Physiological Effects of FHC-Causing K104E Mutation in the Myosin Regulatory Light Chain

Abstract

We have studied the physiological effects of the Lysine 104 to Glutamic Acid (K104E) mutation in the ventricular myosin regulatory light chain (RLC), shown to cause familial hypertrophic cardiomyopathy (FHC). In vitro and in vivo experiments were performed using transgenic (Tg) mouse cardiac muscle preparations carrying the K104E-RLC mutation. We observed a slight but significant decrease in maximal force (ΔFmax≅8kN/m2) and an increase in the Ca2+-sensitivity (ΔpCa50≅0.1) of isometric contraction in glycerinated skinned muscle fibers from Tg-K104E compared to Tg-WT mice. No mutant related changes in rigor binding of transgenic K104E mouse myosin to pyrene-actin were observed. Likewise, no changes in actomyosin or myofibrillar ATPase activities were monitored. Histological examination of Masson's trichrome stained Tg-hearts showed signs of fibrosis in 8 mo-old Tg-K104E mice compared to age matched Tg-WT. These observed changes in myocyte organization in Tg-K104E mice could be due to ∼4-fold decrease in the myosin heavy chain-RLC interaction determined by the binding of bacterially expressed K104E mutant to RLC-depleted porcine myosin. Echocardiography examination of senescent Tg-K104E mice confirmed a hypetrophic phenotype and showed a significantly enlarged interventricular septum and LVPWd (left ventricular posterior wall in diasole), ∼1.6-fold increase in LV mass and significantly decreased LVIDs (LV inner diameter in systole) compared to Tg-WT mice. However, EF (ejection fraction) was higher in Tg-K104E mice (73±8%) compared to 61±7% observed in Tg-WT mice. In addition, Doppler E velocity was also 1.3-fold higher in Tg-K104E mice compared to Tg-WT. These results confirm a mutation induced hypertrophic phenotype in Tg-K104E mice, similar to the patients carrying this FHC-mutation. No drastic changes at the level of actomyosin interaction or in cardiac function assessed in Tg-K104E mice suggest that the mutation might be associated with good prognosis. Supported by NIH-HL071778 (DSC).