Hypertrophic cardiomyopathy (HCM) is the most common primary cardiomyopathy, however, most current therapeutic options are unspecific and do not target the underlying molecular mechanisms, which are frequently caused by mutations in sarcomeric subunits. In recent years, circular RNAs (circRNAs) have emerged as a novel functional class of non-coding RNAs. Since several circRNAs have been described in a pathological context, they represent a promising target for novel therapeutic strategies, inter alia in the cardiovascular field. 

By performing next generation sequencing (NGS) screens, we identified a large number of circRNAs that were differentially expressed between healthy and hypertrophic (HCM) human heart tissue. One of these candidates, subsequently termed ‘circ-DN1’, successfully passed structural validation, was highly conserved between human, pig, mouse, rat, and was specifically expressed in cardiomyocytes, suggesting potential cardiomyocytes-specific functions.

Circ-DN1 showed to be regulated in both in vitro and in vivo models of cardiac hypertrophy. Specifically, circ-DN1 was significantly downregulated in human and rodent cardiomyocytes upon induction of hypertrophy by phenylephrine, isoproterenol, leukemia inhibitory factor or cardiotrophin-1, as well as in mice with transverse aortic constriction (TAC) after 13 weeks. In line, siRNA mediated specific circ-DN1 knockdown, leaving the linear host gene expression unaffected, induced cardiomyocyte dysfunction on different levels, characteristic for cardiac hypertrophy. Silencing of circ-DN1 in neonatal rat cardiomyocytes caused a significant increase in cell size. While Seahorse XF Cell Mito Stress Test revealed metabolic dysfunction. Knockdown of circ-DN1 in human induced pluripotent stem cell-derived cardiomyocytes let to an overall decrease in the mitochondrial oxygen consumption rate, significant reduction of ATP production and spare respiratory capacity as well as a characteristic metabolic shift from fatty acid oxidation towards glycolysis, indicated by a reduced extracellular acidification rate. Furthermore, the absence of circ-DN1 caused significantly decreased cell viability and elevated caspase activity and cytotoxicity. 

In summary, circ-DN1 might be a potential regulator of the HCM phenotype. Since its knockdown induces cardiomyocyte hypertrophy and toxicity, we hypothesise that its overexpression might play a protective role and serve as novel approach for HCM treatment, which is currently subject of further investigations.