Résumé :
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Introduction: Limb-girdle muscular dystrophy type 2A (LGMD2A) is an autosomal recessive disorder caused by mutations in the CAPN3 gene. This gene is preferentially expressed in muscle tissue, but we have recently described that four different CAPN3 transcripts produced by the alternative splicing of exon 6, 15 and 16 are also expressed in White Blood Cells (WBCs) from healthy controls. The sequencing of these transcripts could be applied for LGMD2A diagnosis. Patients and Methods: In the present work, the CAPN3 mRNA expression level was measured in muscle and White blood cells from 3 LGMD2A patients and 2 healthy controls by real-time quantitative RT-PCR (RTQ-PCR). Three different Taqman probes designed in the exon junction of different constitutive exons of the CAPN3 pre-mRNA were used. TBP (TATA box binding protein) expression level was used to normalize the starting quantity of mRNA. The expression level of healthy controls was taken as the reference value. Results: In WBCs the expression level of the CAPN3 mRNA in the three LGMD2A patients was similar to the expression level reported in controls. In muscle, however, LGMD2A patients showed a reduced expression when compared to controls. These reduction was two-fold in the patient who carried a frameshift and a missense mutation, whereas it was up to ten-fold in the two patients who carried a frameshift mutation in each allele. Discussion: These results confirm that in muscle the CAPN3 transcripts which carry a mutation which introduces a premature-termination-codon (PTC) are subjected to non-sense mediated mRNA decay (NMD) as previously reported by Stehliková et al. By contrast, in peripheral blood the CAPN3 transcripts present a reduced sensitivity to mRNA degradation. These differences might result from the interaction between elements in transcripts that are subjected to NMD and factors in the NMD machinery.
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