The immunological contribution to heterotopic ossification disorders

Ossifications in Albright Hereditary Osteodystrophy: Role of Genotype, Inheritance, Sex, Age, Hormonal Status, and BMI

CONTEXT

Albright hereditary osteodystrophy (AHO) is caused by heterozygous inactivating mutations in GNAS. Depending on the parental origin of the mutated allele, patients develop either pseudohypoparathyroidism type 1A (PHP1A), with multihormone resistance and severe obesity, or pseudopseudohypoparathyroidism (PPHP), without hormonal abnormalities or marked obesity. Subcutaneous ossifications (SCOs) are a source of substantial morbidity in both PHP1A and PPHP.

OBJECTIVE

This study investigated the previously undetermined prevalence of SCO formation in PHP1A vs PPHP as well as possible correlations with genotype, sex, age, hormonal resistance, and body mass index (BMI).

DESIGN

This study evaluated patients with AHO for SCOs by physical examination performed by one consistent physician over 16 years.

SETTING

Albright Clinic, Kennedy Krieger Institute; Institute for Clinical and Translational Research, Johns Hopkins Hospital; Albright Center, Connecticut Children's Medical Center.

PATIENTS

We evaluated 67 patients with AHO (49 with PHP1A, 18 with PPHP) with documented mutations in GNAS.

MAIN OUTCOME MEASURES

Relationships of SCOs to genotype, sex, age, hormonal resistance, and BMI.

RESULTS

Forty-seven of 67 participants (70.1%) had SCOs. Patients with PHP1A and PPHP had similar prevalences and degrees of ossification formation. Patients with frameshift and nonsense mutations had much more extensive SCOs than those with missense mutations. Males were affected more than females. There was no correlation with hormonal status or BMI.

CONCLUSIONS

There is a similar prevalence of SCOs in PHP1A and PPHP, and the extent of SCO formation correlates with the severity of the mutation. Males are affected more extensively than females, and the SCOs tend to worsen with age.

Peripheral Blood Mononuclear Cell Immunophenotyping in Fibrodysplasia Ossificans Progressiva Patients: Evidence for Monocyte DNAM1 Up-regulation

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder caused by sporadic heterozygous mutations in ACVR1 gene which progressively leads to severe heterotopic ossification. FOP is characterized by episodic flare-ups triggered by different factors such as viral infections, tissue injuries, vaccinations, or occurring without a recognizable cause. The sporadic course of the disease, the documented presence of an important inflammatory reaction in early lesions and the partial response to corticosteroids support the idea that the immune system, and in particular the innate component, may play a role in FOP pathogenesis. However, an extensive expression profile of the peripheral blood mononuclear cells (PBMC) of FOP patients has never been done.

METHODS

In this study, we carried out a wide PBMC immunophenotyping on a cohort of FOP patients and matching controls by multiparametric analysis of the expression of a panel of 37 markers associated with migration, adhesion, inhibition, activation, and cell death of circulating immune cells.

RESULTS

We observed a statistically significant increase of the expression of DNAM1 receptor in patients' monocytes as compared to controls, and little but significant differences in the expression profile of CXCR1 (CD181), CD62L, CXCR4 (CD184), and HLA-DR molecules.

CONCLUSIONS

DNAM1 had been previously shown to play a pivotal role in monocyte migration through the endothelial barrier and the increased expression detected in patients' monocytes might suggest a role of this surface receptor during the early phases of FOP flare-ups in which the activation of the immune response is believed to represent a crucial event. © 2017 International Clinical Cytometry Society.

Relationship between heterotopic ossification and traumatic brain injury: Why severe traumatic brain injury increases the risk of heterotopic ossification

Heterotopic ossification (HO) is a pathological phenomenon in which ectopic lamellar bone forms in soft tissues. HO involves many predisposing factors, including congenital and postnatal factors. Postnatal HO is usually induced by fracture, burn, neurological damage (brain injury and spinal cord injury) and joint replacement. Recent studies have found that patients who suffered from bone fracture combined with severe traumatic brain injury (S-TBI) are at a significantly increased risk for HO occurrence. Thus, considerable research focused on the influence of S-TBI on fracture healing and bone formation, as well as on the changes in various osteogenic factors with S-TBI occurrence. Brain damage promotes bone formation, but the exact mechanisms underlying bone formation and HO after S-TBI remain to be clarified. Hence, this article summarises the findings of previous studies on the relationship between S-TBI and HO and discusses the probable causes and mechanisms of HO caused by S-TBI. The translational potential of this article: A better understanding of the probable causes of traumatic brain injury-induced HO can provide new perspectives and ideas in preventing HO and may support to design more targeted therapies to reduce HO or enhance the bone formation.

BMP-7 accelerates the differentiation of rabbit mesenchymal stem cells into cartilage through the Wnt/β-catenin pathway

Mesenchymal stem cells (MSCs) are able to differentiate into adipocytes, chondroblasts or cartilage under different stimulation conditions. Identifying a mechanism that triggers the differentiation of MSCs into cartilage may help the development of novel therapeutic approaches for heterotopic ossification, the pathological formation of lamellar bone in soft tissue outside the skeleton that may lead to debilitating immobility. Bone morphogenetic proteins (BMPs), including BMP-7, are the most potent growth factors for enhancing bone formation. The current study aimed to understand the potential involvement of the Wnt/β-catenin signaling pathway in the BMP-7-induced growth of rabbit MSCs (rMSCs). Different concentrations of BMP-7 were applied to cultured rMSCs, and proliferation was evaluated by MTT assay. Changes in the phosphorylation state of glycogen synthase kinase (GSK)-3β, in addition to the expression levels of alkaline phosphatase, β-catenin and runt-related transcription factor 2 were observed by western blot analysis. Following treatment with BMP-7, the phosphorylation of GSK-3β was stimulated and the expression of β-catenin, ALP and Runx2 was increased. Furthermore, inhibiting β-catenin signaling with XAV-939 suppressed the BMP-7-mediated changes. The results indicated that the BMP-7-induced differentiation of rMSCs into cartilage was promoted primarily by the Wnt/β-catenin pathway.

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.

Acquired heterotopic ossification of the temporomandibular joint

Although the risk factors and diagnosis of heterotopic ossification (HO) are discussed in the orthopedics literature, the etiology of HO, as well as its prevention and management, remain theoretical. Furthermore, there is a lack of information in the literature regarding HO in temporomandibular joint replacement (TMJR). This article provides a qualitative review of information relative to the etiology, diagnosis, and management of HO to inform and encourage further investigation in TMJR. The orthopedic HO literature considered for this qualitative review was drawn from a comprehensive examination of the subject published previously by one of the authors. Using the key words "heterotopic ossification" or "heterotopic bone", citations in the PubMed database from both the dental and oral and maxillofacial surgery literature were reviewed. Based on this, it appears that the etiology, diagnosis, imaging, laboratory testing, risk factors, prophylaxis, and non-surgical and surgical options available for the management of TMJR-related HO are similar to those for orthopedic HO, but further elucidation is required for TMJR. There is a lack of published information in the literature on TMJR. Therefore, using the literature from this review as a foundation, studies should be developed and reported so that alloplastic TMJ surgeons have evidence-based protocols that will lead to the early detection and potential prevention of HO.

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.

Peroxidase Enzymes Regulate Collagen Biosynthesis and Matrix Mineralization by Cultured Human Osteoblasts

The early recruitment of inflammatory cells to sites of bone fracture and trauma is a critical determinant in successful fracture healing. Released by infiltrating inflammatory cells, myeloperoxidase (MPO) and eosinophil peroxidase (EPO) are heme-containing enzymes, whose functional involvement in bone repair has mainly been studied in the context of providing a mechanism for oxidative defense against invading microorganisms. We report here novel findings that show peroxidase enzymes have the capacity to stimulate osteoblastic cells to secrete collagen I protein and generate a mineralized extracellular matrix in vitro. Mechanistic studies conducted using cultured osteoblasts show that peroxidase enzymes stimulate collagen biosynthesis at a post-translational level in a prolyl hydroxylase-dependent manner, which does not require ascorbic acid. Our studies demonstrate that osteoblasts rapidly bind and internalize both MPO and EPO, and the catalytic activity of these peroxidase enzymes is essential to support collagen I biosynthesis and subsequent release of collagen by osteoblasts. We show that EPO is capable of regulating osteogenic gene expression and matrix mineralization in culture, suggesting that peroxidase enzymes may play an important role not only in normal bone repair, but also in the progression of pathological states where infiltrating inflammatory cells are known to deposit peroxidases.

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.

Macrophages derived from THP-1 promote the osteogenic differentiation of mesenchymal stem cells through the IL-23/IL-23R/β-catenin pathway

Abnormal bone formation is a clinically significant dilemma for many conditions in response to injury, inflammation or genetic disease. However, the effects of inflammation on the osteogenic differentiation of mesenchymal stem cells (MSCs) remain unclear. IL-23 secretion from macrophages might contribute to the development of bone formation. Here, we investigated the stimulatory effects of THP-1 macrophage conditioned medium (MΦ CM) on the osteogenic differentiation of human MSCs and the associated signaling pathways. The osteogenic differentiation of MSCs was induced after exposure to osteogenic differentiation medium (OM). MΦ CM significantly increased alkaline phosphate (ALP) activity and calcium mineralization in MSCs. Osteogenic marker genes, including RUNX2, ALP and osteocalcin (OCN), were also up-regulated in MSCs after exposure to MΦ CM. Moreover, western blotting revealed that MΦ CM treatment induced STAT3 and β-catenin activation in MSCs. Furthermore, blockade of IL-23 in MΦ CM not only impaired the osteogenic-promotion effects of macrophage but also decreased the expression of osteogenic maker genes. However, IL-23R silencing suppressed MΦ CM-induced calcium mineralization and osteogenic maker gene expression in MSCs. These data suggest that macrophages derived from THP-1 promote the osteoblastic differentiation of MSCs through the IL-23/IL-23R/β-catenin pathway and macrophages might contribute to the development of bone formation in inflammation.