New Congenital Disorders of Glycosylation: Therapy and Models

点击翻译按钮获取中文摘要

PROJECT SUMMARY We discover and sometimes treat rare Congenital Disorders of Glycosylation (CDG) with simple sugars (monosaccharides). But why they work so efficiently is a puzzle. One clue comes from our recent discoveries showing that one activated sugar, GDP-fucose, does not reside in a single homogenous pool, but instead occurs in multiple, distinct, non-homogenous pools determined by whether fucose comes from a de novo pathway or salvage/diet. These pools selectively feed the synthesis of different glycans. Using stable isotope-labeled sugars, we want to understand the mechanisms that underlie the import of fucose into the cell and how it is utilized for selected glycans. The same technology can be applied to the study of galactose, which is currently being used to treat other CDGs. In galactosemia patients, galactose is toxic and must be eliminated from their diet. Yet compliant patients also have glycosylation deficiencies and some persistent symptoms. Small amounts of galactose improve glycosylation, but there is no data on how different glycosylation pathways use exogenous galactose. We hypothesize that galactose also resides in highly complex, non-homogenous pools that feed different glycosylation pathways. We discovered two new CDGs, each caused by recurrent de novo mutations in genes that have well- known, but very different, autosomal recessive presentations. We want to understand the underlying basis of the new disorders and propose models to study the novel phenotypes. Saul Wilson Syndrome (SWS) patients have a progeroid dwarfism and normal intelligence due to a recurrent de novo mutation in the CDG gene, COG4. Patients with bi-allelic COG4 mutations have a very severe, often lethal phenotype with altered N-glycosylation. SWS does not alter N-glycosylation, but selectively effects chondroitin sulfate proteoglycan modification and collagen accumulation/secretion. This suggests defects in the extracellular matrix at the growth plate. We propose series of cellular models to assess the molecular details of COG4 dysregulation in SWS that can explain the phenotypes. We discovered patients with a unique coagulopathy and abnormal serum N-glycosylation. These patients have a single de novo mutation in SLC37A4. The gene encodes the ER-localized glucose-6-P transporter used for glucose homeostasis. When autosomal recessive, it causes glycogen storage disorder, GSD-Ib. In the de novo disorder, it alters only liver-derived protein N-glycosylation. Patient iPS cells show relocation of a portion of SLC37A4 from ER to the Golgi. We hypothesize that the mutation eliminates a critical ER-retention signal, mis- localizing half of the fully functional transporter. In the new location Glc-6-P accumulates in the Golgi lumen. If G6PC, its ER-localized glucose-6-phosphatase partner, comes along, Pi might also accumulate and alter glycosylation, by limiting solubility/availability of critical cations for multiple enzymes. We will create CRISPR- derived model systems to explore and validate this hypothesis. Potential small molecule therapies already exist.