FLNC Gene Splice Mutations Cause Dilated Cardiomyopathy

Arrhythmic Risk Stratification of Carriers of Filamin C Truncating Variants

Importance

Filamin C truncating variants (FLNCtv) are a rare cause of cardiomyopathy with heterogeneous phenotypic presentations. Despite a high incidence of life-threatening ventricular arrhythmias and sudden cardiac death (SCD), reliable risk predictors to stratify carriers of FLNCtv are lacking.

Objective

To determine factors predictive of SCD/major ventricular arrhythmias (MVA) in carriers of FLNCtv.

Design, Setting, and Participants

This was an international, multicenter, retrospective cohort study conducted from February 2023 to June 2024. The Filamin C Registry Consortium included 19 referral centers for genetic cardiomyopathies worldwide. Participants included carriers of pathogenic or likely pathogenic FLNCtv. Phenotype negative was defined as the absence of any pathological findings detected by 12-lead electrocardiogram (ECG), Holter ECG monitoring, echocardiography, or cardiac magnetic resonance.

Exposures

Composite of SCD and MVA in carriers of FLNCtv.

Main Outcomes and Measures

The primary outcome was a composite of SCD and MVA, the last including aborted SCD, sustained ventricular tachycardia, and appropriate implantable cardioverter-defibrillator (ICD) interventions.

Results

Among 308 individuals (median [IQR] age, 45 [33-56] years; 160 male [52%]) with FLNCtv, 112 (36%) were probands, and 72 (23%) were phenotype negative. Median (IQR) left ventricular ejection fraction (LVEF) was 51% (38%-59%); 89 participants (34%) had LVEF less than 45%, and 50 (20%) had right ventricular dysfunction. During a median (IQR) follow-up of 34 (8-63) months, 57 individuals (19%) experienced SCD/MVA, with an annual incidence rate of 4 cases per 100 person-years (95% CI, 3-6). Incidence rates were higher in probands vs nonprobands and in phenotype-positive vs phenotype-negative individuals. A predictive model estimating SCD/MVA risk was derived from multivariable analysis, which included older age, male sex, previous syncope, nonsustained ventricular tachycardia, and LVEF with a time-dependent area under the curve (AUC) ranging between 0.76 (95% CI, 0.67-0.86) at 12 months and 0.78 (95% CI, 0.70-0.86) at 72 months. Notably, the association of LVEF with the SCD/MVA risk was not linear, showing significant lower risk for values of LVEF greater than 58%, and no increase for values less than 58%. Internal validation with bootstrapping confirmed good accuracy and calibration of the model. Results were consistent in subgroups analysis (ie, phenotype-positive carriers and phenotype-positive carriers without MVA at onset).

Conclusions and Relevance

Results suggest that the risk of SCD/MVA in phenotype-positive carriers of FLNCtv was high. A 5-variable predictive model derived from this cohort allows risk estimation and could support clinicians in the shared decision for prophylactic ICD implantation. External cohort validation is warranted.

G protein regulation by RGS proteins in the pathophysiology of dilated cardiomyopathy

Regulators of G protein signaling (RGS) proteins fine-tune signaling via heterotrimeric G proteins to maintain physiologic homeostasis in various organ systems of the human body including the brain, kidney, heart, and vasculature. Impaired regulation of G protein signaling by RGS proteins is implicated in the pathogenesis of several human diseases including various forms of cardiomyopathy such as hypertrophic cardiomyopathy and dilated cardiomyopathy (DCM). Both genetic and nongenetic changes that impinge on G protein signaling in cardiomyocytes are implicated in the etiology of DCM, and there is accumulating evidence that such genetic and nongenetic changes affecting G protein signaling in cell types other than cardiomyocytes could serve as a DCM trigger in humans. This review discusses and highlights mammalian RGS proteins and their roles in cardiac physiology and disease, with a specific focus on the current understanding of the etiology of DCM and the pathogenic roles of RGS proteins that are prominently expressed in the cardiovascular system. Growing evidence suggests that defects in G protein regulation by RGS proteins in the cardiovascular system likely contribute to cardiomyocyte structural damage and decreased contractile function that hallmark DCM. Further studies that enhance the understanding of the dynamics of G protein regulation by RGS proteins in several cell types in the myocardium and the vasculature are critical to gaining more insight into the etiology of DCM and heart failure, and to the identification of novel therapeutic targets.

Connecting intermediate phenotypes to disease using multi-omics in heart failure

Heart failure (HF) is one of the most common, complex, heterogeneous diseases in the world, with over 1-3% of the global population living with the condition. Progression of HF can be tracked via MRI measures of structural and functional changes to the heart, namely left ventricle (LV), including ejection fraction, mass, end-diastolic volume, and LV end-systolic volume. Moreover, while genome-wide association studies (GWAS) have been a useful tool to identify candidate variants involved in HF risk, they lack crucial tissue-specific and mechanistic information which can be gained from incorporating additional data modalities. This study addresses this gap by incorporating transcriptome-wide and proteome-wide association studies (TWAS and PWAS) to gain insights into genetically-regulated changes in gene expression and protein abundance in precursors to HF measured using MRI-derived cardiac measures as well as full-stage all-cause HF. We identified several gene and protein overlaps between LV ejection fraction and end-systolic volume measures. Many of the overlaps identified in MRI-derived measurements through TWAS and PWAS appear to be shared with all-cause HF. We implicate many putative pathways relevant in HF associated with these genes and proteins via gene-set enrichment and protein-protein interaction network approaches. The results of this study (1) highlight the benefit of using multi-omics to better understand genetics and (2) provide novel insights as to how changes in heart structure and function may relate to HF.

Unveiling the Spectrum of Minor Genes in Cardiomyopathies: A Narrative Review

Hereditary cardiomyopathies (CMPs), including arrhythmogenic cardiomyopathy (ACM), dilated cardiomyopathy (DCM), and hypertrophic cardiomyopathy (HCM), represent a group of heart disorders that significantly contribute to cardiovascular morbidity and mortality and are often driven by genetic factors. Recent advances in next-generation sequencing (NGS) technology have enabled the identification of rare variants in both well-established and minor genes associated with CMPs. Nowadays, a set of core genes is included in diagnostic panels for ACM, DCM, and HCM. On the other hand, despite their lesser-known status, variants in the minor genes may contribute to disease mechanisms and influence prognosis. This review evaluates the current evidence supporting the involvement of the minor genes in CMPs, considering their potential pathogenicity and clinical significance. A comprehensive analysis of databases, such as ClinGen, ClinVar, and GeneReviews, along with recent literature and diagnostic guidelines provides a thorough overview of the genetic landscape of minor genes in CMPs and offers guidance in clinical practice, evaluating each case individually based on the clinical referral, and insights for future research. Given the increasing knowledge on these less understood genetic factors, future studies are essential to clearly assess their roles, ultimately leading to improved diagnostic precision and therapeutic strategies in hereditary CMPs.

Role of Filamin C in Muscle Cells

Filamin C (FLNC) is a member of a high-molecular weight protein family, which bind actin filaments in the cytoskeleton of various cells. In human genome FLNC is encoded by the FLNC gene located on chromosome 7 and is expressed predominantly in striated skeletal and cardiac muscle cells. Filamin C is involved in organization and stabilization of thin actin filaments three-dimensional network in sarcomeres, and is supposed to play a role of mechanosensor transferring mechanical signals to different protein targets. Under mechanical stress FLNC can undergo unfolding that increases the risk of its aggregation. FLNC molecules with an impaired native structure could be eliminated by the BAG3-mediated chaperone-assisted selective autophagy. Mutations in the FLNC gene could be accompanied by the changes in FLNC interaction with its protein partners and could lead to formation of aggregates, which overload the autophagy and proteasome protein degradation systems, thus facilitating development of various pathological processes. Molecular mechanisms of the FLNC-associated congenital disorders, called filaminopathies, remain poorly understood. This review is devoted to analysis of the structure and mechanisms of filamin C function in muscle and heart cells in normal state and in the FLNC-associated pathologies. The presented data summarize the results of research at the molecular, cellular, and tissue levels and allow us to outline promising ways for further investigation of pathogenetic mechanisms in filaminopathies.

Connecting intermediate phenotypes to disease using multi-omics in heart failure

Heart failure (HF) is one of the most common, complex, heterogeneous diseases in the world, with over 1-3% of the global population living with the condition. Progression of HF can be tracked via MRI measures of structural and functional changes to the heart, namely left ventricle (LV), including ejection fraction, mass, end-diastolic volume, and LV end-systolic volume. Moreover, while genome-wide association studies (GWAS) have been a useful tool to identify candidate variants involved in HF risk, they lack crucial tissue-specific and mechanistic information which can be gained from incorporating additional data modalities. This study addresses this gap by incorporating transcriptome-wide and proteome-wide association studies (TWAS and PWAS) to gain insights into genetically-regulated changes in gene expression and protein abundance in precursors to HF measured using MRI-derived cardiac measures as well as full-stage all-cause HF. We identified several gene and protein overlaps between LV ejection fraction and end-systolic volume measures. Many of the overlaps identified in MRI-derived measurements through TWAS and PWAS appear to be shared with all-cause HF. We implicate many putative pathways relevant in HF associated with these genes and proteins via gene-set enrichment and protein-protein interaction network approaches. The results of this study (1) highlight the benefit of using multi-omics to better understand genetics and (2) provide novel insights as to how changes in heart structure and function may relate to HF.

Filamin C (FLNC) truncating mutation in a fatal arrhythmogenic left ventricular cardiomyopathy (ALVC)

Forensic pathologists are frequently asked to investigate cases of sudden death (SD), and identifying the cause of death can be of particular importance, especially where it may be necessary to perform family screening among the relatives of the victim. A multidisciplinary approach inclusive of genetic analysis is therefore strongly recommended. According to forensic practice, arrhythmogenic cardiomyopathy (ACM) is a well-known cause of SD. However, cases of SD caused by a left ventricular pattern of ACM diagnosed at autopsy are rarely reported in the literature. We present the case of an apparently healthy, 37-year-old male found dead at his home. At autopsy, multiple foci of epicardial and mid-wall fibrous and fibro-adipose tissue were observed within the left ventricle and, to a lesser extent, within the interventricular septum. Toxicology was negative, whereas a filamin C truncating mutation was detected through genetic analysis. To our knowledge, this is the first instance of arrhythmogenic left ventricular cardiomyopathy being diagnosed at autopsy.

Reduction of Filamin C Results in Altered Proteostasis, Cardiomyopathy, and Arrhythmias

BACKGROUND

Many cardiomyopathy-associated FLNC pathogenic variants are heterozygous truncations, and FLNC pathogenic variants are associated with arrhythmias. Arrhythmia triggers in filaminopathy are incompletely understood.

METHODS AND RESULTS

We describe an individual with biallelic FLNC pathogenic variants, p.Arg650X and c.970-4A>G, with peripartum cardiomyopathy and ventricular arrhythmias. We also describe clinical findings in probands with FLNC variants including Val2715fs87X, Glu2458Serfs71X, Phe106Leu, and c.970-4A>G with hypertrophic and dilated cardiomyopathy, atrial fibrillation, and ventricular tachycardia. Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were generated. The FLNC truncation, Arg650X/c.970-4A>G, showed a marked reduction in filamin C protein consistent with biallelic loss of function mutations. To assess loss of filamin C, gene editing of a healthy control iPSC line was used to generate a homozygous FLNC disruption in the actin binding domain. Because filamin C has been linked to protein quality control, we assessed the necessity of filamin C in iPSC-CMs for response to the proteasome inhibitor bortezomib. After exposure to low-dose bortezomib, FLNC-null iPSC-CMs showed an increase in the chaperone proteins BAG3, HSP70 (heat shock protein 70), and HSPB8 (small heat shock protein B8) and in the autophagy marker LC3I/II. FLNC null iPSC-CMs had prolonged electric field potential, which was further prolonged in the presence of low-dose bortezomib. FLNC null engineered heart tissues had impaired function after low-dose bortezomib.

CONCLUSIONS

FLNC pathogenic variants associate with a predisposition to arrhythmias, which can be modeled in iPSC-CMs. Reduction of filamin C prolonged field potential, a surrogate for action potential, and with bortezomib-induced proteasome inhibition, reduced filamin C led to greater arrhythmia potential and impaired function.

Variable clinical expression of a novel FLNC truncating variant in a large family

BACKGROUND

Variants in Filamin-C (FLNC) have been associated with various hereditary cardiomyopathies. Recent literature reports a prevalence of sudden cardiac death (SCD) of 13-25% among carriers of truncating-variants, with mean age of 42±15 years for first SCD event. This study reports two familial cases of SCD and the results of cascade screening of their large family.

METHODS

Molecular-autopsy of the SCD victims revealed a novel truncating-variant in the FLNC gene (chr 7:128496880 [hg19]; NM_001458.5; c.7467_7474del; p.(Ser2490fs)). We screened thirty-two family members following genetic counseling, and variant carriers underwent a comprehensive workup followed by consultation with a cardiologist with expertise in the genetics of cardiac diseases.

RESULTS

Seventeen variant carriers were identified: ages between 9 and 85 (mean 47±26). Fifteen underwent clinical evaluation. To date, none of the identified carriers has had major adverse events. In evaluated patients, ECG showed right-axis deviation in 60% (n = 9). Holter recorded frequent premature ventricular contractions (PVCs) (991±2030 per 24 h) in 33% (n = 5) with 4 patients having polymorphic PVC morphology. Three carriers had echocardiographic evidence of mild left-ventricular (LV) systolic dysfunction and another with mild LV dilatation. Cardiac magnetic-resonance (CMR) exhibited late‑gadolinium-enhancement in 10 out of 11 exams, mainly in the mid-myocardium and sub-epicardium, frequently involving the septum and the inferior-lateral wall.

CONCLUSION

This large FLNC truncating variant carrier family exhibits high cardiomyopathy penetrance, best diagnosed by CMR, with variable clinical expressions. These findings present a challenge in SCD prevention management and underscoring the imperative for better risk stratification measures.

Defective Biomechanics and Pharmacological Rescue of Human Cardiomyocytes with Filamin C Truncations

Actin-binding filamin C (FLNC) is expressed in cardiomyocytes, where it localizes to Z-discs, sarcolemma, and intercalated discs. Although FLNC truncation variants (FLNCtv) are an established cause of arrhythmias and heart failure, changes in biomechanical properties of cardiomyocytes are mostly unknown. Thus, we investigated the mechanical properties of human-induced pluripotent stem cells-derived cardiomyocytes (hiPSC-CMs) carrying FLNCtv. CRISPR/Cas9 genome-edited homozygous FLNCKO-/- hiPSC-CMs and heterozygous knock-out FLNCKO+/- hiPSC-CMs were analyzed and compared to wild-type FLNC (FLNCWT) hiPSC-CMs. Atomic force microscopy (AFM) was used to perform micro-indentation to evaluate passive and dynamic mechanical properties. A qualitative analysis of the beating traces showed gene dosage-dependent-manner "irregular" peak profiles in FLNCKO+/- and FLNCKO-/- hiPSC-CMs. Two Young's moduli were calculated: E1, reflecting the compression of the plasma membrane and actin cortex, and E2, including the whole cell with a cytoskeleton and nucleus. Both E1 and E2 showed decreased stiffness in mutant FLNCKO+/- and FLNCKO-/- iPSC-CMs compared to that in FLNCWT. The cell adhesion force and work of adhesion were assessed using the retraction curve of the SCFS. Mutant FLNC iPSC-CMs showed gene dosage-dependent decreases in the work of adhesion and adhesion forces from the heterozygous FLNCKO+/- to the FLNCKO-/- model compared to FLNCWT, suggesting damaged cytoskeleton and membrane structures. Finally, we investigated the effect of crenolanib on the mechanical properties of hiPSC-CMs. Crenolanib is an inhibitor of the Platelet-Derived Growth Factor Receptor α (PDGFRA) pathway which is upregulated in FLNCtv hiPSC-CMs. Crenolanib was able to partially rescue the stiffness of FLNCKO-/- hiPSC-CMs compared to control, supporting its potential therapeutic role.

Pathogenic truncating filamin C mutations presenting as acute myocarditis: a case series with insights from cardiac magnetic resonance and histological analysis

Background

The exact mechanisms underlying the pathogenesis of myocarditis are not always understood, but there is emerging evidence to suggest that genetic factors may play a significant role.

Case summary

Herein, we present six cases in which clinical, biochemical, and cardiovascular magnetic resonance data were consistent with myocarditis, and genetic testing subsequently revealed pathogenic filamin C (FLNC) mutations. Three patients presented with ventricular arrhythmias, two with severe biventricular dysfunction, and two suffered sudden cardiac arrest. Three received an implantable cardioverter defibrillator, and one underwent heart transplantation. Cascade testing was useful in identifying other relatives with FLNC mutation. We also present relevant histology results of myocardial specimens showing the presence of lymphocytic infiltration and inflammation, further supporting the potential association between FLNC mutations and a myocarditis phenotype.

Discussion

Genetic testing of affected individuals for FLNC mutations and cascade screening in the setting of acute myocarditis may be considered in selected clinical context, such as in acute myocarditis accompanied by severe left ventricular systolic dysfunction, biventricular failure, significant ventricular arrhythmias, or right ventricular involvement.

Filamin C Deficiency Impairs Sarcomere Stability and Activates Focal Adhesion Kinase through PDGFRA Signaling in Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Truncating mutations in filamin C (FLNC) are associated with dilated cardiomyopathy and arrhythmogenic cardiomyopathy. FLNC is an actin-binding protein and is known to interact with transmembrane and structural proteins; hence, the ablation of FLNC in cardiomyocytes is expected to dysregulate cell adhesion, cytoskeletal organization, sarcomere structural integrity, and likely nuclear function. Our previous study showed that the transcriptional profiles of FLNC homozygous deletions in human pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are highly comparable to the transcriptome profiles of hiPSC-CMs from patients with FLNC truncating mutations. Therefore, in this study, we used CRISPR-Cas-engineered hiPSC-derived FLNC knockout cardiac myocytes as a model of FLNC cardiomyopathy to determine pathogenic mechanisms and to examine structural changes caused by FLNC deficiency. RNA sequencing data indicated the significant upregulation of focal adhesion signaling and the dysregulation of thin filament genes in FLNC-knockout (FLNCKO) hiPSC-CMs compared to isogenic hiPSC-CMs. Furthermore, our findings suggest that the complete loss of FLNC in cardiomyocytes led to cytoskeletal defects and the activation of focal adhesion kinase. Pharmacological inhibition of PDGFRA signaling using crenolanib (an FDA-approved drug) reduced focal adhesion kinase activation and partially normalized the focal adhesion signaling pathway. The findings from this study suggest the opportunity in repurposing FDA-approved drug as a therapeutic strategy to treat FLNC cardiomyopathy.

Multicenter clinical and functional evidence reclassifies a recurrent noncanonical filamin C splice-altering variant

BACKGROUND

Truncating variants in filamin C (FLNC) can cause arrhythmogenic cardiomyopathy (ACM) through haploinsufficiency. Noncanonical splice-altering variants may contribute to this phenotype.

OBJECTIVE

The purpose of this study was to investigate the clinical and functional consequences of a recurrent FLNC intronic variant of uncertain significance (VUS), c.970-4A>G.

METHODS

Clinical data in 9 variant heterozygotes from 4 kindreds were obtained from 5 tertiary health care centers. We used in silico predictors and functional studies with peripheral blood and patient-specific induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). Isolated RNA was studied by reverse transcription polymerase chain reaction. iPSC-CMs were further characterized at baseline and after nonsense-mediated decay (NMD) inhibition, using quantitative polymerase chain reaction (qPCR), RNA-sequencing, and cellular electrophysiology. American College of Medical Genetics and Genomics (ACMG) criteria were used to adjudicate variant pathogenicity.

RESULTS

Variant heterozygotes displayed a spectrum of disease phenotypes, spanning from mild ventricular dysfunction with palpitations to severe ventricular arrhythmias requiring device shocks or progressive cardiomyopathy requiring heart transplantation. Consistent with in silico predictors, the c.970-4A>G FLNC variant activated a cryptic splice acceptor site, introducing a 3-bp insertion containing a premature termination codon. NMD inhibition upregulated aberrantly spliced transcripts by qPCR and RNA-sequencing. Patch clamp studies revealed irregular spontaneous action potentials, increased action potential duration, and increased sodium late current in proband-derived iPSC-CMs. These findings fulfilled multiple ACMG criteria for pathogenicity.

CONCLUSION

Clinical, in silico, and functional evidence support the prediction that the intronic c.970-4A>G VUS disrupts splicing and drives ACM, enabling reclassification from VUS to pathogenic.

Genetic characterization of dilated cardiomyopathy patients undergoing heart transplantation in the Chinese population by whole-exome sequencing

BACKGROUND

Dilated cardiomyopathy (DCM) is one of the most frequent causes of heart failure and heart transplantation (HTx). The genetic basis of DCM among patients undergoing HTx remains to be further studied. This study aimed to characterize the genetic basis of DCM HTx in the Chinese population.

METHODS

In total, 208 unrelated DCM patients who underwent HTx at Fuwai Hospital between June 2004 and June 2017 were included in this study. Whole-exome sequencing (WES) was performed for all patients. Gene burden analysis, variant classification, and genotype-phenotype correlation analysis were subsequently performed.

RESULTS

After completing the bioinformatics analysis, gene burden analysis suggested that titin (TTN), filamin C (FLNC) and lamin A/C (LMNA) were significantly enriched with rare protein-altering variants. The frequencies of TTN and FLNC truncating variants in our cohort were 18.8% and 8.7%, respectively. Among the 165 rare variants in high evidence DCM-related genes, 27 (16.4%) and 59 (35.8%) were interpreted as pathogenic (P) and likely pathogenic (LP), respectively. In addition, 41 (47.7%) and 16 (18.6%) of these 86 P/LP variants are located in TTN and FLNC, respectively. The FLNC group contained more patients with NYHA class IV than the P/LP-negative group (FLNC, 16/18 vs. P/LP-negative, 81/123, P = 0.049).

CONCLUSIONS

Based on WES, we provided a primary genetic spectrum of DCM patients undergoing HTx in the Chinese population. TTN and FLNC harbour the most P/LP variants. FLNC truncation may lead to severe clinical symptoms in DCM patients.

Role of non-coding variants in cardiovascular disease

Cardiovascular diseases (CVDs) constitute one of the significant causes of death worldwide. Different pathological states are linked to CVDs, which despite interventions and treatments, still have poor prognoses. The genetic component, as a beneficial tool in the risk stratification of CVD development, plays a role in the pathogenesis of this group of diseases. The emergence of genome-wide association studies (GWAS) have led to the identification of non-coding parts associated with cardiovascular traits and disorders. Variants located in functional non-coding regions, including promoters/enhancers, introns, miRNAs and 5'/3' UTRs, account for 90% of all identified single-nucleotide polymorphisms associated with CVDs. Here, for the first time, we conducted a comprehensive review on the reported non-coding variants for different CVDs, including hypercholesterolemia, cardiomyopathies, congenital heart diseases, thoracic aortic aneurysms/dissections and coronary artery diseases. Additionally, we present the most commonly reported genes involved in each CVD. In total, 1469 non-coding variants constitute most reports on familial hypercholesterolemia, hypertrophic cardiomyopathy and dilated cardiomyopathy. The application and identification of non-coding variants are beneficial for the genetic diagnosis and better therapeutic management of CVDs.

ROD2 domain filamin C missense mutations exhibit a distinctive cardiac phenotype with restrictive/hypertrophic cardiomyopathy and saw-tooth myocardium

INTRODUCTION AND OBJECTIVES

Missense mutations in the filamin C (FLNC) gene have been reported as cause of inherited cardiomyopathy. Knowledge of the pathogenicity and genotype-phenotype correlation remains scarce. Our aim was to describe a distinctive cardiac phenotype related to rare missense FLNC variants in the ROD2 domain.

METHODS

We recruited 21 unrelated families genetically evaluated because of hypertrophic cardiomyopathy (HCM)/restrictive cardiomyopathy (RCM) phenotype carrying rare missense variants in the ROD2 domain of FLNC (FLNC-mRod2). Carriers underwent advanced cardiac imaging and genetic cascade screening. Myocardial tissue from 3 explanted hearts of a missense FLNC carrier was histologically analyzed and compared with an FLNC-truncating variant heart sample and a healthy control. Plasmids independently containing 3 FLNC missense variants were transfected and analyzed using confocal microscopy.

RESULTS

Eleven families (52%) with 20 assessed individuals (37 [23.7-52.7]) years showed 15 cases with a cardiac phenotype consisting of an overlap of HCM-RCM and left ventricular hypertrabeculation (saw-tooth appearance). During a median follow-up of 6.49 years, they presented with advanced heart failure: 16 (80%) diastolic dysfunction, 3 heart transplants, 3 heart failure deaths) and absence of cardiac conduction disturbances or skeletal myopathy. A total of 6 families had moderate genotype-phenotype segregation, and the remaining were de novo variants. Differential extracellular matrix remodeling and FLNC distribution among cardiomyocytes were confirmed on histology. HT1080 and H9c2 cells did not reveal cytoplasmic aggregation of mutant FLNC.

CONCLUSIONS

FLNC-mRod2 variants show a high prevalence of an overlapped phenotype comprising RCM, HCM and deep hypertrabeculation with saw-tooth appearance and distinctive cardiac histopathological remodeling.

A Novel Nonsense Pathogenic TTN Variant Identified in a Patient with Severe Dilated Cardiomyopathy

Both genetic and environmental factors contribute to the development of dilated cardiomyopathy. Among the genes involved, TTN mutations, including truncated variants, explain 25% of DCM cases. We performed genetic counseling and analysis on a 57-year-old woman diagnosed with severe DCM and presenting relevant acquired risk factors for DCM (hypertension, diabetes, smoking habit, and/or previous alcohol and cocaine abuse) and with a family history of both DCM and sudden cardiac death. The left ventricular systolic function, as assessed by standard echocardiography, was 20%. The genetic analysis performed using TruSight Cardio panel, including 174 genes related to cardiac genetic diseases, revealed a novel nonsense TTN variant (TTN:c.103591A > T, p.Lys34531*), falling within the M-band region of the titin protein. This region is known for its important role in maintaining the structure of the sarcomere and in promoting sarcomerogenesis. The identified variant was classified as likely pathogenic based on ACMG criteria. The current results support the need of genetic analysis in the presence of a family history, even when relevant acquired risk factors for DCM may have contributed to the severity of the disease.

Structural and signaling proteins in the Z-disk and their role in cardiomyopathies

The sarcomere is the smallest functional unit of muscle contraction. It is delineated by a protein-rich structure known as the Z-disk, alternating with M-bands. The Z-disk anchors the actin-rich thin filaments and plays a crucial role in maintaining the mechanical stability of the cardiac muscle. A multitude of proteins interact with each other at the Z-disk and they regulate the mechanical properties of the thin filaments. Over the past 2 decades, the role of the Z-disk in cardiac muscle contraction has been assessed widely, however, the impact of genetic variants in Z-disk proteins has still not been fully elucidated. This review discusses the various Z-disk proteins (alpha-actinin, filamin C, titin, muscle LIM protein, telethonin, myopalladin, nebulette, and nexilin) and Z-disk-associated proteins (desmin, and obscurin) and their role in cardiac structural stability and intracellular signaling. This review further explores how genetic variants of Z-disk proteins are linked to inherited cardiac conditions termed cardiomyopathies.

Cardiovascular Involvement in Pediatric FLNC Variants: A Case Series of Fourteen Patients

Filamin C is a protein specifically expressed in myocytes and cardiomyocytes and is involved in several biological functions, including sarcomere contractile activity, signaling, cellular adhesion, and repair. FLNC variants are associated with different disorders ranging from striated muscle (myofibrillar distal or proximal) myopathy to cardiomyopathies (CMPs) (restrictive, hypertrophic, and dilated), or both. The outcome depends on functional consequences of the detected variants, which result either in FLNC haploinsufficiency or in an aberrant protein, the latter affecting sarcomere structure leading to protein aggregates. Cardiac manifestations of filaminopathies are most often described as adult onset CMPs and limited reports are available in children or on other cardiac spectrums (congenital heart defects-CHDs, or arrhythmias). Here we report on 13 variants in 14 children (2.8%) out of 500 pediatric patients with early-onset different cardiac features ranging from CMP to arrhythmias and CHDs. In one patient, we identified a deletion encompassing FLNC detected by microarray, which was overlooked by next generation sequencing. We established a potential genotype-phenotype correlation of the p.Ala1186Val variant in severe and early-onset restrictive cardiomyopathy (RCM) associated with a limb-girdle defect (two new patients in addition to the five reported in the literature). Moreover, in three patients (21%), we identified a relatively frequent finding of long QT syndrome (LQTS) associated with RCM (n = 2) and a hypertrabeculated left ventricle (n = 1). RCM and LQTS in children might represent a specific red flag for FLNC variants. Further studies are warranted in pediatric cohorts to delineate potential expanding phenotypes related to FLNC.

Rare clinical phenotype of filaminopathy presenting as restrictive cardiomyopathy and myopathy in childhood

Background

FLNC is one of the few genes associated with all types of cardiomyopathies, but it also underlies neuromuscular phenotype. The combination of concomitant neuromuscular and cardiac involvement is not often observed in filaminopathies and the impact of this on the disease prognosis has hitherto not been analyzed.

Results

Here we provide a detailed clinical, genetic, and structural prediction analysis of distinct FLNC-associated phenotypes based on twelve pediatric cases. They include early-onset restrictive cardiomyopathy (RCM) in association with congenital myopathy. In all patients the initial diagnosis was established during the first year of life and in five out of twelve (41.7%) patients the first symptoms were observed at birth. RCM was present in all patients, often in combination with septal defects. No ventricular arrhythmias were noted in any of the patients presented here. Myopathy was confirmed by neurological examination, electromyography, and morphological studies. Arthrogryposes was diagnosed in six patients and remained clinically meaningful with increasing age in three of them. One patient underwent successful heart transplantation at the age of 18 years and two patients are currently included in the waiting list for heart transplantation. Two died due to congestive heart failure. One patient had ICD instally as primary prevention of SCD. In ten out of twelve patients the disease was associated with missense variants and only in two cases loss of function variants were detected. In half of the described cases, an amino acid substitution A1186V, altering the structure of IgFLNc10, was found.

Conclusions

The present description of twelve cases of early-onset restrictive cardiomyopathy with congenital myopathy and FLNC mutation, underlines a distinct unique phenotype that can be suggested as a separate clinical form of filaminopathies. Amino acid substitution A1186V, which was observed in half of the cases, defines a mutational hotspot for the reported combination of myopathy and cardiomyopathy. Several independent molecular mechanisms of FLNC mutations linked to filamin structure and function can explain the broad spectrum of FLNC-associated phenotypes. Early disease presentation and unfavorable prognosis of heart failure demanding heart transplantation make awareness of this clinical form of filaminopathy of great clinical importance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02477-5.

Genetic Background and Clinical Features in Arrhythmogenic Left Ventricular Cardiomyopathy: A Systematic Review

In recent years a phenotypic variant of Arrhythmogenic cardiomyopathy has been described, characterized by predominant left ventricular (LV) involvement with no or minor right ventricular abnormalities, referred to as Arrhythmogenic left ventricular cardiomyopathy (ALVC). Different disease-genes have been identified in this form, such as Desmoplakin (DSP), Filamin C (FLNC), Phospholamban (PLN) and Desmin (DES). The main purpose of this critical systematic review was to assess the level of knowledge on genetic background and clinical features of ALVC. A search (updated to April 2022) was run in the PubMed, Scopus, and Web of Science electronic databases. The search terms used were “arrhythmogenic left ventricular cardiomyopathy” OR “arrhythmogenic cardiomyopathy” and “gene” OR “arrhythmogenic dysplasia” and “gene”. The most represented disease-gene turned out to be DSP, accounting for half of published cases, followed by FLNC. Overall, ECG abnormalities were reported in 58% of patients. Major ventricular arrhythmias were recorded in 26% of cases; an ICD was implanted in 29% of patients. A total of 6% of patients showed heart failure symptoms, and 15% had myocarditis-like episodes. DSP is confirmed to be the most represented disease-gene in ALVC patients. An analysis of reported clinical features of ALVC patients show an important degree of electrical instability, which frequently required an ICD implant. Moreover, myocarditis-like episodes are common.

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.

Activation of PDGFRA signaling contributes to filamin C-related arrhythmogenic cardiomyopathy

FLNC truncating mutations (FLNCtv) are prevalent causes of inherited dilated cardiomyopathy (DCM), with a high risk of developing arrhythmogenic cardiomyopathy. We investigated the molecular mechanisms of mutant FLNC in the pathogenesis of arrhythmogenic DCM (a-DCM) using patient-specific induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). We demonstrated that iPSC-CMs from two patients with different FLNCtv mutations displayed arrhythmias and impaired contraction. FLNC ablation induced a similar phenotype, suggesting that FLNCtv are loss-of-function mutations. Coimmunoprecipitation and proteomic analysis identified β-catenin (CTNNB1) as a downstream target. FLNC deficiency induced nuclear translocation of CTNNB1 and subsequently activated the platelet-derived growth factor receptor alpha (PDGFRA) pathway, which were also observed in human hearts with a-DCM and FLNCtv. Treatment with the PDGFRA inhibitor, crenolanib, improved contractile function of patient iPSC-CMs. Collectively, our findings suggest that PDGFRA signaling is implicated in the pathogenesis, and inhibition of this pathway is a potential therapeutic strategy in FLNC-related cardiomyopathies.

Clinical and genetic features of arrhythmogenic cardiomyopathy: diagnosis, management and the heart failure perspective

Background

Arrhythmogenic cardiomyopathy (ACM) is an emerging new concept of a life-threatening heart muscle disorder due not only to desmosome gene mutations, but also to non-desmosome genes, such as filamin C, lamin A/C, phospholamban, transmembrane protein 43, titin, SCN5A and RNA binding motif protein 20.Multi-modality imaging along with genetic testing are important tools for risk stratification to tailor treatment to a single patient. Cardiac magnetic resonance imaging (CMR) with late gadolinium enhancement (LGE) is the gold standard for evaluating left and right ventricular structure and function, edema, and fibrosis. The identification of regional fibrosis with LGE has prognostic value. The management of ACM involves several aspects: treatment of arrhythmias and heart failure, risk stratification, implantable cardioverter-defibrillator (ICD) placement, exercise restrictions, and life-style changes. The decision for ICD placement in ACM patients is not well established and should be made weighing risks and benefits. However, the presence of specific genotypes can allow a precision medicine approach. In ACM patients with only mild left ventricular dysfunction but phospholamban, filamin C or lamin A/C mutations, an ICD is now considered a reasonable approach.

Aim of Review

We sought to provide an overview of clinical and genetic feature of arrhythmogenic cardiomyopathy providing epidemiology, imaging, diagnostic and treatment information, using a systematic genetic approach.

The Arrhythmic Phenotype in Cardiomyopathy

In the wide phenotypic spectrum of cardiomyopathies, sudden cardiac death (SCD) has always been the most visible and devastating disease complication. The introduction of implantable cardioverter-defibrillators for SCD prevention by the late 1980s has moved the question from how to whom we should protect from SCD, leaving clinicians with a measure of uncertainty regarding the most reliable option to guide identification of the highest-risk patients. In this review, we will go through all the available evidence in the field of arrhythmic expression and arrhythmic risk stratification in the different phenotypes of cardiomyopathies to provide practical suggestions in daily clinical management.

Generation of CRISPR-Cas9 edited human induced pluripotent stem cell line carrying FLNC exon skipping variant

Loss-of-function (LoF) mutations in FLNC are strongly associated with dilated cardiomyopathy (DCM). Using CRISPR/Cas9 mediated edition in an healthy donor derived iPSC (ICAN-403.3) we subcloned 1 iPSC line harboring LoF mutation in FLNC. All lines are fully pluripotent and isogenic except at edited site where it presents a homozygous (ICAN-FLNC42.1) deletion of splice site leading to skipping of exon 42 traduced into a short filamin form with reduced expression in derived cardiomyocytes. This line would serve for FLNC mutation DCM modeling after differentiation into cardiocytes or beating organoids.

Phenotypic Expression, Natural History, and Risk Stratification of Cardiomyopathy Caused by Filamin C Truncating Variants

BACKGROUND

Filamin C truncating variants (FLNCtv) cause a form of arrhythmogenic cardiomyopathy: the mode of presentation, natural history, and risk stratification of FLNCtv remain incompletely explored. We aimed to develop a risk profile for refractory heart failure and life-threatening arrhythmias in a multicenter cohort of FLNCtv carriers.

METHODS

FLNCtv carriers were identified from 10 tertiary care centers for genetic cardiomyopathies. Clinical and outcome data were compiled. Composite outcomes were all-cause mortality/heart transplantation/left ventricle assist device (D/HT/LVAD), nonarrhythmic death/HT/LVAD, and sudden cardiac death/major ventricular arrhythmias. Previously established cohorts of 46 patients with LMNA and 60 with DSP-related arrhythmogenic cardiomyopathies were used for prognostic comparison.

RESULTS

Eighty-five patients carrying FLNCtv were included (42±15 years, 53% men, 45% probands). Phenotypes were heterogeneous at presentation: 49% dilated cardiomyopathy, 25% arrhythmogenic left dominant cardiomyopathy, 3% arrhythmogenic right ventricular cardiomyopathy. Left ventricular ejection fraction was <50% in 64% of carriers and 34% had right ventricular fractional area changes (RVFAC=(right ventricular end-diastolic area - right ventricular end-systolic area)/right ventricular end-diastolic area) <35%. During follow-up (median time 61 months), 19 (22%) carriers experienced D/HT/LVAD, 13 (15%) experienced nonarrhythmic death/HT/LVAD, and 23 (27%) experienced sudden cardiac death/major ventricular arrhythmias. The sudden cardiac death/major ventricular arrhythmias incidence of FLNCtv carriers did not significantly differ from LMNA carriers and DSP carriers. In FLNCtv carriers, left ventricular ejection fraction was associated with the risk of D/HT/LVAD and nonarrhythmic death/HT/LVAD.

CONCLUSIONS

Among patients referred to tertiary referral centers, FLNCtv arrhythmogenic cardiomyopathy is phenotypically heterogeneous and characterized by a high risk of life-threatening arrhythmias, which does not seem to be associated with the severity of left ventricular dysfunction.

Filamin C Cardiomyopathy Variants Cause Protein and Lysosome Accumulation

[Figure: see text].

Filamin C in cardiomyopathy: from physiological roles to DNA variants

Cardiomyopathy affects approximately 1 in 500 adults and is the leading cause of death. Familial cases are common, and mutations in many genes are involved in cardiomyopathy, especially those in genes encoding cytoskeletal, sarcomere, and nuclear envelope proteins. Filamin C is an actin-binding protein encoded by filamin C (FLNC) gene and participates in sarcomere stability maintenance. FLNC was first demonstrated to be a causal gene of myofibrillar myopathy; recently, it has been found that FLNC mutation plays a critical role in the pathogenesis of cardiomyopathy. In this review, we summarized the physiological roles of filamin C in cardiomyocytes and the genetic evidence for links between FLNC mutations and cardiomyopathies. Truncated FLNC is enriched in dilated cardiomyopathy and arrhythmogenic right ventricular cardiomyopathy. Non-truncated FLNC is enriched in hypertrophic cardiomyopathy and restrictive cardiomyopathy. Two major pathomechanisms in FLNC-related cardiomyopathy have been described: protein aggregation resulting from non-truncating mutations and haploinsufficiency triggered by filamin C truncation. Therefore, it is important to understand the cellular biology and molecular regulation of FLNC to design new therapies to treat patients with FLNC-related cardiomyopathy.

Genetics of Cardiomyopathy: Clinical and Mechanistic Implications for Heart Failure

Genetic cardiomyopathies are an important cause of sudden cardiac death across all age groups. Genetic testing in heart failure clinics is useful for family screening and providing individual prognostic insight. Obtaining a family history of at least three generations, including the creation of a pedigree, is recommended for all patients with primary cardiomyopathy. Additionally, when appropriate, consultation with a genetic counsellor can aid in the success of a genetic evaluation. Clinical screening should be performed on all first-degree relatives of patients with genetic cardiomyopathy.

Genetics has played an important role in the understanding of different cardiomyopathies, and the field of heart failure (HF) genetics is progressing rapidly. Much research has also focused on distinguishing markers of risk in patients with cardiomyopathy using genetic testing. While these efforts currently remain incomplete, new genomic technologies and analytical strategies provide promising opportunities to further explore the genetic architecture of cardiomyopathies, afford insight into the early manifestations of cardiomyopathy, and help define the molecular pathophysiological basis for cardiac remodeling. Cardiovascular physicians should be fully aware of the utility and potential pitfalls of incorporating genetic test results into pre-emptive treatment strategies for patients in the preliminary stages of HF. Future work will need to be directed towards elucidating the biological mechanisms of both rare and common gene variants and environmental determinants of plasticity in the genotype-phenotype relationship. This future research should aim to further our ability to identify, diagnose, and treat disorders that cause HF and sudden cardiac death in young patients, as well as prioritize improving our ability to stratify the risk for these patients prior to the onset of the more severe consequences of their disease.

Sex Differences, Genetic and Environmental Influences on Dilated Cardiomyopathy

Dilated cardiomyopathy (DCM) is characterized by dilatation of the left ventricle and impaired systolic function and is the second most common cause of heart failure after coronary heart disease. The etiology of DCM is diverse including genetic pathogenic variants, infection, inflammation, autoimmune diseases, exposure to chemicals/toxins as well as endocrine and neuromuscular causes. DCM is inherited in 20–50% of cases where more than 30 genes have been implicated in the development of DCM with pathogenic variants in TTN (Titin) most frequently associated with disease. Even though male sex is a risk factor for heart failure, few studies have examined sex differences in the pathogenesis of DCM. We searched the literature for studies examining idiopathic or familial/genetic DCM that reported data by sex in order to determine the sex ratio of disease. We found 31 studies that reported data by sex for non-genetic DCM with an average overall sex ratio of 2.5:1 male to female and 7 studies for familial/genetic DCM with an overall average sex ratio of 1.7:1 male to female. No manuscripts that we found had more females than males in their studies. We describe basic and clinical research findings that may explain the increase in DCM in males over females based on sex differences in basic physiology and the immune and fibrotic response to damage caused by mutations, infections, chemotherapy agents and autoimmune responses.

Omics phenotyping in heart failure: the next frontier

This state-of-the-art review aims to provide an up-to-date look at breakthrough omic technologies that are helping to unravel heart failure (HF) disease mechanisms and heterogeneity. Genomics, transcriptomics, proteomics, and metabolomics in HF are reviewed in depth. In addition, there is a thorough, expert discussion regarding the value of omics in identifying novel disease pathways, advancing understanding of disease mechanisms, differentiating HF phenotypes, yielding biomarkers for diagnosis or prognosis, or identifying new therapeutic targets in HF. The combination of multiple omics technologies may create a more comprehensive picture of the factors and physiology involved in HF than achieved by either one alone and provides a rich resource for predictive phenotype modelling. However, the successful translation of omics tools as solutions to clinical HF requires that the observations are robust and reproducible and can be validated across multiple independent populations to ensure confidence in clinical decision-making.

Genetics of dilated cardiomyopathy

PURPOSE OF REVIEW

Dilated cardiomyopathy (DCM), which include genetic and nongenetic forms, is the most common form of cardiomyopathy. DCM is characterized by left ventricular or biventricular dilation with impaired contraction. In the United States, DCM is a burden to healthcare that accounts for approximately 10,000 deaths and 46,000 hospitalizations annually. In this review, we will focus on the genetic forms of DCM and on recent advances in the understanding of cytoskeletal, sarcomeric, desmosomal, nuclear membrane, and RNA binding genes that contribute to the complexity and genetic heterogeneity of DCM.

RECENT FINDINGS

Although mutations in TTN remain the most common identifiable cause of genetic DCM, there is a growing appreciation for arrhythmogenic-prone DCM due to mutations in LMNA, desmosomal genes, and the recently described FLNC gene encoding the structural filamin C protein. Mutations in RBM20 highlight the relevance of RNA splicing regulation in the pathogenesis of DCM. Although expanded genetic testing has improved access to genetic diagnostic studies for many patients, the molecular mechanisms in the pathogenesis of the disease remained largely unknown.

SUMMARY

: The identification of the molecular causes and subsequent insight into the molecular mechanisms of DCM is expanding our understanding of DCM pathogenesis and highlights the complexity of DCM and the need to develop multifaceted strategies to treat the various causes of DCM.

The Postmortem Interpretation of Cardiac Genetic Variants of Unknown Significance in Sudden Death in the Young: A Case Report and Review of the Literature

Sudden cardiac death (SCD) in adolescents and young adults is a major traumatic event for families and communities. In these cases, it is not uncommon to have a negative autopsy with structurally and histologically normal heart. Such SCD cases are generally attributed to channelopathies, which include long QT syndrome, short QT syndrome, Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia. Our understanding of the causes for SCDs has changed significantly with the advancements in molecular and genetic studies, where many mutations are now known to be associated with certain channelopathies. Postmortem analysis provides great value in informing decision-making with regard to screening tests and prophylactic measures that should be taken to prevent sudden death in first degree relatives of the decedent. As this is a rapidly advancing field, our ability to identify genetic mutations has surpassed our ability to interpret them. This led to a unique challenge in genetic testing called variants of unknown significance (VUS). VUSs present a diagnostic dilemma and uncertainty for clinicians and patients with regard to next steps. Caution should be exercised when interpreting VUSs since misinterpretation can result in mismanagement of patients and their families. A case of a young adult man with drowning as his proximate cause of death is presented in circumstances where cardiac genetic testing was indicated and undertaken. Eight VUSs in genes implicated in inheritable cardiac dysfunction were identified and the interpretation of VUSs in this scenario is discussed.

The Role of Z-disc Proteins in Myopathy and Cardiomyopathy

The Z-disc acts as a protein-rich structure to tether thin filament in the contractile units, the sarcomeres, of striated muscle cells. Proteins found in the Z-disc are integral for maintaining the architecture of the sarcomere. They also enable it to function as a (bio-mechanical) signalling hub. Numerous proteins interact in the Z-disc to facilitate force transduction and intracellular signalling in both cardiac and skeletal muscle. This review will focus on six key Z-disc proteins: α-actinin 2, filamin C, myopalladin, myotilin, telethonin and Z-disc alternatively spliced PDZ-motif (ZASP), which have all been linked to myopathies and cardiomyopathies. We will summarise pathogenic variants identified in the six genes coding for these proteins and look at their involvement in myopathy and cardiomyopathy. Listing the Minor Allele Frequency (MAF) of these variants in the Genome Aggregation Database (GnomAD) version 3.1 will help to critically re-evaluate pathogenicity based on variant frequency in normal population cohorts.

The p.Ala2430Val mutation in filamin C causes a "hypertrophic myofibrillar cardiomyopathy"

Hypertrophic cardiomyopathy (HCM) often leads to heart failure. Mutations in sarcomeric proteins are most frequently the cause of HCM but in many patients the gene defect is not known. Here we report on a young man who was diagnosed with HCM shortly after birth. Whole exome sequencing revealed a mutation in the FLNC gene (c.7289C > T; p.Ala2430Val) that was previously shown to cause aggregation of the mutant protein in transfected cells. Myocardial tissue from patients with this mutation has not been analyzed before and thus, the underlying etiology is not well understood. Myocardial tissue of our patient obtained during myectomy at the age of 23 years was analyzed in detail by histochemistry, immunofluorescence staining, electron microscopy and western blot analysis. Cardiac histology showed a pathology typical for myofibrillar myopathy with myofibril disarray and abnormal protein aggregates containing BAG3, desmin, HSPB5 and filamin C. Analysis of sarcomeric and intercalated disc proteins showed focally reduced expression of the gap junction protein connexin43 and Xin-positive sarcomeric lesions in the cardiomyocytes of our patient. In addition, autophagy pathways were altered with upregulation of LC3-II, WIPI1 and HSPB5, 6, 7 and 8. We conclude that the p.Ala2430Val mutation in FLNC most probably is associated with HCM characterized by abnormal intercalated discs, disarray of myofibrils and aggregates containing Z-disc proteins similar to myofibrillar myopathy, which supports the pathological effect of the mutation.

Cardiac Filaminopathies: Illuminating the Divergent Role of Filamin C Mutations in Human Cardiomyopathy

Over the past decades, there has been tremendous progress in understanding genetic alterations that can result in different phenotypes of human cardiomyopathies. More than a thousand mutations in various genes have been identified, indicating that distinct genetic alterations, or combinations of genetic alterations, can cause either hypertrophic (HCM), dilated (DCM), restrictive (RCM), or arrhythmogenic cardiomyopathies (ARVC). Translation of these results from “bench to bedside” can potentially group affected patients according to their molecular etiology and identify subclinical individuals at high risk for developing cardiomyopathy or patients with overt phenotypes at high risk for cardiac deterioration or sudden cardiac death. These advances provide not only mechanistic insights into the earliest manifestations of cardiomyopathy, but such efforts also hold the promise that mutation-specific pathophysiology might result in novel “personalized” therapeutic possibilities. Recently, the FLNC gene encoding the sarcomeric protein filamin C has gained special interest since FLNC mutations were found in several distinct and possibly overlapping cardiomyopathy phenotypes. Specifically, mutations in FLNC were initially only linked to myofibrillar myopathy (MFM), but are now increasingly found in various forms of human cardiomyopathy. FLNC thereby represents another example for the complex genetic and phenotypic continuum of these diseases.

FLNC and MYLK2 Gene Mutations in a Chinese Family with Different Phenotypes of Cardiomyopathy

Mutations in the sarcomeric protein filamin C (FLNC) gene have been linked to hypertrophic cardiomyopathy (HCM), as they have been determined to increase the risk of ventricular arrhythmia and sudden death. Thus, in this study, we identified a novel missense mutation of FLNC in a Chinese family with HCM, and, interestingly, a second novel truncating mutation of MYLK2 was discobered in one family member with different phenotype.We performed whole-exome sequencing in a Chinese family with HCM of unknown cause. To determine and confirm the function of a novel mutation of FLNC, we introduced the mutant and wild-type gene into AC16 cells (human cardiomyocytes): we then used western blotting to analyze the expression of FLNC in subcellular fractions, and confocal microscope to observe the subcellular distribution of the protein.As per our findings, we were able to identify a novel missense single nucleotide variant (FLNC c.G5935A [p.A1979T]) in the family, which segregates with the disease. FLNC expression levels were observed to be equivalent in both wild-type and p.A1979T cardiomyocytes. However, the expression of the mutant protein has resulted in cytoplasmic protein aggregations, in contrast to wild-type FLNC, which was distributed in the cytoplasm and did not form aggregates. Unexpectedly, a second truncating mutation, NM_033118:exon8:c.G1138T:p.E380X of the MYLK2 gene, was identified in the mother of the proband with dilated cardiomyopathy, which was not found in other subjects.We then identified the FLNC A1979T mutation as a novel pathogenic variant associated with HCM in a Chinese family as well as a second causal mutation in a family member with a distinct phenotype. The possibility that there is more than one causal mutation in cardiomyopathy warrants clinical attention, especially for patients with atypical clinical features.

Genetic Association Between Hypoplastic Left Heart Syndrome and Cardiomyopathies

BACKGROUND

Hypoplastic left heart syndrome (HLHS) with risk of poor outcome has been linked to MYH6 variants, implicating overlap in genetic etiologies of structural and myopathic heart disease.

METHODS

Whole genome sequencing was performed in 197 probands with HLHS, 43 family members, and 813 controls. Data were filtered for rare, segregating variants in 3 index families comprised of an HLHS proband and relative(s) with cardiomyopathy. Whole genome sequencing data from cases and controls were compared for rare variant burden across 56 cardiomyopathy genes utilizing a weighted burden test approach, accounting for multiple testing using a Bonferroni correction.

RESULTS

A pathogenic MYBPC3 nonsense variant was identified in the first proband who underwent cardiac transplantation for diastolic heart failure, her father with left ventricular noncompaction, and 2 fourth-degree relatives with hypertrophic cardiomyopathy. A likely pathogenic RYR2 missense variant was identified in the second proband, a second-degree relative with aortic dilation, and a fourth-degree relative with dilated cardiomyopathy. A pathogenic RYR2 exon 3 in-frame deletion was identified in the third proband diagnosed with catecholaminergic polymorphic ventricular tachycardia and his father with left ventricular noncompaction and catecholaminergic polymorphic ventricular tachycardia. To further investigate HLHS-cardiomyopathy gene associations in cases versus controls, rare variant burden testing of 56 genes revealed enrichment in MYH6 (P=0.000068). Rare, predicted-damaging MYH6 variants were identified in 10% of probands in our cohort-4 with familial congenital heart disease, 4 with compound heterozygosity (3 with systolic ventricular dysfunction), and 4 with MYH6-FLNC synergistic heterozygosity.

CONCLUSIONS

Whole genome sequencing in multiplex families, proband-parent trios, and case-control cohorts revealed defects in cardiomyopathy-associated genes in patients with HLHS, which may portend impaired functional reserve of the single-ventricle circulation.

Clinical Implications of the Genetic Architecture of Dilated Cardiomyopathy

PURPOSE OF REVIEW

Dilated cardiomyopathy (DCM) frequently involves an underlying genetic etiology, but the clinical approach for genetic diagnosis and application of results in clinical practice can be complex.

RECENT FINDINGS

International sequence databases described the landscape of genetic variability across populations, which informed guidelines for the interpretation of DCM gene variants. New evidence indicates that loss-of-function mutations in filamin C (FLNC) contribute to DCM and portend high risk of ventricular arrhythmia. A clinical framework aids in referring patients for DCM genetic testing and applying results to patient care. Results of genetic testing can change medical management, particularly in a subset of genes that increase risk for life-threatening ventricular arrhythmias, and can influence decisions for defibrillator therapy. Clinical screening and cascade genetic testing of family members should be diligently pursued to identify those at risk of developing DCM.

Homozygous expression of the myofibrillar myopathy-associated p.W2710X filamin C variant reveals major pathomechanisms of sarcomeric lesion formation

Filamin C (FLNc) is mainly expressed in striated muscle cells where it localizes to Z-discs, myotendinous junctions and intercalated discs. Recent studies have revealed numerous mutations in the FLNC gene causing familial and sporadic myopathies and cardiomyopathies with marked clinical variability. The most frequent myopathic mutation, p.W2710X, which is associated with myofibrillar myopathy, deletes the carboxy-terminal 16 amino acids from FLNc and abolishes the dimerization property of Ig-like domain 24. We previously characterized "knock-in" mice heterozygous for this mutation (p.W2711X), and have now investigated homozygous mice using protein and mRNA expression analyses, mass spectrometry, and extensive immunolocalization and ultrastructural studies. Although the latter mice display a relatively mild myopathy under normal conditions, our analyses identified major mechanisms causing the pathophysiology of this disease: in comparison to wildtype animals (i) the expression level of FLNc protein is drastically reduced; (ii) mutant FLNc is relocalized from Z-discs to particularly mechanically strained parts of muscle cells, i.e. myotendinous junctions and myofibrillar lesions; (iii) the number of lesions is greatly increased and these lesions lack Bcl2-associated athanogene 3 (BAG3) protein; (iv) the expression of heat shock protein beta-7 (HSPB7) is almost completely abolished. These findings indicate grave disturbances of BAG3-dependent and -independent autophagy pathways that are required for efficient lesion repair. In addition, our studies reveal general mechanisms of lesion formation and demonstrate that defective FLNc dimerization via its carboxy-terminal domain does not disturb assembly and basic function of myofibrils. An alternative, more amino-terminally located dimerization site might compensate for that loss. Since filamins function as stress sensors, our data further substantiate that FLNc is important for mechanosensing in the context of Z-disc stabilization and maintenance.

Current Understanding of the Role of Cytoskeletal Cross-Linkers in the Onset and Development of Cardiomyopathies

Cardiomyopathies affect individuals worldwide, without regard to age, sex and ethnicity and are associated with significant morbidity and mortality. Inherited cardiomyopathies account for a relevant part of these conditions. Although progresses have been made over the years, early diagnosis and curative therapies are still challenging. Understanding the events occurring in normal and diseased cardiac cells is crucial, as they are important determinants of overall heart function. Besides chemical and molecular events, there are also structural and mechanical phenomena that require to be investigated. Cell structure and mechanics largely depend from the cytoskeleton, which is composed by filamentous proteins that can be cross-linked via accessory proteins. Alpha-actinin 2 (ACTN2), filamin C (FLNC) and dystrophin are three major actin cross-linkers that extensively contribute to the regulation of cell structure and mechanics. Hereby, we review the current understanding of the roles played by ACTN2, FLNC and dystrophin in the onset and progress of inherited cardiomyopathies. With our work, we aim to set the stage for new approaches to study the cardiomyopathies, which might reveal new therapeutic targets and broaden the panel of genes to be screened.

The intercalated disc: a mechanosensing signalling node in cardiomyopathy

Cardiomyocytes, the cells generating contractile force in the heart, are connected to each other through a highly specialised structure, the intercalated disc (ID), which ensures force transmission and transduction between neighbouring cells and allows the myocardium to function in synchrony. In addition, cardiomyocytes possess an intrinsic ability to sense mechanical changes and to regulate their own contractile output accordingly. To achieve this, some of the components responsible for force transmission have evolved to sense changes in tension and to trigger a biochemical response that results in molecular and cellular changes in cardiomyocytes. This becomes of particular importance in cardiomyopathies, where the heart is exposed to increased mechanical load and needs to adapt to sustain its contractile function. In this review, we will discuss key mechanosensing elements present at the intercalated disc and provide an overview of the signalling molecules involved in mediating the responses to changes in mechanical force.

First clinical and myopathological description of a myofibrillar myopathy with congenital onset and homozygous mutation in FLNC

Filamin C (encoded by the FLNC gene) is a large actin-cross-linking protein involved in shaping the actin cytoskeleton in response to signaling events both at the sarcolemma and at myofibrillar Z-discs of cross-striated muscle cells. Multiple mutations in FLNC are associated with myofibrillar myopathies of autosomal-dominant inheritance. Here, we describe for the first time a boy with congenital onset of generalized muscular hypotonia and muscular weakness, delayed motor development but no cardiac involvement associated with a homozygous FLNC mutation c.1325C>G (p.Pro442Arg). We performed ultramorphological, proteomic, and functional investigations as well as immunological studies of known marker proteins for dominant filaminopathies. We show that the mutant protein is expressed in similar quantities as the wild-type variant in control skeletal muscle fibers. The proteomic signature of quadriceps muscle is altered and ultrastructural perturbations are evident. Moreover, filaminopathy marker proteins are comparable both in our homozygous and a dominant control case (c.5161delG). Biochemical investigations demonstrate that the recombinant mutant protein is less stable and more prone to degradation by proteolytic enzymes than the wild-type variant. The unusual congenital presentation of the disease clearly demonstrates that homozygosity for mutations in FLNC severely aggravates the phenotype.

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.

Structure and Function of Filamin C in the Muscle Z-Disc

Filamin C (FLNC) is one of three filamin proteins (Filamin A (FLNA), Filamin B (FLNB), and FLNC) that cross-link actin filaments and interact with numerous binding partners. FLNC consists of a N-terminal actin-binding domain followed by 24 immunoglobulin-like repeats with two intervening calpain-sensitive hinges separating R15 and R16 (hinge 1) and R23 and R24 (hinge-2). The FLNC subunit is dimerized through R24 and calpain cleaves off the dimerization domain to regulate mobility of the FLNC subunit. FLNC is localized in the Z-disc due to the unique insertion of 82 amino acid residues in repeat 20 and necessary for normal Z-disc formation that connect sarcomeres. Since phosphorylation of FLNC by PKC diminishes the calpain sensitivity, assembly, and disassembly of the Z-disc may be regulated by phosphorylation of FLNC. Mutations of FLNC result in cardiomyopathy and muscle weakness. Although this review will focus on the current understanding of FLNC structure and functions in muscle, we will also discuss other filamins because they share high sequence similarity and are better characterized. We will also discuss a possible role of FLNC as a mechanosensor during muscle contraction.

A mutation update for the FLNC gene in myopathies and cardiomyopathies

Filamin C (FLNC) variants are associated with cardiac and muscular phenotypes. Originally, FLNC variants were described in myofibrillar myopathy (MFM) patients. Later, high‐throughput screening in cardiomyopathy cohorts determined a prominent role for FLNC in isolated hypertrophic and dilated cardiomyopathies (HCM and DCM). FLNC variants are now among the more prevalent causes of genetic DCM. FLNC‐associated DCM is associated with a malignant clinical course and a high risk of sudden cardiac death. The clinical spectrum of FLNC suggests different pathomechanisms related to variant types and their location in the gene. The appropriate functioning of FLNC is crucial for structural integrity and cell signaling of the sarcomere. The secondary protein structure of FLNC is critical to ensure this function. Truncating variants with subsequent haploinsufficiency are associated with DCM and cardiac arrhythmias. Interference with the dimerization and folding of the protein leads to aggregate formation detrimental for muscle function, as found in HCM and MFM. Variants associated with HCM are predominantly missense variants, which cluster in the ROD2 domain. This domain is important for binding to the sarcomere and to ensure appropriate cell signaling. We here review FLNC genotype–phenotype correlations based on available evidence.

Emerging concepts in arrhythmogenic dilated cardiomyopathy

Dilated cardiomyopathy (DCM) represents one of the primary cardiomyopathies and may lead to heart failure and sudden death. Until recently, ventricular arrhythmias were considered to be a direct consequence of the systolic dysfunction of the left ventricle (LV) and guidelines for implantable cardioverter defibrillator implantation were established on this basis. However, the identification of heritable dilated cardiomyopathy phenotypes that presented with mildly impaired or moderate LV dysfunction, with or without chamber dilatation, and ventricular arrhythmias exceeding the degree of the underlying morphological abnormalities lead to the identification of the arrhythmogenic phenotypes and genotypes of DCM. This subset of DCM patients presents phenotypic and in many cases genotypic overlaps with left dominant arrhythmogenic cardiomyopathy (LDAC). LMNA, SCN5A, FLNC, TTN, and RBM20 are the main genes responsible for arrhythmogenic DCM. Moreover, desmosomal genes such as DSP and other non-desmosomal such as DES and PLN have been associated with both LDAC and arrhythmogenic DCM. The aim of this review is to highlight the importance of genetic profiling among DCM patients with disproportionate arrhythmic burden and the significance of the electrocardiogram, cardiac magnetic resonance, Holter monitoring, detailed family history, and other assays in order to identify red flags for arrhythmogenic DCM and proceed to an early preventive approach for sudden cardiac death. A special consideration was given to the phenotypic and genotypic overlap with LDAC. The role of myocarditis as a common disease expression of LDAC and arrhythmogenic DCM is also analyzed supporting the premise of their phenotypic overlap.