We selected the N,O-glycosylated proteoglycan bikunin as a model to establish a chemoenzymatic approach to defined proteoglycans using native chemical ligation. Overexpression of the human linkage region glycosyltransferases B4GalT7, B3GalT6 and B3GlcAT-1 as N-terminal SUMO-fusions gave high yields of soluble and active enzymes in E. coli. When starting with xylosylated bikunin peptides the transferases performed well in enzymatic cascade reactions and provided the desired linkage region tetrasaccharide glycopeptides. B3GalT6 and B3GlcAT-1 led to side products with N,O-glycosylated bikunin peptides revealing unexpected promiscuity of both enzymes towards complex type N-glycans. Additionally, B3GalT6 was found to synthesize short poly-β3 Gal structures. B3GlcAT-1 can slowly convert the biosynthetic intermediate Gal-Xyl to the non-canonical trisaccharide GlcA-Gal-Xyl. This reaction independently confirmed the recently detected biosynthetic bypass to GAGs in the case of dysfunctional B3GalT6 (spondylodysplastic Ehlers-Danlos-syndrome). The three linkage region glycosyltransferases B4GalT7, B3GalT6 and B3GlcAT-1 were dimeric in solution and the crystal structure of B3GalT6 was solved showing a covalent dimer linked by a disulfide in the center of the large dimerization domain. This motif appears to be conserved in higher organisms and reinforces the concept of dimeric glycosyltransferases lining the Golgi.

Ehlers-Danlos syndrome (EDS) is a heritable connective tissue disorder characterized by a varying degree of skin hyperextensibility and joint hypermobility. EDS is classified into 13 subtypes according to the most recent classification. These subtypes are clinically and genetically heterogenous. The spondylodysplastic subvariety of EDS (spEDS) is caused by homozygous mutations in B4GALT7, B3GALT6 and SLC39A13. To date, 13 individuals with molecularly diagnosed SLC39A13-related spEDS have been reported. The spEDS caused by biallelic pathogenic SLC39A13 variants are characterized by short stature, protuberant eyes with bluish sclera, finely wrinkled palms, hypermobile joints, hyperextensible skin and characteristic radiological findings. Herein, we report a case of 7-year-old-female child with spEDS associated with novel homozygous (pathogenic/likely pathogenic) missense variation of the SLC39A13 gene.

Spondylodysplastic Ehlers-Danlos syndrome (spEDS) is a rare subtype of the heritable connective tissue disorder characterized in the 2017 Ehlers-Danlos syndrome (EDS) nosology. Three biallelic mutations, B4GALT7, B3GALT6, and SLC39A13, confirm the diagnosis of spEDS. Hypophosphatasia (HPP) is a heritable disorder caused by a genetic sequence variation in the ALPL gene affecting bone mineralization. Common symptoms in the adult form of HPP are joint pain, muscle hypotonia, and metatarsal fractures. Here we present a case of spEDS and HPP in a patient. A 38-year-old woman was evaluated for chronic diffuse joint pain and a low alkaline phosphatase level of 27 U/L (reference, 31-125 U/L). In addition, she presented with a history of hypermobility, limb bowing, and hyperextensible skin, prompting genetic testing for EDS and HPP. The results returned significant for a synonymous sequence variant at c.441G>A in the B4GALT7 gene indicative of spEDS. HPP was clinically diagnosed by a repeat low alkaline phosphatase level of 23 U/L and high vitamin B6 level of 24.4 ng/mL (reference, 2.1-21.7 ng/mL), despite the absence of the ALPL gene sequence variation on genetic testing. Remarkable personal and family history of this patient suggest that co-occurrence of EDS and HPP is not merely coincidental. Given the overlapping features of muscle hypotonia and joint pain between the 2 heritable disorders, a possible relationship between the 2 may have been previously overlooked. Further investigation in the relationship and management of the 2 heritable diseases is warranted as enzyme replacement therapy, asfotase alfa, approved for infantile and juvenile onset of HPP may improve the symptoms shared with EDS.

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.

The spondylodysplastic Ehlers-Danlos subtype (OMIM #130070) is a rare connective tissue disorder characterized by a combination of connective tissue symptoms, skeletal features and short stature. It is caused by variants in genes encoding for enzymes involved in the proteoglycan biosynthesis or for a zinc transporter. We report two brothers with a similar phenotype of short stature, joint hypermobility, distinct craniofacial features, developmental delay and severe hypermetropia indicative for a spondylodysplastic Ehlers-Danlos subtype. One also suffered from a recurrent pneumothorax. Gene panel analysis identified two compound heterozygous variants in the B4GALT7 gene: c.641G > A and c.723 + 4A > G. B4GALT7 encodes for galactosyltransferase I, which is required for the initiation of glycosaminoglycan side chain synthesis of proteoglycans. This is a first full report on two cases with spondylodysplastic Ehlers-Danlos syndrome and the c.723 + 4A > G variant of B4GALT7. The recurrent pneumothoraces observed in one case expand the variable phenotype of the syndrome.