Abstract 2926: Nomination and characterization of TTK for radiosensitization in basal-like breast cancers
Authors: Chandler Benjamin C. , Moubadder Leah , Ritter Cassie , Wilder-Romans Kari , Cameron Meleah , Liu Meilan , Nyati Shyam , Pesch Andrea , Michmerhuizen Anna , Olsen Eric , Niknafs Yashar , Chinnaiyan Arul , Speers Corey 
Published: August 2019

Abstract
Background: Increased rates of locoregional recurrence have been observed in basal-like breast cancer despite the use of radiation therapy (RT), therefore approaches that result in radiosensitization of basal-like breast cancer are critically needed. Studies detailing the poor response of basal-like tumors to adjuvant RT underscore the biologic differences and as of yet undefined oncogenic drivers of these particular types of breast cancer.
Methods: 4 independent datasets were used to correlate gene expression with local recurrence (LR) after 3 years. Kaplan-Meier analysis was used to validate the impact of TTK expression on LR. The TCGA and institutional breast cancer dataset were used to determine TTK expression in BC subtypes. TTK RNA and protein levels were measured using qPCR and western blot at baseline and correlated to intrinsic radiosensitivity. Clonogenic survival assays were used to determine the radiosensitization of cell lines after TTK inhibition (TTKi). Mice models were used to assess TTKi in combination with RT in vivo. DNA damage was quantified using γH2AX staining. HR and NHEJ efficiency assays were performed using HR/NHEJ specific reporter systems. HR competency was also assessed using RAD51 foci formation assays.
Results: Ten genes were found to significantly correlate with early LR (≤3 years) across 4 distinct datasets (N=896 pts) (OR of recurrence &gt; 2, p-value &lt;0.000001), with TTK, a cell cycle kinase, ranked the highest. Kaplan-Meier survival analysis in 3 cohorts demonstrated that higher than median TTK expression correlates with decrease LR free survival after RT (HR 1.70-2.42, p&lt;0.01 for all 3 cohorts). Subtype association analysis demonstrated that TTK expression was most elevated in basal-like breast cancer (p&lt;0.0001). Clonogenic survival assays, using inducible shRNA models, show the combination of TTK knockdown and RT increases radiosensitivity in MDA-MB-231 and BT-549 basal-like breast cancer cell lines (rER 1.21-1.63). Additionally, TTKi using, Bayer 1161909 (B909), enhanced radiosensitivity in both cell lines (rER 1.10-1.39). In vivo, TTKi, using shRNA and B909 in combination with RT led to a significant increased time to tumor tripling. Using γH2AX foci formation assays, increased DNA damage was found after combination treatment of TTKi and RT compared to RT alone, indicating that DNA damage repair mechanisms may be compromised by TTKi. We found that the efficiency of the double strand DNA damage repair mechanism, homologous recombination (HR), but not non-homologous end joining (NHEJ), was reduced upon TTKi in HR/NHEJ specific reporter systems. Additionally, RAD51 foci formation was reduced by TTKi after RT compared to RT alone.
Conclusion: These data support TTK inhibition as a rationale radiosensitizing strategy for clinical development in basal-like breast cancer patients and that the mechanism of radiosensitization is, at least in part, through impaired HR repair.
Citation Format: Benjamin C. Chandler, Leah Moubadder, Cassie Ritter, Kari Wilder-Romans, Meleah Cameron, Meilan Liu, Shyam Nyati, Andrea Pesch, Anna Michmerhuizen, Eric Olsen, Yashar Niknafs, Arul Chinnaiyan, Corey Speers. Nomination and characterization of TTK for radiosensitization in basal-like breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2926.