Regional Variation in RBM20 Causes a Highly Penetrant Arrhythmogenic Cardiomyopathy

New Insights in RBM20 Cardiomyopathy

PURPOSE OF REVIEW

This review aims to give an update on recent findings related to the cardiac splicing factor RNA-binding motif protein 20 (RBM20) and RBM20 cardiomyopathy, a form of dilated cardiomyopathy caused by mutations in RBM20.

RECENT FINDINGS

While most research on RBM20 splicing targets has focused on titin (TTN), multiple studies over the last years have shown that other splicing targets of RBM20 including Ca²⁺/calmodulin-dependent kinase IIδ (CAMK2D) might be critically involved in the development of RBM20 cardiomyopathy. In this regard, loss of RBM20 causes an abnormal intracellular calcium handling, which may relate to the arrhythmogenic presentation of RBM20 cardiomyopathy. In addition, RBM20 presents clinically in a highly gender-specific manner, with male patients suffering from an earlier disease onset and a more severe disease progression. Further research on RBM20, and treatment of RBM20 cardiomyopathy, will need to consider both the multitude and relative contribution of the different splicing targets and related pathways, as well as gender differences.

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.

Contemporary and Future Approaches to Precision Medicine in Inherited Cardiomyopathies: JACC Focus Seminar 3/5

Inherited cardiomyopathies are commonly occurring myocardial disorders that are associated with substantial morbidity and mortality. Clinical management strategies have focused on treatment of heart failure and arrhythmic complications in symptomatic patients according to standardized guidelines. Clinicians are now being urged to implement precision medicine, but what does this involve? Advances in understanding of the genetic underpinnings of inherited cardiomyopathies have brought new possibilities for interventions that are tailored to genes, specific variants, or downstream mechanisms. However, the phenotypic variability that can occur with any given pathogenic variant suggests that factors other than single driver gene mutations are often involved. This is propelling a new imperative to elucidate the nuanced ways in which individual combinations of genetic variation, comorbidities, and lifestyle may influence cardiomyopathy phenotypes. Here, Part 3 of a 5-part precision medicine Focus Seminar series reviews the current status and future opportunities for precision medicine in the inherited cardiomyopathies.

Precision Medicine in Cardiovascular Disease: Genetics and Impact on Phenotypes: JACC Focus Seminar 1/5

Our understanding of the genetic basis of cardiovascular diseases (CVDs) has evolved rapidly. This has resulted from a combination of dedicated research in well phenotyped CVD patients, the sequencing of the human genome, and the ready accessibility and decreasing cost of next-generation sequencing technologies. This increased knowledge of the genetic basis of CVDs has heralded the era of precision medicine. This encompasses many elements including improved diagnosis, family screening, assistance with reproductive decisions, targeted therapeutics guided by both phenotype and genotype, and providing important insights into risk stratification and prognosis. Furthermore, novel insights into genetic mechanisms, clinical rollout of polygenic risk scores for common CVDs, and the promise of genome editing approaches to effectively cure disease represent some of the exciting future endeavors that will change established clinical approaches. This Part 1 of a 5-part series focuses on the underpinnings and fundamental aspects of precision medicine.

CaMKIIδ Splice Variants in the Healthy and Diseased Heart

RNA splicing has been recognized in recent years as a pivotal player in heart development and disease. The Ca²⁺/calmodulin dependent protein kinase II delta (CaMKIIδ) is a multifunctional Ser/Thr kinase family and generates at least 11 different splice variants through alternative splicing. This enzyme, which belongs to the CaMKII family, is the predominant family member in the heart and functions as a messenger toward adaptive or detrimental signaling in cardiomyocytes. Classically, the nuclear CaMKIIδB and cytoplasmic CaMKIIδC splice variants are described as mediators of arrhythmias, contractile function, Ca²⁺ handling, and gene transcription. Recent findings also put CaMKIIδA and CaMKIIδ9 as cardinal players in the global CaMKII response in the heart. In this review, we discuss and summarize the new insights into CaMKIIδ splice variants and their (proposed) functions, as well as CaMKII-engineered mouse phenotypes and cardiac dysfunction related to CaMKIIδ missplicing. We also discuss RNA splicing factors affecting CaMKII splicing. Finally, we discuss the translational perspective derived from these insights and future directions on CaMKIIδ splicing research in the healthy and diseased heart.

The Role of Genetic Testing in the Evaluation of Dilated Cardiomyopathies

We present an adolescent African American male admitted to the cardiac intensive care unit with cardiogenic shock and acute respiratory failure. Through an overview of his presentation, diagnostic workup, and treatment, we demonstrate the clinical utility of genetic testing in the evaluation of unexplained dilated cardiomyopathies.

Malignant Arrhythmogenic Role Associated with RBM20: A Comprehensive Interpretation Focused on a Personalized Approach

The RBM20 gene encodes the muscle-specific splicing factor RNA-binding motif 20, a regulator of heart-specific alternative splicing. Nearly 40 potentially deleterious variants in RBM20 have been reported in the last ten years, being found to be associated with highly arrhythmogenic events in familial dilated cardiomyopathy. Frequently, malignant arrhythmias can be a primary manifestation of disease. The early recognition of arrhythmic genotypes is crucial in avoiding lethal episodes, as it may have an impact on the adoption of personalized preventive measures. Our study performs a comprehensive update of data concerning rare variants in RBM20 that are associated with malignant arrhythmogenic phenotypes with a focus on personalized medicine.

Promise and Peril of Population Genomics for the Development of Genome-First Approaches in Mendelian Cardiovascular Disease

The rich tradition of cardiovascular genomics has placed the field in prime position to extend our knowledge toward a genome-first approach to diagnosis and therapy. Population-scale genomic data has enabled exponential improvements in our ability to adjudicate variant pathogenicity based on allele rarity, and there has been a significant effort to employ these sizeable data in the investigation of rare disease. Certainly, population genomics data has great potential to aid the development of a genome-first approach to Mendelian cardiovascular disease, but its use in the clinical and investigative decision making is limited by the characteristics of the populations studied, and the evolutionary constraints on human Mendelian variation. To truly empower clinicians and patients, the successful implementation of a genome-first approach to rare cardiovascular disease will require the nuanced incorporation of population-based discovery with detailed investigation of rare disease cohorts and prospective variant evaluation.

RBM20-Associated Ventricular Arrhythmias in a Patient with Structurally Normal Heart

RBM20 (RNA-binding motif protein 20) is a splicing factor targeting multiple cardiac genes, and its mutations cause cardiomyopathies. Originally, RBM20 mutations were discovered to cause the development of dilated cardiomyopathy by erroneous splicing of the gene TTN (titin). Titin is a giant protein found in a structure of the sarcomere that functions as a molecular spring and provides a passive stiffness to the cardiomyocyte. Later, RBM20 mutations were also described in association with arrhythmogenic right ventricular cardiomyopathy and left ventricular noncompaction cardiomyopathy. Here, we present a clinical case of a rare arrhythmogenic phenotype and no structural cardiac abnormalities associated with a RBM20 genetic variant of uncertain significance.

Emerging Techniques for Risk Stratification in Nonischemic Dilated Cardiomyopathy: JACC Review Topic of the Week

Dilated cardiomyopathy (DCM) is a common condition, which carries significant mortality from sudden cardiac death and pump failure. Left ventricular ejection fraction has conventionally been used as a risk marker for sudden cardiac death, but has performed poorly in trials. There have been significant advances in the areas of cardiac magnetic resonance imaging and genetics, which are able to provide useful rick prediction in DCM. Biomarkers and cardiopulmonary exercise testing are well validated in the prediction of risk in heart failure; however, they have been tested less specifically in the DCM setting. This review will discuss these methods with a view toward multiparametric risk assessment in DCM with the hope of creating parametric risk models to predict sudden cardiac death and pump failure in the DCM population.

Dysregulated ribonucleoprotein granules promote cardiomyopathy in RBM20 gene-edited pigs

Ribonucleoprotein (RNP) granules are biomolecular condensates-liquid-liquid phase-separated droplets that organize and manage messenger RNA metabolism, cell signaling, biopolymer assembly, biochemical reactions and stress granule responses to cellular adversity. Dysregulated RNP granules drive neuromuscular degenerative disease but have not previously been linked to heart failure. By exploring the molecular basis of congenital dilated cardiomyopathy (DCM) in genome-edited pigs homozygous for an RBM20 allele encoding the pathogenic R636S variant of human RNA-binding motif protein-20 (RBM20), we discovered that RNP granules accumulated abnormally in the sarcoplasm, and we confirmed this finding in myocardium and reprogrammed cardiomyocytes from patients with DCM carrying the R636S allele. Dysregulated sarcoplasmic RBM20 RNP granules displayed liquid-like material properties, docked at precisely spaced intervals along cytoskeletal elements, promoted phase partitioning of cardiac biomolecules and fused with stress granules. Our results link dysregulated RNP granules to myocardial cellular pathobiology and heart failure in gene-edited pigs and patients with DCM caused by RBM20 mutation.

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.

High prevalence of subtle systolic and diastolic dysfunction in genotype-positive phenotype-negative relatives of dilated cardiomyopathy patients

BACKGROUND

The early diagnosis of genetically determined dilated cardiomyopathy (DCM) could improve the prognosis in mutation carriers. Left ventricular global longitudinal strain (LV GLS) and peak left atrial longitudinal strain (PALS) are promising techniques for the detection of subtle systolic and diastolic dysfunction. We sought to evaluate the prevalence of subtle systolic and diastolic dysfunction by LV GLS and PALS in a cohort of genotype-positive phenotype-negative (GPFN) DCM relatives.

METHODS AND RESULTS

In this retrospective study, we analyzed echocardiograms of forty-one GPFN relatives of DCM patients. They were compared with age and sex matched healthy individuals (control group). Reduced LV GLS and PALS were defined as >18% and <23.1%, respectively. GPFN relatives (37 ± 14 years, 48.8% male) and controls were similar according to standard echocardiographic measurements. Conversely, LV GLS was -18.8 ± 2.7% in the GPFN group vs. -24.0 ± 1.8% in the control group (p < 0.001). Twenty subjects (48.8%) in the GPFN group and no subjects in the control group had a reduced LV GLS. PALS was 29.2 ± 6.7% in the GPFN group vs. 40.8 ± 8.5% in the control group (p < 0.001). Seven subjects (18.4%) in the GPFN group and one (2%) in the control group had a reduced PALS. A cohort of 17 genotype-negative phenotype-negative relatives showed higher values of LV GLS compared to GPFN.

CONCLUSIONS

Despite standard echocardiographic parameters are within the normal range, LV GLS and PALS are lower in GPFN relatives of DCM patients when compared to healthy individuals, suggesting a consistent proportion of subtle systolic and diastolic dysfunction in this population.

State of the Art Review on Genetics and Precision Medicine in Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is an inherited cardiomyopathy characterised by ventricular arrhythmia and an increased risk of sudden cardiac death (SCD). Numerous genetic determinants and phenotypic manifestations have been discovered in ACM, posing a significant clinical challenge. Further to this, wider evaluation of family members has revealed incomplete penetrance and variable expressivity in ACM, suggesting a complex genotype-phenotype relationship. This review details the genetic basis of ACM with specific genotype-phenotype associations, providing the reader with a nuanced perspective of this condition; whilst also proposing a future roadmap to delivering precision medicine-based management in ACM.

Structural basis of UCUU RNA motif recognition by splicing factor RBM20

The vertebrate splicing factor RBM20 (RNA binding motif protein 20) regulates protein isoforms important for heart development and function, with mutations in the gene linked to cardiomyopathy. Previous studies have identified the four nucleotide RNA motif UCUU as a common element in pre-mRNA targeted by RBM20. Here, we have determined the structure of the RNA Recognition Motif (RRM) domain from mouse RBM20 bound to RNA containing a UCUU sequence. The atomic details show that the RRM domain spans a larger region than initially proposed in order to interact with the complete UCUU motif, with a well-folded C-terminal helix encoded by exon 8 critical for high affinity binding. This helix only forms upon binding RNA with the final uracil, and removing the helix reduces affinity as well as specificity. We therefore find that RBM20 uses a coupled folding-binding mechanism by the C-terminal helix to specifically recognize the UCUU RNA motif.

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.

Genetic Risk of Arrhythmic Phenotypes in Patients With Dilated Cardiomyopathy

BACKGROUND

Genotype-phenotype correlations in dilated cardiomyopathy (DCM) and, in particular, the effects of gene variants on clinical outcomes remain poorly understood.

OBJECTIVES

The purpose of this study was to investigate the prognostic role of genetic variant carrier status in a large cohort of DCM patients.

METHODS

A total of 487 DCM patients were analyzed by next-generation sequencing and categorized the disease genes into functional gene groups. The following composite outcome measures were assessed: 1) all-cause mortality; 2) heart failure-related death, heart transplantation, or destination left ventricular assist device implantation (DHF/HTx/VAD); and 3) sudden cardiac death/sustained ventricular tachycardia/ventricular fibrillation (SCD/VT/VF).

RESULTS

A total of 183 pathogenic/likely pathogenic variants were found in 178 patients (37%): 54 (11%) Titin; 19 (4%) Lamin A/C (LMNA); 24 (5%) structural cytoskeleton-Z disk genes; 16 (3.5%) desmosomal genes; 46 (9.5%) sarcomeric genes; 8 (1.6%) ion channel genes; and 11 (2.5%) other genes. All-cause mortality was no different between variant carriers and noncarriers (p = 0.99). A trend toward worse SCD/VT/VF (p = 0.062) and DHF/HTx/VAD (p = 0.061) was found in carriers. Carriers of desmosomal and LMNA variants experienced the highest rate of SCD/VT/VF, which was independent of the left ventricular ejection fraction.

CONCLUSIONS

Desmosomal and LMNA gene variants identify the subset of DCM patients who are at greatest risk for SCD and life-threatening ventricular arrhythmias, regardless of the left ventricular ejection fraction.

Desmoplakin Cardiomyopathy, a Fibrotic and Inflammatory Form of Cardiomyopathy Distinct From Typical Dilated or Arrhythmogenic Right Ventricular Cardiomyopathy

BACKGROUND

Mutations in desmoplakin (DSP), the primary force transducer between cardiac desmosomes and intermediate filaments, cause an arrhythmogenic form of cardiomyopathy that has been variably associated with arrhythmogenic right ventricular cardiomyopathy. Clinical correlates of DSP cardiomyopathy have been limited to small case series.

METHODS

Clinical and genetic data were collected on 107 patients with pathogenic DSP mutations and 81 patients with pathogenic plakophilin 2 (PKP2) mutations as a comparison cohort. A composite outcome of severe ventricular arrhythmia was assessed.

RESULTS

DSP and PKP2 cohorts included similar proportions of probands (41% versus 42%) and patients with truncating mutations (98% versus 100%). Left ventricular (LV) predominant cardiomyopathy was exclusively present among patients with DSP (55% versus 0% for PKP2, P<0.001), whereas right ventricular cardiomyopathy was present in only 14% of patients with DSP versus 40% for PKP2 (P<0.001). Arrhythmogenic right ventricular cardiomyopathy diagnostic criteria had poor sensitivity for DSP cardiomyopathy. LV late gadolinium enhancement was present in a primarily subepicardial distribution in 40% of patients with DSP (23/57 with magnetic resonance images). LV late gadolinium enhancement occurred with normal LV systolic function in 35% (8/23) of patients with DSP. Episodes of acute myocardial injury (chest pain with troponin elevation and normal coronary angiography) occurred in 15% of patients with DSP and were strongly associated with LV late gadolinium enhancement (90%), even in cases of acute myocardial injury with normal ventricular function (4/5, 80% with late gadolinium enhancement). In 4 DSP cases with 18F-fluorodeoxyglucose positron emission tomography scans, acute LV myocardial injury was associated with myocardial inflammation misdiagnosed initially as cardiac sarcoidosis or myocarditis. Left ventricle ejection fraction <55% was strongly associated with severe ventricular arrhythmias for DSP cases (P<0.001, sensitivity 85%, specificity 53%). Right ventricular ejection fraction <45% was associated with severe arrhythmias for PKP2 cases (P<0.001) but was poorly associated for DSP cases (P=0.8). Frequent premature ventricular contractions were common among patients with severe arrhythmias for both DSP (80%) and PKP2 (91%) groups (P=non-significant).

CONCLUSIONS

DSP cardiomyopathy is a distinct form of arrhythmogenic cardiomyopathy characterized by episodic myocardial injury, left ventricular fibrosis that precedes systolic dysfunction, and a high incidence of ventricular arrhythmias. A genotype-specific approach for diagnosis and risk stratification should be used.

Risk Stratification for Sudden Cardiac Death in Non-Ischaemic Dilated Cardiomyopathy

Purpose of Review

Non-ischaemic dilated cardiomyopathy (DCM) occurs in 1 in 2500 individuals in the general population and is associated with considerable morbidity and mortality. Studies involving large numbers of unselected DCM patients have led to consensus guidelines recommending implantable cardioverter-defibrillator (ICD) implantation for protection against sudden cardiac death (SCD) in those with LVEF ≤35%. The purpose of this article is to review the literature for other potential markers including serological, electrocardiographic, echocardiographic, cardiac magnetic resonance, ambulatory ECG and genetic data, to highlight other potential markers that may optimise risk stratification for SCD in this cohort and thereby allow a more personalized approach to ICD-implantation.

Recent Findings

Recent studies including the Danish study to assess the efficacy of ICDs in patients with non-ischemic systolic heart failure on mortality (DANISH) trial have questioned the benefits of ICD implantation in this group of patients with no changes in all-cause mortality. Recent pooled cohorts of patients with genetic DCM and in particular in those with Lamin A/C (LMNA) mutations have identified patients at increased risk of SCD and allowed the creation of algorithms to prognosticate SCD risk in mutation carriers. Furthermore, genetic testing has identified other DCM-causing genes including filamin C (FLNC) and RBM20 which may be associated with higher rates of ventricular arrhythmia.

Summary

To date, risk-stratification for SCD has been hampered by the utilisation of heterogenous subsets of idiopathic DCM patients and by use of static risk models where predictions are based on a single time point with a lack of consideration of disease progression. The current focus of personalised risk-stratification for SCD is shifting towards better characterisation of underlying DCM aetiology and the development of multi-parametric risk-stratification models that incorporate time-dependent disease characteristics and novel biomarkers.

Human Induced Pluripotent Stem-Cell-Derived Cardiomyocytes as Models for Genetic Cardiomyopathies

In the last few decades, many pathogenic or likely pathogenic genetic mutations in over hundred different genes have been described for non-ischemic, genetic cardiomyopathies. However, the functional knowledge about most of these mutations is still limited because the generation of adequate animal models is time-consuming and challenging. Therefore, human induced pluripotent stem cells (iPSCs) carrying specific cardiomyopathy-associated mutations are a promising alternative. Since the original discovery that pluripotency can be artificially induced by the expression of different transcription factors, various patient-specific-induced pluripotent stem cell lines have been generated to model non-ischemic, genetic cardiomyopathies in vitro. In this review, we describe the genetic landscape of non-ischemic, genetic cardiomyopathies and give an overview about different human iPSC lines, which have been developed for the disease modeling of inherited cardiomyopathies. We summarize different methods and protocols for the general differentiation of human iPSCs into cardiomyocytes. In addition, we describe methods and technologies to investigate functionally human iPSC-derived cardiomyocytes. Furthermore, we summarize novel genome editing approaches for the genetic manipulation of human iPSCs. This review provides an overview about the genetic landscape of inherited cardiomyopathies with a focus on iPSC technology, which might be of interest for clinicians and basic scientists interested in genetic cardiomyopathies.