Extensive Hypoglycosylation of Serum N‐Glycoproteins in <scp>SRD5A3</scp> Deficiency

Polyprenal reductase is an enzyme encoded by the SRD5A3 gene, which is involved in the synthesis of dolichol from polyprenol. Dolichol serves as a carrier for glycan precursors or monosaccharides in N-linked glycosylation. Pathogenic variants in SRD5A3 can result in a congenital disorder of glycosylation (CDG), SRD5A3-CDG, which is inherited in an autosomal recessive manner. Most plasma proteins are glycosylated and changes in the glycosylation of several glycoproteins are associated with pathological consequences. Despite the critical role of SRD5A3 in glycosylation, the impact of its deficiency on the glycosylation of serum proteins remains largely unexplored. In this study, we used tandem mass tag-based multiplexed quantitative approach to analyze serum N-glycoproteomics and proteomics in SRD5A3-CDG patients and controls. We quantified 2200 serum N-glycopeptides from 359 N-glycosites from 204 serum proteins. Extensive hypoglycosylation of serum proteins was observed in patients, with 245 of 291 altered glycopeptides decreased in SRD5A3-CDG. Altered glycopeptides included those derived from haptoglobin, plasma serine protease inhibitor, alpha-1-B glycoprotein, alpha-2-macroglobulin, and ceruloplasmin. Some of these proteins have previously been reported to be associated with liver dysfunction, anemia, and coagulopathy, which could underlie similar clinical features observed in SRD5A3-CDG patients. Overall, our study provides novel insights into alterations in the glycosylation status of specific serum proteins in SRD5A3-CDG. Some of these alterations could be further pursued to develop glycopeptide-based biomarkers as the current diagnosis of SRD5A3-CDG by screening assays remains challenging. In addition, knowledge of altered glycoproteins could enhance our understanding of the disease spectrum and potentially unveil additional therapeutic avenues.