Urinary Cystine Excretion Research Articles

#2278 EUROCYS, a European cystinuria registry

Abstract Background and Aims Cystinuria (OMIM 220100) is a rare autosomal recessive disorder in which high urinary cystine excretion leads to formation of cystine stones due to its low solubility at normal urinary pH. Retrospective series have been published but precise data to guide long term optimal management of these patients are lacking. Our goal was to collect consistent and comparative prospective clinical, biological, genetic and radiological data from patients (children and adults) with cystinuria followed in European centers. Method We developed EUROCYS, a multidisciplinary, European, multicenter registry of patients with cystinuria in which demographic, clinical, surgical, biological, genetic, radiological and therapeutic data are collected longitudinally. These data are recorded via the European Rare Kidney Disease Registry (ERKReg) platform. Results Enrolment started in July 2021. To date, 269 patients have been included at 29 different centers in 8 European countries. Mean age at diagnosis was 18.7 years (SD 17.3) and mean age at inclusion was 35.4 years (20.2). 86.9% of patients had less than 9 calculi extractions reported, 11.0% had between 10 to 19, and 2.0% had more than 20. 61.4% (418/680) were treated by ureteroscopy, 16.2% (110/680) by shock wave lithotripsy and 16.0% (109/680) by percutaneous nephrolithotomy. 39% of patients had a new stone expulsion or surgery during the last 12 months. At the inclusion visit, mean urinary pH was below the recommended level at 6, 8 (0.8). Only 16.3% of patients had a urinary pH between 7.5 and 8: 77.2% of patients had a urinary pH below 7.5 and 6.3% above 8. In adults, mean 24-hour urinary volume was 2653 mL (1077). 34.9% had a target urine volume above 3000 ml. Under treatment, spot urine cystine concentration was 345.5 mg/l (255.2) and 44.8% of patients had a concentration below target concentration (250 mg/l). Mean sodium urinary excretion was 147 mmol/24 h (67). Only 29% of patients had a sodium urinary excretion below 100 mmol/24 h. As alkalinizing agent, 87.2% of patients received potassium citrate, 16.3% sodium bicarbonate and 9.1% potassium bicarbonate. As cystine binding drug, 71 patients (26.7%) were treated with Tiopronin and 3 experienced serious adverse events (hematological, cutaneous, proteinuria). 15 (5.6%) were treated with D-Penicillamine and 2 had adverse events (proteinuria and gastrointestinal). In adults, mean eGFR at inclusion was 82.1 ml/min/1.73 m² (26.3): 40.9% of patients were at chronic kidney disease (CKD) stage 1, 40.1% at CKD2, 16.7% at CKD3, 0.7% at CKD 4 and 1.5% at CKD5. Conclusion The EUROCYS registry shows real life data on cystinuria patients followed in European centers. Despite alkalinizing treatment, most patients do not reach the recommended urinary pH level and only one third have an adequate urinary volume. This registry will allow analyzing the impact of adhesion to recommendations on disease activity.

Cystinuria: clinical practice recommendation

Cystinuria (OMIM 220100) is an autosomal recessive hereditary disorder in which high urinary cystine excretion leads to the formation of cystine stones because of the low solubility of cystine at normal urinary pH. We developed clinical practice recommendation for diagnosis, surgical and medical treatment, and follow-up of patients with cystinuria. Elaboration of these clinical practice recommendations spanned from June 2018 to December 2019 with a consensus conference in January 2019. Selected topic areas were chosen by the co-chairs of the conference. Working groups focusing on specific topics were formed. Group members performed systematic literature review using MEDLINE, drafted the statements, and discussed them. They included geneticists, medical biochemists, pediatric and adult nephrologists, pediatric and adult urologists experts in cystinuria, and the Metabolic Nephropathy Joint Working Group of the European Reference Network for Rare Kidney Diseases (ERKNet) and eUROGEN members. Overall 20 statements were produced to provide guidance on diagnosis, genetic analysis, imaging techniques, surgical treatment (indication and modalities), conservative treatment (hydration, dietetic, alkalinization, and cystine-binding drugs), follow-up, self-monitoring, complications (renal failure and hypertension), and impact on quality of life. Because of the rarity of the disease and the poor level of evidence in the literature, these statements could not be graded. This clinical practice recommendation provides guidance on all aspects of the management of both adults and children with cystinuria, including diagnosis, surgery, and medical treatment.

Tolvaptan treatment of cystine urolithiasis in a mouse model of cystinuria.

Cystinuria is an inherited disease characterized by increased urinary cystine excretion and recurrent cystine stones. Current treatment regimens have limited effectiveness in preventing stone recurrence and are often poorly tolerated. The aim of this study was to evaluate the effect of tolvaptan, a vasopressin receptor 2 (V2) antagonist, on cystine stone volume in mice with cystinuria. Tolvaptan (0.4mg per mouse) or placebo was delivered by gavage daily for 30days. Urinary amino acids and cystine stones were analyzed to assess drug efficacy in preventing L-cystine stone growth using several analytical methods. Data were entered into SPSS and analyzed by paired sample T test. p value < 0.05 was considered significant. Compared with control group, the liquid intake and urine volume in tolvaptan-treated mice were significantly increased. The urinary cystine concentrations in group tolvaptan was lower than the baseline concentration before the experiment. After treatment, mice treated with tolvaptan had significantly delayed stone growth, exhibited lower overall stone volume accumulation, compared with control group. The increased stone volume of tolvaptan group was less than control group (8.00 ± 4.93 mm3 vs 27.90 ± 4.48 mm3, p<0.001). The serum creatinine in the control group (11.75 ± 1.634μmol/L) was higher than that in the tolvaptan group (7.625 ± 1.401μmol/L) (p = 0.0759). In addition, tolvaptan significantly inhibited the formation and growth of stones in mice after cystolithotomy. The present study indicated that tolvaptan's efficacy in preventing L-cystine stone growth through increased liquid intake and urine volume of cystinuric mice.

The effect of additional drug therapy as metaphylaxis in patients with cystinuria: a systematic review

To systematically review the effect of additional drug therapy as metaphylaxis in patients with cystinuria. A literature search of three databases (MEDLINE, Embase and the Cochrane Library) was performed according to the PRISMA-guidelines enclosing articles published up to May 2019. A total of 1117 articles were screened. Thirty-four publications met the inclusion criteria for this review. Male-female ratio in the studied cohorts was 49.9% - 50.1%. The majority of studies showed a positive effect in reducing stone events and/or urinary cystine excretion. D-Penicillamine showed success in 13/14 (92%) studies, whereas Tiopronin-treatment showed a reduction in all (8/8; 100%) studies. All studies on Captopril (4/4) showed a decrease, however not all significant. The same is true for studies on Thiols in combination with Captopril (2/2). Furthermore, Tiopronin showed less side effects compared to D-penicillamine, respectively 30% and 37%. Captopril showed the least adverse events, with one event in nine patients. The evidence on benefit of additional drug therapy in patients with cystinuria is scarce. All studied medications showed an effect on stone event and urinary cystine excretion, when used in addition to hyperhydration, alkalization and a diet low on methionine. Based on this systematic review, no drug can be preferred over another. An important aspect in the choice of drug is the risk of side effects. Therefore, the choice of additional drug should be personalized for every patient where the risk of side effects should be taken into consideration.

Genetic and Clinical Analyses of 13 Chinese Families With Cystine Urolithiasis and Identification of 15 Novel Pathogenic Variants in SLC3A1 and SLC7A9.

BackgroundCystinuria is a rare genetic disorder characterized by defective renal reabsorption of cystine, ornithine, arginine, and lysine. The increased urinary excretion of cystine results in the development of cystine urolithiasis (CU). The mutated SLC3A1 and SLC7A9 genes are the cause of CU, a global disorder. Its frequency and mutation spectrum vary between different populations. In Asia, the data for CU are limited.MethodUrinary stones were collected from patients of a single center over a five-year period and analyzed via Fourier transform infrared spectroscopy. Genomic DNA was isolated from 13 patients with CU and their parents and from 26 controls affected by calcium oxalate dihydrate stones. The coding regions and the exon–intron boundaries of SLC3A1 and SLC7A9 were subjected to PCR amplification and then sequenced via traditional Sanger sequencing. Genetic variants were functionally annotated using the InterVar, ClinVar, gnom AD, and HGMD databases.ResultsFrom the 232 samples of urinary stones, we identified 13 patients with CU (10 males and 3 females). The onset age was from 7 months to 9 years. The CU stones varied from 0.26 cm3 to 18.67 cm3. Sanger sequencing detected a total of 14 SLC3A1 (nine were novel) and 10 SLC7A9 (six were novel) rare variants from the 13 CU families. All variants, including 15 novel variants, were pathogenic, disease-causing, or damaging.ConclusionAll 13 pediatric CU families harbored SLC3A1 or/and SLC7A9 rare variants. A total of 15 novel pathogenic variants in SLC3A1 and SLC7A9 were identified. This study expanded the known mutational spectrum of CU in the Chinese population.

Pathophysiology and Treatment of Cystinuria

Cystinuria is a relatively rare autosomal recessive disorder that manifests early in life and is associated with the development of kidney stones composed of cystine. It is due to mutations in two genes that are involved in the transport of cystine, neutral, and dibasic amino acids in the proximal tubule of the kidney. Patients are at risk for developing chronic kidney disease. Diagnosis is typically established with stone analysis and quantitative urinary cystine excretion. These patients may form extremely large stones requiring percutaneous nephrolithotomy. Stone-prevention strategies include dietary modifications (increased fluid intake and limitation of sodium and animal protein consumption), urine pH manipulation, and thiol-binding agents. These patients should be followed closely, and preemptive stone removal with ureteroscopy should be considered to limit the necessity for more invasive procedures. This review contains 2 figures and 38 references. Key Words: a-mercaptopropionyl glycine, amino acid transport, chronic kidney disease, cystinuria, SLC3A1, SLC7A9, thiol-binding agent, urinary pH manipulation

Clinical Outcomes for Cystinuria Patients with Unilateral Versus Bilateral Cystine Stone Disease.

Cystinuria is a genetic disorder marked by elevated urinary cystine excretion and recurrent cystine nephrolithiasis. Interestingly, despite seemingly similar contralateral renal anatomy, a subset of cystinuric patients consistently form stones in only one kidney. The aim of this study is to evaluate clinical outcomes in unilateral vs bilateral cystine stone formers. We performed a retrospective case-control study of cystinuric patients evaluated and treated at the University of California, San Francisco between 1994 and 2015 and categorized patients as either unilateral or bilateral stone formers. Clinical presentation, baseline patient demographics, stone procedures, medical therapy regimens, and long-term renal function were compared between the two groups. A total of 42 cystine stone patients (22 female, 20 male) were included in the analysis. The median age at first presentation was 18.5 years and median age at study conclusion was 45.5 years. Two-thirds of patients (n = 28) had a history of bilateral stones, whereas one-third (n = 14) had unilateral stones. Medical therapy regimens were similar between groups. Despite an increased average number of lifetime surgeries (7.5 sessions for bilateral vs 3.7 sessions for unilateral, p < 0.05), there was no significant difference in medians of the most recent glomerular filtration rate when compared with unilateral stone formers (81.5 vs 95 mL/min, respectively; p = 0.28). The majority of cystinuric patients within our cohort form stones bilaterally during their lifetime, and require more surgical interventions than unilateral stone formers. Despite this, overall renal function is well preserved in unilateral and bilateral cystinuric stone formers treated with minimally invasive stone extraction procedures.

Clinical and molecular characterization of cystinuria in a French cohort: relevance of assessing large-scale rearrangements and splicing variants.

BackgroundCystinuria is an autosomal recessive disorder of dibasic amino acid transport in the kidney and the intestine leading to increased urinary cystine excretion and nephrolithiasis. Two genes, SLC3A1 and SLC7A9, coding respectively for rBAT and b0,+AT, account for the genetic basis of cystinuria.MethodsThis study reports the clinical and molecular characterization of a French cohort including 112 cystinuria patients and 25 relatives from 99 families. Molecular screening was performed using sequencing and Quantitative Multiplex PCR of Short Fluorescent Fragments analyses. Functional minigene‐based assays have been used to characterize splicing variants.ResultsEighty‐eight pathogenic nucleotide changes were identified in SLC3A1 (63) and SLC7A9 (25) genes, of which 42 were novel. Interestingly, 17% (15/88) and 11% (10/88) of the total number of variants correspond, respectively, to large‐scale rearrangements and splicing mutations. Functional minigene‐based assays were performed for six variants located outside the most conserved sequences of the splice sites; three variants affect splice sites, while three others modify exonic splicing regulatory elements (ESR), in good agreement with a new in silico prediction based on ΔtESRseq values.ConclusionThis report expands the spectrum of SLC3A1 and SLC7A9 variants and supports that digenic inheritance is unlikely. Furthermore, it highlights the relevance of assessing large‐scale rearrangements and splicing mutations to fully characterize cystinuria patients at the molecular level.

Cystinuria in a patient with 19q12q13.1 deletion.

Cystinuria is a genetic cause of kidney stones with a prevalence of 1 in 7000 births. So far, two genes have been described responsible for this disorder (SLC3A1 and SLC7A9). We report a patient with an SLC7A9 gene mutation located in 19q13.1 on one allele and with a 19q12q13 region deletion on the other allele. The characteristic clinical features of the 19q13.1 microdeletion syndrome include facial dysmorphism, signs of ectodermal dysplasia, growth retardation, neurologic features and genitourinary anomalies. Cystinuria has not yet been described as part of this syndrome, although one of its responsible genes (SLC7A9) is in the same genomic location. The index case is a 6-year-old male presented with distinctive facial features, cutis aplasia of the scalp, rudimentary teeth, microcephaly, intrauterine and postnatal growth retardation, psychomotor developmental delay, speech delay, epilepsy, inguinal hernias and cystinuria. An array-CGH analysis was performed, finding a large deletion of the 19q12q13.11 cytobands, which affects 19 genes. Two of them are involved in the 19q13.11 deletion syndrome and another affected gene is SLC7A9, responsible for type B cystinuria. Sanger sequencing was performed as well, detecting a heterozygous mutation of the SLC7A9 gene, located in 19q13.1. As far as we know, this is the first described case of cystinuria in a patient with SLC7A9 gene mutation located in 19q13.1 on one allele and with 19q12q13 region deletion on the other allele. Although this patient can be classified as a type B heterozygote and, therefore, his renal prognosis is not severe, the occasional nephrolithiasis found in such patients justifies a close follow-up with regular testing of urinary cystine excretion. We suggest that the recessive behavior of this case, explains the clinical features regarding cystinuria. We propose that in the face of patients affected of a phenotype matchable with 19q13.11 syndrome and cystinuria, a mutational or sequencing study of the SLC7A9 gene should be performed to allow an early onset of diagnosis and treatment.

Mutation analysis of SLC3A1 and SLC7A9 genes in patients with cystinuria.

Cystinuria is an autosomal inherited disorder of renal reabsorption of cystine, arginine, lysine, and ornithine. Increased urinary excretion of cystine results in the formation of kidney stones. Considering the few studies on the genetic basis of the cystinuria in the Middle East and the population-specific distribution of mutations in the SLC3A1 and SLC7A9 genes, in the present study, mutation analysis of these two genes was performed in a cohort of Iranian patients with cystinuria. Thirty unrelated cystinuria patients were analyzed for four of the most common mutations using ARMS-PCR (M467T, T216M) and RFLP-PCR (G105R, R333W) methods. For negative sample, two exons of both genes, which harbor many mutations, were subject to DNA sequencing. Eight variants were identified including missense, polymorphism, intron variant, and a novel variant. The most frequent mutations were not detected in our patients and only G105R was found. Since the molecular genetic testing results may influence the therapy and prognosis of cystinuria, this paper contributes to understanding of the molecular basis of cystinuria in the Iranian patients.

The Interaction of Thiol Drugs and Urine pH in the Treatment of Cystinuria

Pharmacological therapy for cystinuria consists of alkali salts to increase urine pH and thiol drugs to form soluble cysteine-drug complexes. The effect of alkalinizing urine on thiol drug activity has not been well studied. Urine samples were obtained from 5 healthy subjects and pH was adjusted (range 6.0 to 8.0) in each urine aliquot. Urine samples were incubated with cystine crystals at 37C for 5, 15 and 60 minutes, and 48 hours. We compared cystine solubility in urine samples spiked with thiol drugs at a final concentration of 2 mM to that in control urine samples at the various pH levels and time points. In samples incubated for 48 hours, which is the standard time frame for solubility studies, cystine solubility more than doubled with thiol drugs compared to control and did not depend on pH. However, when incubation time was shortened to 5 minutes, representing the dwell time of urine in the renal pelvis, the effect of thiol drugs to solubilize cystine greatly depended on urine pH with less cystine dissolved at lower pH. Increasing urine pH greatly increased the efficacy of thiol drugs to solubilize cystine in a clinically relevant time frame. These findings suggest that to maximize the benefit of thiol drugs alkali therapy should be used in conjunction with thiol drugs with the goal of keeping urine pH at 7.5 or above. Clinical trials are needed to confirm these in vitro findings.

An uncommon cause of acute kidney injury in young children: Cystinuria

Bilateral obstructive nephrolithiasis is a rare cause of acute kidney injury (AKI) in early childhood. As soon as the identification of AKI secondary to ureteral stone is made, it will necessitate an emergency treatment. PatientsWe report three infants with AKI caused by bilateral obstructive ureteral cystine stones. They were diagnosed with acute post-renal injury due to obstructive bilateral ureteral stones based on ultrasound scan findings. Immediately, bilateral ureteral stents were inserted for urinary drainage. Once renal function recovered to normal, each patient underwent ureteroscopy and percutaneous nephrolithotomy at the same session. Cystinuria was diagnosed by stone analysis and increased urinary excretion of cystine. Patients were advised to maintain a high fluid intake and were treated with potassium citrate in addition to tiopronin. ConclusionsWith these three cases we would like to emphasize the importance of urolithiasis in the differential diagnosis of acute renal failure in young children, since urolithiasis may only cause nonspecific symptoms in this population. An early diagnosis with prompt treatment and a close follow-up are the key for achieving the best long-term outcome in cystinuria.

2256 CLINICAL VALIDATION OF A NOVEL ASSAY USED FOR MONITORING TREATMENT OF PATIENTS WITH CYSTINURIA

characterize contemporary cohort of patients with TZ cystinuria and compare them with a concurrent cohort of homozygous MZ cystinuria. METHODS: A retrospective review of prospectively collected data was performed for 42 consecutive patients presenting with positive cyanide-nitroprasside test for cystinuria between September 2009 and September 2011. Demographic and clinical data were collected together with metabolic stone work-up, including two 24-hour urine collections with quantitative 24-hour urinary cystine excretions. RESULTS: Of the 42 patients, 36 (85.7%) and 6 (14.3%) were found to have TZ and MZ cystinuria, respectively. TZ patients were significantly older than MZ patients at first stone episode [46 (6-67) vs. 18.8 (14-44) years, p 0.01]. As expected, the median 24-hr urinary cystine excretion was significantly higher in the MZ group [1085 (6901773) vs. 392 (1461984) umol; p 0.01). Suboptimal 24-hr collection was detected in 25 (69.4%) of TZ patients and 5 (83.3%) of MZ patients with median (range) volumes of 1425 (7003400) mL and 2050 (1000-3000)mL respectively. Hyperurecemia was significantly higher in MZ group (16.7% vs. 66.7%, p 0.02). Stone composition was significantly different in both groups. Whereas 83.3% of MZ group formed pure cystine stones, 47.2%, 19.4% and 8.3% of the TZ patients formed calcium oxalate, uric acid, and cystine stones, respectively (p 0.004). Interestingly, 12 (33.3%) of TZ patients had false negative results on the cyanide-nitroprasside test. CONCLUSIONS: There were significant differences between the heterozygous (TZ) and homozygous (MZ) cystinuria patients in terms of age at first stone episode, male to female ratio, incidence of hyperuricemia and urinary stone compositions. The clinical significance remains to be elucidated.

Cystone in cystine stone formers

This is in response to a trial conducted by Dr. Erickson at Mayo Clinic in a paper titled ‘‘Cystone for one year did not change urine chemistry or decrease stone burden in cystine stone formers’’ [1]. This paper describes the results of clinical evaluation of Cystone in 10 patients. One of the important shortcomings of the study is inclusion of patients with cystine stones of long-standing duration, and probably had undergone every possible treatment. The authors mention that these were refractory patients particularly difficult to treat. When these patients did not respond to medical treatment for sufficient duration, it may not be appropriate to include them in this type of study. It is almost impossible to dislodge an impacted stone with medication and invariably requires surgical intervention. Despite a small number of patients included in this study, the biochemical data is available only for 12 out of 20 observations indicating that the data collection is insufficient for statistical evaluation. A careful analysis of this limited data does indicate that there is a trend in diuresis and reduction in urinary excretion of cystine, though it does not meet the statistical significance. This study was initiated because the authors met a patient with cystine stone who had experienced a near miraculous cure. Probably the author was curious to investigate whether Cystone is a ‘‘Miracle Cure’’. Miracle cure rarely exists in science, but makes no mistake, once a miracle is published in a peer-reviewed medical journal; the damage has already been done [2]. Cystone has never been claimed as a miracle cure. It is claimed to be effective in the management of nephrolithiasis wherein the stone size is \1.5 cm [3, 4]. The question is not about the efficacy of Cystone, but the way the clinical trial was designed and executed leading to ambiguous and conflicting results. A well-designed study should include a larger population size which follow Gaussian distribution baseline so as to increase the statistical strength in discriminating the efficacy or otherwise.

820 Cystinuria Over Two Generations

Cystinuria is an autosomal recessive disorder that is characterized by an impaired tubular transport of cystine and dibasic aminoacids, resulting in cystine urolithiasis. The transport of these amino acids is mediated by the rBAT/b0,+ AT transporter, the subunits of which are encoded by the genes SLC3A1, located on chromosome 2p16.3-21, and SLC7A9, located on chromosome 19q12-13.1. In this work we present a family in which cystinuria was manifested in two generation. The index case is a female, aged 9 years when she was diagnosed with a kidney stone and underwent surgery. Nitroprusside test was positive and urinary excretion of cystine was high (127.5 mM/M creatinine, normal 0.6-20.0 mM/M creatinine). The family screening revealed that two siblings (sister and brother) had positive nitroprusside test and increased urinary excretion of cystine. The mother had history of surgery for nephrolithiasis, had positive nitroprusside test and increased urinary excretion of cystine. The mutational analysis of the SLC3A1 gene revealed that the index case and the mother were compound heterozygote (T216M/M467T), while the female and the male siblings were homozygote for M467T mutation. The father was heterozygote for M467T mutation. The ethnic origin of the patients is Egyptian. To the best of our knowledge this is the first report of cystinuria occurring over two generations.

Five Novel Mutations in Cystinuria Genes SLC3A1 and SLC7A9

Five Novel Mutations in Cystinuria Genes SLC3A1 and SLC7A9Cystinuria is an autosomal recessive disorder that is characterized by impaired transport of cystine, lysine, ornithine and arginine in the proximal renal tubule and epithelial cells of the gastrointestinal tract. The transport of these amino acids is mediated by the rBAT/b0,+AT transporter, the subunits of which are encoded by the genes SLC3A1, located on chromosome 2p16.3-21, and SLC7A9, located on chromosome 19q12-13.1. Based on the urinary cystine excretion patterns of obligate heterozygotes, cystinuria is classified into type I (normal amino acid urinary pattern in heterozygotes) and non type I (a variable degree of urinary hyper excretion of cystine and dibasic amino acids in heterozygotes). On the basis of genetic aspects, cystinuria is classified into type A, is caused by mutations in both alleles of SLC3A1; type B, caused by mutations in both alleles of SLC7A9 and type AB, is caused by one mutation in SLC3A1 and one mutation in SLC7A9. Here we present two novel mutations in the SLC3A1 gene (C242R and L573X), which were found in patients from Serbia, and three in the SLC7A9 gene (G73R, V375I, 1048-1051 delACTC), found in patients from Serbia, Macedonia and Turkey, respectively.

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.

Type I cystinuria and its genetic basis in a population of Turkish school children

Cystinuria is a common inherited disorder characterized by an abnormal urinary excretion of cystine and dibasic amino acids resulting in nephrolithiasis. The SLC3A1 gene, which encodes a dibasic amino acid transporter protein, is involved in the pathogenesis of cystinuria. In the present study we aimed to investigate the prevalence of cystinuria among children in Sivas province (Central Anatolia, Turkey) and to study M467T and M467K mutations and 231T/A polymorphism in patients with cystinuria. A total of 8500 children were screened for cystinuria. The cyanide-nitroprusside test was applied to urine samples of all children. Children having a positive cyanide-nitroprusside test were further analyzed. M467T and M467K mutations (exon 8) and 231T/A polymorphism (exon 1) in the SCL3A1 gene were studied using a restriction fragment length polymorphism (RFLP) assay. We have found that the prevalence of cystinuria (11 cystinuric patients) is 1/772 in our population. Results of mutation analysis in the patients with cystinuria showed that M467T was the only mutation that was found in six cystinuric patients. One patient was homozygous and five were heterozygous for this mutation. The frequency of cystinuria in Sivas Province is the highest among the other populations studied to date. The frequency of M467T mutation is relatively higher than those reported for most populations. High frequency of cystinuria in this region could be due to consanguineous marriages.

Slc7a9knockout mouse is a good cystinuria model for antilithiasic pharmacological studies

Cystinuria is a hereditary disorder caused by a defect in the apical membrane transport system for cystine and dibasic amino acids in renal proximal tubules and intestine, resulting in recurrent urolithiasis. Mutations in SLC3A1 and SLC7A9 genes, that codify for rBAT/b(0,+)AT transporter subunits, cause type A and B cystinuria, respectively. In humans, cystinuria treatment is based on the prevention of calculi formation and its dissolution or breakage. Persistent calculi are treated with thiols [i.e., d-penicillamine (DP) and mercaptopropionylglycine (MPG)] for cystine solubilization. We have developed a new protocol with DP to validate our Slc7a9 knockout mouse model for the study of the therapeutic effect of drugs in the treatment of cystine lithiasis. We performed a 5-wk treatment of individually caged lithiasic mutant mice with a previously tested DP dose. To appraise the evolution of lithiasis throughout the treatment a noninvasive indirect method of calculi quantification was developed: calculi mass was quantified by densitometry of X-ray images from cystinuric mice before and after treatment. Urine was collected in metabolic cage experiments to quantify amino acids in DP-treated and nontreated, nonlithiasic mutant mice. We found significant differences between DP-treated and nontreated knockout mice in calculi size and in urinary cystine excretion. Histopathological analysis showed that globally nontreated mutant mice had more severe and diffuse urinary system damage than DP-treated mice. Our results validate the use of this mouse model for testing the efficacy of potential new drugs against cystinuria.

Urinary cystine excretion and capacity in patients with cystinuria

The treatment of cystinuria is hampered by methods used to measure urinary lithogenicity. Most cystine assays cannot reliably distinguish cystine from soluble thiol drug-cysteine complexes. We used a solid-phase assay of urinary cystine capacity in a large sample of patients with cystinuria. A known amount of solid-phase cystine is added to urine. In supersaturated urine, cystine precipitates onto added crystals, so the solid phase recovered after incubation will be greater than that added. We studied the effect of cystine-binding thiol drugs (CBTD) to solubilize cystine and determined correlates of cystine capacity in patients who were and were not taking CBTD. Increasing concentrations of D-penicillamine, tiopronin and captopril dissolved cystine in urine with similar efficacy. A general linear model in which 24 h cystine excretion was the dependent variable showed that creatinine, urea nitrogen, and sodium excretions were associated with cystine excretion (P<0.02, all three). Urine volume, pH, and cystine excretion strongly correlated with cystine capacity (P<0.001). Tiopronin had no effect on supersaturation in a cross-sectional analysis. A subset of supersaturated samples, with negative cystine capacity, occurred mainly among women not taking CBTD. For this subset, capacity differed significantly between CBTD users and non-users; use of CBTD avoided extremes of supersaturation. Female enrichment in the supersaturated group was accounted for in part by underprescription of CBTD to women. This assay of cystine capacity was reliable in the presence of CBTD. It should be useful in monitoring patients' response to dietary interventions and administration of fluid, citrate, and CBTD.