Genetic predictors for the response to therapy in colorectal cancer cells

The general goal of this thesis is to create a better insight into the genes and signaling proteins that influence the response of colorectal cancer cells to MEK-targeted therapy and chemotherapy (5-FU and oxaliplatin). In chapter 2 of this thesis we set out to explore the mechanisms of resistance to inhibition of the MEK/ERK pathway. We found that mutant KRAS allows colorectal cancer cells to compensate for the loss of ERK activity by activating an alternative signaling pathway. Chemotherapeutics induce DNA-damage by various mechanisms, which prompts the tumor suppressor gene TP53 to halt the cell cycle and either repair the damage or induce programmed cell death. In order to recommence the cell cycle following DNA repair,cells have to remain ‘competent’ for continued cell cycle progression. In chapter 3 we show that the wild type p53-induced phosphatase (Wip1) enables cells to resume the cell cycle after DNA repair has been completed. We and others previously observed an increased sensitivity of KRAS mutated colorectal cancer cells to 5-FU and oxaliplatin (53;54). This suggested that mutant KRAS could be of predictive value in the context of conventional chemotherapy treatment. In chapter 4 we elucidated the mechanism underlying this observation. The pro-apoptoticprotein Noxa (Latin for damage) was a crucial factor in sensitizing tumor cells to apoptosis. We also show that both mutant KRAS and wild type p53 are required for Noxa induction and for sensitization to apoptosis. In chapter 5 we studied whether the combination of wildtype p53 and oncogenic KRAS was associated with increased response rates to oxaliplatin and fluoropyrimidine based chemotherapy in a cohort of colorectal cancer patients. We performed mutation analysis for both KRAS and TP53 in patients and correlated them with response to chemotherapy. After our discovery that upregulation of the pro-apoptotic gene Noxa (and hence neutralization of Mcl-1 and A1) was essential for oxaliplatin-induced apoptosis (chapter 4), we hypothesized that neutralization of the other Bcl-2 pro-survival proteins could synergize with oxaliplatin in inducing tumor cell apoptosis. Indeed in chapter 6, we show that the small molecule Bcl-2/Bcl-w/Blc-xL-inhibitor ABT-737 and oxaliplatin induce apoptosis of colorectal tumor cells in a synergistic fashion, both in vitro and in tumor xenografts