Glycosaminoglycan fragments as a measure of disease burden in the mucopolysaccharidosis type I mouse

Glycosaminoglycan (GAG) catabolism involves endo-hydrolysis of polysaccharides followed by the sequential removal of the non-reducing end residue from the resulting oligosaccharides by exo-enzymes. In the inherited metabolic disorder, mucopolysaccharidosis type I (MPS I), a deficiency in the exo-enzyme, alpha-L-iduronidase, prevents removal of alpha-L-iduronic acid residues from the non-reducing end of the GAGs, heparan sulphate (HS) and dermatan sulphate (DS). The excretion of partially degraded HS and DS in urine of MPS I patients has long been recognized, but the question of whether they do indeed reflect GAG load in a particular tissue has not been addressed - an important issue in the context of biomarkers for assessment of disease burden in MPS I. Therefore, we measured specific low molecular weight HS and DS oligosaccharides with terminal alpha-L-iduronic acid residues, in the brain, liver, kidney, serum and urine, and correlated these findings with total GAG in the MPS I mouse model. Six oligosaccharides were identified in the urine, ranging from di- to pentasaccharides. Of these, five were observed in the kidney, four in the liver and brain, with the three most abundant in urine also seen in serum. These oligosaccharides accounted for just 0.1-2% of total GAG, with a disaccharide showing the best correlation with total GAG. The oligosaccharides and total GAG were most abundant in the liver, with the least observed in the brain. The concentration of oligosaccharides as a percentage of total GAG in urine was similar to that observed in the kidney, and both revealed a similar ratio of HS:DS, suggesting that the oligosaccharide storage pattern in urine is a reflection of that in the kidney. Serum, liver and brain had a similar ratio of HS:DS, which was lower to that seen in the urine and kidney. The distribution of oligosaccharides when ranked from most to least abundant, was also the same between serum, liver and brain suggesting that serum more closely reflects the oligosaccharides of the brain and liver and may therefore be a more informative measurement of disease burden than urine. The accumulation of HS and DS oligosaccharides was observed in the brain as early as one month of age, with the disaccharide showing a continuous increase with age. This demonstrates the progressive nature of the disease and as such this disaccharide could prove to be a useful biomarker to measure disease burden in MPS I.