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Cardiovascular Research Apr 2015Genetic studies in the 1980s and 1990s led to landmark discoveries that sarcomere mutations cause both hypertrophic and dilated cardiomyopathies. Sarcomere mutations... (Review)
Review
Genetic studies in the 1980s and 1990s led to landmark discoveries that sarcomere mutations cause both hypertrophic and dilated cardiomyopathies. Sarcomere mutations also likely play a role in more complex phenotypes and overlap cardiomyopathies with features of hypertrophy, dilation, diastolic abnormalities, and non-compaction. Identification of the genetic cause of these important conditions provides unique opportunities to interrogate and characterize disease pathogenesis and pathophysiology, starting from the molecular level and expanding from there. With such insights, there is potential for clinical translation that may transform management of patients and families with inherited cardiomyopathies. If key pathways for disease development can be identified, they could potentially serve as targets for novel disease-modifying or disease-preventing therapies. By utilizing gene-based diagnostic testing, we can identify at-risk individuals prior to the onset of clinical disease, allowing for disease-modifying therapy to be initiated early in life, at a time that such treatment may be most successful. In this section, we review the current application of genetics in clinical management, focusing on hypertrophic cardiomyopathy as a paradigm; discuss state-of-the-art genetic testing technology; review emerging knowledge of gene expression in sarcomeric cardiomyopathies; and discuss both the prospects, as well as the challenges, of bringing genetics to medicine.
Topics: Animals; Cardiomyopathies; Computational Biology; DNA Mutational Analysis; Genetic Markers; Genetic Predisposition to Disease; Genetic Testing; Genetic Therapy; Humans; Mutation; Phenotype; Predictive Value of Tests; Risk Factors; Sarcomeres
PubMed: 25634555
DOI: 10.1093/cvr/cvv025 -
Trends in Cardiovascular Medicine Jan 2021Hypertrophic cardiomyopathy (HCM) has a variable clinical presentation due to the diversity of causative genetic mutations. Animal models allow in vivo study of... (Review)
Review
Hypertrophic cardiomyopathy (HCM) has a variable clinical presentation due to the diversity of causative genetic mutations. Animal models allow in vivo study of genotypic expression through non-invasive imaging, pathologic sampling, and force analysis. This review focuses on the spontaneous and induced mutations in various animal models affecting mainly sarcomere proteins. The sarcomere is comprised of thick (myosin) filaments and related proteins including myosin heavy chain and myosin binding protein-C; thin (actin) filament proteins and their associated regulators including tropomyosin, troponin I, troponin C, and troponin T. The regulatory milieu including transcription factors and cell signaling also play a significant role. Animal models provide a layered approach of understanding beginning with the causative mutation as a foundation. The functional consequences of protein energy utilization and calcium sensitivity in vivo and ex vivo can be studied. Beyond pathophysiologic disruption of sarcomere function, these models demonstrate the clinical sequalae of diastolic dysfunction, heart failure, and arrhythmogenic death. Through this cascade of understanding the mutation followed by their functional significance, targeted therapies have been developed and are briefly discussed.
Topics: Animals; Animals, Genetically Modified; Cardiomyopathy, Hypertrophic; Disease Models, Animal; Gene Expression Regulation; Gene Targeting; Genetic Predisposition to Disease; Genetic Therapy; Humans; Molecular Targeted Therapy; Mutation; Phenotype; Sarcomeres; Signal Transduction; Species Specificity; Transcription Factors
PubMed: 31862214
DOI: 10.1016/j.tcm.2019.11.009 -
Circulation Research Jun 2022Heart development relies on tight spatiotemporal control of cardiac gene expression. Genes involved in this intricate process have been identified using animals and...
BACKGROUND
Heart development relies on tight spatiotemporal control of cardiac gene expression. Genes involved in this intricate process have been identified using animals and pluripotent stem cell-based models of cardio(myo)genesis. Recently, the repertoire of cardiomyocyte differentiation models has been expanded with iAM-1, a monoclonal line of conditionally immortalized neonatal rat atrial myocytes (NRAMs), which allows toggling between proliferative and differentiated (ie, excitable and contractile) phenotypes in a synchronized and homogenous manner.
METHODS
In this study, the unique properties of conditionally immortalized NRAMs (iAMs) were exploited to identify and characterize (lowly expressed) genes with an as-of-yet uncharacterized role in cardiomyocyte differentiation.
RESULTS
Transcriptome analysis of iAM-1 cells at different stages during one cycle of differentiation and subsequent dedifferentiation identified ≈13 000 transcripts, of which the dynamic changes in expression upon cardiomyogenic differentiation mostly opposed those during dedifferentiation. Among the genes whose expression increased during differentiation and decreased during dedifferentiation were many with known (lineage-specific) functions in cardiac muscle formation. Filtering for cardiac-enriched low-abundance transcripts, identified multiple genes with an uncharacterized role during cardio(myo)genesis including Sbk2 (SH3 domain binding kinase family member 2). Sbk2 encodes an evolutionarily conserved putative serine/threonine protein kinase, whose expression is strongly up- and downregulated during iAM-1 cell differentiation and dedifferentiation, respectively. In neonatal and adult rats, the protein is muscle-specific, highly atrium-enriched, and localized around the A-band of cardiac sarcomeres. Knockdown of Sbk2 expression caused loss of sarcomeric organization in NRAMs, iAMs and their human counterparts, consistent with a decrease in sarcomeric gene expression as evinced by transcriptome and proteome analyses. Interestingly, co-immunoprecipitation using Sbk2 as bait identified possible interaction partners with diverse cellular functions (translation, intracellular trafficking, cytoskeletal organization, chromatin modification, sarcomere formation).
CONCLUSIONS
iAM-1 cells are a relevant and suitable model to identify (lowly expressed) genes with a hitherto unidentified role in cardiomyocyte differentiation as exemplified by Sbk2: a regulator of atrial sarcomerogenesis.
Topics: Animals; Cell Differentiation; Heart Atria; Myocardium; Myocytes, Cardiac; Rats; Sarcomeres
PubMed: 35587025
DOI: 10.1161/CIRCRESAHA.121.319300 -
Journal of the American Heart... Dec 2023Mutations to the co-chaperone protein BAG3 (B-cell lymphoma-2-associated athanogene-3) are a leading cause of dilated cardiomyopathy (DCM). These mutations often impact...
BACKGROUND
Mutations to the co-chaperone protein BAG3 (B-cell lymphoma-2-associated athanogene-3) are a leading cause of dilated cardiomyopathy (DCM). These mutations often impact the C-terminal BAG domain (residues 420-499), which regulates heat shock protein 70-dependent protein turnover via autophagy. While mutations in other regions are less common, previous studies in patients with DCM found that co-occurrence of 2 variants (P63A, P380S) led to worse prognosis. However, the underlying mechanism for dysfunction is not fully understood.
METHODS AND RESULTS
In this study, we used proteomics, Western blots, and myofilament functional assays on left ventricular tissue from patients with nonfailing, DCM, and DCM with to determine how these mutations impact protein quality control and cardiomyocyte contractile function. We found dysregulated autophagy and increased protein ubiquitination in patients with compared with nonfailing and DCM, suggesting impaired protein turnover. Expression and myofilament localization of BAG3-binding proteins were also uniquely altered in the including abolished localization of the small heat shock protein CRYAB (alpha-crystallin B chain) to the sarcomere. To determine whether these variants impacted sarcomere function, we used cardiomyocyte force-calcium assays and found reduced maximal calcium-activated force in DCM and . Interestingly, myofilament calcium sensitivity was increased in DCM but not with , which was not explained by differences in troponin I phosphorylation.
CONCLUSIONS
Together, our data support that the disease-enhancing mechanism for variants outside of the BAG domain is through disrupted protein turnover leading to compromised sarcomere function. These findings suggest a shared mechanism of disease among pathogenic variants, regardless of location.
Topics: Humans; Sarcomeres; Calcium; Apoptosis Regulatory Proteins; Cardiomyopathy, Dilated; Heart Failure; Autophagy; Adaptor Proteins, Signal Transducing
PubMed: 38108245
DOI: 10.1161/JAHA.123.029938 -
Journal of Neuromuscular Diseases 2017Nemaline myopathy (NM) is among the most common non-dystrophic congenital myopathies (incidence 1:50.000). Hallmark features of NM are skeletal muscle weakness and the... (Review)
Review
Nemaline myopathy (NM) is among the most common non-dystrophic congenital myopathies (incidence 1:50.000). Hallmark features of NM are skeletal muscle weakness and the presence of nemaline bodies in the muscle fiber. The clinical phenotype of NM patients is quite diverse, ranging from neonatal death to normal lifespan with almost normal motor function. As the respiratory muscles are involved as well, severely affected patients are ventilator-dependent. The mechanisms underlying muscle weakness in NM are currently poorly understood. Therefore, no therapeutic treatment is available yet.Eleven implicated genes have been identified: ten genes encode proteins that are either components of thin filament, or are thought to contribute to stability or turnover of thin filament proteins. The thin filament is a major constituent of the sarcomere, the smallest contractile unit in muscle. It is at this level of contraction - thin-thick filament interaction - where muscle weakness originates in NM patients.This review focusses on how sarcomeric gene mutations directly compromise sarcomere function in NM. Insight into the contribution of sarcomeric dysfunction to muscle weakness in NM, across the genes involved, will direct towards the development of targeted therapeutic strategies.
Topics: Animals; Humans; Myopathies, Nemaline; Sarcomeres
PubMed: 28436394
DOI: 10.3233/JND-160200 -
International Journal of Molecular... Aug 2021Hypertrophic cardiomyopathy (HCM) is the most common inherited cardiovascular disorder, affecting 1 in 500 people in the general population. Although characterized by... (Review)
Review
Hypertrophic cardiomyopathy (HCM) is the most common inherited cardiovascular disorder, affecting 1 in 500 people in the general population. Although characterized by asymmetric left ventricular hypertrophy, cardiomyocyte disarray, and cardiac fibrosis, HCM is in fact a highly complex disease with heterogenous clinical presentation, onset, and complications. While HCM is generally accepted as a disease of the sarcomere, variable penetrance in families with identical genetic mutations challenges the monogenic origin of HCM and instead implies a multifactorial cause. Furthermore, large-scale genome sequencing studies revealed that many genes previously reported as causative of HCM in fact have little or no evidence of disease association. These findings thus call for a re-evaluation of the sarcomere-centered view of HCM pathogenesis. Here, we summarize our current understanding of sarcomere-independent mechanisms of cardiomyocyte hypertrophy, highlight the role of extracellular signals in cardiac fibrosis, and propose an alternative but integrated model of HCM pathogenesis.
Topics: Cardiomyopathy, Hypertrophic; Genetic Predisposition to Disease; Humans; Phenotype; Sarcomeres
PubMed: 34445638
DOI: 10.3390/ijms22168933 -
Annual Review of Physiology Feb 2018The thin and thick filaments of muscle sarcomeres are interconnected by the giant protein titin, which is a scaffolding filament, signaling platform, and provider of... (Review)
Review
The thin and thick filaments of muscle sarcomeres are interconnected by the giant protein titin, which is a scaffolding filament, signaling platform, and provider of passive tension and elasticity in myocytes. This review summarizes recent insight into the mechanisms behind how titin gene mutations cause hereditary cardiomyopathy and how titin protein is mechanically active in skeletal and cardiac myocytes. A main theme is the evolving role of titin as a modulator of contraction. Topics include strain-sensing via titin in the sarcomeric A-band as the basis for length-dependent activation, titin elastic recoil and refolding of titin domains as an energy source, and Ca-dependent stiffening of titin stretched during eccentric muscle contractions. Findings suggest that titin stiffness is a principal regulator of the contractile behavior of striated muscle. Physiological or pathological changes to titin stiffness therefore affect contractility. Taken together, titin emerges as a linker element between passive and active myocyte properties.
Topics: Animals; Cardiomyopathies; Connectin; Humans; Muscle Contraction; Muscle, Skeletal; Myocardium; Sarcomeres
PubMed: 29131758
DOI: 10.1146/annurev-physiol-021317-121234 -
Journal of Molecular and Cellular... Jan 2019Morphology underlies subdivision of the primary/heritable sarcomeric cardiomyopathies (CMs) into hypertrophic (HCM) and dilated (DCM). Next-generation DNA-sequencing... (Review)
Review
Morphology underlies subdivision of the primary/heritable sarcomeric cardiomyopathies (CMs) into hypertrophic (HCM) and dilated (DCM). Next-generation DNA-sequencing (NGS) has identified important disease-variants, improving CM diagnosis, management, genetic screening, and prognosis. Although monogenic (Mendelian) analyses directly point at downstream studies, they disregard coexisting genomic variations and gene-by-gene interactions molding detailed CM-phenotypes. In-place of polygenic models, in accounting for observed defective genotype-phenotype correlations, fuzzy concepts having gradations of significance and unsharp domain-boundaries are invoked, including pleiotropy, genetic-heterogeneity, incomplete penetrance, and variable expressivity. HCM and DCM undoubtedly entail cooperativity of unidentified/elusive causative genomic-variants. Modern genomics can exploit comprehensive electronic/digital health records, facilitating consideration of multifactorial variant-models. Genome-wide association studies entailing high-fidelity solid-state catheterization, multimodal-imaging, molecular cardiology, systems biology and bioinformatics, will decipher accurate genotype-phenotype correlations and identify novel therapeutic-targets, fostering personalized medicine/cardiology. This review surveys successes and challenges of genetic/genomic approaches to CMs, and their impact on current and future clinical care.
Topics: Biological Variation, Population; Biomechanical Phenomena; Cardiomyopathies; Humans; Multifactorial Inheritance; Sarcomeres; Translational Research, Biomedical
PubMed: 30423317
DOI: 10.1016/j.yjmcc.2018.10.024 -
Journal of Molecular and Cellular... Aug 2018
Topics: Heart; Humans; Myocardial Contraction; Myofibrils; Sarcomeres
PubMed: 29908919
DOI: 10.1016/j.yjmcc.2018.06.003 -
Physiological Reports Mar 2021The force-length relation of the skeletal muscles is an important factor influencing the joint torque at a given joint angle. We aimed to clarify the relationship...
The force-length relation of the skeletal muscles is an important factor influencing the joint torque at a given joint angle. We aimed to clarify the relationship between the resting sarcomere length and knee joint angle in the vastus intermedius (VI) and to compare it with that of the vastus lateralis (VL). The left and right legs were fixed at knee joint angles of 0° and 90°, respectively, in seven cadavers (age at the time of death: 70-91 years). Muscle tissues were dissected by necropsy of the VL and the VI, and electron microscopy images were obtained to calculate the sarcomere length. At knee joint angles of 0° and 90°, the VL sarcomere length was 2.28 ± 0.49 μm and 2.30 ± 0.48 μm, respectively, and the VI sarcomere length was 2.19 ± 0.35 μm and 2.46 ± 0.53 μm, respectively, with a significant difference between the two (p = 0.028). The magnitude of sarcomere length changes with knee joint angle changes was significantly greater for the VI (0.27 ± 0.20 μm) than for the VL (0.02 ± 0.09 μm) (p = 0.009). Thus, knee joint angle changes may affect the passive and active tension produced by the VI more than those produced by the VL.
Topics: Biomechanical Phenomena; Cadaver; Humans; Knee Joint; Muscle Contraction; Muscle, Skeletal; Quadriceps Muscle; Range of Motion, Articular; Sarcomeres
PubMed: 33650805
DOI: 10.14814/phy2.14771