Osteopetrosis and osteoporosis: two sides of the same coin

Together, osteoporosis and osteopetrosis comprise a substantial proportion of the bone diseases that severely affect humans. In order to understand an d effectively treat these disorders, an understanding of the mechanisms con trolling bone remodelling is essential. While numerous animal models of bon e disease have been generated, the lack of correlation between these animal models and human disease has limited their utility in terms of defining th erapeutic strategies. The generation and analysis of cathepsin K knockout m ice has resulted in a model for pycnodysostosis, a rare human osteopetrotic disease, and is now providing considerable insights into both osteoclast f unction and potential therapeutic strategies for the treatment of bone dise ase. This review highlights the importance of genes such as cathepsin K in understanding bone remodelling and illustrates a new trend towards understa nding bone disease as a complete entity rather than as a series of unrelate d disorders.