Cystinuria: an inborn cause of urolithiasis

Genetic defects underlying renal stone disease

Renal stones are common and are usually secondary to risk factors affecting the solubility of substances in the urinary tract. Primary, that is genetic, causes are rare but nevertheless are important to recognise so that appropriate treatments can be instigated and the risks to other family members acknowledged. A brief overview of the investigation of renal stones from a biochemical point of view is presented with emphasis on the problems that can arise. The genetic basis of renal stone disease caused by (i) derangement of a metabolic pathway, (ii) diversion to an insoluble product, (iii) failure of transport and (iv) renal tubular acidosis is described by reference to the disorders of adenine phosphoribosyl transferase (APRT) deficiency, primary hyperoxaluria, cystinuria and autosomal dominant distal renal tubular acidosis.

Cystinuria Associated with Different SLC7A9 Gene Variants in the Cat

Cystinuria is a classical inborn error of metabolism characterized by a selective proximal renal tubular defect affecting cystine, ornithine, lysine, and arginine (COLA) reabsorption, which can lead to uroliths and urinary obstruction. In humans, dogs and mice, cystinuria is caused by variants in one of two genes, SLC3A1 and SLC7A9, which encode the rBAT and bo,+AT subunits of the bo,+ basic amino acid transporter system, respectively. In this study, exons and flanking regions of the SLC3A1 and SLC7A9 genes were sequenced from genomic DNA of cats (Felis catus) with COLAuria and cystine calculi. Relative to the Felis catus-6.2 reference genome sequence, DNA sequences from these affected cats revealed 3 unique homozygous SLC7A9 missense variants: one in exon 5 (p.Asp236Asn) from a non-purpose-bred medium-haired cat, one in exon 7 (p.Val294Glu) in a Maine Coon and a Sphinx cat, and one in exon 10 (p.Thr392Met) from a non-purpose-bred long-haired cat. A genotyping assay subsequently identified another cystinuric domestic medium-haired cat that was homozygous for the variant originally identified in the purebred cats. These missense variants result in deleterious amino acid substitutions of highly conserved residues in the bo,+AT protein. A limited population survey supported that the variants found were likely causative. The remaining 2 sequenced domestic short-haired cats had a heterozygous variant at a splice donor site in intron 10 and a homozygous single nucleotide variant at a branchpoint in intron 11 of SLC7A9, respectively. This study identifies the first SLC7A9 variants causing feline cystinuria and reveals that, as in humans and dogs, this disease is genetically heterogeneous in cats.

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.

Molecular and Clinical Investigation of Cystinuria in the Greek-Cypriot Population

BACKGROUND AND AIMS

Cystinuria represents 3% of nephrolithiasis in humans. Two genes have been identified as the main genetic causes of cystinuria, SLC3A1 and SLC7A9, with an autosomal recessive mode of inheritance. In the present study, we studied for the first time, genetically and clinically, all the cystinuric families identified so far in the Greek-Cypriot population.

METHODS

Discovery of mutations was performed through polymerase chain reaction (PCR)-single analysis and DNA resequencing. New families were investigated through PCR-RFLPs. Clinical data were collected through the hospital patients' records and analytical follow-up of the families.

RESULTS AND DISCUSSION

We found a total of five mutations in 28 Greek-Cypriot cystinuric patients belonging in 12 families. The most frequent mutation among the 28 Greek-Cypriot patients is the SLC3A1-p.T216M, which is also the second most frequent mutation in Europe, representing a genetic founder effect. Sixteen of the 28 patients are homozygous for this mutation. Even though a consanguinity loop was obvious in only one family, other patients were from families in small villages where endogamy was practiced for many centuries. Timely clinical and genetic diagnosis, accompanied by early treatment, is significant for the good health of most of our patients. Only ∼14% of them developed chronic renal failure, and only one reached end-stage renal disease (ESRD).

CONCLUSION

Five SLC3A1 and SLC7A9 mutations appear to be responsible for the genetic basis of cystinuria in the Greek-Cypriot patients; having such a limited number of causative mutations will simplify diagnostics for this population.

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.

Clinical and genetic analysis of patients with cystinuria in the United Kingdom

BACKGROUND AND OBJECTIVES

Cystinuria is a rare inherited renal stone disease. Mutations in the amino acid exchanger System b(0,+), the two subunits of which are encoded by SLC3A1 and SLC7A9, predominantly underlie this disease. The work analyzed the epidemiology of cystinuria and the influence of mutations in these two genes on disease severity in a United Kingdom cohort.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Prevalent patients were studied from 2012 to 2014 in the northeast and southwest of the United Kingdom. Clinical phenotypes were defined, and genetic analysis of SLC3A1 and SLC7A9 combining Sanger sequencing and multiplex ligation probe-dependent amplification was performed.

RESULTS

In total, 76 patients (42 men and 34 women) were studied. All subjects had proven cystine stones. Median age of presentation (first stone episode) was 24 years old, but 21% of patients presented after 40 years old. Patients had varied clinical courses, with 37% of patients having ≥10 stone episodes; 70% had evidence of CKD, and 9% had reached ESRD as a result of cystinuria and its complications. Patients with cystinuria received a variety of different therapies, with no obvious treatment consensus. Notably, 20% of patients had staghorn calculi, with associated impaired renal function in 80% of these patients. Genetic analysis revealed that biallelic mutations were present in either SLC3A1 (n=27) or SLC7A9 (n=20); 22 patients had only one mutated allele detected (SLC3A1 in five patients and SLC7A9 in 17 patients). In total, 37 different mutant variant alleles were identified, including 12 novel mutations; 22% of mutations were caused by large gene rearrangements. No genotype-phenotype association was detected in this cohort.

CONCLUSIONS

Patients with cystinuria in the United Kingdom often present atypically with staghorn calculi at ≥40 years old and commonly develop significant renal impairment. There is no association of clinical course with genotype. Treatments directed toward reducing stone burden need to be rationalized and developed to optimize patient care.

CKD and Its Risk Factors among Patients with Cystinuria

BACKGROUND AND OBJECTIVES

Cystinuria is an autosomal recessive disorder affecting renal cystine reabsorption; it causes 1% and 8% of stones in adults and children, respectively. This study aimed to determine epidemiologic and clinical characteristics as well as comorbidities among cystinuric patients, focusing on CKD and high BP.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

This retrospective study was conducted in France, and involved 47 adult and pediatric nephrology and urology centers from April 2010 to January 2012. Data were collected from 442 cystinuric patients.

RESULTS

Median age at onset of symptoms was 16.7 (minimum to maximum, 0.3-72.1) years and median diagnosis delay was 1.3 (0-45.7) years. Urinary alkalinization and cystine-binding thiol were prescribed for 88.8% and 52.2% of patients, respectively, and 81.8% had at least one urological procedure. Five patients (1.1%, n=4 men) had to be treated by dialysis at a median age of 35.0 years (11.8-70.7). Among the 314 patients aged ≥16 years, using the last available plasma creatinine, 22.5% had an eGFR≥90 ml/min per 1.73 m(2) (calculated by the Modification of Diet in Renal Disease equation), whereas 50.6%, 15.6%, 7.6%, 2.9%, and 0.6% had an eGFR of 60-89, 45-59, 30-44, 15-29, and <15, respectively. Among these 314 patients, 28.6% had high BP. In multivariate analysis, CKD was associated with age (odds ratio, 1.05 [95% confidence interval, 1.03 to 1.07]; P<0.001), hypertension (3.30 [1.54 to 7.10]; P=0.002), and severe damage of renal parenchyma defined as a past history of partial or total nephrectomy, a solitary congenital kidney, or at least one kidney with a size <10 cm in patients aged ≥16 years (4.39 [2.00 to 9.62]; P<0.001), whereas hypertension was associated with age (1.06 [1.04 to 1.08]; P<0.001), male sex (2.3 [1.3 to 4.1]; P=0.003), and an eGFR<60 ml/min per 1.73 m(2) (2.7 [1.5 to 5.1]; P=0.001).

CONCLUSIONS

CKD and high BP occur frequently in patients with cystinuria and should be routinely screened.

Feline cystinuria caused by a missense mutation in the SLC3A1 gene

Background

Cystinuria is an inherited metabolic disease that is relatively common in dogs, but rare in cats and is characterized by defective amino acid reabsorption, leading to cystine urolithiasis.

Objectives

The aim of this study was to report on a mutation in a cystinuric cat.

Animals

A male domestic shorthair (DSH) cat with cystine calculi, 11 control cats from Wyoming, and 54 DSH and purebred control cats from elsewhere in the United States.

Methods

Exons of the SLC3A1 gene were sequenced from genomic DNA of the cystinuric cat and a healthy cat. Genetic screening for the discovered polymorphisms was conducted on all cats.

Results

A DSH cat showed stranguria beginning at 2 months of age, and cystine calculi were removed at 4 months of age. The cat was euthanized at 6 months of age because of neurological signs possibly related to arginine deficiency. Twenty‐five SLC3A1 polymorphisms were observed in the sequenced cats when compared to the feline reference sequence. The cystinuric cat was homozygous for 5 exonic and 8 noncoding SLC3A1 polymorphisms, and 1 of them was a unique missense mutation (c.1342C>T). This mutation results in a deleterious amino acid substitution (p.Arg448Trp) of a highly conserved arginine residue in the rBAT protein encoded by the SLC3A1 gene. This mutation was found previously in cystinuric human patients, but was not seen in any other tested cats.

Conclusions and Clinical Importance

This study is the first report of an SLC3A1 mutation causing cystinuria in a cat, and could be used to characterize other cystinuric cats at the molecular level.

Novel cystine ester mimics for the treatment of cystinuria-induced urolithiasis in a knockout mouse model

OBJECTIVE

To assess the effectiveness of l-cystine dimethyl ester (CDME), an inhibitor of cystine crystal growth, for the treatment of cystine urolithiasis in an Slc3a1 knockout mouse model of cystinuria.

MATERIALS AND METHODS

CDME (200 μg per mouse) or water was delivered by gavage daily for 4 weeks. Higher doses by gavage or in the water supply were administered to assess organ toxicity. Urinary amino acids and cystine stones were analyzed to assess drug efficacy using several analytical methods.

RESULTS

Treatment with CDME led to a significant decrease in stone size compared with that of the water group (P = .0002), but the number of stones was greater (P = .005). The change in stone size distribution between the 2 groups was evident by micro computed tomography. Overall, cystine excretion in urine was the same between the 2 groups (P = .23), indicating that CDME did not interfere with cystine metabolism. Scanning electron microscopy analysis of cystine stones from the CDME group demonstrated a change in crystal habit, with numerous small crystals. l-cysteine methyl ester was detected by ultra-performance liquid chromatography-mass spectrometer in stones from the CDME group only, indicating that a CDME metabolite was incorporated into the crystal structure. No pathologic changes were observed at the doses tested.

CONCLUSION

These data demonstrate that CDME promotes formation of small stones but does not prevent stone formation, consistent with the hypothesis that CDME inhibits cystine crystal growth. Combined with the lack of observed adverse effects, our findings support the use of CDME as a viable treatment for cystine urolithiasis.

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.

SLC3A1 and SLC7A9 mutations in autosomal recessive or dominant canine cystinuria: a new classification system

BACKGROUND

Cystinuria, one of the first recognized inborn errors of metabolism, has been reported in many dog breeds.

HYPOTHESIS/OBJECTIVES

To determine urinary cystine concentrations, inheritance, and mutations in the SLC3A1 and SLC7A9 genes associated with cystinuria in 3 breeds.

ANIMALS

Mixed and purebred Labrador Retrievers (n = 6), Australian Cattle Dogs (6), Miniature Pinschers (4), and 1 mixed breed dog with cystine urolithiasis, relatives and control dogs.

METHODS

Urinary cystinuria and aminoaciduria was assessed and exons of the SLC3A1 and SLC7A9 genes were sequenced from genomic DNA.

RESULTS

In each breed, male and female dogs, independent of neuter status, were found to form calculi. A frameshift mutation in SLC3A1 (c.350delG) resulting in a premature stop codon was identified in autosomal-recessive (AR) cystinuria in Labrador Retrievers and mixed breed dogs. A 6 bp deletion (c.1095_1100del) removing 2 threonines in SLC3A1 was found in autosomal-dominant (AD) cystinuria with a more severe phenotype in homozygous than in heterozygous Australian Cattle Dogs. A missense mutation in SLC7A9 (c.964G>A) was discovered in AD cystinuria in Miniature Pinschers with only heterozygous affected dogs observed to date. Breed-specific DNA tests were developed, but the prevalence of each mutation remains unknown.

CONCLUSIONS AND CLINICAL IMPORTANCE

These studies describe the first AD inheritance and the first putative SLC7A9 mutation to cause cystinuria in dogs and expand our understanding of this phenotypically and genetically heterogeneous disease, leading to a new classification system for canine cystinuria and better therapeutic management and genetic control in these breeds.

New insights into the pathogenesis of renal calculi

The pathophysiology of the various forms of urinary stone disease remains a complex topic. Epidemiologic research and the study of urine and serum chemistries have created an abundance of data to help drive the formulation of pathophysiologic theories. This article addresses the associations of urinary stone disease with hypertension, cardiovascular disease, atherosclerosis, obesity, dyslipidemia, diabetes, and other disease states. Findings regarding the impact of dietary calcium and the formation of Randall's plaques are also explored and their implications discussed. Finally, further avenues of research are explored, including genetic analyses and the use of animal models of urinary stone disease.