Molecular cloning of the BCL-6 gene, a transcriptional repressor for B-cell differentiation, in torafugu (Takifugu rubripes)

B-cell lymphoma-6 (BCL-6) is a transcriptional repressor that prevents the terminal differentiation of mature B-cells to plasma cells, and is essential for germinal center formation in the primary lymphoid organs of mammals. In this study, we identified the BCL-6 gene in torafugu (Takifugu rubripes) using the torafugu genome database, and analyzed the expression of BCL-6 mRNA in various tissues of torafugu, using RT-PCR. The BCL-6 gene consisted of eight exons and seven introns spanning a genome of ca. 3.3 kb. BCL-6 mRNA contained a 2112 bp open reading frame encoding 703 amino acids, with a predicted protein size of 78.8 kDa. The predicted torafugu BCL-6 primary structure contains two conserved specific motifs, the BTB/POZ domain at the N-terminus and the sixC2H2-type zinc finger motifs at the C-terminal region. The homology of torafugu BCL-6 to those of zebrafish (Danio rerio), Xenopus laevis, mouse (Mus musculus) and human (Homo sapiens) is 76, 59, 60 and 60%, respectively. RT-PCR analysis revealed that BCL-6 mRNA is highly expressed in pronephros, thymus, intestine, ovary, brain, nasal cavity and muscle. These results imply that torafugu BCL-6 is involved in regulation of B-cell differentiation in torafugu.

Backbone resonance assignment of the BCL6-BTB/POZ domain

BCL6 is a transcriptional repressor. Two domains of the protein, the N-terminal BTB-POZ domain and the RD2 domain are responsible for recruitment of co-repressor molecules and histone deacetylases. The BTB-POZ domain is found in a large and diverse range of proteins that play important roles in development, homeostasis and neoplasia. Crystal structures of several BTB-POZ domains, including BCL6 have been determined. The BTB-POZ domain of BCL6 not only mediates dimerisation but is also responsible for recruitment of co-repressors such as SMRT, NCOR and BCOR. Interestingly both SMRT and BCOR bind to the same site within the BCL6 BTB-POZ domain despite having very different primary sequences. Since both peptides and small molecules have been shown to bind to the co-repressor binding site it would suggest that the BTB_POZ domain is a suitable target for drug discovery. Here we report near complete backbone ¹⁵N, ¹³C and ¹H assignments for the BTB-POZ domain of BCL6 to assist in the analysis of binding modes for small molecules.

BTBBCL6 dimers as building blocks for reversible drug-induced protein oligomerization

Here, we characterize the BTB domain of the transcription factor BCL6 (BTBBCL6) as a small-molecule-controlled, reversible oligomerization switch, which oligomerizes upon BI-3802 treatment and de-oligomerizes upon addition of BI-3812. We show that the magnitude of oligomerization can be controlled in vitro by BI-3802 concentration and exposure time. In cellular models, exposure to BI-3802/BI-3812 can drive multiple cycles of foci formation consisting of BTBBCL6 fused to EGFP, which are not degraded due to the lack of a degron. We generated an epidermal growth factor receptor (EGFR)-BTBBCL6 fusion. Treatment with BI-3802, as an ON switch, induced EGFR-BTBBCL6 phosphorylation and activation of downstream effectors, which could in part be reversed by the addition of BI-3812, as an OFF switch. Finally, BI-3802-induced oligomerization of the EGFR-BTBBCL6 fusion enhanced proliferation of an EGF-dependent cell line in absence of EGF. These results demonstrate the successful application of small-molecule-induced, reversible oligomerization as a switch for synthetic biology.

Discovery of an In Vivo Chemical Probe for BCL6 Inhibition by Optimization of Tricyclic Quinolinones

B-cell lymphoma 6 (BCL6) is a transcriptional repressor and oncogenic driver of diffuse large B-cell lymphoma (DLBCL). Here, we report the optimization of our previously reported tricyclic quinolinone series for the inhibition of BCL6. We sought to improve the cellular potency and in vivo exposure of the non-degrading isomer, CCT373567, of our recently published degrader, CCT373566. The major limitation of our inhibitors was their high topological polar surface areas (TPSA), leading to increased efflux ratios. Reducing the molecular weight allowed us to remove polarity and decrease TPSA without considerably reducing solubility. Careful optimization of these properties, as guided by pharmacokinetic studies, led to the discovery of CCT374705, a potent inhibitor of BCL6 with a good in vivo profile. Modest in vivo efficacy was achieved in a lymphoma xenograft mouse model after oral dosing.