Selective inhibition of BET bromodomains

Epigenetic proteins are intently pursued targets in ligand discovery. So far, successful efforts have been limited to chromatin modifying enzymes, or so-called epigenetic ‘writers’ and ‘erasers’. Potent inhibitors of histone binding modules have not yet been described. Here we report a cell-permeable small molecule (JQ1) that binds competitively to acetyl-lysine recognition motifs, or bromodomains. High potency and specificity towards a subset of human bromodomains is explained by co-crystal structures with bromodomain and extra-terminal (BET) family member BRD4, revealing excellent shape complementarity with the acetyl-lysine binding cavity. Recurrent translocation of BRD4 is observed in a genetically-defined, incurable subtype of human squamous carcinoma. Competitive binding by JQ1 displaces the BRD4 fusion oncoprotein from chromatin, prompting squamous differentiation and specific antiproliferative effects in BRD4-dependent cell lines and patient-derived xenograft models. These data establish proof-of-concept for targeting protein–protein interactions of epigenetic ‘readers’, and provide a versatile chemical scaffold for the development of chemical probes more broadly throughout the bromodomain family. Small molecules that perturb chromatin proteins are an emerging focus of current biomedical research. Two groups reporting in this issue have targeted bromodomain-containing BET proteins that bind acetylated lysine residues during gene activation, arriving at cell-permeable small molecule compounds with similar structures based on fused triazole-diazepine rings. James Bradner and colleagues report the development of a compound named JQ1. The BET protein BRD4, with two bromodomains, is implicated in human squamous cell carcinoma. JQ1 inhibits the growth of BRD4-dependent tumours in mouse models. Alexander Tarakhovsky and colleagues' inhibitor, I-BET, is shown to interfere with the binding of certain BET family members to acetylated histones. It inhibits activation of pro-inflammatory genes in macrophages and has immunomodulatory activity in a mouse model of inflammatory disease. A new approach is used to target BET family bromodomains which are found in transcriptional regulators where they mediate the recognition of acetyl-lysine chromatin marks. Structural data reveal how the compound JQ1 binds to the bromodomain of BRD4. BRD4 has been implicated in a subtype of human squamous carcinomas, and JQ1 is found to inhibit the growth of BRD4 dependent tumours in mouse models.