Fibrodysplasia ossificans progressiva: a human genetic disorder of extraskeletal bone formation, or--how does one tissue become another?

Current and emerging treatment modalities for fibrodysplasia ossificans progressiva

INTRODUCTION

Heterotopic ossification (HO), acquired or hereditary, is a diverse pathological condition defined by the production of extraskeletal bone in muscles, soft tissues, and connective tissues. Acquired HO is relatively prevalent and develops mostly in response to trauma, although its etiology is unknown. Genetic forms provide insight into the pathobiological mechanisms of this disorder. Fibrodysplasia ossificans progressiva (FOP) is a rare hereditary form of HO that can have a significant impact on affected individuals. FOP steadily weakens affected subjects and increases their risk of death.

AREAS COVERED

The U.S. Food and Drug Administration has recently approved the retinoid palovarotene as the first compound to treat heterotopic ossification in patients with FOP. This review provides a comprehensive overview of current and potential future pharmacotherapeutic options and their modes of action. The online databases PubMed, Cochrane Library, Web of Science, and ClinicalTrials.gov were searched using the terms 'heterotopic ossification' and 'fibrodysplasia ossificans progressiva' or synonyms, with a special focus over the last 5 years of publications.

EXPERT OPINION

Approval of palovarotene, as the first retinoid indicated for reduction in the volume of new HO, may revolutionize the therapeutic landscape. However, long-term safety and efficacy data for palovarotene are currently lacking.

Animal models of tendon calcification: Past, present, and future

Tendon calcification is a common clinical condition that frequently occurs as a complication after tendon injury and surgery, or as an expression of fibrodysplasia ossificans progressiva. This condition can be referred to by various names in clinical practice and literature, including tendon ossification, tendon mineralization, heterotopic ossification, and calcific tendonitis. The exact pathogenesis of tendon calcification remains uncertain, but current mainstream research suggests that calcification is mostly cell mediated. To further elucidate the pathogenesis of tendon calcification and to better simulate the overall process, selecting appropriate experimental animal models is important. Numerous animal models have been utilized in various clinical studies, each with its own set of advantages and limitations. In this review, we have discussed the advancements made in research on animal models of tendon calcification, with a focus on the selection of experimental animals, the sites of injury in these models, and the methods employed for modeling.

The genetic background determines material-induced bone formation through the macrophage-osteoclast axis

Osteoinductive materials are characterized by their ability to induce bone formation in ectopic sites. Thus, osteoinductive materials hold promising potential for repairing bone defects. However, the mechanism of material-induced bone formation remains unknown, which limits the design of highly potent osteoinductive materials. Here, we demonstrated a genetic background link among macrophage polarization, osteoclastogenesis and material-induced bone formation. The intramuscular implantation of an osteoinductive material in FVB/NCrl (FVB) mice resulted in more M2 macrophages at week 1, more osteoclasts at week 2 and increased bone formation after week 4 compared with the results obtained in C57BL/6JOlaHsd (C57) mice. Similarly, in vitro, with a greater potential to form M2 macrophages, monocytes derived from FVB mice formed more osteoclasts than those derived from C57 mice. A transcriptomic analysis identified Csf1, Cxcr4 and Tgfbr2 as the main genes controlling macrophage-osteoclast coupling, which were further confirmed by related inhibitors. With such coupling, macrophage polarization and osteoclast formation of monocytes in vitro successfully predicted in vivo bone formation in four other mouse strains. Considering material-induced bone formation as an example of acquired heterotopic bone formation, the current findings shed a light on precision medicine for both bone regeneration and the treatment of pathological heterotopic bone formation.

The natural history of fibrodysplasia ossificans progressiva: A prospective, global 36-month study

PURPOSE

We report the first prospective, international, natural history study of the ultra-rare genetic disorder fibrodysplasia ossificans progressiva (FOP). FOP is characterized by painful, recurrent flare-ups, and disabling, cumulative heterotopic ossification (HO) in soft tissues.

METHODS

Individuals aged ≤65 years with classical FOP (ACVR1^R206H variant) were assessed at baseline and over 36 months.

RESULTS

In total, 114 individuals participated; 33 completed the study (mean follow up: 26.8 months). Median age was 15.0 (range: 4-56) years; 54.4% were male. During the study, 82 (71.9%) individuals reported 229 flare-ups (upper back: 17.9%, hip: 14.8%, shoulder: 10.9%). After 84 days, 14 of 52 (26.9%) imaged flare-ups had new HO at the flare-up site (mean new HO volume: 28.8 × 10³ mm³). Mean baseline low-dose whole-body computed tomography (excluding head) HO volume was 314.4 × 10³ mm³; lowest at 2 to <8 years (68.8 × 10³ mm³) and increasing by age (25-65 years: 575.2 × 10³ mm³). The mean annualized volume of new HO was 23.6 × 10³ mm³/year; highest at 8 to <15 and 15 to <25 years (21.9 × 10³ and 41.5 × 10³ mm³/year, respectively) and lowest at 25 to 65 years (4.6 × 10³ mm³/year).

CONCLUSION

Results from individuals receiving standard care for up to 3 years in this natural history study show the debilitating effect and progressive nature of FOP cross-sectionally and longitudinally, with greatest progression during childhood and early adulthood.

Fibrodysplasia Ossificans Progressiva and Pregnancy: A Case Series and Review of the Literature

Objective

To evaluate maternal and fetal outcomes in pregnant patients with fibrodysplasia ossificans progressiva (FOP; OMIM#135100), an ultrarare genetic disorder characterized by progressive heterotopic ossification of soft tissues and cumulative disability.

Methods

This is a retrospective case series of three patients with FOP who were admitted to Grady Memorial Hospital in Atlanta, Georgia, from to February 2011 to July 2021.

Results

Three women delivered preterm infants at our institution. These cases posed unique anesthetic and obstetric technical challenges, particularly when securing the airway and performing cesarean delivery. Importantly, each patient received perioperative glucocorticoids for prevention of further heterotopic ossification.

Conclusion

FOP is a unique clinical diagnosis encountered by obstetricians and requires multidisciplinary management for optimal outcomes.

Long-term radial extracorporeal shock wave therapy for neurogenic heterotopic ossification after spinal cord injury: A case report

Context: Heterotopic ossification is characterized by abnormal growth of bone in soft tissues. Neurogenic heterotopic ossification is also closely related to central nervous system injuries and has been reported to respond to radial extracorporeal shock wave therapy.Findings: In this case, a radial extracorporeal shock wave therapy (five times per week, lasted for almost one year) was applied to a patient with neurogenic heterotopic ossification on the left hip as a result of spinal cord injury. Throughout the treatment session, the heterotopic ossification lesion was gradually diminished, associated with the increase in joint range of motion, pain mitigation and decrease in serum alkaline phosphatase level.Conclusion/clinical relevance: Long-term radial extracorporeal shock wave therapy offers a promising therapeutic alternative for neurogenic heterotopic ossification.

Heterotopic Ossification After Revision Carpal Tunnel Release Causing Mixed Ulnar and Median Compression Neuropathy

We report a case of heterotopic ossification formation 6 years after a revision carpal tunnel release in a 46-year-old woman, causing new-onset mixed ulnar and median nerve compression symptoms. The patient underwent excision of the heterotopic ossification mass along with decompression of the median and ulnar nerves, and postoperative radiation. Four years after treatment, the patient was completely asymptomatic with full range of motion in her hand and wrist.

Modeling the ACVR1R206H mutation in human skeletal muscle stem cells

Abnormalities in skeletal muscle repair can lead to poor function and complications such as scarring or heterotopic ossification (HO). Here, we use fibrodysplasia ossificans progressiva (FOP), a disease of progressive HO caused by ACVR1^R206H (Activin receptor type-1 receptor) mutation, to elucidate how ACVR1 affects skeletal muscle repair. Rare and unique primary FOP human muscle stem cells (Hu-MuSCs) isolated from cadaveric skeletal muscle demonstrated increased extracellular matric (ECM) marker expression, showed skeletal muscle-specific impaired engraftment and regeneration ability. Human induced pluripotent stem cell (iPSC)-derived muscle stem/progenitor cells (iMPCs) single-cell transcriptome analyses from FOP also revealed unusually increased ECM and osteogenic marker expression compared to control iMPCs. These results show that iMPCs can recapitulate many aspects of Hu-MuSCs for detailed in vitro study; that ACVR1 is a key regulator of Hu-MuSC function and skeletal muscle repair; and that ACVR1 activation in iMPCs or Hu-MuSCs may contribute to HO by changing the local tissue environment.

Desloratadine inhibits heterotopic ossification by suppression of BMP2-Smad1/5/8 signaling

Heterotopic ossification (HO) is a pathological condition in which ectopic bone forms within soft tissues such as skeletal muscle. Human platelet-derived growth factor receptor α positive (PDGFRα+) cells, which were proved to be the original cells of HO were incubated in osteogenic differentiation medium with Food and Drug Administration-approved compounds. Alkaline phosphatase activity was measured as a screening to inhibit osteogenic differentiation. For the compounds which inhibited osteogenic differentiation of PDGFRα+ cells, we examined dose dependency of its effect using alizarin red S staining and its cell toxicity using WST-8. In addition, regulation of bone morphogenetic proteins (BMP)-Smad signaling which is the major signal of osteogenic differentiation was investigated by Western blotting to elucidate the mechanism of osteogenesis inhibitory effect by the compound. In vivo experiment, complete transverse incision of Achilles tendons in mice was made and mice were fed the compound by mixing with drinking water after operation. Ten weeks after operation, we assessed and quantified HO by micro-computed tomography scan. Intriguingly, we discovered desloratadine inhibited osteogenic differentiation of PDGFRα+ cells using the drug repositioning method. Desloratadine inhibited osteogenic differentiation of the cells dose dependently without cell toxicity. Desloratadine suppressed phosphorylation of Smad1/5/8 induced by BMP2 in PDGFRα+ cells. In Achilles tenotomy mice model, desloratadine treatment significantly inhibited ectopic bone formation compared with control. In conclusion, we discovered desloratadine inhibited osteogenic differentiation using human PDGFRα+ cells and proved its efficacy using Achilles tenotomy ectopic bone formation model in vivo. Our study paved the way to inhibit HO in early clinical use because of its guaranteed safety.

Diagnostic Value of Magnetic Resonance Imaging in Fibrodysplasia Ossificans Progressiva

Using [¹⁸F] Sodium Fuoride (NaF) Positron Emission Tomography (PET) it is not only possible to identify the ossifying potency of a flare-up, but also to identify an asymptomatic chronic stage of fibrodysplasia ossificans progressiva (FOP). The purpose of this study was to investigate the diagnostic role of a more widely available imaging modality, Magnetic Resonance Imaging (MRI), which is of special interest for studies in pediatric FOP patients. MRI and [¹⁸F]NaF PET/CT images at time of inclusion and subsequent follow-up CT scans of 4 patients were analyzed retrospectively. Presence, location, and intensity of edema identified by MRI were compared with activity on [¹⁸F]NaF PET. Occurrence or progression of heterotopic ossification (HO) was examined on the follow-up CT images. Thirteen different lesions in various muscle groups were identified: five lesions with only edema, five lesions with both edema and increased [¹⁸F]NaF uptake, one lesion with only increased [¹⁸F]NaF uptake, and two lesions with neither edema nor uptake of [¹⁸F]NaF. Mild edema, found in three lesions, was present at asymptomatic sites, which did not show increased [¹⁸F] NaF uptake or progression of HO on consecutive CT images. Moderate edema was found in three symptomatic lesions, with increased [¹⁸F]NaF on PET and progression of HO on CT. Severe edema was identified in four lesions. Interestingly, two of these lesions did not develop HO during follow-up; one of these two even gave obvious symptoms of a flare-up. MRI can identify whether symptoms are the result of an acute flare-up by the presence of moderate to severe edema. The occurrence of severe edema on MRI was not always related to an ossifying lesion. The additional diagnostic value of MRI requires further investigation, but MRI does not seem to fully replace the diagnostic characteristics of [¹⁸F]NaF PET/CT in FOP. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Bioinformatics Analysis of the Molecular Mechanism of Late-Stage Heterotopic Ossification

Background

Heterotopic ossification (HO) is a common disease happened in soft tissues after injury. The present study utilized the bioinformatics method to analyze the HO samples in a mouse burn/tenotomy-induced HO model to identify the possible key points and treatment targets.

Methods

The transcriptome profiles of GSE126118 were obtained from the Gene Expression Omnibus (GEO) database. The study was based on a mouse burn/tenotomy-induced HO model, and 2 tenotomy samples and 3 uninjured contralateral hindlimb tendon samples were collected at 3 weeks after injury for further analysis. The transcripts per million approach was performed for background correction and normalization; then, the differentially expressed genes (DEGs) were detected using the limma R package with the settings p < 0.01 and ∣log2FC∣ > 2.0. The Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment, and the protein-protein interaction (PPI) network analysis were performed with the detected DEGs.

Results

A total of 74 DEGs were upregulated, and 159 DEGs were downregulated between the tenotomy and uninjured tendon group. Pathway and process enrichment analyses demonstrated that the upregulated DEGs were mainly associated with terms related to ECM remodeling, ossification, angiogenesis, inflammation, etc., and the downregulated DEGs were mainly associated with oxidative phosphorylation, metabolic process, etc.

Conclusion

The results of GO, KEGG, and PPI network analyses suggested that the ECM remodeling, ossification, angiogenesis, and inflammation processes were markedly upregulated in the tenotomy site. And the oxidative phosphorylation and metabolic processes were markedly downregulated. These findings provide valuable clues for highlighting the characteristics of late-stage HO and investigating possible treatments.

Pediatric syndromes with noncraniofacial anomalies impacting the airways

Syndromes with noncraniofacial abnormalities can be a real challenge in terms of airway management. The key to success is effective preparation, presence of personnel with expertise in difficult pediatric airway management, regular training and familiarity with difficult intubation equipment, and teamwork. Considering that there are a very large number of syndromes, with variable phenotypic expression, the management strategy of every case will be dictated by the anatomical and functional airway as assessed on physical examination and subsidiary examinations before induction of anesthesia.

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.

Early stage fibrodysplasia ossificans progressiva: A case report

Fibrodysplasia ossificans progressiva is a very rare autosomal dominant genetic connective tissue disease with a progressive ectopic ossification of muscle (intramuscular) or perimuscular connective tissue such as tendons or joint capsules. The osseous masses produced will form bridges that abnormally connect sections of the skeleton, causing disfiguration and normal motor function inhibition. We reported a 5-year-old girl with multiple hard nodules on the back region which initially present as a painful soft mass on the posterior neck region. As the pain subsided, the mass hardened and also appeared in other parts of her back. We decided not to do a biopsy or excisional surgery to prevent flaring up of the disease. Early diagnosis prevents catastrophic diagnostic and treatment procedures. The progressive nature of this disease is difficult to stop but we should delay it as much as possible by preventing muscle trauma, giving disease modifying agent and long-term physiotherapy to counter further disabilities which will eventually develop.

Heterotopic Ossification: A Comprehensive Review

Heterotopic ossification (HO) is a diverse pathologic process, defined as the formation of extraskeletal bone in muscle and soft tissues. HO can be conceptualized as a tissue repair process gone awry and is a common complication of trauma and surgery. This comprehensive review seeks to synthesize the clinical, pathoetiologic, and basic biologic features of HO, including nongenetic and genetic forms. First, the clinical features, radiographic appearance, histopathologic diagnosis, and current methods of treatment are discussed. Next, current concepts regarding the mechanistic bases for HO are discussed, including the putative cell types responsible for HO formation, the inflammatory milieu and other prerequisite “niche” factors for HO initiation and propagation, and currently available animal models for the study of HO of this common and potentially devastating condition. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

ACVR1R206H FOP mutation alters mechanosensing and tissue stiffness during heterotopic ossification

An activating bone morphogenetic proteins (BMP) type I receptor ACVR1 (ACVR1^R206H) mutation enhances BMP pathway signaling and causes the rare genetic disorder of heterotopic (extraskeletal) bone formation fibrodysplasia ossificans progressiva. Heterotopic ossification frequently occurs following injury as cells aberrantly differentiate during tissue repair. Biomechanical signals from the tissue microenvironment and cellular responses to these physical cues, such as stiffness and rigidity, are important determinants of cell differentiation and are modulated by BMP signaling. We used an Acvr1^R206H/+ mouse model of injury-induced heterotopic ossification to examine the fibroproliferative tissue preceding heterotopic bone and identified pathologic stiffening at this stage of repair. In response to microenvironment stiffness, in vitro assays showed that Acvr1^R206H/+ cells inappropriately sense their environment, responding to soft substrates with a spread morphology similar to wild-type cells on stiff substrates and to cells undergoing osteoblastogenesis. Increased activation of RhoA and its downstream effectors demonstrated increased mechanosignaling. Nuclear localization of the pro-osteoblastic factor RUNX2 on soft and stiff substrates suggests a predisposition to this cell fate. Our data support that increased BMP signaling in Acvr1^R206H/+ cells alters the tissue microenvironment and results in misinterpretation of the tissue microenvironment through altered sensitivity to mechanical stimuli that lowers the threshold for commitment to chondro/osteogenic lineages.

Fibrodysplasia ossificans progressiva: a case report

Fibrodysplasia Ossificans Progressiva is a rare debilitating disorder of the musculoskeletal system affecting one in two million individuals. It is characterized by progressive extraskeletal ossification of soft tissues resulting in the original skeleton being encased in unyielding new bone leading to disability and ultimately death from cardiorespiratory failure. The present case brings to light the delays and potential pitfalls in diagnosis as a result of the rarity of the condition.

Diffuse Intrinsic Pontine Glioma: Time for Cautious Optimism

Diffuse intrinsic pontine glioma is a lethal brain cancer that arises in the pons of children. The median survival for children with diffuse intrinsic pontine glioma is less than 1 year from diagnosis, and no improvement in survival has been realized in more than 30 years. Currently, the standard of care for diffuse intrinsic pontine glioma is focal radiation therapy, which provides only temporary relief. Recent genomic analysis of tumors from biopsies and autopsies, have resulted in the discovery of K27M H3.3/H3.1 mutations in 80% and ACVR1 mutations in 25% of diffuse intrinsic pontine gliomas, providing renewed hope for future success in identifying effective therapies. In addition, as stereotactic tumor biopsies at diagnosis at specialized centers have been demonstrated to be safe, biopsies have now been incorporated into several prospective clinical trials. This article summarizes the epidemiology, clinical presentation, diagnosis, prognosis, molecular genetics, current treatment, and future therapeutic directions for diffuse intrinsic pontine glioma.

High-throughput screening for modulators of ACVR1 transcription: discovery of potential therapeutics for fibrodysplasia ossificans progressiva

The ACVR1 gene encodes a type I receptor of bone morphogenetic proteins (BMPs). Activating mutations in ACVR1 are responsible for fibrodysplasia ossificans progressiva (FOP), a rare disease characterized by congenital toe malformation and progressive heterotopic endochondral ossification leading to severe and cumulative disability. Until now, no therapy has been available to prevent soft-tissue swelling (flare-ups) that trigger the ossification process. With the aim of finding a new therapeutic strategy for FOP, we developed a high-throughput screening (HTS) assay to identify inhibitors of ACVR1 gene expression among drugs already approved for the therapy of other diseases. The screening, based on an ACVR1 promoter assay, was followed by an in vitro and in vivo test to validate and characterize candidate molecules. Among compounds that modulate the ACVR1 promoter activity, we selected the one showing the highest inhibitory effect, dipyridamole, a drug that is currently used as a platelet anti-aggregant. The inhibitory effect was detectable on ACVR1 gene expression, on the whole Smad-dependent BMP signaling pathway, and on chondrogenic and osteogenic differentiation processes by in vitro cellular assays. Moreover, dipyridamole reduced the process of heterotopic bone formation in vivo. Our drug repositioning strategy has led to the identification of dipyridamole as a possible therapeutic tool for the treatment of FOP. Furthermore, our study has also defined a pipeline of assays that will be useful for the evaluation of other pharmacological inhibitors of heterotopic ossification.

Summary: We describe the identification of dipyridamole as a potential therapeutic tool for FOP, through a series of in vitro and in vivo assays to screen and validate FDA-approved compounds.

Osteochondromas in fibrodysplasia ossificans progressiva: a widespread trait with a streaking but overlooked appearance when arising at femoral bone end

Metaphyseal bony outgrowths are a well-recognized feature of fibrodysplasia ossificans progressiva (FOP) phenotype, but its genuine frequency, topographic distribution, morphological aspect, and potential implications are not fully established. To better ascertain the frequency and characteristics of osteocartilaginous exostoses in FOP disease, we conducted a cross-sectional radiological study based on all the traceable cases identified in a previous comprehensive national research. Metaphyseal exostoses were present in all the 17 cases of FOP studied. Although most often arising from the distal femoral (where metaphyseal exostoses adopt a peculiar not yet reported appearance) and proximal tibial bones, we have found that they are not restricted to these areas, but rather can be seen scattered at a variety of other skeletal sites. Using nuclear magnetic resonance imaging, we show that these exophytic outgrowths are true osteochondromas. As a whole, these results are in agreement with data coming from the literature review. Our study confirms the presence of metaphyseal osteochondromas as a very frequent trait of FOP phenotype and an outstanding feature of its anomalous skeletal developmental component. In line with recent evidences, this might imply that dysregulation of BMP signaling, in addition to promoting exuberant heterotopic ossification, could induce aberrant chondrogenesis and osteochondroma formation. Unveiling the molecular links between these physiopathological pathways could help to illuminate the mechanisms that govern bone morphogenesis.

Fibrodysplasia ossificans progressiva: diagnosis in primary care

OBJECTIVE

To show that it is possible to diagnose fibrodysplasia ossificans progressiva in the primary health care.

CASE DESCRIPTION

A 10-year-old female patient that has developed, since the age of 4, progressive stiffness of the joints and spine and ossification of soft tissues, often associated with trauma. The hallux valgus deviation of both toes was present from birth. X-ray showed the presence of heterotopic ossification.

COMMENTS

This disease is likely to be diagnosed with the resources available in primary health care, since it is based on clinical findings. Currently, there is no cure for this disease, but high doses of corticosteroids and the use of nonsteroidal anti-inflammatory drugs, which are available in the primary care level, may limit the development of new calcifications and mitigate the pain, improving the quality of life of these patients.

Glast-expressing progenitor cells contribute to heterotopic ossification

Heterotopic ossification (HO), acquired or hereditary, is the formation of true bone outside the normal skeleton. Although the lineages of cells contributing to bone formation during normal development are well defined, the precise lineages of cells that contribute to HO are not clear. This study utilized Cre-lox based genetic lineage tracing to examine the contribution to HO of cells that expressed either FoxD1 or Glast. Both lineages contributed broadly to different normal tissues, and FoxD1-cre labeled cells contributed to normal bone formation. Despite the similarity in labeling patterns of normal tissues, and the significant contribution of FoxD1-cre labeled cells to normal bone, only Glast-creERT labeled progenitors contributed significantly to HO at all stages, suggesting that the cell populations that normally contribute to physiological bone formation, such as the Foxd1-cre labeled cells, may not participate in pathological HO. Further, identification of Glast-expressing cells as precursors that give rise to HO should help with the molecular targeting of this population both for the prevention and for the treatment of HO.