Background Pompe disease is an inherited glycogen storage disease caused by a deficiency of the lysosomal −glucosidase (GAA). The treatment by enzyme replacement therapy (ERT) is helpful but it is not as successful as anticipated. Aim To assess the actions of enzyme replacement therapy on stored glycogen in tissue. Methods Enzyme assay results were analyzed by high performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) and glycogen determination involved KOH/HCl extraction, amyloglucosidase degradation and enzymatic glucose determination. Results Glucose is released and glucan oligosaccharides masked by protein are released to the circulation. A biomarker, a glycosylated protein degradation product is present in the circulation of controls and patients on ERT but it is not present in Pompe mice that are not on ERT.
It is
also important to assess ERT in Pompe mice. One problem is the glycogen assay which in many cases has not included glycogen in the precipitate of a tissue homogenate and the masking has reduced the resultant quantities as well. It has been known for forty years that the only way to quantitatively extract glycogen from muscle tissue is by extraction by KOH in a boiling bath. We found the glycogen masking varies among different tissues and organs. With these improvements to glycogen determination, it is important to determine to what extent glycogen degradation is possible by ERT. The differences determined among different tissues may not necessarily be a function of enzyme uptake. To circumvent this potential problem of ERT we have incubated Tissue homogenates with rhGAA assess the maximal glycogen degradation by rhGAA which is a function of the glycogen masking and/or other factors which limit the enzyme access in tissues. Conclusion Identification of proteins masking glycogen prevents ERT from efficiently clearing glycogen. Understanding this component will help us better understand the ERT mechanism.
Funding in part by Sanofi Genzyme GZ-2017-11679, and NIH/NIAMS R21AR080972