Tandem mass spectrometric assay of N-acetylglucosamine-6-sulfatase for multiplex analysis of mucopolysaccharidosis-IIID in dried blood spots

Previously we developed a multiplex liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay using dried blood spots for all subtypes of mucopolysaccharidoses (MPS) except MPS-IIID. Here we show that the MPS-IIID enzyme N-acetylglucosamine-6-sulfatase (GNS) is inhibited in dried blood spot (DBS) extracts, but activity can be recovered if the extract is diluted to reduce the concentrations of endogenous inhibitors. The new GNS assay displays acceptable characteristics including linearity in product formation with incubation time and amount of enzyme, low variability, and ability to distinguish MPS-IIID-affected from healthy patients using DBS. The assay can be added to the LC-MS/MS multiplex panel for all MPS subtypes requiring ∼2 min per newborn for the LC-MS/MS run.