Alternative titles; symbols
HGNC Approved Gene Symbol: SLC17A5
SNOMEDCT: 34566007, 87074006;
Cytogenetic location: 6q13 Genomic coordinates (GRCh38) : 6:73,593,379-73,653,992 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 6q13 | Salla disease | 604369 | Autosomal recessive | 3 |
| Sialic acid storage disorder, infantile | 269920 | Autosomal recessive | 3 |
The SLC17A5 gene encodes a vesicular excitatory amino acid transporter (VEAT) with dual physiologic functions. When present in synaptic vesicles in the central nervous system, sialin is responsible for vesicular storage and subsequent exocytosis of aspartate and glutamate. When present in lysosomes, it acts as an H(+)-coupled sialic acid exporter (Miyaji et al., 2008).
Using a positional cloning approach in a search for the gene that is mutant in sialic acid storage diseases (see 269920 and 604369) that map to chromosome 6q14-q15, Verheijen et al. (1999) identified the SLC17A5 gene. The isolated sequence encoded a predicted 495-residue protein with homology to members of the anion/cation symporter (ACS) family of transporters. This family contains eukaryotic inorganic anion transporters (such as Na+/phosphate cotransporters) as well as prokaryotic organic anion transporters (including H+/acid sugar symporters for hexuronate and glucarate). Verheijen et al. (1999) suggested that the product of the SLC17A5 gene be designated 'sialin' because of its relation to sialic acid storage diseases. The sialin protein contains a characteristic motif in the fourth transmembrane-spanning domain that is present in all members of the ACS family. They could demonstrate homology of sialin with human Na+/phosphate symporters by sequence alignment. For example, sialin shows 34% sequence identity with NPT1 (SLC17A1; 182308). Only the N- and C-terminal regions do not show homology. Verheijen et al. (1999) found extensive homology of human sialin with proteins in other species. On Northern blot analysis of human tissues, Verheijen et al. (1999) found ubiquitous expression of an approximately 4.5-kb major transcript of SLC17A5, and an additional transcript of approximately 3.5 kb. They also observed a ubiquitously expressed 1.8-kb band after very long exposures. They suspected that these different transcripts are due to multiple poly(A) addition sites. The SLC17A5 gene is also known as AST.
Mancini et al. (1991) demonstrated a proton-driven carrier for sialic acid in human lysosomal membranes. This transporter had similar properties to those previously identified in rat liver. By measuring the uptake kinetics of labeled glucuronic acid, they excluded the existence of more than 1 acidic monosaccharide carrier. Uptake studies with labeled sialic acid and glucuronic acid in lysosomal membrane vesicles from cultured fibroblasts from patients with different clinical forms of sialic acid storage disease showed defective carrier-mediated transport for both sugars. Further evidence that the defective transport of acidic sugars represents the primary genetic defect in sialic acid storage diseases was provided by the observation of reduced, half-normal transport rates in lymphoblast-derived lysosomal membrane vesicles from 5 unrelated obligate heterozygotes. This was the first observation of a human lysosomal transport defect for multiple physiologic compounds.
Havelaar et al. (1998) studied the lysosomal sialic acid transporter as a first step toward understanding the molecular defect(s) in the clinically heterogeneous forms of sialic acid storage disease. They purified the sialic acid transporter from lysosomal membranes of rat liver to apparent homogeneity and compared its functional properties with those of other monocarboxylate transporters present in the plasma membrane of various mammalian cells. They found striking biochemical and structural similarities of the sialic acid transporter with the known monocarboxylate transporters of the plasma membrane: MCT1 (600682), MCT2 (603654), and MCT3 (610409).
Studies by Havelaar et al. (1999) indicated that the mammalian sialic acid carrier, which is defective in the transport of sialic acid through the lysosomal membrane in sialic acid storage disease, is a carrier for other organic anions.
Sialic acid storage diseases are autosomal recessive neurodegenerative disorders that may present as a severe infantile form (ISSD; 269920) or a slowly progressive adult form, which is prevalent in Finland and referred to as Salla disease (SD; 604369), because of its geographic distribution. The main symptoms are hypotonia, cerebellar ataxia, and mental retardation; visceromegaly and coarse features are also present in the infantile cases. Progressive cerebellar atrophy and dysmyelination have been documented by magnetic resonance imaging. Enlarged lysosomes are seen on electron microscopic studies, and patients excrete large amounts of free sialic acid in the urine. The locus for Salla disease was assigned to a region of approximately 200 kb on 6q14-q15 in a linkage study using Finnish families (Haataja et al., 1994; Schleutker et al., 1995). Salla disease and ISSD were further shown to be allelic disorders (Schleutker et al. (1995)). By a functional and positional candidate gene approach, Verheijen et al. (1999) traced mutations responsible for both Salla disease and ISSD to mutations in SLC17A5. They found a homozygous SLC17A5 mutation (arg39 to cys; 604322.0001) in 5 Finnish families with Salla disease and 6 other SLC17A5 mutations in either homozygous or compound heterozygous state in 6 patients of other ethnic origins.
Aula et al. (2000) identified a large number of mutations in SLC17A5 in patients presenting with either Salla disease or the infantile sialic acid storage disorder. All 80 Finnish patients with Salla disease had the R39C mutation (604322.0001); 91% of them were homozygous for this old founder mutation. The compound heterozygous patients, with the founder mutation in only 1 allele, presented with a more severe phenotype than did the homozygous patients. The same R39C mutation was also found in most of the Swedish patients with SD and in heterozygous form in 5 patients from central Europe who presented with an unusually severe (intermediate) SD phenotype. Ten different mutations, including deletions, insertions, and missense and nonsense mutations, were identified in patients with the most severe ISSD phenotype.
Kleta et al. (2003) presented 2 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 included a 148-bp deletion of exon 9 (604322.0003) and a 15-bp deletion in exon 6 (604322.0007). 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. She was compound heterozygous for the SLC17A5 mutation commonly found in Finnish Salla disease patients, R39C (604322.0001), and a 15-bp deletion found in ISSD patients (604322.0007).
Biancheri et al. (2005) identified homozygosity for the K136E (604322.0009) mutation in an Italian patient with a severe form of Salla disease. The patient demonstrated psychomotor delay and nystagmus within the first months of life. He later showed severe hypomyelination on brain MRI and peripheral nerve involvement. Functional expression studies showed that the R39C and K136E mutant proteins retained some residual transport activity (Morin et al., 2004 and Wreden et al., 2005).
Strauss et al. (2005) evaluated an Old Order Mennonite child for gross motor delay, truncal ataxia, and slow linear growth. Recognition of a similarly affected second cousin prompted a genomewide homozygosity mapping study using high-density SNP arrays. SNP genotypes from 2 affected individuals and their parents were used to localize the disorder to a 14.9-Mb region on chromosome 6. This region contained 55 genes, including SLC17A5. Direct sequencing of SLC17A5 in the proband demonstrated homozygosity for the R39C sequence variant (604322.0001), which is the common cause of Salla disease in Finland. Three additional Mennonite individuals, ages 8 months to 50 years, were subsequently identified by direct molecular genetic testing. Strauss et al. (2005) emphasized that this small-scale mapping study was rapid, inexpensive, and analytically simple. Previous diagnostic evaluation, which included subspecialty consultations, neuroimaging, and metabolic testing, was long, costly, and did not yield a diagnosis. In families with shared genetic heritage, genomewide SNP arrays with relatively high marker density allow disease gene mapping studies to be incorporated into routine diagnostic evaluations. The oldest affected Mennonite patient was bedridden, spastic, and dystonic at age 51 years. Cognitive and motor functions had been relatively stable until the end of her fifth decade, after which she deteriorated rapidly over a period of 2 to 3 years. An affected younger sister died at age 46 years.
In a patient with Salla disease, Shinawi et al. (2025) identified compound heterozygous mutations in the SLC17A5 gene: a maternally inherited 1-bp deletion (c.533delC; 604322.0002) and a paternally inherited mosaic 184-bp deletion (604322.0010) encompassing the 3-prime end of exon 3, predicted to result in skipping of exon 3. The 184-bp deletion was not detected in the patient's blood by whole-exome or whole-genome sequencing, but was identified in RNAseq in the patient's fibroblasts. Long-read sequencing showed that the 184-bp deletion was present in 25% of paternally inherited alleles in fibroblasts and in 42% of paternally inherited alleles in muscle, but was not present in blood or buccal cells. Coexpression of wildtype SLC17A5 and SLC17A5 with skipping of exon 3 in HEK293 cells resulted in decreased expression of both SLC17A5 constructs, suggesting a dominant-negative effect of the exon 3-skipped constructs. Although both SLC17A5 mutations identified in this patient were predicted to cause a severe sialic storage disorder presentation, Shinawi et al. (2025) hypothesized that the mosaicism of the 184-bp deletion led to a milder Salla disease presentation.
Miyaji et al. (2008) presented evidence that the SLC17A5 gene acts as a vesicular aspartate transporter in the brain. Sialin was found to be present in rat hippocampal synaptic vesicles and synaptic-like microvesicles in the pineal gland. RNA interference of sialin expression decreased exocytosis of aspartate and glutamate in pinealocytes. In addition, proteoliposomes containing purified sialin actively accumulated aspartate and glutamate, consistent with a transporter function. The mouse sialin mutant R39C (604322.0001), which is found in patients with Salla disease (604369), was completely inactive in the energy-dependent uptake of aspartate and glutamic acid, but retained 34% of wildtype sialic acid/H+ cotransport activity. In contrast, mouse sialin mutant H183R (604322.0004), which is found in the severe infantile form of the human disorder ISSD (269920), showed active energy-dependent transport, but inactive H+/sialic acid cotransport. Miyaji et al. (2008) suggested that impaired aspartergic and glutamatergic neurotransmission could explain the severe CNS manifestations in patients with Salla disease who survive to adulthood. The results strongly suggested that sialin possesses dual physiologic functions as a vesicular transporter involved in neurotransmission and as a lysosomal transporter.
In 5 Finnish patients with classic Salla disease (SD; 604369), Verheijen et al. (1999) found an arg39-to-cys (R39C) missense mutation caused by a homozygous C-to-T transition at nucleotide 115 of the SLC17A5 gene. Aula et al. (2000) found that the homozygous R39C mutation was associated with a milder phenotype (Salla disease).
In monozygotic twin girls living in North America and apparently without Finnish ancestry, Martin et al. (2003) described Salla disease caused by the arg39-to-cys mutation. Their clinical histories were typical of the insidious onset and protracted course of Salla disease as described in Finnish patients with the R39C mutation. Their neonatal period was normal and hypotonia with mild delays and gross motor abilities became obvious only at the age of approximately 1 year. The hypotonia progressed to spasticity, as is often the case in Salla disease. Ocular nystagmus, often described as a common childhood feature of Salla disease, was absent in the twins. However, both girls demonstrated truncal ataxia, another prominent feature of Salla disease. Initial language delay and later language loss provided evidence of the progressive nature of the disorder. Neurologic progression often becomes evident as intelligence declines into adulthood to a point at which the IQ rarely exceeds 20. Although the facies of the twins appeared somewhat coarse, facial coarseness is not typically described in Salla disease until adulthood. Generalized skeletal abnormalities such as dysostosis multiplex, typical of many other lysosomal storage diseases including infantile free sialic acid storage disease, are not seen in Salla disease.
Kleta et al. (2003) found the R39C mutation typical of Salla disease in compound heterozygous state with the 15-bp deletion (604322.0007) typical of ISSD, in a North American patient with 'intermediate severe' Salla disease.
Functional expression studies showed that the R39C mutant protein retained some residual transport activity (Morin et al., 2004 and Wreden et al., 2005).
Infantile Sialic Acid Storage Disorder
In a French patient with infantile sialic acid storage disease (ISSD; 269920), Verheijen et al. (1999) found compound heterozygosity for 2 frameshift mutations in the SLC17A5 gene: 533delC, deleting 1 bp from codon 178 and producing a frameshift resulting in 33 new amino acids, then premature termination; and a 148-bp deletion (1112-1259; 604322.0003) producing a frameshift resulting in 27 new amino acids, then premature termination. This French patient, designated DR, was reported earlier by Mancini et al. (1991).
Salla Disease
In a patient with Salla disease (SD; 604369), Shinawi et al. (2025) identified compound heterozygous mutations in the SLC17A5 gene: a maternally inherited c.533delC mutation (c.533delC, NM_012434.5) in exon 4, resulting in a frameshift and premature termination (Thr178AsnfsTer34), and a paternally inherited mosaic 184-bp deletion (604322.0010) encompassing the 3-prime end of exon 3, predicted to result in skipping of exon 3. The mutations were identified by whole-exome sequencing and whole-genome sequencing in patient blood, RNAseq, and targeted capture with massively parallel sequencing in patient fibroblasts. Long-read sequencing showed that the 184-bp deletion was present in 25% of paternally inherited alleles in fibroblasts and in 42% of paternally inherited alleles in muscle, and was not present in blood or buccal cells. The 533delC mutation was present in the gnomAD database at an allele frequency of 93/1178886.
For discussion of the 148-bp deletion in the SLC17A5 gene (1112-1259) that was found in compound heterozygous state in a patient with infantile sialic acid storage disease (ISSD; 269920) by Verheijen et al. (1999), see 604322.0002.
Kleta et al. (2003) determined that the 148-bp deletion of exon 9, due to a G-to-A splice site mutation in position 1 of intron 9, was found in compound heterozygous state with a 15-bp deletion (del801-815) in exon 6 (604322.0007) in a North American patient with a severe clinical course typical of ISSD.
In a Yugoslavian patient with infantile sialic acid storage disease (ISSD; 269920), Verheijen et al. (1999) found compound heterozygosity for 2 missense mutations: his183 to arg (H183R) and pro334 to arg (P334R; 604322.0005), occurring in transmembrane domains 4 and 8, respectively. This patient had previously been reported by Tondeur et al. (1982).
For discussion of the pro334-to-arg (P334R) mutation in the SLC17A5 gene that was found in compound heterozygous state in a patient with infantile sialic acid storage disease (ISSD; 269920) by Verheijen et al. (1999), see 604322.0004.
In an Italian patient with infantile sialic acid storage disease (ISSD; 269920), Verheijen et al. (1999) found homozygosity for a 500-bp insertion after nucleotide 978 or 979. This patient had been reported by Berra et al. (1995).
In 2 French Canadian patients with infantile sialic acid storage disease (ISSD; 269920) reported by Lemyre et al. (1999) and in 1 English patient reported by Cameron et al. (1990), Verheijen et al. (1999) found a 15-bp deletion (nucleotides 802-816) resulting in the deletion of amino acids serine, serine, leucine, arginine, and asparagine in the cytosolic loop between transmembrane domains 6 and 7 of the SLC17A5 gene product.
Biancheri et al. (2002) described 2 Italian brothers with sialic acid storage disease that resembled Salla disease as observed in the Finnish population (SD; 604369) rather than ISSD. Both brothers showed moderate intellectual disability, spastic ataxic syndrome, hypomyelination and cerebellar ataxia on MRI, and lysosomal storage, all typical of Salla disease. In one of the alleles of the younger brother, Biancheri et al. (2002) found the same 15-bp deletion in exon 6 that had been found by Verheijen et al. (1999). No R39C mutation (604322.0001) was found. The older brother had died at the age of 20 years and DNA testing was not performed. The second mutation in the younger brother was presumed to lie in a noncoding area of the gene.
Kleta et al. (2003) detected this mutation in compound heterozygous state with an R39C substitution (604322.0001) in a North American patient with Salla disease. Kleta et al. (2003) described this mutation as 801 815del15 and stated that they believed the mutation detected by them was the same as that described by Verheijen et al. (1999).
In a patient with Salla disease (SD; 604369), Aula et al. (2000) identified compound heterozygosity for 2 mutations in the SLC17A5 gene: the common R39C (604322.0001) mutation and a 406A-G transition in exon 3, resulting in a lys136-to-glu (K136E) substitution in the cytosolic loop just before the third transmembrane domain.
Biancheri et al. (2005) found homozygosity for the K136E mutation in an Italian patient with a severe form of Salla disease. The patient demonstrated psychomotor delay and nystagmus within the first months of life. He later showed severe hypomyelination on brain MRI and peripheral nerve involvement. Functional expression studies showed that the K136E mutant protein retained some residual transport activity (Morin et al., 2004 and Wreden et al., 2005).
For discussion of the mosaic 184-bp deletion (chr6.73,641,566-73,641,749, GRCh38) encompassing the 3-prime end of exon 3 in the SLC17A5 gene, predicted to result in skipping of exon 3, that was identified in compound heterozygous state in a patient with Salla disease (SD; 604369) by Shinawi et al. (2025), see 604322.0002.
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