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A new gene, encoding an anion transporter, is mutated in sialic acid storage diseases.

Sialic acid storage diseases (SASD, MIM 269920) are autosomal recessive neurodegenerative disorders that may present as a severe infantile form (ISSD) or a slowly progressive adult form, which is prevalent in Finland (Salla disease). The main symptoms are hypotonia, cerebellar ataxia and mental retardation; visceromegaly and coarse features are also present in infantile cases. Progressive cerebellar atrophy and dysmyelination have been documented by magnetic resonance imaging (ref. 4). Enlarged lysosomes are seen on electron microscopic studies and patients excrete large amounts of free sialic acid in urine. A H+/anionic sugar symporter mechanism for sialic acid and glucuronic acid is impaired in lysosomal membranes from Salla and ISSD patients. The locus for Salla disease was assigned to a region of approximately 200 kb on chromosome 6q14-q15 in a linkage study using Finnish families. Salla disease and ISSD were further shown to be allelic disorders. A physical map with P1 and PAC clones was constructed to cover the 200-kb area flanked by the loci D6S280 and D6S1622, providing the basis for precise physical positioning of the gene. Here we describe a new gene, SLC17A5 (also known as AST), encoding a protein (sialin) with a predicted transport function that belongs to a family of anion/cation symporters (ACS). We found a homozygous SLC17A5 mutation (R39C) in five Finnish patients with Salla disease and six different SLC17A5 mutations in six ISSD patients of different ethnic origins. Our observations suggest that mutations in SLC17A5 are the primary cause of lysosomal sialic acid storage diseases.

Adult↗

Quantification of free sialic acid in urine by HPLC-electrospray tandem mass spectrometry: a tool for the diagnosis of sialic acid storage disease.

BACKGROUND: Sialic acid storage diseases (SSDs) are severe autosomal recessive neurodegenerative disorders caused by a transport defect across the lysosomal membrane, which leads to accumulation of sialic acid in tissues, fibroblasts, and urine. Defective free sialic acid transport can be established by quantification of free sialic acid in urine. METHODS: Urine sample size was adjusted to the equivalent of 100 nmol of creatinine. After addition of 2-keto-3-deoxy-d-glycero-d-galactonononic acid as internal standard, samples were diluted with water to an end volume of 250 microL. We used 10 microL for HPLC-tandem mass spectrometric analysis in the negative electrospray ionization mode, monitoring transitions m/z 308.3-->m/z 86.9 (sialic acid) and m/z 267.2-->m/z 86.9 (internal standard). The overall method was validated and studied for ion suppression, interfering compounds, and pH effects. Samples from controls (n = 72) and SSD patients (n = 3) were analyzed. RESULTS: The limit of detection was 3 micromol/L. Intraassay imprecision (CV; n = 10) was 6%, 3%, and 2% at 30, 130, and 1000 mmol/mol creatinine, respectively; corresponding interassay CV (n = 10) were 5%, 5%, and 2%. Recovery was 109% (100-1000 mmol/mol creatinine). The mean (SD) [range] excretion rates (mmol/mol creatinine) were 31.3 (16.6) [0.7-56.9] at 0-1 year (n = 20), 21.2 (9.8) [6.3-38.3] at 1-3 years (n = 15), 14.4 (8.2) [1.7-32.9] at 3-10 years (n = 25), and 4.6 (2.6) [0-9.8] above age 10 years (n = 12). SSD patients 1.2, 3.9, and 12 years of age had concentrations of 111.5, 54.2, and 36.1 mmol/mol creatinine, respectively. CONCLUSIONS: The HPLC-tandem MS method for free sialic acid in urine is more rapid, accurate, sensitive, selective, and robust than earlier methods and may serve as a candidate reference method for free sialic acid in diagnosis of SSD.

Child↗

Transport of organic anions by the lysosomal sialic acid transporter: a functional approach towards the gene for sialic acid storage disease.

Transport of sialic acid through the lysosomal membrane is defective in the human sialic acid storage disease. The mammalian sialic acid carrier has a wide substrate specificity for acidic monosaccharides. Recently, we showed that also non-sugar monocarboxylates like L-lactate are substrates for the carrier. Here we report that other organic anions, which are substrates for carriers belonging to several anion transporter families, are recognized by the sialic acid transporter. Hence, the mammalian system reveals once more novel aspects of solute transport, including sugars and a wide array of non-sugar compounds, apparently unique to this system. These data suggest that the search for the sialic acid storage disease gene can be initiated by a functional selection of genes from a limited number of anion transporter families. Among these, candidates will be identified by mapping to the known sialic acid storage disease locus.

Animals↗

Clinical spectrum of infantile free sialic acid storage disease.

Infantile free sialic acid storage disease (ISSD) is a rare autosomal recessive metabolic disorder caused by a lysosomal membrane transport defect, resulting in accumulation of free sialic acid within lysosomes. Only a few cases have been described. We report on three new cases of ISSD with different modes of presentation: an infant with nephrotic syndrome, a case of fetal and neonatal ascites with heart failure, and a case of fetal ascites with esophageal atresia type III. From these patients and a review of the literature (27 cases total) we draw the following conclusions. 1) "Coarse facies," fair complexion, hepatosplenomegaly, and severe psychomotor retardation are constant findings in this disorder. 2) Nephrotic syndrome occurred in most cases (four in seven) in which renal evaluation was performed. Therefore, ISSD is an important cause of nephrosis in infants with a storage disorder phenotype. 3) Fetal/neonatal ascites or hydrops was the mode of presentation in 13 (60%) of 21 cases. Thus, ISSD enters in the differential diagnosis of hydrops fetalis with a storage disease phenotype. 4) Cardiomegaly was evident in nine cases. 5) Corneae were always clear, and albinoid fundi were reported in five cases. 6) Dysostosis multiplex was not prominent. 7) Bone marrow aspiration could be negative. 8) Death ensued in early infancy with a mean age of 13.1 months. All reported deaths were caused by respiratory infections.

Ascites↗

Infantile sialic acid storage disease associated with renal disease.

A child with infantile sialic acid storage disease is reported. Ultrasonography demonstrated fetal ascites. At birth, the infant appeared hydropic and presented with numerous dysmorphic features, including sparse white hair, coarse facies, hypertelorism, epicanthal folds, anteverted nostrils, and a long philtrum. In addition, he had visceromegaly, bilateral inguinal hernias, and a slight gibbus deformity. Lymphocytes were vacuolated and bone marrow contained large numbers of foam cells. There were generalized vacuolations of both reticuloendothelial and parenchymal cells in the examined tissues. Neuropathologic studies revealed wide-spread neuronal storage, myelin loss, axonal spheroids, and gliosis. Neurons, endothelial cells, and Kupffer cells stained with wheat germ agglutinin indicated an accumulation of sialic acid. Free sialic acid was significantly increased in urine and serum, as well as in liver, heart, and brain tissues. The alpha-neuraminidase activity was normal. It is assumed that the basic defect of infantile sialic acid storage disease lies in impaired transport of sialic acid across the lysosomal membrane.

Brain↗

Clinical, morphological, and molecular aspects of sialic acid storage disease manifesting in utero.

BACKGROUND: Sialic acid storage diseases (SASDs) are caused by the defective transport of free sialic acid outside the lysosome. Apart from the Salla presentation in Finland, SASD is a very rare form of lysosomal storage disease (LSD) with approximately 35 cases, all diagnosed after birth, having been reported worldwide. We report a series of 12 French patients with very early manifestations, including eight fetuses diagnosed in utero. RESULTS: Ultrasound examination, fetal autopsy, or clinical examination showed prominent ascites, rarely progressing to complete hydrops, and highlighted the early severity of bone disease. Dramatic increase of free sialic acid in various biological samples confirmed the diagnosis in all cases. Storage staining affinities and storage distribution in placenta and fetal organs allowed differential diagnosis from other LSDs but cannot differentiate between SASD, sialidosis, and galactosialidosis. Fourteen different mutations were identified, showing the molecular heterogeneity of SASD in the French population. We found that the previously described p.Y306X mutation generated two different transcripts, and we identified seven novel mutations: three deletions (del exon 7, del exons10+11 and c.1296delT), one splice site mutation (c.1350+1G-->T) one nonsense mutation (p.W339X), and two missense mutations (p.R57C and p.G127E). CONCLUSIONS: The severity of our patients' genotypes is in agreement with their phenotypes but not with the importance and early appearance of the very frequent in utero manifestations. Minimal fetal disease in some patients and a reported case of heterogeneity of fetal involvement within a family suggest that factors other than the genotype influence fetal manifestations.

Female↗

Infantile sialic acid storage disease: biochemical studies.

Infantile free sialic acid storage disease (ISSD), is an inherited metabolic disorder characterized by hyperexcretion of free sialic acid in the urine and by its storage in the lysosomes of different tissues. In order to obtain more reliable data on the amount of total and free sialic acid, we analyzed the urine, brain, cerebellum, liver, spleen, and kidneys from a 3-month-old baby who died with a diagnosis of ISSD. The lysosomal nature of the disease was confirmed by an electron microscopic study of cells in culture. No significant abnormalities were found involving cholesterol, total phospholipids, glycolipids, and gangliosides in the tissues examined. However, differences in the tissue distribution of individual glycolipids and gangliosides were observed. The amount of free and total sialic acid was markedly increased, due to the storage of free sialic acid accompanied by its hyperexcretion in the urine. These results demonstrate and confirm that only acid monosaccharide transport from the lysosome compartment is involved in the pathogenesis of ISSD.

Brain Chemistry↗

Nephrosis in two siblings with infantile sialic acid storage disease.

The diagnosis of infantile sialic acid storage disease (ISSD) was established in two siblings on the basis of typical clinical signs and the biochemical findings of hyperexcretion and intracellular storage of free sialic acid. A severe, steroid resistant nephrosis occurred in both siblings. The activities of lysosomal enzymes, including sialidase, were normal. A combined detection method for sialic acids with Limax flavus agglutinin labelling and phosphotungstic acid staining showed severely alterated sialic acid components in epithelial kidney cells and indicate a causal relationship between the nephrosis and the underlying biochemical defect. Further observations of ISSD patients with renal involvement will prove if a separate nephropathic phenotype exists.

Carbohydrate Metabolism, Inborn Errors↗

An Italian severe Salla disease variant associated with a SLC17A5 mutation earlier described in infantile sialic acid storage disease.

The present study reports two Italian brothers affected by severe Salla disease (sialic acid storage disease), a slowly progressive autosomal recessive neurodegenerative disorder prevalent in the Finnish population. Mutations of the SLC17A5 gene, which encodes a protein called sialin, are the primary cause of both Salla disease and infantile sialic acid storage disease (ISSD), a clinically distinct severe disorder. All Finnish patients with Salla disease show a R39C mutation. Both patients showed moderate intellectual disability, spastic ataxic syndrome, hypomyelination and cerebellar atrophy on magnetic resonance imaging (MRI), and lysosomal storage, all typical of Salla disease. Mutation analysis of the SLC17A5 gene in the younger brother revealed no R39C mutation, but a 15-bp deletion in exon 6 on one of the alleles. This mutation is the same described in French-Canadian patients with ISSD. Salla disease must be suspected in patients with unexplained psychomotor retardation associated with ataxia and/or pyramidal symptoms, and MRI findings consistent with cerebral hypomyelination, irrespective of the patient's ethnic origin. A mutation screening based on R39C change does not exclude Salla disease outside Finland. Conversely, mutations found in ISSD can be expected, even in patients showing the Salla phenotype (e.g. symptoms at the milder end of the spectrum).

Adolescent↗

Defective lysosomal release of glycoprotein-derived sialic acid in fibroblasts from patients with sialic acid storage disease.

Fibroblasts from patients with sialic acid storage disease (SASD), sialidosis, mucolipidosis II, and from normal controls, were incubated in the presence of the glycoprotein fetuin that was tritium-labelled in its sialic acid residues by the periodate/[3H]borohydride reduction method, and the fate of the intracellular radioactive sialic acid (C7-sialic acid) followed in pulse-chase experiments. The model glycoprotein was readily endocytosed and degraded, more than 90% of the radioactivity being trichloroacetic acid (TCA)-soluble after 4 days of incubation. In all of the patients' fibroblasts, there was an increased accumulation of TCA-soluble radioactivity and, upon chase, a much lower rate of elimination than in normal controls. Gel chromatography of the material from the chase experiment showed that, in normal cells, most of the radioactivity at zero time behaved as free C7-sialic acid. This, as well as material of larger size (sialyloligosaccharides), was very much diminished by 48 h. In cells from two patients with SASD, there were large peaks both in the sialic acid and oligosaccharide positions; whereas the oligosaccharides were somewhat decreased by the end of the chase period, the sialic acid was essentially unchanged. In sialidosis fibroblasts, the radioactive material consisted of oligosaccharides, but very little C7-sialic acid; the elimination of the oligosaccharides was retarded. In normal cells, about 80% of the radioactivity released into the medium after 48 h chase behaved as free C7-sialic acid upon gel chromatography and t.l.c. Subcellular fractionation in Percoll gradients showed that the radioactive C7-sialic acid remaining in normal cells after 48 h of chase was mainly localized in the cytosol. In SASD cells, on the other hand, it was associated with lysosomal fractions which, unexpectedly, exhibited an abnormally low density. Our findings demonstrate that SASD fibroblasts degrade the sialoglycoprotein but, unlike normal cells, accumulate the liberated C7-sialic acid along with sialyloligosaccharides in their lysosomes. The results therefore support the concept of a defective transport system for sialic acid in the lysosomal membrane of patients with SASD.

Biological Transport↗

Prenatal detection of free sialic acid storage disease: genetic and biochemical studies in nine families.

Sialic acid storage disorders, Salla disease (SD) and a severe infantile form of disease (ISSD), are recessively inherited allelic lysosomal storage disorders due to impaired egress of free sialic acid from lysosomes. Fourteen pregnancies at risk of adult-type free sialic acid storage disease, SD, were monitored by sialic acid assays, genetic linkage or mutation detection analyses using chorionic villus samples. Three affected and 12 unaffected fetuses were identified. The first studies were based on the sialic acid assays alone, but the location of the gene enabled the use of genetic linkage analysis and, more recently, the identification of the SLC17A5 gene and disease-causing mutations added yet another possibility for prenatal studies. A missense mutation 115C-->T (R39C) is present in 95% of all Finnish SD alleles, providing an easy and reliable means of diagnostic studies. Both molecular and biochemical (sialic acid assay) studies can be used for prenatal diagnosis of free sialic acid storage diseases.

Adult↗

Clinical, biochemical, and molecular diagnosis of a free sialic acid storage disease patient of moderate severity.

The allelic autosomal recessive lysosomal storage disorders Salla disease and infantile free sialic acid storage disease (ISSD) result from mutations in SLC17A5. This gene codes for sialin, a lysosomal membrane protein that transports the charged sugar, N-acetylneuraminic acid (sialic acid), out of lysosomes. ISSD has a severe phenotype with infantile onset, while the Finnish variant, Salla disease, has a milder phenotype with later onset. Both disorders cause developmental delay, and ISSD is generally fatal in early childhood. We describe a 30-month old non-Finnish, Caucasian child with global developmental delay of postnatal onset, language, and motor skills stagnant at a 3-4 month level, hypotonia, and mild but progressive coarsening of facial features. Urinary excretion of free sialic acid was elevated 4.5 times above control. EM of a skin biopsy revealed enlarged secondary lysosomes consistent with oligosaccharide storage. Free sialic acid in fibroblasts was 3.8+/-0.9 nmol/mg protein (concurrent normal controls, 0.5+/-0.1); differential centrifugation indicated a lysosomal location. Genomic analysis revealed compound heterozygosity for two new SLC17A5 mutations. This child's clinical manifestations of a lysosomal free sialic acid storage disease are consistent with her sialin mutations and biochemical findings. The differential diagnosis of postnatal developmental delay should include free sialic acid storage disorders such as ISSD and Salla disease.

Base Sequence↗

Biochemical and molecular analyses of infantile free sialic acid storage disease in North American children.

The differential diagnosis of developmental delays and growth retardation in early childhood includes the allelic lysosomal sialic acid storage disorders, Salla disease and infantile free sialic acid storage disease (ISSD). These diseases, due to defective free sialic acid transport out of lysosomes, derive from mutations in the SLC17A5 gene coding for the protein sialin. We present two patients with clinical, biochemical, and molecular data indicative of lysosomal free sialic acid storage disorders. One patient, with a severe clinical course typical of ISSD, had 86-fold elevated levels of fibroblast free sialic acid, with 62% in the lysosomal fraction. His SLC17A5 mutations include a 148-bp deletion of exon 9, due to a G >A splice site mutation in position 1 of intron 9, and a 15-bp deletion (del 801-815) in exon 6. Another patient, with "intermediate severe" Salla disease, had 9-fold elevated levels of free sialic acid in cultured fibroblasts, of which 87% resided in the lysosomal fraction. This girl is compound heterozygous for the SLC17A5 mutation commonly found in Finnish Salla disease patients (R39C) and a 15-bp deletion found in ISSD patients (del 801-815). These observations emphasize the importance of considering free sialic acid disorders in infants with developmental delays and growth retardation, regardless of whether they are of Finnish ancestry.

Base Sequence↗

Sialic acid storage disease and related disorders.

This paper gives an overview of the two sialic acid storage disorders, Salla disease and infantile sialic acid storage disease, and the related disorders cystinosis, sialuria, sialidosis, and galactosialidosis. Sialic acid storage disease and cystinosis are models for a deficient lysosomal transport of monosaccharides and amino acids, respectively. Several gene mutations leading to the production of the faulty membrane proteins sialin and cystinosin have been identified in recent years. Knowledge of the underlying pathophysiology is a prerequisite for future research projects, which will focus on the expression of the disease genes in living systems and the physical characterization of these proteins by X-ray crystallography and nuclear magnetic resonance spectroscopy.

Amino Acid Transport Systems, Neutral↗

Prenatal diagnosis of infantile sialic acid storage disease in a twin pregnancy.

A diagnosis of infantile sialic acid storage disease was made in an infant who died aged 17 months. In the mother's next pregnancy no morphological or biochemical abnormality was found in chorionic villi, amniotic fluid, cultured amniotic fluid cells or fetal blood and a normal boy was born. In the subsequent pregnancy an ultrasound scan revealed a twin pregnancy. Chorionic villus samples were obtained from both twins and microscopic and biochemical analysis indicated one twin to be affected with sialic acid storage disease. Selective fetocide was performed. The unaffected twin proceeded to term.

Carbohydrate Metabolism, Inborn Errors↗

A Japanese case of infantile sialic acid storage disease.

We report a 4-year-old Japanese girl with infantile sialic acid storage disease. She presented with failure to thrive, coarse facial features, hepatosplenomegaly, severe mental retardation and spastic quadriplegia. Electron microscopic examination of cultured skin fibroblasts revealed multiple vacuoles and inclusion material representing distended lysosomes, thus suggesting a lysosomal storage disorder. A high concentration of free sialic acid was present in the urine and cultured fibroblasts, but bound sialic acid was not increased. The activity of a variety of lysosomal enzymes was not diminished. The MRI findings included brain atrophy and a diffuse high signal in the cerebral white matter and low signal in the basal ganglia on T2-weighted images. To our knowledge, this is the first case of infantile sialic acid storage disease described in a non-Caucasian family.

Atrophy↗

The genetic locus for free sialic acid storage disease maps to the long arm of chromosome 6.

Salla disease (SD), or adult-type free sialic acid storage disease, is an autosomal recessive lysosomal storage disorder characterized by impaired transport of free sialic acid across the lysosomal membrane and severe psychomotor retardation. Random linkage analysis of a sample of 27 Finnish families allowed us to localize the SD locus to the long arm of chromosome 6. The highest lod score of 8.95 was obtained with a microsatellite marker of locus D6S286 at theta = .00. Evidence for linkage disequilibrium was observed between the SD locus and the alleles of three closely linked markers, suggesting that the length of the critical region for the SD locus is in the order of 190 kb.

Alleles↗

Infantile sialic acid storage disease (ISSD). Report on first case in Czech Republic with biopsy and autopsy findings.

The first case of infantile sialic acid storage disease in Czech Republic is presented in a four-and-half year-old girl. The clinical phenotype consisted of moderate hepatosplenomegaly and skin hypopigmentation, early psychomotoric and developmental arrest, associated with truncal ataxia and lower extremities spasticity, extinguished acoustic and visual perception (optic atrophy without macular alteration) and remarkable automutilation phenomena. The appearance was normosomatic and there were minimal dysostotic changes. Skin and liver biopsy displayed moderate amount of lucent storage lysosomes in epithelial, mesenchymal, and neural elements. Alder-Reily granules were found in the bone marrow and peripheral blood cells. The urinary excretion of mucopolysaccharides and oligosaccharides was not increased. The autopsy showed heterogenous neuronal and glial brain storage (lucent lysosomes, lipopigment, membranous cytoplasmic bodies), severe hypomyelination and severe storage in the splenic sinus endothelium. Diagnosis was made by proving thirteen fold increase of free sialic acid in the fibroblast culture. It is pointed out that in the case of a mucopolysaccharidosis-like storage disease unexplainable by a hydrolytic enzyme deficiency, it is the enzyme product storage which must be suspected. At present, the only candidate is the sialic acid storage disease.

Child, Preschool↗