Transient neonatal cystinuria

Tubular Diseases and Stones Seen From Pediatric and Adult Nephrology Perspectives

The tubular system of the kidneys is a complex series of morphologic and functional units orchestrating the content of tubular fluid as it flows along the nephron and collecting ducts. Renal tubules maintain body water, regulate electrolytes and acid-base balance, reabsorb precious organic solutes, and eliminate specific metabolites, toxins, and drugs. In addition, decisive mechanisms to adjust blood pressure are governed by the renal tubules. Genetic as well as acquired disorders of these tubular functions may cause serious diseases that manifest both in childhood and adulthood. This article addresses a selection of tubulopathies and the underlying pathomechanisms, while highlighting the important differences in pediatric and adult nephrology care. These range from rare monogenic conditions such as nephrogenic diabetes insipidus, cystinosis, and Bartter syndrome that present in childhood, to the genetic and acquired tubular pathologies causing hypertension or nephrolithiasis that are more prevalent in adults. Both pediatric and adult nephrologists must be aware of these conditions and the age-dependent manifestations that warrant close interaction between the two subspecialties.

[Cystinuria]

Cystinuria is the most common monogenic nephrolithiasis disorder. Because of its poor solubility at a typical urine pH of less than 7, cystine excretion results in recurrent urinary cystine stone formation. A high prevalence of high blood pressure and of chronic kidney disease has been reported in these patients. Alkaline hyperdiuresis remains the cornerstone of the preventive medical treatment. To reach a urine pH between 7.5 and 8 and a urine specific gravity less than or equal to 1.005 should be the goal of medical treatment. D-penicillamine and tiopronin, two cysteine-binding thiol agents, should be considered as second line treatments with frequent adverse events that should be closely monitored.

Tissue distribution, hormonal regulation, ontogeny, diurnal expression, and induction of mouse cystine transporters Slc3a1 and Slc7a9

Slc7a11 (xCT) and Slc3a1 (rBAT) are cystine uptake transporters that maintain intracellular concentrations of cysteine, the rate-limiting amino acid in glutathione synthesis. This study was conducted to first determine the tissue distribution of the two transporters in male and female mice. Because Slc3a1 was the primary cystine transporter in liver, its sex-divergent expression, ontogeny, diurnal rhythm and whether its mRNA expression is altered by transcription factors (AhR, CAR, PXR, PPARα, and Nrf2) was also investigated. Slc7a11 was expressed highest in brain and gonads. Slc3a1 was expressed highest in kidney and intestine, followed by liver. Duodenal and hepatic Slc3a1 was higher in females than males. Hepatic Slc3a1 was high during darkness and low during daytime. Hepatic Scl3a1 was lowest pre-birth, increased to near maximal levels at birth, decreased back to pre-birth levels between Days 3-10, and then returned to peak levels by Day 45. Except for CAR, activation of transcription factors did not increase hepatic mRNA expression of Slc3a1. Chemical activation of CAR significantly induced Slc3a1 1.4-fold in wild-type but not CAR-null mice. Slc3a1 mRNA was higher in livers of AhR- and Nrf2-null mice compared to wild-type mice. High doses of diquat but not acetaminophen induced Slc3a1, suggesting Slc3a1 may respond to oxidative stress but not necessarily to GSH depletion. Overall, Slc7a11 is mainly expressed in brain and gonads, whereas Slc3a1 is mainly expressed in kidney, small intestine and liver, and its hepatic expression is regulated by diurnal rhythm and certain xenobiotic treatments.

The developmental programme for genesis of the entire kidney is recapitulated in Wilms tumour

Wilms tumour (WT) is an embryonal tumour that recapitulates kidney development. The normal kidney is formed from two distinct embryological origins: the metanephric mesenchyme (MM) and the ureteric bud (UB). It is generally accepted that WT arises from precursor cells in the MM; however whether UB-equivalent structures participate in tumorigenesis is uncertain. To address the question of the involvement of UB, we assessed 55 Wilms tumours for the molecular features of MM and UB using gene expression profiling, immunohistochemsitry and immunofluorescence. Expression profiling primarily based on the Genitourinary Molecular Anatomy Project data identified molecular signatures of the UB and collecting duct as well as those of the proximal and distal tubules in the triphasic histology group. We performed immunolabeling for fetal kidneys and WTs. We focused on a central epithelial blastema pattern which is the characteristic of triphasic histology characterized by UB-like epithelial structures surrounded by MM and MM-derived epithelial structures, evoking the induction/aggregation phase of the developing kidney. The UB-like epithelial structures and surrounding MM and epithelial structures resembling early glomerular epithelium, proximal and distal tubules showed similar expression patterns to those of the developing kidney. These observations indicate WTs can arise from a precursor cell capable of generating the entire kidney, such as the cells of the intermediate mesoderm from which both the MM and UB are derived. Moreover, this provides an explanation for the variable histological features of mesenchymal to epithelial differentiation seen in WT.

Spectrum of mutations in cystinuria patients presenting with prenatal hyperechoic colon

Cystinuria is a heterogeneous, rare but important cause of inherited kidney stone disease due to mutations in 2 genes: SLC3A1 and SLC7A9. Antenatal hyperechoic colon (HEC) has been reported in some patients as a non-pathological consequence of the intestinal transport defect. We report 83 patients affected by cystinuria: 44 presented prenatally with a HEC (HEC group) and 39 with a classical postnatal form (CC group). SLC3A1 and SLC7A9 were sequenced. All patients were fully genotyped, and the relationship between the genotype and clinical features was analyzed. We identified mutations in SLC3A1 in 80% of the HEC group and in only 49% of the CC group. The SLC3A1 p.Thr216Met mutation was found in 21% of the alleles in the HEC group but was never found in the CC group. Most of the mutations found in the HEC group were considered severe mutations in contrast with the CC group. Twenty-five novel mutations were reported. This study shows a relationship between genotype and the clinical form of cystinuria, suggesting that only the patients with the most severe mutations presented with an HEC. These results emphasized the need for prenatal cystinuria screening using classical third-trimester ultrasound scan and the early management of suspected newborns.

Synergistic mutations in SLC3A1 and SLC7A9 leading to heterogeneous cystinuria phenotypes: pitfalls in the diagnostic workup

BACKGROUND

Cystinuria is an inherited disorder of a renal tubular amino acid transporter and leads to increased cystine excretion with the risk of urinary stone formation. Phenotypical classification is based on urinary amino acid concentration as type I (silent), type non-I (hyper-excretors), mixed or untyped. Genotypic classification is based on mutations in SLC3A1 (type A) or SLC7A9 (type B).

CASE-DIAGNOSIS/TREATMENT

We present six family members with a complex phenotypic profile based on mutations in both genes. The index patient presents a known homozygous mutation (p.T189M) in SLC3A1 and a homozygous mutation (c.225C > T) in SLC7A9. Based on a bioinformatics analysis and published findings, we considered p.T189M to be pathogenic and initially classified c.225C > T as a silent variant. However, segregation analysis detected homozygosity for p.T189M also in non-affected individuals, whereas homozygous c.225C > T segregated with the phenotype. RNA studies confirmed c.225C > T to cause aberrant splicing.

CONCLUSIONS

Based on our findings, we conclude that c.225C > T in SLC7A9 determines the clinical phenotype in this family, whereas additional SLC3A1 mutations aggravate the phenotype in heterozygotes for c.225C > T in SLC7A9 without resulting in cystinuria in the homozygous state. Our results underline the need for careful biochemical characterization of family members of an index case of cystinuria. Genetic analysis of both cystinuria genes may be necessary due to the synergistic effects of mutations in two genes.

Hereditary causes of kidney stones and chronic kidney disease

Adenine phosphoribosyltransferase (APRT) deficiency, cystinuria, Dent disease, familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC), and primary hyperoxaluria (PH) are rare but important causes of severe kidney stone disease and/or chronic kidney disease in children. Recurrent kidney stone disease and nephrocalcinosis, particularly in pre-pubertal children, should alert the physician to the possibility of an inborn error of metabolism as the underlying cause. Unfortunately, the lack of recognition and knowledge of the five disorders has frequently resulted in an unacceptable delay in diagnosis and treatment, sometimes with grave consequences. A high index of suspicion coupled with early diagnosis may reduce or even prevent the serious long-term complications of these diseases. In this paper, we review the epidemiology, clinical features, diagnosis, treatment, and outcome of patients with APRT deficiency, cystinuria, Dent disease, FHHNC, and PH, with an emphasis on childhood manifestations.

Cystinuria: an inborn cause of urolithiasis

Cystinuria (OMIM 220100) is an inborn congenital disorder characterised by a defective cystine metabolism resulting in the formation of cystine stones. Among the heterogeneous group of kidney stone diseases, cystinuria is the only disorder which is exclusively caused by gene mutations. So far, two genes responsible for cystinuria have been identified: SLC3A1 (chromosome 2p21) encodes the heavy subunit rBAT of a renal b^0,+ transporter while SLC7A9 (chromosome 19q12) encodes its interacting light subunit b^0,+AT. Mutations in SLC3A1 are generally associated with an autosomal-recessive mode of inheritance whereas SLC7A9 variants result in a broad clinical variability even within the same family. The detection rate for mutations in these genes is larger than 85%, but it is influenced by the ethnic origin of a patient and the pathophysiological significance of the mutations. In addition to isolated cystinuria, patients suffering from the hypotonia-cystinuria syndrome have been reported carrying deletions including at least the SLC3A1 and the PREPL genes in 2p21. By extensive molecular screening studies in large cohort of patients a broad spectrum of mutations could be identified, several of these variants were functionally analysed and thereby allowed insights in the pathology of the disease as well as in the renal trafficking of cystine and the dibasic amino acids. In our review we will summarize the current knowledge on the physiological and the genetic basis of cystinuria as an inborn cause of kidney stones, and the application of this knowledge in genetic testing strategies.

Isolated hyperechoic fetal colon before 36 weeks' gestation reveals cystinuria

OBJECTIVES

To determine whether there is an association between the fetal ultrasound finding of hyperechoic colon and the gestational age at which it presents and cystinuria.

METHODS

A prospective national survey was performed in France including all observations of isolated fetal hyperechoic colon detected at routine second- and third-trimester ultrasound over a 2-year period. Collected images were reviewed by experts. Colon was defined as being hyperechoic when its echogenicity was at least equal to that of the iliac bone. It was diagnosed when large tubular echogenic portions of the colon, without a focal mass and without posterior acoustic shadows, were observed at the periphery of the abdomen. Urinary amino acid analysis was performed after birth in the cases identified to test for cystinuria.

RESULTS

Nineteen fetuses with ultrasound findings of hyperechoic colon were included, and the mothers of 16 of these agreed to participate in the study. In eight of nine cases of hyperechoic colon observed before 36 weeks' gestation cystinuria was confirmed at birth. In the seven remaining cases, observed after 36 weeks, none was found to have cystinuria and all had normal images at previous routine ultrasound scans at 22 and 33 weeks. When present, no difference in the sonographic appearance of hyperechoic colon was noted between the two groups. In the cystinuria-affected cases, the length of the hyperechoic mass appeared to increase with gestational age.

CONCLUSIONS

In our experience, the presence of a hyperechoic colon at routine ultrasound scan before 36 weeks' gestation should prompt screening for cystinuria at birth, while later observation (> 36 weeks) of this finding does not appear to be related to any disease.

Nephrolithiasis related to inborn metabolic diseases

Nephrolithiasis associated with inborn metabolic diseases is a very rare condition with some common characteristics: early onset of symptoms, family history, associated tubular impairment, bilateral, multiple and recurrent stones, and association with nephrocalcinosis. The prognosis of such diseases may lead to life threatening conditions, not only because of unabated kidney damage but also because of progressive extra-renal involvement, either in a systemic form (e.g. primary hyperoxaluria type 1, requiring combined liver and kidney transplantation), or in a neurological form (Lesch-Nyhan syndrome leading to auto-mutilation and disability, phosphoribosyl pyrophosphate synthetase superactivity, which is associated with mental retardation). Patients with other inborn metabolic diseases present only with recurrent stone formation, such as cystinuria, adenine phosphoribosyl-transferase deficiency, xanthine deficiency. Finally, nephrolithiasis may be secondarily part of some other metabolic diseases, such as glycogen storage disease type 1 or inborn errors of metabolism leading to Fanconi syndrome (nephropathic cystinosis, tyrosinaemia type 1, fructose intolerance, Wilson disease, respiratory chain disorders, etc.). The diagnosis is based on highly specific investigations, including crystal identification, biochemical analyses and DNA study. The treatment of nephrolithiasis requires hydration as well as specific measures. Compliance is a major issue regarding the progression of renal damage, but the overall outcome mainly depends on extra-renal involvement in relation to the metabolic defect.

Cystinuria

Cystinuria is an inherited disorder characterized by the impaired reabsorption of cystine in the proximal tubule of the nephron and the gastrointestinal epithelium. The only clinically significant manifestation is recurrent nephrolithiasis secondary to the poor solubility of cystine in urine. Although cystinuria is a relatively common disorder, it accounts for no more than 1% of all urinary tract stones. Thus far, mutations in 2 genes, SLC3A1 and SLC7A9, have been identified as being responsible for most cases of cystinuria by encoding defective subunits of the cystine transporter. With the discovery of mutated genes, the classification of patients with cystinuria has been changed from one based on phenotypes (I, II, III) to one based on the affected genes (I and non-type I; or A and B). Most often this classification can be used without gene sequencing by determining whether the affected individual's parents have abnormal urinary cystine excretion. Clinically, insoluble cystine precipitates into hexagonal crystals that can coalesce into larger, recurrent calculi. Prevention of stone formation is the primary goal of management and includes hydration, dietary restriction of salt and animal protein, urinary alkalinization, and cystine-binding thiol drugs.

[Inherited monogenic kidney stone diseases: recent diagnostic and therapeutic advances]

Hereditary monogenic kidney stone diseases are rare diseases, since they account for nearly 2% of nephrolithiasis cases in adults and 10% in children. Most of them are severe, because they frequently are associated with nephrocalcinosis and lead to progressive impairment of renal function unless an early and appropriate etiologic treatment is instituted. Unfortunately, treatment is often lacking or started too late since they are often misdiagnosed or overlooked. The present review reports the genotypic and phenotypic characteristics of monogenic nephrolithiases, with special emphasis on the recent advances in the field of diagnosis and therapeutics. Monogenic stone diseases will be classified into three groups according to their mechanism: (1) inborn errors of the metabolism of oxalate (primary hyperoxalurias), uric acid (hereditary hyperuricemias) or other purines (2,8-dihydroxyadeninuria), which, in addition to stone formation, result in crystal deposition in the renal parenchyma; (2) congenital tubulopathies affecting the convoluted proximal tubule (such as Dent's disease, Lowe syndrome or hypophosphatemic rickets), the thick ascending limb of Henlé's loop (such as familial hypomagnesemia and Bartter's syndromes) or the distal past of the nephron (congenital distal tubular acidosis with or without hearing loss), which are frequently associated with nephrocalcinosis, phosphatic stones and extensive tubulointerstitial fibrosis; (3) cystinuria, an isolated defect in tubular reabsorption of cystine and dibasic aminoacids, which results only in the formation of stones but requires a cumbersome treatment. Analysis of stones appears of crucial value for the early diagnosis of these diseases, as in several of them the morphology and composition of stones is specific. In other cases, especially if nephrocalcinosis, phosphatic stones or proteinuria are present, the evaluation of blood and urine chemistry, especially with regard to calcium, phosphate and magnesium, is the key of diagnosis. Search for mutations is now increasingly performed in as much as genetic counselling is important for the detection of heterozygotes in autosomic recessive diseases and of carrier women in X-linked diseases. In conclusion, better awareness to the rare monogenic forms of nephrolithiasis and/or nephrocalcinosis should allow early diagnosis and treatment which are needed to prevent or substantially delay progression of end-stage renal disease. Analysis of every first stone both in children and in adults should never be neglected, in order to early detect unusual forms of nephrolithiasis requiring laboratory evaluation and deep etiologic treatment.

Evidence for association of SLC7A9 gene haplotypes with cystinuria manifestation in SLC7A9 mutation carriers

Cystinuria is a complex genetic disorder. In the present study, we report on the strict linkage disequilibrium of SLC7A9 mutations with the wild type SLC7A9 haplotype of 15 single nucleotide polymorphisms (SNPs) and their effect on cystinuria manifestation and classification. Specifically, screening for mutations and polymorphisms was performed in the family members of ten cystinuric patients with SLC7A9 gene mutations. The molecular genetic and clinical data of cystinuric patients and their relatives were combined to construct the SLC7A9 SNP haplotypes and evaluate the manifestation of the disorder in carriers for a SLC7A9 gene mutation. It was found that all carriers of a SLC7A9 mutation manifested cystinuria if their normal allele had non-wild type nucleotides in two or more of the identified polymorphic sites. Subsequently, the polymorphic background of the SLC7A9 gene probably affects the expression of the disorder in SLC7A9 mutation carriers and points to a revised genetic classification of cystinuric patients.

Heterogeneous mutations in the SLC3A1 and SLC7A9 genes in Chinese patients with cystinuria

Cystinuria is a recessively inherited aminoaciduria that leads to recurrent urolithiasis. It is caused by the defective transport of cystine and dibasic amino acids in the proximal renal tubules and intestinal epithelium. Two genes responsible for this, SLC3A1 and SLC7A9, are known. Patients with two SLC3A1 mutations are classified as type A cystinuria, whereas patients with two SLC7A9 mutations are classified as type B cystinuria. Few clinical and molecular data have been reported for Asian cystinuria patients. In this study, we determined the molecular basis of cystinuria in eight unrelated Chinese subjects. Coding exons and flanking introns of the SLC3A1 and SLC7A9 genes were directly sequenced after amplification by polymerase chain reaction. Five different SLC3A1 mutations were found. Two missense mutations, D210G and S547L, were novel. The other three SLC3A1 mutations (IVS6+2T>C, R181Q and R365W) have been described previously. In addition, four novel SLC7A9 mutations, C137R, c.730delG, IVS10+2_3delTG and IVS12+3insT, together with two previously reported mutations (A70V and G195R) were found. All patients except one carried compound heterozygous mutations. IVS12+3insT was detected in patients from two families. This is the first molecular genetic study on Chinese cystinuria patients. Three patients with type A cystinuria, two with type B cystinuria, and three carriers of type B cystinuria were identified. Our results suggest that the molecular basis of cystinuria is heterogeneous in our local population.