Bone Abstracts

Hereditary multiple osteochondromas (HMO) is an autosomal dominant rare genetic disorder due to LOF heterozygous mutations in EXT1 or EXT2 genes. HMO is an unmet medical condition where patients often requiring multiple surgeries. HMO is characterized by painful cartilaginous capped bony outgrowths at the growth plate (GP) regions of long bones, ribs and other skeletal elements. The molecular mechanism by which these mutations lead to disease is unknown. Mutations in EXT1 and 2 cause impaired heparan sulfate (HS) chain elongation, resulting in short chain HS and abnormal cartilage matrix in bone, which may affect various growth factors that are modulated by HS binding. Previous studies have shown that human HMO lesions express heparanase in chondrocytes of all layers of the GP in contrast to normal GP, where it is restricted to hypertrophic chondrocytes. Heparanase upregulation is thought to induce HS cleavage and promote ectopic BMP signaling, resulting in osteochondromas development. We tested the hypothesis that heparanase inhibition would abrogate osteochondroma development in double heterozygous Ext1 and Ext2 mice. We developed a mouse model of HMO by crossing Ext1 with Ext2 heterozygous mice. Ext1+/−, Ext2+/− and double heterozygous were born alive without any gross abnormalities and overall skeletal development was indistinguishable from control litter mates. Single and double heterozygous mice developed osteochondromas in the ribs, however double heterozygous mice have higher incidence (Ext1+/− 50%, Ext2+/− 38% and Ext1+/− and Ext2+/− 67% of mice developed osteochondromas at 20 weeks of age). Development of osteochondromas is also increased over time in double heterozygous mice, 41% of 8 week old mice developed osteochondromas, increasing to 83% at 24–28 weeks. To determine role of heparanase in the development of osteochondromas, we generated heparanase knockout mice and crossed with Ext1 and Ext2 double heterozygous mice to delete heparanase in the HMO mouse model. We observed that genetic loss of heparanase did not inhibit development of rib osteochondromas, 88% of heparanase KO HMO mice and 92% of HMO mice developed osteochondromas at 33–34 weeks of age. Our mice genetics study revealed that loss of heparanase did not affect osteochondromas development.

Disclosure: All the authors are employed by Regeneron Pharmaceuticals Inc and owners of company stock.