Résumé :
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Communication n° 240 Cardiac myosin-binding protein C (cMyBP-C) gene mutations are frequently involved in familial hypertrophic cardiomyopathy (FHC). Some missense mutations have been associated with a severe phenotype (significant cardiac hypertrophy) and bad prognosis (several sudden deaths in a given family), but their consequence at molecular and cellular levels have not been well investigated yet. In the present study, we generated a novel targeted mouse model of FHC carrying a human missense cMyBP-C mutation and characterized the phenotype in both homozygotes and heterozygotes at 3 mo of age. The mouse model was based upon the targeted nucleotide replacement (G->A transition) in the cMyBP-C gene, which gives rise to a E258K mutant cMyBP-C. Both homozygous and heterozygous cMyBP-C mice appeared normal in all respects and were viable. Heterozygous mice did not exhibit any specific changes when compared to wild-type (WT). In contrast, echocardiography evidenced significant left ventricular hypertrophy in homozygous vs. WT mice (PWd, 1.13Æ0.31 vs. 0.64Æ0.11 mm), left ventricular dilation (LVDd, 4.53Æ0.36 vs. 3.49Æ0.31 mm) and reduced fractional shortening (FS, 24Æ3 vs. 37Æ6 %). Preliminary morphometric and contractility analyses in intact cardiac myocytes showed increase in cell length and decrease in velocity of contraction and relaxation in homozygotes vs. WT mice. RT-PCR analysis showed the presence of mutant mRNA in both heterozygous and homozygous mice. Western-blot analysis showed a significant decrease in mutant cMyBP-C level in homozygotes when compared to WT protein detected in WT mice. These data provide evidence that E258K cMyBP-C mutant protein resulting from a FHC missense mutation is unstable in mouse cardiac tissue. Further analyses will determine whether this results from mRNA instability or rapid degradation of the mutant protein by the ubiquitin-proteasome system. The presence of low levels of mutant cMyBP-C may in turn explain myocyte and cardiac dysfunctions of homozygous cMyBP-C mutant mice.
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