OR30-5 Germline Ablation of G-Protein Subunit Alpha-11 in Mice Causes Hypercalcemia That Is Rectified by Treatment with Cinacalcet

Abstract

G-protein subunit α-11 (Gα11) is a signalling partner of the calcium-sensing receptor (CaSR), which is a G-protein coupled receptor that plays a major role in calcium homeostasis by regulating parathyroid hormone (PTH) secretion, urinary calcium excretion and skeletal development. Although loss-of-function point mutations of the GNA11 gene, which encodes Gα11, have been shown to increase plasma calcium concentrations and cause familial hypocalciuric hypercalcemia type 2 (FHH2), the overall contribution of Gα11 to the parathyroid, bone and renal regulation of calcium homeostasis remains to be established. The aim of this study was to characterise the impact of selective germline ablation of Gα11 on plasma concentrations of albumin-adjusted calcium (adj-calcium) and PTH, 24-hour urinary calcium excretion, and bone mineral density (BMD) in adult male and female heterozygous and homozygous Gna11 null C57BL/6N mice (n=13-16 mice per genotype), which had been previously generated by the International Mouse Phenotyping Consortium (IMPC). These mice were also treated with cinacalcet to assess whether this CaSR positive allosteric modulator can rectify any alterations in calcium homeostasis. All studies were conducted in accordance with institutional welfare guidelines. Male and female heterozygous Gna11 null (Gna11+/-) mice were viable and significantly hypercalcemic (plasma adj-calcium = 2.47±0.02 mM) compared to wild-type (WT) mice (plasma adj-calcium = 2.33±0.01 mM, p<0.0001). Gna11+/- mice showed no alterations in plasma PTH or 24-hour urinary calcium. In contrast, homozygous Gna11 null (Gna11-/-) mice showed reduced viability and the proportion of viable Gna11-/- mice was >25% less than would be expected from a Mendelian pattern of inheritance (p=0.01, Chi-squared analysis). Moreover, male and female Gna11-/- mice had significantly reduced body weight and were significantly more hypercalcemic (plasma adj-calcium = 2.70±0.02 mM) than Gna11+/- or WT mice (p<0.0001). Female Gna11-/- mice also showed significant increases in plasma PTH and 24-hour urinary calcium compared to female Gna11+/- and WT mice. However, no alterations in BMD were observed in male and female Gna11+/- or Gna11-/- mice. Administration of a single oral 30mg/kg cinacalcet dose significantly reduced plasma adj-calcium and PTH in Gna11+/- and Gna11-/- mice, and normalised plasma adj-calcium concentrations in Gna11-/- mice at 1-hour post-dose. Thus, these findings demonstrate that Gα11 plays a major role in regulating plasma calcium concentrations, but does not influence BMD, nor represent a key mediator of urinary calcium excretion. Furthermore, cinacalcet rectifies the hypercalcemia of Gna11 null mice, thus indicating that this CaSR-targeted compound may modulate parathyroid gland function in a Gα11-independent manner.