**Charles Tharp, Luisa Mestroni and Matthew Taylor \***

Adult Medical Genetics Program, Cardiovascular Institute, University of Colorado Anschutz Medical Campus, CO 80045, USA; charles.tharp@cuanschutz.edu (C.T.); luisa.mestroni@cuanschutz.edu (L.M.)

**\*** Correspondence: matthew.taylor@cuanschutz.edu; Tel.: +1-303-724-1400

Received: 22 July 2020; Accepted: 24 August 2020; Published: 26 August 2020

**Abstract:** Titin is the largest human protein and an essential component of the cardiac sarcomere. With multiple immunoglobulin(Ig)-like domains that serve as molecular springs, titin contributes significantly to the passive tension, systolic function, and diastolic function of the heart. Mutations leading to early termination of titin are the most common genetic cause of dilated cardiomyopathy. Modifications of titin, which change protein length, and relative stiffness affect resting tension of the ventricle and are associated with acquired forms of heart failure. Transcriptional and post-translational changes that increase titin's length and extensibility, making the sarcomere longer and softer, are associated with systolic dysfunction and left ventricular dilation. Modifications of titin that decrease its length and extensibility, making the sarcomere shorter and stiffer, are associated with diastolic dysfunction in animal models. There has been significant progress in understanding the mechanisms by which titin is modified. As molecular pathways that modify titin's mechanical properties are elucidated, they represent therapeutic targets for treatment of both systolic and diastolic dysfunction. In this article, we review titin's contribution to normal cardiac physiology, the pathophysiology of titin truncation variations leading to dilated cardiomyopathy, and transcriptional and post-translational modifications of titin. Emphasis is on how modification of titin can be utilized as a therapeutic target for treatment of heart failure.

**Keywords:** titin; RNA binding motif protein 20 (RBM20); sarcomere; systolic dysfunction; diastolic dysfunction; dilated cardiomyopathy; phosphorylation; non-sense mRNA decay; mammalian target of rapamycin (mTOR) complex-1
