No abstract

Mutations in the aquaporin-2 ( AQP2 ) water channel gene cause autosomal recessive nephrogenic diabetes insipidus (NDI). Here we report the first patient with an autosomal dominant form of NDI, which is caused by a G866A transition in the AQP2 gene of one allele, resulting in a E258K substitution in the C-tail of AQP2. To define the molecular cause of NDI in this patient, AQP2-E258K was studied in Xenopus oocytes. In contrast to wild-type AQP2, AQP2-E258K conferred a small increase in water permeability, caused by a reduced expression at the plasma membrane. Coexpression of wild-type AQP2 with AQP2-E258K, but not with an AQP2 mutant in recessive NDI (AQP2-R187C), revealed a dominant-negative effect on the water permeability conferred by wild-type AQP2. The physiologically important phosphorylation of S256 by protein kinase A was not affected by the E258K mutation. Immunoblot and microscopic analyses revealed that AQP2-E258K was, in contrast to AQP2 mutants in recessive NDI, not retarded in the endoplasmic reticulum, but retained in the Golgi compartment. Since AQPs are thought to tetramerize, the retention of AQP2-E258K together with wild-type AQP2 in mixed tetramers in the Golgi compartment is a likely explanation for the dominant inheritance of NDI in this patient. ( J. Clin. Invest. 1998. 102:57-66.)

In many mendelian diseases, some mutations result in the synthesis of misfolded proteins that cannot reach a transportcompetent conformation. In X-linked nephrogenic diabetes insipidus, most of the mutant vasopressin 2 (V2) receptors are trapped in the endoplasmic reticulum and degraded. They are unable to reach the plasma membrane and promote water reabsorption through the principal cells of the collecting ducts. Herein is reported two types of experiments: In vivo studies to assess clinically a short-term treatment with a nonpeptide V1a receptor antagonist (SR49059) and in vitro studies in cultured cell systems. In patients, SR49059 decreased 24-h urine volume (11.9 ؎ 2.3 to 8.2 ؎ 2.0 L; P ‫؍‬ 0.005) and water intake (10.7 ؎ 1.9 to 7.2 ؎ 1.6 L; P < 0.05). Maximum increase in urine osmolality was observed on day 3 (98 ؎ 22 to 170 ؎ 52 mOsm/kg; P ‫؍‬ 0.05). Sodium, potassium, and creatinine excretions and plasma sodium were constant throughout the study. A lthough the activities of the protein synthesis quality control systems are generally advantageous to the cell, on occasion this stringent monitoring process can lead to intracellular retention of salvageable proteins. In recent years, its has been observed that a group of diseases stem from mutations that promote such retention and are collectively referred to as conformational or protein-misfolding diseases (1,2). Nephrogenic diabetes insipidus (NDI) (3,4), which is characterized by a loss of arginine vasopressin (AVP)-mediated antidiuresis, is one of these diseases. In congenital NDI that results from mutations in the AVPR2 gene that encodes the V2 receptor, most missense mutations are misfolded, trapped in the endoplasmic reticulum, and unable to reach the basolateral cell surface to engage the circulating antidiuretic hormone, AVP (5-14).The natural history of untreated X-linked NDI includes hypernatremia, hyperthermia, mental retardation, and repeated episodes of dehydration in early infancy (15,16). In five new patients who were younger than 1 year and were from North America and in whom we provided molecular testing over the past 12 mo, plasma sodium was in every case Ͼ155 mEq/L at the time of diagnosis. We and others initially thought that close monitoring of infants whose AVPR2 mutations were diagnosed pre-or perinatally not only would prevent episodes of dehydration but also would permit close to normal growth and development. Although a low-sodium diet and distal tubule diuretics prescribed to these patients may achieve a 20 to 30% decrease in urine output (17), the low-sodium diet is difficult to follow, and affected children continue to drink large amounts of water. As a result of a physiologic gastroesophageal reflux and to the large amount of water in their stomach, these children often vomit, and, as a consequence, their nutritional intake is not optimal. There is a need, therefore, for a safe further reduction in urine output. We recently used pharmacologic compounds to rescue misfolded mutant V2 receptors by demonstrating in cultured cells t...

Antidiuretic hormone (arginine vasopressin) binds to and activates V2 receptors in renal collecting tubule cells. Subsequent stimulation of the Gs/adenylyl cyclase system promotes insertion of water pores into the luminal membrane and thereby reabsorption of fluid. In congenital nephrogenic diabetes insipidus (CNDI), an X-linked recessive disorder, the kidney fails to respond to arginine vasopressin. Here we report that an affected male of a family with CNDI has a deletion in the open reading frame of the V2 receptor gene, causing a frame shift and premature termination of translation in the third intracellular loop of the receptor protein. A normal receptor gene was found in the patient's brother. Both the normal and the mutant allele were detected in his mother. A different mutation, causing a codon change in the third transmembrane domain of the V2 receptor, was found in the open reading frame of an affected male but not in the unaffected brother belonging to another family suffering from CNDI.