Examining the effect of CBP on the E2A-PBX1 and HOXB4 interaction
Menezes, Sean Christopher
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The E2A-PBX1 fusion gene results from the t(1;19) chromosomal translocation that is found in 25% of pre-B-cell cases of acute lymphoblastic leukemia (ALL). The resulting encoded product contains the transactivation domains of E2A, a Class I basic helix-loop-helix transcription factor, and most of PBX1. PBX1 is a major cofactor for most members of the HOX family of homeodomain proteins and is necessary for regulating the essential role that HOX proteins play in development and tissue homeostasis. We have identified an interaction between the E2A-encoded portion of E2A-PBX1 and the CREB-binding domain (KIX) of the transcriptional coactivator CBP and demonstrated a requirement for this interaction in leukemia induction. Others have shown that HOX proteins and CBP also interact directly, with resulting inhibitory effects on the DNA-binding ability of HOX proteins and on the acetylation of substrate proteins by CBP. Several publications have also identified the interaction of HOX proteins with the PBX1 portion of E2A-PBX1 and the result is a potent transcriptional activator at PBX1/HOX target sequences. In an attempt to develop a molecular model for the induction of ALL by E2A-PBX1, we hypothesize that the addition of CBP interactive peptide elements encoded by E2A to PBX1 allows E2A-PBX1 to stabilize a ternary complex involving E2A-PBX1, HOX, and CBP resulting in the deregulated expression of critical PBX1 or HOX target genes. I demonstrate using in vitro protein-protein interactions that this ternary complex involving E2A-PBX1, HOXB4 (chosen as a representative member of the HOX family), and CBP does form. This direct interaction appears to reduce transcriptional activation by E2A-PBX1/HOXB4 heterodimers from PBX1/HOX enhancer elements. I also show that this suppression of transactivation appears to involve CBP antagonism of DNA binding by E2A-PBX1/HOXB4 heterodimers. My results are consistent with the idea that E2A-PBX1 contributes to ALL induction by promoting the redistribution of CBP away from DNA sites bound by E2A-PBX1/HOXB4 heterodimers and in favour of those sites bound by E2A-PBX1 homodimers.