Usefulness of cyanide-nitroprusside test in detecting incomplete recessive heterozygotes for cystinuria: a standardized dilution procedure

The Saudi urological association guidelines on urolithiasis

Aims

The Saudi Urolithiasis Guidelines are a set of recommendations for diagnosing, evaluating, and treating urolithiasis in the Saudi population. These guidelines are based on the latest evidence and expert consensus to improve patient outcomes and optimize care delivery. They cover the various aspects of urolithiasis, including risk factors, diagnosis, medical and surgical treatments, and prevention strategies. By following these guidelines, health-care professionals can improve care quality for individuals with urolithiasis in Saudi Arabia.

Panel

The Saudi Urolithiasis Guidelines Panel consists of urologists specialized in endourology with expertise in urolithiasis and consultation with a guideline methodologist. All panelists involved in this document have submitted statements disclosing any potential conflicts of interest.

Methods

The Saudi Guidelines on Urolithiasis were developed by relying primarily on established international guidelines to adopt or adapt the most appropriate guidance for the Saudi context. When necessary, the panel modified the phrasing of recommendations from different sources to ensure consistency within the document. To address areas less well covered in existing guidelines, the panel conducted a directed literature search for high quality evidence published in English, including meta analyses, randomized controlled trials, and prospective nonrandomized comparative studies. The panel also searched for locally relevant studies containing information unique to the Saudi Arabian population. The recommendations are formulated with a direction and strength of recommendation based on GRADE terminology and interpretation while relying on existing summaries of evidence from the existing guidelines.

Phenotypic characterization of a pediatric cohort with cystinuria and usefulness of newborn screening

BACKGROUND

Cystinuria is an inherited metabolic disease involving the defective transport of cystine and the dibasic amino acids in the renal proximal tubules that causes the formation of stones in the urinary system. In our regional child health program, cystinuria is included in newborn metabolic screening. Our objectives are the phenotypic characterization of our cystinuric pediatric cohort and to present our experience in neonatal cystinuria screening.

METHODS

The study of clinical cases of pediatric patients diagnosed with cystinuria over a period of 32 years. All patients were studied at demographic, clinical, laboratory, radiological, and therapeutic levels.

RESULTS

We diagnosed 86 pediatric patients with cystinuria; 36% of them had the homozygous biochemical phenotype. 95.3% of the patients were detected by neonatal metabolic screening. We performed urine biochemical analyses of parents with additional diagnoses of 63 adult patients. The mean follow-up time was 16.8 ± 8.5 years. 11.6% of patients developed one or more episodes of urinary tract infection during that period. Chronic kidney disease, proteinuria, and hypertension were uncommon (1.2%). 10.5% developed kidney stones at the mean age of presentation of 7.78 ± 7.6 years; 33% were recurrent. The risk of developing lithiasis was higher for homozygous biochemical-phenotype patients. Hypercalciuria was a significant risk factor in the development of lithiasis.

CONCLUSIONS

Our clinical data suggest that diagnosing cystinuria through neonatal screening could be a useful strategy for the detection of presymptomatic cases, in order to establish preventive measures, as well as for the detection of relatives at risk. A higher resolution version of the Graphical abstract is available as Supplementary information.

Cystinuria: an update on pathophysiology, genetics, and clinical management

Cystinuria is the most common genetic cause of nephrolithiasis in children. It is considered a heritable aminoaciduria as the genetic defect affects the reabsorption of cystine and three other amino acids (ornithine, lysine, and arginine) in the renal proximal tubule. Patients affected by this condition have elevated excretion of cystine in the urine, and because of this amino acid's low solubility at normal urine pH, patients tend to form cystine calculi. To date, two genes have been identified as disease-causative: SLC3A1 and SLC7A9, encoding for the two subunits of the heterodimeric transporter. The clinical features of this condition are solely related to nephrolithiasis. The diagnosis is usually made during infancy or adolescence, but cases of late diagnosis are common. The goal of therapy is to reduce excretion and increase the solubility of cystine, through both modifications of dietary habits and pharmacological treatment. However, therapeutic interventions are not always sufficient, and patients often have to undergo several surgical procedures during their lives to treat recurrent nephrolithiasis. The goal of this literature review is to synthesize the available evidence on diagnosis and management of patients affected by cystinuria in order to provide physicians with a practical tool that can be used in daily clinical practice. This review also aims to shed some light on new therapy directions with the aim of ameliorating kidney outcomes while improving adherence to treatment and quality of life of cystinuric patients.

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.

PREVALENCE OF CYSTINURIA IN SERVALS ( LEPTAILURUS SERVAL) IN THE UNITED STATES

Cystinuria is a condition caused by defects in amino acid transport within the kidneys and small intestines. It has been reported in humans, dogs, domestic cats, ferrets, nondomestic canids, and nondomestic felids, including servals ( Leptailurus serval). Genetic mutations have been identified in dogs, humans, and domestic cats. Cystinuria usually follows an autosomal recessive inheritance, although it can be autosomal dominant and sex linked. The primary objective of this study was to screen urine samples dried on filter paper from captive servals in the United States for cystinuria by using the cyanide-nitroprusside screening test. A second objective was to determine whether cystinuria is inheritable in servals. Servals were initially recruited for the study by survey. Owners and institutions interested in participating were sent a second survey and filter paper for collecting urine samples. Samples were collected from 25 servals. One additional serval with confirmed cystine urolithiasis was added for a total sample size of 26 servals. Twenty-seven percent (7/26) were positive, 54% (14/26) were weakly positive, and 19% (5/26) were negative. Sex, reproductive status, and urine collection method had no significant association with test results. This condition is likely underreported in servals and should be ruled out in any serval with nonspecific signs of illness; neurologic signs such as lethargy, ataxia, or seizures; ptyalism; or signs of lower urinary tract disease such as dysuria, hematuria, stranguria, pollakiuria, or urethral obstructions.

Cyanide-Nitroprusside Colorimetric Assay: A Rapid Colorimetric Screen for Urinary Cystine

BACKGROUND

Cystinuria is an autosomal recessive disorder resulting in poor proximal tubule reabsorption of cystine in the nephron, increasing the risk of cystine stone formation. A fast, inexpensive assay to screen for urinary cystine is needed because cystine stones are difficult to noninvasively differentiate from more common calcium-containing ones. Tandem mass spectrometry (MS/MS) is sensitive and specific but is labor-intensive and costly. Alternatively, a colorimetric assay is fast and cost-effective; however, creatinine interference is an issue.

METHODS

A published cyanide-nitroprusside colorimetric assay was modified for a high-throughput microplate format. Creatinine interference was reduced using 0.1 mol/L PBS and a standard reaction time of 60 s and was further corrected using a formula derived from the slope of multiple creatinine standard curves.

RESULTS

The limit of blank was determined to be 2.6 mg/L, the limit of detection 11.9 mg/L, and the limit of quantitation 15.3 mg/L. The analytic measurement range was established as 15.3-100 mg/L cystine. Intraassay and interassay CV was calculated to be 9.6% and 8.0%, respectively, for a high-level cystine concentration (83.6 mg/L). Low-level cystine (36.4 mg/L) intraassay and interassay CV was determined to be 18.1% and 17.6%, respectively. Passing-Bablok regression analysis of colorimetric vs LC-MS/MS results revealed a slope of 1.10 and y intercept of -7.14 mg/L, with an overall bias of 2% by Bland-Altman plot analysis.

CONCLUSIONS

We analytically validated a rapid colorimetric assay suitable to quantify urinary cystine. The effect of thiol drugs on this assay remains to be determined.

Infrared vibrational spectroscopy: a rapid and novel diagnostic and monitoring tool for cystinuria

Cystinuria is the commonest inherited cause of nephrolithiasis (~1% in adults; ~6% in children) and is the result of impaired cystine reabsorption in the renal proximal tubule. Cystine is poorly soluble in urine with a solubility of ~1 mM and can readily form microcrystals that lead to cystine stone formation, especially at low urine pH. Diagnosis of cystinuria is made typically by ion-exchange chromatography (IEC) detection and quantitation, which is slow, laboursome and costly. More rapid and frequent monitoring of urinary cystine concentration would significantly improve the diagnosis and clinical management of cystinuria. We used attenuated total reflection - Fourier transform infrared spectroscopy (ATR-FTIR) to detect and quantitate insoluble cystine in 22 cystinuric and 5 healthy control urine samples. Creatinine concentration was also determined by ATR-FTIR to adjust for urinary concentration/dilution. Urine was centrifuged, the insoluble fraction re-suspended in 5 μL water and dried on the ATR prism. Cystine was quantitated using its 1296 cm⁻¹ absorption band and levels matched with parallel measurements made using IEC. ATR-FTIR afforded a rapid and inexpensive method of detecting and quantitating insoluble urinary cystine. This proof-of-concept study provides a basis for developing a high-throughput, cost-effective diagnostic method for cystinuria, and for point-of-care clinical monitoring

How should patients with cystine stone disease be evaluated and treated in the twenty-first century?

Cystinuria continues to be one of the most challenging stone diseases. During the latest decades our knowledge of the molecular basis of cystinuria has expanded. Today 160 different mutations in the SLC3A1 gene and 116 in the SLC7A9 gene are listed. The full implications of type A, B or AB status are not yet fully understood but may have implications for prognosis, management and treatment. Despite better understanding of the molecular basis of cystinuria the principles of recurrence prevention have remained essentially the same through decades. No curative treatment of cystinuria exists, and patients will have a life long risk of stone formation, repeated surgery, impaired renal function and quality of life. Therapy to reduce stone formation is directed towards lowering urine cystine concentration and increasing cystine solubility. Different molecules that could play a role in promoting nucleation and have a modulating effect on cystine solubility may represent new targets for cystinuria research. Investigation of newer thiol-containing drugs with fewer adverse effects is also warranted. Determining cystine capacity may be an effective tool to monitor the individual patient's response. Compliance in cystinuric patients concerning both dietary and pharmacological intervention is poor. Frequent clinical follow-up visits in dedicated centres seem to improve compliance. Cystinuric patients should be managed in dedicated centres offering the complete range of minimal invasive treatment modalities, enabling a personalized treatment approach in order to reduce risk and morbidity of multiple procedures.

Cystinuria-a urologist's perspective

Cystinuria is a genetic disease that leads to frequent formation of stones. In patients with recurrent stone formation, particularly patients <30 years old or those who have siblings with stone disease, urologists should maintain a high index of suspicion of the diagnosis of cystinuria. Patients with cystinuria require frequent follow-up and a multidisciplinary approach to diagnosis, prevention and management. Patients have reported success in preventing stone episodes by maintaining dietary changes using a tailored review from a specialist dietician. For patients who do not respond to conservative lifestyle measures, medical therapy to alkalinize urine and thiol-binding drugs can help. A pre-emptive approach to the surgical management of cystine stones is recommended by treating smaller stones with minimally invasive techniques before they enlarge to a size that makes management difficult. However, a multimodal approach can be required for larger complex stones. Current cystinuria research is focused on methods of monitoring disease activity, novel drug therapies and genotype-phenotype studies. The future of research is collaboration at a national and international level, facilitated by groups such as the Rare Kidney Stone Consortium and the UK Registry of Rare Kidney Diseases.

Characterization of patients with heterozygous cystinuria

OBJECTIVE

To characterize a contemporary cohort of patients with heterozygous (TZ) cystinuria and compare them with a concurrent cohort of patients with homozygous (MZ) cystinuria.

METHODS

A retrospective review of prospectively collected data was performed for 42 consecutive patients presenting with a positive cyanide-nitroprusside test from September 2009 to September 2011. Clinical data were collected, including the findings from a detailed metabolic stone workup that included two 24-hour urine collections with quantitative cystine. The patients were divided into 2 groups: those with TZ (30-400 mg/d) and those with MZ (>400 mg/d) cystinuria.

RESULTS

One patient was excluded because the cystine excretion was within the normal range (<30 mg/d), 35 (83.3%) and 6 (14.3%) had TZ and MZ cystinuria, respectively. Compared with those with TZ cystinuria, those with MZ cystinuria were significantly younger at the first stone episode (median 48 years, range 14-67, vs 17, range 6-44, P = .002), more were female (20% vs 66.7%; P = .03), and more patients had bilateral stones (8.6% vs 50%; P = .03). Finally, the patients with MZ cystinuria had more stone episodes than those with TZ cystinuria (3 vs 1; P = .04). From the detailed metabolic stone evaluation, the incidence of hyperuricemia was significantly greater in the MZ patients (17.1% vs 66.7%; P = .02). Although all the MZ patients developed pure cystine stones, 18 (51.4%), 7 (20.0%), and 3 (8.6%) of the TZ patients developed calcium oxalate, uric acid, and cystine stones, respectively (P < .001). In the TZ group, 11 patients (31.4%) had false-negative results on subsequent cyanide-nitroprusside testing.

CONCLUSION

Significant differences were found between the patients with TZ and MZ cystinuria in terms of age at the first stone episode, male/female ratio, incidence of hyperuricemia, and stone composition. The clinical significance remains to be elucidated.

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.

Management of Cystinuria

Cystinuria is a monogenic disorder in which there is a transepithelial transport defect of di-basic amino acids, including cystine, ornithine, lysine, and arginine (COLA). This results in diminished reabsorption of these amino acids in both the intestine and renal proximal tubule. This article describes the disorder, reviews the mechanisms of normal COLA renal transport, and summarizes issues related to the disorder, such as the role of mutations, associated diseases, clinical manifestations, therapies, the renal impact, and handling of pediatric patients.

Cystinuria in childhood and adolescence: recommendations for diagnosis, treatment, and follow-up

Cystinuria, an autosomal-recessive disorder of a renal tubular amino acid transporter, is the cause of about 10% of all kidney stones observed in children. Different genetic characteristics are not represented by different phenotypes. The stones are formed of cystine, which is relatively insoluble at the physiological pH of urine. Without any preventive measures, the patients will suffer from recurrent stone formation throughout their life. Even with medical management, long-term outcome is poor due to insufficient efficacy and low patient compliance. Many patients suffer from renal insufficiency as a result of recurrent stone formation and repeated interventions. However, regular follow-up and optimal pharmacotherapy significantly increase stone-free intervals. Medical management is mainly based on hyperhydration and urine alkalinization. Sulfhydryl agents such as tiopronin can be added. Recurrent stone formation necessitates repeated urological interventions. These mostly minimally invasive procedures carry the risk of impairment of renal function. In adults, extracorporeal shockwave lithotripsy (SWL) as well as intracorporeal lithotripsy is often unsuccessful. However, in children SWL shows excellent results for cystine stones. In cases with large stone burden, percutaneous nephrolithotripsy (PNL) or even open surgical nephrolithotomy are preferred. This review discusses the underlying pathogenetic mechanisms and provides guidance for the diagnosis, therapy, and management of cystinuria following the recommendations of the International Cystinuria Consortium and the European Association of Urology.

[Sensitivity, specificity and predictive value of the genetic analysis of SLC3A1 gene variants used for the diagnosis of cystinuria among the spanish population]

OBJECTIVE

To test the clinical usefulness of the analysis of point mutations R452W, M467T, 114C > A, 231T > A, 1136 + 3delT and 1332 + 7T > C in the gene SLC3A1 as well as their possible haplotypes used for the diagnosis of cystinuria in the mediterranean spanish population.

MATERIAL AND METHODS

A total of 48 patients with cystinuria, 44 relatives without cystinuria, and 81 healthy controls were studied. A genetic analysis was conducted in order to identify variants in the gene SLC3A1. The sensitivity, specificity, and predictive value for each genetic variant and for the possible haplotypes were calculated.

RESULTS

The specificity of mutations M467T, R452W, and 231T > A used for the diagnosis of cystinuria in the general population or for the different subtypes of cystinuria in involved families, was higher than 90%; nevertheless, none of the analysed variants reached a sensitivity higher than 80%. In the study of haplotypes, the highest sensitivity was obtained with the haplotype CTTT (83.8%); however, its specificity and predictive value were low (20.6% and 53.4%, respectively).

CONCLUSIONS

The studied genetic variants did not show enough clinical usefulness.

Pilot screening programme for cystinuria in the Valencian community

Cystinuria is an autosomal recessive disorder of the kidneys and small intestine, affecting a luminal transport mechanism shared by cystine, ornithine, arginine and lysine. When cystine exceeds its solubility at low pH, the risk of stone formation increases. The data reported in the literature show a variation for the incidence of cystinuria, from 1 in 600 to 1 in 17,000, depending on the definition of cystinuria and the method used for screening the population. We set up a pilot screening programme to determine the incidence of cystinuria in the population of the Valencian Community. Urine filter paper samples submitted for the neonatal screening programme from 33,995 newborns (5-10 days old) were used for the study. Thin layer chromatography (TLC) was performed to screen cystinuric patients. To confirm positive filter paper samples, liquid samples were requested and TLC as well as the cyanide-nitroprusside test (CNT) were performed. Final diagnosis was achieved by quantifying cystine, lysine, ornithine and arginine using high-performance liquid chromatography (HPLC) in children's urine samples which remained positive for TLC and CNT for more than 1 year. We conclude that the incidence of subjects at risk for cystine stones in the Valencian Community is 1:1887. TLC is shown as a reliable method to perform newborn screening in large population to detect cystinuric subjects. Additional studies, including characterization of appropriate haplotypes, should be carried out for a more precise identification of the frequency of the different types of cystinuria in our population.