Serum transferrin carrying the xeno‐tetrasaccharide NeuAc‐Gal‐GlcNAc₂ is a biomarker of ALG1‐CDG

Abstract

Summary ALG1-CDG (formerly CDG-Ik) is a subtype of congenital disorders of glycosylation (CDG) where the genetic defect disrupts the synthesis of the lipid-linked oligosaccharide precursor required for N-glycosylation. The initial step in the investigation for these disorders involves the demonstration of hypoglycosylated serum transferrin (TF). There are no specific biomarkers of this CDG subtype known to date. An LC/MS approach was used to analyze sera from patients with ALG1-CDG, PMM2-CDG, suspected CDG and individuals with alcohol abuse. We show mass spectrometric data combined with data from enzymatic digestions that suggest the presence of a tetrasaccharide consisting of two N-acetylglucosamines, one galactose and one sialic acid, appearing on serum TF, is a biomarker of this particular CDG subtype. This is the first time analysis of serum TF can suggest a specific CDG Type I subtype and we suggest this tetrasaccharide be used in the clinic to guide the ALG1-CDG diagnostic process.

ALG1-CDG: Clinical and Molecular Characterization of 39 Unreported Patients

“…These observations in a model system have become more important because ALG1‐CDG patients are known to accumulate a novel N‐linked xeno‐tetrasaccharide NeuAc‐Gal‐GlcNAc 2 [2 N‐acetylglucosamine (GlcNAc), 1 galactose (Gal), 1 sialic acid (NeuAc)] on both total serum glycoproteins and on ∼2%–8% of purified serum Tf [Bengtson et al., ; Zhang et al., ]. The presence of this tetrasaccharide shows that Dol‐PP‐GlcNAc 2 is flipped from the cytoplasmic face into the luminal face of the ER and subsequently transferred to proteins via the oligosaccharyltransferase complex.…”

“…Once transferred, modified proteins enter the Golgi where GlcNAc 2 serves as an acceptor substrate for β1,4 galactosyltransferase (MIM# 137060). Finally, Gal‐GlcNAc 2 is capped with a NeuAc added by 2,6 sialyltransferase to form the final product NeuAc2,6‐Gal β1,4GlcNAc β1,4GlcNAcβ [Bengtson et al., ; Zhang et al., ] (Supp. Fig.…”

“…S2). Importantly, since this tetrasaccharide does not normally occur in mammals and is primarily detected in ALG1‐CDG cases (trace amounts are detected in PMM2‐CDG and MPI‐CDG cases), it serves as a biomarker for either detecting or confirming a diagnosis of ALG1‐CDG [Bengtson et al., ; Zhang et al., ]. We tested a limited number of type I CDG samples and cannot exclude the possibility that other type I's could have this tetrasaccharide.…”

“…As previously described [Bengtson et al., ; Zhang et al., ]. Biological samples tested for the xeno‐tetrasaccharide included serum and/or plasma and primary fibroblast.…”

Recognizable phenotypes in CDG

“…A few of these disorders are treatable, therefore early diagnosis is very important (Dorre et al, 2015, Riley et al, 2017). Some subtypes are detectable by transferrin isoform screening or other forms of glycosylation analysis, helping the diagnostic crusade (Buczkowska et al, 2015, Bengston et al, 2016). But is there a specific clinical pattern in CDGs, which could point the clinician towards the correct diagnosis?…”