Abstract Body: Background: A defining characteristic of heart failure is cardiac gene reprogramming. Heart failure-induced pathological stressors activate transcription factors, including Gata4, Tbx5, and Mef2, required for cardiogenesis. These transcription factors activate fetal cardiac genes in the failing adult heart. The mechanism and role of such cardiac gene reprogramming in response to heart failure remain not fully characterized. Here, we leverage transcriptional reprogramming induced by thyroid hormone deficiency as a model system to understand adult heart failure at the single-cell level.

Methods: We engineered a novel Myh6/Myh7 reporter mouse line. Hypothyroidism was induced in mice with propylthiouracil (PTU). PTU treatment led to global cardiac gene reprogramming. To profile gene expression and chromatin accessibility during cardiac gene reprogramming, single nuclei multiomic sequencing was conducted. Echocardiographs on control and PTU-treated mice evaluated cardiac function changes associated with reprogramming. Cardiac tissues were collected to validate both known and newly identified candidates involved in cardiac gene reprogramming.

Results: Characterization of hypothyroidic mice as a model for cardiac reprogramming revealed that the ventricles of PTU-treated hearts significantly switch from Myh6 to dominantly express Myh7. Evaluation of echocardiographs showed that PTU-treated hearts are significantly atrophic, have chamber dilatation, and reduced cardiac function. Analysis of multiome datasets revealed a unique gene set that is differentially expressed and accessible during cardiac gene reprogramming. Upregulated and more accessible genes in PTU-treated cardiomyocytes (CM) are involved in muscle cell differentiation, muscle hypertrophy, and mitochondrion disassembly (Tbx5, Sox4, Wnt2, Rgs2, Hey2, Mfn2). Downregulated and less accessible genes in PTU-treated CMs are related to heart contraction, regulation of heart rate, and ion membrane transport (Kcnd3, Jarid2, Epas1, Atp2a2, Scn5a, Slc25a4).

Conclusion: Induction of cardiac gene reprogramming by thyroid inhibition can be applied to understand adult heart failure mechanisms. Hypothyroidism effectively induces global remodeling in the adult heart. Single nuclei multiomic analysis reveals new mechanistic insights into cardiac gene reprogramming that alters the expression and accessibility of genes essential for heart development and maturation.