Abstract B06: CDK7: A marker of poor prognosis and tractable therapeutic target in triple-negative breast cancer
Authors: Li Bo , Chonghaile Triona Ni , Fan Yue , Klinger Rut , O'Connor Aisling E. , Conroy Emer , Tarrant Finbarr , O'Hurley Gillian , Udupi Girish Mallya , Gaber Alexander , Chin Suet-Feung , Bardwell Helen A. , Schouten Philip C. , Dubois Thierry , Linn Sabine , Jirstrom Karin , Caldas Carlos , Bernards Rene , O'Connor Darran P. , Gallagher William M. 
Published: May 2016

Abstract
Triple-negative breast cancer (TNBC) represents a heterogeneous subgroup of breast cancer with substantial genotypic and phenotypic diversity. TNBC patients commonly exhibit poor prognosis and high relapse rates at early stages after conventional treatments. Currently, there is a lack of biomarkers and targeted therapies for the management of TNBC. During tumor development and progression, alterations in cellular behavior are frequently linked with kinase expression and activity. Here, we aimed to identify novel kinase targets that may play a pivotal role in the progression of TNBC and, thus, offer new therapeutic vantage points.
We initially focused on identifying kinases linked to differential outcome. Using publicly available transcriptomic data from a collated set of TNBC patients (n = 483), we identified 9 kinases that were associated with survival at the mRNA level. From this screen, CDK7 (cyclin-dependent kinase 7) was found to be correlated with poor relapse-free survival. CDK7's trait as a marker of poor prognosis was further validated within another TNBC cohort (n=109) via assessment of a tissue microarray generated as part of the RATHER Consortium (www.ratherproject.com). At the protein level, high CDK7 expression was associated with poor breast cancer-specific survival, recurrence-free survival and distance recurrence-free survival.
To evaluate CDK7 as a therapeutic target in TNBC, two TNBC breast cancer cell lines (BT-549 and MDA-MB-231) were selected to evaluate phenotypic alterations post shRNA-mediated CDK7 knockdown. CDK7 silencing led to decreased cell proliferation, colony formation and migration in vitro. CDK7 down-regulation also increased TNBC cell sensitivity to the chemotherapeutic agent doxorubicin. BS-181 and THZ1, two highly specific CDK7 inhibitors, attenuated TNBC tumor growth by inducing G2/M cell cycle arrest and global RNA transcription. Moreover, the covalent CDK7 inhibitor THZ1 demonstrated 1000-fold higher potency than BS-181. Inhibition of global RNA transcription preferentially affects proteins with short half-lives. Accordingly, we detected a reduction in the expression of the anti-apoptotic protein MCL-1 in both cell lines. Next, we assessed anti-apoptotic dependence in MDA-MB-231 cells following treatment with THZ1 via the BH3 profiling technology, and observed an increased response to the BAD and HRK peptides, inferring an elevated survival dependence on BCL-2/BCL-XL.
We subsequently evaluated the combination of the BCL-2/BCL-XL inhibitor ABT-263 with THZ1 and discovered synergistic responses in cell growth inhibition and apoptosis. In vivo testing of the efficacy of the CDK7 inhibitor, THZ1, in a xenograft model of TNBC was also carried out. Preliminary data revealed that tumor growth was suppressed following daily treatment with 10mg/kg of THZ1.
Our data implicate that high CDK7 expression is a promising biomarker of poor prognosis in TNBC. Moreover, these findings suggest that targeting CDK7, combined with BCL-2/BCL-XL inhibitor ABT-263, may be a useful therapeutic strategy for TNBC.
Citation Format: Bo Li, Triona Ni Chonghaile, Yue Fan, Rut Klinger, Aisling E. O'Connor, Emer Conroy, Finbarr Tarrant, Gillian O'Hurley, Girish Mallya Udupi, Alexander Gaber, Suet-Feung Chin, Helen A. Bardwell, Philip C. Schouten, Thierry Dubois, Sabine Linn, Karin Jirstrom, Carlos Caldas, Rene Bernards, Darran P. O'Connor, William M. Gallagher. CDK7: A marker of poor prognosis and tractable therapeutic target in triple-negative breast cancer. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr B06.