Titin-truncating mutations in dilated cardiomyopathy: the long and short of it

Structural domain in the Titin N2B-us region binds to FHL2 in a force-activation dependent manner

Titin N2B unique sequence (N2B-us) is a 572 amino acid sequence that acts as an elastic spring to regulate muscle passive elasticity. It is thought to lack stable tertiary structures and is a force-bearing region that is regulated by mechanical stretching. In this study, the conformation of N2B-us and its interaction with four-and-a-half LIM domain protein 2 (FHL2) are investigated using AlphaFold2 predictions and single-molecule experimental validation. Surprisingly, a stable alpha/beta structural domain is predicted and confirmed in N2B-us that can be mechanically unfolded at forces of a few piconewtons. Additionally, more than twenty FHL2 LIM domain binding sites are predicted to spread throughout N2B-us. Single-molecule manipulation experiments reveals the force-dependent binding of FHL2 to the N2B-us structural domain. These findings provide insights into the mechano-sensing functions of N2B-us and its interactions with FHL2.

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.

Phenotyping heart failure by genetics and associated conditions

Heart failure is a highly heterogeneous disease, and genetic testing may allow phenotypic distinctions that are incremental to those obtainable from imaging. Advances in genetic testing have allowed for the identification of deleterious variants in patients with specific heart failure phenotypes (dilated cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy, and hypertrophic cardiomyopathy), and many of these have specific treatment implications. The diagnostic yield of genetic testing in heart failure is modest, and many rare variants are associated with incomplete penetrance and variable expressivity. Environmental factors and co-morbidities have a large role in the heterogeneity of the heart failure phenotype. Future endeavours should concentrate on the cumulative impact of genetic polymorphisms in the development of heart failure.

[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.

Titin-truncating variants in hiPSC cardiomyocytes induce pathogenic proteinopathy and sarcomere defects with preserved core contractile machinery

Titin-truncating variants (TTNtv) are the single largest genetic cause of dilated cardiomyopathy (DCM). In this study we modeled disease phenotypes of A-band TTNtv-induced DCM in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) using genome editing and tissue engineering technologies. Transcriptomic, cellular, and micro-tissue studies revealed that A-band TTNtv hiPSC-CMs exhibit pathogenic proteinopathy, sarcomere defects, aberrant Na⁺ channel activities, and contractile dysfunction. These phenotypes establish a dual mechanism of poison peptide effect and haploinsufficiency that collectively contribute to DCM pathogenesis. However, TTNtv cellular defects did not interfere with the function of the core contractile machinery, the actin-myosin-troponin-Ca²⁺ complex, and preserved the therapeutic mechanism of sarcomere modulators. Treatment of TTNtv cardiac micro-tissues with investigational sarcomere modulators augmented contractility and resulted in sustained transcriptomic changes that promote reversal of DCM disease signatures. Together, our findings elucidate the underlying pathogenic mechanisms of A-band TTNtv-induced DCM and demonstrate the validity of sarcomere modulators as potential therapeutics.

Titin-related Cardiomyopathy: Is it a Distinct Disease?

PURPOSE OF REVIEW

Truncating TTN variants (TTNtv) are the most common genetic cause of dilated cardiomyopathy (DCM), but the underlying mechanisms are incompletely understood and effective therapeutic strategies are lacking. Here we review recent data that shed new light on the functional consequences of TTNtv and how these effects may vary with mutation location.

RECENT FINDINGS

Whether TTNtv act by haploinsufficiency or dominant negative effects has been hotly debated. New evidence now implicates both mechanisms in TTNtv-related DCM, showing reduced titin content and persistent truncated titin that may be incorporated into protein aggregates. The extent to which aggregate formation and protein quality control defects differ with TTNtv location and contribute to contractile dysfunction is unresolved. TTNtv-associated DCM has a complex etiology that involves varying combinations of wild-type titin deficiency and dominant negative effects of truncated mutant titin. Therapeutic strategies to improve protein handling may be beneficial in some cases.

Next-Generation Sequencing Reveals Novel Genetic Variants for Dilated Cardiomyopathy in Pediatric Chinese Patients

Dilated cardiomyopathy (DCM) is a myocardial disease characterized by bilateral or left ventricular cardiac dilation and systolic dysfunction that can lead to heart failure and sudden cardiac death in children. Many studies have focused on genetic variation in DCM-related genes in adult populations; however, the mutational landscape in pediatric DCM patients remains undetermined, especially in the Chinese population. We applied next-generation sequencing (NGS) technology to genetically analyze 46 pediatric DCM patients to reveal genotype-phenotype correlations. Our results indicated DCM-associated pathogenic mutations in 10 genes related to the structure or function of the sarcomere, desmosome, and cytoskeleton. We also identified 6 pathogenic mutations (5 novel) in the Titin (TTN) gene that resulted in truncated TTN variants in 6 (13%) out of 46 patients. Correlations between TTN mutations and clinical outcomes were assessed. Our data indicate that one-third of pediatric DCM cases are caused by genetic mutations. The role of TTN variants should not be underestimated in pediatric DCM and age-dependent pathogenic penetrance of these mutations should be considered for familial DCM cases. We argue that genetic testing of DCM cases is valuable for predicting disease severity, prognosis, and recurrence risk, and for screening first-degree relatives.

Genetic basis and molecular biology of cardiac arrhythmias in cardiomyopathies

Cardiac arrhythmias are common, often the first, and sometimes the life-threatening manifestations of hereditary cardiomyopathies. Pathogenic variants in several genes known to cause hereditary cardiac arrhythmias have also been identified in the sporadic cases and small families with cardiomyopathies. These findings suggest a shared genetic aetiology of a subset of hereditary cardiomyopathies and cardiac arrhythmias. The concept of a shared genetic aetiology is in accord with the complex and exquisite interplays that exist between the ion currents and cardiac mechanical function. However, neither the causal role of cardiac arrhythmias genes in cardiomyopathies is well established nor the causal role of cardiomyopathy genes in arrhythmias. On the contrary, secondary changes in ion currents, such as post-translational modifications, are common and contributors to the pathogenesis of arrhythmias in cardiomyopathies through altering biophysical and functional properties of the ion channels. Moreover, structural changes, such as cardiac hypertrophy, dilatation, and fibrosis provide a pro-arrhythmic substrate in hereditary cardiomyopathies. Genetic basis and molecular biology of cardiac arrhythmias in hereditary cardiomyopathies are discussed.

Dilated cardiomyopathy: a new insight into the rare but common cause of heart failure

Heart failure is a global health burden responsible for high morbidity and mortality with a prevalence of greater than 60 million individuals worldwide. One of the major causes of heart failure is dilated cardiomyopathy (DCM), characterized by associated systolic dysfunction. During the last few decades, there have been remarkable advances in our understanding about the genetics of dilated cardiomyopathy. The genetic causes were initially thought to be associated with mutations in genes encoding proteins that are localized to cytoskeleton and sarcomere only; however, with the advancement in mechanistic understanding, the roles of ion channels, Z-disc, mitochondria, nuclear proteins, cardiac transcription factors (e.g., NKX-2.5, TBX20, GATA4), and the factors involved in calcium homeostasis have also been identified and found to be implicated in both familial and sporadic DCM cases. During past few years, next-generation sequencing (NGS) has been established as a diagnostic tool for genetic analysis and it has added significantly to the existing candidate gene list for DCM. The animal models have also provided novel insights to develop a better treatment strategy based on phenotype-genotype correlation, epigenetic and phenomic profiling. Most of the DCM biomarkers that are used in routine genetic and clinical testing are structural proteins, but during the last few years, the role of mi-RNA has also emerged as a biomarker due to their accessibility through noninvasive methods. Our increasing genetic knowledge can improve the clinical management of DCM by bringing clinicians and geneticists on one platform, thereby influencing the individualized clinical decision making and leading to precision medicine.

Making sense of missense variants in TTN-related congenital myopathies

Mutations in the sarcomeric protein titin, encoded by TTN, are emerging as a common cause of myopathies. The diagnosis of a TTN-related myopathy is, however, often not straightforward due to clinico-pathological overlap with other myopathies and the prevalence of TTN variants in control populations. Here, we present a combined clinico-pathological, genetic and biophysical approach to the diagnosis of TTN-related myopathies and the pathogenicity ascertainment of TTN missense variants. We identified 30 patients with a primary TTN-related congenital myopathy (CM) and two truncating variants, or one truncating and one missense TTN variant, or homozygous for one TTN missense variant. We found that TTN-related myopathies show considerable overlap with other myopathies but are strongly suggested by a combination of certain clinico-pathological features. Presentation was typically at birth with the clinical course characterized by variable progression of weakness, contractures, scoliosis and respiratory symptoms but sparing of extraocular muscles. Cardiac involvement depended on the variant position. Our biophysical analyses demonstrated that missense mutations associated with CMs are strongly destabilizing and exert their effect when expressed on a truncating background or in homozygosity. We hypothesise that destabilizing TTN missense mutations phenocopy truncating variants and are a key pathogenic feature of recessive titinopathies that might be amenable to therapeutic intervention.

Diagnosis and Risk Prediction of Dilated Cardiomyopathy in the Era of Big Data and Genomics

Dilated cardiomyopathy (DCM) is a leading cause of heart failure and life-threatening ventricular arrhythmias (LTVA). Work-up and risk stratification of DCM is clinically challenging, as there is great heterogeneity in phenotype and genotype. Throughout the last decade, improved genetic testing of patients has identified genotype-phenotype associations and enhanced evaluation of at-risk relatives leading to better patient prognosis. The field is now ripe to explore opportunities to improve personalised risk assessments. Multivariable risk models presented as "risk calculators" can incorporate a multitude of clinical variables and predict outcome (such as heart failure hospitalisations or LTVA). In addition, genetic risk scores derived from genome/exome-wide association studies can estimate an individual's lifetime genetic risk of developing DCM. The use of clinically granular investigations, such as late gadolinium enhancement on cardiac magnetic resonance imaging, is warranted in order to increase predictive performance. To this end, constructing big data infrastructures improves accessibility of data by using electronic health records, existing research databases, and disease registries. By applying methods such as machine and deep learning, we can model complex interactions, identify new phenotype clusters, and perform prognostic modelling. This review aims to provide an overview of the evolution of DCM definitions as well as its clinical work-up and considerations in the era of genomics. In addition, we present exciting examples in the field of big data infrastructures, personalised prognostic assessment, and artificial intelligence.

Malignant Arrhythmogenic Role Associated with RBM20: A Comprehensive Interpretation Focused on a Personalized Approach

The RBM20 gene encodes the muscle-specific splicing factor RNA-binding motif 20, a regulator of heart-specific alternative splicing. Nearly 40 potentially deleterious variants in RBM20 have been reported in the last ten years, being found to be associated with highly arrhythmogenic events in familial dilated cardiomyopathy. Frequently, malignant arrhythmias can be a primary manifestation of disease. The early recognition of arrhythmic genotypes is crucial in avoiding lethal episodes, as it may have an impact on the adoption of personalized preventive measures. Our study performs a comprehensive update of data concerning rare variants in RBM20 that are associated with malignant arrhythmogenic phenotypes with a focus on personalized medicine.

Cardiomyopathies in children: classification, diagnosis and treatment

PURPOSE OF REVIEW

Cardiomyopathies are rare in the pediatric population, but significantly impact on morbidity and mortality. The present review aims to provide an overview of cardiomyopathies in children and some practical guidelines for their prognostic stratification and management.

RECENT FINDINGS

Pediatric cardiomyopathies may present as isolated cardiac muscle disease or in the context of complex clinical syndromes. The etiologic characterization represents an important step in the diagnosis and treatment of cardiomyopathies because of its impact on prognosis and on therapeutic measures. Indeed, replacement therapy is nowadays widely available and changes the natural history of the disease. More complex is the management of isolated cardiomyopathies, which lack specific therapies, mainly aimed at symptomatic relief. In this context, heart transplantation shows excellent outcomes in children, but wait-list mortality is still very high. Device therapy for sudden cardiac death prevention and the use of mechanical assist devices are becoming more common in the clinical practice and may help to reduce mortality.

SUMMARY

Providing insight into pediatric cardiomyopathies classification helps in the prognostication and management of such diseases. Recent years witnessed a significant improvement in mortality, but future research is still needed to improve quality of life and life expectations in the pediatric population.

Mortality Risk Associated With Truncating Founder Mutations in Titin

Background Truncating titin variants (TTNtv) are the most prevalent genetic cause of dilated cardiomyopathy, found in ≤25% of familial cases. Moreover, TTNtv associated with dilated cardiomyopathy are estimated to be present in 0.5% of the general population. The prognosis of asymptomatic carriers of TTNtv is poorly understood because TTNtv are associated with a highly variable phenotype. We aim to assess the natural history and clinical relevance of TTNtv by analyzing standardized mortality ratios (SMR) in multigenerational pedigrees and in close relatives of present-day patients. Methods Haplotype and genealogical analyses were performed on 3 recurrent TTNtv. Subsequently, the family tree mortality ratio method was used to compare all-cause mortality of subjects at an a priori 50% risk of carrying TTNtv to the general Dutch population. SMRs were stratified for sex, age, and calendar period. Subgroups were compared with Poisson regression. Similarly, SMRs were calculated in parents of 128 present-day dilated cardiomyopathy probands with TTNtv using the reverse parent-offspring method. Results The TTNtv were established as founder mutations and traced to 18th century ancestors. In 20 522 person-years, overall mortality was not significantly increased (SMR, 1.06; 95% CI, 0.95-1.18; P=0.162). However, mortality was significantly increased in subjects living after 1965 (SMR, 1.27; 95% CI, 1.04-1.53; P=0.009) and aged ≥60 years (SMR, 1.17; 95% CI, 1.01-1.35; P=0.02). The reverse parent-offspring analysis showed overall excess mortality (SMR, 1.26; 95% CI, 1.07-1.48; P=0.003), driven by subjects aged ≥60 years. Conclusions The natural history of the analyzed TTNtv shows a relatively mild disease course with significant excess mortality in elderly patients. With increasing life expectancy, TTNtv-associated morbidity and mortality will likely become more prevalent.

Prevalence of TTN mutations in patients with dilated cardiomyopathy : A meta-analysis

A meta-analysis was performed to assess the prevalence of TTN mutations in patients with dilated cardiomyopathy (DCM). Prevalence point estimates and 95% confidence intervals were computed using the logit transformation formula. The prevalence of TTN mutations in patient with DCM, familial dilated cardiomyopathy (FDCM), and sporadic dilated cardiomyopathy (SDCM) was 0.17 (95% CI: 0.14-0.19), 0.23 (95% CI: 0.20-0.26), and 0.16 (95% CI: 0.12-0.21), respectively. No individual study had a marked influence on the pooled prevalence in the meta-analysis. Meta-regression analysis between the logit event for prevalence and sample size explained 32% of between-study variance (p < 0.05). Cumulative meta-analysis confirmed the influence of sample size on the reported prevalence among the different studies. In conclusion, the present analysis suggests that TTN mutations are familial in DCM patients. More attention should be paid to TTN mutations in clinical examinations.

Titin mutations and muscle disease

The introduction of next-generation sequencing technology has revealed that mutations in the gene that encodes titin (TTN) are linked to multiple skeletal and cardiac myopathies. The most prominent of these myopathies is dilated cardiomyopathy (DCM). Over 60 genes are linked to the etiology of DCM, but by far, the leading cause of DCM is mutations in TTN with truncating variants in TTN (TTNtvs) associated with familial DCM in ∼ 20% of the cases. Titin is a large (3-4 MDa) and abundant protein that forms the third myofilament type of striated muscle where it spans half the sarcomere, from the Z-disk to the M-line. The underlying mechanisms by which titin mutations induce disease are poorly understood and targeted therapies are not available. Here, we review what is known about TTN mutations in muscle disease, with a major focus on DCM. We highlight that exon skipping might provide a possible therapeutic avenue to address diseases that arise from TTNtvs.

Alternative Splicing Regulator RBM20 and Cardiomyopathy

RBM20 is a vertebrate-specific RNA-binding protein with two zinc finger (ZnF) domains, one RNA-recognition motif (RRM)-type RNA-binding domain and an arginine/serine (RS)-rich region. RBM20 has initially been identified as one of dilated cardiomyopathy (DCM)-linked genes. RBM20 is a regulator of heart-specific alternative splicing and Rbm20^ΔRRM mice lacking the RRM domain are defective in the splicing regulation. The Rbm20^ΔRRM mice, however, do not exhibit a characteristic DCM-like phenotype such as dilatation of left ventricles or systolic dysfunction. Considering that most of the RBM20 mutations identified in familial DCM cases were heterozygous missense mutations in an arginine-serine-arginine-serine-proline (RSRSP) stretch whose phosphorylation is crucial for nuclear localization of RBM20, characterization of a knock-in animal model is awaited. One of the major targets for RBM20 is the TTN gene, which is comprised of the largest number of exons in mammals. Alternative splicing of the TTN gene is exceptionally complicated and RBM20 represses >160 of its consecutive exons, yet detailed mechanisms for such extraordinary regulation are to be elucidated. The TTN gene encodes the largest known protein titin, a multi-functional sarcomeric structural protein specific to striated muscles. As titin is the most important factor for passive tension of cardiomyocytes, extensive heart-specific and developmentally regulated alternative splicing of the TTN pre-mRNA by RBM20 plays a critical role in passive stiffness and diastolic function of the heart. In disease models with diastolic dysfunctions, the phenotypes were rescued by increasing titin compliance through manipulation of the Ttn pre-mRNA splicing, raising RBM20 as a potential therapeutic target.

Takotsubo as Initial Manifestation of Non-Myopathic Cardiomyopathy Due to the Titin Variant c.1489G > T

Background: Whether patients with subclinical cardiomyopathy (CMP) are more prone to experience Takotsubo syndrome (TTS) than patients without CMP, is unknown. We present a patient with TTS as the initial manifestation of a hitherto unrecognized genetic CMP. Method: case report. Results: At age 55 after the unexpected death of her father, a now 61-year-old female had developed precordial pressure. Work-up revealed moderately reduced systolic function, dyskinesia of the interventricular septum, and indications for a TTS. Coronary angiography was normal but ventriculography showed TTS. Cardiac MRI confirmed reduced systolic function and TTS. TTS resolved without treatment and sequelae. At age 57 atrial fibrillation was recorded. After deterioration of systolic function at age 59 dilated CMP was diagnosed. Despite application of levosimendan, sacubitril, valsartan, and ivabradine, complete remission could not be achieved. Upon genetic work-up by means of a gene panel, the heterozygous mutation c.1489G > T (p. E497X) in exon 9 of the titin gene was detected and made responsible for the phenotype. Neurological work-up precluded involvement of the skeletal muscles. The further course was complicated by ventricular arrhythmias, requiring implantation of an implantable cardioverter defibrillator (ICD). Conclusions: previously subclinical CMP may initially manifest as TTS. Since patients with titin CMP are at risk of developing ventricular arrhythmias and thus to experience sudden cardiac death, appropriate anti-arrhythmic therapy needs to be established.

MEF2C loss-of-function mutation associated with familial dilated cardiomyopathy

Background:

The MADS-box transcription factor myocyte enhancer factor 2C (MEF2C) is required for the cardiac development and postnatal adaptation and in mice-targeted disruption of the MEF2C gene results in dilated cardiomyopathy (DCM). However, in humans, the association of MEF2C variation with DCM remains to be investigated.

Methods:

The coding regions and splicing boundaries of the MEF2C gene were sequenced in 172 unrelated patients with idiopathic DCM. The available close relatives of the index patient harboring an identified MEF2C mutation and 300 unrelated, ethnically matched healthy individuals used as controls were genotyped for MEF2C. The functional effect of the mutant MEF2C protein was characterized in contrast to its wild-type counterpart by using a dual-luciferase reporter assay system.

Results:

A novel heterozygous MEF2C mutation, p.Y157X, was detected in an index patient with adult-onset DCM. Genetic screen of the mutation carrier’s family members revealed that the mutation co-segregated with DCM, which was transmitted as an autosomal dominant trait with complete penetrance. The non-sense mutation was absent in 300 control individuals. Functional analyses unveiled that the mutant MEF2C protein had no transcriptional activity. Furthermore, the mutation abolished the synergistic transactivation between MEF2C and GATA4 as well as HAND1, two other transcription factors that have been associated with DCM.

Conclusions:

This study indicates MEF2C as a new gene responsible for human DCM, which provides novel insight into the mechanism underpinning DCM, suggesting potential implications for development of innovative prophylactic and therapeutic strategies for DCM, the most prevalent form of primary myocardial disease.

Molecular mechanisms in cardiomyopathy

Cardiomyopathies represent a heterogeneous group of diseases that negatively affect heart function. Primary cardiomyopathies specifically target the myocardium, and may arise from genetic [hypertrophic cardiomyopathy (HCM), arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D), mitochondrial cardiomyopathy] or genetic and acquired [dilated cardiomyopathy (DCM), restrictive cardiomyopathy (RCM)] etiology. Modern genomics has identified mutations that are common in these populations, while in vitro and in vivo experimentation with these mutations have provided invaluable insight into the molecular mechanisms native to these diseases. For example, increased myosin heavy chain (MHC) binding and ATP utilization lead to the hypercontractile sarcomere in HCM, while abnormal protein–protein interaction and impaired Ca2+ flux underlie the relaxed sarcomere of DCM. Furthermore, expanded access to genetic testing has facilitated identification of potential risk factors that appear through inheritance and manifest sometimes only in the advanced stages of the disease. In this review, we discuss the genetic and molecular abnormalities unique to and shared between these primary cardiomyopathies and discuss some of the important advances made using more traditional basic science experimentation.