In Vivo and In Vitro Characterization of Neonatal Hyperparathyroidism Resulting from a De Novo, Heterozygous Mutation in the Ca

We characterized the in vivo, cellular and molecular pathophysiology of a case of neonatal hyperparathyroidism (NHPT) resulting from a de novo, heterozygous missense mutation in the gene for the extracellular Ca 2 1 ( )-sensing receptor (CaR). The female neonate presented with moderately severe hypercalcemia, markedly undermineralized bones, and multiple metaphyseal fractures. Subtotal parathyroidectomy was performed at 6 wk; hypercalcemia recurred rapidly but the bone disease improved gradually with reversion to an asymptomatic state resembling familial benign hypocalciuric hypercalcemia (FBHH). Dispersed parathyroid cells from the resected tissue showed a set-point (the level of half maximally inhibiting PTH secretion) substantially higher than for normal human parathyroid cells ( z 1.8 vs. z 1.0 mM, respectively); a similar increase in set-point was observed in vivo. The proband’s CaR gene showed a missense mutation (R185Q) at codon 185, while her normocalcemic parents were homozygous for wild type (WT) CaR sequence. Transient expression of the mutant R185Q CaR in human embryonic kidney (HEK293) cells revealed a substantially attenuated -evoked accumulation of total inositol phosphates (IP), while cotransfection of normal and mutant receptors showed an EC 50 (the level of eliciting a half-maximal increase in IPs) 37% higher than for WT CaR alone (6.3 6 0.4 vs. 4.6 6 0.3 mM , respectively). Thus this de novo, heterozygous CaR mutation may exert a dominant negative action on the normal CaR, producing NHPT and more severe hypercalcemia than typically seen with FBHH. Moreover, normal maternal calcium homeostasis promoted additional secondary hyperparathyCa o