Homozygous Founder Mutation in Desmocollin-2 ( DSC2 ) Causes Arrhythmogenic Cardiomyopathy in the Hutterite Population

A Systematic Analysis of the Clinical Outcome Associated with Multiple Reclassified Desmosomal Gene Variants in Arrhythmogenic Right Ventricular Cardiomyopathy Patients

The presence of multiple pathogenic variants in desmosomal genes (DSC2, DSG2, DSP, JUP, and PKP2) in patients with arrhythmogenic right ventricular cardiomyopathy (ARVC) has been linked to a severe phenotype. However, the pathogenicity of variants is reclassified frequently, which may result in a changed clinical risk prediction. Here, we present the collection, reclassification, and clinical outcome correlation for the largest series of ARVC patients carrying multiple desmosomal pathogenic variants to date (n = 331). After reclassification, only 29% of patients remained carriers of two (likely) pathogenic variants. They reached the composite endpoint (ventricular arrhythmias, heart failure, and death) significantly earlier than patients with one or no remaining reclassified variant (hazard ratios of 1.9 and 1.8, respectively). Periodic reclassification of variants contributes to more accurate risk stratification and subsequent clinical management strategy. Graphical Abstract.

The Many Faces of Arrhythmogenic Cardiomyopathy: An Overview

Arrhythmogenic cardiomyopathy (AC) is a disease that involves electromechanical uncoupling of cardiomyocytes. This leads to characteristic histologic changes that ultimately lead to the arrhythmogenic clinical features of the disease. Initially thought to affect the right ventricle predominantly, more recent data show that it can affect both the ventricles or the left ventricle alone. Throughout the recent era, diagnostic modalities and criteria for AC have continued to evolve and our understanding of its clinical features in different age groups as well as the genotype to the phenotype correlations have improved. In this review, we set out to detail the epidemiology, etiologies, presentations, evaluation, and management of AC across the age continuum.

Accurate Classification of Non-ischemic Cardiomyopathy

PURPOSE OF REVIEW

This article aims to review the accurate classification of non-ischemic cardiomyopathy, including the methods, basis, subtype characteristics, and prognosis, especially the similarities and differences between different classifications.

RECENT FINDINGS

Non-ischemic cardiomyopathy refers to a myocardial disease that excludes coronary artery disease or ischemic injury and has a variety of etiologies and high incidence. Recent studies suggest that traditional classification methods based on primary/mixed/acquired or genetic/non-genetic cannot meet the precise needs of contemporary clinical management. This article systematically describes the history of classifications of cardiomyopathy and presents etiological and genetic differences between cardiomyopathies. The accurate classification is described from the perspective of morphology, function, and genomics in hypertrophic cardiomyopathy, dilated cardiomyopathy, restrictive cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy, left ventricular noncompaction, and partially acquired cardiomyopathy. The different clinical characteristics and treatment needs of these cardiomyopathies are elaborated. Some single-gene mutant cardiomyopathies have unique phenotypes, and some cardiomyopathies have mixed phenotypes. These special classifications require personalized precision treatment, which is worthy of independent research. This article describes recent advances in the accurate classification of non-ischemic cardiomyopathy from clinical phenotypes and causative genes, discusses the advantages and usage scenarios of each classification, compares the differences in prognosis and patient management needs of different subtypes, and summarizes common methods and new exploration directions for accurate classification.

Understanding Arrhythmogenic Cardiomyopathy: Advances through the Use of Human Pluripotent Stem Cell Models

Cardiomyopathies (CMPs) represent a significant healthcare burden and are a major cause of heart failure leading to premature death. Several CMPs are now recognized to have a strong genetic basis, including arrhythmogenic cardiomyopathy (ACM), which predisposes patients to arrhythmic episodes. Variants in one of the five genes (PKP2, JUP, DSC2, DSG2, and DSP) encoding proteins of the desmosome are known to cause a subset of ACM, which we classify as desmosome-related ACM (dACM). Phenotypically, this disease may lead to sudden cardiac death in young athletes and, during late stages, is often accompanied by myocardial fibrofatty infiltrates. While the pathogenicity of the desmosome genes has been well established through animal studies and limited supplies of primary human cells, these systems have drawbacks that limit their utility and relevance to understanding human disease. Human induced pluripotent stem cells (hiPSCs) have emerged as a powerful tool for modeling ACM in vitro that can overcome these challenges, as they represent a reproducible and scalable source of cardiomyocytes (CMs) that recapitulate patient phenotypes. In this review, we provide an overview of dACM, summarize findings in other model systems linking desmosome proteins with this disease, and provide an up-to-date summary of the work that has been conducted in hiPSC-cardiomyocyte (hiPSC-CM) models of dACM. In the context of the hiPSC-CM model system, we highlight novel findings that have contributed to our understanding of disease and enumerate the limitations, prospects, and directions for research to consider towards future progress.

Cardiomyopathy related desmocollin-2 prodomain variants affect the intracellular cadherin transport and processing

Background

Arrhythmogenic cardiomyopathy can be caused by genetic variants in desmosomal cadherins. Since cardiac desmosomal cadherins are crucial for cell-cell-adhesion, their correct localization at the plasma membrane is essential.

Methods

Nine desmocollin-2 variants at five positions from various public genetic databases (p.D30N, p.V52A/I, p.G77V/D/S, p.V79G, p.I96V/T) and three additional conserved positions (p.C32, p.C57, p.F71) within the prodomain were investigated in vitro using confocal microscopy. Model variants (p.C32A/S, p.V52G/L, p.C57A/S, p.F71Y/A/S, p.V79A/I/L, p.I96l/A) were generated to investigate the impact of specific amino acids.

Results

We revealed that all analyzed positions in the prodomain are critical for the intracellular transport. However, the variants p.D30N, p.V52A/I and p.I96V listed in genetic databases do not disturb the intracellular transport revealing that the loss of these canonical sequences may be compensated.

Conclusion

As disease-related homozygous truncating desmocollin-2 variants lacking the transmembrane domain are not localized at the plasma membrane, we predict that some of the investigated prodomain variants may be relevant in the context of arrhythmogenic cardiomyopathy due to disturbed intracellular transport.

Genetic Insights from Consanguineous Cardiomyopathy Families

Inherited cardiomyopathies are a prevalent cause of heart failure and sudden cardiac death. Both hypertrophic (HCM) and dilated cardiomyopathy (DCM) are genetically heterogeneous and typically present with an autosomal dominant mode of transmission. Whole exome sequencing and autozygosity mapping was carried out in eight un-related probands from consanguineous Middle Eastern families presenting with HCM/DCM followed by bioinformatic and co-segregation analysis to predict the potential pathogenicity of candidate variants. We identified homozygous missense variants in TNNI3K, DSP, and RBCK1 linked with a dilated phenotype, in NRAP linked with a mixed phenotype of dilated/hypertrophic, and in KLHL24 linked with a mixed phenotype of dilated/hypertrophic and non-compaction features. Co-segregation analysis in family members confirmed autosomal recessive inheritance presenting in early childhood/early adulthood. Our findings add to the mutational spectrum of recessive cardiomyopathies, supporting inclusion of KLHL24, NRAP and RBCK1 as disease-causing genes. We also provide evidence for novel (recessive) modes of inheritance of a well-established gene TNNI3K and expand our knowledge of the clinical heterogeneity of cardiomyopathies. A greater understanding of the genetic causes of recessive cardiomyopathies has major implications for diagnosis and screening, particularly in underrepresented populations, such as those of the Middle East.

Meeting report - Desmosome dysfunction and disease: Alpine desmosome disease meeting

Desmosome diseases are caused by dysfunction of desmosomes, which anchor intermediate filaments (IFs) at sites of cell-cell adhesion. For many decades, the focus of attention has been on the role of actin filament-associated adherens junctions in development and disease, especially cancer. However, interference with the function of desmosomes, their molecular constituents or their attachments to IFs has now emerged as a major contributor to a variety of diseases affecting different tissues and organs including skin, heart and the digestive tract. The first Alpine desmosome disease meeting (ADDM) held in Grainau, Germany, in October 2022 brought together international researchers from the basic sciences with clinical experts from diverse fields to share and discuss their ideas and concepts on desmosome function and dysfunction in the different cell types involved in desmosome diseases. Besides the prototypic desmosomal diseases pemphigus and arrhythmogenic cardiomyopathy, the role of desmosome dysfunction in inflammatory bowel diseases and eosinophilic esophagitis was discussed.

Towards a Better Understanding of Genotype-Phenotype Correlations and Therapeutic Targets for Cardiocutaneous Genes: The Importance of Functional Studies above Prediction

Genetic variants in gene-encoding proteins involved in cell−cell connecting structures, such as desmosomes and gap junctions, may cause a skin and/or cardiac phenotype, of which the combination is called cardiocutaneous syndrome. The cardiac phenotype is characterized by cardiomyopathy and/or arrhythmias, while the skin particularly displays phenotypes such as keratoderma, hair abnormalities and skin fragility. The reported variants associated with cardiocutaneous syndrome, in genes DSP, JUP, DSC2, KLHL24, GJA1, are classified by interpretation guidelines from the American College of Medical Genetics and Genomics. The genotype−phenotype correlation, however, remains poorly understood. By providing an overview of variants that are assessed for a functional protein pathology, we show that this number (n = 115) is low compared to the number of variants that are assessed by in silico algorithms (>5000). As expected, there is a mismatch between the prediction of variant pathogenicity and the prediction of the functional effect compared to the real functional evidence. Aiding to improve genotype−phenotype correlations, we separate variants into ‘protein reducing’ or ‘altered protein’ variants and provide general conclusions about the skin and heart phenotype involved. We conclude by stipulating that adequate prognoses can only be given, and targeted therapies can only be designed, upon full knowledge of the protein pathology through functional investigation.

Arrhythmogenic right ventricular cardiomyopathy in a Japanese patient with a homozygous founder variant of DSG2 in the East Asian population

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a hereditary cardiomyopathy that results in fatal arrhythmias and heart failure. Herein, we report a Japanese patient with ARVC whose parents were blood relatives. Genetic testing identified a homozygous rare variant, c.1592T > G (p.Phe531Cys), of DSG2 that is presumed to be a founder variant among East Asians. Genetic counseling sessions with precise risk assessment and appropriate follow-up programs were provided to the patient and family members.

Spectrum of Rare and Common Genetic Variants in Arrhythmogenic Cardiomyopathy Patients

Arrhythmogenic cardiomyopathy (ACM) is a rare inherited disorder, whose genetic cause is elusive in about 50–70% of cases. ACM presents a variable disease course which could be influenced by genetics. We performed next-generation sequencing on a panel of 174 genes associated with inherited cardiovascular diseases on 82 ACM probands (i) to describe and classify the pathogenicity of rare variants according to the American College of Medical Genetics and Genomics both for ACM-associated genes and for genes linked to other cardiovascular genetic conditions; (ii) to assess, for the first time, the impact of common variants on the ACM clinical disease severity by genotype-phenotype correlation and survival analysis. We identified 15 (likely) pathogenic variants and 66 variants of uncertain significance in ACM-genes and 4 high-impact variants in genes never associated with ACM (ABCC9, APOB, DPP6, MIB1), which deserve future consideration. In addition, we found 69 significant genotype-phenotype associations between common variants and clinical parameters. Arrhythmia-associated polymorphisms resulted in an increased risk of arrhythmic events during patients’ follow-up. The description of the genetic framework of our population and the observed genotype-phenotype correlation constitutes the starting point to address the current lack of knowledge in the genetics of ACM.

Overlap phenotypes of the left ventricular noncompaction and hypertrophic cardiomyopathy with complex arrhythmias and heart failure induced by the novel truncated DSC2 mutation

Background

The left ventricular noncompaction cardiomyopathy (LVNC) is a rare subtype of cardiomyopathy associated with a high risk of heart failure (HF), thromboembolism, arrhythmia, and sudden cardiac death.

Methods

The proband with overlap phenotypes of LVNC and hypertrophic cardiomyopathy (HCM) complicates atrial fibrillation (AF), ventricular tachycardia (VT), and HF due to the diffuse myocardial lesion, which were diagnosed by electrocardiogram, echocardiogram and cardiac magnetic resonance imaging. Peripheral blood was collected from the proband and his relatives. DNA was extracted from the peripheral blood of proband for high-throughput target capture sequencing. The Sanger sequence verified the variants. The protein was extracted from the skin of the proband and healthy volunteer. The expression difference of desmocollin2 was detected by Western blot.

Results

The novel heterozygous truncated mutation (p.K47Rfs*2) of the DSC2 gene encoding an important component of desmosomes was detected by targeted capture sequencing. The western blots showed that the expressing level of functional desmocollin2 protein (~ 94kd) was lower in the proband than that in the healthy volunteer, indicating that DSC2 p.K47Rfs*2 obviously reduced the functional desmocollin2 protein expression in the proband.

Conclusion

The heterozygous DSC2 p.K47Rfs*2 remarkably and abnormally reduced the functional desmocollin2 expression, which may potentially induce the overlap phenotypes of LVNC and HCM, complicating AF, VT, and HF.

Insights Into Genetics and Pathophysiology of Arrhythmogenic Cardiomyopathy

Purpose of Review

Arrhythmogenic cardiomyopathy (ACM) is a genetic disease characterized by life-threatening ventricular arrhythmias and sudden cardiac death (SCD) in apparently healthy young adults. Mutations in genes encoding for cellular junctions can be found in about half of the patients. However, disease onset and severity, risk of arrhythmias, and outcome are highly variable and drug-targeted treatment is currently unavailable.

Recent Findings

This review focuses on advances in clinical risk stratification, genetic etiology, and pathophysiological concepts. The desmosome is the central part of the disease, but other intercalated disc and associated structural proteins not only broaden the genetic spectrum but also provide novel molecular and cellular insights into the pathogenesis of ACM. Signaling pathways and the role of inflammation will be discussed and targets for novel therapeutic approaches outlined.

Summary

Genetic discoveries and experimental-driven preclinical research contributed significantly to the understanding of ACM towards mutation- and pathway-specific personalized medicine.

International Evidence Based Reappraisal of Genes Associated With Arrhythmogenic Right Ventricular Cardiomyopathy Using the Clinical Genome Resource Framework

BACKGROUND

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited disease characterized by ventricular arrhythmias and progressive ventricular dysfunction. Genetic testing is recommended, and a pathogenic variant in an ARVC-associated gene is a major criterion for diagnosis according to the 2010 Task Force Criteria. As incorrect attribution of a gene to ARVC can contribute to misdiagnosis, we assembled an international multidisciplinary ARVC Clinical Genome Resource Gene Curation Expert Panel to reappraise all reported ARVC genes.

METHODS

Following a comprehensive literature search, six 2-member teams conducted blinded independent curation of reported ARVC genes using the semiquantitative Clinical Genome Resource framework.

RESULTS

Of 26 reported ARVC genes, only 6 (PKP2, DSP, DSG2, DSC2, JUP, and TMEM43) had strong evidence and were classified as definitive for ARVC causation. There was moderate evidence for 2 genes, DES and PLN. The remaining 18 genes had limited or no evidence. RYR2 was refuted as an ARVC gene since clinical data and model systems exhibited a catecholaminergic polymorphic ventricular tachycardia phenotype. In ClinVar, only 5 pathogenic/likely pathogenic variants (1.1%) in limited evidence genes had been reported in ARVC cases in contrast to 450 desmosome gene variants (97.4%).

CONCLUSIONS

Using the Clinical Genome Resource approach to gene-disease curation, only 8 genes (PKP2, DSP, DSG2, DSC2, JUP, TMEM43, PLN, and DES) had definitive or moderate evidence for ARVC, and these genes accounted for nearly all pathogenic/likely pathogenic ARVC variants in ClinVar. Therefore, only pathogenic/likely pathogenic variants in these 8 genes should yield a major criterion for ARVC diagnosis. Pathogenic/likely pathogenic variants identified in other genes in a patient should prompt further phenotyping as variants in many of these genes are associated with other cardiovascular conditions.

Hemi- and Homozygous Loss-of-Function Mutations in DSG2 (Desmoglein-2) Cause Recessive Arrhythmogenic Cardiomyopathy with an Early Onset

About 50% of patients with arrhythmogenic cardiomyopathy (ACM) carry a pathogenic or likely pathogenic mutation in the desmosomal genes. However, there is a significant number of patients without positive familial anamnesis. Therefore, the molecular reasons for ACM in these patients are frequently unknown and a genetic contribution might be underestimated. Here, we used a next-generation sequencing (NGS) approach and in addition single nucleotide polymor-phism (SNP) arrays for the genetic analysis of two independent index patients without familial medical history. Of note, this genetic strategy revealed a homozygous splice site mutation (DSG2–c.378+1G>T) in the first patient and a nonsense mutation (DSG2–p.L772X) in combination with a large deletion in DSG2 in the second one. In conclusion, a recessive inheritance pattern is likely for both cases, which might contribute to the hidden medical history in both families. This is the first report about these novel loss-of-function mutations in DSG2 that have not been previously identi-fied. Therefore, we suggest performing deep genetic analyses using NGS in combination with SNP arrays also for ACM index patients without obvious familial medical history. In the future, this finding might has relevance for the genetic counseling of similar cases.

Rare Variants Associated with Arrhythmogenic Cardiomyopathy: Reclassification Five Years Later

Genetic interpretation of rare variants associated with arrhythmogenic cardiomyopathy (ACM) is essential due to their diagnostic implications. New data may relabel previous variant classifications, but how often reanalysis is necessary remains undefined. Five years ago, 39 rare ACM-related variants were identified in patients with features of cardiomyopathy. These variants were classified following the American College of Medical Genetics and Genomics’ guidelines. In the present study, we reevaluated these rare variants including novel available data. All cases carried one rare variant classified as being of ambiguous significance (82.05%) or likely pathogenic (17.95%) in 2016. In our comprehensive reanalysis, the classification of 30.77% of these variants changed, mainly due to updated global frequencies. As in 2016, nowadays most variants were classified as having an uncertain role (64.1%), but the proportion of variants with an uncertain role was significantly decreased (17.95%). The percentage of rare variants classified as potentially deleterious increased from 17.95% to 23.07%. Moreover, 83.33% of reclassified variants gained certainty. We propose that periodic genetic reanalysis of all rare variants associated with arrhythmogenic cardiomyopathy should be undertaken at least once every five years. Defining the roles of rare variants may help clinicians obtain a definite diagnosis.

Cardiomyopathies and Genetic Testing in Heart Failure: Role in Defining Phenotype-Targeted Approaches and Management

Cardiomyopathies represent an important cause of heart failure, often affecting young individuals, and have important implications for relatives. Genetic testing for cardiomyopathies is an established care pathway in contemporary cardiology practice. The primary cardiomyopathies where genetic testing is indicated are hypertrophic, dilated, arrhythmogenic, and restrictive cardiomyopathies, with left ventricular noncompaction as a variant phenotype. Early identification and initiation of therapies in patients with inherited cardiomyopathies allow for targeting asymptomatic and presymptomatic patients in stages A and B of the American College of Cardiology/American Heart Association classification of heart failure. The current approach for genetic testing uses gene panel-based testing with the ability to extend to whole-exome and whole-genome sequencing in rare instances. The central components of genetic testing include defining the genetic basis of the diagnosis, providing prognostic information, and the ability to screen and risk-stratify relatives. Genetic testing for cardiomyopathies should be coordinated by a multidisciplinary team including adult and pediatric cardiologists, genetic counsellors, and geneticists, with access to expertise in cardiac imaging and electrophysiology. A pragmatic approach for addressing genetic variants of uncertain significance is important. In this review, we highlight the indications for genetic testing in the various cardiomyopathies, the value of early diagnosis and treatment, family screening, and the care process involved in genetic counselling and testing.

A homozygous DSC2 deletion associated with arrhythmogenic cardiomyopathy is caused by uniparental isodisomy

AIMS

We aimed to unravel the genetic, molecular and cellular pathomechanisms of DSC2 truncation variants leading to arrhythmogenic cardiomyopathy (ACM).

METHODS AND RESULTS

We report a homozygous 4-bp DSC2 deletion variant c.1913_1916delAGAA, p.Q638LfsX647^hom causing a frameshift carried by an ACM patient. Whole exome sequencing and comparative genomic hybridization analysis support a loss of heterozygosity in a large segment of chromosome 18 indicating segmental interstitial uniparental isodisomy (UPD). Ultrastructural analysis of the explanted myocardium from a mutation carrier using transmission electron microscopy revealed a partially widening of the intercalated disc. Using qRT-PCR we demonstrated that DSC2 mRNA expression was substantially decreased in the explanted myocardial tissue of the homozygous carrier compared to controls. Western blot analysis revealed absence of both full-length desmocollin-2 isoforms. Only a weak expression of the truncated form of desmocollin-2 was detectable. Immunohistochemistry showed that the truncated form of desmocollin-2 did not localize at the intercalated discs. In vitro, transfection experiments using induced pluripotent stem cell derived cardiomyocytes and HT-1080 cells demonstrated an obvious absence of the mutant truncated desmocollin-2 at the plasma membrane. Immunoprecipitation in combination with fluorescence measurements and Western blot analyses revealed an abnormal secretion of the truncated desmocollin-2.

CONCLUSION

In summary, we unraveled segmental UPD as the likely genetic reason for a small homozygous DSC2 deletion. We conclude that a combination of nonsense mediated mRNA decay and extracellular secretion is involved in DSC2 related ACM.

Molecular mechanisms of arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy is a genetic disorder characterized by the risk of life-threatening arrhythmias, myocardial dysfunction and fibrofatty replacement of myocardial tissue. Mutations in genes that encode components of desmosomes, the adhesive junctions that connect cardiomyocytes, are the predominant cause of arrhythmogenic cardiomyopathy and can be identified in about half of patients with the condition. However, the molecular mechanisms leading to myocardial destruction, remodelling and arrhythmic predisposition remain poorly understood. Through the development of animal, induced pluripotent stem cell and other models of disease, advances in our understanding of the pathogenic mechanisms of arrhythmogenic cardiomyopathy over the past decade have brought several signalling pathways into focus. These pathways include canonical and non-canonical WNT signalling, the Hippo-Yes-associated protein (YAP) pathway and transforming growth factor-β signalling. These studies have begun to identify potential therapeutic targets whose modulation has shown promise in preclinical models. In this Review, we summarize and discuss the reported molecular mechanisms underlying the pathogenesis of arrhythmogenic cardiomyopathy.

G790del mutation in DSC2 alone is insufficient to develop the pathogenesis of ARVC in a mouse model

Background

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited heart disease that causes heart failure and/or sudden cardiac death. Several desmosomal genes (DSC2, PKG, PKP2, DSP, and RyR2) are thought to be the causative gene involved in ARVC. Out of them, DSC2 mutations account for 2% of ARVC genetic abnormalities. This study aimed to clarify the effect of G790del mutation in DSC2 on the arrhythmogenic mechanism and cardiac function in a mouse model.

Result

Neither the heterozygous +/G790del nor homozygous G790del/G790del mice showed structural and functional defects in the right ventricle (RV) or lethal arrhythmia. The homozygous G790del/G790del 6-month-old mice slightly showed left ventricular (LV) dysfunction. Cell shortening decreased with prolongation of intracellular Ca2+ transient in cardiomyocytes isolated from the homozygous G790del/G790del mice, and spontaneous Ca²⁺ transients were frequently observed in response to isoproterenol.

Conclusions

G790del mutation in DSC2 was not relevant to the pathogenesis of ARVC, but showed a slight contractile dysfunction and Ca²⁺ dysregulation in the LV.

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.

Clinical Diagnosis, Imaging, and Genetics of Arrhythmogenic Right Ventricular Cardiomyopathy/Dysplasia: JACC State-of-the-Art Review

Arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D) is an inherited cardiomyopathy that can lead to sudden cardiac death and heart failure. Our understanding of its pathophysiology and clinical expressivity is continuously evolving. The diagnosis of ARVC/D remains particularly challenging due to the absence of specific unique diagnostic criteria, its variable expressivity, and incomplete penetrance. Advances in genetics have enlarged the clinical spectrum of the disease, highlighting possible phenotypes that overlap with arrhythmogenic dilated cardiomyopathy and channelopathies. The principal challenges for ARVC/D diagnosis include the following: earlier detection of the disease, particularly in cases of focal right ventricular involvement; differential diagnosis from other arrhythmogenic diseases affecting the right ventricle; and the development of new objective electrocardiographic and imaging criteria for diagnosis. This review provides an update on the diagnosis of ARVC/D, focusing on the contribution of emerging imaging techniques, such as echocardiogram/magnetic resonance imaging strain measurements or computed tomography scanning, new electrocardiographic parameters, and high-throughput sequencing.

2019 HRS expert consensus statement on evaluation, risk stratification, and management of arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is an arrhythmogenic disorder of the myocardium not secondary to ischemic, hypertensive, or valvular heart disease. ACM incorporates a broad spectrum of genetic, systemic, infectious, and inflammatory disorders. This designation includes, but is not limited to, arrhythmogenic right/left ventricular cardiomyopathy, cardiac amyloidosis, sarcoidosis, Chagas disease, and left ventricular noncompaction. The ACM phenotype overlaps with other cardiomyopathies, particularly dilated cardiomyopathy with arrhythmia presentation that may be associated with ventricular dilatation and/or impaired systolic function. This expert consensus statement provides the clinician with guidance on evaluation and management of ACM and includes clinically relevant information on genetics and disease mechanisms. PICO questions were utilized to evaluate contemporary evidence and provide clinical guidance related to exercise in arrhythmogenic right ventricular cardiomyopathy. Recommendations were developed and approved by an expert writing group, after a systematic literature search with evidence tables, and discussion of their own clinical experience, to present the current knowledge in the field. Each recommendation is presented using the Class of Recommendation and Level of Evidence system formulated by the American College of Cardiology and the American Heart Association and is accompanied by references and explanatory text to provide essential context. The ongoing recognition of the genetic basis of ACM provides the opportunity to examine the diverse triggers and potential common pathway for the development of disease and arrhythmia.

Incorporation of desmocollin-2 into the plasma membrane requires N-glycosylation at multiple sites

Desmocollin‐2 (DSC2) is a desmosomal protein of the cadherin family. Desmosomes are multiprotein complexes, which are involved in cell adhesion of cardiomyocytes and of keratinocytes. The molecular structure of the complete extracellular domain (ECD) of DSC2 was recently described, revealing three disulfide bridges, four N‐glycosylation sites, and four O‐mannosylation sites. However, the functional relevance of these post‐translational modifications for the protein trafficking of DSC2 to the plasma membrane is still unknown. Here, we generated a set of DSC2 mutants, in which we systematically exchanged all N‐glycosylation sites, O‐mannosylation sites, and disulfide bridges within the ECD and investigated the resulting subcellular localization by confocal laser scanning microscopy. Of note, all single and double N‐glycosylation‐ deficient mutants were efficiently incorporated into the plasma membrane, indicating that the absence of these glycosylation sites has a minor effect on the protein trafficking of DSC2. However, the exchange of multiple N‐glycosylation sites resulted in intracellular accumulation. Colocalization analysis using cell compartment trackers revealed that N‐glycosylation‐ deficient DSC2 mutants were retained within the Golgi apparatus. In contrast, elimination of the four O‐mannosylation sites or the disulfide bridges in the ECD has no obvious effect on the intracellular protein processing of DSC2. These experiments underscore the importance of N‐glycosylation at multiple sites of DSC2 for efficient intracellular transport to the plasma membrane.

Mutations in PERP Cause Dominant and Recessive Keratoderma

Investigation of genetic determinants of Mendelian skin disorders has substantially advanced understanding of epidermal biology. Here we show that mutations in PERP, encoding a crucial component of desmosomes, cause both dominant and recessive human keratoderma. Heterozygosity for a C-terminal truncation, which produces a protein that appears to be unstably incorporated into desmosomes, causes Olmsted syndrome with severe periorificial and palmoplantar keratoderma in multiple unrelated kindreds. Homozygosity for an N-terminal truncation ablates expression and causes widespread erythrokeratoderma, with expansion of epidermal differentiation markers. Both exhibit epidermal hyperproliferation, immature desmosomes lacking a dense midline observed via electron microscopy, and impaired intercellular adhesion upon mechanical stress. Localization of other desmosomal components appears normal, which is in contrast to other conditions caused by mutations in genes encoding desmosomal proteins. These discoveries highlight the essential role of PERP in human desmosomes and epidermal homeostasis and further expand the heterogeneous spectrum of inherited keratinization disorders.

A founder homozygous DSG2 variant in East Asia results in ARVC with full penetrance and heart failure phenotype

BACKGROUND

Variants in the desmoglein-2 (DSG2) gene account for a significant proportion of patients with Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC). The aim of this study was to evaluate the genetic epidemiology of DSG2 and the impact of a frequent homozygous DSG2 variant in East Asia.

METHODS

Genetic screening of 14 ARVC related genes was performed in 118 unrelated index patients using next-generation sequencing. Following that, family screening, clinical evaluation and haplotype analysis were performed among eight probands who carry the same homozygous DSG2 variant. We also examined the histopathology and protein expression using immunofluorescence staining on the myocardial tissue of two probands undergoing heart transplant.

RESULTS

Eighteen (15.2%) patients bear rare putatively deleterious variants in DSG2, among which 8 patients shared the homozygous DSG2 p.Phe531Cys variant. Family screening demonstrated that only homozygous variant carriers exhibited definite ARVC phenotype with 100% penetrance, while heterozygous variant carriers were either unaffected or only presented mild ARVC related symptoms in 25% relatives. Left ventricular involvement and bi-ventricular failure were common among homozygous p. Phe531Cys variant patients even at early age. Haplotype analysis demonstrated p. Phe531Cys was a founder variant in East Asia population with an allele frequency of 0.12%.

CONCLUSIONS

We identified, for the first time, a homozygous founder variant of DSG2 in East Asia, which was at surprisingly high frequency of 8.47% among Chinese ARVC patients with a full penetrance. This result suggested an urgent demand of genetic counseling for the probands and their relatives with heterozygous variant.

Transgenic mice overexpressing desmocollin-2 (DSC2) develop cardiomyopathy associated with myocardial inflammation and fibrotic remodeling

Background

Arrhythmogenic cardiomyopathy is an inherited heart muscle disorder leading to ventricular arrhythmias and heart failure, mainly as a result of mutations in cardiac desmosomal genes. Desmosomes are cell-cell junctions mediating adhesion of cardiomyocytes; however, the molecular and cellular mechanisms underlying the disease remain widely unknown. Desmocollin-2 is a desmosomal cadherin serving as an anchor molecule required to reconstitute homeostatic intercellular adhesion with desmoglein-2. Cardiac specific lack of desmoglein-2 leads to severe cardiomyopathy, whereas overexpression does not. In contrast, the corresponding data for desmocollin-2 are incomplete, in particular from the view of protein overexpression. Therefore, we developed a mouse model overexpressing desmocollin-2 to determine its potential contribution to cardiomyopathy and intercellular adhesion pathology.

Methods and results

We generated transgenic mice overexpressing DSC2 in cardiac myocytes. Transgenic mice developed a severe cardiac dysfunction over 5 to 13 weeks as indicated by 2D-echocardiography measurements. Corresponding histology and immunohistochemistry demonstrated fibrosis, necrosis and calcification which were mainly localized in patches near the epi- and endocardium of both ventricles. Expressions of endogenous desmosomal proteins were markedly reduced in fibrotic areas but appear to be unchanged in non-fibrotic areas. Furthermore, gene expression data indicate an early up-regulation of inflammatory and fibrotic remodeling pathways between 2 to 3.5 weeks of age.

Conclusion

Cardiac specific overexpression of desmocollin-2 induces necrosis, acute inflammation and patchy cardiac fibrotic remodeling leading to fulminant biventricular cardiomyopathy.

Development of a diagnostic DNA chip to screen for 30 autosomal recessive disorders in the Hutterite population

Background

The Hutterites are a religious isolate living in colonies across the North American prairies. This population originated from approximately 90 founders, resulting in a number of genetic diseases that are overrepresented, underrepresented, or unique. The founder effect in this population increases the likelihood that Hutterite couples carry the same recessive mutations. We have designed a diagnostic chip on a fee‐for‐service basis with Asper Biotech to provide Hutterites with the option of comprehensive carrier screening.

Methods

A total of 32 disease‐causing mutations in 30 genes were selected and primers were designed for array primer extension‐based testing. Selected mutations were limited to those leading to autosomal recessive disorders, maintaining its primary use as a test for determining carrier status.

Results

The DNA chip was developed and validated using 59 DNA controls for all but one of the mutations, for which a synthetic control was used. All mutations were readily detected except for a duplication causing restrictive dermopathy where heterozygotes and homozygotes could only be distinguished by sequencing. Blinded testing of 12 additional samples from healthy Hutterites was performed by Asper Biotech using chip testing. All known mutations from previous molecular testing were detected on the chip. As well, additional mutations identified by the chip in these 12 samples were subsequently verified by a second method.

Conclusions

Our analysis indicates that the chip is a sensitive and specific means of carrier testing in the Hutterite population and can serve as a model for other founder populations.

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.

Hutterite-type cataract maps to chromosome 6p21.32-p21.31, cosegregates with a homozygous mutation in LEMD2, and is associated with sudden cardiac death

BACKGROUND

Juvenile-onset cataracts are known among the Hutterites of North America. Despite being identified over 30 years ago, this autosomal recessive condition has not been mapped, and the disease gene is unknown.

METHODS

We performed whole exome sequencing of three Hutterite-type cataract trios and follow-up genotyping and mapping in four extended kindreds.

RESULTS

Trio exomes enabled genome-wide autozygosity mapping, which localized the disease gene to a 9.5-Mb region on chromosome 6p. This region contained two candidate variants, LEMD2 c.T38G and MUC21 c.665delC. Extended pedigrees recruited for variant genotyping revealed multiple additional relatives with juvenile-onset cataract, as well as six deceased relatives with both cataracts and sudden cardiac death. The candidate variants were genotyped in 84 family members, including 17 with cataracts; only the variant in LEMD2 cosegregated with cataracts (LOD = 9.62). SNP-based fine mapping within the 9.5 Mb linked region supported this finding by refining the cataract locus to a 0.5- to 2.9-Mb subregion (6p21.32-p21.31) containing LEMD2 but not MUC21. LEMD2 is expressed in mouse and human lenses and encodes a LEM domain-containing protein; the c.T38G missense mutation is predicted to mutate a highly conserved residue within this domain (p.Leu13Arg).

CONCLUSION

We performed a genetic and genomic study of Hutterite-type cataract and found evidence for an association of this phenotype with sudden cardiac death. Using combined genetic and genomic approaches, we mapped cataracts to a small portion of chromosome 6 and propose that they result from a homozygous missense mutation in LEMD2.

Homozygous Desmocollin-2 Mutations and Arrhythmogenic Cardiomyopathy

Dominant mutations in desmocollin-2 (DSC2) gene cause arrhythmogenic cardiomyopathy (ACM), a progressive heart muscle disease characterized by ventricular tachyarrhythmias, heart failure, and risk of juvenile sudden death. Recessive mutations are rare and are associated with a cardiac or cardiocutaneous phenotype. Here, we evaluated the impact of a homozygous founder DSC2 mutation on clinical expression of ACM. An exon-by-exon analysis of the DSC2 coding region was performed in 94 ACM index patients. The c.536A>G (p.D179G) mutation was identified in 5 patients (5.3%), 4 of which resulted to be homozygous carriers. The 5 subjects shared a conserved haplotype, strongly indicating a common founder. Genetic and clinical investigation of probands' families revealed that p.D179G homozygous carriers displayed severe forms of biventricular cardiomyopathy without hair or skin abnormalities. The only heterozygous proband, who carried an additional variant of unknown significance in αT-catenin gene, showed a mild form of ACM without left ventricular involvement. All heterozygous family members were clinically asymptomatic. In conclusion, this is the first homozygous founder mutation in DSC2 gene identified among Italian ACM probands. Our findings provide further evidence of the occurrence of recessive DSC2 mutations in patients with ACM predominantly presenting with biventricular forms of the disease.

An estimate of the average number of recessive lethal mutations carried by humans

The effects of inbreeding on human health depend critically on the number and severity of recessive, deleterious mutations carried by individuals. In humans, existing estimates of these quantities are based on comparisons between consanguineous and nonconsanguineous couples, an approach that confounds socioeconomic and genetic effects of inbreeding. To overcome this limitation, we focused on a founder population that practices a communal lifestyle, for which there is almost complete Mendelian disease ascertainment and a known pedigree. Focusing on recessive lethal diseases and simulating allele transmissions, we estimated that each haploid set of human autosomes carries on average 0.29 (95% credible interval [0.10, 0.84]) recessive alleles that lead to complete sterility or death by reproductive age when homozygous. Comparison to existing estimates in humans suggests that a substantial fraction of the total burden imposed by recessive deleterious variants is due to single mutations that lead to sterility or death between birth and reproductive age. In turn, comparison to estimates from other eukaryotes points to a surprising constancy of the average number of recessive lethal mutations across organisms with markedly different genome sizes.

Phenotypic analysis of arrhythmogenic cardiomyopathy in the Hutterite population: role of electrocardiogram in identifying high-risk desmocollin-2 carriers

Background

The p.Gln554X mutation in desmocollin‐2 (DSC2) is prevalent in ≈10% of the Hutterite population. While the homozygous mutation causes severe biventricular arrhythmogenic right ventricular cardiomyopathy, the phenotypic features and prognosis of heterozygotes remain incompletely understood.

Methods and Results

Eleven homozygotes (mean age 32±8 years, 45% female), 28 heterozygotes (mean age 40±15 years, 50% female), and 22 mutation‐negatives (mean age 43±17 years, 41% female) were examined. Diagnostic testing was performed as per the arrhythmogenic right ventricular cardiomyopathy modified Task Force Criteria. Inverted T waves in the right precordial leads on ECG were seen in all homozygotes but not in their counterparts (P<0.001). Homozygotes had higher median daily premature ventricular complex burden than did heterozygotes or mutation‐negatives (1407 [IQR 1080 to 2936] versus 2 [IQR 0 to 6] versus 6 [IQR 0 to 214], P=0.0002). Ventricular tachycardia was observed in 60% of homozygotes but in none of the remaining individuals (P<0.001). On cardiac magnetic resonance imaging, homozygotes had significantly larger indexed end‐diastolic volumes (right ventricular: 122±24 versus 83±17 versus 83±12 mL/m², P<0.0001; left ventricular: 93±18 versus 76±13 versus 80±11 mL/m², P=0.0124) and lower ejection fraction values compared with heterozygotes and mutation‐negatives (right ventricular ejection fraction: 41±9% versus 59±9% versus 61±6%, P<0.0001; left ventricular ejection fraction: 53±8% versus 65±5% versus 64±5%, P<0.0001). Most affected individuals lacked right ventricular wall motion abnormalities. Thus, few met cardiac magnetic resonance imaging task force criteria.

Conclusions

The ECG reliably identifies homozygous p.Gln554X carriers and may be useful as an initial step in the screening of high‐risk Hutterites. The cardiac phenotype of heterozygotes appears benign, but further prospective follow‐up of their arrhythmic risk is needed.

Clinical interpretation of genetic variants in arrhythmogenic right ventricular cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy is an inherited cardiac entity characterized by right ventricular, or biventricular, fibrofatty replacement of myocardium. Structural alterations may lead to sudden cardiac death, mainly in young males during exercise. Autosomal dominant pattern of inheritance is reported in most parts of pathogenic genetic variations identified. Currently, 13 genes have been associated with the disease but nearly 40 % of clinically diagnosed cases remain without a genetic diagnosis. New genetic technologies allow further genetic analysis, generating a significant amount of genetic data in novel genes, which is often classified as of ambiguous significance. We focus on genetic advances of arrhythmogenic right ventricular cardiomyopathy, helping clinicians to interpret and translate genetic data into clinical practice.

Disclosure of genetic research results to members of a founder population

There is currently extensive discussion and debate in the literature on how, when, and to whom genetic research results should be returned (see Genetics in Medicine, April 2012 issue). Here, we describe our experience in disclosing genetic information on Mendelian disorders discovered during the course of our research in the Hutterites. We first assessed attitudes toward the disclosure of carrier results, which revealed that many individuals wanted carrier information and that many intended to use the information in family planning. Based on this information, we developed a pilot study to test and disclose cystic fibrosis (CF) carrier status. Next, a larger scale project was developed in order to disclose genetic research results for 14 diseases to those interested in receiving the information. We developed brochures, offered a live interactive educational program, conducted a consent process, and disclosed results in letters mailed to the consented individuals. Overall, ~80% of individuals who participated in the educational program signed consent forms for the release of their results for 14 diseases. We describe our experience with returning individual genetic research results to participants in a population-based research study.

Protein LUMA is a cytoplasmic plaque constituent of various epithelial adherens junctions and composite junctions of myocardial intercalated disks: a unifying finding for cell biology and cardiology

In a series of recent reports, mutations in the gene encoding a protein called LUMA (or TMEM43), widely speculated to be a tetraspan transmembrane protein of the nuclear envelope, have been associated with a specific subtype of cardiomyopathy (arrhythmogenic cardiomyopathies) and cases of sudden death. However, using antibodies of high specificity in immunolocalization experiments, we have discovered that, in mammals, LUMA is a component of zonula adhaerens and punctum adhaerens plaques of diverse epithelia and epithelial cell cultures and is also located in (or in some species associated with) the plaques of composite junctions (CJs) in myocardiac intercalated disks (IDs). In CJs, LUMA often colocalizes with several other CJ marker proteins. In all these cells, LUMA has not been detected in the nuclear envelope. Surprisingly, under certain conditions, similar CJ localizations have also been seen with some antibodies commercially available for some time. The identification of LUMA as a plaque component of myocardiac CJs leads to reconsiderations of the molecular composition and architecture, the development, the functions, and the pathogenic states of CJs and IDs. These findings now also allow the general conclusion that LUMA has to be added to the list of mutations of cardiomyocyte junction proteins that may be involved in cardiomyopathies.

Almanac 2014: cardiomyopathies

Cardiomyopathies are myocardial disorders that are not explained by abnormal loading conditions and coronary artery disease. They are classified into a number of morphological and functional phenotypes that can be caused by genetic and non-genetic mechanisms. The dominant themes in papers published in 2012-2013 are similar to those reported in Almanac 2011, namely, the use (and interpretation) of genetic testing, development and application of novel non-invasive imaging techniques and use of serum biomarkers for diagnosis and prognosis. An important innovation since the last Almanac is the development of more sophisticated models for predicting adverse clinical events.

Arrhythmogenic right ventricular cardiomyopathy with recessive inheritance related to a new homozygous desmocollin-2 mutation

Arrhythmogenic right ventricular cardiomyopathy/dysplasia is an inherited cardiomyopathy that is transmitted in autosomal dominant and autosomal recessive forms and involves mutations in desmosomal and extradesmosomal genes. We present a case of arrhythmogenic right ventricular cardiomyopathy that cosegregates in a Lebanese family with a previously unreported desmocollin-2 mutation (c.712_714delGAT). We believe this newly described genetic variant displays autosomal recessive inheritance without the cutaneous manifestations expected in recessive genotypes, and represents the latest addition to the compendium of desmosomal mutations with pathogenic potential.

The novel desmin mutant p.A120D impairs filament formation, prevents intercalated disk localization, and causes sudden cardiac death

BACKGROUND

The intermediate filament protein desmin is encoded by the gene DES and contributes to the mechanical stabilization of the striated muscle sarcomere and cell contacts within the cardiac intercalated disk. DES mutations cause severe skeletal and cardiac muscle diseases with heterogeneous phenotypes. Recently, DES mutations were also found in patients with arrhythmogenic right ventricular cardiomyopathy. Currently, the cellular and molecular pathomechanisms of the DES mutations leading to this disease are not exactly known.

METHODS AND RESULTS

We identified the 2 novel variants DES-p.A120D (c.359C>A) and DES-p.H326R (c.977A>G), which were characterized by cell culture experiments and atomic force microscopy. Family analysis indicated a broad spectrum of cardiomyopathies with a striking frequency of arrhythmias and sudden cardiac deaths. The in vitro experiments of desmin-p.A120D reveal a severe intrinsic filament formation defect causing cytoplasmic aggregates in cell lines and of the isolated recombinant protein. Model variants of codon 120 indicated that ionic interactions contribute to this filament formation defect. Ex vivo analysis of ventricular tissue slices revealed a loss of desmin staining within the intercalated disk and severe cytoplasmic aggregate formation, whereas z-band localization was not affected. The functional experiments of desmin-p.H326R did not demonstrate any differences from wild type.

CONCLUSIONS

Because of the functional in vivo and in vitro characterization, DES-p.A120D has to be regarded as a pathogenic mutation and DES-p.H326R as a rare variant with unknown significance. Presumably, the loss of the desmin-p. A120D filament localization at the intercalated disk explains its clinical arrhythmogenic potential.