The evolution of centrosome and chromosome number in newly formed tetraploid human cells

Tetraploidy – the presence of four copies of the haploid chromosome complement – is common in cancer. There is evidence that ~40% of tumors pass through a tetraploid stage at some point during their development, and tetraploid cells injected in mice are more tumorigenic than their diploid counterparts. However, the reason for this increased tumorigenicity of tetraploid cells is not well established. Most routes by which cells may become tetraploid also confer cells with double the number of centrosomes, the small membraneless organelle that organizes the cell's microtubule cytoskeleton and mitotic spindle apparatus. Centrosome number homeostasis is crucial for health, and recent studies have shown inducing extra centrosomes in cells can induce tumor formation in mice. This has led some researchers to propose that the extra centrosomes that arise together with tetraploidy may be the reason that tetraploid cells are more tumorigenic. However, several anecdotal reports have found that tetraploid clones generated and grown in vitro appear to lose their extra centrosomes. Here, I investigate the population dynamics of the loss of extra centrosomes in newly formed tetraploid cells generated via cytokinesis failure. I uncover the mechanism driving the process and build a mathematical model that captures the experimentally observed dynamics. Next, I investigate karyotypic heterogeneity in newly formed tetraploid cells and their counterparts that are grown for 12 days under standard culture conditions and find that karyotypic heterogeneity has increased after 12 days of growth after tetraploidization. The day 12 'evolved' population with increased heterogeneity formed larger colonies in soft agar than newly formed tetraploid cells or diploid parental precursors and karyotype analysis of the largest soft agar colonies revealed recurrent aneuploidies shared by a subset of colonies. Finally, I investigate the effects of different culture conditions - meant to mimic various conditions in the tumor microenvironment - on the evolution of centrosome and chromosome number in newly formed tetraploid cells and identify a small subset of conditions that altered centrosome homeostasis or the fitness of tetraploid cells.