Apoptosis — Programmed Cell Death in Cancer

Apoptosis is a tightly regulated physiological sequence characterised by distinct morphological changes, including cell shrinkage, nuclear fragmentation, chromatin condensation, and membrane blebbing. Unlike necrosis, which causes chaotic cell swelling and inflammation, apoptotic cells break down into membrane-bound apoptotic bodies that are rapidly consumed by phagocytes without damaging surrounding tissue.

This process occurs via two primary cascades: the intrinsic (mitochondrial) pathway, triggered by internal cellular stress or DNA damage, and the extrinsic (death receptor) pathway, initiated by extracellular ligands. Both cascades converge on the activation of executioner caspase enzymes. Cancer cells frequently evade apoptosis by overexpressing anti-apoptotic proteins, such as BCL-2, or inactivating tumour suppressor genes like TP53.

The evasion of apoptosis is a fundamental hallmark of cancer. Understanding whether a tumour can still undergo cell death helps researchers design treatments that overcome therapeutic resistance. When a patient's cancer cells resist apoptosis, standard therapies may become less effective. Conversely, therapies engineered to reactivate apoptosis can induce significant tumour shrinkage, offering durable disease control across diverse malignancies.

Apoptosis is evaluated in research and pathology via molecular assays. Immunohistochemistry tests for high levels of BCL-2 or mutated TP53 proteins. Next-generation sequencing detects TP53 mutations that stop cells sensing DNA damage. Specialist laboratories also use TUNEL staining or flow cytometry to directly observe fragmented DNA and measure apoptotic activity in tumour tissue.

Apoptosis evasion is relevant to virtually all cancers. It plays a particularly prominent role in chronic lymphocytic leukaemia (CLL), follicular lymphoma, and acute myeloid leukaemia, where the BCL-2 protein frequently blocks cell death. It is equally critical in solid tumours, such as colorectal, breast, and non-small cell lung cancers displaying TP53 alterations.

Most traditional cancer treatments, including cytotoxic chemotherapy and radiotherapy, induce lethal DNA damage to force cells into apoptosis. Newer targeted therapies act more directly: for example, BCL-2 inhibitors like venetoclax directly neutralise anti-apoptotic proteins in leukaemia. When tumours develop resistance by blocking apoptosis pathways, clinicians may switch to combinations that trigger alternative cell death mechanisms.