Tetraploid state induces p53-dependent arrest of nontransformed mammalian cells in G1

A "spindle assembly" checkpoint has been described that arrests cells in G1 following inappropriate exit from mitosis in the presence of microtubule i nhibitors. We have here addressed the question of whether the resulting tet raploid state itself, rather than failure of spindle function or induction of spindle damage, acts as a checkpoint to arrest cells in G1. Dihydrocytoc halasin B induces cleavage failure in cells where spindle function and chro matid segregation are both normal. Notably, we show here that nontransforme d REF-52 cells arrest indefinitely in tetraploid G1 following cleavage fail ure. The spindle assembly checkpoint and the tetraploidization checkpoint t hat we describe here are likely to be equivalent. Both involve arrest in GI with inactive cdk2 kinase, hypophosphorylated retinoblastoma protein, and elevated levels of p21(WAF1) and cyclin E. Furthermore, both require p53. W e show that failure to arrest in G1 following tetraploidization rapidly res ults in aneuploidy. Similar tetraploid G1 arrest results have been obtained with mouse NIH3T3 and human IMR-90 cells. Thus, we propose that a general checkpoint control acts in G1 to recognize tetraploid cells and induce thei r arrest and thereby prevents the propagation of errors of late mitosis and the generation of aneuploidy. As such, the tetraploidy checkpoint may be a critical activity of p53 in its role of ensuring genomic integrity.