NOVEL GLYCOSYLATION INSIGHTS IN ALG12-CDG

Congenital disorders of glycosylation (CDG) are inherited metabolic diseases affecting the glycan biosynthesis of glycoconjugates [1]. They represent an expanding group of multisystemic diseases with variable phenotypes and prevalent neurological involvement. Among the genetic disorders associated to the protein N-glycosylation pathway, CDG type I (CDG-I) occur in the cytosol or in the endoplasmic reticulum (ER) affecting dolichol-linked oligosaccharide synthesis, whereas CDG type II (CDG-II) involve the N-linked oligosaccharide processing in the Golgi. ALG12-CDG is caused by deficiency of ALG12 ?1,6-mannosyltransferase that adds the eighth mannose residue on the dolichol-PP-oligosaccharide precursor in the ER, thus ensuring its correct shape and branching before the oligosaccharyl-transferase (OST) complex action. We investigated the glyco-phenotype of a novel ALG12-CDG patient with unreported variants, sharing most of the clinical signs of ALG12 deficiency, including recurrent infections and hypogammaglobulinemia. By a combination of consolidated and innovative mass spectrometry-based protocols [2-4], we analyzed serum native transferrin and profiled serum IgG and total serum N-glycans. MALDI MS analysis of intact transferrin showed underoccupancy of N-glycosylation sites, a typical feature of CDG-I. Moreover, N-glycome analysis either on total serum N-glycan pool and on IgG released N-glycans, revealed the abnormal occurrence of high-mannose and hybrid N-glycan species. These accumulating glycoforms, further analyzed by UHPLC-ESI MS, corresponded to unbranched structures with ?1,2-terminal mannose residues, previously identified in serum of patients with MAN1B1-CDG (CDG-II defect) [5]. Glycosylation analyses on the observed ALG12-CDG patient revealed a combination of CDG-I and CDG-II defects, these last associated with abnormal IgG N-glycan profile, consistent with the immunophenotype. As a whole, glycan characterization of target glycoproteins may endorse the molecular defect unraveling the complex clinical phenotype of CDG patients.