cystinuria leads to defects in transepithelial transporters for dibasic amino acids , including cystine , ornithine , lysine , and arginine . although the urine concentration of all dibasic amino acids is elevated in cystinuric patients , only cystine leads to stone disease as it is relatively insoluble at physiological ph ( 1234 ) . more than 50% of patients with cystinuria suffer from stone formation throughout their lifetime , with variable onset ages as well as a high rate of recurrence of up to 60% ( 5 ) . furthermore , since stones are likely to be formed bilaterally in more than three - quarters of cystinuric patients ( 4 ) , they are at high risk of renal dysfunction and consequent poor quality of life ( 6 ) . cystinuria types i , ii , and iii are traditional subgroups divided according to the urinary phenotype in heterozygotes ( 7 ) . while type i heterozygotes show a normal amino aciduria , non - type i ( type ii and iii ) heterozygotes show high and moderate hyperexcretion of cystine and dibasic amino acids , respectively . patients with type iii , a mixed type , inherit type i and non - type i alleles from either parent . however , the international cystinuria consortium developed a new classification system to reflect the genetic and functional characteristics of the disease after the discovery of 2 genes responsible for cystinuria . type a cystinuria is due to biallelic slc3a1 mutations , type b cystinuria is due to biallelic slc7a9 mutations , and type ab cystinuria is due to single heterozygous mutations in both genes ( 8) . slc3a1 encodes a heavy subunit of the renal cystine transport system , rbat , and was identified as the cause of type i cystinuria . slc7a9 encodes b amino acid transporter ( bat ) , a light subunit of the renal cystine transport system and responsible for type ii cystinuria . to date , 163 disease - causing mutations in slc3a1 and 118 disease - causing mutations in slc7a9 have been listed in the human gene mutation database ( hgmd professional 2016.2 , https://portal.biobase-international.com/hgmd/pro/start.php ) . there have been several clinical case reports and studies of korean patients with cystinuria ( 691011121314 ) . however , only 2 case reports published include genetic studies : one was that of a 13-year - old boy with a single heterozygous slc7a9 mutation ( c.517g > a , p.g173r ) ( 11 ) , and the other was of an 8-month - old girl with a homozygous slc3a1 mutation ( c.1820delt , p.l607fs ) ( 10 ) . here , we report a genotype - phenotype study of 8 korean pediatric patients with cystinuria . eight patients diagnosed with cystine stones during the period between january 2003 and june 2016 in 2 hospitals ( seoul national university children 's hospital , seoul , korea and asan medical center children 's hospital , seoul , korea ) were recruited . genomic dna was extracted from nucleated cells in peripheral blood using a commercial kit ( qiaamp dna blood mini kit ; qiagen , hilden , germany ) . all coding exons and flanking introns of the slc3a1 and the slc7a9 genes were amplified using polymerase chain reaction followed by direct sequencing ( primer sequences are available upon request ) . this study was approved by the institutional review board at seoul national university hospital ( irb no . informed consent was obtained from all individual patients included in this study or their parents . eight patients diagnosed with cystine stones during the period between january 2003 and june 2016 in 2 hospitals ( seoul national university children 's hospital , seoul , korea and asan medical center children 's hospital , seoul , korea ) were recruited . genomic dna was extracted from nucleated cells in peripheral blood using a commercial kit ( qiaamp dna blood mini kit ; qiagen , hilden , germany ) . all coding exons and flanking introns of the slc3a1 and the slc7a9 genes were amplified using polymerase chain reaction followed by direct sequencing ( primer sequences are available upon request ) . this study was approved by the institutional review board at seoul national university hospital ( irb no . informed consent was obtained from all individual patients included in this study or their parents . the median ages at onset and diagnosis were 1.5 years ( range , 0.313.6 years ) and 2.6 years ( range , 0.716.7 years ) , respectively . they were followed up for a median period of 7.7 years ( range , 3.414.0 years ) . the presenting symptoms were gross hematuria ( n = 3 ) or urinary tract infection ( n = 3 ) in association with renal stones , while renal stones were detected incidentally in two patients . family history of nephrolithiasis was absent in all patients , except for one ( patient 7 ) , whose maternal uncle had a history of nephrolithiasis of unidentified composition . the initial renal function was normal ( estimated glomerular filtration rate [ egrf calculated using the schwartz formula ]  90 ml / min/1.73 m ) in 2 patients , and the remaining 6 patients had various degrees of renal dysfunction . seven patients were treated with tiopronin ( thiola ) with or without potassium citrate ( n = 6 ) and captopril ( n = 1 ) . the remaining patient ( patient 5 ) had a single heterozygous slc3a1 mutation and was treated with oral sodium bicarbonate only . however , all patients suffered from recurrent attacks of symptomatic nephrolithiasis , which lead to urologic interventions . at the last follow - up , 3 patients had a mild - to - moderate degree of renal dysfunction . ghu = gross hematuria , egfr = estimated glomerular filtration rate ( calculated using the schwartz formula ) , kc = potassium citrate , capto = captopril , nab = sodium bicarbonate , swl = shock wave lithotripsy , pnl = percutaneous nephrolithotomy , ul = ureteroscopic lithotripsy . urinary excretion of cystine and dibasic amino acids were markedly increased in all patients , except for patient 5 who exhibited only a mildly increased level of cystine . the cyanide nitroprusside test was performed and positive in 2 patients ( table 2 ) . mutational analyses revealed biallelic slc3a1 mutations ( aa genotype ) in 4 patients , a single heterozygous slc3a1 mutation ( a- genotype ) in one patient , biallelic slc7a9 mutations ( bb genotype ) in one patient , and a single heterozygous slc7a9 mutation ( b- genotype ) in one patient . in one remaining patient the c.1820delt slc3a1 mutation was detected in 3 of 5 patients with an aa genotype ( 4 of 10 alleles ) . reference values : cystine 27150 m / g creatinine , ornithine 044 m / g creatinine , lysine 62513 m / g creatinine , arginine 044 m / g creatinine . novel mutations ; a nucleotide variation of uncertain pathogenicity ; this large deletion causes a total deletion of exon 12 . urinary cystine excretion levels did not correlate with any clinical parameters or genotypes of the patients . we compared the clinical and laboratory findings of the 4 patients with the aa genotype to those of the others . the only difference observed was an older onset age in patients with the aa genotype . urinary excretion levels of cystine did not correlate with any clinical parameters in the 4 patients with the aa genotype . patients with the aa genotype had older onset ages than those with non - aa genotypes . several previous studies examining genotype - phenotype correlations in cystinuria did not show any correlation between patients with type a genotype and patients with non - a genotypes ( 81516 ) . in a uk study ( 15 ) , patients with at least one missense mutation in slc3a1 had significantly lower levels of lysine , arginine , and ornithine , but not cystine , than patients with all other types of slc3a1 mutations . another uk study ( 16 ) revealed no difference between patients with type aa and patients with type bb in a variety of clinical parameters . in addition , patients with a single mutated allele also had variable disease severity and could not be differentiated from patients with 2 mutated alleles ( 16 ) . in our study , the only difference between patients with the aa genotype and those with non - aa genotypes was the later age of onset in the former group . of the 4 patients in our study with the aa genotype , patient 4 had 2 truncating mutations and the youngest onset age . however , urinary excretion levels of cystine were highest in patient 2 , who had 2 missense mutations . other clinical parameters , including long - term prognosis , showed no correlation with the different genotypes . this mutation is common in south - eastern european ( 1718 ) , gypsy ( 19 ) , and greek populations ( 20 ) but has not been found in chinese ( 21 ) and japanese ( 22 ) patients . conversely , the slc7a9 p.p482l mutation is common in japanese patients ( 22 ) , but has not been found in european populations . in patient 5 , we found a nucleotide variation , c.1976a > c ( p.q659p ) , in slc3a1 . using mutation taster ( http://www.mutationtaster.org/ ) , this variation was predicted to be a polymorphism , but was not found in exac ( http://exac.broadinstitute.org/ ) or 1000 g ( http://www.1000genomes.org/ ) . mild but abnormal hyperexcretion of cystine and early onset nephrolithiasis in patient 5 suggested that the p.q659p variation may be a hypomorphic mutation . a single heterozygous slc7a9 mutation was detected in patient 7 . however , urinary excretion levels of branched amino acids in the patient were very high . furthermore , her mother was heterozygous for the same mutation but was clinically silent , although urinary amino acid excretion levels were not measured in the mother therefore , it is quite possible that patient 7 had another pathogenic mutation , not detected by sanger sequencing , in slc3a1 ( ab genotype ) or slc7a9 ( bb genotype ) . the genotype of patient 8 was unavailable , but family study revealed type i cystinuria . progression to chronic kidney disease in patients with cystinuria has been reported to be between 5 and 17% ( 232425 ) , and the prevalence of end stage renal disease has been reported to be up to 5% ( 26 ) . in a recent uk study ( 16 ) , the level of renal impairment observed was similar across all genotypes . recently , a recent large retrospective study conducted by the french cystinuria group ( 27 ) showed that 5 ( 1.1% ) of 442 patients with cystinuria progressed to end stage renal disease at a median age of 35.0 ( 11.870.7 ) years . multivariate analyses revealed that progression to chronic kidney disease was associated with age , hypertension , severe damage of renal parenchymal defined as a history of partial or total nephrectomy , and so on ( 27 ) . in our study , 3 of 8 patients had a mild degree of renal dysfunction at the last follow - up . in conclusion , we have analyzed the genotypes and phenotypes of cystinuria in 8 pediatric patients and identified two novel mutations . we did not observe an association between clinical course and genotype , except for earlier onset age in patients with non - aa genotypes .
cystinuria is an inherited disorder characterized by defective renal reabsorption of cystine and dibasic amino acids leading to nephrolithiasis . 
 this study was conducted to analyze the genotypes and phenotypes of pediatric patients with cystinuria . 
 eight children from seoul national university hospital and asan medical center presenting with cystinuria from january 2003 to june 2016 were retrospectively analyzed . 
 mutational studies were performed by direct sequencing . 
 two of the 8 were male and 6 were female . 
 the median ages at onset and diagnosis were 1.5 ( range , 0.313.6 ) and 2.6 ( range , 0.716.7 ) years , respectively . 
 the median followed up was 7.7 ( range , 3.414.0 ) years . 
 mutational analyses were performed in 7 patients and revealed biallelic slc3a1 mutations ( aa genotype ) in 4 patients , a single heterozygous slc3a1 mutation ( a- genotype ) in 1 patient , biallelic slc7a9 mutations ( bb genotype ) in 1 patient , and a single heterozygous slc7a9 mutation ( b- genotype ) in 1 patient . 
 two of the mutations were novel . 
 no genotype - phenotype correlations were observed , except for earlier onset age in patients with non - aa genotypes than in patients with the aa genotype . 
 all patients suffered from recurrent attacks of symptomatic nephrolithiasis , which lead to urologic interventions . at the last follow - up , 3 patients had a mild - to - moderate degree of renal dysfunction . 
 this is the first study of genotypic and phenotypic analyses of patients with cystinuria in korea .