BMI-1 expression is enhanced through transcriptional and posttranscriptional regulation during the progression of chronic myeloid leukemia

BMI-1 plays a critical role in regulating the activity of hematopoietic stem and progenitor cells. Patients with chronic myeloid leukemia (CML) are at a risk of developing blastic crisis (BC) even after the emergence of imatinib mesylate. In this study, to determine the relevance of BMI-1 to BC, we investigated the expression of BMI-1 in CD34⁺ cells at each of the chronic phase (CP), the accelerated phase (AP), and BC by flow cytometry. Interestingly, the level of BMI-1 expression was significantly higher in CP than in controls and was further increased during the course of the disease progression (control—5.66%; CP—36.93%; AP and BC—76.41%). Curiously, mRNA levels for BMI-1 were almost consistent during the disease progression from CP to BC (control—2.21; CP—9.77; AP and BC—9.70 (BMI-1/glyceraldehyde-3-phosphate dehydrogenase ratio)). Since we further found that overexpression of BCR–ABL in human embryonic kidney-293 cells enhanced BMI-1 expression and that BMI-1 expression was increased in K562 cells, derived from patients with BC, in the presence of proteasomal inhibitors, BMI-1 was presumed to be positively regulated by BCR–ABL and further by posttranscriptional modification in the course of the disease progression. We suggest the usefulness of BMI-1 expression in CD34⁺ cells as a molecular marker for monitoring patients with CML.