Hitting cancer where it hurts most

The development of cancer is an evolutionary process with accumulating (epi)genetic alterations underlying the transformation of a normal cell into a potentially malignant population. Cancer cells may continue to depend on the presence of such (epi)genetic alterations for maintenance of their malignant phenotype. Concurrently, cancer cells may develop an increased dependency on the function of genes that are not affected by (epi)genetic alterations. These dependencies, named oncogene and non-oncogene addiction, are discussed in this thesis. The identification of genetic dependencies that are specific to cancer can guide the development of therapeutic agents for the treatment of cancer. This thesis describes three distinct studies in which functional genomic screens based on RNA interference (RNAi) have been employed to identify such cancer cell specific dependencies, genes involved in therapy response and regulators of a key signalling network that is often deregulat ed in cancer. The three studies presented in this thesis illustrate the versatility of RNAi-based screens in cancer research. In addition, they provided us with new insight in the regulation of NF-kB signalling and candidate drug targets in cancers driven by the yet intractable RAS oncoproteins. Hence, KIF18A represents a novel candidate drug target for the treatment of cancers driven by RAS oncoproteins. In the first study we performed an RNAi screen for regulators of TNFa-induced activation of NF-kb, a family of transcription factors implicated in immunity, proliferation, differentiation and apoptosis. We describe the identification of FBXO7, a component of SCF-ubiquitin ligase complexes, as a negative regulator of NF-kB signalling. We show that FBXO7 binds to, and mediates ubiquitination of, cIAP1 and TRAF2, resulting in decreased ubiquitination of RIP1 kinase and lowered NF-kB signalling activity. In the second study presented in this thesis, we screened for genes involved in ce llular response to cisplatin, a cytotoxic agent commonly used to treat a variety of cancers. Although we did not discover novel regulators of cellular response mechanisms to cisplatin treatment, our results confirm previous findings and underscore the importance of DNA damage response signalling, homologous recombination and translesion synthesis in the repair of cisplatin-induced DNA damage. The third and last study presented in this thesis involves a screen for novel drug targets in cancer. Despite ample evidence that cancer cells positive for RAS oncoproteins may depend on RAS activity for their survival, there are currently no treatment strategies that specifically target RAS oncoproteins. Therefore, we performed an RNAi screen to identify genetic dependencies associated with the expression of oncogenic RAS. We identified several genes, including kinesin family member 18A (KIF18A), whose suppression results in impaired proliferation and survival of human fibroblasts that express th e H-, K- or N-RAS oncogene. We found that silencing of KIF18A causes a mitotic arrest and that is associated with an increase in apoptosis. More important, we found a correlation between the effects of KIF18A silencing and the RAS mutation status in a panel of Non-Small-Cell Lung Cancer cell lines, suggesting that the increased dependency on KIF18A function associated with the expression of oncogenic RAS is not restricted to fibroblasts. Hence, KIF18A represents a novel candidate drug target for the treatment of cancers driven by RAS oncoproteins.