Abstract
We report here on the three phenotypes of cardiomyopathy: hypertrophic (HCM), dilated (DCM) and restrictive (RCM) cardiomyopathy, linked to mutations in myosin regulatory (RLC) and essential (ELC) light chains. The D166V (Aspartic acid to Valine)-HCM, D94A (Aspartic acid to Alanine)-DCM mutations were found in the RLC protein (MYL2 gene), and E143K (Glutamic acid to Lysine)-RCM mutation, in the myosin ELC protein (MYL3 gene). The hearts of D166V mice, expressing human ventricular D166V-HCM mutant of RLC, present with a severe hypertrophic phenotype with increased left ventricular wall thickness and decreased inner ventricular diameter in systole and diastole. Abnormalities of diastolic and systolic function have been confirmed via in vivo studies. The D94A-transgenic RLC mouse model, expressing human ventricular D94A-DCM RLC variant, represents a different cardiomyopathy phenotype characterized by severe ventricular dilation with an enlargement of the inner ventricular cavity diameters. The systolic dysfunction in D94A mice was confirmed by low ejection fraction. Interestingly, in both RLC mutant mice the replacement of the negatively charged amino acid (Aspartic acid) by two different non-charged amino acids (Alanine and Valine) resulted in a different phosphorylation status of the RLC molecule. The reduction of RLC phosphorylation, observed in D166V-HCM mice correlated with lower contractile force as observed in D166V-skinned papillary muscle fibers. The D94A-DCM model exhibited normal RLC phosphorylation and no changes in contractile force generation were observed. The X-ray diffraction studies revealed two distinct structural phenotypes in HCM and DCM fibers that most likely contributed to the two distinct cardiac phenotypes observed in mice in vivo. On the other hand, the hearts of E143K-ELC mice, expressing human cardiac E143K-RCM ELC variant, showed a severe RCM phenotype due to the hypercontractile state of myosin affecting its interaction with actin and force production. Observed abnormal stiffness of the left ventricle might be the result of the high affinity of myosin to actin also reflected by enhanced maximal force generation in E143K hearts. The X-ray structure study revealed a decrease in the lattice spacing for E143K fibers vs. WT controls, which correlates with higher maximal tension observed in E143K vs. WT hearts. In conclusion, all three mutations in human cardiac myosin RLC and ELC cause specific disruptions of the sarcomere structure in mice which are then followed by functional defects leading to the distinct HCM, DCM and RCM cardiomyopathy phenotypes.