Elucidating The Role Of Kmt2d As A Novel Therapeutic Target For Resistant Her2+Breast Cancer

HER2 targeted agents have improved clinical outcomes for women with HER2+ breast cancer. However, drug resistance remains a critical challenge for curing HER2+ disease. Cancer stem cells (CSCs)-enriched during anti-HER2 therapy promote resistance and recurrence. Therefore, novel targets that drive survival of CSCs are needed. Recently, the histone-lysine N-methyltransferase 2D (KMT2D) has been shown to be a prognostic marker for poor recurrence free survival in women with HER2+ breast cancer. Preliminary data showed that KMT2D RNA was increased in HER2+ CSC-enriched mammospheres compared to bulk cells. Further, KMT2D protein is increased in resistant compared to sensitive cells. These data suggest that KMT2D is necessary for survival of CSCs and resistance to anti-HER2 therapy. To address this hypothesis, RNAi-mediated knockdown of KMT2D was performed in trastuzumab sensitive and resistant cells. KMT2D depletion in sensitive cells resulted in almost complete blockade of proliferation in 2-D culture and CSC survival in 3-D culture. In contrast, proliferation and CSC survival was only moderately inhibited by KMT2D depletion in resistant cells. ChIP studies confirmed that KMT2D is enriched on the MYC enhancer in sensitive cells but only moderately in resistant cells. Based on these data, KMT2D is necessary for growth of HER2+ breast cancer and is possibly inactivated during acquisition of resistance. To determine the mechanism, RNA sequencing was performed in both sensitive and resistance cells expressing or depleted for KMT2D. Several differentially expressed genes that were positively or negatively regulated by KMT2D were identified. Specifically, transcript levels of ITGB6 and CLIC3 were decreased upon KMT2D knockdown, while PGR and AR transcripts were increased. Initial results showed that ITGB6 depletion resulted in decreased bulk cell proliferation and survival of CSCs similar to KMT2D knockdown. These results suggest that ITGB6 could be a direct target gene of KMT2D and thus mediates the phenotypic effects of KMT2D. Overall, our results suggest that KMT2D is necessary for both proliferation and CSC survival in HER2+ breast cancer sensitive to anti-HER2 therapy and a combination approach of targeting both HER2 and KMT2D could prevent enrichment of CSCs and acquired resistance. Citation Format: EMILY MA, Andrei Zlobin, Debra Wyatt, Jeffrey Ng, Andrew Dingwall, Clodia Osipo. Elucidating the role of KMT2D as a novel therapeutic target for resistant HER2+ breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1859.