Mutations in the Vasopressin V2 Receptor Gene in Two Families with Nephrogenic Diabetes Insipidus

Title: Mutations in the Vasopressin V2 Receptor Gene in Two Families with Nephrogenic Diabetes Insipidus
Authors: Holtzman, M.D., Eliezer J.; Kolakowski, PhD, Lee F.; Geifman-Holtzman, Ossie; O'Brien, David; Rasoulpour, MD, Majid; Guillot, Ann P.; Ausiello, M.D., Dennis A.
Publisher: Journal of the American Society of Nephrology
Date Published: August 01, 1994
Reference Number: 96
Congenital nephrogenic diabetes insipidus (CNDI) is a rare X-linked disorder in which the renal collecting duct is unresponsive to arginine vasopressin, and thus, the urine is consistently hypotonic to plasma. As a result, affected individuals are unable to concentrate urine and suffer from episodes of severe dehydration and hypernatremia. Recently, the association between arginine vasopressin V2 receptor gene mutations and CNDI has been demonstrated. In this report, two additional novel molecular defects of the arginine vasopressin V2 receptor gene in CNDI families are described. In one family, the affected individual demonstrated a G-->T transversion causing a nonsense mutation in codon 231. This mutation results in a glutamic acid becoming a termination codon, causing premature termination and truncation of the encoded receptor protein. This mutation causes a NciI site within the gene to be abolished and a BsaWI site to be created. In the second family, affected individuals showed a 28-basepair duplicating insertion in the very beginning of exon 2 down-stream of the splice acceptor site. It was hypothesized that an insertion mutagenesis mechanism involves the formation of a stem-loop structure within the newly synthesized DNA strand, followed by a slipped mispairing. This may be a general mechanism for the deletion or insertion of repeated sequences within the genome. Recent data show that G-protein-coupled receptors are susceptible to many different mutations that often result in the loss of function, causing a similar clinical phenotype.

This translation by the NDI Foundation is to assist the lay reader. To provide a clear, accessible interpretation of the original article, we eliminated or simplified some technical detail and complicated scientific language. We concentrated our translation on those aspects of the article dealing directly with NDI. The NDI Foundation thanks the researchers for their work toward understanding and more effectively treating this disorder.
© Copyright NDI Foundation 2007 (JC)

In this article, Holtzman, et al., report on finding and analyzing two mutated vasopressin-2 receptor (V2R) genes that had never been previously reported in the literature. To visualize a normal V2R, picture it as a long, beaded string (each bead is an amino acid). The majority of the V2R lies in seven folded clumps within the cell membrane, a thin strip of tissue that separates the inside of the cell from the outside. These clumps are called transmembrane helices (or domains, or regions) 1 - 7. Part of the V2R snakes outside the cell forming three curves that are known as extracellular loops 1 - 3. Part of it snakes inside the cell forming three loops called intracellular loops 1 - 3. One end, called the amino terminus, lies outside the cell with the extracellular loops. The other end, called the carboxy terminus, lies inside the cell with the intracellular loops. (You can look at a diagram of a V2R here.)

Mutations in the V2R gene produce V2Rs with altered structures incapable of performing their assigned functions. For example, the first V2R gene mutation analyzed by the authors caused a premature termination of the V2R, allowing it to only develop 231 amino acids instead of the normal 371. This resulted in the V2Rs that were synthesized from this mutated gene to only develop five transmembrane helices instead of the usual seven and left them lacking their entire carboxy termini. The site of the mutation corresponds to the beginning of intracellular loop 3. The authors did not analyze how these structural alterations translated to functional defects, but that the alterations were responsible for defects was clear because the patient had nephrogenic diabetes insipidus (NDI), the molecular basis of which is most commonly mutations in the V2R gene.

The second V2R gene the authors analyzed came from a family lineage where two male cousins had congenital NDI. This mutation results in the shortest V2R so far described in the literature - only 18 amino acids long.

This mutation was formed by the insertion of a specific DNA sequence into the beginning of the gene's second coding region. (Gene's have regions, called exons, that contain the gene's genetic code, and regions, called introns, that do not contain any genetic code.) The authors feel that this type of insertion may be a general mutation mechanism for the deletion or insertion of repeated genetic sequences within the totality of the genetic material of a cell.