Recently, bromodomain proteins, and BRD4 in particular, have received a significant degree of attention because of the discovery of two small molecules capable of disrupting the interaction between BRD bromodomains and acetylated histones. These compounds are designed to mimic acetylated histone tails in order to efficiently compete for binding to the BET but not other bromodomains.
One of the compounds, I-BET (GSK525762A), binds BET proteins with an affinity of about 55 nM and was initially shown to have anti-inflammatory potential through interference with inflammatory gene expression [6
]. However, the major potential for these compounds lies in targeting cancer because of the function of BRD4 in mitotic progression by marking genes for timely activation at the end of mitosis and in early G1 [5
] and, through its association with pTEFb, in enhancing the transcript levels of c-myc
and other oncogenes [4
]. The amino-terminal half of BRD4, including the bromodomains, is also found as a fusion with NUT (NUclear protein in Testis) as a result of a chromosomal translocation associated with squamous carcinoma [4
JQ1, the second prototypical BET domain inhibitor, has an affinity comparable to I-BET, and likewise displaces acetylated histone peptides from the BRD4 bromodomains and from fluorescently tagged versions of both BRD4 and the BRD4-NUT fusion from chromatin in cells [7
]. Proliferation of cells derived from BRD4-NUT-dependent midline carcinoma was diminished upon JQ1 treatment, leading to tumor regression and prolonged survival in mouse models [7
]. Several strands of evidence, from a number of patient-derived cancer cell lines and xenograft models, have further shown that JQ1 has potent antitumor efficacy on myc
-dependent cancer cells, and it has been proposed that this is due to reduced expression of c-myc
and its downstream transcriptional programs [8
]. An elegant, recent study from the Kouzarides laboratory uncovered interactions of BET proteins with mixed lineage leukemia (MLL) fusion proteins as part of the super elongation complex and the polymerase-associated factor (PAF) complex [10
]. BET inhibitors had strong antiproliferative efficacy against cell lines harboring MLL fusions and had profound benefits in mouse models of MLL [10
], expanding the therapeutic spectrum of BET inhibitors from midline carcinoma and c-myc-
dependent cancers to MLL fusion-driven cancers.
The finding by Zhao et al
. that BRD4 functions as a gene expression bookmark prompts the question of whether the mode of action of BET inhibitors extends to the disruption of gene bookmarking in addition to the impairment of transcriptional elongation. Zhao et al
. addressed this question by examining the impact of JQ1 on the output of their assay, finding that JQ1 treatment generally increased the mobility of BRD4 at actively transcribing reporter cassettes [2
]; this is in agreement with previous studies [7
] and indicative of a weakened interaction with chromatin. Notably, post-mitotic reactivation was markedly slowed down in the presence of JQ1, whereas interphase induction kinetics remained unaffected [2
]. These results are consistent with a pronounced impact of JQ1 (and likely other BET inhibitors) on gene bookmarking.