Novel missense mutations in the human lysosomal sialidase gene in sialidosis patients and prediction of structural alterations of mutant enzymes

Abstract

Three novel missense mutations in the human lysosomal sialidase gene causing amino acid substitutions (P80L, W240R, and P316S) in the coding region were identified in two Japanese sialidosis patients. One patient with a severe, congenital form of type 2 sialidosis was a compound heterozygote for 239C-to-T (P80L) and 718T-to-C (W240R). The other patient with a mild juvenile-onset phenotype (type 1) was a homozygote for the base substitution of 946C-to-T (P316S). None of these mutant cDNA products showed enzymatic activity toward an artificial substrate when coexpressed in galactosialidosis fibroblastic cells together with protective protein/cathepsin A (PPCA). All mutants showed a reticular immunofluorescence distribution when coexpressed with the PPCA gene in COS-1 cells, suggesting that the gene products were retained in the endoplasmic reticulum/Golgi area or rapidly degraded in the lysosomes. Homology modeling of the structural changes introduced by the mutations predicted that the P80L and P316S transversions cause large conformational changes including the active site residues responsible for binding the sialic acid carboxylate group. The W240R substitution was deduced to influence the molecular surface structure of a limited region of the constructed models, which was also influenced by previously identified V217M and G243R transversions.

Clinical and Serial MRI Findings of a Sialidosis Type I Patient with a Novel Missense Mutation in the <i>NEU1</i> Gene

“…The c.239C>T (p.P80L) mutation has been previously reported in a Japanese patient with severe congenital sialidosis type II (12). The P80 residue is situated in a conserved FRIP motif that is located at the N-terminus of the first strand of the first β-sheet unit.…”

“…Although the NEU1 genes of the parents were not analyzed, the results suggested that the patient was compound heterozygous for p.P80L and p.D135N. The c.239C> T (p.P80L) mutation has been reported previously in a Japanese sialidosis type II patient (12). On the other hand, the c.403G>A (p.D135N) mutation has not been previously described (HGMD Professional 2011.3, Human Gene Mutation Database, Biobase, Beverly, MA, USA and SNPs reported in the database (dbSNP) of the National Center for Biotechnology information (NCBI) (http://www.ncbi.nlm.nih.gov/ SNP/)).…”

“…Sialidosis type II is the early-onset form and is associated with macular cherry-red spots, the Hurler-like phenotype, dysostosis multiplex, short stature, developmental delays, mental retardation and hepatosplenomegaly (1,8,9). The age of onset and severity of the clinical manifestations are correlated with NEU1 mutations (10,11) and the level of residual neuraminidase activity (10,12,13), indicating the existence of considerable genotype-phenotype correlation in this disease. To date, more than 40 mutations within the NEU1 gene have been identified in patients with sialidosis type I or type II (2,3,(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24).…”

Molecular pathology of NEU1 gene in sialidosis

“…Using homology modeling, a potential effect of missense mutations on the sialidase tertiary structure was estimated and was correlated with the residual activity, folding, and intracellular localization of the enzyme, as well as with the clinical phenotype [Bonten et al, 2000;Lukong et al, 2000Lukong et al, , 2001Naganawa et al, 2000;Itoh et al, 2002]. For example, Lukong et al [2000] built the structural model of human sialidase using the atomic coordinates of homologous sialidases from Micromonospora viridifaciens, Salmonella typhimurium, and Vibrio cholerae.…”

“…Sialidosis type II patients are classified as those having the infantile-onset form who are relatively normal at birth, and those having the congenital-onset form that manifests prenatally and is associated with ascites and hydrops fetalis [Aylsworth et al, 1980;Beck et al, 1984]. Some authors have also used the term ''juvenile sialidosis'' to describe a form that manifests in late childhood, with a relatively mild clinical phenotype [Itoh et al, 2002;Bonten et al, 2000].…”