Sustained mitotic block elicits DNA breaks: one-step alteration of ploidy and chromosome integrityin mammalian cells

Following prolonged mitotic spindle disruption bymicrotubule poisons, mammalian cells delaythei r entryinto anaphase, then progressivelysli p out of mitosis and become tetraploid. Normal cells then stop cycling before S-phase onset, but the mechanisms underlying this arrest are still unclear. Here we show that a double block prevents endo-reduplication. First, cells that exit mitosis without a functional microtubule network are driven toward G0. Reconstitution of the network unmasks a second block that relies on DNA double-strand breaks occurring earlyin the G1 phase that follows the mitotic block. We propose that a stress signal elicited upon mitotic impairment triggers breakage, which couples the leakyspindle checkpoint to the stringent DNA damage response. Consistent with this finding, cells defective for the damage response continue cycling and acquire, within a single cell cycle, both chromosome rearrangements and abnormal chromosome numbers that remarkablymimic the complex genetic hallmark of tumorigenesis.