The mitotic stopwatch synergizes with mild p53 activation to halt cell proliferation

Abstract

The mitotic stopwatch suppresses proliferation of cell lineages experiencing prolonged mitosis that are prone to chromosome missegregation and tumorigenesis. It converts extended mitotic duration into heritable USP28-53BP1 complexes that stabilize p53 and accumulate over generations. To identify genes whose knockout activates the stopwatch, we performed a CRISPR/Cas9 screen comparing dropout kinetics of essential-gene gRNAs in cells lacking versus possessing the stopwatch. Two classes of knockouts emerged: one (27/60 top hits) that prolonged mitosis, and another (33/60 top hits) that mildly elevated p53 without significant mitotic defects, indicating that the stopwatch synergizes with mild p53 activation to halt proliferation. Mild p53 elevation lowered the stopwatch complex threshold for daughter cell arrest and slightly prolonged mitosis. Integrated over successive divisions, the cumulative effect of multiple short mitotic extensions triggered stopwatch-dependent arrest. Thus, the mitotic stopwatch endows the p53 network with a durable lineage memory of modest stress, explaining its tumor-suppressive role.