Identification of a G-Protein Subunit-[alpha]11 Gain-of-Function Mutation, Val340Met, in a Family With Autosomal Dominant Hypocalcemia Type 2 (ADH2)

Autosomal dominant hypocalcemia (ADH) is characterized by hypocalcemia, inappropriately low serum parathyroid hormone concentrations and hypercalciuria. ADH is genetically heterogeneous with ADH type 1 (ADH1), the predominant form, being caused by germline gain-of-function mutations of the G-protein coupled calcium-sensing receptor (CaSR), and ADH2 caused by germline gain-of-function mutations of G-protein subunit [alpha]-11 (G[alpha]11). To date G[alpha]11 mutations causing ADH2 have been reported in only five probands. We investigated a multigenerational nonconsanguineous family, from Iran, with ADH and keratoconus which are not known to be associated, for causative mutations by whole-exome sequencing in two individuals with hypoparathyroidism, of whom one also had keratoconus, followed by cosegregation analysis of variants. This identified a novel heterozygous germline Val340Met G[alpha]11 mutation in both individuals, and this was also present in the other two relatives with hypocalcemia that were tested. Three-dimensional modeling revealed the Val340Met mutation to likely alter the conformation of the C-terminal [alpha]5 helix, which may affect G-protein coupled receptor binding and G-protein activation. In vitro functional expression of wild-type (Val340) and mutant (Met340) G[alpha]11 proteins in HEK293 cells stably expressing the CaSR, demonstrated that the intracellular calcium responses following stimulation with extracellular calcium, of the mutant Met340 G[alpha]11 led to a leftward shift of the concentration-response curve with a significantly (p