Hereditary Multiple Exostoses (HME) is a rare autosomal dominant disorder that affects thousands of children worldwide. HME is characterized by cartilaginous-bony tumors called osteochondromas that form within perichondrium along growth plates and protrude into and collide with surrounding tissues. The tumors can thus cause skeletal deformities, compression of nerves and blood vessels and chronic pain, and become malignant in about 2-3% of patients. Current therapies are limited, and patients struggle with pain and limited mobility and undergo multiple surgeries through life. Most HME patients bear a heterozygous mutation in EXT1 or EXT2 that are responsible for heparan sulfate (HS) synthesis, thus causing a partial systemic HS deficiency. The HS chains -and the proteoglycans of which they are part- regulate and distinctly modulate many processes. Notably, they interact with signaling proteins including bone morphogenetic proteins (BMPs) and hedgehog family members and most often restrict and delimit protein distribution, availability and activity. However, it is not clear whether and which of these mechanisms may be deranged in HME and how it could lead to tumor formation. In the previous funding period, we found that conditional ablation of Ext1 caused an increase in pro-chondrogenic BMP signaling in perichondrium and a concurrent decrease in anti- chondrogenic pERK1/2 and Noggin, deranging normal homeostatic mechanisms that normally maintain the perichondrium phenotype. In preliminary studies, we have aimed to clarify how the osteochondromas acquire a growth plate-like organization, are able to grow unidirectionally against surrounding tissues and thus create damage and havoc. We have obtained evidence for the establishment of an IHH-PTHrP axis driving tumor outgrowth. Our central hypotheses is that osteochondroma development and outgrowth are driven by: (i) a steep local deficiency in HS; (ii) increased BMP signaling; and (iii) establishment of a neo IHH-PTHrP loop. As a result, we posit that osteochondroma development and growth are amenable to drug treatments directed against components of those regulatory circuits. We will use genetic, biochemical and cellular approaches and transgenic mouse models that closely mimic human disease progression and burden. The project will continue to provide fundamentally new insights into cellular and molecular mechanisms of tumor formation as well as normal functioning of those mechanisms in standard perichondrial and growth plate cells. It will also test possible therapies based on those insights and thus has major translational medicine value and implications. The number of HME patients is relatively small, but the community of their families is large. This project will thus provide a renewed sense of hope to patients and families alike that this disease will continue to be actively studied and a cure may one day be found.