Mutations in the V2 Vasopressin Receptor Gene are Associated with X-linked Nephrogenic Diabetes Insipidus
|Title:||Mutations in the V2 Vasopressin Receptor Gene are Associated with X-linked Nephrogenic Diabetes Insipidus|
|Authors:||Pan, Yang; Metzenberg, Ph.D., Aida; Das, Ph.D., Soma; Jing, Biqi; Gitschier, Ph.D., Jane|
|Date Published:||October 01, 1992|
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)
Yang Pan, et al., analyzed the V2R genes of five unrelated NDI patients. They found that each of the patients had a mutation in their V2R gene (one patient had two), and that each mutation was different from the other.
To picture a normal V2R (the protein synthesized from a normal V2R gene), imagine a long beaded string. (Each bead is an amino acid; there are 371 of them.) The bulk of the string lies folded in seven clumps, called transmembrane regions 1 - 7, within the cell membrane, a thin strip of tissue that separates the inside of the cell from the outside. Part of the V2R snakes outside the cell to form three curves called extracellular loops 1 - 3; part of it snakes inside the cell to form intracellular loops 1 - 3. One end of the V2R, 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 V2R here.)
Geneticists are interested in the structure/function relationship of proteins. That is, the structure of a protein allows it to carry out its intended function; an alteration of the structure could affect the protein's ability to function. In the current study, the authors found a nonsense mutation, which causes a premature termination of the protein synthesis and generally results in nonfunctional proteins; three missense mutations. These change the structure of part of the gene so that it creates a different amino acid than normal in one part of the V2R; a frameshift mutation, which results in a completely different set of amino acids being made into the V2R than normal, and an in-frame mutation which results in the loss of some amino acids from the V2R.
The nonsense mutation of the V2R gene caused it to stop producing the V2R in the midst of the third transmembrane region. This resulted in a completely nonfunctional protein. Interestingly, the patient with this mutation had the worst NDI symptoms of the group studied. One of the missense mutations resulted in an arginine amino acid being in the place where a proline amino acid was supposed to be. Prolines are believed to create hormone binding pockets by providing a kink in transmembrane region six. Since the function of V2R is to bind with AVP, this mutation would definitely prevent it from performing its function.
Another of the missense mutations resulted in an arginine amino acid taking the place of a cysteine amino acid on extracellular loop 2, near the cell membrane. The authors speculated that this substitution could disrupt the correct disulphide bonding between extracellular loops 1 and 2.
One NDI patient showed no mutation of their V2R gene, which suggests that there may be another gene that, when mutated, can also result in NDI. On the whole, however, these findings provide extremely strong evidence that X-linked NDI results from defects in the V2R gene. These findings also disprove the hypothesis that stated all North American NDI families have a common mutational origin, since four out of the five patients with different mutations were from North American/European origin.