An anti-ACVR1 antibody exacerbates heterotopic ossification by fibro/adipogenic progenitors in fibrodysplasia ossificans progressiva mice

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease characterized by progressive and catastrophic heterotopic ossification (HO) of skeletal muscle and associated soft tissues. FOP is caused by dominantly acting mutations in the bone morphogenetic protein (BMP) type I receptor, ACVR1 (also known as ALK2), the most prevalent of which is an arginine to histidine substitution [ACVR1(R206H)] in the glycine-serine rich intracellular domain of the receptor. A fundamental pathological consequence of FOP-causing ACVR1 receptor mutations is to enable activin A to initiate canonical BMP signaling in responsive progenitors, which drives skeletogenic commitment and HO. With the clear targets of activin A and ACVR1 identified, development of antibody therapeutics to prevent ligand-receptor interactions is an interventional approach currently being explored. Here, we developed a monoclonal blocking antibody (JAB0505) to the extracellular domain of ACVR1 and tested its ability to inhibit HO in established FOP mouse models. JAB0505 inhibited BMP-dependent gene expression in wild-type and ACVR1(R206H)-overexpressing cell lines. Strikingly, however, JAB0505 treatment markedly exacerbated injury-induced HO in two independent FOP mouse models in which ACVR1(R206H) was either broadly expressed, or more selectively expressed in fibro/adipogenic progenitors (FAPs). JAB0505 drove HO even under conditions of activin A inhibition, indicating that JAB0505 has receptor agonist activity. JAB0505-treated mice exhibited multiple, distinct foci of heterotopic lesions, suggesting an atypically broad anatomical domain of FAP recruitment to endochondral ossification. In addition, skeletogenic differentiation was both delayed and prolonged, and this was accompanied by dysregulation of FAP population growth. Collectively, alterations in the growth and differentiative properties of FAPs and FAP-derived skeletal cells are implicated in the aggravated HO phenotype. These data raise serious safety and efficacy concerns for the use of anti-ACVR1 antibodies to treat FOP patients.