Glycomics in rare diseases: from diagnosis tomechanism.

The National Institutes of Health (NIH) Undiagnosed Diseases Program (UDP) studies rare genetic disorders not only to achieve diagnoses, but to understand human biology. To ascertain the contribution of protein glycosylation to rare diseases, the NIH UDP used mass spectrometry to agnostically identify abnormalities of N-linked and O-linked glycans in plasma and free oligosaccharides in the urine of 207 patients. 60% of UDP patients had a glycome profile that deviated from control values in at least 1 fluid. Additional evaluation of the fibroblast glycome in 66 patients with abnormalities in plasma and/or urine revealed a consistent glycome phenotype in 83% of these cases. Many of these patients may have secondary glycosylation defects, since it is unlikely that they all have congenital disorders of glycosylation (CDGs). In fact, whole exome sequencing revealed only a few patients with CDGs, along with several others having disorders indirectly altering glycosylation. In summary, we describe a biochemical phenotyping screen to identify defects in protein glycosylation that can elucidate mechanisms of disease among NIH UDP patients.

Disruption of Golgi morphology and altered protein glycosylation in PLA2G6-associated neurodegeneration.

BACKGROUND: Mutations in PLA2G6, which encodes the calcium-independent phospholipase A2 group VI, cause neurodegeneration and diffuse cortical Lewy body formation by a yet undefined mechanism. We assessed whether altered protein glycosylation due to abnormal Golgi morphology might be a factor in the pathology of this disease. METHODS: Three patients presented with PLA2G6-associated neurodegeneration (PLAN); two had infantile neuroaxonal dystrophy (INAD) and one had adult-onset dystonia-parkinsonism. We analysed protein N-linked and O-linked glycosylation in cerebrospinal fluid, plasma, urine, and cultured skin fibroblasts using high performance liquid chromatography (HPLC) and matrix-assisted laser desorption ionization--time of flight/mass spectrometry (MALDI-TOF/MS). We also assessed sialylation and Golgi morphology in cultured fibroblasts by immunofluorescence and performed rescue experiments using a lentiviral vector. RESULTS: The patients with INAD had PLA2G6 mutations NM_003560.2: c.[950G>T];[426-1077dup] and c.[1799G>A];[2221C>T] and the patient with dystonia-parkinsonism had PLA2G6 mutations NM_003560.2: c.[609G>A];[2222G>A]. All three patients had altered Golgi morphology and abnormalities of protein O-linked glycosylation and sialylation in cultured fibroblasts that were rescued by lentiviral overexpression of wild type PLA2G6. CONCLUSIONS: Our findings add altered Golgi morphology, O-linked glycosylation and sialylation defects to the phenotypical spectrum of PLAN; these pathways are essential for correct processing and distribution of proteins. Lewy body and Tau pathology, two neuropathological features of PLAN, could emerge from these defects. Therefore, Golgi morphology, O-linked glycosylation and sialylation may play a role in the pathogenesis of PLAN and perhaps other neurodegenerative disorders.

Glycosylation Analysis for Congenital Disorders of Glycosylation.

Congenital disorders of glycosylation (CDG) are a group of diseases with highly variable phenotypes and inconsistent clinical features. Since the first description of a CDG in 1980, approximately 100 disorders have been identified. Most of these are defects in protein glycosylation, although an increasing number are defects of glycolipid or proteoglycan biosynthesis. A group of biochemical markers has been used to characterize protein glycosylation abnormalities in CDG. This unit describes three protocols that can be used to measure plasma or serum carbohydrate deficient transferrin (CDT) profile, N-glycan profile, and O-glycan profile by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) or liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS). The quantification of particular biomarkers, such as T antigens or sialylated T antigens, could also be achieved by liquid chromatography-tandem mass spectrometry (LC-MS/MS). These techniques can be used to identify a majority of patients with defects in protein glycosylation, although different techniques, such as flow cytometry with immunostaining, are necessary to detect defects in glycolipid or proteoglycan biosynthesis which is not included in this unit.