Chromosome 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 is 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 are associated with impaired postnatal growth and survival in mice. Our studies support a model in which the alignment of mitotic chromosomes promotes proper organization of chromosomes into a single nucleus and continued proliferation by ensuring that chromosomes segregate as a compact mass during anaphase.

Chromosomal instability (CIN) is a hallmark of tumor cells caused by changes in the dynamics and control of microtubules that compromise the mitotic spindle. Thus, CIN cells may respond differently than diploid cells to treatments that target mitotic spindle regulation. Here, we test this idea by inhibiting a subset of kinesin motor proteins involved in mitotic spindle control. KIF18A is required for proliferation of CIN cells derived from triple negative breast cancer or colorectal cancer tumors but is not required in near-diploid cells. Following KIF18A inhibition, CIN tumor cells exhibit mitotic delays, multipolar spindles, and increased cell death. Sensitivity to KIF18A knockdown is strongly correlated with centrosome fragmentation, which requires dynamic microtubules but does not depend on bipolar spindle formation or mitotic arrest. Our results indicate the altered spindle microtubule dynamics characteristic of CIN tumor cells can be exploited to reduce the proliferative capacity of CIN cells.

Kif18A and Kif4A display a similar ability to attenuate the dynamics of microtubules but function to control the lengths of distinct subsets of spindle microtubules during mitosis. Kif18A and Kif4A are not functionally equivalent for chromosome alignment, and Kif18A's function in this process depends on its loop2 region.

Chromosomal instability (CIN), characterized by frequent missegregation of chromosomes during mitosis, is a hallmark of tumor cells caused by changes in the dynamics and control of microtubules that comprise the mitotic spindle 1-3 . Thus, CIN tumor cells may respond differently than normal diploid cells to treatments that target mitotic spindle regulation. We tested this idea by inhibiting a subset of kinesin motor proteins that control spindle microtubule dynamics and mechanics but are not required for the proliferation of near-diploid cells. Our results indicate that KIF18A is required for proliferation of CIN cells derived from triple negative breast cancer or colorectal cancer tumors but not normal breast epithelial cells or near-diploid colorectal cancer cells exhibiting microsatellite instability. CIN tumor cells exhibit mitotic delays, multipolar spindles due to centrosome fragmentation, and increased cell death following inhibition of KIF18A. These mitotic defects were further enhanced by increasing the activity of the microtubule depolymerizing kinesin KIF2C/MCAK and are reminiscent of the phenotypes that result from clinically relevant doses of the chemotherapeutic drug paclitaxel 4 . Our results indicate that the altered spindle microtubule dynamics characteristic of CIN tumor cells can be exploited to reduce their proliferative capacity.

The alignment of chromosomes during metaphase is a hallmark of mitosis. For this reason, chromosome alignment has served as an informative functional assay for evaluating mitotic fidelity. The common approach of quantifying the number of mitotic cells with unaligned chromosomes within a population has led to the identification of many proteins required for this conserved process. However, more sensitive assays are now required to dissect the complex molecular control of chromosome alignment. In this chapter, we describe a microscopy-based method for objectively quantifying the distribution of fluorescently labeled chromosomes within the mitotic spindle that can be used to evaluate the extent of chromosome alignment within individual mitotic cells.