The Giant Protein Titin’s Role in Cardiomyopathy: Genetic, Transcriptional, and Post-translational Modifications of TTN and Their Contribution to Cardiac Disease

Abstract

Dilated cardiomyopathy (DCM) is a leading cause of heart failure, sudden cardiac death and heart transplant. DCM is inherited in approximately 50% of cases, in which the most frequent genetic defects are truncation variants of the titin gene (TTNtv). TTN encodes titin, which is the largest protein in the body and is an essential component of the sarcomere. Titin serves as a biological spring, spanning half of the sarcomere and connecting the Z-disk to the M-line, with scaffold and signaling functions. Truncations of titin are believed to lead to either haploinsufficiency and loss-of-function, or to a “poison peptide” effect. However, other titin mechanisms are postulated to influence cardiac function including post-translational modifications, in particular changes in titin phosphorylation that alters the stiffness of the protein, and diversity of alternative splicing that generates different titin isoforms. In this article, we review the role of TTN mutations in development of DCM, how differential expression of titin isoforms relate to DCM pathophysiology, and discuss how post-translational modifications of titin can affect cardiomyocyte function. Current research efforts aim to elucidate the contribution of titin to myofibril assembly, stability, and signal transduction, and how mutant titin leads to cardiac dysfunction and human disease. Future research will need to translate this knowledge toward novel therapeutic approaches that can modulate titin transcriptional and post-translational defects to treat DCM and heart failure.HIGHLIGHTS- Titin (TTN) truncation variants are the most frequent cause of dilated cardiomyopathy, one of the main causes of heart failure and heart transplant. Titin is a giant protein, and the mechanisms causing the disease are both complex and still incompletely understood.- This review discusses the role of titin in myocardial function and in disease. In particular, we discuss TTN gene structure, the complexity of genotype-phenotype correlation in human disease, the physiology of TTN and the role of post-translation modification.- Additional studies will be required to clarify whether missense variants are associated with cardiac disease. While initial studies suggested a role of non-synonymous variants in arrhythmogenic cardiomyopathy, confirmatory investigations have been hampered by the complexity of the protein structure and function.

Modifications of Titin Contribute to the Progression of Cardiomyopathy and Represent a Therapeutic Target for Treatment of Heart Failure

“…This was assigned a value, proportion spliced in (PSI) from 0 to 1, with a PSI > 0.9 imparting a 93% probability of pathogenicity [ 23 ]. The association of PSI and phenotype is further supported by pathogenicity data from the Clinvar ( ) database where exon locations of high PSI mutations are correlated with known pathogenic mutations [ 17 ]. This is an essential finding because TTN mutations are common, therefore predicting whether a given cardiac phenotype is attributable to a TTNtv mutation or an alternative cause is helpful for clinical prognostication.…”

“…As the sarcomere expands in diastole, the springy Ig-like domains of titin’s I-band elongate, thereby increasing tension that is released as restorative force during systole. This increased diastolic tension also modulates actomyosin interactions to increase the force of contraction during systole [ 16 , 17 ]. Because titin spans the entirety of the sarcomere, its length and elasticity are major contributors to the passive tension of the ventricle including diastolic tension and diastolic volume [ 18 ].…”

“…Based on proteomic analysis, there are hundreds of predicted phosphorylation sites on titin [ 77 , 78 , 79 ]. Only a few of these potential phosphosites have been studied and demonstrated to impart functional changes on titin with a majority of the studied sites located in the spring-like I-band domain [ 17 ]. Phosphorylation within different regions of the I-band have different effects on the functional properties of titin.…”

“…This is a positively charged region, therefore phosphorylation increases repulsion forces leading to elongation and softening of titin [ 84 ]. Kinases capable of phosphorylating this element include protein kinase A (PKA), extracellular signal-regulated kinase 2 (ERK2), protein kinase G (PKG), and calcium/calmodulin-dependent protein kinase II delta (CaMKIIδ) [ 17 ]. Dephosphorylation of the N2Bus has been demonstrated by protein phosphatase 5 (PP5) [ 85 ].…”

Conserved cysteines in titin sustain the mechanical function of cardiomyocytes

“…However, the primordial pathomechanisms remain elusive, reflecting our incomplete knowledge of the function of titin (12)(13)(14). Akin to other sarcomeric proteins in which both truncations and missense variants lead to cardiomyopathy (9), it has been speculated that rare missense mutations in TTN could be the cause of DCM in some of the ~50% genotype-negative patients (7,15,16).…”