Relative quantification of plasma N-glycans in type II congenital disorder of glycosylation patients by mass spectrometry

Background: Type II Congenital Disorders of Glycosylation (CDG-II) are a group of diseases with challenging diagnostics characterized by defects in the processing of glycans in the Golgi apparatus. Mass Spectrometry (MS) has been a valuable tool in the definition of CDG-II subtypes. While some CDG-II subtypes are associated with specific N-glycan structures, others only produce changes in relative levels, reinforcing the demand for quantification methods. Methods: Plasma samples from control individuals were pooled, derivatized with deuterated iodomethane (I-CD3), and used as internal standards for controls and patients whose glycans were derivatized with iodomethane (I-CH3), followed by MALDI MS, LC-MS and -MS/MS analyses. Results: Total N-glycans from fifteen CDG-II patients were evaluated, and 4 cases with molecular diagnosis were considered in detail: 2ATP6V0A2-CDG siblings, and 2 MAN1B1-CDG patients, one of them carrying a previously undescribed p.Gly536Val mutation. Conclusions: Our methodology offers a feasible alternative to the current methods for CDG-II diagnosis by MS, which quantify glycan structures as fractions of the total summed signal across a mass spectrum, a strategy that lowers the variability of minor components. Moreover, given its sensitivity for less concentrated yet biologically relevant structures, it might assist the uncovering of novel diagnostic glycans in other CDG-II subtypes.