Children with I-cell disease ("I" for "Inclusion bodies") synthesize perfectly good lysosomal enzymes, but secrete them outside the cell instead of sorting them to lysosomes. The mistake occurs because the cells lack GlcNAc-P-transferase, which is required to create the mannose-6-phosphate marker that is essential for proper delivery of hydrolytic enzymes into the lysosomes. In principle, I-cell disease could also be caused by deficiencies in two other proteins: the phosphoglycosidase that removes GlcNAc to expose mannose-6-phosphate, and the mannose 6-phosphate receptor itself.
These three potential kinds of I-cell disease could be distinguished by the ability of various culture supernatants to correct defects in mutant cells. Imagine that you have cell lines from three hypothetical I-cell patients (A, B, and C) that give the results below:
A. The supernatant from normal cells corrects the defects in B and C, but not the defect in A.
B. Hurler's disease is due to the failure to make one particular lysosomal enzyme. The supernatant from A corrects the defect in Hurler's cells, but supernatants from B and C do not.
C. If the supernatants from the mutant cells are first treated with the phosphoglycosidase that removes GlcNac, then the supernatants from A and C correct the defect in Hurler's cells, but the supernatant from B does not.
From these results, deduce the nature of the defect in each of the three mutant cell lines (A, B, C).