Cardiac Myosin Binding Protein-C Mutations in Families With Hypertrophic Cardiomyopathy

Cardiac myosin binding protein-C: a novel sarcomeric target for gene therapy

Through its ability to interact with both the thick and thin filament proteins within the sarcomere, cardiac myosin binding protein-C (cMyBP-C) regulates the contractile properties of the myocardium. The central regulatory role of cMyBP-C in heart function is emphasized by the fact that a large proportion of inherited hypertrophic cardiomyopathy cases in humans are caused by mutations in cMyBP-C. The primary dysfunction in cMyBP-C-related cardiomyopathies is likely to be abnormal myofilament contractile function; however, currently, there are no effective therapies for ameliorating these contractile defects. Thus, there is a compelling need to design novel therapies to restore normal contractile function in cMyBP-C-related cardiomyopathies. To this end, concepts gleaned from various structural, functional, and biochemical studies can now be utilized to engineer cMyBP-C proteins that, when incorporated into the sarcomere, can significantly improve contractile function. In this review, we discuss the rationale for cMyBP-C-based gene therapies that can be utilized to treat contractile dysfunction in inherited and acquired cardiomyopathies.

Where genotype is not predictive of phenotype: towards an understanding of the molecular basis of reduced penetrance in human inherited disease

Some individuals with a particular disease-causing mutation or genotype fail to express most if not all features of the disease in question, a phenomenon that is known as 'reduced (or incomplete) penetrance'. Reduced penetrance is not uncommon; indeed, there are many known examples of 'disease-causing mutations' that fail to cause disease in at least a proportion of the individuals who carry them. Reduced penetrance may therefore explain not only why genetic diseases are occasionally transmitted through unaffected parents, but also why healthy individuals can harbour quite large numbers of potentially disadvantageous variants in their genomes without suffering any obvious ill effects. Reduced penetrance can be a function of the specific mutation(s) involved or of allele dosage. It may also result from differential allelic expression, copy number variation or the modulating influence of additional genetic variants in cis or in trans. The penetrance of some pathogenic genotypes is known to be age- and/or sex-dependent. Variable penetrance may also reflect the action of unlinked modifier genes, epigenetic changes or environmental factors. At least in some cases, complete penetrance appears to require the presence of one or more genetic variants at other loci. In this review, we summarize the evidence for reduced penetrance being a widespread phenomenon in human genetics and explore some of the molecular mechanisms that may help to explain this enigmatic characteristic of human inherited disease.

MicroRNA transcriptome profiling in cardiac tissue of hypertrophic cardiomyopathy patients with MYBPC3 mutations

Hypertrophic cardiomyopathy (HCM) is predominantly caused by mutations in genes encoding sarcomeric proteins. One of the most frequent affected genes is MYBPC3, which encodes the thick filament protein cardiac myosin binding protein C. Despite the prevalence of HCM, disease pathology and clinical outcome of sarcomeric mutations are largely unknown. We hypothesized that microRNAs (miRNAs) could play a role in the disease process. To determine which miRNAs were changed in expression, miRNA arrays were performed on heart tissue from HCM patients with a MYBPC3 mutation (n=6) and compared with hearts of non-failing donors (n=6). 532 out of 664 analyzed miRNAs were expressed in at least one heart sample. 13 miRNAs were differentially expressed in HCM compared with donors (at p<0.01, fold change ≥ 2). The genomic context of these differentially expressed miRNAs revealed that miR-204 (fold change 2.4 in HCM vs. donor) was located in an intron of the TRPM3 gene, encoding an aspecific cation channel involved in calcium entry. RT-PCR analysis revealed a trend towards TRPM3 upregulation in HCM compared with donor myocardium (fold change 2.3, p=0.078). In silico identification of mRNA targets of differentially expressed miRNAs showed a large proportion of genes involved in cardiac hypertrophy and cardiac beta-adrenergic receptor signaling and we showed reduced phosphorylation of cardiac troponin I in the HCM myocardium when compared with donor. HCM patients with MYBPC3 mutations have a specific miRNA expression profile. Downstream mRNA targets reveal possible involvement in cardiac signaling pathways.

Current perspectives in genetic cardiovascular disorders: from basic to clinical aspects

We summarize recent advances in the clinical genetics of hypercholesterolemia, hypertrophic cardiomyopathy (HCM), and lethal arrhythmia, all of which are monogenic cardiovascular diseases being essential to understanding the heart and circulatory pathophysiology. Among the issues of hypercholesterolemia which play a pivotal role in development of vascular damages, familial hypercholesterolemia is the common genetic cardiovascular disease; in addition to identifying the gene mutation coding low-density lipoprotein receptor, lipid kinetics in autosomal recessive hypercholesterolemia as well as in proprotein convertase subtilisin/kexin 9 gene mutation were recently demonstrated. As for HCM, some gene mutations were identified to correlate with clinical manifestations. Additionally, a gene polymorphism of the renin-angiotensin system in development of heart failure was identified as a modifier gene. The lethal arrhythmias such as sudden death syndromes, QT prolongation, and Brugada syndrome were found to exhibit gene mutation coding potassium and/or sodium ion channels. Interestingly, functional analysis of these gene mutations helped to identify the role of each gene mutation in developing these cardiovascular disorders. We suggest considering the genetic mechanisms of cardiovascular diseases associated with hyperlipidemia, myocardial hypertrophy, or lethal arrhythmia in terms of not only clinical diagnosis but also understanding pathophysiology of each disease with therapeutic aspects.

Using genetic testing to guide therapeutic decisions in cardiomyopathy

OPINION STATEMENT

Genetic analysis of human cardiomyopathy has rapidly transitioned from a strictly research endeavor to a diagnostic tool readily available to clinicians across the globe. In contemporary practice, genetic testing improves the efficiency of family evaluations and clarifies the etiology of ambiguous clinical presentations. The great promise of genetic diagnosis is to enable preventative therapies for individuals at high risk of future disease development, a strategy that is under active clinical investigation. However, in the present and future, careful interpretation of DNA sequence variation is critical, and can be ensured by referral to a specialized cardiovascular genetics clinic.

Genetic testing in the contemporary diagnosis of cardiomyopathy

The heritable cardiomyopathies are relatively common conditions that can lead to heart failure and sudden cardiac death. Family history collection, genetic testing and genetic counseling are recommended for these patients and families in multiple practice guidelines and consensus statements. Research discoveries and rapidly dropping costs of DNA sequencing technologies have resulted in the availability of multiple cardiomyopathy genetic testing panels. Genetic testing not only helps in determining the underlying etiology of idiopathic and familial cardiomyopathies, but is also a powerful tool in the determination of which relatives are at-risk and which are not. Both pre- and post-test genetic counseling is an imperative component of genetic testing, as there are many benefits and limitations of genetic testing that need discussed with each patient undergoing this process.

Detection of a large duplication mutation in the myosin-binding protein C3 gene in a case of hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a cardiovascular disease with autosomal dominant inheritance caused by mutations in genes coding for sarcomeric and/or regulatory proteins expressed in cardiomyocytes. In a small cohort of HCM patients (n=8), we searched for mutations in the two most common genes responsible for HCM and found four missense mutations in the MYH7 gene encoding cardiac β-myosin heavy chain (R204H, M493V, R719W, and R870H) and three mutations in the myosin-binding protein C3 gene (MYBPC3) including one missense (A848V) and two frameshift mutations (c.3713delTG and c.702ins26bp). The c.702ins26bp insertion resulted from the duplication of a 26-bp fragment in a 54-year-old female HCM patient presenting with clinical signs of heart failure due to diastolic dysfunction. Although such large duplications (>10 bp) in the MYBPC3 gene are very rare and have been identified only in 4 families reported so far, the identical duplication mutation was found earlier in a Dutch patient, demonstrating that it may constitute a hitherto unknown founder mutation in central European populations. This observation underscores the significance of insertions into the coding sequence of the MYBPC3 gene for the development and pathogenesis of HCM.

Discrete effects of A57G-myosin essential light chain mutation associated with familial hypertrophic cardiomyopathy

The functional consequences of the familial hypertrophic cardiomyopathy A57G (alanine-to-glycine) mutation in the myosin ventricular essential light chain (ELC) were assessed in vitro and in vivo using previously generated transgenic (Tg) mice expressing A57G-ELC mutant vs. wild-type (WT) of human cardiac ELC and in recombinant A57G- or WT-protein-exchanged porcine cardiac muscle strips. Compared with the Tg-WT, there was a significant increase in the Ca²⁺ sensitivity of force (ΔpCa₅₀ ≅ 0.1) and an ~1.3-fold decrease in maximal force per cross section of muscle observed in the mutant preparations. In addition, a significant increase in passive tension in response to stretch was monitored in Tg-A57G vs. Tg-WT strips indicating a mutation-induced myocardial stiffness. Consistently, the hearts of Tg-A57G mice demonstrated a high level of fibrosis and hypertrophy manifested by increased heart weight-to-body weight ratios and a decreased number of nuclei indicating an increase in the two-dimensional size of Tg-A57G vs. Tg-WT myocytes. Echocardiography examination showed a phenotype of eccentric hypertrophy in Tg-A57G mice, enhanced left ventricular (LV) cavity dimension without changes in LV posterior/anterior wall thickness. Invasive hemodynamics data revealed significantly increased end-systolic elastance, defined by the slope of the pressure-volume relationship, indicating a mutation-induced increase in cardiac contractility. Our results suggest that the A57G allele causes disease by means of a discrete modulation of myofilament function, increased Ca²⁺ sensitivity, and decreased maximal tension followed by compensatory hypertrophy and enhanced contractility. These and other contributing factors such as increased myocardial stiffness and fibrosis most likely activate cardiomyopathic signaling pathways leading to pathologic cardiac remodeling.

A systematic review and meta-analysis of genotype-phenotype associations in patients with hypertrophic cardiomyopathy caused by sarcomeric protein mutations

BACKGROUND

The genetic basis of familial hypertrophic cardiomyopathy (HCM) is well described, but the relation between genotype and clinical phenotype is still poorly characterised.

OBJECTIVE

To summarise and critically review the current literature on genotype-phenotype associations in patients with HCM and to perform a meta-analysis on selected clinical features.

DATA SOURCES

PubMed/Medline was searched up to January 2013. Retrieved articles were checked for additional publications.

SELECTION CRITERIA

Observational, cross-sectional and prospectively designed English language human studies that analysed the relationship between the presence of mutations in sarcomeric protein genes and clinical parameters.

DATA EXTRACTION AND ANALYSIS

The pooled analysis was confined to studies reporting on cohorts of unrelated and consecutive patients in which at least two sarcomere genes were sequenced. A random effect meta-regression model was used to determine the overall prevalence of predefined clinical features: age at presentation, gender, family history of HCM, family history of sudden cardiac death (SCD), and maximum left ventricular wall thickness (MLVWT). The I(2) statistic was used to estimate the proportion of total variability in the prevalence data attributable to the heterogeneity between studies.

RESULTS

Eighteen publications (corresponding to a total of 2459 patients) were selected for the pooled analysis. The presence of any sarcomere gene mutation was associated with a younger age at presentation (38.4 vs 46.0 years, p<0.0005), a family history of HCM (50.6% vs 23.1%, p<0.0005), a family history of SCD (27.0% vs 14.9%, p<0.0005) and greater MLVWT (21.0 vs 19.3 mm, p=0.03). There were no differences when the two most frequently affected genes, MYBPC3 and MYH7, were compared. A total of 53 family studies were also included in the review. These were characterised by pronounced variability and the majority of studies reporting on outcomes analysed small cross-sectional cohorts and were unsuitable for pooled analyses.

CONCLUSIONS

The presence of a mutation in any sarcomere gene is associated with a number of clinical features. The heterogeneous nature of the disease and the inconsistency of study design precludes the establishment of more precise genotype-phenotype relationships. Large scale studies examining the relation between genotype, disease severity, and prognosis are required.

Impaired contractile function due to decreased cardiac myosin binding protein C content in the sarcomere

Mutations in cardiac myosin binding protein C (MyBP-C) are a common cause of familial hypertrophic cardiomyopathy (FHC). The majority of MyBP-C mutations are expected to reduce MyBP-C expression; however, the consequences of MyBP-C deficiency on the regulation of myofilament function, Ca²⁺ homeostasis, and in vivo cardiac function are unknown. To elucidate the effects of decreased MyBP-C expression on cardiac function, we employed MyBP-C heterozygous null (MyBP-C+/-) mice presenting decreases in MyBP-C expression (32%) similar to those of FHC patients carrying MyBP-C mutations. The levels of MyBP-C phosphorylation were reduced 53% in MyBP-C+/- hearts compared with wild-type hearts. Skinned myocardium isolated from MyBP-C+/- hearts displayed decreased cross-bridge stiffness at half-maximal Ca²⁺ activations, increased steady-state force generation, and accelerated rates of cross-bridge recruitment at low Ca²⁺ activations (<15% and <25% of maximum, respectively). Protein kinase A treatment abolished basal differences in rates of cross-bridge recruitment between MyBP-C+/- and wild-type myocardium. Intact ventricular myocytes from MyBP-C+/- hearts displayed abnormal sarcomere shortening but unchanged Ca²⁺ transient kinetics. Despite a lack of left ventricular hypertrophy, MyBP-C+/- hearts exhibited elevated end-diastolic pressure and decreased peak rate of LV pressure rise, which was normalized following dobutamine infusion. Furthermore, electrocardiogram recordings in conscious MyBP-C+/- mice revealed prolonged QRS and QT intervals, which are known risk factors for cardiac arrhythmia. Collectively, our data show that reduced MyBP-C expression and phosphorylation in the sarcomere result in myofilament dysfunction, contributing to contractile dysfunction that precedes compensatory adaptations in Ca²⁺ handling, and chamber remodeling. Perturbations in mechanical and electrical activity in MyBP-C+/- mice could increase their susceptibility to cardiac dysfunction and arrhythmia.

Genetic complexity in hypertrophic cardiomyopathy revealed by high-throughput sequencing

Background

Clinical interpretation of the large number of rare variants identified by high throughput sequencing (HTS) technologies is challenging. The aim of this study was to explore the clinical implications of a HTS strategy for patients with hypertrophic cardiomyopathy (HCM) using a targeted HTS methodology and workflow developed for patients with a range of inherited cardiovascular diseases. By comparing the sequencing results with published findings and with sequence data from a large-scale exome sequencing screen of UK individuals, we sought to quantify the strength of the evidence supporting causality for detected candidate variants.

Methods and results

223 unrelated patients with HCM (46±15 years at diagnosis, 74% males) were studied. In order to analyse coding, intronic and regulatory regions of 41 cardiovascular genes, we used solution-based sequence capture followed by massive parallel resequencing on Illumina GAIIx. Average read-depth in the 2.1 Mb target region was 120. Rare (frequency<0.5%) non-synonymous, loss-of-function and splice-site variants were defined as candidates. Excluding titin, we identified 152 distinct candidate variants in sarcomeric or associated genes (89 novel) in 143 patients (64%). Four sarcomeric genes (MYH7, MYBPC3, TNNI3, TNNT2) showed an excess of rare single non-synonymous single-nucleotide polymorphisms (nsSNPs) in cases compared to controls. The estimated probability that a nsSNP in these genes is pathogenic varied between 57% and near certainty depending on the location. We detected an additional 94 candidate variants (73 novel) in desmosomal, and ion-channel genes in 96 patients (43%).

Conclusions

This study provides the first large-scale quantitative analysis of the prevalence of sarcomere protein gene variants in patients with HCM using HTS technology. Inclusion of other genes implicated in inherited cardiac disease identifies a large number of non-synonymous rare variants of unknown clinical significance.

Genetics of heart failure

Heart failure (HF) occurs when the cardiac output, no longer compensated by endogenous mechanisms, fails to meet the metabolic demands of the body. In most populations, the prevalence of heart failure continues to rise, constituting a major public health burden, especially in developed countries. There is some evidence that the risk of HF in the general population depends on genetic predisposition, necessarily characterised by a very complex architecture. In a small, but probably underestimated proportion, HF is caused by Mendelian inherited forms of myocardial disease. The genetic background of these genetic conditions is a matter of intensive research that is already shedding light onto the genetics of common sporadic forms of HF. In this review, we briefly review the insights provided by candidate gene and genome-wide association approaches in common HF and then describe the main genetic causes of inherited heart muscle disease. Finally we present the current challenges and future research needs for both forms of HF. This article is part of a Special Issue entitled: Heart failure pathogenesis and emerging diagnostic and therapeutic interventions.

Subtle abnormalities in contractile function are an early manifestation of sarcomere mutations in dilated cardiomyopathy

BACKGROUND

Sarcomere mutations cause both dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM); however, the steps leading from mutation to disease are not well described. By studying mutation carriers before a clinical diagnosis develops, we characterize the early manifestations of sarcomere mutations in DCM and investigate how these manifestations differ from sarcomere mutations associated with HCM.

METHODS AND RESULTS

Sixty-two genotyped individuals in families with sarcomeric DCM underwent clinical evaluation including strain echocardiography. The group included 12 subclinical DCM mutation carriers with normal cardiac dimensions and left ventricular ejection fraction (LVEF ≥55%), 21 overt DCM subjects, and 29 related mutation (-) normal controls. Results were compared with a previously characterized cohort of 60 subclinical HCM subjects (sarcomere mutation carriers without left ventricular hypertrophy). Systolic myocardial tissue velocity, longitudinal, circumferential, and radial strain, and longitudinal and radial strain rate were reduced by 10%-23% in subclinical DCM mutation carriers compared with controls (P<0.001 for all comparisons), after adjusting for age and family relations. No significant differences in diastolic parameters were identified comparing the subclinical and control cohorts. The opposite pattern of contractile abnormalities with reduced diastolic but preserved systolic function was seen in subclinical HCM.

CONCLUSIONS

Subtle abnormalities in systolic function are present in subclinical DCM mutation carriers, despite normal left ventricular size and ejection fraction. In contrast, impaired relaxation and preserved systolic function appear to be the predominant early manifestations of sarcomere mutations that lead to HCM. These findings support the theory that the mutation's intrinsic impact on sarcomere function influences whether a dilated or hypertrophic phenotype develops.