Abstract 2941: Targeting MCL1-dependent cancers through RNA splicing modulation
Authors: Park Eun Sun , Aicher Michelle , Aird Daniel , Buonamici Silvia , Chan Betty , Eifert Cheryl , Fekkes Peter , Furman Craig , Gerard Baudouin , Karr Craig , Keaney Gregg , Kunii Kaiko , Lee Linda , Pazolli Ermira , Prajapati Sudeep , Satoh Takashi , Smith Peter , Wang John Yuan , Wang Karen , Warmuth Markus , Yu Lihua , Zhu Ping , Mizui Yoshiharu , Corson Laura B. 
Published: August 2015

Abstract
Myeloid cell leukemia 1 (MCL1) is a member of the BCL2-family of proteins governing the apoptosis pathway and is one of the most frequently amplified genes in cancer. MCL1 overexpression often results in dependence on MCL1 for survival and is linked to resistance to anticancer therapies. However, the development of direct MCL1 inhibitors has proven challenging and thus far has been unsuccessful. Alternative splicing of MCL1 converts the anti-apoptotic MCL1 long (MCL1-L) isoform to the BH3-only containing MCL1 short (MCL1-S) isoform. As a potential approach for targeting MCL1-dependent cancers, we explored the use of MCL1 splicing modulators.
We screened a unique chemical library of compounds that span a range of splicing activities on various substrates in an in vitro assay. Interestingly, we found a subset of general splicing modulators, as well as a subset of SF3B1 inhibitors, that are capable of driving the distinctive alterations in MCL1 splicing that in turn can trigger preferential killing of MCL1-dependent cell lines. The best modulators induce a prominent down-regulation of MCL1-L, up-regulation of MCL1-S, and accumulation of intron-retained MCL1 transcript.
Somewhat surprisingly, several additional avenues of investigation pointed to MCL1-L down-regulation rather than MCL1-S up-regulation as the driver of preferential killing of MCL1-dependent cells. This includes the fact that compound-induced cytotoxicity can be rescued by expression of a MCL1-L cDNA and MCL1-L specific shRNA knockdown is sufficient to kill MCL1-dependent cells. On the other hand, overexpression of MCL1-S cDNA had no significant effect on cells and splicing modulators that induced very high levels of MCL1-S mRNA in the absence potent MCL1-L down-regulation exhibit minimal cytotoxicity. Biochemical characterization and understanding of these MCL1 splicing modulators has enabled further optimization of compounds that can induce potent and preferential killing of MCL1-dependent cancer cell lines in vitro. Preliminary studies in mice bearing MCL1-dependent NSCLC xenografts confirmed current lead compounds can indeed induce rapid down-regulation of MCL1-L, induction of apoptosis, and antitumor activity.
Collectively these data yield insight into mechanisms of MCL1 splicing modulation that can trigger acute apoptosis in MCL1-dependent cancers and provides support for the idea of using splicing modulators to target difficult-to-drug oncogenic drivers such as MCL1.
Citation Format: Eun Sun Park, Michelle Aicher, Daniel Aird, Silvia Buonamici, Betty Chan, Cheryl Eifert, Peter Fekkes, Craig Furman, Baudouin Gerard, Craig Karr, Gregg Keaney, Kaiko Kunii, Linda Lee, Ermira Pazolli, Sudeep Prajapati, Takashi Satoh, Peter Smith, John Yuan Wang, Karen Wang, Markus Warmuth, Lihua Yu, Ping Zhu, Yoshiharu Mizui, Laura B. Corson. Targeting MCL1-dependent cancers through RNA splicing modulation. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2941. doi:10.1158/1538-7445.AM2015-2941