1237 Escape form adaptive drug tolerance through OGT and TET1 mediated H3K4me3 remodeling in MAPKi-resistant melanoma

Abstract

Genetic alterations linked to acquired BRAF inhibitor resistance are absent in about 40% of relapsed melanoma patients suggesting the involvement of epigenetic changes. We investigated epigenetic remodeling in BRAF mutant melanoma upon BRAF/MEK inhibition. Long term treatment of more than 45 days enables the cells to escape the slow cycling state which results in proliferating cellular clusters (drug-tolerant persister colonies) with stem-like characteristics that regain global H3K4me3. H3K4me3 ChIP-seq of colonies compared to parental cells revealed differential marking at promotor regions of several target genes involved in MAPKi resistance, including ARAF, BRAF, and CRAF. H3K4me3 remodeling corresponded to increased gene expression and susceptibility to pan-RAF inhibitors. Two enzymes, OGT and TET1 that are both linked to H3K4me3 regulation are significantly upregulated in persister colonies and tumor tissue of PDXs from BRAF mutant melanoma patients under MEK1/2 inhibition. A shift in OGT nuclear localization and O-linked glycosylation patterns was observed in colonies compared to parental cells and OGT ChIP-PCR confirmed a set of genes with exclusively H3K4me3 marking in colonies. shRNA mediated knockdown of OGT and TET1 blocked H3K4me3 increase in colonies, prevented colony formation and delayed tumor relapse in BRAF mutant xenografts. OGT and TET1 are promising targets to combat treatment failure and prolong overall survivial.