Abstract 2660: Downregulation of Bora gene expression in human tumor cells results in radioresistance by promoting repair of double strand breaks

Abstract

Abstract DNA double-strand breaks (DSBs) are highly cytotoxic lesions caused by genotoxin such as ionizing radiation (IR). Following DSB, cells activate the DNA-damage response pathway by activating protein kinases such as ATM, ATR and DNA-PK, which then trigger histone H2AX phosphorylation and the accumulation of proteins such as Mediator of DNA damage checkpoint protein 1 (MDC1), 53BP1, and BRCA1 into IR-induced foci. This process initiates the repair of DSB. Previously we have identified a novel biomarker for radiation response, Bora, by using a genome-wide association approach using data generated with 300 human lymphoblastoid cell liens. Bora is known as a cofactor of Aurora-A, which is involved in mitosis regulation. During G2, Aurora-A, in complex with Bora, phosphorylates Plk1 at Thr-210. Activated Plk1 phosphorylates Cdc25C and Wee1, which induces activation of cyclin B-cdk1 complexes and promotes mitotic entry. In this study, we want to determine the underlying mechanisms by which Bora regulates response to IR. We found that knockdown of Bora in human cancer cell lines including HupT3, MIAPaCa-2 and Hela (siBora) exhibit increased clonogenic survival rate in response to IR, an observation that is opposite from the effect of Plk1 knockdown, suggesting a different mechanism involved. Knockdown of Bora also desensitize cancer cells to chemotherapeutic agents including gemcitabine and oxiplatin. This enhanced resistance to genotoxin is primarily due to increased DNA DSB repair. More than 90% of DSBs were repaired in siBora cells compared with 60% in control cells 8 hours after cell exposure to 2 Gy IR. Upon IR, more cells with Bora knockdown retained in G2-M phase compared with control. Furthermore, Bora directly interacts with BRCT domain of MDC1 via its C terminus, and over expression of Bora can abolish MDC1 foci formation induced by IR. All of these observations suggested a role of Bora in DNA damage response pathway and in turn, in chemo-radio resistance. Indeed, overexpression of the full length or C-terminus of Bora sensitized cancer cells to IR treatment, while the N-terminal Bora did not have significant effect on IR response. In summary, we have identified a novel function of Bora involved in DNA damage and repair pathway, which indicates a broad role of Bora in both tumorgenesis and chemo-radio resistance. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2660. doi:10.1158/1538-7445.AM2011-2660