Background

Research in the epigenetic field has recently highlighted the central role of BRD4 in cancer progression. BRD4 is an acetyl-lysine reader of the BET (bromodomain and extraterminal domain) family (Dey et al., 2003; Filippakopoulos et al., 2012; Wang et al., 2012) able to bind to acetylated histones at promoter and enhancer regions (Dey et al., 2003; Filippakopoulos et al., 2012; Nagarajan et al., 2014). The mechanism of action of this epigenetic reader consists in the activation of gene promoters and enhancers by recruiting several transcription factors, cofactors and RNA polymerase II (RNApol II), which results in modulating, mostly enhancing, the transcription of certain target genes. The BRD4-histone module has been described to play a key role regulating cell cycle progression (Dey et al., 2003; Wu and Chiang, 2007; Yang et al., 2008; Devaiah and Singer, 2013) and genomic structure and stability (Wu and Chiang, 2007; Floyd et al., 2013); for those reasons, BRD4 has frequently been associated with cancer development and progression (Yang et al., 2008; Zuber et al., 2011; Nagarajan et al., 2014; Wu et al., 2015).

Chromatin reporter cell lines have been already developed in order to identify modulators of position effect variegation (Tchasovnikarova et al., 2015) or to discover new chromatin-targeting compound (Johnson et al., 2008; Best et al., 2011; Wang et al., 2013). In contrast to previous approaches, we wanted to develop a protocol for the generation of a reporter cell line able to monitor the BRD4-dependent heterochromatization of a generic reporter. To achieve that, we used a common retroviral vector carrying an RFP (Red Fluorescent Protein) gene, and selected clones that integrated it in fully repressed genomic regions specifically reactivated by BRD4 inhibition. The haploid nature of the cell line used (KBM7 [Andersson et al., 1995]), makes the reporter easily amenable not only to chemical screens, but also to genetic screens. Both methods can be used for the identification of new BRD4 direct and functional partners, and results from these approaches will provide further insights into BRD4 biology.