A congenital disorder of glycosylation (previously called carbohydrate-deficient glycoprotein syndrome) is one of several rare inborn errors of metabolism in which glycosylation of a variety of tissue proteins and/or lipids is deficient or defective. Congenital disorders of glycosylation are sometimes known as CDG syndromes. They often cause serious, sometimes fatal, malfunction of several different organ systems (especially the nervous system, muscles, and intestines) in affected infants. The most common sub-type is PMM2-CDG (formerly known as CDG-Ia) where the genetic defect leads to the loss of phosphomannomutase 2 (PMM2), the enzyme responsible for the conversion of mannose-6-phosphate into mannose-1-phosphate.<ref name="pmid9425221"
Presentation Clinical features depend on the molecular pathology of the particular CDG subtype. Common manifestations include ataxia; seizures; retinopathy; liver disease; coagulopathies; failure to thrive (FTT); dysmorphic features (e.g., inverted nipples and subcutaneous fat pads); pericardial effusion, skeletal abnormalities, and hypotonia. If an MRI is obtained, cerebellar hypoplasia is a common finding. Some CDG subtypes, like SSR4-CDG 1y, have been classified as connective tissue disorders.
Ocular abnormalities of PMM2-CDG include: myopia, infantile esotropia, delayed visual maturation, peripheral neuropathy (PN), strabismus, nystagmus, optic disc pallor, and reduced rod function on electroretinography. Three CDG subtypes PMM2-CDG, PMI-CDG, ALG6-CDG can cause congenital hyperinsulinism with hyperinsulinemic hypoglycemia in infancy. Because glycoproteins are involved in many central nervous system processes important during early development, intellectual disability and developmental delays are also common in CDG
N-Glycosylation and known defects A biologically very important group of carbohydrates is the asparagine (Asn)-linked, or N-linked, oligosaccharides. Their biosynthetic pathway is very complex and involves a hundred or more glycosyltransferases, glycosidases, transporters and synthases. This plethora allows for the formation of a multitude of different final oligosaccharide structures, involved in protein folding, intracellular transport/localization, protein activity, and degradation/half-life. A vast amount of carbohydrate binding molecules (lectins) depend on correct glycosylation for appropriate binding; the selectins, involved in leukocyte extravasation, is a prime example. Their binding depends on a correct fucosylation of cell surface glycoproteins. Lack thereof leads to leukocytosis and increased sensitivity to infections as seen in SLC35C1-CDG(CDG-IIc); caused by a GDP-fucose (Fuc) transporter deficiency. All N-linked oligosaccharides originate from a common lipid-linked oligosaccharide (LLO) precursor, synthesized in the ER on a dolichol-phosphate (Dol-P) anchor. The mature LLO is transferred co-translationally to consensus sequence Asn residues in the nascent protein, and is further modified by trimming and re-building in the Golgi.
Deficiencies in the genes involved in N-linked glycosylation constitute the molecular background of most CDGs. Type I defects involve the synthesis and transfer of the LLO Type II defects impair the modification process of protein-bound oligosaccharides.
Type I
Description Disorder Product The formation of the LLO is initiated by the synthesis of the polyisoprenyl dolichol from farnesyl, a precursor of cholesterol biosynthesis. This step involves at least three genes, DHDDS (encoding dehydrodolichyl diphosphate synthase that is a cis-prenyl transferase), DOLPP1 (a pyrophosphatase) and SRD5A3, encoding a reductase that completes the formation of dolichol. Recently, exome sequencing showed that mutations in DHDDS cause a disorder with a retinal phenotype (retinitis pigmentosa, a common finding in CDG patients. Further, the intermediary reductase in this process (encoded by SRD5A3), is deficient in SRD5A3-CDG (CDG-Iq). Dol is then activated to Dol-P via the action of Dol kinase in the ER membrane. This process is defective in DOLK-CDG (CDG-Im). Consecutive N-acetylglucosamine (GlcNAc)- and mannosyltransferases use the nucleotide sugar donors UDP-GlcNAc and GDP-mannose (Man) to form a pyrophosphate-linked seven sugar glycan structure (Man5GlcNAc2-PP-Dol) on the cytoplasmatic side of the ER. Some of these steps have been found deficient in patients. Deficiency in GlcNAc-1-P transferase causes DPAGT1-CDG (CDG-Ij) Loss of the first mannosyltransferase causes ALG1-CDG (CDG-Ik) Loss of the second mannosyltransferase (adds Man II and III) causes ALG2-CDG (CDG-Ii). Loss of the third mannosyltransferase (adds Man IV and V) causes ALG11-CDG (CDG-Ip) Mutations in the other genes involved in these steps (ALG13 and ALG14) are yet to be described. Man5GlcNAc2-PP-Dol The M5GlcNAc2-structure is then flipped to the ER lumen, via the action of a "flippase" This is deficient in RFT1-CDG (CDG-In). Finally, three mannosyltransferases and three glucosyltransferases complete the LLO structure Glc3Man9GlcNAc2-PP-Dol using Dol-P-Man and Dol-P-glucose (Glc) as donors. There are five known defects: mannosyltransferase VI deficiency causes ALG3-CDG (CDG-Id) mannosyltransferase VII/IX deficiency causes ALG9-CDG (CDG-IL) mannosyltransferase VIII deficiency causes ALG12-CDG (CDG-Ig) glucosyltransferase I deficiency causes ALG6-CDG (CDG-Ic) glucosyltransferase II deficiency causes ALG8-CDG (CDG-Ih). Glc3Man9GlcNAc2-PP-Dol A protein with hitherto unknown activity, MPDU-1, is required for the efficient presentation of Dol-P-Man and Dol-P-Glc. Its deficiency causes MPDU1-CDG (CDG-If). The synthesis of GDP-Man is crucial for proper N-glycosylation, as it serves as donor substrate for the formation of Dol-P-Man and the initial Man5GlcNAc2-P-Dol structure. GDP-Man synthesis is linked to glycolysis via the interconversion of fructose-6-P and Man-6-P, catalyzed by phosphomannose isomerase (PMI). This step is deficient in MPI-CDG (CDG-Ib), which is the only treatable CDG-I subtype. Man-1-P is then formed from Man-6-P, catalyzed by phosphomannomutase (PMM2), and Man-1-P serves as substrate in the GDP-Man synthesis. Mutations in PMM2 cause PMM2-CDG (CDG-Ia), the most common CDG subtype. Dol-P-Man is formed via the action of Dol-P-Man synthase, consisting of three subunits; DPM1, DPM2, and DPM3. Mutations in DPM1 causes DPM1-CDG (CDG-Ie). Mutations in DPM2 (DPM2-CDG) and DPM3 (DPM3-CDG (CDG-Io)) cause syndromes with a muscle phenotype resembling an a-dystroglycanopathy, possibly due to lack of Dol-P-Man required for O-mannosylation. The final Dol-PP-bound 14mer oligosaccharides (Glc3Man9GlcNAc2-PP-Dol) are transferred to consensus Asn residues in the acceptor proteins in the ER lumen, catalyzed by the oligosaccharyltransferase(OST). The OST is composed by several subunits, including DDOST, TUSC3, MAGT1, KRTCAP2 and STT3a and -3b. Three of these genes have hitherto been shown to be mutated in CDG patients, DDOST (DDOST-CDG (CDG-Ir)), TUSC3 (TUSC3-CDG) and MAGT1 (MAGT1-CDG).
Type II The mature LLO chain is next transferred to the growing protein chain, a process catalysed by the oligosaccharyl transferase (OST) complex. Once transferred to the protein chain, the oligosaccharide is trimmed by specific glycosidases. This process is vital since the lectin chaperones calnexin and calreticulin, involved in protein quality, bind to the Glc1Man9GlcNAc-structure and assure proper folding. Lack of the first glycosidase (GCS1) causes CDG-IIb. Removal of the Glc residues and the first Man residue occurs in the ER. The glycoprotein then travels to the Golgi, where a multitude of different structures with different biological activities are formed. Mannosidase I creates a Man5GlcNAc2-structure on the protein, but note that this has a different structure than the one made on LLO. Next, a GlcNAc residue forms GlcNAc1Man5GlcNAc2, the substrate for a-mannosidase II (aManII). aManII then removes two Man residues, creating the substrate for GlcNAc transferase II, which adds a GlcNAc to the second Man branch. This structure serves as substrate for additional galactosylation, fucosylation and sialylation reactions. Additionally, substitution with more GlcNAc residues can yield tri- and tetra-antennary molecules.
Not all structures are fully modified, some remain as high-mannose structures, others as hybrids (one unmodified Man branch and one modified), but the majority become fully modified complex type oligosaccharides.
In addition to glycosidase I, mutations have been found: in MGAT2, in GlcNAc transferase II (CDG-IIa) in SLC35C1, the GDP-Fuc transporter (CDG-IIc) in B4GALT1, a galactosyltransferase (CDG-IId) in COG7, the conserved oligomeric Golgi complex-7 (CDG-IIe) in SLC35A1, the CMP-sialic acid (NeuAc) transporter (CDG-IIf)
However, since at least 1% of the genome is involved in glycosylation, it is likely that many more defects remain to be found.
Diagnosis
Classification Historically, CDGs are classified as Types I and II (CDG-I and CDG-II), depending on the nature and location of the biochemical defect in the metabolic pathway relative to the action of oligosaccharyltransferase. The most commonly used screening method for CDG, analysis of transferrin glycosylation status by isoelectric focusing, ESI-MS, or other techniques, distinguish between these subtypes in so called Type I and Type II patterns.
Currently, over 130 subtypes of CDG have been described.
Since 2009, most researchers use a different nomenclature based on the gene defect (e.g. CDG-Ia = PMM2-CDG, CDG-Ib = PMI-CDG, CDG-Ic = ALG6-CDG etc.).Jaeken, J., Hennet, T., Matthijs, G., and Freeze, H.H. (2009) CDG nomenclature: time for a change! Biochim Biophys Acta. 1792, 825-6. The reason for the new nomenclature was the fact that proteins not directly involved in glycan synthesis (such as members of the COG-familyWu, X., Steet, R.A., Bohorov, O., Bakker, J., Newell, J., Krieger, M., Spaapen, L., Kornfeld, S., and Freeze, H.H. Mutation of the COG complex subunit gene COG7 causes a lethal congenital disorder. (2004) Nat. Med. 10, 518-23. and vesicular H+-ATPase) were found to be causing the glycosylation defect in some CDG patients.
Also, defects disturbing other glycosylation pathways than the N-linked one are included in this classification. Examples are the α-dystroglycanopathies (e.g. POMT1/POMT2-CDG (Walker–Warburg syndrome and uscle–eye–brain syndrome)) with deficiencies in O-mannosylation of proteins; synthesis defects (EXT1/EXT2-CDG (hereditary multiple exostoses) and B4GALT7-CDG (Ehlers–Danlos syndrome, progeroid variant)); synthesis (B3GALTL-CDG (Peter's plus syndrome) and LFNG-CDG (spondylocostal dysostosis III)).
Type I Type I disorders involve disrupted synthesis of the lipid-linked oligosaccharide precursor (LLO) or its transfer to the protein.
Types include:
Type OMIM Gene Locus Ia (PMM2-CDG) PMM2 16p13.3-p13.2 Ib (MPI-CDG) MPI 15q22-qter Ic (ALG6-CDG) ALG6 1p22.3 Id (ALG3-CDG) ALG3 3q27 Ie (DPM1-CDG) DPM1 20q13.13 If (MPDU1-CDG) MPDU1 17p13.1-p12 Ig (ALG12-CDG) ALG12 22q13.33 Ih (ALG8-CDG) ALG8 11pter-p15.5 Ii (ALG2-CDG) ALG2 9q22 Ij (DPAGT1-CDG) DPAGT1 11q23.3 Ik (ALG1-CDG) ALG1 16p13.3 1L (ALG9-CDG) ALG9 11q23 Im (DOLK-CDG) DOLK 9q34.11 In (RFT1-CDG) RFT1 3p21.1 Io (DPM3-CDG) DPM3 1q12-q21 Ip (ALG11-CDG) ALG11 13q14.3 Iq (SRD5A3-CDG) SRD5A3 4q12 Ir (DDOST-CDG) DDOST 1p36.12 It (PGM1-CDG) (formerly GSD-XIV)
Phosphoglucomutase deficiency614921PGM11p31.3 DPM2-CDG DPM2 9q34.13 TUSC3-CDG TUSC3 8p22 MAGT1-CDG MAGT1 X21.1 DHDDS-CDG DHDDS 1p36.11 I/IIx n/a n/a
Type II Type II disorders involve malfunctioning trimming/processing of the protein-bound oligosaccharide chain.
Types include: