Abstract 3064: MicroRNA-21 enhances the effect of ionizing radiation via alteration of the DNA damage response
Authors: Dan Tu , Palagani Ajay , DeAngelis Tiziana , Han Sunny , Liotta Lance , Pestell Richard , Simone Nicole 
Published: August 2015

Abstract
Micro-RNA 21 (miR-21) is a well-known onco-miR found to be up-regulated in over 18 major cancer types and is linked to radiation and chemotherapy resistance. It is hypothesized that dysregulation of miR-21 may contribute to treatment resistance through multiple DNA damage response pathways by enhancing DNA repair and increasing DNA damage tolerance. Given its involvement in multiple resistance pathways, miR-21 appears to be an attractive target for overcoming resistance with radiation therapy.
In vitro, we demonstrated that knockdown of miR-21 revealed an increase in tumor cell death via clonogenic cell survival curves following exposure to ionizing radiation in multiple cancer cell lines including triple negative breast cancer cells, GBM, and lung cancer. A miR-21 knockout (KO) mouse model was then generated based on deletion of a miR-21 upstream promoter element. To determine if mir-21 KO would lead to radiation sensitivity, miR-21 deficient mice were exposed to 6.5Gy total body irradiation (TBI) and compared to control wild type (WT) mice. miR-21 KO mice died at an average of 12 days after TBI vs. over 100 days survival in wild-type mice. Tissue analysis revealed an increase in double strand breaks measured by elevated γ-H2AX in miR-21 deficient mice. An associated decrease in DNA repair genes such as MSH2 and STAG2 was also seen.
Therefore, we hypothesize deletion or silencing of miR-21 results in a radiation sensitive phenotype due to a decreased DNA repair capacity, particularly in the repair of double-strand breaks. In silico analysis predicted miR-21 targets involved in the DNA repair response including rad21 and stag2, which are both members of the cohesin complex. The cohesion complex is a ring-shaped protein complex involved in homologous repair and the DNA damage-response. When exposed to ionizing radiation, rad21 and stag2 expression appears inversely correlated with miR-21 levels in vivo and in vitro. Similarly, in the setting of miR-21 knockdown, rad21 and stag2 expression are increased, suggesting dysregulation in the function of the cohesin complex may contribute to genomic instability and a DNA damage-permissive phenotype.
In the current study, we propose a novel mechanism for increasing radiation sensitivity through miRNA-mediated pathways controlling cellular DNA-repair. Future studies investigating miR-21 expression in patients may identify subgroups of patients who are at risk for resistance to standard cytotoxic therapies and may open avenues for targeted therapies involving mir-21 mediated DNA repair pathways.
Citation Format: Tu Dan, Ajay Palagani, Tiziana DeAngelis, Sunny Han, Lance Liotta, Richard Pestell, Nicole Simone. MicroRNA-21 enhances the effect of ionizing radiation via alteration of the DNA damage response. [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 3064. doi:10.1158/1538-7445.AM2015-3064