Authors Manuel Palacín    Paul Goodyer    Virginia Nunes    Paolo Gasparini   
Abstract

Cystinuria (MIM 220200) is a disorder of amino acid transport affecting epithelial cells of the renal tubule and the gastrointestinal tract. The defective transport of cystine, lysine, arginine, and ornithine is transmitted as an autosomal recessive trait. According to the level of urinary excretion of cystine and dibasic amino acids in obligate heterozygotes, two types of cystinuria are envisaged: type I, the fully recessive form, and non-type I (type II, type III), the incomplete recessive form. In the latter type, the affected amino acids are excreted by heterozygotes in urine at levels greater than normal but less than in the homozygous state.

The only proven clinical manifestation of cystinuria is urolithiasis, due to the low solubility of cystine at low pH. Distinctive hexagonal cystine crystals appear in the urine and radiopaque cystine stones develop repeatedly in affected individuals. Cystinuria is diagnosed by demonstrating selective hyperexcretion of cystine and dibasic amino acids in urine. Stones generally form in acidic urine when urinary cystine concentration exceeds 300 mg cystine per liter (1200 μM). Prevention of urolithiasis is directed at high fluid intake and alkalinizing the urine to maximize cystine solubility. Oral sulfhydryl agents such as D-penicillamine and mercaptopropionylglycine may be used to form soluble mixed disulfides of cystine in the urine. Although effective, these agents are not risk-free and are usually reserved for patients who fail to respond to conservative therapy.

The corresponding small intestinal transport mechanism for absorption of cystine and dibasic amino acids is also defective in many cystinuria patients; oral loading tests and in vitro studies of jejunal biopsies demonstrate this. However, there are no gastrointestinal symptoms and, under conditions of normal protein intake, plasma amino acid levels are normal. This is presumably due to alternative absorptive mechanisms in the intestine, including direct uptake of dipeptides.

The renal clearance of cystine varies widely among affected individuals. In both humans and canine mutant phenotypes, fractional excretion of cystine may exceed the glomerular filtration rate, indicating net secretion, or back-leak, of cystine into the tubular fluid. This is particularly true in the first months of life when renal amino acid transport is immature. Thus, some heterozygous infants may initially appear to have homozygous cystinuria and should not be classified until at least 1 year of age.

In vitro studies of rat renal and intestinal brush-border membrane vesicles and renal tubular fragments show that cystine and dibasic amino acids share a high-affinity (low Km) transport mechanism. This transport system, also shared with neutral amino acids, is detected at the luminal surface of epithelial cells in the proximal straight tubule (S3 segment). A comparable system (bo,+-like) for cystine, dibasic, and neutral amino acids has been demonstrated in the apical membrane of an opossum kidney (OK) cell line. The bo,+-like system is an amino acid exchanger that normally mediates influx of cystine and dibasic amino acids and efflux of neutral amino acids. A separate low-affinity (high-Km) system for cystine, not shared with dibasic amino acids, is located in the proximal convoluted tubule. The molecular basis and the physiologic role of this transporter is unknown.

Direct evidence for a defect in the shared transport mechanism for cystine and the dibasic amino acids derives from in vitro studies of renal cortical slices and jejunal mucosa biopsies from cystinuric patients.

Expression cloning was used to identify a renal cDNA (related to bo,+ amino acid transporter (rBAT)) that induces the bo,+ system when expressed in Xenopus oocytes. In oocytes, the rBAT (bo,+-like) system acts as a tertiary active transport mechanism. A recently cloned "light" subunit (bo,+AT) coexpresses system bo,+ activity in heterologous expression systems. Both proteins are expressed in brush-border membranes of proximal straight tubule and small intestinal mucosa. Whereas all renal bo,+AT is disulfide bound with rBAT, there is an excess of rBAT covalently bound to an unidentified "light" subunit (X) in renal brush-border membranes. The rBAT/bo,+AT heterodimer shows a gradient of expression along the proximal tubule: higher in the convoluted tubule and lower in the straight tubule. In contrast, the rBAT/X heterodimer has the opposite gradient of expression along the proximal tubule.

To date, 59 unique SLC3A1 /2p16.3-p21) mutations have been identified in cystinuria patients. Missense mutations show loss of transport function in oocytes, apparently due to trafficking defects during transfer from endoplasmic reticulum to plasma membrane. Null SLC3A1 mutations are fully recessive in cystinuria heterozygotes.

Mutational analysis and linkage studies have demonstrated genetic heterogeneity in cystinuria. The SLC3A1 gene is only associated with type I (fully recessive) cystinuria. A second locus, accounting for types II and III cystinuria (incomplete recessive forms), has been identified by linkage analysis at chromosome 19q13.1-13.2. The gene for the non-type I cystinuria (SLC7A9) codes for the "light" subunit (bo,+AT) of rBAT, where 37 unique cystinuria-specific mutations have been identified. This strongly supports the notion that rBAT/bo,+AT heterodimer (system bo,+) is the main apical reabsorption system for cystine.

In non-type I cystinuria, the urinary hyperexcretion of cystine and dibasic amino acids among heterozygous carriers of SLC7A9 mutations correlates well with the severity of defective amino acid transport in vitro. In some cases, mild SLC7A9 mutations account for heterozygous type I cystinuria. Patients with the mixed form of cystinuria (type I/III) excrete slightly lower levels of cystine and appear to have a lower risk of nephrolithiasis in the first decade. An SLC3A1 mutation is rarely identified on the fully recessive allele in these patients.

After nearly 200 years, the molecular basis of cystinuria, one of the oldest recognized inborn errors, is finally being unraveled. However, the molecular physiology of the renal cystine transport mechanism still needs clarification. In particular, we need to understand the physiologic role of the SLC3A1 gene, why mutations in the "heavy" subunit (rBAT) of system bo,+ produce a silent phenotype in carriers whereas mutations in the "light" subunit (bo,+AT) produce a dominant negative effect, and which genes modulate urolithiasis in patients with cystinuria.