High BCR::ABL1 Expression Defines CD34+ Cells with Significant Alterations in Signal Transduction, Short-Proliferative Potential and Self-Renewal Ability

Purpose: Chronic Myeloid Leukemia (CML) is a clonal disorder of the hematopoietic stem cell caused by expression of the BCR:: ABL1 oncoprotein. High BCR::ABL1 levels have been associated to proliferative advantage of leukemic cells, blast crisis progression and tyrosine kinase inhibitors (TKIs) inefficacy. We have previously shown that high BCR::ABL1/GUSIS transcripts measured at diagnosis are associated with inferior responses to standard dose Imatinib (IM). However, the mechanisms underlying the higher rates of disease progression and development of TKIs resistance dependent on elevated BCR::ABL1 levels remain unclear. Methods: Leukemic cells were collected from CML patients showing, at diagnosis, high or low BCR::ABL1/GUSIS. BCR::ABL1 expression levels were measured using real-time PCR. Short-term culture and long-term culture-initiating cells assays were employed to investigate the role of BCR::ABL1 gene-expression levels on proliferation, clonogenicity, signal transduction, TKIs responsiveness and self-renewal ability. Cell division was performed by carboxyfluorescein-succinimidyl ester (CFSE) assay. Results: We found that BCR::ABL1 oncogene expression levels correlate in both PMNs and CD34+ cells. Furthermore, high oncogene levels increased both proliferation and anti-apoptotic signaling via ERK and AKT phosphorylation. Moreover, high BCR::ABL1 expression reduced the clonogenicity of leukemic CD34+ cells and increased their sensitivity to high doses IM but not to those of dasatinib. Furthermore, we observed that high BCR::ABL1 levels are associated with a reduced self-renewal of primitive leukemic cells and, also, that these cells showed comparable TKIs responsiveness with cells expressing lower BCR::ABL1 levels. Interestingly, we found a direct correlation between high BCR::ABL1 levels and reduced number of quiescent leukemic cells caused by increasing their cycling. Conclusion: Higher BCR::ABL1 levels improving the proliferation, anti-apoptotic signaling and reducing self-renewal properties cause an increased expansion of leukemic clone.