Understanding the Action of RARγ Agonists on Human Osteochondroma Explants

Hereditary multiple exostoses: an educational review

Hereditary multiple exostoses (HME), an autosomal dominant disorder with an incidence of 1:50,000 to 1:100,000, is characterised by the formation of multiple osteochondromas arising from the metaphyses of long and flat bones. These osteochondromas often present as painless palpable lumps, though some cases are symptomatic due to mechanical compression or bursitis. Diagnosis of HME is typically clinical and radiological. WHO diagnostic criteria include ≥ 2 radiological osteochondromas in the juxta-epiphyseal region of the long bones. Genetic testing is reserved for ambiguous cases. HME is associated with mutations in the EXT-1 (exostosin-1) and EXT-2 (exostosin-2) genes. Imaging techniques, including conventional radiography, CT, MRI, ultrasound, and nuclear medicine, play a crucial role in diagnosing and assessing HME, with each modality offering distinct advantages in visualising the lesions and associated complications. Common complications include skeletal deformities, fractures, bursitis, as well as neural and vascular abnormalities. Notably, there is a 10% risk of malignant transformation into secondary chondrosarcoma in HME patients, compared to only a 1% risk in those with solitary osteochondromas. Malignant transformation should be suspected in patients with new-onset pain or specific imaging features in an osteochondroma, such as growth of de cartilaginous cap. In these cases, an MRI should be performed to assess the cartilage cap thickness. Advances in imaging techniques and genetic understanding have improved the management and prognosis of HME. Follow-up is essential to rule out malignant transformation. This review summarises current knowledge on the clinical presentation, pathogenesis, imaging characteristics, complications, and treatment of HME. CRITICAL RELEVANCE STATEMENT: HME is a disorder characterised by the formation of osteochondromas arising from long and flat bones. Multi-modality imaging characteristics, clinical presentation, complications, and treatment are highlighted to familiarise the readers with this entity and offer optimal patient care. KEY POINTS: HME is characterised by multiple osteochondromas on long and flat bones. Imaging for HME includes radiography, CT, MRI, ultrasound, and nuclear medicine studies. Complications include non-malignant complications, such as bone deformities and malignant transformation. Cartilage-cap measurement with MRI or US is key to exclude malignancy. Follow-up is essential to rule out malignant transformation of the osteochondromas.

Palovarotene (Sohonos), a synthetic retinoid for reducing new heterotopic ossification in fibrodysplasia ossificans progressiva: history, present, and future

Retinoids are metabolic derivatives of vitamin A and play crucial roles in the regulation of various tissues and organs during prenatal and postnatal development. Active retinoids, like all-trans-retinoic acid, are synthesized in the cytoplasm and subsequently interact with nuclear retinoic acid receptors (RARα, RARβ, and RARγ) to enhance transcription of specific genes. In the absence of retinoids, RARs can still bind to response elements of target genes but repress their transcription. Chondrogenic cell differentiation and cartilage maturation in the growth plate require the absence of retinoid signaling and transcriptional repression by unliganded RARs. This led to the hypothesis that synthetic retinoid agonists may be pharmacological agents to inhibit those cellular processes and counter the excessive formation of cartilage and bone in conditions like heterotopic ossification (HO). HO can be instigated by diverse culprits including trauma, invasive surgeries, inflammatory disorders, or genetic conditions. One such genetic disease is fibrodysplasia ossificans progressiva (FOP), a rare disorder driven by activating mutations in the ACVR1 gene. Patients with FOP have severe and progressive HO formation in soft tissues, leading to extensive permanent loss of mobility and increased mortality. Synthetic retinoid agonists selective for RARα or RARγ showed efficacy against injury-induced and genetic HO in mouse models. The RARγ agonists showed the highest effectiveness, with palovarotene being selected for clinical trials in patients with FOP. Post hoc analyses of phase II and phase III clinical trials showed that palovarotene has significant disease-modifying effects for FOP, but with significant risks such as premature growth plate closure in some younger subjects. This review provides an overview of retinoid and RAR roles in skeletal development and discusses the identification of palovarotene as a potential FOP therapy, the clinical data supporting its regulatory approval in some countries, and the potential applications of this drug for other relevant disorders besides FOP.

Intra-Articular Osteochondroma in the Elbow: Diagnosis and Surgical Treatment in an 8-Year-Old Boy

BACKGROUND Osteochondroma is the most common bone tumor and is a surface bone lesion that includes cortical and medullary bone with a hyaline cartilage cap. Benign osteochondroma is a common tumor in children that may be asymptomatic but can cause pain and limit joint movement when arising in a joint. This report describes the presentation, diagnosis, and management of an intra-articular osteochondroma of the right elbow joint in an 8-year-old boy. CASE REPORT An 8-year-old boy experienced persistent right elbow pain and limited motion, which were unresponsive to conservative measures. Examination revealed a firm swelling in the right cubital fossa. Radiographic and advanced imaging confirmed an osteochondroma originating from the capitellum and trochlea. Surgical exploration via a lateral approach and capsulotomy excised a lobulated intra-articular mass (5×2×1.5 cm). Histopathology showed a hyaline cartilage cap with typical chondrocytes and endochondral ossification, and normal fatty marrow and hematopoietic elements in the stroma. The procedure restored normal elbow function. This case is the first documented instance of an elbow joint intra-articular osteochondroma. CONCLUSIONS This report has highlighted the importance of surgical removal and histopathology in the diagnosis of this common bone lesion to exclude the differential diagnoses of intra-articular masses that include a foreign body, enchondroma, chondroblastoma, periosteal chondroma, chondromyxoid fibroma, or malignant chondrosarcoma.

Paediatric Calcaneal Osteochondroma: A Case Report and a Literature Review

BACKGROUND

Heel pain in children is a common condition. The aetiology can be ascribed to fractures, osteochondrosis, tendinitis, calcaneal-navicular or talo-calcaneal coalition, osteomyelitis, rheumatic diseases, anatomic variants, malignant tumours (osteosarcoma, Ewing's sarcoma), and benign lesions (bone cyst, aneurismal bone cyst, osteoid osteoma, or exostosis). In particular, this manuscript focuses on a case of calcaneal exostosis in the paediatric age, aiming to highlight its rarity. Osteochondromas are benign tumours of the surface of the bone and the overlying cartilage. They grow until skeletal maturity and can cause stiffness, pain, cosmetic alterations, tendinitis, and neuro-vascular compression. The calcaneus is an extremely rare site for these tumours. Only two case reports of paediatric exostosis of the calcaneus bone are available.

METHODS

We describe a case of a girl of 16 years of age, affected by multiple cartilaginous exostosis, who presented with a painful mass on the inferior margin of the foot in the calcaneal region, which was diagnosed as an exostosis. The neoformation was excised, and the girl underwent clinical follow-up.

RESULTS

The patient was promptly discharged in good condition, and on the 25th postoperative day, she was completely pain-free and allowed weight bearing.

CONCLUSIONS

In the case of heel pain resistant to conservative treatment, the presence of an osteochondroma should be considered after excluding more common causes. If symptomatic, calcaneal osteochondromas could require surgical excision.

Analysis of the Actions of RARγ Agonists on Growing Osteochondromas in a Mouse Model

The actions of the retinoic acid nuclear receptor gamma (RARγ) agonist, palovarotene, on pre-existing osteochondromas were investigated using a mouse multiple osteochondroma model. This approach was based on the knowledge that patients often present to the clinic after realizing the existence of osteochondroma masses, and the findings from preclinical investigations are the effects of drugs on the initial formation of osteochondromas. Systemic administration of palovarotene, with increased doses (from 1.76 to 4.0 mg/kg) over time, fully inhibited tumor growth, keeping the tumor size (0.31 ± 0.049 mm3) similar to the initial size (0.27 ± 0.031 mm3, p = 0.66) while the control group tumor grew (1.03 ± 0.23 mm3, p = 0.023 to the drug-treated group). Nanoparticle (NP)-based local delivery of the RARγ agonist also inhibited the growth of osteochondromas at an early stage (Control: 0.52 ± 0.11 mm3; NP: 0.26 ± 0.10, p = 0.008). Transcriptome analysis revealed that the osteoarthritis pathway was activated in cultured chondrocytes treated with palovarotene (Z-score = 2.29), with the upregulation of matrix catabolic genes and the downregulation of matrix anabolic genes, consistent with the histology of palovarotene-treated osteochondromas. A reporter assay performed in cultured chondrocytes demonstrated that the Stat3 pathway, but not the Stat1/2 pathway, was stimulated by RARγ agonists. The activation of Stat3 by palovarotene was confirmed using immunoblotting and immunohistochemistry. These findings suggest that palovarotene treatment is effective against pre-existing osteochondromas and that the Stat3 pathway is involved in the antitumor actions of palovarotene.

Retinoic Acid Receptor Gamma (RARγ) Promotes Cartilage Destruction through Positive Feedback Activation of NF-κB Pathway in Human Osteoarthritis

Osteoarthritis (OA) is a severe inflammation-related disease which leads to cartilage destruction. The retinoic acid receptor gamma (RARγ) has been indicated to be involved in many inflammation processes. However, the role and mechanism of RARγ in cartilage destruction caused by inflammation in OA are still unknown. Here, we demonstrated that the RARγ was highly expressed in chondrocytes of OA patients compared with healthy people and was positively correlated with the damage degree of cartilage in OA. Cytokine TNF-α promoted the transcription and expression of RARγ through activating the NF-κB pathway in OA cartilage. In addition, the overexpression of RARγ resulted in the upregulation of matrix degradation and inflammation associated genes and downregulation of differentiation and collagen production genes in human normal chondrocyte C28/I2 cells. Mechanistically, overexpression of RARγ could increase the level of p-IκBα and p-P65 to regulate the expression of downstream genes. RARγ and IκBα also could interact with each other and had the same localization in C28/I2 cells. Moreover, the SD rats OA model induced by monosodium iodoacetate indicated that CD437 (RARγ agonist) and TNF-α accelerated the OA progression, including more severe cartilage layer destruction, larger knee joint diameter, and higher serum ALP levels, while LY2955303 (RARγ inhibitor) showed the opposite result. RARγ was also highly expressed in OA group and even higher in TNF-α group. In conclusion, RARγ/NF-κB positive feedback loop was activated by TNF-α in chondrocyte to promote cartilage destruction. Our data not only propose a novel and precise molecular mechanism for OA disease but also provide a prospective strategy for the treatment.

Structural development of silicon-containing retinoids: structure-activity relationship study of the hydrophobic pharmacophore of retinobenzoic acids using silyl functionalities

We designed and synthesized a series of retinobenzoic acids bearing various silyl functionalities in order to explore in detail the structure-activity relationship (SAR) at the hydrophobic moiety of retinoids. Among the synthesized compounds, 24c bearing a t -butyldimethylsilyl (TBS) group at the hydrophobic site exhibited potent retinoid activity comparable to that of the lead compound Am555S ( 4 ). Compound 24c exhibited transcription-promoting activity towards all three subtypes of retinoic acid receptor (RAR), but showed the highest activity towards RARγ, in contrast to the high RARα-selectivity of Am80 ( 3 ) and Am555S ( 4 ). The SARs presented here should be helpful in the development of subtype-selective retinoids, and in particular 24c might be a promising lead compound for new RARγ ligands.

Osteochondroma Pathogenesis: Mouse Models and Mechanistic Insights into Interactions with Retinoid Signaling

Osteochondromas are cartilage-capped tumors that arise near growing physes and are the most common benign bone tumor in children. Osteochondromas can lead to skeletal deformity, pain, loss of motion, and neurovascular compression. Currently, surgery is the only available treatment for symptomatic osteochondromas. Osteochondroma mouse models have been developed to understand the pathology and the origin of osteochondromas and develop therapeutic drugs. Several cartilage regulatory pathways have been implicated in the development of osteochondromas, such as bone morphogenetic protein, hedgehog, and WNT/β-catenin signaling. Retinoic acid receptor-γ is an important regulator of endochondral bone formation. Selective agonists for retinoic acid receptor-γ, such as palovarotene, have been investigated as drugs for inhibition of ectopic endochondral ossification, including osteochondromas. This review discusses the signaling pathways involved in osteochondroma pathogenesis and their possible interactions with the retinoid pathway.

Administration of a selective retinoic acid receptor-γ agonist improves neuromuscular strength in a rodent model of volumetric muscle loss

PURPOSE

Volumetric muscle loss is a uniquely challenging pathology that results in irrecoverable functional deficits. Furthermore, a breakthrough drug or bioactive factor has yet to be established that adequately improves repair of these severe skeletal muscle injuries. This study sought to assess the ability of an orally administered selective retinoic acid receptor-γ agonist, palovarotene, to improve recovery of neuromuscular strength in a rat model of volumetric muscle loss.

METHODS

An irrecoverable, full thickness defect was created in the tibialis anterior muscle of Lewis rats and animals were survived for 4 weeks. Functional recovery of the tibialis anterior muscle was assessed in vivo via neural stimulation and determination of peak isometric torque. Histological staining was performed to qualitatively assess fibrous scarring of the defect site.

RESULTS

Treatment with the selective retinoic acid receptor-γ agonist, palovarotene, resulted in a 38% improvement of peak isometric torque in volumetric muscle loss affected limbs after 4 weeks of healing compared to untreated controls. Additionally, preliminary histological assessment suggests that oral administration of palovarotene reduced fibrous scarring at the defect site.

CONCLUSIONS

These results highlight the potential role of selective retinoic acid receptor-γ agonists in the design of regenerative medicine platforms to maximize skeletal muscle healing. Additional studies are needed to further elucidate cellular responses, optimize therapeutic delivery, and characterize synergistic potential with adjunct therapies.

Osteochondromas: An Updated Review of Epidemiology, Pathogenesis, Clinical Presentation, Radiological Features and Treatment Options

Osteochondroma, the most common benign bone tumor, is a projection on the external surface of the bone, which can be sessile or pedunculated. 85% of osteochondromas present as solitary lesions, while 15% occur in the context of hereditary multiple exostoses (HME), a genetic disorder that is inherited in an autosomal dominant manner. Although often asymptomatic, symptoms may eventuate from compression of adjacent vessels or nerves, fractures, osseous deformities, bursa formation, or malignant transformation. Cartilage cap thickness >2 cm in adults or >3 cm in children as well as new onset of pain or growth, or rapid growth of the lesion, especially after the closure of the growth plate, might reflect cancerous transformation. Surgical resection is indicated for symptomatic lesions, complications, cosmetic reasons or malignant transformation. Excision of the tumor with free margin is the treatment of choice. Local recurrence is less than 2% if complete resection is achieved.