Identification of a G-Protein Subunit-α11 Gain-of-Function Mutation, Val340Met, in a Family With Autosomal Dominant Hypocalcemia Type 2 (ADH2)

Abstract

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. © 2016 The Authors. Journal of Bone and Mineral Research Published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research (ASBMR).

Knockin mouse with mutant Gα11 mimics human inherited hypocalcemia and is rescued by pharmacologic inhibitors

“…This gainof-function effect of the Gα 11 mutant on CASR signaling is consistent with the effects observed for other Gα 11 mutations identified in ADH2 and, mechanistically, can explain the hypocalcemic and hyperphosphatemic disturbances seen in ADH2 patients (10)(11)(12)(13)(14), which are fully recapitulated in the GNA11 R60C mutant mice. The results, thus, establish that the mutation is sufficient to cause ADH2 and provide a plausible mechanistic basis of the disease.…”

“…In principle, ADH2 patients would also be expected to exhibit elevated levels of calcium excretion, but data are limited and variable (10)(11)(12)(13)(14). In our estimates of urinary fractional excretion of calcium, we detected no difference between WT and the hypocalcemic heterozygous and homozygous mutant ADH2 mice at baseline while fasting.…”

“…Recently, we and others identified germline gain-of-function mutations in Gα 11 , encoded by the GNA11 gene, in families with autosomal-dominant hypocalcemia type 2 (ADH2) (10)(11)(12)(13)(14). To date, 6 such mutations have been uncovered in ADH2 patients (10)(11)(12)(13)(14).…”

“…To date, 6 such mutations have been uncovered in ADH2 patients (10)(11)(12)(13)(14). These mutations show weak effects in vitro when compared with the highly activating mutations discussed above (12).…”

“…Studies in transfected cells show that the gain-of-function Gα 11 mutations of ADH2 each result in a small but significant increase in the sensitivity with which the CASR responds to extracellular calcium, an effect that likely accounts for the perturbations in PTH secretion and calcium homeostasis seen in patients (11)(12)(13). Short stature has been reported in affected individuals from two families (12,14), suggesting possible effects on growth.…”