| DNA can be damaged by many endogenous and exogenous factors. In response to DNA damage, cells activate cell cycle checkpoints to stop the cell cycle and allow time for repair of the damage. Defects in these pathways allow replication to continue in the face of DNA damage which can lead to mutations that contribute to cancer. These defects can also make cancer cells more sensitive to chemotherapeutic DNA damaging agents as cell cycle progression without repair can be lethal to cells. In tumors in which these pathways are intact, inhibiting checkpoint proteins and DNA repair molecules is an attractive strategy for sensitizing tumors to existing chemotherapeutics. Two key players in these pathways are the Mre11/Rad50/Nbs1 (MRN) complex and ATM. This research aims to determine whether inhibitors of the MRN complex or ATM can provide effective therapeutic approaches when combined with DNA damage, as well as to find drugs that preferentially target cancer cells with defects in the MRN complex and the S-phase checkpoint. We screened a panel of 59 cells lines for MRN protein levels and the ability to S-phase arrest in response to the topoisomerase I inhibitor, SN38. We then compared the results to a database of the known sensitivity of the cell lines to ~100,000 compounds. We found several unclassified compounds which correlated with MRN status. In addition, sensitivity to a class of compounds, called benzothiazoles, correlated with a defective S-phase arrest. We also found that the Mre11 inhibitor, mirin, sensitizes cells to ionizing radiation but not cisplatin or bleomycin. The ATM inhibitor, KU55933, sensitizes cells to SN38 but not to hydroxyurea, whereas inhibition of another checkpoint protein, Chk1, sensitizes cells to hydroxyurea but not SN38. These results demonstrate that the specific type of DNA damage is very important for the ability of these targeted inhibitors of checkpoints and repair to be effective. These findings, as well as a more detailed understanding of the exact mechanisms of action of these combinations, will facilitate the rational combination of DNA inhibitors and DNA damaging agents in the clinic. |
