BH3-mimetic toolkit guides the respective use of BCL2 and MCL1 BH3-mimetics in myeloma treatment: Disease progression favors MCL1 priming in myeloma

BH3-mimetics are promising drugs for hematologic malignancies that trigger cell death by promoting the release of pro-apoptotic BCL2 family members from anti-apoptotic proteins. Multiple myeloma is considered to be a disease dependent mainly on MCL1 for survival based mostly on studies using cell lines. We used a BH3-mimetic toolkit to study the dependency on BCL2, BCLXL or MCL1 in malignant plasma cells from 60 patients. Dependencies were analyzed using an unbiased BH3-mimetics cell-death clustering by k-means. In the whole cohort of patients, BCL2 dependency was mostly found in the CCND1 subgroup (83%). Of note, MCL1 dependence significantly increased from 33% at diagnosis to 69% at relapse, suggesting a plasticity of the cellular dependency favoring MCL1 dependencies at relapse. In addition, 35% of overall patient samples showed co-dependencies on either BCL2/MCL1 or BCLXL/MCL1. Finally, we identified a group of patients not targeted by any of the BH3-mimetics, predominantly at diagnosis in patients not presenting the common recurrent translocations. Mechanistically, we demonstrated that BAK is crucial for cell death induced by MCL1 mimetic A1210477, according to the protection from cell death observed by BAK knock-down as well as the complete and early disruption of MCL1/BAK complexes upon A1210477 treatment. Interestingly, this complex was also dissociated in A1210477 resistant cells, but free BAK was simultaneously recaptured by BCLXL, supporting the role of BCLXL in A1210477 resistance. In conclusion, our study opens the way to rationally use venetoclax and/or MCL1 BH3-mimetics for clinical evaluation in myeloma both at diagnosis and relapse.