A Kinetic Insight into Troponin T Mutations Related To Dilated and Hypertrophic Cardiomyopathies

Dilated and Hypertrophic Cadiomyopathy can be caused by mutations of genes encoding sarcomeric proteins. Mutations in cTnT are of particular interest since they are generally associated with mild or no ventricular hypertrophy but a high incidence of sudden death. Previous investigations have focused on steady state parameters such as maximal activation and inhibition of actomyosin ATPase and force and Ca2+ sensitivity. We have aimed to use transient kinetics to investigate the effects of 7 cTnT mutations on the dynamics of thin filament switching. We have studied two DCM mutations (R141W, ∆K210) and five HCM (∆E160, S179F, K273E, ∆14, ∆28+7) mutations present in two functional domains of TnT (T1 and T2). Overall circular dichroism studies showed that the structure of these mutant proteins is not grossly affected although minor changes in the α–helical content were found for cTnT mutants K273E, ∆14, ∆28+7 and ∆E160. Co-sedimentation with actin suggested that most of cTnT mutations do not interfere with the association between cTn and thin filament except for the truncated mutations. Cooperativity along thin filament was changed for all deletion mutations (∆K210, ∆E160, ∆14 and ∆28+7) but unchanged by the point mutations. In this study we also demonstrated that the equilibrium constant between the blocked and closed states (KB) for DCM mutations were unchanged but increased dramatically for HCM mutations suggesting loss of blocked state specifically for those in the T2 region. We assessed Ca2+ binding of the regulatory site of cardiac TnC using IAANS attached to C35 and C84 of cTnC. Ca2+ binding affinity (pCa50 =6.65) of reconstituted Tn complex was unaffected by all mutations with the exception of ∆28+7 which caused a decrease (pCa50 0.34). In contrast when incorporated into thin filament, all HCM mutations and DCM ∆K210 showed increased Ca2+ affinity. The observed rate constant of Ca2+ dissociation was unchanged for all mutations except for ∆28+7. In conclusion, we have observed multiple structural and functional consequences from different TnT mutations that occur in different regions of the molecule. Overall the data suggests that it is the functional changes caused by mutations that are critical in developing the disease and not the specific location of the mutation.