B1 variant of GM2 gangliosidosis in a 12-year-old patient.

A girl aged 12 y, 9 mo, suffered from a progressive neurodegenerative disorder marked by ataxia, extrapyramidal symptoms, and convulsions. A skin biopsy showed axonal pathology that emphasized axonal segments enlarged by mitochondria, dense bodies, and lysosomal residual bodies of the membranous cytoplasmic body type. This ultrastructural pathology suggested GM2 gangliosidosis which was shown to be a B1 variant by specific biochemical studies, although conventional techniques had failed to detect GM2 gangliosidosis. The B1 variant is marked by a deficient activity of beta-hexosaminidase A towards one substrate, and by an almost normal activity towards another. Both parents showed a diminished activity towards the sulfated substrate, suggesting a heterozygous state, and almost normal activity with the second substrate type.