Mannose supplementation corrects GDP-mannose deficiency in cultured fibroblasts from some patients with congenital disorders of glycosylation (CDG)

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 Glc_0-3Man₉GlcNAc₂-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. [2000] 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.