A zebrafish model of congenital disorders of glycosylation with phosphomannose isomerase deficiency reveals an early opportunity for corrective mannose supplementation.

Individuals with congenital disorders of glycosylation (CDG) have recessive mutations in genes required for protein N-glycosylation, resulting in multi-systemic disease. Despite the well-characterized biochemical consequences in these individuals, the underlying cellular defects that contribute to CDG are not well understood. Synthesis of the lipid-linked oligosaccharide (LLO), which serves as the sugar donor for the N-glycosylation of secretory proteins, requires conversion of fructose-6-phosphate to mannose-6-phosphate via the phosphomannose isomerase (MPI) enzyme. Individuals who are deficient in MPI present with bleeding, diarrhea, edema, gastrointestinal bleeding and liver fibrosis. MPI-CDG patients can be treated with oral mannose supplements, which is converted to mannose-6-phosphate through a minor complementary metabolic pathway, restoring protein glycosylation and ameliorating most symptoms, although liver disease continues to progress. Because Mpi deletion in mice causes early embryonic lethality and thus is difficult to study, we used zebrafish to establish a model of MPI-CDG. We used a morpholino to block mpi mRNA translation and established a concentration that consistently yielded 13% residual Mpi enzyme activity at 4 days post-fertilization (dpf), which is within the range of MPI activity detected in fibroblasts from MPI-CDG patients. Fluorophore-assisted carbohydrate electrophoresis detected decreased LLO and N-glycans in mpi morphants. These deficiencies resulted in 50% embryonic lethality by 4 dpf. Multi-systemic abnormalities, including small eyes, dysmorphic jaws, pericardial edema, a small liver and curled tails, occurred in 82% of the surviving larvae. Importantly, these phenotypes could be rescued with mannose supplementation. Thus, parallel processes in fish and humans contribute to the phenotypes caused by Mpi depletion. Interestingly, mannose was only effective if provided prior to 24 hpf. These data provide insight into treatment efficacy and the broader molecular and developmental abnormalities that contribute to disorders associated with defective protein glycosylation.

Seizures and stupor during intravenous mannose therapy in a patient with CDG syndrome type 1b (MPI-CDG).

MPI-CDG (formally called CDG 1b), caused by phosphomannose isomerase (MPI) deficiency, leads to hypoglycaemia, protein losing enteropathy, hepatopathy, and thrombotic events, whereas neurologic development remains unaffected. Dietary supplementation of mannose can reverse clinical symptoms by entering the N-glycosylation pathway downstream of MPI. When oral intake of mannose in patients with MPI-CDG is not possible, e.g. due to surgery, mannose has to be given intravenously. We report a patient with MPI-CDG on intravenous mannose therapy that showed severe depression of consciousness and seizures without apparent cause. EEG and cranial MRI findings were compatible with metabolic coma whereas extended laboratory examinations including repeated blood glucose measurements were normal. Importantly, an intravenous bolus of glucose immediately led to clinical recovery and EEG improvement. Mannose did not interfere with glucose measurement in our assay. We suggest that in patients with MPI-CDG, intravenous mannose infusion can lead to intracellular ATP deprivation due to several mechanisms: (1) in MPI deficiency, mannose 6-P cannot be isomerised to fructose 6-P and therefore is unavailable for glycolysis; (2) animal data has shown that accumulating intracellular mannose 6-P inhibits glycolysis; and (3) elevated intracellular mannose 6-P may induce an ATP wasting cycle of dephosphorylation and rephosphorylation ("honey bee effect"). The mannose-induced metabolic inhibition may be overcome by high-dose glucose treatment. We caution that, in patients with MPI-CDG, life-threatening central nervous system disturbances may occur with intravenous mannose treatment. These may be due to intracellular energy failure. Clinical symptoms of energy deficiency should be treated early and aggressively with intravenous glucose regardless of blood glucose levels.