Initial perturbations triggered by p53 loss in stem cells.

p53 is frequently mutated in human cancers, but how it normally acts to prevent transformation is not fully understood. The initial events triggered by p53 loss are commonly extrapolated from cell lines, patient tumors, or mouse models in which p53 has been absent for long durations (e.g., months, years). In these contexts, collateral changes, competitive selection, and secondary adaptations may obscure the immediate impact of p53 elimination. Therefore, to inspect the direct consequences triggered by p53 loss in real time, we developed a platform that enables conditional removal of p53 in unstressed mouse embryonic stem cells. Within 48 h, activated germline programs were accompanied by altered chromatin profiles and downregulated canonical p53 targets. At the single-cell level, extensive heterogeneity was observed in the form of erupting retroelements, increased SINE accessibility, features of Warburg metabolism, and functional germ cell effectors (e.g., Dmrt1 and PGC7/Dppa3). Together, these findings expose ground state functions for p53 and establish that germline transitions, activated mobile elements, and metabolic reprogramming define acute perturbations caused by p53 loss.