The link between heparan sulfate and hereditary bone disease: finding a function for the EXT family of putative tumor suppressor proteins

Although genetic linkage analysis is a vital tool for identifying disease genes, further study is often hindered by the lack of a known function for the corresponding gene products. In the case of hereditary multiple exostoses (HME), a dominantly inherited genetic disorder characterized by the formation of multiple cartilaginous tumors, extensive genetic analyses of affected families linked HME to mutations in two members of a novel family of putative tumor suppressor genes, EXT1 and EXT2. The biological function of the corresponding proteins, exostosin-1 (EXT1) and exostosin-2 (EXT2), has emerged in part by way of a serendipitous discovery made in the study of herpes simplex virology, which revealed that the pathogenesis of HME is linked to a defect in heparan sulfate (HS) biosynthesis. Biochemical analysis shows that EXT1 and EXT2 are type II transmembrane glycoproteins and form a Golgi-localized hetero-oligomeric complex that catalyzes the polymerization of HS. In this Perspective we will review the identification and characterization of the EXT family, with a particular focus on the biology of the EXT proteins in vivo, and we will explore their possible role(s) in both normal bone development and the formation of exostoses. Hereditary multiple exostoses Hereditary multiple exostoses (HME), an autosomal dominant bone disorder, is the most common type of benign bone tumor, with an estimated occurrence of 1 in 50,000‐100,000 in Western populations. It is