Diseases of GAG biosynthesis

The polymerization of heparan sulfate chains is mediated by a heterodimer of the EXT1 and EXT2 proteins. Three other a1-4 GlcNAc-transferases called EXTL1, EXTL2 and EXTL3 also participate in the initiation of chain polymerization. The abbreviations EXT and EXTL are derived from the term exostosis, which designates a type of benign bone tumor. Mutations in the EXT and EXTL genes are associated with increased exostosis formation, which is why these genes also belong to the family of tumor suppressor genes.

FIG: EXT COMPLEX

Exostoses are protrusions of bone material mainly emerging at the end of long bones and originating from benign cartilage tumors. Exostoses are quite frequent with an estimated prevalence of 1 to 50’000 in most ethnic groups. The inclusion of a mutant EXT1 or EXT2 subunit to the heterodimer inactivates the enzyme, so explaining the dominant negative effect. Theoretically, this situation causes a decreased enzymatic activity to 50% of normal levels, which significantly decreases heparan sulfate abundance in tissues. In bone tissue, heparan sulfate chains are required for the proper localization of several morphogenic factors, like FGFs and BMPs, which regulate endochondral ossification, a process characterized by a directional sequence of chondrocyte proliferation and differentiation. Notably, the complete loss of the EXT activity is not compatible with life, explaining why homozygous or combined heterozygous mutations in the EXT genes are not reported. With a frequency of about 5%, exostoses can transform into malignant chondrosarcomas.

FIG: CHONDROCYTE DIFFERENTIATION GRADIENTS

Another defect of GAG chain biosynthesis is caused by mutations in the xylosylprotein b1-4 Gal-transferase XGPT1, which catalyzes the transfer of the first Gal residue to the GAG core GlcA(b1-3)Gal(b1-3)Gal(b1-4)Xyl(b1-O)Ser. The resulting disease, called progeroid form of the Ehlers-Danlos syndrome (OMIM 130070), is characterized by mental retardation, short stature and connective tissues abnormalities such as loose skin, wrinkled facies, osteopenia and joint hypermobility. The disease is very rare, less than 10 cases have been reported to date. Little is known about the extent of the GAG chain biosynthetic defect in the affected patients. Analysis of skin fibroblasts from patients indicated that some proteoglycan proteins are secreted free of GAG chains.

Mutations in the b1-3 GlcA-transferase B3GAT3 gene (encoding GlcAT-I) are found in patients featuring congenital heart defects and joint dislocations. Decreased GlcAT-I activity reduces the levels of heparan, chondroitin and dermatan sulfate chains on proteoglycans. Some proteoglycans like decorin are secreted devoid of its single dermatan sulfate chain. The disease in question has been named Larsen-like syndrome, B3GAT3 type (OMIM 245600). At least three diseases are associated to defective sulfotransferases. The first disease, named Omani type of spondylepiphyseal dysplasia (OMIM 608637), is caused by a deficiency of the chondroitin 6-O sulfotransferase enzyme CHST3 that catalyzes the 6-O sulfation of GalNAc found in chondroitin and dermatan sulfate GAG chains. The symptoms of Omani type of spondylepiphyseal dysplasia are typical of connective tissue disorders, i.e. short stature, skeletal dysplasia, kyphoscoliosis and arthritic joints. Patients show a normal intelligence. Mutations in the CHST14 dermatan 4-sulfotransferase-1 gene are associated to the musculocontractural type of Ehlers-Danlos syndrome (OMIM 601776), which is characterized by severe manifestations such as craniofacial dysmorphism, kyphoscoliosis, hypotonia and joint hypermobility. The CHST14 enzyme transfers sulfate to the 4-O position of GalNAc flanked on both sides by IdoA. This strict acceptor recognition explains why this sulfotransferase is specific for dermatan sulfate and does not affect chondroitin sulfate chains.

The third sulfation defect affects the GlcNAc-6-sulfotransferase CHST6 gene, which is involved in the sulfation of keratan sulfate chains. The CHST6 gene is expressed in many cell types but the clinical features of GlcNAc-6-O sulfotransferase deficiency are limited to the eye, resulting in a progressive opacity of the cornea, a condition referred to as macular corneal dystrophy (OMIM 217800). Keratan sulfate chains are important for the solubility and water retention of corneal proteoglycans and collagens. The decreased level of sulfation caused by CHST6 deficiency leads to aggregation of corneal proteins and thereby to increased opacity of the cornea.