Functional Genomics of Multiple Hereditary Exostoses

Multiple hereditary exostoses (MHE) is an autosomal dominant disorder that affects one in every 50,000 children worldwide and is characterized by the formation of cartilage-capped osteochondromas, known as exostoses, adjacent to the growth plates of long bones and other skeletal elements. Due to their location and size, exostoses

can cause skeletal deformities, growth retardation, chronic pain, and undergo malignant transformation in ~5% of the patients. Currently, there are no approved treatments for this disorder besides surgery and pain management; therefore, there is an urgent need to discover novel therapeutic targets to prevent and/or slow the

progression of the disease. Greater than 90% of cases are caused by heterozygous loss-of-function genetic mutations in exostosin-1 (EXT1) or exostosin-2 (EXT2), genes that encode enzymes responsible for the biosynthesis of heparan sulfate (HS), which can lead to truncation of the HS chains and a consequent decrease

in HS levels in various tissues. Current evidence suggests that a decrease in HS content disrupts multiple signaling pathways through which growth factors regulate the organization and function of chondrocytes in the growth plate. Regardless of the mechanism, the primary defect is haploinsufficiency in EXT1 or EXT2, which

results in lower HS levels and leads to the formation of exostoses. To date, very little is known about the upstream and downstream factors that regulate the expression and function of EXT1 and EXT2 in human cells. Interestingly, there are also numerous patients lacking mutations in these genes, suggesting that heritable

alterations in other factors contribute to exostoses formation in MHE patients. Our previous studies have revealed defined transcriptional and epigenetic regulatory mechanisms of HS biosynthesis in cells. The central hypothesis of this proposal is that other factors exist that regulate EXT expression and are viable drug targets

for the restoration of HS levels and reduction of exostoses in MHE patients. Since a deficiency in HS is a key component of MHE, we propose that over-stimulating the expression of the normal EXT allele to compensate for the activity of the mutant allele could be a novel therapeutic approach to restore functionally normal levels of HS

in cells. The goal of this proposal is to leverage our experience in uncovering regulatory factors of HS formation to adapt genome-wide screening assays to identify genes that control EXT1/EXT2 expression for drug target discovery for MHE. To accomplish this goal, we aim to (i) adapt genome-wide screening assays to search for

novel factors that regulate EXT1 and/or EXT2 expression in human cells, and (ii) validate prioritized hits from the screens in primary human chondrocytes using commercially available drugs and/or RNAi methods. The successful completion of these aims will provide new pharmacological targets for enhancing EXT expression

and HS levels in cells and may also uncover previously unknown genes associated with MHE. Importantly, this project will lead to future detailed hypothesis-driven studies that will bring us closer to finding a cure for this debilitating disorder.