Characterization of the mutant β-subunit of β-hexosaminidase for dimer formation responsible for the adult form of Sandhoff disease with the motor neuron disease phenotype

The adult form of Sandhoff disease with the motor neuron disease phenotype is a rare neurodegenerative disorder caused by mutations in HEXB encoding the β-subunit of β-hexosaminidase, yet the properties of mutant β-subunits of the disease have not been fully determined. We identified a novel mutation (H235Y) in the β-sheet of the (β/α)8-barrel domain, in addition to the previously reported P417L mutation that causes aberrant splicing, in a Japanese patient with the motor neuron disease phenotype. Enzyme assays, gel filtration studies and immunoprecipitation studies with HEK293 cells transiently expressing mutant β-subunits demonstrated that the H235Y mutation abolished both α–β and β–β dimer formation without increasing β-hexosaminidase activity, whereas other reported mutant β-subunits (Y456S, P504S or R533H) associated with the motor neuron disease phenotype formed dimers. Structural analysis suggested that the H235Y mutation in the β-sheet of the (β/α)8-barrel domain changed the conformation of the β-subunit by causing a clash with the E288 side chain. In summary, H235Y is the first mutation in the β-sheet of the (β/α)8-barrel domain of the β-subunit that abolishes α–β and β–β dimer formation; the presented patient is the second patient to exhibit the motor neuron disease phenotype with P417L and a non-functional allele of HEXB.