Mass spectrometry contribution to the diagnosis and characterization of Congenital Disorders of Glycosylation (CDG): our experience in the EUROGLYCANET European network

Congenital Disorders of Glycosylation (CDG) comprise an ever increasing group of inborn errors in the proteins glycosylation pathway, representing a paradigm as they disclose the direct connection between glycosylation changes and human diseases. Since the first description in 1980 [1], about 40 CDG forms were discovered, each with a variable clinical spectrum, from multisystem diseases to single organ involvements, causing high morbidity and even mortality. Sharing information and competences is an essential step in order to identify new CDG types and to promote awareness and early diagnosis. Mass spectrometry contributed significantly to this research field to identify possible underglycosylation profiles of intact individual glycoproteins and/or to map glycan population of the sample under study, typically serum or plasma. Here we report on serum glycosylation analyses by MALDI-MS in patients with CDG recruited in the context of Euroglycanet, an European network focused on diagnosis and understanding CDG [2]. During the last five years (2005-2010), almost 3000 samples of Italian patients with clinical suspicion of CDG were analyzed by serum Transferrin (Tf) isoelectric focusing (IEF) in a clinical setting at the University Paediatric Hospital in Catania. Patient with abnormal IEF pattern underwent MALDI MS glycosylation analyses at the ICTP-CNR in Catania, where a specific method for intact glycoprotein investigation and N-glycan analyses was set up [3]. Eighteen patients showed by MS the Tf abnormal glycosylation profile characteristic of type I CDG, encompassing defects in the assembly of dolichol-linked oligosaccharide in the Endoplasmic reticulum. Among these, PMM2-CDG (or CDGIa) was most frequently found (11 cases). Of particular importance, one patient affected with epileptic encephalopathy, had a newly identified CDG type due to DPM2 gene mutation [4]. Moreover, MS glycosylation analysis was performed on seventy-one serum samples with type II IEF pattern, collected from the Euroglycanet partners. Two of these samples showed recognizable glycan processing defects, respectively caused by gene mutations of subunit 5 and subunit 7 of the Conserved Oligomeric Golgi (COG) complex (COG5-CDG and COG7-CDG) [5-6]. Four more samples with combined Type I and Type II Tf MS profiles were also found; this significant information makes disease comprehension at molecular basis and diagnosis particularly challenging in these patients.