The Rapidly Evolving Role of Titin in Cardiac Physiology and Cardiomyopathy

Late Pleistocene polar bear genomes reveal the timing of allele fixation in key genes associated with Arctic adaptation

The polar bear (Ursus maritimus) occupies a relatively narrow ecological niche, with many traits adapted for cold temperatures, movement across snow, ice and open water, and for consuming highly lipid-dense prey species. The divergence of polar bears from brown bears (Ursus arctos) and their adaptation to their Arctic lifestyle is a well-known example of rapid evolution. Previous research investigating whole genomes uncovered twelve key genes that are highly differentiated between polar and brown bears, show signatures of selection in the polar bear lineage, and are associated with polar bear adaptation to the Arctic environment. Further research suggested fixed derived alleles in these genes arose from selection on both standing variation and de novo mutations in the evolution of polar bears. Here, we reevaluate these findings based on a larger and geographically more representative dataset of 119 polar bears and 135 brown bears, and assess the timing of derived allele fixation in polar bears by incorporating the genomes of two Late Pleistocene individuals (aged 130-100,000 years old and 100-70,000 years old). In contrast with previous results, we found no evidence of derived alleles fixed in present-day polar bears within the key genes arising from de novo mutation. Most derived alleles fixed in present-day polar bears were also fixed in the Late Pleistocene polar bears, suggesting selection occurred prior to 70,000 years ago. However, some derived alleles fixed in present-day polar bears were not fixed in the two Late Pleistocene polar bears, including at sites within APOB, LYST, and TTN. These three genes are associated with cardiovascular function, metabolism, and pigmentation, suggesting selection may have acted on different loci at different times.

Proteolytic degradation of atrial sarcomere proteins underlies contractile defects in atrial fibrillation

Atrial fibrillation (AFib) is the most common cardiac rhythm disturbance, often treated via electrical cardioversion. Following rhythm restoration, a period of depressed mechanical function known as atrial stunning occurs, suggesting that defects in contractility occur in AFib and are revealed upon restoration of rhythm. This project aims to define the contractile remodeling that occurs in AFib. To assess contractile function, we used a canine atrial tachypacing model of induced AFib. Mass spectrometry analysis showed dysregulation of contractile proteins in samples from AFib compared with sinus rhythm atria. Atrial cardiomyocytes show reduced force of contraction, decreased resting tension, and increased calcium sensitivity in skinned single cardiomyocyte studies. These alterations correlated with degradation of myofilament proteins including myosin heavy chain altering force of contraction, titin altering resting tension, and troponin I altering calcium sensitivity. We measured degradation of other myofilament proteins, including cardiac myosin binding protein C and actinin, that show degradation products in the AFib samples that are absent in the sinus rhythm atria. Many of the degradation products appeared as discrete cleavage products that are generated by calpain proteolysis. We assessed calpain activity and found it to be significantly increased. These results provide an understanding of the contractile remodeling that occurs in AFib and provide insight into the molecular explanation for atrial stunning and the increased risk of atrial thrombus and stroke in AFib.NEW & NOTEWORTHY Atrial fibrillation is the most common cardiac rhythm disorder, and remodeling during atrial fibrillation is highly variable between patients. This study has defined the biophysical changes in contractility that occur in atrial fibrillation along with identifying potential molecular mechanisms that may drive this remodeling. This includes proteolysis of several myofilament proteins including titin, troponin I, myosin heavy chain, myosin binding protein C, and actinin, which is consistent with the observed contractile deficits.

Allelic heterogeneity of TTNtv dilated cardiomyopathy can be modeled in adult zebrafish

Allelic heterogeneity (AH) has been noted in truncational TTN-associated (TTNtv-associated) dilated cardiomyopathy (DCM); i.e., mutations affecting A-band-encoding exons are pathogenic, but those affecting Z-disc-encoding exons are likely benign. The lack of an in vivo animal model that recapitulates AH hinders the deciphering of the underlying mechanism. Here, we explored zebrafish as a candidate vertebrate model by phenotyping a collection of zebrafish ttntv alleles. We noted that cardiac function and sarcomere structure were more severely disrupted in ttntv-A than in ttntv-Z homozygous embryos. Consistently, cardiomyopathy-like phenotypes were present in ttntv-A but not ttntv-Z adult heterozygous mutants. The phenotypes observed in ttntv-A alleles were recapitulated in null mutants with the full titin-encoding sequences removed. Defective autophagic flux, largely due to impaired autophagosome-lysosome fusion, was also noted only in ttntv-A but not in ttntv-Z models. Moreover, we found that genetic manipulation of ulk1a restored autophagy flux and rescued cardiac dysfunction in ttntv-A animals. Together, our findings presented adult zebrafish as an in vivo animal model for studying AH in TTNtv DCM, demonstrated TTN loss of function is sufficient to trigger ttntv DCM in zebrafish, and uncovered ulk1a as a potential therapeutic target gene for TTNtv DCM.

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.

The TTN p. Tyr4418Ter mutation causes cardiomyopathy in human and mice

OBJECTIVE

To generate a mouse model carrying TTNtv Y4370* simulating the newly discovered human heterozygous nonsense TTNtv c.13254T>G (p.Tyr4418Ter) to supplement and improve the functional evidence of pathogenic mutation TTNtv c.13254T>G on the pathogenic type of dilated cardiomyopathy.

METHODS

We generated 4 mice carrying TTNtv p. Y4370* through CRISPR/Cas-mediated genome engineering. Monthly serological detection, bimonthly echocardiography, and histology evaluation were carried out to observe and compare alterations of cardiac structure and function between 4 TTN+/- mice and 4 wild-type (WT) mice.

RESULTS

For the two-month-old TTN+/- mice, serum glutamic-oxalacetic transaminase (AST), lactic dehydrogenase (LDH), and creatine kinase (CK) were significantly increased, the diastolic Left Ventricular Systolic Anterior Wall (LVAW), and the LV mass markedly rose, with the left ventricular volume displaying an increasing trend and Ejection Fraction (EF) and Fractional Shortening (FS) showing a decreasing trend. Besides, the histological evaluation showed that cardiac fibrosis level and positive rate of cardiac mast cell of TTN+/- mice were obviously increased compared with WT mice.

CONCLUSIONS

TTNtv Y4370* could lead to cardiac structure and function alterations in mice, supplementing the evidence of TTNtv c.13254T>G pathogenicity in human.

Proteolytic degradation of atrial sarcomere proteins underlies contractile defects in atrial fibrillation

Aims

Atrial fibrillation (AFib) is the most common cardiac rhythm disturbance. Treatment of AFib involves restoration of the atrial electrical rhythm. Following rhythm restoration, a period of depressed mechanical function known as atrial stunning occurs that involves decreased blood flow velocity and reduced atrial contractility. This suggests that defects in contractility occur in AFib and are revealed upon restoration of rhythm. The aim of this project is to define the contractile remodeling that occurs in AFib.

Methods and Results

To assess contractile function, we used a canine atrial tachypacing model of induced AFib. Mass spectrometry analysis showed dysregulation of contractile proteins in samples from AFib compared to sinus rhythm atria. Atrial cardiomyocytes showed reduced force of contraction in skinned single cardiomyocyte calcium-force studies. There were no significant differences in myosin heavy chain isoform expression. Resting tension is decreased in the AFib samples correlating with reduced full-length titin in the sarcomere. We measured degradation of other myofilament proteins including cMyBP-C, actinin, and cTnI, showing significant degradation in the AFib samples compared to sinus rhythm atria. Many of the protein degradation products appeared as discrete cleavage products that are generated by calpain proteolysis. We assessed calpain activity and found it to be significantly increased. Skinned cardiomyocytes from AFib atria showed decreased troponin I phosphorylation, consistent with the increased calcium sensitivity that was found within these cardiomyocytes.

Conclusions

With these results it can be concluded that AFib causes alterations in contraction that can be explained by both molecular changes occurring in myofilament proteins and overall myofilament protein degradation. These results provide an understanding of the contractile remodeling that occurs in AFib and provides insight into the molecular explanation for atrial stunning and the increased risk of atrial thrombus and stroke in AFib.

[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.

Use of animal models to understand titin physiology and pathology

In recent years, increasing attention has been paid to titin (TTN) and its mutations. Heterozygous TTN truncating variants (TTNtv) increase the risk of a cardiomyopathy. At the same time, TTNtv and few missense variants have been identified in patients with mainly recessive skeletal muscle diseases. The pathogenic mechanisms underlying titin‐related diseases are still partly unknown. Similarly, the titin mechanical and functional role in the muscle contraction are far from being exhaustively clarified. In the last few years, several animal models carrying variants in the titin gene have been developed and characterized to study the structural and mechanical properties of specific titin domains or to mimic patients' mutations. This review describes the main animal models so far characterized, including eight mice models and three fish models (Medaka and Zebrafish) and discusses the useful insights provided by a thorough characterization of the cell‐, tissue‐ and organism‐phenotypes in these models.

Genetic Insights into Primary Restrictive Cardiomyopathy

Restrictive cardiomyopathy is a rare cardiac disease causing severe diastolic dysfunction, ventricular stiffness and dilated atria. In consequence, it induces heart failure often with preserved ejection fraction and is associated with a high mortality. Since it is a poor clinical prognosis, patients with restrictive cardiomyopathy frequently require heart transplantation. Genetic as well as non-genetic factors contribute to restrictive cardiomyopathy and a significant portion of cases are of unknown etiology. However, the genetic forms of restrictive cardiomyopathy and the involved molecular pathomechanisms are only partially understood. In this review, we summarize the current knowledge about primary genetic restrictive cardiomyopathy and describe its genetic landscape, which might be of interest for geneticists as well as for cardiologists.

Developmental Perspectives on Arterial Fate Specification

Blood vessel acquisition of arterial or venous fate is an adaptive phenomenon in response to increasing blood circulation during vascular morphogenesis. The past two decades of effort in this field led to development of a widely accepted paradigm of molecular regulators centering on VEGF and Notch signaling. More recent findings focused on shear stress-induced cell cycle arrest as a prerequisite for arterial specification substantially modify this traditional understanding. This review aims to summarize key molecular mechanisms that work in concert to drive the acquisition of arterial fate in two distinct developmental settings of vascular morphogenesis: de novo vasculogenesis of the dorsal aorta and postnatal retinal angiogenesis. We will also discuss the questions and conceptual controversies that potentially point to novel directions of investigation and possible clinical relevance.

Mechanisms of TTNtv-Related Dilated Cardiomyopathy: Insights from Zebrafish Models

Dilated cardiomyopathy (DCM) is a common heart muscle disorder characterized by ventricular dilation and contractile dysfunction that is associated with significant morbidity and mortality. New insights into disease mechanisms and strategies for treatment and prevention are urgently needed. Truncating variants in the TTN gene, which encodes the giant sarcomeric protein titin (TTNtv), are the most common genetic cause of DCM, but exactly how TTNtv promote cardiomyocyte dysfunction is not known. Although rodent models have been widely used to investigate titin biology, they have had limited utility for TTNtv-related DCM. In recent years, zebrafish (Danio rerio) have emerged as a powerful alternative model system for studying titin function in the healthy and diseased heart. Optically transparent embryonic zebrafish models have demonstrated key roles of titin in sarcomere assembly and cardiac development. The increasing availability of sophisticated imaging tools for assessment of heart function in adult zebrafish has revolutionized the field and opened new opportunities for modelling human genetic disorders. Genetically modified zebrafish that carry a human A-band TTNtv have now been generated and shown to spontaneously develop DCM with age. This zebrafish model will be a valuable resource for elucidating the phenotype modifying effects of genetic and environmental factors, and for exploring new drug therapies.

Genetic Animal Models for Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy has been clinically defined since the 1980s and causes right or biventricular cardiomyopathy associated with ventricular arrhythmia. Although it is a rare cardiac disease, it is responsible for a significant proportion of sudden cardiac deaths, especially in athletes. The majority of patients with arrhythmogenic cardiomyopathy carry one or more genetic variants in desmosomal genes. In the 1990s, several knockout mouse models of genes encoding for desmosomal proteins involved in cell-cell adhesion revealed for the first time embryonic lethality due to cardiac defects. Influenced by these initial discoveries in mice, arrhythmogenic cardiomyopathy received an increasing interest in human cardiovascular genetics, leading to the discovery of mutations initially in desmosomal genes and later on in more than 25 different genes. Of note, even in the clinic, routine genetic diagnostics are important for risk prediction of patients and their relatives with arrhythmogenic cardiomyopathy. Based on improvements in genetic animal engineering, different transgenic, knock-in, or cardiac-specific knockout animal models for desmosomal and nondesmosomal proteins have been generated, leading to important discoveries in this field. Here, we present an overview about the existing animal models of arrhythmogenic cardiomyopathy with a focus on the underlying pathomechanism and its importance for understanding of this disease. Prospectively, novel mechanistic insights gained from the whole animal, organ, tissue, cellular, and molecular levels will lead to the development of efficient personalized therapies for treatment of arrhythmogenic cardiomyopathy.

N2A Titin: Signaling Hub and Mechanical Switch in Skeletal Muscle

Since its belated discovery, our understanding of the giant protein titin has grown exponentially from its humble beginning as a sarcomeric scaffold to recent recognition of its critical mechanical and signaling functions in active muscle. One uniquely useful model to unravel titin’s functions, muscular dystrophy with myositis (mdm), arose spontaneously in mice as a transposon-like LINE repeat insertion that results in a small deletion in the N2A region of titin. This small deletion profoundly affects hypertrophic signaling and muscle mechanics, thereby providing insights into the function of this specific region and the consequences of its dysfunction. The impact of this mutation is profound, affecting diverse aspects of the phenotype including muscle mechanics, developmental hypertrophy, and thermoregulation. In this review, we explore accumulating evidence that points to the N2A region of titin as a dynamic “switch” that is critical for both mechanical and signaling functions in skeletal muscle. Calcium-dependent binding of N2A titin to actin filaments triggers a cascade of changes in titin that affect mechanical properties such as elastic energy storage and return, as well as hypertrophic signaling. The mdm phenotype also points to the existence of as yet unidentified signaling pathways for muscle hypertrophy and thermoregulation, likely involving titin’s PEVK region as well as the N2A signalosome.

Decreased atrioventricular plane displacement after acute myocardial infarction yields a concomitant decrease in stroke volume

Acute myocardial infarction (AMI) can progress to heart failure, which has a poor prognosis. Normally, 60% of stroke volume (SV) is attributed to the longitudinal ventricular shortening and lengthening evident in the atrioventricular plane displacement (AVPD) during the cardiac cycle, but there is no information on how the relationship changes between SV and AVPD before and after AMI. Therefore, the aim of this study was to determine how SV depends on AVPD before and after AMI in two swine models. Serial cardiac magnetic resonance imaging was carried out before and 1–2 h after AMI in a microembolization model (n = 12) and an ischemia-reperfusion model (n = 14). A subset of pigs (n = 7) were additionally imaged at 24 h and at 7 days. Cine and late gadolinium enhancement images were analyzed for cardiac function, AVPD measurements and infarct size estimation, respectively. AVPD decreased (P < 0.05) in all myocardial regions after AMI, with a concomitant SV decrease (P < 0.001). The ischemia-reperfusion model affected SV to a higher degree and had a larger AVPD decrease than the microembolization model (−29 ± 14% vs. −15 ± 18%; P < 0.05). Wall thickening decreased in infarcted areas (P < 0.001), and A-wave AVPD remained unchanged (P = 0.93) whereas E-wave AVPD decreased (P < 0.001) after AMI. We conclude that AVPD is coupled to SV independent of infarct type but likely to a greater degree in ischemia-reperfusion infarcts compared with microembolization infarcts. AMI reduces diastolic early filling AVPD but not AVPD from atrial contraction. These findings shed light on the physiological significance of atrioventricular plane motion when assessing acute and subacute myocardial infarction.

NEW & NOTEWORTHY The link between cardiac longitudinal motion, measured as atrioventricular plane displacement (AVPD), and stroke volume (SV) is investigated in swine after acute myocardial infarction (AMI). This cardiac magnetic resonance study demonstrates a close coupling between AVPD and SV before and after AMI in an experimental setting and demonstrates that this connection is present in ischemia-reperfusion and microembolization infarcts, acutely and during the first week. Furthermore, AVPD is equally and persistently depressed in infarcted and remote myocardium after AMI.

Targeted Next Generation Sequencing for Genetic Mutations of Dilated Cardiomyopathy

BACKGROUND

Approximately one-third of cases of dilated cardiomyopathy (DCM) are caused by genetic mutations. With new sequencing technologies, numerous variants have been associated with this inherited cardiomyopathy, however the prevalence and genotype-phenotype correlations in different ethnic cohorts remain unclear. This study aimed to investigate the variants in Chinese DCM patients and correlate them with clinical presentations and prognosis.

METHODS AND RESULTS

From September 2013 to December 2016, 70 index patients underwent DNA sequencing for 12 common disease-causing genes with next generation sequencing. Using a bioinformatics filtering process, 12 rare truncating variants (7 nonsense variants, 4 frameshift variants, and 1 splice site variant) and 29 rare missense variants were identified. Of these, 3 patients were double heterozygotes and 10 patients were compound heterozygotes. Overall, 47.1% (33/70) of the index patients had the seputatively pathogenic variants. The majority (33/41, 80.4%) of these variants were located in titin (TTN). More than 80% of the TTN variants (27/33, 81.8%) were distributed in the A band region of the sarcomere. Patients carrying these variants did not have a different phenotype in disease severity, clinical outcome and reversibility of ventricular function compared with non-carriers.

CONCLUSIONS

Several new rare variants were identified in a Chinese population in this study, indicating that there are ethnic differences in genetic mutations in DCM patients. TTN remains the major disease-causing gene. Our results could be a reference for future genetic tests in Chinese populations. No specific genotype-phenotype correlations were found, however a prospective large cohort study may be needed to confirm our findings.

Interpreting Genetic Variants in Titin in Patients With Muscle Disorders

Importance

Mutations in the titin gene (TTN) cause a wide spectrum of genetic diseases. The interpretation of the numerous rare variants identified in TTN is a difficult challenge given its large size.

Objective

To identify genetic variants in titin in a cohort of patients with muscle disorders.

Design, Setting, and Participants

In this case series, 9 patients with titinopathy and 4 other patients with possibly disease-causing variants in TTN were identified. Titin mutations were detected through targeted resequencing performed on DNA from 504 patients with muscular dystrophy, congenital myopathy, or other skeletal muscle disorders. Patients were enrolled from 10 clinical centers in April 2012 to December 2013. All of them had not received a diagnosis after undergoing an extensive investigation, including Sanger sequencing of candidate genes. The data analysis was performed between September 2013 and January 2017. Sequencing data were analyzed using an internal custom bioinformatics pipeline.

Main Outcomes and Measures

The identification of novel mutations in the TTN gene and novel patients with titinopathy. We performed an evaluation of putative causative variants in the TTN gene, combining genetic, clinical, and imaging data with messenger RNA and/or protein studies.

Results

Of the 9 novel patients with titinopathy, 5 (55.5%) were men and the mean (SD) age at onset was 25 (15.8) years (range, 0-46 years). Of the 4 other patients (3 men and 1 woman) with possibly disease-causing TTN variants, 2 (50%) had a congenital myopathy and 2 (50%) had a slowly progressive distal myopathy with onset in the second decade. Most of the identified mutations were previously unreported. However, all the variants, even the already described mutations, require careful clinical and molecular evaluation of probands and relatives. Heterozygous truncating variants or unique missense changes are not sufficient to make a diagnosis of titinopathy.

Conclusions and Relevance

The interpretation of TTN variants often requires further analyses, including a comprehensive evaluation of the clinical phenotype (deep phenotyping) as well as messenger RNA and protein studies. We propose a specific workflow for the clinical interpretation of genetic findings in titin.

Sequence variants in muscle tissue-related genes may determine the severity of muscle contractures in cerebral palsy

Muscle contractures are a common complication to cerebral palsy (CP). The purpose of this study was to evaluate whether individuals with CP carry specific gene variants of important structural genes that might explain the severity of muscle contractures. Next-generation-sequencing (NGS) of 96 candidate genes associated with muscle structure and metabolism were analyzed in 43 individuals with CP (Gross Motor Function classification system [GMFCS] I, n=10; GMFCS II, n=14; GMFCS III, n=19) and four control participants. In silico analysis of the identified variants was performed. The variants were classified into four categories ranging from likely benign (VUS0) to highly likely functional effect (VUS3). All individuals with CP were classified and grouped according to their GMFCS level: Statistical comparisons were made between GMFCS groups. Kruskal-Wallis tests showed significantly more VUS2 variants in the genes COL4 (GMFCS I-III; 1, 1, 5, respectively [p < .04]), COL5 (GMFCS I-III; 1, 1, 5 [p < .04]), COL6 (GMFCS I-III; 0, 4, 7 [p < .003]), and COL9 (GMFCS I-III; 1, 1, 5 [p < .04]), in individuals with CP within GMFCS Level III when compared to the other GMFCS levels. Furthermore, significantly more VUS3 variants in COL6 (GMFCS I-III; 0, 5, 2 [p < .01]) and COL7 (GMFCS I-III; 0, 3, 0 [p < .04]) were identified in the GMFCS II level when compared to the other GMFCS levels. The present results highlight several candidate gene variants in different collagen types with likely functional effects in individuals with CP.

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.

Identification of a Novel Titin Variant Underlying Myocardial Involvement in Neurofibromatosis Type 1

Because of the rare co-occurrence, it remains a question whether cardiomyopathy is a true association of neurofibromatosis type 1. A boy with café-au-lait spots manifested restrictive cardiomyopathy. Whole exome sequencing confirmed the genetic diagnosis of neurofibromatosis and further identified a novel titin (TTN) missense variant. The significance of the variant is supported by its de novo origin, in silico predictions, and evolutionary conservation. Modern genetics raises an intriguing explanation for the unexpected phenotype and adds to the evolving role of TTN variants in cardiomyopathy.

Calcium increases titin N2A binding to F-actin and regulated thin filaments

Mutations in titin are responsible for many cardiac and muscle diseases, yet the underlying mechanisms remain largely unexplained. Numerous studies have established roles for titin in muscle function, and Ca²⁺-dependent interactions between titin and actin have been suggested to play a role in muscle contraction. The present study used co-sedimentation assays, dynamic force spectroscopy (DFS), and in vitro motility (IVM) assays to determine whether the N2A region of titin, overlooked in previous studies, interacts with actin in the presence of Ca²⁺. Co-sedimentation demonstrated that N2A – F-actin binding increases with increasing protein and Ca²⁺ concentration, DFS demonstrated increased rupture forces and decreased k_off in the presence of Ca²⁺, and IVM demonstrated a Ca²⁺-dependent reduction in motility of F-actin and reconstituted thin filaments in the presence of N2A. These results indicate that Ca²⁺ increases the strength and stability of N2A – actin interactions, supporting the hypothesis that titin plays a regulatory role in muscle contraction. The results further support a model in which N2A – actin binding in active muscle increases titin stiffness, and that impairment of this mechanism contributes to the phenotype in muscular dystrophy with myositis. Future studies are required to determine whether titin – actin binding occurs in skeletal muscle sarcomeres in vivo.

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.

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.

Genetic Pathogenesis of Hypertrophic and Dilated Cardiomyopathy

Sarcomere cardiomyopathies are genetic diseases that perturb contractile function and lead to hypertrophic or dilated myocardial remodeling. Identification of preclinical mutation carriers has yielded insights into the earliest biomechanical defects that link pathogenic variants to cardiac dysfunction. Understanding this early molecular pathophysiology can illuminate modifiable pathways to reduce the emergence of overt cardiomyopathy and curb adverse outcomes. Here, the authors review current understandings of how human hypertrophic cardiomyopathy- and hypertrophic dilated cardiomyopathy-linked mutations disrupt the normal structure and function of the sarcomere.

Biophysical Derangements in Genetic Cardiomyopathies

This article focuses on three "bins" that comprise sets of biophysical derangements elicited by cardiomyopathy-associated mutations in the myofilament. Current therapies focus on symptom palliation and do not address the disease at its core. We and others have proposed that a more nuanced classification could lead to direct interventions based on early dysregulation changing the trajectory of disease progression in the preclinical cohort. Continued research is necessary to address the complexity of cardiomyopathic progression and develop efficacious therapeutics.

The complexity of titin splicing pattern in human adult skeletal muscles

BACKGROUND

Mutations in the titin gene (TTN) cause a large spectrum of diseases affecting skeletal and/or cardiac muscle. TTN includes 363 coding exons, a repeated region with a high degree of complexity, isoform-specific elements, and metatranscript-only exons thought to be expressed only during fetal development. Although three main classes of isoforms have been described so far, alternative splicing events (ASEs) in different tissues or in different developmental and physiological states have been reported.

METHODS

To achieve a comprehensive view of titin ASEs in adult human skeletal muscles, we performed a RNA-Sequencing experiment on 42 human biopsies collected from 12 anatomically different skeletal muscles of 11 individuals without any skeletal-muscle disorders.

RESULTS

We confirmed that the skeletal muscle N2A isoforms are highly prevalent, but we found an elevated number of alternative splicing events, some at a very high level. These include previously unknown exon skipping events and alternative 5' and 3' splice sites. Our data suggests the partial inclusion in the TTN transcript of some metatranscript-only exons and the partial exclusion of canonical N2A exons.

CONCLUSIONS

This study provides an extensive picture of the complex TTN splicing pattern in human adult skeletal muscle, which is crucial for a proper clinical interpretation of TTN variants.

Increasing Role of Titin Mutations in Neuromuscular Disorders

The TTN gene with 363 coding exons encodes titin, a giant muscle protein spanning from the Z-disk to the M-band within the sarcomere. Mutations in the TTN gene have been associated with different genetic disorders, including hypertrophic and dilated cardiomyopathy and several skeletal muscle diseases.

Before the introduction of next generation sequencing (NGS) methods, the molecular analysis of TTN has been laborious, expensive and not widely used, resulting in a limited number of mutations identified. Recent studies however, based on the use of NGS strategies, give evidence of an increasing number of rare and unique TTN variants. The interpretation of these rare variants of uncertain significance (VOUS) represents a challenge for clinicians and researchers.

The main aim of this review is to describe the wide spectrum of muscle diseases caused by TTN mutations so far determined, summarizing the molecular findings as well as the clinical data, and to highlight the importance of joint efforts to respond to the challenges arising from the use of NGS. An international collaboration through a clinical and research consortium and the development of a single accessible database listing variants in the TTN gene, identified by high throughput approaches, may be the key to a better assessment of titinopathies and to systematic genotype– phenotype correlation studies.

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.

Cardiomyocytes of the Heart and Pulmonary Veins: Novel Contributors to Asthma?

Recent genome-wide association studies have implicated both cardiac and pulmonary vein-related genes in the pathogenesis of asthma. Since cardiac cells are not present in lung airways or viewed to affect the immune system, interpretation of these findings in the context of more well-established contributors to asthma has remained challenging. However, cardiomyocytes are present in the lung, specifically along pulmonary veins, and recent murine models suggest that cardiac cells lining the pulmonary veins may contribute to allergic airway disease. Notably, the cardiac cell-junction protein αT-catenin (αT-cat, CTNNA3), which is implicated in occupational and steroid-resistant asthma by clinical genetic data, appears to play an important role in regulating inflammation around the cardiac cells of pulmonary veins. Beyond the potential contribution of pulmonary veins, clinical data directly examining cardiac function through echocardiography have found strong associations between asthmatic phenotypes and the mechanical properties of the heart. Together, these data suggest that targeting the function of cardiac cells in the pulmonary veins and/or heart may allow for novel and potentially efficacious therapies for asthma, particularly in challenging cases of steroid-resistant asthma.

Abnormal arterial-venous fusions and fate specification in mouse embryos lacking blood flow

The functions of blood flow in the morphogenesis of mammalian arteries and veins are not well understood. We examined the development of the dorsal aorta (DA) and the cardinal vein (CV) in Ncx1 -/- mutants, which lack blood flow due to a deficiency in a sodium calcium ion exchanger expressed specifically in the heart. The mutant DA and CV were abnormally connected. The endothelium of the Ncx1 -/- mutant DA lacked normal expression of the arterial markers ephrin-B2 and Connexin-40. Notch1 activation, known to promote arterial specification, was decreased in mutant DA endothelial cells (ECs), which ectopically expressed the venous marker Coup-TFII. These findings suggest that flow has essential functions in the DA by promoting arterial and suppressing venous marker expression. In contrast, flow plays a lesser role in the CV, because expression of arterial-venous markers in CV ECs was not as dramatically affected in Ncx1 -/- mutants. We propose a molecular mechanism by which blood flow mediates DA and CV morphogenesis, by regulating arterial-venous specification of DA ECs to ensure proper separation of the developing DA and CV.

Titin-Truncating Variants Increase the Risk of Cardiovascular Death in Patients With Hypertrophic Cardiomyopathy

BACKGROUND

Titin-truncating variants (TTNtv) have been detected in a variety of cardiomyopathies and represent the most common cause of dilated cardiomyopathy. However, their significance in hypertrophic cardiomyopathy (HCM) is still unclear.

METHODS

The titin gene (TTN) was sequenced for truncating variants in a cohort of 529 Chinese patients with HCM and 307 healthy controls. Baseline and follow-up clinical data (for 4.7 ± 3.2 years) from these patients were obtained.

RESULTS

We identified 13 and 8 TTNtv in patients with HCM (13 of 529 [2.5%]) and controls (8 of 307 [2.6%]), respectively. The prevalence of TTNtv in patients with HCM and in healthy controls was comparable (P = 0.895). There were no significant differences in baseline characteristics between patients with and those without TTNtv. However, during follow-up, patients with TTNtv (3 of 13 [23.1%]) were more likely to experience cardiovascular death compared with those without TTNtv (39 of 516 [7.6%]) [adjusted hazard ratio, 6.88; 95% confidence interval, 2.04-23.20; P = 0.002).

CONCLUSIONS

Our study suggests that TTNtv might be a genetic modifier of HCM and confer an increased risk for cardiovascular death.

Nuances of electrophoresis study of titin/connectin

Almost 40 years has passed since the discovery of giant elastic protein titin (also known as connectin) of striated and smooth muscles using gel electrophoresis. Sodium dodecyl sulfate polyacrylamide gel electrophoresis is a major technique for studying the isoform composition and content of titin. This review provides historical insights into the technical aspects of the electrophoresis methods used to identify titin and its isoforms. We particularly focus on the nuances of the technique that improve the preservation of its primary structure so that its high molecular weight isoforms can be visualized.

Potential prognostic biomarkers of cardiovascular disease in fetal macrosomia: the impact of gestational diabetes

OBJECTIVE

Fetal macrosomia is associated with cardiac hypertrophy and increased cardiovascular risk. Cardiac biomarkers may play diagnostic/prognostic role in cardiovascular disease. We tested whether cardiac biomarkers are differentially expressed in cord blood samples of full-term singleton large-for-gestational-age (LGA), as compared to appropriate-for-gestational-age (AGA) pregnancies.

METHODS

Cardiotrophin-1 (CT-1), Titin, pentraxin (PTX-3) and soluble CD36 (sCD36) concentrations were determined in 80 cord blood samples from a) LGA pregnancies due to maternal diabetes (n = 8), overweight/obese (n = 11), excessive weight gain (n = 7), without specific pathology (n = 14), b) AGA normal pregnancies (controls, n = 40). Neonates were classified as LGA or AGA based on customized birth weight (BW) standards.

RESULTS

CT-1 and Titin concentrations were higher in LGA than AGA pregnancies (p < .001 and p = .023, respectively). A subgroup analysis (in the LGA group) showed increased CT-1 concentrations only in diabetic pregnancies. PTX-3 and sCD36 concentrations were similar in LGA and AGA fetuses. In the LGA group, PTX-3 concentrations positively correlated with birth-weight (r = .416, p = .008) and respective sCD36 concentrations (r = .443, p = .004).

CONCLUSION

Higher Titin concentrations in LGAs possibly represent a candidate molecular mechanism underlying the association between fetal macrosomia and cardiomyocyte/diastolic dysfunction. CT-1 is up-regulated only in LGAs exposed to maternal diabetes. PTX-3 and sCD36 are probably not affected by excessive fetal growth.

Neonatal complex arrhythmias possibly related to a TTN mutation

We describe a neonate born with complex arrhythmias that included concurrent atrial and ventricular tachycardias. Genetic testing demonstrated a mutation in the TTN gene, which codes for titin, a large protein found in striated muscle sarcomeres. The complex arrhythmias were successfully treated with amiodarone and flecainide. The patient remains asymptomatic with normal biventricular function. We speculate that the complex arrhythmias and TTN gene mutation may be related.

Combination of Whole Genome Sequencing, Linkage, and Functional Studies Implicates a Missense Mutation in Titin as a Cause of Autosomal Dominant Cardiomyopathy With Features of Left Ventricular Noncompaction

Supplemental Digital Content is available in the text.

Background—

High throughput next-generation sequencing techniques have made whole genome sequencing accessible in clinical practice; however, the abundance of variation in the human genomes makes the identification of a disease-causing mutation on a background of benign rare variants challenging.

Methods and Results—

Here we combine whole genome sequencing with linkage analysis in a 3-generation family affected by cardiomyopathy with features of autosomal dominant left ventricular noncompaction cardiomyopathy. A missense mutation in the giant protein titin is the only plausible disease-causing variant that segregates with disease among the 7 surviving affected individuals, with interrogation of the entire genome excluding other potential causes. This A178D missense mutation, affecting a conserved residue in the second immunoglobulin-like domain of titin, was introduced in a bacterially expressed recombinant protein fragment and biophysically characterized in comparison to its wild-type counterpart. Multiple experiments, including size exclusion chromatography, small-angle x ray scattering, and circular dichroism spectroscopy suggest partial unfolding and domain destabilization in the presence of the mutation. Moreover, binding experiments in mammalian cells show that the mutation markedly impairs binding to the titin ligand telethonin.

Conclusions—

Here we present genetic and functional evidence implicating the novel A178D missense mutation in titin as the cause of a highly penetrant familial cardiomyopathy with features of left ventricular noncompaction. This expands the spectrum of titin’s roles in cardiomyopathies. It furthermore highlights that rare titin missense variants, currently often ignored or left uninterpreted, should be considered to be relevant for cardiomyopathies and can be identified by the approach presented here.

CARP interacts with titin at a unique helical N2A sequence and at the domain Ig81 to form a structured complex

The cardiac ankyrin repeat protein (CARP) is up-regulated in the myocardium during cardiovascular disease and in response to mechanical or toxic stress. Stress-induced CARP interacts with the N2A spring region of the titin filament to modulate muscle compliance. We characterize the interaction between CARP and titin-N2A and show that the binding site in titin spans the dual domain UN2A-Ig81. We find that the unique sequence UN2A is not structurally disordered, but that it has a stable, elongated α-helical fold that possibly acts as a constant force spring. Our findings portray CARP/titin-N2A as a structured node and help to rationalize the molecular basis of CARP mechanosensing in the sarcomeric I-band.

The Role of Genetic Testing in the Identification of Young Athletes with Inherited Primitive Cardiac Disorders at Risk of Exercise Sudden Death

Although relatively rare, inherited primitive cardiac disorders (IPCDs) in athletes have a deep social impact since they often present as sudden cardiac death (SCD) of young and otherwise healthy persons. The diagnosis of these conditions is likely underestimated due to the lack of shared clinical criteria and to the existence of several borderline clinical pictures. We will focus on the clinical and molecular diagnosis of the most common IPCDs, namely hypertrophic cardiomyopathies, long QT syndrome, arrhythmogenic right ventricular cardiomyopathy, and left ventricular non-compaction. Collectively, these conditions account for the majority of SCD episodes and/or cardiologic clinical problems in athletes. In addition to the clinical and instrumental tools for the diagnosis of IPCD, the viral technological advances in genetic testing have facilitated the molecular confirmation of these conditions. However, genetic testing presents several issues: the limited sensitivity (globally, around 50%), the low prognostic predictive value, the probability to find pathogenic variants in different genes in the same patient, and the risk of non-interpretable results. In this review, we will analyze the pros and cons of the different clinical approaches for the presymptomatic identification, the diagnosis and management of IPCD athletes, and we will discuss the indications to the genetic testing for patients and their relatives, particularly focusing on the most complex scenarios, such as presymptomatic tests, uncertain results, and unexpected findings.

Whole-Exome Sequencing Identifies Two Novel TTN Mutations in Chinese Families with Dilated Cardiomyopathy

OBJECTIVES

Dilated cardiomyopathy (DCM) is a leading cause of sudden cardiac death. So far, only 127 mutations of Titin(TTN) have been reported in patients with different phenotypes such as isolated cardiomyopathies, purely skeletal muscle phenotypes or complex overlapping disorders of muscles.

METHODS

We applied whole-exome sequencing (WES) to investigate cardiomyopathy patients and a cardiomyopathy-related gene-filtering strategy was used to analyze the disease-causing mutations. Sanger sequencing was applied to confirm the mutation cosegregation in the affected families.

RESULTS

A nonsense mutation (c.12325C>T/p.R4109X) and a missense mutation (c.17755G>C/p.G5919R) of TTN were identified in 2 Chinese DCM families, respectively. Both mutations were cosegregated in all affected members of both families. The nonsense mutation is predicted to result in a truncated TTN protein and the missense mutation leads to a substitution of glycine by arginine. Both variants may cause the structure changes of titin protein.

CONCLUSIONS

We employed WES to detect the mutations of DCM patients and identified 2 novel mutations. Our study expands the spectrum of TTN mutations and offers accurate genetic testing information for DCM patients who are still clinically negative.

A Review of the Giant Protein Titin in Clinical Molecular Diagnostics of Cardiomyopathies

Titin (TTN) is known as the largest sarcomeric protein that resides within the heart muscle. Due to alternative splicing of TTN, the heart expresses two major isoforms (N2B and N2BA) that incorporate four distinct regions termed the Z-line, I-band, A-band, and M-line. Next-generation sequencing allows a large number of genes to be sequenced simultaneously and provides the opportunity to easily analyze giant genes such as TTN. Mutations in the TTN gene can cause cardiomyopathies, in particular dilated cardiomyopathy (DCM). DCM is the most common form of cardiomyopathy, and it is characterized by systolic dysfunction and dilation of the left ventricle. TTN truncating variants have been described as the most common cause of DCM, while the real impact of TTN missense variants in the pathogenesis of DCM is still unclear. In a recent population screening study, rare missense variants potentially pathogenic based on bioinformatic filtering represented only 12.6% of the several hundred rare TTN missense variants found, suggesting that missense variants are very common in TTN and are frequently benign. The aim of this review is to understand the clinical role of TTN mutations in DCM and in other cardiomyopathies. Whereas TTN truncations are common in DCM, there is evidence that TTN truncations are rare in the hypertrophic cardiomyopathy (HCM) phenotype. Furthermore, TTN mutations can also cause arrhythmogenic right ventricular cardiomyopathy (ARVC) with distinct clinical features and outcomes. Finally, the identification of a rare TTN missense variant cosegregating with the restrictive cardiomyopathy (RCM) phenotype suggests that TTN is a novel disease-causing gene in this disease. Clinical diagnostic testing is currently able to analyze over 100 cardiomyopathy genes, including TTN; however, the size and presence of extensive genetic variation in TTN presents clinical challenges in determining significant disease-causing mutations. This review discusses the current knowledge of TTN genetic variations in cardiomyopathies and the impact of the diagnosis of TTN pathogenic mutations in the clinical setting.