free storage

Infantile free sialic acid storage disease (ISSD) is a lysosomal storage disease. ISSD occurs when sialic acid is unable to be transported out of the lysosomal membrane and instead accumulates in the tissue, causing free sialic acid to be 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).
Approximately ~250 individuals with FSASD have been reported in the literature, of which the majority (> 160 cases) are of Finnish or Swedish ancestry. Individuals with FSASD may go misdiagnosed or undiagnosed, making it difficult to determine the true frequency of the disease in the general population.

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