# Congenital disorders of glycosylation, type II
Congenital disorders of glycosylation type II (CDG-II) are a group of rare inborn errors of metabolism in which the processing of protein-bound N-linked oligosaccharides is defective. Whereas type I CDG arise from faulty assembly and transfer of the lipid-linked oligosaccharide (LLO) precursor in the endoplasmic reticulum, type II defects impair the trimming and rebuilding of glycans already attached to proteins, mostly in the [Golgi apparatus](https://www.edgepedia.org/golgi-apparatus).[1](https://ncbi.nlm.nih.gov/books/NBK453041/)
| Key facts | Detail |
|---|---|
| Biochemical defect | Impaired trimming and processing of protein-bound N-glycans, mainly in the Golgi apparatus[1](https://ncbi.nlm.nih.gov/books/NBK453041/) |
| Diagnostic hallmark | Type II transferrin pattern: increased di- and asialotransferrin plus tri- and/or monosialotransferrin bands[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC6331365/) |
| Broader category | Also includes COG complex, activated sugar transport (SLC35C1), monosaccharide synthesis (FCSK), and V-ATPase (ATP6AP2) defects[3](https://link.springer.com/article/10.1186/s13023-023-02879-z) |
| Clinical contrast with type I | More severe psychomotor retardation, no peripheral neuropathy; cerebellar hypoplasia is characteristic of type I (e.g., PMM2-CDG) rather than a distinguishing feature of type II[4](https://www.kegg.jp/entry/H00119) • [7](https://pmc.ncbi.nlm.nih.gov/articles/PMC7827962/) |
| Treatment | No general treatment; dietary fucose has partially helped some SLC35C1-CDG patients[1](https://ncbi.nlm.nih.gov/books/NBK453041/) |
## Biochemical basis
All N-linked oligosaccharides begin as a common lipid-linked oligosaccharide assembled on a dolichol-phosphate anchor in the endoplasmic reticulum and transferred to asparagine residues in a growing protein. After transfer, the glycan is trimmed by specific glycosidases and rebuilt in the Golgi by glycosyltransferases that add GlcNAc, galactose, fucose, and sialic acid, producing high-mannose, hybrid, or complex-type structures.
**Type II defects** fall along this processing pathway. Removal of the glucose residues is catalyzed by glucosidase I; deficiency of the enzyme encoded by MOGS causes CDG-IIb, in which residual glucosidase I activity is under 3% of control values.[5](https://omim.org/entry/606056) In the Golgi, MGAT2 (GlcNAc transferase II, CDG-IIa), and B4GALT1 (β1-4 galactosyltransferase I, CDG-IId) act on branching and galactosylation; loss of B4GALT1 activity removes both galactose and sialic acid from transferrin glycans.[1](https://ncbi.nlm.nih.gov/books/NBK453041/)
Other type II defects affect the supply of activated sugars or Golgi organization rather than a single enzyme. SLC35C1 encodes the GDP-fucose transporter (CDG-IIc), and SLC35A1 the CMP-sialic acid transporter (CDG-IIf).[6](https://doi.org/10.1373/clinchem.2010.153635) The conserved oligomeric Golgi (COG) complex, an eight-subunit protein involved in Golgi trafficking, is another source of type II disease; COG7-CDG (CDG-IIe) was discovered first, and mutations have since been found in all COG subunits except COG3.[1](https://ncbi.nlm.nih.gov/books/NBK453041/) Defects in V-ATPase pumps such as ATP6AP2-CDG and in monosaccharide synthesis such as FCSK-CDG are also grouped with type II mechanisms.[3](https://link.springer.com/article/10.1186/s13023-023-02879-z)
## Clinical features
Compared with type I patients, type II patients show more severe psychomotor retardation and absence of peripheral neuropathy, but cerebellar hypoplasia is not a distinguishing feature, being a recognized finding in PMM2-CDG (type Ia).[4](https://www.kegg.jp/entry/H00119) • [7](https://pmc.ncbi.nlm.nih.gov/articles/PMC7827962/) [Individual](https://www.edgepedia.org/individual) subtypes add characteristic findings. Loss of fucosylation of cell-surface glycoproteins in SLC35C1-CDG, historically known as leukocyte adhesion deficiency type II, impairs selectin binding and produces leukocytosis and increased sensitivity to infections; these patients have normal transferrin sialylation, and some have responded to dietary fucose therapy.[1](https://ncbi.nlm.nih.gov/books/NBK453041/) In CDG-IIb, patient cells show decreased entry of enveloped viruses including HIV and influenza A (H1N1) compared with controls.[5](https://omim.org/entry/606056)
## Diagnosis and classification
The standard screening test analyzes the glycosylation status of the plasma protein transferrin by isoelectric focusing, ESI-MS, or related techniques. A type II pattern shows increased di- and asialotransferrin bands together with tri- and/or monosialotransferrin bands, indicating defective N-glycan processing in the Golgi; a type I pattern instead reflects loss of whole glycans from defective precursor synthesis and transfer.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC6331365/)
Current classification schemes place CDG into four broader categories: [N-linked glycosylation](https://www.edgepedia.org/n-linked-glycosylation) defects, [O-linked glycosylation](https://www.edgepedia.org/o-linked-glycosylation) defects, combined N- and O-linked or multiple glycosylation defects, and lipid and GPI anchor biosynthesis defects.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC6331365/) Within this framework, type II corresponds to the processing side of the N-linked pathway.[1](https://ncbi.nlm.nih.gov/books/NBK453041/)
## Treatment
No treatment is available for most of these disorders. Mannose supplementation relieves most symptoms of MPI-CDG, a type I disorder, though hepatic fibrosis may persist, and fucose supplementation has had a partial effect on some SLC35C1-CDG patients.[1](https://ncbi.nlm.nih.gov/books/NBK453041/)
## References
1. [Chapter 45 Genetic Disorders of Glycosylation - NCBI Bookshelf](https://ncbi.nlm.nih.gov/books/NBK453041/)
2. [Congenital disorders of glycosylation (review) - PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC6331365/)
3. [Congenital disorders of glycosylation (CDG): state of the art in 2022 - Orphanet Journal of Rare Diseases](https://link.springer.com/article/10.1186/s13023-023-02879-z)
4. [KEGG DISEASE: Congenital disorders of glycosylation type II](https://www.kegg.jp/entry/H00119)
5. [OMIM #606056 - Congenital disorder of glycosylation, type IIb](https://omim.org/entry/606056)
6. [Plasma N-Glycan Profiling by Mass Spectrometry for Congenital Disorders of Glycosylation Type II, doi.org](https://doi.org/10.1373/clinchem.2010.153635)
7. [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC7827962/)
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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Metabolism and metabolic pathways › Inborn errors of metabolism (biochemical scope) › Carbohydrate and glycosylation pathway defects › Congenital disorders of glycosylation, type II*
*Initially written Sep 17, 2026 · Edited: Oct 15, 2026; Oct 25, 2026*
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