Clinical presentation of genetically defined patients with hypokalemic salt-losing tubulopathies.

PURPOSE: Hypokalemic salt-losing tubulopathies (Bartter-like syndromes) comprise a set of clinically and genetically distinct inherited renal disorders. Mutations in four renal membrane proteins involved in electrolyte reabsorption have been identified in these disorders: the furosemide-sensitive sodium-potassium-chloride cotransporter NKCC2, the potassium channel ROMK, the chloride channel ClC-Kb, and the thiazide-sensitive sodium-chloride cotransporter NCCT. The aim of this study was to characterize the clinical features associated with each mutation in a large cohort of genetically defined patients. PATIENTS AND METHODS: The phenotypic characteristics of 65 patients with molecular defects in NKCC2, ROMK, ClC-Kb, or NCCT were collected retrospectively. RESULTS: ROMK and NKCC2 patients presented with polyhydramnios, nephrocalcinosis, and hypo- or isosthenuria. Hypokalemia was less severe in the ROMK patients compared with the NKCC2 patients. In contrast, NCCT patients had hypocalciuria, hypomagnesemia, and marked hypokalemia. While this dissociation of renal calcium and magnesium handling was also observed in some ClC-Kb patients, a few ClC-Kb patients presented with hypercalciuria and hypo- or isosthenuria. CONCLUSIONS: ROMK, NKCC2, and NCCT mutations usually have uniform clinical presentations, whereas mutations in ClC-Kb occasionally lead to phenotypic overlaps with the NCCT or, less commonly, with the ROMK/NKCC2 cohort. Based on these results, we propose an algorithm for the molecular diagnosis of hypokalemic salt-losing tubulopathies.

Increased systolic blood pressure with rofecoxib in congenital furosemide-like salt loss.

BACKGROUND: To analyse whether congenital furosemide- or thiazide-like renal salt loss protects against the potential prohypertensive effects of two cyclooxygenase (COX) inhibitors: rofecoxib, a COX-2 selective inhibitor, and indomethacin, an unselective COX-inhibitor. METHODS: In a retrospective analysis, the effects of rofecoxib and indomethacin on blood pressure (bp: transformed into age-independent standard deviation scores (SDS) values), creatinine clearance (CRC), fractional excretion of sodium (FeNa), and renal excretion of systemic prostaglandins were studied in 28 patients with a genetically proven congenital hypokalaemic salt-losing tubulopathy (SLT) (11 female and 17 male, age: 2-25 years), 19 with a furosemide-like SLT, and nine with a thiazide-like SLT. RESULTS: In furosemide-like SLT patients, systolic SDS bp values were significantly higher with rofecoxib (1.03 +/- 0.16 SDS, n = 107) compared with indomethacin (0.56 +/- 0.09 SDS, n = 282; P = 0.007, 95% CI: 0.12-0.8). Without the drugs, systolic SDS bp values were elevated by 0.63 +/- 0.11 SDS, n = 164. Both drugs reduced renin and aldosterone-plasma levels to a similar extent. SDS values were significantly correlated with the excretion of the vasoconstrictor thromboxane (T x B2) (R2: 0.038, P = 0.021, n: 159), but not with CRC or FeNa. Blood pressure was not increased in thiazide-like SLT patients treated with rofecoxib. CONCLUSION: Congenital furosemide-like renal salt loss does not protect against the prohypertensive effects of rofecoxib. The positive correlation between bp SDS values with T x B2 but not with FeNa or CRC point towards an altered vascular homeostasis as one mechanism increasing blood pressure.

Late-onset manifestation of antenatal Bartter syndrome as a result of residual function of the mutated renal Na+-K+-2Cl- co-transporter.

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.

Salt handling in the distal nephron: lessons learned from inherited human disorders.

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.

Induction of microsomal prostaglandin E2 synthase in the macula densa in children with hypokalemic salt-losing tubulopathies.

In hyperprostaglandin E syndrome (HPGES) and classic Bartter syndrome (cBS), tubular salt and water losses stimulate renin secretion, which is dependent on enhanced cyclooxygenase-2 (COX-2) enzymatic activity. In contrast to other renal COX metabolites, only prostaglandin E(2) (PGE(2)) is selectively up-regulated in these patients. To determine the intrarenal source of PGE(2) synthesis, we analyzed the expression of microsomal PGE(2) synthase (mPGES; EC: 5.3.99.3), whose product PGE(2) has been shown to stimulate renin secretion in vitro. Expression of mPGES was analyzed by immunohistochemistry in eight patients with HPGES, in two patients with cBS, and in six control subjects. Expression of mPGES immunoreactive protein was observed in cells of the macula densa in five of eight HPGES patients and in one of two cBS patients. Expression of mPGES immunoreactive protein was not observed in cells associated with the macula densa in kidneys from control subjects without a history consistent with activation of the renin angiotensin system. Co-induction of COX-2 and mPGES in cells of the macula densa suggests that PGE(2) activates renin secretion in humans.

Role of cyclooxygenase-2 in hyperprostaglandin E syndrome/antenatal Bartter syndrome.

BACKGROUND: Hyperprostaglandin E syndrome/antenatal Bartter syndrome (HPS/aBS) is a congenital salt-losing tubulopathy with an induced expression of cyclooxygenase-2 (COX-2) in the macula densa probably leading to hyperreninemia. Inhibition of stimulated prostaglandin E2 (PGE2) formation with indomethacin results in a significant improvement of clinical symptoms and is therefore standard therapy. Using the COX-2 selective inhibitor rofecoxib, we investigated the role of COX-2 in the pathophysiology of HPS/aBS. METHODS: Six clinically well-characterized patients with HPS/aBS (3 girls) were enrolled into the study. Four patients had mutations in the renal potassium channel ROMK, one patient in the furosemide-sensitive cotransporter NKCC2, whereas in one patient no molecular abnormality could be detected. Median age was 15.8 years (range: 9.1 to 19.0 years). Patients were evaluated on indomethacin treatment, 3 days after indomethacin withdrawal, and after 4 days of treatment with rofecoxib. Therapeutic drug monitoring was performed. RESULTS: COX-2-selectivity of rofecoxib was confirmed in vivo and ex vivo. Both indomethacin and rofecoxib ameliorated clinical symptoms, the typical laboratory findings, and significantly suppressed PGE2 and PGE-M excretion to normal values while it was elevated under withdrawal conditions. Rofecoxib suppressed hyperreninemia to a similar extent as indomethacin. CONCLUSION: 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.