CNS demyelination in fibrodysplasia ossificans progressiva.

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder of progressive heterotopic ossification (HO) caused by a recurrent activating mutation of ACVR1/ALK2, a bone morphogenetic protein (BMP) type I receptor. FOP is characterized by progressive HO, which is associated with inflammation in the setting of dysregulated BMP signaling, however, a variety of atypical neurologic symptoms are also reported by FOP patients. The main objective of this study is to investigate the potential underlying mechanism that is responsible for the observed atypical neurologic symptoms. We evaluated two mouse models of dysregulated BMP signaling for potential CNS pathology through non-invasive magnetic resonance imaging (MRI) studies and histological and immunohistochemical approaches. In one model, BMP4 is over-expressed under the control of the neuron-specific enolase promoter; the second model is a knock-in of a recurrent FOP mutation of ACVR1/ALK2. We also retrospectively examined MRI scans of four FOP patients. We consistently observed demyelinated lesions and focal inflammatory changes of the CNS in both mouse models but not in wild-type controls, and also found CNS white matter lesions in each of the four FOP patients examined. These findings suggest that dysregulated BMP signaling disturbs normal homeostasis of target tissues, including CNS where focal demyelination may manifest as the neurologic symptoms frequently observed in FOP.