Abstract 5179: Context-specific dependence of ovarian cancer on IKKε and CHEK1

Background: The IκB kinase, IKKe, was previously identified as an oncogene in breast cancer and was associated with poor clinical outcome in ovarian cancer. Recently, we demonstrated that IKKe is a key regulator of invasion and metastasis in ovarian cancer. A highly specific inhibitor of IKKe is not readily available for use as a tool compound to study pathway interactions in the preclinical setting. Therefore, we utilized dual shRNA technique to create IKKe-matched cell line pairs and performed sensitization screens to identify the key molecular targets influenced by the loss of IKKe expression. Results: We screened a previously established shRNA library targeting the human kinome, and identified 65 genes that further inhibited the survival of ovarian cancer cells in combination with IKKe depletion. In order to evaluate potential clinical significance associated with their overexpression, we examined the expression levels of these 65 genes in TCGA ovarian serous cystadenocarcinoma dataset. Strikingly, CHEK1 was overexpressed in 100% of TCGA ovarian cancers with a cut-off of 1.5 fold change compared to normal ovarian surface epithelium. Interestingly, CHEK1 loss also sensitized IKKe depleted A2780, a wild-type p53 containing ovarian cancer cell line. Next, we investigated whether chemical inhibitors could recapitulate the effects of shRNAs, although they are less specific than RNAi and are unable to interrogate kinase-independent functions of cellular proteins. Consistent with the initial screening methodology, inhibition of IKKe and CHEK1 activities by BX795 and PF00477736, respectively, in 6 ovarian cancer cell lines was critical to the cooperative decrease in cell viability. Thus, the co-inhibition of IKKe and CHEK1 activities, regardless of p53 status, was more effective in killing ovarian cancer cells than single treatment. The inhibition of IKKe activity alone induced dramatic G2/M arrest in all 6 cell lines without inducing a DNA damage response, while CHEK1 inhibition resulted in significant DNA damage response with abnormal S phase profile in cell cycle analysis and an increase in gamma-H2A.X level. This combined inhibition dramatically decreased CHEK1 level and further increased apoptosis compared to CHEK1 inhibitor alone. Conclusion: Our dual shRNA technique efficiently identified CHEK1 as an IKKe synthetic lethal gene in ovarian cancer. CHEK1 inhibitor as a single agent has not been successful in clinic, and this may be due to increased pro-survival signaling mediated by IKKe, which may contribute to resistance and subsequent relapse. Identification of IKKe dependent signaling will lead to development of context-specific therapeutics in the poor prognostic group of patients with a high level of IKKe expression. Citation Format: Marianne K. H. Kim, Dong Joon Min, George Wright, Christina M. Annunziata. Context-specific dependence of ovarian cancer on IKKe and CHEK1. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5179. doi:10.1158/1538-7445.AM2014-5179