Biography and research overview
Dr Stephen Pettitt is a Staff Scientist in the Gene Function Team. He uses experimental genetic approaches to investigate the function of genes that are mutated in cancer. 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 where mutations are introduced individually removes some of this complexity associated with analysing cancer genomes.
Dr Pettitt uses CRISPR mutagenesis, where an enzyme that cuts DNA can be programmed to introduce mutations at a site specified by a short RNA molecule that binds to the enzyme and guides it to its DNA target site. This has a wide variety of uses in research, such as in genetic screens, where each of the roughly 20,000 genes is mutated individually using a different guide RNA. This results in a “library” of cells containing mutants for each gene – if any of these mutants show different properties, it is likely that this is due to the mutated gene.
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. Dr Pettitt is particularly interested in drugs targeting the DNA damage response, such as PARP inhibitors. He has used CRISPR mutant libraries to isolate PARP inhibitor resistant cells, identifying several new genes that result in PARP inhibitor resistance when mutated. He has also developed techniques for dense CRISPR mutagenesis, focused on a single gene, to map resistance-causing mutations in fine detail.
In parallel with his laboratory studies, Dr Pettitt analyses DNA sequencing data from patients involved in clinical trials of new agents to see if predictions of drug resistance or sensitivity from lab experiments can be informative in the clinic. These studies also generate new hypotheses to be tested in the lab.
Pettitt SJ et 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.
Pettitt, SJ, Krastev, DB et al., Genome-wide and high-density CRISPR-Cas9 screens identify point mutations in PARP1 causing PARP inhibitor resistance. bioRxiv, 203224 (2017)