Abstract B065: Enhancing the therapeutic gain of proton radiotherapy in Locally Advanced Rectal Cancer (LARC) through manipulation of WSB1, ATM, and β-Catenin/c-Myc pathway Ubiquitination
Authors: Dragojevic Sonja , Callaghan Cameron M. 
Published: December 2023

Abstract
Just 20% of patients with LARC attain a complete response to preoperative chemoradiotherapy which portends an excellent prognosis and opportunity to preserve the rectum through nonoperative surveillance. Moreover, normal tissue radiation toxicity can lead to significant morbidity, and becomes an even more important consideration in non-operative approaches where irradiated tissue is not resected. The geometry of conventional proton beams is already being used clinically to spare patient’s normal tissue, but the biology of entrance (ENT) vs. Bragg Peak (BP) protons has yet to be fully exploited to enhance tumor efficacy. Protons, unlike conventional x-ray radiation, spare normal tissue by lacking an exit dose - but they also exhibit a Bragg Peak (BP), which causes more complex DNA damage and is more biologically effective. ATM is critical in the repair of double strand DNA breaks (DSBs) and our lab has demonstrated that inhibitors of ATM (iATM) preferentially radiosensitize BP protons in CRC (Fig. 1A). Overactivation of the Wnt/β-catenin pathway via loss of function of APC is the most common genetic mutation in colorectal cancer (75-80%) and β-catenin over-expression negatively correlates with prognosis. WSB1 has been discovered by our lab to regulate ATM ubiquitination and degradation, it decreases metastates-free survival in CRC, and also forms a positive feedback loop with β-catenin and c-Myc that drives oncogenic progression. It is unknown if this upregulation of WSB1 promotes ATM degredation in LARC specifically, and a deubiquitinase (DUB) for ATM (which would counter WSB1) has yet to be identified. We preformed a comprehensive database search (UbiBrowser 2.0) for the most likely predicted DUBs for ATM (Fig. 1B), several of which have been identified as regulators of β-catenin/c-Myc, DSB repair pathways, and correlated with poor prognosis in CRC. We have also performed a screen of DUBs predicted to interact with ATM, and have preliminary evidence suggesting USP14 as the worlds first identified DUB of ATM (Fig 1C). USP14 expression is upregulated in CRC compared to surrounding normal tissue, promotes radioresistance, and promotes Wnt/β-catenin signalling. Higher USP14 expression in CRC is associated with liver and lymph node metastases and worse overall survival. USP14 regulates β-catenin by deubiquitinating K63-linked polyubiquitin chains of Dvl, and several small molecule inhibitors of USP14 have been identified. Our lab has preliminary data demonstrating USP14 is preferentially activated after BP proton irradiation (Fig. 1D), suggesting ATM may be necessary for repair of complex DSBs. While more work will be necessary to confirm these findings, many of the other predicted DUBs of ATM have overlapping functions regulating DSB repair and/or β-catenin/c-Myc signalling. We hypothesize that targeting ATM ubiquitination will open a therapeutic window in LARC by disrupting β-catenin/c-Myc signalling and selectively radiosensitizing BP protons.
Citation Format: Sonja Dragojevic, Cameron M. Callaghan. Enhancing the therapeutic gain of proton radiotherapy in Locally Advanced Rectal Cancer (LARC) through manipulation of WSB1, ATM, and β-Catenin/c-Myc pathway Ubiquitination [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr B065.