The genomes of eukaryotic organisms are highly organised within the nucleus of the cell. The long strands of duplex DNA are wrapped around an octomer of histone proteins (most usually comprising two copies of histones H2A, H2B, H3 and H4) to form a nucleosome. This basic unit is then further compressed by the aggregation and folding of nucleosomes to form a highly condensed chromatin structure. A range of different states of condensation are possible, and the tightness of this structure varies during the cell cycle, being most compact during the process of cell division. Chromatin structure plays a critical role in regulating gene transcription, which cannot occur efficiently from highly condensed chromatin. The chromatin structure is controlled by a series of post translational modifications to histone proteins, notably histones H3 and H4, and most commonly within the histone tails which extend beyond the core nucleosome structure. These modifications include acetylation, methylation, phosphorylation, ubiquitinylation, SUMOylation. These epigenetic marks are written and erased by specific enzymes, which place the tags on specific residues within the histone tail, thereby forming an epigenetic code, which is then interpreted by the cell to allow gene specific regulation of chromatin structure and thereby transcription.
Histone acetylation is most usually associated with the activation of gene transcription, as the modification loosens the interaction of the DNA and the histone octomer by changing the electrostatics. In addition to this physical change, specific proteins recognise and bind to acetylated lysine residues within histones to read the epigenetic code. Bromodomains are small (˜110 amino acid) distinct domains within proteins that bind to acetylated lysine residues commonly but not exclusively in the context of histones. There is a family of around 50 proteins known to contain bromodomains, and they have a range of functions within the cell.
The BET family of bromodomain containing proteins comprises 4 proteins (BRD2, BRD3, BRD4 and BRDT) which contain tandem bromodomains capable of binding to two acetylated lysine residues in close proximity, increasing the specificity of the interaction. Numbering from the N-terminal end of each BET protein the tandem bromodomains are typically labelled Binding Domain 1 (BD1) and Binding Domain 2 (BD2) (Chung et al, J Med. Chem, 2011, 54, 3827-3838).
Chan et al. report that BET bromodomain inhibition suppresses transcriptional responses to cytokine-Jak-STAT signalling in a gene-specific manner in human monocytes, which suggests that BET inhibition reduces inflammation partially through suppression of cytokine activity. (Chan et al., Eur. J. Immunol., 2015, 45: 287-297).
Klein et al. report that the bromodomain protein inhibitor I-BET151 suppresses expression of inflammatory genes and matrix degrading enzymes in rheumatoid arthritis synovial fibroblasts, which suggests a therapeutic potential in the targeting of epigenetic reader proteins in rheumatoid arthritis. (Klein et al., Ann. Rheum. Dis., 2014, 0:1-8).
Park-Min et al. report that I-BET151 that targets bromo and extra-terminal (BET) proteins that ‘read’ chromatin states by binding to acetylated histones, strongly suppresses osteoclastogenesis. (Park-Min et al. Nature Communications, 2014, 5, 5418).