Germline-Derived Gain-of-Function Variants of Gsα-Coding GNAS Gene Identified in Nephrogenic Syndrome of Inappropriate Antidiuresis
Authors: Miyado Mami , Fukami Maki , Takada Shuji , Terao Miho , Nakabayashi Kazuhiko , Hata Kenichiro , Matsubara Yoichi , Tanaka Yoko , Sasaki Goro , Nagasaki Keisuke , Shiina Masaaki , Ogata Kazuhiro , Masunaga Youhei , Saitsu Hirotomo , Ogata Tsutomu 
Published: April 2019

Significance Statement
The stimulatory G-protein α-subunit GNAS-Gsα mediates signal transduction of multiple G protein–coupled receptors, including arginine vasopressin receptor 2 (AVPR2). Specific gain-of-function variants in AVPR2 are known causes of nephrogenic syndrome of inappropriate antidiuresis (NSIAD), an arginine vasopressin–independent antidiuresis. In two families with NSIAD, after excluding AVPR2 gain-of-function variants, the authors identified two novel germline-derived variants of GNAS-Gsα. They also showed that both of the GNAS-Gsα variants had gain-of-function effects that were milder than those of specific somatic GNAS-Gsα variants reported in McCune–Albright syndrome, a condition that may result in renal phosphate wasting. The results refute the widely believed concept that a germline-derived GNAS-Gsα gain-of-function variant is absent because of embryonic lethality and reveal the genetic heterogeneity in NSIAD.

Background
The stimulatory G-protein α-subunit encoded by GNAS exons 1–13 (GNAS-Gsα) mediates signal transduction of multiple G protein–coupled receptors, including arginine vasopressin receptor 2 (AVPR2). Various germline-derived loss-of-function GNAS-Gsα variants of maternal and paternal origin have been found in pseudohypoparathyroidism type Ia and pseudopseudohypoparathyroidism, respectively. Specific somatic gain-of-function GNAS-Gsα variants have been detected in McCune–Albright syndrome and may result in phosphate wasting. However, no germline-derived gain-of-function variant has been identified, implying that such a variant causes embryonic lethality.

Methods
We performed whole-exome sequencing in two families with dominantly inherited nephrogenic syndrome of inappropriate antidiuresis (NSIAD) as a salient phenotype after excluding a gain-of-function variant of AVPR2 and functional studies for identified variants.

Results
Whole-exome sequencing revealed two GNAS-Gsα candidate variants for NSIAD: GNAS-Gsα p.(F68_G70del) in one family and GNAS-Gsα p.(M255V) in one family. Both variants were absent from public and in-house databases. Of genes with rare variants, GNAS-Gsα alone was involved in AVPR2 signaling and shared by the families. Protein structural analyses revealed a gain-of-function–compatible conformational property for p.M255V-Gsα, although such assessment was not possible for p.F68_G70del-Gsα. Both variants had gain-of-function effects that were significantly milder than those of McCune–Albright syndrome–specific somatic Gsα variants. Model mice for p.F68_G70del-Gsα showed normal survivability and NSIAD-compatible phenotype, whereas those for p.M255V-Gsα exhibited severe failure to thrive.

Conclusions
This study shows that germline-derived gain-of-function rare variants of GNAS-Gsα exist and cause NSIAD as a novel Gsα-mediated genetic disease. It is likely that AVPR2 signaling is most sensitive to GNAS-Gsα’s gain-of-function effects.