Identification of a G-Protein Subunit-α11 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 α-11 (Gα₁₁). To date Gα₁₁ 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 Gα₁₁ 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 α5 helix, which may affect G-protein coupled receptor binding and G-protein activation. In vitro functional expression of wild-type (Val340) and mutant (Met340) Gα₁₁ proteins in HEK293 cells stably expressing the CaSR, demonstrated that the intracellular calcium responses following stimulation with extracellular calcium, of the mutant Met340 Gα₁₁ led to a leftward shift of the concentration-response curve with a significantly (p < 0.0001) reduced mean half-maximal concentration (EC₅₀) value of 2.44 mM (95% CI, 2.31 to 2.77 mM) when compared to the wild-type EC₅₀ of 3.14 mM (95% CI, 3.03 to 3.26 mM), consistent with a gain-of-function mutation. A novel His403Gln variant in transforming growth factor, beta-induced (TGFBI), that may be causing keratoconus was also identified, indicating likely digenic inheritance of keratoconus and ADH2 in this family. In conclusion, our identification of a novel germline gain-of-function Gα₁₁ mutation, Val340Met, causing ADH2 demonstrates the importance of the Gα₁₁ C-terminal region for G-protein function and CaSR signal transduction.