The spondylodysplastic type of
Ehlers-Danlos syndrome (spEDS) is caused by genetic defects in the B4GALT7 or B3GALT6 genes both deranging the biosynthesis of the
glycosaminoglycan linkage region of
chondroitin/
dermatan sulfate and
heparan sulfate proteoglycans. In this study, we have analyzed the linkage regions of urinary
chondroitin sulfate proteoglycans of three siblings, diagnosed with spEDS and carrying biallelic pathogenic variants of the B3GALT6 gene.
Proteoglycans were digested with
trypsin,
glycopeptides enriched on
anion-exchange columns, depolymerized with
chondroitinase ABC, and analyzed by nLC-MS/MS. In urine of the unaffected mother, the dominating
glycopeptide of bikunin/
protein AMBP appeared as only one dominating (99.9%) peak with the canonical tetrasaccharide linkage region modification. In contrast, the samples of the three affected siblings contained two different
glycopeptide peaks, corresponding to the canonical tetrasaccharide and to the non-canonical
trisaccharide linkage region modifications in individual ratios of 61/38, 73/27, and 59/41. We propose that the relative distribution of
glycosaminoglycan linkage regions of urinary bikunin
glycopeptides may serve as a phenotypic
biomarker in a diagnostic test but also as a
biomarker to follow the effect of future
therapies in affected individuals.