[Fibrodysplasia ossificans progressiva and osteoimmunology]

Fibrodysplasia ossificans progressive (FOP) is a genetic disorder characterized by progressive heterotopic ossification (HO) in skeletal muscle, tendons and ligaments. FOP is caused by gain-of-function mutations of ALK2, a receptor of bone morphogenetic proteins. Immune responses have been suggested to be involved in HO in FOP, because muscle trauma induces acute HO in patients with FOP. Recently, Activin A, a non-osteogenic ligand, was identified as a ligand of the mutated ALK2 to induce HO. It was suggested that Activin A is a novel interface between FOP and osteoimmunology.