Classical galactosaemia: novel insights in IgG N -glycosylation and N -glycan biosynthesis

Classical galactosaemia (OMIM #230400), a rare disorder of carbohydrate metabolism, is caused by a deficient activity of galactose-1-phosphate uridyltransferase (EC 2.7.7.12). The pathophysiology of the long-term complications, mainly cognitive, neurological and female fertility problems remains poorly understood. The lack of validated biomarkers to determine prognosis, monitor disease progression and responses to new therapies, pose a huge challenge. We report the detailed analysis of an automated robotic hydrophilic interaction ultra-performance liquid chromatography N -glycan analytical method of high glycan peak resolution applied to serum IgG. This has revealed specific N -glycan processing defects observed in 40 adult galactosaemia patients (adults and adolescents), in comparison with 81 matched healthy controls. We have identified a significant increase in core fucosylated neutral glycans ( P <0.0001) and a significant decrease in core fucosylated ( P <0.001), non-fucosylated ( P <0.0001) bisected glycans and, of specific note, decreased N -linked mannose-5 glycans ( P <0.0001), in galactosaemia patients. We also report the abnormal expression of a number of related relevant N -glycan biosynthesis genes in peripheral blood mononuclear cells from 32 adult galactosaemia patients. We have noted significant dysregulation of two key N -glycan biosynthesis genes: ALG9 upregulated ( P <0.001) and MGAT1 downregulated ( P <0.01) in galactosaemia patients, which may contribute to its ongoing pathophysiology. Our data suggest that the use of IgG N -glycosylation analysis with matched N -glycan biosynthesis gene profiles may provide useful biomarkers for monitoring response to therapy and interventions. They also indicate potential gene modifying steps in this N -glycan biosynthesis pathway, of relevance to galactosaemia and related N -glycan biosynthesis disorders.