Chromatin accessibility combined with enhancer clusters activation mediates heterogeneous response to dexamethasone in myeloma cells

Glucocorticoids (GC) effects occur through binding to the GC receptor (GR) which, once translocated to the nucleus, binds to GC response elements (GREs) to activate or repress target genes. Among GCs, dexamethasone (Dex) is widely used in treatment of multiple myeloma (MM), mainly in combination regimens. However, despite a definite benefit, all patients relapse. Moreover, while GC efficacy can be largely attributed to lymphocyte-specific apoptosis, its molecular basis remains elusive. To determine the functional role of GR binding in myeloma cells, we generated bulk and single cell multi-omic data and high-resolution contact maps of active enhancers and target genes. We show that a minority (6%) of GR binding sites are associated with enhancer activity gains and increased interaction loops. We find that enhancers contribute to regulate gene activity through combinatorial assembly of large stretches of enhancers and/or enhancer cliques. Furthermore, one enhancer, proximal to GR-responsive genes, is predominantly associated with increased chromatin accessibility and higher H3K27ac occupancy. Finally, we show that Dex exposure leads to co-accessibility changes between predominant enhancer and other regulatory regions of the interaction network. Notably, these epigenomic changes are associated with cell-to-cell transcriptional heterogeneity. As consequences, BIM critical for GR-induced apoptosis and CXCR4 protective from chemotherapy-induced apoptosis are rather upregulated in different cells. In summary, our work provides new insights into the molecular mechanisms involved in Dex escape. ### Competing Interest Statement The authors have declared no competing interest. * GC : Glucocorticoids GR : Glucocorticoid Receptor GRE : Glucocorticoid response element MM : Multiple Myeloma Dex : Dexamethasone IMiDs : immunomodulatory drugs PIs : Proteasome Inhibitors TF : Transcription Factor SE : Super-Enhancers ISRE : Interferon Signaling Response Element scDAGs : single cell Dex-Activated Genes RRG : Ratio of Responsive Genes TSS : Transcription Start Site kb : kilobases IQR : Inter-Quartile Range FPC : Fraction of Positive Cells FDR : False Discovery Rate LogFC:Log Fold Change.