MUTATION SCREENING OF THE EXT1 AND EXT2 GENES IN PATIENTS WITH HEREDITARY MULTIPLE EXOSTOSES

Hereditary multiple exostoses (HME), the most frequent of all skeletal dysplasias, is an autosomal dominant disorder characterized by the pr esence of multiple exostoses localized mainly at the end of long bones . HME is genetically heterogeneous, with at least three loci, on 8q24. 1 (EXT1), 11p11-p13 (EXT2), and 19p (EXT3). Both the EXT1 and EXT2 gen es have been cloned recently and define a new family of potential tumo r suppressor genes. This is the first study in which mutation screenin g has been performed for both the EXT1 and EXT2 genes prior to any lin kage analysis. We have screened 17 probands with the HME phenotype, fo r alterations in all translated exons and nanking intronic sequences, in the EXT1 and EXT2 genes, by conformation-sensitive gel electrophore sis. We found the disease-causing mutation in 12 families (70%), 7 (41 %) of which have EXT1 mutations and 5 (29%) EXT2 mutations. Together w ith the previously described 1-bp deletion in exon 6, which is present in 2 of our families, we report five new mutations in EXT1, Two are m issense mutations in exon 2 (G339D and R340C), and the other three alt erations (a nonsense mutation, a frameshift, and a splicing mutation) are likely to result in truncated non-functional proteins. Four new mu tations are described in EXT2. A missense mutation (D227N) was found i n 2 different families; the other three alterations (two non-sense mut ations and one frameshift mutation) lead directly or indirectly to pre mature stop codons. The mis-sense mutations in EXT1 and EXT2 may pinpo int crucial domains in both proteins and therefore give clues for the understanding of the pathophysiology of this skeletal disorder.