Genetic advances in sarcomeric cardiomyopathies: state of the art

Focus on cardiac troponin complex: From gene expression to cardiomyopathy

The cardiac troponin complex (cTn) is a regulatory component of sarcomere. cTn consists of three subunits: cardiac troponin C (cTnC), which confers Ca2+ sensitivity to muscle; cTnI, which inhibits the interaction of cross-bridge of myosin with thin filament during diastole; and cTnT, which has multiple roles in sarcomere, such as promoting the link between the cTnI-cTnC complex and tropomyosin within the thin filament and influencing Ca2+ sensitivity of cTn and force development during contraction. Conditions that interfere with interactions within cTn and/or other thin filament proteins can be key factors in the regulation of cardiac contraction. These conditions include alterations in myofilament Ca2+ sensitivity, direct changes in cTn function, and triggering downstream events that lead to adverse cardiac remodeling and impairment of heart function. This review describes gene expression and post-translational modifications of cTn as well as the conditions that can adversely affect the delicate balance among the components of cTn, thereby promoting contractile dysfunction.

Family Screening in Hypertrophic Cardiomyopathy: Identification of Relatives With Low Yield From Systematic Follow-Up

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is a common inherited cardiac disease, and clinical and genetic family screening is recommended by guidelines.

OBJECTIVES

This study sought to investigate the diagnostic yield of screening relatives of HCM patients and identify predictive factors for HCM development during long-term follow-up in relatives from gene-elusive families.

METHODS

This was a retrospective cohort study of families screened at clinics for inherited cardiomyopathies in Eastern Denmark, from 2006 to 2023.

RESULTS

We included 1,230 relatives (55% female; age: 42 ± 17 years) from 531 families. The combined clinical and genetic yield at baseline was 26% (n = 321). After 7 years (mean) of follow-up (6,762 person-years), 43 (4%) additional relatives developed HCM. The strongest predictors of developing HCM were carrying a likely pathogenic/pathogenic variant (HR: 4.58; 95% CI: 2.50-8.40; P < 0.001) and larger left ventricular maximum wall thickness (MWT) (HR: 2.21 per mm; 95% CI: 1.76-2.77 per mm; P < 0.001). In gene-elusive families, we found that an MWT of ≥10 mm represented the optimal classification threshold for developing HCM (area under the curve: 0.80), with only 2 (0.4%) relatives from gene-elusive families with an MWT of <10 mm developing HCM during follow-up.

CONCLUSIONS

In HCM, the diagnostic yield of a single screening visit was 1 in 4, and the additional yield during 7 years of follow-up was 4%. Gene carriers and relatives from gene-elusive families with a baseline MWT of ≥10 mm were at the highest risk of developing HCM during follow-up. These findings may inform future recommendations on the management of relatives of HCM patients.

Novel Multiplexed Plasma Biomarker Panel Has Diagnostic and Prognostic Potential in Children With Hypertrophic Cardiomyopathy

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is defined clinically by pathological left ventricular hypertrophy. We have previously developed a plasma proteomics biomarker panel that correlates with clinical markers of disease severity and sudden cardiac death risk in adult patients with HCM. The aim of this study was to investigate the utility of adult biomarkers and perform new discoveries in proteomics for childhood-onset HCM.

METHODS

Fifty-nine protein biomarkers were identified from an exploratory plasma proteomics screen in children with HCM and augmented into our existing multiplexed targeted liquid chromatography-tandem/mass spectrometry-based assay. The association of these biomarkers with clinical phenotypes and outcomes was prospectively tested in plasma collected from 148 children with HCM and 50 healthy controls. Machine learning techniques were used to develop novel pediatric plasma proteomic biomarker panels.

RESULTS

Four previously identified adult HCM markers (aldolase fructose-bisphosphate A, complement C3a, talin-1, and thrombospondin 1) and 3 new markers (glycogen phosphorylase B, lipoprotein a and profilin 1) were elevated in pediatric HCM. Using supervised machine learning applied to training (n=137) and validation cohorts (n=61), this 7-biomarker panel differentiated HCM from healthy controls with an area under the curve of 1.0 in the training data set (sensitivity 100% [95% CI, 95-100]; specificity 100% [95% CI, 96-100]) and 0.82 in the validation data set (sensitivity 75% [95% CI, 59-86]; specificity 88% [95% CI, 75-94]). Reduced circulating levels of 4 other peptides (apolipoprotein L1, complement 5b, immunoglobulin heavy constant epsilon, and serum amyloid A4) found in children with high sudden cardiac death risk provided complete separation from the low and intermediate risk groups and predicted mortality and adverse arrhythmic outcomes (hazard ratio, 2.04 [95% CI, 1.0-4.2]; P=0.044).

CONCLUSIONS

In children, a 7-biomarker proteomics panel can distinguish HCM from controls with high sensitivity and specificity, and another 4-biomarker panel identifies those at high risk of adverse arrhythmic outcomes, including sudden cardiac death.

Integrating Clinical Phenotype With Multiomics Analyses of Human Cardiac Tissue Unveils Divergent Metabolic Remodeling in Genotype-Positive and Genotype-Negative Patients With Hypertrophic Cardiomyopathy

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is caused by sarcomere gene mutations (genotype-positive HCM) in ≈50% of patients and occurs in the absence of mutations (genotype-negative HCM) in the other half of patients. We explored how alterations in the metabolomic and lipidomic landscape are involved in cardiac remodeling in both patient groups.

METHODS

We performed proteomics, metabolomics, and lipidomics on myectomy samples (genotype-positive N=19; genotype-negative N=22; and genotype unknown N=6) from clinically well-phenotyped patients with HCM and on cardiac tissue samples from sex- and age-matched and body mass index-matched nonfailing donors (N=20). These data sets were integrated to comprehensively map changes in lipid-handling and energy metabolism pathways. By linking metabolomic and lipidomic data to variability in clinical data, we explored patient group-specific associations between cardiac and metabolic remodeling.

RESULTS

HCM myectomy samples exhibited (1) increased glucose and glycogen metabolism, (2) downregulation of fatty acid oxidation, and (3) reduced ceramide formation and lipid storage. In genotype-negative patients, septal hypertrophy and diastolic dysfunction correlated with lowering of acylcarnitines, redox metabolites, amino acids, pentose phosphate pathway intermediates, purines, and pyrimidines. In contrast, redox metabolites, amino acids, pentose phosphate pathway intermediates, purines, and pyrimidines were positively associated with septal hypertrophy and diastolic impairment in genotype-positive patients.

CONCLUSIONS

We provide novel insights into both general and genotype-specific metabolic changes in HCM. Distinct metabolic alterations underlie cardiac disease progression in genotype-negative and genotype-positive patients with HCM.

Cardiomyopathy in children: a single-centre, retrospective study of genetic and clinical characteristics

OBJECTIVES

This study aimed to describe the genetic and clinical characteristics of paediatric cardiomyopathy in a cohort of Chinese patients.

METHODS

We retrospectively reviewed the clinical history and mutation spectrum of 75 unrelated Chinese paediatric patients who were diagnosed with cardiomyopathy and referred to our hospital between January 2016 and December 2022.

RESULTS

Seventy-five children with cardiomyopathy were enrolled, including 32 (42.7%) boys and 43 (57.3%) girls. Dilated cardiomyopathy was the most prevalent cardiomyopathy (61.3%) in the patients, followed by hypertrophic cardiomyopathy (17.3%), ventricular non-compaction (14.7%), restrictive cardiomyopathy (5.3%) and arrhythmogenic right ventricular cardiomyopathy (1.3%). Whole-exome sequencing and targeted next-generation sequencing identified 34 pathogenic/likely pathogenic variants and 1 copy number variant in 14 genes related to cardiomyopathy in 30 children, accounting for 40% of all patients. TNNC1 p.Asp65Asn and MYH7 p.Glu500Lys have not been reported previously. The follow-up time ranged from 2 months to 6 years. Twenty-two children died (mortality rate 29%).

CONCLUSIONS

Comprehensive genetic testing was associated with a 40% yield of causal genetic mutations in Chinese cardiomyopathy cases. We found diversity in the mutation profile in different patients, which suggests that the mutational background of cardiomyopathy in China is heterogeneous, and the findings may be helpful to those counselling patients and families.

Emerging Themes in Genetics of Hypertrophic Cardiomyopathy: Current Status and Clinical Application

Hypertrophic cardiomyopathy (HCM), defined clinically by the presence of unexplained left ventricular hypertrophy (LVH), with wall thickness ≥ 1.5 cm, is a phenotype in search of a diagnosis, which is most often a genetically determined, cardiac exclusive, or systemic disorder. Familial evaluation and genetic testing are required for definitive diagnosis. The role of genetic findings in predicting development of disease, outcomes, and increasingly to guide management is evolving with access to larger data sets. The specific mutation and sex of the patient are important determinants that ultimately are likely to guide management. The genetic/familial evaluation is influenced by the accuracy of the clinical diagnosis and the extent/expertise of the genetic laboratory. Genetic testing in a patient with unexplained LVH without systemic manifestations will yield a definite/likely pathogenetic mutation in a sarcomere (30%-50%), regulatory/functional (10%-15%) or metabolic/syndromic (< 5%) gene associated with Mendelian inheritance. The importance of oligo- and polygenic determinants, usually in the absence of Mendelian inheritance, is under investigation with important implications, particularly related to familial evaluation and definition of risk of disease development in relatives of probands. The results of genetic testing are increasingly important in management strategies related to the use of the implantable cardioverter defibrillator for prevention of sudden death, use of myosin inhibitors for refractory symptoms in patients with and without outflow tract obstruction, and-on the immediate horizon-gene therapy. This review will focus on genetic and outcome data in sarcomeric HCM, and minor causative genes with robust evidence of their association will also be considered.

Hypertrophic cardiomyopathy dysfunction mimicked in human engineered heart tissue and improved by sodium-glucose cotransporter 2 inhibitors

AIMS

Hypertrophic cardiomyopathy (HCM) is the most common inherited cardiomyopathy, often caused by pathogenic sarcomere mutations. Early characteristics of HCM are diastolic dysfunction and hypercontractility. Treatment to prevent mutation-induced cardiac dysfunction is lacking. Sodium-glucose cotransporter 2 inhibitors (SGLT2i) are a group of antidiabetic drugs that recently showed beneficial cardiovascular outcomes in patients with acquired forms of heart failure. We here studied if SGLT2i represent a potential therapy to correct cardiomyocyte dysfunction induced by an HCM sarcomere mutation.

METHODS AND RESULTS

Contractility was measured of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) harbouring an HCM mutation cultured in 2D and in 3D engineered heart tissue (EHT). Mutations in the gene encoding β-myosin heavy chain (MYH7-R403Q) or cardiac troponin T (TNNT2-R92Q) were investigated. In 2D, intracellular [Ca2+], action potential and ion currents were determined. HCM mutations in hiPSC-CMs impaired relaxation or increased force, mimicking early features observed in human HCM. SGLT2i enhance the relaxation of hiPSC-CMs, to a larger extent in HCM compared to control hiPSC-CMs. Moreover, SGLT2i-effects on relaxation in R403Q EHT increased with culture duration, i.e. hiPSC-CMs maturation. Canagliflozin's effects on relaxation were more pronounced than empagliflozin and dapagliflozin. SGLT2i acutely altered Ca2+ handling in HCM hiPSC-CMs. Analyses of SGLT2i-mediated mechanisms that may underlie enhanced relaxation in mutant hiPSC-CMs excluded SGLT2, Na+/H+ exchanger, peak and late Nav1.5 currents, and L-type Ca2+ current, but indicate an important role for the Na+/Ca2+ exchanger. Indeed, electrophysiological measurements in mutant hiPSC-CM indicate that SGLT2i altered Na+/Ca2+ exchange current.

CONCLUSION

SGLT2i (canagliflozin > dapagliflozin > empagliflozin) acutely enhance relaxation in human EHT, especially in HCM and upon prolonged culture. SGLT2i may represent a potential therapy to correct early cardiac dysfunction in HCM.

ERRγ agonist under mechanical stretching manifests hypertrophic cardiomyopathy phenotypes of engineered cardiac tissue through maturation

Engineered cardiac tissue (ECT) using human induced pluripotent stem cell-derived cardiomyocytes is a promising tool for modeling heart disease. However, tissue immaturity makes robust disease modeling difficult. Here, we established a method for modeling hypertrophic cardiomyopathy (HCM) malignant (MYH7 R719Q) and nonmalignant (MYBPC3 G115∗) pathogenic sarcomere gene mutations by accelerating ECT maturation using an ERRγ agonist, T112, and mechanical stretching. ECTs treated with T112 under 10% elongation stimulation exhibited more organized and mature characteristics. Whereas matured ECTs with the MYH7 R719Q mutation showed broad HCM phenotypes, including hypertrophy, hypercontraction, diastolic dysfunction, myofibril misalignment, fibrotic change, and glycolytic activation, matured MYBPC3 G115∗ ECTs displayed limited phenotypes, which were primarily observed only under our new maturation protocol (i.e., hypertrophy). Altogether, ERRγ activation combined with mechanical stimulation enhanced ECT maturation, leading to a more accurate manifestation of HCM phenotypes, including non-cardiomyocyte activation, consistent with clinical observations.

Novel Therapeutic Avenues for Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a complicated, heterogeneous genetic condition that causes left ventricular hypertrophy, fibrosis, hypercontractility, and decreased compliance. Despite the advances made over the past 3 decades in understanding the molecular and cellular mechanisms aggravating HCM, the relationship between pathophysiological stress stimuli and distinctive myocyte growth profiles is still imprecise. Currently, mavacamten, a selective and reversible inhibitor of cardiac myosin ATPase, is the only drug approved by the US FDA for the treatment of HCM. Thus, there is an unmet need for developing novel disease-specific therapeutic approaches. This article provides an overview of emerging therapeutic targets for the treatment of HCM based on various molecular pathways and novel developments that are hopefully soon to enter the clinical study. These newly discovered targets include the dual specificity tyrosine-phosphorylation-regulated kinase 1B, the absence of the melanoma 1 inflammasome, the leucine-rich repeat kinase 2 enzyme, and the cluster of differentiation 147.

Proteomic and phosphoproteomic analyses of myectomy tissue reveals difference between sarcomeric and genotype-negative hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a genetically heterogenous condition with about half of cases remaining genetically elusive or non-genetic in origin. HCM patients with a positive genetic test (HCMSarc) present earlier and with more severe disease than those with a negative genetic test (HCMNeg). We hypothesized these differences may be due to and/or reflect proteomic and phosphoproteomic differences between the two groups. TMT-labeled mass spectrometry was performed on 15 HCMSarc, 8 HCMNeg, and 7 control samples. There were 243 proteins differentially expressed and 257 proteins differentially phosphorylated between HCMSarc and HCMNeg. About 90% of pathways altered between genotypes were in disease-related pathways and HCMSarc showed enhanced proteomic and phosphoproteomic alterations in these pathways. Thus, we show HCMSarc has enhanced proteomic and phosphoproteomic dysregulation observed which may contribute to the more severe disease phenotype.

Nucleus Mechanosensing in Cardiomyocytes

Cardiac muscle contraction is distinct from the contraction of other muscle types. The heart continuously undergoes contraction-relaxation cycles throughout an animal's lifespan. It must respond to constantly varying physical and energetic burdens over the short term on a beat-to-beat basis and relies on different mechanisms over the long term. Muscle contractility is based on actin and myosin interactions that are regulated by cytoplasmic calcium ions. Genetic variants of sarcomeric proteins can lead to the pathophysiological development of cardiac dysfunction. The sarcomere is physically connected to other cytoskeletal components. Actin filaments, microtubules and desmin proteins are responsible for these interactions. Therefore, mechanical as well as biochemical signals from sarcomeric contractions are transmitted to and sensed by other parts of the cardiomyocyte, particularly the nucleus which can respond to these stimuli. Proteins anchored to the nuclear envelope display a broad response which remodels the structure of the nucleus. In this review, we examine the central aspects of mechanotransduction in the cardiomyocyte where the transmission of mechanical signals to the nucleus can result in changes in gene expression and nucleus morphology. The correlation of nucleus sensing and dysfunction of sarcomeric proteins may assist the understanding of a wide range of functional responses in the progress of cardiomyopathic diseases.

Regulated cell death pathways in cardiomyopathy

Heart disease is a worldwide health menace. Both intractable primary and secondary cardiomyopathies contribute to malignant cardiac dysfunction and mortality. One of the key cellular processes associated with cardiomyopathy is cardiomyocyte death. Cardiomyocytes are terminally differentiated cells with very limited regenerative capacity. Various insults can lead to irreversible damage of cardiomyocytes, contributing to progression of cardiac dysfunction. Accumulating evidence indicates that majority of cardiomyocyte death is executed by regulating molecular pathways, including apoptosis, ferroptosis, autophagy, pyroptosis, and necroptosis. Importantly, these forms of regulated cell death (RCD) are cardinal features in the pathogenesis of various cardiomyopathies, including dilated cardiomyopathy, diabetic cardiomyopathy, sepsis-induced cardiomyopathy, and drug-induced cardiomyopathy. The relevance between abnormity of RCD with adverse outcome of cardiomyopathy has been unequivocally evident. Therefore, there is an urgent need to uncover the molecular and cellular mechanisms for RCD in order to better understand the pathogenesis of cardiomyopathies. In this review, we summarize the latest progress from studies on RCD pathways in cardiomyocytes in context of the pathogenesis of cardiomyopathies, with particular emphasis on apoptosis, necroptosis, ferroptosis, autophagy, and pyroptosis. We also elaborate the crosstalk among various forms of RCD in pathologically stressed myocardium and the prospects of therapeutic applications targeted to various cell death pathways.

Basic science methods for the characterization of variants of uncertain significance in hypertrophic cardiomyopathy

With the advent of next-generation whole genome sequencing, many variants of uncertain significance (VUS) have been identified in individuals suffering from inheritable hypertrophic cardiomyopathy (HCM). Unfortunately, this classification of a genetic variant results in ambiguity in interpretation, risk stratification, and clinical practice. Here, we aim to review some basic science methods to gain a more accurate characterization of VUS in HCM. Currently, many genomic data-based computational methods have been developed and validated against each other to provide a robust set of resources for researchers. With the continual improvement in computing speed and accuracy, in silico molecular dynamic simulations can also be applied in mutational studies and provide valuable mechanistic insights. In addition, high throughput in vitro screening can provide more biologically meaningful insights into the structural and functional effects of VUS. Lastly, multi-level mathematical modeling can predict how the mutations could cause clinically significant organ-level dysfunction. We discuss emerging technologies that will aid in better VUS characterization and offer a possible basic science workflow for exploring the pathogenicity of VUS in HCM. Although the focus of this mini review was on HCM, these basic science methods can be applied to research in dilated cardiomyopathy (DCM), restrictive cardiomyopathy (RCM), arrhythmogenic cardiomyopathy (ACM), or other genetic cardiomyopathies.

A Family with Myh7 Mutation and Different Forms of Cardiomyopathies

BACKGROUND

Hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) are common heart muscle disorders that are caused by pathogenic variants in sarcomere protein genes. In this study, we describe a variant in the MHY7 gene, segregating in a family having three different phenotypes of cardiomyopathies. MYH7 encodes for the myosin heavy-chain β (MHC-β) isoform involved in cardiac muscle contractility.

METHOD AND RESULTS

We present the case of a family with four members diagnosed with HCM and four members with DCM. The proband is a 42-year-old man diagnosed with HCM. He has an extended family of eight siblings; two of them are diagnosed with HCM and are implantable cardioverter-defibrillator (ICD) carriers. One of the siblings died at the age of 23 after suffering a sudden cardiac arrest and DCM of unknown etiology which was diagnosed at autopsy. Another brother was diagnosed with DCM during a routine echocardiographic exam. Genetic testing was performed for the proband and two of his siblings and a niece of the proband, who suffered a cardiac arrest at the age of nine, all being MYH7 mutation positive. For all four of them, cardiac imaging was performed with different findings. They are ICD carriers as well.

CONCLUSIONS

Our results reveal three variants in phenotypes of cardiomyopathies in a family with MYH7 mutation associated with high SCD risk and ICD needed for primary and secondary prevention.

Role of non-coding variants in cardiovascular disease

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

Genetic profile and genotype-phenotype correlations in childhood cardiomyopathy

BACKGROUND

Genetic cardiomyopathy is a rare disease in childhood.

AIMS

To analyse clinical and genetic aspects of a paediatric cardiomyopathy population, and to establish genotype-phenotype correlations.

METHODS

We performed a retrospective study of all patients with idiopathic cardiomyopathy aged<18years in Southeast France. Secondary causes of cardiomyopathy were excluded. All data (clinical, echocardiography, genetic testing) were collected retrospectively. Patients were classified into six groups: hypertrophic cardiomyopathy; dilated cardiomyopathy; restrictive cardiomyopathy; left ventricular non-compaction; arrhythmogenic right ventricular dysplasia; and mixed cardiomyopathy. Patients who did not have a complete genetic test according to current scientific developments had another deoxyribonucleic acid blood sample during the study time. Genetic tests were considered positive if the variant found was classified as pathogenic, likely pathogenic or a variant of uncertain significance.

RESULTS

Eighty-three patients were included between 2005 and 2019. Most patients had hypertrophic cardiomyopathy (39.8%) or dilated cardiomyopathy (27.7%). The median age at diagnosis was 1.28years (interquartile range: 0.27-10.48years). Heart transplantation was performed in 30.1% of patients, and 10.8% died during follow-up. Among 64 patients with a complete genetic analysis, 64.1% had genetic anomalies, mostly in MYH7 (34.2%) and MYBPC3 (12.2%) genes. There were no differences in the whole cohort between genotype-positive and genotype-negative patients. In the hypertrophic cardiomyopathy group, 63.6% had a positive genetic test. Patients with a positive genetic test more often had extracardiac impact (38.1% vs. 8.3%; P=0.009), and more often required an implantable cardiac defibrillator (23.8% vs. 0%; P=0.025) or a heart transplant (19.1% vs. 0%; P=0.047).

CONCLUSIONS

In our population, children with cardiomyopathy had a high positive genetic test rate. Hypertrophic cardiomyopathy with a positive genetic test is associated with a worse outcome.

Genetic Testing and Counselling in Hypertrophic Cardiomyopathy: Frequently Asked Questions

Genetic counselling and genetic testing in hypertrophic cardiomyopathy (HCM) represent an integral part of the diagnostic algorithm to confirm the diagnosis, distinguish it from phenocopies, and suggest tailored therapeutic intervention strategies. Additionally, they enable cascade genetic testing in the family. With the implementation of Next Generation Sequencing technologies (NGS), the interpretation of genetic data has become more complex. In this regard, cardiologists play a central role, aiding geneticists to correctly evaluate the pathogenicity of the identified genetic alterations. In the ideal setting, geneticists and cardiologists must work side by side to diagnose HCM as well as convey the correct information to patients in response to their many questions and concerns. After a brief overview of the role of genetics in the diagnosis of HCM, we present and discuss the frequently asked questions by HCM patients throughout our 20-year genetic counselling experience. Appropriate communication between the team and the families is key to the goal of delivering the full potential of genetic testing to our patients.

Slower Calcium Handling Balances Faster Cross-Bridge Cycling in Human MYBPC3 HCM

BACKGROUND

The pathogenesis of MYBPC3-associated hypertrophic cardiomyopathy (HCM) is still unresolved. In our HCM patient cohort, a large and well-characterized population carrying the MYBPC3:c772G>A variant (p.Glu258Lys, E258K) provides the unique opportunity to study the basic mechanisms of MYBPC3-HCM with a comprehensive translational approach.

METHODS

We collected clinical and genetic data from 93 HCM patients carrying the MYBPC3:c772G>A variant. Functional perturbations were investigated using different biophysical techniques in left ventricular samples from 4 patients who underwent myectomy for refractory outflow obstruction, compared with samples from non-failing non-hypertrophic surgical patients and healthy donors. Human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes and engineered heart tissues (EHTs) were also investigated.

RESULTS

Haplotype analysis revealed MYBPC3:c772G>A as a founder mutation in Tuscany. In ventricular myocardium, the mutation leads to reduced cMyBP-C (cardiac myosin binding protein-C) expression, supporting haploinsufficiency as the main primary disease mechanism. Mechanical studies in single myofibrils and permeabilized muscle strips highlighted faster cross-bridge cycling, and higher energy cost of tension generation. A novel approach based on tissue clearing and advanced optical microscopy supported the idea that the sarcomere energetics dysfunction is intrinsically related with the reduction in cMyBP-C. Studies in single cardiomyocytes (native and hiPSC-derived), intact trabeculae and hiPSC-EHTs revealed prolonged action potentials, slower Ca2+ transients and preserved twitch duration, suggesting that the slower excitation-contraction coupling counterbalanced the faster sarcomere kinetics. This conclusion was strengthened by in silico simulations.

CONCLUSIONS

HCM-related MYBPC3:c772G>A mutation invariably impairs sarcomere energetics and cross-bridge cycling. Compensatory electrophysiological changes (eg, reduced potassium channel expression) appear to preserve twitch contraction parameters, but may expose patients to greater arrhythmic propensity and disease progression. Therapeutic approaches correcting the primary sarcomeric defects may prevent secondary cardiomyocyte remodeling.

Homology-directed repair of an MYBPC3 gene mutation in a rat model of hypertrophic cardiomyopathy

Variants in myosin-binding protein C3 (MYBPC3) gene are a main cause of hypertrophic cardiomyopathy (HCM), accounting for 30% to 40% of the total number of HCM mutations. Gene editing represents a potential permanent cure for HCM. The aim of this study was to investigate whether genome editing of MYBPC3 using the CRISPR/Cas9 system in vivo could rescue the phenotype of rats with HCM. We generated a rat model of HCM ("1098hom") that carried an Mybpc3 premature termination codon mutation (p.W1098x) discovered in a human HCM pedigree. On postnatal day 3, the CRISPR/Cas9 system was introduced into rat pups by a single dose of AAV9 particles to correct the variant using homology-directed repair (HDR). Analysis was performed 6 months after AAV9 injection. The 1098hom rats didn't express MYBPC3 protein and developed an HCM phenotype with increased ventricular wall thickness and diminished cardiac function. Importantly, CRISPR HDR genome editing corrected 3.56% of total mutations, restored MYBPC3 protein expression by 2.12%, and normalized the HCM phenotype of 1098hom rats. Our work demonstrates that the HDR strategy is a promising approach for treating HCM associated with MYBPC3 mutation, and that CRISPR technology has great potential for treating hereditary heart diseases.

Mitochondrial dysfunction in human hypertrophic cardiomyopathy is linked to cardiomyocyte architecture disruption and corrected by improving NADH-driven mitochondrial respiration

AIMS

Genetic hypertrophic cardiomyopathy (HCM) is caused by mutations in sarcomere protein-encoding genes (i.e. genotype-positive HCM). In an increasing number of patients, HCM occurs in the absence of a mutation (i.e. genotype-negative HCM). Mitochondrial dysfunction is thought to be a key driver of pathological remodelling in HCM. Reports of mitochondrial respiratory function and specific disease-modifying treatment options in patients with HCM are scarce.

METHODS AND RESULTS

Respirometry was performed on septal myectomy tissue from patients with HCM (n = 59) to evaluate oxidative phosphorylation and fatty acid oxidation. Mitochondrial dysfunction was most notably reflected by impaired NADH-linked respiration. In genotype-negative patients, but not genotype-positive patients, NADH-linked respiration was markedly depressed in patients with an indexed septal thickness ≥10 compared with <10. Mitochondrial dysfunction was not explained by reduced abundance or fragmentation of mitochondria, as evaluated by transmission electron microscopy. Rather, improper organization of mitochondria relative to myofibrils (expressed as a percentage of disorganized mitochondria) was strongly associated with mitochondrial dysfunction. Pre-incubation with the cardiolipin-stabilizing drug elamipretide and raising mitochondrial NAD+ levels both boosted NADH-linked respiration.

CONCLUSION

Mitochondrial dysfunction is explained by cardiomyocyte architecture disruption and is linked to septal hypertrophy in genotype-negative HCM. Despite severe myocardial remodelling mitochondria were responsive to treatments aimed at restoring respiratory function, eliciting the mitochondria as a drug target to prevent and ameliorate cardiac disease in HCM. Mitochondria-targeting therapy may particularly benefit genotype-negative patients with HCM, given the tight link between mitochondrial impairment and septal thickening in this subpopulation.

Cardiac troponins: Mechanisms of release and role in healthy and diseased subjects

The cardiac troponins (cTns), cardiac troponin C (cTnC), cTnT, and cTnI are key elements of myocardial apparatus, fixed as protein complex on the thin filament of sarcomere and are involved in the regulation of excitation-contraction coupling of cardiomyocytes in the presence of Ca²⁺ . Circulating cTnT and cTnI (cTns) increase following cardiac tissue necrosis, and they are consolidated biomarkers of acute myocardial infarction (AMI). However, the use of high sensitivity (hs)-immunoassay tests for cTnT and cTnI has made it possible to identify a multitude of other clinical conditions associated with increased circulating levels of cTns. cTns can be measured also in the peripheral circulation of healthy subjects or athletes, suggesting that different mechanisms are involved in the release of cTns in the blood independently of cardiac cell necrosis. In this review, the molecular/cellular mechanisms involved in cTns release in blood and the exploitation of cTnI and cTnT as biomarkers of cardiac adverse events, in addition to cardiac necrosis, are discussed.

Current and emerging perspectives on pathophysiology, diagnosis, and management of hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is the most common genetically inherited cardiomyopathy with an autosomal dominant inheritance pattern. A disease-causing gene is found between 34% and >60% of the times and the two most frequently mutated genes, which encode sarcomeric proteins, are MYBPC3 and MYH7. HCM is a diagnosis of exclusion since secondary causes of left ventricular hypertrophy should first be ruled out. These include hypertension, aortic stenosis, infiltrative disease, metabolic and endocrine disorders, mitochondrial cardiomyopathies, neuromuscular disorders, malformation syndromes and some chronic drug use. The disease is characterized by great heterogeneity of its clinical manifestations, however diastolic dysfunction and increased ventricular arrhythmogenesis are commonly seen. Current HCM therapies focus on symptom management and prevention of sudden cardiac death. Symptom management includes the use of pharmacological agents, elimination of medication promoting outflow track obstruction, control of comorbid conditions and invasive procedures, whereas in the prevention of sudden cardiac death, implantable cardiac defibrillators and antiarrhythmic drugs are used. A targeted therapy for LVOTO represented by allosteric cardiac myosin inhibitors has been developed. In terms of sport participation, a more liberal approach is recently recommended, after careful evaluation and common-shared decision. The application of the current therapies has lowered HCM mortality rates to <1.0%/year, however it appears to have shifted focus to heart failure and atrial fibrillation, as the predominant causes of disease-related morbidity and mortality and, therefore, unmet treatment need. With improved understanding of the genetic and molecular basis of HCM, the present decade will witness novel treatments for disease prevention and modification.

Genotype-Phenotype Associations with Restrictive Cardiomyopathy Induced by Pathogenic Genetic Mutations

Restrictive cardiomyopathy (RCM) is an uncommon cardiac muscle disease characterized by impaired ventricular filling and severe diastolic dysfunction with or without systolic dysfunction. The patients with RCM present poor prognosis and high prevalence of sudden cardiac death, especially in the young. The etiology of RCM may be idiopathic, familial or acquired predispositions from various systemic diseases. The genetic background of familial RCM is often caused by mutations in genes encoding proteins of sarcomeres and a significant minority by mutations in non-sarcomeric proteins and transthyretin proteins. It is important to identify the associations between genotype and phenotype to guide clinical diagnosis and treatment. Here, we have summarized the reported index cases with RCM involving genetic etiology to date and highlighted the most significant phenotype results.

Muscle LIM Protein Force-Sensing Mediates Sarcomeric Biomechanical Signaling in Human Familial Hypertrophic Cardiomyopathy

BACKGROUND

Familial hypertrophic cardiomyopathy (HCM) is the most common inherited cardiac disease and is typically caused by mutations in genes encoding sarcomeric proteins that regulate cardiac contractility. HCM manifestations include left ventricular hypertrophy and heart failure, arrythmias, and sudden cardiac death. How dysregulated sarcomeric force production is sensed and leads to pathological remodeling remains poorly understood in HCM, thereby inhibiting the efficient development of new therapeutics.

METHODS

Our discovery was based on insights from a severe phenotype of an individual with HCM and a second genetic alteration in a sarcomeric mechanosensing protein. We derived cardiomyocytes from patient-specific induced pluripotent stem cells and developed robust engineered heart tissues by seeding induced pluripotent stem cell-derived cardiomyocytes into a laser-cut scaffold possessing native cardiac fiber alignment to study human cardiac mechanobiology at both the cellular and tissue levels. Coupled with computational modeling for muscle contraction and rescue of disease phenotype by gene editing and pharmacological interventions, we have identified a new mechanotransduction pathway in HCM, shown to be essential in modulating the phenotypic expression of HCM in 5 families bearing distinct sarcomeric mutations.

RESULTS

Enhanced actomyosin crossbridge formation caused by sarcomeric mutations in cardiac myosin heavy chain (MYH7) led to increased force generation, which, when coupled with slower twitch relaxation, destabilized the MLP (muscle LIM protein) stretch-sensing complex at the Z-disc. Subsequent reduction in the sarcomeric muscle LIM protein level caused disinhibition of calcineurin-nuclear factor of activated T-cells signaling, which promoted cardiac hypertrophy. We demonstrate that the common muscle LIM protein-W4R variant is an important modifier, exacerbating the phenotypic expression of HCM, but alone may not be a disease-causing mutation. By mitigating enhanced actomyosin crossbridge formation through either genetic or pharmacological means, we alleviated stress at the Z-disc, preventing the development of hypertrophy associated with sarcomeric mutations.

CONCLUSIONS

Our studies have uncovered a novel biomechanical mechanism through which dysregulated sarcomeric force production is sensed and leads to pathological signaling, remodeling, and hypertrophic responses. Together, these establish the foundation for developing innovative mechanism-based treatments for HCM that stabilize the Z-disc MLP-mechanosensory complex.

The mechanics of the heart: zooming in on hypertrophic cardiomyopathy and cMyBP-C

Hypertrophic cardiomyopathy (HCM), a disease characterized by cardiac muscle hypertrophy and hypercontractility, is the most frequently inherited disorder of the heart. HCM is mainly caused by variants in genes encoding proteins of the sarcomere, the basic contractile unit of cardiomyocytes. The most frequently mutated among them is MYBPC3, which encodes cardiac myosin-binding protein C (cMyBP-C), a key regulator of sarcomere contraction. In this review, we summarize clinical and genetic aspects of HCM and provide updated information on the function of the healthy and HCM sarcomere, as well as on emerging therapeutic options targeting sarcomere mechanical activity. Building on what is known about cMyBP-C activity, we examine different pathogenicity drivers by which MYBPC3 variants can cause disease, focussing on protein haploinsufficiency as a common pathomechanism also in nontruncating variants. Finally, we discuss recent evidence correlating altered cMyBP-C mechanical properties with HCM development.

Distinct Metabolomic Signatures in Preclinical and Obstructive Hypertrophic Cardiomyopathy

Hypertrophic Cardiomyopathy (HCM) is a common inherited heart disease with poor risk prediction due to incomplete penetrance and a lack of clear genotype–phenotype correlations. Advanced imaging techniques have shown altered myocardial energetics already in preclinical gene variant carriers. To determine whether disturbed myocardial energetics with the potential to serve as biomarkers are also reflected in the serum metabolome, we analyzed the serum metabolome of asymptomatic carriers in comparison to healthy controls and obstructive HCM patients (HOCM). We performed non-quantitative direct-infusion high-resolution mass spectrometry-based untargeted metabolomics on serum from fasted asymptomatic gene variant carriers, symptomatic HOCM patients and healthy controls (n = 31, 14 and 9, respectively). Biomarker panels that discriminated the groups were identified by performing multivariate modeling with gradient-boosting classifiers. For all three group-wise comparisons we identified a panel of 30 serum metabolites that best discriminated the groups. These metabolite panels performed equally well as advanced cardiac imaging modalities in distinguishing the groups. Seven metabolites were found to be predictive in two different comparisons and may play an important role in defining the disease stage. This study reveals unique metabolic signatures in serum of preclinical carriers and HOCM patients that may potentially be used for HCM risk stratification and precision therapeutics.

Ion Channel Impairment and Myofilament Ca2+ Sensitization: Two Parallel Mechanisms Underlying Arrhythmogenesis in Hypertrophic Cardiomyopathy

Life-threatening ventricular arrhythmias are the main clinical burden in patients with hypertrophic cardiomyopathy (HCM), and frequently occur in young patients with mild structural disease. While massive hypertrophy, fibrosis and microvascular ischemia are the main mechanisms underlying sustained reentry-based ventricular arrhythmias in advanced HCM, cardiomyocyte-based functional arrhythmogenic mechanisms are likely prevalent at earlier stages of the disease. In this review, we will describe studies conducted in human surgical samples from HCM patients, transgenic animal models and human cultured cell lines derived from induced pluripotent stem cells. Current pieces of evidence concur to attribute the increased risk of ventricular arrhythmias in early HCM to different cellular mechanisms. The increase of late sodium current and L-type calcium current is an early observation in HCM, which follows post-translation channel modifications and increases the occurrence of early and delayed afterdepolarizations. Increased myofilament Ca²⁺ sensitivity, commonly observed in HCM, may promote afterdepolarizations and reentry arrhythmias with direct mechanisms. Decrease of K⁺-currents due to transcriptional regulation occurs in the advanced disease and contributes to reducing the repolarization-reserve and increasing the early afterdepolarizations (EADs). The presented evidence supports the idea that patients with early-stage HCM should be considered and managed as subjects with an acquired channelopathy rather than with a structural cardiac disease.

Targeted exome analysis of Russian patients with hypertrophic cardiomyopathy

Background

Hypertrophic cardiomyopathy (HCM), described as the presence of hypertrophy of left ventricular, is the most prevalent heritable cardiovascular disease with predominantly an autosomal dominant type of inheritance. However, pathogenic alleles are not identified in at least 25% of patients with HCM, and the spectrum of pathogenic variants that contribute to the development of HCM in Russia has not been fully described. Therefore, the goal of our study was to identify genetic variants associated with the etiopathogenesis of HCM in Russian patients.

Methods

The study cohort included 98 unrelated adult patients with HCM. We performed targeted exome sequencing, an analysis using various algorithms for prediction of the impact of variants on protein structure and the prediction of pathogenicity using ACMG Guidelines.

Results

The frequency of pathogenic and likely pathogenic variants in all HCM‐related genes was 8% in our patients. We also identified 20 variants of uncertain significance in all HCM‐related genes.

Conclusions

The prevalence of individual pathogenic variants in HCM‐related genes in Russian population appears to be lower than in general European population, which could be explained by ethnic features of Russian population, age characteristics of our sample, or unidentified pathogenic variants in genes previously not linked with HCM.

Genetic Testing in Patients with Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a common inherited heart disease with an estimated prevalence of up to 1 in 200 individuals. In the majority of cases, HCM is considered a Mendelian disease, with mainly autosomal dominant inheritance. Most pathogenic variants are usually detected in genes for sarcomeric proteins. Nowadays, the genetic basis of HCM is believed to be rather complex. Thousands of mutations in more than 60 genes have been described in association with HCM. Nevertheless, screening large numbers of genes results in the identification of many genetic variants of uncertain significance and makes the interpretation of the results difficult. Patients lacking a pathogenic variant are now believed to have non-Mendelian HCM and probably have a better prognosis than patients with sarcomeric pathogenic mutations. Identifying the genetic basis of HCM creates remarkable opportunities to understand how the disease develops, and by extension, how to disrupt the disease progression in the future. The aim of this review is to discuss the brief history and recent advances in the genetics of HCM and the application of molecular genetic testing into common clinical practice.

Ouabain worsens diastolic sarcomere length in myocytes from a cardiomyopathy mouse model

Diastolic dysfunction is a major feature of hypertrophic cardiomyopathy (HCM). Data from patient tissue and animal models associate increased Ca²⁺ sensitivity of myofilaments with altered Na⁺ and Ca²⁺ ion homeostasis in cardiomyocytes with diastolic dysfunction. In this study, we tested the acute effects of ouabain on ventricular myocytes of an HCM mouse model. The effects of ouabain on contractility and Ca²⁺ transients were tested in intact adult mouse ventricular myocytes (AMVMs) of Mybpc3-targeted knock-in (KI) and wild-type (WT) mice. Concentration-response assessment of contractile function revealed low sensitivity of AMVMs to ouabain (10 μM) compared to literature data on human cardiomyocytes (100 nM). Three hundred μM ouabain increased contraction amplitude (WT ~1.8-fold; KI ~1.5-fold) and diastolic intracellular Ca²⁺ in both WT and KI (+12-18%), but further decreased diastolic sarcomere length in KI cardiomyocytes (-5%). Western Blot analysis of whole heart protein extracts revealed 50% lower amounts of Na⁺/K⁺ ATPase (NKA) in KI than in WT. Ouabain worsened the diastolic phenotype of KI cardiomyocytes at concentrations which did not impair WT diastolic function. Ouabain led to an elevation of intracellular Ca²⁺, which was poorly tolerated in KI showing already high cytosolic Ca²⁺ at baseline due to increased myofilament Ca²⁺ sensitivity. Lower amounts of NKA in KI could amplify the need to exchange excessive intracellular Na⁺ for Ca²⁺ and thereby explain the general tendency to higher diastolic Ca²⁺ in KI.

Replication Stress Response Modifies Sarcomeric Cardiomyopathy Remodeling

Background

Sarcomere gene mutations lead to cardiomyocyte hypertrophy and pathological myocardial remodeling. However, there is considerable phenotypic heterogeneity at both the cellular and the organ level, suggesting modifiers regulate the effects of these mutations. We hypothesized that sarcomere dysfunction leads to cardiomyocyte genotoxic stress, and this modifies pathological ventricular remodeling.

Methods and Results

Using a murine model deficient in the sarcomere protein, Mybpc3^−/− (cardiac myosin‐binding protein 3), we discovered that there was a surge in cardiomyocyte nuclear DNA damage during the earliest stages of cardiomyopathy. This was accompanied by a selective increase in ataxia telangiectasia and rad3‐related phosphorylation and increased p53 protein accumulation. The cause of the DNA damage and DNA damage pathway activation was dysregulated cardiomyocyte DNA synthesis, leading to replication stress. We discovered that selective inhibition of ataxia telangiectasia and rad3 related or cardiomyocyte deletion of p53 reduced pathological left ventricular remodeling and cardiomyocyte hypertrophy in Mybpc3^−/− animals. Mice and humans harboring other types of sarcomere gene mutations also had evidence of activation of the replication stress response, and this was associated with cardiomyocyte aneuploidy in all models studied.

Conclusions

Collectively, our results show that sarcomere mutations lead to activation of the cardiomyocyte replication stress response, which modifies pathological myocardial remodeling in sarcomeric cardiomyopathy.

X Chromosome Inactivation in Carriers of Fabry Disease: Review and Meta-Analysis

Anderson-Fabry disease is an X-linked inborn error of glycosphingolipid catabolism caused by a deficiency of α-galactosidase A. The incidence ranges between 1: 40,000 and 1:117,000 of live male births. In Italy, an estimate of incidence is available only for the north-western Italy, where it is of approximately 1:4000. Clinical symptoms include angiokeratomas, corneal dystrophy, and neurological, cardiac and kidney involvement. The prevalence of symptomatic female carriers is about 70%, and in some cases, they can exhibit a severe phenotype. Previous studies suggest a correlation between skewed X chromosome inactivation and symptoms in carriers of X-linked disease, including Fabry disease. In this review, we briefly summarize the disease, focusing on the clinical symptoms of carriers and analysis of the studies so far published in regards to X chromosome inactivation pattern, and manifesting Fabry carriers. Out of 151 records identified, only five reported the correlation between the analysis of XCI in leukocytes and the related phenotype in Fabry carriers, in particular evaluating the Mainz Severity Score Index or cardiac involvement. The meta-analysis did not show any correlation between MSSI or cardiac involvement and skewed XCI, likely because the analysis of XCI in leukocytes is not useful for predicting the phenotype in Fabry carriers.

Global and regional echocardiographic strain to assess the early phase of hypertrophic cardiomyopathy due to sarcomeric mutations

AIMS

Hypertrophic cardiomyopathy (HCM) is a genetic disease with delayed cardiac expression. Our objective was to characterize left ventricular (LV) myocardial strain by two-dimensional echocardiography in sarcomeric mutation carriers before the hypertrophic stage.

METHODS AND RESULTS

We studied 140 adults [derivation cohort (n = 79), validation cohort (n = 61)]. The derivation cohort comprised 38 confirmed HCM patients with hypertrophy (LVH+/Gen+), 20 mutation carriers without LV hypertrophy (LVH-/Gen+), and 21 healthy controls. LV global longitudinal strain was not different in LVH-/Gen+ compared with controls [20.6%, interquartile (IQ): 18.3/24.2 vs. 22.9%, IQ: 20.9/26.8] but was reduced in LVH+/Gen+ patients (14.1%, IQ: 11.8/18.5, P < 0.001). Regional peak longitudinal strain was significantly decreased in LVH-/Gen+ when compared with controls in four segments: basal anteroseptal (BAS) wall (P = 0.018), basal inferoseptal wall (P = 0.047), basal inferior wall (P = 0.006), and mid anteroseptal wall (P = 0.022). Receiver operating characteristic analysis identified that BAS strain <16.5% had a sensitivity (Se), specificity (Sp), positive and negative predictive values (PPV, NPV) of 57%, 90%, 82%, and 67%, respectively, to differentiate LVH-/G+ patients from controls. Similarly, the accuracy of a ratio between basal inferoseptal/basal anterolateral (BIS/BAL) strain <0.76 was 73%, 92%, 82%, and 64%, respectively (Se/Sp/PPV/NPV). In the validation cohort, the accuracy of BAS and BIS/BAL was 39%/93%/87%/57% and 55%/96%/95%/64% (Se/Sp/PPV/NPV), respectively, to differentiate the LVH-/Gen+ group from controls.

CONCLUSION

Regional longitudinal strain, but not global strain, was significantly reduced at the early stage of HCM before LV hypertrophy. This suggests that the inclusion of strain (BAS < 16.5%; BIS/BAL < 0.76) in the evaluation of HCM relatives would help identify mutation carriers and early LV abnormalities.

Pathophysiology and Treatment of Hypertrophic Cardiomyopathy: New Perspectives

PURPOSE OF REVIEW

We provide a state of the art of therapeutic options in hypertrophic cardiomyopathy (HCM), focusing on recent advances in our understanding of the pathophysiology of sarcomeric disease.

RECENT FINDINGS

A wealth of novel information regarding the molecular mechanisms associated with the clinical phenotype and natural history of HCM have been developed over the last two decades. Such advances have only recently led to a number of controlled randomized studies, often limited in size and fortune. Recently, however, the allosteric inhibitors of cardiac myosin adenosine triphosphatase, countering the main pathophysiological abnormality associated with HCM-causing mutations, i.e. hypercontractility, have opened new management perspectives. Mavacamten is the first drug specifically developed for HCM used in a successful phase 3 trial, with the promise to reach symptomatic obstructive patients in the near future. In addition, the fine characterization of cardiomyocyte electrophysiological remodelling has recently highlighted relevant therapeutic targets. Current therapies for HCM focus on late disease manifestations without addressing the intrinsic pathological mechanisms. However, novel evidence-based approaches have opened the way for agents targeting HCM molecular substrates. The impact of these targeted interventions will hopefully alter the natural history of the disease in the near future.

Computational prediction of protein subdomain stability in MYBPC3 enables clinical risk stratification in hypertrophic cardiomyopathy and enhances variant interpretation

Purpose

Variants in MYBPC3 causing loss of function are the most common cause of hypertrophic cardiomyopathy (HCM). However, a substantial number of patients carry missense variants of uncertain significance (VUS) in MYBPC3. We hypothesize that a structural-based algorithm, STRUM, which estimates the effect of missense variants on protein folding, will identify a subgroup of HCM patients with a MYBPC3 VUS associated with increased clinical risk.

Methods

Among 7,963 patients in the multicenter Sarcomeric Human Cardiomyopathy Registry (SHaRe), 120 unique missense VUS in MYBPC3 were identified. Variants were evaluated for their effect on subdomain folding and a stratified time-to-event analysis for an overall composite endpoint (first occurrence of ventricular arrhythmia, heart failure, all-cause mortality, atrial fibrillation, and stroke) was performed for patients with HCM and a MYBPC3 missense VUS.

Results

We demonstrated that patients carrying a MYBPC3 VUS predicted to cause subdomain misfolding (STRUM+, ΔΔG ≤ −1.2 kcal/mol) exhibited a higher rate of adverse events compared with those with a STRUM- VUS (hazard ratio = 2.29, P = 0.0282). In silico saturation mutagenesis of MYBPC3 identified 4,943/23,427 (21%) missense variants that were predicted to cause subdomain misfolding.

Conclusion

STRUM identifies patients with HCM and a MYBPC3 VUS who may be at higher clinical risk and provides supportive evidence for pathogenicity.

Phenotypic Diversity of Cardiomyopathy Caused by an MYBPC3 Frameshift Mutation in a Korean Family: A Case Report

Restrictive cardiomyopathy (RCM) is one of the rarest cardiac disorders, with a very poor prognosis, and heart transplantation is the only long-term treatment of choice. We reported that a Korean family presented different cardiomyopathies, such as idiopathic RCM and hypertrophic cardiomyopathy (HCM), caused by the same MYBPC3 mutation in different individuals. A 74-year-old male was admitted for the evaluation of exertional dyspnea, palpitations, and pitting edema in both legs for several months. Transthoracic echocardiography (TTE) showed RCM with biatrial enlargement and pericardial effusion. Cardiac magnetic resonance (CMR) images revealed normal left ventricular chamber size, borderline diffuse left ventricular hypertrophy and very large atria. In contrast to the proband, CMR images showed asymmetric septal hypertrophy of the left ventricle, consistent with a diagnosis of HCM in the proband’s two daughters. Of the five heterozygous variants identified as candidate causes of inherited cardiomyopathy by whole exome sequencing in the proband, Sanger sequencing confirmed the presence of a heterozygous frameshift mutation (NM_000256.3:c.3313_3314insGG; p.Ala1105Glyfs*85) in MYBPC3 in the proband and his affected daughters, but not in his unaffected granddaughter. There is clinical and genetic overlap of HCM with restrictive physiology and RCM, especially when HCM is combined with severe myocardial fibrosis. Family screening with genetic testing and CMR imaging could be excellent tools for the evaluation of idiopathic RCM.

Sex-Related Differences in Protein Expression in Sarcomere Mutation-Positive Hypertrophic Cardiomyopathy

Background: Sex-differences in clinical presentation contribute to the phenotypic heterogeneity of hypertrophic cardiomyopathy (HCM) patients. While disease prevalence is higher in men, women present with more severe diastolic dysfunction and worse survival. Until today, little is known about the cellular differences underlying sex-differences in clinical presentation.

Methods: To define sex-differences at the protein level, we performed a proteomic analysis in cardiac tissue obtained during myectomy surgery to relieve left ventricular outflow tract obstruction of age-matched female and male HCM patients harboring a sarcomere mutation (n = 13 in both groups). Furthermore, these samples were compared to 8 non-failing controls. Women presented with more severe diastolic dysfunction.

Results: Out of 2099 quantified proteins, direct comparison of male, and female HCM samples revealed only 46 significantly differentially expressed proteins. Increased levels of tubulin and heat shock proteins were observed in female compared to male HCM patients. Western blot analyses confirmed higher levels of tubulin in female HCM samples. In addition, proteins involved in carbohydrate metabolism were significantly lower in female compared to male samples. Furthermore, we found lower levels of translational proteins specifically in male HCM samples. The disease-specificity of these changes were confirmed by a second analysis in which we compared female and male samples separately to non-failing control samples. Transcription factor analysis showed that sex hormone-dependent transcription factors may contribute to differential protein expression, but do not explain the majority of protein changes observed between male and female HCM samples.

Conclusion: In conclusion, based on our proteomics analyses we propose that increased levels of tubulin partly underlie more severe diastolic dysfunction in women compared to men. Since heat shock proteins have cardioprotective effects, elevated levels of heat shock proteins in females may contribute to later disease onset in woman, while reduced protein turnover in men may lead to the accumulation of damaged proteins which in turn affects proper cellular function.

Genome editing in cardiovascular diseases

Genetic modification at the molecular level in somatic cells, germline, and animal models requires for different purposes, such as introducing desired mutation, deletion of alleles, and insertion of novel genes in the genome. Various genome-editing tools are available to accomplish these alterations, such as zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR associated (Cas) system. CRISPR-Cas system is an emerging technology, which is being used in biological and medical sciences, including in the cardiovascular field. It assists to identify the mechanism of various cardiovascular disease occurrence, such as hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy (DCM), and arrhythmogenic cardiomyopathy (ACM). Furthermore, it has been advantages to edit various genes simultaneously and can also be used to treat and prevent several human diseases. This chapter explores the use of the scientific and therapeutic potential of a CRISPR-Cas system to edit the various cardiovascular disease-associated genes to understand the pathways involved in disease progression and treatment.

Regional myocardial function at preclinical disease stage of hypertrophic cardiomyopathy in female gene variant carriers

We recently showed more severe diastolic dysfunction at the time of myectomy in female compared to male patients with obstructive hypertrophic cardiomyopathy. Early recognition of aberrant cardiac contracility using cardiovascular magnetic resonance (CMR) imaging may identify women at risk of cardiac dysfunction. To define myocardial function at an early disease stage, we studied regional cardiac function using CMR imaging with tissue tagging in asymptomatic female gene variant carriers. CMR imaging with tissue tagging was done in 13 MYBPC3, 11 MYH7 and 6 TNNT2 gene carriers and 16 age-matched controls. Regional peak circumferential strain was derived from tissue tagging images of the basal and midventricular segments of the septum and lateral wall. Left ventricular wall thickness and global function were comparable between MYBPC3, MYH7, TNNT2 carriers and controls. MYH7 gene variant carriers showed a different strain pattern as compared to the other groups, with higher septal peak circumferential strain at the basal segments compared to the lateral wall, whereas MYBPC3, TNNT2 carriers and controls showed higher strain at the lateral wall compared to the septum. Only subtle gene-specific changes in strain pattern occur in the myocardium preceding development of cardiac hypertrophy. Overall, our study shows that there are no major contractile deficits in asymptomatic females carrying a pathogenic gene variant, which would justify the use of CMR imaging for earlier diagnosis.

To lie or not to lie: Super-relaxing with myosins

Since the discovery of muscle in the 19th century, myosins as molecular motors have been extensively studied. However, in the last decade, a new functional super-relaxed (SRX) state of myosin has been discovered, which has a 10-fold slower ATP turnover rate than the already-known non-actin-bound, disordered relaxed (DRX) state. These two states are in dynamic equilibrium under resting muscle conditions and are thought to be significant contributors to adaptive thermogenesis in skeletal muscle and can act as a reserve pool that may be recruited when there is a sustained demand for increased cardiac muscle power. This report provides an evolutionary perspective of how striated muscle contraction is regulated by modulating this myosin DRX↔SRX state equilibrium. We further discuss this equilibrium with respect to different physiological and pathophysiological perturbations, including insults causing hypertrophic cardiomyopathy, and small-molecule effectors that modulate muscle contractility in diseased pathology.

Mutation location of HCM-causing troponin T mutations defines the degree of myofilament dysfunction in human cardiomyocytes

BACKGROUND

The clinical outcome of hypertrophic cardiomyopathy patients is not only determined by the disease-causing mutation but influenced by a variety of disease modifiers. Here, we defined the role of the mutation location and the mutant protein dose of the troponin T mutations I79N, R94C and R278C.

METHODS AND RESULTS

We determined myofilament function after troponin exchange in permeabilized single human cardiomyocytes as well as in cardiac patient samples harboring the R278C mutation. Notably, we found that a small dose of mutant protein is sufficient for the maximal effect on myofilament Ca2+-sensitivity for the I79N and R94C mutation while the mutation location determines the magnitude of this effect. While incorporation of I79N and R94C increased myofilament Ca2+-sensitivity, incorporation of R278C increased Ca2+-sensitivity at low and intermediate dose, while it decreased Ca2+-sensitivity at high dose. All three cTnT mutants showed reduced thin filament binding affinity, which coincided with a relatively low maximal exchange (50.5 ± 5.2%) of mutant troponin complex in cardiomyocytes. In accordance, 32.2 ± 4.0% mutant R278C was found in two patient samples which showed 50.0 ± 3.7% mutant mRNA. In accordance with studies that showed clinical variability in patients with the exact same mutation, we observed variability on the functional single cell level in patients with the R278C mutation. These differences in myofilament properties could not be explained by differences in the amount of mutant protein.

CONCLUSIONS

Using troponin exchange in single human cardiomyocytes, we show that TNNT2 mutation-induced changes in myofilament Ca2+-sensitivity depend on mutation location, while all mutants show reduced thin filament binding affinity. The specific mutation-effect observed for R278C could not be translated to myofilament function of cardiomyocytes from patients, and is most likely explained by other (post)-translational troponin modifications. Overall, our studies illustrate that mutation location underlies variability in myofilament Ca2+-sensitivity, while only the R278C mutation shows a highly dose-dependent effect on myofilament function.

Hypertrophic Cardiomyopathy: Diverse Pathophysiology Revealed by Genetic Research, Toward Future Therapy

Hypertrophic cardiomyopathy (HCM) is an intractable disease that causes heart failure mainly due to unexplained severe cardiac hypertrophy and diastolic dysfunction. HCM, which occurs in 0.2% of the general population, is the most common cause of sudden cardiac death in young people. HCM has been studied extensively using molecular genetic approaches. Genes encoding cardiac β-myosin heavy chain, cardiac myosin-binding protein C, and troponin complex, which were originally identified as causative genes, were subsequently reported to be frequently implicated in HCM. Indeed, HCM has been considered a disease of sarcomere gene mutations. However, fewer than half of patients with HCM have mutations in sarcomere genes. The others have been documented to have mutations in cardiac proteins in various other locations, including the Z disc, sarcoplasmic reticulum, plasma membrane, nucleus, and mitochondria. Next-generation sequencing makes it possible to detect mutations at high throughput, and it has become increasingly common to identify multiple cardiomyopathy-causing gene mutations in a single HCM patient. Elucidating how mutations in different genes contribute to the disease pathophysiology will be a challenge. In studies using animal models, sarcomere mutations generally tend to increase myocardial Ca²⁺ sensitivity, and some mutations increase the activity of myosin ATPase. Clinical trials of drugs to treat HCM are ongoing, and further new therapies based on pathophysiological analyses of the causative genes are eagerly anticipated.

T-tubule remodeling in human hypertrophic cardiomyopathy

The highly organized transverse T-tubule membrane system represents the ultrastructural substrate for excitation–contraction coupling in ventricular myocytes. While the architecture and function of T-tubules have been well described in animal models, there is limited morpho-functional data on T-tubules in human myocardium. Hypertrophic cardiomyopathy (HCM) is a primary disease of the heart muscle, characterized by different clinical presentations at the various stages of its progression. Most HCM patients, indeed, show a compensated hypertrophic disease (“non-failing hypertrophic phase”), with preserved left ventricular function, and only a small subset of individuals evolves into heart failure (“end stage HCM”). In terms of T-tubule remodeling, the “end-stage” disease does not differ from other forms of heart failure. In this review we aim to recapitulate the main structural features of T-tubules during the “non-failing hypertrophic stage” of human HCM by revisiting data obtained from human myectomy samples. Moreover, by comparing pathological changes observed in myectomy samples with those introduced by acute (experimentally induced) detubulation, we discuss the role of T-tubular disruption as a part of the complex excitation–contraction coupling remodeling process that occurs during disease progression. Lastly, we highlight how T-tubule morpho-functional changes may be related to patient genotype and we discuss the possibility of a primitive remodeling of the T-tubule system in rare HCM forms associated with genes coding for proteins implicated in T-tubule structural integrity, formation and maintenance.

Modeling Inherited Cardiomyopathies in Adult Zebrafish for Precision Medicine

Cardiomyopathies are a highly heterogeneous group of heart muscle disorders. More than 100 causative genes have been linked to various cardiomyopathies, which explain about half of familial cardiomyopathy cases. More than a dozen candidate therapeutic signaling pathways have been identified; however, precision medicine is not being used to treat the various types of cardiomyopathy because knowledge is lacking for how to tailor treatment plans for different genetic causes. Adult zebrafish (Danio rerio) have a higher throughout than rodents and are an emerging vertebrate model for studying cardiomyopathy. Herein, we review progress in the past decade that has proven the feasibility of this simple vertebrate for modeling inherited cardiomyopathies of distinct etiology, identifying effective therapeutic strategies for a particular type of cardiomyopathy, and discovering new cardiomyopathy genes or new therapeutic strategies via a forward genetic approach. On the basis of this progress, we discuss future research that would benefit from integrating this emerging model, including discovery of remaining causative genes and development of genotype-based therapies. Studies using this efficient vertebrate model are anticipated to significantly accelerate the implementation of precision medicine for inherited cardiomyopathies.

Unraveling the Genotype-Phenotype Relationship in Hypertrophic Cardiomyopathy: Obesity-Related Cardiac Defects as a Major Disease Modifier

Hypertrophic cardiomyopathy (HCM) is the most common inherited cardiomyopathy and is characterized by asymmetric septal thickening and diastolic dysfunction. More than 1500 mutations in genes encoding sarcomere proteins are associated with HCM. However, the genotype‐phenotype relationship in HCM is incompletely understood and involves modification by additional disease hits. Recent cohort studies identify obesity as a major adverse modifier of disease penetrance, severity, and clinical course. In this review, we provide an overview of these clinical findings. Moreover, we explore putative mechanisms underlying obesity‐induced sensitization and aggravation of the HCM phenotype. We hypothesize obesity‐related stressors to impact on cardiomyocyte structure, metabolism, and homeostasis. These may impair cardiac function by directly acting on the primary mutation‐induced myofilament defects and by independently adding to the total cardiac disease burden. Last, we address important clinical and pharmacological implications of the involvement of obesity in HCM disease modification.

Impact of functional studies on exome sequence variant interpretation in early-onset cardiac conduction system diseases

AIMS

The genetic cause of cardiac conduction system disease (CCSD) has not been fully elucidated. Whole-exome sequencing (WES) can detect various genetic variants; however, the identification of pathogenic variants remains a challenge. We aimed to identify pathogenic or likely pathogenic variants in CCSD patients by using WES and 2015 American College of Medical Genetics and Genomics (ACMG) standards and guidelines as well as evaluating the usefulness of functional studies for determining them.

METHODS AND RESULTS

We performed WES of 23 probands diagnosed with early-onset (<65 years) CCSD and analysed 117 genes linked to arrhythmogenic diseases or cardiomyopathies. We focused on rare variants (minor allele frequency < 0.1%) that were absent from population databases. Five probands had protein truncating variants in EMD and LMNA which were classified as 'pathogenic' by 2015 ACMG standards and guidelines. To evaluate the functional changes brought about by these variants, we generated a knock-out zebrafish with CRISPR-mediated insertions or deletions of the EMD or LMNA homologs in zebrafish. The mean heart rate and conduction velocities in the CRISPR/Cas9-injected embryos and F2 generation embryos with homozygous deletions were significantly decreased. Twenty-one variants of uncertain significance were identified in 11 probands. Cellular electrophysiological study and in vivo zebrafish cardiac assay showed that two variants in KCNH2 and SCN5A, four variants in SCN10A, and one variant in MYH6 damaged each gene, which resulted in the change of the clinical significance of them from 'Uncertain significance' to 'Likely pathogenic' in six probands.

CONCLUSION

Of 23 CCSD probands, we successfully identified pathogenic or likely pathogenic variants in 11 probands (48%). Functional analyses of a cellular electrophysiological study and in vivo zebrafish cardiac assay might be useful for determining the pathogenicity of rare variants in patients with CCSD. SCN10A may be one of the major genes responsible for CCSD.

MYBPC3 Haplotype Linked to Hypertrophic Cardiomyopathy in Rhesus Macaques (Macaca mulatta)

In humans, abnormal thickening of the left ventricle of the heart clinically defines hypertrophic cardiomyopathy (HCM), a common inherited cardiovascular disorder that can precede a sudden cardiac death event. The wide range of clinical presentations in HCM obscures genetic variants that may influence an individual's susceptibility to sudden cardiac death. Although exon sequencing of major sarcomere genes can be used to detect high-impact causal mutations, this strategy is successful in only half of patient cases. The incidence of left ventricular hypertrophy (LVH) in a managed research colony of rhesus macaques provides an excellent comparative model in which to explore the genomic etiology of severe HCM and sudden cardiac death. Because no rhesus HCM-associated mutations have been reported, we used a next-generation genotyping assay that targets 7 sarcomeric rhesus genes within 63 genomic sites that are orthologous to human genomic regions known to harbor HCM disease variants. Amplicon sequencing was performed on 52 macaques with confirmed LVH and 42 unrelated, unaffected animals representing both the Indian and Chinese rhesus macaque subspecies. Bias-reduced logistic regression uncovered a risk haplotype in the rhesus MYBPC3 gene, which is frequently disrupted in both human and feline HCM; this haplotype implicates an intronic variant strongly associated with disease in either homozygous or carrier form. Our results highlight that leveraging evolutionary genomic data provides a unique, practical strategy for minimizing population bias in complex disease studies.

Myocardial Energy Metabolism in Non-ischemic Cardiomyopathy

As the most metabolically demanding organ in the body, the heart must generate massive amounts of energy adenosine triphosphate (ATP) from the oxidation of fatty acids, carbohydrates and other fuels (e.g., amino acids, ketone bodies), in order to sustain constant contractile function. While the healthy mature heart acts omnivorously and is highly flexible in its ability to utilize the numerous fuel sources delivered to it through its coronary circulation, the heart’s ability to produce ATP from these fuel sources becomes perturbed in numerous cardiovascular disorders. This includes ischemic heart disease and myocardial infarction, as well as in various cardiomyopathies that often precede the development of overt heart failure. We herein will provide an overview of myocardial energy metabolism in the healthy heart, while describing the numerous perturbations that take place in various non-ischemic cardiomyopathies such as hypertrophic cardiomyopathy, diabetic cardiomyopathy, arrhythmogenic cardiomyopathy, and the cardiomyopathy associated with the rare genetic disease, Barth Syndrome. Based on preclinical evidence where optimizing myocardial energy metabolism has been shown to attenuate cardiac dysfunction, we will discuss the feasibility of myocardial energetics optimization as an approach to treat the cardiac pathology associated with these various non-ischemic cardiomyopathies.

Spatial and Functional Distribution of MYBPC3 Pathogenic Variants and Clinical Outcomes in Patients With Hypertrophic Cardiomyopathy

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Pathogenic variants in MYBPC3, encoding cardiac MyBP-C (myosin binding protein C), are the most common cause of familial hypertrophic cardiomyopathy. A large number of unique MYBPC3 variants and relatively small genotyped hypertrophic cardiomyopathy cohorts have precluded detailed genotype-phenotype correlations.

Genetic Testing for Inherited Cardiovascular Diseases: A Scientific Statement From the American Heart Association

Advances in human genetics are improving the understanding of a variety of inherited cardiovascular diseases, including cardiomyopathies, arrhythmic disorders, vascular disorders, and lipid disorders such as familial hypercholesterolemia. However, not all cardiovascular practitioners are fully aware of the utility and potential pitfalls of incorporating genetic test results into the care of patients and their families. This statement summarizes current best practices with respect to genetic testing and its implications for the management of inherited cardiovascular diseases.