Background: Increased rates of locoregional recurrence have been observed in TNBC despite the use of RT, therefore approaches that result in radiosensitizaton in TNBC are critically needed. Our previous work identified one such novel molecular target and we sought to investigate the impact of MELK expression on radiation response and patient outcomes.

Methods: MELK expression was measured with RNA-seq and RPPA in 2,061 breast tumor samples and 51 breast cancer cell (BCC) lines. Clonogenic survival assays were used to quantify the degree of radiosensitivity after MELK inhibition. Mass spectrometry was utilized to identify proteins associated with MELK. NHEJ assays utilizing linearized pEYFP vector with FACS and qRT-PCR assessed NEHJ competency. We measured in vivo tumor growth and doubling time using xenografts in mice (16-20 tumors/group) treated with vehicle, RT, MELK-inhibitor, or combination. Kaplan-Meier analysis assessed the clinical impact of MELK on LR and OS. A Cox proportional hazards model was constructed to identify potential factors of LRF-survival in UVA and MVA.

Results: MELK expression is absent in normal tissues and only expressed in cancerous tissue, suggesting a favorable therapeutic index. MELK expression was significantly elevated in human TNBC (p-value 7.5e-21) compared to non-TNBC. MELK expression was significantly correlated with radioresistance (R: 0.62, p-value 0.003). Inhibition of MELK using both siRNA and a small molecule inhibitor induced radiation sensitivity in vitro with an enhancement ratio (ER) of 1.5-1.6 through impaired dsDNA repair and cell cycle arrest mechanisms. MELK protein interacts with Ku70 and Ku80 to mediate NHEJ which is significantly impaired at 1, 3, and 6 hours after MELK inhibitor treatment (p-value<0.01). MELK inhibition either with gene knock-down or small molecule inhibitor significantly radiosensitized TNBC xenografts in mouse models and markedly delayed tumor doubling time and tumor growth (median tumor doubling 7.95 vs. 29.1 days for RT vs. MELK inhibition+RT, p-value <0.0001). Breast cancer patients whose tumors have high expression of MELK had markedly higher rates of LR after RT and an overall poorer prognosis than patients with low expression of MELK (HR for LR 1.89-2.23, p-value<0.001; HR for OS 1.46-3.3; p-value<0.001 in 3 independent datasets). In multivariable analysis only MELK expression and grade were significantly associated with worse LRF survival (HR:1.35,p-value<0.01).

Conclusion: Our results support the rationale for developing clinical strategies to inhibit MELK as a novel target to increase radiosensitivity in TNBC.

Citation Format: Corey Speers, Alyssa Santola, Shuang G. Zhao, Meilan Liu, Vishal Kothari, Kari Wilder-Romans, Theodore S. Lawrence, Powel H. Brown, Lori J. Pierce, Felix Y. Fang. Maternal embryonic leucine zipper kinase (MELK): A novel target for radiosensitization that is independently prognostic in triple-negative breast cancers. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-010.