(505c) Harnessing p53 to Stabilize Accelerated, Dual-Phase Reprogramming

The tumor-suppressor protein p53 regulates proliferation and transcription, binds nucleosome-dense regions, triggers apoptosis, and maintains genomic integrity, serving as a guardian of cellular identity. In reprogramming, inhibition and knockdown of p53 enhances reprogramming suggesting that p53 functions as an inhibitor of cell fate transitions. However, we find that maintaining p53 levels enhances reprogramming to motor neurons. This paradox may be resolved by our observation that cells proceed through two distinct phases of reprogramming: a proliferation-dominant phase and a transcription-dominant phase. We define these phases as separated by an inflection point observed in non-monotonic changes in proliferation, transcription, and chromatin accessibility. Our previous work indicates that transcription and proliferation represent dual competing demands. Cells that rapidly proliferate and establish high transcription rates at early timepoints reprogram near-deterministically across diverse protocols. Interference with wildtype p53 via a p53 mutant increases proliferation and the p53 presence in the nucleus. Loss of either proliferation or p53 reduces the population of hyperproliferating, hypertranscribing cells (HHCs) and reprogramming. In vitro and cell lines, p53 activity controls genome stability via Topoisomerase I (Top1). We find that loss of Top1 phenocopies loss of p53 in reducing transcription, HHCs, and reprogramming while increasing genomic stress. Together our data suggest a model in which p53 serves dual roles in inhibiting and promoting reprogramming through regulation of cell cycle and transcription, respectively. This more nuanced perspective of p53 highlights temporally distinct molecular mechanisms by which p53 restricts or confers cellular plasticity.