SLC33A1 and hereditary spastic paraplegia: More interestingly, the bone morphogenic protein (BMP) signaling pathway has progressively emerged as a common pathogenic mechanism in at least six HSP subtypes: SPG6 (NIPA1), SPG3A (atlastin-1), SPG4 (spastin), SPG20 (spartin), SPG39 (PNPLA6), SPG42 (SLC33A1) [26,27,34,35,36,49,51,52,96].