Reevaluation of a genetic model for the development of exostosis in hereditary multiple exostosis

EXT1 and EXT2 are genes that have been shown to cause hereditary multiple exostosis (HME), a syndrome marked by the formation of bony growths juxtaposed to the growth plate. These genes are members of a growing family of proteins with glycosyltransferase activity required for the synthesis of heparan sulfate chains. This protein activity is predicted to play a role in the expression of proteoglycans on the cell surface and in the extracellular matrix. We and others have previously suggested that a two-hit mutational model applies to the development of an exostosis where a germline mutation coupled with a somatic mutation results in the loss of EXT1 or EXT2 function and subsequent tumor formation. We report the direct sequencing and loss of heterozygosity (LOH) analysis of 12 exostoses from 10 HME families, 4 solitary exostoses, and their corresponding constitutional DNA. Of the 16 exostoses screened, we find only one solitary case in which two somatic mutations, a deletion and an LOH, are present. This provides limited support for the two-hit hypothesis involving the EXT1 and EXT2 genes for the development of an exostosis. Alternative models are developed based on the functional significance of EXT proteins in heparan sulfate biosynthesis.