Inherited Proteoglycan Biosynthesis Defects—Current Laboratory Tools and Bikunin as a Promising Blood Biomarker

Abstract

Proteoglycans consist of proteins linked to sulfated glycosaminoglycan chains. They constitute a family of macromolecules mainly involved in the architecture of organs and tissues as major components of extracellular matrices. Some proteoglycans also act as signaling molecules involved in inflammatory response as well as cell proliferation, adhesion, and differentiation. Inborn errors of proteoglycan metabolism are a group of orphan diseases with severe and irreversible skeletal abnormalities associated with multiorgan impairments. Identifying the gene variants that cause these pathologies proves to be difficult because of unspecific clinical symptoms, hardly accessible functional laboratory tests, and a lack of convenient blood biomarkers. In this review, we summarize the molecular pathways of proteoglycan biosynthesis, the associated inherited syndromes, and the related biochemical screening techniques, and we focus especially on a circulating proteoglycan called bikunin and on its potential as a new biomarker of these diseases.

Interplay of heparan sulfate chains with the core proteins of syndecans 2 and 4

“…The sugar residues of the linker can be phosphorylated, sulfated, and sialylated, and a noncanonical trisaccharide linker has been reported for bikunin 43 . It would be interesting to determine if the modifications and size of this linker affect the conformations of the core proteins of syndecans and/or the flexibility of the GAG chains, inasmuch as defective synthesis of the linker is associated with rare heritable connective tissue diseases known as linkeropathies 44,45 …”

Alterations in glycosaminoglycan biosynthesis associated with the Ehlers-Danlos syndromes

“…(111,126) In addition, the detection of increased amounts of GAG-free bikunin core protein in serum by western blot analysis could function as a biomarker for spEDS. (117,118,140) Important gaps remain in our understanding of how these GAG defects give rise to the corresponding phenotype. It is still unclear to what extent different aspects of GAG chain biosynthesis are affected in spEDS and mcEDS, including (i) the length and modification of GAG chains, (ii) modification and composition of the linker region, (iii) if preferential production of certain proteoglycans occurs when GAG biosynthesis is restricted and (iv) whether these alterations occur in a tissue-specific manner.…”

Glycosaminoglycan linkage region of urinary bikunin as a potentially useful biomarker for β3GalT6‐deficient spondylodysplastic Ehlers–Danlos syndrome

“…In a recent publication, the Bruneel group reviewed the status of known proteoglycan inherited biosynthesis defects and the analytical tools available for diagnosis. 7 They also summarized and commented on their own work on using Western blot, or two‐dimensional electrophoresis, for efficiently identifying different glycoforms of serum bikunin as screening methods for such diseases. 6 However, as also commented on in their review, these methods will, in contrast to what becomes available by mass spectrometry, not provide accurate structural data on the GAG chains or on the glycopeptides of relevant proteoglycans as presented here.…”

“…The relative distribution of glycosaminoglycan linkage regions of urinary bikunin glycopeptides may thus serve not only as a phenotypic biomarker in a diagnostic test but possibly also as a biomarker to follow the effect of future therapies (e.g., enzyme replacement therapies) in affected children. In a recent publication, the Bruneel group reviewed the status of known proteoglycan inherited biosynthesis defects and the analytical tools available for diagnosis 7 . They also summarized and commented on their own work on using Western blot, or two‐dimensional electrophoresis, for efficiently identifying different glycoforms of serum bikunin as screening methods for such diseases 6 .…”