The Role of Dysregulated Transcription Factors in Drug Resistant CML

The kinase activity of BCR-ABL1 is central to chronic myeloid leukaemia (CML) pathology. Clinical treatment of CML with the kinase inhibitor Imatinib Mesylate (IM) is a fundamental success. However, 15-20% of CML patients develop drug resistance. For the majority of patients, the challenge is to understand how leukaemic cells can survive in the absence of BCR-ABL1 activity. To address this issue, a model of CML drug resistance was previously established using KCL22 cells. Here, the drug resistant cells recapitulate clinical findings and survive in a BCR-ABL1 kinase independent manner. Noting that the oncogene is a multi-domain complex protein, the functional relevance of the other domains of BCR-ABL1 in compensating for the loss of kinase activity was explored. In contrast to the parental KCL22 cells, depletion of the oncogene within the drug-resistant clones failed to induce apoptosis but rather the cells underwent growth arrest. A novel model is proposed for how these cells maintain a leukaemic phenotype in kinase independent CML. Here, the other domains of BCR-ABL1 regulate cell proliferation, the Src kinases inhibit apoptosis while transcription factors block cellular differentiation.
The proposed mechanism of how cells can survive in the absent of BCR-ABL1 kinase activity is based on the findings from a single cell line. To compliment the KCL22 model, a second drug resistance CML line (EM2 cells) was generated. Interestingly, while both KCL22 and EM2 models were kinase independent, they contrasted in terms of the transcription factors deregulated upon drug resistance. Lineage phenotyping of these cells identify them as erythroid and GMP respectively and prompted the ‘cell of origin’ hypothesis whereby the identity of the transformed progenitor dictates the nature of the transcriptome changes acquired upon drug resistance. Furthermore, clonal analysis of different EM2 single cell clones highlighted the heterogeneity of drug resistant CML and suggested that the mechanism of resistance is innately established within the cells prior to any exposure to drug.
The deregulated expression of transcription factors is a key contributing factor in the pathology of many blood cancers. Targeting these factors with the aims of restoring their normal functional activity has significant potential in the therapy of leukaemias. Notably, the expression of the SOX4 transcription factor is strongly induced upon KCL22 drug resistance. Depletion of SOX4 within drug resistant KCL22 cells failed to confer any overt phenotypic changes. However, these studies unravel a possible redundant network among the SOXC family members (SOX4, SOX11 and SOX12).
In summary, the work herein provides a greater understanding of CML and identifies, for the first time, the potential importance of the other domains of BCR-ABL1 in establishing kinase independency. Moreover, this study suggests a personalised stratification of CML patients where future treatment of drug resistance could depend on the lineage identity of the original transformed clone.