Characterization of the sialidase molecular defects in sialidosis patientssuggests the structural organization of the lysosomal multienzyme complex

Sialidosis is an autosomal recessive disease caused by the genetic deficien cy of lysosomal sialidase, which catalyzes the hydrolysis of sialoglycoconj ugates, The disease is associated with progressive impaired vision, macular cherry-red spots and myoclonus (sialidosis type I) or with skeletal dyspla sia, Hurler-like phenotype, dysostosis multiplex, mental retardation and he patosplenomegaly (sialidosis type II). We have analyzed the genomic DNA fro m nine sialidosis patients of multiple ethnic origin in order to find mutat ions responsible for the enzyme deficiency. The activity of the identified variants was studied by transgenic expression. One patient had a frameshift mutation (G623delG deletion), which introduced a stop codon, truncating 11 3 amino acids. All others had missense mutations: G679G-->A (Gly227Arg), C8 93C-->T (Ala298Val), G203G-->T (Gly68Val), A544A-->G (Ser182Gly) C808C-->T (Leu270Phe) and G982G-->A (Gly328Ser), We have modeled the three-dimensiona l structure of sialidase based on the atomic coordinates of the homologous bacterial sialidases, located the positions of mutations and estimated thei r potential effect. This analysis showed that five mutations are clustered in one region on the surface of the sialidase molecule. These mutations dra matically reduce the enzyme activity and cause a rapid intralysosomal degra dation of the expressed protein. We hypothesize that this region may be inv olved in the interface of sialidase binding with lysosomal cathepsin A and/ or beta-galactosidase in their high-molecular-weight complex required for t he expression of sialidase activity in the lysosome.