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Identification of resistance mechanisms for therapy of venetoclax-refractory high-risk CLLs

Applicant Dr. Lukas Frenzel
Subject Area Hematology, Oncology
Term from 2016 to 2021
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 226262100
 

Final Report Abstract

The BCL2 inhibitor venetoclax (VEN) has been approved to treat different hematological malignancies. Deciphering the evolution of cancer cells under therapeutic pressure is a crucial step to understand the mechanisms that lead to treatment resistance. To this end, we analyzed whole-exome sequencing data of eight chronic lymphocytic leukemia (CLL) patients that developed resistance upon BCL2-inhibition by VEN. Here, we report recurrent mutations in BTG1 (2 patients) and homozygous deletions affecting CDKN2A/B (3 patients) that developed during treatment, as well as a mutation in BRAF and a highlevel focal amplification of CD274 (PD-L1) that might pinpoint molecular aberrations offering structures for further therapeutic interventions. Since there is no common genetic alteration causing resistance to VEN in CLL and B cell lymphoma, we asked if epigenetic events might be involved in VEN resistance. Therefore, we employed whole exome sequencing, methylated DNA immunoprecipitation sequencing and genome wide CRISPR/Cas9 screening to investigate VEN resistance in aggressive lymphoma and high-risk CLL patients. We identified a regulatory CpG island within the PUMA promoter which is methylated upon VEN treatment, mediating PUMA downregulation on transcript and protein level. PUMA expression and sensitivity towards VEN can be restored by inhibition of methyltransferases. We can demonstrate that loss of PUMA results in metabolic reprogramming with higher OXPHOS and ATP production, resembling the metabolic phenotype that is seen upon VEN resistance. While PUMA loss is specific for acquired VEN resistance but not for acquired MCL1 resistance and is not seen in CLL patients after chemotherapy-resistance, BAX is key for sensitivity towards both VEN and MCL1 inhibition. As we found loss of BAX in Richter syndrome patients with TP53-loss after VEN failure, we defined BAX-mediated apoptosis to be critical for drug resistance but not for disease progression of CLL into aggressive DLBCL in vivo. A compound screen revealed TRAIL-mediated apoptosis as a target to overcome BAX deficiency and antibody or CAR redirected T cells eliminated VEN resistant lymphoma cells paving a clinically applicable way to overcome VEN resistance.

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