Genetic and acquired heterotopic ossification are driven by a self‐amplifying positive feedback loop of Hedgehog signaling

Abstract

Heterotopic ossification (HO), which refers to extra-skeletal pathological bone formation, occurs as a common complication after injury or in genetic disorders. However, limited by poor understanding of the underlying cellular and molecular mechanisms, there is currently no effective treatment and surgical eviction often results in recurrence. We have found that in both genetic and injury-induced HO, activated ectopicexpression of a hedgehog ligand drives HO progression by forming a positive feedback loop with Yap activation that results in non-cell-autonomous and self-propagation of osteoblast differentiation of wild type cells. In mouse models of progressive osseous heteroplasia (POH), a human disease caused by null mutations in GNASthat encodes Gαs, we found previously that Hedgehog signaling was activated, which is both necessary and sufficient to induce HO. We recently further found that progressively expanded ectopic bone was formed by progressively recruited wild type cells. Mechanistically, the Gnas-/-mesenchymal cells differentiate into osteoblasts and recruit wild type cells to form bone by activatingexpression of Hedgehog ligands. Secreted Hedgehog further induces Yap activation that directly activating more Hedgehogligand expression in surrounding wild type cells. Activated Yap and Hedgehog signaling induces osteoblast differentiation and this self-propagating positive feedback loop is both necessary and sufficient for ectopic bone formation and expansion and can act independently of Gαs. Importantly, Hedgehog activation was also found in fibrodysplasia ossificans progressive (FOP), Achilles tendon puncture (ATP)-induced HO mouse models and in human HO samples. Genetic removal of Hedgehog and Yap abolished HO not only in POH, but also in FOP and acquired HO mouse models. We therefore identify a Hedgehog and Yap-driven, self- amplifying and self-propagating osteoblast differentiation as a common cellular and molecular mechanism underlying HO initiation and expansion. Our work highlights the importance of rare genetic disease studies in identifying a shared core pathological mechanism underlying a class of diseases. Our results further suggest that the Hedgehog pathway is a promising new therapeutic target for HO without affecting the normal bone.