Independent Stochastic Burst-Like Transcription of Mutant and Wildtype Alleles as Mechanism for Cell-to-Cell Functional Imbalance in Hypertrophic Cardiomyopathy

Abstract

Hypertrophic Cardiomyopathy (HCM) is an inherited disease mainly linked to mutations in genes coding for sarcomeric proteins. Most of these mutations result in a similar phenotype which is characterized by hypertrophy, cellular and myofibrillar disarray and interstitial fibrosis within the myocardium. The similar phenotype suggests an as yet unknown trigger for HCM-development common to different mutations.In our previous work on HCM-mutations in MYH7 we found a large cell-to-cell functional variability in HCM-tissue samples, presumably due to cell-to-cell variability in the expression of mutant protein. This was supported by a large cell-to-cell variability in mutant mRNA. We hypothesized that this observed functional imbalance among individual cardiomyocytes in the cellular network of the myocardium results in hypertrophy, disarray and interstitial fibrosis, i.e., triggers development of the HCM phenotype. Furthermore, FISH-based visualization of active transcription sites showed a substantial proportion of nuclei without active transcription, indicating discontinuous, burst-like transcription of the MYH7-alleles.To test whether independent, stochastic, burst-like transcription of the mutant and wildtype MYH7-alleles could indeed account for all these observations, we programmed a numerical model where the transcription of both alleles is switched on and off stochastically, resulting in sequential generation of pre-mRNA, mRNA and myosin molecules with defined rate constants. Rate constants for generation and decay of pre-mRNA, mRNA and myosin were taken from the literature. The model shows that independent, stochastic, burst-like transcription can indeed account for all of our above mentioned experimental results by adjusting the on and off rates of the transcription yielding the large cell-to-cell variability in mutant mRNA and protein. The fraction of nuclei without active transcription sites is determined by the on/off rates of transcription and modulated by the pre-mRNA to mRNA transition.