Review Article Cystinuria: Current Diagnosis and Management Panagiotis Saravakos, Vasiliki Kokkinou, and Evangelos Giannatos Cystinuria is an inherited disorder of the dibasic amino acid transport system in the proximal tubule and the small intestine. Two responsible genes have been identified, the SLC3A1 on chromosome 2 and the SLC7A9 on chromosome 19. The inability of renal tubules to reabsorb cystine and the relative insolubility of cystine at physiological urine pH lead to stone formation. Cornerstone of the treatment remains stone prevention with hyperhydration, urinary alkalization, and pharmacologic therapy. Repeated stone formation necessitates urologic interventions, which mainly include minimally invasive procedures. The appropriate management of cystinuria is often challenging and requires close followup of the patient. UROLOGY 83: 693e699, 2014. 2014 Elsevier Inc.
C
ystinuria is an autosomal recessive disorder that accounts for up to 10% of all pediatric stone disease.1 The disease causes recurrent stone formation, which leads to an impairment of renal function and quality of life.2 Although the mainstay of the therapy is conservative management, its side effects, poor patient compliance and its limited efficacy, lead to repeated urologic interventions.1,3
EPIDEMIOLOGY The worldwide prevalence of cystinuria is estimated at 1:7000, although it is regionally variable, ranging from 1:2500 among Libyan Jews to 1:100,000 persons in Sweden.1,4 In the Mediterranean East Coast, cystinuria occurs in approximately 1:1887 persons.4 The first symptoms of cystine stones typically present between ages 2 and 40 years, whereas the peak age of onset of stones is in the third decade of life.5 Cystine stones account for 1%-2% of urinary calculi, but the prevalence is 6%-10% of all renal calculi in children.6 More than 50% of cystinuric patients will develop cystine stones during their lifetime, with a high likelihood of bilateral stone formation.1 Incidence is equal between genders, but male patients tend to have a more aggressive course of the disease with a significant higher number of stones and an earlier onset of symptoms.7,8 The diagnosis of cystinuria should be made carefully before age 2 years, as heterozygotic carriers might have falsely elevated urine cystine levels, because of immaturity of the amino acid transporters within the renal tubule during the first 2 years of life.9 Financial Disclosure: The authors declare that they have no relevant financial interests. From the Department of General Surgery, General Hospital of Kefalonia, Argostoli, Greece; and the Dialysis Department, General Hospital of Kefalonia, Argostoli, Greece Reprint requests: Panagiotis Saravakos, M.D., Department of General Surgery, General Hospital of Kefalonia, Soudias Street, GR-28100 Argostoli, Kefalonia, Greece. E-mail: [email protected] Submitted: August 13, 2013, accepted (with revisions): October 8, 2013
ª 2014 Elsevier Inc. All Rights Reserved
PATHOPHYSIOLOGY AND GENETICS Cystinuria is an autosomal recessive genetic disorder of the transepithelial transporters for the dibasic amino acids COLA (cystine, ornithine, lysine, and arginine), resulting in an impairment of their reabsorption in the renal proximal tubule and the small intestine.4,7 In normal individuals, about 99% of the amino acids, including cystine, filtered by the glomerulus, are reabsorbed by the end of the proximal tubule. Cystinuria results from a defective amino acid transport system, which consists of a high-affinity-low-capacity system, located almost exclusively in the apical membrane of the third segment (S3) of the proximal tubule, and which is responsible for 10% of cystine reabsorption, and a low-affinity-high-capacity system, found in the S1 and S2 segments of the proximal tubule and responsible for the 90% of cystine reabsorption.10 The cystine transporter system b0,þ is a heterodimer composed of 2 subunits linked by disulfide bridge, belonging to the family of heterodimeric amino acid transporters. The heavy subunit rBAT is the glycosylated protein produced by the SLC3A1 gene sequence on chromosome 2, whereas the light subunit b0,þ AT is encoded by SLC7A9 gene in chromosome 19.10 The b0,þ AT protein (neutral and dibasic amino acid transporter) is the transport channel, whereas the rBAT protein (related to basic amino transporter) modulates the activity of b0,þ AT protein.7,11 The intracellular cystine transportation is mediated by a sodium-independent manner, unlike other neutral amino acids and solutes such as glucose, but sodium-dependent amino acid transporters at the apical and basolateral membrane favor the transport of dibasic amino acids into the cell.10 After absorption, cystine, which is an amino acid composed of 2 cysteine molecules joined by a disulfide bond, is intracellulary hydrolyzed to 2 molecules of cysteine, which then exit the cell across the basolateral membrane. In persons with cystinuria, the movement of cystine from the tubular cells into the blood is not affected (Fig. 1).7,10,12 0090-4295/14/$36.00 http://dx.doi.org/10.1016/j.urology.2013.10.013
693
Figure 1. Transport of cystine and other dibasic amino acids in the proximal tubule. Normally, cystine is reabsorbed by the apical membrane of the proximal tubule by the transport system b0,þ. The transport system b0,þ is a heterodimer composed of 2 subunits, the light subunit b0,þ AT, which is encoded by the SLC7A9 gene sequence, and the heavy subunit rBAT, which is encoded by the SLC3A1 gene sequence. This transport system is affected in patients with cystinuria. Inside the cell, cystine is reduced to 2 molecules of cysteine, which exit the cell across the basolateral membrane. (Color version available online.)
Table 1. Genetic classification of cystinuria4 Classification
Type A
Type B
Type AB
Frequency 45% 53% 2% Gene locus Chromosome 2 (SLC3A1) Chromosome 19 (SLC7A9) Chromosomes 2 and 19 (SLC3A1/SLC7A9) rBAT/ b0,þ AT Subunit affected rBAT (heavy subunit) b0,þ AT protein (light subunit)
The result of mutations in the aforementioned genes is a defective dibasic amino acid transporter, a reduced reabsorption in the proximal tubule, and elevated levels of dibasic amino acid secretion in the urine. Although all dibasic amino acids reach high concentrations in the urine, only cystine is relatively insoluble at physiological pH levels of 5-7, with a pKa level of 8,3.1,7 Up to pH 7, solubility of cystine is approximately 250 mg/L urine, whereas at a urine pH level of 7.5 this will double to 500 mg/L and triple at pH 8 or higher. Other factors also seem to influence cysteine excretion, such as dietary salt and animal protein intake, especially the intake of cystine’s precursor methionine.13,14
CLASSIFICATION Cystinuria is an autosomal recessive disease, but some heterozygote carriers have an autosomal dominant, incomplete penetrance appearance with elevated, but typically nonpathologic urinary cystine excretion. Consequently, the first classification by Rosenberg et al15 in 1966 was the phenotypic classification and was based on the amount of urinary cystine excreted of the patient’s parents, dividing 694
the disease into 3 subgroups: type I, type II, and type III (the 2 latter are also referred as non-type-I). Type I heterozygotes, an autosomal recessive disorder, show a normal urinary cystine excretion pattern (900 mmol/g creatinine), and type III heterozygotes show a moderate increase (100-900 mmol/g creatinine). Non-type-I heterozygotes are considered to have an autosomal dominant disorder with incomplete penetrance and present a slightly increased risk of stone formation.7,10,15 Genetic studies of patients with cystinuria have provided new insights in the genetic characteristics of the disease, and, on the basis of the type of the mutation, Dello Strologo et al8 proposed a new classification of the disease into type A, type B, and type AB. In type A cystinuria (45% of the cases), the disease is caused by mutations in both inherited alleles of the SLC3A1 gene in chromosome 2. In type B heterozygotes (53% of the cases) the mutation is located in the SLC7A9 gene on chromosome 19. Patients with the rare type AB cystinuria UROLOGY 83 (4), 2014
(2% of the cases) have 2 mutated alleles in the same gene in addition to a mutated allele in the other gene and so are actually AAB or ABB.4,16 So far, 133 mutations have been identified in SLC3A1 and 95 in SLC7A9.17 Table 1 summarizes the genetic types, the responsible mutations, and the affected protein for each type.
CLINICAL MANIFESTATIONS The clinically significant manifestations are related to stone formation and might include nausea, flank pain, hematuria, recurrent urinary tract infections, and rarely acute or chronic renal failure.10 A high index of suspicion should be held in patients with family history of cystinuria or stone disease, recurrent crystalluria or stone formation in the first 2 decades of life.3 Cystinuria should also be suspected in patients with large staghorn calculi filling the collecting system and requiring surgical management.18 There is no pathologic correlation between the urinary excretion of the other dibasic amino acid ornithine, lysine, arginine.7 The gastrointestinal implications of impaired absorption of cystine have no clinical significance, because cystine absorption is maintained by the ability of the small intestine to absorb oligopeptides containing cystine, resulting in comparable serum levels with normal people.10
DIAGNOSTIC EVALUATION In 2006, Dello Strologo and Rizzoni19 proposed a definition of cystinuria. A diagnosis can be made in patients with cystine stones, if an increased urinary excretion of dibasic amino acids is noted or mutations on both alleles of 1 of the 2 genes involved are identified. A patient can also be considered as having cystinuria in the absence of a urinary stone if urine cystine excretion exceeds 1300 mmol/g creatinine (150 mmol/mmol creatinine) or the sum of COLA (cystine, ornithine, lysine, and arginine) excretion through the urine exceeds 5900 mmol/g creatinine (670 mmol/mmol creatinine) in a 24-hour urine sample (Table 2). Laboratory Investigations The initial test of choice is microscopic morning evaluation of the urine and might reveal the presence of the pathognomonic hexagonal cystine crystals in up to 25% of patients with cystinuria.20 Because cystine is one of the sulfur-containing amino acids, the urine might have the characteristic odor of rotten eggs.3 The sodium-cyanide-nitroprusside test offers a rapid qualitative determination of cystine concentration. Cyanide reduces cystine to cysteine, which then binds to nitroprusside, causing a red-purple color change in 2-10 minutes.3,10,21 This test detects cystine levels higher than 75 mg/g of creatinine, with a sensitivity of 72% and specificity of 95%. It is propagated as a cheap screening test for homozygotes, but is not specific, showing false positive results in patients with Fanconi syndrome, homocystinuria, acetonuria, in heterozygous carriers of UROLOGY 83 (4), 2014
Table 2. Definition of cystinuria19 Demonstration Increased urinary Or Identification of mutations on of a cystine excretion of both alleles of stone dibasic amino 1 of the 2 acids involved genes No present Excretion in 24-hour urine cystine (mmol/g creatinine) stones Only cystine Or Sum of dibasic >1300 amino acids >5900
cystinuria, or patients taking sulfur-containing medications, ampicillin or N-acetylcysteine. The test is preferred to be performed at age 2 years or older, when renal tubules have matured and excrete cystine at levels comparable with their adult counterparts.3,10,21,22 A positive cyanide-nitroprusside test must be followed by an ion-exchange chromatographic quantitative analysis of a 24-hour collected urine sample, to confirm the diagnosis. The normal rate of cystine excretion is up to 30 mg/ day (0.13 mmol/day), when homozygotes usually excrete more than 400 mg/day (1.7 mmol/day), non-type-I heterozygotes 200-400 mg/day (0.85-1.7 mmol/day), and heterozygotes type I 1000 HU.27 However, these results are extrapolated from studies performed mostly in vitro, and further observations are required.
TREATMENT Medical Treatment and Conservative Measures The primary goal of the treatment is to prevent cystine stone formation by improving the solubility of urinary cystine. The treatment approaches include mainly dietary measures, measures that decrease urinary cystine concentration and drugs leading to reduction of cystine to the more soluble cysteine. Initial management methods should always include hydration and urinary alkalization. If the patient does not respond to this conservative approach, additional drugs, such as D-penicillamine, alpha-MPG (mercaptopropionylglycine or tiopronin), and captopril can be used.1,3 Hydration. The most important preventive technique is proper hydration and hyperdiuresis to decrease urinary cystine concentration. Fluid intake should reach at least 3 L/day in children and 4-5 L/day in adults, to preserve a urine volume of >2 L/1.73 m2 in children and >2.5 L/ day in adults. The individual rate of cystine excretion can be a guide to estimate the optimal urine output to achieve a urine concentration of
C
ystinuria is an autosomal recessive disorder that accounts for up to 10% of all pediatric stone disease.1 The disease causes recurrent stone formation, which leads to an impairment of renal function and quality of life.2 Although the mainstay of the therapy is conservative management, its side effects, poor patient compliance and its limited efficacy, lead to repeated urologic interventions.1,3
EPIDEMIOLOGY The worldwide prevalence of cystinuria is estimated at 1:7000, although it is regionally variable, ranging from 1:2500 among Libyan Jews to 1:100,000 persons in Sweden.1,4 In the Mediterranean East Coast, cystinuria occurs in approximately 1:1887 persons.4 The first symptoms of cystine stones typically present between ages 2 and 40 years, whereas the peak age of onset of stones is in the third decade of life.5 Cystine stones account for 1%-2% of urinary calculi, but the prevalence is 6%-10% of all renal calculi in children.6 More than 50% of cystinuric patients will develop cystine stones during their lifetime, with a high likelihood of bilateral stone formation.1 Incidence is equal between genders, but male patients tend to have a more aggressive course of the disease with a significant higher number of stones and an earlier onset of symptoms.7,8 The diagnosis of cystinuria should be made carefully before age 2 years, as heterozygotic carriers might have falsely elevated urine cystine levels, because of immaturity of the amino acid transporters within the renal tubule during the first 2 years of life.9 Financial Disclosure: The authors declare that they have no relevant financial interests. From the Department of General Surgery, General Hospital of Kefalonia, Argostoli, Greece; and the Dialysis Department, General Hospital of Kefalonia, Argostoli, Greece Reprint requests: Panagiotis Saravakos, M.D., Department of General Surgery, General Hospital of Kefalonia, Soudias Street, GR-28100 Argostoli, Kefalonia, Greece. E-mail: [email protected] Submitted: August 13, 2013, accepted (with revisions): October 8, 2013
ª 2014 Elsevier Inc. All Rights Reserved
PATHOPHYSIOLOGY AND GENETICS Cystinuria is an autosomal recessive genetic disorder of the transepithelial transporters for the dibasic amino acids COLA (cystine, ornithine, lysine, and arginine), resulting in an impairment of their reabsorption in the renal proximal tubule and the small intestine.4,7 In normal individuals, about 99% of the amino acids, including cystine, filtered by the glomerulus, are reabsorbed by the end of the proximal tubule. Cystinuria results from a defective amino acid transport system, which consists of a high-affinity-low-capacity system, located almost exclusively in the apical membrane of the third segment (S3) of the proximal tubule, and which is responsible for 10% of cystine reabsorption, and a low-affinity-high-capacity system, found in the S1 and S2 segments of the proximal tubule and responsible for the 90% of cystine reabsorption.10 The cystine transporter system b0,þ is a heterodimer composed of 2 subunits linked by disulfide bridge, belonging to the family of heterodimeric amino acid transporters. The heavy subunit rBAT is the glycosylated protein produced by the SLC3A1 gene sequence on chromosome 2, whereas the light subunit b0,þ AT is encoded by SLC7A9 gene in chromosome 19.10 The b0,þ AT protein (neutral and dibasic amino acid transporter) is the transport channel, whereas the rBAT protein (related to basic amino transporter) modulates the activity of b0,þ AT protein.7,11 The intracellular cystine transportation is mediated by a sodium-independent manner, unlike other neutral amino acids and solutes such as glucose, but sodium-dependent amino acid transporters at the apical and basolateral membrane favor the transport of dibasic amino acids into the cell.10 After absorption, cystine, which is an amino acid composed of 2 cysteine molecules joined by a disulfide bond, is intracellulary hydrolyzed to 2 molecules of cysteine, which then exit the cell across the basolateral membrane. In persons with cystinuria, the movement of cystine from the tubular cells into the blood is not affected (Fig. 1).7,10,12 0090-4295/14/$36.00 http://dx.doi.org/10.1016/j.urology.2013.10.013
693
Figure 1. Transport of cystine and other dibasic amino acids in the proximal tubule. Normally, cystine is reabsorbed by the apical membrane of the proximal tubule by the transport system b0,þ. The transport system b0,þ is a heterodimer composed of 2 subunits, the light subunit b0,þ AT, which is encoded by the SLC7A9 gene sequence, and the heavy subunit rBAT, which is encoded by the SLC3A1 gene sequence. This transport system is affected in patients with cystinuria. Inside the cell, cystine is reduced to 2 molecules of cysteine, which exit the cell across the basolateral membrane. (Color version available online.)
Table 1. Genetic classification of cystinuria4 Classification
Type A
Type B
Type AB
Frequency 45% 53% 2% Gene locus Chromosome 2 (SLC3A1) Chromosome 19 (SLC7A9) Chromosomes 2 and 19 (SLC3A1/SLC7A9) rBAT/ b0,þ AT Subunit affected rBAT (heavy subunit) b0,þ AT protein (light subunit)
The result of mutations in the aforementioned genes is a defective dibasic amino acid transporter, a reduced reabsorption in the proximal tubule, and elevated levels of dibasic amino acid secretion in the urine. Although all dibasic amino acids reach high concentrations in the urine, only cystine is relatively insoluble at physiological pH levels of 5-7, with a pKa level of 8,3.1,7 Up to pH 7, solubility of cystine is approximately 250 mg/L urine, whereas at a urine pH level of 7.5 this will double to 500 mg/L and triple at pH 8 or higher. Other factors also seem to influence cysteine excretion, such as dietary salt and animal protein intake, especially the intake of cystine’s precursor methionine.13,14
CLASSIFICATION Cystinuria is an autosomal recessive disease, but some heterozygote carriers have an autosomal dominant, incomplete penetrance appearance with elevated, but typically nonpathologic urinary cystine excretion. Consequently, the first classification by Rosenberg et al15 in 1966 was the phenotypic classification and was based on the amount of urinary cystine excreted of the patient’s parents, dividing 694
the disease into 3 subgroups: type I, type II, and type III (the 2 latter are also referred as non-type-I). Type I heterozygotes, an autosomal recessive disorder, show a normal urinary cystine excretion pattern (900 mmol/g creatinine), and type III heterozygotes show a moderate increase (100-900 mmol/g creatinine). Non-type-I heterozygotes are considered to have an autosomal dominant disorder with incomplete penetrance and present a slightly increased risk of stone formation.7,10,15 Genetic studies of patients with cystinuria have provided new insights in the genetic characteristics of the disease, and, on the basis of the type of the mutation, Dello Strologo et al8 proposed a new classification of the disease into type A, type B, and type AB. In type A cystinuria (45% of the cases), the disease is caused by mutations in both inherited alleles of the SLC3A1 gene in chromosome 2. In type B heterozygotes (53% of the cases) the mutation is located in the SLC7A9 gene on chromosome 19. Patients with the rare type AB cystinuria UROLOGY 83 (4), 2014
(2% of the cases) have 2 mutated alleles in the same gene in addition to a mutated allele in the other gene and so are actually AAB or ABB.4,16 So far, 133 mutations have been identified in SLC3A1 and 95 in SLC7A9.17 Table 1 summarizes the genetic types, the responsible mutations, and the affected protein for each type.
CLINICAL MANIFESTATIONS The clinically significant manifestations are related to stone formation and might include nausea, flank pain, hematuria, recurrent urinary tract infections, and rarely acute or chronic renal failure.10 A high index of suspicion should be held in patients with family history of cystinuria or stone disease, recurrent crystalluria or stone formation in the first 2 decades of life.3 Cystinuria should also be suspected in patients with large staghorn calculi filling the collecting system and requiring surgical management.18 There is no pathologic correlation between the urinary excretion of the other dibasic amino acid ornithine, lysine, arginine.7 The gastrointestinal implications of impaired absorption of cystine have no clinical significance, because cystine absorption is maintained by the ability of the small intestine to absorb oligopeptides containing cystine, resulting in comparable serum levels with normal people.10
DIAGNOSTIC EVALUATION In 2006, Dello Strologo and Rizzoni19 proposed a definition of cystinuria. A diagnosis can be made in patients with cystine stones, if an increased urinary excretion of dibasic amino acids is noted or mutations on both alleles of 1 of the 2 genes involved are identified. A patient can also be considered as having cystinuria in the absence of a urinary stone if urine cystine excretion exceeds 1300 mmol/g creatinine (150 mmol/mmol creatinine) or the sum of COLA (cystine, ornithine, lysine, and arginine) excretion through the urine exceeds 5900 mmol/g creatinine (670 mmol/mmol creatinine) in a 24-hour urine sample (Table 2). Laboratory Investigations The initial test of choice is microscopic morning evaluation of the urine and might reveal the presence of the pathognomonic hexagonal cystine crystals in up to 25% of patients with cystinuria.20 Because cystine is one of the sulfur-containing amino acids, the urine might have the characteristic odor of rotten eggs.3 The sodium-cyanide-nitroprusside test offers a rapid qualitative determination of cystine concentration. Cyanide reduces cystine to cysteine, which then binds to nitroprusside, causing a red-purple color change in 2-10 minutes.3,10,21 This test detects cystine levels higher than 75 mg/g of creatinine, with a sensitivity of 72% and specificity of 95%. It is propagated as a cheap screening test for homozygotes, but is not specific, showing false positive results in patients with Fanconi syndrome, homocystinuria, acetonuria, in heterozygous carriers of UROLOGY 83 (4), 2014
Table 2. Definition of cystinuria19 Demonstration Increased urinary Or Identification of mutations on of a cystine excretion of both alleles of stone dibasic amino 1 of the 2 acids involved genes No present Excretion in 24-hour urine cystine (mmol/g creatinine) stones Only cystine Or Sum of dibasic >1300 amino acids >5900
cystinuria, or patients taking sulfur-containing medications, ampicillin or N-acetylcysteine. The test is preferred to be performed at age 2 years or older, when renal tubules have matured and excrete cystine at levels comparable with their adult counterparts.3,10,21,22 A positive cyanide-nitroprusside test must be followed by an ion-exchange chromatographic quantitative analysis of a 24-hour collected urine sample, to confirm the diagnosis. The normal rate of cystine excretion is up to 30 mg/ day (0.13 mmol/day), when homozygotes usually excrete more than 400 mg/day (1.7 mmol/day), non-type-I heterozygotes 200-400 mg/day (0.85-1.7 mmol/day), and heterozygotes type I 1000 HU.27 However, these results are extrapolated from studies performed mostly in vitro, and further observations are required.
TREATMENT Medical Treatment and Conservative Measures The primary goal of the treatment is to prevent cystine stone formation by improving the solubility of urinary cystine. The treatment approaches include mainly dietary measures, measures that decrease urinary cystine concentration and drugs leading to reduction of cystine to the more soluble cysteine. Initial management methods should always include hydration and urinary alkalization. If the patient does not respond to this conservative approach, additional drugs, such as D-penicillamine, alpha-MPG (mercaptopropionylglycine or tiopronin), and captopril can be used.1,3 Hydration. The most important preventive technique is proper hydration and hyperdiuresis to decrease urinary cystine concentration. Fluid intake should reach at least 3 L/day in children and 4-5 L/day in adults, to preserve a urine volume of >2 L/1.73 m2 in children and >2.5 L/ day in adults. The individual rate of cystine excretion can be a guide to estimate the optimal urine output to achieve a urine concentration of
