Multiple Hereditary Exostoses (MHE) is an autosomal dominant disorder characterized by the formation of ectopic cartilage-capped growth plate-like exostoses next to long bones and other skeletal elements. MHE results from mutations in the genes Ext1 or Ext2, which diminish the capacity of cells in the growth plate and the surrounding perichondrium to make heparan sulfate. The mechanism by which a change in heparan sulfate content causes ectopic osteochondromas is unknown, but evidence suggests that the decrease in heparan sulfate affects multiple signaling pathways through which growth factors regulate the organization and proliferation of chondrocytes in the growth plate. Regardless of the mechanism, the primary defect is in the assembly of heparan sulfate, suggesting that restoring the level of heparan sulfate would diminish the frequency of exostoses. All cells make heparan sulfate through a common mechanism. Thus, we propose to use Chinese hamster ovary (CHO) cells that are functionally hemizygous for Ext1 and to employ a primary cell- based screen to find potential drug candidates that augment heparan sulfate expression. Pilot studies have been done in collaboration with the High Content Screening Core in the Conrad Prebys Center for Chemical Genomics at the Sanford-Burnham Institute. Assay optimization, validation and final implementation of the proposed image-based high-throughput screening assay will be accomplished. The Cheminformatics and Informatics Core at Sanford-Burnham will assist in data analysis, artifact filtering, replicate hit confirmation, and generation of dose response profiles. Secondary assays will test positive hits for their impact on heparan sulfate content and structure. Tertiary assays will measure if the hits modulate heparan sulfate expression in mouse chondrocytes and perichondrial cells and in human chondrocytes. The resultant rank ordering of potency of confirmed hit sets and chemotypes merged with additional secondary and tertiary assay results will aid in hit-to-lead identification. The Cheminformatics Core will search for commercially available analogs to support limited structure-activity profiling. Agents that enhance heparan sulfate synthesis will be evaluated by formulation, stability, pharmacokinetics, and toxicity and their capacity to reduce exostoses in Ext1+/-;Ext2+/- mice. The central hypothesis is that altering key enzymes involved in heparan sulfate metabolism can result in restoration of functionally normal levels of heparan sulfate and reduction of exostoses in mice, which would serve as a proof-of-principle for pharmacological manipulation of exostosis formation in MHE patients.
Multiple Hereditary Exostoses (MHE) has been linked to mutations in two genes involved in heparan sulfate biosynthesis. The objective of this project is to screen for low molecular weight drug-like agents to restore heparan sulfate formation in cellular and murine models of the disease. This project is of direct relevance to human disease because the agents might prove of therapeutic value for treating this orphan disorder.