Disorders of Protein O-Glycosylation

What Is Glycosylation?

Glycosylation, the addition of Carbohydrate chains to proteins, is the most common post-translational and cotranslational modification. It is initiated by the cytosolic synthesis of activated sugars (except CMP-N-acetylneuraminic acid) that are subsequently transported to the endoplasmic reticulum (ER) and Golgi apparatus, where glycans are assembled and modified on proteins. Glycosylation affects many aspects of protein function, including protein folding, enzyme activity, and cell-to-cell and cell-to-extracellular matrix (ECM) interactions. Therefore, it is not surprising that glycosylation disorders present a broad range of clinical phenotypes.

There are two main types of Protein Glycosylation: N-glycosylation and O-glycosylation. Over 100 human congenital disorders of glycosylation (CDG) have been described. Of these, about 30% reside in the O-glycosylation pathway. O-glycosylation requires a variety of different enzymes to activate, modify and attach sugars to proteins and can be classified by the first sugar that is attached to the amino acid in the protein. In humans, seven different types of O-glycans are known that are classified based on the first sugar residue attached to amino acid residues.

Characteristics of O-Glycosylation Disorders

O-glycosylation disorders are characterized by high phenotypic variability, reflecting the large diversity of O-glycan structures. O-glycans (O-linked saccharides) in O-glycoproteins are covalently linked to the hydroxyl group of serine or threonine (or hydroxylysine and hydroxyproline) of the protein. In contrast to N-glycans, O-glycans are built on different protein glycan linkage and have extremely diverse structures. O-glycosylation is a very complex process involving several enzymes that are encoded by multiple genes. Mutations in these genes are the main cause of enzyme deficiency and lead to defects in the biosynthesis of different types of O-glycans. These defects are responsible for several diseases named congenital disorders of O-glycosylation.

Types of O-Glycosylation Disorders

Disorders of O-glycosylation can be grouped based on the type of O-glycan that is affected. For example, if a deficient enzyme prevents the formation of a normal glycan structure that is attached to a protein by a mannose sugar, then it is categorized as a disorder of O-mannosylation. Each class of O-glycosylation relies on a variety of different types of enzymes, and disorders of O-glycosylation are caused by mutations in the genes encoding these enzymes. O-linked CDG that have been identified to date include disorders of O-mannosylation, disorders of O-xylosylation, disorders of O-fucosylation, disorders of O-GalNAcylation, disorders of O-GLCNAcylation, disorders of O-glucosylation.

Characterization of O-glycosylation disorders.Fig.1 Characterization of O-glycosylation disorders. (Van, et al., 2019)

Symptoms of O-Glycosylation Disorders

Like all CDG, disorders of O-glycosylation vary widely in their clinical presentation but typically manifest symptoms in multiple systems. Common presentations include neurological abnormalities, developmental/cognitive delay, failure to thrive, poor muscle tone (hypotonia), muscular dystrophy, and clotting abnormalities. Cardiac, ophthalmological, dermatological, and renal abnormalities may also be present.

Diagnosis of O-linked CDG

Diagnosis of O-linked CDG is achieved by conducting a series of tests that can help narrow down which part of the glycosylation pathway is affected and potentially identify individual CDG. O-linked glycosylation disorders cannot be detected with transferrin analysis, as the transferrin protein is only modified with N-glycans (glycans linked to proteins through a nitrogen atom on asparagine residues). However, patients may be assessed for abnormalities with O-glycosylated proteins, such as apolipoprotein C-III, or by analysis of total O-glycan content present in plasma or serum. O-glycan defects linked to alpha-dystroglycanopathies (disorders of O-mannosylation), can be identified by specialized staining techniques that use Antibodies to bind to glycosylated alpha-dystroglycan. Disorders affecting GAG biosynthesis (disorders of O-xylosylation) can also be identified using experiments that use antibodies to detect defects in O-linked proteoglycans. However, diagnosis of suspected individuals with O-linked glycosylation disorders typically relies on genetic testing strategies.

In contrast to N-glycosylation disorders, a generic biochemical screening test is lacking, which limits the identification of novel O-glycosylation disorders. The emergence of next-generation sequencing (NGS) and O-glycoproteomics technologies have changed this situation, resulting in significant progress to link disease phenotypes with underlying biochemical mechanisms.

Glycosylation is not only associated with pathology, but it also has an impact on the efficacy and safety of many biological drugs. CD BioGlyco has developed a perfect Drug Glycosylation Characterization solution to help customers explore glycosylation modification. We provide our customers with high-quality and fast services. If you are interested in our services, please feel free to contact us, we are looking forward to your inquiry.


  1. Van, T.W.; et al. O-glycosylation disorders pave the road for understanding the complex human O-glycosylation machinery. Curr Opin Struct Biol. 2019, 56: 107-118.
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