Children with I-cell Disease 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 cause by deficiencies in two other proteins; the phosphoglycosidase that removes GlcNac to expose mannose-6-phosphate, and the mannose-6-phosphate receptor itself.
These 3 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 3 hypothetical I-cell patients (A, B and C) that give the results below:
1. The supernatant from normal cells corrects the defects in B and C, but not the defect in A.
2. 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.
3. 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 3 mutant cell lines (A,B, and C). State your reasoning.