RESUMO
Hereditary inclusion body myopathy (HIBM) is a neuromuscular disorder, caused by mutations in UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase, the key enzyme of sialic acid biosynthesis. In Middle Eastern patients a single homozygous mutation occurs, converting methionine-712 to threonine. Recombinant expression of the mutated enzyme revealed slightly reduced N-acetylmannosamine kinase activity, in agreement with the localization of the mutation within the kinase domain. B lymphoblastoid cell lines derived from patients expressing the mutated enzyme also display reduced UDP-N-acetylglucosamine 2-epimerase activity. Nevertheless, no reduced cellular sialylation was found in those cells by colorimetric assays and lectin analysis, indicating that HIBM is not directly caused by an altered overall expression of sialic acids.
Assuntos
Carboidratos Epimerases/genética , Carboidratos Epimerases/metabolismo , Miosite de Corpos de Inclusão/metabolismo , Ácido N-Acetilneuramínico/metabolismo , Fosfotransferases (Aceptor do Grupo Álcool)/genética , Fosfotransferases (Aceptor do Grupo Álcool)/metabolismo , Substituição de Aminoácidos , Animais , Linfócitos B/enzimologia , Linhagem Celular , Membrana Celular/metabolismo , Citosol/enzimologia , Proteínas de Escherichia coli/genética , Proteínas de Escherichia coli/metabolismo , Citometria de Fluxo , Homozigoto , Humanos , Miosite de Corpos de Inclusão/enzimologia , Proteínas Recombinantes/genética , Proteínas Recombinantes/metabolismo , Spodoptera/citologia , Células Tumorais CultivadasRESUMO
Hereditary inclusion body myopathy (HIBM) is a unique group of neuromuscular disorders characterized by adult-onset, slowly progressive distal and proximal muscle weakness, which is caused by mutations in UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE), the key enzyme in the biosynthetic pathway of sialic acid. In order to investigate the consequences of the mutated GNE enzyme in muscle cells, we have established cell cultures from muscle biopsies carrying either kinase or epimerase mutations. While all myoblasts carrying a mutated GNE gene show a reduction in their epimerase activity, only the cells derived from the patient carrying a homozygous epimerase mutation present also a significant reduction in the overall membrane bound sialic acid. These results indicate that although mutations in each of the two GNE domains result in an impaired enzymatic activity and the same HIBM phenotype, they do not equally affect the overall sialylation of muscle cells. This lack of correlation suggests that the pathological mechanism of the disease may not be linked solely to the well-characterized sialic acid pathway.