Biography and research overview
Dr Steve Pettitt is a postdoctoral research fellow and recipient of a Dean’s Pathways to Independence Award. He uses experimental genetics in mouse embryonic stem cells to investigate the function of genes that are mutated in cancer. One property that Dr Pettitt is particularly interested in is drug sensitivity or resistance – if the genetic profiles that are responsible for these responses can be identified, this information can be used to better target cancer drugs.
Cancer cells can have thousands of mutations in their DNA when compared to normal cells, making it difficult to know which mutated genes are responsible for their different characteristics. Using an experimental approach removes much of the complexity associated with analysing cancer genomes. Embryonic stem cells have a stable, unmutated genotype without the chromosome rearrangements commonly observed in cancers. In addition, a haploid embryonic stem cell line is available, and since this has only one copy of each chromosome, it is very easy to make loss-of-function mutations.
Dr Pettitt’s strategy is to make random mutations in this cell line and analyse them for sensitivity or resistance to commonly used cancer drugs. This approach, known as a forward genetic screen, is commonplace in model organisms such as yeast and fruit flies, but has only recently been made possible in mammalian cells by the isolation of haploid cell lines. Because there is only one mutated gene per cell, analysis is much easier than in cells derived from a tumour. This allows genes and cellular processes that are important for drug toxicity to be identified.
Another advantage of embryonic stem cells is that advanced techniques for knocking out specific genes are available. Dr Pettitt has designed engineered versions of genes commonly mutated or lost in cancer that can be turned off on demand. He is combining these with his forward genetic screening approach to find genes that become essential for survival only when the cancer gene is turned off. The products of these genes represent good drug targets for cancers with the corresponding mutation.
Pettitt SJet al., A Genetic Screen Using the PiggyBac Transposon in Haploid Cells Identifies Parp1 as a Mediator of Olaparib Toxicity. PloS One (2013) 8: e61520, doi:10.1371/journal.pone.0061520.