A vesicular transporter that mediates aspartate and glutamate neurotransmission.

Aspartate, an excitatory amino acid, is known to be stored in synaptic vesicles and exocytosed from some neurons to perform aspartergic neurotransmission. Through in vitro reconstitution, we found that sialin, a lysosomal sialic acid exporter, is responsible for the vesicular storage of aspartate in hippocampal neurons and pinealocytes. Mutations found in Salla disease cause decreased aspartate transport activity without affecting sialic acid transport. Thus, sialin is a multifunctional transporter. It is possible that people with Salla disease lose the ability of aspartergic neurotransmission, and this could explain why Salla disease involves severe neurological defects.

Sialin, an anion transporter defective in sialic acid storage diseases, shows highly variable expression in adult mouse brain, and is developmentally regulated.

Sialin is a lysosomal membrane protein encoded by the SLC17A5 gene, which is mutated in patients with sialic acid storage diseases (SASD). To further understand the role of sialin in normal CNS development and in the progressive neuronal atrophy and dysmyelination seen in SASD, we investigated its normal cellular distribution in adult and developing mice. Overall, sialin showed granular immunoreactivity, consistent with a vesicular protein. Adult mice showed widespread sialin expression, including in the brain, heart, lung, and liver. High-level immunoreactivity was seen in the neuropil of the hippocampus, striatum, and cerebral cortex, as well as in the perikarya of cerebellar Purkinje cells, globus pallidus, and certain thalamic and brainstem nuclei. In mouse embryos, the highest levels of expression were observed in the nervous system. We discuss the possible role of sialin in normal development and in SASD pathogenesis, as a framework for further investigation of its function in these contexts.

Neurocognitive profiles in Salla disease.

Salla disease, a free sialic acid storage disorder, is one of the 36 currently known disorders in Finland that form the Finnish disease heritage. Salla disease leads to learning disability* with a wide clinical variation. Two main categories of the disease have been classified: a conventional subtype and a severe subtype with more severe defects. We present detailed neurocognitive profiles of 41 Finnish patients with Salla disease (19 females, 22 males; age range 11mo to 63y, median 19y). The neurocognitive development of patients with Salla disease was assessed by psychological and neuropsychological testing. All patients were also examined by a paediatric neurologist and a speech therapist. The characteristic cognitive profile consisted of a lower non-verbal performance (mean developmental age 13mo) compared with linguistic skills (mean developmental age 17mo). In particular, spatial and visual-constructive impairments were typical of these patients. Tactile and visual discrimination of forms was poor. Tasks demanding hand-eye coordination, maintenance of visual attention, and those requiring short-term visual memory and executive skills were performed better. Receptive language skills were notably better compared with expressive speech. The patients' interactive and non-verbal communication skills were quite strong. Another typical pattern with Salla disease was severe motor disability. After the second decade of life, the decline in these skills was more pronounced than patients' cognitive deterioration. Our results indicate that even though there is a considerable variation in the clinical findings of patients with Salla disease, the characteristic neurocognitive profile of the disease can be outlined.

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.

A novel mutation in the SLC17A5 gene causing both severe and mild phenotypes of free sialic acid storage disease in one inbred Bedouin kindred.

Four members of an extended consanguineous Bedouin family presented with different phenotypic variants of an autosomal recessive lysosomal free sialic acid storage disease. One affected individual had congenital ascites followed by rapid clinical deterioration and death, a presentation concordant with the clinical course of infantile free sialic acid storage disorder. His three first cousins had a more slowly progressive neurodegenerative disease, in line with the clinical phenotype of the milder form (Salla type) of this lysosomal disorder. Diagnosis of free sialic acid storage disease was based on clinical findings, histology, and biochemical assays of sialic acid. Molecular studies showed that all four affected individuals were homozygous for the same novel 983G > A mutation in exon 8 of the SLC17A5 gene, replacing glycine with glutamic acid at position 328 of the sialin protein. This family demonstrates the significant phenotypic variability of the disease in affected members of a single inbred kindred with precisely the same mutation, suggesting a role for modifier genes or environmental factors. It also highlights the need to consider this rare disorder in the differential diagnosis of congenital ascites and of unexplained psychomotor retardation, ataxia, and hypomyelination in infancy.

Sialin expression in the CNS implicates extralysosomal function in neurons.

SLC17A5 encodes a lysosomal membrane protein, sialin, which transports sialic acid from lysosomes. Mutations in sialin result in neurodegenerative sialic acid storage disorders, Salla disease (SD) and infantile sialic acid storage disease (ISSD). Here we analyzed sialin in mouse central nervous system (CNS) and primary cortical and hippocampal neurons and glia. In the CNS, sialin was predominantly expressed in neurons, especially in the proliferative zone of the prospective neocortex and the hippocampus in developing brain. In nonneuronal cells and primary glial cell cultures, mouse sialin was localized into lysosomes but interestingly, in primary neuronal cultures sialin was not targeted into lysosomes but rather revealed a punctate staining along the neuronal processes and was also seen in the plasma membrane. These data demonstrate a nonlysosomal localization of sialin in neurons and would imply a role for sialin in the secretory processes of neuronal cells.

Phenotypic spectrum of Salla disease, a free sialic acid storage disorder.

Salla disease (MIM 269920) represents the mildest phenotype among recessively inherited lysosomal-free sialic acid storage disorders. Although the vast majority of Salla disease patients in Finland share the same founder mutation, R39C in the SLC17A5 gene, there still is a wide clinical variation among mentally retarded, ataxic patients. We evaluated neurologic and neurocognitive findings of Salla disease in a cross-sectional study of 41 Finnish patients who were 11 months to 63 years of age (median = 19.5 years). The phenotype of Salla disease could be classified into two main categories. The majority of patients (90%) had so-called conventional phenotype, including a subgroup of seven patients with relatively mild symptoms. All but two patients with conventional phenotype were homozygous for the Finnish founder mutation. Four severely disabled, profoundly mentally retarded patients, 15-28 years of age, clearly could be clinically delineated as a separate group, likely reflecting the underlying compound heterozygous genotype. A typical developmental pattern could be outlined in the conventional type of the disease, emphasizing a strong motor handicap in Salla disease. The cognitive profile consisted of better verbal ability, especially speech comprehension, compared with nonverbal functioning in all patients. Our results indicate a partial genotype-phenotype correlation, although factors other than the molecular background are also involved in the phenotypic manifestation of Salla disease.

Clinical course and biochemistry of sialuria.

Sialuria is a rare inborn error of metabolism in which excessive free sialic acid (N-acetylneuraminic acid, NeuAc) is synthesized. A defect in the feedback inhibition of UDP-N-acetylglucosamine (UDP-GlcNAc) 2-epimerase by the end-product of the sialic acid synthetic pathway, CMP-NeuAc, is the mechanism underlying this overproduction. Recent evidence suggests that sialuria is an autosomal dominant disorder. Only five patients have been documented to have such an enzymatic defect. We report a longitudinal study of one of the original sialuria patients, to age 11 years. Although he has coarse features and massive hepatomegaly, he has shown normal growth and relatively normal development. Pulmonary function testing showed minimal small airway obstruction. At 11 years, he developed intermittent abdominal pain and transient transaminase elevation above his baseline. Sialuria should be considered in the differential diagnosis of a patient with a phenotype suggestive of a mucopolysaccharidosis or oligosaccharidosis in the absence of developmental regression or prominent dysostosis multiplex. We recommend close monitoring of liver and pulmonary function in sialuria patients.