Novel mutations in the thiazide-sensitive NaCl cotransporter gene in patients with Gitelman syndrome with predominant localization to the C-terminal domain

Novel paracellin-1 mutations in 25 families with familial hypomagnesemia with hypercalciuria and nephrocalcinosis

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is an autosomal recessive tubular disorder that is frequently associated with progressive renal failure. The primary defect is related to impaired tubular reabsorption of magnesium and calcium in the thick ascending limb of Henle's loop. Mutations in PCLN-1, which encodes the renal tight junction protein paracellin-1 (claudin-16), were identified as the underlying genetic defects. Comprehensive clinical data and the results of PCLN-1 mutation analysis of 25 FHHNC families with 33 affected individuals are presented. Patients presented mainly with urinary tract infections, polyuria, and hematuria at a median age of 3.5 yr. At the time of diagnosis, the GFR was already decreased to <60 ml/min per 1.73 m(2) for 11 patients. Twelve patients exhibited progression to end-stage renal disease, at a median age of 14.5 yr. Treatment with magnesium salts and thiazides seemed to have no effect on the progression of the disease. Genotype analysis revealed PCLN-1 mutations in all except three mutant alleles (94%). Fifteen different mutations were observed, including eight novel mutations. The accumulation of mutations affecting the first extracellular loop was striking, with 48% of all mutant alleles exhibiting a Leu151Phe exchange. Haplotype analysis strongly suggested a founder effect among patients with FHHNC who originated from Germany or eastern European countries. In 13 of 23 families, hypercalciuria and/or nephrolithiasis were observed in otherwise unaffected family members, indicating a possible role of heterozygous PCLN-1 mutations in yielding hypercalciuric stone-forming conditions.

Abnormalities of Gq-mediated cell signaling in Bartter and Gitelman syndromes

BACKGROUND

The constitutive endothelial isoform of nitric oxide synthase (ecNOS) and nitric oxide (NO) production are increased in patients with Bartter syndrome (BS) and Gitelman (GS) syndrome and may reduce vascular tone. Moreover, these patients present an abnormal cell signaling [reduced stimulated intracellular calcium ([Ca(2+)](i)) and inositol-1,4,5,triphosphate ([IP(3)](i)) in neutrophils], suggesting the presence of a generalized reduction of protein kinase C (PKC) and cell reactivity. Since PKC regulates ecNOS gene expression, we evaluated the signal transduction system involving Gq protein, PKC, and ecNOS in circulating nucleated cells from patients with BS/GS.

METHODS

Nucleated blood cells from 2 BS and 7 GS and from 10 controls (C) were used. PKC activity was evaluated in neutrophils by radioenzymatic assay; PKC alpha concentration was evaluated in monocytes by Western blot analysis. ecNOS and G alpha q mRNA production was evaluated in monocytes by reverse transcription-polymerase chain reaction (RT-PCR) analysis using specific primers and quantitated by PCR-based semiquantitative analysis of ecNOS and G alpha q mRNA expression.

RESULTS

Cytosol and membrane basal PKC activity were similar in neutrophils from BS/GS and C (70 +/- 3 vs. 80 +/- 2; 37 +/- 3 vs. 46 +/- 2 pmol/min/mg protein, respectively), while fMLP-stimulated membrane PKC activity increased to a lower extent in BS/GS (from 43 +/- 2 to 53 +/- 3 vs. 38 +/- 2 to 66 +/- 3 pmol/min/mg protein, P < 0.05 for the difference). Membrane PKC alpha expression was similar in basal conditions (8.5 +/- 1.5 vs. 12.4 +/- 4.0 densitometric units), but increased after fMLP was reduced in BS/GS (4.5 +/- 1.4 vs. 9.5 +/- 2.1, P < 0.01). In BS/GS, PKC stimulation with PMA dose dependently reduced ecNOS gene expression (from 0.80 +/- 0.05 to 0.78 +/- 0.03 densitometric units; PMA 50 nmol/L, P = NS; to 0.55 +/- 0.07, PMA 100 nmol/L, P < 0.001) to an undetectable expression (PMA 200 nmol/L). Qualitatively similar effects were seen in monocytes from control subjects. Incubation of monocytes from patients and controls with the PKC inhibitor GF109203X increased ecNOS mRNA, with no difference between patients and controls. G alpha q mRNA was reduced in BS/GS versus controls (0.87 +/- 0.013 vs. 0.98 +/- 0.005 densitometric units, P < 0.0004).

CONCLUSION

An abnormal G alpha q expression blunts cell signaling and PKC production in BS/GS. A reduced PKC up-regulated NO system may contribute to the vascular hyporeactivity of BS/GS.

Chronic renal failure, end-stage renal disease, and peritoneal dialysis in Gitelman's syndrome

The chronic state of hypovolemia, hypotension, and hypokalemia found in Bartter's syndrome has been shown to lead to a chronic nephropathy, which then can progress toward end-stage renal disease and dialysis. This progression, however, has never been reported for Gitelman's syndrome, a variant of Bartter's syndrome that shows a milder clinical picture. This report is the first to document this progression (ie, the development of end-stage renal disease in Gitelman's syndrome) as well as the first report of the use of peritoneal dialysis in either Bartter's syndrome or Gitelman's syndrome. The clinical course highlights the importance of and the need for careful control of hemodynamic status in these patients to slow the progression of renal injury. The hemodynamic alterations that characterize Bartter's syndrome and Gitelman's syndrome patients suggest that for patients requiring renal replacement therapy, peritoneal dialysis is a more appropriate treatment because of its less severe impact on these parameters.

Gitelman's syndrome: an overlooked cause of chronic hypokalemia and hypomagnesemia in adults

In 1966, Gitelman described a benign variant of classical Bartter's syndrome in adults characterized by consistent hypomagnesemia and hypocalciuria, hypokalemic metabolic alkalosis and hyperreninemic hyperaldosteronism with normal blood pressure. A specific gene has been found responsible for this disorder, encoding the thiazide-sensitve Na-Cl coporter (TSC) in the distal convoluted tubule. Mutant alleles result in loss of normal TSC function and the phenotype is identical to patients with chronic use of thiazide diuretics. Gitelman's syndrome is a more common cause of chronic hypokalemia than Bartter's syndrome, with which it is often confused. The distinguishing features between both syndromes are discussed.

Gitelman's syndrome revisited: an evaluation of symptoms and health-related quality of life

BACKGROUND

Gitelman's syndrome (GS), also called Gitelman's variant of Bartter's syndrome, is an autosomal recessive renal disorder characterized by hypokalemia, hypomagnesemia, metabolic alkalosis, and hypocalciuria. GS is caused by inactivating mutations in the thiazide-sensitive sodium chloride cotransporter gene (NCCT). It is also known as the "milder" form of Bartter's syndrome, as patients with GS are usually diagnosed in adulthood during routine investigation. Symptoms reported in the literature range from asymptomatic, to mild symptoms of cramps and fatigue, to severe manifestations such as tetany, paralysis, and rhabdomyolysis. This is the first systematic evaluation of a large group of patients with genetically defined GS.

METHODS

We evaluated the symptoms and quality of life (QOL) in 50 adult GS patients with confirmed mutations in NCCT, using a standardized questionnaire. This cohort was compared with 25 age- and sex-matched controls.

RESULTS

GS patients were significantly more symptomatic than controls. The most common symptoms were salt craving, with musculoskeletal symptoms such as cramps, muscle weakness, and aches and constitutional symptoms such as fatigue, generalized weakness and dizziness, and nocturia and polydipsia. Forty-five percent of GS patients consider their symptoms a moderate to big problem. Measures of health-related QOL were significantly lower in GS patients compared with controls, particularly in terms of role limitations caused by physical health, emotion, level of energy, and general health perception.

CONCLUSIONS

This descriptive study indicates that GS is not an asymptomatic disease and adversely affects QOL in these patients. Further studies are needed to assess the impact of therapy on symptoms and QOL.

Mutations in the chloride channel gene, CLCNKB, leading to a mixed Bartter-Gitelman phenotype

Gitelman syndrome is an inherited renal disorder characterized by impaired NaCl reabsorption in the distal convoluted tubule and secondary hypokalemic alkalosis. In clinical practice, it is distinguished from other hypokalemic tubulopathies by the presence of both hypomagnesemia and normocalcemic hypocalciuria. To date, only mutations in a single gene encoding the thiazide-sensitive NaCl cotransporter have been found as the molecular basis of GS. We describe three unrelated patients presenting with the typical laboratory findings of GS. Mutational analysis in these patients revealed no abnormality in the SLC12A3 gene. Instead, all patients were found to carry previously described mutations in the CLCNKB gene, which encodes the kidney-specific chloride channel ClC-Kb, raising the possibility of genetic heterogeneity. Review of the medical histories revealed manifestation of the disease within the first year of life in all cases. Clinical presentation included episodes of dehydration, weakness, and failure to thrive, much more suggestive of classic Bartter syndrome than of GS. The coexistence of hypomagnesemia and hypocalciuria was not present from the beginning. In the follow-up, however, a drop of both parameters below normal range was a consistent finding reflecting a transition from cBS to GS phenotype. The phenotypic overlap may indicate a physiologic cooperation of the apical thiazide-sensitive NaCl cotransporter and the basolateral chloride channel for salt reabsorption in the distal convoluted tubule.

Genetic variants of thiazide-sensitive NaCl-cotransporter in Gitelman's syndrome and primary hypertension

Gitelman's syndrome is an autosomal recessive disorder characterized by electrolyte disturbances and low blood pressure. The disease is caused by homozygous or compound heterozygous inactivating mutations in the thiazide-sensitive NaCl-cotransporter gene leading to reduced renal sodium reabsorption. We report 4 patients with Gitelman's syndrome from southern Sweden, all in whom we identified compound heterozygous mutations in the thiazide-sensitive NaCl-cotransporter gene (Gly439Ser, Gly731Arg, Gly741Arg, Thr304Pro, and 2745insAGCA), of which the latter 2 have not been described before. We hypothesized that such mutations in their heterozygous form protect against primary hypertension in the general population and that the gene may also harbor activating mutations that increase the risk for primary hypertension. Accordingly, the gene was screened for mutations in 20 patients with primary hypertension and in 20 normotensive subjects by single-strand conformation polymorphism and direct DNA sequencing. The Arg904Gln, Gly264Ala, and C1420T variants, found in the mutation screening of subjects without Gitelman's syndrome, were studied further. Population genotype frequencies were determined in 292 unrelated patients with primary hypertension and 264 unrelated normotensive subjects from southern Sweden. Gln904 homozygotes were overrepresented in hypertensive patients compared with normotensive subjects (5 of 292 versus 0 of 264; P:=0.03). In conclusion, we confirm that Gitelman's syndrome is caused by mutations in the thiazide-sensitive NaCl-cotransporter gene. Our results further suggest that subjects homozygous for the Gln904 variant have an increased risk for development of primary hypertension.

Mutations in the chloride channel gene CLCNKB as a cause of classic Bartter syndrome

ABSTRACT.: Inherited hypokalemic renal tubulopathies are differentiated into at least three clinical subtypes: (1) the Gitelman variant of Bartter syndrome (GS); (2) hyperprostaglandin E syndrome, the antenatal variant of Bartter syndrome (HPS/aBS); and (3) the classic Bartter syndrome (cBS). Hypokalemic metabolic alkalosis and renal salt wasting are the common characteristics of all three subtypes. Hypocalciuria and hypomagnesemia are specific clinical features of Gitelman syndrome, while HPS/aBS is a life-threatening disorder of the newborn with polyhydramnios, premature delivery, hyposthenuria, and nephrocalcinosis. The Gitelman variant is uniformly caused by mutations in the gene for the thiazide-sensitive NaCl-cotransporter NCCT (SLC12A3) of the distal tubule, while HPS/aBS is caused by mutations in the gene for either the furosemide-sensitive NaK-2Cl-cotransporter NKCC2 (SLC12A1) or the inwardly rectifying potassium channel ROMK (KCNJ1). Recently, mutations in a basolateral chloride channel CLC-Kb (CLCNKB) have been described in a subset of patients with a Bartter-like phenotype typically lacking nephrocalcinosis. In this study, the screening for CLCNKB mutations showed 20 different mutations in the affected children from 30 families. The clinical characterization revealed a highly variable phenotype ranging from episodes of severe volume depletion and hypokalemia during the neonatal period to almost asymptomatic patients diagnosed during adolescence. This study adds 16 novel mutations to the nine already described, providing further evidence that mutations in the gene for the basolateral chloride channel CLC-Kb are the molecular basis of classic Bartter syndrome. Interestingly, the phenotype elicited by CLCNKB mutations occasionally includes HPS/aBS, as well as a Gitelman-like phenotype.

The molecular basis of renal tubular transport disorders

Sodium and water homeostasis are key to the survival of organisms. Reabsorption of sodium and water occurs throughout the tubule structure of the nephron, the basic functional unit of the kidney, by various transport mechanisms. Altered transport protein function can lead to renal tubular disorders resulting in metabolic alkalosis, hypokalemia, hypertension, and decreased capacity to concentrate urine, for instance. However, recent advances in molecular physiology, molecular genetics and expression cloning systems have aided in unraveling the molecular basis of some renal tubular disorders. This review will examine the molecular basis of Bartter's syndrome, Gitelman's syndrome, Liddle's syndrome, and autosomal nephrogenic diabetes insipidus. An understanding of the molecular basis of these disorders of the human kidney can give us a better understanding of basic renal function of lower mammals and other vertebrates.

Pharmacology of diuretics

The diuretics in our therapeutic armamentarium have predictable effects based on their nephron sites of action. All but spironolactone must reach the lumen or urinary side of the nephron to exert their effects. Thus, in settings of decreased renal function, doses must be increased to deliver more diuretic into the urine. In other edematous disorders, such as congestive heart failure (CHF) and cirrhosis, adequate amounts of diuretic reach the site of action if renal function is satisfactory. Diminished response in these conditions is caused by a decrease in the sensitivity of the nephron to the diuretic, the mechanism of which is unknown. Rather than using large single doses of diuretic in CHF and cirrhosis, multiple doses and/or combinations of diuretics should be used. Therefore, thiazide diuretics coupled with loop diuretics are most logical because they affect different nephron sites and the thiazide counteracts distal nephron hypertrophy that may occur with loop diuretics alone. Ample studies have shown that such combinations can result in a truly synergistic response. Using pharmacokinetics and pharmacodynamics of diuretics, we can design therapeutic regimens in which satisfactory control of fluid and electrolyte homeostasis can be achieved in the vast majority of patients.

Novel mutations in thiazide-sensitive Na-Cl cotransporter gene of patients with Gitelman's syndrome

Gitelman's syndrome (GS) is an autosomal recessive disorder characterized by metabolic alkalosis, hypokalemia, hypomagnesemia, and hypocalciuria that has recently been reported to be linked to thiazide-sensitive Na-Cl cotransporter (TSC) gene mutations. In this study, possible mutations in the TSC gene of six Japanese patients clinically diagnosed with GS were investigated. Twenty-six exons encoding TSC were amplified by PCR and then completely sequenced by the direct sequencing method. Patient A showed a missense mutation of Arg 642 to Cys on the paternal allele and a missense mutation of Val 578 to Met and a 2-bp deletion (nucleotide 2543-2544) on the maternal allele. This deletion results in a frameshift that alters codon 837 to encode a stop signal rather than phenylalanine, and it is predicted to lead to loss of the latter half of the intracellular carboxy terminus. In the second family, two affected sisters, patients B and C, had a homozygous missense mutation of Thr 180 to Lys. Both of their parents, who are consanguineously married, have a heterozygous Thr180Lys mutation. Patient D has a homozygous mutation Thr180Lys, which is the same as the second family. Haplotype analysis indicates that patients B and C are not related to patient D. In patients E and F, we could identify only one mutant allele; Ala569Glu and Leu849His, respectively. All of the mutations identified are novel except for the Arg642Cys mutation, which has been found in a Japanese GS patient. Although further in vitro study is required to prove that the mutations are responsible for GS, it is possible that Thr180Lys and Arg642Cys mutations might be common mutations in Japanese GS.

Mammalian distal tubule: physiology, pathophysiology, and molecular anatomy

The distal tubule of the mammalian kidney, defined as the region between the macula densa and the collecting duct, is morphologically and functionally heterogeneous. This heterogeneity has stymied attempts to define functional properties of individual cell types and has led to controversy concerning mechanisms and regulation of ion transport. Recently, molecular techniques have been used to identify and localize ion transport pathways along the distal tubule and to identify human diseases that result from abnormal distal tubule function. Results of these studies have clarified the roles of individual distal cell types. They suggest that the basic molecular architecture of the distal nephron is surprisingly similar in mammalian species investigated to date. The results have also reemphasized the role played by the distal tubule in regulating urinary potassium excretion. They have clarified how both peptide and steroid hormones, including aldosterone and estrogen, regulate ion transport by distal convoluted tubule cells. Furthermore, they highlight the central role that the distal tubule plays in systemic calcium homeostasis. Disorders of distal nephron function, such as Gitelman's syndrome, nephrolithiasis, and adaptation to diuretic drug administration, emphasize the importance of this relatively short nephron segment to human physiology. This review integrates molecular and functional results to provide a contemporary picture of distal tubule function in mammals.

Magnesium in disease: a review with special emphasis on the serum ionized magnesium

This review deals with the six main clinical situations related to magnesium or one of its fractions, including ionized magnesium: renal disease, hypertension, pre-eclampsia, diabetes mellitus, cardiac disease, and the administration of therapeutic drugs. Issues addressed are the physiological role of magnesium, eventual changes in its levels, and how these best can be monitored. In renal disease mostly moderate hypermagnesemia is seen; measuring ionized magnesium offers minimal advantage. In hypertension magnesium might be lowered but its measurement does not seem relevant. In the prediction of severe pre-eclampsia, elevated ionized magnesium concentration may play a role, but no unequivocal picture emerges. Low magnesium in blood may be cause for, or consequence of, diabetes mellitus. No special fraction clearly indicates magnesium deficiency leading to insulin resistance. Cardiac diseases are related to diminished magnesium levels. During myocardial infarction, serum magnesium drops. Total magnesium concentration in cardiac cells can be predicted from levels in sublingual or skeletal muscle cells. Most therapeutic drugs (diuretics, chemotherapeutics, immunosuppressive agents, antibiotics) cause hypomagnesemia due to increased urinary loss. It is concluded that most of the clinical situations studied show hypomagnesemia due to renal loss, with exception of renal disease. Keeping in mind that only 1% of the total body magnesium pool is extracellular, no simple measurement of the real intracellular situation has emerged; measuring ionized magnesium in serum has little added value at present.

Defective processing and expression of thiazide-sensitive Na-Cl cotransporter as a cause of Gitelman's syndrome

Gitelman's syndrome is an autosomal recessive disorder of salt wasting and hypokalemia caused by mutations in the thiazide-sensitive Na-Cl cotransporter. To investigate the pathogenesis of Gitelman's syndrome, eight disease mutations were introduced into the mouse thiazide-sensitive Na-Cl cotransporter and studied by functional expression in Xenopus oocytes. Sodium uptake into oocytes that expressed the wild-type clone was more than sevenfold greater than uptake into control oocytes. Uptake into oocytes that expressed the mutated transporters was not different from control. Hydrochlorothiazide reduced Na uptake by oocytes expressing the wild-type gene to control values but had no effect on oocytes expressing the mutant clones. Western blots of oocytes injected with the wild-type clone showed bands representing glycosylated (125 kDa) and unglycosylated (110 kDa) forms of the transport protein. Immunoblot of oocytes expressing the mutated clones showed only the unglycosylated protein, indicating that protein processing was disrupted. Immunocytochemistry with an antibody against the transport protein showed intense membrane staining of oocytes expressing the wild-type protein. Membrane staining was completely absent from oocytes expressing mNCC(R948X); instead, diffuse cytoplasmic staining was evident. In summary, the results show that several mutations that cause Gitelman's syndrome are nonfunctional because the mutant thiazide-sensitive Na-Cl cotransporter is not processed normally, probably activating the "quality control" mechanism of the endoplasmic reticulum.

Gitelman disease associated with growth hormone deficiency, disturbances in vasopressin secretion and empty sella: a new hereditary renal tubular-pituitary syndrome?

Gitelman disease was diagnosed in two unrelated children with hypokalemic metabolic alkalosis and growth failure (a boy and a girl aged 7 mo and 9.5 y, respectively, at clinical presentation) on the basis of mutations detected in the gene encoding the thiazide-sensitive NaCl cotransporter of the distal convoluted tubule. GH deficiency was demonstrated by specific diagnostic tests in both children. Hypertonic saline infusion tests showed a partial vasopressin deficiency in the girl and delayed secretion of this hormone in the boy. Magnetic resonance imaging revealed an empty sella in both cases. Up to now, hypomagnesemia and hypocalciuria have been considered obligatory criteria for the diagnosis of Gitelman disease; however, our two patients had hypomagnesemia and hypocalciuria in less than half the determinations. GH replacement treatment was associated with a good clinical response in both children. It appears that these cases represent a new phenotype, not previously described in Gitelman disease, and that the entity may be considered a new complex hereditary renal tubular-pituitary syndrome.