Regulation of BCR-ABL expression via its 3’untranslated region.

Abstract

In patients with chronic myeloid leukaemia (CML), a translocation between chromosomes 9 and 22 fuses the BCR gene with the ABL1 gene, and gives rise to the BCR-ABL gene. Expression of the BCR-ABL protein initiates and drives CML. The level of BCR-ABL expression is associated with disease progression and response to therapy, yet control of BCR-ABL expression is poorly understood. This study has added to this limited knowledgebase by investigating the role that the BCR-ABL 3‟untranslated region (3‟UTR) plays in controlling BCR-ABL expression. Due to the nature of the translocation, the BCR-ABL 3‟UTR is contributed by the ABL1 gene. We found that ABL1 and BCR-ABL have similar, but shorter half-lives than BCR. This suggests that the ABL1 moiety influences the stability of the BCR-ABL transcript. Addition of the ABL1 3‟UTR to a Renilla reporter gene strongly repressed reporter expression. Furthermore, insertion of a premature poly-adenylation site in the 3‟UTR resulted in a rescue of reporter expression, demonstrating that the 3‟UTR is required in the transcript for full activity, and thus is indicative of post-transcriptional control. Generation of ABL1 reporters containing various regions of the 3‟UTR revealed that discrete regions of the 3‟UTR could strongly influence gene expression. Following these results, we attempted to identify factors involved in the regulation of BCR-ABL expression. We focused on microRNAs-29, 30, 125, 141, 196 and 203, predicted by bioinformatics to interact with the 3‟UTR. Although some of these microRNAs interacted with ABL1 reporters, they did not modulate endogenous BCR-ABL expression. In parallel, we developed an assay that was aimed at identifying RNA-binding proteins that bind to the ABL1 3‟UTR. Finally, using publically-available datasets, we found data suggesting that RNA-binding proteins; TTP, hnRNP-C, ELAVL-1, TIA-1 and TIAL-1 interact with functional regulatory regions of the ABL1 3‟UTR. Taken together, we have shown that the BCR-ABL 3‟UTR sequence is repressive, and contains discrete regions that can influence gene expression. The 3‟UTR is located within a region of the BCR-ABL transcript that is responsible for controlling BCR-ABL mRNA stability. Although many attempts were made to discover functional ABL1 3‟UTR binding factors, their identities remain unknown. Further research is required to identify the binding factors involved. We envisage that a comprehensive understanding of how BCR-ABL is controlled will contribute to a better understanding of the biology of CML, and pave the way for innovative forms of targeted therapy.