Online

Sepaniac, Leslie Anne

Cellular, Molecular, and Biomedical Sciences Graduate Program

2021

Ph. D.

Regulated and repeated cell division is necessary for the development, growth, and reproduction of multicellular organisms. A central purpose of mitosis is to faithfully pass hereditary information from one cell onto two genetically identical daughter cells, thus maintaining genomic stability. Cells employ several mechanisms for maintaining genomic stability, including well-characterized cell cycle checkpoints. However, chromosome segregation errors can occur in spite of these regulatory mechanisms. Such errors can result in an improper number of chromosomes being distributed to daughter cells -- termed aneuploidy -- or improper localization of chromosomes into separate satellite nuclei -- termed micronuclei. What, if any, additional mechanisms may prevent aneuploidy and micronuclear formation? One long-standing hypothesis in the field is that chromosome alignment at the mitotic spindle equator prior to segregation may play an essential role in facilitating the equal portioning of chromosomes to daughter cells. To test this, we analyzed cells and mice lacking the function of KIF18A, a kinesin family motor protein required for chromosome alignment. We found that alignment-deficient cells continue to maintain normal copy numbers in vitro and in vivo, suggesting that alignment of chromosomes is not required for maintaining euploidy. However, cells lacking KIF18A displayed interchromosomal compaction defects during anaphase, which in turn lead to abnormal nuclear morphology and micronucleus formation. Micronuclei contain whole or fragmented chromosomes spatially separated from the main nucleus and are associated with genomic instability and tumorigenesis. Paradoxically, we found that Kif18a mutant mice produce micronuclei but do not develop spontaneous tumors. Furthermore, loss of Kif18a had modest or no effect on survival of Trp53 homozygotes and heterozygotes, which form thymic lymphoma. Our analyses indicate that micronuclei in Kif18a deficient cells form stable nuclear envelopes characterized by (1) increased recruitment of nuclear envelope components, and (2) successful expansion of chromatin, which must decondense as cells exit mitosis, compared to the envelopes of micronuclei which form following nocodazole washout or radiation exposure. We provide evidence that this stability occurs as a result of lagging chromosomes being positioned closer to the main chromatin masses in KIF18A KO cells, suggesting a potential positive regulation of nuclear envelope formation. These data suggest that not all micronuclei actively promote tumorigenesis and raise important questions about the impact of micronuclei on genomic stability in vivo.