Germline-derived GNAS-Gsα variants associated with both gain-of-function and loss-of-function phenotypes.

Objective: Heterozygous germline inactivating mutations in GNAS can cause hormonal resistance, while activating mutations, usually somatic, result in constitutive cyclic adenosine monophosphate (cAMP) stimulation. Recent research has described germline activating variants leading to nephrogenic syndrome of inappropriate antidiuresis (NSIAD). The present study aims to characterise 4 families with an unusual combination of symptoms indicative of loss of Gsα function and a tendency to hyponatraemia compatible with NSIAD.

Design: Clinical, genetic, structural, and functional characterization of GNAS variants identified.

Methods: We performed GNAS sequencing followed by in vitro functional studies by dual luciferase assays and protein structural analyses of the identified variants and the previously described GNAS variant c.166A>T, p.(Ile56Phe), and correlated these data with clinical manifestations.

Results: Genetic tests identified 2 heterozygous variants in GNAS: c.592C>T p.(Leu198Phe) in 1 family and c.501C>G p.(Asn167Lys) in other 2. Parental analyses revealed that the variants had been maternally inherited. One of the mothers, with the variant in her paternal allele, presented NSIAD. The baseline luciferase studies in the arginine vasopressin receptor 2 (AVPR2)-AVP system revealed mildly but significantly higher activity for p.(Ile56Phe) and p.(Asn167Lys) than for wildtype (WT), while statistical significance for p.(Leu198Phe) was not reached. Parathyroid hormone (PTH)-stimulated luciferase activity was lower for the 3-variant Gsα proteins than for WT-Gsα. Protein structural analyses suggest that the 3 variants could have distinct effects on the interactions with AVPR2 and PTH 1 receptor.

Conclusions: This study provides further evidence in favour of the existence of germline variants that can cause clinical manifestations of both gain and loss of Gsα function.