Abstract P1006: Cross-lineage Potential Of Ascl1 Uncovered By Comparing Diverse Reprogramming Regulatomes

Direct reprogramming has revolutionized the fields of stem cell biology and regenerative medicine, making it possible to interconvert somatic cell fates without intermediary pluripotency. Although studies have identified different transcription factor (TF) cocktails that can reprogram fibroblasts into different cell types, the common mechanisms governing how reprogramming cells undergo transcriptome and epigenome remodeling (i.e., regulatome remodeling) have not been investigated. Here, by characterizing early changes in the regulatome of three different types of direct reprogramming - induced neurons, induced hepatocytes, and induced cardiomyocytes - we identify lineage-specific features as well as common regulatory transcription factors that act in a context-dependent manner for each type of reprogramming. Of particular interest, we discover that the neuronal factor Ascl1 possesses cross-lineage potential; together with Mef2c, it navigates reprogramming fibroblasts through a targeted and potent iCM trajectory. Single-cell multi-omics reveal that A+M reprogramming terminates in a more mature iCM phenotype than MGT. Finally, through ChIP-seq and RNA-seq, we find that Mef2c drives the shift in Ascl1 binding away from neuronal genes towards cardiac genes, guiding their co-operative epigenetic and transcription activities. Given the relatively short DNA coding sequence of Ascl1 (696bp) and Mef2c (1299bp), our novel A+M reprogramming cocktail has the potential to be delivered within a single AAV vector. Such a strategy is not feasible with classic cocktail MGT (4182bp). Together, our study demonstrates the existence of common regulators of different direct reprogramming processes, argues against the premise that TFs possess only lineage-specific capabilities for altering cell fate - the basic premise used to develop direct reprogramming approaches and discover a novel cardiac reprogramming cocktail showing great potential for future clinical application.