Rare diseases have a significant impact on Canadian families affecting 1 in 12 Canadians with great cost to our society. Ninety percent of rare diseases are life-limiting and have no treatment, and 50% affect children (http://igniteproject.ca/). Our group is actively engaged in work focusing on rare disease research, with specific attention to congenital disorders of glycosylation (CDG) which results from faulty N-linked glycosylation.
N-linked glycosylation is a critical process involved in the post-translational modification of newly synthesized proteins. This process is responsible for the formation of glycoproteins by the attachment of a sugar molecule (known as a glycan) to proteins. The resulting linkage influences protein structure and function. Glycosylated proteins play a vital role in essential cellular processes and regulate developmental pathways through embryogenesis and beyond.
The biosynthesis pathway of N-linked glycoproteins begins in the endoplasmic reticulum (ER) of the cell and ends in the Golgi apparatus. As shown below in Figure 1 from Hum Mutat 2009;30:1628-41, the pathway begins on the cytoplasmic side of the ER and is catalysed by glycosyltransferase enzymes that utilize nucleotide activated sugars (UDP-GlcNAc and UDP-Glc) to form Man5GlcNAc2-PP-Dol. The evolving glycan is then translocated to the luminal side of the ER membrane where it is further processed by luminally-oriented glucosyl and mannosyltransferases to form the fourteen sugar precursor, Glc3Man9GlcNAc2. Once the lipid-linked oligosaccharide (LLO) is assembled, it is then transferred to an asparagine side chain of a nascent growing protein, which bears a consensus sequence for glycosylation (Asn-X-Ser/Thr). Once the glycoproteins have folded properly, they move to the Golgi complex where they are subjected to further trimming and modification to form mature glycoproteins.
N-linked glycosylation is an essential process for maintaining normal biological activity of proteins. Impairment in this process may lead to formation of proteins with reduced or lost function. In general, disturbances of glycosylation may be either congenital or acquired. Disorders of glycosylation are rare though identified sub-types and cases of CDG are growing rapidly. CDG is a devastating disorder impacting numerous organ systems in developing children resulting in severe cognitive and physical developmental delay and early death in 20% of cases. (Acta Biochimica Polonica 2013;60:151-161.)
There is currently no treatment for CDG; however, with a growing understanding of the cellular pathways and the associated genetic mutations that result in CDG sub-types, the avenue for therapeutic research has now been opened widely. The approach to therapeutic research involves the development of a disease model with critical measurable biomarkers. Glycosylation studies are currently underway at the Dalhousie Medicine Research Laboratory in Saint John, New Brunswick. Current disease models being used and developed include the yeast Saccharmomyces cerevisiae, the Zebrafish Danio rerio, and human fibroblasts.
The Zebrafish have become an invaluable tool for studying developmental human disorders. This vertebrate organism develops from embryonic stages to a fully grown adult within days and its transparent nature allows for clear observation of cellular functions and organogenesis. Through CRISPR/Cas9 technology, the zebrafish genetic structure may be manipulated and organisms with CDG constructed. Subsequently, these constructs may be utilized as disease models and potential therapeutic strategies studied. With assistance from Prof Aiebi's lab in Zurich, our group has successfully completed preliminary work utilizing transformed yeast strains, which serve as disease models for CDG. With this background work we are now prepared to develop a vertebrate disease model.