Mass spectrometry measurements of glycosaminoglycans to assess disease-modifying treatments for the mucopolysaccharidoses: Methodological considerations.
Abstract
The mucopolysaccharidoses are rare lysosomal disorders caused by defects in glycosaminoglycan (GAG) degradation, resulting in progressive multisystem disease. Biomarker measurement is central to assessing disease burden and therapeutic response, particularly as novel enzyme replacement, transplantation, gene and substrate-directed therapies are increasingly available. Liquid chromatography - tandem mass spectrometry (LC-MS/MS) has enabled specific measurement of GAG-derived biomarkers; however, the analytical measurands and their biological interpretation vary substantially between methodologies. This review examines four LC-MS/MS approaches currently used for GAG measurement: endogenous non-reducing end (NRE) oligosaccharides, enzymatically generated internal disaccharides, enzymatic NRE oligosaccharides, and methanolysis-derived disaccharides. Each method provides a surrogate or representative measure of GAG burden rather than a direct measurement of intact or "total" GAG. Differences in depolymerisation chemistry, analyte specificity, calibration, internal standards, chromatographic separation and susceptibility to structural heterogeneity influence quantitative results and their interpretation. In particular, measurements reported as "total" HS, DS, CS or KS may comprise different analytes across methods and laboratories and should not be assumed to be interchangeable. These considerations are especially important when GAG biomarkers are used as pharmacodynamic or surrogate endpoints in clinical trials. Appropriate validation, method-specific reference intervals and clinical decision limits, harmonized nomenclature, certified reference materials, stable isotope-labelled internal standards and interlaboratory quality assessment are required to improve comparability. Clear identification of the actual measurand is essential to ensure that observed biomarker changes are appropriately interpreted as evidence of therapeutic response.