Specific interactions of BCL-2 family proteins mediate sensitivity to BH3-mimetics in diffuse large B-cell lymphoma.

The BCL-2 specific inhibitor, venetoclax/ABT-199 has exhibited remarkable clinical activity in nearly all cases of chronic lymphocytic leukemia. In contrast, responses are usually much less in diffuse large B-cell lymphoma, despite high level expression of BCL-2 in over 40% of cases, indicating that co-expression of related anti-apoptotic BCL-2 family proteins may limit activity. We have investigated the roles of the BCL-2 proteins in diffuse large B-cell lymphoma cells using a panel of specific BCL-2 Homology 3 mimetics and identified subgroups of diffuse large B-cell lymphoma cells that exhibited marked and specific dependency on either BCL-2, BCL-XL or MCL-1 for survival. Dependency was associated with a sequestration of the pro-apoptotic proteins BIM, BAX and BAK selectively by the specific anti-apoptotic BCL-2 protein that was important for cellular survival. Sensitivity to BCL-2 Homology 3 mimetics was independent of genetic alterations involving the BCL-2 family and only partially correlated with protein expression levels. Treatment with ABT-199 displaced BAX and BIM from BCL-2, leading subsequently to BAK activation and apoptosis. In contrast, apoptosis induced by inhibiting BCL-XL with A1331852 was associated with a displacement of both BAX and BAK from BCL-XL and occurred independently of BIM. Finally, the MCL-1 inhibitor S63845 induced mainly BAX-dependent apoptosis mediated by a displacement of BAK, BIM and NOXA from MCL-1. In conclusion, our study indicates that in diffuse large B-cell lymphoma, the heterogeneous response to BCL-2 Homology 3 mimetics is mediated by selective interactions between BAX, BAK and anti-apoptotic BCL-2 proteins.