Somatic mutations in the RAS oncogenes are the most common activating lesions in human cancers. These mutations are frequently associated with poor responses to standard cancer therapies and over the last three decades intense effort has been directed towards the identification of compounds that can bind to RAS and alter its activity. As yet this has been met with little success and mutant RAS is considered by many to be ‘undruggable’. Thus identifying therapeutic vulnerabilities in cancer cells that harbour RAS mutations has the potential to greatly improve outcomes for patients with cancers of diverse tissue origin.
Chronic activation of RAS signalling places persistently high metabolic demands on cells to sustain robust cancer cell growth. This prompted us to investigate whether the genes underlying the endodermal phenotypes in a collection of zebrafish mutants we identified in the LiverPlus screen (1) might be required to sustain the rapid growth and proliferation of cells fuelled by oncogenic Ras expression. We have shown that these genes are required to maintain the high proliferative activity of intestinal epithelial cells and hepatocytes between 3 and 7 days of zebrafish development, and that they encode components of multi-subunit complexes that perform essential cellular functions, including transcription, RNA processing and nuclear pore assembly. Using a doxycycline inducible zebrafish hepatocellular carcinoma (HCC) model (2), we identified genetic interactions between oncogenic krasG12V and three of our cloned zebrafish genes. Transgenic zebrafish, Tg(fabp10:rtTA2s-M2;TRE2:EGFP-KrasG12V), hereafter TO(krasG12V), were exposed to doxycycline (20μg/ml) at 2 dpf and again at 5 dpf, prior to analysis of liver volume at 7 dpf using two photon microscopy. This regime produced robust expression of EGFP-KrasG12V and hepatocyte overgrowth, resulting in a 7.5 fold increase in the volume of the liver between 3-7 dpf. When this experiment was conducted with TO(krasG12V) larvae carrying heterozygous mutations in genes important for U12-type splicing, ribosome biogenesis or nuclear pore formation, liver volume in this HCC model was reduced by up to 50%. Since these heterozygous mutations do not affect normal liver development, these experiments suggest that drugs designed to disrupt U12-type splicing, ribosome biogenesis and nuclear pore formation may provide a therapeutic window that could be exploited clinically to restrict the growth of cancer cells without affecting normal proliferative compartments.
1. Ober et al., Mech Dev (2003) 120:5-18
2. Chew et al., Oncogene (2014) 33:2717-27.