Galactose supplementation in phosphoglucomutase-1 deficiency; review and outlook for a novel treatable CDG

We recently redefined phosphoglucomutase-1 deficiency not only as an enzyme defect, involved in normal glycogen metabolism, but also an inborn error of protein glycosylation. Phosphoglucomutase-1 is a key enzyme in glycolysis and glycogenesis by catalyzing in the bidirectional transfer of phosphate from position 1 to 6 on glucose. Glucose-1-P and UDP-glucose are closely linked to galactose metabolism. Normal PGM1 activity is important for effective glycolysis during fasting. Activated glucose and galactose are essential for normal protein glycosylation. The complex defect involving abnormal concentrations of activated sugars leads to hypoglycemia and two major phenotypic presentations, one with primary muscle involvement and the other with severe multisystem disease. The multisystem phenotype includes growth delay and malformations, like cleft palate or uvula, and liver, endocrine and heart function with possible cardiomyopathy. The patients have normal intelligence. Decreased transferrin galactosylation is a characteristic finding on mass spectrometry. Previous in vitro studies in patient fibroblasts showed an improvement of glycosylation on galactose supplements. Four patients with PGM1 deficiency have been trialed on d-galactose (compassionate use), and showed improvement of serum transferrin hypoglycosylation. There was a parallel improvement of liver function, endocrine abnormalities and a decrease in the frequency of hypoglycemic episodes. No side effects have been observed. Galactose supplementation didn't seem to resolve all clinical symptoms. Adding complex carbohydrates showed an additional clinical amelioration. Based on the available clinical data we suggest to consider the use of 0.5-1g/kg/day d-galactose and maximum 50 g/day oral galactose therapy in PGM1-CDG. The existing data on galactose therapy have to be viewed as preliminary observations. A prospective multicenter trial is ongoing to evaluate the efficacy and optimal d-galactose dose of galactose supplementation.