EZH2 Inhibition Reshapes 3D Chromatin Architecture to Induce Immunogenic Phenotype in Small Cell Lung Cancer

BackgroundThe histone methyltransferase EZH2, enzymatic core of the trimeric polycomb repressive complex 2 (PRC2), has been shown to promote small cell lung cancer (SCLC) survival through epigenetic silencing of multiple targets including Class I MHC molecules (HLA-A/B) and DNA repair factors (SLFN11). Treatment of SCLC cells with EZH2 inhibitors in vitro can reactivate expression of these genes and result in therapeutic response to immune checkpoint inhibition (ICI) and chemotherapy. Here, we investigate the impact of EZH1/2 dual inhibition on 3D chromatin structure and its relationship to transcriptional regulation in neuroendocrine (NE) SCLC. ResultsEmploying Micro-C, a micrococcal nuclease-based 3D genome mapping technique, we show that EZH1/2 inhibition with Valemetostat induced significant changes at multiple genome organizational levels (compartment, topological associated domain, and chromatin loop) without incurring cell death in NE SCLC. Alterations in 3D genome permissive for transcriptional activation were correlated with increased chromatin accessibility (ATAC-sequencing) and expression of target genes (transcriptome profiling). Known transcription factor motif discovery revealed enrichment of non-NE motifs (e.g., REST) in regions with gained chromatin accessibility in Valemetostat-treated cells, consistent with results from gene set enrichment analysis demonstrating NE to non-neuroendocrine lineage shift. Notably, EZH1/2 inhibition reactivated Class I MHC expression by facilitating enhancer-promoter looping. ConclusionOur results demonstrate that repression of a subset of EZH2 targets including Class I MHC genes is affected through modulation of 3D genome structure to the level of chromatin looping and further support clinical investigation of EZH2 inhibition in boosting therapeutic efficacy of ICI in SCLC patients.