Human cardiac
Troponin I (cTnI) is the first sarcomeric
protein for which mutations have been associated with
restrictive cardiomyopathy. To determine whether five mutations in cTnI (L144Q, R145W, A171T, K178E, and R192H) associated with
restrictive cardiomyopathy were distinguishable from
hypertrophic cardiomyopathy-causing mutations in cTnI,
actomyosin ATPase activity and skinned fiber studies were carried out. All five mutations investigated showed an increase in the Ca2+ sensitivity of force development compared with wild-type cTnI. The two mutations with the worst clinical phenotype (K178E and R192H) both showed large increases in Ca2+ sensitivity (deltapCa50 = 0.47 and 0.36, respectively). Although at least one of these mutations is not in the known inhibitory regions of cTnI, all of the mutations investigated caused a decrease in the ability of cTnI to inhibit
actomyosin ATPase activity. Mixtures of wild-type and mutant cTnI showed that cTnI mutants could be classified into three different groups: dominant (L144Q, A171T and R192H), equivalent (K178E), or weaker (R145W) than wild-type cTnI in
actomyosin ATPase assays in the absence of Ca2+. Although most of the mutants were able to activate
actomyosin ATPase similarly to wild-type cTnI, L144Q had significantly lower maximal
ATPase activities than any of the other mutants or wild-type cTnI. Three mutants (L144Q, R145W, and K178E) were unable to fully relax contraction in the absence of Ca2+. The inability of the five cTnI mutations investigated to fully inhibit
ATPase activity/force development and the generally larger increases in Ca2+ sensitivity than observed for most
hypertrophic cardiomyopathy mutations would likely lead to severe diastolic dysfunction and may be the major physiological factors responsible for causing the
restrictive cardiomyopathy phenotype in some of the genetically affected individuals.