Introduction:
The enhanced radiation sensitivity of HPV+ HNSCC is also observed on the cellular level when comparing HPV+ and HPV- HNSCC cell lines. We could show that the underlying mechanism is a defect in DNA double-strand break repair associated with a profound and sustained G2-arrest. This defect was exploited by additionally targeting the DNA damage response of these cells resulting in a further enhancement of their radiation sensitivity. We now tested a novel approach of combined targeting of PARP1 and the intra-S/G2 cell cycle checkpoints to achieve highly efficient radiosensitization.
Methods:
Western blot, immunofluorescence microscopy, colony formation assay, assessment of cell cycle distribution and flow cytometric assessment of γH2AX. PARP1 was inhibited using olaparib; intra-S/G2 checkpoint inhibition was performed using the Wee1-inhibitor AZD1775.
Results:
Enhancing CDK1/2 activity through AZD1775 resulted in reduced proliferation rates and severe replication stress. The latter was apparent from an accumulation of cells in the S-phase as well a strong increase in the replication stress & DNA damage marker γH2AX in S-phase cells. Addition of olaparib had little effect on these endpoints but resulted in a clearly enhanced radiosensitization as compared to single inhibitor usage.
Conclusion:
Combined inhibition of PARP1 and the intra-S/G2 checkpoint is a highly effective approach for radiosensitization of HPV+ HNSCC cells. It may therefore represent a viable alternative for the current standard of concomitant cisplatin-based chemotherapy and may allow for a reduction in radiation dose. The exact mechanisms of radiosensitization through this combined approach are currently being further investigated.
