ZFYVE27 and hereditary spastic paraplegia: Mutations of the human protrudin gene—ZFYVE27, also known as SPG33 (spastic paraplegia gene 33)—give rise to hereditary spastic paraplegia (HSP), an axonopathy characterized by progressive spasticity and weakness of the lower limbs due to degeneration of the long axons of corticospinal tract motor neurons [12–16].