Infantile free sialic acid storage disease (ISSD) is a lysosomal storage disease caused when sialic acid, a kind of charged sugar, is unable to be transported out of the lysosomal membrane and instead, accumulates in the tissue and free sialic acid is excreted in the urine. Mutations in the SLC17A5 (solute carrier family 17 (anion/sugar transporter), member 50) gene cause all forms of sialic acid storage disease. The SLC17A5 gene is located on the long (q) arm of chromosome 6 between positions 14 and 15. This gene provides instructions for producing a protein called sialin that is located mainly on the membranes of lysosomes, compartments in the cell that digest and recycle materials.
ISSD is the most severe form of the sialic acid storage diseases. Babies with this condition have severe developmental delay, weak muscle tone (hypotonia), and failure to gain weight and grow at the expected rate (failure to thrive). They may have unusual facial features that are often described as "coarse," seizures, bone malformations, enlarged liver and spleen (hepatosplenomegaly), and an enlarged heart (cardiomegaly).
ISSD is a rare autosomal recessive disorder and affects 1 in 528,000 live births worldwide.
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