Introduction

Fibrodysplasia ossificans progressiva (FOP; OMIM 135100) is a rare genetic disorder with autosomal dominant transmission (Kaplan et al. 2002). FOP is the most severe disorder of heterotopic ossification and results in the postnatal formation of an ectopic skeleton (Cohen et al. 1993; Kaplan et al. 1993, 2004). Heterotopic ossification in FOP begins in childhood and can be induced most often by trauma. The ossification is episodic and progressive, leading to extra-articular ankylosis of all major joints of the axial and appendicular skeleton (Cohen et al. 1993; Kaplan et al. 1993, 2004), rendering movement impossible. Congenital malformation of the great toes is common.

Recently, Shore et al. (2006) mapped FOP to chromosome 2q23-24 by linkage analysis and identified a heterozygous mutation (617G>A; R206H) in the glycine-serine (GS) activation domain of ACVR1, encoding a BMP type I receptor. They found the mutation in 39 families examined, including African–American, European–American, European (UK), Korean, and Native Brazilian descents. Furthermore, a Taiwanese patient with a de novo ACVR1 617G>A mutation has been reported (Lin et al. 2006). Only the 617G>A mutation has been found in FOP so far. To investigate whether ACVR1 617G>A mutation is common and recurrent, we examined the ACVR1 gene in Japanese FOP patients. We found the 617G>A mutation in all patients examined.

Materials and methods

The diagnosis of FOP was based on clinical and radiographic findings. The phenotypes of the three patients are summarized in Table 1. All patients were sporadic. Peripheral blood or fingernails were obtained with informed consent from the patients. Genomic DNA was extracted using standard procedures. We amplified exon 4 by polymerase chain reaction (PCR) using primer sequences 5′-CCAGTCCTTCTTCCTTCTTCC-3′ and 5′-AGCAGATTTTCCAAGTTCCATC-3′ (Shore et al. 2006). PCR products were sequenced directly using an ABI Prism 3700 automated sequencer (Applied Biosystems, Foster City, CA).

Table 1 Clinical summary of the patients with fibrodysplasia ossificans progressiva (FOP)
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Results and discussion

Direct sequence analysis of genomic DNA demonstrated the presence of a single heterozygous 617G>A mutation in all three patients with FOP (Fig. 1). No other mutation was identified. Thus, we found that Japanese patients with FOP also had the same ACVR1 617G>A mutation. These results suggest that the mutation is common and recurrent in FOP, regardless of ethnic background. The presence of the mutation hot spot facilitates molecular diagnosis in clinical practice. Early diagnosis is very critical for FOP patients to avoid ossification-causing stimuli, including medical intervention.

Fig. 1
figure 1

A direct sequence of genomic DNA from a Japanese patient with fibrodysplasia ossificans progressiva (FOP), showing the common ACVR1 mutation 617G>A (R206H) (arrow)

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Accumulation of mutation in a particular single nucleotide is most famous in achondroplasia, the most common skeletal dysplasia (OMIM 100800). Mutations in the transmembrane domain of fibroblast growth factor receptor 3 (FGFR3) are identified in achondroplasia, and more than 99% of the FGFR3 mutation occurs in a particular single nucleotide, 1138G (1138G>A/C (G380R) mutation). To date, this represents the most mutable single nucleotide reported in the human genome. In both achondroplasia and FOP, the mutations are CpG dinucleotide changes, and paternal age effects in sporadic cases have been reported (Penrose 1955; Tuente et al. 1967). In achondroplasia, the initial and subsequent studies (Shiang et al. 1994; Rousseau et al. 1994; Bellus et al. 1995) found only G380R mutation in more than 200 subjects, which led some researchers to conclude that achondroplasia is defined by recurrent G380R mutation (Bellus et al. 1995). However, the different mutation in FGFR3 has been found later in slightly atypical cases (Ikegawa et al. 1995; Superti-Furga et al. 1995). The mutation G375C is also in the transmembrane domain. The phenotypic difference between the common and uncommon mutation are subtle (Nishimura et al. 1995).

In contrast to homogeneity of the achondroplasia phenotype, the clinical manifestation of FOP is considerably diverse, which argues against the homogeneity of the FOP mutation. In a family reported by Shore et al. (2006) who showed ambiguous FOP feature, the ACVR1 617G>A mutation was not identified. More extensive search for the FOP/ACVR1 mutation that includes patients with atypical FOP phenotypes is necessary to conclude that the single mutation determines the FOP phenotype.