Ext-mutation analysis in Italian sporadic and hereditary osteochondromas

Osteochondromas represent the largest group of benign tumors of bone. Multiple osteochondromatosis or hereditary multiple exostoses (EXT) is an autosomal dominant inherited disorder characterized by the presence of multiple benign cartilage-capped exostoses. EXT is genetically heterogeneous with at least 3 chromosomal loci: EXT1 (8q24.1), EXT2 (11p11-p13), and EXT3 (19p). In <5% of EXT patients, the inactivation of both copies of EXT alleles (LOH) is associated with malignant transformation. We have analyzed the EXT1 and EXT2 genes in 9 unrelated EXT families and in a patient with a sporadic osteochondroma, all originating from Italy. Four families show an EXT1 mutation, consisting of a small deletion in 3 of them and a small insertion in the 4th. All these mutations lead to premature termination of translation and thus a truncated EXT1 protein. Three families presented EXT2 mutations consisting of nucleotide substitutions leading to alterations of the third intron splice-site, to an amino acid substitution and to a nonsense mutation. All these mutations cosegregate with the disease phenotype. The sporadic osteochondroma patient carried a novel missense mutation in exon 11 of EXT2 gene, leading to an amino acid substitution. Seven of these mutations have never been described before. EXT2 missense mutations were also confirmed by amino acids conservation between human and mouse and by analysis of a healthy control population. In conclusion, our study provide further evidence that loss of function of the EXT1 or EXT2 gene is the main cause of EXT supporting the putative tumor-suppressor function of these genes.