Do endothelial cells belong to the primitive stem leukemic clone in CML? Role of extracellular vesicles

The expression of BCR-ABL in hematopoietic stem cells is a well-defined primary event in chronic myeloid leukemia (CML). Some reports have described the presence of BCR-ABL on endothelial cells from CML patients, suggesting the origin of the disease in a primitive hemangioblastic cell. On the other hand, extracellular vesicles (EVs) released by CML leukemic cells are involved in the angiogenesis modulation process. In the current work we hypothesized that EVs released from BCR-ABL(+) cells may carry inside the oncogene that can be transferred to endothelial cells leading to the expression of both BCR-ABL transcript and the oncoprotein. EVs from K562 cells and plasma of newly diagnosed CML patients were isolated by ultracentrifugation. RT-PCR analysis detected the presence of BCR-ABL RNA in the EVs isolated from both K562 cells and plasma of CML patients. The incorporation of these EVs into endothelial cells was demonstrated by flow cytometry and fluorescence microscopy showed that after 24h of incubation most EVs were incorporated. BCR-ABL transcripts were detected in all experiments on endothelial cells incubated with EVs from both sources. The presence of BCR-ABL on endothelial cells incubated with Philadelphia(+) EVs was also confirmed by Western blot assays. In summary, endothelial cells acquire BCR-ABL RNA and the oncoprotein after incubation with EVs released from Ph(+) positive cells (either from K562 cells or from plasma of newly diagnosed CML patients). This results challenge the hypothesis that endothelial cells may be part of the Philadelphia(+) clone in CML.