Abstract 89: Targeting Rho/MRTF regulated gene transcription in drug-resistant melanoma
Authors: Misek Sean A. , Larsen Scott D. , Gallo Kathleen A. , Neubig Richard R. 
Published: July 2017

Abstract
Much of the recent focus of melanoma targeted therapy has been on the ERK pathway, which is aberrantly activated in approximately 90% of melanoma tumors (over half of which express BRAFV600E). Current targeted therapies such as vemurafenib (BRAFV600E inhibitor), or a combination therapy using dabrafenib (BRAFV600E inhibitor) and low dose trametinib (MEK inhibitor) shows profound initial effects in a majority of BRAFV600E expressing tumors. However, these responses are often short-lived and resistances typically develops within months. Resistance to these targeted therapies can arise from multiple mechanisms, including activation of pro-survival signaling pathways parallel to the ERK pathway. The goal of this work is to identify pharmacologically targetable resistance mechanisms so that effective combination therapies can be developed.
Despite the clear role of the RhoA subfamily of Rho GTPases (RhoA/B/C) as melanoma oncogenes, their role in drug resistance is not well understood. It is challenging to develop small molecule inhibitors which directly target the activity of small Rho GTPases, so an alternative approach is to inhibit downstream pathways. Through modulation of the actin cytoskeleton Rho can induce gene transcription through multiple transcriptional co-activators including Myocardin-Related Transcription Factor (MRTF) and Yes-Associated Protein 1 (YAP).
My bioinformatics analysis demonstrates that MRTF-A gene expression is correlated with poor overall survival in a large cohort of cutaneous melanoma patients. Furthermore, expression of a set of 216 MRTF target genes is enriched in dabrafenib/trametinib resistant cutaneous melanoma tumors compared to matched pre-treatment tumors, suggesting that MRTF activation may be involved in drug resistance. Based upon these results I hypothesized that small Rho GTPases may promote resistance to MAPK pathway targeted therapies through activation of MRTF/YAP.
To test this hypothesis, I generated vemurafenib resistant melanoma cells through chronic exposure to vemurafenib. This vemurafenib-resistant cell population is enriched for actin stress fiber positive cells, and these cells have increased Myosin Light Chain 2 (MLC2) phosphorylation, suggesting that there is increased Rho activation. Furthermore, these drug resistant cells are more sensitive to pharmacological inhibition of MRTF activity. These preliminary data suggest that vemurafenib resistant melanoma cells may be re-wired to depend on the Rho-induced gene transcription for their survival, and that a combination therapy simultaneously targeting these two pathways may be an effective treatment strategy for BRAF inhibitor-resistant melanomas.
Citation Format: Sean A. Misek, Scott D. Larsen, Kathleen A. Gallo, Richard R. Neubig. Targeting Rho/MRTF regulated gene transcription in drug-resistant melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 89. doi:10.1158/1538-7445.AM2017-89