Depletion of Mast Cells and Macrophages Impairs Heterotopic Ossification in an Acvr1R206H Mouse Model of Fibrodysplasia Ossificans Progressiva.

Heterotopic ossification (HO) is a clinical condition that often reduces mobility and diminishes quality of life for affected individuals. The most severe form of progressive HO occurs in those with fibrodysplasia ossificans progressiva (FOP; OMIM #135100), a genetic disorder caused by a recurrent heterozygous gain-of-function mutation (R206H) in the bone morphogenetic protein (BMP) type I receptor ACVR1/ALK2. In individuals with FOP, episodes of HO frequently follow injury. The first sign of active disease is commonly an inflammatory "flare-up" that precedes connective tissue degradation, progenitor cell recruitment, and endochondral HO. We used a conditional-on global knock-in mouse model expressing Acvr1R206H (referred to as Acvr1cR206H/+ ) to investigate the cellular and molecular inflammatory response in FOP lesions following injury. We found that the Acvr1 R206H mutation caused increased BMP signaling in posttraumatic FOP lesions and early divergence from the normal skeletal muscle repair program with elevated and prolonged immune cell infiltration. The proinflammatory cytokine response of TNFα, IL-1ß, and IL-6 was elevated and prolonged in Acvr1cR206H/+ lesions and in Acvr1cR206H/+ mast cells. Importantly, depletion of mast cells and macrophages significantly impaired injury-induced HO in Acvr1cR206H/+ mice, reducing injury-induced HO volume by ∼50% with depletion of each cell population independently, and ∼75% with combined depletion of both cell populations. Together, our data show that the immune system contributes to the initiation and development of HO in FOP. Further, the expression of Acvr1R206H in immune cells alters cytokine expression and cellular response to injury and unveils novel therapeutic targets for treatment of FOP and nongenetic forms of HO. © 2017 American Society for Bone and Mineral Research.

Palovarotene Inhibits Heterotopic Ossification and Maintains Limb Mobility and Growth in Mice With the Human ACVR1(R206H) Fibrodysplasia Ossificans Progressiva (FOP) Mutation.

Fibrodysplasia ossificans progressiva (FOP), a rare and as yet untreatable genetic disorder of progressive extraskeletal ossification, is the most disabling form of heterotopic ossification (HO) in humans and causes skeletal deformities, movement impairment, and premature death. Most FOP patients carry an activating mutation in a bone morphogenetic protein (BMP) type I receptor gene, ACVR1(R206H) , that promotes ectopic chondrogenesis and osteogenesis and, in turn, HO. We showed previously that the retinoic acid receptor γ (RARγ) agonist palovarotene effectively inhibited HO in injury-induced and genetic mouse models of the disease. Here we report that the drug additionally prevents spontaneous HO, using a novel conditional-on knock-in mouse line carrying the human ACVR1(R206H) mutation for classic FOP. In addition, palovarotene restored long bone growth, maintained growth plate function, and protected growing mutant neonates when given to lactating mothers. Importantly, palovarotene maintained joint, limb, and body motion, providing clear evidence for its encompassing therapeutic potential as a treatment for FOP. © 2016 American Society for Bone and Mineral Research.

The immunological contribution to heterotopic ossification disorders.

The formation of bone outside the endogenous skeleton is a significant clinical event, rendering affected individuals with immobility and a diminished quality of life. This bone, termed heterotopic ossification (HO), can appear in patients following invasive surgeries and traumatic injuries, as well as progressively manifest in several congenital disorders. A unifying feature of both genetic and nongenetic episodes of HO is immune system involvement at the early stages of disease. Activation of the immune system sets the stage for the downstream anabolic events that eventually result in ectopic bone formation, rendering the immune system a particularly appealing site of early therapeutic intervention for optimal management of disease. In this review, we will discuss the immunological contributions to HO disorders, with specific focus on contributing cell types, signaling pathways, relevant in vivo animal models, and potential therapeutic targets.

An Acvr1 R206H knock-in mouse has fibrodysplasia ossificans progressiva.

Fibrodysplasia ossificans progressiva (FOP; MIM #135100) is a debilitating genetic disorder of dysregulated cellular differentiation characterized by malformation of the great toes during embryonic skeletal development and by progressive heterotopic endochondral ossification postnatally. Patients with these classic clinical features of FOP have the identical heterozygous single nucleotide substitution (c.617G > A; R206H) in the gene encoding ACVR1/ALK2, a bone morphogenetic protein (BMP) type I receptor. Gene targeting was used to develop an Acvr1 knock-in model for FOP (Acvr1(R206H/+)). Radiographic analysis of Acvr1(R206H/+) chimeric mice revealed that this mutation induced malformed first digits in the hind limbs and postnatal extraskeletal bone formation, recapitulating the human disease. Histological analysis of murine lesions showed inflammatory infiltration and apoptosis of skeletal muscle followed by robust formation of heterotopic bone through an endochondral pathway, identical to that seen in patients. Progenitor cells of a Tie2(+) lineage participated in each stage of endochondral osteogenesis. We further determined that both wild-type (WT) and mutant cells are present within the ectopic bone tissue, an unexpected finding that indicates that although the mutation is necessary to induce the bone formation process, the mutation is not required for progenitor cell contribution to bone and cartilage. This unique knock-in mouse model provides novel insight into the genetic regulation of heterotopic ossification and establishes the first direct in vivo evidence that the R206H mutation in ACVR1 causes FOP.