Abstract 5845: Roles for MLL2/ KMT2D or MLL3/ KMT2C in HER+ breast cancer stem cells

A major clinical challenge for the treatment of breast cancer (BC) is drug resistance. Cancer stem cells (CSCs) may contribute to drug resistance despite therapy. The HER2-positive subtype of BC contains a gene amplification for the ERBB2 proto-oncogene and these tumors are treated with anti-HER2 therapy. Anti-HER2 therapy, such as trastuzumab or lapatinib, increases Notch signaling and is required for HER2+ breast tumor resistance and tumor recurrence. Notch promotes survival of BC CSCs cells. MLL2/KMT2D (Lysine Methyltransferase 2D) and MLL3/KMT2C (Lysine Methyltransferase 2C) are histone methyltransferases that directly control gene enhancer activity through methylation of histone3 lysine4 within enhancer nucleosomes. These complexes collaborate with transcription factors, such as Notch, to drive gene expression by helping to shape the epigenetic landscape of a cell. Recently, it was shown that lapatinib or a PI3K inhibitor increases MLL2 activity and this increase in MLL2 contributes to resistance. Both MLL2 and MLL3 were found to be required for CSCs differentiation, while MLL2 was required for upregulation of c-Myc in HER2+ cells. Recent breast cancer xeno-engraftment studies found that clonal selection and expansion of primary and metastatic breast tumors was associated with amplification of MLL3 in 80% of breast cancer xenografts, suggesting that increased MLL3 expression may contribute to the survival of CSCs in breast tumors. High expression of MLL2 and/or MLL3 predict poor outcome for women with HER2+ breast cancer from cBio portal and Kaplan-Meier Plotter datasets. We measured the expression levels of MLL2 and MLL3 transcripts in both bulk HER2+ breast cancer cells that are trastuzumab sensitive, cells that have acquired resistance to trastuzumab, and cancer stem-enriched mammospheres. Results from PCR analysis showed that MLL2 and MLL3 transcripts were increased in mammospheres compared to bulk cells. Mammosphere forming efficiency of trastuzumab sensitive cells was decreased when MLL2 or MLL3 was knocked down using RNAi. Mammosphere forming efficiency of anti-HER2 therapy resistant (BT474 TR) cells was significantly decreased upon MLL3 knockdown, while HCC1954 mammospheres were inhibited by either MLL2 or MLL3 knockdown. We next measured transcript levels of Notch genes and other stem, mesenchymal, and luminal genes to determine whether MLL2 or MLL3 is necessary for the proper expression of cell fate genes. We found that Notch3 transcripts were decreased in both BT474 and HCC1954 cells upon MLL2 knockdown. SNAIL and Notch1 transcripts were also decreased, suggesting that MLL2 is required for the expression of genes involved in regulating cancer stem cell fates. These results suggest that MLL2/KMT2D and its paralog MLL3/KMT2C are increased in CSCs-derived from HER2+ breast cancer and may regulate CSC genes such Notch1 and Notch3 to promote resistance to anti-HER2 based therapy. Citation Format: Andrei Zlobin, Debra Wyatt, Mary Varsanik, Andrew Dingwall, Clodia Osipo.. Roles for MLL2/ KMT2D or MLL3/ KMT2C in HER+ breast cancer stem cells [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 5845.