[Juvenile GM2 gangliosidosis with progressive spinal muscular atrophy onset].

GM2 gangliosidosis are caused by a beta-hexosaminidase A enzyme deficiency. Mutations in the gene leaving residual enzyme activity give rise to juvenile and adult forms of the disease which have a great clinical heterogeneity. We report three cases which have been considered for some time as Kugelberg-Welander disease. beta-hexosaminidase A was determined with the sulfated synthetic substrate, 4-méthylumbelliferyl-N-acetylglucosamine 6-sulfate (4-MUGS), which allowed the diagnosis. Two of these cases from one family had normal values of hexosaminidase A in serum as found in the B1 variant. Compound mutations were detected. The B1 variants had a classical B1 mutation (G533-->A) and a new mutation located on exon 11. The patient of the second family had the classical mutation of adult GM2 gangliosidosis (Gly269-->Ser) and a new mutation on exon 1, at the initiation codon.