Abnormal protein N-glycosylation has been associated with a wide variety of human diseases, such as cancer, diabetes and neurodegenerative disease. The underlying biological mechanisms that control the precise glycan structural abnormalities, in a protein- and tissue-specific manner, are still very poorly understood. Important novel insights in the biochemical mechanisms of protein N-glycosylation have originated from a group of monogenic defects, the Congenital Disorders of Glycosylation (CDG). After initial discovery of many defects in glycosyltransferases, a growing group of additional genetic causes is being identified. These factors cluster in two main groups: defects of Golgi homeostasis1-3 and defects in cellular sugar metabolism4,5.
In our group, we focus on the understanding of protein-specific glycosylation abnormalities via development of N-glycoproteomics, as well as on the tissue-specific mechanisms in sugar metabolism by targeted mass spectrometry. All known sugar metabolites can be sensitively detected in cells, tissues and organisms, for example confirming the presence of CMP-sialic acid in Drosophila. In addition, this resulted in the identification of CDP-ribitol as 10th human nucleotide sugar known to date6. Thus far, ribitol-phosphate had only been shown to be present in bacterial polysaccharides.
We are currently applying this technology to unravel surprising and novel mechanisms in the synthesis and use of human CMP-sialic acid. For example, SLC35A1, the Golgi transporter of CMP-sialic acid, was shown to be also required for O-mannosylation of alpha-Dystroglycan, in a process that is independent from sialic acid7,8. Clinical phenotypes of genetic defects in the sialic acid pathway are contradicting, with an adult myopathy due to GNE mutations, a neurological syndrome due to NANS mutations9, the next enzyme in this pathway, and a neuromuscular disease due to NPL mutations, required for catabolism of sialic acid10. Metabolic tracing with 13C-labeled sugars and chemically modified sugar derivatives, ManNPoc and SiaNPoc is carried out in knock-out cells of the sialic acid pathway, generated via CRISPR/Cas9 genome editing. These studies are generating the first novel insights of tissue-specific mechanisms in the sialic acid pathway, that might explain the contrasting tissue-specific clinical outcome in patients.
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- Wen XY et al. JCI insight 2018;3:122373