Molecular, biochemical and structural analysis of ten novel lysosomal neuraminidase mutations: correlation with clinical severity in sialidosis. E. Bonten, A. d.Azzo. Genetics, St Jude Child Res Hosp, Memphis, TN.

   Sialidosis is a severe neurodegenerative lysosomal disorder caused by structural lesions in the lysosomal neuraminidases gene. Type I sialidosis (normophormic) is a mild form of the disease usually diagnosed in the second decade of life and characterized by the cherry-red spot-myoclonus phenotype and progressive impaired vision, but absence of dismorphic features. Type II sialidosis is a severe form of the disease, which can be subdivided into three forms: (I) congenital or hydropic (in utero), (II) infantile (0-12 months), and (III) juvenile (2-20 years). All type II patients eventually develop progressive mucopolysaccharidosis-like features, including coarse facies, visceromegaly, dysostosis multiplex, vertrebal deformities and severe mental retardation. We have identified 12 mutations, 10 of which novel, in the mRNA and genomic DNA of 9 unrelated sialidosis patients and some of their parents. The majority of the mutations were base substitutions resulting in single amino acid changes. Five patients were compound heterozygotes, while the others were homozygotes. To study the biochemical properties of the individual mutant proteins we expressed their encoded cDNA's in COS-1 cells or deficient sialidosis fibroblasts. Transfected cells were analyzed for residual neuraminidase activity and subcellular localization of the mutant enzyme. Based on these biochemical studies we grouped the mutations in three distinct catagories. Five mutant proteins lacked residual neuraminidase activity and were not localized in lysosomes. Three lacked residual activity, but had weak lysosomal localization. In contrast, the remaining three had residual activity (30-60%) and clear lysosomal localization. Interestingly, only the first category carried mutations on residues that are conserved within the family of bacterial/mammalian neuraminidases. Furthermore, we could make a clear correlation between the impact of the mutation on the neuraminidase function and the disease phenotype. In future the assignment of mutations to one of the groups might be helpful for determining the clinical prognosis of sialidosis patients.
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