Yeast genome-wide telomere screens and insights into cancer

Abstract

Telomeres are the very ends of linear eukaryotic chromosomes and when too short or dysfunctional they can trigger senescence (ageing). If the cell can bypass senescence, it can lead to genetic instability or cancer. Telomere capping proteins such as the CST (Cdc13, Stn1 and Ten1) complex and Yku70 are essential for the telomeres not to be recognised as double strand breaks. In this thesis I have used published yeast genome-wide screens to identify genes that are relevant to cancer and telomere biology. Overall 14 out of 19 genetic interactions identified by genome-wide screens could be confirmed by small scale experiments. This work mainly focuses on the telomeric roles of VPS74 and the PAF1 complex. Here, I show that the Golgi gene VPS74, whose human orthologue, GOLPH3, is an oncogene, genetically interacts with telomere capping genes and DNA damage response genes. I demonstrate that Vps74 is important for cell fitness of yku70Δ cells and that the low fitness of Vps74 depleted cells is dependent on the presence of DNA damage checkpoint proteins. I have also systematically investigated the roles of PAF1 complex (Cdc73, Paf1, Ctr9, Leo1 and Rtf1, in yeast) components on telomere biology. The conserved PAF1 complex affects RNA abundance in eukaryotes. I demonstrate that individual PAF1 complex components perform different functions at telomeres. I show that loss of Cdc73 improves fitness of telomere defective yeast cells, while loss of other PAF1 components has the opposite effect. Moreover, I show that Paf1 and Ctr9 strongly reduce telomeric repeat-containing non-coding RNA (TERRA), while Cdc73, Leo1 and Rtf1 have little effect. Paf1 and Ctr9 function independently of Sir4 to regulate TERRA and this is because they stimulate TERRA decay, as well as decay of other RNAs. Additionally, I found that Paf1 and Ctr9 decrease TEN1 and STN1 mRNA levels. I suggest that the PAF1 complex plays a specialized role at telomeres, with Paf1 and Ctr9 maintaining telomere integrity and Cdc73 decreasing the fitness of telomere defective cells.