Lineage Plasticity in The Establishment of CML Drug Resistance

Abstract

The pathogenesis of chronic myeloid leukaemia (CML) is the epitome of oncogene addiction. The disease is iniLated by the tyrosine kinase BCR::ABL1 oncoprotein. Clinical treatment with the kinase inhibitor Imatinib Mesylate, has been an outstanding success and clearly demonstrates the addiction of CML upon BCR::ABL1 activity. However, up to 20% of patients will develop drug resistance and there is no current successful treatment. A greater understanding of how cells can escape drug treatment is needed, and this is more-so urgent in cases when the leukaemic cells have adapted to be kinase-independent rendering the continued use of targeted therapy redundant. Previous work in the lab has established a kinase-independent model of CML drug resistance using the KCL22 cells, defining a novel mechanism of transdifferentiation (erythroid to myeloid switch) associated with this process. To understand the regulators of this transdifferentiation, the cellular reprogramming of cells by the master C/EBPα (myeloid) and GATA1 (erythroid) regulators was attempted (Chapter 3). While the KCL22 model provides novel insight into kinase-independent CML and lineage switching, it suffers the concern that it may be a KCL22-specific phenomenon. To address this, the cellular reprogramming associated with CML drug resistance was validated within an independent K562 model. Moreover, a common set of regulated transcription factors was identified between the KCL22 and K562 models, suggesting a core ‘drug resistant’ gene network (Chapter 4). Complexity of the kinase-independent mechanism of drug resistance was highlighted by the establishment of an additional KCL22 model that did not undergo cellular reprogramming (Chapter 5). Nevertheless, despite the establishment of a stable cell state, the drug resistant KCL22 cells were susceptible to further clonal evolution with a genetic variant characterised having gross over-amplified BCR::ABL1 protein expression (Chapter 6). The extent of cellular plasticity within leukaemic cells was further exemplified as this new variant had reprogrammed the myeloid drug resistant cells back towards an erythroid lineage. Finally, dynamic changes in the alternative splicing of RNA transcripts in the establishment of CML drug resistant was characterised (Chapter 7). These lineage-dependent changes in RNA splicing events could have significant impact on the proteome and contribute to the establishment, and maintenance, of drug resistant CML cells.