MOLECULAR ANALYSIS OF A GM2-ACTIVATOR DEFICIENCY IN 2 PATIENTS WITH GM2-GANGLIOSIDOSIS AB VARIANT

Lysosomal degradation of ganglioside GM2 by beta-hexosaminidase A (hex A) requires the presence of the GM2 activator protein (GM2AP) as an e ssential cofactor. A deficiency of the GM2 activator causes the AB var iant of GM2 gangliosidosis, a recessively inherited disorder character ized by excessive neuronal accumulation of GM2 and related glycolipids . Two novel mutations in the GM2 activator gene (GM2A) have been ident ified by the reverse-transcriptase-PCR method-a three-base deletion, A AG(262-264) resulting in a deletion of Lys(88), and a single-base dele tion, A(410), that causes a frameshift. The latter results in substitu tion of 33 amino acids and the loss of another 24 amino acid residues. Both patients are homoallelic for their respective mutations inherite d from their parents, who are heteroallelic at the GM2A locus. Althoug h the cultured fibroblasts of both patients produce normal levels of a ctivator mRNA, they lack a lysosomal form of GM2AP. Pulse/chase labeli ng of cultured fibroblasts of the patients, in presence and absence of brefeldin A, indicates a premature degradation of both-mutant and tru ncated-GM2APs in the endoplasmic reticulum or Golgi. These results wer e supported by in vitro translation experiments and expression of the mutated proteins. When the mutated GM2APs were expressed in Escherichi a coli, both mature GM2AP forms turned proved to exhibit only residual activities in an in vitro assay.