Oncotarget | Inducible knock-out of BCL6 in lymphoma cells results in tumor stasis

The cover for issue 9 of Oncotarget features Figure 6, "BCL6 knock-out in a DLBCL xenograft induces tumor stasis," by Schlager, et al.

Read more: 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. 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.

Dr. Manfred Koegl from Boehringer Ingelheim RCV GmbH & Co KG, Vienna, Austria said, "DLBCL is an aggressive and genetically diverse B-cell neoplasm in adults resulting in a biologically and clinically heterogeneous disease."

"DLBCL is an aggressive and genetically diverse B-cell neoplasm in adults resulting in a biologically and clinically heterogeneous disease"

- Dr. Manfred Koegl, Boehringer Ingelheim RCV GmbH & Co KG

Such genetic alterations include translocations that fuse its coding sequence to heterologous promoters, point mutations in BCL6 promoter negative regulatory elements or mutations that affect BCL6 transcription, acetylation-mediated BCL6 inactivation or BCL6 degradation.

Constitutive BCL6 expression within GC B-cells leads to the development of DLBCL in mice that mimics that observed in patients suggesting that BCL6 is sufficient to initiate cancer.

A variety of BCL6 inhibitors have been previously reported, several of which have demonstrated that the BTB domain of BCL6 is amenable to targeting with peptide and small molecule inhibitors as well as PROTACs.

Figure 6: BCL6 knock-out in a DLBCL xenograft induces tumor stasis. Tumor xenografts were established in C.B-17 SCID mice by subcutaneous injection of inducible SU-DHL-4 Cas9 BCL6 and control sgRNA cells. Mice were randomized to receive drinking water with DOX (2 mg/kg) plus 5% sucrose (DOX on) or 5% sucrose only (DOX off). (A) After 5 days DOX treatment tumors from four mice were harvested and analyzed for Cas9 GFP induction using flow cytometry. Cas9-GFP-induced cells are indicated in green, non-induced cells in red. (B–E) Tumor-bearing mice were treated with DOX for 8 days after which tumors from control and BCL6 knock-out tumors were harvested 17/20 days after start of DOX treatment, respectively. Tumor volumes from (B) BCL6 sgRNA tumors (n = 10 DOX off, n = 7 DOX on) and (C) control (n = 10 DOX off, n = 8 DOX on) were measured. ^*p < 0.05; ^***p ≤ 0.001. (D) Tumor BCL6 protein levels were determined using IHC analysis. Representative images of BCL6 IHC staining in SU-DHL-4 tumors are shown. Scale bars 100 μm. (E) Quantification of BCL6 positive cells in SU-DHL-4 BCL6 sgRNA tumor sections after vehicle (DOX off) and DOX treatment (5 days and 20 days after start of DOX treatment). Data are shown as means ± SD relative to DOX off (n = 4 – 10)

Importantly, we found that the anti-proliferative activity of BCL6 degraders such as BI-3802 on tissue culture cells is generally higher than that of BCL6 inhibitors despite their equipotent BCL6 binding affinities.

Addressing this question, we report on the establishment of an inducible BCL6 knock-out DLBCL model, which allows studying the phenotype of BCL6 loss in DLBCL xenografts in vivo.

The Koegl research team concluded in their Oncotarget Research Paper, "our findings have important implications for understanding the impact of BCL6-targeted therapies in DLBCL. According to our studies, it is reasonable to predict that the treatment of DLBCL with BCL6 degraders results in significant tumor growth inhibition and at least tumor stasis. The observed magnitude of effects of BCL6 blockade in monotherapy might provide a rationale for therapeutic combinations with other targeted and/or chemotherapeutic agents. Our CRISPR/Cas9 BCL6 knock-out model represents a valuable pre-clinical tool to evaluate such combination approaches."

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DOI - https://doi.org/10.18632/oncotarget.27506

Full text - https://www.oncotarget.com/article/27506/text/

Correspondence to - Manfred Koegl - manfred.koegl@boehringer-ingelheim.com

Keywords - BCL6, DLBCL, lymphoma, inducible CRISPR/Cas9, in vivo xenograft

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