Tandem Mass Spectrometry for the Direct Assay of Lysosomal Enzymes in Dried Blood Spots: Application to Screening Newborns for Mucopolysaccharidosis I

BACKGROUND: Treatments now available for Mucopolysaccharidosis I require early detection for optimum therapy. Therefore, we have developed an assay appropriate for newborn screening of the activity of the relevant enzyme, α-l-iduronidase. METHODS: We synthesized a new α-l-iduronidase substrate that can be used to assay the enzyme by use of tandem mass spectrometry together with an internal standard. The assay uses a dried blood spot on a newborn screening card as the enzyme source. The assay protocol uses a simple liquid-liquid extraction step prior to mass spectrometry. We optimized enzyme reaction conditions and procedures for the assay, including the concentration of substrate, the reaction pH, the incubation time, and mass spectrometer operation. We also assessed inter- and intra-assay imprecision. RESULTS: When the assay was tested on dried blood spots, the α-l-iduronidase activity measured for 5 patients with Mucopolysaccharidosis I was well below the interval found for 10 randomly chosen newborns. Inter- and intra-assay imprecision were less than 10 %. The synthesis of the α-l-iduronidase substrate is practical on a scale needed to support newborn screening demands. CONCLUSIONS: This newly developed tandem mass spectrometry assay has the potential to be adopted for newborn screening of Mucopolysaccharidosis I. It presents advantages compared to the one previously published from this laboratory and has the potential to be performed in a multiplex fashion with several lysosomal enzymes relevant to treatable lysosomal storage diseases.