Defective glycosylation of decorin and biglycan, altered collagen structure, and abnormal phenotype of the skin fibroblasts of an Ehlers–Danlos syndrome patient carrying the novel Arg270Cys substitution in galactosyltransferase I (β4GalT-7)
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- Seidler, D.G., Faiyaz-Ul-Haque, M., Hansen, U. et al. J Mol Med (2006) 84: 583. doi:10.1007/s00109-006-0046-4
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The Ehlers–Danlos syndrome (EDS) is a heterogeneous group of connective tissue disorders affecting skin and joint function. Molecular defects in extracellular matrix proteins, including collagen (type I, III, and V) and tenascin X are associated with different forms of EDS. Compound heterozygous mutations in the B4GALT7 gene, resulting in aberrant glycosylation of the dermatan sulfate proteoglycan decorin, had been described in a single patient affected with the progeroid form of EDS. We have studied the molecular phenotype of decorin, biglycan, and collagen type I containing fibrils in skin fibroblasts of a patient carrying the novel homozygous C808T point mutation in the B4GALT7 gene, which causes an Arg270Cys substitution in β4GalT-7. Compared to control fibroblasts, galactosyltransferase activity in β4GalT-7Arg270Cys cells was approximately three times reduced over a temperature range of 25–41°C. Pulse-chase experiments and confocal microscopy demonstrated that synthesis and secretion of decorin were normal in β4GalT-7Arg270Cys cells. However, about 50% of decorin were synthesized as a protein core in addition to its proteoglycan form. Biglycan was found in a monoglycanated form in addition to its mature form. Glycosaminoglycan chains were of the dermatan/chondroitin sulfate type both in β4GalT-7Arg270Cys and control cells, and epimerization was reduced for decorin and biglycan. Compared to control cells, β4GalT-7Arg270Cys cells showed altered, highly spread or stretched phenotypes and decreased proliferation rates. At the ultrastructural level, an intracellular accumulation of multiple secondary lysosomes and degenerative vacuoles was seen in β4GalT-7Arg270Cys cells. Furthermore, the collagen suprastructures were altered in the β4GalT-7Arg270Cys cells. The reduced β4GalT-7 activity resulting in defective glycosylation of decorin and biglycan may be responsible for the complex molecular pathology in β4GalT-7 deficient EDS patients, given the role of these proteoglycans in bone formation, collagen fibrillogenesis, and skeletal muscle development.