Metabolism of lysosomal sialidase and molecular basis of sialidosis

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To gain insight into the pathogenesis of sialidosis, we performed molecular investigations of four unrelated Japanese patients, and identified five novel missense mutations causing amino acid substitutions, P80L, V217M,W240R, G243R and P316S, respectively. Using homology modeling, the structure of human lysosomal sialidase was constructed and the structural changes caused by these mutations were deduced. The results showed that the P80L and P316S transversions cause large conformational changes including the active site residues responsible for binding the sialic acid carboxylate group. The V217M, W240R and G243R substitutions were deduced to influence the molecular surface structure of a limited region of the constructed models, involving in the interaction with the protective protein/cathepsin A. The predicted change due to V217M was smaller than that caused by W240R or G243R, the latters resulting in a drastic, widespread alteration. The overexpressed gene products containing these mutations had the same molecular weight as that of the wild type, although the amounts of the products were moderately decreased. A biochemical study demonstrated that the expressed sialidase containing a V217M mutation was partly transported to lysosomes and showed residual enzyme activity, although other mutants were retained in the endoplasmic reticulum/Golgi area and had lost the enzyme activity. Considering the data, we surmise that the V217M substitution may be closely associated with the phenotype of sialidosis with a late onset and moderate clinical course