Cytoskeletal abnormalities in chondrocytes with EXT1 and EXT2 mutations

The EXT genes are a group of putative tumor suppressor genes that previousl y have been shown to participate in the development of hereditary multiple exostoses (HME), HME-associated and isolated chondrosarcomas, Two HME disea se genes, EXT1 and EXT2, have been identified and are expressed ubiquitousl y, However, the only known effect of mutations in the EXT genes is on chond rocyte function as evidenced by aberrant proliferation of chondrocytes lead ing to formation of bony, cartilage-capped projections (exostoses), In this study, we have characterized exostosis chondrocytes from three patients wi th HME (one with EXT1 and two with EXT2 germline mutations) and from one in dividual with a non-HME, isolated exostosis, At the Light microscopic level , exostosis chondrocytes have a stellate appearance with elongated inclusio ns in the cytoplasm. Confocal and immunofluorescence of in vitro and in viv o chondrocytes showed that these massive accumulations are composed of acti n bundled by 1.5-mu m repeat cross-bridges of alpha-actinin. Western blot a nalysis shows that exostosis chondrocytes from two out of three patients ab errantly produce high levels of muscle-specific. alpha-actin, whereas beta- actin levels are similar to normal chondrocytes. These findings suggest tha t mutations in the EXT genes cause abnormal processing of cytoskeleton prot eins in chondrocytes.