Molecular Effects of cTnC DCM Mutations on Calcium Sensitivity and Myofilament ActivationAn Integrated Multiscale Modeling Study

Mutations
in cardiac troponin C (D75Y, E59D, and G159D), a key
regulatory protein of myofilament contraction, have been associated
with dilated cardiomyopathy (DCM). Despite reports of altered myofilament
function in these mutants, the underlying molecular alterations caused
by these mutations remain elusive. Here we investigate in
silico the intramolecular mechanisms by which these mutations
affect myofilament contraction. On the basis of the location of cardiac
troponin C (cTnC) mutations, we tested the hypothesis that intramolecular
effects can explain the altered myofilament calcium sensitivity of
force development for D75Y and E59D cTnC, whereas altered cardiac
troponin C–troponin I (cTnC–cTnI) interaction contributes
to the reported contractile effects of the G159D mutation. We employed
a multiscale approach combining molecular dynamics (MD) and Brownian
dynamics (BD) simulations to estimate cTnC calcium association and
hydrophobic patch opening. We then integrated these parameters into
a Markov model of myofilament activation to compute the steady-state
force–pCa relationship. The analysis showed that myofilament
calcium sensitivity with D75Y and E59D can be explained by changes
in calcium binding affinity of cTnC and the rate of hydrophobic patch
opening, if a partial cTnC interhelical opening angle (110°)
is sufficient for cTnI switch peptide association to cTnC. In contrast,
interactions between cTnC and cTnI within the cardiac troponin complex
must also be accounted for to explain contractile alterations due
to G159D. In conclusion, this is the first multiscale in silico study to elucidate how direct molecular effects of genetic mutations
in cTnC translate to altered myofilament contractile function.