Raised CK and acute kidney injury following intense exercise in three patients with a history of exercise intolerance due to homozygous mutations in SLC2A9

Acute rhabdomyolysis (AR) leading to acute kidney injury has many underlying etiologies, however, when the primary trigger is exercise, the most usual underlying cause is either a genetic muscle disorder or unaccustomed intense exercise in a healthy individual. Three adult men presented with a history of exercise intolerance and episodes of acute renal impairment following intense exercise, thought to be due to AR in the case of two, and dehydration in one. The baseline serum CK was mildly raised between attacks in all three patients and acutely raised during attacks in two of the three patients. Following referral to a specialized neuromuscular centre, further investigation identified very low serum urate (<12 umol/L). In all three men, genetic studies confirmed homozygous mutations in SLC2A9, which encodes for facilitated glucose transporter member 9 (GLUT9), a major regulator of urate homeostasis. Hereditary hypouricaemia should be considered in people presenting with acute kidney injury related to intense exercise. Serum urate evaluation is a useful screening test best undertaken after recovery.

New SLC22A12 (URAT1) Variant Associated with Renal Hypouricemia Identified by Whole-Exome Sequencing Analysis and Bioinformatics Predictions

Renal hypouricemia (RHUC) is a rare hereditary disorder caused by loss-of-function mutations in the SLC22A12 (RHUC type 1) or SLC2A9 (RHUC type 2) genes, encoding urate transporters URAT1 and GLUT9, respectively, that reabsorb urate in the renal proximal tubule. The characteristics of this disorder are low serum urate levels, high renal fractional excretion of urate, and occasional severe complications such as nephrolithiasis and exercise-induced acute renal failure. In this study, we report two Spanish (Caucasian) siblings and a Pakistani boy with clinical characteristics compatible with RHUC. Whole-exome sequencing (WES) analysis identified two homozygous variants: a novel pathogenic SLC22A12 variant, c.1523G>A; p.(S508N), in the two Caucasian siblings and a previously reported SLC2A9 variant, c.646G>A; p.(G216R), in the Pakistani boy. Our findings suggest that these two mutations cause RHUC through loss of urate reabsorption and extend the SLC22A12 mutation spectrum. In addition, this work further emphasizes the importance of WES analysis in clinical settings.

Clinical significance of hypouricemia in children and adolescents

BACKGROUND

Although hyperuricemia is a widely studied condition with well-known effects on the kidneys, hypouricemia is usually considered a biochemical abnormality of no clinical significance despite the fact that it can be a sign or major finding of serious metabolic or genetic diseases affecting kidney health. In this study, we aimed to investigate and emphasize the clinical significance of hypouricemia.

METHODS

Patients were evaluated retrospectively for persistent hypouricemia defined as serum uric acid concentrations of < 2 mg/dL on at least 3 different occasions. According to the blood and urine uric acid (UA) levels, the patients were classified as having hypouricemia due to UA underproduction vs. overexcretion. Demographic, clinical, and genetic characteristics were noted for analysis.

RESULTS

Fourteen patients (n = 14; M/F 8/6) with persistent hypouricemia were identified. Hypouricemia due to underproduction was the cause of 42.8% of these cases. All of the patients with a uric acid level of 0 mg/dL (n = 4) had hypouricemia due to underproduction. The median serum uric acid level was 0.85 (0-1.6) mg/dL. Isolated hypouricemia and hypouricemia with metabolic acidosis were equally distributed. Among the patients with hypouricemia due to underproduction, the final diagnoses were xanthine dehydrogenase deficiency (n = 5) and alkaptonuria (n = 1). In the overexcretion group, the final diagnoses were nephropathic cystinosis (n = 6), distal renal tubular acidosis (n = 1), and hereditary renal hypouricemia (n = 1). The diagnostic lag was longer for patients with isolated hypouricemia compared to other patients (p = 0.001).

CONCLUSIONS

Hypouricemia may reflect underlying genetic or metabolic diseases, early diagnosis of which could help preserve kidney function. A higher resolution version of the Graphical abstract is available as Supplementary information.

Cubilin-, megalin-, and Dab2-dependent transcription revealed by CRISPR/Cas9 knockout in kidney proximal tubule cells

The multiligand receptors megalin (Lrp2) and cubilin (Cubn) and their endocytic adaptor protein Dab2 (Dab2) play essential roles in maintaining the integrity of the apical endocytic pathway of proximal tubule (PT) cells and have complex and poorly understood roles in the development of chronic kidney disease. Here, we used RNA-sequencing and CRISPR/Cas9 knockout (KO) technology in a well-differentiated cell culture model to identify PT-specific transcriptional changes that are directly consequent to the loss of megalin, cubilin, or Dab2 expression. KO of Lrp2 had the greatest transcriptional effect, and nearly all genes whose expression was affected in Cubn KO and Dab2 KO cells were also changed in Lrp2 KO cells. Pathway analysis and more granular inspection of the altered gene profiles suggested changes in pathways with immunomodulatory functions that might trigger the pathological changes observed in KO mice and patients with Donnai-Barrow syndrome. In addition, differences in transcription patterns between Lrp2 and Dab2 KO cells suggested the possibility that altered spatial signaling by aberrantly localized receptors contributes to transcriptional changes upon the disruption of PT endocytic function. A reduction in transcripts encoding sodium-glucose cotransporter isoform 2 was confirmed in Lrp2 KO mouse kidney lysates by quantitative PCR analysis. Our results highlight the role of megalin as a master regulator and coordinator of ion transport, metabolism, and endocytosis in the PT. Compared with the studies in animal models, this approach provides a means to identify PT-specific transcriptional changes that are directly consequent to the loss of these target genes.NEW & NOTEWORTHY Megalin and cubilin receptors together with their adaptor protein Dab2 represent major components of the endocytic machinery responsible for efficient uptake of filtered proteins by the proximal tubule (PT). Dab2 and megalin expression have been implicated as both positive and negative modulators of kidney disease. We used RNA sequencing to knock out CRISPR/Cas9 cubilin, megalin, and Dab2 in highly differentiated PT cells to identify PT-specific changes that are directly consequent to knockout of each component.

Hypomagnesemia with Hypercalciuria Leading to Nephrocalcinosis, Amelogenesis Imperfecta, and Short Stature in a Child Carrying a Homozygous Deletion in the CLDN16 Gene

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare autosomal recessive disease caused by mutations in the CLDN16 or CLDN19 gene; however, few cases develop classical amelogenesis imperfecta. Herein, we report the case of a boy with early clinical renal manifestations that started at 1 year of age and presenting with dental hypoplasia and growth delay. The patient presented with vomiting, polyuria, and polydipsia. Apart from recurrent sterile leukocyturia, erroneously treated as infectious, he was normal, except for short stature and amelogenesis imperfecta with gradually discolored teeth. Laboratory tests revealed hyperparathyroidism, hypomagnesemia, severe hypercalciuria, and hypermagnesuria on 24-h urine testing. Helical computed tomography confirmed nephrocalcinosis. We performed whole-exome sequencing (WES) to test the hypothesis of FHHNC and oligogenic inheritance of amelogenesis. Analysis of the WES binary sequence alignment/map file revealed the presence of exon 1 of the CLDN16 and absence of the other exons [c.325_c918*? (E2_E5del)]. We confirmed a CLDN16 E2_E5 homozygous deletion by multiplex ligation-dependent probe amplification and polymerase chain reaction assays. Although most mutations causing FHHNC are missense and nonsense mutations in the CLDN16 or CLDN19 gene, large deletions occur and may be misled by WES, which is generally used for genetic screening of oligogenic disorders. The patient received cholecalciferol, magnesium oxide and potassium citrate. Later, the combination with hydrochlorothiazide plus amiloride was prescribed, with a good response during follow-up. Our report broadens the phenotype of FHHNC, including severe early-onset amelogenesis and short stature, and reinforces the phenotype-genotype correlation of the large deletion found in CLDN16.

Novel variant in the CNNM2 gene associated with dominant hypomagnesemia

The maintenance of magnesium (Mg2+) homeostasis is essential for human life. The Cystathionine-β-synthase (CBS)-pair domain divalent metal cation transport mediators (CNNMs) have been described to be involved in maintaining Mg2+ homeostasis. Among these CNNMs, CNNM2 is expressed in the basolateral membrane of the kidney tubules where it is involved in Mg2+ reabsorption. A total of four patients, two of them with a suspected disorder of calcium metabolism, and two patients with a clinical diagnosis of primary tubulopathy were screened for mutations by Next-Generation Sequencing (NGS). We found one novel likely pathogenic variant in the heterozygous state (c.2384C>A; p.(Ser795*)) in the CNNM2 gene in a family with a suspected disorder of calcium metabolism. In this family, hypomagnesemia was indirectly discovered. Moreover, we observed three novel variants of uncertain significance in heterozygous state in the other three patients (c.557G>C; p.(Ser186Thr), c.778A>T; p.(Ile260Phe), and c.1003G>A; p.(Asp335Asn)). Our study shows the utility of Next-Generation Sequencing in unravelling the genetic origin of rare diseases. In clinical practice, serum Mg2+ should be determined in calcium and PTH-related disorders.

Inherited Renal Tubulopathies-Challenges and Controversies

Electrolyte homeostasis is maintained by the kidney through a complex transport function mostly performed by specialized proteins distributed along the renal tubules. Pathogenic variants in the genes encoding these proteins impair this function and have consequences on the whole organism. Establishing a genetic diagnosis in patients with renal tubular dysfunction is a challenging task given the genetic and phenotypic heterogeneity, functional characteristics of the genes involved and the number of yet unknown causes. Part of these difficulties can be overcome by gathering large patient cohorts and applying high-throughput sequencing techniques combined with experimental work to prove functional impact. This approach has led to the identification of a number of genes but also generated controversies about proper interpretation of variants. In this article, we will highlight these challenges and controversies.

In-Depth Bioinformatic Study of the CLDN16 Gene and Protein: Prediction of Subcellular Localization to Mitochondria

Background and Objectives: The defects in the CLDN16 gene are a cause of primary hypomagnesemia (FHHNC), which is characterized by massive renal magnesium wasting, resulting in nephrocalcinosis and renal failure. The mutations occur throughout the gene’s coding region and can impact on intracellular trafficking of the protein or its paracellular pore forming function. To gain more understanding about the mechanisms by which CLDN16 mutations can induce FHHNC, we performed an in-depth computational analysis of the CLDN16 gene and protein, focusing specifically on the prediction of the latter’s subcellular localization. Materials and Methods: The complete nucleotide or amino acid sequence of CLDN16 in FASTA format was entered and processed in 14 databases. Results: One CpG island was identified. Twenty five promoters/enhancers were predicted. The CLDN16 interactome was found to consist of 20 genes, mainly involved in kidney diseases. No signal peptide cleavage site was identified. A probability of export to mitochondria equal to 0.9740 and a cleavable mitochondrial localization signal in the N terminal of the CLDN16 protein were predicted. The secondary structure prediction was visualized. Νo phosphorylation sites were identified within the CLDN16 protein region by applying DISPHOS to the functional class of transport. The KnotProt database did not predict any knot or slipknot in the protein structure of CLDN16. Seven putative miRNA binding sites within the 3’-UTR region of CLDN16 were identified. Conclusions: This is the first study to identify mitochondria as a probable cytoplasmic compartment for CLDN16 localization, thus providing new insights into the protein’s intracellular transport. The results relative to the CLDN16 interactome underline its role in renal pathophysiology and highlight the functional dependence of CLDNs-10, 14, 16, 19. The predictions pertaining to the miRNAs, promoters/enhancers and CpG islands of the CLDN16 gene indicate a strict regulation of its expression both transcriptionally and post-transcriptionally.

Establishment of urinary exosome-like vesicles isolation protocol for FHHNC patients and evaluation of different exosomal RNA extraction methods

BACKGROUND

Molecular and cellular pathophysiological events occurring in the majority of rare kidney diseases remain to be elucidated. Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare autosomal recessive disorder caused by mutations in either CLDN16 or CLDN19 genes. This disease is characterized by massive urinary wasting of magnesium and calcium, osmosis deregulation and polyuria. Patients with p.G20D homozygous mutation in CLDN19 gene exhibit different progression to kidney failure suggesting that beyond the pathogenic mutation itself, other molecular events are favoring disease progression. Due to the fact that biopsy is not clinically indicated in these patients, urinary exosome-like vesicles (uEVs) can be envisioned as a valuable non-invasive source of information of events occurring in the kidney. Exosome research has increased notably to identify novel disease biomarkers but there is no consensus standardized protocols for uEVs isolation in patients with polyuria. For this reason, this work was aimed to evaluate and refine different uEVs isolation methods based on differential centrifugation, the gold standard method.

RESULTS

Characterization by NTA, cryo-TEM and immunoblotting techniques identified the most appropriate protocol to obtain the highest yield and purest uEVs enriched fraction possible from urine control samples and FHHNC patients. Moreover, we tested five different RNA extraction methods and evaluated the miRNA expression pattern by qRT-PCR.

CONCLUSIONS

In summary, we have standardized the conditions to proceed with the identification of differentially expressed miRNAs in uEVs of FHHNC patients, or other renal diseases characterized by polyuria.

Genetic causes of hypomagnesemia, a clinical overview

Magnesium is essential to the proper functioning of numerous cellular processes. Magnesium ion (Mg2+) deficits, as reflected in hypomagnesemia, can cause neuromuscular irritability, seizures and cardiac arrhythmias. With normal Mg2+ intake, homeostasis is maintained primarily through the regulated reabsorption of Mg2+ by the thick ascending limb of Henle's loop and distal convoluted tubule of the kidney. Inadequate reabsorption results in renal Mg2+ wasting, as evidenced by an inappropriately high fractional Mg2+ excretion. Familial renal Mg2+ wasting is suggestive of a genetic cause, and subsequent studies in these hypomagnesemic families have revealed over a dozen genes directly or indirectly involved in Mg2+ transport. Those can be classified into four groups: hypercalciuric hypomagnesemias (encompassing mutations in CLDN16, CLDN19, CASR, CLCNKB), Gitelman-like hypomagnesemias (CLCNKB, SLC12A3, BSND, KCNJ10, FYXD2, HNF1B, PCBD1), mitochondrial hypomagnesemias (SARS2, MT-TI, Kearns-Sayre syndrome) and other hypomagnesemias (TRPM6, CNMM2, EGF, EGFR, KCNA1, FAM111A). Although identification of these genes has not yet changed treatment, which remains Mg2+ supplementation, it has contributed enormously to our understanding of Mg2+ transport and renal function. In this review, we discuss general mechanisms and symptoms of genetic causes of hypomagnesemia as well as the specific molecular mechanisms and clinical phenotypes associated with each syndrome.

A Novel Homozygous SLC2A9 Mutation Associated with Renal-Induced Hypouricemia

BACKGROUND

Hereditary renal hypouricemia (RHUC) is a genetically heterogenous disorder characterized by defective uric acid (UA) reabsorption resulting in hypouricemia and increased fractional excretion of UA; acute kidney injury (AKI) and nephrolithiasis are recognized complications. Type 1 (RHUC1) is caused by mutations in the SLC22A12 gene, whereas RHUC2 is caused by mutations in the SLC2A9 gene. Patient ethnicity is diverse but only few Caucasian families with an SLC2A9 mutation have been reported.

METHODS

The current report describes the clinical history, biochemical and molecular genetics findings of a native Austrian family with RHUC2. The propositus presented with 2 episodes of exercise-induced AKI and exhibited profound hypouricemia. Mutational screening of the SLC22A12 and SLC2A9 genes was performed.

RESULTS

The molecular analyses revealed the homozygous c.512G>A transition that leads to the p.Arg171His missense substitution in SLC2A9, confirming the diagnosis of RHUC2. Segregation study of the causal mutation revealed that the mother and elder sister were heterozygous carriers, whereas the younger sister was found to be homozygous.

CONCLUSION

We report the identification of a novel mutation in SLC2A9 as the cause of RHUC2 in a native Austrian family. We show that glucose transporter 9 mutations cause severe hypouricemia in homozygous individuals and confirm the high risk of AKI in male individuals harbouring these mutations. In our literature review, we provide an overview of the putative underlying pathophysiology, potential renal complications, findings on kidney biopsy as well as potential long-time renal sequelae.

Hypomagnesemia due to two novel TRPM6 mutations

BACKGROUND

Although most hypocalcemia with hypomagenesemia in the neonatal period is due to transient neonatal hypoparathyroidism, magnesium channel defects should also be considered.

CASE

We report a case of persistent hypomagnesemia in an 8-day-old Hispanic male who presented with generalized seizures. He was initially found to have hypomagnesemia, hypocalcemia, hyperphosphatemia and normal parathyroid hormone. Serum calcium normalized with administration of calcitriol and calcium carbonate. Serum magnesium improved with oral magnesium sulfate. However, 1 week after magnesium was discontinued, serum magnesium declined to 0.5 mg/dL. Magnesium supplementation was immediately restarted, and periodic seizure activity resolved after serum magnesium concentration was maintained above 0.9 mg/dL. The child was eventually weaned off oral calcium and calcitriol with persistent normocalemia. However, supraphysiologic oral magnesium doses were necessary to prevent seizures and maintain serum magnesium at the low limit of normal.

METHODS AND RESULTS

As his clinical presentation suggested primary renal magnesium wastage, TRPM6 gene mutations were suspected; subsequent genetic testing revealed the child to be compound heterozygous for TRPM6 mutations.

CONCLUSION

Two novel TRPM6 mutations are described with a new geographic and ethnic origin. This case highlights the importance of recognizing disorders of magnesium imbalance and describing new genetic mutations.

The long-term complications of the inherited tubulopathies: an adult perspective

The inherited tubulopathies are lifelong disorders and their clinical features and complications may present quite different challenges in adulthood from those in childhood. In this review we outline the pathophysiology and documented complications (including the late and unusual) of the monogenic tubulopathies from the perspective of the adult nephrologist.

Distal convoluted tubule

The distal convoluted tubule (DCT) is a short nephron segment, interposed between the macula densa and collecting duct. Even though it is short, it plays a key role in regulating extracellular fluid volume and electrolyte homeostasis. DCT cells are rich in mitochondria, and possess the highest density of Na+/K+-ATPase along the nephron, where it is expressed on the highly amplified basolateral membranes. DCT cells are largely water impermeable, and reabsorb sodium and chloride across the apical membrane via electroneurtral pathways. Prominent among this is the thiazide-sensitive sodium chloride cotransporter, target of widely used diuretic drugs. These cells also play a key role in magnesium reabsorption, which occurs predominantly, via a transient receptor potential channel (TRPM6). Human genetic diseases in which DCT function is perturbed have provided critical insights into the physiological role of the DCT, and how transport is regulated. These include Familial Hyperkalemic Hypertension, the salt-wasting diseases Gitelman syndrome and EAST syndrome, and hereditary hypomagnesemias. The DCT is also established as an important target for the hormones angiotensin II and aldosterone; it also appears to respond to sympathetic-nerve stimulation and changes in plasma potassium. Here, we discuss what is currently known about DCT physiology. Early studies that determined transport rates of ions by the DCT are described, as are the channels and transporters expressed along the DCT with the advent of molecular cloning. Regulation of expression and activity of these channels and transporters is also described; particular emphasis is placed on the contribution of genetic forms of DCT dysregulation to our understanding.

Purine disorders with hypouricemia

Hypouricemia is defined as a serum urate levels less than 2 mg/dL (119 µmol/L). Primary hypouricemia is caused by disorders of purine metabolism and transport. This laboratory finding is sometimes overlooked and, following two genetic defects, should be considered in differential diagnosis of unexplained hypouricemia. Hereditary xanthinuria is autosomal recessive and due to mutations in xanthine oxidase, leading to over-production of xanthine and minimal production of urate. Patients have very low serum urate levels and suffer from elevated levels of xanthine in the urine, leading to xanthine stones, haematuria, and sometimes occult chronic kidney failure. Hypouricemia is the key to diagnosis. Hereditary renal hypouricemia is a new genetic defect of renal transport of uric acid. Two types were distinguished: a) renal hypouricemia type 1, caused by the defects in the SLC22A12 gene coding the human urate transporter 1 (hURAT1) and b) renal hypouricemia type 2, caused by the defects in the SLC2A9 gene, which encodes GLUT9 transporter. This disorder predisposes patients to exercise-induced acute renal failure and/or nephrolithiasis. Diagnosis is based on two markers: hypouricemia (<119 µmol/L) and increased fractional excretion of uric acid (>10%). Over one hundred cases were identified in Japan and and this number is unique worldwide. Several patients were described in Macedonia. We were able to detect four Czech families with hereditary xanthinuria and eight cases of hereditary renal hypouricemia. In conclusion, hereditary xanthinuria and hereditary renal hypouricemia are still unrecognized conditions. Patients with unexplained hypouricemia need detailed purine metabolic investigations.

Posterior reversible encephalopathy syndrome with exercise-induced acute kidney injury in renal hypouricemia type 1

Renal hypouricemia type 1 is caused by mutations in the SLC22A12 gene, whereas type 2 is caused by defects in the SLC2A9 gene. Although both subtypes predispose to exercise-induced acute kidney injury (EIAKI), posterior reversible encephalopathy syndrome (PRES) occurring with this disorder is an uncommon phenomenon that has only been reported to date in a patient with renal hypouricemia type 2. We describe a 13-year-old boy with renal hypouricemia type 1 (serum uric acid, 0.9 mg/dL) with a homozygous W258X mutation in the SLC22A12 gene, presenting with EIAKI and PRES. On admission, his body weight was 61 kg (11 kg above the dry weight), and blood pressure was 153/88 mmHg. Cranial magnetic resonance imaging revealed high-intensity areas in the cortical and subcortical white matter of the occipital lobe. After admission, the patient responded well to a combination of hemodialysis and intravenous nicardipine. This is the first case of concurrent PRES and EIAKI in a patient with renal hypouricemia type 1. We suggest that PRES is not due to severe hypouricemia caused by SLC2A9 mutation but is a manifestation of severe EIAKI associated with renal hypouricemia.

Molecular background of urate transporter genes in patients with exercise-induced acute kidney injury

BACKGROUND/AIMS

Exercise-induced acute renal failure [exercise-induced acute kidney injury (EI-AKI)] is defined as AKI due to heavy anaerobic exercise. Although hypouricemia is known to be a risk factor for the onset of EI-AKI, a direct causal link between EI-AKI and serum uric acid has not been established. This study aimed to analyze urate transporter genes in patients with EI-AKI and its molecular mechanism.

METHODS

Genomic DNA and total RNA were isolated from peripheral blood leukocytes of patients with a history of EI-AKI. Mutations were analyzed by PCR and a direct sequencing method. We first analyzed the SLC22A12 gene, and then the SLC2A9 gene if no mutations were found in SLC22A12.

RESULTS

Seventeen patients were enrolled in this study and 16 had mutations: 15 in SLC22A12 and 1 in SLC2A9. Fourteen (82.4%) patients showed hypouricemia, and all of the patients with hypouricemia had either homozygous or compound heterozygous mutations in SLC22A12 or SLC2A9, which confirmed that all of them had renal hypouricemia. Two patients had heterozygous mutations of SLC22A12, and they were not accompanied by hypouricemia. One patient was found to have no mutations in SLC22A12 or SLC2A9.

CONCLUSION

We were able to determine the genetic background of urate transporter genes in patients with EI-AKI. Decreased function of urate transporters, rather than decreased serum uric acid levels, may be of great importance for the onset of EI-AKI.

Novel TRPM6 mutations in familial hypomagnesemia with secondary hypocalcemia

BACKGROUND

Familial hypomagnesemia with secondary hypocalcemia (HSH) is a rare autosomal recessive disease characterized by severe hypomagnesemia and hypocalcemia associated with neurological symptoms, including generalized seizures, tetany and muscle spasms, which are refractory to anticonvulsant treatment. The pathophysiological hallmarks of HSH are the impaired intestinal absorption of magnesium accompanied by renal magnesium wasting as a result of a reabsorption defect in the distal convoluted tubule. Mutations in TRPM6, the gene encoding the transient receptor potential cation channel subfamily member 6, have been found to be responsible for this disease. In the present study, we report a Chinese family with 2 sisters affected with severe HSH, and elucidate the characteristics of TRPM6 gene mutations in these 2 patients.

METHODS

We evaluated the clinical, laboratory, and radiographic findings. All 39 TRPM6 exons and flanking exon-intron junctions from genomic DNA were amplified and sequenced in 2 affected members suffering from HSH and their family.

RESULTS

We found two novel mutations in the family, one frameshift mutation (c.1196delC) and one non-sense mutation (c.4577G>A). These mutations were predicted to result in a complete loss of function of TRPM6. Both of the sisters were compound heterozygotes for these mutations.

CONCLUSION

Our results suggested that the compound heterozygous mutations in TRPM6 were responsible for HSH in the Chinese family.

Acute kidney injury in two children caused by renal hypouricaemia type 2

BACKGROUND

Renal hypouricaemia is a heterogeneous inherited disorder characterized by impaired tubular uric acid transport with severe complications, such as acute kidney injury and nephrolithiasis. Type 1 is caused by a loss-of-function mutation in the SLC22A12 gene (OMIM #220150), while type 2 is caused by defects in the SLC2A9 gene (OMIM #612076).

CASE-DIAGNOSIS/TREATMENT

The cases of two children, a 12- and a 14-year-old boy with acute kidney injury (proband 1: urea 9.4 mmol/l, creatinine 226 μmol/l; proband 2: urea 11.7 mmol/l, creatinine 202 μmol/l) are described. Both are offspring of nonconsanguineous couples in the UK. The concentrations of serum uric acid were consistently below the normal range (0.03 and 0.04 mmol/l) and expressed as an increase in the fractional excretion of uric acid (46 and 93 %).

CONCLUSIONS

A sequencing analysis of the coding region of uric acid transporters SLC22A12 and SLC2A9 was performed. Analysis of genomic DNA revealed two unpublished missense transitions, p.G216R and p.N333S in the SLC2A9 gene. No sequence variants in SLC22A12 were found. Our findings suggest that homozygous and/or compound heterozygous loss-of-function mutations p.G216R and p.N333S cause renal hypouricaemia via loss of uric acid absorption and do lead to acute kidney injury.

Clinical and functional characterization of URAT1 variants

Idiopathic renal hypouricaemia is an inherited form of hypouricaemia, associated with abnormal renal handling of uric acid. There is excessive urinary wasting of uric acid resulting in hypouricaemia. Patients may be asymptomatic, but the persistent urinary abnormalities may manifest as renal stone disease, and hypouricaemia may manifest as exercise induced acute kidney injury. Here we have identified Macedonian and British patients with hypouricaemia, who presented with a variety of renal symptoms and signs including renal stone disease, hematuria, pyelonephritis and nephrocalcinosis. We have identified heterozygous missense mutations in SLC22A12 encoding the urate transporter protein URAT1 and correlate these genetic findings with functional characterization. Urate handling was determined using uptake experiments in HEK293 cells. This data highlights the importance of the URAT1 renal urate transporter in determining serum urate concentrations and the clinical phenotypes, including nephrolithiasis, that should prompt the clinician to suspect an inherited form of renal hypouricaemia.

A case of exercise-induced acute renal failure with G774A mutation in SCL22A12 causing renal hypouricemia

Acute renal failure with severe loin pain which develops after anaerobic exercise is rare. One of predisposing factors of exercise-induced acute renal failure is renal hypouricemia. Idiopathic renal hypouricemia is a genetic disorder characterized by hypouricemia with abnormally high renal tubular uric acid excretion. The mutation in SCL22A12 gene which encodes renal uric acid transporter, URAT1, is the known major cause of this disorder. We here described a 25-yr-old man showing idiopathic renal hypouricemia with G774A mutation in SCL22A12 who presented exercise-induced acute renal failure. There have been a few reports of mutational analysis in Korean idiopathic renal hypouricemia without acute renal failure. This is the first report of genetically diagnosed idiopathic renal hypouricemia with exercise-induced acute renal failure in Korea.

Novel homozygous insertion in SLC2A9 gene caused renal hypouricemia

Renal hypouricemia is a heterogeneous inherited disorder characterized by impaired uric acid handling in the renal tubules. Patients are usually asymptomatic; however, some may experience urolithiasis and/or acute kidney injury. Most of the described patients (compound heterozygous and/or homozygous) are Japanese with mutations in the SLC22A12 gene (OMIM #220150). Four patients with renal hypouricemia caused by heterozygous defects and two families with homozygous mutations in the SLC2A9 gene have been recently described (OMIM #612076). We describe the clinical history, biochemical and molecular genetics findings of a Czech family with renal hypouricemia. The concentration of serum uric acid in the proband (16-year-old Czech girl with unrelated parents) was 0.17 ± 0.05 mg/dl and expressed as an increase in the fractional excretion of uric acid (194 ± 99%). The sequencing analysis of the coding region of uric acid transporters SLC22A12, SLC2A9, SLC17A3, ABCC4 and ABCG2, was performed. Analysis of genomic DNA revealed novel one nucleotide homozygote insertion in exon 3 in the SLC2A9 gene in proband and her brother resulting in a truncated protein (p.Ile118HisfsX27). No sequence variants in other candidate uric acid transporter were found. Homozygous loss-of-function mutations cause massive renal hypouricemia via total loss of uric acid absorption; however, they do not necessarily lead to nephrolithiasis and acute kidney injury. In contrast to previously reported heterozygous patients with renal hypouricemia type 2, we did not find even slight hypouricemia and found no decrease in the FE-UA of the heterozygous parents of the reported siblings.

SLC22A12 W258X frequency according to serum uric acid level among Japanese health checkup examinees

Although the SLC22A12 (uric acid transporter 1) 258X allele is known to cause hypouricemia, the genotype frequency according to the serum uric acid (SUA) level has not been reported. This study investigated the SLC22A12 258WX frequency according to SUA levels among Japanese health-checkup examinees. In addition, the changes were reported in SUA levels during five years for individuals with 258WX. Subjects were 746 Japanese aged 39-86 years in 2003. Their SUA records were linked during the five years from 2003 to 2007. SLC22A12 W258X was genotyped using a polymerase chain reaction with confronting two-pair primers. The 258X allele comprised 1.9% (95% CI, 1.3-2.8%) of all the subjects. Among those with SUA <3.0 mg/dL, 258WX was more common in males (66.7%, 95% CI, 22.2-95.7%) than in females (39.3%, 95% CI, 21.5-59.4%). Among subjects with a SUA of 3.0-4.9 mg/dL, those with 258WX totaled 10.7% (95% CI, 4.0-21.9%) and 2.6% (95% CI, 1.1-5.0%), respectively. There were no subjects with 258WX among those with a SUA of 5.0 mg/dL or more. During the five years from 2003 to 2007, the changes in SUA among 23 individuals with 258WX were found to be similar to those among 258WW subjects (n=536). This study indicated that SLC22A12 258WX was more common among those with a lower serum uric acid concentration. The observed SUA level changes in individuals with 258WX suggested that lifestyle factors could influence the levels of those with 258WX.

A novel mutation of the claudin 16 gene in familial hypomagnesemia with hypercalciuria and nephrocalcinosis mimicking rickets

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is caused by a mutation in the gene CLDN16, which encodes paracellin 1 (claudin-16), atight junction protein mediating paracellular transport which is expressed in the thick ascending loop of Henle and in the distal convoluted tubule, where reabsorption of magnesium occurs. We present a 4 years old Turkish female child with a chief complaint of hypocalcemic tetany. A diagnosis of FHHNC was confirmed by genetic testing for a mutation in claudin 16 gene. Claudin 16 gene revealed homozygosity for the p.K183E(AAA>GAA) C. 547A>G indicating the diagnosis of hypomagnesemia with hypercalciuria and nephrocalcinosis. To our knowledge, this is the first case of FHHNC reported in Turkish population diagnosed at molecular level.

Epilepsy, ataxia, sensorineural deafness, tubulopathy, and KCNJ10 mutations

BACKGROUND

Five children from two consanguineous families presented with epilepsy beginning in infancy and severe ataxia, moderate sensorineural deafness, and a renal salt-losing tubulopathy with normotensive hypokalemic metabolic alkalosis. We investigated the genetic basis of this autosomal recessive disease, which we call the EAST syndrome (the presence of epilepsy, ataxia, sensorineural deafness, and tubulopathy).

METHODS

Whole-genome linkage analysis was performed in the four affected children in one of the families. Newly identified mutations in a potassium-channel gene were evaluated with the use of a heterologous expression system. Protein expression and function were further investigated in genetically modified mice.

RESULTS

Linkage analysis identified a single significant locus on chromosome 1q23.2 with a lod score of 4.98. This region contained the KCNJ10 gene, which encodes a potassium channel expressed in the brain, inner ear, and kidney. Sequencing of this candidate gene revealed homozygous missense mutations in affected persons in both families. These mutations, when expressed heterologously in xenopus oocytes, caused significant and specific decreases in potassium currents. Mice with Kcnj10 deletions became dehydrated, with definitive evidence of renal salt wasting.

CONCLUSIONS

Mutations in KCNJ10 cause a specific disorder, consisting of epilepsy, ataxia, sensorineural deafness, and tubulopathy. Our findings indicate that KCNJ10 plays a major role in renal salt handling and, hence, possibly also in blood-pressure maintenance and its regulation.

Impaired basolateral sorting of pro-EGF causes isolated recessive renal hypomagnesemia

Primary hypomagnesemia constitutes a rare heterogeneous group of disorders characterized by renal or intestinal magnesium (Mg(2+)) wasting resulting in generally shared symptoms of Mg(2+) depletion, such as tetany and generalized convulsions, and often including associated disturbances in calcium excretion. However, most of the genes involved in the physiology of Mg(2+) handling are unknown. Through the discovery of a mutation in the EGF gene in isolated autosomal recessive renal hypomagnesemia, we have, for what we believe is the first time, identified a magnesiotropic hormone crucial for total body Mg(2+) balance. The mutation leads to impaired basolateral sorting of pro-EGF. As a consequence, the renal EGFR is inadequately stimulated, resulting in insufficient activation of the epithelial Mg(2+) channel TRPM6 (transient receptor potential cation channel, subfamily M, member 6) and thereby Mg(2+) loss. Furthermore, we show that colorectal cancer patients treated with cetuximab, an antagonist of the EGFR, develop hypomagnesemia, emphasizing the significance of EGF in maintaining Mg(2+) balance.

When EGF is offside, magnesium is wasted

Our understanding of magnesium (Mg(2+)) regulation has recently been catapulted forward by the discovery of several disease loci for monogenic disorders of Mg(2+) homeostasis. In this issue of the JCI, Groenestege et al. report that their study of a rare inherited Mg(2+) wasting disorder in consanguineous kindred shows that EGF acts as an autocrine/paracrine magnesiotropic hormone (see the related article beginning on page 2260). EGF stimulates Mg(2+) reabsorption in the renal distal convoluted tubule (DCT) via engagement of its receptor on the basolateral membrane of DCT cells and activation of the Mg(2+) channel TRPM6 (transient receptor potential cation channel, subfamily M, member 6) in the apical membrane. These authors show that a point mutation in pro-EGF retains EGF secretion to the apical but not the basolateral membrane, disrupting this cascade and causing renal Mg(2+) wasting. This work is another seminal example of the power of the study of monogenic disorders in the quest to understand human physiology.

Novel OCRL1 mutations in patients with the phenotype of Dent disease

BACKGROUND

Dent disease is an X-linked tubulopathy frequently caused by mutations affecting the voltage-gated chloride channel and chloride/proton antiporter ClC-5. A recent study showed that defects in OCRL1, encoding a phosphatidylinositol 4,5-bisphosphate 5-phosphatase (Ocrl) and usually found mutated in patients with Lowe syndrome, also can provoke a Dent-like phenotype (Dent 2 disease).

METHODS

We investigated 20 CLCN5-negative males from 17 families with a phenotype resembling Dent disease for defects in OCRL1.

RESULTS

In our complete series of 35 families with a phenotype of Dent disease, a mutation in the OCRL1 gene was detected in 6 kindreds. All were novel frameshift (Q70RfsX88 and T121NfsX122, detected twice) or missense mutations (I257T and R476W). None of our patients had cognitive or behavioral impairment or cataracts, 2 classic hallmarks of Lowe syndrome. All patients had mild increases in lactate dehydrogenase and/or creatine kinase levels, which rarely is observed in CLCN5-positive patients, but frequently found in patients with Lowe syndrome. To explain the phenotypic heterogeneity caused by OCRL1 mutations, we performed extensive data-bank mining and extended reverse-transcriptase polymerase chain reaction analysis, which provided no evidence for yet unknown (tissue-specific) alternative OCRL1 transcripts.

CONCLUSION

Mutations in the OCRL1 gene are found in approximately 23% of kindreds with a Dent phenotype. Defective protein sorting/targeting of Ocrl might be the reason for mildly elevated creatine kinase and lactate dehydrogenase serum concentrations in these patients and a clue to suspect Dent disease unrelated to CLCN5 mutations. It remains to be elucidated why the various OCRL1 mutations found in patients with Dent 2 disease do not cause cataracts.

Stones, bones, and heredity

Genetic disorders of mineral metabolism cause urolithiasis, renal disease, and osteodystrophy. Most are rare, such that the full spectrum of clinical expression is difficult to appreciate. Diagnosis is further complicated by overlap of clinical features. Dent's disease and primary hyperoxaluria, inherited causes of calcium urolithiasis, are both associated with nephrocalcinosis and urolithiasis in early childhood and renal failure that can occur at any age but is seen more often in adulthood. Bone disease is an inconsistent feature of each. Dent's disease is caused by mutations of the CLCN-5 gene with impaired kidney-specific CLC-5 chloride channel expression in the proximal tubule, thick ascending limb of Henle, and the collecting ducts. Resulting hypercalciuria and proximal tubule dysfunction, including phosphate wasting, are primarily responsible for the clinical manifestations. Low-molecular-weight proteinuria is characteristic. Definitive diagnosis is made by DNA mutation analysis. Primary hyperoxaluria, type I, is due to mutations of the AGXT gene leading to deficient hepatic alanine-glyoxylate aminotransferase activity. Marked overproduction of oxalate by hepatic cells results in the hyperoxaluria responsible for clinical features. Definitive diagnosis is by liver biopsy with measurement of enzyme activity, with DNA mutation analysis used increasingly as mutations and their frequency are defined. These disorders of calcium urolithiasis illustrate the value of molecular medicine for diagnosis and the promise it provides for innovative and more effective future treatments.

Roles of organic anion transporters (OATs) and a urate transporter (URAT1) in the pathophysiology of human disease

Renal proximal and distal tubules are highly polarized epithelial cells that carry out the specialized directional transport of various solutes. This renal function, which is essential for homeostasis in the body, is achieved through the close pairing of apical and basolateral carriers expressed in the renal epithelial cells. The family of organic anion transporters (OATs), which belong to the major facilitator superfamily (SLC22A), are expressed in the renal epithelial cells to regulate the excretion and reabsorption of endogenous and exogenous organic anions. We now understand that these OATs are crucial components in the renal handling of drugs and their metabolites, and they are implicated in various clinically important drug interactions, and their adverse reactions. In recent years, the molecular entities of these transporters have been identified, and their function and regulatory mechanisms have been partially clarified. Workers in this field have identified URAT1 (urate transporter 1), a novel member of the OAT family that displays unique and selective substrate specificity compared with other multispecific OATs. In the OAT family, URAT1 is the main transporster responsible for human genetic diseases. In this review, we introduce and discuss some novel aspects of OATs, with special emphasis on URAT1, in the context of their biological significance, functional regulation, and roles in human disease.

[From gene to disease; mutations in the SLC12A3 gene as the cause of Gitelman's syndrome]

Gitelman's syndrome is characterised by persistent hypokalaemia, hypomagnesaemia and hypocalciuria (OMIM 263800). This rare autosomal recessive disorder is caused by renal Na+, Cl-, K+ and Mg2+ wasting. Other typical features include hypocalciuria and an intact renal concentrating ability. Gitelman's syndrome is caused by mutations in the SLC12A3 gene, encoding the thiazide-sensitive sodium-chloride co-transporter (NCC). NCC is located in the distal convoluted tubule of the kidney, a segment known to play an important role in active magnesium reabsorption in the nephron. The exact mechanisms underlying hypomagnesaemia and hypocalciuria in Gitelman's syndrome are still poorly understood, but point to enhanced proximal Na+ and Ca2+ reabsorption and apoptosis of distal convoluted tubule cells.

Enhanced passive Ca2+ reabsorption and reduced Mg2+ channel abundance explains thiazide-induced hypocalciuria and hypomagnesemia

Thiazide diuretics enhance renal Na+ excretion by blocking the Na+-Cl- cotransporter (NCC), and mutations in NCC result in Gitelman syndrome. The mechanisms underlying the accompanying hypocalciuria and hypomagnesemia remain debated. Here, we show that enhanced passive Ca2+ transport in the proximal tubule rather than active Ca2+ transport in distal convolution explains thiazide-induced hypocalciuria. First, micropuncture experiments in mice demonstrated increased reabsorption of Na+ and Ca2+ in the proximal tubule during chronic hydrochlorothiazide (HCTZ) treatment, whereas Ca2+ reabsorption in distal convolution appeared unaffected. Second, HCTZ administration still induced hypocalciuria in transient receptor potential channel subfamily V, member 5-knockout (Trpv5-knockout) mice, in which active distal Ca2+ reabsorption is abolished due to inactivation of the epithelial Ca2+ channel Trpv5. Third, HCTZ upregulated the Na+/H+ exchanger, responsible for the majority of Na+ and, consequently, Ca2+ reabsorption in the proximal tubule, while the expression of proteins involved in active Ca2+ transport was unaltered. Fourth, experiments addressing the time-dependent effect of a single dose of HCTZ showed that the development of hypocalciuria parallels a compensatory increase in Na+ reabsorption secondary to an initial natriuresis. Hypomagnesemia developed during chronic HCTZ administration and in NCC-knockout mice, an animal model of Gitelman syndrome, accompanied by downregulation of the epithelial Mg2+ channel transient receptor potential channel subfamily M, member 6 (Trpm6). Thus, Trpm6 downregulation may represent a general mechanism involved in the pathogenesis of hypomagnesemia accompanying NCC inhibition or inactivation.

Mutations in human urate transporter 1 gene in presecretory reabsorption defect type of familial renal hypouricemia

To date, 11 loss of function mutations in the human urate transporter 1 (hURAT1) gene have been identified in subjects with idiopathic renal hypouricemia. In the present studies we investigated the clinical features and the mutations in the hURAT1 gene in seven families with presecretory reabsorption defect-type renal hypouricemia and in one family with the postsecretory reabsorption defect type. Twelve affected subjects and 26 family members were investigated. Mutations were analyzed by PCR and the direct sequencing method. Urate-transporting activities of wild-type and mutant hURAT1 were determined by [14C]urate uptake in Xenopus oocytes. Mutational analysis revealed three previously reported mutations (G774A, A1145T, and 1639-1643 del-GTCCT) and a novel mutation (T1253G) in families with the presecretory reabsorption defect type. Neither mutations in the coding region of hURAT1 gene nor significant segregation patterns of the hURAT1 locus were detected in the postsecretory reabsorption defect type. All hURAT1 mutants had significantly reduced urate-transporting activities compared with wild type (P < 0.05; n = 12), suggesting that T1253G is a loss of function mutation, and hURAT1 is responsible for the presecretory reabsorption defect-type familial renal hypouricemia. Future studies are needed to identify a responsible gene for the postsecretory reabsorption defect-type familial renal hypouricemia.

Dent Disease with mutations in OCRL1

Dent disease is an X-linked renal proximal tubulopathy associated with mutations in the chloride channel gene CLCN5. Lowe syndrome, a multisystem disease characterized by renal tubulopathy, congenital cataracts, and mental retardation, is associated with mutations in the gene OCRL1, which encodes a phosphatidylinositol 4,5-bisphosphate (PIP(2)) 5-phosphatase. Genetic heterogeneity has been suspected in Dent disease, but no other gene for Dent disease has been reported. We studied male probands in 13 families, all of whom met strict criteria for Dent disease but lacked mutations in CLCN5. Linkage analysis in the one large family localized the gene to a candidate region at Xq25-Xq27.1. Sequencing of candidate genes revealed a mutation in the OCRL1 gene. Of the 13 families studied, OCRL1 mutations were found in 5. PIP(2) 5-phosphatase activity was markedly reduced in skin fibroblasts cultured from the probands of these five families, and protein expression, measured by western blotting, was reduced or absent. Slit-lamp examinations performed in childhood or adulthood for all five probands showed normal results. Unlike patients with typical Lowe syndrome, none of these patients had metabolic acidosis. Three of the five probands had mild mental retardation, whereas two had no developmental delay or behavioral disturbance. These findings demonstrate that mutations in OCRL1 can occur with the isolated renal phenotype of Dent disease in patients lacking the cataracts, renal tubular acidosis, and neurological abnormalities that are characteristic of Lowe syndrome. This observation confirms genetic heterogeneity in Dent disease and demonstrates more-extensive phenotypic heterogeneity in Lowe syndrome than was previously appreciated. It establishes that the diagnostic criteria for disorders resulting from mutations in the Lowe syndrome gene OCRL1 need to be revised.

Dent’s disease: identification of a novel mutation in the renal chloride channel CLCN5

Dent's disease is an inherited tubulopathy caused by a mutation in the CLCN5 chloride channel gene. It is characterized by low-molecular weight proteinuria, hypercalciuria, nephrolithiasis or nephrocalcinosis, rickets and eventual-progressive renal failure. Onset of clinical symptoms show a great variability, making a diagnosis at an early stage of the disease often difficult. Given the variably clinical picture, genetic analysis can provide a reliable method to confirm the diagnosis. Here, we report on the case of a patient with progressive renal failure showing signs of a tubular lesion and symptoms of Dent's disease. Although this rare disease was suspected by means of the clinical features, it was genetic analysis that confirmed the diagnosis and revealed a novel mutation in the CLCN5 gene.

Autosomal recessive renal proximal tubulopathy and hypercalciuria: a new syndrome

BACKGROUND

The best described primary inherited proximal tubulopathies include X-linked hypercalciuric nephrolithiasis (XLHN), caused by a mutation in the chloride channel gene CLCN5, and classic Fanconi's syndrome, the genetic basis of which is unknown. The aim of this study is to examine the clinical, biochemical, and genetic characteristics of a highly consanguineous Druze family with autosomal recessive proximal tubulopathy and hypercalciuria (ARPTH), a syndrome not reported previously.

METHODS

Three children (2 girls, 1 boy) of the family referred for evaluation of renal glycosuria and hypercalciuria and 10 of their close relatives were evaluated clinically and biochemically. All study participants underwent genetic analysis to exclude involvement of the CLCN5 gene.

RESULTS

Evaluation of the 3 affected children showed glycosuria, generalized aminoaciduria, hypouricemia, uricosuria, low molecular weight (LMW) proteinuria, and hypercalciuria in all 3 children and phosphaturia in 2 children. They had no metabolic acidosis or renal insufficiency. One affected girl had nephrocalcinosis. Two children had a history of growth retardation and radiological findings of metabolic bone disease. Parathyroid hormone and 1,25-dihydroxyvitamin D [1,25(OH)2Vit D] blood levels in affected children were normal. Unaffected family members examined had no renal tubular defects or LMW proteinuria. Genetic linkage analysis excluded cosegregation of the ARPTH phenotype with the CLCN5 locus.

CONCLUSION

ARPTH is a new syndrome characterized by nonacidotic proximal tubulopathy, hypercalciuria, metabolic bone disease, and growth retardation. It can be distinguished from XLHN by its autosomal recessive mode of inheritance and normal serum levels of calciotropic hormones, as well as the absence of LMW proteinuria in obligate carriers. The gene mutated in ARPTH remains to be identified.

Identification of novel mutations in Na-Cl cotransporter gene in a Korean patient with atypical Gitelman's syndrome

The authors report the case of a 20-year-old man with unexplained hypokalemia and metabolic alkalosis suggesting hypokalemic tubulopathy. Interestingly, he showed a mixed phenotype of Gitelman's syndrome (GS) and Bartter's syndrome, which includes normomagnesemia, normal renal magnesium excretion, and hypocalciuria. Renal clearance study showed the presence of a critical defect in the distal nephron rather than loop of Henle. Further family study showed that his mother had a definitive phenotype of GS. By the molecular genetic analysis of these patients, 7 different mutations of the NCCT gene were identified consisting of 3 missense, 1 splice site, and 3 silent mutations. Four of these mutations were novel. The authors emphasize that the combination of a molecular genetic approach and renal clearance study could be of practical benefit in confusing clinical setting and support new diagnostic criteria in GS.

[Hypokalemic metabolic alkalosis: apropos of a case of Gitelman's syndrome]

We present a case of Gitelman's Syndrome in a 20 year-old woman who came to our service with weakness, asthenia, leg cramps and tetany. Laboratory studies revealed metabolic alkalosis with hypokalemia, hypomagnesemia and low calcium in a 24-hour urine test. The diagnosis of this syndrome is made in some cases during adult life because this syndrome is asymptomatic over several years. Gitelman's Syndrome is autosomal recessive as is Bartter's Syndrome. The gene is located in chromosome 16q, which encodes the cotransporter Na/Cl sensitive to thiazide in the distal convoluted tubule. The defect of cotransporter produces an alteration of sodium reabsorption that causes electrolytic disorders typical of this Syndrome and different from Bartter's Syndrome. The typical electrolytic alterations are hypocalciuria and hypomagnesemia secondary to high urinary magnesium excretion. The prognosis of this syndrome is excellent and treatment consists in correction of serum electrolytes with oral administration of magnesium and potassium. In spite of this treatment, in some cases it is very difficult to reach normal serum levels of magnesium because of the high doses of oral magnesium, which produce common crises of diarrhea that increase magnesium gastrointestinal losses.