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

Genetic Markers of Cardiovascular Disease

Cardiovascular Disorders (CVDs) are the leading cause mortality in developed as well as developing nations, and has now emerged as one of the leading causes of disability and mortality around the globe. According to the World Health Organization, four out of every five patients with cardiovascular disease die from a myocardial infarction each year. Numerous genes have been linked to coronary artery disease, influencing mechanisms such as blood pressure regulation, lipid metabolism, inflammation, and cardiac activity. Genetic variations or mutations in these genes can affect lipid metabolism, blood pressure management, and heart function, increasing the risk of obesity, metabolic disorders, and resulting in the development of cardiovascular disease. Understanding the role of genes and related complications are essential for the identification, management, and prevention of cardiovascular conditions. Performing a genetic test for variations in the gene may help identify people as well as their families who are at a greater risk of heart disease, which enables risk identification and timely intervention. . This article investigates the applications of genetic biomarkers in cardiac disorders such as coronary artery disease, hypertension, arrhythmias, cardiomyopathy, and heart failure, with an emphasis on individual genes and their effects on mutation.

Bruno 1/CELF regulates splicing and cytoskeleton dynamics to ensure correct sarcomere assembly in Drosophila flight muscles

Muscles undergo developmental transitions in gene expression and alternative splicing that are necessary to refine sarcomere structure and contractility. CUG-BP and ETR-3-like (CELF) family RNA-binding proteins are important regulators of RNA processing during myogenesis that are misregulated in diseases such as Myotonic Dystrophy Type I (DM1). Here, we report a conserved function for Bruno 1 (Bru1, Arrest), a CELF1/2 family homolog in Drosophila, during early muscle myogenesis. Loss of Bru1 in flight muscles results in disorganization of the actin cytoskeleton leading to aberrant myofiber compaction and defects in pre-myofibril formation. Temporally restricted rescue and RNAi knockdown demonstrate that early cytoskeletal defects interfere with subsequent steps in sarcomere growth and maturation. Early defects are distinct from a later requirement for bru1 to regulate sarcomere assembly dynamics during myofiber maturation. We identify an imbalance in growth in sarcomere length and width during later stages of development as the mechanism driving abnormal radial growth, myofibril fusion, and the formation of hollow myofibrils in bru1 mutant muscle. Molecularly, we characterize a genome-wide transition from immature to mature sarcomere gene isoform expression in flight muscle development that is blocked in bru1 mutants. We further demonstrate that temporally restricted Bru1 rescue can partially alleviate hypercontraction in late pupal and adult stages, but it cannot restore myofiber function or correct structural deficits. Our results reveal the conserved nature of CELF function in regulating cytoskeletal dynamics in muscle development and demonstrate that defective RNA processing due to misexpression of CELF proteins causes wide-reaching structural defects and progressive malfunction of affected muscles that cannot be rescued by late-stage gene replacement.

Assessing Cardiac Contractility From Single Molecules to Whole Hearts

Fundamentally, the heart needs to generate sufficient force and power output to dynamically meet the needs of the body. Cardiomyocytes contain specialized structures referred to as sarcomeres that power and regulate contraction. Disruption of sarcomeric function or regulation impairs contractility and leads to cardiomyopathies and heart failure. Basic, translational, and clinical studies have adapted numerous methods to assess cardiac contraction in a variety of pathophysiological contexts. These tools measure aspects of cardiac contraction at different scales ranging from single molecules to whole organisms. Moreover, these studies have revealed new pathogenic mechanisms of heart disease leading to the development of novel therapies targeting contractility. In this review, the authors explore the breadth of tools available for studying cardiac contractile function across scales, discuss their strengths and limitations, highlight new insights into cardiac physiology and pathophysiology, and describe how these insights can be harnessed for therapeutic candidate development and translational.

State-of-the-art document on optimal contemporary management of cardiomyopathies

Cardiomyopathies represent significant contributors to cardiovascular morbidity and mortality. Over the past decades, a progress has occurred in characterization of the genetic background and major pathophysiological mechanisms, which has been incorporated into a more nuanced diagnostic approach and risk stratification. Furthermore, medications targeting core disease processes and/or their downstream adverse effects have been introduced for several cardiomyopathies. Combined with standard care and prevention of sudden cardiac death, these novel and emerging targeted therapies offer a possibility of improving the outcomes in several cardiomyopathies. Therefore, the aim of this document is to summarize practical approaches to the treatment of cardiomyopathies, which includes the evidence-based novel therapeutic concepts and established principles of care, tailored to the individual patient aetiology and clinical presentation of the cardiomyopathy. The scope of the document encompasses contemporary treatment of dilated, hypertrophic, restrictive and arrhythmogenic cardiomyopathy. It was based on an expert consensus reached at the Heart Failure Association online Workshop, held on 18 March 2021.

Structure determination and analysis of titin A-band fibronectin type III domains provides insights for disease-linked variants and protein oligomerisation

Titin is the largest protein found in nature and spans half a sarcomere in vertebrate striated muscle. The protein has multiple functions, including in the organisation of the thick filament and acting as a molecular spring during the muscle contraction cycle. Missense variants in titin have been linked to both cardiac and skeletal myopathies. Titin is primarily composed of tandem repeats of immunoglobulin and fibronectin type III (Fn3) domains in a variety of repeat patterns; however, the vast majority of these domains have not had their high-resolution structure determined experimentally. Here, we present the crystal structures of seven wild type titin Fn3 domains and two harbouring rare missense variants reported in hypertrophic cardiomyopathy (HCM) patients. All domains present the typical Fn3 fold, with the domains harbouring variants reported in HCM patients retaining the wild-type conformation. The effect on domain folding and stability were assessed for five rare missense variants found in HCM patients: four caused thermal destabilization of between 7 and 13 °C and one prevented the folding of its domain. The structures also allowed us to locate the positions of residues whose mutations have been linked to congenital myopathies and rationalise how they convey their deleterious effects. We find no evidence of physiological homodimer formation, excluding one hypothesised mechanism as to how titin variants could exert pathological effects.

[Recent studies on dilated cardiomyopathy caused by TTN mutations in children]

The mutations of TTN gene that encodes titin are the most common mutation type among the genetic causes of dilated cardiomyopathy (DCM). This article reviews the worldwide studies on potential molecular pathogenesis (transcription, post-translational modification, etc.), clinical phenotypes, and gene therapies of pediatric DCM caused by TTN mutations, with the hope of providing a reference for the precision treatment of pediatric DCM caused by TTN mutations.

RBM20, a Therapeutic Target to Alleviate Myocardial Stiffness via Titin Isoforms Switching in HFpEF

Increased myocardial stiffness is critically involved in heart diseases with impaired cardiac compliance, especially heart failure with preserved ejection fraction (HFpEF). Myocardial stiffness mainly derives from cardiomyocyte- and extracellular matrix (ECM)-derived passive stiffness. Titin, a major component of sarcomeres, participates in myocardial passive stiffness and stress-sensitive signaling. The ratio of two titin isoforms, N2BA to N2B, was validated to influence diastolic dysfunction via several pathways. RNA binding motif protein 20 (RBM20) is a well-studied splicing factor of titin, functional deficiency of RBM20 in mice profile improved cardiac compliance and function, which indicated that RBM20 functions as a potential therapeutic target for mitigating myocardial stiffness by modulating titin isoforms. This minor review summarized how RBM20 and other splicing factors modify the titin isoforms ratio, therefore providing a promising target for improving the myocardial compliance of HFpEF.

The ryanodine receptor stabilizer S107 ameliorates contractility of adult Rbm20 knockout rat cardiomyocytes

RNA binding motif 20 (RBM20) cardiomyopathy has been detected in approximately 3% of populations afflicted with dilated cardiomyopathy (DCM). It is well conceived that RBM20 cardiomyopathy is provoked by titin isoform switching in combination with resting Ca2+ leaking. In this study, we characterized the cardiac function in Rbm20 knockout (KO) rats at 3-, 6-, 9-, and 12-months of age and examined the effect of the ryanodine receptor stabilizer S107 on resting intracellular levels and cardiomyocyte contractile properties. Our results revealed that even though Rbm20 depletion promoted expression of larger titin isoform and reduced myocardial stiffness in young rats (3 months of age), the established DCM phenotype required more time to embellish. S107 restored elevated intracellular Ca2+ to normal levels and ameliorated cardiomyocyte contractile properties in isolated cardiomyocytes from 6-month-old Rbm20 KO rats. However, S107 failed to preserve cardiac homeostasis in Rbm20 KO rats at 12 months of age, unexpectedly, likely due to the existence of multiple pathogenic mechanisms. Taken together, our data suggest the therapeutic promises of S107 in the management of RBM20 cardiomyopathy.

The Sarcomeric Spring Protein Titin: Biophysical Properties, Molecular Mechanisms, and Genetic Mutations Associated with Heart Failure and Cardiomyopathy

PURPOSE OF REVIEW

The giant protein titin forms the "elastic" filament of the sarcomere, essential for the mechanical compliance of the heart muscle. Titin serves a biological spring, and therefore structural modifications of titin affect function of the myocardium and are associated with heart failure and cardiomyopathy.

RECENT FINDINGS

In this review, we discuss the current understanding of titin's biophysical properties and how modifications contribute to cardiac function and heart failure. In addition, we review the most recent data on the clinical impact and phenotype heterogeneity of TTN truncating variants, including diseases involving striated muscles, and prospects for future therapies. Because of the giant structure of the titin protein and the complexity of its function, titin's role in health and disease is not yet completely understood. Future research efforts need to focus on novel therapeutic approaches able to modulate titin transcriptional and post-translational modification.

Urinary Titin N-Fragment as a Biomarker of Muscle Atrophy, Intensive Care Unit-Acquired Weakness, and Possible Application for Post-Intensive Care Syndrome

Titin is a giant protein that functions as a molecular spring in sarcomeres. Titin interconnects the contraction of actin-containing thin filaments and myosin-containing thick filaments. Titin breaks down to form urinary titin N-fragments, which are measurable in urine. Urinary titin N-fragment was originally reported to be a useful biomarker in the diagnosis of muscle dystrophy. Recently, the urinary titin N-fragment has been increasingly gaining attention as a novel biomarker of muscle atrophy and intensive care unit-acquired weakness in critically ill patients, in whom titin loss is a possible pathophysiology. Furthermore, several studies have reported that the urinary titin N-fragment also reflected muscle atrophy and weakness in patients with chronic illnesses. It may be used to predict the risk of post-intensive care syndrome or to monitor patients' condition after hospital discharge for better nutritional and rehabilitation management. We provide several tips on the use of this promising biomarker in post-intensive care syndrome.

Clinical Considerations for a Family with Dilated Cardiomyopathy, Sudden Cardiac Death, and a Novel TTN Frameshift Mutation

Dilated cardiomyopathy (DCM) is the leading indication for heart transplantation. TTN gene truncating mutations account for about 25% of familial DCM cases and for 18% of sporadic DCM cases. The clinical relevance of specific variants in TTN has been difficult to determine because of the sheer size of the protein for which TTN encodes, as well as existing extensive genetic variation. Clinicians should communicate novel clinically-relevant variants and genotype–phenotype associations, so that animal studies evaluating the molecular mechanisms are always conducted with a focus on clinical significance. In the present study, we report for the first time the novel truncating heterozygous variant NM_001256850.1:c.72777_72783del (p.Phe24259Leufs*51) in the TTN gene and its association with DCM in a family with sudden death. This variant occurs in the A-band region of the sarcomere, in a known mutational hotspot of the gene. Truncating titin variants that occur in this region are the most common cause of DCM and have been rarely reported in asymptomatic individuals, differently from other pathogenic TTN gene variants. Further studies are warranted to better understand this particular clinically-relevant variant.