Altered regulatory function of two familial hypertrophic cardiomyopathy troponin T mutants

Mutations in the gene encoding human cardiac troponin T can cause familial hypertrophic cardiomyopathy, a disease that is characterized by ventricular hypertrophy and sudden, premature death. Troponin T is the tropomyosin-bin ding subunit of troponin required for thin filament regulation of contracti on. One mutation, a change in the intron 15 splice donor site, results in t wo truncated forms of troponin T [Thierfelder et al. (1994) Cell 77, 701-71 2]. In one form, the mRNA skips exon 16 that encodes the C-terminal 14 amin o acids; in the other, seven novel residues replace the exon 15- and 16-enc oded C-terminal 28 amino acids. The two troponin T cDNAs were expressed in Escherichia coli for functional analysis. Both C-terminal deletion mutants formed a complex with cardiac troponin C and troponin I that exhibited the same concentration dependence as wild-type for regulation of the actomyosin MgATPase. However, both mutants showed severely reduced activation of the regulated actomyosin in the presence of Ca2+, though the inhibition in the absence of Ca2+ and the Ca2+-dependence of activation were not altered. The C-terminal deletions reduce the effectiveness of Ca2+-troponin to switch t he thin filament from the "off" to the "'on" state. Both mutant troponin Ts have reduced affinity for; troponin; the shorter mutant is at least 6-fold weaker than wild-type. The low level of activation of the ATPase would be consistent with reduced contractile performance, and the results suggest re duced troponin I affinity may be the molecular basis for the disease.