Specific Peptide Disruption of the Bcl-6 Repression Complex Reveals Its Transcriptional Mechanism of Action in Normal and Malignant B-Cells and Is a Novel Therapeutic Approach for Diffuse Large B-Cell Lymphoma.
Bibliographic record
Abstract
Abstract The Bcl-6 transcriptional repressor is required for the establishment of germinal centers during B-cell maturation. Regulatory elements of the BCL6 gene are frequently mutated in diffuse large B-cell lymphomas (DLBCL), leading to inappropriately timed expression of Bcl-6. However, it is unknown whether Bcl-6 is involved in lymphomagenesis. Within Bcl-6, the N-terminal BTB domain is involved in mediating transcriptional repression. We investigated the mechanism of action of the Bcl-6 BTB domain using X-ray crystallography and functional assays. We found that the SMRT, N-CoR and BCoR corepressors bind directly to Bcl-6 through a 16 residue “BBD motif”, which fits into a highly specific “lateral groove” surface feature of the Bcl-6 BTB domain. To determine the contribution of BTB lateral groove recruitment of corepressors to transcriptional and biological functions of Bcl-6, we engineered small Bcl-6 BTB peptide inhibitors (BPI). BPI penetrate cells and localize to the nuclei where they specifically bound to Bcl-6 and blocked its interaction and co-localization with co-repressors. This resulted in dose-dependent blockade of repression by Bcl-6 in reporter assays and reactivation of endogenous Bcl-6 target genes in Bcl-6 expressing B-cells measured by real-time PCR. Loss of repression was caused by disruption of the endogenous Bcl-6 transcriptional repression complex. We found that BPI specifically excluded corepressors from the promoters of endogenous Bcl-6 target genes, resulting in a switch from repressed to activated histone code settings with consequent gene reactivation. From the biological standpoint, BPI injection into mice reproduced the B-cell phenotype of Bcl-6 null animals, as germinal center formation in response to T-cell dependent antigens was abrogated. To address the question of whether Bcl-6 is required to maintain the malignant phenotype of DLBCL, a panel of Bcl-6 positive and negative DLBCL cells were exposed to BPI. BPI induced growth suppression, cell cycle arrest and apoptosis in Bcl-6 positive but not negative DLBCL cells. Expression array analysis showed immediate upregulation of checkpoint genes involved in proliferation and apoptosis, allowing us to identify novel direct Bcl-6 target genes. In contrast, BPI did not upregulate differentiation related genes and did not induce differentiation in lymphoma cells, indicating that the Bcl-6 BTB domain lateral groove controls proliferation and survival pathways but not differentiation. Finally, BPI completely suppressed growth of Bcl-6 positive human DLBCL xenotransplants without any toxicity to other organs. In summary, specific disruption of the Bcl-6 BTB domain repression mechanism allowed us to reveal the contribution of specific corepressor recruitment to the activities of Bcl-6 in B-cells and to demonstrate that Bcl-6 is in fact an oncogene, required to maintain the malignant phenotype of Bcl-6 positive DLBCL. Our pre-clinical studies indicate that Bcl-6 is a bona fide therapeutic target and that BPI is a specific, potent and non-toxic anti-lymphoma therapeutic agent.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".