FBH1-deficient cells exhibit a dampened stress response to WEE1 inhibition

Cancer is defined by uncontrolled growth from irregular activity by cell cycle proteins; recently, some cell cycle regulators have become attractive targets for cancer therapies. One of these targets is WEE1, a tyrosine kinase that phosphorylates tyrosine 15 (Y15) on CDK1 to stop the progression from G2 into mitosis. FBH1 is a DNA helicase critical for the induction of apoptosis in response to replication stress. Importantly, FBH1 is lost in many melanomas. FBH1 cooperates with MUS81 to create double-strand breaks and induce cell death. We hypothesized that the replication stress response in FBH1-deficient cells depends on the WEE1-dependent G2/M checkpoint to repair the damage before entering mitosis; therefore, it could be used as a potential chemotherapeutic option. We examined genome stability in response to the inhibition of WEE1 (AZD1775) by measuring nuclear abnormalities and gamma-H2AX staining. We also measured double-strand break formation after WEE1i treatment using the TUNEL assay. Lastly, we measured the sensitivity of U2OS cells to WEE1i treatment using the Cell titer blue assay. We found that FBH1-deficient U2OS cells possessed increased double-strand breaks, increased mitotic catastrophe, and increased pan-nuclear gamma-H2AX staining in abnormal nuclei. However, this dampened response did not lead to resistance to WEE1 inhibition, as FBH1-deficient cells were more sensitive to WEE1 inhibition. These data suggest that FBH1-deficient cells are more sensitive to WEE1i treatment and ultimately depend on the G2/M checkpoint to repair damages. Future studies will address the response of FBH1 to WEE1i when combined with other treatments like hydroxyurea