Abstract 677: TTK: A novel target for radiosensitization in triple-negative breast cancers
Authors: Chandler Benjamin C. , Moubadder Leah , Ritter Cassie , Niknafs Yashar , Olsen Eric , Cameron Meleah , Liu Meilan , Wilder-Romans Kari , Nyati Shyam , Brown Powel , Chinnaiyan Arul , Speers Corey 
Published: August 2018

Abstract
Background: Increased rates of locoregional recurrence leading to poorer clinical outcomes have been observed in triple-negative breast cancer (TNBC) despite the use of radiation therapy (RT), therefore approaches that result in radiosensitizaton in TNBC are critically needed. Our previous work identified a group of cell cycle kinases differentially expressed in estrogen receptor negative (ER-) versus estrogen receptor positive (ER+) breast cancer (BC). Here we described the impact of elevated expression of one of these identified kinases, TTK, on radiation response and patient outcomes in TNBC.
Methods: TCGA breast cancer datasets were used to determine TTK expression in the intrinsic subtypes of BC. TTK RNA and protein levels were measured using qPCR and western blot at baseline and after radiation treatment. Clonogenic survival assays were used to determine the radiosensitization in several cells after TTK inhibition. DNA damage was quantified using γH2AX staining in TNBC cell lines. Kaplan-Meier analysis was used to determine the impact of TTK expression on locoregional recurrence (LRR) and overall survival (OS). A Cox proportional hazards model was constructed to identify potential factors of LRR-free survival in univariate (UVA) and multivariable analyses (MVA).
Results: TTK expression is elevated in breast cancer tissue compared to normal breast tissue (Q-VAL: 8.70 E-291) and is most highly expressed in basal-like tumors. TTK is overexpressed in ER- versus ER+ tumors in both breast cancers patient samples (p-value: &lt;0.0001), and cells lines (p-value: &lt;0.0001). TTK expression is significantly correlated with intrinsic radioresistance in a panel of 23 BC cell lines (R=0.58, p-value=0.0035). TTK RNA and protein levels are significantly increased at 12 and 24 hours after RT in TNBC cell lines. Genomic (siRNA knockdown) or pharmacologic (NMS-P715) inhibition of TTK increased radiosensitivity in vitro in 3 different TNBC cell lines. This sensitization is mediated, at least in part, through impaired DNA damage repair. Increased γH2AX foci was found after combination treatment of RT and TTK inhibition compared to only TTK inhibition, or RT. Clinically, breast cancer patients treated with breast conserving surgery and RT whose tumors have higher than median expression of TTK had worse local-recurrence free (LRF) survival and overall survival (HR for local recurrence 1.7 as continuous variable, p-value 0.004) compared to patients with higher than median expression, and this pattern held when considering quartile expression. TTK expression was associated with poorer LRF survival in UVA and in MVA only TTK expression and grade were significantly associated with worse LRF survival in 3 independent datasets.
Conclusion: Our results support the rationale for developing translational clinical trials to investigate TTK inhibition as a novel radiosensitizing target in TNBC.
Citation Format: Benjamin C. Chandler, Leah Moubadder, Cassie Ritter, Yashar Niknafs, Eric Olsen, Meleah Cameron, Meilan Liu, Kari Wilder-Romans, Shyam Nyati, Powel Brown, Arul Chinnaiyan, Corey Speers. TTK: A novel target for radiosensitization in triple-negative breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 677.