Abstract Body: Introduction and Research Question: The impact of post-transcriptional regulation of gene expression, including N6-methyladenosine (m6A) messenger RNA methylation, on cardiac function has not yet been fully understood. Our study found that the absence of the m6A eraser, Fat mass and obesity-associated protein (Fto), in cardiomyocytes of adult mice led to cardiac dysfunction and cardiac hypertrophy.

Methods and Results: To evaluate Fto physiological functions, we generated a tamoxifen-inducible cardiac-specific Fto knockout mouse model (Fto cKO). This resulted in a substantial reduction of Fto at both mRNA and protein levels in the whole heart, leading to an almost complete absence of Fto within cardiomyocytes. Echocardiography analyses showed that Fto cKO mice exhibit impaired cardiac functions, indicated by a 30% reduction in ejection fraction and 40% decrease in fractional shortening after one month of deletion of cardiac Fto (p<0.01). We found a significant increase in heart weight to body weight ratios in Fto cKO animals (p<0.05). This result corresponded with an increased expression of hypertrophic markers including Nppb and Myh7 mRNA in the absence of Fto (p<0.01). We noted an increase in the length of Fto cKO cardiomyocytes (p<0.05) in isolated cardiomyocytes, a characteristic feature of eccentric pathologic hypertrophy. Electron microscopy analysis of intact hearts showed ultrastructure abnormalities in cardiomyocytes from Fto cKO mice including Z-line density loss and disarray in cristae alignment of mitochondria. Moreover, cardiomyocytes from Fto cKO mice demonstrated defects in sarcomeric shortening including a prolongation of time to 90% peak, time to 90% rest, and decay time constant of relaxation for sarcomere shortening (p<0.001), compared to control mice. Mechanistically, we found down-regulation of Serca2a and Ryr2 mRNA in cardiomyocytes lacking Fto. We further demonstrated that Fto binds Serca2a mRNA and controls its cytoplasmic-nuclear shuttling. Fto deficiency led to accumulation of Serca2a mRNA in the nucleus.

Conclusion: These findings suggest that Fto plays a pivotal role as a crucial regulator for cardiac function. Consequently, our results unveil a significant and novel mechanism through which Fto-dependent regulation contributes to the preservation of cardiac homeostasis.