Proteoglycans are important components of cell plasma membranes and extracellular matrices of connective tissues. They consist of glycosaminoglycan chains attached to a core protein via a tetrasaccharide linkage, whereby the addition of the third residue is catalyzed by galactosyltransferase II (β3GalT6), encoded by B3GALT6. Homozygosity mapping and candidate gene sequence analysis in three independent families, presenting a severe autosomal-recessive connective tissue disorder characterized by skin fragility, delayed wound healing, joint hyperlaxity and contractures, muscle hypotonia, intellectual disability, and a spondyloepimetaphyseal dysplasia with bone fragility and severe kyphoscoliosis, identified biallelic B3GALT6 mutations, including homozygous missense mutations in family 1 (c.619G>C [p.Asp207His]) and family 3 (c.649G>A [p.Gly217Ser]) and compound heterozygous mutations in family 2 (c.323_344del [p.Ala108Glyfs(∗)163], c.619G>C [p.Asp207His]). The phenotype overlaps with several recessive Ehlers-Danlos variants and spondyloepimetaphyseal dysplasia with joint hyperlaxity. Affected individuals' fibroblasts exhibited a large decrease in ability to prime glycosaminoglycan synthesis together with impaired glycanation of the small chondroitin/dermatan sulfate proteoglycan decorin, confirming β3GalT6 loss of function. Dermal electron microcopy disclosed abnormalities in collagen fibril organization, in line with the important regulatory role of decorin in this process. A strong reduction in heparan sulfate level was also observed, indicating that β3GalT6 deficiency alters synthesis of both main types of glycosaminoglycans. In vitro wound healing assay revealed a significant delay in fibroblasts from two index individuals, pointing to a role for glycosaminoglycan defect in impaired wound repair in vivo. Our study emphasizes a crucial role for β3GalT6 in multiple major developmental and pathophysiological processes.

Objective To assess the variations of galactose‐ β‐1,3‐glucuronosyltransferase I (GlcAT‐I) expression related to the decrease in proteoglycan synthesis mediated by interleukin‐1β (IL‐1β) in rat chondrocytes, and to evaluate the influence of glucosamine on the effects elicited by this proinflammatory cytokine. Methods Rat articular chondrocytes in primary monolayer cultures or encapsulated into alginate beads were treated with recombinant IL‐1β in the absence or presence (1.0–4.5 gm/liter) of glucosamine. Variations of GlcAT‐I and expression of stromelysin 1 (matrix metalloproteinase 3 [MMP‐3]) messenger RNA (mRNA) were evaluated by quantitative multistandard reverse transcriptase–polymerase chain reaction. In vitro enzymatic activity of GlcAT‐I was measured by thin‐layer chromatography, with radiolabeled UDP‐glucuronic acid and a digalactoside derivative as substrates. Proteoglycan synthesis was determined by ex vivo incorporation of Na2‐35SO4. Nitric oxide synthase and cyclooxygenase activities were monitored by the evaluation of nitrite (NO −2) and prostaglandin E2 (PGE2) produced in the culture medium, respectively. Results IL‐1β treatment resulted in a marked inhibition of GlcAT‐I mRNA expression and in vitro catalytic activity, together with a decrease in proteoglycan synthesis. In addition, glucosamine was able to prevent, in a dose‐dependent manner, the inhibitory effects of IL‐1β. In the same way, the amino sugar reduced NO −2 and PGE2production induced by IL‐1β. Finally, the up‐regulation of stromelysin 1 (MMP‐3) mRNA expression by IL‐1β was fully prevented by glucosamine. Conclusion The results of this study suggest that the deleterious effect of IL‐1β on the anabolism of proteoglycan could involve the repression of GlcAT‐I, a key enzyme in the biosynthesis of glycosaminoglycan. Glucosamine was highly effective in preventing these IL‐1β–mediated suppressive effects. The amino sugar also prevented the production of inflammatory mediators induced by the cytokine. This action could account for a possible beneficial effect of glucosamine on osteoarthritic articular cartilage.