Fibrodysplasia ossificans progressiva in two half-sisters: evidence for maternal mosaicism

A case of Fibrodysplasia Ossificans Progressiva associated with a novel variant of the ACVR1 gene

Background

Fibrodysplasia Ossificans Progressiva (FOP) is a rare autosomal dominant disease characterized by congenital malformation of the great toes and progressive heterotopic ossification of soft tissues leading to cumulative disability. The genetic cause of FOP are mutations in the ACVR1 gene that encodes a type I receptor of Bone Morphogenetic Proteins. The most recurrent mutation in FOP patients is R206H affecting the Glycine‐Serine rich domain and causing the hyper‐activation of the receptor and the responsivity to the non‐canonical ligand, Activin A.

In the present study, we described a 3‐years old child with early and highly suggestive clinical features of FOP who was found negative for the recurrent p.R206H substitution.

Methods

Molecular screening of the whole ACVR1 coding sequence and functional characterization in transfection‐based assays.

Results and Conclusions

We identified a novel, de novo variant in the fifth ACVR1 coding exon (NM_001111067.4:c.772A>T; NP_001104537.1:p.(R258W)). This substitution, never reported in association with FOP, affects a conserved arginine residue in the kinase domain of the protein. In silico analysis predicted the pathogenicity of this substitution, demonstrated by in vitro assays showing that the p.R258W ACVR1 mutated receptor acquires the ability to transduce the aberrant Activin A‐mediated signaling, as observed for the gene variants associated with FOP.

Genomic Context and Mechanisms of the ACVR1 Mutation in Fibrodysplasia Ossificans Progressiva

Basic research in Fibrodysplasia Ossificans Progressiva (FOP) was carried out in the various fields involved in the disease pathophysiology and was important for designing therapeutic approaches, some of which were already developed as ongoing or planned clinical trials. Genetic research was fundamental in identifying the FOP causative mutation, and the astonishing progress in technologies for genomic analysis, coupled to related computational methods, now make possible further research in this field. We present here a review of molecular and cellular factors which could explain why a single mutation, the R206H in the ACVR1 gene, is absolutely prevalent in FOP patients. We also address the mechanisms by which FOP expressivity could be modulated by cis-acting variants in the ACVR1 genomic region in human chromosome 2q. Finally, we also discuss the general issue of genetic modifiers in FOP.

Genetics and future therapy prospects of fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare autosomal dominant genetic condition characterised by progressive extra-skeletal bone formation in connective tissues. Over time, heterotopic ossification entombs patients within a second skeleton, drastically impairing their mobility and autonomy. Mutations in the ACVR1 gene have been identified as the cause of FOP. The single nucleotide missense mutation in ACVR1, c.617G > A, causes a single amino acid substitution, p.R206H, and is found in >90% of all patients. Heterotopic bone formation in FOP mimics embryonic skeletal endochondral ossification, with cartilage forming after fibroproliferative tissue condensation as an intermediate stage prior to osteogenesis and tissue ossification. In contrast to normal embryonic endochondral ossification, heterotopic ossification in FOP involves an inflammatory phase that precedes cartilage and bone formation. New insights into the mechanisms of action of heterotopic bone formation in FOP have led to the discovery of new potential treatment targets including inhibitors of BMP signalling, activin A inhibitors, and mTOR inhibitors. This review summarises the current knowledge on mutations causing FOP, as well as the molecular basis of heterotopic ossification and the therapeutic options that result from these discoveries.

From mysteries to medicines: drug development for fibrodysplasia ossificans progressive

INTRODUCTION

Fibrodysplasia ossificans progressiva (FOP) is the most disabling disorder of skeletal metamorphosis in humans and leads to the formation of a second skeleton of heterotopic bone. Presently, there is no effective treatment.

AREAS COVERED

In this review, the authors discuss heterozygous activating mutations in Activin receptor A, type I/ Activin-like kinase 2 (ACVR1/ALK2), a bone morphogenetic protein (BMP) type I receptor that are the genetic cause of FOP and reveal a promising pharmacologic target in the BMP signaling pathway. Despite these germline mutations, episodic disease activation is induced by soft tissue injury and resultant inflammatory triggers that are dependent on responding progenitor cells and a tissue microenvironment that supports heterotopic ossification.

EXPERT OPINION

Here we review opportunities and challenges for the development of effective therapeutics for FOP. There are many potential approaches that may eventually be used to harness FOP. The long-term treatment of FOP is likely to involve not one, but several concomitant approaches that acknowledge molecular mechanisms involved in the induction and progression of the disease.

Fibrodysplasia ossificans progressiva: mechanisms and models of skeletal metamorphosis

Fibrodysplasia ossificans progressiva (FOP; MIM #135100) is a debilitating genetic disorder of connective tissue metamorphosis. It is characterized by malformation of the great (big) toes during embryonic skeletal development and by progressive heterotopic endochondral ossification (HEO) postnatally, which leads to the formation of a second skeleton of heterotopic bone. Individuals with these classic clinical features of FOP have the identical heterozygous activating mutation (c.617G>A; R206H) in the gene encoding ACVR1 (also known as ALK2), a bone morphogenetic protein (BMP) type I receptor. Disease activity caused by this ACVR1 mutation also depends on altered cell and tissue physiology that can be best understood in the context of a high-fidelity animal model. Recently, we developed such a knock-in mouse model for FOP (Acvr1^R206H/+) that recapitulates the human disease, and provides a valuable new tool for testing and developing effective therapies. The FOP knock-in mouse and other models in Drosophila, zebrafish, chickens and mice provide an arsenal of tools for understanding BMP signaling and addressing outstanding questions of disease mechanisms that are relevant not only to FOP but also to a wide variety of disorders associated with regenerative medicine and tissue metamorphosis.

Mutation Analysis and Prenatal Exclusion of Fibrodysplasia Ossificans Progressiva in a Chinese Fetus

Aims: Fibrodysplasia ossificans progressiva (FOP) is a rare and severely disabling autosomal dominant disorder characterized by congenital malformations of the great toes and progressive postnatal heterotopic ossification. A point mutation in the activin receptor IA (ACVR1) gene is the cause of FOP. Most of the reported cases of FOP are sporadic and caused by de novo mutations; however, some rare cases can also result from parental germline mosaicism associated with a greater risk of recurrence in successive pregnancies. Therefore, once the pathogenic mutation has been identified in the proband, it is relative cheaper and important to perform prenatal diagnostic tests to exclude the recurrence risk of FOP in subsequent pregnancies. In this study, we first investigated the mutation in the ACVR1 gene in a Chinese FOP patient and then performed prenatal tests to exclude the risk of recurrence in the patient's unborn sibling. Methods: A couple visited our clinic with their 4-year-old son, who was clinically diagnosed with FOP, for genetic counseling. Genetic testing was performed by amplifying all the nine exons of the ACVR1 gene using the conventional polymerase chain reaction. Further, DNA sequencing was used to determine the mutation based on the results of a mutation screening using denaturing high-performance liquid chromatography. Subsequently, a prenatal test was performed using the same technique as that used for the proband. Results: A recurrent single nucleotide mutation c.617 G>A (R206H) of the ACVR1 gene was identified in the patient; however, both the parents had a normal ACVR1 gene. Prenatal tests showed that the fetus did not carry the pathogenic mutation. Conclusion: The results confirmed that a recurrent single nucleotide mutation c.617 G>A (R206H) was the genetic cause of FOP and explored the utility of prenatal testing in excluding the risk of recurrence in the successive pregnancy.

Osteochondral diseases and fibrodysplasia ossificans progressiva

Osteochondrodysplasias like thanatophoric dysplasia, osteogenesis imperfecta, achondroplasia, and other genetic skeletal disorders like fibrodysplasia ossificans progressiva are infrequently seen in clinical practice. In cases of sporadic achondroplasia as well as in fibrodysplasia ossificans progressiva, there is a strong association with paternal age, a relationship that is less evident in other genetic osteochondral diseases. No other constitutional or environmental factor has proven to be associated with these disorders. The use of prenatal ultrasonography as a routine component of prenatal care is crucial in the early suspicion of osteochondrodysplasias whereas definitive diagnosis is usually obtained by pre-natal molecular analysis. In the case of fibrodysplasia ossificans progressiva, recognition of congenital great toe malformations associated with rapidly-appearing soft tissue swelling is sufficient to make the proper clinical diagnosis, which can be confirmed by genetic testing. Large regional centres will improve diagnosis performance, provide accurate genetic counselling, and ensure an integral assistance for these often severe and incapacitating conditions.

Early diagnosis of fibrodysplasia ossificans progressiva

BACKGROUND

Fibrodysplasia ossificans progressiva is a rare and disabling genetic condition characterized by congenital malformation of the great toes and by progressive heterotopic ossification in specific anatomic patterns. Most patients with fibrodysplasia ossificans progressiva are misdiagnosed early in life before the appearance of heterotopic ossification and undergo diagnostic procedures that can cause lifelong disability. Recently, the genetic cause of fibrodysplasia ossificans progressiva was identified, and definitive genetic testing for fibrodysplasia ossificans progressiva is now available before the appearance of heterotopic ossification.

METHODS

We recently evaluated 7 children for diagnosis of fibrodysplasia ossificans progressiva before the onset of heterotopic ossification. A medical history, physical examination, and skeletal survey were obtained on all of the patients, as well as clinical genetic testing for the canonical fibrodysplasia ossificans progressiva mutation.

RESULTS

All 7 of the children (4 girls and 3 boys; ages 3 months to 6 years) had congenital malformations of the great toes, but none had radiographic evidence of heterotopic ossification at the time of evaluation. Five of the 7 children had soft tissue lesions of the neck and back, suggestive of early fibrodysplasia ossificans progressiva flare-ups, 3 of whom had undergone invasive diagnostic procedures that exacerbated their condition. Two children had no history or signs of soft tissue swelling or flare-ups. DNA sequence analysis found that all 7 of the children had the recurrent fibrodysplasia ossificans progressiva missense mutation, a single nucleotide substitution (c.617G>A) at codon 206 in the glycine-serine activation domain of activin receptor IA, a bone morphogenetic protein type 1 receptor.

CONCLUSION

Clinical suspicion of fibrodysplasia ossificans progressiva early in life on the basis of malformed great toes can lead to early clinical diagnosis, confirmatory diagnostic genetic testing, and the avoidance of additional harmful diagnostic and treatment procedures. This is the first report of genetic confirmation of fibrodysplasia ossificans progressiva before the appearance of heterotopic ossification. Pediatricians should be aware of the early diagnostic features of fibrodysplasia ossificans progressiva, even before the appearance of heterotopic ossification. This awareness should prompt early genetic consultation and testing and the institution of assiduous precautions to prevent iatrogenic harm.

Linkage exclusion and mutational analysis of the noggin gene in patients with fibrodysplasia ossificans progressiva (FOP)

Fibrodysplasia ossificans progressiva (FOP) is an extremely rare and disabling genetic disorder characterized by congenital malformation of the great toes and by progressive heterotopic endochondral ossification in predictable anatomical patterns. Although elevated levels of bone morphogenetic protein 4 (BMP4) occur in lymphoblastoid cells and in lesional cells of patients with FOP, mutations have not been identified in the BMP4 gene, suggesting that the mutation in FOP may reside in a BMP4-interacting factor or in another component of the BMP4 pathway. A powerful antagonist of BMP4 is the secreted polypeptide noggin. A recent case report described a heterozygous 42-bp deletion in the protein-coding region of the noggin gene in a patient with FOP. In order to determine if noggin mutations are a widespread finding in FOP, we examined 31 families with 1 or more FOP patients. Linkage analysis with an array of highly polymorphic microsatellite markers closely linked to the noggin gene was performed in four classically-affected multigenerational FOP families and excluded linkage of the noggin locus to FOP (the multipoint lod score was -2 or less throughout the entire range of markers). We sequenced the noggin gene in affected members of all four families, as well as in 18 patients with sporadic FOP, and failed to detect any mutations. Single-strand conformation polymorphism (SSCP) analysis of 4 of these patients plus an additional 9 patients also failed to reveal any mutations. Among the samples analyzed by SSCP and DNA sequencing was an independently obtained DNA sample from the identical FOP patient previously described with the 42-bp noggin deletion; no mutation was detected. Examination of the DNA sequences of 20 cloned noggin PCR products, undertaken to evaluate the possibility of a somatic mutation in the noggin gene which could be carried by a small subset of white blood cells, also failed to detect the presence of the reported 42-bp deletion. We conclude that mutations in the coding region of noggin are not associated with FOP.

Osteogenic induction in hereditary disorders of heterotopic ossification

The formation of heterotopic bone within soft connective tissue is a common feature of at least three distinct genetic disorders of osteogenesis in humans: fibrodysplasia ossificans progressiva; progressive osseous heteroplasia; and Albright hereditary osteodystrophy. The pathobiologic characteristics of osteogenic induction, the histopathologic features of osteogenesis, the anatomic distribution of heterotopic lesions, and the developmental patterns of disease progression differ among all three conditions. The molecular and cellular basis of redirecting a mature connective tissue phenotype to form bone is a remarkable biological phenomenon with enormous implications for the control of bone regeneration, fracture healing, and disorders of osteogenesis.