Over-expression of wild-type ACVR1 in fibrodysplasia ossificans progressiva mice rescues perinatal lethality and inhibits heterotopic ossification

Fibrodysplasia ossificans progressiva (FOP) is a devastating disease of progressive heterotopic bone formation for which effective treatments are currently unavailable. FOP is caused by dominant gain-of-function mutations in the receptor ACVR1 (also known as ALK2), which render the receptor inappropriately responsive to activin ligands. In previous studies, we developed a genetic mouse model of FOP that recapitulates most clinical aspects of the disease. In this model, genetic loss of the wild-type Acvr1 allele profoundly exacerbated heterotopic ossification, suggesting the hypothesis that the stoichiometry of wild-type and mutant receptors dictates disease severity. Here, we tested this model by producing FOP mice that conditionally over-express human wild-type ACVR1. Injury-induced heterotopic ossification (HO) was completely blocked in FOP mice when expression of both the mutant and wild-type receptor were targeted to Tie2-positive cells, which includes fibro/adipogenic progenitors (FAPs). Perinatal lethality of Acvr1R206H/+ mice was rescued by constitutive ACVR1 over-expression and these mice survived to adulthood at predicted Mendelian frequencies. Constitutive over-expression of ACVR1 also provided protection from spontaneous HO, and the incidence and severity of injury-induced HO in these mice was dramatically reduced. Analysis of pSMAD1/5/8 signaling both in cultured cells and in vivo indicates that ACVR1 over-expression functions cell-autonomously by reducing osteogenic signaling in response to activin A. Manipulating the stoichiometry of FOP-causing and wild-type ACVR1 receptors may provide the foundation for novel therapeutic strategies to treat this devastating disease.