Prognostic Impact and Targeting of BMI-1 in Acute Myeloid Leukemia

A minor fraction of leukemia cells, leukemia stem cells, have been shown to be highly resistant to current therapies and thought to be responsible for recurrence. BMI-1, a part of polycomb repressive complex 1 (PRC1) is essential for the self-renewal of normal hematopoietic and leukemia stem cells. PTC-209 is a novel selective transcriptional inhibitor of BMI-1, which has been shown to have antitumor activity against cancer-initiating cells in colorectal cancer. We investigated the prognostic significance of BMI-1 in acute myeloid leukemia (AML) using reversed phase protein array and effects of the BMI-1 inhibitor PTC-209 on primary and leukemia cell lines. BMI-1 protein expression was determined in bulk AML blasts from 511 newly diagnosed patients. BMI-1 expression was higher in unfavorable cytogenetics (n=252, median 0.068) compared to intermediate (n=225, median -0.116, P = 0.017) or favorable cytogenetics (n=34, median -0.338, P = 0.0007 versus unfavorable, 0.05 versus intermediate). Higher BMI-1 levels were associated with shorter median overall survival (42.8 versus 55.3 weeks, P = 0.046 Log Rank test). There was no correlation between BMI-1 levels and percentages of CD34 -positive cells (r = 0.07). A total of 6 AML (MOLM-13, OCI-AML3, MV4-11, NB4, HL60 and U-937) and 5 ALL (Reh, NALM6, Jurkat, Raji and MOLT-4) cell lines were exposed to PTC-209 for 48 hours. PTC-209 exhibited dose- and time-dependent anti-proliferative and cytotoxic activities. The IC₅₀ values (concentration at which cell growth is inhibited by 50% at 48 hours of exposure) were 0.33 ± 0.04 µM (mean ± SEM) for AML and 0.55 ± 0.09 µM for ALL, indicating potent anti-proliferative effects. In contrast, PTC-209 showed differential cytotoxic effects between AML and ALL cells. The ED₅₀ values (effective concentration inducing 50% killing as measured by Annexin V positivity) were no more than 2.5 µM in 5 of 6 AML lines while they were higher than 10 µM in 3 out of 5 ALL cell lines, implicating that BMI-1 is more critical in AML than ALL. Treatment with PTC-209 triggered several molecular events consistent with induction of apoptosis in sensitive lines (e.g. MV4-11 and MOLM-13): conformational change of BAX (i.e., BAX activation), loss of mitochondrial membrane potential (MMP), caspase-3 activation and DNA fragmentation in addition to phosphatidylserine (PS) externalization. Eighteen-hour treatment of MV4-11 cells with 2.5 µM PTC-209 led to compound-specific induction of conformationally active BAX (31%), MMP loss (80%) and caspase-3 cleavage (38%). qRT-PCR showed reduced transcript level of BMI-1 (61% reduction) after 6-hour PTC-209 exposure in MV4-11 cells. PTC-209 induced PS externalization in primary AML cells (82.5 ± 4.3% after 48-hour treatment with 2 µM PTC-209, n = 6) and to a lesser degree, in ALL cells (33.7 ± 13.4%, n = 4, p < 0.05). Importantly, CD34+CD38– AML progenitor cells were as sensitive to PTC-209 as C34– more mature AML cells. Normal lymphocytes were resistant to PTC-209 (9.1 ± 4.6% even at 10 µM). Collectively, BMI-1 inhibition by small molecule inhibitors could be developed into a novel therapeutic strategy.