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Congenital disorders of glycosylation

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Introduction• The importance of glycosylation in human cells• Clinical impact and complexity of congenital disorders of

glycosylation (CDG)• Diagnostic challenge of CDG• Highlight of Mayo experience• CDG testing strategies• Resources for physicians and families

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Glycosylation• Post-translational modification of proteins and lipids with sugars

(glycans)• Results in glycoconjugates• Complexity in assembly and processing of each glycoconjugate• Common eukaryotic types: proteoglycans, sphingolipids, GPI-

anchored, N- and O-linked• Categorized per the nature of the linkage to the protein or lipid• Mediate a wide variety of cellular metabolic processes• Increases the functional diversity of an organism

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N-Glycosylation Pathway

Reprinted from ScienceDirect, 34/6, Ng B and Freeze H, Perspectives on glycoslyation and its congenital disorders, 466-476., Copyright (2018), with permission from Elsevier.

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What are CDGs?• First described in 1980 by Jaak

Jaeken• Group of genetic disorders due to

defective glycosylation• Classified according to isoform

patterns in Tf analysis: CDG-type I, II, or mixed type

• Expanded criteria• Clinical features and severity are

variable• The majority are AR

Reprinted from ScienceDirect, 34/6, Ng B and Freeze H, Perspectives on glycoslyation and its congenital disorders, 466-476., Copyright (2018), with permission from Elsevier.

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Clinical Spectrum of CDG• Neurologic

• Dev delay• Hypotonia• Seizures

• Multi-organ failure• Skeletal and facial dysmorphism• Cutaneous

• Cutis laxa• Ichthyosis

• Non-specific lab findings• Abnormal LFTs, coagulation• Endocrine abnormalities • Protein losing enteropathy• Hypoglycemia • Elevated CK

Reprinted from ScienceDirect, 11/3, Aebi M and Hennet T, Trends in cell biology, 136-141, Copyright (2001), with permission from Elsevier.

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Biochemical Diagnostic Work-Up• Begin with serum transferrin and apolipoprotein-CIII isoform

simultaneous analysis by ESI-MS (MCL test: CDG)• Advantages:

• High throughput, fast, small sample volume• Detects abnormal N- and mucin-type core-1 O-glycosylation• Possible to detect abnormal glycans transferred onto Tf and Apo-CIII

• Disadvantages• Profile may be abnormal due to secondary causes• Rare cases have a normal Tf profile or can normalize over time

• Total serum N-glycan analysis (MCL test CDGN)• Free oligosaccharides analysis (MCL test OLIGU)

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Transferrin and Apolipoprotein-CIll Isoform Analysis by LC/MS

Calculated Tf isoforms Calculated Apo-CIII isoforms

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Transferrin Profile

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Apo-CIII Profile

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Total N-Glycan Analysis by MALDI-TOF

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Oligosaccharide Analysis by MALDI-TOF

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Mayo Clinic Experience• Two decades of performing CDG analysis• Average over 4,000/year since 2012• Different types of CDG patterns based on >300 confirmed

positive patient profiles• CLIR bioinformatic software to establish normal transferrin and

Apo-CIII reference ranges• Reference range established using >19,000 samples

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Cases by Type

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Normal Reference Range (n=19,000)*

*CLIR: Collaborative LaboratoryIntegrated Reports

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CDG Type I Profile: ALG1-CDG

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CDG Type II Profile: SLC35A2-CDGApolipoprotein-CIII Serum intact transferrinNon-informative Informative

Serum total N-glycans Informative

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CDG Mixed (I,II) Profile: PGM1-CDG

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CDG w/Abn’l Apo-CIII/Normal Tf Profile: GALNT2-CDG

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CDG w/Abn’l Total N-glycan/Normal Tf/Apo-CIII: SLC35C1-CDG

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CDG w/Abn’l Oligosaccharide/Normal Tf/Apo CIII/N-glycan: MOGS-CDG

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CDG Screening Strategy

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Coordinated Care for CDG

https://www.mayoclinic.org/departments-centers/clinical-genomics/overview/specialty-groups/cdg-clinic

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Frontiers in CDG Consortium

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References1. Apweiler R, Hermjakob H, Sharon N. On the frequency of protein glycosylation, as deduced from analysis of the SWISS-

PROT database Biochim Biophys Acta. 1999;1473(1):4–8.

2. Ng B, Freeze H. Perspectives on glycosylation and its congenital disorders. Trends in Genetics. 2018;34(6):466-476.

3. Aebi M, Hennet T. Congenital disorders of glycosylation: genetic model systems lead the way. Trends in Cell Biology. 2001;11(3):136-141.

4. Ajit Varki, Cummings RD, Esko JD, et al. Essentials of glycobiology.3rd edition. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 2015-2017. Accessed October 16, 2019. Available from: https://www.ncbi.nlm.nih.gov/books/NBK310274/.

5. ALG1-congenital disorder of glycosylation: MedlinePlus Genetics. medlineplus.gov. Published December 1, 2017. Accessed October 8, 2020. https://medlineplus.gov/genetics/condition/alg1-congenital-disorder-of-glycosylation/.

6. Peanne R, de Lonlay P, Foulquier F, et al. Congenital disorders of glycosylation (CDG): Quo vadis? Eur J Med Genet. 2018;61(11):643–663.

7. Clinical and basic investigations into congenital disorders of flycosylation. Mayo Clinic. https://www.mayo.edu/research/clinical-trials/cls-20467386?_ga=2.149775990.1126976891.1596547642-1452676687.1548426765