Résumé :
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Communication n° 200 Hereditary inclusion body myopathy (HIBM) is a unique group of neuromuscular disorders characterized by adult-onset, slowly progressive distal and proximal muscle weakness, caused by mutations in UDP-N-acetylglucosamine 2-epimerase/ N-acetylmannosamine kinase (GNE), the key enzyme in the biosynthetic pathway of sialic acid. Persian Jewish and Middle Eastern patients present a homozygous mutation in the kinase domain of GNE but most of the HIBM affected individuals of various ethnicities are compound heterozygotes, where all combinations of mutations among the domains epimerase/epimerase, kinase/kinase and epimerase/kinase, are found. To investigate the consequences of the mutated GNE enzyme in muscle cells, we established cell cultures from muscle biopsies from 11 HIBM patients (9 patients carrying the Middle Eastern kinase homozygous mutation, 1 patient carrying an epimerase homozygous mutation and 1 compound heterozygote patient with one mutation in each domain) and from 9 controls. Myoblasts carrying the homozygous mutation M712T had a 30% reduction in epimerase activity. This activity was reduced to 50% in cells from the compound heterozygote patient as well as the patient carrying the homozygous epimerase mutation. Membrane bound sialic acid measurements showed that myoblasts carrying either the kinase/kinase or the epimerase/kinase mutations, exhibit no significant reduction of the bound sialic acid amount, while cells with the epimerase/epimerase mutations in GNE had a 60% reduction. No differences were observed in the amount of membrane bound sialic acid glycoproteins either by Western blot or by flow cytometry in any of the mutants analyzed. These results show that in spite of the feed back inhibition mechanism of CMP-sialic acid on GNE epimerase activity, reduced enzyme activities of mutated GNE may decrease the amount of sialic acids in muscle cells. However different mutations do not equally affect overall sialylation although leading to similar phenotypes. This lack of correlation may indicate that the pathological mechanism of the disease may not be linked to the well characterized sialic acid pathway.
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