The new disorder, called Nephrogenic Syndrome of Inappropriate Antidiuresis (NSIAD), is described in the May 5 issue of The New England Journal of Medicine.
"This discovery gives better insights into treating these patients and potentially many others," said Stephen Gitelman, MD, principal author of the study and professor of clinical pediatrics at the University of California, San Francisco. "It sheds new light on the mechanisms that the body uses to maintain fluid homeostasis -- the correct balance of fluids needed for health and life."
Gitelman and a team of fellow pediatrician scientists at UCSF took their findings about the two patients to the laboratory, working with colleagues to isolate the genetic mutations responsible for their disruption in water balance.
They found that each infant has a different mutation in a specific gene, AVPR2, that encodes the V2 receptor (V2R) for vasopressin, a hormone that instructs the kidneys to retain water. Neither patient was producing measurable levels of vasopressin, yet the V2 receptor on cells in the collecting duct of the kidneys remained activated as if it was binding to the hormone.
Both mutations activate the receptor by changing a single amino acid, arginine, located on the gene at position 137. This location, R137, already is known to scientists who study fluid homeostasis. A third, different mutation had been previously shown to block the V2 receptor, causing a condition opposite to NSIAD called Nephrogenic Diabetes Insipidus (NDI). In that condition, instead of retaining fluids, the kidneys excrete water excessively, leading to severe dehydration.