A novel titin mutation in adult-onset familial dilated cardiomyopathy

Exploring TTN variants as genetic insights into cardiomyopathy pathogenesis and potential emerging clues to molecular mechanisms in cardiomyopathies

The giant protein titin (TTN) is a sarcomeric protein that forms the myofibrillar backbone for the components of the contractile machinery which plays a crucial role in muscle disorders and cardiomyopathies. Diagnosing TTN pathogenic variants has important implications for patient management and genetic counseling. Genetic testing for TTN variants can help identify individuals at risk for developing cardiomyopathies, allowing for early intervention and personalized treatment strategies. Furthermore, identifying TTN variants can inform prognosis and guide therapeutic decisions. Deciphering the intricate genotype-phenotype correlations between TTN variants and their pathologic traits in cardiomyopathies is imperative for gene-based diagnosis, risk assessment, and personalized clinical management. With the increasing use of next-generation sequencing (NGS), a high number of variants in the TTN gene have been detected in patients with cardiomyopathies. However, not all TTN variants detected in cardiomyopathy cohorts can be assumed to be disease-causing. The interpretation of TTN variants remains challenging due to high background population variation. This narrative review aimed to comprehensively summarize current evidence on TTN variants identified in published cardiomyopathy studies and determine which specific variants are likely pathogenic contributors to cardiomyopathy development.

Exploring the Potential of Symmetric Exon Deletion to Treat Non-Ischemic Dilated Cardiomyopathy by Removing Frameshift Mutations in TTN

Non-ischemic dilated cardiomyopathy (DCM) is one of the most frequent pathologies requiring cardiac transplants. Even though the etiology of this disease is complex, frameshift mutations in the giant sarcomeric protein Titin could explain up to 25% of the familial and 18% of the sporadic cases of DCM. Many studies have shown the potential of genome editing using CRISPR/Cas9 to correct truncating mutations in sarcomeric proteins and have established the grounds for myoediting. However, these therapies are still in an immature state, with only few studies showing an efficient treatment of cardiac diseases. This publication hypothesizes that the Titin (TTN)-specific gene structure allows the application of myoediting approaches in a broad range of locations to reframe TTNtvvariants and to treat DCM patients. Additionally, to pave the way for the generation of efficient myoediting approaches for DCM, we screened and selected promising target locations in TTN. We conceptually explored the deletion of symmetric exons as a therapeutic approach to restore TTN’s reading frame in cases of frameshift mutations. We identified a set of 94 potential candidate exons of TTN that we consider particularly suitable for this therapeutic deletion. With this study, we aim to contribute to the development of new therapies to efficiently treat titinopathies and other diseases caused by mutations in genes encoding proteins with modular structures, e.g., Obscurin.

Whole-exome sequencing identified compound heterozygous variants in the TTN gene causing Salih myopathy with dilated cardiomyopathy in an Iranian family

BACKGROUND

Salih myopathy, characterised by both congenital myopathy and fatal dilated cardiomyopathy, is an inherited muscle disorder that affects skeletal and cardiac muscles. TTN has been identified as the main cause of this myopathy, the enormous size of this gene poses a formidable challenge to molecular genetic diagnostics.

METHOD

In the present study, whole-exome sequencing, cardiac MRI, and metabolic parameter assessment were performed to investigate the genetic causes of Salih myopathy in a consanguineous Iranian family who presented with titinopathy involving both skeletal and heart muscles in an autosomal recessive inheritance pattern.

RESULTS

Two missense variants of TTN gene (NM_001267550.2), namely c.61280A>C (p. Gln20427Pro) and c.54970G>A (p. Gly18324Ser), were detected and segregations were confirmed by polymerase chain reaction-based Sanger sequencing.

CONCLUSIONS

The compound heterozygous variants, c.61280A>C, (p. Gln20427Pro) and c.54970G>A, (p. Gly18324Ser) in the TTN gene appear to be the cause of Salih myopathy and dilated cardiomyopathy in the family presented. Whole-exome sequencing is an effective molecular diagnostic tool to identify the causative genetic variants of large genes such as TTN.

Muscular dystrophies

Muscular dystrophies are primary diseases of muscle due to mutations in more than 40 genes, which result in dystrophic changes on muscle biopsy. Now that most of the genes responsible for these conditions have been identified, it is possible to accurately diagnose them and implement subtype-specific anticipatory care, as complications such as cardiac and respiratory muscle involvement vary greatly. This development and advances in the field of supportive medicine have changed the standard of care, with an overall improvement in the clinical course, survival, and quality of life of affected individuals. The improved understanding of the pathogenesis of these diseases is being used for the development of novel therapies. In the most common form, Duchenne muscular dystrophy, a few personalised therapies have recently achieved conditional approval and many more are at advanced stages of clinical development. In this Seminar, we concentrate on clinical manifestations, molecular pathogenesis, diagnostic strategy, and therapeutic developments for this group of conditions.

Functional abnormalities in induced Pluripotent Stem Cell-derived cardiomyocytes generated from titin-mutated patients with dilated cardiomyopathy

Aims

Dilated cardiomyopathy (DCM), a myocardial disorder that can result in progressive heart failure and arrhythmias, is defined by ventricular chamber enlargement and dilatation, and systolic dysfunction. Despite extensive research, the pathological mechanisms of DCM are unclear mainly due to numerous mutations in different gene families resulting in the same outcome—decreased ventricular function. Titin (TTN)—a giant protein, expressed in cardiac and skeletal muscles, is an important part of the sarcomere, and thus TTN mutations are the most common cause of adult DCM. To decipher the basis for the cardiac pathology in titin-mutated patients, we investigated the hypothesis that induced Pluripotent Stem Cell (iPSC)-derived cardiomyocytes (iPSC-CM) generated from patients, recapitulate the disease phenotype. The hypothesis was tested by 3 Aims: (1) Investigate key features of the excitation-contraction-coupling machinery; (2) Investigate the responsiveness to positive inotropic interventions; (3) Investigate the proteome profile of the AuP cardiomyocytes using mass-spectrometry (MS).

Methods and results

iPSC were generated from the patients' skin fibroblasts. The major findings were: (1) Sarcomeric organization analysis in mutated iPSC-CM showed defects in assembly and maintenance of sarcomeric structure. (2) Mutated iPSC-CM exhibited diminished inotropic and lusitropic responses to β-adrenergic stimulation with isoproterenol, increased [Ca²⁺]_out and angiotensin-II. Additionally, mutated iPSC-CM displayed prolonged recovery in response to caffeine. These findings may result from defective or lack of interactions of the sarcomeric components with titin through its kinase domain which is absent in the mutated cells.

Conclusions

These findings show that the mutated cardiomyocytes from DCM patients recapitulate abnormalities of the inherited cardiomyopathies, expressed as blunted inotropic response.

Usefulness of Urinary N-Terminal Fragment of Titin to Predict Mortality in Dilated Cardiomyopathy

Titin is associated with myocardial stiffness and hypertrophy, and mutations in its gene have been identified in cardiac myopathies such as dilated cardiomyopathy (DC). It has recently been reported that in damaged muscle, the N-terminal fragment of titin (Titin-N) is cleaved by calpain-3, and urinary Titin-N (U-TN) could be a marker of sarcomere damage. We aimed to investigate the impact of U-TN on prognosis of DC. We measured urinary levels of Titin-N/creatinine ratio (U-TN/Cr; pmol/mg/dl) in 102 patients with DC, and followed up all the patients (mean 1,167 days). The patients were divided into 3 groups based on the U-TN/Cr: first (U-TN/Cr <3.35, n = 34), second (3.35 ≤ U-TN/Cr <7.26, n = 34), and third (7.26 ≤ U-TN/Cr, n = 34) tertiles. In the Kaplan-Meier analysis, cardiac and all-cause mortality progressively increased from the first to the second and third groups (p <0.05, respectively). In the Cox proportional hazard analyses, U-TN/Cr was a predictor of cardiac and all-cause mortality in patients with DC (p <0.05, respectively). U-TN, a possible marker of sarcomere damage, can identify high-risk patients with DC.

Thick Filament Protein Network, Functions, and Disease Association

Sarcomeres consist of highly ordered arrays of thick myosin and thin actin filaments along with accessory proteins. Thick filaments occupy the center of sarcomeres where they partially overlap with thin filaments. The sliding of thick filaments past thin filaments is a highly regulated process that occurs in an ATP-dependent manner driving muscle contraction. In addition to myosin that makes up the backbone of the thick filament, four other proteins which are intimately bound to the thick filament, myosin binding protein-C, titin, myomesin, and obscurin play important structural and regulatory roles. Consistent with this, mutations in the respective genes have been associated with idiopathic and congenital forms of skeletal and cardiac myopathies. In this review, we aim to summarize our current knowledge on the molecular structure, subcellular localization, interacting partners, function, modulation via posttranslational modifications, and disease involvement of these five major proteins that comprise the thick filament of striated muscle cells. © 2018 American Physiological Society. Compr Physiol 8:631-709, 2018.

Genetic epidemiology of titin-truncating variants in the etiology of dilated cardiomyopathy

Heart failure (HF) is a complex clinical syndrome defined by the inability of the heart to pump enough blood to meet the body's metabolic demands. Major causes of HF are cardiomyopathies (diseases of the myocardium associated with mechanical and/or electrical dysfunction), among which the most common form is dilated cardiomyopathy (DCM). DCM is defined by ventricular chamber enlargement and systolic dysfunction with normal left ventricular wall thickness, which leads to progressive HF. Over 60 genes are linked to the etiology of DCM. Titin (TTN) is the largest known protein in biology, spanning half the cardiac sarcomere and, as such, is a basic structural and functional unit of striated muscles. It is essential for heart development as well as mechanical and regulatory functions of the sarcomere. Next-generation sequencing (NGS) in clinical DCM cohorts implicated truncating variants in titin (TTNtv) as major disease alleles, accounting for more than 25% of familial DCM cases, but these variants have also been identified in 2-3% of the general population, where these TTNtv blur diagnostic and clinical utility. Taking into account the published TTNtv and their association to DCM, it becomes clear that TTNtv harm the heart with position-dependent occurrence, being more harmful when present in the A-band TTN, presumably with dominant negative/gain-of-function mechanisms. However, these insights are challenged by the depiction of position-independent toxicity of TTNtv acting via haploinsufficient alleles, which are sufficient to induce cardiac pathology upon stress. In the current review, we provide an overview of TTN and discuss studies investigating various TTN mutations. We also present an overview of different mechanisms postulated or experimentally validated in the pathogenicity of TTNtv. DCM-causing genes are also discussed with respect to non-truncating mutations in the etiology of DCM. One way of understanding pathogenic variants is probably to understand the context in which they may or may not affect protein-protein interactions, changes in cell signaling, and substrate specificity. In this regard, we also provide a brief overview of TTN interactions in situ. Quantitative models in the risk assessment of TTNtv are also discussed. In summary, we highlight the importance of gene-environment interactions in the etiology of DCM and further mechanistic studies used to delineate the pathways which could be targeted in the management of DCM.

Prevalence of Titin Truncating Variants in General Population

Background

Truncating titin (TTN) mutations, especially in A-band region, represent the most common cause of dilated cardiomyopathy (DCM). Clinical interpretation of these variants can be challenging, as these variants are also present in reference populations. We carried out systematic analyses of TTN truncating variants (TTNtv) in publicly available reference populations, including, for the first time, data from Exome Aggregation Consortium (ExAC). The goal was to establish more accurate estimate of prevalence of different TTNtv to allow better clinical interpretation of these findings.

Methods and Results

Using data from 1000 Genomes Project, Exome Sequencing Project (ESP) and ExAC, we estimated the prevalence of TTNtv in the population. In the three population datasets, 52–54% of TTNtv were not affecting all TTN transcripts. The frequency of truncations affecting all transcripts in ExAC was 0.36% (0.32% - 0.41%, 95% CI) and 0.19% (0.16% - 0.23%, 95% CI) for those affecting the A-band. In the A-band region, the prevalences of frameshift, nonsense and essential splice site variants were 0.057%, 0.090%, and 0.047% respectively. Cga/Tga (arginine/nonsense–R/*) transitional change at CpG mutation hotspots was the most frequent type of TTN nonsense mutation accounting for 91.3% (21/23) of arginine residue nonsense mutation (R/*) at TTN A-band region. Non-essential splice-site variants had significantly lower proportion of private variants and higher proportion of low-frequency variants compared to essential splice-site variants (P = 0.01; P = 5.1 X 10⁻⁴, respectively).

Conclusion

A-band TTNtv are more rare in the general population than previously reported. Based on this analysis, one in 500 carries a truncation in TTN A-band suggesting the penetrance of these potentially harmful variants is still poorly understood, and some of these variants do not manifest as autosomal dominant DCM. This calls for caution when interpreting TTNtv in individuals and families with no history of DCM. Considering the size of TTN, expertise in DNA library preparation, high coverage NGS strategies, validated bioinformatics approach, accurate variant assessment strategy, and confirmatory sequencing are prerequisites for reliable evaluation of TTN in clinical settings, and ideally with the inclusion of mRNA and/or protein level assessment for a definite diagnosis.

Atypical case of post-partum cardiomyopathy: an overlap syndrome with arrhythmogenic right ventricular cardiomyopathy?

A middle-aged female patient presented with increasing dyspnoea following delivery of her second child. Echocardiography showed left ventricular (LV) dilatation and severe global impairment of systolic function (ejection fraction < 10%) but normal right ventricular (RV) dimensions. Plasma B-type natriuretic peptide level was elevated. Post-partum cardiomyopathy (PPCM) was considered and after initiating appropriate heart failure pharmacotherapy, her symptoms improved significantly. Cardiovascular MR showed RV free wall dyskinesia and aneurysms at the LV apex, RV free wall and RV outflow tract. Genetic analysis showed a C11842T substitution in the titin gene (TTN). This is the first case to propose an overlap syndrome of PPCM and arrhythmogenic RV cardiomyopathy.

Ethnic differences among implantable cardioverter defibrillators recipients in Israel

Heart failure is an increasingly common condition arising from a variety of different pathophysiological processes. Little is known about the unique features of Israeli Arabs who present with heart failure and who undergo cardiac device implantation. The study population comprised of 4,671 patients who were enrolled in the national Israeli Implantable Cardioverter Defibrillator registry. We compared demographic, clinical, and echocardiographic characteristics; device-related indications; and outcomes between Israeli Arabs (n = 733) and Jews (n = 3,938), who were enrolled in the registry from July 2010 through December 2013. Israeli Arabs constituted 15.7% of the study population. They were younger at presentation compared with Jews (57 ± 15 vs 66 ± 12 years, respectively; p <0.001), with a greater burden of co-morbidities, including diabetes mellitus and chronic obstructive lung disease and smoking. In addition, Arab patients had a greater frequency of non-ischemic cardiomyopathy (40.2% vs 24.6%, respectively; p <0.001), which was associated with a greater frequency of familial history of sudden cardiac death. During 15 ± 9 month follow-up, the mortality rates and appropriate device therapy were similar in both ethnic groups. In conclusion, Israeli Arab patients implanted with implantable cardioverter defibrillators display unique clinical features with greater prevalence of non-ischemic cardiomyopathy characterized by an early-onset and rapid deterioration.

Cardiac titin and heart disease

The giant sarcomeric protein titin is a key determinant of myocardial passive stiffness and stress-sensitive signaling. Titin stiffness is modulated by isoform variation, phosphorylation by protein kinases, and, possibly, oxidative stress through disulfide bond formation. Titin has also emerged as an important human disease gene. Early studies in patients with dilated cardiomyopathy (DCM) revealed shifts toward more compliant isoforms, an adaptation that offsets increases in passive stiffness based on the extracellular matrix. Similar shifts are observed in heart failure with preserved ejection fraction. In contrast, hypophosphorylation of PKA/G sites contributes to a net increase in cardiomyocyte resting tension in heart failure with preserved ejection fraction. More recently, titin mutations have been recognized as the most common etiology of inherited DCM. In addition, some DCM-causing mutations affect RBM20, a titin splice factor. Titin mutations are a rare cause of hypertrophic cardiomyopathy and also underlie some cases of arrhythmogenic right ventricular dysplasia. Finally, mutations of genes encoding proteins that interact with and/or bind to titin are responsible for both DCM and hypertrophic cardiomyopathy. Targeting titin as a therapeutic strategy is in its infancy, but it could potentially involve manipulation of isoforms, posttranslational modifications, and upregulation of normal protein in patients with disease-causing mutations.

A rising titan: TTN review and mutation update

The 364 exon TTN gene encodes titin (TTN), the largest known protein, which plays key structural, developmental, mechanical, and regulatory roles in cardiac and skeletal muscles. Prior to next-generation sequencing (NGS), routine analysis of the whole TTN gene was impossible due to its giant size and complexity. Thus, only a few TTN mutations had been reported and the general incidence and spectrum of titinopathies was significantly underestimated. In the last months, due to the widespread use of NGS, TTN is emerging as a major gene in human-inherited disease. So far, 127 TTN disease-causing mutations have been reported in patients with at least 10 different conditions, including isolated cardiomyopathies, purely skeletal muscle phenotypes, or infantile diseases affecting both types of striated muscles. However, the identification of TTN variants in virtually every individual from control populations, as well as the multiplicity of TTN isoforms and reference sequences used, stress the difficulties in assessing the relevance, inheritance, and correlation with the phenotype of TTN sequence changes. In this review, we provide the first comprehensive update of the TTN mutations reported and discuss their distribution, molecular mechanisms, associated phenotypes, transmission pattern, and phenotype-genotype correlations, alongside with their implications for basic research and for human health.

Gigantic business: titin properties and function through thick and thin

The giant protein titin forms a unique filament network in cardiomyocytes, which engages in both mechanical and signaling functions of the heart. TTN, which encodes titin, is also a major human disease gene. In this review, we cover the roles of cardiac titin in normal and failing hearts, with a special emphasis on the contribution of titin to diastolic stiffness. We provide an update on disease-associated titin mutations in cardiac and skeletal muscles and summarize what is known about the impact of protein-protein interactions on titin properties and functions. We discuss the importance of titin-isoform shifts and titin phosphorylation, as well as titin modifications related to oxidative stress, in adjusting the diastolic stiffness of the healthy and the failing heart. Along the way we distinguish among titin alterations in systolic and in diastolic heart failure and ponder the evidence for titin stiffness as a potential target for pharmacological intervention in heart disease.

Titin mutation in familial restrictive cardiomyopathy

BACKGROUND

Familial restrictive cardiomyopathy (RCM) caused by a single gene mutation is the least common of the inherited cardiomyopathies. Only a few RCM-causing mutations have been described. Most mutations causing RCM are located in sarcomere protein genes which also cause hypertrophic cardiomyopathy (HCM). Other genes associated with RCM include the desmin and familial amyloidosis genes. In the present study we describe familial RCM with severe heart failure triggered by a de novo mutation in TTN, encoding the huge muscle filament protein titin.

METHODS AND RESULTS

Family members underwent physical examination, ECG and Doppler echocardiogram studies. The family comprised 6 affected individuals aged 12-35 years. Linkage to candidate loci was performed, followed by gene sequencing. Candidate loci/gene analysis excluded 18 candidate genes but showed segregation with a common haplotype surrounding the TTN locus. Sequence analysis identified a de novo mutation within exon 266 of the TTN gene, resulting in the replacement of tyrosine by cysteine. p.Y7621C affects a highly conserved region in the protein within a fibronectin-3 domain, belonging to the A/I junction region of titin. No other disease-causing mutation was identified in cardiomyopathy genes by whole exome sequencing.

CONCLUSIONS

Our study shows, for the first time, that mutations in TTN can cause restrictive cardiomyopathy. The giant filament titin is considered to be a determinant of a resting tension of the sarcomere and this report provides genetic evidence of its crucial role in diastolic function.

Muscular dystrophies

Muscular dystrophies are a heterogeneous group of inherited disorders that share similar clinical features and dystrophic changes on muscle biopsy. An improved understanding of their molecular bases has led to more accurate definitions of the clinical features associated with known subtypes. Knowledge of disease-specific complications, implementation of anticipatory care, and medical advances have changed the standard of care, with an overall improvement in the clinical course, survival, and quality of life of affected people. A better understanding of the mechanisms underlying the molecular pathogenesis of several disorders and the availability of preclinical models are leading to several new experimental approaches, some of which are already in clinical trials. In this Seminar, we provide a comprehensive review that integrates clinical manifestations, molecular pathogenesis, diagnostic strategy, and therapeutic developments.

Doubly heterozygous LMNA and TTN mutations revealed by exome sequencing in a severe form of dilated cardiomyopathy

Familial dilated cardiomyopathy (DCM) is a heterogeneous disease; although 30 disease genes have been discovered, they explain only no more than half of all cases; in addition, the causes of intra-familial variability in DCM have remained largely unknown. In this study, we exploited the use of whole-exome sequencing (WES) to investigate the causes of clinical variability in an extended family with 14 affected subjects, four of whom showed particular severe manifestations of cardiomyopathy requiring heart transplantation in early adulthood. This analysis, followed by confirmative conventional sequencing, identified the mutation p.K219T in the lamin A/C gene in all 14 affected patients. An additional variant in the gene for titin, p.L4855F, was identified in the severely affected patients. The age for heart transplantation was substantially less for LMNA:p.K219T/TTN:p.L4855F double heterozygotes than that for LMNA:p.K219T single heterozygotes. Myocardial specimens of doubly heterozygote individuals showed increased nuclear length, sarcomeric disorganization, and myonuclear clustering compared with samples from single heterozygotes. In conclusion, our results show that WES can be used for the identification of causal and modifier variants in families with variable manifestations of DCM. In addition, they not only indicate that LMNA and TTN mutational status may be useful in this family for risk stratification in individuals at risk for DCM but also suggest titin as a modifier for DCM.