Answer: Marrow stem cell transplantation in fibrodysplasia ossificans progressiva

Hematopoietic stem-cell contribution to ectopic skeletogenesis

BACKGROUND

Fibrodysplasia ossificans progressiva is a rare genetic disorder of ectopic skeletogenesis associated with dysregulation of bone morphogenetic protein (BMP) signaling. Hematopoietic cells have been implicated in the ectopic skeletogenesis of fibrodysplasia ossificans progressiva, and their replacement has been postulated as a possible cure. However, the definitive contribution of hematopoietic cells to the pathogenesis of ectopic skeletogenesis remains obscure.

METHODS

We employed both careful clinical observation and in vivo murine transplantation studies to more precisely determine the contribution of hematopoietic cells to ectopic skeletogenesis. We identified a patient with fibrodysplasia ossificans progressiva who had undergone bone marrow transplantation for the treatment of intercurrent aplastic anemia twenty-five years earlier and investigated whether the clinical course of the fibrodysplasia ossificans progressiva had been influenced by bone marrow replacement or immunosuppression, or both. In complementary studies, we transplanted hematopoietic stem cells from constitutively expressing LacZ transgenic mice to identify the contribution of hematopoietic cells to BMP4-induced heterotopic ossification, a histopathologic model of fibrodysplasia ossificans progressiva.

RESULTS

We found that replacement of hematopoietic cells was not sufficient to prevent ectopic skeletogenesis in the patient with fibrodysplasia ossificans progressiva but pharmacologic suppression of the apparently normal donor immune system following transplantation in the new host modulated the activity of the fibrodysplasia ossificans progressiva and diminished the expression of skeletal ectopia. In complementary murine transplantation studies, we found that cells of hematopoietic origin contributed to the early inflammatory and late marrow-repopulating stages of BMP4-induced heterotopic ossification but were not represented in the fibroproliferative, chondrogenic, or osteogenic stages of heterotopic ossification. Interestingly, both recombinant human BMP4 induction in an animal model and the dysregulated BMP signaling pathway in a patient with fibrodysplasia ossificans progressiva were sufficient to recruit at least two populations of cells, one of hematopoietic origin and at least one of non-hematopoietic origin, that contribute to the formation of an ectopic skeleton.

CONCLUSIONS

Taken together, these findings demonstrate that bone marrow transplantation did not cure fibrodysplasia ossificans progressiva in the patient in this study, most likely because the hematopoietic cell population is not the site, or at least not the dominant site, of the intrinsic dysregulation of the BMP signaling pathway in fibrodysplasia ossificans progressiva. However, following transplantation of bone marrow from a presumably normal donor, immunosuppression of the immune system appeared to ameliorate activation of ectopic skeletogenesis in a genetically susceptible host. Thus, cells of hematopoietic origin may contribute to the formation of an ectopic skeleton, although they are not sufficient to initiate the process alone.

Consideration of Rituximab for fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a severe, progressive disease of the musculoskeletal system. Muscles, tendons and other connective tissues ossify after minor trauma, and patients often become encased in a second immobile skeleton. There is no known cure or treatment for FOP. It has been found that lymphocytes from FOP patients elaborate excess levels of bone morphogenic protein-4 (BMP-4). Given this, it has been suggested that allogenic bone marrow transplantation (BMT) possibly could be a cure for FOP, and drawn attention to a previously unappreciated case of an FOP patient who had successful BMT for aplastic anemia with apparent short- and medium-term arresting of the FOP disease process. However, BMT has non-trivial associated morbidity and mortality. Here, it is noted that if B cells are found to be the lymphocytes responsible for excess BMP-4 production in FOP, use of Rituximab, a monoclonal anti-CD20 antibody which effectively targets B cells, could be a less permanent and less risky treatment alternative for FOP.

Heterotopic ossification

Heterotopic ossification, the formation of bone in soft tissue, requires inductive signaling pathways, inducible osteoprogenitor cells, and a heterotopic environment conducive to osteogenesis. Little is known about the molecular pathogenesis of this condition. Research into two rare heritable and developmental forms, fibrodysplasia ossificans progressiva and progressive osseous heteroplasia, has provided clinical, pathologic, and genetic insights. In fibrodysplasia ossificans progressiva, overexpression of bone morphogenetic protein 4 and underexpression of multiple antagonists of this protein highlight the potential role of a potent morphogenetic gradient. Research on fibrodysplasia ossificans progressiva also has led to the identification of the genetic cause of progressive osseous heteroplasia: inactivating mutations in the alpha subunit of the gene coding for the stimulatory G protein of adenylyl cyclase. Better understanding of the complex developmental and molecular pathology of these disorders may lead to more effective strategies to prevent and treat other, more common forms of heterotopic ossification.