Evidence for non‐lysosomal storage of N‐acetylneuraminic acid (sialic acid) in sialuria fibroblasts

The results of the investigations reported here indicate that patients affected with the infantile sialic acid storage disorder (ISSD) and the original French sialuria patient suffer from distinct and fundamentally different disorders. While phase microscopy and immunochcmical studies demonstrated abnormal storage within intracellular inclusions in ISSD cells, no morphological evidence of storage within any subcellular organelles was found in the sialuria cells. Moreover, comparative subcellular fractionation studies on gradients of colloidal silica showed the excess sialic acid in ISSD cells to be located within the light (buoyant) lysosomal fraction, while the excessive, free sialic acid in the sialuria cells was found in the cytoplasmic fraction with no increased storage within the lysosomal tractions. It is concluded that the sialic acid abnormalities in ISSD and the French type of sialuria are the result of very different biochemical and genetically unrelated abnormalities.