Inherited human diseases of heterotopic bone formation

Abstract

Human disorders of hereditary and nonhereditary heterotopic ossification are conditions in which osteogenesis occurs outside of the skeleton, within soft tissues of the body. The resulting extraskeletal bone is normal. The aberration lies within the mechanisms that regulate cell-fate determination, directing the inappropriate formation of cartilage or bone, or both, in tissues such as skeletal muscle and adipose tissue. Specific gene mutations have been identified in two rare inherited disorders that are clinically characterized by extensive and progressive extraskeletal bone formation-fibrodysplasia ossificans progressiva and progressive osseous heteroplasia. In fibrodysplasia ossificans progressiva, activating mutations in activin receptor type-1, a bone morphogenetic protein type I receptor, induce heterotopic endochondral ossification, which results in the development of a functional bone organ system that includes skeletal-like bone and bone marrow. In progressive osseous heteroplasia, the heterotopic ossification leads to the formation of mainly intramembranous bone tissue in response to inactivating mutations in the GNAS gene. Patients with these diseases variably show malformation of normal skeletal elements, identifying the causative genes and their associated signaling pathways as key mediators of skeletal development in addition to regulating cell-fate decisions by adult stem cells.

Figure 1

Characteristic clinical features of FOP. a | Extensive heterotopic bone formation typical of FOP seen in a three-dimensional reconstructed CT scan of the back of a 12-year-old child. Flare-ups of FOP arise and progress in a well-defined spatial pattern, resulting in ribbons, sheets and plates of bone that fuse the joints of the axial and appendicular skeletons. b | Anteroposterior radiograph of the feet of a 3-year-old child, showing symmetrical big toe malformations of metatarsals and proximal phalanges together with microdactyly, fused interphalangeal joints and hallux valgus deviations at the metatarsophalangeal joints (circled). Abbreviation: FOP, fibrodysplasia ossificans progressiva. Permission obtained for Figure 1a,b from Nature Publishing Group © Shore, E. M. et al. Nat. Genet. 38, 525-527 (2006).

Figure 2

Generalized schematic representation of the BMP signaling pathway. Type I and type II BMP receptors span the cell membrane and bind extracellular BMP ligand. Ligand binding to BMP heterotetrameric receptor complexes activates signaling through type II-receptor-mediated phosphorylation of the type I receptor on the GS domain. Type I receptor phosphorylation is accompanied by decreased binding to the GS domain of proteins that prevent receptor signaling in the absence of ligand binding. Activated type I receptors phosphorylate cytoplasmic signal transduction proteins such as R-SMADs and MAPKs (including JNK, ERK and p38), which, in turn, directly or indirectly regulate the transcription of target genes in the nucleus. Abbreviations: BMP, bone morphogenetic protein; BMPR, bone morphogenetic protein receptor; Co-SMAD, common-mediator SMAD; ERK, extracellular signal-regulated kinase; GS domain, glycine–serine domain; JNK, c-Jun N-terminal kinase; MAPK, mitogen-activated protein kinase; R-SMAD, receptor-regulated SMAD.

Figure 3

Schematic histologic representation of the stages of endochondral heterotopic ossification in FOP. Lesion formation in FOP involves inflammation and the destruction of connective tissues (phase 1) followed by a replacement phase of new tissue development (phase 2). The initial histologic evidence of lesion induction is the presence of abundant perivascular lymphocytes (stage 1A) in connective tissue such as skeletal muscle. Lymphocytes expand into the tissue (stage 1B) and loss of the connective tissue structure follows (stage 1C). As the tissue is degraded, it is rapidly replaced by fibroproliferative cells (stage 2A). Angiogenesis and vascularization occur (stage 2B), followed by chondrogenesis and osteogenesis (stage 2C) and the formation of heterotopic bone. Abbreviation: FOP, fibrodysplasia ossificans progressiva.

Figure 4

A working model for altered BMP signaling in FOP. The ACVR1 mutation that is present in the vast majority of patients with FOP is an activating mutation that can stimulate signaling, at least in part in the absence of BMP, but this activity might only be moderately ‘on’ under basal in vivo conditions, effectively raising the BMP pathway activation set-point and reducing the amount of additional activation that is needed to stimulate endochondral ossification. Triggering events, such as tissue injury and associated changes in the tissue microenvironment, enhance signaling and overcome the reduced amount of ‘activation energy’ that is needed to stimulate cartilage and bone cell differentiation; these events might induce a stronger response of longer duration than is normal, owing to the mechanism of mutant receptor activation or an impaired negative feedback response, or both. In normal, non-FOP tissues, the set point is sufficiently low such that the equivalent activation of BMP signaling does not have sufficient activation energy to overcome the threshold that leads to cartilage and bone formation, and normal tissue repair occurs, for example, in response to injury. Abbreviation: ACVR1, activin receptor type-1; BMP, bone morphogenetic protein; FOP, fibrodysplasia ossificans progressiva.

Figure 5

Heterotopic ossification in POH. Radiographic appearance of heterotopic ossification. Lateral serial X-rays of the lower leg of a child, showing progression of heterotopic ossification from the ages of 18 months a | to 30 months b | to 8 years (c | amputation specimen). Extensive ossification of the soft tissues of the superficial and deep posterior compartments of the leg, disuse osteopenia and anterior bowing of the tibia can be seen. Abbreviation: POH, progressive osseous heteroplasia. Permission obtained from The Journal of Bone and Joint Surgery, Inc. © Kaplan, F. S. et al. J. Bone Joint Surg. 76, 425-436 (1994).

Figure 6

Histopathology of a POH lesion. Lower power image (left, magnification ×50) shows the epidermis and dermis. Irregular deposits of bone within the dermal tissue (shown at higher power on the right, magnification ×200) are often observed in proximity to subcutaneous adipose tissue. Both images were obtained following hematoxylin and eosin staining. Abbreviation: POH, progressive osseous heteroplasia.

Figure 7

The GNAS gene locus. GNAS is a complex gene, encoding multiple transcripts that are expressed from several promoters within the locus. In the figure, exons are depicted by boxes, with arrows indicating transcriptional activity and direction. The first exons of Nesp55, XLαs, A/Band Gsα splice to a set of common downstream exons (exons 2–13). The antisense transcript (AS-1) is also indicated. Genomic imprinting differentially marks the maternally inherited and paternally inherited GNAS alleles by DNA methylation (indicated by asterisks) in a reciprocal pattern (differentially methylated regions, DMrs). Methylation is associated with transcriptional silencing.