The role of the pentose phosphate pathway in TMPyP4 resistance and the telomere uncapping response

Abstract

G-quadruplex stabilising ligands are of interest as potential anti-cancer drugs. Gquadruplexes are folded DNA structures which can form in guanine-rich regions of DNA or RNA, and stabilisation of these structures at telomeres can result in the inhibition of telomerase activity. In this thesis, Saccharomyces cerevisiae was used as a model to examine the in vivo cellular response to treatment with the G-quadruplex stabilising ligand TMPyP4. The findings indicate that the pentose phosphate pathway (PPP) is key for resistance to TMPyP4, since the absence of PPP genes resulted in increased sensitivity to treatment with the ligand. However, the TMPyP4-sensitivity exhibited by pppΔ strains is most likely due to oxidative stress caused by the photosensitivity of the porphyrin. There are also potential links between PPP activity, the response to uncapped telomeres and the DNA damage response (DDR). The cdc13- 1 mutant strain, in which the telomere binding protein Cdc13 is defective, was used to explore these connections. Here, I demonstrate that deletion of key PPP genes results in suppression of the temperature-sensitive growth phenotype of cdc13-1. In addition, the activity of the enzyme which catalyses the initial step of the oxidative phase of the PPP, Zwf1, increases in cdc13-1 strains. Studies of Zwf1 activity in strains arrested in late anaphase and G1, however, suggest that the increase in Zwf1 activity is due to the phase of the cell cycle in which the strains are arrested, rather than the DDR. The work described here demonstrates that the PPP is intrinsically linked to the response to a variety of cellular stresses. Due to its role in the Warburg effect, a metabolic shift observed in tumour cells, examination of PPP function is important not only for the study of normal tissues but also immortal cancer cells, and lessons in budding yeast can lead to important insights.