Endocrine Abstracts

The calcium-sensing receptor (CaSR) is a guanine-nucleotide-binding protein (G-protein)-coupled receptor that has a central role in calcium homeostasis. Loss-of-function mutations of the CaSR result in familial hypocalciuric hypercalcemia type 1 (FHH1) and loss-of-function coding mutations in the CaSR-associated G-protein subunit Gα₁₁ have been reported to cause FHH2 in only two patients to date. The aim of our study was therefore to characterise additional GNA11 mutations associated with FHH2. We undertook DNA sequence analysis of the 1077-bp coding region, 12 exon–intron boundaries and 5′-UTR of GNA11 in 40 unrelated hypercalcaemic patients who did not have CaSR mutations. This identified in one patient, a 40-bp deletion in the 5′-UTR, encompassing positions −43 to −4 with respect to the ATG, of GNA11. To investigate the effect of this mutation on the translational efficiency of GNA11, luciferase (luc) reporter expression vectors were constructed containing luc under the control of either the WT GNA11 promoter and 5′-UTR or the WT GNA11 promoter and mutant 5′-UTR. Transient transfection of HEK293 cells stably expressing the CaSR with these constructs demonstrated that the mutant 5′-UTR sequence resulted in a >80% reduction in luciferase activity compared to the WT 5′ UTR sequence (WT, luciferase activity fold change compared to untransfected cells=70.81±3.77; 5′ UTR deletion, luciferase activity fold change compared to untransfected cells=12.89±0.72, P<0.0001). Thus, our results demonstrate that the FHH2-associated 40-bp deletion in the 5′-UTR of GNA11 significantly decreases the translational efficiency of Gα₁₁ and that FHH2 may be due to haploinsufficiency rather than a dominant-negative effect. These studies, which have identified the first non-coding FHH2-causing mutation in GNA11, provide new insights into the role of Gα₁₁ in CaSR signalling and the mechanisms causing FHH2.