Diffuse large B-cell lymphoma (DLBCL) is the most common subtype of lymphoma in adults. Although a significant proportion of patients are cured with chemoimmunotherapy, those with relapsed or refractory disease have poor outcomes. Concurrent rearrangements of c-MYCand either BCL-2, BCL-6(double-hit lymphoma), or both (triple-hit lymphoma), as well as protein overexpression of c-MYC in combination with overexpression of BCL-2, are associated with chemotherapy resistance and low overall survival rates. Given that approximately 10 percent and 30 percent of cases of DLBCL harbor rearrangements of c-MYC or protein overexpression of c-MYC, respectively, novel therapeutic approaches targeting these poor-prognosis populations are urgently needed.
Recent studies have demonstrated that inhibition of the bromodomain and extraterminal (BET) family of proteins leads to suppression of c-MYC expression. BET proteins fall into the category of epigenetic “readers” because they contain tandem protein modules (bromodomains) that “read” (i.e., bind to) acetylated lysines in histone tails. BRD4, a member of the BET family, facilitates transcription of acetylated chromatin via its capacity to recruit protein complexes, including the mediator complex and the super elongation complex. Through this mechanism, BRD4 contributes to malignant transformation by increasing expression of oncogenes such as c-MYC. In the current article, Dr. Sally Trabucco and colleagues from the University of Massachusetts School of Medicine studied JQ1, a small molecule inhibitor of BRD4 and other bromodomain proteins.
First, a panel of 11 human DLBCL cells lines (four classified by gene expression profiling as activated B-cell subtype [ABC] and seven as germinal center [GCB] B-cell subtype) were treated with JQ1. At doses between 25 and 160 nM, cell proliferation was inhibited by at least 50 percent in all cell lines compared with DMSO-treated controls. By evaluating cell-cycle distribution and caspase 3/7 activity, the authors determined that cell lines underwent either apoptosis or prolonged cell cycle arrest without cell death in response to the drug. In addition, in the cell lines displaying senescence, growth arrest persisted even after the drug was discontinued.
Mutational status did not predict response to the BET inhibitor. Treatment with JQ1 resulted in decreased levels of c-MYC protein expression in 10 of 11 cell lines, regardless of whether the cell line had normal c-MYC loci, c-MYCtranslocation, or c-MYC gene amplification. In an attempt to distinguish further between JQ1 induced apoptosis versus senescence among the cell lines, the authors measured levels of the antiapoptotic protein BCL-XL; cell cycle regulators p16, p21, and Rb; and p53 in response to treatment with JQ1. No changes in BCL-XL were detected; p16, p21, and Rb levels fell or were not measurable; and p53 protein levels were not affected. Thus, the apoptosis vs. senescence phenotype was not distinguishable based on the parameters examined in this study.
Lastly, the effect of JQ1 in DLBCL murine xenografts was examined using both subcutaneous and intraperitoneal injection of tumor cells derived from one of the ABC cell lines. Compared with vehicle-treated controls, the mice receiving JQ1 for 21 days experienced a moderate, yet statistically significant, increase in survival time in both models.
In Brief
The studies of Dr. Trabucco and colleagues suggest that targeting BRD proteins with JQ1 in both ABC and GCB subtypes of DLBCL cell lines results in c-MYC–dependent cell cycle arrest or cell death. The JQ1-mediated decrease in the levels of c-MYC protein expression was independent of the molecular subtype of c-MYC (normal, amplified, or rearranged). In patients with double- or triple-hit lymphomas and in those with concurrent overexpression of c-MYC and BCL-2, the outcome of treatment with both standard and high-dose chemotherapy is uniformly dismal. Arguably, these subsets of DLBCL represent the highest unmet clinical need in lymphoma management. Although single-agent therapy is unlikely to result in significant improvements in outcome, targeting BRD proteins with drugs such as JQ1 in combination with chemoimmunotherapy or other novel agents holds exciting promise for improving outcomes in poor-prognosis DLBCL.
Competing Interests
Dr. LaCasce indicates no relevant conflicts of interest.
