Mediator subunit MED1 is required for E2A-PBX1–mediated oncogenic transcription and leukemic cell growth
Menée in vitro, cette étude met en évidence le rôle de la sous-unité MED1 du complexe "Médiateur" dans la croissance des cellules leucémiques et la transcription oncogène induite par le complexe protéique E2A-PBX1
Current challenges in B cell acute lymphoblastic leukemia (B-ALL) include a comprehensive understanding of mechanistic effects of associated oncogenic factors, the development of efficacious therapeutic regimens, and the identification of B-ALL subgroups with characteristic molecular features that can be targeted in cancer treatment. Here we show that MED1 is required for a identified interaction between the Mediator coactivator complex and oncogenic E2A-PBX1, for the proliferation, specifically, of E2A-PBX1–driven leukemic cells and for the activation of E2A-PBX1 target genes. RUNX1 directs the recruitment of E2A-PBX1 to target genes, including cell cycle regulatory E2F5 and survival signaling pre-B cell receptor genes. These results provide insights into mechanisms underlying E2A-PBX1–mediated leukemogenesis and MED1–E2A-PBX1 interactions as potential therapeutic targets.The chimeric transcription factor E2A-PBX1, containing the N-terminal activation domains of E2A fused to the C-terminal DNA-binding domain of PBX1, results in 5% of pediatric acute lymphoblastic leukemias (ALL). We recently have reported a mechanism for RUNX1-dependent recruitment of E2A-PBX1 to chromatin in pre-B leukemic cells; but the subsequent E2A-PBX1 functions through various coactivators and the general transcriptional machinery remain unclear. The Mediator complex plays a critical role in cell-specific gene activation by serving as a key coactivator for gene-specific transcription factors that facilitates their function through the RNA polymerase II transcriptional machinery, but whether Mediator contributes to aberrant expression of E2A-PBX1 target genes remains largely unexplored. Here we show that Mediator interacts directly with E2A-PBX1 through an interaction of the MED1 subunit with an E2A activation domain. Results of MED1 depletion by CRISPR/Cas9 further indicate that MED1 is specifically required for E2A-PBX1–dependent gene activation and leukemic cell growth. Integrated transcriptome and cistrome analyses identify pre-B cell receptor and cell cycle regulatory genes as direct cotargets of MED1 and E2A-PBX1. Notably, complementary biochemical analyses also demonstrate that recruitment of E2A-PBX1 to a target DNA template involves a direct interaction with DNA-bound RUNX1 that can be further stabilized by EBF1. These findings suggest that E2A-PBX1 interactions with RUNX1 and MED1/Mediator are of functional importance for both gene-specific transcriptional activation and maintenance of E2A-PBX1–driven leukemia. The MED1 dependency for E2A-PBX1–mediated gene activation and leukemogenesis may provide a potential therapeutic opportunity by targeting MED1 in E2A-PBX1+ pre-B leukemia.ChIP-seq and RNA-seq data data have been deposited in the European Bioinformatics Institute (EMBL-EBI), https://www.ebi.ac.uk/ (accession nos. E-MTAB-8177, E-MTAB-8178, and E-MTAB-8179).