Impaired Right Ventricular Calcium Cycling Is an Early Risk Factor in R14del-Phospholamban Arrhythmias

Early consequences of the phospholamban mutation PLN-R14del+/- in a transgenic mouse model

AIMS

The heterozygous phospholamban (PLN) mutation R14del (PLN R14del+/- ) is associated with a severe arrhythmogenic cardiomyopathy (ACM) developing in the adult. "Superinhibition" of SERCA2a by PLN R14del is widely assumed to underlie the pathogenesis, but alternative mechanisms such abnormal energy metabolism have also been reported. This work aims to (1) to evaluate Ca2+ dynamics and energy metabolism in a transgenic (TG) mouse model of the mutation prior to cardiomyopathy development; (2) to test whether they are causally connected.

METHODS

Ca2+ dynamics, energy metabolism parameters, reporters of mitochondrial integrity, energy, and redox homeostasis were measured in ventricular myocytes of 8-12 weeks-old, phenotypically silent, TG mice. Mutation effects were compared to pharmacological PLN antagonism and analyzed during modulation of sarcoplasmic reticulum (SR) and cytosolic Ca2+ compartments. Transcripts and proteins of relevant signaling pathways were evaluated.

RESULTS

The mutation was characterized by hyperdynamic Ca2+ handling, compatible with a loss of SERCA2a inhibition by PLN. All components of energy metabolism were depressed; myocyte energy charge was preserved under quiescence but reduced during stimulation. Cytosolic Ca2+ buffering or SERCA2a blockade reduced O2 consumption with larger effect in the mutant. Signaling changes suggest cellular adaptation to perturbed Ca2+ dynamics and response to stress.

CONCLUSIONS

(1) PLN R14del+/- loses its ability to inhibit SERCA2a, which argues against SERCA2a superinhibition as a pathogenetic mechanism; (2) depressed energy metabolism, its enhanced dependency on Ca2+ and activation of signaling responses point to an early involvement of metabolic stress in the pathogenesis of this ACM model.

ECG-only explainable deep learning algorithm predicts the risk for malignant ventricular arrhythmia in phospholamban cardiomyopathy

BACKGROUND

Phospholamban (PLN) p.(Arg14del) variant carriers are at risk for development of malignant ventricular arrhythmia (MVA). Accurate risk stratification allows timely implantation of intracardiac defibrillators and is currently performed with a multimodality prediction model.

OBJECTIVE

This study aimed to investigate whether an explainable deep learning-based approach allows risk prediction with only electrocardiogram (ECG) data.

METHODS

A total of 679 PLN p.(Arg14del) carriers without MVA at baseline were identified. A deep learning-based variational auto-encoder, trained on 1.1 million ECGs, was used to convert the 12-lead baseline ECG into its FactorECG, a compressed version of the ECG that summarizes it into 32 explainable factors. Prediction models were developed by Cox regression.

RESULTS

The deep learning-based ECG-only approach was able to predict MVA with a C statistic of 0.79 (95% CI, 0.76-0.83), comparable to the current prediction model (C statistic, 0.83 [95% CI, 0.79-0.88]; P = .054) and outperforming a model based on conventional ECG parameters (low-voltage ECG and negative T waves; C statistic, 0.65 [95% CI, 0.58-0.73]; P < .001). Clinical simulations showed that a 2-step approach, with ECG-only screening followed by a full workup, resulted in 60% less additional diagnostics while outperforming the multimodal prediction model in all patients. A visualization tool was created to provide interactive visualizations (https://pln.ecgx.ai).

CONCLUSION

Our deep learning-based algorithm based on ECG data only accurately predicts the occurrence of MVA in PLN p.(Arg14del) carriers, enabling more efficient stratification of patients who need additional diagnostic testing and follow-up.

Deep neural network-based clustering of deformation curves reveals novel disease features in PLN pathogenic variant carriers

Echocardiographic deformation curves provide detailed information on myocardial function. Deep neural networks (DNNs) may enable automated detection of disease features in deformation curves, and improve the clinical assessment of these curves. We aimed to investigate whether an explainable DNN-based pipeline can be used to detect and visualize disease features in echocardiographic deformation curves of phospholamban (PLN) p.Arg14del variant carriers. A DNN was trained to discriminate PLN variant carriers (n = 278) from control subjects (n = 621) using raw deformation curves obtained by 2D-speckle tracking in the longitudinal axis. A visualization technique was used to identify the parts of these curves that were used by the DNN for classification. The PLN variant carriers were clustered according to the output of the visualization technique. The DNN showed excellent discriminatory performance (C-statistic 0.93 [95% CI 0.87-0.97]). We identified four clusters with PLN-associated disease features in the deformation curves. Two clusters showed previously described features: apical post-systolic shortening and reduced systolic strain. The two other clusters revealed novel features, both reflecting delayed relaxation. Additionally, a fifth cluster was identified containing variant carriers without disease features in the deformation curves, who were classified as controls by the DNN. This latter cluster had a very benign disease course regarding development of ventricular arrhythmias. Applying an explainable DNN-based pipeline to myocardial deformation curves enables automated detection and visualization of disease features. In PLN variant carriers, we discovered novel disease features which may improve individual risk stratification. Applying this approach to other diseases will further expand our knowledge on disease-specific deformation patterns. Overview of the deep neural network-based pipeline for feature detection in myocardial deformation curves. Firstly, phospholamban (PLN) p.Arg14del variant carriers and controls were selected and a deep neural network (DNN) was trained to detect the PLN variant carriers. Subsequently, a clustering-based approach was performed on the attention maps of the DNN, which revealed 4 distinct phenotypes of PLN variant carriers with different prognoses. Moreover, a cluster without features and a benign prognosis was detected.

Therapeutic Targets and Personalized Medicine in Cardiac Disease

Despite extensive research that has achieved notable advancements over the last decades, cardiovascular disease (CVD) remains the leading cause of death worldwide, with millions affected around the world [...].

Evolving cardiovascular genetic counseling needs in the era of precision medicine

In the era of Precision Medicine the approach to disease diagnosis, treatment, and prevention is being transformed across medical specialties, including Cardiology, and increasingly involves genomics approaches. The American Heart Association endorses genetic counseling as an essential component in the successful delivery of cardiovascular genetics care. However, with the dramatic increase in the number of available cardiogenetic tests, the demand, and the test result complexity, there is a need not only for a greater number of genetic counselors but more importantly, for highly specialized cardiovascular genetic counselors. Consequently, there is a pressing need for advanced cardiovascular genetic counseling training, along with innovative online services, telemedicine, and patient-facing digital tools, as the most effective way forward. The speed of implementation of these reforms will be of essence in the translation of scientific advancements into measurable benefits for patients with heritable cardiovascular disease and their families.

Dilated cardiomyopathy variant R14del increases phospholamban pentamer stability, blunting dynamic regulation of cardiac calcium handling

The sarco(endo)plasmic reticulum Ca ²⁺ ATPase (SERCA) is a membrane transporter that creates and maintains intracellular Ca ²⁺ stores. In the heart, SERCA is regulated by an inhibitory interaction with the monomeric form of the transmembrane micropeptide phospholamban (PLB). PLB also forms avid homo-pentamers, and dynamic exchange of PLB between pentamers and the regulatory complex with SERCA is an important determinant of cardiac responsiveness to exercise. Here, we investigated two naturally occurring pathogenic mutations of PLB, a cysteine substitution of arginine 9 (R9C) and an in-frame deletion of arginine 14 (R14del). Both mutations are associated with dilated cardiomyopathy. We previously showed that the R9C mutation causes disulfide crosslinking and hyperstabilization of pentamers. While the pathogenic mechanism of R14del is unclear, we hypothesized that this mutation may also alter PLB homo-oligomerization and disrupt the PLB-SERCA regulatory interaction. SDS-PAGE revealed a significantly increased pentamer:monomer ratio for R14del-PLB when compared to WT-PLB. In addition, we quantified homo-oligomerization and SERCA-binding in live cells using fluorescence resonance energy transfer (FRET) microscopy. R14del-PLB showed an increased affinity for homo-oligomerization and decreased binding affinity for SERCA compared to WT, suggesting that, like R9C, the R14del mutation stabilizes PLB in its pentameric form, decreasing its ability to regulate SERCA. Moreover, the R14del mutation reduces the rate of PLB unbinding from the pentamer after a transient Ca ²⁺ elevation, limiting the rate of re-binding to SERCA. A computational model predicted that hyperstabilization of PLB pentamers by R14del impairs the ability of cardiac Ca ²⁺ handling to respond to changing heart rates between rest and exercise. We postulate that impaired responsiveness to physiological stress contributes to arrhythmogenesis in human carriers of the R14del mutation.

Myofilament Alterations Associated with Human R14del-Phospholamban Cardiomyopathy

Phospholamban (PLN) is a major regulator of cardiac contractility, and human mutations in this gene give rise to inherited cardiomyopathies. The deletion of Arginine 14 is the most-prevalent cardiomyopathy-related mutation, and it has been linked to arrhythmogenesis and early death. Studies in PLN-humanized mutant mice indicated an increased propensity to arrhythmias, but the underlying cellular mechanisms associated with R14del-PLN cardiac dysfunction in the absence of any apparent structural remodeling remain unclear. The present study addressed the specific role of myofilaments in the setting of R14del-PLN and the long-term effects of R14del-PLN in the heart. Maximal force was depressed in skinned cardiomyocytes from both left and right ventricles, but this effect was more pronounced in the right ventricle of R14del-PLN mice. In addition, the Ca2+ sensitivity of myofilaments was increased in both ventricles of mutant mice. However, the depressive effects of R14del-PLN on contractile parameters could be reversed with the positive inotropic drug omecamtiv mecarbil, a myosin activator. At 12 months of age, corresponding to the mean symptomatic age of R14del-PLN patients, contractile parameters and Ca2+ transients were significantly depressed in the right ventricular R14del-PLN cardiomyocytes. Echocardiography did not reveal any alterations in cardiac function or remodeling, although histological and electron microscopy analyses indicated subtle alterations in mutant hearts. These findings suggest that both aberrant myocyte calcium cycling and aberrant contractility remain specific to the right ventricle in the long term. In addition, altered myofilament activity is an early characteristic of R14del-PLN mutant hearts and the positive inotropic drug omecamtiv mecarbil may be beneficial in treating R14del-PLN cardiomyopathy.

Phospholamban R14del disease: The past, the present and the future

Arrhythmogenic cardiomyopathy affects significant number of patients worldwide and is characterized by life-threatening ventricular arrhythmias and sudden cardiac death. Mutations in multiple genes with diverse functions have been reported to date including phospholamban (PLN), a key regulator of sarcoplasmic reticulum (SR) Ca²⁺ homeostasis and cardiac contractility. The PLN-R14del variant in specific is recognized as the cause in an increasing number of patients worldwide, and extensive investigations have enabled rapid advances towards the delineation of PLN-R14del disease pathogenesis and discovery of an effective treatment. We provide a critical overview of current knowledge on PLN-R14del disease pathophysiology, including clinical, animal model, cellular and biochemical studies, as well as diverse therapeutic approaches that are being pursued. The milestones achieved in <20 years, since the discovery of the PLN R14del mutation (2006), serve as a paradigm of international scientific collaboration and patient involvement towards finding a cure.

Gene editing reverses arrhythmia susceptibility in humanized PLN-R14del mice: modelling a European cardiomyopathy with global impact

AIMS

A mutation in the phospholamban (PLN) gene, leading to deletion of Arg14 (R14del), has been associated with malignant arrhythmias and ventricular dilation. Identifying pre-symptomatic carriers with vulnerable myocardium is crucial because arrhythmia can result in sudden cardiac death, especially in young adults with PLN-R14del mutation. This study aimed at assessing the efficiency and efficacy of in vivo genome editing, using CRISPR/Cas9 and a cardiotropic adeno-associated virus-9 (AAV9), in improving cardiac function in young adult mice expressing the human PLN-R14del.

METHODS AND RESULTS

Humanized mice were generated expressing human wild-type (hPLN-WT) or mutant (hPLN-R14del) PLN in the heterozygous state, mimicking human carriers. Cardiac magnetic resonance imaging at 12 weeks of age showed bi-ventricular dilation and increased stroke volume in mutant vs. WT mice, with no deficit in ejection fraction or cardiac output. Challenge of ex vivo hearts with isoproterenol and rapid pacing unmasked higher propensity for sustained ventricular tachycardia (VT) in hPLN-R14del relative to hPLN-WT. Specifically, the VT threshold was significantly reduced (20.3 ± 1.2 Hz in hPLN-R14del vs. 25.7 ± 1.3 Hz in WT, P < 0.01) reflecting higher arrhythmia burden. To inactivate the R14del allele, mice were tail-vein-injected with AAV9.CRISPR/Cas9/gRNA or AAV9 empty capsid (controls). CRISPR-Cas9 efficiency was evaluated by droplet digital polymerase chain reaction and NGS-based amplicon sequencing. In vivo gene editing significantly reduced end-diastolic and stroke volumes in hPLN-R14del CRISPR-treated mice compared to controls. Susceptibility to VT was also reduced, as the VT threshold was significantly increased relative to controls (30.9 ± 2.3 Hz vs. 21.3 ± 1.5 Hz; P < 0.01).

CONCLUSIONS

This study is the first to show that disruption of hPLN-R14del allele by AAV9-CRISPR/Cas9 improves cardiac function and reduces VT susceptibility in humanized PLN-R14del mice, offering preclinical evidence for translatable approaches to therapeutically suppress the arrhythmogenic phenotype in human patients with PLN-R14del disease.

Aberrant PLN-R14del Protein Interactions Intensify SERCA2a Inhibition, Driving Impaired Ca2+ Handling and Arrhythmogenesis

Phospholamban (PLN), a key modulator of Ca2+-homeostasis, inhibits sarcoplasmic reticulum (SR) calcium-ATPase (SERCA2a) and regulates cardiac contractility. The human PLN mutation R14del has been identified in arrhythmogenic cardiomyopathy patients worldwide and is currently extensively investigated. In search of the molecular mechanisms mediating the pathological phenotype, we examined PLN-R14del associations to known PLN-interacting partners. We determined that PLN-R14del interactions to key Ca2+-handling proteins SERCA2a and HS-1-associated protein X-1 (HAX-1) were enhanced, indicating the super-inhibition of SERCA2a’s Ca2+-affinity. Additionally, histidine-rich calcium binding protein (HRC) binding to SERCA2a was increased, suggesting the inhibition of SERCA2a maximal velocity. As phosphorylation relieves the inhibitory effect of PLN on SERCA2a activity, we examined the impact of phosphorylation on the PLN-R14del/SERCA2a interaction. Contrary to PLN-WT, phosphorylation did not affect PLN-R14del binding to SERCA2a, due to a lack of Ser-16 phosphorylation in PLN-R14del. No changes were observed in the subcellular distribution of PLN-R14del or its co-localization to SERCA2a. However, in silico predictions suggest structural perturbations in PLN-R14del that could impact its binding and function. Our findings reveal for the first time that by increased binding to SERCA2a and HAX-1, PLN-R14del acts as an enhanced inhibitor of SERCA2a, causing a cascade of molecular events contributing to impaired Ca2+-homeostasis and arrhythmogenesis. Relieving SERCA2a super-inhibition could offer a promising therapeutic approach for PLN-R14del patients.

Generation of human induced 4x44 stem cell (iPSC) lines derived from five patients carrying the pathogenic phospholamban-R14del (PLN-R14del) variant and three non-carrier family members

The R14del pathogenic variant in the phospholamban (PLN) gene (PLN-R14del), has been identified in families with hereditary cardiomyopathy, including dilated and arrhythmogenic cardiomyopathies. Here we have generated human iPSC lines from five PLN-R14del carriers and three non-carrier family members. Peripheral blood mononuclear cells (PBMC) were obtained from the eight individuals and reprogrammed using Sendai viral vector system carrying the Yamanaka factors. All eight lines show typical iPSC morphology, normal karyotype, high expression of pluripotency markers, and possess the ability to differentiate into all three germ layers. These lines represent valuable resources for studying the pathophysiological mechanisms of PLN-R14del associated cardiomyopathy.