Abstract 3332: Aberrant expression of CSF1R in melanoma is driven through an endogenous viral promoter and it contributes to malignant growth and the acquisition of resistance against BRAF inhibition

Epigenetic changes in cancer are thought to contribute to regulation of invasion and metastasis. To study this at a genome-wide level in melanoma we analyzed the methylome of 44 cases of malignant melanoma. We saw widespread demethylation in melanoma occurring preferentially outside of CpG islands. Comparison of primary and metastatic lesions showed demethylation occurs early during carcinogenesis with few additional alterations in advanced tumors. The colony stimulating factor-1 receptor was aberrantly expressed and hypomethylated in nearly all cases. The expression of CSF1R was validated by IHC on primary tumors and by qPCR and Western blotting in BRAF mutant and WT cell lines. CSF1R can be aberrantly expressed via an upstream LTR element in Hodgkin’s lymphoma. After analyzing our patient samples and the cell lines, we have found this aberrant transcript may be the dominant form in melanoma as well. Expression of one of its ligands IL34 was also shown in the cell lines by both ELISA and qPCR pointing to a potential autocrine regulatory loop. The effects of a small molecule inhibitor, PLX3397 as well as shRNA-mediated knockdown of the receptor were investigated in 2D and 3D cell culture. We saw inhibition of cell growth, smaller colony size, increased apoptosis and decreased invasiveness - suggesting a functional role for CSF1R in melanoma. Treatment of melanoma with small molecule inhibitors of BRAF V600E is effective for a time, but resistance invariably develops. The feedback activation of EGFR, BRAF amplification, BRAF splice variants and others are known to aid in the acquisition of resistance and the rebound activation of the MAPK-pathway. We are suggesting a role for CSF1R in this process. In Western experiments, the rebound of phospho-ERK after BRAF inhibitor treatment was accelerated with the addition of CSF1R ligands, or delayed with PLX3397, also attenuating AKT phosphorylation. Melanoma cells stably expressing shRNA against CSF1R recapitulated the effects of the inhibitor. Assaying the cells at different time points during a long-term V600E inhibitory experiment, we saw increasing levels of the transcription factor RUNX1, followed by increasing levels of IL34 and of the receptor, as well as its maturation, evidenced by the appearance of the high MW form. Utilizing shRNA-mediated knockdown of RUNX1 resulted in lower levels of the CSF1R and IL34 transcripts and delayed the rebound. Analysis of primary RNA-Seq data showed an increase in RUNX1, CSF1R and IL34 expression in resistant tumors. Co-inhibition of CSF1R and BRAF was also tested and resulted in synergistic blockade of cell growth in vitro and xenograft growth in vivo. The CSF1R inhibitor, PLX3397 is currently in clinical trials for glioblastoma, prostate, breast cancers and other cancers. These data present a preclinical rationale for its study in malignant melanoma. Citation Format: Orsolya Giricz, Yongkai Mo, Caroline Y. Hu, Yiting Yu, Kith Pradhan, Matthias Bartenstein, Nandini Ramachandra, Veronika Polishchuck, Kimberly B. Dahlman, Tushar Bhagat, Hoa Nguyen, Bernice Matusow, Rafe Shellooe, Elizabeth Burton, Paraic Kenny, John Greally, Jeffrey Sosman, Gideon Bollag, Brian West, Amit Verma. Aberrant expression of CSF1R in melanoma is driven through an endogenous viral promoter and it contributes to malignant growth and the acquisition of resistance against BRAF inhibition [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 3332. doi:10.1158/1538-7445.AM2017-3332