Abstract PO-020: Telomerase reverse transcriptase inhibitor NU-1 enhances radiation and chemotherapy sensitivity by disrupting DNA repair and accelerating cellular senescence
Authors: Liu Yue , Kron Stephen J. , Betori Rick C. , Scheidt Karl A. , Frost Grant , Wu Ding , Wolfgeher Don J. , Efimova Elena V. , Cohen Scott B. 
Published: April 2021

Abstract
Upon reactivation in cancer, telomerase adds TTAGGG DNA repeats to the ends of chromosomes to prevent telomere erosion and senescence despite deregulated cell proliferation. The telomerase catalytic core consists of telomerase reverse transcriptase (TERT) and the template RNA, telomerase RNA (TR). While TR is broadly expressed, TERT is silenced in somatic cells, leading to progressive telomere shortening with each cell division leading to replicative senescence or apoptosis. That most cancers can maintain telomere integrity despite continued cell division has made TERT an attractive target for cancer therapy. We recently reported NU-1, a novel covalent telomerase inhibitor derived from the natural product chrolactomycin, that irreversibly blocks the telomerase activity of TERT. In examining mechanisms of action, our RNA-seq analysis found that NU-1 affects not only telomere maintenance but also DNA damage response (DDR) pathways. TERT has previously been implicated in multiple cancer-related pathways, including cell survival and repair of DNA double strand breaks (DSBs). To further explore the effects of TERT inhibition on the DDR, we used the human ER+ breast cancer cell line MCF7 to examine radiation response in the presence or absence of TERT inhibitors, including NU-1, chrolactomycin, BIBR1532 and MST-312. TERT inhibition was sufficient to delay both the resolution of γH2AX foci, a marker for DDR signaling, and DSB repair based on neutral comet assay. Surviving cells displayed increased cellular senescence after TERT inhibition. Importantly, only a minor fraction of the persistent γH2AX foci induced by TERT inhibitors colocalized with telomeres, based on binding of PNA probes. Examining mechanisms of DSB repair, NU-1 or BIBR1532 appeared to primarily inhibit the non-homologous end joining (NHEJ) DSB repair pathway versus less effect on homologous recombination (HR) repair, using Traffic Light Reporter assays. Similarly, NU-1 enhanced loss of cell proliferation after treatment with irinotecan, doxorubicin, etoposide, or paclitaxel in multiple TERT-expressing cancer cell lines. This effect was absent in the TERT negative Saos2 cell line. These results establish NU-1 as a chemical probe to explore TERT catalytic activity as a determinant of DNA damage tolerance and suggest telomerase inhibitors as tumor-specific radio/chemo-sensitizers.
Citation Format: Yue Liu, Stephen J. Kron, Rick C. Betori, Karl A. Scheidt, Grant Frost, Ding Wu, Don J. Wolfgeher, Elena V. Efimova, Scott B. Cohen. Telomerase reverse transcriptase inhibitor NU-1 enhances radiation and chemotherapy sensitivity by disrupting DNA repair and accelerating cellular senescence [abstract]. In: Proceedings of the AACR Virtual Special Conference on Radiation Science and Medicine; 2021 Mar 2-3. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(8_Suppl):Abstract nr PO-020.