Finding Balance in the Chaos: Causes and Consequences of Karyotype Deviations

This thesis explores the intricate relationship between chromosomal instability (CIN) and aneuploidy, two closely linked phenomena. While aneuploidy results from CIN, it is unclear if aneuploidy itself drives CIN. Chapter 2 systematically tests the impact of various aneuploidies on chromosomal stability using aneuploid clones with whole chromosome and segmental aneuploidies. By evaluating clones with monosomies and comparing them to those with trisomies, we assess the differences between losing and gaining genetic material and their effects on CIN. Our findings indicate that gained genes drive aneuploidy-induced CIN, as monosomy clones remain chromosomally stable. The strongest correlation between the number of gained genes and CIN suggests that gene gain, rather than loss, is pivotal. Proteotoxic stress unique to trisomies may underlie this observed CIN, along with an elevated inflammatory response attributed to CIN presence. Segmental aneuploidies can also lead to ongoing breakage-fusion-bridge (BFB) cycles due to dicentric chromosome formation, resulting in high CIN levels. These results enhance our understanding of the short-term relationship between aneuploidy and CIN.

Chapter 3 shifts focus to the long-term effects of aneuploidy on cells. Despite aneuploidy's detrimental impact on cellular physiology, it is prevalent in cancer cells, a phenomenon known as the aneuploidy paradox. We investigate potential adaptation mechanisms that cells use to cope with aneuploidy, using clones that initially exhibit slow growth. Long-term culturing reveals that all clones show improved cellular fitness and increased proliferation rates, indicating adaptation. This improved fitness correlates with reduced CIN and inflammation, consistently observed across all clones. A subset of adapted clones recurrently shows amplification of chromosome 12p, which harbors the oncogene KRAS. Parental RPE-1 cells contain one mutated KRAS allele, and the adapted clones with 12p amplification acquire an additional copy of mutant KRAS. Overexpression of mutant KRAS in early clones enhances proliferation, suggesting its role in adaptation.

Chapter 4 investigates the link between hypoxia, a common feature in tumors, and CIN. We expose various cell lines to hypoxic conditions and assess their instability levels, consistently identifying hypoxia as a driver of CIN. This instability is dependent on hypoxia-inducible factor 1-alpha (HIF1α). To test the relevance of this finding in tumor biology, we examine xenograft models and find that hypoxia-induced CIN is present in tumor regions, confirming the impact of hypoxia on CIN in vivo.

In conclusion, this thesis provides insights into the direct consequences of aneuploidy on chromosomal instability and their interconnectedness. We also explore the long-term effects of aneuploidy, revealing cellular adaptation mechanisms that mitigate initial detriments. Finally, we uncover the interplay between hypoxia and CIN, enhancing our understanding of factors contributing to aneuploidy and CIN in tumor contexts. This research contributes to a deeper comprehension of the complexities surrounding CIN and aneuploidy, with implications for cancer biology and potential therapeutic strategies.