Neofunction of ACVR1 in fibrodysplasia ossificans progressiva

Abstract

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease characterized by extraskeletal bone formation through endochondral ossification. FOP patients harbor point mutations in ACVR1 (also known as ALK2), a type I receptor for bone morphogenetic protein (BMP). Two mechanisms of mutated ACVR1 (FOP-ACVR1) have been proposed: ligand-independent constitutive activity and liganddependent hyperactivity in BMP signaling. Here, by using FOP patient-derived induced pluripotent stem cells (FOP-iPSCs), we report a third mechanism, where FOP-ACVR1 abnormally transduces BMP signaling in response to Activin-A, a molecule that normally transduces TGF-β signaling but not BMP signaling. Activin-A enhanced the chondrogenesis of induced mesenchymal stromal cells derived from FOPiPSCs (FOP-iMSCs) via aberrant activation of BMP signaling in addition to the normal activation of TGF-β signaling in vitro, and induced endochondral ossification of FOP-iMSCs in vivo. These results uncover a novel mechanism of extraskeletal bone formation in FOP and provide a potential new therapeutic strategy for FOP.iPSC | fibrodysplasia ossificans progressiva | heterotopic ossification | BMP | TGF H eterotopic ossification (HO) is defined as bone formation in soft tissue where bone normally does not exist. It can be the result of surgical operations, trauma, or genetic conditions, one of which is fibrodysplasia ossificans progressiva (FOP). FOP is a rare genetic disease characterized by extraskeletal bone formation through endochondral ossification (1-6). The responsive mutation for classic FOP is 617G > A (R206H) in the intracellular glycineand serine-rich (GS) domain (7) of ACVR1 (also known as ALK2), a type I receptor for bone morphogenetic protein (BMP) (8-10). ACVR1 mutations in atypical FOP patients have been found also in other amino acids of the GS domain or protein kinase domain (11,12). Regardless of the mutation site, mutated ACVR1 (FOP-ACVR1) has been shown to activate BMP signaling without exogenous BMP ligands (constitutive activity) and transmit much stronger BMP signaling after ligand stimulation (hyperactivity) (12-25).To reveal the molecular nature of how FOP-ACVR1 activates BMP signaling, cells overexpressing FOP-ACVR1 (12-20), mouse embryonic fibroblasts derived from Alk2 R206H/+ mice (21, 22), and cells from FOP patients, such as stem cells from human exfoliated deciduous teeth (23), FOP patient-derived induced pluripotent stem cells (FOP-iPSCs) (24, 25) and induced mesenchymal stromal cells (iMSCs) from FOP-iPSCs (FOP-iMSCs) (26) have been used as models. Among these cells, Alk2 R206H/+ mouse embryonic fibroblasts and FOP-iMSCs are preferred because of their accessibility and expression level of FOP-ACVR1 using an endogenous promoter. In these cells, however, the constitutive activity and hyperactivity is not strong (within twofold normal levels) (22,26). In addition, despite the essential role of BMP signaling in development (27-31), the pre-and postnatal development and growth of FOP patients are almost normal, and H...

Towards a cure for Fibrodysplasia ossificans progressiva

“…Using a new mouse model fully recapitulating the phenotype of the disease, that is, spontaneous induced HO mediated by altered BMP signaling, the authors demonstrated that activin A is capable of inducing Smad 1/5/8 phosphorylation and downstream transcriptional activation exclusively in the presence of the mutant ALK2 receptor. This finding was confirmed shortly thereafter by an independent group (14). Although this message probably summarizes the major translational finding, there is other relevant information provided within the manuscript.…”

“…These are secreted antagonists characterized by an activin binding site, although they can also interact with other ligands of the TGF-β family (23). Indeed, Hino et al tested a number of follistatin-like proteins to prevent activin A induced chondrogenesis in FOP iPSCs-derived MSCs (14). Finally, mutual antagonism is usually observed between members of the TGF-β family, by means of competing for the same receptors at the membrane or intracellular regulators that mediate gene transcription (24).…”

TGF-β Family Signaling in Connective Tissue and Skeletal Diseases

“…The efficacy of palovarotene is currently tested in a phase II clinical trial (ClinicalTrials.gov, NCT02190747). Moreover, a monoclonal antibody against activin A was reported to impair HO in a mouse model and in iPS cells with the FOP mutation (Hatsell et al 2015;Hino et al 2015); although the cellular mechanism for this mode of action has not been determined, the data suggest that activin A may be a therapeutic target for FOP.…”

Peripheral Blood Mononuclear Cell Immunophenotyping in Fibrodysplasia Ossificans Progressiva Patients: Evidence for Monocyte DNAM1 Up‐regulation

“…As suggested by previous in vitro studies, these mutations have a "gain of function" effect on the receptor's activity with dysregulation of the downstream pathway and increased responsivity to BMPs (6,(9)(10)(11)(12). Very recently, two seminal works have demonstrated that these mutations cause a modification of ACVR1 ligand binding properties, by conferring to the mutated receptor the ability to respond to Activin-A, a molecule that normally transduces TGF-b signaling through the Smad2/3 cascade (13,14). In FOP, the binding of Activin A to the mutated receptor, improperly induces the activation of the canonical, ACVR1-mediated, Smad1/5/8 pathway thus committing resident, tissue-specific, multipotent progenitors to the formation of ectopic bone (15).…”