The correct number of chromosomes is important for the maintenance of healthy cells and organisms. Maintenance of a correct chromosome number depends on the accurate distribution of chromosomes to the daughter cells during cell division, and errors in chromosome segregation result in abnormal chromosome numbers, or aneuploidy. Aneuploidy is typically associated with deleterious effects on organismal and cellular fitness; however, aneuploidy has also been associated with enhanced cellular growth in certain contexts, such as cancer. Another type of deviation from the normal chromosome number can occur when entire sets of chromosomes are added to the normal (diploid) chromosome number, resulting in polyploidy. Whereas polyploidy is found in certain normal tissues and organisms, tetraploidy (four sets of chromosomes) is associated with a number of precancerous lesions and is believed to promote aneuploidy and tumorigenesis. While it is clear that chromosome mis-segregation causes aneuploidy, the effect of aneuploidy on chromosome segregation is less clear. Similarly, it is unclear whether and how tetraploidy may affect chromosome segregation. The work described here shows that aneuploidy can cause chromosome mis-segregation and induces chromosome-specific phenotypic effects. In contrast, tetraploidy does not per se induce chromosome mis-segregation, but enables the accumulation of aneuploidy thanks to a "genetic buffer" effect that allows tetraploid cells to tolerate aneuploidy better than diploid cells. / Ph. D.

At any given time there are on the order of one hundred million cells undergoing mitosis in the human body. To accurately segregate chromosomes, the cell forms the bipolar mitotic spindle, a molecular machine that distributes chromosomes equally to the daughter cells. To this end, microtubules of the mitotic spindle must appropriately attach the kinetochores: protein structures that form on each chromatid of each mitotic chromosome. The majority of the time correct kinetochore microtubule attachments are formed. However, mis-attachments can and do form. Mis-attachments that are not corrected before chromosome segregation can give rise to aneuploidy, an incorrect number of chromosomes. Aneuploidy occurring in the germ line can cause both miscarriage and genetic diseases. Furthermore, aneuploidy is a major characteristic of cancer cells, and aneuploid cancer cells frequently mis-segregate chromosomes at high rates, a phenotype termed chromosomal instability (CIN). CIN has been correlated with both advanced tumorigenesis and poor patient prognosis and over the years there have been many hypotheses for what causes CIN. In this study, we identified two distinct mechanisms that are responsible for CIN. Both of these mechanisms cause a transient, abnormal geometric arrangement of the mitotic spindle. Specifically, cancer cells possess supernumerary centrosomes, which lead to the assembly of multipolar spindles during early mitosis when attachments between kinetochores and microtubules are forming. Supernumerary centrosomes facilitate the formation of merotelic attachments, in which a single kinetochore binds microtubules from more than one centrosome. As mitosis progresses the supernumerary centrosomes cluster, giving rise to a bipolar spindle by the time of chromosome segregation. However, the high rates of merotelic attachments formed during the transient multipolar stage result in high rates of chromosome mis-segregation. The second geometric defect characterized is caused by failure of centrosomes to separate before kinetochore-microtubule attachments begin to form. This mechanism, too, leads to high rates of kinetochore mis-attachment formation and high rates of chromosome mis-segregation. Finally, this study shows that the mechanisms characterized here are prevalent in human cancer cells from multiple organ sites, thus revealing that both mechanisms are a common cause of CIN. / Ph. D.