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Burkart V, Kowalski K, Aldag-Niebling D, Beck J, Frick DA, Holler T, Radocaj A, Piep B, Zeug A, Hilfiker-Kleiner D, dos Remedios CG, van der Velden J, Montag J, Kraft T. Transcriptional bursts and heterogeneity among cardiomyocytes in hypertrophic cardiomyopathy. Front Cardiovasc Med 2022; 9:987889. [PMID: 36082122 PMCID: PMC9445301 DOI: 10.3389/fcvm.2022.987889] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/06/2022] [Accepted: 08/02/2022] [Indexed: 12/01/2022] Open
Abstract
Transcriptional bursting is a common expression mode for most genes where independent transcription of alleles leads to different ratios of allelic mRNA from cell to cell. Here we investigated burst-like transcription and its consequences in cardiac tissue from Hypertrophic Cardiomyopathy (HCM) patients with heterozygous mutations in the sarcomeric proteins cardiac myosin binding protein C (cMyBP-C, MYBPC3) and cardiac troponin I (cTnI, TNNI3). Using fluorescence in situ hybridization (RNA-FISH) we found that both, MYBPC3 and TNNI3 are transcribed burst-like. Along with that, we show unequal allelic ratios of TNNI3-mRNA among single cardiomyocytes and unequally distributed wildtype cMyBP-C protein across tissue sections from heterozygous HCM-patients. The mutations led to opposing functional alterations, namely increasing (cMyBP-Cc.927−2A>G) or decreasing (cTnIR145W) calcium sensitivity. Regardless, all patients revealed highly variable calcium-dependent force generation between individual cardiomyocytes, indicating contractile imbalance, which appears widespread in HCM-patients. Altogether, we provide strong evidence that burst-like transcription of sarcomeric genes can lead to an allelic mosaic among neighboring cardiomyocytes at mRNA and protein level. In HCM-patients, this presumably induces the observed contractile imbalance among individual cardiomyocytes and promotes HCM-development.
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Affiliation(s)
- Valentin Burkart
- Institute for Molecular and Cell Physiology, Hannover Medical School, Hannover, Germany
- Valentin Burkart
| | - Kathrin Kowalski
- Institute for Molecular and Cell Physiology, Hannover Medical School, Hannover, Germany
| | - David Aldag-Niebling
- Institute for Molecular and Cell Physiology, Hannover Medical School, Hannover, Germany
| | - Julia Beck
- Institute for Molecular and Cell Physiology, Hannover Medical School, Hannover, Germany
| | - Dirk Alexander Frick
- Institute for Molecular and Cell Physiology, Hannover Medical School, Hannover, Germany
| | - Tim Holler
- Institute for Molecular and Cell Physiology, Hannover Medical School, Hannover, Germany
| | - Ante Radocaj
- Institute for Molecular and Cell Physiology, Hannover Medical School, Hannover, Germany
| | - Birgit Piep
- Institute for Molecular and Cell Physiology, Hannover Medical School, Hannover, Germany
| | - Andre Zeug
- Institute for Cellular Neurophysiology, Hannover Medical School, Hannover, Germany
| | | | - Cristobal G. dos Remedios
- Mechanosensory Biophysics Laboratory, Victor Chang Cardiac Research Institute, Darlinghurst, NSW, Australia
| | | | - Judith Montag
- Institute for Molecular and Cell Physiology, Hannover Medical School, Hannover, Germany
- *Correspondence: Judith Montag
| | - Theresia Kraft
- Institute for Molecular and Cell Physiology, Hannover Medical School, Hannover, Germany
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Montag J, Burkart V, Beck J, Aldag-Niebling D, Piep B, Keyser B, Dos Remedios C, Hilfigker-Kleiner D, Van Der Velden J, Kraft T. P1617Contractile imbalance as trigger for HCM pathogenesis: evidence from mutations in different sarcomeric proteins. Eur Heart J 2019. [DOI: 10.1093/eurheartj/ehz748.0376] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Abstract
Hypertrophic cardiomyopathy (HCM) is mostly caused by mutations in sarcomeric proteins. About 90% of mutation-positive patients have mutations in one of four proteins: the β-myosin heavy chain (β-MyHC, MYH7), cardiac myosin binding protein C (cMyBP-C, MYBPC3), cardiac troponin I (cTnI, TNNI3), and cardiac troponin T. Almost all patients are heterozygous; they express the wildtype and the mutant protein isoform. For patients with β-MyHC missense mutations we have reported previously that individual cardiomyocytes show a significant variability in force generation and calcium-sensitivity, ranging from essentially donor–like to highly altered function. We provided evidence that the MYH7-alleles are switched on and off stochastically and independently from each other in each cell. This burst-like expression leads to highly variable fractions of mutant and wildtype mRNA between the cardiomyocytes, presumably causing variable fractions of mutant protein. We assume that this variability underlies the determined contractile imbalance leading to stronger cells that over-contract and over-stretch weaker cells. This could trigger development of HCM-hallmarks like myocyte disarray, fibrosis and hypertrophy.
To test whether contractile imbalance may provide a common mechanism of HCM-development, we extended our analysis to additional sarcomeric proteins with HCM-mutations. Analysis of cardiomyocytes from a patient with missense mutation R145W in cTnI revealed highly variable calcium-sensitivity between individual cardiomyocytes, substantially higher than for donor cardiomyocytes. This functional heterogeneity was associated with highly variable fractions of mutant TNNI3-mRNA from cell-to-cell. This suggests that not only missense mutations in β-MyHC but also in cTnI induce hallmarks of HCM via the contractile imbalance mechanism.
In contrast to missense mutations, truncation mutations in cMyBP-C presumably cause HCM via haploinsufficiency. Degradation of truncated proteins causes a lack of functional cMyBP-C and thereby alters function of the sarcomere. We hypothesized that different levels of haploinsufficiency from cell-to-cell may also cause contractile imbalance. Therefore we examined a patient with truncation mutation c.927–2A>G in cMyBP-C. Western blot analysis revealed no truncated protein and reduced levels of wildtype-cMyBP-C, consistent with haploinsufficiency. We also observed a significantly higher variability in fluorescence intensity ratio (MyBPC/Alpha-Actinin) for cardiomyocytes of the HCM-patient than in donor cardiomyocytes. The patchy distribution of cMyBP-C in histological tissue section indicated variable levels of functional protein from cell-to-cell. Functional analysis revealed significantly more variable isometric force generation from cell to cell of patient cardiomyocytes compared to donor, suggesting contractile imbalance.
We conclude that contractile imbalance may be a potential common mechanism of HCM pathogenesis.
Acknowledgement/Funding
German Research Foundation (DFG)
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Affiliation(s)
- J Montag
- Hannover Medical School, Hannover, Germany
| | - V Burkart
- Hannover Medical School, Hannover, Germany
| | - J Beck
- Hannover Medical School, Hannover, Germany
| | | | - B Piep
- Hannover Medical School, Hannover, Germany
| | - B Keyser
- Hannover Medical School, Hannover, Germany
| | | | | | | | - T Kraft
- Hannover Medical School, Hannover, Germany
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Aldag-Niebling D, Radocaj A, Hilfiker-Kleiner D, dos Remedios C, Brenner B, Kraft T. Hypertrophic Cardiomyopathy: Variable Expression of Myosin-Binding Protein C from Cell-To-Cell and Functional Imbalance among Individual Cardiomyocytes. Biophys J 2018. [DOI: 10.1016/j.bpj.2017.11.1762] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022] Open
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