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Synthetic oligonucleotides containing GC-rich triplet sequences were used in a scanning strategy to identify unstable genetic sequences at the myotonic dystrophy (DM) locus. A highly polymorphic GCT repeat was identified and found to be unstable, with an increased number of repeats occurring in DM patients. In the case of severe congenital DM, the paternal triplet allele was inherited unaltered while the maternal, DM-associated allele was unstable. These studies suggest that the mutational mechanism leading to DM is triplet amplification, similar to that occurring in the fragile X syndrome. The triplet repeat sequence is within a gene (to be referred to as myotonin-protein kinase), which has a sequence similar to protein kinases.

SUMMARYThe syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a common cause of hyponatremia. We describe two infants whose clinical and laboratory evaluations were consistent with the presence of SIADH, yet who had undetectable arginine vasopressin (AVP) levels. We hypothesized that they had gain-of-function mutations in the V2 vasopressin receptor (V2R). DNA sequencing of each patient's V2R gene (AVPR2) identified missense mutations in both, with resultant changes in codon 137 from arginine to cysteine or leucine. These novel mutations cause constitutive activation of the receptor and are the likely cause of the patients' SIADH-like clinical picture, which we have termed "nephrogenic syndrome of inappropriate antidiuresis."Fluid homeostasis depends on proper water intake, governed by an intact thirst mechanism, and on urinary excretion of free water, mediated by appropriate secretion of arginine vasopressin (AVP) (also known as antidiuretic hormone). 1 AVP exerts its antidiuretic action by binding to the V2 vasopressin receptor (V2R), a G protein-coupled receptor, on the basolateral membrane of epithelial cells in the collecting duct of the kidney. Ligand binding activates the V2R, stimulating adenylate cyclase by means of G s proteins. The resulting increase in intracellular cyclic AMP (cAMP) promotes shuttling of intracellular vesicles containing the water channel aquaporin-2 to the apical membrane of the collecting-duct cells, thereby increasing water permeability and inducing antidiuresis.Clinical disorders of water balance are common, and alterations in many steps of this pathway have been described. 1 Urinary concentrating defects associated with diabetes insipidus may result from a deficiency of AVP or from nephrogenic causes, such as Xlinked, inactivating mutations in the V2R or autosomal recessive or autosomal dominant lesions in aquaporin-2. 2 Conversely, the syndrome of inappropriate antidiuretic hormone secretion (SIADH) manifests as an inability to excrete a free water load, with inappropriately concentrated urine and resultant hyponatremia, hypo-osmolality, and We describe two unrelated male infants whose clinical presentation was consistent with the presence of chronic SIADH but who had undetectable AVP levels. We postulated that novel activating mutations of the V2R might account for their unique presentation. Evaluation revealed novel activating mutations of the V2R leading to what we term "nephrogenic syndrome of inappropriate antidiuresis" (NSIAD). HHS Public Access CASE REPORTSPatient 1 presented at 3 months of age with irritability, and Patient 2 presented at 2.5 months of age with two generalized seizures. Both children had had unremarkable early neonatal courses. Both were exclusively bottle-fed formula (7 mmol of sodium per liter). Both infants had mild systolic hypertension with otherwise normal physical examinations. Initial laboratory evaluations demonstrated hyponatremia with normal serum levels of potassium and bicarbonate (Table 1). Both children had serum hypo-o...