Abstract LB-215: Epigenetic loss-of-function BRCA1 mediates tumor cure by single dose radiotherapy
Authors: CAMPAGNE Cecile G. , Thin Tin H. , Fuller John D. , Ackerstaff Ellen , Koutcher Jason A. , Haimovitz-Friedman Adriana , Powell Simon N. , Kolesnick Richard N. , Fuks Zvi 
Published: August 2015

Abstract
The mechanism of tumor cure by ionizing radiation is regarded tumor cell autonomous, effected by misrepair of radiation-induced DNA double strand breaks (DSBs), mainly via the function of error prone non-homologous end joining (NHEJ). Genomic instability yields mitosis-dependent buildup of potentially lethal chromosomal aberrations, with repeated radiation exposures required for tumor ablation. Here we show that at the high dose range (&gt;10Gy), single dose radiotherapy (SDRT) engages an alternative dual target model, inducing in addition to DSBs also an early wave of acid sphingomyelinase (ASMase)-mediated microcirculatory ischemia/reperfusion injury.
DSB repair was analyzed in situ by quantitative assessment of the time-dependent buildup and resolution of ionizing radiation-induced foci (IRIF) of specific NHEJ or homology-driven repair (HDR) mediators. Effect of SDRT on the tumor microvasculature was assessed by dynamic contrast-enhanced magnetic resonance imaging. Engagement of microvascular dysfunction in DSB repair was assessed using ASMase-deficient mice, refractory to vascular endothelial injury. Western blot analysis of Small Ubiquitin-like Modifiers (SUMO) in tumor extracts and studies of SUMO conjugating enzymes IRIF in situ were used to evaluate effects of SDRT on SUMOylation.
SDRT concomitantly induces DSBs in tumor cells and an early wave (&lt;1 hour) of ASMase-mediated microcirculatory ischemia/reperfusion, synthetically coupling parenchymal tumor cell DNA damage response to re-program tumor lethality. Ischemia/reperfusion induced in reperfused tumor clonogens an oxidative stress, dysfunctioning therein SUMO conjugating enzymes that are critically required for assembly of the inherent DSB repair codex, leading to catastrophic reprograming of DSB repair. While Ku- and 53BP1-mediated NHEJ were not affected, although 53BP1 resolution was delayed, HDR was aborted. We show that SUMO 2/3 dysfunction, specifically induced post reperfusion, impairs recruitment of RAP80, BRCA1, RPA and RAD51 into DSB repair foci. The epigenetic loss-of-function BRCA1/HDR diverted DSB repair to an aberrant lethal NHEJ pathway, yielding massive lethal chromosomal aberrations, reproductive cell death and tumor cure.
The dual target microvascular/tumor clonogen model, described here, which functions exclusively at high-dose radiation exposures, constitutes a functional alternative to the classical single target mechanism operating at the low dose range, and provides new targets for modulation of the radiation response, with a potential for yielding new options for tumor ablation in the clinical management of human cancer.
Citation Format: Cecile G. CAMPAGNE, Tin H. Thin, John D. Fuller, Ellen Ackerstaff, Jason A. Koutcher, Adriana Haimovitz-Friedman, Simon N. Powell, Richard N. Kolesnick, Zvi Fuks. Epigenetic loss-of-function BRCA1 mediates tumor cure by single dose radiotherapy. [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 LB-215. doi:10.1158/1538-7445.AM2015-LB-215