After the identification of
ACVR1 as the causative gene for FOP, the door opened to investigations into the mechanisms of action of the disease. ACVR1 is a BMP type I receptor that is usually activated upon binding of BMP ligands. Together with BMP type II receptors, ACVR1 activates canonical Smad1/5/8 signalling, but also non-canonical BMP signalling, such as through MAPK, has been identified. The gain-of-function mutations in the GS domain of the
ACVR1 lead to two distinct features of the protein: first, they result in the constitutive activation of the receptor, meaning that it can activate downstream Smad signalling, even in the absence of ligand [
32‐
34], and second, it may alter the binding properties of ligands. As such, activin A, which normally transduces its signal via Smad2/3 through the ACVR2/ACVR1B complex, gains enhanced ability to activate the mutated ACVR1, induce Smad1/5/8 signalling, and promote heterotopic ossification similar to typical osteogenic TGF-beta family members (i.e. BMP2, BMP4, BMP6, BMP7, BMP9) [
22,
27,
35,
36]. Besides Smad1/5/8 signalling, mutated ACVR1 has also been shown to induce mTOR and phosphatidylinositol 3‑kinase α (PI3Kα) signalling [
37,
38]. Based on these findings, various treatment strategies have been explored, either blocking excessive BMP/Smad signalling or inhibiting activin A or activin A‑induced downstream signalling. These are discussed in more detail in the following sections.
Blocking hyperactive BMP signalling
Various approaches to blocking excessive BMP signalling have been examined. These include RNA interference approaches, blocking the activity of the mutant R206H ACVR1 receptor [
39,
40]. These approaches decreased the increased Smad1/5/8 signalling in mesenchymal cells derived from FOP patients or in muscle and bone cells overexpressing mutant ACVR1. Other approaches were directed at blocking ACVR1, which belongs to the family of BMP type I receptors using dorsomorphin or other synthetic molecules (LDN-193189) [
41,
42]. All of these approaches significantly reduced heterotopic ossification in mouse models of activated ACVR1 activity. However, these approaches block all three BMP type I receptors (BMPR1A/ALK3, BMPR1B/ALK6, ACVR1/ALK2), and thus do not confer specificity to the FOP mutations, possibly causing adverse effects.
Additional approaches include the inhibition of ligand binding to ACVR1. As such, ligand traps have been used, including ACVR2B-Fc and ACVR2A-Fc. These ligand traps are made of the extracellular domain of ACVR2B and ACVR2A and are fused to the constant region of IgG1 [
43]. Both molecules blocked heterotopic ossification in mice, indicating that one or more of the ligands that are blocked by these traps are required to induce heterotopic ossification in FOP [
21]. In another study, the BMP binding properties of transferrin receptor 2 were used as a ligand trap and also showed efficacy in reducing BMP-2-induced heterotopic ossification [
44]. However, this latter approach requires further validation in mouse models of FOP.
Finally, one of the most promising approaches in blocking BMP signalling is palovarotene, a retinoic acid receptor agonist that has been identified to indirectly block Smad1/5/8 signalling by activating the retinoid signalling pathway through RARγ receptors and inhibiting chondrogenesis and subsequent heterotopic endochondral ossification [
45,
46]. This agent showed great success in reducing the volume of new heterotopic ossifications in patients with FOP (>70%) in a phase II trial and is currently being tested in the MOVE trial (NCT03312634), a phase III clinical study. Even though palovarotene is generally well tolerated and showed the expected side effect profile of retinoids (e.g. mucocutaneous side effects, liver toxicity and abnormalities of serum lipid profiles), high levels of daily intraperitoneal treatment with palovarotene in juvenile animals carrying the R206H mutation suggested synovial joint overgrowth and long bone growth plate ablation [
47]. However, lower doses of an orally administered RARγ agonist inhibited HO formation in juvenile
Acvr1R206H/+ mice without these negative effects and additionally partially rescued growth plate defects induced by the FOP mutation [
46]. Long-term follow-ups and monitoring of patients will be necessary to fully evaluate the spectrum of effects of palovarotene on children and adults with FOP.
Inhibiting activin A signalling
The discovery that the R206H mutation in the ACVR1 gene leads to increased Smad1/5/8 responsiveness to activin A has led to the development of neutralising antibodies for activin A as a treatment for heterotopic ossification in FOP [
21]. This approach been tested in cells carrying the R206H ACVR1 mutation and in mouse models of FOP and showed great efficacy in reducing heterotopic ossification. Moreover, pSmad1/5/8 signalling by activin A has also been demonstrated in induced pluripotent stem cells (iPS) from FOP patients [
22]. This antibody is now also under investigation in clinical trials (REGN2477, NCT03188666). Activin A is important in several cell and tissues, including immune cells and the reproductive system; therefore, as for palovarotene, careful monitoring and patient follow-up will provide information about the long-term effects of this treatment.
Additional treatment strategies
Another strategy to block aberrant ACVR1 signalling is focused on mTOR signalling. This pathway was identified in a large chemical screen on FOP-iPSc, which shows increased chondrogenic and osteogenic potential in vitro [
48]. Blocking mTOR signalling with rapamycin prevented activin A‑induced chondrogenic and osteogenic differentiation and inhibited heterotopic bone formation in vivo [
37]. In addition, BYL719, an inhibitor of PI3Kα, inhibited Smad, Akt and mTOR signalling in cells or mice carrying ACVR1 R206H or Q207D mutations [
38]. Thus, these studies highlight the potential of blocking mTOR signalling for the treatment of FOP. However, mTOR signalling exerts direct functions on the skeleton that will need to be considered in the safety profile [
49].
Finally, besides directly targeting aberrant pathways, the inflammatory phase of FOP is also a critical mechanism preceding heterotopic ossification and thus has been examined as a therapeutic intervention point. In particular, lesions are frequently heavily infiltrated with monocytes, macrophages and mast cells that may enhance the inflammatory immune response. Mast cells and macrophages from FOP patients have been shown to have an increased production of cytokines (IL‑3, IL‑7, IL‑8 and IL-10) and chemokines (CCL5, CCR7 and CXCL10), as well as enhanced TGF-beta, NF-κB and MAPK signalling [
50,
51]. Depletion of macrophages and mast cells decreased heterotopic ossification in a mouse model of FOP, suggesting that this might be an effective approach to halting ossification in FOP patients [
51].
Considering that particularly the early, hypoxic inflammatory stages of FOP flare-ups involve—amongst others—activation of HIF1-alpha, c‑KIT, PDGFR-alpha and multiple MAP kinases, an off-label attempt has been made in a small series of seven children using the tyrosine kinase inhibitor imatinib, basically developed and approved for chronic myeloid leukaemia. Although not controlled study data, the results support the notion that treatment was well-tolerated and a decrease in the intensity of flare-ups was achieved in six of the patients [
52]. Another approach that is being made concerns the development of a small molecule inhibitor with sufficient specificity for the ALK2 receptor to control undesired receptor activity [
53].