Angiotensin II type 1 receptor gene abnormality in a patient with Bartter's syndrome

Rho kinase and PAI-1 in Bartter's/Gitelman's syndromes

OBJECTIVE

Angiotensin II (Ang II)-mediated activation of Rho kinase (ROK) is involved in the pathophysiology of hypertension and cardiovascular remodeling. ROK also controls plasminogen activator inhibitor-1 (PAI-1) which promotes vascular fibrosis contributing to atherogenesis. Bartter's and Gitelman's syndromes (BS/GS) are useful models to investigate abnormalities of vascular tone regulation, due to their reduced short- and long-term signaling pathways of Ang II. This study evaluated, using BS/GS as a model, ROK and PAI-1 gene and protein expression and the effect of Ang II co-incubation on ROK and PAI-1 gene and protein expression.

DESIGN, METHODS AND RESULTS

We measured ROK and PAI-1 gene and protein expression [reverse transcription-polymerase chain reaction (RT-PCR) and Western blot] in mononuclear cells (PBM) from one BS and eight GS patients. The effect of Ang II on ROK and PAI-1 gene and protein expression was also evaluated and compared with 10 controls. ROK gene and protein expression was reduced in BS/GS [0.47 +/- 0.11 densitometric units (d.u.) versus 0.70 +/- 0.04 d.u., P = 0.0038 and 0.39 +/- 0.07 d.u. versus 0.55 +/- 0.07 d.u., P = 0.0026, respectively]. The basal level of PAI-1 gene and protein expression did not differ (0.40 +/- 0.03 d.u. versus 0.39 +/- 0.02 d.u. and 0.81 +/- 0.02 d.u. versus 0.83 +/- 0.02 d.u., respectively). Ang II increased ROK and PAI-1 gene and protein expression only in controls: from 0.70 +/- 0.04 to 0.90 +/- 0.06 d.u., P = 0.007 (ROK mRNA); from 0.55 +/- 0.07 to 0.86 +/- 0.07 d.u., P = 0.0005 (ROK protein); from 0.40 +/- 0.02 to 0.63 +/- 0.03 d.u., P = 0.001 (PAI-1 mRNA); and from 0.83 +/- 0.02 to 1.34 +/- 0.16 d.u., P = 0.0023 (PAI-1 protein).

CONCLUSIONS

This study confirms BS/GS as a human model to investigate interrelated systems involved in the pathophysiology of hypertension and throws more light on the cellular mechanisms of BS/GS reduced Ang II short- and long-term signaling pathways.

Bartter syndrome complicated by immune complex nephropathy. Case report and literature review

The unusual coincidence of Bartter syndrome and C1q nephropathy is described and the literature reviewed. An African-American girl presented at 4 years of age with acute hyponatremic dehydration and failure to thrive. Persistent hypokalemic alkalosis and secondary hyperaldosteronism were found. The case was atypical for Bartter syndrome in that proteinuria (0.19 g/day) was present. Renal biopsy showed juxtaglomerular hyperplasia and C1q nephropathy. Molecular analysis showed deletion of the renal chloride channel gene (CLCNKB) typical of autosomal recessive childhood Bartter syndrome. Chronic sodium and potassium chloride replacement therapy together with indomethacin normalized her metabolic status, and she experienced catch-up growth. Proteinuria persisted, however. This is the first documentation of C1q nephropathy, in mild form, complicating autosomal recessive Bartter syndrome. This case shows the importance of the renal biopsy and of molecular analysis in delineating the cause of atypical nephropathy associated with Bartter syndrome. These findings add to the evidence of a possible association between the congenital syndrome and acquired immune complex nephropathy.

Inherited primary renal tubular hypokalemic alkalosis: a review of Gitelman and Bartter syndromes

Inherited hypokalemic metabolic alkalosis, or Bartter syndrome, comprises several closely related disorders of renal tubular electrolyte transport. Recent advances in the field of molecular genetics have demonstrated that there are four genetically distinct abnormalities, which result from mutations in renal electrolyte transporters and channels. Neonatal Bartter syndrome affects neonates and is characterized by polyhydramnios, premature delivery, severe electrolyte derangements, growth retardation, and hypercalciuria leading to nephrocalcinosis. It may be caused by a mutation in the gene encoding the Na-K-2Cl cotransporter (NKCC2) or the outwardly rectifying potassium channel (ROMK), a regulator of NKCC2. Classic Bartter syndrome is due to a mutation in the gene encoding the chloride channel (CLCNKB), also a regulator of NKCC2, and typically presents in infancy or early childhood with failure to thrive. Nephrocalcinosis is typically absent despite hypercalciuria. The hypocalciuric, hypomagnesemic variant of Bartter syndrome (Gitelman syndrome), presents in early adulthood with predominantly musculoskeletal symptoms and is due to mutations in the gene encoding the Na-Cl cotransporter (NCCT). Even though our understanding of these disorders has been greatly advanced by these discoveries, the pathophysiology remains to be completely defined. Genotype-phenotype correlations among the four disorders are quite variable and continue to be studied. A comprehensive review of Bartter and Gitelman syndromes will be provided here.

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.

The pathophysiological and molecular basis of Bartter's and Gitelman's syndromes

Molecular defects affecting the transport of sodium, potassium and chloride in the nephron through the ROMK K+ channel, Na+/K+/2Cl- cotransporter, the Na+/Cl- cotransporter and chloride channel have been identified in patients with Bartter's and Gitelman's syndromes. Defects of the angiotensin II type I receptor and CFTR have also being described. These defects are simple (i.e., most are single amino acid substitutions) but affect key elements in tubular transport. The simplicity of the genetic defects may explain why the inheritance of these conditions remains unclear in most kindreds (i.e., not just recessive or dominant) and emphasises the crucial importance of the conformational structure of these channels. Application of this molecular information will allow the early genetic identification of patients with these syndromes and enable us to differentiate between the various disorders at a functional level. It may also identify a subgroup in which the heterozygous form may make patients potentially exquisitely sensitive to diuretics.

Bartter syndrome: unraveling the pathophysiologic enigma

Familial hypokalemic, hypochloremic metabolic alkalosis, or Bartter syndrome, is not a single disorder but rather a set of closely related disorders. These Bartter-like syndromes share many of the same physiologic derangements, but differ with regard to the age of onset, the presenting symptoms, the magnitude of urinary potassium (K) and prostaglandin excretion, and the extent of urinary calcium excretion. At least three clinical phenotypes have been distinguished: (1) classic Bartter syndrome; (2) the hypocalciuric-hypomagnesemic Gitelman variant; and (3) the antenatal hypercalciuric variant (also termed hyperprostaglandin E syndrome). The fundamental pathogenesis of this complex set of disorders has long fascinated and stymied investigators. Physiologic investigations have suggested numerous pathogenic models. The cloning of genes encoding renal transport proteins has provided molecular tools to begin testing these hypotheses. To date, molecular genetic analyses have determined that mutations in the gene encoding the thiazide-sensitive sodium-chloride (Na-Cl) cotransporter underlie the pathogenesis of the Gitelman variant. In comparison, the antenatal variant is genetically heterogeneous with mutations in the genes encoding either the bumetanide-sensitive sodium-potassium-chloride (Na-K-2Cl) cotransporter or the luminal, ATP-regulated, K channel. With these data, investigators have begun to unravel the pathophysiologic enigma of the Bartter-like syndromes. Further studies will help refine pathogenic models for this set of disorders as well as provide new insights into the normal mechanisms of renal electrolyte transport.

Linkage of infantile Bartter syndrome with sensorineural deafness to chromosome 1p

Bartter syndrome (BS) is a family of disorders manifested by hypokalemic hypochloremic metabolic alkalosis with normotensive hyperreninemic hyperaldosteronism. We evaluated a unique, inbred Bedouin kindred in which sensorineural deafness (SND) cosegregates with an infantile variant of the BS phenotype. Using a DNA-pooling strategy, we screened the human genome and successfully demonstrated linkage of this unique syndrome to chromosome 1p31. The genes for two kidney-specific chloride channels and a sodium/hydrogen antiporter, located near this region, were excluded as candidate genes. Although the search for the disease-causing gene in this family continues, this linkage further demonstrates the genetic heterogeneity of BS. In addition, the cosegregation of these phenotypes allows us to postulate that a single genetic alteration may be responsible for the SND and the BS phenotype. The identification and characterization of this gene would lead to a better understanding of the normal physiology of the kidney and the inner ear.

Hypokalemic metabolic alkalosis with hypomagnesuric hypermagnesemia and severe hypocalciuria: a new syndrome?

Bartter's and Gitelman's syndromes are characterized by hypokalemia, urinary potassium wasting, elevated plasma renin activity and aldosterone levels, normotension, and prostaglandinuria. They differ in that hypomagnesemia and hypocalciuria are universal in Gitelman's syndrome; 20% of cases of Bartter's syndrome have hypomagnesemia and hypercalciuria. We present a 44-year-old white man referred for hypokalemia. Clinical evaluation was unremarkable. He had hypokalemia (P(K), 2.8 to 3.0 mEq/L), hypochloremic metabolic alkalosis, mild azotemia (serum creatinine, 1.4 to 1.8 mg/dL; creatinine clearance, 59 mL/min), normocalcemia, marked persistent hypocalciuria (FE(Ca), 0.08% to 0.09%), and normal intact parathyroid hormone levels (51 pg/mL) and glucosuria. He had persistent hypermagnesemia (P(Mg), 2.1 to 2.8 mEq/L) with relative hypomagnesuria (FE(Mg), 3.2% to 5.2%) given the level of renal impairment and hypermagnesemia. Supine plasma renin activity and aldosterone levels were high (11 ng/mL/hr and 43 ng/dL, respectively). An excessive dietary intake of magnesium, including medications, was excluded. Studies were performed after withdrawing all medications for 8 days. A maximum water diuresis was established (an oral load of 20 mL/kg; stable Uosm, 120 mOsm/kg), and free water and solute clearances were studied at baseline and after sequential intravenous injections of 125 mg chlorothiazide and 40 mg furosemide. The patient had moderate renal impairment (technetium diethylene triamine pentacetic acid [DTPA] clearance, 35.4 mL/min/1.73 m2) and, in contradistinction to Bartter's and Gitelman's syndromes, sodium and water handling in the thick ascending limb of the loop of Henle and the distal tubule (fractional distal solute reabsorption) was normal, but there was evidence of a defect in the proximal tubule reabsorption (glucosuria, supranormal C(H2O) and high distal delivery). Hypomagnesuria and hypocalciuria appeared to be secondary to an increase in their absorption in the loop of Henle (increased excretion following furosemide). In conclusion, this combination of metabolic abnormalities has never been described. We postulate a proximal tubular defect in the absorption of NaCl leading to hypocalciuria, hypomagnesuria, and potassium wasting. Whether the tubular defect is primary or secondary to a renal parenchymal disease is, however, unclear.

Impaired response to furosemide in hyperprostaglandin E syndrome: evidence for a tubular defect in the loop of Henle

In hyperprostaglandin E syndrome (HPS) renal wasting of electrolytes and water is consistently associated with enhanced synthesis of prostaglandin E2. In contrast to Bartter or Gitelman syndrome (BS/GS), HPS is characterized by its severe prenatal manifestation, leading to fetal polyuria, development of polyhydramnios, and premature birth. This disorder mimics furosemide treatment with hypokalemic alkalosis, hypochloremia, isosthenuria, and impaired renal conservation of both calcium and magnesium. Therefore the thick ascending limb of the loop of Henle seems to be involved in HPS. To characterize the tubular defect we investigated the response to furosemide (2 mg/kg) in HPS (n = 8) and BS/GS (n = 3) 1 week after discontinuation of long-term indomethacin treatment. Sensitivity to furosemide was completely maintained in patients with BS/GS. The diuretic, saluretic, and hormonal responses were similar to those of a control group of healthy children (n = 13), indicating an intact function of the thick ascending limb of the loop of Henle in BS/GS. In contrast, patients with HPS had a marked resistance to this loop diuretic. Furosemide treatment increased urine output by 7.5 +/- 0.7 ml/kg per hour in healthy control subjects but only by 4.4 +/- 1.2 ml/kg per hour (p < 0.5) in children with HPS. In parallel, the latter also had a markedly impaired saluretic response (delta Cl(urine) 0.14 +/- 0.04 mmol/kg per hour vs 0.85 +/- 0.09 mmol/kg per hour, p < 0.001; delta Na(urine) 0.23 +/- 0.06 mmol/kg per hour vs 0.77 +/- 0.09 mmol/kg per hour, p < 0.001). Furosemide therapy further enhanced prostaglandin E2 excretion in patients with HPS (54 +/- 17 to 107 +/- 28 ng/hr per 1.73 m2, p < 0.05), whereas no significant effect was observed in healthy children (20 +/- 3 to 12 +/- 3 ng/hr per 1.73 m2). We conclude that a defect of electrolyte reabsorption in the thick ascending limb of the loop of Henle plays a major role in HPS.

Gitelman's variant of Bartter's syndrome, inherited hypokalaemic alkalosis, is caused by mutations in the thiazide-sensitive Na-Cl cotransporter

Maintenance of fluid and electrolyte homeostasis is critical for normal neuromuscular function. Bartter's syndrome is an autosomal recessive disease characterized by diverse abnormalities in electrolyte homeostasis including hypokalaemic metabolic alkalosis; Gitelman's syndrome represents the predominant subset of Bartter's patients having hypomagnesemia and hypocalciuria. We now demonstrate complete linkage of Gitelman's syndrome to the locus encoding the renal thiazide-sensitive Na-Cl cotransporter, and identify a wide variety of non-conservative mutations, consistent with loss of function alleles, in affected subjects. These findings demonstrate the molecular basis of Gitelman's syndrome. We speculate that these mutant alleles lead to reduced sodium chloride reabsorption in the more common heterozygotes, potentially protecting against development of hypertension.

Bartter's syndrome: the unsolved puzzle

Bartter's syndrome is a congenital abnormality characterized by metabolic alkalosis [corrected], hyperreninemic hyperaldosteronism, and hypokalemia. Most patients present early in life with symptoms such as muscle weakness and polyuria, which may be attributed to potassium depletion. Despite the hyperaldosteronism, the patients tend to be normotensive, which is at least partially explained by vascular hyporesponsiveness to pressor hormones. Numerous studies have documented increased renal excretion of prostaglandins. Several different patterns of aberrant renal ion transport have been observed in patients with the syndrome, suggesting that it actually may represent a family of related but distinct tubular disorders. Therapeutic approaches to Bartter's syndrome include potassium supplementation, prostaglandin synthesis inhibitors (nonsteroidal anti-inflammatory agents), aldosterone antagonists, and converting enzyme inhibitors. During the first two decades following its initial description, Bartter's syndrome was the focus of widespread interest, based on the likelihood that its investigation might provide insight into the normal functioning of the renin-angiotensin-aldosterone and prostanoid hormone systems. During the past decade, however, little additional progress has been made in Bartter's syndrome, and its patho-physiology remains poorly understood.