Symmetrical glial hyperplasia in the brainstem of fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare autosomal dominant disease, characterized by the progressive ossification of skeletal muscles, fascia, tendons, and ligaments. In most cases, the great toes of patients show symmetrical congenital malformations. The causative gene for FOP has been identified as the activin A receptor, type 1 (ACVR1) gene (ACVR1). The ACVR1 R206H mutation is the most common mutation among FOP patients, and the ACVR1 G356D mutation has been identified as a rare mutation in a Japanese FOP patient with slow progression. In addition to musculoskeletal abnormalities, a series of autopsy studies described one FOP case, without genetic testing to identify ACVR1 mutation, showing nodular heterotopia at the edge of the fourth ventricle. Here, we report the general autopsy findings for a 75-year-old man with FOP, caused by the ACVR1 G356D mutation, including the precise examination of brainstem lesions. Postmortem examination revealed unique symmetrical glial hyperplasia of the pons and medulla oblongata. Microscopically, lesions of the pons involving residual neurons and lesions of the medulla oblongata consisted of subependymal cells. Immunohistochemical analysis of these lesions revealed developmental anomalies, with different cellular components. In this report, for the first time, we present the neuropathological description of a patient with genetically confirmed FOP and symmetrical glial hyperplasia of the pons and medulla oblongata. The presented pathological findings, in conjunction with previous reports implying that the glial hyperplasia of the brainstem is common in FOP, suggest that ACVR1 may play an unclarified developmental role in the human brainstem.