Etiological point mutations in the hereditary multiple exostoses gene EXT1: a functional analysis of heparan sulfate polymerase activity

Order out of chaos: assembly of ligand binding sites in heparan sulfate

Virtually every cell type in metazoan organisms produces heparan sulfate. These complex polysaccharides provide docking sites for numerous protein ligands and receptors involved in diverse biological processes, including growth control, signal transduction, cell adhesion, hemostasis, and lipid metabolism. The binding sites consist of relatively small tracts of variably sulfated glucosamine and uronic acid residues in specific arrangements. Their formation occurs in a tissue-specific fashion, generated by the action of a large family of enzymes involved in nucleotide sugar metabolism, polymer formation (glycosyltransferases), and chain processing (sulfotransferases and an epimerase). New insights into the specificity and organization of the biosynthetic apparatus have emerged from genetic studies of cultured cells, nematodes, fruit flies, zebrafish, rodents, and humans. This review covers recent developments in the field and provides a resource for investigators interested in the incredible diversity and specificity of this process.

Evolution of glycosaminoglycans and their glycosyltransferases: Implications for the extracellular matrices of animals and the capsules of pathogenic bacteria

Glycosaminoglycans (linear polysaccharides with a repeating disaccharide backbone containing an amino sugar) are essential components of extracellular matrices of animals. These complex molecules play important structural, adhesion, and signaling roles in mammals. Direct detection of glycosaminoglycans has been reported in a variety of organisms, but perhaps more definitive tests for the glycosyltransferase genes should be utilized to clarify the distribution of glycosaminoglycans in metazoans. Recently, glycosyltransferases that form the hyaluronan, heparin/heparan, or chondroitin backbone were identified at the molecular level. The three types of glycosyltransferases appear to have evolved independently based on sequence comparisons and other characteristics. All metazoans appear to possess heparin/heparan. Chondroitin is found in some worms, arthropods, and higher animals. Hyaluronan is found only in two of the three main branches of chordates. The presence of several types of glycosaminoglycans in the body allows multiple communication channels and adhesion systems to operate simultaneously. Certain pathogenic bacteria produce extracellular coatings, called capsules, which are composed of glycosaminoglycans that increase their virulence during infection. The capsule helps shield the microbe from the host defenses and/or modulates host physiology. The bacterial and animal polysaccharides are chemically identical or at least very similar. Therefore, no immune response is generated, in contrast to the vast majority of capsular polymers from other bacteria. In microbial systems, it appears that in most cases functional convergent evolution of glycosaminoglycan glycosyltransferases occurred, rather than direct horizontal gene transfer from their vertebrate hosts.

Hereditary multiple exostoses: one center's experience and review of etiology

Hereditary multiple exostosis is a genetic disorder characterized by multiple osteochondromas that can cause pain, deformity, and potential malignant degeneration. Linkage analysis has identified a family of EXT genes which, if mutated, can lose heterozygosity and potentially cause osteochondromas. A database was established of 43 patients with hereditary multiple exostoses treated at a tertiary pediatric healthcare system. Twenty patients had a known family history of the disorder. All patients were diagnosed between birth and 13 years. Symptoms or deformity were observed in the forearms of 29 patients, the knees of 37 patients, and the ankles of 28 patients. Valgus knee deformity related to hereditary multiple exostoses, previously reported to be attributable to proximal tibial changes alone, resulted from proximal tibial or distal femoral valgus deformities in this series. Twenty-seven patients required between one and five surgeries to address their lesions. No patient had malignant degeneration of an osteochondroma; however, three patients had first-degree relatives with transformation of an osteochondroma to chondrosarcoma. This database now may be a resource for additional analysis. By correlating specific genetic mutations with clinical manifestations, it may be possible to stratify patients into subtypes of hereditary multiple exostoses and identify genetic markers associated with malignant degeneration.