A mouse model of human familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism

Abstract

Mice lacking the calcium-sensing receptor (Casr) were created to examine the receptor's role in calcium homeostasis and to elucidate the mechanism by which inherited human Casr gene defects cause diseases. Casr+/- mice, analogous to humans with familial hypocalciuric hypercalcemia, had benign and modest elevations of serum calcium, magnesium and parathyroid hormone levels as well as hypocalciuria. In contrast, Casr-/- mice, like humans with neonatal severe hyperparathyroidism, had markedly elevated serum calcium and parathyroid hormone levels, parathyroid hyperplasia, bone abnormalities, retarded growth and premature death. Our findings suggest that Casr mutations cause these human disorders by reducing the number of functional receptor molecules on the cell surface.

The calcium-sensing receptor and 25-hydroxyvitamin D–1α-hydroxylase interact to modulate skeletal growth and bone turnover

“…The derivation of the two parental strains of Casr À/À mice and 1a(OH)ase À/À mice by homologous recombination in embryonic stem cells was described previously by Ho and colleagues (4) and Panda and colleagues, (9) respectively. Briefly, a neomycin resistance gene was inserted into exon 5 of the mouse Casr gene.…”

“…Western blot analysis of kidney membrane protein extract from homozygous Casr À/À mice confirmed that no detectable protein is expressed from this allele. (4) A neomycin resistance gene replaced exons VI, VII, and VIII of the mouse 1a(OH)ase gene (Cyp27b1), removing both the ligand-and hemebinding domains. Lack of 1a(OH)ase mRNA expression in kidney and of circulating concentrations of 1,25(OH) 2 D has been demonstrated previously.…”

“…Lack of 1a(OH)ase mRNA expression in kidney and of circulating concentrations of 1,25(OH) 2 D has been demonstrated previously. (9) Mice heterozygous for the null Casr allele were described previously as being fertile, (4) as were mice heterozygous for the null 1a(OH)ase allele. (9) Offspring heterozygous at both loci then were mated with one another in order to generate pups homozygous for both the Casr and 1a(OH)ase null alleles [CasR À/À 1a(OH)ase À/À ].…”

“…(3) Targeted deletion of the Casr gene in mice produced a phenocopy of the human condition resulting from inactivating CASR mutations. (4) 1,25(OH) 2 D is synthesized via the action of the enzyme 25-hydroxyvitamin D-1a-hydroxylase [1(OH)ase or cyp27B1] to convert 25(OH)D to 1,25(OH) 2 D (5) and acts predominantly on a nuclear vitamin D receptor (VDR). (6) Loss-of-function mutations in 1(OH)ase in humans produce pseudodeficiency rickets or vitamin D-deficiency rickets type 1, (7) and loss-of-function mutations in the VDR gene in humans produce vitamin Ddeficiency rickets type 2.…”

“…(12) Mice with chondrocyte-specific deletion of Casr displayed delayed growth plate development. (12) The early lethality in the neonatal period of mice with homozygous Casr deletion (Casr À/À ) (4) precludes assessment of the progression of skeletal abnormalities in this model. This early lethality is, however, corrected by crossing these mice with mice that manifest hypoparathyroidism, suggesting that the early lethality could be corrected by eliminating the hypercalcemia and hyperparathyroidism.…”