Checkpoint Inhibitor sensitizes human Cancer Cells

When actively growing cells are exposed to radiation, two important checkpoints are activated to block further progression through the cell cycle, allowing the cell to repair any damaged DNA. Drugs inhibiting these checkpoints could prevent DNA repair pathways and eventually lead to cell death, making them ideal targets for altering a cell’s susceptibility to radiation. Unlike normal cells, half of human tumors have altered p53, an important protein that controls the first checkpoint. James Mitchell, Ph.D. and researchers from the Radiation Biology and Radiation Oncology Branches decided to focus their efforts on a novel inhibitor of the protein Chk1, which regulates the second checkpoint.
Their study, published in Clinical Cancer Research, showed that treating various cancer cell lines expressing p53 mutations with the Chk1 inhibitor significantly increased cell death following exposure to radiation. Interestingly, cells with normal p53 showed the same survival rate after radiation with or without the drug. This suggests that cells with altered p53 become sensitized to radiation in the presence of the drug while normal cells do not.
Mitchell and colleagues then wanted to understand how the inhibitor sensitized the p53 mutant cells. They first investigated whether the drug affected the cell cycle arrest triggered by radiation exposure. Measuring the amount of DNA in p53 normal and mutant cells treated with radiation and the inhibitor together demonstrated that the drug prevented cell cycle arrest in both cell types.
Since the Chk1 inhibitor caused universal cell cycle progression, the researchers next determined the ability of the cells to repair radiation-induced DNA damage in the presence of the agent. In p53 normal cells, immediately following radiation and drug treatment, there was an increase in a protein modification indicative of DNA damage. After 24 hours the modified protein returned to pre-radiation levels demonstrating the completion of DNA repair. In the mutant p53 cells, however, there was an extended up-regulation of the modified protein, even at 24 hours, suggesting a block in DNA damage repair. This inhibition of repair resulted in increased nuclear condensation, a marker of cell death.