2002 Global Researcher Conference Proceeding

April 26 - 28, 2002

Conference: 2002 Global Researcher Conference
Title: Cell biological and functional analysis of five new Aquaporin-2 gene missense mutations in recessive Nephrogenic Diabetes Insipidus
Authors: Robben, Joris; Marr, Nannette; Bichet, Daniel G.; Hoefs, Susan; Arthus, Marie-Francoise; Lonergan, Michele; Fujiwara, T. Mary; Knoers, Nine; Landau, MD, Daniel; Balfe, J.W.; Oksche, Alexander; Rosenthal, Walter; Muller, Dominik; van Os, Carel; Deen, Peter M.T.
Institutions: University of Nijmegen, Institut fr Kardiovaskul re Physiologie, Hopital du Sacre-Coeur de Montreal, University Hospital Nijmegen, Montreal General Hospital, University Medical Centre Nijmegen, Soroka University Medical Center, Institut Fuer Pharmakologie, Charite - Universitatsmedizin Berlin, Charite Department of Pediatric Nephrology
Robben Mutations in the renal water channel Aquaporin-2 have been shown to cause autosomal Nephrogenic Diabetes Insipidus (NDI), a disease in which the kidney is unable to concentrate urine in response to vasopressin. Most encoded AQP2 missense mutants in recessive NDI are retained in the endoplasmic reticulum (ER), but AQP2-T125M and AQP2-G175R were reported to be non-functional channels unimpaired in their routing to the plasma membrane. In five families, we identified seven novel AQP2 gene mutations and analyzed the cell-biological basis for their recessive NDI. The patients of four families were homozygous for mutations, encoding AQP2-L28P, AQP2-A47V, AQP2-V71M or AQP2-P185A. Expression in oocytes revealed that all these mutants, but also AQP2-T125M and AQP2-G175R, conferred a reduced water permeability compared to wt-AQP2, which was due to ER retardation. The patient of the fifth family had a G>A nucleotide switch in the splice donor site of one allele, expected to result in an out of frame protein. The other allele encoded a cC652 nucleotide deletion and a V194I missense mutation. Since the routing and function of AQP2-V194I in oocytes was not different from wt-AQP2, it is likely that the cC652 deletion, leading to an out of frame protein, is the NDI-causing mutation of the second allele. Our study indicates that misfolding and ER retention is the main, and possibly only, cell-biological basis for recessive NDI caused by missense AQP2 proteins. In addition, the reduced single channel water permeability of AQP2-A47V (40%) and AQP2-T125M (25%) might become of therapeutic value when chemical chaperones can be found that restore their routing to the plasma membrane.
Robben, et al., found seven new Aquaporin 2 (AQP2) mutations that result in NDI. They analyzed five of these mutations (all were missense mutations, which means the gene produces a protein where an amino acid appears in a position it should not be.) They concluded that four out of five mutated AQP2s were improperly shaped and retained in the cell's quality control segment, the endoplasmic reticulum. Because the AQP2s are retained, they cannot get to the cellular membrane to let water pass through it. Hence, the manifestation of NDI in patients carrying any of these mutations in their AQP2 genes is causative for the disease. The fifth mutated AQP2, in which valine 194 was replaced by an isoleucine, routed normally to the plasma membrane, and yielded a normal water transport. Most likely, two other mutations (not discussed in detail here) identified in this patient are causative for NDI in this patient.