Challenges and Opportunities for Drug Repositioning in Fibrodysplasia Ossificans Progressiva

Advancements in mechanisms and drug treatments for fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder characterized by congenital bilateral malformation of the large toe and progressive, extensive, and irreversible heterotopic ossification (HO) of soft tissues throughout the body, leading to severe disabilities. FOP is caused primarily by mutations in activin A receptor type 1 (ACVR1), also known as activin-like kinase 2 (ALK2), which encodes a receptor belonging to the bone morphogenetic protein (BMP) type I family. However, the continuous and complex process of HO in FOP is not yet fully understood, which has impeded the development of therapeutic drugs. Despite surgical removal of HO, which often results in recurrence and expansion of ossification, there is currently no definitive drug treatment available to completely prevent, halt, or reverse the progression of HO in FOP. Currently, researchers are intensively studying the pathogenesis of FOP at various stages and developing promising drug candidates, including saracatinib, palovarotene, and rapamycin. This review provides an overview of progress in understanding the mechanism of FOP and the development of therapeutic drugs, with the goal of providing insights for further research and the development of new treatment methods.

Interplay between ALK2R206H mutant receptor and autophagy signaling regulates receptor stability and its chondrogenic functions

Heterozygous mutations in the Bone morphogenetic protein (BMP) type I receptor ACVR1, encoding activin-like kinase 2 (ALK2), underlie all cases of the rare genetic musculoskeletal disorder Fibrodysplasia Ossificans Progressiva (FOP). The most commonly found mutant ALK2 p.R206H receptor variant exhibits loss of auto inhibition of BMP signaling and can be activated by Activins, while wild-type receptors remain unresponsive. Consequently, the downstream chondrogenic signaling is enhanced, thus driving heterotopic ossification within soft connective tissues. Despite several investigational treatments being evaluated in clinical trials, no cure for FOP exists today. The cellular and molecular mechanisms underlying disease progression are still being deciphered. In this study, we show a close interplay between the mutant ALK2R206H receptor signaling and dysregulation of the autophagic flux triggered by hypoxia. Mechanistically, reduced autophagic flux correlates with increased stability of ALK2R206H, resulting in sustained signaling. Of note, we demonstrated that Rapamycin, under clinical investigation as a treatment for FOP, inhibits chondrogenic differentiation in an autophagy-dependent manner. Consistently, other pharmacological autophagy inducers, like Spermidine, can reduce ALK2R206H driven chondrogenic differentiation in vitro. These results were verified in FOP patient-derived cells. In conclusion, this study shows that aberrant autophagic flux mediates sustained ALK2R206H signaling, introducing a novel druggable target in FOP by reactivating autophagy.

Fibrodysplasia ossificans progressiva complicated with post traumatic and infectious myositis ossificans in masseter: A case report

RATIONALE

Myositis ossificans (MO) is characterized by benign heterotopic ossificans in soft tissues like muscles, which can be classified into nonhereditary MO and fibrodysplasia ossificans progressiva (FOP). Nonhereditary MO is characterized by ossification of the soft tissues after acute or repetitive trauma, burns, or surgical intervention. FOP is a rare and crippling disease characterized by congenital malformation of the big toe and heterotopic ossification in muscle. The majority of FOP's musculoskeletal traits are associated with dysregulated chondrogenesis. The diagnosis is mainly based on clinical manifestation, imaging examination, and genetic analysis. There is still no effective treatment to cure or slow its progression. The best approach remains early diagnosis, conservative drug treatment, and injury prevention to avoid local ossification.

PATIENT CONCERNS

A 34-year-old male presented at our hospital because of trismus caused by ossification of the masseter muscle. In addition, he had serious stiffness and multiple bony masses throughout the body, which led to limited movement.

DIAGNOSES

Based on the clinical manifestation of movement restriction, characteristic radiographic images of ossification of soft tissues, the genetic test showing a heterozygous molecule (c.974G > C, p.G325A) of the activin A receptor type I, the patient was diagnosed as FOP complicated with localized MO in masseter after trauma and infection.

INTERVENTIONS

The patient underwent the surgical resection of ossification in the masseter muscle, he was instructed to insist on mouth-opening exercises and take glucocorticoids and nonsteroidal anti-inflammatory medications after surgery.

OUTCOMES

The symptoms of trismus are relieved, and eating can be basically achieved after surgery, while the symptoms of trismus recurred 2 years later.

LESSONS

Although FOP has unique clinical manifestations, its diagnosis may be difficult because of its rarity. Gene analysis is the main standard for diagnosis, while patients with different genotypic variations may show different clinical symptoms. Therapeutic interventions are still supportive and preventive, and surgery is not recommended except under certain circumstances.

iMSC-mediated delivery of ACVR2B-Fc fusion protein reduces heterotopic ossification in a mouse model of fibrodysplasia ossificans progressiva

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease caused by a gain-of-function mutation in ACVR1, which is a bone morphogenetic protein (BMP) type I receptor. Moreover, it causes progressive heterotopic ossification (HO) in connective tissues. Using FOP patient-derived induced pluripotent stem cells (FOP-iPSCs) and mouse models, we elucidated the underlying mechanisms of FOP pathogenesis and identified a candidate drug for FOP.

METHODS

In the current study, healthy mesenchymal stem/stromal cells derived from iPSCs (iMSCs) expressing ACVR2B-Fc (iMSCACVR2B-Fc), which is a neutralizing receptobody, were constructed. Furthermore, patient-derived iMSCs and FOP mouse model (ACVR1R206H, female) were used to confirm the inhibitory function of ACVR2B-Fc fusion protein secreted by iMSCACVR2B-Fc on BMP signaling pathways and HO development, respectively.

RESULTS

We found that secreted ACVR2B-Fc attenuated BMP signaling initiated by Activin-A and BMP-9 in both iMSCs and FOP-iMSCs in vitro. Transplantation of ACVR2B-Fc-expressing iMSCs reduced primary HO in a transgenic mouse model of FOP. Notably, a local injection of ACVR2B-Fc-expressing iMSCs and not an intraperitoneal injection improved the treadmill performance, suggesting compound effects of ACVR2B-Fc and iMSCs.

CONCLUSIONS

These results offer a new perspective for treating FOP through stem cell therapy.

How Activin A Became a Therapeutic Target in Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder characterized by episodic yet cumulative heterotopic ossification (HO) of skeletal muscles, tendons, ligaments, and fascia. FOP arises from missense mutations in Activin Receptor type I (ACVR1), a type I bone morphogenetic protein (BMP) receptor. Although initial findings implicated constitutive activity of FOP-variant ACVR1 (ACVR1FOP) and/or hyperactivation by BMPs, it was later shown that HO in FOP requires activation of ACVR1FOP by Activin A. Inhibition of Activin A completely prevents HO in FOP mice, indicating that Activin A is an obligate driver of HO in FOP, and excluding a key role for BMPs in this process. This discovery led to the clinical development of garetosmab, an investigational antibody that blocks Activin A. In a phase 2 trial, garetosmab inhibited new heterotopic bone lesion formation in FOP patients. In contrast, antibodies to ACVR1 activate ACVR1FOP and promote HO in FOP mice. Beyond their potential clinical relevance, these findings have enhanced our understanding of FOP's pathophysiology, leading to the identification of fibroadipogenic progenitors as the cells that form HO, and the discovery of non-signaling complexes between Activin A and wild type ACVR1 and their role in tempering HO, and are also starting to inform biological processes beyond FOP.

Navigating the Complex Landscape of Fibrodysplasia Ossificans Progressiva: From Current Paradigms to Therapeutic Frontiers

Fibrodysplasia ossificans progressiva (FOP) is an enigmatic, ultra-rare genetic disorder characterized by progressive heterotopic ossification, wherein soft connective tissues undergo pathological transformation into bone structures. This incapacitating process severely limits patient mobility and poses formidable challenges for therapeutic intervention. Predominantly caused by missense mutations in the ACVR1 gene, this disorder has hitherto defied comprehensive mechanistic understanding and effective treatment paradigms. This write-up offers a comprehensive overview of the contemporary understanding of FOP's complex pathobiology, underscored by advances in molecular genetics and proteomic studies. We delve into targeted therapy, spanning genetic therapeutics, enzymatic and transcriptional modulation, stem cell therapies, and innovative immunotherapies. We also highlight the intricate complexities surrounding clinical trial design for ultra-rare disorders like FOP, addressing fundamental statistical limitations, ethical conundrums, and methodological advancements essential for the success of interventional studies. We advocate for the adoption of a multi-disciplinary approach that converges bench-to-bedside research, clinical expertise, and ethical considerations to tackle the challenges of ultra-rare diseases like FOP and comparable ultra-rare diseases. In essence, this manuscript serves a dual purpose: as a definitive scientific resource for ongoing and future FOP research and a call to action for innovative solutions to address methodological and ethical challenges that impede progress in the broader field of medical research into ultra-rare conditions.

Novel Therapeutic Targets for Fibrodysplasia Ossificans Progressiva: Emerging Strategies and Future Directions

Fibrodysplasia ossificans progressiva (FOP), also known as Stoneman syndrome, is a rare genetic disorder characterized by abnormal bone development caused by activating mutations of the ACVR1 gene. FOP affects both the developmental and postnatal stages, resulting in musculoskeletal abnormalities and heterotopic ossification. Current treatment options for FOP are limited, emphasizing the need for innovative therapeutic approaches. Challenges in the development of management criteria for FOP include difficulties in recruitment due to the rarity of FOP, disease variability, the absence of reliable biomarkers, and ethical considerations regarding placebo-controlled trials. This narrative review provides an overview of the disease and explores emerging strategies for FOP treatment. Gene therapy, particularly the CRISPR-Cas9 (clustered regularly interspaced short palindromic repeats-associated protein 9) system, holds promise in treating FOP by specifically targeting the ACVR1 gene mutation. Another gene therapy approach being investigated is RNA interference, which aims to silence the mutant ACVR1 gene. Small molecule inhibitors targeting glycogen synthase kinase-3β and modulation of the bone morphogenetic protein signaling pathway are also being explored as potential therapies for FOP. Stem cell-based approaches, such as mesenchymal stem cells and induced pluripotent stem cells, show potential in tissue regeneration and inhibiting abnormal bone formation in FOP. Immunotherapy and nanoparticle delivery systems provide alternative avenues for FOP treatment.

The role of miRNA and lncRNA in heterotopic ossification pathogenesis

Heterotopic ossification (HO) is the formation of bone in non-osseous tissues, such as skeletal muscles. The HO could have a genetic or a non-genetic (acquired) background, that is, it could be caused by musculoskeletal trauma, such as burns, fractures, joint arthroplasty (traumatic HO), or cerebral or spinal insult (neurogenetic HO). HO formation is caused by the differentiation of stem or progenitor cells induced by local or systemic imbalances. The main factors described so far in HO induction are TGFβ1, BMPs, activin A, oncostatin M, substance P, neurotrophin-3, and WNT. In addition, dysregulation of noncoding RNAs, such as microRNA or long noncoding RNA, homeostasis may play an important role in the development of HO. For example, decreased expression of miRNA-630, which is responsible for the endothelial-mesenchymal transition, was observed in HO patients. The reduced level of miRNA-421 in patients with humeral fracture was shown to be associated with overexpression of BMP2 and a higher rate of HO occurrence. Down-regulation of miRNA-203 increased the expression of runt-related transcription factor 2 (RUNX2), a crucial regulator of osteoblast differentiation. Thus, understanding the various functions of noncoding RNAs can reveal potential targets for the prevention or treatment of HO.

Overexpression of Wild-Type ACVR1 in Fibrodysplasia Ossificans Progressiva Mice Rescues Perinatal Lethality and Inhibits Heterotopic Ossification

Fibrodysplasia ossificans progressiva (FOP) is a devastating disease of progressive heterotopic bone formation for which effective treatments are currently unavailable. FOP is caused by dominant gain-of-function mutations in the receptor ACVR1 (also known as ALK2), which render the receptor inappropriately responsive to activin ligands. In previous studies, we developed a genetic mouse model of FOP that recapitulates most clinical aspects of the disease. In this model, genetic loss of the wild-type Acvr1 allele profoundly exacerbated heterotopic ossification, suggesting the hypothesis that the stoichiometry of wild-type and mutant receptors dictates disease severity. Here, we tested this model by producing FOP mice that conditionally overexpress human wild-type ACVR1. Injury-induced heterotopic ossification (HO) was completely blocked in FOP mice when expression of both the mutant and wild-type receptor were targeted to Tie2-positive cells, which includes fibro/adipogenic progenitors (FAPs). Perinatal lethality of Acvr1^R206H/+ mice was rescued by constitutive ACVR1 overexpression, and these mice survived to adulthood at predicted Mendelian frequencies. Constitutive overexpression of ACVR1 also provided protection from spontaneous abnormal skeletogenesis, and the incidence and severity of injury-induced HO in these mice was dramatically reduced. Analysis of pSMAD1/5/8 signaling both in cultured cells and in vivo indicates that ACVR1 overexpression functions cell-autonomously by reducing osteogenic signaling in response to activin A. We propose that ACVR1 overexpression inhibits HO by decreasing the abundance of ACVR1(R206H)-containing signaling complexes at the cell surface while increasing the representation of activin-A-bound non-signaling complexes comprised of wild-type ACVR1. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Editorial of Special Issue “Fibrodysplasia Ossificans Progressiva: Studies on Disease Mechanism towards Novel Therapeutic Approaches”

The Special Issue on “Fibrodysplasia Ossificans Progressiva: Studies on Disease Mechanism towards Novel Therapeutic Approaches” has published interesting and useful review articles and original experimental articles on fibrodysplasia ossificans progressiva (FOP), a very rare genetic disorder for which much effort is being devoted to search for a cure. In this editorial, I briefly cite the essential content of all the published articles.

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.