For more than a decade, scientists have been testing a class of cancer drugs known as BET inhibitors with great promise. The science behind them seemed strong. Many tumors depend on oncogenes being switched on with the help of ‘bromo-terminal and distal end domain’ (BET) proteins, so it was hoped that blocking these proteins would slow cancer growth. In laboratory experiments, this approach often worked well. But in real patients, the effects were less impressive: efficacy was modest, side effects were noticeable, and there was no reliable way to predict who would respond.
Now, researchers at the Max Planck Institute for Immunobiology and Epigenetics (MPI-IE) in Freiburg believe they have uncovered an important reason for this gap between theory and reality. Their findings also point to more precise ways to design future treatments.
Reconsidering BET proteins as drug targets
BET inhibitors were designed to block a common function that all BET proteins use to bind to chromatin, the tightly packed structure of DNA and proteins where genes are stored and regulated. The idea was simple and clear. By preventing these proteins from binding to chromatin, we can shut down the mechanisms that activate cancer-causing genes.
This strategy is based on the basic premise that all BET proteins behave similarly. New research from Asifa Akhtar’s lab suggests that that assumption doesn’t hold true. This study shows that two important BET proteins, BRD2 and BRD4, actually perform different tasks at separate stages of gene activation.
BRD4 is involved later in the process. It helps release RNA polymerase II, an enzyme that actively transcribes genes. Most current treatments focus on this stage. In contrast, BRD2 functions earlier, helping to assemble and organize the molecular components needed to initially initiate transcription.
The molecular “stage manager” behind gene activation
Because BRD2 and BRD4 act at different times, blocking both at the same time, as many current drugs do, prevents multiple steps of gene activation. This can have unpredictable and context-dependent effects.
“Think of gene activation like a stage production. BRD2 sets the stage, assembling the props, costumes, and actors to ensure the preparations go smoothly. BRD2 then sends the ‘start’ signal to the actor, BRD4, to begin acting,” says Asifa Akhtar, who led the study at MPI-IE. “Previous research has focused almost entirely on performance. Our data show that the setup work that was done earlier is just as important for gene activation,” explains Asifa Akhtar.
For many years, BRD2 was considered less important than BRD4. New discoveries call that view into question. One reason is how BRD2 responds to intracellular signals. The enzyme MOF places a chemical tag called histone acetylation on the chromatin. These marks act like a guidance system, indicating which genes to activate and where BRD2 should start its work.
BRD2 is particularly sensitive to these “bookmarks.” When MOF is removed, BRD2 no longer remains bound to chromatin, whereas other BET proteins are largely unaffected. “This finding supports the model that acetylated chromatin creates a platform that concentrates regulatory proteins like BRD2 and allows them to prime the transcriptional machinery when needed,” says lead author Umut Erdogdu from Akhtar’s lab.
The role of clustering in gene regulation
BRD2 not only recognizes these signals but also helps organize the physical layout of the transcription machinery. They form clusters at gene sites and bring together the components needed to initiate transcription exactly where they are needed.
“To understand the importance of clustering in gene transcription, we removed only the specific parts of BRD2 involved in clustering, while leaving the rest of the protein intact,” explains Umut Erdogdu.
The results were dramatic. Even though BRD2 was still present in the nucleus, gene transcription was almost as slow as when the entire protein was removed. “This shows that clustering is not a side effect, but a functional feature of transcriptional regulation, and like a stage manager, BRD2 ensures that all performers and all equipment are in the right position before the curtain rises,” says Asifa Akhtar.
Aiming for more accurate cancer treatment
These insights suggest new directions for anticancer drug development. Future therapeutics could focus on the distinct roles of BRD2 and BRD4, rather than broadly blocking all BET proteins through their common chromatin binding ability.
By targeting these proteins more selectively, researchers may be able to develop more effective and predictable treatments. Understanding how each protein contributes to gene activation may help refine strategies that are compatible with the biology of different cancers.
Important points
- Why some cancer drugs are inadequate: Researchers at the Max Planck Institute for Immunobiology and Epigenetics have uncovered why BET inhibitors, despite early promise, have not performed as well in clinical trials.
- Two proteins, two different jobs: The study revealed that the BET proteins BRD2 and BRD4 play different roles in turning on genes. This critical difference helps explain why targeting them together does not work as expected.
- The path to better treatment: Most current drugs block both proteins simultaneously. New findings suggest that targeting BRD2 and BRD4 more precisely may lead to more effective and predictable cancer treatments.
