PU.1 Eviction at Lymphocyte-Specific Chromatin Domains Mediates Glucocorticoid Response in Acute Lymphoblastic Leukemia

Glucocorticoids are an essential part of many multi-agent treatment regimens for lymphoid malignancies where the emergence of glucocorticoid resistance is a significant barrier to a cure. We have previously identified that aberrant chromatin accessibility, at lymphocyte-specific glucocorticoid-response elements (GREs), is associated with drug resistance in acute lymphoblastic leukaemia (ALL). To gain a deeper understanding of regulatory mechanisms leading to these epigenetic alterations, we conducted a multi-omics study, to examine changes in chromatin conformation, following the in-vivo treatment of patient-derived xenografts (PDXs) with glucocorticoid i.e., dexamethasone (DEX). We found that glucocorticoid treatment led to distinct patterns of topological associated domains (TADs) in DEX sensitive and resistant ALLs. Pre-treatment, these TADs were primed by a development-related pioneer transcription factor PU.1, which also occupied GREs in glucocorticoid sensitive but not resistant ALLs. PU.1 binding was associated with lymphocyte-specific activation of GREs and GRE-interacting super-enhancers within the TADs. Furthermore, the PU.1-associated TADs modulated epigenetics and promoted expression of multiple genes, including BIM, ZBTB16, RASA1 and other genes mediating glucocorticoid-induced apoptosis in ALL. The inhibition of PU.1 enhanced sensitivity of ALL cells to glucocorticoid treatment. Overall, this study illustrated a link between the lineage-specific efficiency of glucocorticoids and the developmental epigenetic modulators and provides insights into the role of fundamental epigenetics in clinical practice.