Kristina Keuper

• Chromosomal aberrations are manifold changes in the configuration of the DNA. Each cell in a tumor may accumulate different karyotype changes, making it challenging to determine the causes and consequences of this instability. Therefore, model systems have been developed in the past to generate and study specific genome alterations. In this thesis, I present the results of my studies on three types of chromosomal aberrations, all of which may contribute to tumor development or progression. Chromothripsis is a phenomenon that describes a one-off massive chromosomal disruption and reassembly, perhaps arising via DNA damage micronuclei (MN). MN are small DNA-packed nuclear envelopes. I tested potential causes of DNA damage in MN and found that the rupture of the MN envelope and the entry of cytosolic fractions increase DNA damage in MN. Furthermore, I addressed the question of what physiological consequences cell lines with an additional rearranged chromosome have compared to those with an intact extra chromosome. Strikingly, the cells with more rearrangements showed a functional advantage resulting in an improved fitness potential. However, the engineering of polysomic cell lines with fully intact additional chromosomes increases various cellular stress responses and reduces the proliferation capacity. To investigate how cancer cells overcome the detrimental consequences of aneuploidy, I explored physiological adaptations of model cells with a defined additional chromosome that underwent in vivo and in vitro evolution. Interestingly, unfavorable phenotypes of aneuploid cells, such as the replication stress, were mitigated upon evolution. Furthermore, I examined the replication on single molecule resolution, showing alteration after evolution that might underlie the replication stress bypass or tolerance. In contrast to these unbalanced forms of genomic aberrations, whole genome doubling (WGD) leads to a full doubled chromosome set, which was shown to evolve into aneuploid karyotypes by chromosomal instability (CIN), frequently by losing chromosomes. Cells that underwent WGD accumulate DNA damage in the S phase. I performed a single molecule analysis on the DNA during the first cell cycle after WGD to elucidate how the DNA damage arises and found that the number of active origins is not sufficient to replicate the doubled amount of DNA in the first S phase after WGD faithfully. This starts a genome-destabilizing cascade that eventually promotes tumorigenesis, metastasis, and poor patient outcome. Taken together, these studies provide insights into the causes and consequences of three types of genomic aberrations: chromothripsis, polysomy, and WGD. However different these phenomena may be, they share one common feature – they contribute to tumor development and progression. Therefore, elucidating the aberrant cell functions caused by genomic aberrations contributes to a better understanding of a cancer cell's nature and will perhaps help to find new cancer therapy targets.