NuMA Deficiency Causes Micronuclei via Checkpoint-Insensitive k-Fiber Minus-End Detachment From Mitotic Spindle Poles

Micronuclei resulting from improper chromosome segregation foster chromosomal instability in somatic cell division cycles. To prevent micronuclei formation, bundled kinetochore-microtubules called k-fibers must be properly connected to all sister kinetochores on chromosomes via their plus-ends, whereas k-fiber minus-ends must be clustered at the two opposing spindle poles throughout mitosis. The bipolar attachment between sister kinetochores and k-fiber plus-ends is carefully monitored by the spindle assembly checkpoint and further promoted by error-correction mechanisms. However, how k-fiber minus-end clustering is maintained and monitored remains poorly understood. Here, we show that degradation of the Nuclear Mitotic Apparatus (NuMA) protein by auxin-inducible degron technologies in human cells results in micronuclei formation through k-fiber minus-end detachment from focused spindle poles during metaphase. Importantly, this k-fiber minus-end detachment creates misaligned chromosomes that maintain chromosome biorientation and do not activate the mitotic checkpoint, resulting in lagging chromosomes in anaphase. Moreover, we find that NuMA depletion causes centrosome clustering defects in tetraploid cells, leading to an increased frequency of multipolar divisions. Together, our data indicate that NuMA-mediated minus-end clustering of k-fibers and spindle microtubules is critical for faithful chromosome segregation. Similar to erroneous merotelic kinetochore attachments, detachment of k-fiber minus-ends from metaphase spindle poles evades spindle checkpoint surveillance and may therefore be a source of genomic instability in dividing cells.