Fusion of Early B Cell Factor 1 with Platelet-Derived Growth Factor Receptor Beta in B-ALL disrupts functions necessary for normal B lymphopoiesis

Abstract

Abstract Human B cell Acute Lymphoblastic Leukemia (B-ALL) cells often lack functional genes encoding Early B cell Factor 1 (EBF1). EBF1 is essential for B lineage specification, where it regulates >500 genes necessary for normal B cell development. Here, we describe molecular mechanisms that contribute to EBF1-associated B-ALL. Intra-chromosomal deletions result in fusion of the nearly complete EBF1 gene with the 3′ half of the Platelet-Derived Growth Factor Receptor Beta gene (EBF1:PDGFRB) in a subset of high risk pediatric B-ALL. The fusion protein, EBF1:PDGFRβ, unifies loss of EBF1 function with a proliferative advantage due to unregulated Receptor Tyrosine Kinase (RTK) activity of PDGFRβ. Our laboratory has utilized biochemical, cell-based, and fluorescence microscopy approaches to study DNA binding, transcriptional activation and repression, oligomerization, and localization of EBF1:PDGFRβ in B cells. EBF1:PDGFRβ transforms B cell progenitors by eliminating their dependence on IL-7. Although the fusion protein includes nearly intact EBF1 and its nuclear localization signal, EBF1:PDGFRβ localizes predominately in the cytosol. Localization of EBF1:PDGFRβ is dependent upon the transmembrane (TM) domain of PDGFRβ; however, rather than docking EBF1:PDGFRβ in the plasma membrane, the TM functions as a nuclear export signal. Treatment with the tyrosine kinase inhibitor (TKI) imatinib reduces RTK activity and partially restores transactivation of EBF1 target genes by EBF1:PDGFRβ. This suggests a potentially undesirable effect of TKI chemotherapy, which could allow evasion of leukemic cells and disease relapse. Our data suggests that the TM domain of PDGFRβ could serve as a therapeutic target to supplement TKI chemotherapy.