Foundations · Year 1 · from Foundations
Case 3: I-Cell Disease (Protein Trafficking Defect)
Clinical Image
Source: Wikipedia - Inclusion-cell disease - CC BY-SA 3.0
Case Presentation
A 10-month-old boy is evaluated for coarse facial features, skeletal abnormalities, and developmental delay. He was noted at birth to have an umbilical hernia and has since developed hepatosplenomegaly, limited joint mobility, and progressive developmental regression. Physical examination reveals coarse facial features, gingival hyperplasia, and claw hand deformities. Radiographs show dysostosis multiplex. Fibroblast culture reveals numerous cytoplasmic inclusions (giving the disease its name, "I-cell disease"). Lysosomal enzyme activities are markedly reduced in fibroblasts but elevated in serum, indicating that lysosomal enzymes are being secreted rather than targeted to lysosomes. The diagnosis of mucolipidosis type II (I-cell disease) is confirmed by demonstrating deficient N-acetylglucosamine-1-phosphotransferase activity. This enzyme normally adds mannose-6-phosphate (M6P) tags to lysosomal enzymes in the Golgi apparatus. Without M6P tags, newly synthesized lysosomal enzymes cannot be recognized by M6P receptors and are secreted instead of being delivered to lysosomes. Lysosomes cannot degrade substrates, leading to accumulation of undegraded material. The prognosis is poor, with most patients dying in the first decade from cardiorespiratory complications.
Key Learning Points
- Lysosomal enzymes are tagged with mannose-6-phosphate (M6P) in the Golgi; M6P receptors recognize this tag and direct enzymes to lysosomes via the late endosomal pathway
- I-cell disease results from deficiency of the enzyme that adds the M6P tag (N-acetylglucosamine-1-phosphotransferase); without this signal, lysosomal enzymes are secreted rather than delivered to lysosomes
- This illustrates the critical importance of post-translational modifications and sorting signals in protein trafficking; the name "I-cell" (inclusion cell) refers to the accumulated undegraded material visible as inclusions