Limitations of galactose therapy in phosphoglucomutase 1 deficiency

Abstract

Introduction: Phosphoglucomutase 1 deficiency (PGM1 deficiency) has been identified as both, glycogenosis and congenital disorder of glycosylation (CDG). The phenotype includes hepatopathy, myopathy, oropharyngeal malformations, heart disease and growth retardation. Oral galactose supplementation at a dosage of 1 g per kg body weight per day is regarded as the therapy of choice.

Results: We report on a patient with a novel disease causing mutation, who was treated for 1.5 years with oral galactose supplementation. Initially, elevated transaminases were reduced and protein glycosylation of serum transferrin improved rapidly. Long-term surveillance however indicated limitations of galactose supplementation at the standard dose: 1 g per kg body weight per day did not achieve permanent correction of protein glycosylation. Even increased doses of up to 2.5 g per kg body weight did not result in complete normalization. Furthermore, we described for the first time heart rhythm abnormalities, i.e. long QT Syndrome associated with a glycosylation disorder. Mass spectrometry of IGFBP3, which was assumed to play a major role in growth retardation associated with PGM1 deficiency, revealed no glycosylation abnormalities. Growth rate did not improve under galactose supplementation.

Conclusions: The results of our study indicate that the current standard dose of galactose might be too low to achieve normal glycosylation in all patients. In addition, growth retardation in PGM1 deficiency is complex and multifactorial. Furthermore, heart rhythm abnormalities must be considered when treating patients with PGM1 deficiency.

Keywords: Congenital disorder of glycosylation (CDG); Galactose; Glycogenosis; Glycoprotein profile; Growth retardation; Phosphoglucomutase 1 (PGM1).

Fig. 1

Schematic representation of phosphoglucomutase 1 (PGM1) key role in metabolism and protein interaction. In PGM1 deficiency, malfunction of the enzyme causes a wide range of clinical symptoms such as hepatopathy, uvula bifida, myopathy, cardiomyopathy and growth retardation . PGM1 serves as a binding factor to the ZASP in heart muscle cells. The deficient enzyme causes dilated cardiomyopathy . In PGM1 deficiency, glucose-6-phosphate (blue) cannot be generated from glycogen, which causes hypoglycemia and exercise- induced rhabdomyolysis . By galactose supplementation (yellow) the PGM1 pathway is bypassed and the formerly deficient protein glycosylation is compensated .

Fig. 2

Pedigree of the patient's family. Sanger sequencing performed on the patient and his consanguineous parents revealed a homozygous 1 base deletion in the phosphoglucomutase 1 gene (c.771delT) resulting in a frameshift and a premature stop codon further downstream.

Fig. 3

Changes in glutamate-oxaloacetate transaminase (GOT) levels and protein glycosylation under galactose and temporary uridine supplementation monitored by frequent analysis of aspartate transaminase levels, HPLC and IEF of serum transferrin (Tf). Elevated aspartate transaminase levels decreased with galactose supplementation and rose during interruption of therapy. Deficient glycosylation patterns showed quick improvement after galactose intake, but levels of tetrasialo-Tf (blue line) and hypoglycosylated transferrin isoforms (sum of glycoforms with 0–3 sialic acids, not shown) remained stagnating without reaching physiological rates (reference according to : asialo-Tf: below level of detection, monosial-Tf: below level of detection, disialo-Tf: 1.1 ± 0.72, trisialo-Tf: ± 2.60, tetrasialo-Tf: 89.84 ± 4.16, pentasialo-Tf: 6.4 ± 3.80). Dosage increase led to nearly normalized rates (tetrasialo-Tf range: light blue range), but failed to entirely correct the glycosylation deficiency. We could not show an additional effect of uridine. Interruption of galactose supplementation led to pretreatment glycosylation patterns. Partial recovery of transferrin glycosylation was achieved by a restart of galactose therapy at a daily intake of 1 g per kg body weight.

Fig. 4

Body height entered in growth curves for Turkish children aged 0 to 18 years according to reference values . At the age of 5 6/12 years, the patient was introduced to growth hormone therapy (orange) gradually increasing the dosage up to 1 mg per kg body weight. Growth rate was improved and catch-up growth was observed. Additional galactose intake did not show further improvement (green). The patient's calculated target height is indicated by a blue line.

Fig. 5

Panel A: Schematic representation of IGFBP-3 structure showing glycosylation sites and ALS binding domain. The glycoprotein IGFBP-3 is an important component of the ternary IGF transport complex (IGF-I/IGF-II, IGFBP-3, ALS) determining the bioavailability of the IGFs . The three N-glycosylation sites are located in the non-conserved central or linker domain, while the binding site for ALS is located in a highly conserved C-terminal domain . Hypoglycosylated IGFBP-3 shows increased cell binding activity compared to normally glycosylated protein . Since the same basic amino acids required for cell binding are also responsible for ALS binding, the competition between cell surface molecules and ALS for the IGFBP-3 binding domain presumably leads to dissociation of the ternary IGF-IGFBP-3 complex . Panel B: Mass spectra of IGFBP-3 in patient's serum before and under galactose supplementation. Mass spectrometry of IGFBP-3 did not detect unglycosylated peptides, neither before nor under galactose supplementation.