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Concept
I-cell disease
Résumé
Inclusion-cell (I-cell) disease, also referred to as mucolipidosis II (ML II), is part of the lysosomal storage disease family and results from a defective phosphotransferase (an enzyme of the Golgi apparatus). This enzyme transfers phosphate to mannose residues on specific proteins. Mannose-6-phosphate serves as a marker for proteins to be targeted to lysosomes within the cell. Without this marker, proteins are instead secreted outside the cell, which is the default pathway for proteins moving through the Golgi apparatus. Lysosomes cannot function without these proteins, which function as catabolic enzymes for the normal breakdown of substances (e.g. oligosaccharides, lipids, and glycosaminoglycans) in various tissues throughout the body (i.e. fibroblasts). As a result, a buildup of these substances occurs within lysosomes because they cannot be degraded, resulting in the characteristic I-cells, or "inclusion cells" seen microscopically. In addition, the defective lysosomal enzymes normally found only within lysosomes are instead found in high concentrations in the blood, but they remain inactive at blood pH (around 7.4) because they require the low lysosomal pH 5 to function.
Mucolipidosis II (ML II) is a particularly severe form of ML that has a significant resemblance to another mucopolysaccharidosis called Hurler syndrome. Generally, only
laboratory testing can distinguish the two as the presentation is so similar, with high plasma concentrations of lysosomal enzymes, often fatal in childhood. Typically, by the age of 6 months, failure to thrive and developmental delays are obvious signs of this disorder. Some physical signs, such as abnormal skeletal development, coarse facial features (e.g. bulging scaphocephalic head, flat nose), and restricted joint movement, may be present at birth. Children with ML II usually have enlargement of certain organs, such as the liver (hepatomegaly) or spleen (splenomegaly), and sometimes even the heart valves. Affected children often have stiff claw-shaped hands and fail to grow and develop in the first months of life.
Source officielle
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