Identification of novel therapeutics for the treatment of MMR deficient tumours using high-throughput screens.

Abstract

The DNA Mismatch repair (MMR) pathway is responsible for the repair of base-base mismatches and insertion/deletion loops, formed during DNA replication. Mutations in MMR genes significantly increase the predisposition to cancer with MMR deficiency estimated to be present in 15-17 % of all colorectal cancers. 5-fluorouracil is the main treatment for advanced colorectal cancer however the majority of studies suggest that MMR deficient tumours are more resistant to 5-fluorouracil than MMR proficient tumours. Therefore, there is a critical clinical need to identify novel therapeutics to treat these tumours. To this end, we have performed a high-throughput compound screen, to identify compounds that cause selective lethality in MMR deficient cell lines. We identified the potassium-sparing diuretic drug, Triamterene, as selectively lethal in vitro and in vivo in MMR deficient cell lines. Our data suggest that this selectivity is through its antifolate activity, leading to the accumulation of reactive oxygen species and DNA double strand breaks in MMR deficient cells. Interestingly, we identified a requirement, for thymidylate synthase expression, the only de novo enzyme for dTTP synthesis for the Triamterene cytotoxicity. NRF2 and NRF2-induced antioxidants were regulated upon Triamterene treatment and thymidylate synthase silencing, therefore suggesting a role for the antioxidant response in Triamterene toxicity. Taken together, our results suggest Triamterene as a promising novel therapeutic for the treatment of MMR deficient cancers. In order to identify novel therapeutics to treat MMR deficient tumours, we have also performed a high-throughput siRNA screen, to identify genes that cause selective lethality in MMR deficient cell lines. We identified AURKA gene as synthetically lethal in MSH6 deficient cell lines which suggests AURKA as a promising novel therapeutic target for the treatment of MMR deficient cancers. Taken together, in this PhD thesis we have identified two novel therapeutic strategies for the treatment of MMR deficient cancers.