Abstract 300: Inhibition of oncogenic MAPK signaling in melanoma triggers SOX2-dependent adaptive drug resistance

Treatment of metastatic melanoma with BRAF V600E -specific small molecule inhibitors (BRAFi) provides dramatic clinical relief response but fails to suppress long-term tumor growth in the long-term. Extensive studies have uncovered identified a number of resistance-causing genetic alterations that, which are acquired upon continued drug exposure. However, little is known about the changes in gene expression programs changes during the critical time before drug resistance is manifested on at the genomic level. These changes may lead to adaptive drug resistance and hence, represent vital mechanisms by which tumor cells survive the acute drug insult prior to acquisition of resistance-causing mutations. Using a combination of RNAseq and molecular analyses, we find that acute inhibition of oncogenic MAPK signaling by BRAFi or MEKi in established and primary patient-derived melanoma cell cultures, leads to a rapid induction of the stem cell maintenance and pluripotency factor SOX2. Immunohistochemical analysis of Sox2 expression in melanoma sections derived from Braf V600E /Pten −/− -mice treated for 36 hours with BRAFi/MEKi confirmed the emergence of a Sox2-positive melanoma subpopulation in vivo. SOX2 Ectopic ectopic expression of SOX2 protected melanoma cells from the anti-proliferative effects of BRAFi indicative ofindicating a role for SOX2 in mediating drug resistance. Furthermore, siRNA-mediated depletion or overexpression experiments of SOX2 unveiled revealed that it is necessary and sufficient for the expansion of a drug-tolerant SOX2 + CD24 + cell population. This subpopulation is characterized by increased migratory and invasive behavior as well as an increased proliferative potential in the presence of BRAFi when compared to SOX2 + CD24 − cells. These results imply that BRAFi treatment induces a SOX2-dependent cellular program with features of adaptive drug resistance. Suppression of drug-dependent induction of SOX2 or its downstream effector pathways could potentially extend the clinical efficacy of BRAFi. Citation Format: Stefanie Flueckiger, Andrea Aloia, Lukas Frischknecht, Amanda Tuozzo, Davide Croci, Olga Shakhova, Christian Britschgi, Reinhard Dummer, Mitchell Paul Levesque, Wilhelm Krek. Inhibition of oncogenic MAPK signaling in melanoma triggers SOX2-dependent adaptive drug resistance. [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 300.