ST3GAL5 and Spastic paraplegia: Defects occurring upstream of the GM1 ganglioside biosynthesis, such as the deficit of GM3 synthase and of N-acetyl-beta-glucosaminyl-glycoprotein 4-beta-N-acetylgalactosaminyltransferase 1, responsible for Spastic Paraplegia 2644, could cause a significant depletion of GM1 ganglioside levels.