SUMMARYChromosome alignment at the equator of the mitotic spindle is a highly conserved step during cell division, however, its importance to genomic stability and cellular fitness are not understood. Normal mammalian somatic cells lacking Kif18A function complete cell division without aligning chromosomes. These alignment-deficient cells display normal chromosome copy numbers in vitro and in vivo, suggesting that chromosome alignment is largely dispensable for maintenance of euploidy. However, we find that loss of chromosome alignment leads to interchromosomal compaction defects during anaphase, abnormal organization of chromosomes into a single nucleus at mitotic exit, and the formation of micronuclei in vitro and in vivo. These defects slow cell proliferation and reduce postnatal growth and survival with variable penetrance in mice. Our studies support a model in which the alignment of mitotic chromosomes promotes proper nuclear envelope reassembly and continued proliferation by ensuring that chromosomes segregate as a compact mass during anaphase.

Micronuclei, whole or fragmented chromosomes which are spatially separated from the main nucleus, are strongly associated with genomic instability and have been identified as drivers of tumorigenesis. Paradoxically, Kif18a mutant mice produce micronuclei due to unaligned chromosomes in vivo but do not develop spontaneous tumors, raising questions about whether all micronuclei contribute similarly to genomic instability and cancer. We report here that micronuclei in Kif18a mutant mice form stable nuclear envelopes. Challenging Kif18a mutant mice via deletion of the Trp53 gene led to formation of thymic lymphoma with elevated levels of micronuclei. However, loss of Kif18a had modest or no effect on survival of Trp53 homozygotes and heterozygotes, respectively. To further explore micronuclear envelope stability in KIF18A KO cells, we compared micronuclei induced via different insults in cultured cells. Micronuclei in KIF18A KO cells form stable nuclear envelopes characterized by increased recruitment of core and non-core nuclear envelope components and successful expansion of decondensing chromatin compared to those induced by microtubule drug washout or exposure to radiation. We also observed that lagging chromosomes, which lead to micronucleus formation, were positioned closer to the main chromatin masses, and further from the central spindle, in KIF18A KO cells. Our studies provide in vivo support to models suggesting that micronuclear fate depends on the sub-cellular location of late lagging chromosomes and suggest that not all micronuclei actively promote tumorigenesis.