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.

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.

Bartter's, Gitelman's, and Gordon's syndromes. From physiology to molecular biology and back, yet still some unanswered questions

The molecular basis of many of the inherited disorders of potassium homeostasis has become much clearer in the last two decades. Despite these new insights into the physiology of renal potassium handling, a number of questions remain to be answered. The examples we use to illustrate these issues are Gordon's syndrome, Bartter's syndrome, and Gitelman's syndrome. Our objective is to integrate these new insights into an understanding of the pathophysiology of renal potassium handling. We also propose different ways to think about some of the unresolved issues in this area.

Hypokalemic metabolic alkalosis--three case reports

The two most common forms of inherited normotensive hypokalemic metabolic alkalosis are Bartter's and Gitelman's syndromes. Bartter's syndrome typically present with normal or increased calcium excretion. Hypomagnesemia occurs in only one third of affected individuals. In contrast, hypomagnesemia and hypocalciuria are considered hallmarks of Gitelman's syndrome. In most patients, the symptom of muscle weakness and polyuria occur early in life, which may be attributed to potassium depletion. Despite hyperaldosteronism, the patients tend to be normotensive, which is at least explained by vascular hyperresponsiveness to prostaglandins. Therapeutic approaches to Bartter's and Gitelman's syndromes include potassium supplementation, prostaglandin synthesis inhibitors (nonsteroid anti-inflammatory agents), aldosterone antagonists and converting enzyme inhibitors. Three patients with hypokalemia, normal blood pressure, metabolic alkalosis, hyperreninemia and hyperaldosteronism are described. Two patients had Bartter's syndrome and one patients had Gitelman's syndrome.

Monogenic tubular salt and acid transporter disorders

In this review I will briefly discuss two major categories of inherited renal tubular transport disorders: those primarily affecting sodium and potassium (whether retention or wasting), and the primary renal acidopathies. The kidney does not of course carry out any of its myriad homeostatic functions in a vacuum, and interactions with other systems form an important part of the overall picture. This is perhaps most marked in the context of salt homeostasis, where the endocrine system, particularly the mineralocorticoid axis and aldosterone, are closely dovetailed. While these disorders are in general rare, the insights gained in understanding their molecular bases have added significantly to our understanding of normal human physiology. The hope is that these findings may in the future prove relevant to the understanding and treatment of more common disorders.

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

PURPOSE

Hypokalemic salt-losing tubulopathies (Bartter-like syndromes) comprise a set of clinically and genetically distinct inherited renal disorders. Mutations in four renal membrane proteins involved in electrolyte reabsorption have been identified in these disorders: the furosemide-sensitive sodium-potassium-chloride cotransporter NKCC2, the potassium channel ROMK, the chloride channel ClC-Kb, and the thiazide-sensitive sodium-chloride cotransporter NCCT. The aim of this study was to characterize the clinical features associated with each mutation in a large cohort of genetically defined patients.

PATIENTS AND METHODS

The phenotypic characteristics of 65 patients with molecular defects in NKCC2, ROMK, ClC-Kb, or NCCT were collected retrospectively.

RESULTS

ROMK and NKCC2 patients presented with polyhydramnios, nephrocalcinosis, and hypo- or isosthenuria. Hypokalemia was less severe in the ROMK patients compared with the NKCC2 patients. In contrast, NCCT patients had hypocalciuria, hypomagnesemia, and marked hypokalemia. While this dissociation of renal calcium and magnesium handling was also observed in some ClC-Kb patients, a few ClC-Kb patients presented with hypercalciuria and hypo- or isosthenuria.

CONCLUSIONS

ROMK, NKCC2, and NCCT mutations usually have uniform clinical presentations, whereas mutations in ClC-Kb occasionally lead to phenotypic overlaps with the NCCT or, less commonly, with the ROMK/NKCC2 cohort. Based on these results, we propose an algorithm for the molecular diagnosis of hypokalemic salt-losing tubulopathies.

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.

Hypokalemic salt-losing tubulopathy with chronic renal failure and sensorineural deafness

OBJECTIVE

To characterize a rare inherited hypokalemic salt-losing tubulopathy with linkage to chromosome 1p31.

METHODS

We conducted a retrospective analysis of the clinical data for 7 patients in whom cosegregation of the disease with chromosome 1p31 had been demonstrated. In addition, in 1 kindred, prenatal diagnosis in the second child was established, allowing a prospective clinical evaluation.

RESULTS

Clinical presentation of the patients was homogeneous and included premature birth attributable to polyhydramnios, severe renal salt loss, normotensive hyperreninemia, hypokalemic alkalosis, and excessive hyperprostaglandin E-uria, which suggested the diagnosis of hyperprostaglandin E syndrome/antenatal Bartter syndrome. However, the response to indomethacin was only poor, accounting for a more severe variant of the disease. The patients invariably developed chronic renal failure. The majority had extreme growth retardation, and motor development was markedly delayed. In addition, all patients turned out to be deaf.

CONCLUSION

The hypokalemic salt-losing tubulopathy with chronic renal failure and sensorineural deafness represents not only genetically but also clinically a disease entity distinct from hyperprostaglandin E syndrome/antenatal Bartter syndrome. A pleiotropic effect of a single gene defect is most likely causative for syndromic hearing loss.

Genetic renal tubular disorders of renal ion channels and transporters

This perspective on genetic renal tubular transport disorders selectively reviews the pathophysiology of renal apical Na(+) transport systems. These transporters play an essential role in the control of extracellular fluid volume and blood pressure. Significant advancements in the understanding of the role of these genes in Mendelian forms of extracellular volume homeostatic disorders have been achieved in the recent years. Of even greater importance will be the ongoing definition of the various factors that regulate the expression and activity of the Na(+) transport systems. These regulatory pathways, and the responses to environmental factors such as dietary salt, stress, and so on, may determine the appearance, severity and complexity of the clinical phenotypes that result from genetic disorders of the renal apical Na(+) transporters.

Inherited tubular transport disorders

Inherited abnormalities of renal tubular transport processes encompass a heterogeneous set of disorders due to single gene defects. Elucidating the molecular basis for these generally rare disorders has provided important insights into disease pathogenesis as well as the complexities of normal renal transport physiology. This review focuses on the clinical features and recent molecular advances in cystinuria, X-linked hypophosphatemic rickets, and the Bartter-Gittelman spectrum of disorders. Also addressed are disorders of calcium homeostasis resulting from loss-of-function and gain-of-function mutations of the extracellular calcium-sensing receptor, as well as the single gene defects that cause distal renal tubular acidosis.

A common NKCC2 mutation in Costa Rican Bartter's syndrome patients: evidence for a founder effect

Bartter's syndrome involves an overlapping set of closely related renal tubular disorders that can be subdivided into at least three clinical phenotypes: (1) the hypercalciuric antenatal Bartter variant; (2) the classic Bartter variant; and (3) the hypocalciuric-hypomagnesemic Gitelman variant. Recent data demonstrate that in several phenotypically indistinguishable cohorts, antenatal Bartter's syndrome is genetically heterogeneous. In these patients, mutations in the genes encoding either the bumetanide-sensitive Na-K-2Cl cotransporter (NKCC2) or the ATP-regulated potassium channel ROMK (KCNJI) have been identified. A cohort of 20 Costa Rican patients with a congenital syndrome that bears strong similarities to antenatal Bartter's syndrome but also has several distinct features has recently been described. In this cohort, we have identified a predominant mutation that introduces a premature stop in codon W625 of the NKCC2 gene (SCL12A1). This mutant allele is contained on a single common haplotype, suggesting that the majority of antenatal Bartter's syndrome patients in Costa Rica share a single common ancestor.

[Non-lithiasic hereditary tubulopathies]

Renal tubular disorders include various clinical conditions wherein the renal tubular reabsorption of an ion or organic solute is significantly decreased. It may concern the renal handling of a single substance, such as glucose or phosphate, a group of substances or a combination of ions and organic substances. Close to this group of diseases, it is also possible to consider two conditions due to renal tubular resistance to vasopressin and parathyroid hormone: congenital nephrogenic diabetes insipidus and pseudohypoparathyroidisms, respectively. Clinically, the urinary loss of these substances may be inapparent, like in familial glucosuria, but in most cases, it may be responsible for characteristic disorders such as failure to thrive, dehydration, rickets, etc. Recently, physiological and molecular cloning studies have brought crucial information for testing candidate genes and understanding the underlying molecular bases of these disorders.