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) g ene has been shown to cause familial hypertrophic cardiomyopathy (HCM). In this study, two truncated human cardiac TnTs expected to be produced by thi s mutation were expressed in Escherichia coli and partially (50-55%) exchan ged into rabbit permeabilized cardiac muscle fibers. The fibers into which a short truncated TnT, which lacked the COOH-terminal 21 amino acids becaus e of the replacement of 28 amino acids with 7 novel residues, had been exch anged generated a Ca2+-activated maximum force that was slightly, but stati stically significantly, lower than that generated by fibers into which wild -type TnT had been exchanged when troponin I (TnI) was phosphorylated by cA MP-dependent protein kinase. A long truncated TnT simply lacking the COOH-t erminal 14 amino acids had no significant effect on the maximum force-gener ating capability in the fibers with either phosphorylated or dephosphorylat ed TnI. Both these two truncated TnTs conferred a lower cooperativity and a higher Ca2+ sensitivity on the Ca2+-activated force generation than did wi ld-type TnT, independent of the phosphorylation of TnI by cAMP-dependent pr otein kinase. The results demonstrate that the splice donor site mutation i n 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 coop erativity, of cardiac muscle contraction, which might be involved in the pa thogenesis of HCM.