To identify molecular liabilities associated with specific cancer genotypes, we take genetic and chemical-genetic approaches in the zebrafish system, complemented with validation studies in cultured human cancer cell lines. Like mice, zebrafish are vertebrates that faithfully recapitulate the core pathways of human oncogenesis. Decisively, however, zebrafish also offer the high-throughput capacity and genetic tractability of flies or worms. Another key advantage of the zebrafish model for identifying targets for systemic inhibition in humans is the system’s whole-body capacity, especially at the embryonic stage. Genetic or pharmacologic manipulation of large clutches of externally developing embryos not only enables target discovery, but also affords preclinical studies in which the therapeutic versus toxic effects of drugs can be analyzed with unparalleled spatial resolution in intact animals. Once we identify a promising target, we can unravel its mechanism of action through morpholino-based epistasis analysis in live embryos, and readily assess the human relevance of our findings via RNAi and biochemical studies in cancer cell lines.
Taking this integrated approach, we previously discovered that vertebrate cells deficient in the p53 tumor suppressor gene are hyperdependent on the Chk1 protein kinase for survival after radiation-induced DNA damage (Sidi et al., Cell 133:864-877, 2008). Genetic or pharmacologic inhibition of Chk1 is sufficient to restore an apoptotic response to ionizing radiation (IR) in otherwise radioresistant p53 mutant zebrafish embryos or human cancer cells, thereby overriding a major mechanism by which tumor cells evade radiation therapy (p53 is mutated in over 50% of human solid tumors). Surprisingly, we found that the mechanism by which Chk1 inhibition restores apoptosis in p53-deficient cells does not rely on reactivation of classical mitochondrial or death-receptor signaling downstream of malfunctional p53. Instead, Chk1-inhibited cells appear to trigger a fundamentally new form of apoptosis, designated “Chk1-suppressed” pathway, which involves the DNA damage-response kinases, ATM and ATR, and the highly conserved but poorly understood caspase-2 protease.
A second focus in the lab is to identify novel cancer liabilities by applying the genetic concept of synthetic lethality to cancer-relevant genotypes. As originally described in yeast, two genes are said to be synthetically lethal if mutation of either alone is viable but simultaneous mutation of both genes is lethal. This concept provides an attractive framework for identifying optimal targets for cancer therapy, because targeting a gene synthetically lethal to a cancer lesion should be deadly to cancer cells but harmless to all other cells in the body. The whole-animal and high-throughput capacities of the zebrafish embryo make it an ideal model system in which to identify synthetic lethal interactors of common molecular alterations in cancer, and we recently obtained proof of principle for the strategy. We are currently focusing on identifying synthetic lethal interactors of the PTEN tumor suppressor by examining the zebrafish orthologs of genes essential for the viability of PTEN mutant, but not PTEN wild-type, cultured cancer cell lines, in collaboration with Pr Alan Ashworth (ICR, UK).