Overexpression of an osteogenic morphogen in fibrodysplasia ossificans progressiva

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.

How Activin A Became a Therapeutic Target in Fibrodysplasia Ossificans Progressiva

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

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

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

The serum levels of activin A and bone morphogenetic protein-4 and -6 in patients with fibrodysplasia ossificans progressiva

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP) is an ultrarare and disabling genetic disorder of connective tissue characterized by congenital malformation of the great toes, and progressive heterotopic ossification (HO) in soft connective tissues. A gain-of-function mutation of activin A receptor type I (ACVR1) enables ACVR1 to recognize activin A as an agonist with bone morphogenetic protein (BMP) signalling that leads to HO. Previous studies confirmed that activin A stimulates BMP signalling in vitro and drives HO in mouse models of FOP. However, the roles for BMP4 and BMP6 in FOP are supported only by correlative evidence in vitro. Thus, it remains unclear whether the circulating levels of activin A, BMP4 and BMP6 correlate with flare-ups in FOP patients. Hence, we investigated the protein levels of activin A, BMP4 and BMP6 in the serum of FOP patients.

RESULTS

We recruited 16 untreated FOP patients and 16 age- and sex- matched healthy control subjects in this study. The 16 FOP patients were retrospectively divided into the flare-up group (n = 8) and remission group (n = 8) depending on whether they had flare-ups or worsening of any joint movement in the last 6 months. The serum activin A, BMP4 and BMP6 levels were detected by enzyme-linked immunosorbent assay. The serum activin A, BMP4 and BMP6 levels were slightly higher in FOP patients (median: 434.05 pg/mL, 459.48 pg/mL and 67.84 pg/mL) versus healthy control subjects (median: 364.14 pg/mL, 450.39 pg/mL and 55.36 pg/mL). However, there were no statistically significant differences between the two groups (p > 0.05 for all items), nor were there significant differences between the flare-up and remission groups of FOP (p > 0.05 for all items). Univariate and multivariate logistic regression analyses showed that age, sex, and serum activin A, BMP4 and BMP6 levels were not related to flare-up in FOP patients.

CONCLUSIONS

There were no significant differences in the serum levels of activin A, BMP4 and BMP6 in FOP patients compared with healthy control subjects. Serum activin A, BMP4 and BMP6 proteins might not be the stimulators for FOP flare-up, and may not be biomarkers for FOP diagnosis.

Transcriptional heterogeneity in human diabetic foot wounds

Wound repair requires the coordination of multiple cell types including immune cells and tissue resident cells to coordinate healing and return of tissue function. Diabetic foot ulceration is a type of chronic wound that impacts over 4 million patients in the US and over 7 million worldwide (Edmonds et al., 2021). Yet, the cellular and molecular mechanisms that go awry in these wounds are not fully understood. Here, by profiling chronic foot ulcers from non-diabetic (NDFUs) and diabetic (DFUs) patients using single-cell RNA sequencing, we find that DFUs display transcription changes that implicate reduced keratinocyte differentiation, altered fibroblast function and lineages, and defects in macrophage metabolism, inflammation, and ECM production compared to NDFUs. Furthermore, analysis of cellular interactions reveals major alterations in several signaling pathways that are altered in DFUs. These data provide a view of the mechanisms by which diabetes alters healing of foot ulcers and may provide therapeutic avenues for DFU treatments.

mTORC1 coordinates NF-κB signaling pathway to promote chondrogenic differentiation of tendon cells in heterotopic ossification

Heterotopic ossification (HO) is a pathological bone formation based on endochondral ossification distinguished by ossification within muscles, tendons, or other soft tissues. There has been growing studies focusing on the treatment with rapamycin to inhibit HO, but the mechanism of mTORC1 on HO remains unclear. Tendon cells (TDs) are the first cells to form during tendon heterotopic ossification. Here, we used an in vivo model of HO and an in vitro model of chondrogenesis induction to elucidate the effect and underlying mechanism of mTORC1 in HO. The current study highlights the effect of rapamycin on murine Achilles tenotomy-induced HO and the role of mTORC1 signaling pathway on TDs. Our result showed that mTORC1 was activation in the early stage of HO, whereas the mTORC1 maintained low expression in the mature ectopic cartilage tissue and the ectopic bone formation sites. The use of mTORC1-specific inhibitor (rapamycin) immediately after Achilles tendon injury could suppress the formation of HO; once ectopic cartilage and bone had formed, treatment with rapamycin could not significantly inhibit the progression of HO. Mechanistically, mTORC1 stimulation by silencing of TSC1 promoted the expression of the chondrogenic markers in TDs. In TDs, treated with mTORC1 stimulation by silencing of TSC1, mTORC1 increased the activation of the NF-κB signaling pathway. NF-κB selective inhibitor BAY11-7082 significantly suppressed the chondrogenesis of TDs that treated with mTORC1 stimulation by silencing of TSC1. Together, our findings demonstrated that mTORC1 promoted HO by regulating TDs chondrogenesis partly through the NF-κB signaling pathway; and rapamycin could be a viable HO therapeutic regimen.

Pathophysiology and Emerging Molecular Therapeutic Targets in Heterotopic Ossification

The term heterotopic ossification (HO) describes bone formation in tissues where bone is normally not present. Musculoskeletal trauma induces signalling events that in turn trigger cells, probably of mesenchymal origin, to differentiate into bone. The aetiology of HO includes extremely rare but severe, generalised and fatal monogenic forms of the disease; and as a common complex disorder in response to musculoskeletal, neurological or burn trauma. The resulting bone forms through a combination of endochondral and intramembranous ossification, depending on the aetiology, initiating stimulus and affected tissue. Given the heterogeneity of the disease, many cell types and biological pathways have been studied in efforts to find effective therapeutic strategies for the disorder. Cells of mesenchymal, haematopoietic and neuroectodermal lineages have all been implicated in the pathogenesis of HO, and the emerging dominant signalling pathways are thought to occur through the bone morphogenetic proteins (BMP), mammalian target of rapamycin (mTOR), and retinoic acid receptor pathways. Increased understanding of these disease mechanisms has resulted in the emergence of several novel investigational therapeutic avenues, including palovarotene and other retinoic acid receptor agonists and activin A inhibitors that target both canonical and non-canonical signalling downstream of the BMP type 1 receptor. In this article we aim to illustrate the key cellular and molecular mechanisms involved in the pathogenesis of HO and outline recent advances in emerging molecular therapies to treat and prevent HO that have had early success in the monogenic disease and are currently being explored in the common complex forms of HO.

A self-amplifying loop of YAP and SHH drives formation and expansion of heterotopic ossification

Heterotopic ossification (HO) occurs as a common complication after injury or in genetic disorders. The mechanisms underlying HO remain incompletely understood, and there are no approved prophylactic or secondary treatments available. Here, we identify a self-amplifying, self-propagating loop of Yes-associated protein (YAP)-Sonic hedgehog (SHH) as a core molecular mechanism underlying diverse forms of HO. In mouse models of progressive osseous heteroplasia (POH), a disease caused by null mutations in GNAS, we found that Gnas^-/- mesenchymal cells secreted SHH, which induced osteoblast differentiation of the surrounding wild-type cells. We further showed that loss of Gnas led to activation of YAP transcription activity, which directly drove Shh expression. Secreted SHH further induced YAP activation, Shh expression, and osteoblast differentiation in surrounding wild-type cells. This self-propagating positive feedback loop was both necessary and sufficient for HO expansion and could act independently of Gnas in fibrodysplasia ossificans progressiva (FOP), another genetic HO, and nonhereditary HO mouse models. Genetic or pharmacological inhibition of YAP or SHH abolished HO in POH and FOP and acquired HO mouse models without affecting normal bone homeostasis, providing a previously unrecognized therapeutic rationale to prevent, reduce, and shrink HO.

Accumulated Knowledge of Activin Receptor-Like Kinase 2 (ALK2)/Activin A Receptor, Type 1 (ACVR1) as a Target for Human Disorders

Activin receptor-like kinase 2 (ALK2), also known as Activin A receptor type 1 (ACVR1), is a transmembrane kinase receptor for members of the transforming growth factor-β family. Wild-type ALK2/ACVR1 transduces osteogenic signaling in response to ligand binding. Fifteen years ago, a gain-of-function mutation in the ALK2/ACVR1 gene was detected in patients with the genetic disorder fibro-dysplasia ossificans progressiva, which is characterized by heterotopic ossification in soft tissues. Additional disorders, such as diffuse intrinsic pontin glioma, diffuse idiopathic skeletal hyperostosis, primary focal hyperhidrosis, and congenital heart defects, have also been found to be associated with ALK2/ACVR1. These findings further expand in vitro and in vivo model system research and promote our understanding of the molecular mechanisms of the pathogenesis and development of novel therapeutics and diagnosis for disorders associated with ALK2/ACVR1. Through aggressive efforts, some of the disorders associated with ALK2/ACVR1 will be overcome in the near future.

Reduction of mechanical loading in tendons induces heterotopic ossification and activation of the β-catenin signaling pathway

Background

Tendons are the force transferring tissue that enable joint movement. Excessive mechanical loading is commonly considered as a primary factor causing tendinopathy, however, an increasing body of evidence supports the hypothesis that overloading creates microdamage of collagen fibers resulting in a localized decreased loading on the cell population within the damaged site. Heterotopic ossification is a complication of late stage tendinopathy, which can significantly affect the mechanical properties and homeostasis of the tendon. Here, we the examine the effect of mechanical underloading on tendon ossification and investigate its underlying molecular mechanism.

Method

Rabbit Achilles tendons were dissected and cultured in an underloading environment (3% cyclic tensile stain,0.25 ​Hz, 8 ​h/day) for either 10, 15 or 20 days. Using isolated tendon-derived stem cells (TDSCs) 3D constructs were generated, cultured and subjected to an underloading environment for 6 days. Histological assessments were performed to evaluate the structure of the 3D constructs; qPCR and immunohistochemistry were employed to study TDSC differentiation and the β-catenin signal pathway was investigated by Western blotting. Mechanical testing was used to determine ability of the tendon to withstand force generation.

Result

Tendons cultured for extended times in an environment of underloading showed progressive heterotopic ossification and a reduction in biomechanical strength. qPCR revealed that 3D TDSCs constructs cultured in an underloading environment exhibited increased expression of several osteogenic genes: these include RUNX2, ALP and osteocalcin in comparison to tenogenic differentiation markers (scleraxis and tenomodulin). Immunohistochemical analysis further confirmed high osteocalcin production in 3D TDSCs constructs subject to underloading. Western blotting of TDSC constructs revealed that β-catenin accumulation and translocation were associated with an increase in phosphorylation at Ser552 and decrease phosphorylation at Ser33.

Conclusion

These findings unveil a potential mechanism for heterotopic ossification in tendinopathy due to the underloading of TDSCs at the damage sites, and also that β-catenin could be a potential target for treating heterotopic ossification in tendons.

The Translational potential

Tendon heterotopic ossification detrimentally affect quality of life especially for those who has atheletic career. This study reveals the possible mechanism of heterotpic ossification in tendon related to mechanical loading. This study provided the possible to develop a mechanical stimulation protocol for preventive and therapeutic purpose for tendon heterotopic ossification.

Chondrogenesis mediates progression of ankylosing spondylitis through heterotopic ossification

Ankylosing spondylitis (AS) is chronic inflammatory arthritis with a progressive fusion of axial joints. Anti-inflammatory treatments such as anti-TNF-α antibody therapy suppress inflammation but do not effectively halt the progression of spine fusion in AS patients. Here we report that the autoimmune inflammation of AS generates a microenvironment that promotes chondrogenesis in spine ligaments as the process of spine fusion. Chondrocyte differentiation was observed in the ligaments of patients with early-stage AS, and cartilage formation was followed by calcification. Moreover, a large number of giant osteoclasts were found in the inflammatory environment of ligaments and on bony surfaces of calcified cartilage. Resorption activity by these giant osteoclasts generated marrow with high levels of active TGF-β, which induced new bone formation in the ligaments. Notably, no Osterix+ osteoprogenitors were found in osteoclast resorption areas, indicating uncoupled bone resorption and formation. Even at the late and maturation stages, the uncoupled osteoclast resorption in bony interspinous ligament activates TGF-β to induce the progression of ossification in AS patients. Osteoclast resorption of calcified cartilage-initiated ossification in the progression of AS is a similar pathologic process of acquired heterotopic ossification (HO). Our finding of cartilage formation in the ligaments of AS patients revealed that the pathogenesis of spinal fusion is a process of HO and explained why anti-inflammatory treatments do not slow ankylosing once there is new bone formation in spinal soft tissues. Thus, inhibition of HO formation, such as osteoclast activity, cartilage formation, or TGF-β activity could be a potential therapy for AS.

Genetic and Acquired Heterotopic Ossification: A Translational Tale of Mice and Men

Heterotopic ossification is defined as an aberrant formation of bone in extraskeletal soft tissue, for which both genetic and acquired conditions are known. This pathologic process may occur in many different sites such as the skin, subcutaneous tissue, skeletal muscle and fibrous tissue adjacent to joints, ligaments, walls of blood vessels, mesentery and other. The clinical spectrum of this disorder is wide: lesions may range from small foci of ossification to massive deposits of bone throughout the body, typical of the progressive genetically determined conditions such as fibrodysplasia ossificans progressiva, to mention one of the most severe and disabling forms. The ectopic bone formation may be regarded as a failed tissue repair process in response to a variety of triggers and evolving towards bone formation through a multistage differentiation program, with several steps common to different clinical presentations and distinctive features. In this review, we aim at providing a comprehensive view of the genetic and acquired heterotopic ossification disorders by detailing the clinical and molecular features underlying the different human conditions in comparison with the corresponding, currently available mouse models.

The hypoxic microenvironment: a driving force for heterotopic ossification progression

Heterotopic ossification (HO) refers to the formation of bone tissue outside the normal skeletal system. According to its pathogenesis, HO is divided into hereditary HO and acquired HO. There currently lack effective approaches for HO prevention or treatment. A deep understanding of its pathogenesis will provide promising strategies to prevent and treat HO. Studies have shown that the hypoxia-adaptive microenvironment generated after trauma is a potent stimulus of HO. The hypoxic microenvironment enhances the stability of hypoxia-inducible factor-1α (HIF-1α), which regulates a complex network including bone morphogenetic proteins (BMPs), vascular endothelial growth factor (VEGF), and neuropilin-1 (NRP-1), which are implicated in the formation of ectopic bone. In this review, we summarize the current understanding of the triggering role and underlying molecular mechanisms of the hypoxic microenvironment in the initiation and progression of HO, focusing mainly on HIF-1 and it's influenced genes  BMP, VEGF, and NRP-1. A better understanding of the role of hypoxia in HO unveils novel therapeutic targets for HO that reduce the local hypoxic microenvironment and inhibit HIF-1α activity.

The Morphology of Cross-Beaks and BMP4 Gene Expression in Huiyang Bearded Chickens

Simple Summary

Recently, the emergence of cross-beaks has been reported in several domestic chickens. Despite several candidate genes, bone morphogenetic protein 4 (BMP4) has been suggested as responsible for chicken cross-beaks. The subtypes of the morphology term, etiopathogenesis, and the relationship of the candidate BMP4 gene to cross-beaks are not yet known. The objective of this study was to describe the subtypes of cross-beaks by left or right and upper and lower jaw bones and to figure out the relationship between BMP4 and the development of craniofacial bones in Huiyang bearded chickens.

Abstract

Bird beaks are important for biological purposes such as food intake, removing parasites, and defining phenotypic attributes. Cross-beaks are a threat to poultry health and are harmful to productivity, wasting some units in the poultry industry. However, there is still limited research on subtypes of cross-beaks and the genetic basis of cross-beaks as well. Here, we described the subtypes of cross-beaks in terms of left or right and upper or lower jaw bones. We evaluated the impact of cross-beaks on craniofacial bones and figured out the relationship between bone morphogenetic protein 4 (BMP4) and the development of craniofacial bones in Huiyang bearded chickens. We identified five typical subtypes of cross-beaks by morphological assessment and X-ray scanning. We found that cross-beaks caused certain changes in the facial bone morphology, including changes to the length and width of the bone around the ocular area (p < 0.05). The relative expressions of BMP4 in lacrimal, mandible, premaxilla, frontal, and parietal bones were significantly higher in the severe cross-beak group, followed by that of the medium cross-beak group, weak cross-beak group, and control group (p < 0.05). Overall, we constructed a generally applicable method to classify cross-beaks in term of the angle. The skeleton around the ocular area was affected by the cross-beak. The expression levels of BMP4 in craniofacial bones may provide insight to potential role of BMP4 in the development of cross-beaks.

Inflammation in Fibrodysplasia Ossificans Progressiva and Other Forms of Heterotopic Ossification

PURPOSE OF REVIEW

Heterotopic ossification (HO) is associated with inflammation. The goal of this review is to examine recent findings on the roles of inflammation and the immune system in HO. We examine how inflammation changes in fibrodysplasia ossificans progressiva, in traumatic HO, and in other clinical conditions of HO. We also discuss how inflammation may be a target for treating HO.

RECENT FINDINGS

Both genetic and acquired forms of HO show similarities in their inflammatory cell types and signaling pathways. These include macrophages, mast cells, and adaptive immune cells, along with hypoxia signaling pathways, mesenchymal stem cell differentiation signaling pathways, vascular signaling pathways, and inflammatory cytokines. Because there are common inflammatory mediators across various types of HO, these mediators may serve as common targets for blocking HO. Future research may focus on identifying new inflammatory targets and testing combinatorial therapies based on these results.

Fibrodysplasia Ossificans Progressiva - A Rare Genetic Disorder and the Role of Technetium-99m Methylene Diphosphonate Bone Scan

Fibrodysplasia ossificans progressiva is a rare genetic disease believed to occur in approximately 1 in 2 million people worldwide and is characterized by progressive extraosseous ossification over the course of a lifetime in an inevitable and unpredictable episodic manner, with most patients being confined to a wheelchair by the third decade of life and requiring life-long care. The extraosseous calcification involves ligaments, tendons, muscles, and connective tissue leading to severe restriction of movements. Another hallmark of this condition is abnormal great toes. The diagnosis is often made on clinical and radiological examination, but Technetium-99m methylene diphosphonate (Tc-99m MDP) bone scan is usually indicated to determine the extent of the disease. We hereby present a case series comprising of four patients suffering from this debilitating illness who underwent Tc99m MDP bone scan for initial diagnosis and localizing sites of heterotopic ossification.

Recent Topics in Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease that is characterized by the formation of heterotopic bone tissues in soft tissues, such as skeletal muscle, ligament, and tendon. It is difficult to remove such heterotopic bones via internal medicine or invasive procedures. The identification of activin A receptor, type I (ACVR1)/ALK2 gene mutations associated with FOP has allowed the genetic diagnosis of FOP. The ACVR1/ALK2 gene encodes the ALK2 protein, which is a transmembrane kinase receptor in the transforming growth factor-β family. The relevant mutations activate intracellular signaling in vitro and induce heterotopic bone formation in vivo. Activin A is a potential ligand that activates mutant ALK2 but not wild-type ALK2. Various types of small chemical and biological inhibitors of ALK2 signaling have been developed to establish treatments for FOP. Some of these are in clinical trials in patients with FOP.

Fibrodysplasia ossificans progressiva: a current review of imaging findings

OBJECTIVE

Fibrodysplasia ossificans progressiva is a rare genetic disorder characterized by congenital skeletal deformities and soft tissue masses that progress to heterotopic ossification. Deformities of the great toes are distinctive, and heterotopic ossification in the soft tissues follows an expected anatomic and temporal pattern. In addition to heterotopic ossification, osteochondromata, middle ear ossification, demyelination, lymphedema, and venous thrombosis are characteristic. Awareness of this constellation of findings is important to early diagnosis and surveillance.

CONCLUSIONS

Recognition of the imaging manifestations of fibrodysplasia ossificans progressiva is imperative to early diagnosis in order to appropriately direct patient care and preclude unnecessary biopsies or surgical procedures.

TGF-β Family Signaling in Mesenchymal Differentiation

Mesenchymal stem cells (MSCs) can differentiate into several lineages during development and also contribute to tissue homeostasis and regeneration, although the requirements for both may be distinct. MSC lineage commitment and progression in differentiation are regulated by members of the transforming growth factor-β (TGF-β) family. This review focuses on the roles of TGF-β family signaling in mesenchymal lineage commitment and differentiation into osteoblasts, chondrocytes, myoblasts, adipocytes, and tenocytes. We summarize the reported findings of cell culture studies, animal models, and interactions with other signaling pathways and highlight how aberrations in TGF-β family signaling can drive human disease by affecting mesenchymal differentiation.

Activin A amplifies dysregulated BMP signaling and induces chondro-osseous differentiation of primary connective tissue progenitor cells in patients with fibrodysplasia ossificans progressiva (FOP)

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP), is caused by mutations in the type I BMP receptor ACVR1 that lead to increased activation of the BMP-pSmad1/5/8 signaling pathway. Recent findings suggest that Activin A (Act A) promiscuously stimulates the bone morphogenetic protein (BMP) signaling pathway in vitro and mediates heterotopic ossification (HO) in mouse models of FOP, but primary data from FOP patient cells are lacking.

OBJECTIVE

To examine BMP-pSmad1/5/8 pathway signaling and chondro-osseous differentiation in response to endogenous and exogenous Act A in primary connective tissue progenitor cells [CTPCs; also known as stem cells from human exfoliated deciduous teeth (SHED) cells] from patients with FOP. These cells express the common FOP mutation, ACVR1 (R206H).

RESULTS

We found that Act A amplifies dysregulated BMP pathway signaling in human FOP primary CTPCs cells through the Smad1/5/8 pathway and induces chondro-osseous differentiation. Amplification of BMP-pSmad1/5/8 signaling was inhibited by Follistatin and by a neutralizing antibody to Activin A. The increased basal pSmad1/5/8 activity, as well as the hypoxia-induced stimulation of FOP CTPCs cells, were BMP4 and Act A independent. Importantly, either BMP4 or Act A stimulated pSmad1/5/8 pathway signaling but BMP4 signaling was not dependent on Activin A and vice versa. Circulating plasma levels of Act A or BMP4 are similar in controls compared to FOP patients, and suggest the potential for an autocrine or paracrine route for pathological signaling.

CONCLUSIONS

The mutated FOP receptor [ACVR1 (R206H)] is hypersensitive to BMP4 and uniquely sensitive (compared to the wild type receptor) to Act A. Both canonical and non-canonical ligands have a synergistic effect on BMP-pSmad1/5/8 signaling in FOP CTPCs and may cooperate to alter the threshold for HO in FOP. Our findings in primary human FOP CTPCs have important implications for the design of clinical trials to inhibit dysregulated BMP pathway signaling in humans who have FOP.

Shared ACVR1 mutations in FOP and DIPG: Opportunities and challenges in extending biological and clinical implications across rare diseases

Gain-of-function mutations in the Type I Bone Morphogenic Protein (BMP) receptor ACVR1 have been identified in two diseases: Fibrodysplasia Ossificans Progressiva (FOP), a rare autosomal dominant disorder characterized by genetically driven heterotopic ossification, and in 20-25% of Diffuse Intrinsic Pontine Gliomas (DIPGs), a pediatric brain tumor with no effective therapies and dismal median survival. While the ACVR1 mutation is causal for FOP, its role in DIPG tumor biology remains under active investigation. Here, we discuss cross-fertilization between the FOP and DIPG fields, focusing on the biological mechanisms and principles gleaned from FOP that can be applied to DIPG biology. We highlight our current knowledge of ACVR1 in both diseases, and then describe the growing opportunities and barriers to effectively investigate ACVR1 in DIPG. Importantly, learning from other seemingly unrelated diseases harboring similar mutations may uncover novel mechanisms or processes for future investigation.

The obligatory role of Activin A in the formation of heterotopic bone in Fibrodysplasia Ossificans Progressiva

Fibrodysplasia Ossificans Progressiva (FOP) is a rare genetic disorder that presents at birth with only minor patterning defects, but manifests its debilitating pathology early in life with episodic, yet progressive and cumulative, heterotopic ossification (HO) of ligaments, tendons, and a subset of major skeletal muscles. The resulting HO lesions are endochondral in nature, and appear to be linked to inflammatory stimuli arising in association with known injuries, or from inflammation linked to normal tissue repair. FOP is caused by gain-of-function mutations in ACVR1, which encodes a type I BMP receptor. Initial studies on the pathogenic mechanism of FOP-causing mutations in ACVR1 focused on the enhanced function of this receptor in response to certain BMP ligands, or independently of ligands, but did not directly address the fact that HO in FOP is episodic and inflammation-driven. Recently, we and others demonstrated that Activin A is an obligate factor for the initiation of HO in FOP, signaling aberrantly via mutant ACVR1 to transduce osteogenic signals and trigger heterotopic bone formation (Hatsell et al., 2015; Hino et al., 2015). Subsequently, we identified distinct tissue-resident mesenchymal progenitor cells residing in muscles and tendons that recognize Activin A as a pro-osteogenic signal (solely in the context of FOP-causing mutant ACVR1), and give rise to the cartilaginous anlagen that form heterotopic bone (Dey et al., 2016). During the course of these studies, we also found that the activity of FOP-causing ACVR1 mutations does not by itself explain the triggered or inflammatory nature of HO in FOP, suggesting the importance of other, inflammation-introduced, factors or processes. This review presents a synthesis of these findings with a focus on the role of Activin A and inflammation in HO, and lays out perspectives for future research.

Variable signaling activity by FOP ACVR1 mutations

Most patients with fibrodysplasia ossificans progressiva (FOP), a rare genetic disorder of heterotopic ossification, have the same causative mutation in ACVR1, R206H. However, additional mutations within the ACVR1 BMP type I receptor have been identified in a small number of FOP cases, often in patients with disease of lesser or greater severity than occurs with R206H mutations. Genotype-phenotype correlations have been suggested in patients, resulting in classification of FOP mutations based on location within different receptor domains and structural modeling. However while each of the mutations induces increased signaling through the BMP-pSmad1/5/8 pathway, the molecular mechanisms underlying functional differences of these FOP variant receptors remained undetermined. We now demonstrate that FOP mutations within the ACVR1 receptor kinase domain are more sensitive to low levels of BMP than mutations in the ACVR1 GS domain. Our data additionally confirm responsiveness of cells with FOP ACVR1 mutations to both BMP and Activin A ligands. We also have determined that constructs with FOP ACVR1 mutations that are engineered without the ligand-binding domain retain increased BMP-pSmad1/5/8 pathway activation relative to wild-type ACVR1, supporting that the mutant receptors can function through ligand-independent mechanisms either directly through mutant ACVR1 or through indirect mechanisms.

Heterotopic bone induction via BMP signaling: Potential therapeutic targets for fibrodysplasia ossificans progressiva

More than 50years ago, Marshal M. Urist detected "heterotopic bone-inducing activity" in demineralized bone matrix. This unique activity was referred to as "bone morphogenetic protein (BMP)" because it was sensitive to trypsin digestion. Purification of the bone-inducing activity from demineralized bone matrix using a bone-inducing assay in vivo indicated that the original "BMP" consisted of a mixture of new members of the transforming growth factor-β (TGF-β) family. The establishment of new in vitro assay systems that reflect the bone-inducing activity of BMPs in vivo have revealed the functional receptors and downstream effectors of BMPs. Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder characterized by progressive heterotopic bone formation in soft tissues similar to the event induced by the transplantation of BMPs in skeletal muscle. In patients with FOP, genetic mutations have been identified in the ACVR1 gene, which encodes the BMP receptor ALK2. The mutations in ALK2 associated with FOP are hypersensitive to type II receptor kinases. Recently, activin A, a non-osteogenic member of the TGF-β family, was identified as the ligand of the mutant ALK2 in FOP, and various types of signaling inhibitors for mutant ALK2 are currently under development to establish effective treatments for FOP.

Stem cells and heterotopic ossification: Lessons from animal models

Put most simply, heterotopic ossification (HO) is the abnormal formation of bone at extraskeletal sites. HO can be classified into two main subtypes, genetic and acquired. Acquired HO is a common complication of major connective tissue injury, traumatic central nervous system injury, and surgical interventions, where it can cause significant pain and postoperative disability. A particularly devastating form of HO is manifested in the rare genetic disorder, fibrodysplasia ossificans progressiva (FOP), in which progressive heterotopic bone formation occurs throughout life, resulting in painful and disabling cumulative immobility. While the central role of stem/progenitor cell populations in HO is firmly established, the identity of the offending cell type(s) remains to be conclusively determined, and little is known of the mechanisms that direct these progenitor cells to initiate cartilage and bone formation. In this review, we summarize current knowledge of the cells responsible for acquired HO and FOP, highlighting the strengths and weaknesses of animal models used to interrogate the cellular origins of HO.

Inhibition of overactive TGF-β attenuates progression of heterotopic ossification in mice

Acquired heterotopic ossification (HO) is a painful and debilitating disease characterized by extraskeletal bone formation after injury. The exact pathogenesis of HO remains unknown. Here we show that TGF-β initiates and promotes HO in mice. We find that calcified cartilage and newly formed bone resorb osteoclasts after onset of HO, which leads to high levels of active TGF-β that recruit mesenchymal stromal/progenitor cells (MSPCs) in the HO microenvironment. Transgenic expression of active TGF-β in tendon induces spontaneous HO, whereas systemic injection of a TGF-β neutralizing antibody attenuates ectopic bone formation in traumatic and BMP-induced mouse HO models, and in a fibrodysplasia ossificans progressive mouse model. Moreover, inducible knockout of the TGF-β type II receptor in MSPCs inhibits HO progression in HO mouse models. Our study points toward elevated levels of active TGF-β as inducers and promoters of ectopic bone formation, and suggest that TGF-β might be a therapeutic target in HO.

Abundant heterotopic bone formation following use of rhBMP-2 in the treatment of acetabular bone defects during revision hip arthroplasty

Revision hip arthroplasty in the setting of periacetabular bone loss presents a significant challenge, as options for restoring bone loss are limited. Recombinant human bone morphogenetic protein-2 may offer a solution by promoting bone growth to restore bone stock before implant reimplantation. Here we present a case of a patient with a periprosthetic acetabulum fracture, resulting in pelvic discontinuity as the result of significant periacetabular bone loss. Using a staged approach, periacetabular bone stock was nearly entirely reconstituted using recombinant BMPs and allograft, which resulted in stable fixation, but with abundant heterotopic bone formation. Recombinant BMP-2 offers a useful tool for restoring bone stock in complex hip arthroplasty revision cases with periacetabular bone loss; however, caution must be used as overabundant bone growth as heterotopic ossification may result.

TGF-β Family Signaling in Connective Tissue and Skeletal Diseases

The transforming growth factor β (TGF-β) family of signaling molecules, which includes TGF-βs, activins, inhibins, and numerous bone morphogenetic proteins (BMPs) and growth and differentiation factors (GDFs), has important functions in all cells and tissues, including soft connective tissues and the skeleton. Specific TGF-β family members play different roles in these tissues, and their activities are often balanced with those of other TGF-β family members and by interactions with other signaling pathways. Perturbations in TGF-β family pathways are associated with numerous human diseases with prominent involvement of the skeletal and cardiovascular systems. This review focuses on the role of this family of signaling molecules in the pathologies of connective tissues that manifest in rare genetic syndromes (e.g., syndromic presentations of thoracic aortic aneurysm), as well as in more common disorders (e.g., osteoarthritis and osteoporosis). Many of these diseases are caused by or result in pathological alterations of the complex relationship between the TGF-β family of signaling mediators and the extracellular matrix in connective tissues.

The traumatic bone: trauma-induced heterotopic ossification

Heterotopic ossification (HO) is a common occurrence after multiple forms of extensive trauma. These include arthroplasties, traumatic brain and spinal cord injuries, extensive burns in the civilian setting, and combat-related extremity injuries in the battlefield. Irrespective of the form of trauma, heterotopic bone is typically endochondral in structure and is laid down via a cartilaginous matrix. Once formed, the heterotopic bone typically needs to be excised surgically, which may result in wound healing complications, in addition to a risk of recurrence. Refinements of existing diagnostic modalities, like micro- and nano-CT are being adapted toward early intervention. Trauma-induced HO is a consequence of aberrant wound healing, systemic and local immune system activation, infections, extensive vascularization, and innervation. This intricate molecular crosstalk culminates in activation of stem cells that initiate heterotopic endochondral ossification. Development of animal models recapitulating the unique traumatic injuries has greatly facilitated the mechanistic understanding of trauma-induced HO. These same models also serve as powerful tools to test the efficacy of small molecules which specifically target the molecular pathways underlying ectopic ossification. This review summarizes the recent advances in the molecular understanding, diagnostic and treatment modalities in the field of trauma-induced HO.

Fibrodysplasia ossificans progressiva. A case report and focus on the BMP signaling pathway

Fibrodysplasia ossificans progressiva is a very rare heritable disease characterized by a progressive heterotopic endochondal ossification, occurring in the first decade of life, and leading thereafter to a severe ankylosis of the spine, limbs and jaw, with a progressive and severe functional disability. To date the cause of the disease remains unknown and no medical treatment has been proved efficient. It has recently been shown that a recurrent mutation in activation domain of the activin-receptor IA (ACVR1), a BMP receptor, could lead to an abnormal signalling pathway of BMP-4 and contribute to the occurrence of the devastating lesions characteristic of the disease.

Myositis ossificans progressive: case report

Myositis ossificans progressiva (MOP) is an autosomal dominant disorder. There is a progressive ectopic ossification and skeletal malformation, mainly in the connective tissue of muscle. The diagnosis is based on the clinical findings and radiological demonstration of the skeletal malformations. A 38-year-old female patient was admitted to our department with progressive increase of the thigh. Results of laboratory studies were normal. The radiography of the right thigh showed multiple intramuscular calcifications. Myositis ossificans progressiva should be diagnosed as early as possible and non-invasively, based upon history, clinical and radiological findings. Early and correct diagnosis is fundamental for indication of proper management of the disease.

The Fibrodysplasia Ossificans Progressiva (FOP) mutation p.R206H in ACVR1 confers an altered ligand response

Patients with Fibrodysplasia Ossificans Progressiva (FOP) suffer from ectopic bone formation, which progresses during life and results in dramatic movement restrictions. Cause of the disease are point mutations in the Activin A receptor type 1 (ACVR1), with p.R206H being most common. In this study we compared the signalling responses of ACVR1^WT and ACVR1^R206H to different ligands. ACVR1^WT, but not ACVR1^R206H inhibited BMP signalling of BMP2 or BMP4 in a ligand binding domain independent manner. Likewise, the basal BMP signalling activity of the receptor BMPR1A or BMPR1B was inhibited by ACVR1^WT, but enhanced by ACVR1^R206H. In comparison, BMP6 or BMP7 activated ACVR1^WT and caused a hyper-activation of ACVR1^R206H. These effects were dependent on an intact ligand binding domain. Finally, the neofunction of Activin A in FOP was tested and found to depend on the ligand binding domain for activating ACVR1^R206H. We conclude that the FOP mutation ACVR1^R206H is more sensitive to a number of natural ligands. The mutant receptor apparently lost some essential inhibitory interactions with its ligands and co-receptors, thereby conferring an enhanced ligand-dependent signalling and stimulating ectopic bone formation as observed in the patients.

Heterotopic Ossification: A Review of Current Understanding, Treatment, and Future

Heterotopic ossification is the formation of bone at extraskeletal sites. The incidence of heterotopic ossification in military amputees from recent operations in Iraq and Afghanistan has been demonstrated to be as high as 65%. Heterotopic ossification poses problems to wound healing, rehabilitation, and prosthetic fitting. This article details the current evidence regarding its etiology, prevention, management, and research strategies.

Role of osteoclasts in heterotopic ossification enhanced by fibrodysplasia ossificans progressiva-related activin-like kinase 2 mutation in mice

Fibrodysplasia ossificans progressiva (FOP) is a disorder of skeletal malformations and progressive heterotopic ossification. The constitutively activating mutation (R206H) of the bone morphogenetic protein type 1 receptor, activin-like kinase 2 (ALK2), is responsible for the pathogenesis of FOP. Although transfection of the causal mutation of FOP into myoblasts enhances osteoclast formation by transforming growth factor-β (TGF-β), the role of osteoclasts in heterotopic ossification is unknown. We therefore examined the effects of alendronate, SB431542 and SB203580 on heterotopic ossification induced by the causal mutation of FOP. Total bone mineral content as well as numbers of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated and alkaline phosphatase (ALP)-positive cells in heterotopic bone were significantly higher in muscle tissues implanted with ALK2 (R206H)-transfected mouse myoblastic C2C12 cells than in the tissues implanted with empty vector-transfected cells in nude mice. Alendronate, an aminobisphosphonate, did not affect total mineral content or numbers of TRAP-positive multinucleated and ALP-positive cells in heterotopic bone, which were enhanced by the implantation of ALK2 (R206H)-transfected C2C12 cells, although it significantly decreased serum levels of cross-linked C-telopeptide of type I collagen, a bone resorption index. Moreover, neither SB431542, an inhibitor of TGF-β receptor type I kinase, nor SB203580, an inhibitor of p38 mitogen-activated protein kinase, affected the increase in heterotopic ossification due to the implantation of ALK2 (R206H)-transfected C2C12 cells. In conclusion, the present study indicates that osteoclast inhibition does not affect heterotopic ossification enhanced by FOP-related mutation.

Myositis ossificans after navigated knee surgery: A report of two cases and literature review

Computer-assisted surgery is now a common procedure in the field of orthopedics. We present two patients who developed myositis ossificans, a rare complication after navigated knee surgery. Due to careful follow-up and conservative treatment, surgical excision was avoided in both patients with good clinical results. Development of myositis ossificans after navigated knee surgery should be considered in cases of thigh pain and restricted range of motion of the knee.

Bone Morphogenetic Proteins

Bone morphogenetic proteins (BMPs), originally identified as osteoinductive components in extracts derived from bone, are now known to play important roles in a wide array of processes during formation and maintenance of various organs including bone, cartilage, muscle, kidney, and blood vessels. BMPs and the related "growth and differentiation factors" (GDFs) are members of the transforming growth factor β (TGF-β) family, and transduce their signals through type I and type II serine-threonine kinase receptors and their intracellular downstream effectors, including Smad proteins. Furthermore, BMP signals are finely tuned by various agonists and antagonists. Because deregulation of the BMP activity at multiple steps in signal transduction is linked to a wide variety of human diseases, therapeutic use of activators and inhibitors of BMP signaling will provide potential avenues for the treatment of the human disorders that are caused by hypo- and hyperactivation of BMP signals, respectively.

Trauma-induced heterotopic bone formation and the role of the immune system: A review

Extremity trauma, spinal cord injuries, head injuries, and burn injuries place patients at high risk of pathologic extraskeletal bone formation. This heterotopic bone causes severe pain, deformities, and joint contractures. The immune system has been increasingly implicated in this debilitating condition. This review summarizes the various roles immune cells and inflammation play in the formation of ectopic bone and highlights potential areas of future investigation and treatment. Cell types in both the innate and adaptive immune system such as neutrophils, macrophages, mast cells, B cells, and T cells have all been implicated as having a role in ectopic bone formation through various mechanisms. Many of these cell types are promising areas of therapeutic investigation for potential treatment. The immune system has also been known to also influence osteoclastogenesis, which is heavily involved in ectopic bone formation. Chronic inflammation is also known to have an inhibitory role in the formation of ectopic bone, whereas acute inflammation is necessary for ectopic bone formation.

New Protocol to Optimize iPS Cells for Genome Analysis of Fibrodysplasia Ossificans Progressiva

Successful in vitro disease-recapitulation using patient-specific induced pluripotent stem cells (iPSCs) requires two fundamental technical issues: appropriate control cells and robust differentiation protocols. To investigate fibrodysplasia ossificans progressiva (FOP), a rare genetic disease leading to extraskeletal bone formation through endochondral ossification, gene-corrected (rescued) iPSC clones (resFOP-iPSC) were generated from patient-derived iPSC (FOP-iPSC) as genetically matched controls, and the stepwise induction method of mesenchymal stromal cells (iMSCs) through neural crest cell (NCC) lineage was used to recapitulate the disease phenotype. FOP-iMSCs possessing enhanced chondrogenic ability were transcriptionally distinguishable from resFOP-iMSCs and activated the SMAD1/5/8 and SMAD2/3 pathways at steady state. Using this method, we identified MMP1 and PAI1 as genes responsible for accelerating the chondrogenesis of FOP-iMSCs. These data indicate that iMSCs through NCC lineage are useful for investigating the molecular mechanism of FOP and corresponding drug discovery.

Inhibition of TGFβ signaling decreases osteogenic differentiation of fibrodysplasia ossificans progressiva fibroblasts in a novel in vitro model of the disease

Fibrodysplasia ossificans progressiva is a rare genetic disorder characterized by progressive heterotopic ossification. FOP patients develop soft tissue lumps as a result of inflammation-induced flare-ups which leads to the irreversible replacement of skeletal muscle tissue with bone tissue. Classical FOP patients possess a mutation (c.617G>A; R206H) in the ACVR1-encoding gene which leads to dysregulated BMP signaling. Nonetheless, not all FOP patients with this mutation exhibit equal severity in symptom presentation or disease progression which indicates a strong contribution by environmental factors. Given the pro-inflammatory role of TGFβ, we studied the role of TGFβ in the progression of osteogenic differentiation in primary dermal fibroblasts from five classical FOP patients based on a novel method of platelet lysate-based osteogenic transdifferentiation. During the course of transdifferentiation the osteogenic properties of the cells were evaluated by the mRNA expression of Sp7/Osterix, Runx2, Alp, OC and the presence of mineralization. During transdifferentiation the expression of osteoblast markers Runx2 (p<0.05) and Alp were higher in patient cells compared to healthy controls. All cell lines exhibited increase in mineralisation. FOP fibroblasts also expressed higher baseline Sp7/Osterix levels (p<0.05) confirming their higher osteogenic potential. The pharmacological inhibition of TGFβ signaling during osteogenic transdifferentiation resulted in the attenuation of osteogenic transdifferentiation in all cell lines as shown by the decrease in the expression of Runx2 (p<0.05), Alp and mineralization. We suggest that blocking of TGFβ signaling can decrease the osteogenic transdifferentiation of FOP fibroblasts.

Granting immunity to FOP and catching heterotopic ossification in the Act

The progressive transformation of one organ system into another is a fundamental signature of fibrodysplasia ossificans progressiva (FOP), the most catastrophic form of extraskeletal bone formation in humans. In all affected individuals, FOP is caused by heterozygous missense gain-of-function mutations in Activin receptor A type I (ACVR1), a bone morphogenetic protein (BMP) type I receptor. Loss of autoinhibition of the mutant receptor (mACVR1) results in dysregulated BMP pathway signaling, and is necessary for the myriad developmental features of FOP, but does not appear sufficient to induce the episodic flare-ups that lead to disabling post-natal heterotopic endochondral ossification (HEO) and that are a hallmark of the disease. Post-natal FOP flare-ups strongly implicate an underlying immunological trigger involving inflammation and the innate immune system. Recent studies implicate canonical and non-canonical TGFβ/BMP family ligands in the amplification of mACVR1 signaling leading to the formation of FOP lesions and resultant HEO. BMP and Activin ligands that stimulate mACVR1 signaling also have critical regulatory functions in the immune system. Cross-talk between the morphogenetic and immunological pathways that regulate tissue maintenance and wound healing identifies potential robust therapeutic targets for FOP. Here we review current evidence for an immunological trigger for flare-ups and HEO in FOP, propose a working schema for the pathophysiology of observed phenomena, and highlight outstanding questions under investigation.

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

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

Emerging from the rib: resolving the turtle controversies

Two of the major controversies in the present study of turtle shell development involve the mechanism by which the carapacial ridge initiates shell formation and the mechanism by which each rib forms the costal bones adjacent to it. This paper claims that both sides of each debate might be correct-but within the species examined. Mechanism is more properly "mechanisms," and there is more than one single way to initiate carapace formation and to form the costal bones. In the initiation of the shell, the rib precursors may be kept dorsal by either "axial displacement" (in the hard-shell turtles) or "axial arrest" (in the soft-shell turtle Pelodiscus), or by a combination of these. The former process would deflect the rib into the dorsal dermis and allow it to continue its growth there, while the latter process would truncate rib growth. In both instances, though, the result is to keep the ribs from extending into the ventral body wall. Our recent work has shown that the properties of the carapacial ridge, a key evolutionary innovation of turtles, differ greatly between these two groups. Similarly, the mechanism of costal bone formation may differ between soft-shell and hard-shell turtles, in that the hard-shell species may have both periosteal flattening as well as dermal bone induction, while the soft-shelled turtles may have only the first of these processes.

Capsular closure does not affect development of heterotopic ossification after hip arthroscopy

PURPOSE

The purpose of this study was to evaluate the role of capsular closure after hip arthroscopy in reduction of the incidence of heterotopic ossification (HO).

METHODS

One hundred (50 study group, 50 control group) consecutive hip arthroscopy procedures with radiographic follow-up of more than 9 weeks were included in the study. The study group consisted of 50 patients in whom capsular closure with 2 No. 1 polydioxanone (PDS) sutures was performed, and a control group consisted of 50 patients in whom the capsule remained open after capsulotomy. HO was assessed by radiographs using the Brooker classification. Statistical analysis of the data was carried out with the χ-square or Fisher exact test and Student t test, when appropriate, at a significance level of .05.

RESULTS

Thirty-six (36%) patients had radiographic evidence of postoperative HO (14 patients in the capsular closure group). No significant difference was found regarding sex, side of operation, age, or HO rate between the study and the control groups (P = .778, P = .123, P = .744, and P = .144, respectively). Furthermore, no significant difference was found in the rate of HO with potential clinical significance (Brooker classification > I) between the control and study groups (P = .764).

CONCLUSIONS

Capsular closure did not seem to alter the rate of HO when compared with a control group of patients in whom the capsulotomy was not repaired.

LEVEL OF EVIDENCE

Level III, retrospective comparative study.

Risk factors for the development of heterotopic ossification after knee dislocation

BACKGROUND

Results of treatment for acute knee dislocations and multiligament knee injuries may be influenced by a multitude of patient- and injury-related factors, including neurologic function, vascular status, ipsilateral fractures, and joint stability. The development of heterotopic ossification (HO) may nullify any benefits of reconstruction, because it can cause stiffness and discomfort. Identifying factors associated with HO after knee dislocation may help identify patients who might benefit from prophylaxis.

QUESTIONS/PURPOSES

The purposes of this study were (1) to identify specific risk factors for the development of HO in patients with knee dislocation; and (2) to elucidate the relationship between the presence of absence of HO and postoperative range of motion.

METHODS

Between 2005 and 2010, we performed 101 multiligament reconstructions for patients with knee dislocations, of which 91 (90%) in 91 patients were available for followup at a minimum of 6 months (mean, 18 months; range, 6-44 months), and were reviewed here. AP and lateral radiographs were reviewed for all patients and HO was classified according to the Mills and Tejwani classification system. This knee dislocation cohort was separated into two groups based on the presence or absence of HO for comparison. Using a significance level of p < 0.05 for factors in the univariate analyses, we identified potential variables for a multivariate logistic regression model to identify risk factors predicting development of HO in patients with multiligament knee injuries; multivariate analysis then was performed to mitigate the influence of potentially confounding variables. Thirty patients (34%) developed HO after multiligament knee injury in our series.

RESULTS

Posterior cruciate ligament reconstruction was the only independent predictor of HO that we identified (odds ratio, 6.3; 95% confidence interval, 1.2-34.6). Patients who developed HO were more likely to develop stiff knees and undergo surgery (50%; 15 of 30 patients) versus those without HO (12%; seven of 58 patients) to attempt to restore functional range of motion (p < 0.001).

CONCLUSIONS

HO is a common complication after knee dislocation and can diminish range of motion and cause patients to undergo further surgery. Posterior cruciate ligament reconstruction is an independent risk factor for the development of HO. Strategies to identify risk factors for, and safe prevention of, HO after multiple ligament injury and surgery should be investigated going forward.

LEVEL OF EVIDENCE

Level III, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

The inhibition effects of insulin on BMP2-induced muscle heterotopic ossification

Bone morphogenetic proteins (BMPs) play an important role in regulating osteoblastic differentiation and bone formation. But the diffuse of BMPs into muscle tissues around bone injury sites often leads to heterotopic ossification, which has been regarded as one of major side-effects of BMP implementation in bone defect patients. It raises great demands for exploring effective methods that preventing BMP-induced heterotopic ossification while not interrupting the osteoinductive activity of BMPs for in situ bone defect repair. Here we found insulin, a positive regulator for bone regeneration, inhibited BMP2-induced muscle heterotopic ossification by suppressing the expression of bone transcription factor Osterix. By analyzing downstream molecules of insulin pathway, we found AKT/mTOR/GSK3 signaling was responsible for the inhibition of insulin on BMP2-induced ossification, and GSK3 inhibitor SB216763 attenuated BMP2-induced muscle heterotopic ossification. The data might shed light on developing effective clinical therapy for inhibiting muscle heterotopic ossification when BMPs were used bone defect repair.

Osteogenic gene expression correlates with development of heterotopic ossification in war wounds

BACKGROUND

Heterotopic ossification (HO) is a frequent complication of modern wartime extremity injuries. The biological mechanisms responsible for the development of HO in traumatic wounds remain elusive.

QUESTION/PURPOSES

The aims of our study were to (1) characterize the expression profile of osteogenesis-related gene transcripts in traumatic war wounds in which HO developed; and (2) determine whether expression at the mRNA level correlated with functional protein expression and HO formation.

METHODS

Biopsy specimens from 54 high-energy penetrating extremity wounds obtained at the initial and final surgical débridements were evaluated. The levels of selected osteogenic-related gene transcripts from RNA extracts were assessed by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. As a result of its key role in osteogenesis, the concentration of BMP-2 in the effluent of 29 wounds also was determined.

RESULTS

The transcripts of 13 genes (ALPL [p = 0.006], BMP-2 [p < 0.001], BMP-3 [p = 0.06], COL2A1 [p < 0.001], COLL10A1 [p < 0.001], COL11A1 [p = 0.006], COMP [p = 0.02], CSF2 [p = 0.003], CSF3 [p = 0.012], MMP8 [p < 0.001], MMP9 [p = 0.014], SMAD1 [p = 0.024], and VEGFA [p = 0.017]) were upregulated greater than twofold in wounds in which HO developed compared with wounds in which it did not develop. Gene transcript expression of BMP-2 also correlated directly with functional protein expression in the wounds that formed HO (p = 0.029).

CONCLUSIONS

Important differences exist in the osteogenic gene expression profile of wounds in which HO developed compared with wounds in which it did not develop. The upregulation of multiple osteogenesis-related gene transcripts indicates the presence of a proosteogenic environment necessary for ectopic bone formation in traumatic wounds.

CLINICAL RELEVANCE

Understanding the osteogenic environment associated with war wounds may allow for the development of novel therapeutic strategies for HO.