Cardiac troponin T mutations: correlation between the type of mutation andthe nature of myofilament dysfunction in transgenic mice

1. The heterogenic nature of familial hypertrophic cardiomyopathy (FHC) in humans suggests a link between the type of mutation and the nature of patho -physiological alterations in cardiac myocytes. Exactly how FHC-associated mutations in cardiac troponin T (cTnT) lead to impaired cardiac function is unclear. 2. We measured steady-state isometric force and ATPase activity in detergen t-skinned cardiac fibre bundles from three transgenic (TG) mouse hearts in which 50, 92 and 6% of the native cTnT was replaced by the wild type (WT) c TnT, R92Q mutant cTnT (R92Q) and the C-terminal deletion mutant of cTnT (cT nT(DEL)), respectively. 3. Normalized pCa-tension relationships of R92Q and cTnT(DEL) fibres demons trated a significant increase in sensitivity to Ca2+ at short (2.0 mum) and long (2.3 mum) sarcomere lengths (SL). At short SL, the pCa(50) values, re presenting the midpoint of the pCa-tension relationship, were 5.69 +/- 0.01 , 5.96 +/- 0.01 and 5.81 +/- 0.01 for WT, R92Q and cTnTDEL fibres, respecti vely. At long SL, the pCa(50) values were 5.81 +/- 0.01, 6.08 +/- 0.01 and 5.95 +/- 0.01 for WT, R92Q and cTnT(DEL) fibres, respectively. 4. The difference in pCa required for half-maximal activation (Delta pCa(50 )) at short and long SL was 0.12 +/- 0.01 for the R92Q (92%) TG fibres, whi ch is significantly less than the previously reported ApCa(50) value of 0.2 9 +/- 0.02 for R92Q (67%) TG fibres. 5. At short SL, Ca2+-activated maximal tension in both R92Q and cTnT(DEL) f ibres decreased significantly (24 and 21%, respectively; P < 0.005), with n o corresponding decrease in Ca2+-activated maximal ATPase activity. Therefo re, at short SL, the tension cost in R92Q and cTnTDEL fibres increased by 3 5 and 29%, respectively (P < 0.001). 6. The fibre bundles reconstituted with the recombinant mutant cTnT(DEL) pr otein developed only 37% of the Ca2+-activated maximal force developed by r ecombinant WT cTnT reconstituted fibre bundles, with no apparent changes in Ca2+ sensitivity. 7. Our data indicate that an important mutation-linked effect on cardiac fu nction is the result of an inefficient use of ATP at the myofilament level. Furthermore, the extent of the mutation-induced dysfunction depends not on ly on the nature of the mutation, but also on the concentration of the muta nt protein in the sarcomere.