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

Mechanisms coupling sodium and magnesium reabsorption in the distal convoluted tubule of the kidney

Hypomagnesaemia is a common feature of renal Na+ wasting disorders such as Gitelman and EAST/SeSAME syndrome. These genetic defects specifically affect Na+ reabsorption in the distal convoluted tubule, where Mg2+ reabsorption is tightly regulated. Apical uptake via TRPM6 Mg2+ channels and basolateral Mg2+ extrusion via a putative Na+ -Mg2+ exchanger determines Mg2+ reabsorption in the distal convoluted tubule. However, the mechanisms that explain the high incidence of hypomagnesaemia in patients with Na+ wasting disorders of the distal convoluted tubule are largely unknown. In this review, we describe three potential mechanisms by which Mg2+ reabsorption in the distal convoluted tubule is linked to Na+ reabsorption. First, decreased activity of the thiazide-sensitive Na+ /Cl- cotransporter (NCC) results in shortening of the segment, reducing the Mg2+ reabsorption capacity. Second, the activity of TRPM6 and NCC are determined by common regulatory pathways. Secondary effects of NCC dysregulation such as hormonal imbalance, therefore, might disturb TRPM6 expression. Third, the basolateral membrane potential, maintained by the K+ permeability and Na+ -K+ -ATPase activity, provides the driving force for Na+ and Mg2+ extrusion. Depolarisation of the basolateral membrane potential in Na+ wasting disorders of the distal convoluted tubule may therefore lead to reduced activity of the putative Na+ -Mg2+ exchanger SLC41A1. Elucidating the interconnections between Mg2+ and Na+ transport in the distal convoluted tubule is hampered by the currently available models. Our analysis indicates that the coupling of Na+ and Mg2+ reabsorption may be multifactorial and that advanced experimental models are required to study the molecular mechanisms.

Gitelman syndrome combined with growth hormone deficiency: Three cases report

RATIONALE

Gitelman syndrome (GS) is a rare autosomal recessive hereditary salt-losing tubulopathy caused by loss-of-function mutations in the SLC12A3 gene. It is usually characterized by hypokalemia, metabolic alkalosis, hypomagnesemia, and hypocalciuria. There are only a few reports on GS combined with growth hormone deficiency (GHD).

PATIENT CONCERNS

Three patients presented with weakness, spasm, and growth retardation, respectively.

DIAGNOSES

GS was diagnosed based on the clinical symptoms, laboratory test results, and genetic analysis. GH stimulation tests were performed when the magnesium level returned to normal under magnesium oxide (MgO) therapy.

INTERVENTIONS

Initially, all patients received oral replacement of MgO and potassium chloride, and 2 of them received simultaneous spironolactone therapy. Recombinant human growth hormone (rhGH) therapy was initiated after they were diagnosed with GHD.

OUTCOMES

All 3 patients exhibited satisfactory growth velocity and normal serum magnesium level, although the potassium level was still slightly lower than normal.

LESSONS

We suggest that all GS patients should undergo genetic evaluation, especially regarding SLC12A3 gene mutation. GHD should be considered if these patients have short stature. rhGH therapy is useful for stimulating the patients' growth, and it may increase the serum magnesium level.

Growth hormone deficiency in children with antenatal Bartter syndrome

Background Bartter syndrome is a group of rare autosomal-recessive renal disorders characterized by hypokalemic hypochloremic metabolic alkalosis associated with severe growth failure; the exact causes for growth retardation are unclear. GH deficiency (GHD) has been reported in a few cases of Bartter syndrome. The aim of our study was to determine the prevalence of GHD in children with antenatal Bartter syndrome and to assess their response to GH therapy. Methods Ten patients aged 1.5-14.5 years and diagnosed with antenatal Bartter syndrome were enrolled. Seven children with short stature underwent GH stimulation tests. Results Common presenting symptoms were failure to thrive and polyuria. The mean patient height at study entry was -2.7 standard deviation (SD) (range 0.89 to -5.95) and mean weight (SD) was -1.7 (range 1.89 to -4.11). A decline in height and weight (SD) was observed over the years. GHD was diagnosed in four children and GH therapy was started in all of them. Two patients responded very well and gained >1 SD in height, one patient stopped therapy due to non-adherence and one had a poor response. Conclusions In addition to other important causes for poor growth in antenatal Bartter syndrome, our findings suggest that GHD should also be considered as a cause of growth retardation and therefore, clinical assessment of the GH axis is recommended. GH therapy has a role in the treatment of growth failure in some individuals with Bartter syndrome.

Gitelman Syndrome: A Rare Cause of Seizure Disorder and a Systematic Review

Gitelman syndrome is one of the few inherited causes of metabolic alkalosis due to salt losing tubulopathy. It is caused by tubular defects at the level of distal convoluted tubules, mimicking a thiazide-like tumor. It usually presents in late childhood or in teenage as nonspecific weakness, fatigability, polyuria, and polydipsia but very rarely with seizures. It is classically associated with hypokalemia, hypomagnesemia, hypocalciuria, hyperreninemia, and hyperaldosteronism. However, less frequently, it can present with normal magnesium levels. It is even rarer to find normomagnesemic patients of GS who develop seizures as the main complication since hypomagnesemia is considered the principal etiology of abnormal foci of seizure-related brain activity in GS cases. Interestingly, patients with GS are oftentimes diagnosed during pregnancy when the classic electrolyte pattern consistent with GS is noticed. Our case presents GS with normal serum magnesium in a patient, with seizures being the main clinical presentation. We also did a comprehensive literature review of 122 reported cases to show the prevalence of normal magnesium in GS cases and an overview of clinical and biochemical variability in GS. We suggest that further studies and in-depth analysis are required to understand the pathophysiology of seizures in GS patients with both normal and low magnesium levels.

Classic Bartter syndrome complicated with profound growth hormone deficiency: a case report

INTRODUCTION

Classic Bartter syndrome is a salt-wasting tubulopathy caused by mutations in the CLCNKB (chloride channel Kb) gene. Although growth hormone deficiency has been suggested as a cause for persistent growth failure in patients with classic Bartter syndrome, in our opinion the diagnoses of growth hormone deficiency has been unconvincing in some reports. Moreover, Gitelman syndrome seems to have been confused with Bartter syndrome in some cases in the literature. In the present work, we describe a new case with CLCNKB gene mutations and review the reported cases of classic Bartter syndrome associated with growth hormone deficiency.

CASE PRESENTATION

Our patient was a Japanese boy diagnosed as having classic Bartter syndrome at eight months of age. The diagnosis of Bartter syndrome was confirmed by CLCNKB gene analysis, which revealed compound heterozygous mutations with deletion of exons 1 to 3 (derived from his mother) and ΔL130 (derived from his father). His medical therapy consisted of potassium (K), sodium chloride, spironolactone, and anti-inflammatory agents; this regime was started at eight months of age. Our patient was very short (131.1cm, -4.9 standard deviation) at 14.3 years and showed profoundly impaired growth hormone responses to pharmacological stimulants: 0.15μg/L to insulin-induced hypoglycemia and 0.39μg/L to arginine. His growth response to growth hormone therapy was excellent.

CONCLUSIONS

The present case strengthens the association between classic Bartter syndrome and growth hormone deficiency. We propose that growth hormone status should be considered while treating children with classic Bartter syndrome.

Osteoporotic fractures and persistent non-fusion of the hand epiphyses caused by empty sella syndrome in an adult: a case report

We report a case of primary empty sella syndrome (ESS) resulting in osteoporotic fractures and persistent non-fusion of the hand epiphyses, and discuss the potential pathogenesis of this disease. A 41-year-old man presented with pain in the right hand and back after a fall. X-radiographs revealed persistent epiphyses and severe osteoporosis. Serum phosphorus and prolactin levels were above normal levels, and free triiodothyronine, free thyroxine and testosterone levels were below normal limits. Magnetic resonance imaging of the head revealed empty sella. A lumbar bone mineral density examination indicated severe osteoporosis. ESS caused a systemic hormone disorder in this patient, resulting in osteoporotic fractures and persistent non-fusion of the hand epiphyses. Possible causes of this anomaly are chronic or congenital abnormities of the pituitary gland.

Gitelman syndrome combined with complete growth hormone deficiency

Gitelman syndrome is a rare autosomal recessive hereditary salt-losing tubulopathy, that manifests as hypokalemic metabolic alkalosis, hypomagnesemia, and hypocalciuria. It is caused by mutations in the solute carrier family 12(sodium/chloride transporters), member 3 (SLC12A3) gene encoding the thiazide-sensitive sodium chloride cotransporter channel (NCCT) in the distal convoluted tubule of the kidney. It is associated with muscle weakness, cramps, tetany, vomiting, diarrhea, abdominal pain, and growth retardation. The incidence of growth retardation, the exact cause of which is unknown, is lower than that of Bartter syndrome. Herein, we discuss the case of an overweight 12.9-year-old girl of short stature presenting with hypokalemic metabolic alkalosis. The patient, on the basis of detection of a heterozygous mutation in the SLC12A3 gene and poor growth hormone (GH) responses in two provocative tests, was diagnosed with Gitelman syndrome combined with complete GH deficiency. GH treatment accompanied by magnesium oxide and potassium replacement was associated with a good clinical response.

Gitelman's syndrome with panhypopituitarism: Reno-endocrine interplay

Gitelman's syndrome is an inherited tubulopathy affecting thiazide-sensitive sodium chloride cotransporter, which manifests with hypokalemic alkalosis, hypomagnesemia, and hypocalciuria. Recently few cases have been described having an association of Gitelman's syndrome with pituitary abnormalities on imaging, though with normal hormonal status. We describe the first case of an adult patient having Gitelman's syndrome and hypopituitarism with abnormal pituitary imaging. She presented to us with hypotension, hypokalemia, hypomagnesemia with alkalosis, hypothyroidism, hypocortisolism, and hypogonadism. She was treated with replacement of electrolytes and hormones, to which she showed an excellent response.

Gitelman syndrome in Gypsy paediatric patients carrying the same intron 9 + 1 G>T mutation. Clinical features and impact on quality of life

BACKGROUND

Gitelman syndrome is a primary tubular disorder causing hypokalaemic metabolic alkalosis with hypocalciuria. Its prevalence is high in Gypsies, who harbour an identical mutation, intron 9 + 1 G>T, in the SLC12A3 gene.

METHODS

To better define the Gitelman syndrome in Gypsies, the clinical and biochemical features of 34 Spanish paediatric Gypsy patients were analysed. At diagnosis, symptoms, height and weight as well as serum and urinary biochemical data were collected. During a follow-up of 4.5 ± 2.4 years [X ± standard deviation (SD)], therapy, treatment compliance, symptoms, frequency of hospital admissions and, at the last visit, growth and biochemical work-up of 29 patients followed for at least 6 months were analysed. Quality of life items were also assessed by a questionnaire.

RESULTS

Muscle cramps (41%) and asthenia (35%) were the most frequent presenting symptoms. Biochemical data at diagnosis were serum K 2.76 ± 0.46 mEq/L, serum Mg 1.32 ± 0.28 mg/dL, blood pH 7.45 ± 0.06, serum bicarbonate 28.2 ± 2.9 mEq/L, urinary calcium/creatinine ratio 0.03 ± 0.04 mg/mg, fractional K excretion 24.4 ± 17.1% and fractional Mg excretion 8.9 ± 8.3%. During follow-up, Mg and K supplements were prescribed to 79 and 86% of patients, respectively; compliance with treatment was good in 35%. Hospital admission rate was 0.03/patient/month. Muscle cramps were the symptom most often referred by the patients (45%) during the follow-up, and 71% of patients considered their health status as excellent or good. Twenty-one patients stated that their disease did not adversely interfere with their mood or social relationships. Height and weight of patients at diagnosis were -0.60 ± 1.17 and -0.49 ± 1.32 SD, respectively, and improved to -0.44 ± 1.28 (P < 0.05) and 0.18 ± 1.79 SD (P < 0.01) at the last visit.

CONCLUSIONS

Gypsy children with Gitelman syndrome mostly exhibit muscle symptoms and asthenia although the disease is not particularly severe in this ethnic group. Body growth improves with treatment and close follow-up.

Longitudinal growth in chronic hypokalemic disorders

Growth retardation remains a major complication in children with primary tubular disorders, despite adequate supplemental treatment with electrolytes, water and bicarbonate. Chronic hypokalemia, characteristic of some tubulopathies, impairs growth by mechanisms that are not well known. Association with growth hormone deficiency has been reported in patients with Bartter's or Gitelman's syndrome. Tissue-specific alterations of growth hormone and insulin-like growth factor I axis have been described in experimental models of potassium depletion. Hypokalemic rats gain less body length and weight than pair-fed normokalemic animals and, by contrast, develop renal hypertrophy. These rats have low circulating concentrations of insulin-like growth factor I, depressed messenger ribonucleic acid (mRNA) levels of this peptide in the tibial growth plate, and they are resistant to the longitudinal growth-promoting effects of exogenous growth hormone. The reason for this resistance remains to be defined. No alterations in the intracellular signaling for growth hormone have been found in the liver of hypokalemic rats. However, treatment with high doses of growth hormone is unable to normalize hypertrophy of the epiphyseal cartilage chondrocytes, which are severely disturbed in potassium depletion and likely play an important role in the pathogenia of growth impairment in this condition.

A patient with Bartter syndrome accompanying severe growth hormone deficiency and focal segmental glomerulosclerosis

Bartter syndrome is a rare autosomal recessive, salt-losing disorder characterized by hypokalemic hypochloremic metabolic alkalosis. A 10-year-old boy had severe growth retardation (height standard deviation score -8.15). He had a thin, triangular face, prominent ears and forehead, and big eyes. Megacystis, bilateral hydroureteronephrosis, and residual urine were detected in ultrasonography, but there was no vesicoureteral reflux. Lumbosacral magnetic resonance (MR) showed posterior disc bulging at L4-5. Serum sodium and chloride levels were normal, but mild hypokalemia was overlooked initially. During follow-up, hypokalemic hypochloremic metabolic alkalosis developed, with high urinary chloride and potassium excretion (52 and 43 mEq/L, respectively). The patient, with renal salt loss, was thought to have classic Bartter syndrome due to absence of nephrocalcinosis, presence of persistent hypercalciuria and sensorineural deafness, and presence of relatively mild clinical and laboratory findings, except polyuria initially. The child was treated with indomethacin, spironolactone, and oral potassium in addition to growth hormone (GH). During treatment, he had considerable increase in weight and height compared with the period of GH therapy only. We present this case because, although growth retardation is a major feature of Bartter syndrome, associated GH deficiency is rarely reported in the literature. Diagnosis of Bartter syndrome was made later, as our patient was followed for megacystis and megaureter secondary to the neurogenic bladder and GH deficiency initially; and proteinuria associated with focal segmental glomerulosclerosis responded to treatment for Bartter syndrome.

Alterations of growth plate and abnormal insulin-like growth factor I metabolism in growth-retarded hypokalemic rats: effect of growth hormone treatment

Hypokalemic tubular disorders may lead to growth retardation which is resistant to growth hormone (GH) treatment. The mechanism of these alterations is unknown. Weaning female rats were grouped ( n = 10) in control, potassium-depleted (KD), KD treated with intraperitoneal GH at 3.3 mg·kg−1·day−1 during the last week (KDGH), and control pair-fed with KD (CPF). After 2 wk, KD rats were growth retarded compared with CPF rats, the osseous front advance (±SD) being 67.07 ± 10.44 and 81.56 ± 12.70 μm/day, respectively. GH treatment did not accelerate growth rate. The tibial growth plate of KD rats had marked morphological alterations: lower heights of growth cartilage (228.26 ± 23.58 μm), hypertrophic zone (123.68 ± 13.49 μm), and terminal chondrocytes (20.8 ± 2.39 μm) than normokalemic CPF (264.21 ± 21.77, 153.18 ± 15.80, and 24.21 ± 5.86 μm). GH administration normalized these changes except for the distal chondrocyte height. Quantitative PCR of insulin-like growth factor I (IGF-I), IGF-I receptor, and GH receptor genes in KD growth plates showed downregulation of IGF-I and upregulation of IGF-I receptor mRNAs, without changes in their distribution as analyzed by immunohistochemistry and in situ hybridization. GH did not further modify IGF-I mRNA expression. KD rats had normal hepatic IGF-I mRNA levels and low serum IGF-I values. GH increased liver IGF-I mRNA, but circulating IGF-I levels remained reduced. This study discloses the structural and molecular alterations induced by potassium depletion on the growth plate and shows that the lack of response to GH administration is associated with persistence of the disturbed process of chondrocyte hypertrophy and depressed mRNA expression of local IGF-I in the growth plate.

Reduced urinary excretion of thiazide-sensitive Na-Cl cotransporter in Gitelman syndrome: preliminary data

BACKGROUND

The relationship between SLC12A3 mutations and actual sodium-chloride (Na-Cl) cotransporter (NCC) expression in patients with Gitelman syndrome (GS) was rarely evaluated. Detection of urinary thiazide-sensitive NCC was not tried in patients with GS.

STUDY DESIGN

Case series.

SETTING & PARTICIPANTS

6 patients with GS and 1 patient with surreptitious vomiting.

OUTCOMES & MEASUREMENTS

Renal clearance study, mutation analysis using reverse-transcription polymerase chain reaction and direct sequencing for the SLC12A3 gene, and immunohistochemical staining for NCC, Na-K-2Cl-cotransporter, alpha1-subunit of Na(+),K(+)-ATPase, and calbindin-D(28K) of the renal biopsy specimens were performed. Membrane fractions of urine were obtained by using differential centrifugation and probed with antibodies against human NCC and aquaporin 2.

RESULTS

Results of clearance studies were consistent with GS, showing decreased distal fractional chloride reabsorption with only furosemide. SLC12A3 gene mutations were found in all patients with GS. Immunohistochemistry showed markedly decreased NCC expression in the distal convoluted tubule, whereas expression of other transporters remained intact. Urinary NCC excretion was markedly decreased in patients with GS, but not in the patient with surreptitious vomiting.

LIMITATIONS

Small number of patients and lack of mutation analysis of CLCNKB.

CONCLUSIONS

There were no relations between NCC expression and types of mutations. Detection of urinary NCC might be helpful for the differential diagnosis of GS.

Simultaneous mutations in the CLCNKB and SLC12A3 genes in two siblings with phenotypic heterogeneity in classic Bartter syndrome

Two siblings (brother and sister) with renal tubular hypokalemic alkalosis underwent clinical, biochemical and molecular investigations. Although the biochemical findings were similar (including hypokalemia, metabolic alkalosis, hyperreninemia, hyperaldosteronism and normal blood pressure), the clinical findings were different: the boy, who also presented syndromic signs, developed glomerular proteinuria and renal biopsy revealed focal segmental glomerular sclerosis; the girl showed the typical signs of classic Bartter syndrome. As described in a previous paper, a heterozygous mutation (frameshift 2534delT) was demonstrated in the gene encoding the thiazide-sensitive NaCl co-transporter (SLC12A3) of the distal convoluted tubule; the second molecular analysis revealed a compound heterozygous mutation (A61D/V149E) in the CLCNKB chloride channel gene in both subjects, inherited in trans from the parents. The children were finally diagnosed as having classic Bartter syndrome. These cases represent the first report of the simultaneous presence of heterozygous and compound heterozygous mutations in the SLC12A3 and CLCNKB genes, both of which are involved in renal salt losing tubulopathies, and confirm previous observations regarding classic Bartter syndrome phenotype variability in the same kindred.

A new mutation (intron 9 +1 G>T) in the SLC12A3 gene is linked to Gitelman syndrome in Gypsies

BACKGROUND

Gitel syndrome is an inherited tubular disorder characterized by metabolic alkalosis, hypokalemia, and hypomagnesemia of renal origin and hypocalciuria. The majority of patients with Gitelman syndrome carry inactivating mutations in the SLC12A3 gene encoding the sodium-chloride cotransporter located in the distal convoluted tubule. The purpose of this study was to investigate the underlying mutation in Gitelman syndrome patients of Gypsy race from different geographic origin.

METHODS

Twenty Gypsy patients with clinical and biochemical features of Gitelman syndrome were investigated by mutational analysis. The patients belonged to 12 unrelated Gypsy families living in four different European countries. The parents and unaffected siblings of each patient, as well as the DNA of a population of 200 healthy control patients, were also analyzed.

RESULTS

All patients were homozygous for the same splice site mutation, guanine to thymine in the first position of intron 9 of SLC12A3 gene. This mutation was not found in the control population. Parents were heterozygous for the mutation. Despite sharing a common mutation, the clinical manifestations of the syndrome in the patients varied from lack of symptoms in six children to severe growth retardation in four.

CONCLUSION

Demonstration of a novel point mutation within the SLC12A3 gene in our cohort of Gypsy families with Gitelman syndrome is highly suggestive of a founder effect. This finding will facilitate the identification of the genetic defect in further cases of Gitelman syndrome among the Gypsy population. Our study represents the largest series ever published of patients with Gitelman syndrome having the same underlying mutation, and supports the lack of correlation between genotype and clinical phenotype in this disease.

Pharmacotyping of hypokalaemic salt-losing tubular disorders

Long standing confusion exists in the terminology of hypokalaemic salt-losing tubulopathies (SLTs). SLTs are autosomal recessively transmitted and characterized by normotensive secondary hyperreninism/hyperaldosteronism with hypokalaemic metabolic alkalosis. Historically, four phenotypical variants have been described: (1) the (classic) Bartter syndrome (cBS), (2) the hypomagnesaemic hypocalciuric Gitelman syndrome (GS), (3) the hypercalciuric hyperprostaglandin-E-syndrome (HPS) or antenatal Bartter syndrome (aBS) and (4) the hyperprostaglandin-E-syndrome with sensorineural deafness (HPS + SND). The latter two syndromes are the most severe variants with antenatal manifestation with polyhydramnios and life-threatening course of salt- and water-loss. Defects in five renal membrane proteins involved in electrolyte reabsorption have been identified: In HPS-patients mutations in (1) either the furosemide-sensitive sodium-potassium-chloride cotransporter NKCC2, or (2) in the potassium channel ROMK have been identified, and (3) HPS + SND is caused by mutations in the beta-subunit of the chloride channels ClC-Kb and -Ka (named barttin), all mimicking the major pharmacological effects of furosemide with minor potassium-wasting in ROMK-patients as seen in patients treated with simultaneous furosemide and amiloride, and minor calcium-wasting in Barttin-patients resembling the combination of furosemide and thiazides. (4) cBS is caused by mutations in the chloride channel ClC-Kb with similar clinical characteristics as seen under combination of thiazides and furosemide, (5) GS is caused by mutations in the thiazide-sensitive sodium-chloride cotransporter NCCT resembling the effect of long-term thiazide administration.

CONCLUSION

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.

Depressive state and paresthesia dramatically improved by intravenous MgSO4 in Gitelman's syndrome

A 69-year-old woman was referred to our department for evaluation of hypokalemia, which had been treated by oral potassium for more than ten years. She complained of headache, knee joint pain, sleeplessness and paresthesia in extremities and, most prominently, depression. Laboratory data suggested Gitelman's syndrome, which is caused by mutations in the gene encoding the thiazide-sensitive Na-Cl cotransporter. Direct sequencing of the gene in this patient revealed homozygous mutation R964Q in exon 25. Intravenous supplement of MgSO4 dramatically improved both the depression and the paresthesia, suggesting that hypomagnesemia played a role in the clinical manifestations.

Hereditary disorders of potassium homeostasis

There has been a dramatic recent increase in the understanding of the renal epithelial transport systems with the identification, cloning and characterization of a large number of membrane transport proteins. The aim of this chapter is to integrate this body of knowledge with the understanding of the clinical disorders that accompany gain, loss or dysregulation of function of these transport systems. The specific focus is on the best-defined human clinical syndromes in which there are derangements in potassium (K(+)) homeostasis. The focus is on inherited syndromes, rather than on acquired syndromes due to tubular transport defects, and the therapeutic approaches address chronic derangements of K(+) homeostasis rather than acute interventions directed at life-threatening hyperkalaemia.

Renal genetic disorders related to K+ and Mg2+

The recent knowledge of the renal epithelial transport systems has exploded with the identification, cloning, and characterization of a large number of membrane transport proteins. The fundamental aspects of these transporters are beginning to emerge at the molecular level and are summarized in the accompanying contributions in this volume of the Annual Review of Physiology. The aim of my review is to integrate this body of knowledge with the understanding of the clinical disorders of human mineral homeostasis that accompany gain, loss, or dysregulation of function of these transport systems. The specific focus is on the best defined human clinical syndromes in which there are derangements in K(+) and Mg(2+) homeostasis.

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.

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.

Inherited disorders of renal magnesium handling

The genetic basis and cellular defects of a number of primary magnesium wasting diseases have been elucidated over the past decade. This review correlates the clinical pathophysiology with the primary defect and secondary changes in cellular electrolyte transport. The described disorders include (1) hypomagnesemia with secondary hypocalcemia, an earlyonset, autosomal-recessive disease segregating with chromosome 9q12-22.2; (2) autosomal-dominant hypomagnesemia caused by isolated renal magnesium wasting, mapped to chromosome 11q23; (3) hypomagnesemia with hypercalciuria and nephrocalcinosis, a recessive condition caused by a mutation of the claudin 16 gene (3q27) coding for a tight junctional protein that regulates paracellular Mg(2+) transport in the loop of Henle; (4) autosomal-dominant hypoparathyroidism, a variably hypomagnesemic disorder caused by inactivating mutations of the extracellular Ca(2+)/Mg(2+)-sensing receptor, CASR: gene, at 3q13.3-21 (a significant association between common polymorphisms of the CASR: and extracellular Mg(2+) concentration has been demonstrated in a healthy adult population); and (5) Gitelman syndrome, a recessive form of hypomagnesemia caused by mutations in the distal tubular NaCl cotransporter gene, SLC12A3, at 16q13. The basis for renal magnesium wasting in this disease is not known. These inherited conditions affect different nephron segments and different cell types and lead to variable but increasingly distinguishable phenotypic presentations. No doubt, there are in the general population other disorders that have not yet been identified or characterized. The continued use of molecular techniques to probe the constitutive and congenital disturbances of magnesium metabolism will increase the understanding of cellular magnesium transport and provide new insights into the way these diseases are diagnosed and managed.