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).

Comparison of PET/CT versus CT only in the assessment of new heterotopic ossification bone lesions in patients with fibrodysplasia ossificans progressiva.

Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare disorder characterized by the deposition of bone in soft tissues, known as heterotopic ossification (HO). This post hoc analysis compared the performance of two imaging modalities for the detection and volumetric measurement of new HO lesions. LUMINA-1, a phase 2, randomized, double-blind study (NCT03188666), evaluated the safety and efficacy of garetosmab, an anti-activin A antibody, versus placebo in adult patients with FOP. From baseline through to week 28, 18F-labeled sodium fluoride positron emission tomography (PET)/X-ray computed tomography (CT) and CT-only scans prospectively acquired during the initial placebo-controlled period of the study were independently reviewed by two sets of fixed blinded readers plus an adjudicator for the presence and volume of new HO lesions. The number of patients with new lesions was 14/44 (31.8 %) and 12/44 (27.3 %) as detected by PET/CT and CT only, respectively. The aggregate number/volume of new lesions were very similar both for the placebo and the garetosmab group between PET/CT (27/245.0 cm3 and 3/21.3 cm3, respectively) and CT only (37/261.8 cm3 and 1/0.1 cm3, respectively). The mean (standard deviation) number of new lesions per patient by PET/CT through week 28 was 0.68 (1.57) versus 0.86 (1.95) as detected by CT only. Through week 28, the mean (standard deviation) volume of new lesions per patient detected by PET/CT was 6.05 (14.88) cm3 versus 5.94 (21.13) cm3 by CT only. Moderate agreement between PET/CT and CT-only detection was observed when identifying patients with new lesions, with a kappa coefficient of 0.46 (standard error, 0.146; 95 % confidence interval, 0.17-0.74). CT-only imaging showed similar performance to PET/CT in the detection and characterization of new HO lesions. CT-only imaging therefore is a viable option for the assessment of therapies on new HO in patients with FOP.

Challenges in the diagnosis of fibrodysplasia ossificans progressiva with the ACVR1 mutation (c.774G > C, p.R258S): a case report and review of literature.

The diagnosis of fibrodysplasia ossificans progressiva is missed or delayed because of its insidious precursors, especially in uncharacteristic cases. Fibrodysplasia ossificans progressiva, which mostly displayed the mutation c.617G > A, p.R206H, is characterized by congenital malformation of the great toe and progressive extra-skeletal ossification of ligaments, tendons and muscles. The mutation c.774G > C, p.R258S (HGVS: NC_000002.11:g.158626896 C > G) in activin A receptor type I is an infrequent etiology of fibrodysplasia ossificans progressiva and can present different clinical features. Awareness of these multiple clinical features will help endocrinologists in the early diagnosis of fibrodysplasia ossificans progressiva. We report a case of fibrodysplasia ossificans progressiva with the activin A receptor type I mutation c.774G > C, p.R258S, which was diagnosed before its ossifying period.

Fibrodysplasia ossificans progressiva emerges from obscurity.

Fibrodysplasia ossificans progressiva (FOP), a disorder of congenital skeletal malformations and progressive extraskeletal ossification, is the most severe form of heterotopic ossification (HO) in humans. Gain-of-function pathogenic variants in activin A receptor type I (ACVR1), a bone morphogenetic protein (BMP) type 1 receptor, cause FOP by dramatically altering the normal physiologic functions of ACVR1, impacting BMP signaling and other interacting pathways. These alterations affect various systems, including inflammation, innate immunity, hypoxia sensing, wound healing, aging, temperature and mechanical thresholds, pain sensitivity, skeletal growth, diarthrodial joint patterning, joint function and fate, and HO. This article examines the emergent properties of FOP's diverse phenotypes, proposes a schema for targeting these phenotypes, and highlights outstanding questions and knowledge gaps.

Characterization of flare-ups and impact of garetosmab in adults with fibrodysplasia ossificans progressiva: a post hoc analysis of the randomized, double-blind, placebo-controlled LUMINA-1 trial.

Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare disorder, characterized by progressive heterotopic ossification (HO) and painful soft-tissue inflammatory flare-ups. This was a post hoc analysis from a phase 2 (NCT03188666) trial in which adults with FOP received intravenous anti-activin A antibody garetosmab 10 mg/kg or placebo every 4 wk over 28 wk (Period 1), followed by a 28-wk open-label treatment and extension (Periods 2 and 3). Here we describe flare-ups, their relationship to new HO lesions, and the impact of garetosmab on flare-ups. Volume of new HO lesions was measured by CT. Patient-reported flare-ups were defined by any 2 of the following: new onset of pain, swelling, joint stiffness, decrease in movement, or perceived presence of HO. Flare-ups were experienced by 71% (17/24) of placebo-treated patients, 59% (10/17) of whom developed a new HO lesion irrespective of flare-up location; 24% of flare-ups location-matched new HO lesions. Twenty-nine new HO lesions occurred in the placebo cohort by week 28, of which 12 (41%) occurred in the same location as new or ongoing flare-ups. A higher volume of newly formed heterotopic bone (week 28) occurred in placebo-treated patients who had experienced a prior flare-up vs those without (median [Q1:Q3] of 16.6 [12.0:31.1] vs 3.2 cm3). Garetosmab was previously shown to decrease patient-reported flare-up frequency in Period 1; here, garetosmab reduced the median (Q1:Q3) duration of patient-reported flares (15.0 [6.0:82.0] vs 48.0 [15.0:1.00] d) and the severity of flare-ups vs placebo. Frequency of corticosteroid use was numerically reduced in those treated with garetosmab (40.0%) vs placebo (58.3%). In this analysis, 71% of placebo-treated adults with FOP experienced flare-ups over 28 wk, which were associated with an increased volume of newly formed heterotopic bone. Garetosmab reduced the severity and duration of flare-ups, with effects sustained during the entire trial.

Fibrodysplasia ossificans progressiva (FOP) is a very rare genetic disorder caused by mutations in the ACVR1 gene. People with FOP experience growth of new bone in places where bone does not usually develop. Soft tissues (like skeletal muscles) and connective tissues (like tendons and ligaments) are gradually replaced by bone beyond the normal skeleton­a process called heterotopic ossification (HO). People with FOP experience flare-ups, which are painful swellings of the soft tissues. In this clinical study in people with FOP, we looked at the number of flareups, whether flareups were linked to new HO lesions, and the impact of garetosmab (a monoclonal antibody) on flareups. At random, about half the patients received placebo, or inactive drug, with the other half receiving garetosmab, the study drug. Of the patients who received placebo, 71% had flare-ups and 59% percent of those who had flare-ups also had a new HO lesion, which was not always related to the location of the flare-up. We have previously shown that garetosmab reduces the number of flareups patients report. In this study, we show that garetosmab reduces the length and pain severity of flare-ups too. The treatment effects were maintained for the whole study.

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.

Fibrodysplasia Ossificans Progressiva: A Man Turned to Stone.

Fibrodysplasia ossificans progressiva (FOP) is an exceptionally rare genetic disorder, representing humans' most debilitating form of extraskeletal ossification. It is characterized by progressive postnatal heterotopic ossification of connective tissue and malformations of the big toes. In FOP, ectopic ossification usually begins in the upper paraspinal muscles and then spreads from axial to appendicular regions, cranial to caudal directions, and proximal to distal sites. The mean life expectancy for these patients is typically 40-50 years. Most patients need partial or complete assistance with walking by age 30, and common causes of death include thoracic insufficiency syndrome and pneumonia. We present the case of a patient with an advanced stage of FOP, highlighting its complex and progressive nature. The patient exhibits severe impairment of jaw mobility, swallowing difficulties, speech impediments, and hearing impairment. Additionally, severe kyphoscoliosis, heterotopic ossification of intercostal and paravertebral muscles, and ankylosis of the spine and all major joints of the upper and lower extremities, except the metacarpophalangeal and proximal interphalangeal joints, are evident. We discuss disease presentation, current management options, and rehabilitation challenges. To our knowledge, this is the first reported case of this rare disease from our country.

Long-term use of interleukin-1 inhibitors reduce flare activity in patients with fibrodysplasia ossificans progressiva.

OBJECTIVES: Fibrodysplasia ossificans progressiva (FOP) is one of the most catastrophic forms of genetic heterotopic ossification (HO). FOP is characterized by severe, progressive inflammatory flare-ups, that often lead to HO. The flare-ups are associated with increased inflammatory cytokine production, suggesting auto-inflammatory features driven by IL-1ß. This study describes the short- and long-term responses of FOP patients to anti-IL-1 therapy. METHODS: Previously, we reported that a patient with FOP treated with anti-IL-1 agents showed dramatically lower rates of flare-ups, improved flare-up symptoms, decreased use of glucocorticoids and apparently decreased size of residual lesions. Plasma analyses also showed marked elevation in IL-1ß levels during a FOP flare, further supporting a role of IL-1ß in the pathogenesis of FOP flares. Here, we report results from long-term therapy with IL-1 inhibitors in that patient and describe 3 additional patients, from two medical centres. RESULTS: All 4 patients showed persistent improvement in flare activity during treatment with IL-1 inhibitors, with minimal formation of new HO sites. Two patients who stopped therapy experienced a resurgence of flare activity that was re-suppressed upon re-initiation. These patients had IL-1ß levels comparable to those in IL-1ß-driven diseases. Child Health Assessment Questionnaires confirmed extensive subjective improvements in the pain and general health visual analogue scales. CONCLUSION: This case series demonstrates significant benefits from IL-1 inhibitors for reducing flare activity and improving the general health of patients with FOP. These data provide strong support for additional studies to better understand the function of IL-1 inhibition, primarily in reducing the formation of new HO. FUNDING: RH received support from the International FOP Association ACT grant; ECH received support from NIH/NIAMS R01AR073015 and the UCSF Robert Kroc Chair in Connective Tissue and Rheumatic Diseases III.

Fibrodysplasia Ossificans Progressiva: A Case Report.

Fibrodysplasia ossificans progressiva (FOP) is a rare autosomal dominant genetic disorder characterized by congenital great toe malformations and progressive ectopic ossification. We report a typical case of FOP in a 22-year-old female patient presenting with limited movement of the left knee joint, which began following trauma in 2019. Clinical examination revealed a large mass behind the left knee, bilateral great toe deformities, and no palpable superficial lymph nodes, without systemic pain or other discomfort. Imaging and genetic testing further supported the diagnosis of FOP, demonstrating high-density ossification within soft tissues and a mutation in the ACVR1 gene. Treatment involved a combination of methylprednisolone and alendronate sodium vitamin D3 tablets, which yielded some therapeutic efficacy. The discussion emphasizes clinical diagnosis, pathogenesis, and treatment strategies for FOP, including injury prevention, rehabilitation exercises, and pharmacological interventions. Despite the lack of definitive treatment options, timely diagnosis and comprehensive management can effectively alleviate symptoms and improve the quality of life for affected individuals.

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.

Tracheostomy for the pediatric patient with fibrodysplasia ossificans progressiva: a case report.

BACKGROUND: Fibrodysplasia ossificans progressiva (FOP) is an extremely rare connective tissue disease characterized by subsequent ossification of skeletal muscles, tendons, ligaments, and other fibrous tissues. The ossification of these tissues progresses during childhood and leads to limb and trunk deformities. Since any surgery may trigger subsequent ossification, it is relatively contraindicated for patients with FOP. In this report, we describe our experience in performing tracheostomy in a pediatric patient with FOP who developed a restrictive respiratory disorder due to progressive deformity of the trunk. CASE PRESENTATION: A 12-year-old boy, diagnosed with FOP at the age of one, was referred for a tracheotomy after requiring 2 months of oral intubation and mechanical ventilation due to severe deformity-induced dyspnea. After changing from oral intubation to nasal intubation, we carefully considered the indications and benefits of tracheostomy in patients with FOP. Eventually, tracheostomy was successfully performed using our surgical design: creating a skin incision at the level of the cricoid cartilage that can always be identified, creating inverted U-shaped incision on the anterior tracheal wall to make a flap, and suturing the entire circumference of the tracheotomy and skin. One month after the surgery, he regained normal breathing and pronunciation and returned to school. The patient showed no unfavorable postoperative outcomes over a 4-year follow-up period. CONCLUSIONS: Tracheostomy in our pediatric case of FOP required careful perioperative management. However, it could effectively improve the patient's quality of life.

Fibrodysplasia Ossificans Progressiva: A rare disease due to unawareness, case report and literature review.

INTRODUCTION AND IMPORTANCE: Fibrodysplasia Ossificans Progressiva is an ultra-rare genetic disorder of progressive soft tissue ossification. Due to unawareness and poor clinical suspicion, the rate of misdiagnosis, delay in diagnosis, and unnecessary diagnostic procedures leading to permanent injury and lifelong disability is common. Here we report this rare genetic disorder in a six years old child who was initially misdiagnosed as multiple exostoses and operated on. CASE PRESENTATION: A 6 year old child presented with swellings over the posterior neck and back for four years. The patient was misdiagnosed as a case of multiple exostoses and an excisional biopsy was done a year back. The swelling worsened after the excision; currently, she cannot move her neck from side to side, and flex and extend. Examination revealed multiple hard and slightly tender masses over the posterior neck, para scapular and thoracolumbar para spinal region. She also has hallux valgus deformity that had been present since birth. CT (computed tomography) scan confirmed extensive extra-skeletal soft tissue ossification. CLINICAL DISCUSSION: The progression of heterotopic ossification is characteristically anatomic and orderly, typically initially involving the body's dorsal, axial, cranial, and proximal regions and later in the ventral, appendicular, caudal, and distal regions. Skeletal muscles of the tongue, diaphragm, extra-ocular muscles, cardiac muscles, and smooth muscles are inexplicably spared. CONCLUSION: Early diagnosis prevents potentially harmful diagnostic and therapeutic procedures. The characteristic big toes malformation is the most important and best key for the early suspicion of the diagnosis.

Intersections of Fibrodysplasia Ossificans Progressiva and Traumatic Heterotopic Ossification.

Heterotopic ossification (HO) is a debilitating pathology where ectopic bone develops in areas of soft tissue. HO can develop as a consequence of traumatic insult or as a result of dysregulated osteogenic signaling, as in the case of the orphan disease fibrodysplasia ossificans progressiva (FOP). Traumatic HO (tHO) formation is mediated by the complex interplay of signaling between progenitor, inflammatory, and nerve cells, among others, making it a challenging process to understand. Research into the pathogenesis of genetically mediated HO (gHO) in FOP has established a pathway involving uninhibited activin-like kinase 2 receptor (ALK2) signaling that leads to downstream osteogenesis. Current methods of diagnosis and treatment lag behind pre-mature HO detection and progressive HO accumulation, resulting in irreversible decreases in range of motion and chronic pain for patients. As such, it is necessary to draw on advancements made in the study of tHO and gHO to better diagnose, comprehend, prevent, and treat both.

Immunologic Aspects in Fibrodysplasia Ossificans Progressiva.

BACKGROUND: Inflammation is a major driver of heterotopic ossification (HO), a condition of abnormal bone growth in a site that is not normally mineralized. PURPOSE OF REVIEW: This review will examine recent findings on the roles of inflammation and the immune system in fibrodysplasia ossificans progressiva (FOP). FOP is a genetic condition of aggressive and progressive HO formation. We also examine how inflammation may be a valuable target for the treatment of HO. Rationale/Recent findings: Multiple lines of evidence indicate a key role for the immune system in driving FOP pathogenesis. Critical cell types include macrophages, mast cells, and adaptive immune cells, working through hypoxia signaling pathways, stem cell differentiation signaling pathways, vascular regulatory pathways, and inflammatory cytokines. In addition, recent clinical reports suggest a potential role for immune modulators in the management of FOP. FUTURE PERSPECTIVES: The central role of inflammatory mediators in HO suggests that the immune system may be a common target for blocking HO in both FOP and non-genetic forms of HO. Future research focusing on the identification of novel inflammatory targets will help support the testing of potential therapies for FOP and other related conditions.

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.

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.

A healthy 35-year-old man (patient-R) with the classic fibrodysplasia ossificans progressiva (FOP) mutation and the congenital great toe malformation of FOP had extreme lack of heterotopic ossification (HO) and nearly normal mobility. We hypothesized that patient-R lacked a sufficient inflammatory trigger for HO. Blood tests revealed a reduction in the level of an inflammatory protein called matrix metalloproteinase-9 (MMP-9) compared to other individuals with FOP as well as healthy controls. DNA analysis in patient-R identified mutations in MMP-9, one of which predicted decreased activity of MMP-9 which was confirmed by further testing. Inflammatory cells (macrophages) expressing the MMP-9 mutations identified in patient-R produced significantly less Activin A, an obligate stimulus for HO in FOP. In order to determine if MMP-9 deficiency was a cause of HO prevention in FOP, we inhibited MMP-9 activity by genetic, biologic, or pharmacologic means in FOP mouse models and showed that MMP-9 inhibition prevented or dramatically decreased trauma-induced HO in FOP, locked-up Activin A in the extracellular matrix, and induced regeneration of injured skeletal muscle. Our data show that MMP-9 links inflammation to HO and illustrate that one patient's clinical picture can reveal critical molecular mechanisms of disease that unveil new treatment strategies.

The Activation of the Fibrodysplasia Ossificans Progressiva-Inducing ALK2-R206H Mutant Depends on the Distinct Homo-Oligomerization Patterns of ACVR2B and ACVR2A.

Mutations in activin-like kinase 2 (ALK2), e.g., ALK2-R206H, induce aberrant signaling to SMAD1/5/8, leading to Fibrodysplasia Ossificans Progressiva (FOP). In spite of extensive studies, the underlying mechanism is still unclear. Here, we quantified the homomeric and heteromeric interactions of ACVR2A, ACVR2B, ALK2-WT, and ALK2-R206H by combining IgG-mediated immobilization of one receptor with fluorescence recovery after photobleaching (FRAP) measurements on the lateral diffusion of a co-expressed receptor. ACVR2B formed stable homomeric complexes that were enhanced by Activin A (ActA), while ACVR2A required ActA for homodimerization. ALK2-WT, but not ALK2-R206H, exhibited homomeric complexes unaffected by ActA. ACVR2B formed ActA-enhanced heterocomplexes with ALK2-R206H or ALK2-WT, while ACVR2A interacted mainly with ALK2-WT. The extent of the homomeric complex formation of ACVR2A or ACVR2B was reflected in their ability to induce the oligomerization of ALK2-R206H and ALK2-WT. Thus, ACVR2B, which forms dimers without ligand, induced ActA-independent ALK2-R206H clustering but required ActA for enhancing the oligomerization of the largely dimeric ALK2-WT. In contrast, ACVR2A, which undergoes homodimerization in response to ActA, required ActA to induce ALK2-R206H oligomerization. To investigate whether these interactions are translated into signaling, we studied signaling by the FOP-inducing hyperactive ALK2-R206H mutant, with ALK2-WT signaling as control. The activation of SMAD1/5/8 signaling in cells expressing ALK2-R206H alone or together with ACVR2A or ACVR2B was measured by blotting for pSMAD1/5/8 and by transcriptional activation assays using BRE-Luc reporter. In line with the biophysical studies, ACVR2B activated ALK2-R206H without ligand, while activation by ACVR2A was weaker and required ActA. We propose that the homodimerization of ACVR2B or ACVR2A dictates their ability to recruit ALK2-R206H into higher complexes, enabling the homomeric interactions of ALK2-R206H receptors and, subsequently, their activation.

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.

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.