The regulatory troponin complex regulates skeletal and cardiac muscle contraction. This complex, together with tropomyosin, is located on the actin filament and it is composed of three protein subunits: troponin T (the tropomyosin-binding subunit), troponin I (the inhibitory subunit, which inhibits the ATPase activity of acto-myosin), and troponin C (the Ca2+-binding subunit). Troponins T and I have unique cardiac isoforms, whereas cardiac and skeletal muscle share troponin C. Specifically, three human troponin I isoforms have been described: one is expressed in cardiac muscle (Cardiac Troponin I) and the other two are found in slow-twitch and fast-twitch skeletal muscle fibers (slow sTnI and fast sTnI, respectively). The sequence homology between Cardiac Troponin I and slow sTnI is approximately 40% and somewhat less for fast sTnI. Cardiac Troponin I is 209 amino acid long with a molecular weight of approximately 24 kDa. Mouse and rat Cardiac Troponin I proteins both show 93% amino acid identity to human Cardiac Troponin I. The presence of human Cardiac Troponin I in serum (together with chest pain and electrocardiographic changes) is now considered as one highly specific biochemical marker of myocardial injury, risk stratification of acute coronary syndrome and myocardial infarction. Mutations of Cardiac Troponin I are associated with hereditary cardiomyopathy. Specifically, defects in Cardiac Troponin I are the cause of cardiomyopathy familial hypertrophic type 7 (CMH7). Familial hypertrophic cardiomyopathy is a hereditary heart disorder characterized by ventricular hypertrophy, which is usually asymmetric and often involves the interventricular septum. The symptoms include dyspnea, syncope, collapse, palpitations, and chest pain. They can be readily provoked by exercise. The disorder has inter- and intra-familial variability ranging from benign to malignant forms with high risk of cardiac failure and sudden cardiac death. Defects in Cardiac Troponin I also cause cardiomyopathy familial restrictive type 1 (RCM1). RCM1 is a heart muscle disorder characterized by impaired filling of the ventricles with reduced diastolic volume, in the presence of normal or near normal wall thickness and systolic function. Furthermore, cardiomyopathy dilated type 2A (CMD2A) and cardiomyopathy dilated type 1FF (CMD1FF), disorders characterized by ventricular dilation and impaired systolic function resulting in congestive heart failure and arrhythmia, are caused by defects in Cardiac Troponin I.

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