NEW MUTATIONS IN THE AQP2 GENE IN NEPHROGENIC DIABETES-INSIPIDUS RESULTING IN FUNCTIONAL BUT MISROUTED WATER CHANNELS

Nephrogenic diabetes insipidus (NDI) is characterized by the inability of the kidney to concentrate urine in response to vasopressin. The au tosomal recessive form of NDI is caused by mutations in the AQP2 gene, encoding the vasopressin-regulated water channel of the kidney collec ting duct. This report presents three new mutations in the AQP2 gene t hat cause NDI, resulting in A147T-, T126M-, or N68S-substituted AQP2 p roteins. Expression of the A147T and T126M mutant AQP2 proteins in Xen opus oocytes revealed a relatively small, but significant increase in water permeability, whereas the water permeability of N68S expressing oocytes was not increased. cRNA encoding missense and wild-type AQP2 w ere equally stable in oocytes. Immunoblots of oocyte lysates showed th at only the A147T mutant protein was less stable than wild-type AQP2. The mutant AQP2 proteins showed, in addition to the wild-type 29-kd ba nd, an endoplasmic reticulum-retarded form of AQP2 of approximately 32 kd. Immunoblotting and immunocytochemistry demonstrated only intense labeling of the plasma membranes of oocytes expressing wildtype AQP2. In summary, two mutant AQP2 proteins encoded in NDI are functional wat er channels. Therefore, the major cause underlying autosomal recessive NDI is the misrouting of AQP2 mutant proteins.