AbstractBackgroundThe t(17;19)(q21;p13), which occurs in a small subset of acute lymphoblastic leukemias (ALLs) and is associated with a dismal prognosis, creates a chimeric E2A‐HLF transcription factor with transforming properties.ProcedureWe used representational difference analysis to identify candidate E2A‐HLF target genes. Transient transfection assays and an inducible expression model system were then used to evaluate the ability of E2A‐HLF to modulate target gene expression.ResultsWe identified ABCB1 (MDR1, P‐glycoprotein) as a gene differentially expressed in ALL cell lines with and without E2A‐HLF expression and demonstrated that t(17;19)+ ALL cell lines expressed high levels of ABCB1 protein and had a drug efflux‐positive phenotype. Although ABCB1 transcription is regulated by C/EBPβ via interaction with a DNA response element that shares significant homology with the optimal E2A‐HLF binding site, E2A‐HLF did not directly activate transcription of reporter genes under control of ABCB1 promoter elements in transient transfection assays. However, ABCB1 expression was induced in a DNA‐binding independent manner by E2A‐HLF, E2A‐PBX1, and truncated E2A polypeptides consisting of those portions of E2A present in leukemic fusion proteins.ConclusionsE2A‐HLF‐mediated over‐expression of ABCB1 may play a critical role in defining the clinical phenotype of ALLs with a t(17;19), suggesting pharmacologic modulation of ABCB1 activity as a rational therapeutic strategy for this chemotherapy resistant subtype of ALL. Pediatric Blood Cancer 2006;47:757–764. © 2005 Wiley‐Liss, Inc.