IN-VIVO AND IN-VITRO CHARACTERIZATION OF NEONATAL HYPERPARATHYROIDISMRESULTING FROM A DE-NOVO, HETEROZYGOUS MUTATION IN THE CA2-SENSING RECEPTOR GENE - NORMAL MATERNAL CALCIUM HOMEOSTASIS AS A CAUSE OF SECONDARY HYPERPARATHYROIDISM IN FAMILIAL BENIGN HYPOCALCIURIC HYPERCALCEMIA()

We characterized the in vivo, cellular and molecular pathophysiology o f a case of neonatal hyperparathyroidism (NHPT) resulting from a de no vo, heterozygous missense mutation in the gene for the extracellular C a2+ (Ca-o(2+))-sensing receptor (CaR), The female neonate presented wi th moderately severe hypercalcemia, markedly undermineralized bones, a nd multiple metaphyseal fractures, Subtotal parathyroidectomy was perf ormed at 6 wk; hypercalcemia recurred rapidly but the bone disease imp roved gradually with reversion to an asymptomatic state resembling fam ilial benign hypocalciuric hypercalcemia (FBHH), Dispersed parathyroid cells from the resected tissue showed a set-point (the level of Ca-o( 2+) half maximally inhibiting PTH secretion) substantially higher than for normal human parathyroid cells (similar to 1.8 vs, similar to 1.0 mM, respectively); a similar increase in set-point was observed in vi vo, The proband's CaR gene showed a missense mutation (R185Q) at codon 185, while her normocalcemic parents were homozygous for wild type (W T) CaR sequence. Transient expression of the mutant R185Q CaR in human embryonic kidney (HEK293) cells revealed a substantially attenuated C a-o(2+)-evoked accumulation of total inositol phosphates (IP), while c otransfection of normal and mutant receptors showed an EC(50) (the lev el of Ca-o(2+) eliciting a half-maximal increase in IPs) 37% higher th an for WT CaR alone (6.3 +/- 0.4 vs, 4.6 +/- 0.3 mM Ca-o(2+), respecti vely). Thus this de novo, heterozygous CaR mutation may exert a domina nt negative action on the normal CaR, producing NHPT and more severe h ypercalcemia than typically seen with FBHH. Moreover, normal maternal calcium homeostasis promoted additional secondary hyperparathyroidism in the fetus, contributing to the severity of the NHPT in this case wi th FBHH.