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A haploid genetic screen identifies the G1/S regulatory machinery as a determinant of Wee1 inhibitor sensitivity

Significance Inhibition of Wee1 is considered an attractive anticancer therapy for TP53 mutant tumors. However, additional factors besides p53 inactivation may determine Wee1 inhibitor sensitivity, which we searched for using unbiased functional genetic screening. We discovered that the mutational status of several S-phase genes, including CDK2, determines the cytotoxicity induced by Wee1 inhibition. Notably, we found that Wee1 inhibition induces two distinct phenotypes: accumulation of DNA damage in S phase and karyokinesis/cytokinesis failure during mitosis. Stable depletion of S-phase genes only reversed the formation of DNA damage, but did not rescue karyokinesis/cytokinesis failure upon Wee1 inhibition. Thus, inactivation of nonessential S-phase genes can overcome Wee1 inhibitor resistance, while allowing the survival of genomically instable cancer cells. The Wee1 cell cycle checkpoint kinase prevents premature mitotic entry by inhibiting cyclin-dependent kinases. Chemical inhibitors of Wee1 are currently being tested clinically as targeted anticancer drugs. Wee1 inhibition is thought to be preferentially cytotoxic in p53-defective cancer cells. However, TP53 mutant cancers do not respond consistently to Wee1 inhibitor treatment, indicating the existence of genetic determinants of Wee1 inhibitor sensitivity other than TP53 status. To optimally facilitate patient selection for Wee1 inhibition and uncover potential resistance mechanisms, identification of these currently unknown genes is necessary. The aim of this study was therefore to identify gene mutations that determine Wee1 inhibitor sensitivity. We performed a genome-wide unbiased functional genetic screen in TP53 mutant near-haploid KBM-7 cells using gene-trap insertional mutagenesis. Insertion site mapping of cells that survived long-term Wee1 inhibition revealed enrichment of G1/S regulatory genes, including SKP2, CUL1, and CDK2. Stable depletion of SKP2, CUL1, or CDK2 or chemical Cdk2 inhibition rescued the γ-H2AX induction and abrogation of G2 phase as induced by Wee1 inhibition in breast and ovarian cancer cell lines. Remarkably, live cell imaging showed that depletion of SKP2, CUL1, or CDK2 did not rescue the Wee1 inhibition-induced karyokinesis and cytokinesis defects. These data indicate that the activity of the DNA replication machinery, beyond TP53 mutation status, determines Wee1 inhibitor sensitivity, and could serve as a selection criterion for Wee1-inhibitor eligible patients. Conversely, loss of the identified S-phase genes could serve as a mechanism of acquired resistance, which goes along with development of severe genomic instability.