Pathologic extraskeletal bone formation, or heterotopic ossification (HO), occurs following mechanical trauma, burns, orthopedic operations, and in patients with hyperactivating mutations of the type I bone morphogenetic protein receptor ACVR1 (Activin type 1 receptor). Extraskeletal bone forms through an endochondral process with a cartilage intermediary prompting the hypothesis that hypoxic signaling present during cartilage formation drives HO development and that HO precursor cells derive from a mesenchymal lineage as defined by Paired related homeobox 1 (Prx). Here we demonstrate that Hypoxia inducible factor-1α (Hif1α), a key mediator of cellular adaptation to hypoxia, is highly expressed and active in three separate mouse models: trauma-induced, genetic, and a hybrid model of genetic and trauma-induced HO. In each of these models, Hif1α expression coincides with the expression of master transcription factor of cartilage, Sox9 [(sex determining region Y)-box 9]. Pharmacologic inhibition of Hif1α using PX-478 or rapamycin significantly decreased or inhibited extraskeletal bone formation. Importantly, de novo soft-tissue HO was eliminated or significantly diminished in treated mice. Lineage-tracing mice demonstrate that cells forming HO belong to the Prx lineage. Burn/tenotomy performed in lineage-specific Hif1α knockout mice (Prx-Cre/Hif1α fl:fl ) resulted in substantially decreased HO, and again lack of de novo soft-tissue HO. Genetic loss of Hif1α in mesenchymal cells marked by Prx-cre prevents the formation of the mesenchymal condensations as shown by routine histology and immunostaining for Sox9 and PDGFRα. Pharmacologic inhibition of Hif1α had a similar effect on mesenchymal condensation development. Our findings indicate that Hif1α represents a promising target to prevent and treat pathologic extraskeletal bone.is the pathologic formation of extraskeletal bone in soft tissues. This process occurs in two separate patient populations: those with severe trauma, including large surface-area burns, musculoskeletal injury, orthopedic operations, and even spinal cord injury; and those with a genetic disease known as fibrodysplasia ossificans progressiva (FOP) (1-4). FOP is caused by a hyperactivating mutation in the type I bone morphogenetic protein (BMP) receptor ACVR1 (Activin type 1 receptor), and patients with FOP develop ectopic bone lesions in the absence of any substantial trauma. The clinical sequela of these pathologic ectopic bone formations, whether in the setting of trauma or genetic mutations, include nonhealing wounds, chronic pain, and joint immobility. In the case of FOP, progressive ossification may lead to death as a result of loss of thoracic cage compliance.Treatment options for HO are limited because bone often recurs following surgical resection, and some patients may have nonresectable HO because of its sensitive location. The risk of an operation may outweigh the benefits of excision, especially in the face of recurrence (5). Therefore, there is a need to identify therapeutic options ...

Heterotopic ossification (HO) is a common occurrence after multiple forms of extensive trauma. These include arthroplasties, traumatic brain and spinal cord injuries, extensive burns in the civilian setting, and combat-related extremity injuries in the battlefield. Irrespective of the form of trauma, heterotopic bone is typically endochondral in structure and is laid down via a cartilaginous matrix. Once formed, the heterotopic bone typically needs to be excised surgically, which may result in wound healing complications, in addition to a risk of recurrence. Refinements of existing diagnostic modalities, like micro- and nano-CT are being adapted toward early intervention. Trauma-induced HO is a consequence of aberrant wound healing, systemic and local immune system activation, infections, extensive vascularization, and innervation. This intricate molecular crosstalk culminates in activation of stem cells that initiate heterotopic endochondral ossification. Development of animal models recapitulating the unique traumatic injuries has greatly facilitated the mechanistic understanding of trauma-induced HO. These same models also serve as powerful tools to test the efficacy of small molecules which specifically target the molecular pathways underlying ectopic ossification. This review summarizes the recent advances in the molecular understanding, diagnostic and treatment modalities in the field of trauma-induced HO.

The pathologic development of heterotopic ossification (HO) is well described in patients with extensive trauma or with hyperactivating mutations of the bone morphogenetic protein (BMP) receptor ACVR1. However, identification of progenitor cells contributing to this process remains elusive. Here we show that connective tissue cells contribute to a substantial amount of HO anlagen caused by trauma using post-natal, tamoxifen-inducible, scleraxis-lineage restricted reporter mice (Scx-creERT2/tdTomatofl/fl). When the scleraxis-lineage is restricted specifically to adults prior to injury marked cells contribute to each stage of the developing HO anlagen and co-express markers of endochondral ossification (Osterix, SOX9). Furthermore, these adult pre-injury restricted cells co-expressed mesenchymal stem cell markers including PDGFRα, Sca1, and S100A4 in HO. When constitutively active ACVR1 (caACVR1) was expressed in scx-cre cells in the absence of injury (Scx-cre/caACVR1fl/fl), tendons and joints formed HO. Post-natal lineage-restricted, tamoxifen-inducible caACVR1 expression (Scx-creERT2/caACVR1fl/fl) was sufficient to form HO after directed cardiotoxin-induced muscle injury. These findings suggest that cells expressing scleraxis within muscle or tendon contribute to HO in the setting of both trauma or hyperactive bone morphogenetic protein receptor (e.g. caACVR1) activity.

Tissue regeneration following acute or persistent inflammation can manifest a spectrum of phenotypes ranging from the adaptive to the pathologic. Heterotopic Ossification (HO), the endochondral formation of bone within soft-tissue structures following severe injury serves as a prominent example of pathologic differentiation; and remains a persistent clinical issue incurring significant patient morbidity and expense to adequately diagnose and treat. The pathogenesis of HO provides an intriguing opportunity to better characterize the cellular and cell-signaling contributors to aberrant differentiation. Indeed, recent work has continued to resolve the unique cellular lineages, and causative pathways responsible for ectopic bone development yielding promising avenues for the development of novel therapeutic strategies shown to be successful in analogous animal models of HO development. This review details advances in the understanding of HO in the context of inciting inflammation, and explains how these advances inform the current standards of diagnosis and treatment.