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Fibrodysplasia ossificans progressiva: current concepts from bench to bedside - PubMed

  • ️Wed Jan 01 2020

Review

Fibrodysplasia ossificans progressiva: current concepts from bench to bedside

Arun-Kumar Kaliya-Perumal et al. Dis Model Mech. 2020.

Abstract

Heterotopic ossification (HO) is a disorder characterised by the formation of ectopic bone in soft tissue. Acquired HO typically occurs in response to trauma and is relatively common, yet its aetiology remains poorly understood. Genetic forms, by contrast, are very rare, but provide insights into the mechanisms of HO pathobiology. Fibrodysplasia ossificans progressiva (FOP) is the most debilitating form of HO. All patients reported to date carry heterozygous gain-of-function mutations in the gene encoding activin A receptor type I (ACVR1). These mutations cause dysregulated bone morphogenetic protein (BMP) signalling, leading to HO at extraskeletal sites including, but not limited to, muscles, ligaments, tendons and fascia. Ever since the identification of the causative gene, developing a cure for FOP has been a focus of investigation, and studies have decoded the pathophysiology at the molecular and cellular levels, and explored novel management strategies. Based on the established role of BMP signalling throughout HO in FOP, therapeutic modalities that target multiple levels of the signalling cascade have been designed, and some drugs have entered clinical trials, holding out hope of a cure. A potential role of other signalling pathways that could influence the dysregulated BMP signalling and present alternative therapeutic targets remains a matter of debate. Here, we review the recent FOP literature, including pathophysiology, clinical aspects, animal models and current management strategies. We also consider how this research can inform our understanding of other types of HO and highlight some of the remaining knowledge gaps.

Keywords: ACVR1; Bone morphogenetic protein; Fibrodysplasia ossificans progressiva; Heterotopic ossification; Inflammation.

© 2020. Published by The Company of Biologists Ltd.

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Conflict of interest statement

Competing interestsThe authors declare no competing or financial interests.

Figures

Fig. 1.
Fig. 1.

Schematic representation of human ACVR1, its various domains and locations of the mutations that have been causally linked to FOP. GS, glycine-serine.

Fig. 2.
Fig. 2.

BMP signalling. (A) BMPs bind to complexes of type I and type II serine/threonine kinase BMP receptors, such as ACVR1, on the cell surface to activate intracellular signal transduction via R-SMADs SMAD1/5/9(8). Phosphorylated SMAD1/5/9(8) forms a complex with co-mediator SMAD4 and translocates into the nucleus, where it regulates transcription that drives endochondral ossification. (B) On binding activin A, complexes of type I and type II BMP receptors activate intracellular signal transduction via SMAD2/3, which activates a transcription programme that regulates inflammation. (C) ACVR1 carrying a FOP mutation (most frequently the R206H substitution) in the intracellular glycine-serine domain not only yields enhanced response to various BMP ligands by initiating downstream signalling via SMAD1/5/9(8), but also responds to various activin ligands, thereby favouring endochondral ossification by triggering an osteogenic gene expression programme. BMP, bone morphogenetic protein; Co-SMAD, common partner SMAD; P, phosphorylation; R-SMAD, receptor-regulated SMAD.

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