Identification of novel SLC3A1 gene mutations in Spanish cystinuria families and association with clinical phenotypes

Molecular characterization of cystinuria in south-eastern European countries

Cystinuria is an autosomal recessive disorder caused by defective transport of cystine and dibasic amino acids in the proximal renal tubules and small intestine. So far, more than 128 mutations in SLC3A1 gene, and 93 in SLC7A9 gene have been described as a cause of cystinuria. We present a molecular characterization of the cystinuria in 47 unrelated south-east European families. The molecular methodology included direct sequencing, single strand conformational polymorphism, and restriction fragment length polymorphism. A total of 93 (94.9 %) out of 98 unrelated cystinuric chromosomes have been characterized. Mutations in SLC3A1 gene account for 64.3 % and in SLC7A9 gene for 30.6 % of the cystinuric chromosomes. Ten different mutations in SLC3A1 gene were found, and two of them were novel (C242R and L573X), while in SLC7A9 gene seven mutations were found, of which three were novel (G73R, V375I and c.1048_1051delACTC). The most common mutations in this study were T216M (24.5 %), M467T (16.3 %) and R365L (11.2 %) in SLC3A1 and G105R (21.4 %) in SLC7A9 gene. A population specificity of cystinuria mutations was observed; T216M mutation was the only mutation present among Gypsies, G105R was the most common mutation among Albanians and Macedonians, and R365L among Serbs. The results of this study allowed introduction of rapid, simple and cost-effective genetic diagnosis of cystinuria that enables an early preventive care of affected patients and a prenatal diagnosis in affected families.

Cystinuria: an inborn cause of urolithiasis

Cystinuria (OMIM 220100) is an inborn congenital disorder characterised by a defective cystine metabolism resulting in the formation of cystine stones. Among the heterogeneous group of kidney stone diseases, cystinuria is the only disorder which is exclusively caused by gene mutations. So far, two genes responsible for cystinuria have been identified: SLC3A1 (chromosome 2p21) encodes the heavy subunit rBAT of a renal b^0,+ transporter while SLC7A9 (chromosome 19q12) encodes its interacting light subunit b^0,+AT. Mutations in SLC3A1 are generally associated with an autosomal-recessive mode of inheritance whereas SLC7A9 variants result in a broad clinical variability even within the same family. The detection rate for mutations in these genes is larger than 85%, but it is influenced by the ethnic origin of a patient and the pathophysiological significance of the mutations. In addition to isolated cystinuria, patients suffering from the hypotonia-cystinuria syndrome have been reported carrying deletions including at least the SLC3A1 and the PREPL genes in 2p21. By extensive molecular screening studies in large cohort of patients a broad spectrum of mutations could be identified, several of these variants were functionally analysed and thereby allowed insights in the pathology of the disease as well as in the renal trafficking of cystine and the dibasic amino acids. In our review we will summarize the current knowledge on the physiological and the genetic basis of cystinuria as an inborn cause of kidney stones, and the application of this knowledge in genetic testing strategies.

Distinct classes of trafficking rBAT mutants cause the type I cystinuria phenotype

Most mutations in the rBAT subunit of the heterodimeric cystine transporter rBAT-b(0,+)AT cause type I cystinuria. Trafficking of the transporter requires the intracellular assembly of the two subunits. Without its partner, rBAT, but not b(0,+)AT, is rapidly degraded. We analyzed the initial biogenesis of wild-type rBAT and type I cystinuria rBAT mutants. rBAT was degraded, at least in part, via the ERAD pathway. Assembly with b(0,+)AT within the endoplasmic reticulum (ER) blocked rBAT degradation and could be independent of the calnexin chaperone system. This system was, however, necessary for post-assembly maturation of the heterodimer. Without b(0,+)AT, wild-type and rBAT mutants were degraded with similar kinetics. In its presence, rBAT mutants showed strongly reduced (L89P) or no transport activity, failed to acquire complex N-glycosylation and to oligomerize, suggesting assembly and/or folding defects. Most of the transmembrane domain mutant L89P did not heterodimerize with b(0,+)AT and was degraded. However, the few [L89P]rBAT-b(0,+)AT heterodimers were stable, consistent with assembly, but not folding, defects. Mutants of the rBAT extracellular domain (T216M, R365W, M467K and M467T) efficiently assembled with b(0,+)AT but were subsequently degraded. Together with earlier results, the data suggest a two-step biogenesis model, with the early assembly of the subunits followed by folding of the rBAT extracellular domain. Defects on either of these steps lead to the type I cystinuria phenotype.

Type I cystinuria and its genetic basis in a population of Turkish school children

OBJECTIVES

Cystinuria is a common inherited disorder characterized by an abnormal urinary excretion of cystine and dibasic amino acids resulting in nephrolithiasis. The SLC3A1 gene, which encodes a dibasic amino acid transporter protein, is involved in the pathogenesis of cystinuria. In the present study we aimed to investigate the prevalence of cystinuria among children in Sivas province (Central Anatolia, Turkey) and to study M467T and M467K mutations and 231T/A polymorphism in patients with cystinuria.

METHODS

A total of 8500 children were screened for cystinuria. The cyanide-nitroprusside test was applied to urine samples of all children. Children having a positive cyanide-nitroprusside test were further analyzed. M467T and M467K mutations (exon 8) and 231T/A polymorphism (exon 1) in the SCL3A1 gene were studied using a restriction fragment length polymorphism (RFLP) assay.

RESULTS

We have found that the prevalence of cystinuria (11 cystinuric patients) is 1/772 in our population. Results of mutation analysis in the patients with cystinuria showed that M467T was the only mutation that was found in six cystinuric patients. One patient was homozygous and five were heterozygous for this mutation.

CONCLUSIONS

The frequency of cystinuria in Sivas Province is the highest among the other populations studied to date. The frequency of M467T mutation is relatively higher than those reported for most populations. High frequency of cystinuria in this region could be due to consanguineous marriages.

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.

Analysis of a 1-year-old cystinuric patient with recurrent renal stones

Cystinuria is a hereditary disorder of cystine and dibasic amino acids (lysine, arginine, ornithine) transport across the luminal membrane of renal tubules and intestine, resulting in recurrent nephrolithiasis. Cystine stones frequently occur in the second or third decade of life with an occasional occurrence in infancy and in old age. Herein is presented the case of a 1-year-old girl with cystinuria and recurrent urolithiasis; the genetic basis of the disease was investigated by mutational analysis of the SLC3A1 gene. The data show that the present patient has an increased cystine (923.08 microg/mL) level and was heterozygote for M467T mutation.

[SLC7A9 gene variation: impact of 13 frequent mutations in the etiology of cystinuria in a Spanish Mediterranean population]

BACKGROUND AND OBJECTIVE

The aim of this study was to investigate the presence of the most prevalent mutation in the SLC7A9 gene in families of the Mediterranean Spanish population and their association with clinical phenotypes.

PATIENTS AND METHOD

Twenty cystinuria families were studied (6 type I, 12 non type I, and 2 unknown type), including 48 cystinuria patients and 44 relatives. DNA was isolated and molecular analysis of 13 variations (P52L, N58_G79del22, G63R, G105R, T123M, V170M, A182T, V188M, c.614dupA, G259R, L283F, A316V and R333W) in the SLC7A9 gene was undertaken. Association studies between these mutations and urinary aminoacid concentrations, stones, urinary infections, colics and other clinical traits were carried out.

RESULTS

Of the 13 investigated mutations, the most prevalent mutation in cystinuria patients was c.614dupA (17.1%), which was found in 13 patients in heterozygous state (17.1%) and in 2 relatives, all of them belonging to 4 non type I families. Mutations G105R (9.2%), T123M (3.9%) and N58_G79del22 (2.6%) were detected only in non type I cystinuria patients. Meanwhile, a R333W carrier allele was found in a patient of a unknown family, and a G105R allele in a relative of a non type I family. No mutation was found in type I families and no patients with mutations in both SLC3A1 and SLC7A9 genes were found in any family.

CONCLUSIONS

Although we have not carried out the whole screening of SLC7A9 gene, the detection rate of variations in SLC7A9 gene suggests a greater impact of this gene in the etiology of cystinuria in our population than variations in the previously screened SLC3A1 gene. The wide variation of phenotypical traits in subjects of families with the same mutations suggests that further investigation of other genetic and/or environmental factors should be carried out.

Heterogeneous mutations in the SLC3A1 and SLC7A9 genes in Chinese patients with cystinuria

Cystinuria is a recessively inherited aminoaciduria that leads to recurrent urolithiasis. It is caused by the defective transport of cystine and dibasic amino acids in the proximal renal tubules and intestinal epithelium. Two genes responsible for this, SLC3A1 and SLC7A9, are known. Patients with two SLC3A1 mutations are classified as type A cystinuria, whereas patients with two SLC7A9 mutations are classified as type B cystinuria. Few clinical and molecular data have been reported for Asian cystinuria patients. In this study, we determined the molecular basis of cystinuria in eight unrelated Chinese subjects. Coding exons and flanking introns of the SLC3A1 and SLC7A9 genes were directly sequenced after amplification by polymerase chain reaction. Five different SLC3A1 mutations were found. Two missense mutations, D210G and S547L, were novel. The other three SLC3A1 mutations (IVS6+2T>C, R181Q and R365W) have been described previously. In addition, four novel SLC7A9 mutations, C137R, c.730delG, IVS10+2_3delTG and IVS12+3insT, together with two previously reported mutations (A70V and G195R) were found. All patients except one carried compound heterozygous mutations. IVS12+3insT was detected in patients from two families. This is the first molecular genetic study on Chinese cystinuria patients. Three patients with type A cystinuria, two with type B cystinuria, and three carriers of type B cystinuria were identified. Our results suggest that the molecular basis of cystinuria is heterogeneous in our local population.