Cardiac and skeletal muscle expression of mutant β-myosin heavy chains, degree of functional impairment and phenotypic heterogeneity in hypertrophic cardiomyopathy

Single-cell transcriptomics provides insights into hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a genetic heart disease that is characterized by unexplained segmental hypertrophy that is usually most pronounced in the septum. While sarcomeric gene mutations are often the genetic basis for HCM, the mechanistic origin for the heterogeneous remodeling remains largely unknown. A better understanding of the gene networks driving the cardiomyocyte (CM) hypertrophy is required to improve therapeutic strategies. Patients suffering from HCM often receive a septal myectomy surgery to relieve outflow tract obstruction due to hypertrophy. Using single-cell RNA sequencing (scRNA-seq) on septal myectomy samples from patients with HCM, we identify functional links between genes, transcription factors, and cell size relevant for HCM. The data show the utility of using scRNA-seq on the human hypertrophic heart, highlight CM heterogeneity, and provide a wealth of insights into molecular events involved in HCM that can eventually contribute to the development of enhanced therapies.

AQUA Mutant Protein Quantification of Endomyocardial Biopsy-Sized Samples From a Patient With Hypertrophic Cardiomyopathy

In genetic diseases like hypertrophic cardiomyopathy, reliable quantification of the expression level of mutant protein can play an important role in disease research, diagnosis, treatment and prognosis. For heterozygous β-myosin heavy chain (β-MyHC) mutations it has been shown that disease severity is related to the fraction of mutant protein in the myocardium. Yet, heart tissue from patients with genetically characterized diseases is scarce. Here we asked, if even in the case of small endomyocardial biopsies, single quantifications produce reliable results. Myocardial samples were taken from four different regions of an explanted heart of a patient with hypertrophic cardiomyopathy carrying point mutation p.Gly716Arg in β-MyHC. From both, large samples (15 mg) and small, endomyocardial biopsy-sized samples (≤ 1 mg) myosin was extracted and enzymatically digested to yield a specific peptide of interest that allowed to distinguish mutant and wild-type β-MyHC. Absolute quantification by mass spectrometry (AQUA) of the peptide of interest was performed repeatedly for both sample sizes to determine the fraction of mutant β-MyHC. Fractions of mutant β-MyHC (32% on average) showed only small differences between the four cardiac regions and for large and small samples. The standard deviations were smaller than five percentage points for all cardiac regions. The two quantification methods (large and small sample size) produce results with comparable accuracy and precision. Consequently, with our method even small endomyocardial biopsies allow reliable protein quantification for potential diagnostic purposes.

Critical Evaluation of Current Hypotheses for the Pathogenesis of Hypertrophic Cardiomyopathy

Hereditary hypertrophic cardiomyopathy (HCM), due to mutations in sarcomere proteins, occurs in more than 1/500 individuals and is the leading cause of sudden cardiac death in young people. The clinical course exhibits appreciable variability. However, typically, heart morphology and function are normal at birth, with pathological remodeling developing over years to decades, leading to a phenotype characterized by asymmetric ventricular hypertrophy, scattered fibrosis and myofibrillar/cellular disarray with ultimate mechanical heart failure and/or severe arrhythmias. The identity of the primary mutation-induced changes in sarcomere function and how they trigger debilitating remodeling are poorly understood. Support for the importance of mutation-induced hypercontractility, e.g., increased calcium sensitivity and/or increased power output, has been strengthened in recent years. However, other ideas that mutation-induced hypocontractility or non-uniformities with contractile instabilities, instead, constitute primary triggers cannot yet be discarded. Here, we review evidence for and criticism against the mentioned hypotheses. In this process, we find support for previous ideas that inefficient energy usage and a blunted Frank-Starling mechanism have central roles in pathogenesis, although presumably representing effects secondary to the primary mutation-induced changes. While first trying to reconcile apparently diverging evidence for the different hypotheses in one unified model, we also identify key remaining questions and suggest how experimental systems that are built around isolated primarily expressed proteins could be useful.

Advanced Technologies to Target Cardiac Cell Fate Plasticity for Heart Regeneration

The adult human heart can only adapt to heart diseases by starting a myocardial remodeling process to compensate for the loss of functional cardiomyocytes, which ultimately develop into heart failure. In recent decades, the evolution of new strategies to regenerate the injured myocardium based on cellular reprogramming represents a revolutionary new paradigm for cardiac repair by targeting some key signaling molecules governing cardiac cell fate plasticity. While the indirect reprogramming routes require an in vitro engineered 3D tissue to be transplanted in vivo, the direct cardiac reprogramming would allow the administration of reprogramming factors directly in situ, thus holding great potential as in vivo treatment for clinical applications. In this framework, cellular reprogramming in partnership with nanotechnologies and bioengineering will offer new perspectives in the field of cardiovascular research for disease modeling, drug screening, and tissue engineering applications. In this review, we will summarize the recent progress in developing innovative therapeutic strategies based on manipulating cardiac cell fate plasticity in combination with bioengineering and nanotechnology-based approaches for targeting the failing heart.

Deciphering the Role of Wnt and Rho Signaling Pathway in iPSC-Derived ARVC Cardiomyocytes by In Silico Mathematical Modeling

Arrhythmogenic Right Ventricular cardiomyopathy (ARVC) is an inherited cardiac muscle disease linked to genetic deficiency in components of the desmosomes. The disease is characterized by progressive fibro-fatty replacement of the right ventricle, which acts as a substrate for arrhythmias and sudden cardiac death. The molecular mechanisms underpinning ARVC are largely unknown. Here we propose a mathematical model for investigating the molecular dynamics underlying heart remodeling and the loss of cardiac myocytes identity during ARVC. Our methodology is based on three computational models: firstly, in the context of the Wnt pathway, we examined two different competition mechanisms between β-catenin and Plakoglobin (PG) and their role in the expression of adipogenic program. Secondly, we investigated the role of RhoA-ROCK pathway in ARVC pathogenesis, and thirdly we analyzed the interplay between Wnt and RhoA-ROCK pathways in the context of the ARVC phenotype. We conclude with the following remark: both Wnt/β-catenin and RhoA-ROCK pathways must be inactive for a significant increase of PPARγ expression, suggesting that a crosstalk mechanism might be responsible for mediating ARVC pathogenesis.

Renal sympathetic denervation attenuates left ventricle hypertrophy in spontaneously hypertensive rats by suppressing the Raf/MEK/ERK signaling pathway

OBJECTIVE

To explore the effect of renal sympathetic denervation (RSD) on left ventricle hypertrophy and the Raf/MEK/ERK signaling pathway in spontaneously hypertensive rats (SHRs).

METHODS

SHRs were divided into SHR, SHR + Sham, SHR + RSD and SHR + U0126 groups, with WKY rats as the baseline controls. The blood pressure of rats was observed, while myocardial fibrosis was evaluated through Masson staining. Thereafter, real-time quantitative polymerase chain reaction (qRT-PCR) was carried out to determine the levels of myocardial-hypertrophy-related markers, and Western blotting was used to measure the activity of the Raf/MEK/ERK signaling pathway.

RESULTS

In comparison with the WKY group, significant increases were observed in the systolic pressure and diastolic pressure of rats from the other four groups at different time points after surgery. In addition, rats in these groups had obvious increases in LVMI, renal NE and IVSd and decreases in LVEDd, LVEF and LVFS. In addition, the CVF of myocardial tissues was increased, with the upregulation of ANP, BNP and β-MHC and the downregulation of α-MHC. For the activity of the Raf/MEK/ERK signaling pathway, the levels of p-Raf/Raf, p-MEK/MEK and p-ERK1/2/ERK1/2 were all remarkably elevated (all P < .05). Further comparison with the SHR group showed that the above indexes in the rats were significantly improved in the RSD group and SHR + U0126 group (all P < .05).

CONCLUSION

RSD may decrease blood pressure, mitigate hypertension-induced left ventricle hypertrophy and improve cardiac function efficiently in SHRs via the suppression of the Raf/MEK/ERK signaling pathway.

Hypertrophic cardiomyopathy MYH7 mutation R723G alters mRNA secondary structure

Point mutation R723G in the MYH7 gene causes hypertrophic cardiomyopathy (HCM). Heterozygous patients with this mutation exhibit a comparable allelic imbalance of the MYH7 gene. On average 67% of the total MYH7 mRNA are derived from the MYH7^R723G-allele and 33% from the MYH7^WT allele. Mechanisms underlying mRNA allelic imbalance are largely unknown. We suggest that a different mRNA lifetime of the alleles may cause the allelic drift in R723G patients. A potent regulator of mRNA lifetime is its secondary structure. To test for alterations in the MYH7^R723G mRNA structure we used selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) analysis. We show significantly different SHAPE reactivity of wild-type and MYH7^R723G RNA, which is in accordance with bioinformatically predicted structures. Thus, we provide the first experimental evidence for mRNA secondary structure alterations by the HCM point mutation. We assume that this may result in a prolonged lifetime of MYH7^R723G mRNA in vivo and subsequently in the determined allelic imbalance.

Comparative analysis of functional assay evidence use by ClinGen Variant Curation Expert Panels

BACKGROUND

The 2015 American College of Medical Genetics and Genomics (ACMG) and the Association for Molecular Pathology (AMP) guidelines for clinical sequence variant interpretation state that "well-established" functional studies can be used as evidence in variant classification. These guidelines articulated key attributes of functional data, including that assays should reflect the biological environment and be analytically sound; however, details of how to evaluate these attributes were left to expert judgment. The Clinical Genome Resource (ClinGen) designates Variant Curation Expert Panels (VCEPs) in specific disease areas to make gene-centric specifications to the ACMG/AMP guidelines, including more specific definitions of appropriate functional assays. We set out to evaluate the existing VCEP guidelines for functional assays.

METHODS

We evaluated the functional criteria (PS3/BS3) of six VCEPs (CDH1, Hearing Loss, Inherited Cardiomyopathy-MYH7, PAH, PTEN, RASopathy). We then established criteria for evaluating functional studies based on disease mechanism, general class of assay, and the characteristics of specific assay instances described in the primary literature. Using these criteria, we extensively curated assay instances cited by each VCEP in their pilot variant classification to analyze VCEP recommendations and their use in the interpretation of functional studies.

RESULTS

Unsurprisingly, our analysis highlighted the breadth of VCEP-approved assays, reflecting the diversity of disease mechanisms among VCEPs. We also noted substantial variability between VCEPs in the method used to select these assays and in the approach used to specify strength modifications, as well as differences in suggested validation parameters. Importantly, we observed discrepancies between the parameters VCEPs specified as required for approved assay instances and the fulfillment of these requirements in the individual assays cited in pilot variant interpretation.

CONCLUSIONS

Interpretation of the intricacies of functional assays often requires expert-level knowledge of the gene and disease, and current VCEP recommendations for functional assay evidence are a useful tool to improve the accessibility of functional data by providing a starting point for curators to identify approved functional assays and key metrics. However, our analysis suggests that further guidance is needed to standardize this process and ensure consistency in the application of functional evidence.

Altered force generation and cell-to-cell contractile imbalance in hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is mainly caused by mutations in sarcomeric proteins. Thirty to forty percent of identified mutations are found in the ventricular myosin heavy chain (β-MyHC). A common mechanism explaining how numerous mutations in several different proteins induce a similar HCM-phenotype is unclear. It was proposed that HCM-mutations cause hypercontractility, which for some mutations is thought to result from mutation-induced unlocking of myosin heads from a so-called super-relaxed state (SRX). The SRX was suggested to be related to the "interacting head motif," i.e., pairs of myosin heads folded back onto their S2-region. Here, we address these structural states of myosin in context of earlier work on weak binding cross-bridges. However, not all HCM-mutations cause hypercontractility and/or are involved in the interacting head motif. But most likely, all mutations alter the force generating mechanism, yet in different ways, possibly including inhibition of SRX. Such functional-hyper- and hypocontractile-changes are the basis of our previously proposed concept stating that contractile imbalance due to unequal fractions of mutated and wildtype protein among individual cardiomyocytes over time will induce cardiomyocyte disarray and fibrosis, hallmarks of HCM. Studying β-MyHC-mutations, we found substantial contractile variability from cardiomyocyte to cardiomyocyte within a patient's myocardium, much higher than in controls. This was paralleled by a similarly variable fraction of mutant MYH7-mRNA (cell-to-cell allelic imbalance), due to random, burst-like transcription, independent for mutant and wildtype MYH7-alleles. Evidence suggests that HCM-mutations in other sarcomeric proteins follow the same disease mechanism.

Intrinsic MYH7 expression regulation contributes to tissue level allelic imbalance in hypertrophic cardiomyopathy

HCM, the most common inherited cardiac disease, is mainly caused by mutations in sarcomeric genes. More than a third of the patients are heterozygous for mutations in the MYH7 gene encoding for the β-myosin heavy chain. In HCM-patients, expression of the mutant and the wildtype allele can be unequal, thus leading to fractions of mutant and wildtype mRNA and protein which deviate from 1:1. This so-called allelic imbalance was detected in whole tissue samples but also in individual cells. There is evidence that the severity of HCM not only depends on the functional effect of the mutation itself, but also on the fraction of mutant protein in the myocardial tissue. Allelic imbalance has been shown to occur in a broad range of genes. Therefore, we aimed to examine whether the MYH7-alleles are intrinsically expressed imbalanced or whether the allelic imbalance is solely associated with the disease. We compared the expression of MYH7-alleles in non-HCM donors and in HCM-patients with different MYH7-missense mutations. In the HCM-patients, we identified imbalanced as well as equal expression of both alleles. Also at the protein level, allelic imbalance was determined. Most interestingly, we also discovered allelic imbalance and balance in non-HCM donors. Our findings therefore strongly indicate that apart from mutation-specific mechanisms, also non-HCM associated allelic-mRNA expression regulation may account for the allelic imbalance of the MYH7 gene in HCM-patients. Since the relative amount of mutant mRNA and protein or the extent of allelic imbalance has been associated with the severity of HCM, individual analysis of the MYH7-allelic expression may provide valuable information for the prognosis of each patient.

Impaired cardiac mitochondrial oxidative phosphorylation and enhanced mitochondrial oxidative stress in feline hypertrophic cardiomyopathy

Mitochondrial dysfunction and oxidative stress are important players in the development of various cardiovascular diseases, but their roles in hypertrophic cardiomyopathy (HCM) remain unknown. We examined whether mitochondrial oxidative phosphorylation (OXPHOS) capacity was impaired with enhanced mitochondrial oxidative stress in HCM. Cardiac and skeletal muscles were obtained from 9 domestic cats with spontaneously occurring HCM with preserved left ventricular systolic function and from 15 age-matched control cats. Mitochondrial OXPHOS capacities with nonfatty acid and fatty acid substrates in permeabilized fibers and isolated mitochondria were assessed using high-resolution respirometry. ROS release originating from isolated mitochondria was assessed by spectrofluorometry. Thiobarbituric acid-reactive substances were also measured as a marker of oxidative damage. Mitochondrial ADP-stimulated state 3 respiration with complex I-linked nonfatty acid substrates and with fatty acid substrates, respectively, was significantly lower in the hearts of HCM cats compared with control cats. Mitochondrial ROS release during state 3 with complex I-linked substrates and thiobarbituric acid-reactive substances in the heart were significantly increased in cats with HCM. In contrast, there were no significant differences in mitochondrial OXPHOS capacity, mitochondrial ROS release, and oxidative damage in skeletal muscle between groups. Mitochondrial OXPHOS capacity with both nonfatty acid substrates and fatty acid substrates was impaired with increased mitochondrial ROS release in the feline HCM heart. These findings provide new insights into the pathophysiology of HCM and support the hypothesis that restoration of the redox state in the mitochondria is beneficial in the treatment of HCM.

Structural implications of β-cardiac myosin heavy chain mutations in human disease

Over 500 disease-causing point mutations have been found in the human β-cardiac myosin heavy chain, many quite recently with modern sequencing techniques. This review shows that clusters of these mutations occur at critical points in the sequence and investigates whether the many studies on these mutants reveal information about the function of this protein.

Hypothesis and theory: mechanical instabilities and non-uniformities in hereditary sarcomere myopathies

Familial hypertrophic cardiomyopathy (HCM), due to point mutations in genes for sarcomere proteins such as myosin, occurs in 1/500 people and is the most common cause of sudden death in young individuals. Similar mutations in skeletal muscle, e.g., in the MYH7 gene for slow myosin found in both the cardiac ventricle and slow skeletal muscle, may also cause severe disease but the severity and the morphological changes are often different. In HCM, the modified protein function leads, over years to decades, to secondary remodeling with substantial morphological changes, such as hypertrophy, myofibrillar disarray, and extensive fibrosis associated with severe functional deterioration. Despite intense studies, it is unclear how the moderate mutation-induced changes in protein function cause the long-term effects. In hypertrophy of the heart due to pressure overload (e.g., hypertension), mechanical stress in the myocyte is believed to be major initiating stimulus for activation of relevant cell signaling cascades. Here it is considered how expression of mutated proteins, such as myosin or regulatory proteins, could have similar consequences through one or both of the following mechanisms: (1) contractile instabilities within each sarcomere (with more than one stable velocity for a given load), (2) different tension generating capacities of cells in series. These mechanisms would have the potential to cause increased tension and/or stretch of certain cells during parts of the cardiac cycle. Modeling studies are used to illustrate these ideas and experimental tests are proposed. The applicability of similar ideas to skeletal muscle is also postulated, and differences between heart and skeletal muscle are discussed.

Sarcomere mutation-specific expression patterns in human hypertrophic cardiomyopathy

BACKGROUND

Heterozygous mutations in sarcomere genes in hypertrophic cardiomyopathy (HCM) are proposed to exert their effect through gain of function for missense mutations or loss of function for truncating mutations. However, allelic expression from individual mutations has not been sufficiently characterized to support this exclusive distinction in human HCM.

METHODS AND RESULTS

Sarcomere transcript and protein levels were analyzed in septal myectomy and transplant specimens from 46 genotyped HCM patients with or without sarcomere gene mutations and 10 control hearts. For truncating mutations in MYBPC3, the average ratio of mutant:wild-type transcripts was ≈1:5, in contrast to ≈1:1 for all sarcomere missense mutations, confirming that nonsense transcripts are uniquely unstable. However, total MYBPC3 mRNA was significantly increased by 9-fold in HCM samples with MYBPC3 mutations compared with control hearts and with HCM samples without sarcomere gene mutations. Full-length MYBPC3 protein content was not different between MYBPC3 mutant HCM and control samples, and no truncated proteins were detected. By absolute quantification of abundance with multiple reaction monitoring, stoichiometric ratios of mutant sarcomere proteins relative to wild type were strikingly variable in a mutation-specific manner, with the fraction of mutant protein ranging from 30% to 84%.

CONCLUSIONS

These results challenge the concept that haploinsufficiency is a unifying mechanism for HCM caused by MYBPC3 truncating mutations. The range of allelic imbalance for several missense sarcomere mutations suggests that certain mutant proteins may be more or less stable or incorporate more or less efficiently into the sarcomere than wild-type proteins. These mutation-specific properties may distinctly influence disease phenotypes.