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?