Impaired urinary water excretion in a three-generation family

Germline-Derived Gain-of-Function Variants of Gsα-Coding GNAS Gene Identified in Nephrogenic Syndrome of Inappropriate Antidiuresis

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.

Long-term outcomes in a family with nephrogenic syndrome of inappropriate antidiuresis

We report a familial case of the nephrogenic syndrome of inappropriate antidiuresis (NSIAD), including 30-year followup data on two patients. The proband and one maternal uncle presented in their infancy with severe recurrent hyponatremia, and clinical pictures consistent with the syndrome of inappropriate antidiuretic hormone (SIADH) in the absence of an elevated ADH level. They were both confirmed to be hemizygous for the R137C mutation on the V2R gene (AVPR2), the same locus of the gain of function mutation demonstrated in the original reports of this condition. The proband's mother was identified as an asymptomatic carrier of this X-linked condition. Our case describes a favourable long-term outcome for NSIAD, in particular, successful treatment with oral urea during the infancy period, and with self-regulated precautions on fluid intake into adult life.

Hyponatremia resulting from arginine vasopressin receptor 2 gene mutation

Chronic hyponatremia, unless associated with extracellular fluid volume expansion, is an infrequent electrolyte imbalance in pediatrics. We report an infant with chronic hyponatremia suggestive of a syndrome of inappropriate secretion of antidiuretic hormone (SIADH), in the absence of ADH secretion. A mutation was found in the same codon of the gene that results in a loss-of- function of arginine vasopressin receptor 2 (AVPR2) observed in congenital nephrogenic diabetes insipidus. In this case, a gain-of- function of AVPR 2 was found to be responsible for a SIADH-like state.

Nephrogenic syndrome of inappropriate antidiuresis in adults: high phenotypic variability in men and women from a large pedigree

Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a recently described genetic cause of hyponatremia in male infants. Whether this X-linked condition could be detected in the adult or also could affect women is unknown. A large five-generation family was identified in which the recently described arginine-vasopressin receptor type 2 (AVPR2) mutation that is responsible for NSIAD was segregated. The proband was a 74-yr-old patient who had a syndrome of inappropriate antidiuresis and whose hyponatremia resisted administration of two AVPR2 antagonists. The phenotype of family members who carry the mutation was investigated. Patients with normal serum sodium were subjected to a water-load test. The previously reported activating missense R137C mutation in the AVPR2 gene in three hemizygous male and four heterozygous female individuals was identified. Except in one woman, spontaneous episodes of hyponatremia or abnormal water-load test were identified in all patients with the mutation, whether male or female. Skewed X inactivation was evidenced in the blood of the asymptomatic woman, which is compatible with preferential inactivation of her mutated allele. NSIAD is not limited to male infants. The diagnosis also should be considered in both male and female adults.

Nephrogenic syndrome of inappropriate antidiuresis: a novel disorder in water balance in pediatric patients

The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a common cause of hyponatremia. We report findings in 2 unrelated male infants whose clinical presentation and laboratory findings were consistent with SIADH, but who exhibited unmeasurable arginine vasopressin (AVP) levels on repeated occasions. We hypothesized that these infants had a novel gain of function defect in the AVP-signaling pathway. DNA sequencing of each patient's vasopressin V2 receptor (V2R) gene identified mutations (R137C or R137L) in each. R137H mutations have been reported previously to cause nephrogenic diabetes insipidus. To further characterize the effects of these mutations, we re-created each mutation by site-directed mutagenesis in a vasopressin V2R expression vector and cotransfected COS-7 cells with wild-type and mutant vasopressin V2R vectors and a cyclic adenosine monophosphate-responsive luciferase reporter plasmid. The luciferase activity was induced 7.5-fold (R137L mutant; P = 0.0037) and 4-fold (R137C mutant; P = 0.013) more than the wild-type vasopressin V2R, which is the empty vector or the inactivating R137H mutant. This novel gain of function mutation in the vasopressin V2R can cause constitutive activation of the receptor and resultant hyponatremia. These findings represent a previously unrecognized genetic disease, which was designated as nephrogenic syndrome of inappropriate antidiuresis. A number of questions have emerged, including the following: (1) What is the frequency? (2) Are there nonrenal manifestations? (3) Are heterozygotes affected? (4) What is the optimal therapy? and (5) How do these mutations cause constitutive activation of the receptor?

Oral urea for the treatment of chronic syndrome of inappropriate antidiuresis in children

We report the successful use of oral urea in the management of children with chronic syndrome of inappropriate antidiuretic hormone secretion (SIAD). We performed a retrospective review of four children with chronic SIAD. After initial attempts at management with fluid restriction, each was started on a 30% to 50% oral urea solution, and the dose was titrated until normal serum sodium was achieved. Fluid intake was liberalized after serum sodium normalization. All four children normalized their serum sodium. No side effects or toxicities were experienced. Oral urea is a safe, effective treatment for chronic SIAD in children.