Misaligned chromosomes that satisfy the spindle assembly checkpoint are a strong predictor of micronuclei formation in dividing cancer cells
Chromosome alignment to the spindle equator is a hallmark of mitosis that is thought to promote chromosome segregation fidelity in metazoans. Yet, chromosome alignment is only indirectly supervised by the spindle assembly checkpoint (SAC) as a byproduct of chromosome bi-orientation, and the consequences of defective chromosome alignment remain unclear. Here we investigated how human cells respond to chromosome alignment defects of distinct molecular nature by following the fate of live HeLa cells after RNAi-mediated depletion of 120 proteins previously implicated in chromosome alignment. Surprisingly, in all cases, cells frequently entered anaphase after a delay with chronically misaligned chromosomes. Using depletion of key proteins as prototypes for defective chromosome alignment, we show that chronically misaligned chromosomes often satisfy the SAC and directly missegregate. In-depth analysis of specific molecular perturbations that prevent proper kinetochore-microtubule attachments revealed that chronically misaligned chromosomes that missegregate frequently result in micronuclei. Higher-resolution live-cell imaging indicated that, contrary to most anaphase lagging chromosomes that correct and reintegrate the main nuclei, chronically misaligned chromosomes are a strong predictor of micronuclei formation in a cancer cell model of chromosomal instability, but not in normal near-diploid cells. We provide evidence supporting that intrinsic differences in kinetochore-microtubule attachment stability on misaligned chromosomes account for this distinct outcome. Thus, chronically misaligned chromosomes that satisfy the SAC may represent a previously overlooked mechanism driving chromosomal/genomic instability during cancer cell division, and we unveil genetic conditions predisposing for these events.