Leading Articles Molecular Developments in Renal Tubulopathies

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Arch Dis Child 2000;83:189–191 189 Arch Dis Child: first published as 10.1136/adc.83.3.189 on 1 September 2000. Downloaded from Leading articles

Molecular developments in renal tubulopathies

The renal tubule is responsible for the reabsorption of The proximal renal tubule is the site of the bulk of solute more than 99% of the water and sodium in the glomerular and water reabsorption in the nephron. Approximately ultrafiltrate. Congenital or acquired tubular dysfunction 60% of the filtered sodium is reabsorbed in the proximal can therefore readily cause profound electrolyte and segments, along with water, potassium, bicarbonate, phos- volume disturbance. The tubule also has to regulate acid– phate, amino acids, and low molecular weight proteins. base balance, mineral homoeostasis, and the excretion of Dysfunction of the proximal tubule may be isolated or organic anions and drugs. To fulfil these functions, a large generalised. In contrast, the distal tubule has a specialised number of specialised transporters and channels are role in the final modification of urine. Specialised specifically localised in the tubular cell membranes, some transporters are involved in the regulation of sodium and in the luminal border and others in the plasma membrane potassium reabsorption and in proton secretion. Disorders border (basolateral membrane). In the past decade (and of the distal tubule therefore tend to be isolated to a specific especially in the past five years) advances in molecular transporter. genetic research have revealed the structure, function, and eVects of mutations in these transporters, thereby greatly Proximal tubulopathies increasing our understanding of the function and dysfunc- Proximal RTA in children is usually seen as a part of the tion of the renal tubule. Some renal stone disorders (for renal Fanconi syndrome, consisting of aminoaciduria, gly- example, cystinuria, Dent’s disease) and rare genetic cosuria, bicarbonaturia, phosphaturia, and rickets. Most causes of hypertension (for example, Liddle’s syndrome) paediatric cases occur as part of a metabolic disorder are now known to be caused by mutations in tubular trans- although it may be found in some nephropathies and port systems. following exposure to some drugs and toxins (see table 1). A Fanconi like syndrome can be seen in severe vitamin D deficiency; similar plasma biochemical abnormalities can Presentation also be seen in patients with chronic diarrhoea. Recent evi- Many children with genetic defects in tubular function dence supports the theory that there is a common present in infancy although there are several, less severe pathogenetic mechanism for the Fanconi syndrome. disorders that present later or may be asymptomatic (for Rather than there being a multiplicity of defective example, Gitelman’s syndrome) and may only be detected transporters accounting for the many solutes lost in excess, when the patient has a blood or urine sample taken as part it is far more likely that the various metabolic disorders all, of a routine assessment. As a result the true incidence of in some way, reduce the availability of ATP for the enzyme some of these defects is not known. Many tubulopathies NaK-ATPase, thereby reducing sodium extrusion from the lead to failure to thrive; those causing chronic dehydration, tubular cell and reducing the gradient for solute transport http://adc.bmj.com/ salt wasting, or acidosis will inevitably impair growth, while which is coupled to sodium reabsorption.1 Such general- excessive phosphate wastage will lead to rickets and retard ised dysfunction often causes severe sodium and water bone development. Children with renal stones or who are losses in addition to the other electrolytes (potassium, found to have nephrocalcinosis require investigation of bicarbonate, phosphate, etc). their tubular function. A better understanding of the mechanisms whereby The initial assessment of tubular function is based on the these metabolic disorders aVect energy production in the

results of routine biochemical investigations. The request tubular cell should follow from knowledge of the molecu- on September 30, 2021 by guest. Protected copyright. for plasma biochemistry should include sodium, potas- lar bases of these conditions. The gene for cystinosis has sium, urea, creatinine, bicarbonate, chloride, calcium, recently been isolated and characterised.2 Most aVected phosphate, and magnesium, and a urine sample should be children of European origin share a common 65 kB collected for determination of urine electrolytes (sodium, deletion (which should make molecular diagnosis viable). potassium, chloride, calcium, phosphate), amino acids, The gene (CTNS) codes for a protein, cystinosin, predicted glucose, and creatinine. Documentation of urine pH (by to be located in the lysosomal membrane. This is consist- glass electrode) during an episode of severe acidosis (either ent with the known biochemical basis of defective spontaneous or induced by ammonium chloride) helps to lysosomal cystine transport, but further functional work is diVerentiate between proximal and distal renal tubular aci- needed to prove that cystinosin is indeed a cystine dosis (RTA). In proximal (type II) RTA, at a time of severe transporter. Dent’s disease is an X linked condition in acidosis, the urine pH will fall to less than 5.5, while in dis- which aVected males develop low molecular weight tal (type I) RTA, the urine pH will remain above 6. The proteinuria, hypercalciuria, nephrocalcinosis, generalised osmolality of a sample of the ﬁrst morning urine is a useful proximal tubular dysfunction with rickets, and in adult life, check of renal concentrating ability (normally more than may suVer renal stones and chronic renal failure. The dis- 800 mOsm/kg, but less in infants). Determination of order is caused by mutations in a voltage gated chloride urinary low molecular weight proteins (for example, retinol channel, ClC-5, expressed predominantly in the kidney in 3 binding protein, â2 microglobulin) and enzymes (for exam- the subapical endosomes (precursors of lysosomes). As in ple, N-acetyl glucosaminidase) provides more sensitive and cystinosis, disruption of intracellular organelle function speciﬁc evidence of proximal tubular dysfunction and/or (endosome or lysosome) leads to tubular cell dysfunction. damage. An abdominal ultrasound to check the renal anat- Children aVected with the Fanconi–Bickel syndrome omy and to look for the presence of nephrocalcinosis, present with hepatomegaly and failure to thrive. They have together with a radiograph of the wrists or knees for hepatic glycogenosis, fasting ketonuria, and hypoglycaemia evidence of rickets, completes the basic assessment. (features of decreased mobilisation of glucose) and

www.archdischild.com 190 Leading articles Arch Dis Child: first published as 10.1136/adc.83.3.189 on 1 September 2000. Downloaded from Table 1 Inherited causes of the renal Fanconi syndrome

Disorder Onset/features Defective gene/protein Diagnostic test Speciﬁc treatment

Cystinosis Mid/late infancy, poor growth, CTNS/cystinosin Leucocyte cystine concentration Cysteamine may be blond/fair hair, corneal cystine crystals Tyrosinaemia Infancy, poor growth, hepatic Fumaryl acetoacetate hydrolase Plasma amino acids, urine Nitro-triﬂuoro-benzoyl enlargement and dysfunction organic acids (succinyl acetone) cyclohexidine (NTBC) Lowe’s syndrome Birth, X linked, cataracts, Inositol polyphosphate Clinical and molecular genetic hypotonia, developmental delay 5-phosphatase diagnosis Galactosaemia Birth, jaundice, encephalopathy Galactose 1-phosphate uridyl Red cell galactose 1-phosphate Galactose free diet transferase uridyl transferase Fructosaemia Rapid onset after fructose given, Fructose-1-phosphate aldolase B Hepatic fructose-1-phosphate Fructose and sucrose free vomiting, hypoglycaemia, aldolase B diet hepatomegaly Fanconi–Bickel syndrome Infancy, failure to thrive, GLUT2/Glut2 (facilitated ? Monosaccharide diet hepatomegaly, hypoglycaemia glucose transporter) rickets, glycosuria, galactosuria Dent’s disease Child/adulthood, X-linked, CLC-5/CLCN5 (voltage gated Molecular diagnosis ? Potassium citrate/thiazide hypercalciuria, nephrocalcinosis chloride channel) Mitochondrial disorders Usually in infancy, may be Mitochondrial DNA Lactate, pyruvate, muscle multisystem dysfunction enzymology Wilson’s disease Childhood, hepatic disease, Wc-1/P-type copper transporting Copper, caeruloplasmin D-penicillamine neurological signs, ATPase Kayser–Fleischer rings post-prandial hyperglycaemia, galactosaemia, and galacto- of the tubular cell into the lumen (using a potassium chan- suria (features of decreased utilisation of glucose and nel, ROMK), and transport of chloride across the basola- galactose). These features led workers to speculate that teral membrane (by another chloride channel, ClC-Kb). these children had a defect in monosaccharide transport Mutations in the genes coding for NKCC2, ROMK, and and this has now been established. Patients have mutations ClC-Kb have been identiﬁed in patients with various Bar- in a gene encoding Glut2, one of four facilitated diVusion tter syndromes and there is some correlation between the glucose transporters, expressed in liver, small intestine, and genotype and phenotype.8–10 In general, children with the proximal renal tubule.4 more severe phenotype (neonatal Bartter syndrome) tend X linked hypophosphataemic rickets (XLHR, also to have mutations in the genes coding for NKCC2 or known as vitamin D resistant rickets) is the commonest ROMK while those with the milder “classic” Bartter’s syn- inherited form of rickets. Children present with short stat- drome have mutations aVecting the chloride channel ClC- ure and rickets and investigations show a normal plasma Kb. Individuals with Gitelman’s syndrome are often calcium, phosphaturia, hypophosphataemia, a normal para- asymptomatic but are characterised biochemically by thyroid hormone level, and an increased alkaline phos- hypokalaemic alkalosis, hypomagnesaemia, and hypocal- phatase concentration. The pathogenesis is complex with ciuria. The molecular defect aVects another sodium evidence of abnormal sodium coupled phosphate cotrans- chloride cotransporter (NCCT) but with diVerences to port, abnormal vitamin D metabolism, abnormalities of NKCC2. NCCT is localised in the distal tubule, does not

intracellular bone metabolism, and the presence of an as transport potassium, and is the target for thiazide http://adc.bmj.com/ yet unidentified phosphate regulating hormone diuretics.11 The mechanism of hypomagnesaemia is not “phosphatonin”).5 Patients have mutations in the PHEX proven. gene which codes for a membrane glycoprotein that has endopeptidase activity (endopeptidases have an important regulatory eVect on some hormones6). There are rarer dis- Distal renal tubular acidosis Distal RTA is characterised by a failure of urinary acidifi- orders with hypophosphataemic rickets, inherited in an cation, hypokalaemia, hypercalciuria leading to nephrocal- autosomal pattern (for example, the autosomal recessive cinosis and, potentially, stone formation. In contrast to form associated with hypercalciuria). on September 30, 2021 by guest. Protected copyright. proximal RTA which in children is usually part of the generalised Fanconi syndrome, distal RTA commonly occurs Bartter’s and Gitelman’s syndromes as the only tubular abnormality. Various inheritance Since their description some 30 years ago, there has been patterns are known. An autosomal dominant form, more much debate about the pathogenesis of Bartter’s syndrome obvious in adulthood, appears to be related to mutations in and its variants.7 The presentation varies from polyhy- dramnios, prematurity, and life threatening neonatal electrolyte disturbance, through failure to thrive to asymp- NKCC2 tomatic detection of abnormal biochemistry in later life. co-transporter These disorders share features of hypokalaemic alkalosis in Na+ NaK ATPase the presence of a normal blood pressure, elevation of + K 3 Na+ plasma renin and aldosterone, and increased urinary chlo- 2 CI– 2 K+ ride and potassium loss. Some have hypercalciuria and + nephrocalcinosis, some have hypomagnesaemia and hy- K CI– pocalciuria. Recent work has greatly illuminated our understanding of these disorders and thereby of electrolyte CLCNKB transport in the loop of Henle (see fig 1). Sodium chloride ROMK is absorbed in the thick ascending limb by a sodium– potassium-2 chloride cotransporter (NKCC2), the target for inhibition by loop diuretics (for example, frusemide). Reabsorption of sodium and chloride by NKCC2 depends Lumen Blood in turn on basolateral NaK-ATPase (to generate a gradient Figure 1 Schematic view of a renal tubular cell in the thick ascending for sodium entry into the cell), recycling of potassium out limb of the loop of Henle.

www.archdischild.com Leading articles 191 Arch Dis Child: first published as 10.1136/adc.83.3.189 on 1 September 2000. Downloaded from an anion exchanger (AE1). AE1 (or band 3) is expressed in tubulopathies. There is still much to be done and atypical erythrocytes and in the basolateral membrane of the inter- cases (for example, Bartter’s syndrome variants) will calated cells of the renal tubule.12 A significant number of continue to pose challenges. These genetic studies now patients with autosomal recessive distal RTA also have need to be followed by a return to biochemical research to sensorineural deafness. The pathogenetic basis of this link the mutation to the phenotype. This is especially association has now been revealed by recent molecular relevant in proximal tubular disorders where an insight to studies. Urinary acidification in the distal tubule relies on intracellular dysfunction in single gene disorders is likely to secretion of hydrogen ions which is achieved by H+ATPases reveal the mechanisms of the commoner acquired tubular located in the apical membrane. Mutations in ATP6B1, the dysfunction (for example, in acute tubular necrosis). In gene coding for the B1 subunit of apical by H+ATPases are addition, funding for research initiatives has not yet been found in patients with recessive distal RTA and would lead paralleled by provision of routine molecular genetic to failure of acid secretion.13 B1 subunit containing diagnostic services. The diagnosis of a renal tubulopathy H+ATPases are also expressed in the inner ear epithelia so therefore continues to rely on a high index of suspicion and that it is possible that mutations in the gene coding for this the correct interpretation of plasma and urine biochemical unit could aVect endolymphatic acid–base balance at this data. site and consequently impair hearing.13 WILLIAM G VAN’T HOFF Consultant Paediatric Nephrologist, Abnormalities of the amiloride sensitive epithelial Great Ormond Street Hospital, sodium channel Great Ormond Street, London WC1N 3JH, UK Pseudohypoaldosteronism type 1 (PHA1) is a recessive email: [email protected] disorder of severe urinary salt wasting, presenting in the neonatal period with weight loss, vomiting, dehydration, 1 Foreman J. Cystinosis and Fanconi syndromes. In: Barratt TM, Avner ED, Harmon WE, eds. Pediatric nephrology, 4th edition. Philadelphia: Lippin- and sometimes respiratory distress. It is characterised, bio- cott, Williams and Wilkins, 1999:593–607. chemically, by hyponatraemia, severe hyperkalaemia, and 2 Town M, Jean G, Fuchshuber A, et al. A novel gene encoding an integral membrane protein is mutated in patients with nephropathic cystinosis. Nat raised plasma renin and aldosterone concentrations. Genet 1998;18:319–24. Recent work has shown that PHA1 results from mutations 3 Devuyst O, Christie PT, Courtoy PJ, Beaqens R, Thakker RV. Intra-renal and subcellular distribution of the human chloride channel, ClC-5, reveals in the three subunits of the amiloride sensitive epithelial a pathophysiological basis for Dent’s syndrome. Hum Mol Genet sodium channel.14 Other mutations in subunits of the same 1999;8:247–57. 4 Santer R, Schneppenheim R, Dombrowski A, Gotze H, Steinmann B, sodium channel lead to a rare autosomal dominant form of Schaub J. Mutations in GLUT2, the gene for the liver-type glucose trans- hypertension (Liddle’s syndrome).15 Thus some mutations porter, in patients with Fanconi-Bickel syndrome. Nat Genet 1997;17: 324–6. cause gain of function, increased sodium–potassium 5 Rowe PSN. Molecular biology of hypophosphataemic rickets and osteogenic exchange in the distal tubule, and hypertension (Liddle’s) osteomalacia. Hum Genet 1994;94:457–67. 6 The HYP Consortium. A gene (PEX) with homologies to endopeptidases is whereas loss of function mutations are associated with salt mutated in patients with X-linked hypophosphataemic rickets. Nat Genet loss (PHA1). 1995;11:130–6. 7 Rodriguez-Soriano J. Bartter and related syndromes: the puzzle is almost solved. Pediatr Nephrol 1998;12:315–27. Nephrogenic diabetes insipidus 8 Simon DB, Karet FE, Hamdam JM, Di Pietro A, Sanjad SA, Lifton RP. Bartter’s syndrome, hypokalaemic alkalosis with hypercalciuria, is caused Congenital nephrogenic diabetes insipidus (NDI) is by mutations in the Na-K-2Cl cotransporter NKCC2. Nat Genet 1996;13: usually inherited in an X linked manner and aVected males 183–8. 9 Simon DB, Karet FE, Rodriguez-Soriano J, et al. Genetic heterogeneity of typically present in the newborn period with severe polyu- Bartter’s syndrome revealed by mutations in the K+ channel, ROMK. Nat http://adc.bmj.com/ ria and polydipsia, poor sleep, recurrent vomiting, and Genet 1996;14:152–6. 10 Simon DB, Bindra RS, Nelson-Williams C, et al. Mutations in the chloride constipation. Plasma biochemistry shows hypernatraemia channel ClC-Kb cause Bartter’s syndrome type III. Nat Genet 1997;17: and the urinary sodium is low; plasma arginine vasopressin 171–8. 11 Simon DB, Nelson-Williams C, Bia MJ, et al. Gitelman’s variant of Bartter’s and plasma osmolarity are abnormally raised while the syndrome, inherited hypokalaemic alkalosis, is caused by mutations in the corresponding urine values are inappropriately low. thiazide-sensitive NaCl cotransporter. Nat Genet 1996;12:24–30 12 Bruce LJ, Cope DL, Jones GK, et al. Familial distal renal tubular acidosis is Administration of DDAVP fails to correct the urine associated with mutations in the red cell anion exchanger (Band 3, AE1) concentrating defect. The disorder arises as a result of gene. J Clin Invest 1997;100:1693–707. 13 Karet FE, Finberg KE, Nelson RD, et al. Mutations in the gene encoding B1 mutations in the gene encoding the vasopressin receptor in subunit of H+-ATPase cause renal tubular acidosis with sensorineural on September 30, 2021 by guest. Protected copyright. the collecting duct cells (V2R).16 17 These prevent vaso- deafness. Nat Genet 1999;21:84–90. 14 Chang SS, Grunder S, Hanukoglu A, et al. Mutations in subunits of the epi- pressin binding to the receptor or inhibit the signal thelial sodium channel cause salt wasting with hyperkalaemic acidosis, transduction which normally leads to distribution of pseudohypoaldosteronism type 1. Nat Genet 1996;12:248–53. 15 Shimkets RA, Warnock DG, Bosits CM, et al. Liddle’s syndrome: aquaporin 2 (AQP2) water channels to the apical hereditable human hypertension caused by mutations in the â subunit of membrane of the tubular cell. A rare autosomal recessive the epithelial sodium channel. Cell 1994;79:407–14. 18 16 Van den Ouweland AM, Dreesen JC, Verdijk M, et al. Mutations in the form of NDI is a result of mutations in the AQP2 gene. vasopressin type 2 receptor gene (AVPR2) associated with nephrogenic diabetes insipidus. Nat Genet 1992;2:99–102. 17 Pan Y, Metzenberg A, Das S, Jing B, Gitschier J. Mutations in the V2 vaso- Conclusions pressin receptor gene are associated with X-linked nephrogenic diabetes The recent explosion in studies defining the molecular insipidus. Nat Genet 1992;2:103–6. 18 Van Lieburg AF, Verdiijk M, Knoers NVAM, et al. Patients with autosomal basis of disease has greatly increased our knowledge of the nephrogenic diabetes insipidus homozygous for mutations in the renal tubule and has defined the pathogenesis of many aquaporin-2 water-channel gene. Am J Hum Genet 1994;55:648–52.