ZFYVE26 and hereditary spastic paraplegia: 11 patients with HSP (mean age 37.3 ± 8.1 years; 6 men and 5 women; 8 affected by spastin/SPG4,1 by atlastin1/SPG3a, 1 by paraplegin/SPG7 and 1 by ZFYVE26/SPG15)