Mechanisms of RAS/MAPK Signaling by BCR-ABL1 in Chronic Myeloid Leukemia

Oncogene addiction refers to a cancer cell’s dependence on the continued activity of a single oncogene for survival. This concept has been validated by the success of tyrosine kinase inhibitor (TKI) therapy in chronic myeloid leukemia (CML), a clonal myeloproliferative disorder characterized by the presence of the BCR-ABL1 oncoprotein. BCR-ABL1 is a tyrosine kinase that constitutively activates RAS, which initiates multiple downstream signaling pathways that regulate cell proliferation, differentiation, and apoptosis. Because aberrant regulation of these cellular processes results in disease progression and resistance to therapy, a better understanding of how BCR-ABL1 activates RAS may uncover new therapeutic strategies. Previous studies in our lab have demonstrated that BCR-ABL1 mediated oncogene addiction is facilitated by persistent levels of MEK-dependent negative feedback, which oppose BCR-ABL1-dependent RAS activation. Inhibition of MEK in BCR-ABL1 expressing cells increases RAS-GTP levels, thereby providing a tractable, disease-relevant model to address the question of how BCR-ABL1 activates RAS. Here we identify a biologically and therapeutically important novel mechanism whereby BCR-ABL1 activates the critical downstream effector RAS in CML cells. We find that BCR-ABL1 activates RAS in part through the PLCG1/DAG/RasGRP4 signaling axis. Loss of PLCG1 decreases RAS nucleotide exchange factor activity, decreases cell proliferation, and increases sensitivity to BCR-ABL1 tyrosine kinase inhibitors (TKIs). Collectively, these studies suggest that PLCG inhibitors may re-sensitize some patients to BCR-ABL1 TKIs, and may lead to more effective cytotoxicity when combined with BCR-ABL1 TKIs. Another mechanistic insight of BCR-ABL1 mediated oncogene addiction our lab has reported is that the negative feedback established by BCR-ABL1 persists for an extended period of time following the initiation of TKI treatment, during which CML cells commit to apoptosis. This is in contrast to what has been observed in BRAF-V600E driven malignancies, where MEK-dependent negative feedback rapidly attenuates following BRAF inhibitor treatment. This observation has raised the question of whether negative feedback can be harnessed to affect targeted therapy response in pathologically activated kinases. We sought to identify the critical molecular entities responsible for the prolonged negative feedback in patient-derived CML cell lines. We identified some candidate mediators of prolonged negative feedback in BCR-ABL1-expressing cells and validated that they were able to affect BCR-ABL1 TKI sensitivity. Future studies will need to be performed to determine the extent to which these candidate genes modulate the duration of negative feedback.