Electrospray Ionization Mass Spectrometry of Apolipoprotein CIII to Evaluate O-glycan Site Occupancy and Sialylation in Congenital Disorders of Glycosylation

抄録

Congenital disorders of glycosylation (CDG) are inherited metabolic diseases affecting synthesis of glycoconjugates. Defects of mucin-type O-glycosylation occur independently or in combination with N-glycosylation disorders, and profiling of the O-glycans of apolipoprotein CIII (apoCIII) by mass spectrometry (MS) supports diagnosis. The biomarkers are site occupancy and sialylation levels, which can be indicated by the content of non-glycosylated apoCIII0a isoform and by the ratio of monosialylated apoCIII1 to disialylated apoCIII2 isoforms, respectively. In this report, electrospray ionization (ESI) quadrupole MS of apoCIII was studied focusing on these biomarkers. Among the instrumental parameters, declustering potential (DP) induced fragmentation of the O-glycan moiety including the Thr-GalNAc linkage and thus increased apoCIII0a ions. This incurs the risk of making a false positive for reduced site occupancy. The ratio of apoCIII1 / apoCIII2 was substantially unchanged despite some dissociation of sialic acids. Therefore, proper DP settings are especially important when transferrin, which requires a higher DP, for N-glycosylation disorders is simultaneously analyzed with apoCIII in a single ESI MS measurement. Finally, a reference range of diagnostic biomarkers and mass spectra of apoCIII from patients with SLC35A1-, TRAPPC11-, and ATP6V0A2-CDG were presented.