Identification of the Epigenetic Reader BRD4 As a Novel Potential Target in Ph+ CML

Chronic myelogenous leukemia (CML) is a bone marrow-derived hematopoietic neoplasm in which BCR/ABL1 acts as a major driver of proliferation, differentiation and survival of leukemic cells. In a majority of all patients with CML, leukemic cells can be kept under control by BCR/ABL1 tyrosine kinase inhibitors (TKI), including imatinib, nilotinib, dasatinib, bosutinib, and ponatinib. Nevertheless, resistance or intolerance against one or more of these TKI may occur. Therefore, current research is focusing on novel potential drug targets in CML. A promising class of targets may be epigenetic regulators of cell growth, such as members of the bromodomain and extra-terminal domain (BET) family. The epigenetic reader and BET family member BRD4 has recently been identified as a novel potential drug target in acute myeloid leukemia (AML). However, so far, little is known about the expression and function of BRD4 in CML cells. The aims of the present study were to determine the expression of BRD4 and its downstream target MYC in CML cells and to explore whether BRD4 can serve as a novel drug target in this disease. As determined by qPCR, primary CML cells (chronic phase patients, n=7) as well as the CML cell lines KU812 and K562 expressed BRD4 mRNA. In addition, both CML cell lines stained positive for BRD4 in our immunocytochemistry staining experiments. In one patient with accelerated phase CML, putative leukemic (CD34+/CD38-) stem cells were sorted to near homogeneity and found to express BRD4 mRNA by qPCR. In order to examine the functional role of BRD4 in CML cells, a BRD4-specific shRNA was applied. In these experiments, the shRNA-induced knockdown of BRD4 in KU812 cells and K562 resulted in reduced growth compared to a control shRNA. Furthermore, the BRD4-targeting drug JQ1 was found to inhibit ³H-thymidine uptake and thus proliferation in KU812 cells in a dose-dependent manner (IC₅₀: 0.25-0.75 µM). In addition, we were able to show that JQ1 inhibits growth of primary CML cells with variable IC₅₀ values (0.1-5 µM). However, no substantial growth-inhibitory effects of JQ1 were seen in K562 cells (IC₅₀: >5 µM). As determined by Annexin V/PI staining, JQ1 induced apoptosis in KU812 cells whereas no apoptosis-inducing effect of JQ1 was observed in K562 cells. Nevertheless, we were able to show that both CML cell lines as well as primary CML cells express MYC mRNA, and treatment of KU812 cells or K562 cells with JQ1 resulted in a decreased expression of MYC mRNA and MYC protein. Next, we analyzed whether MYC expression in CML cells can be blocked by BCR/ABL1 TKI. We found that imatinib, nilotinib, dasatinib, and ponatinib decrease MYC mRNA- and MYC protein expression in KU812 and K562 cells. Finally, we found that JQ1 cooperates with imatinib, nilotinib, ponatinib and dasatinib in inhibiting the proliferation of KU812 and K562 cells. Together, our data show that BRD4 serves as a potential new target in CML cells, and that the BRD4 blocker JQ1 cooperates with BCR/ABL1 TKI in inducing growth-inhibition. Whether BRD4 inhibition is a pharmacologically meaningful approach in patients with TKI-resistant CML remains to be determined in clinical trials.