EXTraordinary bones: Functional and genetic analysis of the EXT gene family

Far from serving as an inert skeletal scaffold, bone is a dynamic tissue that cycles through tightly coordinated cycles of developmental growth and regeneration. Bone growth, which determines the overall growth of vertebrates, is well-characterized histologically and increasingly understood at the molecular level. Positional cloning strategies applied to diseases of simple Mendelian inheritance have revealed genes important in the proper formation of bone. Functional studies of these genes, aided considerably by insights provided by studies of orthologs in various model systems, have led to significant advances in our understanding of the pathways of mammalian bone morphogenesis. One such disorder, hereditary multiple exostoses, is caused by members of the EXT tumor suppressor gene family. Progress on the molecular dissection of this disorder, with emphasis on the interplay of genomic, model system, and clinical studies, is reviewed herein. We are now challenged to re-direct the biochemical pathway of chondrogenesis/osteogenesis defined by the EXT genes toward therapeutic control of bone growth and malignancy.