Targeting only the first bromodomain of BRD4 is sufficient for displacement of BRD4 from chromatin.

  • Major finding: Targeting only the first bromodomain of BRD4 is sufficient for displacement of BRD4 from chromatin.

  • Concept: Selective inhibitors of mutant bromodomains can dissect roles of individual BET proteins.

  • Impact: Use of this approach may help determine which BET bromodomains represent therapeutic targets.

Small-molecule inhibitors of the bromo and extraterminal (BET) protein subfamily, which includes BRD2, BRD3, BRD4, and BRDT, have shown potent antitumor activity. However, although these inhibitors are selective for BET protein bromodomains compared with other bromodomains, they are not selective for individual BET family members, making it difficult to discern their mechanism of action in a given tumor type. Baud, Lin-Shiao, Cardote, and colleagues devised a strategy to specifically target individual bromodomains by mutating a conserved residue on an individual BET bromodomain to a smaller amino acid to create a “hole” that could accommodate pan-BET inhibitor derivatives with a sterically bulky “bump.” Such mutant proteins in which a conserved leucine residue required for hydrophobic contacts with pan-BET inhibitors was mutated to an alanine were stable and structurally and functionally similar to their wild-type counterparts. Introduction of a bump into the pan-BET inhibitors I-BET and JQ1 generated a compound that bound mutant bromodomains in individual BET subfamily members with significantly higher affinity than wild-type bromodomains. Fluorescence recovery after photobleaching assays with full-length BRD4 harboring mutations in either or both of its bromodomains showed that the mutant-selective inhibitor specifically displaced mutant BRD4 from chromatin, and that targeting the first bromodomain of BRD4 was sufficient for displacement. These findings illustrate the potential of this approach to provide insight into the biologic role of bromodomains within individual BET proteins and suggest that it may be possible to use such tool compounds to determine which BET proteins are the relevant therapeutic targets in a given cancer cell type.

Baud MG, Lin-Shiao E, Cardote T, Tallant C, Pschibul A, Chan KH, et al. A bump-and-hole approach to engineer controlled selectivity of BET bromodomain chemical probes. Science 2014 Oct 16 [Epub ahead of print].