Abstract 3298: ABL1 is required for Tie2-mediated DNA repair in brain tumor stem cells

Proceedings: AACR 106th Annual Meeting 2015; April 18-22, 2015; Philadelphia, PA Glioblastoma is the most aggressive primary brain tumor and, in spite of surgery and chemoradiotherapy, invariably recurs. The poor prognosis associated with this disease, with a median survival of 15 months, is largely caused by the striking radioresistance of these tumors. The development of new therapeutic strategies for patient with brain tumors requires the identification of key molecular pathways regulating their resistant phenotype. The abnormal function of tyrosine kinase receptors (TKRs) is a hallmark of malignant gliomas. We previously reported the expression of the TKR Tie2 in brain tumor stem cells (BTSCs) and in human surgical glioma specimens in relation to malignancy. In in vivo experiments, consisting of ionizing irradiation (IR) of mice bearing intracranial BTSCs-derived xenografts showed unexpected Tie2 nuclear localization. These results were confirmed by using immunofluorescence studies using confocal microscope and subcellular fractionation followed by Western blots. Of clinical interest, the presence of Tie2 in the nucleus is associated with radioresistance, as observed after mutagenesis of a newly discovered nuclear localization signal. In addition, upon IR, we detected increased levels of Tie2 natural ligand, Angiopoietin1 (Ang1). The blocking of the Ang1/Tie2 interaction, by the use of a soluble receptor, modulated the IR-mediated Tie2 nuclear translocation, indicating Tie2 intracellular trafficking was ligand dependent. Additionally we also found that after IR treatment, Tie2 localized in the DNA-repair foci and complexed with the H2AX, the key DNA repair protein. The data presented here clearly suggested a role of Tie2 in the DNA damage repair machinery. To test our hypothesis, we used a fluorescent reporter construct in which a functional GFP gene was reconstituted following a non-homologous end joining (NHEJ) event (gift from Dr. Gorbunova, University of Rochester), and we observed that Tie2-expressing cells displayed a more efficient NHEJ repair than Tie2 negative counterparts. Based on the recently reported role of ABL1 (cAbl) in the ATM and KAT5 mediated DNA damage repair, we explored the relationship between ABL1 and the Tie2-mediated radioresistance. Our data clearly showed that DNA repair efficiency significantly and specifically decreased by using ABL1 inhibitor but not by knocking down ABL2 expression. We further analyzed the interactions between Tie2 and chromatin and, interestingly, observed that Tie2 complexes with core histones. Collectively, our results should propel the development of preclinical studies on the combination of nuclear Tie2-targeting strategies with radiotherapy for patients with glioblastomas. Citation Format: Mohammad B. Hossain, Rehnuma Shifat, David G. Johnson, Mark T. Bedford, Mien-Chie Hung, Nahir Cortes-Santiago, Konrad Gabrusiewicz, Joy Gumin, Ravesanker Ezhilarasan, Erik P. Sulman, Frederick Lang, Raymond Sawaya, W.K. Alfred Yung, Juan Fueyo, Candelaria Gomez-Manzano. ABL1 is required for Tie2-mediated DNA repair in brain tumor stem cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3298. doi:10.1158/1538-7445.AM2015-3298