B4GALNT1 is an
enzyme essential for the synthesis of complex
gangliosides, whose absence leads to progressive neurodegeneration with aging in mice. Recently, eleven cases of
hereditary spastic paraplegia with mutation in the coding region of B4GALNT1 were reported. However, changes in the enzymatic activity of their products have never been studied. We have constructed expression vectors for individual mutant cDNAs, and examined their activities by cell-free in vitro
enzyme assays, and flow cytometry of cells transfected with their expression vectors. Among them, almost all mutant genes showed the complete loss of B4GALNT1 activity in both the in vitro
enzyme assays and flow cytometry. Two mutants exceptionally showed weak activity. One of them, M4, had a mutation at
amino acid 228 with a
premature termination codon. Interestingly, the intensity of fluorescence of GM2 measured by flow cytometry was equivalent between the WT and M4 mutant, although the positive cell population was relatively small in M4. Western immunoblotting of cell lysates from transfectants with
cDNA plasmids revealed 67-kDa bands except those containing
premature termination codons or frame-shift mutation. Taken together with the clinical findings of patients, loss of
enzyme activity may be responsible for the clinical features of
hereditary spastic paraplegia, whereas the intensity of
neurological disorders was relatively milder than expected. These clinical features of patients including those with male
hypogonadism are very similar to the abnormal phenotypes detected in B4galnt1-deficient mice.