Congenital Disorders of Glycosylation (CDG) are human deficiencies in glycoprotein biosynthesis. Previous studies showed that 1 mM mannose corrects defective protein N-glycosylation in cultured fibroblasts from some CDG patients. We hypothesized that these CDG cells have limited GDP-mannose (GDP-Man) and that exogenous mannose increases the GDP-Man levels. Using a well established method to measure GDP-Man, we found that normal fibroblasts had an average of 23.5 pmol GDP-Man/106 cells, whereas phosphomannomutase (PMM)-deficient fibroblasts had only 2.3-2.7 pmol/106 cells. Adding 1 mM mannose to the culture medium increased the GDP-Man level in PMM-deficient cells to approximately 15.5 pmol/106 cells, but had no significant effect on GDP-Man levels in normal fibroblasts. Similarly, mannose supplementation increased GDP-Man from 4.6 pmol/106 cells to 24.6 pmol/106 cells in phosphomannose isomerase (PMI)-deficient fibroblasts. Based on the specific activity of the GDP-[3H]Man pool present in [2-3H]mannose labeled cells, mannose supplementation also partially corrected the impaired synthesis of mannosylphosphoryldolichol (Man-P-Dol) and Glc0-3Man9GlcNAc2-P-P-Dol. These results confirm directly that deficiencies in PMM and PMI result in lowered cellular GDP-Man levels that are corrected by the addition of mannose. In contrast to these results, GDP-Man levels in fibroblasts from a CDG-Ie patient, who is deficient in Man-P-Dol synthase, were normal and unaffected by mannose supplementation even though mannose addition was found to correct abnormal lipid intermediate synthesis in another study (Kim et al.  J. Clin. Invest., 105, 191-198). The mechanism by which mannose supplementation corrects abnormal protein N-glycosylation in Man-P-Dol synthase deficient cells is unknown, but this observation suggests that the regulation of Man-P-Dol synthesis and utilization may be more complex than is currently understood.
|Number of pages||7|
|State||Published - 2000|
- Congenital disorders of glycosylation (CDG)
- Phosphomannose isomerase
ASJC Scopus subject areas