N-Glycan Testing for Congenital Disorders of Glycosylation
Congenital disorders of glycosylation (CDG) are a large group of rare inherited disorders that cause abnormal protein and lipid glycosylation, multisystemic involvement and a broad spectrum of clinical symptoms. CDG that affect N-linked protein glycosylation are often classified into two groups, based on the pathway of the genetic defect:
- Type I CDG include defects in glycan assembly and abnormal transfer of glycan from lipids to nascent proteins
- Type II CDG include defects in processing of the N-glycan to maturity and can also indirectly affect serine/threonine-linked O-GlcNAc glycosylation
Screening for N-linked and multiple glycosylation disorders may include:
- N-glycan profile analysis
- Carbohydrate deficient transferrin (CDT) profile
At Children’s Hospital of Philadelphia, we offer carbohydrate deficient transferrin and plasma N-Glycan testing for children with suspected CDG, or undiagnosed health problems linked to the way glycan (sugar) building blocks are attached to proteins.
What is N-glycan profile testing?
In N-glycan profile testing, the N-glycans are removed from proteins in a patient’s blood, then are specifically derivatized at the amine group, purified and measured by ESI-Time of-Flight mass spectrometry (ESI-TOF/MS). ESI-TOF/MS is an accurate, high resolution mass spectrometry that precisely identifies diagnostic N-glycans. An isotope labeled “glycopeptide” is added to each sample that allows quantification of N-glycans, thus improving the specificity and sensitivity of the laboratory diagnosis of CDG.
More than 100 glycans can be evaluated and monitored with N-glycan profile testing, providing a comprehensive view of N-glycan metabolism. This is essential in better understanding glycosylation processes in the endoplasmic reticulum (ER) and the Golgi apparatus, which are associated with N-linked glycosylation disorders (50-plus subtypes) and multiple glycosylation disorders (more than 40 subtypes).
At Children’s Hospital of Philadelphia, N-glycan profiling may be ordered in conjunction with a CDT test on the same specimen. The combination of these two blood tests offers the first-tier screening for more than 90 subtypes of CDG. These tests can be used as functional analyses for variants of unknown significance (VOUS) identified from other genetic tests, such as whole exome sequencing or gene panel sequencing tests. They may also be used to monitor the disease progression in a patient with a known diagnosis of CDG and offer additional details to help identify which treatments may be most effective.
Positive biochemical results should be confirmed with molecular analysis. In some cases, secondary glycosylation may be reported in patients with other metabolic diseases including hereditary fructose intolerance, galactosemia, certain gluconeogenesis disorders or some peroxisomal biogenesis disorders.