Abstract
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 knock-out 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.
Highlights
Diffuse large B-cell lymphoma (DLBCL) is an aggressive and genetically diverse B-cell neoplasm in adults resulting in a biologically and clinically heterogeneous disease
Pharmacokinetic properties, limit the use of these B-cell lymphoma 6 (BCL6)-degrading compounds in vivo, such that the effect of BCL6 degradation on in vivo growth of lymphoma cells cannot be studied. 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
We performed gene knock-out studies using the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) system to address the dependency of different DLBCL cell lines on BCL6 (Figure 1)
Summary
DLBCL is an aggressive and genetically diverse B-cell neoplasm in adults resulting in a biologically and clinically heterogeneous disease. Standard of care treatment, which includes a combination of chemotherapy and the monoclonal CD20 antibody rituximab (R-CHOP), results in an initial response but leads to disease recurrence in 30% of patients for whom there remains a high unmet medical need [1]. In a significant subset of lymphoid malignancies chromosomal translocations and mutations lead to BCL6 deregulation. Such genetic alterations include translocations that fuse its coding sequence to heterologous promoters [10], point mutations in BCL6 promoter negative regulatory elements [11, 12] or mutations that affect BCL6 transcription [13], acetylation-mediated BCL6 inactivation [14] or BCL6 degradation [15]
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