Abstract POSTER-BIOL-1313: Loss of INPP4B causes a DNA repair defect and upregulation of EGFR expression and can be targeted with PARP inhibitor treatment

Treatment options for ovarian cancer patients remain limited and overall survival is less than 50 percent despite recent clinical advances. This study aimed to identify treatment options for ovarian tumors deficient in the lipid phosphatase inositol polyphosphate 4-phosphatase type II (INPP4B). INPP4B has been described as a tumor suppressor in the PI3K/Akt pathway with loss of expression found most pronounced in breast, ovarian cancer and melanoma. Using microarray technology we identified a DNA repair defect in INPP4B-deficient cells, which we further characterized by comet assays and quantification of γH2AX, RAD51 and 53BP1 foci formation. INPP4B loss resulted in significantly increased sensitivity towards PARP inhibition, comparable to loss of BRCA1 in two- and three-dimensional in vitro models, as well as in in vivo xenograft models. Mechanistically, we discovered that INPP4B forms a protein complex with the key players of DNA repair, ATR and BRCA1, and INPP4B loss affects BRCA1, ATM and ATR protein stability resulting in the observed DNA repair defect. To identify further synthetic-lethal gene interactions in INPP4B-deficient ovarian cancer cells we employed the GlaxoSmithKline small compound library in a high-throughput screen. We successfully validated 9 out of initial 19 hit compounds which target Akt, VEGFR (Vascular Endothelial Growth Factor Receptor), p38α, LCK (Lymphocyte-specific protein Kinase) and TGFRβ1 (Transforming Growth Factor Receptor beta 1). Interestingly, we found that INPP4B loss resulted in increased efficacy of small compound inhibitors directed against EGFR (Epidermal Growth Factor Receptor), contrary to expectation from the literature given the documented activation of Akt in these cells. Cancer cells with high phosphorylation levels of Akt through PTEN loss or PI3-kinase mutation have been shown to be resistant to EGFR inhibitors. Using cell proliferation, clonogenic and xenograft assays we demonstrated that loss of INPP4B in the human ovarian cancer cell line OVCA433 and Hey resulted in significant sensitivity towards EGFR inhibition using Gefitinib, Erlotinib and Lapatinib compared to PTEN knockdown pools. Inhibition of ErbB2 using Herceptin did not result in synthetic lethality in Ovca433 INPP4B knockdown pools. As anticipated, Ovca429 INPP4B knockdown cell pools showed resistance to Gefitinib treatment due to an inherent PIK3CA mutation in Ovca429 ovarian cancer cells. Given that INPP4B loss has been found in 40% of ovarian cancer patients, this study provides the rationale for establishing INPP4B as a biomarker of PARP and EGFR inhibitor response, and consequently offering novel therapeutic options for a significant patient subgroup.

Citation Format: Christina Anja Gewinner. Loss of INPP4B causes a DNA repair defect and upregulation of EGFR expression and can be targeted with PARP inhibitor treatment [abstract]. In: Proceedings of the 10th Biennial Ovarian Cancer Research Symposium; Sep 8-9, 2014; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(16 Suppl):Abstract nr POSTER-BIOL-1313.