ReviewInsights from a rare genetic disorder of extra-skeletal bone formation, fibrodysplasia ossificans progressiva (FOP)
Introduction
William Harvey, the seventeenth century physician who discovered blood circulation, wrote [1]: “Nature is nowhere accustomed more openly to display her secret mysteries than in cases where she shows traces of her workings apart from the beaten path; nor is there any better way to advance the proper practice of medicine than to give our minds to the discovery of the usual law of Nature by careful investigation of cases of rare forms of disease.”
The primary goal of biomedical research is “to advance the proper practice of medicine” by developing reliable diagnostics, efficient treatments, and effective preventions. As William Harvey's quotation illustrates, the value of rare disorders to help us understand common diseases, as well as understand the normal patterns of development and function, has long been recognized.
An increasingly valuable approach to determining the causes of many commonly occurring human diseases is through identifying common genetic variants (polymorphisms) that cause or pre-dispose an individual to a particular disease in combination with other common genetic variants and/or environmental influences [2]. Such an approach has great potential to understand many human disorders while also providing an opportunity to elucidate the genes and cellular pathways that regulate normal function. However, special opportunities for insight into how our bodies and our cells function are provided by rare genetic diseases that are caused by mutations in single genes. These conditions are rare because such genes are likely of such critical functional importance that mutations in these genes are rarely tolerated. By identifying the genes that cause these genetic disorders, it becomes possible to reveal key components of cellular function that can be targeted for therapeutic intervention — not only for the rare disease under examination, but also for more common conditions that utilize the same cellular pathways.
In this review, we focus on the rare human genetic disease fibrodysplasia ossificans progressiva (FOP; MIM #135100). While our understanding of FOP remains incomplete, we already have gained tremendous insight into cellular mechanisms that regulate bone formation and that, when mis-regulated, cause this disease.
Section snippets
FOP — overview
FOP is a rare condition, occurring at a population frequency of about 1 per 2 million [3], [4]. Most cases of FOP are sporadic, with a single affected person in a family. FOP can be inherited in an autosomal dominant pattern, however due to the severe disability of FOP, only a few cases of inheritance from one generation of a family to the next are known to have occurred [4], [5].
The main characteristic clinical feature of FOP is the formation of extra-skeletal, or heterotopic, bone. In
Clinical description and diagnostic criteria for classic FOP
Two major characteristics define “classic” FOP [6], [8]. Congenital malformation of the great toes (Fig. 1) is the earliest phenotypic feature of FOP [9], [10], [11]. This is the most recognizable skeletal feature of FOP, although other subtle skeletal changes (such as cervical spine fusions, short/broad femoral necks, and osteochondromas) also commonly occur [12], [13], [14].
The second classic feature is extra-skeletal bone formation (Fig. 1) that begins during childhood; formation of
FOP histology
Since surgical trauma to tissues of FOP patients induces additional bone formation, biopsy specimens of developing FOP lesions are extremely rare and have only been obtained before a diagnosis of FOP has been made. The opportunities to observe the stages of lesion formation have therefore been limited, but extremely informative.
FOP heterotopic bone characteristically forms through an endochondral pathway [18], [24]. We have recognized several stages of FOP lesion formation leading to mature FOP
POH — a second rare genetic disease of extra-skeletal bone formation
One of the first insights that FOP provided was that all cases of genetic heterotopic ossification do not have the same underlying cause. Clinical referrals of patients with a preliminary diagnosis of FOP soon revealed that these patients segregated into two clearly distinct patterns of type and location of bone formation, leading us to characterize and identify of a second genetic disorder, progressive osseous heteroplasia (POH) [30].
FOP and POH share significant clinical similarities [31],
FOP experimental approaches
A standard approach to investigating Mendelian genetic diseases is to identify the causative gene through genetic linkage and positional cloning approaches. Although FOP is an inherited disease, through the early stages of our investigations to understand its cause, we were limited in using these genetic strategies by the small numbers of families in which FOP is inherited from one generation to the next.
An alternate approach to studying a rare genetic disease is to identify differences in cell
BMP signaling in FOP
We examined several aspects of BMP pathway signaling in FOP cells in order to identify differences in expression of pathway components and pathway activation. Through an extensive series of experiments, we demonstrated significant changes affecting the BMP pathway in FOP cells, suggesting to us that increased BMP pathway signaling plays a role in FOP pathogenesis.
Early studies used several approaches to examine the expression of BMP family members in cells from FOP patients. One of our first
Genetics of FOP
Concurrently with investigations of the BMP signaling pathway in FOP cells, we continued to work with colleagues world-wide to identify families with inherited FOP in order to conduct an informative whole-genome linkage analysis. An initial linkage study had supported linkage of FOP to a small number of chromosomal regions with the strongest linkage to a region of chromosome 4 [55], containing the Smad1 gene. However, DNA sequence analysis of Smad1 and other candidate genes within the interval
Classic FOP mutations
We examined the ACVR1 gene for mutations in both inherited and sporadic FOP patients with the classic features of FOP by DNA sequencing analysis of ACVR1 coding regions (exons and splice junctions). We discovered that all people with classic features of FOP contain a unique and very specific mutation — the identical heterozygous single nucleotide change at nucleotide position 617 of the cDNA (c.617G > A) [8] (Fig. 2).
This recurrent mutation changes amino acid 206 from arginine to histidine. Amino
Atypical forms of FOP
Following our analysis of individuals with “classic” FOP features, we next turned our attention to more unusual presentations of patients with FOP-like heterotopic ossification [64]. We classified these patients into two groups (Table 1). “FOP-plus” patients have the characteristic classic FOP features of extensive extra-skeletal ossification and toe malformations, but additionally have one or more features that are not typically associated with FOP. “FOP variants” have heterotopic
ACVR1 mutations in FOP — summary
All patients with FOP-type heterotopic ossification that we examined contain heterozygous mutations in ACVR1 [8], [64]. These dominant ACVR1 mutant alleles, affecting the GS or protein kinase domains of the receptor protein, appear to have high penetrance: thus far, the mutations have not been found in any individuals without heterotopic ossification and conversely every person with FOP-like heterotopic ossification has an identified ACVR1 mutation. One immediate benefit of identifying ACVR1
ACVR1/ALK2
ACVR1/ALK2 is one of four type I receptors that mediate signaling through BMPs [66], [67], [68], [69]. In response to BMP ligand binding, these receptors phosphorylate cytoplasmic signal transduction molecules to regulate target gene transcriptional activation or repression. In vivo, specific functional roles for each of the three type I receptors are likely determined by varying efficiencies of ligand binding and by non-overlapping receptor expression patterns in specific tissues and during
Ongoing and future FOP studies
We are currently examining the functional effects of FOP ACVR1 mutations through both cell signaling studies and animal models in order to understand the mechanism of the altered BMP signaling and to identify the wide range of effects on tissues and cell systems.
In vitro studies [75] using ACVR1 expression constructs for the classic FOP mutation (c.617G > A; R206H) showed ligand-independent activation of BMP pathway signaling through Smad phosphorylation and promoter reporter assays. We further
Conclusions
FOP is a rare genetic disorder that has given us, and continues to provide, important insight into the cellular and genetic regulation of skeletal and extra-skeletal bone formation. However, the beneficial lessons that FOP has taught us extend far beyond what the relatively small numbers of affected patients might initially cause one to expect. The heterotopic bone that forms in FOP is normal bone by all evaluated criteria; its aberration lies in the lost regulation of cell fate determination
Acknowledgments
We thank members of our research laboratory and our many collaborators for their contributions. We also thank the NIH/NIAMS-supported Penn Center for Musculoskeletal Disorders (AR050950). This work was supported in part by the Center for Research in FOP and Related Disorders, the International FOP Association (IFOPA), the Ian Cali Endowment, the Weldon Family Endowment, the Isaac and Rose Nassau Professorship of Orthopaedic Molecular Medicine, and by grants from the National Institutes of
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