A novel alpha-tropomyosin mutation associates with dilated and non-compaction cardiomyopathy and diminishes actin binding

Identification of a novel likely pathogenic TPM1 variant linked to hypertrophic cardiomyopathy in a family with sudden cardiac death

AIMS

Hypertrophic cardiomyopathy (HCM) is an autosomal dominant genetic cardiac disorder characterized by unexplained left ventricular hypertrophy. It can cause a wide spectrum of clinical manifestations, ranging from asymptomatic to heart failure and sudden cardiac death (SCD). Approximately half of HCM cases are caused by variants in sarcomeric proteins, including α-tropomyosin (TPM1). In this study, we aimed to characterize the clinical and molecular phenotype of HCM in an Iranian pedigree with SCD.

METHODS AND RESULTS

The proband and available family members underwent comprehensive clinical evaluations, including echocardiography, cardiac magnetic resonance (CMR) imaging and electrocardiography (ECG). Whole-exome sequencing (WES) was performed in all available family members to identify the causal variant, which was validated, and segregation analysis was conducted via Sanger sequencing. WES identified a novel missense variant, c.761A>G:p.D254G (NM_001018005.2), in the TPM1 gene, in the proband, his father and one of his sisters. Bioinformatic analysis predicted it to be likely pathogenic. Clinical features in affected individuals were consistent with HCM.

CONCLUSIONS

The identification of a novel TPM1 variant in a family with HCM and SCD underscores the critical role of genetic screening in at-risk families. Early detection of pathogenic variants can facilitate timely intervention and management, potentially reducing the risk of SCD in individuals with HCM.

Nucleotide sequence variant of the TPM1 gene in a family with different phenotypes of left ventricular non-compaction

Left ventricular non-compaction (LVNC) is a rare, genetically and phenotypically heterogeneous disease, which is often accompanied by diagnostic difficulties.

Aim. To demonstrate several generations of a family with LVNC with various clinical and phenotypic manifestations of the disease (dilated and isolated types of LVNC) with an identified rs397516387 variant of the TPM1 gene.

Material and methods. Based on the multicenter registry "Myocardial Non-compaction", a family with a familial form of LVNC was selected. Next generation sequencing (NGS) was performed on an Ion S5 system (Thermo Fisher Scientific, USA) using Ampliseq technology. Variant was verified using Sanger sequencing on an Applied Biosystem 3500 Genetic Analyzer (Thermo Fisher Scientific, USA). For clinical interpretation, variants in the genes associated with LVNC with a minor allele frequency <0,1% were selected in the gnomAD database (v2.1.1). Results. Variant rs397516387 was found in 5 family members, including the proband. Further examination revealed LVNC in 2 additional family members. The proband and the proband’s uncle had a dilated type of LVNC, and the proband’s mother had an isolated type.

Conclusion. The paper presents several generations of a family with different phenotypic manifestations of LVNC and rs397516387 variant in the TPM1 gene. The beginning of genetic screening from a proband, a thorough collection of a family history and further detailed genetic screening of relatives led to the identification of rs397516387 variant in 4 more family members, which in turn made it possible to conduct an additional examination to confirm the diagnosis and prescribe timely drug therapy.

Identification of a novel missense mutation in the TPM1 gene via exome sequencing in a Chinese family with dilated cardiomyopathy: A case report and literature review

RATIONALE

Dilated cardiomyopathy (DCM) is a cardiovascular disorder characterized by consecutive ventricular dilation and contractile dysfunction, often leading to congestive heart failure. DCM type 1Y (DCM1Y) is caused by a mutation in the TPM1 (tropomyosin 1) gene. To date, about thirty TPM1 gene mutations have been reported to be related to DCM1Y. However, mutational screening of the TPM1 gene is still far from being complete. Identification of TPM1 mutation is particularly important in the diagnosis of DCM1Y and will give more insights into the molecular pathogenesis of DCM1Y.

PATIENT CONCERNS

A Chinese Han family with DCM phenotypes was examined.

DIAGNOSIS

A novel missense mutation, c.340G > C in exon 3 of the TPM1 gene, was identified.

INTERVENTIONS

Next-generation sequencing (NGS) of DNA samples was performed to detect the gene mutation in the proband, which was confirmed by Sanger sequencing.

OUTCOMES

This novel heterozygous mutation results in the substitution of glutamic acid with glutamine (p.E114Q). Based on this finding and clinical manifestations, a final diagnosis of DCM1Y was made.

LESSONS

We present evidence that p.E114Q mutation represents a novel TPM1 mutation in a Chinese Han family with DCM. Our data expand the mutation spectrum of the TPM1 gene and may facilitate the clinical diagnosis of DCM1Y.

Cardiomyocyte Dysfunction in Inherited Cardiomyopathies

Inherited cardiomyopathies form a heterogenous group of disorders that affect the structure and function of the heart. Defects in the genes encoding sarcomeric proteins are associated with various perturbations that induce contractile dysfunction and promote disease development. In this review we aimed to outline the functional consequences of the major inherited cardiomyopathies in terms of myocardial contraction and kinetics, and to highlight the structural and functional alterations in some sarcomeric variants that have been demonstrated to be involved in the pathogenesis of the inherited cardiomyopathies. A particular focus was made on mutation-induced alterations in cardiomyocyte mechanics. Since no disease-specific treatments for familial cardiomyopathies exist, several novel agents have been developed to modulate sarcomere contractility. Understanding the molecular basis of the disease opens new avenues for the development of new therapies. Furthermore, the earlier the awareness of the genetic defect, the better the clinical prognostication would be for patients and the better the prevention of development of the disease.

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.

Reevaluating the Genetic Contribution of Monogenic Dilated Cardiomyopathy

BACKGROUND

Dilated cardiomyopathy (DCM) is genetically heterogeneous, with >100 purported disease genes tested in clinical laboratories. However, many genes were originally identified based on candidate-gene studies that did not adequately account for background population variation. Here we define the frequency of rare variation in 2538 patients with DCM across protein-coding regions of 56 commonly tested genes and compare this to both 912 confirmed healthy controls and a reference population of 60 706 individuals to identify clinically interpretable genes robustly associated with dominant monogenic DCM.

METHODS

We used the TruSight Cardio sequencing panel to evaluate the burden of rare variants in 56 putative DCM genes in 1040 patients with DCM and 912 healthy volunteers processed with identical sequencing and bioinformatics pipelines. We further aggregated data from 1498 patients with DCM sequenced in diagnostic laboratories and the Exome Aggregation Consortium database for replication and meta-analysis.

RESULTS

Truncating variants in TTN and DSP were associated with DCM in all comparisons. Variants in MYH7, LMNA, BAG3, TNNT2, TNNC1, PLN, ACTC1, NEXN, TPM1, and VCL were significantly enriched in specific patient subsets, with the last 2 genes potentially contributing primarily to early-onset forms of DCM. Overall, rare variants in these 12 genes potentially explained 17% of cases in the outpatient clinic cohort representing a broad range of adult patients with DCM and 26% of cases in the diagnostic referral cohort enriched in familial and early-onset DCM. Although the absence of a significant excess in other genes cannot preclude a limited role in disease, such genes have limited diagnostic value because novel variants will be uninterpretable and their diagnostic yield is minimal.

CONCLUSIONS

In the largest sequenced DCM cohort yet described, we observe robust disease association with 12 genes, highlighting their importance in DCM and translating into high interpretability in diagnostic testing. The other genes analyzed here will need to be rigorously evaluated in ongoing curation efforts to determine their validity as Mendelian DCM genes but have limited value in diagnostic testing in DCM at present. This data will contribute to community gene curation efforts and will reduce erroneous and inconclusive findings in diagnostic testing.

The Effect of Tropomyosin Mutations on Actin-Tropomyosin Binding: In Search of Lost Time

The initial binding of tropomyosin onto actin filaments and then its polymerization into continuous cables on the filament surface must be precisely tuned to overall thin-filament structure, function, and performance. Low-affinity interaction of tropomyosin with actin has to be sufficiently strong to localize the tropomyosin on actin, yet not so tight that regulatory movement on filaments is curtailed. Likewise, head-to-tail association of tropomyosin molecules must be favorable enough to promote tropomyosin cable formation but not so tenacious that polymerization precedes filament binding. Arguably, little molecular detail on early tropomyosin binding steps has been revealed since Wegner's seminal studies on filament assembly almost 40 years ago. Thus, interpretation of mutation-based actin-tropomyosin binding anomalies leading to cardiomyopathies cannot be described fully. In vitro, tropomyosin binding is masked by explosive tropomyosin polymerization once cable formation is initiated on actin filaments. In contrast, in silico analysis, characterizing molecular dynamics simulations of single wild-type and mutant tropomyosin molecules on F-actin, is not complicated by tropomyosin polymerization at all. In fact, molecular dynamics performed here demonstrates that a midpiece tropomyosin domain is essential for normal actin-tropomyosin interaction and that this interaction is strictly conserved in a number of tropomyosin mutant species. Elsewhere along these mutant molecules, twisting and bending corrupts the tropomyosin superhelices as they "lose their grip" on F-actin. We propose that residual interactions displayed by these mutant tropomyosin structures with actin mimic ones that occur in early stages of thin-filament generation, as if the mutants are recapitulating the assembly process but in reverse. We conclude therefore that an initial binding step in tropomyosin assembly onto actin involves interaction of the essential centrally located domain.

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.

Plasma microRNA profiling of children with idiopathic dilated cardiomyopathy

CONTEXT

Dilated cardiomyopathy (DCM) is the most common cardiomyopathy in children. MicroRNAs (miRNA) are small RNAs which have regulatory functions in many biological processes.

OBJECTIVE

We aimed to determine miRNA expression levels in plasma of children with DCM.

MATERIALS AND METHODS

Plasma expression levels of 379 miRNAs were compared between 23 DCM and 26 healthy children.

RESULTS

The expression levels of miR-618, miR-875-3p, miR-205, miR-194, miR-302a, miR-147, and miR-544 were found decreased. The expression levels of miR-518f and miR-454 were found increased in DCM patients.

DISCUSSION

miRNA level differences may provide the chance of using these miRNAs as new biomarkers.

Mechanistic heterogeneity in contractile properties of α-tropomyosin (TPM1) mutants associated with inherited cardiomyopathies

The most frequent known causes of primary cardiomyopathies are mutations in the genes encoding sarcomeric proteins. Among those are 30 single-residue mutations in TPM1, the gene encoding α-tropomyosin. We examined seven mutant tropomyosins, E62Q, D84N, I172T, L185R, S215L, D230N, and M281T, that were chosen based on their clinical severity and locations along the molecule. The goal of our study was to determine how the biochemical characteristics of each of these mutant proteins are altered, which in turn could provide a structural rationale for treatment of the cardiomyopathies they produce. Measurements of Ca(2+) sensitivity of human β-cardiac myosin ATPase activity are consistent with the hypothesis that hypertrophic cardiomyopathies are hypersensitive to Ca(2+) activation, and dilated cardiomyopathies are hyposensitive. We also report correlations between ATPase activity at maximum Ca(2+) concentrations and conformational changes in TnC measured using a fluorescent probe, which provide evidence that different substitutions perturb the structure of the regulatory complex in different ways. Moreover, we observed changes in protein stability and protein-protein interactions in these mutants. Our results suggest multiple mechanistic pathways to hypertrophic and dilated cardiomyopathies. Finally, we examined a computationally designed mutant, E181K, that is hypersensitive, confirming predictions derived from in silico structural analysis.

A study of tropomyosin's role in cardiac function and disease using thin-filament reconstituted myocardium

Tropomyosin (Tm) is the key regulatory component of the thin-filament and plays a central role in the cardiac muscle's cooperative activation mechanism. Many mutations of cardiac Tm are related to hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy (DCM), and left ventricular noncompaction (LVNC). Using the thin-filament extraction/reconstitution technique, we are able to incorporate various Tm mutants and protein isoforms into a muscle fiber environment to study their roles in Ca(2+) regulation, cross-bridge kinetics, and force generation. The thin-filament reconstitution technique poses several advantages compared to other in vitro and in vivo methods: (1) Tm mutants and isoforms are placed into the real muscle fiber environment to exhibit their effect on a level much higher than simple protein complexes; (2) only the primary and immediate effects of Tm mutants are studied in the thin-filament reconstituted myocardium; (3) lethal mutants of Tm can be studied without causing a problem; and (4) inexpensive. In transgenic models, various secondary effects (myocyte disarray, ECM fibrosis, altered protein phosphorylation levels, etc.) also affect the performance of the myocardium, making it very difficult to isolate the primary effect of the mutation. Our studies on Tm have demonstrated that: (1) Tm positively enhances the hydrophobic interaction between actin and myosin in the "closed state", which in turn enhances the isometric tension; (2) Tm's seven periodical repeats carry distinct functions, with the 3rd period being essential for the tension enhancement; (3) Tm mutants lead to HCM by impairing the relaxation on one hand, and lead to DCM by over inhibition of the AM interaction on the other hand. Ca(2+) sensitivity is affected by inorganic phosphate, ionic strength, and phosphorylation of constituent proteins; hence it may not be the primary cause of the pathogenesis. Here, we review our current knowledge regarding Tm's effect on the actomyosin interaction and the early molecular pathogenesis of Tm mutation related to HCM, DCM, and LVNC.