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The molecular basis of inherited salt-losing tubular disorders with secondary hypokalemia has become much clearer in the past two decades. Two distinct segments along the nephron turned out to be affected, the thick ascending limb of Henle's loop and the distal convoluted tubule, accounting for two major clinical phenotypes, hyperprostaglandin E syndrome and Bartter-Gitelman syndrome. To date, inactivating mutations have been detected in six different genes encoding for proteins involved in renal transepithelial salt transport. Careful examination of genetically defined patients ("human knockouts") allowed us to determine the individual role of a specific protein and its contribution to the overall process of renal salt reabsorption. The recent generation of several genetically engineered mouse models that are deficient in orthologous genes further enabled us to compare the human phenotype with the animal models, revealing some unexpected interspecies differences. As the first line treatment in hyperprostaglandin E syndrome includes cyclooxygenase inhibitors, we propose some hypotheses about the mysterious role of PGE(2) in the etiology of renal salt-losing disorders.

ABSTRACT.Objective. To characterize a rare inherited hypokalemic salt-losing tubulopathy with linkage to chromosome 1p31.Methods. We conducted a retrospective analysis of the clinical data for 7 patients in whom cosegregation of the disease with chromosome 1p31 had been demonstrated. In addition, in 1 kindred, prenatal diagnosis in the second child was established, allowing a prospective clinical evaluation.Results. Clinical presentation of the patients was homogeneous and included premature birth attributable to polyhydramnios, severe renal salt loss, normotensive hyperreninemia, hypokalemic alkalosis, and excessive hyperprostaglandin E-uria, which suggested the diagnosis of hyperprostaglandin E syndrome/antenatal Bartter syndrome. However, the response to indomethacin was only poor, accounting for a more severe variant of the disease. The patients invariably developed chronic renal failure. The majority had extreme growth retardation, and motor development was markedly delayed. In addition, all patients turned out to be deaf.Conclusion. The hypokalemic salt-losing tubulopathy with chronic renal failure and sensorineural deafness represents not only genetically but also clinically a disease entity distinct from hyperprostaglandin E syndrome/antenatal Bartter syndrome. A pleiotropic effect of a single gene defect is most likely causative for syndromic hearing loss. Pediatrics 2001;108(1). URL: http:// www.pediatrics.org/cgi/content/full/108/1/e5; tubulopathy, Bartter syndrome, polyhydramnios, renal failure, syndromic deafness, pleiotropic gene.ABBREVIATIONS. HPS/aBS, hyperprostaglandin E syndrome/ antenatal Bartter syndrome; TAL, thick ascending limb of Henle's loop; PGE 2 , prostaglandin E 2 ; NKCC2, furosemide-sensitive Na-K-2Cl cotransporter; ROMK, renal outer-medullary potassium channel; SND, sensorineural deafness; GFR, glomerular filtration rate; BERA, brainstem-evoked response audiometry; PPN, partial parenteral nutrition. I nherited salt-losing tubulopathies with hypokalemic alkalosis involve an overlapping set of renal tubular disorders that can be subdivided into at least 3 phenotypes: 1) classic Bartter syndrome, 2) Gitelman syndrome, and 3) hyperprostaglandin E syndrome/antenatal Bartter syndrome (HPS/aBS). 1,2 Whereas patients with classic Bartter syndrome and Gitelman syndrome typically present in early infancy and childhood or adolescence, manifestation of HPS/aBS occurs in utero and the neonatal course is severe. 3 The first clinical sign is maternal polyhydramnios caused by fetal polyuria, which regularly results in premature birth between 28 and 34 weeks of gestation. 4,5 Postnatally, affected infants present with the typical pattern of impaired tubular reabsorption in the thick ascending limb of Henle's loop (TAL), including salt wasting, isosthenuric or hyposthenuric polyuria, and hypercalciuria with subsequent medullary nephrocalcinosis. 6 -8 Characteristically, endogenous formation of prostaglandin E 2 (PGE 2 ) is stimulated markedly, resulting in additional aggravation of saluretic polyuri...

In patients with HPS/aBS, excessive PGE2 synthesis and hyperreninemia is dependent on COX-2 activity. This observation proves the stimulatory role of COX-2 on renin-secretion in salt-depletion in humans. Clinical long-term efficacy and potential side effects of rofecoxib need to be evaluated in a larger cohort of HPS/aBS-patients.

Genetic defects of the Na؉ -K ؉ -2Cl ؊ (NKCC2) sodium potassium chloride co-transporter result in severe, prenatal-onset renal salt wasting accompanied by polyhydramnios, prematurity, and life-threatening hypovolemia of the neonate (antenatal Bartter syndrome or hyperprostaglandin E syndrome). Herein are described two brothers who presented with hyperuricemia, mild metabolic alkalosis, low serum potassium levels, and bilateral medullary nephrocalcinosis at the ages of 13 and 15 yr. Impaired function of sodium chloride reabsorption along the thick ascending limb of Henle's loop was deduced from a reduced increase in diuresis and urinary chloride excretion upon application of furosemide. Molecular genetic analysis revealed that the brothers were compound heterozygotes for mutations in the SLC12A1 gene coding for the NKCC2 co-transporter. Functional analysis of the mutated rat NKCC2 protein by tracer-flux assays after heterologous expression in Xenopus oocytes revealed significant residual transport activity of the NKCC2 p.F177Y mutant construct in contrast to no activity of the NKCC2-D918fs frameshift mutant construct. However, coexpression of the two mutants was not significantly different from that of NKCC2-F177Y alone or wild type. Membrane expression of NKCC2-F177Y as determined by luminometric surface quantification was not significantly different from wild-type protein, pointing to an intrinsic partial transport defect caused by the p.F177Y mutation. The partial function of NKCC2-F177Y, which is not negatively affected by NKCC2-D918fs, therefore explains a mild and late-onset phenotype and for the first time establishes a mild phenotype-associated SLC12A1 gene mutation.

The combination of pharmacology and genetics suggests a new terminology for the above described SLTs: Furosemide-like-SLT for HPS caused by NKCC2-mutations, furosemide/amiloride-like-SLT for HPS caused by ROMK-mutations, furosemide/thiazide-like-SLT for HPS + SND, thiazide/furosemide-like-SLT for cBS, and thiazide-like-SLT for GS.

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