Recent studies have revealed de novo or germline-derived GNAS-Gsα variants with constitutive ligand-independent gain-of-function (GOF) effects on specific G-protein-coupled receptor signalings in patients with nephrogenic syndrome of inappropriate antidiuresis (NSIAD), osteolytic bone disorder with metaphyseal dysplasia, and peripheral precocious puberty. We encountered a Japanese girl with NSIAD and osteolytic bone disorder with metaphyseal dysplasia. Whole genome sequencing identified a de novo ″likely pathogenic″ heterozygous GNAS-Gsα missense variant (NM_000516.7:c.163 A > G:p.(Thr55Ala)) which occurred on the paternally inherited allele. Luciferase assays for p.Thr55Ala showed ligand-independent GOF effects on AVPR2 and PTH1R signalings, and a ligand-dependent loss-of-function (LOF) effect on PTH1R signaling. Protein structural analysis for p.Thr55Ala indicated disruption of the hydrogen bond between p.Thr55 side chain and the α-phosphate group of the bound nucleotide in both GDP-bound inactive form and GTP-bound active form and resultantly reduced affinity of the variant-positive Gsα protein for both GDP and GTP, consistent with the ligand-independent GOF and ligand-dependent LOF effects. The results, in conjunction with the previous findings, indicate that GNAS-Gsα variants with constitutive GOF effects cause clinically distinctive congenital rare disorders including NSIAD and characteristic bone disorder.

Hyponatremia can occur in endocrinological diseases, neoplasms, kidney diseases, and acquired or genetically conditioned disorders of antidiuretic hormone levels. Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a rare X-linked disease caused by a point mutation of the type 2 vasopressin receptor (AVPR2) gene. This mutation results in constitutive activation of the AVPR2 and a low sodium level.We reported the first familial NSIAD in Poland in siblings with hyponatremia. Case 1. A 2.5-year-old boy, during a respiratory tract infection, showed the following laboratory test results: Na 125 mmol/L, serumosmolality 260 mOsm/kg H2O, low uric acid level, and increased fractional sodium and uric acid excretions. Thyroid, adrenal, and renal function were normal. Copeptin level was low. Case 2. A 7-month-old brother presented with reduced activity and muscle tone, a sodium level of 117 mmol/L, and a serum osmolality of 249 mOsm/kg H2O. They were both confirmed to be hemizygous for the R137C mutation on the AVPR2 gene. The boys were advised to restrict their oral fluid intake and supplement sodium orally, aiming for sodium levels of 133-140 mmol/L. Conclusions: Genetic testing for an AVPR2 mutation is crucial in patients with hyponatremia, normovolemia, hypoosmolality, and low copeptin level.

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

Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a rare X-linked disease caused by gain-of-function mutations of arginine vasopressin receptor 2 (V2R). Patients with NSIAD are characterized by the inability to excrete a free water load and by inappropriately increased urinary osmolality despite very low levels of plasma vasopressin, resulting in euvolaemic hyponatraemia. To dissect the signalling downstream V2R constitutively active variants, Flp-In T-REx Madin-Darby canine kidney (FTM) cells, stably transfected with V2R mutants (R137L, R137C and F229V) and AQP2-wt or non-phosphorylatable AQP2-S269A/AQP2-S256A, were used as cellular models. All three activating V2R mutations presented constitutive plasma membrane expression of AQP2-wt and significantly higher basal water permeability. In addition, V2R-R137L/C showed significantly higher activity of Rho-associated kinase (ROCK), a serine/threonine kinase previously suggested to be involved in S269-AQP2 phosphorylation downstream of these V2R mutants. Interestingly, FTM cells expressing V2R-R137L/C mutants and AQP2-S269A showed a significant reduction in AQP2 membrane abundance and a significant reduction in ROCK activity, indicating the crucial importance of S269-AQP2 phosphorylation in the gain-of-function phenotype. Conversely, V2R-R137L/C mutants retained the gain-of-function phenotype when AQP2-S256A was co-expressed. In contrast, cells expressing the F229V mutant and the non-phosphorylatable AQP2-S256A had a significant reduction in AQP2 membrane abundance along with a significant reduction in basal osmotic water permeability, indicating a crucial role of Ser256 for this mutant. These data indicate that the constitutive AQP2 trafficking associated with the gain-of-function V2R-R137L/C mutants causing NSIAD is protein kinase A independent and requires an intact Ser269 in AQP2 under the control of ROCK phosphorylation. KEY POINTS: Nephrogenic syndrome of inappropriate antidiuresis is caused by two constitutively active variant phenotypes of AVPR2, one sensitive to vaptans (V2R-F229V) and the other vaptan resistant (V2R-R137C/L). In renal cells, all three activating arginine vasopressin receptor 2 (V2R) variants display constitutive AQP2 plasma membrane expression and high basal water permeability. In cells expressing V2R-R137L/C mutants, disruption of the AQP2-S269 phosphorylation site caused the loss of the gain-of-function phenotype, which, in contrast, was retained in V2R-F229V-expressing cells. Cells expressing the V2R-F229V mutant were instead sensitive to disruption of the AQP2-S256 phosphorylation site. The serine/threonine kinase Rho-associated kinase (ROCK) was found to be involved in AQP2-S269 phosphorylation downstream of the V2R-R137L/C mutants. These findings might have clinical relevance for patients with nephrogenic syndrome of inappropriate antidiuresis.

Cyclophosphamide (CY) is an alkylating agent often used as a chemotherapeutic agent, with increasing use as an immunosuppressant. Cyclophosphamide has many established adverse effects, including hyponatremia and limited reports of hepatotoxicity, particularly in high-dose treatment. A case of simultaneous hyponatremia and acute liver injury associated with the initiation of cyclophosphamide two weeks prior is discussed here. A 73-year-old male with acquired hemophilia A/factor VIII deficiency presented to the emergency department (ED) with four days of hip pain and was found to have jaundice and confusion. Laboratory evaluation demonstrated hyponatremia and an acute liver injury associated with his recent cyclophosphamide use. With the discontinuation of the offending agent and sodium correction, he made a full recovery. Cyclophosphamide-induced hyponatremia is likely secondary to the nephrogenic syndrome of inappropriate antidiuresis (NSIAD) and is most often associated with high-dose regimens. While the mechanism of hepatotoxicity requires further study, it is likely dose-dependent and related to excess levels of 4-hydroxycyclophosphamide (HCY). The management of cyclophosphamide-induced water toxicity and hepatotoxicity is centered around the discontinuation of medication, the correction of electrolyte abnormalities, and supportive treatment.