WT) and variant CaSRs expressed in human embryonic kidney 293T (HEK293T) cells were determined using enzyme-linked immunosorbent assay, and the pharmacological properties of the receptors were delineated in 2 functional assays.

RESULTS

The Y63C and I81T variations in the extracellular domain (ECD) of CaSR yielded markedly reduced cell surface expression and Ca2+ responsiveness, while Q459R displayed WT-like expression and functional properties. Truncation of the 7-transmembrane domain (7TMD) in W818stop eliminated cell surface expression, whereas R955stop in the intracellular carboxy-terminal yielded modestly increased surface expression and Ca2+ potency compared with WT CaSR. Interestingly, the effectiveness of positive allosteric modulators (PAMs) at the variants varied. Ca2+-mediated signaling through Y63C and I81T was significantly augmented by 7TMD-binding PAMs (NPS R-568 and evocalcet) but not by ECD-binding PAMs (etelcalcetide and Nb4), whereas signaling through Q459R and R955stop were robustly potentiated by all four PAMs.

CONCLUSION

While the molecular phenotypes exhibited by the 5 CaSR variants concord with the clinical phenotypes in individuals harboring them, CASR variant-induced calcium homeostasis disorders clearly arise from diverse molecular origins, and the effectiveness of calcimimetics in these disorders could differ depending on the specific variants.