Abstract 3882: Compromised Cdk1 activity sensitizes BRCA-proficient cancer cells to inhibition of Poly(ADP-ribose) polymerase
Authors: Johnson Neil , Li Yu-Chen , Moreau Lisa A. , Thomas Huw D. , Newell David R. , D'andrea Alan D. , Curtin Nicola J. , Shapiro Geoffrey I. 
Published: February 2011

Abstract
Poly(ADP-ribose) polymerase (PARP)-1 is a sensor of DNA nicks that signals the presence of DNA damage and facilitates repair. When PARP-1 is inhibited, single stranded DNA breaks (SSBs) degenerate to more lethal, double stranded DNA breaks (DSBs). BRCA1 and BRCA2 tumor suppressor proteins mediate the repair of DSBs by homologous recombination (HR). When these proteins are dysfunctional or mutated, cells become HR defective and are hypersensitive to PARP inhibition. However, BRCA-deficient tumors represent only a small fraction of adult cancers, potentially restricting the clinical utility of PARP inhibitor monotherapy. We previously showed that cdk1 phosphorylates BRCA1, an event essential for efficient BRCA1 focus formation and S phase checkpoint control in response to DNA damage (Johnson et al., Mol Cell 2009; 35: 327-39). Here, we show that shRNA-mediated depletion of cdk1 in the non-small cell lung cancer (NSCLC) cell line - NCI-H1299 compromises formation of BRCA1 foci, and consequently Rad51 foci, in response to treatment with the PARP inhibitor AG14361. PARP inhibitor treatment also resulted in increased numbers of chromosome breaks and G2/M accumulation in cdk1 depleted cells, but not in cells with normal cdk1 levels or cdk2 depleted control cells. Cdk1 depleted NCI-H1299 cells were 74-fold (p=0.004) more sensitive than cells with normal cdk1 levels to AG14361 treatment measured by colony formation (LC50 85 nM and 6300 nM, respectively). Similar results were obtained with A549 cells. In addition, cdk1 depleted NCI-H1299 cells demonstrated a substantial delay in tumor xenograft growth with PARP inhibitor treatment compared to cells expressing normal cdk1 levels.
The effects of cdk1 depletion were reproduced by small molecule-mediated cdk1 inhibition. The cdk1 inhibitor RO3306 sensitized a range of BRCA1 proficient cancer cell types to PARP inhibitor treatment. In contrast, RO3306 did not sensitize the non-transformed Retinal Pigmented Epithelial (RPE) cells to PARP inhibition. Because RO3306 has poor pharmacokinetic properties, we combined cdk1 and PARP inhibition in vivo with a clinically relevant cdk1 inhibitor, AG024322. In colony forming assays, NCI-H1299 cells were 10-fold more sensitive to treatment with the PARP inhibitor AG014699 when co-treated with 50 nM AG024322. Similarly, when NCI-H1299 xenograft-bearing mice were treated with both compounds, tumor growth was significantly delayed compared to vehicle exposure or treatment with either compound alone. Because reduced cdk1 activity impairs BRCA1 function and HR repair, cdk1 inhibition represents a plausible strategy for expanding the therapeutic utility of PARP inhibitors to the BRCA-proficient cancer population.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3882.