GM2A

Abstract: The GM2 activator deficiency (also known as the AB variant), Tay–Sachs disease, and Sandhoff disease are the major forms of the GM2 gangliosidoses, disorders caused by defective degradation of GM2 ganglioside. Tay–Sachs and Sandhoff diseases are caused by mutations in the genes (HEXA and HEXB) encoding the subunits of β-hexosaminidase A. The GM2 activator deficiency is caused by mutations in the GM2A gene encoding the GM2 activator protein. For degradation of GM2 ganglioside by β-hexosamindase A, the GM2 activator protein must participate by forming a soluble complex with the ganglioside. In each of the disorders, GM2 ganglioside and related lipids accumulate to pathologic levels in neuronal lysosomes, resulting in clinically similar disorders with an onset in the first year of life, progressive neurodegeneration, and death by early childhood. We previously have described mouse models of Tay–Sachs (Hexa −/−) and Sandhoff (Hexb −/−) diseases with vastly different clinical phenotypes. The Hexa −/− mice were asymptomatic whereas the Hexb −/− mice were severely affected. Through gene disruption in embryonic stem cells we now have established a mouse model of the GM2 activator deficiency that manifests an intermediate phenotype. The Gm2a −/− mice demonstrated neuronal storage but only in restricted regions of the brain (piriform, entorhinal cortex, amygdala, and hypothalamic nuclei) reminiscent of the asymptomatic Tay–Sachs model mice. However, unlike the Tay–Sachs mice, the Gm2a −/− mice displayed significant storage in the cerebellum and defects in balance and coordination. The abnormal ganglioside storage in the Gm2a −/− mice consisted of GM2 with a low amount of GA2. The results demonstrate that the activator protein is required for GM2 degradation and also may indicate a role for the GM2 activator in GA2 degradation.

Abstract: Lysosomal degradation of ganglioside GM2 by beta-hexosaminidase A (hex A) requires the presence of the GM2 activator protein (GM2AP) as an essential cofactor. A deficiency of the GM2 activator causes the AB variant of GM2 gangliosidosis, a recessively inherited disorder characterized by excessive neuronal accumulation of GM2 and related glycolipids. Two novel mutations in the GM2 activator gene (GM2A) have been identified by the reverse-transcriptase-PCR method--a three-base deletion, AAG262-264, resulting in a deletion of Lys88, and a single-base deletion, A410, that causes a frameshift. The latter results in substitution of 33 amino acids and the loss of another 24 amino acid residues. Both patients are homoallelic for their respective mutations inherited from their parents, who are heteroallelic at the GM2A locus. Although the cultured fibroblasts of both patients produce normal levels of activator mRNA, they lack a lysosomal form of GM2AP. Pulse/chase labeling of cultured fibroblasts of the patients, in presence and absence of brefeldin A, indicates a premature degradation of both--mutant and truncated--GM2APs in the endoplasmic reticulum or Golgi. These results were supported by in vitro translation experiments and expression of the mutated proteins. When the mutated GM2APs were expressed in Escherichia coli, both mature GM2AP forms turned proved to exhibit only residual activities in an in vitro assay.