Background— Familial dilated cardiomyopathy can be caused by mutations in the proteins of the muscle thin filament. In vitro, these mutations decrease Ca²⁺ sensitivity and cross-bridge turnover rate, but the mutations have not been investigated in human tissue. We studied the Ca²⁺-regulatory properties of myocytes and troponin extracted from the explanted heart of a patient with inherited dilated cardiomyopathy due to the cTnC G159D mutation.

Methods and Results— Mass spectroscopy showed that the mutant cTnC was expressed approximately equimolar with wild-type cTnC. Contraction was compared in skinned ventricular myocytes from the cTnC G159D patient and nonfailing donor heart. Maximal Ca²⁺-activated force was similar in cTnC G159D and donor myocytes, but the Ca²⁺ sensitivity of cTnC G159D myocytes was higher (EC₅₀ G159D/donor=0.60). Thin filaments reconstituted with skeletal muscle actin and human cardiac tropomyosin and troponin were studied by in vitro motility assay. Thin filaments containing the mutation had a higher Ca²⁺ sensitivity (EC₅₀ G159D/donor=0.55 0.13), whereas the maximally activated sliding speed was unaltered. In addition, the cTnC G159D mutation blunted the change in Ca²⁺ sensitivity when TnI was dephosphorylated. With wild-type troponin, Ca²⁺ sensitivity was increased (EC₅₀ P/unP=4.7 1.9) but not with cTnC G159D troponin (EC₅₀ P/unP=1.2 0.1).

Conclusions— We propose that uncoupling of the relationship between phosphorylation and Ca²⁺ sensitivity could be the cause of the dilated cardiomyopathy phenotype. The differences between these data and previous in vitro results show that native phosphorylation of troponin I and troponin T and other posttranslational modifications of sarcomeric proteins strongly influence the functional effects of a mutation.