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 Analysis of genomic DNA revealed two unpublished missense transitions, p.G216R and p.N333S in the SLC2A9 gene.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.