A study from the Max Planck Institute of Immunobiology and Epigenetics (MPI-IE) in Freiburg, announced on Apr. 9, offers new insight into why a class of drugs known as BET inhibitors have not met expectations in cancer trials.
BET inhibitors were designed to block proteins that help activate oncogenes, with the hope of slowing tumor growth. While these drugs showed promise in laboratory settings, their performance in patients has been limited, with few positive responses and notable side effects.
The research led by Asifa Akhtar at MPI-IE reveals that two key proteins targeted by these drugs—BRD2 and BRD4—play different roles during gene activation. BRD4 is responsible for releasing RNA Polymerase II to start active transcription, which is the focus of most current therapies. In contrast, BRD2 acts earlier by organizing the molecular machinery needed to begin transcription. "Think of gene activation like stage production. BRD2 sets up the stage: assembling the props, costumes and actors to ensure preparations run smoothly. BRD2 then gives BRD4, the actor, the 'start' signal to begin with the performance," said Akhtar.
The study suggests that blocking both proteins at once disrupts multiple steps in gene activation and can lead to unpredictable results depending on context. Akhtar added: "Previous studies had been focused almost entirely on the performance. Our data shows that the setup work happening before is just as critical for gene activation." The research also found that an enzyme called MOF places chemical tags on chromatin which guide BRD2's actions; without MOF's tags, BRD2 cannot function properly while other BET proteins are less affected.
Umut Erdogdu from Akhtar’s lab said: "The findings support a model in which acetylated chromatin creates a platform that allows regulatory proteins like BRD2 to concentrate and prepare the transcription machinery for when it will be needed." The study further discovered that even when present inside cell nuclei but unable to form clusters at gene sites due to missing bookmarks or clustering ability, BRD2 could not support normal transcription activity.
Akhtar concluded: "This demonstrates that clustering is not a side effect, but a functional feature of transcription regulation. And like a stage manager, BRD2 ensures that every performer and every piece of equipment is in place before the curtain rises." These findings suggest future therapies may need more selective approaches targeting specific functions within BET protein family members.
