Pathophysiology of arrhythmogenic cardiomyopathy. Nat Rev Cardiol. 2012;9:223-233. [PMID: 22124316 DOI: 10.1038/nrcardio.2011.173]

Engineering Tissues from Induced Pluripotent Stem Cells

Stem cells hold tremendous promise for replacing or regenerating tissues damaged by injury and disease as well as to study developmental biology and pathomechanisms. The discovery of methods to generate and culture human pluripotent stem cells (hESC and hiPSC) paved the way for producing genetically defined organ and tissue-specific cell types in a controlled laboratory setting. Cell and tissue engineering approaches have proven essential to unlocking the power of human pluripotent stem cells for both disease modeling and regenerative medicine. This editorial summarizes impressive examples of burgeoning research by leading groups that harness cellular and tissue engineering principles to study mechanisms of disease and injury, and in the context of repairing damaged tissue. These studies highlight both the power of these approaches, as well as ongoing challenges in the field.

Mitochondrial Dysfunction as Substrate for Arrhythmogenic Cardiomyopathy: A Search for New Disease Mechanisms

Arrhythmogenic cardiomyopathy (ACM) is a familial heart disease, associated with ventricular arrhythmias, fibrofatty replacement of the myocardial mass and an increased risk of sudden cardiac death (SCD). Malignant ventricular arrhythmias and SCD largely occur in the pre-clinical phase of the disease, before overt structural changes occur. To prevent or interfere with ACM disease progression, more insight in mechanisms related to electrical instability are needed. Currently, numerous studies are focused on the link between cardiac arrhythmias and metabolic disease. In line with that, a potential role of mitochondrial dysfunction in ACM pathology is unclear and mitochondrial biology in the ACM heart remains understudied. In this review, we explore mitochondrial dysfunction in relation to arrhythmogenesis, and postulate a link to typical hallmarks of ACM. Mitochondrial dysfunction depletes adenosine triphosphate (ATP) production and increases levels of reactive oxygen species in the heart. Both metabolic changes affect cardiac ion channel gating, electrical conduction, intracellular calcium handling, and fibrosis formation; all well-known aspects of ACM pathophysiology. ATP-mediated structural remodeling, apoptosis, and mitochondria-related alterations have already been shown in models of PKP2 dysfunction. Yet, the limited amount of experimental evidence in ACM models makes it difficult to determine whether mitochondrial dysfunction indeed precedes and/or accompanies ACM pathogenesis. Nevertheless, current experimental ACM models can be very useful in unraveling ACM-related mitochondrial biology and in testing potential therapeutic interventions.

Integrating Clinical Data and Imputed Transcriptome from GWAS to Uncover Complex Disease Subtypes: Applications in Psychiatry and Cardiology

Classifying subjects into clinically and biologically homogeneous subgroups will facilitate the understanding of disease pathophysiology and development of targeted prevention and intervention strategies. Traditionally, disease subtyping is based on clinical characteristics alone, but subtypes identified by such an approach may not conform exactly to the underlying biological mechanisms. Very few studies have integrated genomic profiles (e.g., those from GWASs) with clinical symptoms for disease subtyping. Here we proposed an analytic framework capable of finding complex diseases subgroups by leveraging both GWAS-predicted gene expression levels and clinical data by a multi-view bicluster analysis. This approach connects SNPs to genes via their effects on expression, so the analysis is more biologically relevant and interpretable than a pure SNP-based analysis. Transcriptome of different tissues can also be readily modeled. We also proposed various evaluation metrics for assessing clustering performance. Our framework was able to subtype schizophrenia subjects into diverse subgroups with different prognosis and treatment response. We also applied the framework to the Northern Finland Birth Cohort (NFBC) 1966 dataset and identified high and low cardiometabolic risk subgroups in a gender-stratified analysis. The prediction strength by cross-validation was generally greater than 80%, suggesting good stability of the clustering model. Our results suggest a more data-driven and biologically informed approach to defining metabolic syndrome and subtyping psychiatric disorders. Moreover, we found that the genes "blindly" selected by the algorithm are significantly enriched for known susceptibility genes discovered in GWASs of schizophrenia or cardiovascular diseases. The proposed framework opens up an approach to subject stratification.

Prevalence and Electronic Health Record-Based Phenotype of Loss-of-Function Genetic Variants in Arrhythmogenic Right Ventricular Cardiomyopathy-Associated Genes

BACKGROUND

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is associated with variants in desmosome genes. Secondary findings of pathogenic/likely pathogenic variants, primarily loss-of-function (LOF) variants, are recommended for clinical reporting; however, their prevalence and associated phenotype in a general clinical population are not fully characterized.

METHODS

From whole-exome sequencing of 61 019 individuals in the DiscovEHR cohort, we screened for putative loss-of-function variants in PKP2, DSC2, DSG2, and DSP. We evaluated measures from prior clinical ECG and echocardiograms, manually over-read to evaluate ARVC diagnostic criteria, and performed a PheWAS (phenome-wide association study). Finally, we estimated expected penetrance using Bayesian inference.

RESULTS

One hundred forty individuals (0.23%; 59±18 years old at last encounter; 33% male) had an ARVC variant (G⁺). None had an existing diagnosis of ARVC in the electronic health record, nor significant differences in prior ECG or echocardiogram findings compared with matched controls without variants. Several G⁺ individuals satisfied major repolarization (n=4) and ventricular function (n=5) criteria, but this prevalence matched controls. PheWAS showed no significant associations of other heart disease diagnoses. Combining our best genetic and disease prevalence estimates yields an estimated penetrance of 6.0%.

CONCLUSIONS

The prevalence of ARVC loss-of-function variants is ≈1:435 in a general clinical population of predominantly European descent, but with limited electronic health record-based evidence of phenotypic association in our population, consistent with a low penetrance estimate. Prospective deep phenotyping and longitudinal follow-up of a large sequenced cohort is needed to determine the true clinical relevance of an incidentally identified ARVC loss-of-function variant.

Filamin C variants are associated with a distinctive clinical and immunohistochemical arrhythmogenic cardiomyopathy phenotype

BACKGROUND

Pathogenic variants in the filamin C (FLNC) gene are associated with inherited cardiomyopathies including dilated cardiomyopathy with an arrhythmogenic phenotype. We evaluated FLNC variants in arrhythmogenic cardiomyopathy (ACM) and investigated the disease mechanism at a molecular level.

METHODS

120 gene-elusive ACM patients who fulfilled diagnostic criteria for arrhythmogenic right ventricular cardiomyopathy (ARVC) were screened by whole exome sequencing. Fixed cardiac tissue from FLNC variant carriers who had died suddenly was investigated by histology and immunohistochemistry.

RESULTS

Novel or rare FLNC variants, four null and five variants of unknown significance, were identified in nine ACM probands (7.5%). In FLNC null variant carriers (including family members, n = 16) Task Force diagnostic electrocardiogram repolarization/depolarization abnormalities were uncommon (19%), echocardiography was normal in 69%, while 56% had >500 ventricular ectopics/24 h or ventricular tachycardia on Holter and 67% had late gadolinium enhancement (LGE) on cardiac magnetic resonance imaging (CMRI). Ten gene positive individuals (63%) had abnormalities on ECG or CMRI that are not included in the current diagnostic criteria for ARVC. Immunohistochemistry showed altered key protein distribution, distinctive from that observed in ARVC, predominantly in the left ventricle.

CONCLUSIONS

ACM associated with FLNC variants presents with a distinctive phenotype characterized by Holter arrhythmia and LGE on CMRI with unremarkable ECG and echocardiographic findings. Clinical presentation in asymptomatic mutation carriers at risk of sudden death may include abnormalities which are currently non-diagnostic for ARVC. At the molecular level, the pathogenic mechanism related to FLNC appears different to classic forms of ARVC caused by desmosomal mutations.

Therapeutic Modulation of the Immune Response in Arrhythmogenic Cardiomyopathy

BACKGROUND

Inflammation is a prominent feature of arrhythmogenic cardiomyopathy (ACM), but whether it contributes to the disease phenotype is not known.

METHODS

To define the role of inflammation in the pathogenesis of ACM, we characterized nuclear factor-κB signaling in ACM models in vitro and in vivo and in cardiac myocytes from patient induced pluripotent stem cells.

RESULTS

Activation of nuclear factor-κB signaling, indicated by increased expression and nuclear accumulation of phospho-RelA/p65, occurred in both an in vitro model of ACM (expression of JUP^2157del2 in neonatal rat ventricular myocytes) and a robust murine model of ACM (homozygous knock-in of mutant desmoglein-2 [Dsg2^mut/mut]) that recapitulates the cardiac manifestations seen in patients with ACM. Bay 11-7082, a small-molecule inhibitor of nuclear factor-κB signaling, prevented the development of ACM disease features in vitro (abnormal redistribution of intercalated disk proteins, myocyte apoptosis, release of inflammatory cytokines) and in vivo (myocardial necrosis and fibrosis, left ventricular contractile dysfunction, electrocardiographic abnormalities). Hearts of Dsg2^mut/mut mice expressed markedly increased levels of inflammatory cytokines and chemotactic molecules that were attenuated by Bay 11-7082. Salutary effects of Bay 11-7082 correlated with the extent to which production of selected cytokines had been blocked. Nuclear factor-κB signaling was also activated in cardiac myocytes derived from a patient with ACM. These cells produced and secreted abundant inflammatory cytokines under basal conditions, and this was also greatly reduced by Bay 11-7082.

CONCLUSIONS

Inflammatory signaling is activated in ACM and drives key features of the disease. Targeting inflammatory pathways may be an effective new mechanism-based therapy for ACM.

Malignant Arrhythmia with Variants of Desmocollin-2 and Desmoplakin Genes

Malignant arrhythmia is a fast cardiac arrhythmia that can lead to a hemodynamic abnormality within a short time, most of which is ventricular tachycardia or ventricular fibrillation (VF), which should be managed in time. Both organic and nonorganic cardiac diseases have the potential to cause malignant arrhythmia. We report a noteworthy case of malignant arrhythmia in a teenager during exercise. Transthoracic echocardiography, cardiac magnetic resonance (CMR), electrophysiological study, magnetic resonance imaging of the brain, electroencephalography, chest X-ray, and blood tests were all normal. Twelve-lead electrocardiography showed incomplete right bundle branch block (IRBBB). Two heterozygous missense variants of the desmocollin-2 gene (DSC2, c.G2446A/p.V816M) and desmoplakin gene (DSP, c.G3620A/p.R1207K) were detected in the peripheral blood of this teenager and his father by genetic testing, which encoded a desmosomal protein that was related to arrhythmogenic right ventricular cardiomyopathy (ARVC). In these two rare variants, DSC2 V816M has been reported but uncertain significance, whereas DSP R1207K is never reported. Therefore, the two site variants in DSC2 and DSP genes are likely to become a new research focus for diagnosis and treatment of ARVC in the future. Meanwhile, this report emphasizes that, in addition to a standard set of laboratory tests and examinations, genetic testing may be useful for analyzing the causes of malignant arrhythmia.

Two pedigrees with arrhythmogenic right ventricular cardiomyopathy linked with R49H and F531C mutation in DSG2

Arrhythmogenic right ventricular cardiomyopathy (ARVC) presents as the progressive fibrofatty replacement of the cardiomyocytes particularly in the right ventricular wall. Here, we report two cases with ARVC. In family A, the proband carries a Desmoglein2 (DSG2) gene complex heterozygous mutation NM_001943.4:c.146G>A/p.(Arg49His)and NM_001943.3:c.1592T>G/p.(Phe531Cys). In family B, the proband carries a homozygous mutation NM_001943.3:c.1592T>G/p.(Phe531Cys).

Update on cardiomyopathies and sudden cardiac death

Sudden cardiac death (SCD) remains a leading mode of death in western countries. Since SCD can be the first and last clinical presentation of the underlying disease, autopsy could be the only medical examination available for early diagnosis and it should be performed according to the guidelines of the Association for European Cardiovascular Pathology. Although the vast majority of SCD are due to coronary artery disease, non-ischemic causes of SCD do exist and are prevalent in young people with structural (i.e. arrhythmogenic, hypertrophic and inflammatory cardiomyopathy) and non-structural (ion channel diseases) cardiomyopathies, accounting for up to one half of cases. A standardized autopsy protocol, in combination with blood sampling to ensure feasibility of postmortem molecular testing if needed, is mandatory. The pathologist is called to provide the correct diagnosis and to advice the relatives on the need of a cascade clinical and genetic screening in the presence of a heredo-familial disease.

Loss of cardiac Wnt/β-catenin signalling in desmoplakin-deficient AC8 zebrafish models is rescuable by genetic and pharmacological intervention

Aims

Arrhythmogenic cardiomyopathy (AC) is an inherited heart disease characterized by life-threatening ventricular arrhythmias and fibro-fatty replacement of the myocardium. More than 60% of AC patients show pathogenic mutations in genes encoding for desmosomal proteins. By focusing our attention on the AC8 form, linked to the junctional protein desmoplakin (DSP), we present here a zebrafish model of DSP deficiency, exploited to identify early changes of cell signalling in the cardiac region.

Methods and results

To obtain an embryonic model of Dsp deficiency, we first confirmed the orthologous correspondence of zebrafish Dsp genes (dspa and dspb) to the human DSP counterpart. Then, we verified their cardiac expression, at embryonic and adult stages, and subsequently we targeted them by antisense morpholino strategy, confirming specific and disruptive effects on desmosomes, like those identified in AC patients. Finally, we exploited our Dsp-deficient models for an in vivo cell signalling screen, using pathway-specific reporter transgenes. Out of nine considered, three pathways (Wnt/β-catenin, TGFβ/Smad3, and Hippo/YAP-TAZ) were significantly altered, with Wnt as the most dramatically affected. Interestingly, under persistent Dsp deficiency, Wnt signalling is rescuable both by a genetic and a pharmacological approach.

Conclusion

Our data point to Wnt/β-catenin as the final common pathway underlying different desmosomal AC forms and support the zebrafish as a suitable model for detecting early signalling pathways involved in the pathogenesis of DSP-associated diseases, possibly responsive to pharmacological or genetic rescue.

Cyclophilin A in Arrhythmogenic Cardiomyopathy Cardiac Remodeling

Arrhythmogenic cardiomyopathy (ACM) is a genetic disorder characterized by the progressive substitution of functional myocardium with noncontractile fibro-fatty tissue contributing to ventricular arrhythmias and sudden cardiac death. Cyclophilin A (CyPA) is a ubiquitous protein involved in several pathological mechanisms, which also characterize ACM (i.e., fibrosis, inflammation, and adipogenesis). Nevertheless, the involvement of CyPA in ACM cardiac remodeling has not been investigated yet. Thus, we first evaluated CyPA expression levels in the right ventricle (RV) tissue specimens obtained from ACM patients and healthy controls (HC) by immunohistochemistry. Then, we took advantage of ACM- and HC-derived cardiac mesenchymal stromal cells (C-MSC) to assess CyPA modulation during adipogenic differentiation. Interestingly, CyPA was more expressed in the RV sections obtained from ACM vs. HC subjects and positively correlated with the adipose replacement extent. Moreover, CyPA was upregulated at early stages of C-MSC adipogenic differentiation and was secreted at higher level over time in ACM- derived C-MSC. Our study provides novel ex vivo and in vitro information on CyPA expression in ACM remodeling paving the way for future C-MSC-based mechanistic and therapeutic investigations.

Desmoglein 2 mutation provokes skeletal muscle actin expression and accumulation at intercalated discs in murine hearts

Arrhythmogenic cardiomyopathy (AC) is an incurable progressive disease that is linked to mutations in genes coding for components of desmosomal adhesions that are localized to the intercalated disc region, which electromechanically couples adjacent cardiomyocytes. To date, the underlying molecular dysfunctions are not well characterized. In two murine AC models, we find an upregulation of the skeletal muscle actin gene (Acta1), which is known to be a compensatory reaction to compromised heart function. Expression of this gene is elevated prior to visible morphological alterations and clinical symptoms, and persists throughout pathogenesis with an additional major rise during the chronic disease stage. We provide evidence that the increased Acta1 transcription is initiated through nuclear activation of the serum response transcription factor (SRF) by its transcriptional co-activator megakaryoblastic leukemia 1 protein (MKL1, also known as MRTFA). Our data further suggest that perturbed desmosomal adhesion causes Acta1 overexpression during the early stages of the disease, which is amplified by transforming growth factor β (TGFβ) release from fibrotic lesions and surrounding cardiomyocytes during later disease stages. These observations highlight a hitherto unknown molecular AC pathomechanism.

Engineered Heart Slice Model of Arrhythmogenic Cardiomyopathy Using Plakophilin-2 Mutant Myocytes

IMPACT STATEMENT

Genetic heart diseases such as arrhythmogenic cardiomyopathy (AC), a common genetic cause of sudden cardiac death, can be modeled using patient-specific induced pluripotent stem cell-derived cardiac myocytes (CMs). However, it is important to culture these cells in a multicellular syncytium with exposure to surrounding matrix cues to create more accurate and robust models of the disease due to the importance of cell-cell and cell-matrix interactions. The engineered heart slice, constructed by seeding CMs on intact decellularized matrix slices, allows molecular and functional studies on an aligned multilayered syncytium of CMs. This study reveals the potential for an improved disease-in-a-dish model of AC.

Heterogeneous distribution of substrates between the endocardium and epicardium promotes ventricular fibrillation in arrhythmogenic right ventricular dysplasia/cardiomyopathy

Aims

Whether the distribution of scar in arrhythmogenic right ventricular cardiomyopathy (ARVC) plays a role in predicting different types of ventricular arrhythmias is unknown. This study aimed to investigate the prognostic value of scar distribution in patients with ARVC.

Methods and results

We studied 80 consecutive ARVC patients (46 men, mean age 47 ± 15 years) who underwent an electrophysiological study with ablation. Thirty-four patients receive both endocardial and epicardial mapping. Abnormal endocardial substrates and epicardial substrates were characterized. Three groups were defined according to the epicardial and endocardial scar gradient (<10%: transmural, 10-20%: intermediate, >20%: horizontal, as groups 1, 2, and 3, respectively). Sinus rhythm electrograms underwent a Hilbert-Huang spectral analysis and were displayed as 3D Simultaneous Amplitude Frequency Electrogram Transformation (SAFE-T) maps, which represented the arrhythmogenic potentials. The baseline characteristics were similar between the three groups. Group 3 patients had a higher incidence of fatal ventricular arrhythmias requiring defibrillation and cardiac arrest during the initial presentation despite having fewer premature ventricular complexes. A larger area of arrhythmogenic potentials in the epicardium was observed in patients with horizontal scar. The epicardial-endocardial scar gradient was independently associated with the occurrence of fatal ventricular arrhythmias after a multivariate adjustment. The total, ventricular tachycardia, and VF recurrent rates were higher in Group 3 during 38 ± 21 months of follow-up.

Conclusion

For ARVC, the epicardial substrate that extended in the horizontal plane rather than transmurally provided the arrhythmogenic substrate for a fatal ventricular arrhythmia circuit.

Assessment of a conduction-repolarisation metric to predict Arrhythmogenesis in right ventricular disorders

BACKGROUND

The re-entry vulnerability index (RVI) is a recently proposed activation-repolarization metric designed to quantify tissue susceptibility to re-entry. This study aimed to test feasibility of an RVI-based algorithm to predict the earliest endocardial activation site of ventricular tachycardia (VT) during electrophysiological studies and occurrence of haemodynamically significant ventricular arrhythmias in follow-up.

METHODS

Patients with Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC) (n = 11), Brugada Syndrome (BrS) (n = 13) and focal RV outflow tract VT (n = 9) underwent programmed stimulation with unipolar electrograms recorded from a non-contact array in the RV.

RESULTS

Lowest values of RVI co-localised with VT earliest activation site in ARVC/BrS but not in focal VT. The distance between region of lowest RVI and site of VT earliest site (Dmin) was lower in ARVC/BrS than in focal VT (6.8 ± 6.7 mm vs 26.9 ± 13.3 mm, p = 0.005). ARVC/BrS patients with inducible VT had lower Global-RVI (RVIG) than those who were non-inducible (-54.9 ± 13.0 ms vs -35.9 ± 8.6 ms, p = 0.005) or those with focal VT (-30.6 ± 11.5 ms, p = 0.001). Patients were followed up for 112 ± 19 months. Those with clinical VT events had lower Global-RVI than both ARVC and BrS patients without VT (-54.5 ± 13.5 ms vs -36.2 ± 8.8 ms, p = 0.007) and focal VT patients (-30.6 ± 11.5 ms, p = 0.002).

CONCLUSIONS

RVI reliably identifies the earliest RV endocardial activation site of VT in BrS and ARVC but not focal ventricular arrhythmias and predicts the incidence of haemodynamically significant arrhythmias. Therefore, RVI may be of value in predicting VT exit sites and hence targeting of re-entrant arrhythmias.

"Primary" dilated hearts

Dilated cardiomyopathy is part of the spectrum of heart failure which is a syndrome with certain morphological and functional characteristics. Although significant progress in the management of those patients has been achieved, seems that risk stratification and future treatments will be related to the specific pathological substrate.

Role of Lamin A/C Gene Mutations in the Signaling Defects Leading to Cardiomyopathies

Nuclear lamin A/C are crucial components of the intricate protein mesh that underlies the inner nuclear membrane and confers mainly nuclear and cytosolic rigidity. However, throughout the years a number of other key physiological processes have been associated with lamins such as modulation of both genes expression and the activity of signaling mediators. To further solidify its importance in cell physiology, mutations in the lamin A/C gene (LMNA) have been associated to diverse pathological phenotypes with skeletal muscles and the heart being the most affected systems. When affected, the heart develops a wide array of phenotypes spanning from dilated cardiomyopathy with conduction defects to arrhythmogenic right ventricular cardiomyopathy. The surprising large number of cardiac phenotypes reflects the equally large number of specific mutations identified in the LMNA gene. In this review, we underlie how mutations in LMNA can impact the activity and the spatial/temporal organization of signaling mediators and transcription factors. We analyzed the ever-increasing amount of findings collected in Lmna^H222P/H222P mice whose cardiomyopathy resemble the most important features of the disease in humans and a number of key evidences from other experimental models. With this mini review, we attempt to combine the newest insights regarding both the pathogenic effects of LMNA mutations in terms of signaling abnormalities and cardiac laminopathies.

Epithelial maturity influences EPEC-induced desmosomal alterations

Desmosomes are junctional protein complexes that confer strong adhesive capacity to adjacent host cells. In a recent study, we showed that enteropathogenic Escherichia coli (EPEC) disrupts desmosomes, weakens cell-cell adhesion and perturbs barrier function of intestinal epithelial (C2_BBe) cells. Desmosomal damage was dependent on the EPEC effector protein EspH and its inhibitory effect on Rho GTPases. EspH-mediated Rho inactivation resulted in retraction of keratin intermediate filaments and degradation of desmosomal cadherins. Immunofluorescence studies of EPEC-infected C2_BBe cells revealed keratin retraction towards the nucleus coincident with significant cytoplasmic redistribution of the desmosomal cadherin desmoglein-2 (DSG2). In this addendum, we expand on how EPEC-induced keratin retraction leads to loss of DSG2 anchoring at the junctions, and show that maturity of the epithelial cell monolayer impacts the fate of desmosomes during infection.

Arrhythmogenic cardiomyopathy: pathology, genetics, and concepts in pathogenesis

Arrhythmogenic cardiomyopathy (ACM) is a rare, heritable heart disease characterized by fibro-fatty replacement of the myocardium and a high degree of electric instability. It was first thought to be a congenital disorder, but is now regarded as a dystrophic heart muscle disease that develops over time. There is no curative treatment and current treatment strategies focus on attenuating the symptoms, slowing disease progression, and preventing life-threatening arrhythmias and sudden cardiac death. Identification of mutations in genes encoding desmosomal proteins and in other genes has led to insights into the disease pathogenesis and greatly facilitated identification of family members at risk. The disease phenotype is, however, highly variable and characterized by incomplete penetrance. Although the reasons are still poorly understood, sex, endurance exercise and a gene-dosage effect seem to play a role in these phenomena. The discovery of the genes and mutations implicated in ACM has allowed animal and cellular models to be generated, enabling researchers to start unravelling it's underlying molecular mechanisms. Observations in humans and in animal models suggest that reduced cell-cell adhesion affects gap junction and ion channel remodelling at the intercalated disc, and along with impaired desmosomal function, these can lead to perturbations in signalling cascades like the Wnt/β-catenin and Hippo/YAP pathways. Perturbations of these pathways are also thought to lead to fibro-fatty replacement. A better understanding of the molecular processes may lead to new therapies that target specific pathways involved in ACM.

Cell models of arrhythmogenic cardiomyopathy: advances and opportunities

Arrhythmogenic cardiomyopathy is a rare genetic disease that is mostly inherited as an autosomal dominant trait. It is associated predominantly with mutations in desmosomal genes and is characterized by the replacement of the ventricular myocardium with fibrous fatty deposits, arrhythmias and a high risk of sudden death. In vitro studies have contributed to our understanding of the pathogenic mechanisms underlying this disease, including its genetic determinants, as well as its cellular, signaling and molecular defects. Here, we review what is currently known about the pathogenesis of arrhythmogenic cardiomyopathy and focus on the in vitro models that have advanced our understanding of the disease. Finally, we assess the potential of established and innovative cell platforms for elucidating unknown aspects of this disease, and for screening new potential therapeutic agents. This appraisal of in vitro models of arrhythmogenic cardiomyopathy highlights the discoveries made about this disease and the uses of these models for future basic and therapeutic research.

Summary:In vitro models of ACM provide insights into the molecular mechanisms of this disease. This reappraisal offers a comprehensive vision of past discoveries and constitutes a tool for future research.

MicroRNA-130a Regulation of Desmocollin 2 in a Novel Model of Arrhythmogenic Cardiomyopathy

BACKGROUND

MicroRNAs are small noncoding RNA molecules that play a critical role in regulating physiological and disease processes. Recent studies have now recognized microRNAs as an important player in cardiac arrhythmogenesis. Molecular insight into arrhythmogenic cardiomyopathy (AC) has primarily focused on mutations in desmosome proteins. To our knowledge, models of AC due to microRNA dysregulation have not been reported. Previously, we reported on miR-130a mediated down-regulation of Connexin43.

OBJECTIVE

Here, we investigate miR-130a-mediated translational repression of Desmocollin2 (DSC2), as it has a predicted target site for miR-130a. DSC2 is an important protein for cell adhesion, which has been shown to be dysregulated in human AC.

METHOD & RESULTS

After induction of miR-130a, transgenic mice demonstrated right ventricular dilation. Surface ECG revealed spontaneous premature ventricular complexes confirming an arrhythmogenic phenotype in αMHC-miR130a mice. Using total protein from whole ventricular lysate, western blot analysis demonstrated an 80% reduction in DSC2 levels in transgenic myocardium. Furthermore, immunofluorescent staining confirmed downregulation of DSC2 in transgenic compared with littermate control myocardium. In transgenic hearts, histologic findings revealed fibrosis and lipid accumulation within both ventricles. To validate DSC2 as a direct target of miR-130a, we performed in vitro target assays in 3T3 fibroblasts, known to express miR-130a. Using a luciferase reporter fused to the 3UTR of DSC2 compared with a control, we found a 42% reduction in luciferase activity with the DSC2 3UTR. This reduction was reversed upon selective inhibition of miR-130a.

CONCLUSION

Overexpression of miR-130a results in a disease phenotype characteristic of AC and therefore, may serve as potential model for microRNA-induced AC.

Endomyocardial biopsy in differential diagnosis between arrhythmogenic right ventricular cardiomyopathy and dilated cardiomyopathy: an in vitro simulated study

Arrhythmogenic right ventricular cardiomyopathy (ARVC) and dilated cardiomyopathy (DCM), despite being two dramatically different entities, have overlapping phenotypes. As it is easy to misdiagnose between ARVC and DCM, there is a need to establish a new differential diagnostic parameter to differentiate the two. We investigated the utility of endomyocardial biopsy (EMB) for the differential diagnosis, and our study had three aims. The first was to verify the EMB high diagnostic efficacy. The second was to investigate the EMB perforation risk at the right ventricle (RV) free wall of end-stage ARVC. The third was to determine the best EMB sampling site in differential diagnosis between ARVC and DCM. Transmural tissues were sampled at six sites on the ventricular free walls and interventricular septum of recipient hearts (35 ARVCs and 35 DCMs). Participants with wall thickness <1.7 mm were included in the subgroup with high perforation risk, and the rest were included in a subgroup with low perforation risk. The best EMB sampling site was determined by the largest area under curve (AUC) among receiver operating characteristic curves. We found significant differences (P<.01) in percentages of tissue components in transmural sections between ARVC and DCM. In the subgroup with high perforation risk, there were 12 ARVCs and no DCMs, and paper-like RV walls and transmural fat replacement were their features in the cardiac enhancement computed tomography images. In the subgroup with low perforation risk, the largest AUC was on the myocardium at the ARV: AUC=0.839, cutoff=74.76%, sensitivity=73.68%, specificity=97.14%. We conclude that EMB high differential diagnostic efficacy is a meaningful fact regardless of limited sampling range, that EMB perforation risk at the RV free wall of end-stage ARVC cannot be neglected, and that the best EMB sampling site is the ARV. Among participants with low perforation risk, ARV is still recommended as an EMB sampling site with good differential diagnostic efficacy.

WNT Signaling in Cardiac and Vascular Disease

WNT signaling is an elaborate and complex collection of signal transduction pathways mediated by multiple signaling molecules. WNT signaling is critically important for developmental processes, including cell proliferation, differentiation and tissue patterning. Little WNT signaling activity is present in the cardiovascular system of healthy adults, but reactivation of the pathway is observed in many pathologies of heart and blood vessels. The high prevalence of these pathologies and their significant contribution to human disease burden has raised interest in WNT signaling as a potential target for therapeutic intervention. In this review, we first will focus on the constituents of the pathway and their regulation and the different signaling routes. Subsequently, the role of WNT signaling in cardiovascular development is addressed, followed by a detailed discussion of its involvement in vascular and cardiac disease. After highlighting the crosstalk between WNT, transforming growth factor-β and angiotensin II signaling, and the emerging role of WNT signaling in the regulation of stem cells, we provide an overview of drugs targeting the pathway at different levels. From the combined studies we conclude that, despite the sometimes conflicting experimental data, a general picture is emerging that excessive stimulation of WNT signaling adversely affects cardiovascular pathology. The rapidly increasing collection of drugs interfering at different levels of WNT signaling will allow the evaluation of therapeutic interventions in the pathway in relevant animal models of cardiovascular diseases and eventually in patients in the near future, translating the outcomes of the many preclinical studies into a clinically relevant context.

Sudden cardiac death owing to arrhythmogenic right ventricular cardiomyopathy: Two case reports and systematic literature review

BACKGROUND AND OBJECTIVE

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is increasingly recognized in forensic practice with controversial diagnosis. Here we described the epidemiological characteristics and reported the pathogenetic mechanism, diagnostic challenges, and forensic implications of Chinese ARVC autopsy cases.

METHODS

Two cases of sudden cardiac death owing to ARVC were reported. Retrospective analysis were performed on such 2 cases and 45 cases of separate ARVC complete autopsy case reports through Chinese literature databases in the last 30 years.

RESULTS

There were 27 males and 20 females, and the mean age at death was 35 years. Sudden cardiac death was the first manifestation observed in most patients, with no previous family and medical history. Exercise, acute stress, increased cardiac workload, and ethanol are frequently involved. The mean heart weight was 393 g (range, 240-590 g), and 10 cases had relative heart hypertrophy. Microscopic abnormalities included replacement of myocardium by adipose infiltration in 68.09% cases and fibroadipose in 31.91% cases; 80.85% cases were restricted to the right ventricle (RV), whereas biventricular subtype was seen in the remaining 19.15% cases. The preliminary quantitative histology showed 60.7% of fat tissues, 12.1% of fibrosis, and 27.2% residual myocytes in RV. Inflammatory cell infiltration was found in 25.53% cases, but myocyte necrosis was found in only 1 case. In 10.64% of cases, cardiac conduction was infiltrated by fibrosis, adipose, or both.

CONCLUSION

In this review, the most characteristic and distinct histopathologic features that are diagnostic or highly suggestive of ARVC for forensic pathologists were identified. Combining gross and histological examinations with postmortem genetic analysis is recommended for identifying ARVC.

Arrhythmogenic Right Ventricular Cardiomyopathy: Risk Stratification and Indications for Defibrillator Therapy

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a genetically determined disease which predisposes to life-threatening ventricular arrhythmias. The main goal of ARVC therapy is prevention of sudden cardiac death (SCD). Implantable cardioverter defibrillator (ICD) is the most effective therapy for interruption of potentially lethal ventricular tachyarrhythmias. Despite its life-saving potential, ICD implantation is associated with a high rate of complications and significant impact on quality of life. Accurate risk stratification is needed to identify individuals who most benefit from the therapy. While there is general agreement that patients with a history of cardiac arrest or hemodynamically unstable ventricular tachycardia are at high risk of SCD and needs an ICD, indications for primary prevention remain a matter of debate. The article reviews the available scientific evidence and guidelines that may help to stratify the arrhythmic risk of ARVC patients and guide ICD implantation. Other therapeutic strategies, either alternative or additional to ICD, will be also addressed.

Arrhythmogenic Cardiomyopathy: the Guilty Party in Adipogenesis

Arrhythmogenic cardiomyopathy (ACM) is a genetic cardiac condition characterized by the replacement of the ventricular myocardium with fibro-fatty tissue, by arrhythmias and sudden death. Adipogenesis in ACM is considered an aberrant remodeling following myocardial loss. Which cell type(s) is (are) responsible for the adipose replacement is still matter of debate. A systematic overview of the different cells that have been, over time, considered as main players in adipose replacement is provided. The comprehension of the cellular component giving rise to arrhythmogenic cardiomyopathy substrate defects may represent both an essential tool for mechanistic studies of disease pathogenesis and a novel possible therapeutic target.

Case reports of two pedigrees with recessive arrhythmogenic right ventricular cardiomyopathy associated with homozygous Thr335Ala variant in DSG2

Background

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited cardiac disease, involving changes in ventricular myocardial tissue and leading to fatal arrhythmias. Mutations in desmosomal genes are thought to be the main cause of ARVC. However, the exact molecular genetic etiology of the disease still remains largely inconclusive, and this along with large variabilities in clinical manifestations complicate clinical diagnostics.

Case presentation

We report two families (n = 20) in which a desmoglein-2 (DSG2) missense variant c.1003A > G, p.(Thr335Ala) was discovered in the index patients using next-generation sequencing panels. The presence of this variant in probands’ siblings and children was studied by Sanger sequencing. Five homozygotes and nine heterozygotes were found with the mutation. Participants were evaluated clinically where possible, and available medical records were obtained. All patients homozygous for the variant fulfilled the current diagnostic criteria for ARVC, whereas none of the heterozygous subjects had symptoms suggestive of ARVC or other cardiomyopathies.

Conclusions

The homozygous DSG2 variant c.1003A > G co-segregated with ARVC, indicating autosomal recessive inheritance and complete penetrance. More research is needed to establish a detailed understanding of the relevance of rare variants in ARVC associated genes, which is essential for informative genetic counseling and rational family member testing.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-017-0442-3) contains supplementary material, which is available to authorized users.

Unique genetic background and outcome of non-Caucasian Japanese probands with arrhythmogenic right ventricular dysplasia/cardiomyopathy

BACKGROUND

Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C) is an inherited cardiomyopathy mainly caused by desmosomal gene mutation. More than half of Caucasian probands have desmosomal mutations, which lead to earlier onset of ventricular arrhythmias. Among non-Caucasians, the genetic background of ARVD/C probands and its prognostic impact remain unclear.

METHODS AND RESULTS

We genotyped 99 unrelated Japanese ARVD/C probands for plakophilin 2 (PKP2), desmoglein 2 (DSG2), desmoplakin (DSP), and desmocollin 2 (DSC2) between 2005 and 2014. Seventy-five probands who fulfilled "definite" category according to the 2010 Task Force Criteria (TFC) were enrolled and followed up for 6.4 years. Sixty-four percent of probands had desmosomal mutations; DSG2 was predominant (48% of mutations) followed by PKP2 (38%). DSG2 mutations were almost missense, whereas over 90% of PKP2 mutations were truncating mutations. Lethal ventricular arrhythmias (VAs, sustained ventricular tachycardia/fibrillation) occurred in 57% of probands as the first manifestation and 71% at the end of follow-up. Five died during follow-up. Truncating mutation carriers exhibited earlier lethal VAs onset compared to missense mutation carriers or mutation negatives (age at onset 35 ± 12, 49 ± 16, and 50 ± 19 years, respectively, P < 0.05 in each). Cox proportional hazard analysis revealed for the first time that, compared to mutation negatives, truncating mutation carriers had higher risk for lethal VAs, and especially for onset by their 40s, in an age-dependent manner (RR = 4.6, P < 0.01 by their 40s; RR = 2.9, P = 0.01 by their 50s).

CONCLUSION

The genetic background of Japanese ARVD/C probands is distinct from that of Caucasian probands, leading to distinct prognosis. The most affected gene mutations in Japanese probands were missense mutations in DSG2 leading to modest outcome, whereas PKP2 truncating mutations were the second most and might be a strong marker for lethal VAs in non-Caucasian Japanese ARVD/C probands.

Comprehensive Myocardial Proteogenomics Profiling Reveals C/EBPα as the Key Factor in the Lipid Storage of ARVC

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is hereditary cardiomyopathy characterized by the fibro-fatty replacement of the myocardium. A small number of noncomprehensive profiling studies based on human cardiac tissues have been conducted and reported; consequently, ARVC's gene expression pattern characteristics remain largely undocumented. Our study applies large-scaled, quantitative proteomics based on TMT-labeled LC-MS/MS to analyze the left and right ventricular myocardium of four ARVC and four DCM explanted hearts to compare them with normal hearts. Our objective is to reveal the characteristic proteome pattern in ARVC compared with DCM as well as nondiseased heart. We also conducted the RNA sequencing of 10 right ventricles from ARVC hearts paired with four nondiseased donor hearts to validate the proteome results. In a manner similar to that of the well-defined DCM heart failure model, the ARVC model demonstrates the downregulation of mitochondrial function proteins and the effects of many heart failure regulators such as TGFB, RICTOR, and KDM5A. In addition, the inflammatory signaling, especially the complement system, was activated much more severely in ARVC than in DCM. Our most significant discovery was the lipid metabolism reprogramming of both ARVC ventricles in accordance with the upregulation of lipogenesis factors such as FABP4 and FASN. We identified the key upstream regulator of lipogenesis as C/EBPα. Transcriptome profiling verified the consistency with proteome alterations. This comprehensive proteogenomics profiling study reveals that an activation of C/EBPα, along with the upregulation of its lipogenesis targets, accounts for lipid storage and acts as a hallmark of ARVC.

Co-inheritance of mutations associated with arrhythmogenic cardiomyopathy and hypertrophic cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) and hypertrophic cardiomyopathy (HCM) are genetically and phenotypically distinct disorders of the myocardium. Here we describe for the first time co-inheritance of mutations in genes associated with ACM or HCM in two families with recurrence of both cardiomyopathies. Among the double heterozygotes for mutations in desmoplakin (DSP) and myosin binding protein C (MYBPC3) genes identified in Family A, two were diagnosed with ACM and two with HCM. In Family B, one patient was identified to carry mutations in α-T-catenin (CTTNA3) and β-myosin (MYH7) genes, but he does not fulfill the current diagnostic criteria neither for ACM nor for HCM. Interestingly, the double heterozygotes showed a variable clinical expression of both cardiomyopathies and they do not exhibit a more severe phenotype than family members carrying only one of the two mutations.

Arrhythmogenic Right Ventricular Cardiomyopathy: Clinical Course and Predictors of Arrhythmic Risk

BACKGROUND

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a leading cause of sudden cardiac death, but its progression over time and predictors of arrhythmias are still being defined.

OBJECTIVES

This study sought to describe the clinical course of ARVC and occurrence of life-threatening arrhythmic events (LAE) and cardiovascular mortality; identify risk factors associated with increased LAE risk; and define the response to therapy.

METHODS

We determined the clinical course of 301 consecutive patients with ARVC using the Kaplan-Meier method adjusted to avoid the bias of delayed entry. Predictors of LAE over 5.8 years of follow-up were determined with Cox multivariable analysis. Treatment efficacy was assessed comparing LAE rates during matched time intervals.

RESULTS

A first LAE occurred in 1.5 per 100 person-years between birth and age 20 years, in 4.0 per 100 person-years between ages 21 and 40 years, and in 2.4 per 100 person-years between ages 41 and 60 years. Cumulative probability of a first LAE at follow-up was 14% at 5 years, 23% at 10 years, and 30% at 15 years. Higher risk of LAE was predicted by atrial fibrillation (hazard ratio [HR]: 4.38; p = 0.002), syncope (HR: 3.36; p < 0.001), participation in strenuous exercise after the diagnosis (HR: 2.98; p = 0.028), hemodynamically tolerated sustained monomorphic ventricular tachycardia (HR: 2.19; p = 0.023), and male sex (HR: 2.49; p = 0.012). No difference was observed in the occurrence of LAE before and after treatment with amiodarone, beta-blockers, sotalol, or ablation. A total of 81 patients received an implantable cardioverter-defibrillator, 34 were successfully defibrillated.

CONCLUSIONS

The high risk of life-threatening arrhythmias in patients with ARVC spans from adolescence to advanced age, reaching its peak between ages 21 and 40 years. Atrial fibrillation, syncope, participation in strenuous exercise after the diagnosis of ARVC, hemodynamically tolerated sustained monomorphic ventricular tachycardia, and male sex predicted lethal arrhythmias at follow-up. The lack of efficacy of antiarrhythmic therapy and the life-saving role of the implantable cardioverter-defibrillator highlight the importance of risk stratification for patient management.

Whole Genome Sequence Identified a Rare Homozygous Pathogenic Mutation of the DSG2 Gene in a Familial Arrhythmogenic Cardiomyopathy Involving Both Ventricles

BACKGROUND

This study was designed to identify the pathogenic mutation in a Chinese family with arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D) using whole genome sequencing (WGS).

METHODS AND RESULTS

Probands II:1 and II:2 underwent routine examinations for diagnosis. Genomic DNA was extracted from the peripheral blood of family members and analyzed using WGS. A total of 60,285 single-nucleotide polymorphisms (SNP) and 13,918 insertions/deletions (InDel) occurring in the exonic regions of genes and predisposing to cardiomyopathies and arrhythmias were identified. When filtered using the 1000 Genomes Project (2014 version), NHLBI ESP6500, and ExAC databases, 12 missense SNP and 2 InDel in exonic regions remained, the allele frequencies of which were <0.01 or unknown. The potentially pathogenic mutations that occurred in the genes DSG2, PKP4, PRKAG2, FOXD4, CTTN, and DMD, which were identified by SIFT or PolyPhen-2 software as "damaging," were validated using Sanger sequencing. Probands II:1 and II:2 shared an extremely rare homozygous mutation in the DSG2 (p.F531C) gene, which was also demonstrated using intersection analysis of WGS data from probands II:1 and II:2. Electron microscopy and histological staining of myocardial biopsies showed widened and destroyed intercalated discs, and interrupted, atrophic, and disarranged myocardial fibers, and hyperplastic interstitial fibers, collagen fibers, and adipocytes were infiltrated and invaded.

CONCLUSIONS

A homozygous mutation of DSG2 p.F531C was identified as the pathogenic mutation in patients with ARVC/D involving both ventricles, as a result of widened and impaired intercalated discs, interrupted myocardial fibers, and abnormally hyperplastic interstitial fibers, collagen fibers, and adipocytes.

Wnt/β-catenin pathway in arrhythmogenic cardiomyopathy

Wnt/β-catenin signaling pathway plays essential roles in heart development as well as cardiac tissue homoeostasis in adults. Abnormal regulation of this signaling pathway is linked to a variety of cardiac disease conditions, including hypertrophy, fibrosis, arrhythmias, and infarction. Recent studies on genetically modified cellular and animal models document a crucial role of Wnt/β-catenin signaling in the molecular pathogenesis of arrhythmogenic cardiomyopathy (AC), an inherited disease of intercalated discs, typically characterized by ventricular arrhythmias and progressive substitution of the myocardium with fibrofatty tissue. In this review, we summarize the conflicting published data regarding the Wnt/β-catenin signaling contribution to AC pathogenesis and we report the identification of a new potential therapeutic molecule that prevents myocyte injury and cardiac dysfunction due to desmosome mutations in vitro and in vivo by interfering in this signaling pathway. Finally, we underline the potential function of microRNAs, epigenetic regulatory RNA factors reported to participate in several pathological responses in heart tissue and in the Wnt signaling network, as important modulators of Wnt/β-catenin signaling transduction in AC.

Elucidation of the precise regulatory mechanism of Wnt/β-catenin signaling in AC molecular pathogenesis could provide fundamental insights for new mechanism-based therapeutic strategy to delay the onset or progression of this cardiac disease.

Genetic and epigenetic regulation of arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (AC) is most commonly characterized as a disease of the intercalated disc that promotes abnormal cardiac conduction. Previously, arrhythmogenic cardiomyopathy was frequently referred to as arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D); however, genotype-phenotype studies have defined a broader phenotypic spectrum; with the identification of left-dominant and biventricular subtypes. Molecular insight into AC has primarily focused on mutations in desmosomal proteins and the downstream signaling pathways; however, desmosomal gene mutations can only be identified in approximately 50% of patients with AC. Animal and cellular studies have shown that in addition to abnormal biomechanical properties from changes in desmosome function, crosstalk from the desmosome to the nucleus, gap junctions, and ion channels are implicated in the pathobiology of AC. In this review, we highlight some of the newly identified genetic and epigenetic mechanisms that may lead to the development of AC including the role of the Hippo pathway and microRNAs. This article is part of a Special Issue entitled: Genetic and epigenetic control of heart failure - edited by Jun Ren & Megan Yingmei Zhang.

Comprehensive multi-modality imaging approach in arrhythmogenic cardiomyopathy-an expert consensus document of the European Association of Cardiovascular Imaging

Arrhythmogenic cardiomyopathy (AC) is a progressive disease with high risk of life-threatening ventricular arrhythmias. A genetic mutation is found in up to 50-60% of probands, mostly affecting desmosomal genes. Diagnosis of AC is made by a combination of data from different modalities including imaging, electrocardiogram, Holter monitoring, family history, genetic testing, and tissue properties. Being a progressive cardiomyopathy, repeated cardiac imaging is needed in AC patients. Repeated imaging is important also for risk assessment of ventricular arrhythmias. This expert consensus document gives clinical recommendations for how to use multi-modality imaging in the different aspects of AC disease, including diagnosis, family screening, follow-up, risk assessment, and differential diagnosis.

Potential new mechanisms of pro-arrhythmia in arrhythmogenic cardiomyopathy: focus on calcium sensitive pathways

Arrhythmogenic cardiomyopathy, or its most well-known subform arrhythmogenic right ventricular cardiomyopathy (ARVC), is a cardiac disease mainly characterised by a gradual replacement of the myocardial mass by fibrous and fatty tissue, leading to dilatation of the ventricular wall, arrhythmias and progression towards heart failure. ARVC is commonly regarded as a disease of the intercalated disk in which mutations in desmosomal proteins are an important causative factor. Interestingly, the Dutch founder mutation PLN R14Del has been identified to play an additional, and major, role in ARVC patients within the Netherlands. This is remarkable since the phospholamban (PLN) protein plays a leading role in regulation of the sarcoplasmic reticulum calcium load rather than in the establishment of intercellular integrity. In this review we outline the intracellular cardiac calcium dynamics and relate pathophysiological signalling, induced by disturbed calcium handling, with activation of calmodulin dependent kinase II (CaMKII) and calcineurin A (CnA). We postulate a thus far unrecognised role for Ca2+ sensitive signalling proteins in maladaptive remodelling of the macromolecular protein complex that forms the intercalated disk, during pro-arrhythmic remodelling of the heart.

Molecular mechanisms in the pathogenesis of arrhythmogenic cardiomyopathy

The article is based on work presented in the Distinguished Achievement Award lecture at the Society for Cardiovascular Pathology meeting in Seattle, WA, in March 2016. It reviews our current understanding of mechanisms responsible for a highly arrhythmogenic, nonischemic cardiomyopathy. It highlights the armamentarium of powerful methods available to the experimental pathologist in efforts to define how complex cardiovascular diseases work. It concludes with acknowledgment of the need for a far more detailed approach as to how we categorize human disease, a task for which pathologists are especially well positioned.

Cardiomyocyte Hypertrophy in Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy (AC) is a hereditary disease leading to sudden cardiac death or heart failure. AC pathology is characterized by cardiomyocyte loss and replacement fibrosis. Our goal was to determine whether cardiomyocytes respond to AC progression by pathological hypertrophy. To this end, we examined tissue samples from AC patients with end-stage heart failure and tissue samples that were collected at different disease stages from desmoglein 2-mutant mice, a well characterized AC model. We find that cardiomyocyte diameters are significantly increased in right ventricles of AC patients. Increased mRNA expression of the cardiac stress marker natriuretic peptide B is also observed in the right ventricle of AC patients. Elevated myosin heavy chain 7 mRNA expression is detected in left ventricles. In desmoglein 2-mutant mice, cardiomyocyte diameters are normal during the concealed disease phase but increase significantly after acute disease onset on cardiomyocyte death and fibrotic myocardial remodeling. Hypertrophy progresses further during the chronic disease stage. In parallel, mRNA expression of myosin heavy chain 7 and natriuretic peptide B is up-regulated in both ventricles with right ventricular preference. Calcineurin/nuclear factor of activated T cells (Nfat) signaling, which is linked to pathological hypertrophy, is observed during AC progression, as evidenced by Nfatc2 and Nfatc3 mRNA in cardiomyocytes and increased mRNA of the Nfat target regulator of calcineurin 1. Taken together, we demonstrate that pathological hypertrophy occurs in AC and is secondary to cardiomyocyte loss and cardiac remodeling.

Next-Generation Sequencing in Post-mortem Genetic Testing of Young Sudden Cardiac Death Cases

Sudden cardiac death (SCD) in the young (<40 years) occurs in the setting of a variety of rare inherited cardiac disorders and is a disastrous event for family members. Establishing the cause of SCD is important as it permits the pre-symptomatic identification of relatives at risk of SCD. Sudden arrhythmic death syndrome (SADS) is defined as SCD in the setting of negative autopsy findings and toxicological analysis. In such cases, reaching a diagnosis is even more challenging and post-mortem genetic testing can crucially contribute to the identification of the underlying cause of death. In this review, we will discuss the current achievements of “the molecular autopsy” in young SADS cases and provide an overview of key challenges in assessing pathogenicity (i.e., causality) of genetic variants identified through next-generation sequencing.

Arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (AC) is a heart muscle disease clinically characterized by life-threatening ventricular arrhythmias and pathologically by an acquired and progressive dystrophy of the ventricular myocardium with fibro-fatty replacement. Due to an estimated prevalence of 1:2000-1:5000, AC is listed among rare diseases. A familial background consistent with an autosomal-dominant trait of inheritance is present in most of AC patients; recessive variants have also been reported, either or not associated with palmoplantar keratoderma and woolly hair. AC-causing genes mostly encode major components of the cardiac desmosome and up to 50 % of AC probands harbor mutations in one of them. Mutations in non-desmosomal genes have been also described in a minority of AC patients, predisposing to the same or an overlapping disease phenotype. Compound/digenic heterozygosity was identified in up to 25 % of AC-causing desmosomal gene mutation carriers, in part explaining the phenotypic variability. Abnormal trafficking of intercellular proteins to the intercalated discs of cardiomyocytes and Wnt/beta catenin and Hippo signaling pathways have been implicated in disease pathogenesis.

AC is a major cause of sudden death in the young and in athletes. The clinical picture may include a sub-clinical phase; an overt electrical disorder; and right ventricular or biventricular pump failure. Ventricular fibrillation can occur at any stage. Genotype-phenotype correlation studies led to identify biventricular and dominant left ventricular variants, thus supporting the use of the broader term AC.

Since there is no “gold standard” to reach the diagnosis of AC, multiple categories of diagnostic information have been combined and the criteria recently updated, to improve diagnostic sensitivity while maintaining specificity. Among diagnostic tools, contrast enhanced cardiac magnetic resonance is playing a major role in detecting left dominant forms of AC, even preceding morpho-functional abnormalities. The main differential diagnoses are idiopathic right ventricular outflow tract tachycardia, myocarditis, sarcoidosis, dilated cardiomyopathy, right ventricular infarction, congenital heart diseases with right ventricular overload and athlete heart. A positive genetic test in the affected AC proband allows early identification of asymptomatic carriers by cascade genetic screening of family members. Risk stratification remains a major clinical challenge and antiarrhythmic drugs, catheter ablation and implantable cardioverter defibrillator are the currently available therapeutic tools. Sport disqualification is life-saving, since effort is a major trigger not only of electrical instability but also of disease onset and progression. We review the current knowledge of this rare cardiomyopathy, suggesting a flowchart for primary care clinicians and geneticists.