Autosomal Dominant Hypocalcemia Caused by a Novel Mutation in the Loop 2 Region of the Human Calcium Receptor Extracellular Domain

We report a novel missense mutation N124K in the extracellular calcium receptor (CaR) identified in two related subjects with the phenotypic features of autosomal dominant hypocalcemia (ADH). Expression of the N124K mutant receptor created by site-directed mutagenesis and transfected into HEK-293 cells was comparable with that of the wild-type (WT) receptor and two other mutant receptors N118K and L125P identified in subjects with ADH. Functional characterization by the extracellular Ca²⁺ ion ([Ca²⁺]₀)-stimulated phosphoinositide (PI) hydrolysis in transfected HEK-293 cells showed that the N124K mutant receptor was left-shifted in Ca²⁺ sensitivity. This biochemical gain-of-function is comparable with that seen in other missense mutations of the CaR identified in subjects with ADH. We tested a series of missense substitutions (R, Q, E, and G) in addition to K for N¹²⁴ and found that only the N124K mutation and to a much lesser extent N124R caused a left shift in Ca²⁺ sensitivity. Thus, a specific substitution, not merely a mutation of the N¹²⁴ residue, is required for receptor activation. The N124K mutation is one of eight naturally occurring mutations in subjects with ADH identified in a short segment A¹¹⁶-C¹²⁹ of the CaR extracellular domain (ECD). We present a hypothesis to explain receptor activation by mutations in this region based on the recently described three-dimensional structure of the related metabotropic glutamate type 1 receptor (mGluR1).