Early clinical observations on the use of imatinib mesylate in FOP: A report of seven cases

Fibrodysplasia Ossificans Progressiva: A Case Report with Pseudo-Ankylosis of the Temporomandibular Joint

Fibrodysplasia ossificans progressiva (FOP) is a rare condition characterized by progressive heterotopic ossifications and congenital hallux valgus deformities. The common underlying genetic cause is an ACVR1 mutation, resulting in altered bone morphogenetic protein (BMP) regulation. Trauma and/or minor procedures aggravate the abnormal bony formation in soft tissues. This report presents a 3-year-old child with this condition who presented pseudo-ankylosis of the temporomandibular joint (TMJ) after minor craniofacial trauma. Abnormal ossification in the medial pterygoid muscle was identified as the causative abnormality for the presentation with trismus.

The HIF-1α and mTOR Pathways Amplify Heterotopic Ossification

Fibrodysplasia ossificans progressiva (FOP; MIM# 135100) is an ultra-rare congenital disorder caused by gain-of-function point mutations in the Activin receptor A type I (ACVR1, also known as ALK2) gene. FOP is characterized by episodic heterotopic ossification (HO) in skeletal muscles, tendons, ligaments, or other soft tissues that progressively causes irreversible loss of mobility. FOP mutations cause mild ligand-independent constitutive activation as well as ligand-dependent bone morphogenetic protein (BMP) pathway hypersensitivity of mutant ACVR1. BMP signaling is also a key pathway for mediating acquired HO. However, HO is a highly complex biological process involving multiple interacting signaling pathways. Among them, the hypoxia-inducible factor (HIF) and mechanistic target of rapamycin (mTOR) pathways are intimately involved in both genetic and acquired HO formation. HIF-1α inhibition or mTOR inhibition reduces HO formation in mouse models of FOP or acquired HO in part by de-amplifying the BMP pathway signaling. Here, we review the recent progress on the mechanisms of the HIF-1α and mTOR pathways in the amplification of HO lesions and discuss the future directions and strategies to translate the targeting of HIF-1α and the mTOR pathways into clinical interventions for FOP and other forms of HO.

Successful experience of tofacitinib treatment in patients with Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressive (FOP) is an ultra-rare genetic disorder that is caused by a mutation in the ACVR1 gene and provokes severe heterotopic ossification. Since flares of the disease are associated with inflammation, it is assumed that JAK inhibitors can control active FOP due to blocking multiple signaling pathways.

Discoidin domain receptor 2 regulates aberrant mesenchymal lineage cell fate and matrix organization

Extracellular matrix (ECM) interactions regulate both the cell transcriptome and proteome, thereby determining cell fate. Traumatic heterotopic ossification (HO) is a disorder characterized by aberrant mesenchymal lineage (MLin) cell differentiation, forming bone within soft tissues of the musculoskeletal system following traumatic injury. Recent work has shown that HO is influenced by ECM-MLin cell receptor signaling, but how ECM binding affects cellular outcomes remains unclear. Using time course transcriptomic and proteomic analyses, we identified discoidin domain receptor 2 (DDR2), a cell surface receptor for fibrillar collagen, as a key MLin cell regulator in HO formation. Inhibition of DDR2 signaling, through either constitutive or conditional Ddr2 deletion or pharmaceutical inhibition, reduced HO formation in mice. Mechanistically, DDR2 perturbation alters focal adhesion orientation and subsequent matrix organization, modulating Focal Adhesion Kinase (FAK) and Yes1 Associated Transcriptional Regulator and WW Domain Containing Transcription Regulator 1 (YAP/TAZ)-mediated MLin cell signaling. Hence, ECM-DDR2 interactions are critical in driving HO and could serve as a previously unknown therapeutic target for treating this disease process.

Monitoring and Management of Fibrodysplasia Ossificans Progressiva: Current Perspectives

Fibrodysplasia ossificans progressiva (FOP), sometimes known as myositis ossificans progressiva, is an ultra-rare disease in which bone is formed in muscular tissue, tendons and ligaments. This is known as heterotopic ossification (HO). FOP is caused by a heterozygous mutation in the highly conserved ACVR1/ALK2 gene which affects about 1 in 1.5–2 million individuals. At birth, patients with the predominant R206H mutation only exhibit a bilateral hallux valgus. During childhood, heterotopic bone formation develops in a typical pattern, affecting the axial muscles first before appendicular body parts are involved. HO can start spontaneously but is often elicited by soft tissue trauma or medical procedures. After soft tissue injury, an inflammatory process called a flare-up can start, followed by the formation of HO. HO leads to a limited range of motion, culminating in complete ankylosis of nearly all joints. As a result of HO surrounding the thorax, patients often suffer from thoracic insufficiency syndrome (TIS). TIS is the most common cause of a limited life expectancy for FOP patients, with a median life expectancy of 56 years. Management is focused on preventing soft-tissue injury that can provoke flare-ups. This includes prevention of iatrogenic damage by biopsies, intramuscular injections and surgery. Anti-inflammatory medication is often started when a flare-up occurs but has a poor basis of evidence. Several forms of potential treatment for FOP are being researched in clinical trials. Progression of the disease is monitored using CT and 18F-NaF PET/CT combined with functional assessments. Patients are regularly evaluated for frequently occurring complications such as restrictive lung disease. Here, we review the current management, monitoring and treatment of FOP.

Macrophages in heterotopic ossification: from mechanisms to therapy

Heterotopic ossification (HO) is the formation of extraskeletal bone in non-osseous tissues. It is caused by an injury that stimulates abnormal tissue healing and regeneration, and inflammation is involved in this process. It is worth noting that macrophages are crucial mediators of inflammation. In this regard, abundant macrophages are recruited to the HO site and contribute to HO progression. Macrophages can acquire different functional phenotypes and promote mesenchymal stem cell (MSC) osteogenic differentiation, chondrogenic differentiation, and angiogenesis by expressing cytokines and other factors such as the transforming growth factor-β1 (TGF-β1), bone morphogenetic protein (BMP), activin A (Act A), oncostatin M (OSM), substance P (SP), neurotrophin-3 (NT-3), and vascular endothelial growth factor (VEGF). In addition, macrophages significantly contribute to the hypoxic microenvironment, which primarily drives HO progression. Thus, these have led to an interest in the role of macrophages in HO by exploring whether HO is a "butterfly effect" event. Heterogeneous macrophages are regarded as the "butterflies" that drive a sequence of events and ultimately promote HO. In this review, we discuss how the recruitment of macrophages contributes to HO progression. In particular, we review the molecular mechanisms through which macrophages participate in MSC osteogenic differentiation, angiogenesis, and the hypoxic microenvironment. Understanding the diverse role of macrophages may unveil potential targets for the prevention and treatment of HO.

Clinical trials in skeletal dysplasia: a paradigm for treating rare diseases

BACKGROUND

Genetic skeletal dysplasia conditions (GSDs) account for 5% of all birth defects. Until recently, targeted treatments were only available for select few conditions; 1 however, opportunities arising from developments in molecular diagnostic technologies are now leading to unparalleled therapeutic advances. This review explores current GSD clinical trials, their challenges and the hopes for the future.

SOURCES OF DATA

A systematic literature search of relevant original articles, reviews and meta-analyses restricted to English was conducted using PubMed up to February 2020 regarding emerging GSD therapies.

AREAS OF AGREEMENT

We discuss current clinical trials for in achondroplasia, osteopetrosis, osteogenesis imperfecta, hypophosphataemic rickets, hypophosphatasia and fibrous ossificans progressiva.

AREAS OF CONTROVERSY

We explore challenges in GSD drug development from clinician input, cost-effectiveness and evidenced-based practice.

GROWING POINTS

We explore opportunities brought by earlier diagnosis, its treatment impact and the challenges of gene editing.

AREAS TIMELY FOR DEVELOPING RESEARCH

We horizon scan for future clinical trials.

Off-on-off-on use of imatinib in three children with fibrodysplasia ossificans progressiva

The compassionate use of available medications with unproven efficacy is often in conflict with their clinical evaluation in placebo-controlled clinical trials. For ultra-rare diseases where no approved treatments exist, such as fibrodysplasia ossificans progressiva (FOP), routine clinical trial enrollment for available medications may be difficult to achieve. Therefore adaptive methods of evaluation are often desirable. Off-on-off-on (O4) approaches offer an opportunity to rapidly assess the potential symptomatic efficacy and tolerability of a medication with a limited number of patients and may aid in the design of more focused clinical trials that are amenable to enrollment. Here we report three children with classic FOP who had recalcitrant flare-ups of the back and who had been treated with an O4 regimen of imatinib. In all three children, fewer flare-ups, decreased swelling and improved function with activities of daily living were reported by the parents and treating physician when the children were "on" imatinib than when they were "off" imatinib. The median time to improvement on imatinib was 2-3 weeks. The anecdotal O4 experience with imatinib reported here in three children with FOP who had recalcitrant flare-ups of the back supports the design of a brief placebo controlled trial to assess the potential efficacy of imatinib in reducing the symptoms in children with refractory flare-ups of FOP. A tool to prospectively measure and quantitate flare-up symptoms is presently being developed and validated and will be used for such a study.

Fibrodysplasia Ossificans Progressiva: A Challenging Diagnosis

Fibrodysplasia ossificans progressiva (FOP) is an ultrarare genetic condition characterized by extraskeletal bone formation. Most of the musculoskeletal characteristics of FOP are related to dysregulated chondrogenesis, with heterotopic ossification being the most typical feature. Activating mutations of activin receptor A type I (ACVR1), a bone morphogenetic protein (BMP) type I receptor, are responsible for the skeletal and nonskeletal features. The clinical phenotype is always consistent, with congenital bilateral hallux valgus malformation and early-onset heterotopic ossification occurring spontaneously or, more frequently, precipitated by trauma. Painful, recurrent soft-tissue swellings (flare-ups) precede localized heterotopic ossification that can occur at any location, typically affecting regions near the axial skeleton and later progressing to the appendicular bones. A diagnosis of FOP is suspected in a proband presenting with hallux valgus malformation, heterotopic ossification, and confirmed by the identification of a heterozygous pathogenic variant in the ACVR1/ALK2 gene. Avoiding unnecessary surgical procedures, prescribing prophylactic corticosteroids, preventing falls, and using protective headgear represent essential interventions for care management. Different classes of medications to contain acute inflammation flare-ups have been proposed, with high dose corticosteroids and nonsteroidal anti-inflammatory drugs usually utilized. Here, we report on two FOP patients, with typical clinical features summarizing the principal aspects of FOP, and we aim to provide comprehensive information outlining some unusual findings, possibly contributing to FOP’s definition and management.

Do Interactions of Vitamin D3 and BMP Signaling Hold Implications in the Pathogenesis of Fibrodysplasia Ossificans Progressiva?

PURPOSE OF REVIEW

Fibrodysplasia ossificans progressiva (FOP) is a debilitating rare disease known for episodic endochondral heterotopic ossification (HO) caused by gain-of-function mutations in ACVR1/ALK2. However, disease severity varies among patients with identical mutations suggesting disease-modifying factors, including diet, may have therapeutic implications. The roles of vitamin D₃ in calcium metabolism and chondrogenesis are known, but its effects on BMP signaling and chondrogenesis are less studied. This review attempts to assess the possibility of vitamin D's effects in FOP by exploring relevant intersections of VD₃ with mechanisms of FOP flares.

RECENT FINDINGS

In vitro and in vivo studies suggest vitamin D suppresses inflammation, while clinical studies suggest that vitamin D₃ protects against arteriosclerosis and inversely correlates with non-genetic intramuscular HO. However, the enhancement of chondrogenesis, BMP signaling, and possibly Activin A expression by vitamin D may be more relevant in FOP. There appears to be little potential for vitamin D to reduce HO in FOP, but testing the potential for excess vitamin D to promote HO may be warranted.

New Strategies in Neurogenic Heterotopic Ossification

The term neurogenic heterotopic ossification (NHO) is used to describe the pathological bone formation in soft tissues, due to spinal cord or brain injury. Commonly is presented with pain and stiffness of the affected joint. NHO affects the quality of life of these patients, delays their rehabilitation and therefore increases morbidity. The aim of this article is to emphasize pathophysiology mechanism and review new molecular treatments of heterotopic ossification (HO). It was demonstrated that potent treatment strategies are based on understanding the molecular mechanisms and aiming to inhibit the pathological process of the HO in various stages. New treatments are targeting several factors such as bone morphogenetic proteins (BMPs), retinoic acid receptors (RARs), hypoxic inhibitors (Hif1-inhibitors, rapamycin), free radical scavengers and immunological agents (imatinib). The endogenous pathways that lead to HO at molecular and cellular levels have been the aim of many studies in recent years. New treatment options for HO should be recommended due to the ineffectiveness of traditional older options, such as anti-inflammatory drugs and radiation, especially in the case of NHO.

Challenges and Opportunities for Drug Repositioning in Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is an ultrarare congenital disease that progresses through intermittent episodes of bone formation at ectopic sites. FOP patients carry heterozygous gene point mutations in activin A receptor type I ACVR1, encoding the bone morphogenetic protein (BMP) type I serine/threonine kinase receptor ALK2, termed activin receptor-like kinase (ALK)2. The mutant ALK2 displays neofunctional responses to activin, a closely related BMP cytokine that normally inhibits regular bone formation. Moreover, the mutant ALK2 becomes hypersensitive to BMPs. Both these activities contribute to enhanced ALK2 signalling and endochondral bone formation in connective tissue. Being a receptor with an extracellular ligand-binding domain and intrinsic intracellular kinase activity, the mutant ALK2 is a druggable target. Although there is no approved cure for FOP yet, a number of clinical trials have been recently initiated, aiming to identify a safe and effective treatment for FOP. Among other targeted approaches, several repurposed drugs have shown promising results. In this review, we describe the molecular mechanisms underlying ALK2 mutation-induced aberrant signalling and ectopic bone formation. In addition, we recapitulate existing in vitro models to screen for novel compounds with a potential application in FOP. We summarize existing therapeutic alternatives and focus on repositioned drugs in FOP, at preclinical and clinical stages.

Clinical Aspects and Current Therapeutic Approaches for FOP

Fibrodysplasia ossificans progressiva (FOP) is an extremely rare heritable disorder of connective tissues characterized by progressive heterotopic ossification in various skeletal sites. It is caused by gain-of-function mutations in the gene encoding activin A receptor type I (ACVR1)/activin-like kinase 2 (ALK2), a bone morphogenetic protein (BMP) type I receptor. Heterotopic ossification is usually progressive leading to severe deformities in the trunk and extremities. Early clinical diagnosis is important to prevent unnecessary iatrogenic harm or trauma. Clinicians should become aware of early detectable skeletal malformations, including great toe deformities, shortened thumb, neck stiffness associated with hypertrophy of the posterior elements of the cervical spine, multiple ossification centers in the calcaneus, and osteochondroma-like lesions of the long bones. Although there is presently no definitive medical treatment to prevent, stop or reverse heterotopic ossification in FOP, exciting advances of novel pharmacological drugs focusing on target inhibition of the activated ACVR1 receptor, including palovarotene, REGN 2477, rapamycin, and saracatinib, have developed and are currently in clinical trials.

Surgical management of bilateral hip fractures in a patient with fibrodysplasia ossificans progressiva treated with the RAR-γ agonist palovarotene: a case report

Background

Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare disorder marked by painful, recurrent flare-ups and heterotopic ossification (HO) in soft and connective tissues, which can be idiopathic or provoked by trauma, illness, inflammation, or surgery. There are currently no effective treatments for FOP, or for patients with FOP who must undergo surgery. Palovarotene, an investigational retinoic acid receptor-γ agonist, offers a potential avenue to prevent HO formation.

Case presentation

The patient is a 32 year-old male, who at age 29 enrolled in a study evaluating palovarotene to prevent HO formation in FOP. One year after starting palovarotene, he fell resulting in a left intertrochanteric fracture. He underwent intramedullary nailing of the femur shaft with screw placement at the distal femur. After surgery, he received palovarotene at 20 mg/day for 4 weeks, then 10 mg/day for 8 weeks. Imaging 12 weeks after surgery showed new bridging HO at the site of intramedullary rod insertion and distal screw.

Nine months after the left hip fracture, the patient had a second fall resulting in a subdural hematoma, left parietal bone fracture, and right intertrochanteric fracture. He underwent intramedullary nailing of the right hip, in a modified procedure which did not require distal screw placement. Palovarotene 20 mg/day was started at fracture occurrence and continued for 4 weeks, then reduced to 10 mg/day for 8 weeks. HO also formed near the insertion site of the intramedullary rod. No HO developed at the right distal intramedullary rod. After each fracture, the patient had prolonged recurrent flare-ups around the hips.

Conclusion

Surgery is only rarely considered in FOP due to the high risks of procedural complications and potential for inducing HO. This case emphasizes the risks of increased flare activity and HO formation from injury and surgery in patients with FOP. The efficacy of HO prevention by palovarotene could not be assessed; however, our observation that palovarotene can be administered in an individual with FOP following surgery with no negative impact on clinical fracture healing, osteointegration, or skin healing will help facilitate future trials testing the role of palovarotene as a therapy for HO.

Fibrodysplasia ossificans progressiva-a rare disease with distinctive features yet still a diagnostic challenge: A case report

RATIONALE

Fibrodysplasia ossificans progressiva (FOP) is rare genetic disease featuring progressive heterotopic ossification of soft tissues of the musculoskeletal system which leads to severe disability and premature death. Recognition of this disease is important since invasive diagnostic procedures can promote disease progression. However, despite its distinctive clinical manifestations, diagnosis can be difficult because of its rarity PATIENT CONCERNS:: A 20-year-old woman was referred to rheumatology clinic for management of "ankylosing spondylitis". The patent had begun to have hard subcutaneous nodules when she was 1 year old, and subsequently developed hip joint pain and flexion contractures of knees and hips leading to disability.

DIAGNOSES

Based on characteristic bilateral great toe deformities and radiographic images of ossification of soft tissues, a clinical diagnosis of FOP was made. This was confirmed by genetic test showing a heterozygous mutation (c.G617A) of the activin receptor 1A gene (ACVR1).

INTERVENTIONS

The patient was treated symptomatically and with supportive measures, and her condition remained stable.

LESSONS

Diagnosis of FOP can be difficult, despite its distinctive clinical manifestations, because of its rarity. Recognition of this disease is important to avoid invasive diagnostic procedures which can promote progression.

Genetics and future therapy prospects of fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare autosomal dominant genetic condition characterised by progressive extra-skeletal bone formation in connective tissues. Over time, heterotopic ossification entombs patients within a second skeleton, drastically impairing their mobility and autonomy. Mutations in the ACVR1 gene have been identified as the cause of FOP. The single nucleotide missense mutation in ACVR1, c.617G > A, causes a single amino acid substitution, p.R206H, and is found in >90% of all patients. Heterotopic bone formation in FOP mimics embryonic skeletal endochondral ossification, with cartilage forming after fibroproliferative tissue condensation as an intermediate stage prior to osteogenesis and tissue ossification. In contrast to normal embryonic endochondral ossification, heterotopic ossification in FOP involves an inflammatory phase that precedes cartilage and bone formation. New insights into the mechanisms of action of heterotopic bone formation in FOP have led to the discovery of new potential treatment targets including inhibitors of BMP signalling, activin A inhibitors, and mTOR inhibitors. This review summarises the current knowledge on mutations causing FOP, as well as the molecular basis of heterotopic ossification and the therapeutic options that result from these discoveries.

The evolving therapeutic landscape of genetic skeletal disorders

BACKGROUND

Rare bone diseases account for 5% of all birth defects yet very few have personalised treatments. Developments in genetic diagnosis, molecular techniques and treatment technologies however, are leading to unparalleled therapeutic advance. This review explores the evolving therapeutic landscape of genetic skeletal disorders (GSDs); the key conditions and there key differentials.

METHODS

A retrospective literature based review was conducted in December 2018 using a systematic search strategy for relevant articles and trials in Pubmed and clinicaltrials.gov respectively. Over 140 articles and 80 trials were generated for review.

RESULTS

Over 20 personalised therapies are discussed in addition to several novel disease modifying treatments in over 25 GSDs. Treatments discussed are at different stages from preclinical studies to clinical trials and approved drugs, including; Burosumab for X-linked hypophosphatemia, Palovarotene for Hereditary Multiple Exostoses, Carbamazepine for Metaphyseal Chondrodysplasia (Schmid type), Lithium carbonate and anti-sclerostin therapy for Osteoporosis Pseudoglioma syndrome and novel therapies for Osteopetrosis. We also discuss therapeutic advances in Achondroplasia, Osteogenesis Imperfecta (OI), Hypophosphotasia (HPP), Fibrodysplasia Ossificans Progressiva, and RNA silencing therapies in preclinical studies for OI and HPP.

DISCUSSION

It is an exciting time for GSD therapies despite the challenges of drug development in rare diseases. In discussing emerging therapies, we explore novel approaches to drug development from drug repurposing to in-utero stem cell transplants. We highlight the improved understanding of bone pathophysiology, genetic pathways and challenges of developing gene therapies for GSDs.

Fibrodysplasia ossificans progressiva: lessons learned from a rare disease

PURPOSE OF REVIEW

Fibrodysplasia ossificans progressiva (FOP) is an extremely rare and severely disabling autosomal dominant disease that is yet to be clearly understood. The purpose of this review is to present recent literature on pathophysiology, clinical features, diagnosis and treatment of FOP.

RECENT FINDINGS

FOP is characterized by congenital great toe deformity and progressive heterotopic ossifications in connective tissue. Heterotopic ossifications occur after painful flare-ups that can arise spontaneously or can be triggered by minor trauma. Each flare-up ultimately causes restriction of related-joint, and along with the others eventually leads to immobility. Death is usually caused by pulmonary complications because of chest wall involvement. The causative gene of FOP is activin A receptor type 1 (ACVR1), a bone morphogenetic protein-signalling component, which normally acts to inhibit osteoblastogenesis. The treatment of FOP is still preventive and supportive.

SUMMARY

Although there are still gaps in the underlying mechanism of FOP, effective treatment options, such as potential pharmacologic targets and cell-based therapies are promising for the future. Some of these were tested without a clinical trial setting, and are currently in the process of evidence-based research.

Elevated plasma RANTES in fibrodysplasia ossificans progressiva - A novel therapeutic target?

Fibrodysplasia ossificans progressiva (FOP) is a rare hereditary disease caused by a mutation in the intracellular domain of the activin A receptor type I and is characterized by episodes (flare-ups) of progressive heterotopic endochondral ossification (HO) in the soft tissues. The mutation alone is not sufficient for the occurrence of HO since flare-ups are triggered by inflammation and activation of the innate immune system. A number of cellular and humoral mediators have been implicated in animal and in vitro models. Observations in humans support the inflammatory nature of the condition, but data on the involved mediators are variable. We hypothesize that for induction of flare-ups in patients with FOP increase in at least one of the pro-inflammatory cytokines is both essential and sufficient to trigger the entire process of the inflammatory cells influx resulting in the novel ectopic bone formation and we suggest that C-C motif ligand 5 (CCL5), a pro-inflammatory chemokine also known as Regulated on activation, normal T-cell expressed and secreted (RANTES), might be the key candidate. CCL5 is a chemoattractant for all cellular types implicated in HO and is produced by the cells of the tissue microenvironment at the sites of HO as well as by the pro-inflammatory cellular mediators. CCL5 induces ossification in cultured human pluripotent mesenchymal cells (hMSCs) and in the primary culture of monocytes from FOP patients (but not from their healthy relatives), stimulation with lipopolysaccharide induces CCL5 expression. Finally, in a pilot study we used a panel of 23 cytokines and chemokines to screen the plasma samples of three subjects: a female patient with FOP during a flare-up; a female patient with hyperostosis corticalis generalisata (van Buchem disease), another rare disease characterized by excessive bone formation at the sites where it regularly occurs that does not include inflammatory events; and a healthy woman without bone disorders. There appeared a rather clear-cut signal of a 2-fold higher level of CCL5 in the FOP patient vs. the healthy subject and the van Buchem patient. Evaluation of the hypothesis would require an international prospective study, with main motivation being the lack of a conclusive treatment as the major unmet need in FOP. A treatment targeting CCL5 receptor already exists and is used in HIV-infected patients.

BMP and mTOR signaling in heterotopic ossification: Does their crosstalk provide therapeutic opportunities?

Heterotopic ossification (HO) refers to the pathological formation of ectopic bone in soft tissues, it occurs following severe trauma or in patients with a rare genetic disorder known as fibrodysplasia ossificans progressiva. The pathological process of HO formation is a two-step mechanism: inflammation and destruction of connective tissues, followed by bone formation. The latter is further subdivided into three stages: fibroproliferation/angiogenesis, chondrogenesis, and osteogenesis. Currently, therapeutic options for HO are limited. New potential therapeutics will most likely arise from a more detailed understanding of the signaling pathways implicated in each stage of ectopic bone formation and molecular targets that may be effective at both the early and late stages of HO. Bone morphogenetic protein (BMP) signaling is believed to play a key role in the overall HO process. Recently, the mammalian target of rapamycin (mTOR) signaling pathway has received attention as a critical pathway for chondrogenesis, osteogenesis, and HO. Inhibition of mTOR signaling has been shown to block trauma-induced and genetic HO. Intriguingly, recent studies have revealed crosstalk between mTOR and BMP signaling. Moreover, mTOR has emerged as a factor involved in the early hypoxic and inflammatory stages of HO. We will summarize the current knowledge of the roles of mTOR and BMP signaling in HO, with a particular focus on the crosstalk between mTOR and BMP signaling. We also discuss the activation of AMP activated protein kinase (AMPK) by the most widely used drug for type 2 diabetes, metformin, which exerts a dual negative regulatory effect on mTOR and BMP signaling, suggesting that metformin is a promising drug treatment for HO. The discovery of an mTOR-BMP signaling network may be a potential molecular mechanism of HO and may represent a novel therapeutic target for the pharmacological control of HO.

Therapeutic advances for blocking heterotopic ossification in fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease in which heterotopic bone forms in muscle and soft tissue, leading to joint dysfunction and significant disability. FOP is progressive and many patients are wheelchair-bound by the 3rd decade of life. FOP is caused by an activating mutation in the ACVR1 gene, which encodes the activin A Type 1 receptor. Aberrant signalling through this receptor leads to abnormal activation of the pSMAD 1/5/8 pathway and triggers the formation of bone outside of the skeleton. There is no curative therapy for FOP; however, exciting advances in novel therapies have developed recently. Here, we review the clinical and translational pharmacology of three drugs that are currently in clinical trials (palovarotene, REGN 2477 and rapamycin) as well as other emerging treatment strategies for FOP.

NF-κB/MAPK activation underlies ACVR1-mediated inflammation in human heterotopic ossification

BACKGROUND

Inflammation helps regulate normal growth and tissue repair. Although bone morphogenetic proteins (BMPs) and inflammation are known contributors to abnormal bone formation, how these pathways interact in ossification remains unclear.

METHODS

We examined this potential link in patients with fibrodysplasia ossificans progressiva (FOP), a genetic condition of progressive heterotopic ossification caused by activating mutations in the Activin A type I receptor (ACVR1/ALK2). FOP patients show exquisite sensitivity to trauma, suggesting that BMP pathway activation may alter immune responses. We studied primary blood, monocyte, and macrophage samples from control and FOP subjects using multiplex cytokine, gene expression, and protein analyses; examined CD14+ primary monocyte and macrophage responses to TLR ligands; and assayed BMP, TGF-β activated kinase 1 (TAK1), and NF-κB pathways.

RESULTS

FOP subjects at baseline without clinically evident heterotopic ossification showed increased serum IL-3, IL-7, IL-8, and IL-10. CD14+ primary monocytes treated with the TLR4 activator LPS showed increased CCL5, CCR7, and CXCL10; abnormal cytokine/chemokine secretion; and prolonged activation of the NF-κB pathway. FOP macrophages derived from primary monocytes also showed abnormal cytokine/chemokine secretion, increased TGF-β production, and p38MAPK activation. Surprisingly, SMAD phosphorylation was not significantly changed in the FOP monocytes/macrophages.

CONCLUSIONS

Abnormal ACVR1 activity causes a proinflammatory state via increased NF-κB and p38MAPK activity. Similar changes may contribute to other types of heterotopic ossification, such as in scleroderma and dermatomyositis; after trauma; or with recombinant BMP-induced bone fusion. Our findings suggest that chronic antiinflammatory treatment may be useful for heterotopic ossification.

Challenges in the treatment of fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP), is a rare autosomal dominant connective tissue disease with a prevalence of 1 in 2 million. It is characterized by congenital foot deformities and multiple heterotopic ossifications in fibrous tissue. It usually starts with painful soft tissue swellings occurring with attacks at the ages of three or four. The attacks develop spontaneously or after minor trauma, and gradually turn into heterotopic ossifications that cause joint limitations, growth defects, skeletal deformities and chronic pain. The average life expectancy is forthy, and most of the patients are lost due to pulmonary complications. FOP is often misdiagnosed as fibromatosis, desmoid tumour or cancer, bunion, myositis, arthritis and rheumatic diseases. After clinical suspicion, confirmatory genetic analysis should be used for the diagnosis. The treatment of FOP is currently supportive. An effective, proven method has not yet been established. Herein, we present an 18-year-old female patient with FOP who underwent different treatment modalities in a 5-year period. This case-based review reveals all available treatment approaches with at least 6-month follow-up for FOP in the literature.

Atypical Presentation of Fibrodysplasia Ossificans Progressiva: A Case Report and Review of Literature

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease characterized by widespread areas of abnormal bone formation in muscles, ligaments, tendons and joint capsules. Typically, the symptoms begin in the first decade of life with episodes of painful inflammatory soft tissue swellings. Gradually, there occurs restriction of motion at various joints, severely limiting the activities of daily living and the quality of life of such patients by the third decade of life. There is no definite cure available for the disease and the current treatment options target symptomatic and palliative management. We describe the case of a 10-year-old child who presented to our institute with a severe disability of upper limbs due to joint contractures along with several bony masses at various locations of the body but without having any prior complaints of painful soft tissue lesions or the characteristic flare-ups of the disease ever. Identification of typical soft tissue ossified masses in the specific anatomic pattern, along with the presence of short and malformed great toes helped us in reaching the diagnosis. Surgical procedures including biopsies should be strictly avoided in such patients to prevent triggering the development of more lesions, which occurred in our patient after inadvertent removal of the first swelling by an orthopaedic specialist.

Hypoxia-selective allosteric destabilization of activin receptor-like kinases: A potential therapeutic avenue for prophylaxis of heterotopic ossification

Heterotopic ossification (HO), the pathological extraskeletal formation of bone, can arise from blast injuries, severe burns, orthopedic procedures and gain-of-function mutations in a component of the bone morphogenetic protein (BMP) signaling pathway, the ACVR1/ALK2 receptor serine-threonine (protein) kinase, causative of Fibrodysplasia Ossificans Progressiva (FOP). All three ALKs (-2, -3, -6) that play roles in bone morphogenesis contribute to trauma-induced HO, hence are well-validated pharmacological targets. That said, development of inhibitors, typically competitors of ATP binding, is inherently difficult due to the conserved nature of the active site of the 500+ human protein kinases. Since these enzymes are regulated via inherent plasticity, pharmacological chaperone-like drugs binding to another (allosteric) site could hypothetically modulate kinase conformation and activity. To test for such a mechanism, a surface pocket of ALK2 kinase formed largely by a key allosteric substructure was targeted by supercomputer docking of drug-like compounds from a virtual library. Subsequently, the effects of docked hits were further screened in vitro with purified recombinant kinase protein. A family of compounds with terminal hydrogen-bonding acceptor groups was identified that significantly destabilized the protein, inhibiting activity. Destabilization was pH-dependent, putatively mediated by ionization of a histidine within the allosteric substructure with decreasing pH. In vivo, nonnative proteins are degraded by proteolysis in the proteasome complex, or cellular trashcan, allowing for the emergence of therapeutics that inhibit through degradation of over-active proteins implicated in the pathology of diseases and disorders. Because HO is triggered by soft-tissue trauma and ensuing hypoxia, dependency of ALK destabilization on hypoxic pH imparts selective efficacy on the allosteric inhibitors, providing potential for safe prophylactic use.

The Horizon of a Therapy for Rare Genetic Diseases: A "Druggable" Future for Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic condition characterized by progressive extra-skeletal ossification leading to cumulative and severe disability. FOP has an extremely variable and episodic course and can be induced by trauma, infections, iatrogenic harms, immunization or can occur in an unpredictable way, without any recognizable trigger. The causative gene is ACVR1, encoding the Alk-2 type I receptor for bone morphogenetic proteins (BMPs). The signaling is initiated by BMP binding to a receptor complex consisting of type I and II molecules and can proceed into the cell through two main pathways, a canonical, SMAD-dependent signaling and a p38-mediated cascade. Most FOP patients carry the recurrent R206H substitution in the receptor Glycine-Serine rich (GS) domain, whereas a few other mutations are responsible for a limited number of cases. Mutations cause a dysregulation of the downstream BMP-dependent pathway and make mutated ACVR1 responsive to a non-canonical ligand, Activin A. There is no etiologic treatment for FOP. However, many efforts are currently ongoing to find specific therapies targeting the receptor activity and the downstream aberrant pathway at different levels or targeting cellular components and/or processes that are important in modifying the local environment leading to bone neo-formation.

Fibrodysplasia ossificans progressiva: Basic understanding and experimental models

Fibrodysplasia ossificans progressive (FOP) is an extremely rare autosomal dominant disorder characterized by congenital malformations of the great toes and progressive heterotopic ossification that can induce a disabling second skeleton. Spontaneously occurring flare-ups can cause inflammatory soft tissue to swell, followed by progressive and disabling heterotopic endochondral ossification. FOP is very rare, with an estimated incidence of one case per two million individuals. There is no definitive treatment for FOP, but the longevity of patients with FOP can be extended by early diagnosis and appropriate prevention of flares-up. Some promising treatment strategies and targets have recently been reported. The current review describes the classical phenotype and genotype of FOP, useful methods of diagnosing the condition, therapeutic approaches and commonly used drugs, and experimental models used to study this disease.