Abstract 1585: DNA damage response contrasts in stem and differentiated cells contributing to differential radiosensitivities.

Abstract The use of ionizing radiation (IR) for radiotherapy is an efficient method of treating cancers albeit often causing unintended damage to normal tissue. Upon induction of DNA double strand breaks, cells activate DNA Damage Response (DDR) which involves sensing of broken DNA, activation of signal transduction pathways and recruitment of repair factors leading to downstream biological responses including DNA repair and checkpoint arrest. We have established that stem cells are highly radiosensitive to IR as compared to their isogenic differentiated progeny both in vivo and in primary culture models. Failure to sufficiently activate or maintain DDR signals in response to DNA damage may be a possible mechanism for radiation-induced cell death in stem cells. We investigated the differences in DDR signaling among stem and differentiated cells within the tissue niches of brain, intestine, testis and skin in addition to cell culture models of embryonic and neural stem cells and their isogenic differentiated counterparts. Our results indicate an attenuation of the DDR selectively in stem cells, in contrast to differentiated cells which robustly activate the DDR in response to IR. Stem cells lack ATM activation and have unique absence of γH2AX IR-induced foci. Differentiation and loss of “stemness” result in robust ATM activation, the presence of γH2AX foci at damaged sites and recruitment of downstream repair factors at the DNA break sites. Interestingly, while ATM-dependent H2AX phosphorylation at break sites is abrogated in stem cells, strong MST1 kinase-dependent pan-nuclear H2AX phosphorylation at the same S-139 site is observed 6hrs after irradiation exclusively in stem cells, eventually leading to apoptotic DNA fragmentation. DDR proficiencies may be strongly impacted by epigenetic status, which may determine access to the damaged DNA and recruitment of repair factors. We therefore screened for histone modifications that could have either a supportive or inhibitory role on DDR signaling in stem and differentiated cells and discovered that acetylation of histone 3 lysine 56 (H3K56ac) is strongly enhanced in stem cells and negatively correlates with the presence of DDR signals. Knockdown and inhibition of the H3K56 acetyltransferase p300 was utilized to determine if changes in acetylation state could produce both global and local changes in recruitment of DDR factors to damaged DNA sites, and whether these changes differed among stem and differentiated cells. Results of our studies will be presented. Citation Format: Keith M. Jacobs, Sandeep Misri, Dennis E. Hallahan, Girdhar G. Sharma. DNA damage response contrasts in stem and differentiated cells contributing to differential radiosensitivities. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1585. doi:10.1158/1538-7445.AM2013-1585