When ganglioside pathways go awry: congenital disorders and experimental insights

Glycosphingolipids comprise a hydrophobic ceramide backbone, consisting of a long-chain base (sphingosine) and a fatty acid, conjugated with a hydrophilic oligosaccharide moiety. These amphipathic molecules are integral constituents of cellular membranes, playing pivotal roles in modulating membrane protein functionality and receptor-mediated signaling. Among glycosphingolipids, gangliosides, defined by their inclusion of sialic acid residues, are abundantly enriched in the central nervous system. Notably, four predominant species, GM1, GD1a, GD1b, and GT1b, constitute the majority of gangliosides in the mammalian brain and are indispensable for neuronal development, synaptic architecture, and signal transduction. These gangliosides are critically involved in neurogenesis, differentiation, membrane stability, and the modulation of receptor function, ion channel activity, and immunological signaling within the nervous system. The biosynthesis of these gangliosides is orchestrated by key enzymes, including GM3 synthase (ST3GAL5) and GM2/GD2 synthase (B4GALNT1) catalyzing the formation of downstream intermediates. Pathogenic variants in ST3GAL5 result in GM3 synthase deficiency (GM3SD), an autosomal recessive disorder characterized by infantile-onset epileptic encephalopathy and profound developmental regression. In contrast, biallelic mutations in B4GALNT1 cause a complex form of hereditary spastic paraplegia (SPG26), marked by progressive spasticity and intellectual impairment. ST3GAL3, another α2,3-sialyltransferase, contributes to the synthesis of GD1a and GT1b, as well as to glycoprotein sialylation. Mutations in this gene underlie neurodevelopmental disorders, including developmental and epileptic encephalopathy type 15 (DEE15). In this review, we summarize the current understanding of the molecular pathogenesis of congenital ganglioside biosynthesis disorders, integrating data from genetically engineered mouse models and affected individuals.