Abstract 918: Evidence of Alterations of the Ubiquitin-Proteasome System in Mouse Models of Familial Hypertrophic Cardiomyopathy

Cardiac myosin-binding protein C (cMyBP-C) gene mutations are frequently involved in familial hypertrophic cardiomyopathy (FHC) and most of them are expected to produce truncated mutants. However, truncated mutants were not detected in myocardial tissue of FHC patients and were degraded by the ubiquitin-proteasome system (UPS) in rat cardiac myocytes. Moreover, the UPS was impaired by truncated cMyBP-C ex vivo . In the present study, we investigated whether UPS alterations contribute to the pathophysiology of FHC in vivo . We therefore developed two mouse models based on a cMyBP-C mutation associated with a severe phenotype and a poor prognosis in humans. It corresponds to a G->A transition on the last nucleotide of exon 6. The first model overexpressed a human myc-tagged cMyBP-C transgene deleted of exon 6 under the control of the alpha-MHC promoter (TG). These mice did not develop any cardiac phenotype up to the age of 13 months. No truncated protein was detected in the TG cardiac tissue despite the presence of mutant mRNA. The TG mice exhibited a slight increase in the steady-state level of ubiquitinated proteins but no change in the chymotrypsin-like activity of the 20S proteasome vs. wild-type (WT) mice. The second model was generated by targeted knock-in of the mutation at the homozygous state (KI). The KI mice exhibited eccentric left ventricular hypertrophy with cardiac dysfunction at 9 wks of age. The level of mutant mRNAs (containing both missense and frameshift cMyBP-C mutations) decreased by 80% in KI vs. WT mice. The level of full-length protein decreased by 90% in KI vs. WT mice and the truncated protein was not detected. The level of ubiquitinated proteins and the chymotrypsin-like activity significantly increased in the KI vs. WT. These data show that truncated mutants are unstable in both mouse models suggesting degradation by the UPS. In addition, the mutants are also regulated at the mRNA level in the KI mice, suggesting that nonsense-mediated mRNA decay is also involved. Finally, both models exhibited alterations of the UPS resulting in an increased level of ubiquitinated proteins. These data suggest that UPS alterations may contribute to the pathophysiology of FHC.