Classic and atypical fibrodysplasia ossificans progressiva (FOP) phenotypes are caused by mutations in the bone morphogenetic protein (BMP) type I receptor ACVR1

Molecular Developmental Biology of Fibrodysplasia Ossificans Progressiva: Measuring the Giant by Its Toe

When a genetic disease is characterized by the abnormal activation of normal molecular pathways and cellular events, it is illuminating to critically examine the places and times of these activities both in health and disease. Therefore, because heterotopic ossification (HO) in fibrodysplasia ossificans progressiva (FOP) is by far the disease's most prominent symptom, attention is also directed toward the pathways and processes of bone formation during skeletal development. FOP is recognizable by effects of the causative mutation on skeletal development even before HO manifests, specifically in the malformation of the great toes. This signature skeletal phenotype is the most highly penetrant, but is only one among several skeletal abnormalities associated with FOP. Patients may present clinically with joint malformation and ankylosis, particularly in the cervical spine and costovertebral joints, as well as characteristic facial features and a litany of less common, non-skeletal symptoms, all stemming from missense mutations in the ACVR1 gene. In the same way that studying the genetic cause of HO advanced our understanding of HO initiation and progression, insight into the roles of ACVR1 signaling during tissue development, particularly in the musculoskeletal system, can be gained from examining altered skeletal development in individuals with FOP. This review will detail what is known about the molecular mechanisms of developmental phenotypes in FOP and the early role of ACVR1 in skeletal patterning and growth, as well as highlight how better understanding these processes may serve to advance patient care, assessments of patient outcomes, and the fields of bone and joint biology.

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.

Faces of Fibrodysplasia Ossificans Progressiva: Lessons from a Clinical Masquerader

OBJECTIVES

To evaluate the natural history and to highlight the possible masqueraders causing diagnostic delay and iatrogenic interventions in Fibrodysplasia Ossificans Progressiva (FOP).

METHODS

Patient details with suspected FOP were retrieved from the patient registry from 2012 through 2021. Clinical records, X-rays, clinical photographs, and molecular testing results were captured. Follow-up was recorded where available.

RESULTS

A total of 16 patients with a clinical diagnosis of FOP were found. Twelve patients with both clinical and molecular records were included in this study. The median age of onset and diagnosis was 1.5 y and 6.5 y respectively with a median diagnostic delay of 3.5 y. The disease course was progressive in ten patients. Seven out of twelve patients were subjected to invasive procedures due to misdiagnosis, which exacerbated their disease progression.

CONCLUSIONS

Clinical suspicion followed by molecular testing is straightforward for a confirmed diagnosis of FOP. It is not only diagnostic, cost-effective, and saves time but also avoids unnecessary interventions in these patients.

Primary cilia mediate skeletogenic BMP and Hedgehog signaling in heterotopic ossification

Heterotopic ossification (HO), defined as the formation of extraskeletal bone in muscle and soft tissues, is a diverse pathological process caused by either genetic mutations or inciting trauma. Fibrodysplasia ossificans progressiva (FOP) is a genetic form of HO caused by mutations in the bone morphogenetic protein (BMP) type I receptor gene activin A receptor type 1 (ACVR1). These mutations make ACVR1 hypersensitive to BMP and responsive to activin A. Hedgehog (Hh) signaling also contributes to HO development. However, the exact pathophysiology of how skeletogenic cells contribute to endochondral ossification in FOP remains unknown. Here, we showed that the wild-type or FOP-mutant ACVR1 localized in the cilia of stem cells from human exfoliated deciduous teeth with key FOP signaling components, including activin A receptor type 2A/2B, SMAD family member 1/5, and FK506-binding protein 12kD. Cilia suppression by deletion of intraflagellar transport 88 or ADP ribosylation factor like GTPase 3 effectively inhibited pathological BMP and Hh signaling, subdued aberrant chondro-osteogenic differentiation in primary mouse or human FOP cells, and diminished in vivo extraskeletal ossification in Acvr1Q207D, Sox2-Cre; Acvr1R206H/+ FOP mice and in burn tenotomy-treated wild-type mice. Our results provide a rationale for early and localized suppression of cilia in affected tissues after injury as a therapeutic strategy against either genetic or acquired HO.

Performance of simplified methods for quantification of [18F]NaF uptake in fibrodysplasia ossificans progressiva

Background

Fibrodysplasia Ossificans Progressiva (FOP) is a rare, genetic disease in which heterotopic bone is formed in muscles, tendons and ligaments throughout the body. Disease progression is variable over time and between individuals. 18F-fluoride uptake in newly formed bone can be evaluated using [18F]NaF (i.e., sodiumfluoride) PET/CT, identifying active areas of bone formation in FOP. The purpose of this study was to assess the performance of various semi-quantitative methods with full kinetic analysis.

Results

Seven patients (age range: 20-31 years) with FOP underwent dynamic [18F]NaF scans at baseline and after one year. [18F]NaF uptake was measured in aorta descendens, vertebrae, heterotopic bone lesions and metabolically active regions on PET, and quantified using nonlinear regression (NLR) analysis together with standardized uptake value (SUV) and target-to-blood ratio (TBR). SUV was on measured the 40-45 min frame of the dynamic sequence (SUV40-45) and on the subsequent static sweep (SUVStatic). Correlations between and SUV40-45 and NLR-derived Ki were comparable when normalized to body weight (r = 0.81, 95% CI 0.64-0.90), lean body mass (r = 0.79, 95% CI 0.61-0.89) and body surface area (r = 0.84, 95% CI 0.70-0.92). Correlation between TBR40-45 and NLR-derived Ki (r = 0.92, 95% CI 0.85-0.96) was higher than for SUV40-45. Correlation between TBR40-45 and NLR-derived Ki was similar at baseline and after one year (r = 0.93 and 0.94). The change in TBR40-45 between baseline measurement and after one year correlated best with the change in NLR-derived Ki in the PET-active lesions (r = 0.87).

Conclusion

The present data supports the use of TBR for assessing fluoride uptake in PET-active lesions in FOP.

Clinical trial registration

Sub-study of the Lumina-1 trial (clinicaltrials.gov, NCT03188666, registered 13-06-2017).

Research trends and hotspots of myositis ossificans: a bibliometric analysis from 1993 to 2022

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). Although MO has been studied for decades, no research reviewed and analyzed the features of publications in this field quantitatively and qualitatively. Using bibliometrics tools (bibliometrix R package, VOSviewer, and CiteSpace), we conducted a bibliometric analysis of 1280 articles regarding MO in the Web of Science Core Collection database from 1993 to 2022. The annual number of publications and related research areas in the MO field increased gradually in the past 20 years. The USA contributed the most percentage (42.58%) of articles. The University of Pennsylvania (UPenn) and the Journal Bone published the most articles among all institutions and journals. Kaplan FS and Shore EM from UPenn were the top two scholars who made the largest contributions to this field. Keyword analysis showed that research hotspots changed from traumatic MO and clinical management of MO to the genetic etiology, pathogenesis, and treatment of FOP. This study can provide new insights into the research trends of MO and helps researchers grasp and determine future study directions more easily.

An ALK2 inhibitor, BLU-782, prevents heterotopic ossification in a mouse model of fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease driven by gain-of-function variants in activin receptor-like kinase 2 (ALK2), the most common variant being ALK2R206H. In FOP, ALK2 variants display increased and dysregulated signaling through the bone morphogenetic protein (BMP) pathway resulting in progressive and permanent replacement of skeletal muscle and connective tissues with heterotopic bone, ultimately leading to severe debilitation and premature death. Here, we describe the discovery of BLU-782 (IPN60130), a small-molecule ALK2R206H inhibitor developed for the treatment of FOP. A small-molecule library was screened in a biochemical ALK2 binding assay to identify potent ALK2 binding compounds. Iterative rounds of structure-guided drug design were used to optimize compounds for ALK2R206H binding, ALK2 selectivity, and other desirable pharmacokinetic properties. BLU-782 preferentially bound to ALK2R206H with high affinity, inhibiting signaling from ALK2R206H and other rare FOP variants in cells in vitro without affecting signaling of closely related homologs ALK1, ALK3, and ALK6. In vivo efficacy of BLU-782 was demonstrated using a conditional knock-in ALK2R206H mouse model, where prophylactic oral dosing reduced edema and prevented cartilage and heterotopic ossification (HO) in both muscle and bone injury models. BLU-782 treatment preserved the normal muscle-healing response in ALK2R206H mice. Delayed dosing revealed a short 2-day window after injury when BLU-782 treatment prevented HO in ALK2R206H mice, but dosing delays of 4 days or longer abrogated HO prevention. Together, these data suggest that BLU-782 may be a candidate for prevention of HO in FOP.

Matrix metalloproteinase-9 deficiency confers resilience in fibrodysplasia ossificans progressiva in a man and mice

Single case studies of extraordinary disease resilience may provide therapeutic insight into conditions for which no definitive treatments exist. An otherwise healthy 35-year-old man (patient-R) with the canonical pathogenic ACVR1R206H variant and the classic congenital great toe malformation of fibrodysplasia ossificans progressiva (FOP) had extreme paucity of post-natal heterotopic ossification (HO) and nearly normal mobility. We hypothesized that patient-R lacked a sufficient post-natal inflammatory trigger for HO. A plasma biomarker survey revealed a reduction in total matrix metalloproteinase-9 (MMP-9) compared to healthy controls and individuals with quiescent FOP. Whole exome sequencing identified compound heterozygous variants in MMP-9 (c.59C > T, p.A20V and c.493G > A, p.D165N). Structural analysis of the D165N variant predicted both decreased MMP-9 secretion and activity that were confirmed by enzyme-linked immunosorbent assay and gelatin zymography. Further, human proinflammatory M1-like macrophages expressing either MMP-9 variant produced significantly less Activin A, an obligate ligand for HO in FOP, compared to wildtype controls. Importantly, MMP-9 inhibition by genetic, biologic, or pharmacologic means in multiple FOP mouse models abrogated trauma-induced HO, sequestered Activin A in the extracellular matrix (ECM), and induced regeneration of injured skeletal muscle. Our data suggest that MMP-9 is a druggable node linking inflammation to HO, orchestrates an existential role in the pathogenesis of FOP, and illustrates that a single patient's clinical phenotype can reveal critical molecular mechanisms of disease that unveil novel treatment strategies.

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.

Oxidative phosphorylation is a pivotal therapeutic target of fibrodysplasia ossificans progressiva

Heterotopic ossification (HO) is a non-physiological bone formation where soft tissue progenitor cells differentiate into chondrogenic cells. In fibrodysplasia ossificans progressiva (FOP), a rare genetic disease characterized by progressive and systemic HO, the Activin A/mutated ACVR1/mTORC1 cascade induces HO in progenitors in muscle tissues. The relevant biological processes aberrantly regulated by activated mTORC1 remain unclear, however. RNA-sequencing analyses revealed the enrichment of genes involved in oxidative phosphorylation (OXPHOS) during Activin A-induced chondrogenesis of mesenchymal stem cells derived from FOP patient-specific induced pluripotent stem cells. Functional analyses showed a metabolic transition from glycolysis to OXPHOS during chondrogenesis, along with increased mitochondrial biogenesis. mTORC1 inhibition by rapamycin suppressed OXPHOS, whereas OXPHOS inhibitor IACS-010759 inhibited cartilage matrix formation in vitro, indicating that OXPHOS is principally involved in mTORC1-induced chondrogenesis. Furthermore, IACS-010759 inhibited the muscle injury-induced enrichment of fibro/adipogenic progenitor genes and HO in transgenic mice carrying the mutated human ACVR1. These data indicated that OXPHOS is a critical downstream mediator of mTORC1 signaling in chondrogenesis and therefore is a potential FOP therapeutic target.

Cell Senescence in Heterotopic Ossification

The formation of bone outside the normal skeleton, or heterotopic ossification (HO), occurs through genetic and acquired mechanisms. Fibrodysplasia ossificans progressiva (FOP), the most devastating genetic condition of HO, is due to mutations in the ACVR1/ALK2 gene and is relentlessly progressive. Acquired HO is mostly precipitated by injury or orthopedic surgical procedures but can also be associated with certain conditions related to aging. Cellular senescence is a hallmark of aging and thought to be a tumor-suppressive mechanism with characteristic features such as irreversible growth arrest, apoptosis resistance, and an inflammatory senescence-associated secretory phenotype (SASP). Here, we review possible roles for cellular senescence in HO and how targeting senescent cells may provide new therapeutic approaches to both FOP and acquired forms of HO.

Cellular and Molecular Mechanisms of Heterotopic Ossification in Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a debilitating genetic disorder characterized by recurrent episodes of heterotopic ossification (HO) formation in muscles, tendons, and ligaments. FOP is caused by a missense mutation in the ACVR1 gene (activin A receptor type I), an important signaling receptor involved in endochondral ossification. The ACVR1R206H mutation induces increased downstream canonical SMAD-signaling and drives tissue-resident progenitor cells with osteogenic potential to participate in endochondral HO formation. In this article, we review aberrant ACVR1R206H signaling and the cells that give rise to HO in FOP. FOP mouse models and lineage tracing analyses have been used to provide strong evidence for tissue-resident mesenchymal cells as cellular contributors to HO. We assess how the underlying mutation in FOP disrupts muscle-specific dynamics during homeostasis and repair, with a focus on muscle-resident mesenchymal cells known as fibro-adipogenic progenitors (FAPs). Accumulating research points to FAPs as a prominent HO progenitor population, with ACVR1R206H FAPs not only aberrantly differentiating into chondro-osteogenic lineages but creating a permissive environment for bone formation at the expense of muscle regeneration. We will further discuss the emerging role of ACVR1R206H FAPs in muscle regeneration and therapeutic targeting of these cells to reduce HO formation in FOP.

"Heterotopic abdominal wall ossification: A case report" information

INTRODUCTION AND IMPORTANCE

Heterotopic ossification is forming a new bone in tissues that do not normally ossify. HO was first reported in 1901 by Askanazy and Lubarsh in a case report study. The range of HO is wide from minute foci to large clinically significant ossification. The incidence of HO in abdominal scars is extremely low.

CASE PRESENTATION

We present an 84-year-old man referred to our hospital after an unsuccessful elective colostomy reversal in a local hospital. The colostomy was made for fecal diversion after sigmoidectomy due to treatment of sigmoid volvulus about three months ago. The patient had a past medical history of hypertension for 8 years under treatment of amiloride.

CLINICAL DISCUSSION

In general appearance, the patient was not ill or toxic. Vital signs were normal. Postoperatively Patient did not defecate. In his physical examination was not found abdominal tenderness or rebound tenderness. The patient underwent laparotomy which revealed significant retroperitoneal adhesion and colostomy was reversed. Accidentally was found a dense structure with bone-like consistency in the abdominal wall close to the scar was resected. The specimen Pathologic examination showed metaplastic bone deposition with mature bone trabeculae and heterotopic ossification was confirmed.

CONCLUSION

We report a rare case of HO that was identified at the abdominal wall. Heterotopic ossification can lead to serious complications. However, in symptomatic patients, surgical excision is an acceptable treatment, unlike in asymptomatic patients.

Hedgehog Signaling Controls Chondrogenesis and Ectopic Bone Formation via the Yap-Ihh Axis

Fibrodysplasia ossificans progressiva (FOP) is a rare congenital disorder characterized by abnormal bone formation due to ACVR1 gene mutations. The identification of the molecular mechanisms underlying the ectopic bone formation and expansion in FOP is critical for the effective treatment or prevention of HO. Here we find that Hh signaling activation is required for the aberrant ectopic bone formation in FOP. We show that the expression of Indian hedgehog (Ihh), a Hh ligand, as well as downstream Hh signaling, was increased in ectopic bone lesions in Acvr1R206H; ScxCre mice. Pharmacological treatment with an Ihh-neutralizing monoclonal antibody dramatically reduced chondrogenesis and ectopic bone formation. Moreover, we find that the activation of Yap in the FOP mouse model and the genetic deletion of Yap halted ectopic bone formation and decreased Ihh expression. Our mechanistic studies showed that Yap and Smad1 directly bind to the Ihh promoter and coordinate to induce chondrogenesis by promoting Ihh expression. Therefore, the Yap activation in FOP lesions promoted ectopic bone formation and expansion in both cell-autonomous and non-cell-autonomous manners. These results uncovered the crucial role of the Yap-Ihh axis in FOP pathogenesis, suggesting the inhibition of Ihh or Yap as a potential therapeutic strategy to prevent and reduce HO.

Pay Attention to the Osteochondromas in Fibrodysplasia Ossificans Progressiva

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP) is an extremely rare disease characterized by malformation of the bilateral great toes and progressive heterotopic ossification. The clinical features of FOP occur due to dysfunction of the bone morphogenetic protein (BMP) signaling pathway induced by the mutant activin A type I receptor/activin-like kinase-2 (ACVR1/ALK2) which contributes to the clinical features in FOP. Dysregulation of the BMP signaling pathway causes the development of osteochondroma. Poor awareness of the association between FOP and osteochondromas always results in misdiagnosis and unnecessary invasive operation.

CASE PRESENTATION

In this study, we present a case of classical FOP involving osteochondroma. An 18-year-old male adolescent, born with deformity of bilateral big toes, complained multiple masses on his back for 1 year. The mass initially emerged with a tough texture and did not cause pain. It was misdiagnosed as an osteochondroma. After two surgeries, the masses became hard and spread around the entire back region. Meanwhile, extensive heterotopic ossification was observed around the back, neck, hip, knee, ribs, and mandible during follow-up. Osteochondromas were observed around the bilateral knees. No abnormalities were observed in the laboratory blood test results. Whole exome sequencing revealed missense mutation of ACVR1/ALK2 (c.617G > A; p.R206H) in the patient and confirmed the diagnosis of FOP.

CONCLUSION

In summary, classical FOP always behaves as a bilateral deformity of the big toes, as well as progressive ectopic ossification and osteochondromas in the distal femur and proximal tibia. An understanding of the association between osteochondromas and FOP aids in diagnosis and avoids unnecessary invasive management in patients.

A fibrodysplasia ossificans progressiva patient with a rare missense mutation in ACVR1 detected on 18F-FDG PET/CT

Fibrodysplasia ossificans progressiva (FOP) is an exceedingly rare human genetic disorder characterized by the progressive and incapacitating formation of ectopic bone outside the skeleton. We report a case of FOP patient with mutations within the ACVR1 gene (c.982G>A; p.G328R). 18F-FDG positron emission tomography/computed tomography (PET/CT) was carried out for disease assessment. Previous studies have shown increased FDG uptake in regions of heterotopic ossification (HO) in FOP. However, in our study, the PET/CT features demonstrate that active ossificans exhibit increased 18F-FDG uptake, whereas end-stage ossifications do not. Collectively, 18F-FDG PET/CT emerges as a prospective approach to evaluate medication efficacy in the early stages, directing early intervention and pharmacological management of FOP before ossifications formation.

Sex as a Critical Variable in Basic and Pre-Clinical Studies of Fibrodysplasia Ossificans Progressiva

Heterotopic ossification (HO) is most dramatically manifested in the rare and severely debilitating disease, fibrodysplasia ossificans progressiva (FOP), in which heterotopic bone progressively accumulates in skeletal muscles and associated soft tissues. The great majority of FOP cases are caused by a single amino acid substitution in the type 1 bone morphogenetic protein (BMP) receptor ACVR1, a mutation that imparts responsiveness to activin A. Although it is well-established that biological sex is a critical variable in a range of physiological and disease processes, the impact of sex on HO in animal models of FOP has not been explored. We show that female FOP mice exhibit both significantly greater and more variable HO responses after muscle injury. Additionally, the incidence of spontaneous HO was significantly greater in female mice. This sex dimorphism is not dependent on gonadally derived sex hormones, and reciprocal cell transplantations indicate that apparent differences in osteogenic activity are intrinsic to the sex of the transplanted cells. By circumventing the absolute requirement for activin A using an agonist of mutant ACVR1, we show that the female-specific response to muscle injury or BMP2 implantation is dependent on activin A. These data identify sex as a critical variable in basic and pre-clinical studies of FOP.

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.

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.

Most Fractures Treated Nonoperatively in Individuals With Fibrodysplasia Ossificans Progressiva Heal With a Paucity of Flareups, Heterotopic Ossification, and Loss of Mobility

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP) is an ultrarare genetic disorder with episodic and progressive heterotopic ossification. Tissue trauma is a major risk factor for flareups, heterotopic ossification (HO), and loss of mobility in patients with FOP. The International Clinical Council on FOP generally recommends avoiding surgery in patients with FOP unless the situation is life-threatening, because soft tissue injury can trigger an FOP flareup. Surprisingly little is known about flareups, HO formation, and loss of mobility after fractures of the normotopic (occurring in the normal place, distinct from heterotopic) skeleton when treated nonoperatively in patients with FOP.

QUESTIONS/PURPOSES

(1) What proportion of fractures had radiographic evidence of union (defined as radiographic evidence of healing at 6 weeks) or nonunion (defined as the radiographic absence of a bridging callus at 3 years after the fracture)? (2) What proportion of patients had clinical symptoms of an FOP flareup because of the fracture (defined by increased pain or swelling at the fracture site within several days after closed immobilization)? (3) What proportion of patients with fractures had radiographic evidence of HO? (4) What proportion of patients lost movement after a fracture?

METHODS

We retrospectively identified 36 patients with FOP from five continents who sustained 48 fractures of the normotopic skeleton from January 2001 to February 2021, who were treated nonoperatively, and who were followed for a minimum of 18 months after the fracture and for as long as 20 years, depending on when they sustained their fracture during the study period. Five patients (seven fractures) were excluded from the analysis to minimize cotreatment bias because these patients were enrolled in palovarotene clinical trials (NCT02190747 and NCT03312634) at the time of their fractures. Thus, we analyzed 31 patients (13 male, 18 female, median age 22 years, range 5 to 57 years) who sustained 41 fractures of the normotopic skeleton that were treated nonoperatively. Patients were analyzed at a median follow-up of 6 years (range 18 months to 20 years), and none was lost to follow-up. Clinical records for each patient were reviewed by the referring physician-author and the following data for each fracture were recorded: biological sex, ACVR1 gene pathogenic variant, age at the time of fracture, fracture mechanism, fracture location, initial treatment modality, prednisone use at the time of the fracture as indicated in the FOP Treatment Guidelines for flare prevention (2 mg/kg once daily for 4 days), patient-reported flareups (episodic inflammatory lesions of muscle and deep soft connective tissue characterized variably by swelling, escalating pain, stiffness, and immobility) after the fracture, follow-up radiographs of the fracture if available, HO formation (yes or no) as a result of the fracture determined at a minimum of 6 weeks after the fracture, and patient-reported loss of motion at least 6 months after and as long as 20 years after the fracture. Postfracture radiographs were available in 76% (31 of 41) of fractures in 25 patients and were independently reviewed by the referring physician-author and senior author for radiographic criteria of fracture healing and HO.

RESULTS

Radiographic healing was noted in 97% (30 of 31) of fractures at 6 weeks after the incident fracture. Painless nonunion was noted in one patient who sustained a displaced patellar fracture and HO. In seven percent (three of 41) of fractures, patients reported increased pain or swelling at or near the fracture site within several days after fracture immobilization that likely indicated a site-specific FOP flareup. The same three patients reported a residual loss of motion 1 year after the fracture compared with their prefracture status. HO developed in 10% (three of 31) of the fractures for which follow-up radiographs were available. Patient-reported loss of motion occurred in 10% (four of 41) of fractures. Two of the four patients reported noticeable loss of motion and the other two patients reported that the joint was completely immobile (ankylosis).

CONCLUSION

Most fractures treated nonoperatively in individuals with FOP healed with few flareups, little or no HO, and preservation of mobility, suggesting an uncoupling of fracture repair and HO, which are two inflammation-induced processes of endochondral ossification. These findings underscore the importance of considering nonoperative treatment for fractures in individuals with FOP. Physicians who treat fractures in patients with FOP should consult with a member of the International Clinical Council listed in the FOP Treatment Guidelines ( https://www.iccfop.org ).

LEVEL OF EVIDENCE

Level IV, therapeutic study.

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.

[Recommendations for the healthcare of patients with FOP]

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP) is a very rare, severe genetic disorder triggered by a gain-of-function mutation in the ACVR1 gene that codes for the type I bone morphogenetic protein (BMP) receptor ACVR1 (activin A receptor-type 1), also known as ALK2 (activin receptor-like kinase-2). It leads to the onset and progression of heterotopic ossification (HO) in soft and connective tissue. HO is often preceded by episodes of soft tissue swelling or flare-ups. Flare-ups, characteristic of FOP, may be induced by trauma, infection, vaccination, or other medications, as well as surgical procedures or may occur spontaneously. As patients age, they develop severe mobility limitations due to progressive HO formation, including immobility, causing a shortened life expectancy. FOP's first characteristic clinical sign is the congenital malformation of one or both big toes with valgus axis deviation, which is present in almost all patients. To confirm the diagnosis, molecular genetic analysis of the ACVR1 gene is possible.

AIM OF THE RECOMMENDATIONS

This white paper aims to provide an overview of the necessary prerequisites and conditions for the care of patients with FOP and positively contribute to patients with FOP by improving the overall availability of knowledge. To achieve this, relevant aspects of the care of the very rare disease FOP are presented, from the initial diagnosis to the care in regular care based on the authors' knowledge (German FOP network) and the international FOP Treatment Guidelines. The recommendations presented here are addressed to all actors and decision-makers in the health care system and are also intended to inform patients and the public.

Reduced GS Domain Serine/Threonine Requirements of Fibrodysplasia Ossificans Progressiva Mutant Type I BMP Receptor ACVR1 in the Zebrafish

Fibrodysplasia ossificans progressiva (FOP) is a rare human genetic condition characterized by altered skeletal development and extraskeletal bone formation. All cases of FOP are caused by mutations in the type I bone morphogenetic protein (BMP) receptor gene ACVR1 that result in overactivation of the BMP signaling pathway. Activation of the wild-type ACVR1 kinase requires assembly of a tetrameric type I and II BMP receptor complex followed by phosphorylation of the ACVR1 GS domain by type II BMP receptors. Previous studies showed that the FOP-mutant ACVR1-R206H required type II BMP receptors and presumptive glycine/serine-rich (GS) domain phosphorylation for overactive signaling. Structural modeling of the ACVR1-R206H mutant kinase domain supports the idea that FOP mutations alter the conformation of the GS domain, but it is unclear how this leads to overactive signaling. Here we show, using a developing zebrafish embryo BMP signaling assay, that the FOP-mutant receptors ACVR1-R206H and -G328R have reduced requirements for GS domain phosphorylatable sites to signal compared to wild-type ACVR1. Further, ligand-independent and ligand-dependent signaling through the FOP-mutant ACVR1 receptors have distinct GS domain phosphorylatable site requirements. ACVR1-G328R showed increased GS domain serine/threonine requirements for ligand-independent signaling compared to ACVR1-R206H, whereas it exhibited reduced serine/threonine requirements for ligand-dependent signaling. Remarkably, while ACVR1-R206H does not require the type I BMP receptor partner, Bmpr1, to signal, a ligand-dependent GS domain mutant of ACVR1-R206H could signal independently of Bmpr1 only when Bmp7 ligand was overexpressed. Of note, unlike human ACVR1-R206H, the zebrafish paralog Acvr1l-R203H does not show increased signaling activity. However, in domain-swapping studies, the human kinase domain, but not the human GS domain, was sufficient to confer overactive signaling to the Acvr1l-R203H receptor. Together these results reflect the importance of GS domain activation and kinase domain functions in regulating ACVR1 signaling and identify mechanisms of reduced regulatory constraints conferred by FOP mutations. © 2023 American Society for Bone and Mineral Research (ASBMR).

TNFα-dependent mTOR activity is required for tenotomy-induced ectopic ossification in mice

INTRODUCTION

Ectopic ossifications often occur in skeletal muscles or tendons following local trauma or internal hemorrhage, and occasionally cause severe pain that limits activities of daily living. However, mechanisms underlying their development remain unknown.

MATERIALS AND METHODS

The right Achilles tendon in 8-week-old female or male mice was dissected. Some mice were injected intraperitoneally either with phosphate-buffered saline, dimethyl sulfoxide, cimetidine, rapamycin, celecoxib or loxoprofen for 10 weeks. One week after surgery, immunohistochemical analysis was performed for mTOR, TNFα or F4/80. Ten weeks after surgery, ectopic ossification at the tenotomy site was detected by 3D micro-CT.

RESULTS

Ectopic ossification was seen at dissection sites in all wild-type mice by dissection of the Achilles tendon. mTOR activation was detected at dissection sites, and development of ectopic ossification was significantly inhibited by administration of rapamycin, an mTOR inhibitor, to wild-type mice. Moreover, administration of the histamine 2 blocker cimetidine, which reportedly inhibits ectopic ossification in tendons, was not effective in inhibiting ectopic ossification in our models. TNFα-expressing F4/80-positive macrophages accumulate at dissection sites and that ectopic ossification of the Achilles tendon dissection was significantly inhibited in TNFα-deficient mice in vivo. Ectopic ossification is significantly inhibited by administration of either celecoxib or loxoprofen, both anti-inflammatory agents, in wild-type mice. mTOR activation by Achilles tendon tenotomy is inhibited in TNFα-deficient mice.

CONCLUSION

The TNFα-mTOR axis could be targeted therapeutically to prevent trauma-induced ectopic ossification in tendons.

Current status and prospects of GREM1 research in cancer (Review)

GREM1 is a secreted protein that antagonizes bone morphogenetic proteins (BMPs) and participates in critical biological processes, including embryonic development, organogenesis and tissue differentiation. Gremlin 1 (GREM1) is also an inhibitor of TGF-β and a ligand for vascular endothelial growth factor receptor 2. In addition, GREM1 can induce cells, participate in the process of epithelial-mesenchymal transition, and then participate in tumor development. GREM1 has a variety of biological functions and can participate in the malignant progression of a variety of tumors through the BMP signaling pathway. GREM1 also can inhibit TGF-β in some tumors, thereby inhibiting tumors, and its involvement in tumor development varies in different types of cancer. The present review examines the role and function of GREM1 in tumors. GREM1 is expressed in a variety of tumor types. GREM1 expression can affect the epithelial-mesenchymal transformation of tumor cells. GREM1 has been studied in breast and colon cancer, and its potential role is to promote cancer. However, in pancreatic cancer, which was found to act differently from other cancer types, overexpression of GREM1 inhibits tumor metastasis. The present review suggests that GREM1 can be a diagnostic and prognostic indicator. In future studies, the study of GREM1 based on single-cell sequencing technology will further clarify its role and function in tumors.

A blocking monoclonal antibody reveals dimerization of intracellular domains of ALK2 associated with genetic disorders

Mutations in activin receptor-like kinase 2 (ALK2) can cause the pathological osteogenic signaling seen in some patients with fibrodysplasia ossificans progressiva and other conditions such as diffuse intrinsic pontine glioma. Here, we report that intracellular domain of wild-type ALK2 readily dimerizes in response to BMP7 binding to drive osteogenic signaling. This osteogenic signaling is pathologically triggered by heterotetramers of type II receptor kinases and ALK2 mutant forms, which form intracellular domain dimers in response to activin A binding. We develop a blocking monoclonal antibody, Rm0443, that can suppress ALK2 signaling. We solve the crystal structure of the ALK2 extracellular domain complex with a Fab fragment of Rm0443 and show that Rm0443 induces dimerization of ALK2 extracellular domains in a back-to-back orientation on the cell membrane by binding the residues H64 and F63 on opposite faces of the ligand-binding site. Rm0443 could prevent heterotopic ossification in a mouse model of fibrodysplasia ossificans progressiva that carries the human R206H pathogenic mutant.

Functional comparison of human ACVR1 and zebrafish Acvr1l FOP-associated variants in embryonic zebrafish

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP), a rare disease characterized by progressive heterotopic ossification of muscle and connective tissues, is caused by autosomal dominant activating mutations in the type I receptor, ACVR1/ALK2. The classic human FOP variant, ACVR1R206H , shows increased bone morphogenetic protein (BMP) signaling and activation by activins.

RESULTS

Here, we performed in vivo functional characterization of human ACVR1R206H and orthologous zebrafish Acvr1lR203H using early embryonic zebrafish dorsoventral patterning as a phenotypic readout for receptor activity. Our results showed that human ACVR1R206H and zebrafish Acvr1lR203H exhibit functional differences in early embryonic zebrafish, and that human ACVR1R206H retained its signaling activity in the absence of a ligand-binding domain (LBD). We also showed, for the first time, that zebrafish Acvr2ba/Acvr2bb receptors are required for human ACVR1R206H signaling in early embryonic zebrafish.

CONCLUSIONS

Together, these data provide new insight into ACVR1R206H signaling pathways that may facilitate the design of new and effective therapies for FOP patients.

Fibrodysplasia ossificans progressiva in Hong Kong-A case report series

Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare condition. The diagnosis could be challenging due to its rarity and non-specific presenting symptoms. However, early diagnosis and appropriate management help in preserving patients' function and quality of life. Herein, we report the diagnostic journeys and clinical courses of 8 patients with FOP in Hong Kong and illustrate the challenges involved.

Transcriptomic Differences Underlying the Activin-A Induced Large Osteoclast Formation in Both Healthy Control and Fibrodysplasia Ossificans Progressiva Osteoclasts

Fibrodysplasia Ossificans Progressiva (FOP) is a very rare genetic disease characterized by progressive heterotopic ossification (HO) of soft tissues, leading to immobility and premature death. FOP is caused by a mutation in the Activin receptor Type 1 (ACVR1) gene, resulting in altered responsiveness to Activin-A. We recently revealed that Activin-A induces fewer, but larger and more active, osteoclasts regardless of the presence of the mutated ACVR1 receptor. The underlying mechanism of Activin-A-induced changes in osteoclastogenesis at the gene expression level remains unknown. Transcriptomic changes induced by Activin-A during osteoclast formation from healthy controls and patient-derived CD14-positive monocytes were studied using RNA sequencing. CD14-positive monocytes from six FOP patients and six age- and sex-matched healthy controls were differentiated into osteoclasts in the absence or presence of Activin-A. RNA samples were isolated after 14 days of culturing and analyzed by RNA sequencing. Non-supervised principal component analysis (PCA) showed that samples from the same culture conditions (e.g., without or with Activin-A) tended to cluster, indicating that the variability induced by Activin-A treatment was larger than the variability between the control and FOP samples. RNA sequencing analysis revealed 1480 differentially expressed genes induced by Activin-A in healthy control and FOP osteoclasts with p(adj) < 0.01 and a Log2 fold change of ≥±2. Pathway and gene ontology enrichment analysis revealed several significantly enriched pathways for genes upregulated by Activin-A that could be linked to the differentiation or function of osteoclasts, cell fusion or inflammation. Our data showed that Activin-A has a substantial effect on gene expression during osteoclast formation and that this effect occurred regardless of the presence of the mutated ACVR1 receptor causing FOP.

A rare challenge: A patient with progressive fibrodysplasia ossificans and acute ischemic stroke treated with mechanical thrombectomy

Fibrodysplasia ossificans progressiva (FOP) is a rare, disabling genetic condition characterized by congenital malformations of the great toes and progressive heterotopic ossification. Here, we briefly describe the case of a 56-year-old male with known FOP and acute ischemic stroke that underwent mechanical thrombectomy with conscious sedation. Treating physicians should be aware of special medical considerations to prevent flare-ups and inflammation that result from any tissue injuries in this disease. Mechanical thrombectomy is a challenging scenario because general anesthesia and injections should be avoided in these patients. The treatment is still preventive and supportive, but this is the first report of the procedure in a patient with FOP.

Transcriptome Profile in the Mouse Brain of Hepatic Encephalopathy and Alzheimer's Disease

Hepatic encephalopathy (HE) is a chronic metabolic disease accompanied by neuropathological and neuropsychiatric features, including memory deficits, psychomotor dysfunction, depression, and anxiety. Alzheimer's disease (AD), the most common neurodegenerative disease, is characterized by tau hyperphosphorylation, excessive amyloid beta (Aβ) accumulation, the formation of fibrillary tangles, hippocampus atrophy, and neuroinflammation. Recent studies have suggested a positive correlation between HE and AD. Some studies reported that an impaired cholesterol pathway, abnormal bile acid secretion, excessive ammonia level, impaired Aβ clearance, astrocytic dysfunction, and abnormal γ-aminobutyric acid GABAergic neuronal signaling in HE may also be involved in AD pathology. However, the mechanisms and related genes involved in AD-like pathology in the HE brain are unclear. Thus, we compared the cortical transcriptome profile between an HE mouse model, bile duct ligation (BDL), and an AD mouse model, the 5×FAD. Our study showed that the expression of many genes implicated in HE is associated with neuronal dysfunction in AD mice. We found changes in various protein-coding RNAs, implicated in synapses, neurogenesis, neuron projection, neuron differentiation, and neurite outgrowth, and non-coding RNAs possibly associated with neuropathology. Our data provide an important resource for further studies to elucidate AD-like pathophysiology in HE patients.

Fibrodysplasia Ossificans Progressiva: Literature Review and Case Report

Background. Fibrodysplasia ossificans progressiva (FOP) is a genetic disease of the heterotopic ossification group associated with the mutation in ACVR1/ALK2 gene. FOP is characterized by progressive heterotopic endochondral ossification of connective tissue that occurs in postnatal period. It leads to formation of qualitatively normal bone in extraskeletal areas. Congenital hallux deformity is typical for this disease. The clinical picture is characterized by aggravations that are usually caused by trauma or viral infections. Formation of Heterotopic ossificate formation can be observed during aggravations. There is no etiological treatment for FOP. Systemic glucocorticosteroids, non-steroidal anti-inflammatory drug (NSAIDs), mast cell stabilisers, antileukotriene drugs and bisphosphonates can be used in these patients. Clinical case description. The child was born with congenital hallux deformity typical for FOP. The disease onset was noted at the age of 2 years 8 months with a tumor-like painful mass on the neck. Oncological (lymphoproliferative) disease was suspected but biopsy from the lesion did not confirm its malignant nature. The child was consulted by pediatric rheumatologist who has diagnosed FOP. Etanercept and zoledronic acid were administrated, though etanercept was later discontinued. For now, the child receives zoledronic acid infusions 2 times per year and daily NSAIDs. Conclusion. The difficulties in FOP diagnosing are associated to its sporadic nature and clinical picture similarity to other diseases. Suspected malignancy leads to biopsy that is highly undesirable in FOP patients due to high risk of iatrogenic complications.

Suppression of heterotopic ossification in fibrodysplasia ossificans progressiva using AAV gene delivery

Heterotopic ossification is the most disabling feature of fibrodysplasia ossificans progressiva, an ultra-rare genetic disorder for which there is currently no prevention or treatment. Most patients with this disease harbor a heterozygous activating mutation (c.617 G > A;p.R206H) in ACVR1. Here, we identify recombinant AAV9 as the most effective serotype for transduction of the major cells-of-origin of heterotopic ossification. We use AAV9 delivery for gene replacement by expression of codon-optimized human ACVR1, ACVR1R206H allele-specific silencing by AAV-compatible artificial miRNA and a combination of gene replacement and silencing. In mouse skeletal cells harboring a conditional knock-in allele of human mutant ACVR1 and in patient-derived induced pluripotent stem cells, AAV gene therapy ablated aberrant Activin A signaling and chondrogenic and osteogenic differentiation. In Acvr1(R206H) knock-in mice treated locally in early adulthood or systemically at birth, trauma-induced endochondral bone formation was markedly reduced, while inflammation and fibroproliferative responses remained largely intact in the injured muscle. Remarkably, spontaneous heterotopic ossification also substantially decreased in in Acvr1(R206H) knock-in mice treated systemically at birth or in early adulthood. Collectively, we develop promising gene therapeutics that can prevent disabling heterotopic ossification in mice, supporting clinical translation to patients with fibrodysplasia ossificans progressiva.

Bioanalysis of INCB000928 in hemodialysate: prevention of nonspecific binding and validation of surrogate matrices

Aim: To develop and validate a bioanalytical method for the quantification of INCB000928 in hemodialysate. Materials & methods: Blank dialysate and phosphate-buffered saline were compared with hemodialysate for surrogate matrix selection. Direct addition of internal standard without analyte extraction and a high-performance LC-MS/MS were used for analysis. Results & conclusion: INCB000928 in hemodialysate exhibited strong nonspecific binding to polypropylene containers. In the presence of 10% isopropyl alcohol, the loss of INCB000928 was fully recovered, regardless of pre- or post-addition of the solvent. Blank dialysate and phosphate-buffered saline were determined to be appropriate surrogate matrices by using a three-way cross-comparison and were subsequently validated in the quantitative analysis of INCB000928 in hemodialysate.

Recapitulation of pro-inflammatory signature of monocytes with ACVR1A mutation using FOP patient-derived iPSCs

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease characterized by progressive heterotopic ossification (HO) in soft tissues due to a heterozygous mutation of the ACVR1A gene (FOP-ACVR1A), which erroneously transduces the BMP signal by Activin-A. Although inflammation is known to trigger HO in FOP, the role of FOP-ACVR1A on inflammatory cells remains to be elucidated.

RESULTS

We generated immortalized monocytic cell lines from FOP-iPSCs (FOP-ML) and mutation rescued iPSCs (resFOP-ML). Cell morphology was evaluated during the monocyte induction and after immortalization. Fluorescence-activated cell sorting (FACS) was performed to evaluate the cell surface markers CD14 and CD16 on MLs. MLs were stimulated with lipopolysaccharide or Activin-A and the gene expression was evaluated by quantitative PCR and microarray analysis. Histological analysis was performed for HO tissue obtained from wild type mice and FOP-ACVR1A mice which conditionally express human mutant ACVR1A gene by doxycycline administration. Without any stimulation, FOP-ML showed the pro-inflammatory signature of CD16+ monocytes with an upregulation of INHBA gene, and treatment of resFOP-ML with Activin-A induced an expression profile mimicking that of FOP-ML at baseline. Treatment of FOP-ML with Activin-A further induced the inflammatory profile with an up-regulation of inflammation-associated genes, of which some, but not all, of which were suppressed by corticosteroid. Experiments using an inhibitor for TGFβ or BMP signal demonstrated that Activin-A-induced genes such as CD16 and CCL7, were regulated by both signals, indicating Activin-A transduced dual signals in FOP-ML. A comparison with resFOP-ML identified several down-regulated genes in FOP-ML including LYVE-1, which is known to suppress matrix-formation in vivo. The down-regulation of LYVE-1 in HO tissues was confirmed in FOP model mice, verifying the significance of the in vitro experiments.

CONCLUSION

These results indicate that FOP-ML faithfully recapitulated the phenotype of primary monocytes of FOP and the combination with resFOP-ML is a useful tool to investigate molecular events at the initial inflammation stage of HO in FOP.

Fibrodysplasia ossificans progressiva in Brazil: challenges and strategies to create assistance and educational networks

Fibrodysplasia ossificans progressiva (FOP) is an ultrarare condition and one of the most impactful disorders associated with progressive heterotopic ossification events. It is estimated that there are 120–150 patients in Brazil; however, currently, fewer than 100 patients have been identified, and the role of a FOP advocacy group (FOP Brazil) has been instrumental for the identification and follow-up of these individuals and families. The aim of this article is to summarize the current status of FOP in Brazil and describe strategies proposed to approach this challenge in a continental size country.

The impact of fibrodysplasia ossificans progressiva (FOP) on patients and their family members: results from an international burden of illness survey

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare, genetic disorder of heterotopic ossification, which transforms soft, connective tissues into bone, resulting in limited joint function and severe disability. We present results from an international burden of illness survey (NCT04665323) assessing physical, quality of life (QoL), and economic impacts of FOP on patients and family members.

METHODS

Patient associations in 15 countries invited their members to participate; individuals with FOP and their family members were eligible. The survey was available online, in 11 languages, from January 18-April 30, 2021. Participants responded to assessments measuring joint function, QoL, healthcare service and living adaptation utilization, out-of-pocket costs, employment, and travel.

RESULTS

The survey received 463 responses (patients, n=219; family members, n=244). For patients, decreased joint function was associated with reduced QoL and greater reliance on living adaptations. Nearly half of primary caregivers experienced a mild to moderate impact on their health/psychological wellbeing. Most primary caregivers and patients (≥18 years) reported that FOP impacted their career decisions.

CONCLUSIONS

Data from this survey will improve understanding of the impact of FOP on patients and family members, which is important for identifying unmet needs, optimizing care, and improving support for the FOP community.

Multi-omics therapeutic perspective on ACVR1 gene: from genetic alterations to potential targeting

Activin A receptor type I (ACVR1), a transmembrane serine/threonine kinase, belongs to the transforming growth factor-β superfamily, which signals via phosphorylating the downstream effectors and SMAD transcription factors. Its central role in several biological processes and intracellular signaling is well known. Genetic variation in ACVR1 has been associated with a rare disease, fibrodysplasia ossificans progressive, and its somatic alteration is reported in rare cancer diffuse intrinsic pontine glioma. Furthermore, altered expression or variation of ACVR1 is associated with multiple pathologies such as polycystic ovary syndrome, congenital heart defects, diffuse idiopathic skeletal hyperostosis, posterior fossa ependymoma and other malignancies. Recent advancements have witnessed ACVR1 as a potential pharmacological target, and divergent promising approaches for its therapeutic targeting have been explored. This review highlights the structural and functional characteristics of receptor ACVR1, associated signaling pathways, genetic variants in several diseases and cancers, protein-protein interaction, gene expression, regulatory miRNA prediction and potential therapeutic targeting approaches. The comprehensive knowledge will offer new horizons and insights into future strategies harnessing its therapeutic potential.

Role of endothelial cells in vascular calcification

Vascular calcification (VC) is active and regulates extraosseous ossification progress, which is an independent predictor of cardiovascular disease (CVD) morbidity and mortality. Endothelial cells (ECs) line the innermost layer of blood vessels and directly respond to changes in flow shear stress and blood composition. Together with vascular smooth muscle cells, ECs maintain vascular homeostasis. Increased evidence shows that ECs have irreplaceable roles in VC due to their high plasticity. Endothelial progenitor cells, oxidative stress, inflammation, autocrine and paracrine functions, mechanotransduction, endothelial-to-mesenchymal transition (EndMT), and other factors prompt ECs to participate in VC. EndMT is a dedifferentiation process by which ECs lose their cell lineage and acquire other cell lineages; this progress coexists in both embryonic development and CVD. EndMT is regulated by several signaling molecules and transcription factors and ultimately mediates VC via osteogenic differentiation. The specific molecular mechanism of EndMT remains unclear. Can EndMT be reversed to treat VC? To address this and other questions, this study reviews the pathogenesis and research progress of VC, expounds the role of ECs in VC, and focuses on the regulatory factors underlying EndMT, with a view to providing new concepts for VC prevention and treatment.

Computational Insights into the Structural and Functional Impacts of nsSNPs of Bone Morphogenetic Proteins

BMPs (bone morphogenetic proteins) are multipurpose (transforming growth factor)TGF-superfamily released cytokines. These glycoproteins, acting as disulfide-linked homo- or heterodimers, are highly potent regulators of bone and cartilage production and repair, cell proliferation throughout embryonic development, and bone homeostasis in the adults. Due to the fact that genetic variation might influence structural functions, this study is aimed to determine the pathogenic effect of nonsynonymous single-nucleotide polymorphisms (nsSNPs) in BMP genes. The implications of these variations, investigated using computational analysis and molecular models of the mature TGF-β domain, revealed the impact of modifications on the function of BMP protein. The three-dimensional (3D) structure analysis was performed on the nsSNP Y316S, V386G, E387G, C389G, and C391G nsSNP in the TGF-β domain of chicken BMP2 and H344P, S347P, V357A nsSNP in the TGF-β domain of chicken BMP4 protein that was anticipated to be harmful and of high risk. The ability of the proteins to perform variety of tasks interact with other molecules depends on their tertiary structural composition. The current analysis revealed the four most damaging variants (Y316S, V386G, E387G, C389G, and C391G), highly conserved and functional and are located in the TGF-beta domain of BMP2 and BMP4. The amino acid substitutions E387G, C389G, and C391G are discovered in the binding region. It was observed that the mutations in the TGF-beta domain caused significant changes in its structural organization including the substrate binding sites. Current findings will assist future research focused on the role of these variants in BMP function loss and their role in skeletal disorders, and this will possibly help to develop practical strategies for treating bone-related conditions.

Protocol paper: a multi-center, double-blinded, randomized, 6-month, placebo-controlled study followed by 12-month open label extension to evaluate the safety and efficacy of Saracatinib in Fibrodysplasia Ossificans Progressiva (STOPFOP)

Background

Fibrodysplasia Ossificans Progressiva (FOP) is a genetic, progressive and devastating disease characterized by severe heterotopic ossification (HO), loss of mobility and early death. There are no FDA approved medications. The STOPFOP team identified AZD0530 (saracatinib) as a potent inhibitor of the ALK2/ACVR1-kinase which is the causative gene for this rare bone disease. AZD0530 was proven to prevent HO formation in FOP mouse models. The STOPFOP trial investigates the repositioning of AZD0530, originally developed for ovarian cancer treatment, to treat patients with FOP.

Methods

The STOPFOP trial is a phase 2a study. It is designed as a European, multicentre, 6-month double blind randomized controlled trial of AZD0530 versus placebo, followed by a 12-month trial comparing open-label extended AZD0530 treatment with natural history data as a control. Enrollment will include 20 FOP patients, aged 18–65 years, with the classic FOP mutation (ALK2 R206H). The primary endpoint is objective change in heterotopic bone volume measured by low-dose whole-body computer tomography (CT) in the RCT phase. Secondary endpoints include ¹⁸F NaF PET activity and patient reported outcome measures.

Discussion

Clinical trials in rare diseases with limited study populations pose unique challenges. An ideal solution for limiting risks in early clinical studies is drug repositioning – using existing clinical molecules for new disease indications. Using existing assets may also allow a more fluid transition into clinical practice.

With positive study outcome, AZD0530 may provide a therapy for FOP that can be rapidly progressed due to the availability of existing safety data from 28 registered clinical trials with AZD0530 involving over 600 patients.

Trial registration

EudraCT, 2019–003324-20. Registered 16 October 2019, https://www.clinicaltrialsregister.eu/ctr-search/trial/2019-003324-20/NL. Clinicaltrials.gov, NCT04307953. Registered 13 March 2020.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-022-05471-x.

Recent progress in drug development for fibrodysplasia ossificans progressiva

Fibrodysplasia Ossificans Progressiva (FOP) is a rare genetic disease caused by heterozygous missense mutations in Activin A receptor type I which is also known as Activin-like kinase 2 (ALK2), a type I receptor of Bone Morphogenetic Proteins(BMP). Patients with FOP usually undergo episodic flare-ups and the heterotopic ossification in soft and connective tissues. Molecular mechanism study indicates that Activin A, the ligand which normally transduces Transforming Growth Factor Beta signaling, abnormally activates BMP signaling through ALK2 mutants in FOP, leading to heterotopic bone formation. To date, effective therapies to FOP are unavailable. However, significant advances have recently been made in the development of FOP drugs. In this article, we review the recent advances in understanding the FOP mechanism and drug development, with a focus on the small-molecular and antibody drugs currently in the clinical trials for FOP treatment.

Two Modulators of Skeletal Development: BMPs and Proteoglycans

During embryogenesis, skeletal development is tightly regulated by locally secreted growth factors that interact with proteoglycans (PGs) in the extracellular matrix (ECM). Bone morphogenetic proteins (BMPs) are multifunctional growth factors that play critical roles in cartilage maturation and bone formation. BMP signals are transduced from plasma membrane receptors to the nucleus through both canonical Smad and noncanonical p38 mitogen-activated protein kinase (MAPK) pathways. BMP signalling is modulated by a variety of endogenous and exogenous molecular mechanisms at different spatiotemporal levels and in both positive and negative manners. As an endogenous example, BMPs undergo extracellular regulation by PGs, which generally regulate the efficiency of ligand-receptor binding. BMP signalling can also be exogenously perturbed by a group of small molecule antagonists, such as dorsomorphin and its derivatives, that selectively bind to and inhibit the intracellular kinase domain of BMP type I receptors. In this review, we present a current understanding of BMPs and PGs functions in cartilage maturation and osteoblast differentiation, highlighting BMP–PG interactions. We also discuss the identification of highly selective small-molecule BMP receptor type I inhibitors. This review aims to shed light on the importance of BMP signalling and PGs in cartilage maturation and bone formation.

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.

Bioanalysis of INCB000928 in human saliva: nonspecific binding and inhomogeneous concentration

Aim: To develop a bioanalytical method for quantifying INCB000928 in human saliva. Materials & methods: Human centrifuged saliva and human whole saliva were compared for matrix selection. Protein precipitation extraction and HPLC-MS/MS was used for analysis. Results & conclusion: Nonspecific binding of INCB000928 was reduced in whole versus centrifuged saliva. Whole saliva was a preferred matrix for INCB000928 bioanalytical method validation. Incurred sample reanalysis (ISR) using a successfully validated method failed in a healthy volunteer study because of inhomogeneous INCB000928 concentration across sample tube depths. Individual mixing of sample tubes followed by immediate aliquoting corrected the ISR failure, with 97.2% of repeats passing versus 41.7% for the same ISR samples.

[Progressive myositis ossificans: Case report]

Background

Myositis ossificans progressiva (MOP) is a low prevalence hereditary connective tissue disease (1:2,000,000 habitants). It is characterized by heterotopic ossification with an uncertain behavior that has been exceptionally related to neoplasms. The objective was to know the coexistence of MOP with neoplasms of mesodermal origin, so that they can be considered in the diagnosis of other patients, as well as formulate hypotheses to clarify their association.

Clinical case

27-year-old female with right gluteal and ischitiobial muscle pain that increased with exercise, without remission with analgesics until limiting the mobility of both extremities. A bone series was requested where areas of heterogeneous radiolucency were evidenced in the region of, both, thighs and pelvis in an irregular manner, similar to bone density, which was compatible with the ultrasound and tomographic findings; we concluded that they were images of myositis ossificans of the hip. The patient reported gastric symptoms and an endoscopy was requested, which histopathologically reported diffuse gastric carcinoma with signet ring cells; cabinet images showed an ovarian tumor.

Conclusion

MOP is a low prevalence disease, which is why its knowledge and suspicion are essential for the diagnosis. We found little literature that involves the three entities; therefore, their pathophysiology and understanding is limited. Regarding MOP, at this moment there is no curative treatment; however, an accurate diagnosis allows to start rehabilitation in a timely manner with an improvement in the quality of life.

Extensive progressive heterotopic ossification post-Covid-19 in a man

Heterotopic ossification (HO) is the formation of extraskeletal bone in muscle and soft tissues and could be genetic or non-genetic. The classic presentation of non-genetic HO is in young adults with a clear history of local trauma, surgery or prolonged immobilization after spinal cord and traumatic brain injuries. Genetic HO has a significant clinical severity compared to non-genetic causes and includes fibrodysplasia ossificans progressiva (FOP). FOP is an extremely rare genetic skeletal disorder characterized by congenital malformations of the great toes and progressive heterotopic ossification that forms qualitatively normal bone in characteristic extraskeletal sites affecting skeletal muscles, fascia, tendons, and ligaments. Previously, it has been reported an association between SARS-CoV-2 infection (COVID-19) and HO or FOP exacerbation with unclear etiopathogenesis. The possible mechanisms could be prolonged immobilization and systemic inflammation. Here, we describe the case of a 55-year-old apparently healthy man who suffered from a severe SARS-CoV-2 infection after that he experienced an extensive and progressive heterotopic ossification around the shoulders, the elbows, the hip, the knees, and the ankles. Because of the clinical severity, the painful soft-tissue swelling, the progressive HO, and the bilateral congenital hallux valgus deformity, a late-onset atypical FOP was suspected. Nevertheless, no variant of clinical significance has been identified in the coding regions and splicing sites in the ACVR1 gene and no deletions and/or duplications have been identified in exonic regions.

Dynamics of skeletal muscle-resident stem cells during myogenesis in fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease in which extraskeletal (heterotopic) bone forms within tissues such as skeletal muscles, often in response to injury. Mutations in the BMP type I receptor ACVR1/ALK2 cause FOP by increasing BMP pathway signaling. In contrast to the growing understanding of the inappropriate formation of bone tissue within the muscle in FOP, much is still unknown about the regenerative capacity of adult diseased muscles. Utilizing an inducible ACVR1^R206H knock-in mouse, we found that injured Acvr1^R206H/+ skeletal muscle tissue regenerates poorly. We demonstrated that while two resident stem cell populations, muscle stem cells (MuSCs) and fibro/adipogenic progenitors (FAPs), have similar proliferation rates after injury, the differentiation potential of mutant MuSCs is compromised. Although MuSC-specific deletion of the ACVR1^R206H mutation does not alter the regenerative potential of skeletal muscles in vivo, Acvr1^R206H/+ MuSCs form underdeveloped fibers that fail to fuse in vitro. We further determined that FAPs from Acvr1^R206H/+ mice repress the MuSC-mediated formation of Acvr1^R206H/+ myotubes in vitro. These results identify a previously unrecognized role for ACVR1^R206H in myogenesis in FOP, via improper interaction of tissue-resident stem cells during skeletal muscle regeneration.

BMP signaling and skeletal development in fibrodysplasia ossificans progressiva (FOP)

Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare genetic disease caused by increased BMP pathway signaling due to mutation of ACVR1, a bone morphogenetic protein (BMP) type 1 receptor. The primary clinical manifestation of FOP is extra-skeletal bone formation (heterotopic ossification) within soft connective tissues. However, the underlying ACVR1 mutation additionally alters skeletal bone development and nearly all people born with FOP have bilateral malformation of the great toes as well as other skeletal malformations at diverse anatomic sites. The specific mechanisms through which ACVR1 mutations and altered BMP pathway signaling in FOP influence skeletal bone formation during development remain to be elucidated; however, recent investigations are providing a clearer understanding of the molecular and developmental processes associated with ACVR1-regulated skeletal formation.