Incomplete nonsense-mediated decay of mutant lamin A/C mRNA provokes dilated cardiomyopathy and ventricular tachycardia

We have identified a family in which several members died of sudden cardiac death or suffer from dilated cardiomyopathy (DCM) and rhythm disturbances. Mutation screening revealed co-segregation of a novel nonsense mutation (pR321X) in the lamin A gene, LMNA, with the disease. Lamin A, and its smaller splice form lamin C are nuclear intermediate filament proteins forming a major part of the lamina, which is underlying the inner nuclear membrane. They are involved in the organization of heterochromatin and both in DNA replication and transcription. Recently, an increasing number of missense mutations in LMNA have been discovered to cause various types of rare diseases. Here, we investigated the causal role of the new nonsense mutation for the disease. Quantification of wild type and mutant lamin A mRNA from explanted myocardial tissue and cultured fibroblasts revealed an up to 30-fold reduction in the relative amount of the mutant transcript indicating that its synthesis was massively down-regulated by nonsense-mediated mRNA decay (NMD). Correspondingly, we did not detect the mutant truncated lamin A by Western blot analysis in extracts of patient fibroblasts and cardiac muscle tissue. Both wild type lamin A and C were present, however, in normal quantities. The immunohistochemical analyses of patient tissues revealed a normal distribution of lamin A/C and of major inner nuclear membrane proteins such as emerin and the lamin B receptor. Moreover, both chromatin distribution and nuclear shape were normal. However, over-expression of truncated lamin A in HeLa cells by transient transfection caused major disturbances of lamin A organization within both the nucleoplasm and the cytoplasm. In addition, after treatment of patient fibroblasts with the proteasome inhibitor epoxomicin, mutant truncated lamin A was detected in relatively high levels by Western blotting demonstrating that it is synthesized in these cells. Therefore, we conclude that NMD is not sufficient to completely prevent the expression of truncated lamin A and that even trace amounts of it may negatively interfere with structural and/or regulatory functions of lamin A/C eventually leading to the development of DCM and rhythm disturbances.