Functional substitution of imatinib-inactivated canonical oncoprotein BCR::ABL1 by overexpressed PLAG1 results in BCR::ABL1-independent resistance in blast phase of chronic myeloid leukemia.

Inactivation of oncogenic BCR::ABL1 by imatinib in the early chronic phase (CP) of chronic myeloid leukemia (CML) has achieved unprecedented clinical success, which eludes the majority of terminal blast crisis (BC) patients, predominantly due to the scantily understood BCR::ABL1-independent mechanism/s of resistance. Our earlier proteomic studies on K562 and KU812 cell lines, which mimicked the BCR::ABL1-independent resistance to imatinib reported in CML patients, demonstrated that atypical activation of signalling downstream of imatinib-inactivated BCR::ABL1, mediated by reduced levels of cell adhesion receptor - integrin ITGB1, caused resistance. In the present study, events upstream of ITGB1 were derived from the transcriptome, methylome, and miRNA profiles of imatinib-resistant cell lines. The analyses identified major alterations in cell adhesion pathway molecules in resistant cells. Significant among them were gene hypomethylation, upregulation of the transcript and protein of ITGB3, known to influence the protein level of ITGB1, which was reduced despite unaltered gene methylation and transcription. Indeed, knockdown (KD) of ITGB3 caused augmentation of ITGB1 protein level. Further, the mediator of ITGB3 overexpression was detected in genomic analysis. A recurrent gain at chromosomal region 8q11.21-12.1 and amplification of the transcription factor PLAG1 residing therein were observed in resistant K562 cells and CD34 + cells from resistant BC patients. KD of PLAG1 not only reversed gene hypomethylation, upregulation of transcript and protein of ITGB3 but also restored ITGB1 level and sensitized resistant cells to imatinib. The study has identified events upstream of ITGB1, thereby advancing the understanding of BCR::ABL1-independent imatinib resistance and has provided explorable therapeutic avenues for imatinib-resistant blast phase CML.