PRDM16 is required for proper specification of the working ventricular myocardium by suppressing alternative cardiomyocyte cell fates.

PRDM16 is a transcription factor with histone methyltransferase activity expressed at the earliest stages of cardiac development. Pathogenic mutations in humans lead to cardiomyopathy, conduction abnormalities and heart failure. PRDM16 is specifically expressed in ventricular but not atrial cardiomyocytes and declines in expression postnatally. Since in other tissues PRDM16 is best known for its role in binary cell fate decisions, we hypothesized a similar decision-making function in cardiomyocytes. Here, we demonstrated that cardiomyocyte-specific deletion of Prdm16 during cardiac development results in contractile dysfunction and abnormal electrophysiology of the postnatal heart, resulting in premature death. By combined RNA+ATAC single-cell sequencing we found that PRDM16 favors ventricular working cardiomyocyte identity, by opposing the activity of master regulators of atrial and ventricular conduction fate. Myocardial loss of Prdm16 during development resulted in hyperplasia of the ventricular conduction system. Hence, PRDM16 plays an indispensable role during cardiac development by driving ventricular working cardiomyocyte identity.