Identification and Characterization of Aquaporin-2 Water Channel Mutations Causing Nephrogenic Diabetes Insipidus with Partial Vasopressin Response
| Title: | Identification and Characterization of Aquaporin-2 Water Channel Mutations Causing Nephrogenic Diabetes Insipidus with Partial Vasopressin Response |
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| Authors: | Moses, Arnold M.; Canfield, Marie C.; Tamarappoo, B.K.; Verkman, Alan S.; Holtzman, M.D., Eliezer J. |
| Publisher: | Human Molecular Genetics |
| Date Published: | October 01, 1997 |
| Reference Number: | 159 |
Congenital nephrogenic diabetes insipidus (NDI) is a rare disease caused most often by mutations in the vasopressin V2 receptor (AVPR2). We studied a family which included a female patient with NDI with symptoms dating from infancy. The patient responded to large doses of desmopressin (dDAVP) which decreased urine volume from 10 to 4 I/day. Neither the parents nor the three sisters were polyuric. The patient was found to be a compound heterozygote for two novel recessive point mutations in the aquaporin-2 (AQP2) gene: L22V in exon 1 and C181W in exon 3. Residue Cys181 in AQP2 is the site for inhibition of water permeation by mercurial compounds and is located near to the NPA motif conserved in all aquaporins. Osmotic water permeability (Pf) in Xenopus oocytes injected with cRNA encoding C181W-AQP2 was not increased over water control, while expression of L22V cRNA increased the Pf to approximately 60% of that for wild-type AQP2. Co-injection of the mutant cRNAs with the wild-type cRNA did not affect the function of the wild-type AQP2. Immunolocalization of AQP2-transfected CHO cells showed that the C181W mutant had an endoplasmic reticulum-like intracellular distribution, whereas L22V and wild-type AQP2 showed endosome and plasma membrane staining. Water permeability assays showed a high Pf in cells expressing wild-type and L22V AQP2. This study indicates that AQP2 mutations can confer partially responsive NDI.
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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)
Canfield, et al., report on a 33-year-old female with congenital NDI who had a partial response to a synthetic AVP called desmopressin (dDAVP). Theirs is the first report describing the molecular mechanisms responsible for partially responsive NDI.
After, as expected, finding no mutations in the AVPR2 gene, the authors found two mutations in the two alleles of the AQP2 gene. Genes come in pairs--one gene originating from the father, the other from the mother. Each pair of genes control the production of a specific protein, and either of the pair of genes may take any of several forms called alleles. So when we speak of one of the gene pairs we can refer to it as an allele. And when we speak of both genes in the gene pair, we can refer to them as alleles. This 33-year-old NDI patient had a mutation in each of the two alleles of her AQP2 gene. One of the mutations, called a L22V, was on one allele and was inherited from her mother. The other mutation, called C181W, was on the other allele and was inherited from her father.
The authors tested the L22V and the C181W mutations to see if they created functional or nonfunctional AQP2s by expressing them into two different cell cultures in the laboratory. They found the L22V-AQP2 increased the water permeability of the cell membranes, but not as much as normal AQP2s, whereas the C181W-AQP2 didn't significantly increase cell membrane permeability. When the authors injected both a normal and the L22V-AQP2 into the cells, the cells displayed an intermediate water permeability, which showed both AQP2s were functioning.
There is a place in the principal cells of the kidney duct that is called the endoplasmic reticulum (ER) where AQP2s wait until signaled to shuttle to the cell membrane. If the AQP2s are misshapen, the ER doesn't let them out to the membrane and they stay in the ER. To test if the two mutant AQP2 alleles were not allowed to leave the ER, the researchers injected them into another cell culture. The results suggested that the C181W mutation was retained in the ER, whereas the L22V mutation was not, and was, in fact, allowed to go to the membrane.
The authors showed that the patient's L22V mutation produced an AQP2 that could leave the ER and reach the cell membrane and, once there, could increase the water permeability to about 60% of what a normal AQP2 could. Therefore, the authors believe that this mutation is what gives the patient the ability to respond to dDAVP.