The BCL2 family: from apoptosis mechanisms to new advances in targeted therapy

Abstract

The B cell lymphoma 2 (BCL2) protein family critically controls apoptosis by regulating the release of cytochrome c from mitochondria. In this cutting-edge review, we summarize the basic biology regulating the BCL2 family including canonical and non-canonical functions, and highlight milestones from basic research to clinical applications in cancer and other pathophysiological conditions. We review laboratory and clinical development of BH3-mimetics as well as more recent approaches including proteolysis targeting chimeras (PROTACs), antibody-drug conjugates (ADCs) and tools targeting the BH4 domain of BCL2. The first BCL2-selective BH3-mimetic, venetoclax, showed remarkable efficacy with manageable toxicities and has transformed the treatment of several hematologic malignancies. Following its success, several chemically similar BCL2 inhibitors such as sonrotoclax and lisaftoclax are currently under clinical evaluation, alone and in combination. Genetic analysis highlights the importance of BCL-X_L and MCL1 across different cancer types and the possible utility of BH3-mimetics targeting these proteins. However, the development of BH3-mimetics targeting BCL-X_L or MCL1 has been more challenging, with on-target toxicities including thrombocytopenia for BCL-X_L and cardiac toxicities for MCL1 inhibitors precluding clinical development. Tumor-specific BCL-X_L or MCL1 inhibition may be achieved by novel targeting approaches using PROTACs or selective drug delivery strategies and would be transformational in many subtypes of malignancy. Taken together, we envision that the targeting of BCL2 proteins, while already a success story of translational research, may in the foreseeable future have broader clinical applicability and improve the treatment of multiple diseases.

Fig. 1

Milestones in the development of BH3-mimetics. a Overview of the anti-apoptotic BCL2 family proteins containing the BH domains and alpha helical structures α1-α9. b General interaction pattern within the BCL2 family, with the anti-apoptotic proteins inhibiting the pore-forming pro-apoptotic proteins BAX and BAK. The BH3-only proteins inhibit the anti-apoptotic BCL2 proteins and may also induce direct activation of BAX and BAK. c Timeline illustrating milestones and achievements in the discovery of BCL2 proteins and the development of BH3-mimetics. a and b Created in BioRender

Fig. 2

Overview of the BCL2 family and the regulation of cytochrome c release. a In unstressed healthy cells, several mechanisms, including canonical and non-canonical functions of BCL2 family members, prevent MOMP and release of cytochrome c from mitochondria, including the retrotranslocation of BAX/BAK and the limited availability of BH3-only proteins at mitochondrial membranes and preventing mitochondrial Ca²⁺ overload. b Upon cellular stress, BH3-only proteins become available at the mitochondria and facilitate BAX/BAK oligomerization, leading to MOMP. This is also facilitated by increased Ca²⁺ signaling including Ca²⁺ transfers from ER stores towards mitochondria. Created in BioRender

Fig. 3

Genetic alterations in cancer. Analysis of genetic modifications involving BCL2, BCL2L1 and MCL1 using cBioportal^– was performed. Four main pan-cancer studies using targeted deep sequencing and encompassing 71,060 samples were selected (MSK-IMPACT, Cancer Therapy and Clonal Hematopoesis, China Pan-Cancer and MSK MetTropism) and analyzed for genetic alterations involving BCL2, BCL2L1 or MCL1 in different cancer types. Thresholds were set for 100 samples / cancer type and a minimal frequency of 0.5%

Fig. 4

BCL2 family dependencies in cancer. Boxplots depicting the Chronos dependency scores of BCL2, BCL2L1 and MCL1 of cancer cell lines according to the DepMap data. Cancer subtypes (primary disease) with data available for n ≥ 5 cell lines were included. If multiple subtypes belonged to one lineage, the lineage was color-coded, the rest are shown in black

Fig. 5

Clinically developed BH3-mimetics. a Overview of current clinically tested BH3-mimetics targeting BCL2, BCL-X_L, MCL1 or multiple anti-apoptotic BCL2 proteins (created in Biorender). b Hydrophobic pockets P1, P2, P3 and P4 mapped onto the surface of the BCL2 structure (PDB-id: 1G5M). c Zoom into SS55746, navitoclax, venetoclax or sonrotoclax bound to BCL2 (PDB-id: 6GL8, PDB-id: 4LVT, PDB-id: 6O0K, PDB-id: 8HOG). d Overlay of venetoclax bound to BCL2 (light grey) and BCL2-G101V (yellow). Mutation of glycine 101 (light grey spheres) to valine (yellow spheres) effects conformational change (indicated by black arrow) of the adjacent E152 (side chains as sticks in light grey and yellow, respectively), pushing it towards the chlorine (green) of venetoclax and slightly displacing it. (PDBids: 6O0K and 6O0L). e 2D sketches of inhibitors shown in B-E, and pelcitoclax and lisaftoclax

Fig. 6

Prediction of BH3-mimetic responses. Linear regression analyses between cancer cell line response to BH3-mimetics from the PRISM Drug Repurposing Library and BCL2 RNA interference (green), BCL2 dependence score (pink) and BCL2 gene expression (blue). a Linear regression analyses between response to ABT-199 (venetoclax) and BCL2 (myeloid and lymphoid malignancies were not included in PRISM repurposing screening for ABT-199). b Linear regression analyses between response to ABT-263 (navitoclax) and BCL2. c Linear regression analyses between response to ABT-263 (navitoclax) and BCL2L1/BCL-X_L. d Linear regression analyses between response to S63845 and MCL1. The most significant correlation is a negative one between BCL2L1 expression and navitoclax response (c), however the R-squared value is only 0.06