Hiroyuki Nakaura

Functional changes in troponin T by a splice donor site mutation that causes hypertrophic cardiomyopathy.

A splice donor site mutation in intron 15 of the cardiac troponin T (TnT) gene has been shown to cause familial hypertrophic cardiomyopathy (HCM). In this study, two truncated human cardiac TnTs expected to be produced by this mutation were expressed in Escherichia coli and partially (50-55%) exchanged into rabbit permeabilized cardiac muscle fibers. The fibers into which a short truncated TnT, which lacked the COOH-terminal 21 amino acids because of the replacement of 28 amino acids with 7 novel residues, had been exchanged generated a Ca2+-activated maximum force that was slightly, but statistically significantly, lower than that generated by fibers into which wild-type TnT had been exchanged when troponin I (TnI) was phosphorylated by cAMP-dependent protein kinase. A long truncated TnT simply lacking the COOH-terminal 14 amino acids had no significant effect on the maximum force-generating capability in the fibers with either phosphorylated or dephosphorylated TnI. Both these two truncated TnTs conferred a lower cooperativity and a higher Ca2+ sensitivity on the Ca2+-activated force generation than did wild-type TnT, independent of the phosphorylation of TnI by cAMP-dependent protein kinase. The results demonstrate that the splice donor site mutation in the cardiac TnT gene impairs the regulatory function of the TnT molecule, leading to an increase in the Ca2+ sensitivity, and a decrease in the cooperativity, of cardiac muscle contraction, which might be involved in the pathogenesis of HCM.A splice donor site mutation in intron 15 of the cardiac troponin T (TnT) gene has been shown to cause familial hypertrophic cardiomyopathy (HCM). In this study, two truncated human cardiac TnTs expected to be produced by this mutation were expressed in Escherichia coli and partially (50-55%) exchanged into rabbit permeabilized cardiac muscle fibers. The fibers into which a short truncated TnT, which lacked the COOH-terminal 21 amino acids because of the replacement of 28 amino acids with 7 novel residues, had been exchanged generated a Ca(2+)-activated maximum force that was slightly, but statistically significantly, lower than that generated by fibers into which wild-type TnT had been exchanged when troponin I (TnI) was phosphorylated by cAMP-dependent protein kinase. A long truncated TnT simply lacking the COOH-terminal 14 amino acids had no significant effect on the maximum force-generating capability in the fibers with either phosphorylated or dephosphorylated TnI. Both these two truncated TnTs conferred a lower cooperativity and a higher Ca(2+) sensitivity on the Ca(2+)-activated force generation than did wild-type TnT, independent of the phosphorylation of TnI by cAMP-dependent protein kinase. The results demonstrate that the splice donor site mutation in the cardiac TnT gene impairs the regulatory function of the TnT molecule, leading to an increase in the Ca(2+) sensitivity, and a decrease in the cooperativity, of cardiac muscle contraction, which might be involved in the pathogenesis of HCM.

Long-Term Treatment with Valsartan Improved Cyclic Variation of the Myocardial Integral Backscatter Signal and Diastolic Dysfunction in Hypertensive Patients : The Echocardiographic Assessment

Myocardial fibrosis is the major determinant of diastolic property of the left ventricle (LV). Experimental and clinical studies have suggested that angiotensin receptor blockers attenuate myocardial fibrosis in various heart diseases. The integrated backscatter signal (IBS) represents a promising ultrasonic method for assessing the characterization of myocardial tissue: cardiac cycle–dependent variation of the IBS (IBS-CV) is negatively correlated with myocardial collagen deposition in hypertensive hearts. Using non-invasive echocardiographic techniques, we performed a prospective, multi-center trial to examine whether long-term treatment with valsartan would improve myocardial fibrosis and diastolic dysfunction in hypertensives. This study included 43 hypertensive patients who had impaired diastolic function (transmitral Doppler flow early to late filling velocity ratio [E/A ratio] <1.0) and preserved systolic function (LV ejection fraction [LVEF] >50%). Twelve-month valsartan treatment reduced blood pressure (BP) and LV mass index. Valsartan significantly increased not only IBS-CV but also E/A ratio without changing LVEF. The effects of valsartan were compared between two subgroups: one with low IBS-CV (IBS-CV <5.08 dB [the average of 43 patients at baseline]), the other with high IBS-CV (IBS-CV >5.08 dB). At baseline, BP, LV mass index, LVEF, and E/A ratio were similar in the two groups. Valsartan significantly increased IBS-CV and E/A ratio in the low IBS-CV group, but not in the high IBS-CV group, despite comparable reductions in BP and LV mass. In conclusion, long-term valsartan treatment attenuated myocardial fibrosis and improved diastolic dysfunction in hypertensives. It is suggested that in the low IBS-CV group, improvement of diastolic dysfunction by valsartan may be caused by attenuation of myocardial fibrosis, and not by regression of LV hypertrophy.