Robert J Caron

Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania, United States

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Publications (5)17.23 Total impact

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    ABSTRACT: Fibrodysplasia ossificans progressiva (FOP; MIM #135100) is a debilitating genetic disorder of dysregulated cellular differentiation characterized by malformation of the great toes during embryonic skeletal development and by progressive heterotopic endochondral ossification postnatally. Patients with these classic clinical features of FOP have the identical heterozygous single nucleotide substitution (c.617G > A; R206H) in the gene encoding ACVR1/ALK2, a bone morphogenetic protein (BMP) type I receptor. Gene targeting was used to develop an Acvr1 knock-in model for FOP (Acvr1(R206H/+)). Radiographic analysis of Acvr1(R206H/+) chimeric mice revealed that this mutation induced malformed first digits in the hind limbs and postnatal extraskeletal bone formation, recapitulating the human disease. Histological analysis of murine lesions showed inflammatory infiltration and apoptosis of skeletal muscle followed by robust formation of heterotopic bone through an endochondral pathway, identical to that seen in patients. Progenitor cells of a Tie2(+) lineage participated in each stage of endochondral osteogenesis. We further determined that both wild-type (WT) and mutant cells are present within the ectopic bone tissue, an unexpected finding that indicates that although the mutation is necessary to induce the bone formation process, the mutation is not required for progenitor cell contribution to bone and cartilage. This unique knock-in mouse model provides novel insight into the genetic regulation of heterotopic ossification and establishes the first direct in vivo evidence that the R206H mutation in ACVR1 causes FOP.
    Journal of bone and mineral research: the official journal of the American Society for Bone and Mineral Research 04/2012; 27(8):1746-56. · 6.04 Impact Factor
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    ABSTRACT: The study of FOP, a disabling genetic disorder of progressive heterotopic ossification, is hampered by the lack of readily available connective tissue progenitor cells. We isolated such cells from discarded primary teeth of patients with FOP and controls and discovered dysregulation of BMP signaling and rapid osteoblast differentiation in FOP cells compared with control cells. Fibrodysplasia ossificans progressiva (FOP), the most disabling condition of progressive heterotopic ossification in humans, is caused by a recurrent heterozygous missense mutation in activin receptor IA (ACVR1), a bone morphogenetic protein (BMP) type I receptor, in all classically affected individuals. A comprehensive understanding of FOP has been limited, in part, by a lack of readily available connective tissue progenitor cells in which to study the molecular pathology of this disorder. We derived connective tissue progenitor cells from discarded primary teeth (SHED cells) of patients with FOP and controls and examined BMP signaling and osteogenic differentiation in these cells. SHED cells transmitted BMP signals through both the SMAD and p38 mitogen-activated protein kinase (MAPK) pathways and responded to BMP4 treatment by inducing BMP responsive genes. FOP cells showed ligand-independent BMP signaling and ligand-dependent hyper-responsiveness to BMP stimulation. Furthermore, FOP cells showed more rapid differentiation to an osteogenic phenotype than control cells. This is the first study of BMP signaling and osteogenic differentiation in connective tissue progenitor cells from patients with FOP. Our data strongly support both basal and ligand-stimulated dysregulation of BMP signaling consistent with in silico studies of the mutant ACVR1 receptor in this condition. This study substantially extends our understanding of dysregulated BMP signaling in a progenitor cell population relevant to the pathogenesis of this catastrophic disorder of progressive ectopic ossification.
    Journal of bone and mineral research: the official journal of the American Society for Bone and Mineral Research 04/2008; 23(3):305-13. · 6.04 Impact Factor
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    ABSTRACT: Bone morphogenetic proteins (BMPs) are a highly conserved class of signaling molecules that induce ectopic cartilage and bone formation in vivo. Dysregulated expression of bone morphogenetic protein 4 (BMP4) is found in the cells of patients who have fibrodysplasia ossificans progressiva (FOP), a genetic disorder of axial and appendicular skeletal malformation and progressive heterotopic ossification. Loss of function mutations in the bone morphogenetic protein 5 (bmp5) gene leading to under-expression of BMP5 cause the murine short ear syndrome, characterized by small malformed ears and a broad range of axial skeletal malformations. We found features reminiscent of both the short ear mouse and FOP in a child with malformed external ears, multiple malformations of the axial skeleton, and progressive heterotopic ossification in the neck and back. We examined BMP mRNA expression in transformed lymphocytes by semi-quantitative RT-PCR and protein expression by ELISA assays and immunohistochemistry. Elevated levels of BMP4 and BMP5 mRNA and protein were detected in the patient's cells while levels of BMP2 mRNA were unchanged. Our data suggest that dysregulated expression of BMP4 and BMP5 genes is associated with an array of human axial skeletal abnormalities similar to the short ear mouse and FOP.
    American Journal of Medical Genetics Part A 05/2007; 143(7):699-706. · 2.30 Impact Factor
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    ABSTRACT: Fibrodysplasia ossificans progessiva (FOP) is a genetic disease of progressive, heterotopic ossification, resulting in profound decreased mobility. To investigate the pathophysiology of this condition, lymphoblastoid cells (LCLs) derived from patients with FOP or unaffected family members were implanted subcutaneously into athymic (nude) mice. Cells from unaffected individuals persisted as small masses with little evidence of a fibrotic or angiogenic response. In contrast, cells from patients with FOP gave rise to palpable, solid, fibrotic cellular masses in the animals. Histological and immunohistochemical evaluation revealed that FOP cells proliferated in the host and induced a fibrotic and angiogeneic response similar to the early-stage FOP lesions in patients, but did not progress to form ettopic cartilage or bone. These results de monstrate that lymphoblastoid cells from patients with FOP induce early preosseous FOP-like lesions in mice, but are not sufficient to induce heterotopic ossification in immunocompromised host animals. Implantation of FOP-derived cells in nude mice provides a useful model system for examining the earliest stages of the disease.
    Clinical Reviews in Bone and Mineral Metabolism 01/2005; 3(3):225-228.
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    ABSTRACT: Fibrodysplasia ossificans progressiva (FOP) is a catastrophic genetic disorder of progressive heterotopic ossification associated with dysregulated production of bone morphogenetic protein 4 (BMP4), a potent osteogenic morphogen. Postnatal heterotopic ossification in FOP is often heralded by hectic episodes of severe post-traumatic connective tissue swelling and intramuscular edema, followed by an intense and highly angiogenic fibroproliferative mass. The abrupt appearance, intense size, and rapid intrafascial spread of the edematous preosseous fibroproliferative lesions implicate a dysregulated wound response mechanism and suggest that cells and mediators involved in inflammation and tissue repair may be conscripted in the growth and progression of FOP lesions. The central and coordinate role of inflammatory mast cells and their mediators in tissue edema, wound repair, fibrogenesis, angiogenesis, and tumor invasion prompted us to investigate the potential involvement of mast cells in the pathology of FOP lesions. We show that inflammatory mast cells are present at every stage of the development of FOP lesions and are most pronounced at the highly vascular fibroproliferative stage. Mast cell density at the periphery of FOP lesional tissue is 40- to 150-fold greater than in normal control skeletal muscle or in uninvolved skeletal muscle from FOP patients and 10- to 40-fold greater than in any other inflammatory myopathy examined. These findings document mobilization and activation of inflammatory mast cells in the pathology of FOP lesions and provide a novel and previously unrecognized target for pharmacologic intervention in this extremely disabling disease.
    Human Pathlogy 09/2001; 32(8):842-8. · 2.84 Impact Factor