Apoptosis — programmed cell death — is the cell's ultimate tumour-suppressive mechanism, eliminating cells with irreparable DNA damage, oncogenic mutations, or aberrant signalling. The intrinsic (mitochondrial) apoptosis pathway is governed by a dynamic balance between anti-apoptotic BCL2-family proteins (BCL2, BCL-XL, MCL1) and pro-apoptotic counterparts (BAX, BAK, BIM, PUMA, NOXA). Cancer cells exploit this balance with extraordinary sophistication: BCL2 overexpression, MCL1 amplification, p53 mutation, and AKT hyperactivation each tip the scales toward survival, often creating targetable dependencies on specific anti-apoptotic proteins.
The decision to undergo intrinsic apoptosis is made at the outer mitochondrial membrane (OMM). Anti-apoptotic BCL2-family proteins (BCL2, BCL-XL, MCL1) sequester pro-apoptotic proteins in their hydrophobic BH3-binding groove. When stress signals exceed a threshold, BH3-only proteins (BIM, PUMA, BAD, NOXA) overwhelm BCL2/BCL-XL sequestration capacity, freeing BAX/BAK to oligomerise and form pores in the OMM. This mitochondrial outer membrane permeabilisation (MOMP) releases cytochrome c, triggering apoptosome formation and caspase cascade activation — a one-way door to cellular demolition.
The decision to undergo intrinsic apoptosis is made at the outer mitochondrial membrane (OMM). Anti-apoptotic BCL2-family proteins (BCL2, BCL-XL, MCL1) sequester pro-apoptotic proteins in their hydrophobic BH3-binding groove. When stress signals exceed a threshold, BH3-only proteins (BIM, PUMA, BAD, NOXA) overwhelm BCL2/BCL-XL sequestration capacity, freeing BAX/BAK to oligomerise and form pores in the OMM. This mitochondrial outer membrane permeabilisation (MOMP) releases cytochrome c, triggering apoptosome formation and caspase cascade activation — a one-way door to cellular demolition.
Intrinsic Apoptosis Pathway
Anti-apoptotic BCL2, BCL-XL, and MCL1 occupy the OMM via C-terminal transmembrane helices. Their hydrophobic BH3-binding grooves sequester pro-apoptotic BAX monomers and BH3-only proteins (BIM, PUMA, BAD, NOXA), preventing spontaneous mitochondrial pore formation.
Unsequestered BH3-only proteins and 'activator' proteins (tBID, BIM) directly contact BAX and BAK, inducing conformational changes that expose their transmembrane domains. BAX/BAK oligomerise into lipidic pores in the OMM — mitochondrial outer membrane permeabilisation (MOMP) — an irreversible commitment to apoptosis.
MOMP releases cytochrome c, SMAC/DIABLO, HTRA2/Omi, and AIF from the intermembrane space. Cytochrome c binds APAF1, which (with dATP) oligomerises into a heptameric wheel — the apoptosome. The apoptosome recruits and activates pro-caspase-9.
Active caspase-9 cleaves and activates effector caspases-3 and -7. Caspase-3 cleaves >500 cellular substrates including PARP1, lamins, β-actin, and CAD (caspase-activated DNase). CAD fragments nuclear DNA at internucleosomal linkers, producing the characteristic DNA ladder of apoptosis.
Constitutively active AKT phosphorylates BAD (Ser136), maintaining BAD–14-3-3 sequestration and preventing BCL2 displacement. AKT also degrades p53 via MDM2, suppressing transcription of PUMA and NOXA. PTEN loss therefore creates profound apoptotic resistance through constitutive AKT activity.
Some haematologic malignancies are strongly dependent on BCL2 because pro-apoptotic proteins are sequestered near the mitochondrial death threshold. Other tumours rely more on MCL1, BCL-XL or several proteins at once, and expression does not automatically reveal dependency. TP53 loss can alter stress-induced apoptosis in some contexts but does not make all TP53-mutant tumours broadly resistant to chemotherapy.
Venetoclax is a BCL2-directed BH3 mimetic with current FDA-labelled uses in defined CLL/SLL populations and, in combination, a defined newly diagnosed AML population; it is not FDA-approved for multiple myeloma. BH3 profiling measures mitochondrial response to selected peptides and is primarily a research and translational assay rather than a universal clinical predictor. Agents targeting MCL1, BCL-XL or multiple anti-apoptotic proteins remain indication- and trial-specific.
What is MOMP and why is it the point of no return in apoptosis?
MOMP (mitochondrial outer membrane permeabilisation) is the formation of BAX/BAK pores in the outer mitochondrial membrane. Once MOMP occurs, cytochrome c is released and the apoptosome assembles irreversibly. Even if caspases are inhibited after MOMP, cells undergo caspase-independent death through AIF and EndoG. This makes MOMP the irreversible apoptotic commitment event.
Why do MYC-overexpressing cells need BCL2 to survive?
MYC overexpression paradoxically activates the apoptotic programme as a tumour-suppressive safeguard — through ARF-mediated p53 stabilisation and direct BIM upregulation. For MYC-driven cancer cells to survive this oncogene-induced apoptosis, they must co-acquire BCL2 overexpression, p53 loss, or MCL1 amplification to suppress the apoptotic cascade, explaining the co-occurrence of MYC and BCL2 alterations in aggressive B-cell lymphomas.
How does venetoclax work so quickly in some patients?
Venetoclax works instantly because it directly occupies BCL2's hydrophobic groove, releasing pre-loaded BH3-only proteins (particularly BIM) that are already bound to BCL2 in primed tumour cells. In highly BCL2-dependent CLL, the mitochondria are 'primed for death' — saturated with bound pro-apoptotic proteins — requiring only displacement of the BCL2 plug to trigger instant cytochrome c release and caspase activation.
Understand how tumor suppressor genes — including TP53, BRCA1, PTEN, and RB1 — act as the genome's brakes, and what happens when they are lost in cancer.
How BCL2-family proteins control the mitochondrial apoptosis decision, how cancer cells exploit BCL2/MCL1 overexpression for survival, and how venetoclax exploits BCL2 dependency.
Understand TP53 loss of function, dominant-negative effects, hotspot mutations, p53 immunohistochemistry limits, inherited-risk context and experimental strategies.
Understand sensitising EGFR mutations in NSCLC, how molecular testing is interpreted, and how T790M, C797S, MET amplification and other resistance mechanisms differ.
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