The transcription factor BCL6 is a known driver of oncogenesis in lymphoid malignancies, including diffuse large B cell lymphoma (DLBCL). Disruption of its interaction with transcriptional repressors interferes with the oncogenic effects of BCL6. We used a structure-based drug design to develop highly potent compounds that block this interaction. A subset of these inhibitors also causes rapid ubiquitylation and degradation of BCL6 in cells. These compounds display significantly stronger induction of expression of BCL6-repressed genes and anti-proliferative effects than compounds that merely inhibit co-repressor interactions. This work establishes the BTB domain as a highly druggable structure, paving the way for the use of other members of this protein family as drug targets. The magnitude of effects elicited by this class of BCL6-degrading compounds exceeds that of our equipotent non-degrading inhibitors, suggesting opportunities for the development of BCL6-based lymphoma therapeutics.
Diffuse large B-cell lymphoma (DLBCL) is the most common type of non-Hodgkin lymphomas worldwide and is characterized by a high diversity of genetic and molecular alterations. Chromosomal translocations and mutations leading to deregulated expression of the transcriptional repressor BCL6 occur in a significant fraction of DLBCL patients. An oncogenic role of BCL6 in the initiation of DLBCL has been shown as the constitutive expression of BCL6 in mice recapitulates the pathogenesis of human DLBCL. However, the role of BCL6 in tumor maintenance remains poorly investigated due to the absence of suitable genetic models and limitations of pharmacological inhibitors. Here, we have utilized tetracycline-inducible CRISPR/Cas9 mutagenesis to study the consequences of BCL6 deletion in established DLBCL models in culture and in vivo. We show that BCL6 knockout in SU-DHL-4 cells in vitro results in an anti-proliferative response 4-7 days after Cas9 induction that was characterized by cell cycle (G1) arrest. Conditional BCL6 deletion in established DLBCL tumors in vivo induced a significant tumor growth inhibition with initial tumor stasis followed by slow tumor growth kinetics. Our findings support a role of BCL6 in the maintenance of lymphoma growth and showcase the utility of inducible CRISPR/ Cas9 systems for probing oncogene addiction. Recent comprehensive sequencing studies in a large cohort of DLBCL patients highlight the heterogeneity of alterations including somatic mutations, copy number alterations, and structural variants [2-4]. Among the most frequently rearranged genes are IGH, BCL2, BCL6, and MYC, with 40%, 21%, 19%, and 8% of cases affected, respectively [5-8]. BCL6 is a DNA-binding protein that represses gene transcription in Germinal Center (GC) B-cells through the recruitment of corepressor proteins. In GCs, BCL6 inhibits DNA damage response pathways and thereby prevents cell cycle arrest and apoptosis during class switch recombination and somatic hypermutation required for antibody maturation in B-cells. Subsequent BCL6 downregulation is crucial
Background: Engagement of Tumor Necrosis Factor-α (TNF-α) with its receptor can lead to dramatically different cellular outcomes ranging from regulating cell survival and inflammation to induction of programmed forms of cell death. A critical proximal checkpoint determining the nature of TNF-α signaling is put in place by the cellular inhibitor of apoptosis proteins (cIAPs). In the context of cancer therapy these constitute an attractive target as they (1) block the TNF-α induced activation of apoptotic/necroptotic cues and (2) are negatively regulated by a highly selective endogenous ligand (i.e. SMAC), which served as a blueprint for the development of small molecule inhibitors of IAP (so called SMAC mimetics). Methods: Here we investigated the efficacy of SMAC mimetic BI891065 in enhancing targeted and chemotherapeutic approaches in preclinical mouse cancer models and describe immune-modulatory effects in syngeneic settings. To identify responding indications, a large pan-cancer cell line panel screening comprising 246 cell lines was performed (Eurofins). Proliferation of cells treated with increasing concentrations of BI 891065 combined with a fixed concentration of TNF-α was assessed by high-content screening. Furthermore, to gain a better understanding of the molecular determinants associated with sensitivity to SMAC mimetic treatment, genome-wide CRISPR/Cas9 drug modifier screens were performed. Results: Here we present key data demonstrating antitumor activity of BI891065 in preclinical models, our efforts towards understanding of genetic determinants of SMAC sensitivity and of potential responsive indications. By using genome-wide CRISPR/Cas9 drug modifier screens we not only demonstrated the feasibility of such unbiased approaches, as we identified many known (e.g. TNF Receptor 1, RIPK1, Caspase 8 and members of the NFκB signaling pathways) - but also potentially novel - regulators of TNF-α/SMAC mimetic induced cell death. In addition, to identify potential responsive indications to BI891065, extensive profiling of in vitro drug sensitivity across a large set of cancer cell types was performed. As a result of this, colorectal cancer (n=56) was identified as a promising indication: 5% of cell lines were found to be sensitive to BI 891065 single treatment. This could be further extended by the exogenous supply of TNF-α to BI 891065, increasing the number of sensitive cells to 21%. Conclusion: The presented data demonstrate the potential of BI 891065 to facilitate tumor cell death and to enhance anti-tumor immune responses, and nominate the compound as an attractive combination partner in cancer therapy. Our results led to the identification of potentially novel modulators of SMAC mimetic sensitivity via genome-wide CRISPR/Cas9 drug sensitizer screens and suggest colorectal cancer as a promising indication for clinical positioning. Citation Format: Martin Aichinger, Valeria Santoro, Ksenija Slavic-Obradovic, Stefanie Ruhland, Andreas Wernitznig, Andrea Neudolt, Markus Schaefer, Sabine Kallenda, Daniel Zach, Sabine Olt, Carina Salomon, Sarah Rieser, Martina Weissenboeck, Florian Ebner, Andreas Schlattl, Melanie Talata De Almeida, Rebecca Langlois, Martina Sykora, Markus Reschke, Thomas Zichner, Daniel Gerlach, Julian Jude, Michaela Fellner, Dirk Scharn, Norbert Kraut, Juergen Moll, Johannes Zuber, Sebastian Carotta, Maria Antonietta Impagnatiello, Ulrike Tontsch-Grunt. Targeting IAP in cancer: BI 891065 a potent small molecule SMAC mimetic that synergizes with immune checkpoint inhibition [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6221.
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