Dysregulated BMP signaling through ACVR1 impairs digit joint development in fibrodysplasia ossificans progressiva (FOP)

Fibrodysplasia ossificans progressiva emerges from obscurity

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

Molecular Developmental Biology of Fibrodysplasia Ossificans Progressiva: Measuring the Giant by Its Toe

When a genetic disease is characterized by the abnormal activation of normal molecular pathways and cellular events, it is illuminating to critically examine the places and times of these activities both in health and disease. Therefore, because heterotopic ossification (HO) in fibrodysplasia ossificans progressiva (FOP) is by far the disease's most prominent symptom, attention is also directed toward the pathways and processes of bone formation during skeletal development. FOP is recognizable by effects of the causative mutation on skeletal development even before HO manifests, specifically in the malformation of the great toes. This signature skeletal phenotype is the most highly penetrant, but is only one among several skeletal abnormalities associated with FOP. Patients may present clinically with joint malformation and ankylosis, particularly in the cervical spine and costovertebral joints, as well as characteristic facial features and a litany of less common, non-skeletal symptoms, all stemming from missense mutations in the ACVR1 gene. In the same way that studying the genetic cause of HO advanced our understanding of HO initiation and progression, insight into the roles of ACVR1 signaling during tissue development, particularly in the musculoskeletal system, can be gained from examining altered skeletal development in individuals with FOP. This review will detail what is known about the molecular mechanisms of developmental phenotypes in FOP and the early role of ACVR1 in skeletal patterning and growth, as well as highlight how better understanding these processes may serve to advance patient care, assessments of patient outcomes, and the fields of bone and joint biology.

Pay Attention to the Osteochondromas in Fibrodysplasia Ossificans Progressiva

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP) is an extremely rare disease characterized by malformation of the bilateral great toes and progressive heterotopic ossification. The clinical features of FOP occur due to dysfunction of the bone morphogenetic protein (BMP) signaling pathway induced by the mutant activin A type I receptor/activin-like kinase-2 (ACVR1/ALK2) which contributes to the clinical features in FOP. Dysregulation of the BMP signaling pathway causes the development of osteochondroma. Poor awareness of the association between FOP and osteochondromas always results in misdiagnosis and unnecessary invasive operation.

CASE PRESENTATION

In this study, we present a case of classical FOP involving osteochondroma. An 18-year-old male adolescent, born with deformity of bilateral big toes, complained multiple masses on his back for 1 year. The mass initially emerged with a tough texture and did not cause pain. It was misdiagnosed as an osteochondroma. After two surgeries, the masses became hard and spread around the entire back region. Meanwhile, extensive heterotopic ossification was observed around the back, neck, hip, knee, ribs, and mandible during follow-up. Osteochondromas were observed around the bilateral knees. No abnormalities were observed in the laboratory blood test results. Whole exome sequencing revealed missense mutation of ACVR1/ALK2 (c.617G > A; p.R206H) in the patient and confirmed the diagnosis of FOP.

CONCLUSION

In summary, classical FOP always behaves as a bilateral deformity of the big toes, as well as progressive ectopic ossification and osteochondromas in the distal femur and proximal tibia. An understanding of the association between osteochondromas and FOP aids in diagnosis and avoids unnecessary invasive management in patients.

Functional Testing of Bone Morphogenetic Protein (BMP) Pathway Variants Identified on Whole-Exome Sequencing in a Patient with Delayed-Onset Fibrodysplasia Ossificans Progressiva (FOP) Using ACVR1R206H -Specific Human Cellular and Zebrafish Models

Bone morphogenetic protein (BMP) signaling is critical in skeletal development. Overactivation can trigger heterotopic ossification (HO) as in fibrodysplasia ossificans progressiva (FOP), a rare, progressive disease of massive HO formation. A small subset of FOP patients harboring the causative ACVR1^R206H mutation show strikingly mild or delayed-onset HO, suggesting that genetic variants in the BMP pathway could act as disease modifiers. Whole-exome sequencing of one such patient identified BMPR1A^R443C and ACVR2A^V173I as candidate modifiers. Molecular modeling predicted significant structural perturbations. Neither variant decreased BMP signaling in ACVR1^R206H HEK 293T cells at baseline or after stimulation with BMP4 or activin A (AA), ligands that activate ACVR1^R206H signaling. Overexpression of BMPR1A^R443C in a Tg(ACVR1-R206Ha) embryonic zebrafish model, in which overactive BMP signaling yields ventralized embryos, did not alter ventralization severity, while ACVR2A^V173I exacerbated ventralization. Co-expression of both variants did not affect dorsoventral patterning. In contrast, BMPR1A knockdown in ACVR1^R206H HEK cells decreased ligand-stimulated BMP signaling but did not affect dorsoventral patterning in Tg(ACVR1-R206Ha) zebrafish. ACVR2A knockdown decreased only AA-stimulated signaling in ACVR1^R206H HEK cells and had no effect in Tg(ACVR1-R206Ha) zebrafish. Co-knockdown in ACVR1^R206H HEK cells decreased basal and ligand-stimulated signaling, and co-knockdown/knockout (bmpr1aa/ab; acvr2aa/ab) decreased Tg(ACVR1-R206Ha) zebrafish ventralization phenotypes. Our functional studies showed that knockdown of wild-type BMPR1A and ACVR2A could attenuate ACVR1^R206H signaling, particularly in response to AA, and that ACVR2A^V173I unexpectedly increased ACVR1^R206H -mediated signaling in zebrafish. These studies describe a useful strategy and platform for functionally interrogating potential genes and genetic variants that may impact the BMP signaling pathway. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Suppression of heterotopic ossification in fibrodysplasia ossificans progressiva using AAV gene delivery

Heterotopic ossification is the most disabling feature of fibrodysplasia ossificans progressiva, an ultra-rare genetic disorder for which there is currently no prevention or treatment. Most patients with this disease harbor a heterozygous activating mutation (c.617 G > A;p.R206H) in ACVR1. Here, we identify recombinant AAV9 as the most effective serotype for transduction of the major cells-of-origin of heterotopic ossification. We use AAV9 delivery for gene replacement by expression of codon-optimized human ACVR1, ACVR1R206H allele-specific silencing by AAV-compatible artificial miRNA and a combination of gene replacement and silencing. In mouse skeletal cells harboring a conditional knock-in allele of human mutant ACVR1 and in patient-derived induced pluripotent stem cells, AAV gene therapy ablated aberrant Activin A signaling and chondrogenic and osteogenic differentiation. In Acvr1(R206H) knock-in mice treated locally in early adulthood or systemically at birth, trauma-induced endochondral bone formation was markedly reduced, while inflammation and fibroproliferative responses remained largely intact in the injured muscle. Remarkably, spontaneous heterotopic ossification also substantially decreased in in Acvr1(R206H) knock-in mice treated systemically at birth or in early adulthood. Collectively, we develop promising gene therapeutics that can prevent disabling heterotopic ossification in mice, supporting clinical translation to patients with fibrodysplasia ossificans progressiva.

Current challenges and opportunities in the care of patients with fibrodysplasia ossificans progressiva (FOP): an international, multi-stakeholder perspective

Background

Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare, disabling genetic disorder characterized by congenital malformations of the great toes and progressive heterotopic ossification of soft and connective tissues. Assiduous attention to the unmet needs of this patient community is crucial to prevent potential iatrogenic harm and optimize care for individuals with FOP.

Objective

To gather international expert opinion and real-world experience on the key challenges for individuals with FOP and their families, highlight critical gaps in care, communication, and research, and provide recommendations for improvement.

Methods

An international group of expert clinicians, patients and patient advocates, caregivers and representatives from the international FOP community participated in a virtual, half-day meeting on 22 March 2021 to discuss the key unmet needs of individuals with FOP.

Results

Individuals with FOP often face the frustration of long diagnostic journeys, the burden of self-advocacy and the navigation of novel care pathways. Globally, patients with FOP are also confronted with inequities in access to diagnosis and specialist care, and consequently, unequal access to registries, clinical trials, and essential support from patient associations. Organizations such as the International FOP Association, the International Clinical Council on FOP, and national FOP organizations work to provide information, facilitate access to expert clinical guidance, nurture patient empowerment, fund FOP research and/or foster meaningful collaborations with the research community. The non-profit Tin Soldiers Global FOP Patient Search program aims to identify and provide a pathway to diagnosis and care for individuals with FOP, particularly in underserved communities. Such global initiatives and the increasingly widespread use of telemedicine and digital platforms offer opportunities to improve vital access to care and research.

Conclusions

This multi-stakeholder perspective highlights some of the unmet needs of individuals with FOP and their families. Regional and international organizations play an important role in improving the quality of life of those they reach in the global FOP community. However, globally, fundamental issues remain around raising awareness of FOP among healthcare professionals, identifying individuals with FOP, reducing time to diagnosis, and ensuring access to best practice in care, support, and clinical research. Medical writing support was industry-sponsored.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02224-w.

BMP signaling and skeletal development in fibrodysplasia ossificans progressiva (FOP)

Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare genetic disease caused by increased BMP pathway signaling due to mutation of ACVR1, a bone morphogenetic protein (BMP) type 1 receptor. The primary clinical manifestation of FOP is extra-skeletal bone formation (heterotopic ossification) within soft connective tissues. However, the underlying ACVR1 mutation additionally alters skeletal bone development and nearly all people born with FOP have bilateral malformation of the great toes as well as other skeletal malformations at diverse anatomic sites. The specific mechanisms through which ACVR1 mutations and altered BMP pathway signaling in FOP influence skeletal bone formation during development remain to be elucidated; however, recent investigations are providing a clearer understanding of the molecular and developmental processes associated with ACVR1-regulated skeletal formation.

Nonclassic fibrodysplasia ossificans progressiva: A child from Angola with an ACVR1G328E variant

Little is known about FOP in Africa and few cases of nonclassic fibrodysplasia ossificans progressiva (FOP) have been reported on the continent. Here we report a three-year-old girl from Angola with a nonclassic FOP clinical presentation that is characterized by complex malformations of the toes and fingers, reduction defects of the digits, absence of nails, progressive heterotopic ossification, and a confirmed heterozygous ACVR1 variant at c.983G > A. Emerging knowledge of FOP can serve as a catalyst for increasing awareness of FOP in under-represented medical communities by achieving a correct FOP diagnosis, improving access of individuals with FOP to clinical trial recruitment, and enhancing the ability of affected individuals to be part of and interact with the international FOP community.