The mitotic stopwatch limits the proliferation of whole genome doubled cells

Whole genome doubling (WGD) is a frequent event in tumourigenesis that promotes chromosomal instability and tumour evolution. WGD is sensed indirectly due to the presence of extra centrosomes, which activate the PIDDosome to induce a p53-dependent G1-arrest. Here we uncouple WGD from centrosome amplification and show that p53 still arrests tetraploid cells, but via the mitotic stopwatch; a p53/53BP1/USP28-dependent pathway that causes G1-arrest following an extended mitotic delay. Mitotic timing is unaffected by WGD, but the threshold mitotic delay needed to invoke a G1-arrest is reduced. This sensitivity to the stopwatch mechanism is not associated with altered levels of mitotic stopwatch components, but instead, is associated with enhanced p21 concentrations prior to mitosis. Similar effects are observed in diploid cells treated with CDK4/6 inhibitors to double their size, implicating G1 delays and cell size as key determinants of stopwatch sensitivity. This ability of the mitotic stopwatch to arrest proliferation after increases to genome or cell size has important implications for the progression and treatment of cancer. It also demonstrates that the stopwatch pathway can sense more than just mitotic delays.