The Hallmarks of Cancer

1. The Hanahan-Weinberg Framework

In a 2000 review in Cell that has since been cited over 60,000 times, Douglas Hanahan and Robert Weinberg proposed that despite cancer’s extraordinary diversity, all malignant tumours share a small set of acquired hallmarks — functional capabilities that distinguish them from normal tissue. The original list contained six; updated reviews in 2011 and 2022 added two more hallmarks plus four enabling characteristics.

The framework is more than a tidy list: it is the diagnostic and therapeutic backbone of contemporary oncology. Each hallmark identifies a vulnerability — a capability that, if blocked, kills or stalls the cancer. Modern targeted therapies and immunotherapies map one-to-one onto specific hallmarks.

The Eight Hallmarks at a Glance

2. Sustained Proliferative Signalling

Normal tissue tightly controls the production and release of growth-promoting signals that instruct cells to enter the cell cycle. Cancer cells become autonomous: they manufacture their own growth-factor ligands (autocrine), induce stromal cells to supply them (paracrine), or constitutively activate the receptors and downstream signalling cascades.

The ligand-receptor pairing is the entry point of every growth-factor signal. A soluble ligand (a growth factor such as EGF, PDGF, FGF, insulin, or IGF-1) binds a membrane receptor; the bound complex changes conformation, dimerises or oligomerises, and converts the extracellular event into an intracellular biochemical signal — kinase activation, GTPase loading, or second-messenger release. The signal is amplified through a cascade and ultimately drives the transcription of cell-cycle and growth genes (CCND1, MYC, ribosomal RNA loci).

Cancer cells subvert this circuit at several distinct points. First, the production of growth-factor ligands can be amplified: the tumour cell itself, or stromal cells it recruits, secretes more ligand than normal tissue would. By analogy, if the ligand is a basketball and the receptor a hoop, increasing the number of basketballs increases the chance of scoring — independent of any change to the receptor. PDGF-α and TGF-α autocrine loops in gliomas and squamous carcinomas are textbook examples.

Second, the number of receptors on the cell surfacemay be increased — often by gene amplification. More hoops means a baseline ligand concentration that would have left a normal cell quiet now drives a tumour cell to divide. HER2 (ERBB2) amplification in 15–20% of breast cancers produces 10–100× normal receptor density; the trastuzumab (Herceptin) antibody was designed against this surplus.

Third, the structure of the receptor itself can be alteredso that it signals without ligand at all — or signals far more strongly when ligand is present. EGFR-vIII (a deletion variant in glioblastoma) lacks part of the extracellular domain and is constitutively active. EGFR L858R and exon-19 deletion variants in NSCLC stabilise the active kinase conformation. BRAF V600E in melanoma and KRAS G12 mutations across many cancers achieve the same outcome at the cytoplasmic-kinase and small-GTPase levels respectively. In all three modes the cell acts as if it were continuously hearing a growth instruction that nobody is sending.

Mechanisms

• Autocrine ligand production — e.g. PDGF in glioma, TGF-α in carcinomas.
• Receptor overexpression — HER2 amplification in 15–20% of breast cancers (basis of trastuzumab/Herceptin).
• Activating receptor mutations — EGFR L858R / del-19 in NSCLC, FLT3-ITD in AML.
• Constitutive downstream activation — KRAS G12 mutations (~25% of all cancers, ~90% of pancreatic), BRAF V600E in melanoma, PIK3CA hotspots.
• Loss of negative feedback — NF1 loss disinhibits Ras; PTEN loss disinhibits PI3K/Akt.

The Ras-Raf-MEK-ERK and PI3K-Akt-mTOR axes are the two dominant downstream pathways. Both are introduced in detail in Cell Physiology — Signaling; their dysregulation is the substrate of most targeted cancer drugs.

Mechanisms

• Autocrine ligand production — e.g. PDGF in glioma, TGF-α in carcinomas.
• Receptor overexpression — HER2 amplification in 15–20% of breast cancers (basis of trastuzumab/Herceptin).
• Activating receptor mutations — EGFR L858R / del-19 in NSCLC, FLT3-ITD in AML.
• Constitutive downstream activation — KRAS G12 mutations (~25% of all cancers, ~90% of pancreatic), BRAF V600E in melanoma, PIK3CA hotspots.
• Loss of negative feedback — NF1 loss disinhibits Ras; PTEN loss disinhibits PI3K/Akt.

The Ras-Raf-MEK-ERK and PI3K-Akt-mTOR axes are the two dominant downstream pathways. Both are introduced in detail in Cell Physiology — Signaling; their dysregulation is the substrate of most targeted cancer drugs.

3. Evading Growth Suppressors

Two tumour-suppressor circuits dominate growth control: the RB pathway (cell-cycle entry) and the p53 pathway (genomic-stress response). Both are inactivated, directly or indirectly, in essentially every human cancer.

4. Resisting Cell Death

Apoptosis is the genetically encoded suicide programme that culls damaged or superfluous cells. Its core decision is governed by the BCL-2 protein family, whose anti-apoptotic members (BCL-2, BCL-xL, MCL-1) restrain the pro-apoptotic effectors (BAX, BAK), with BH3-only proteins (BIM, BAD, PUMA, NOXA) as the pro-death triggers.

• BCL-2 overexpression — the t(14;18) translocation in follicular lymphoma drives BCL-2 under the IgH enhancer.
• Loss of pro-apoptotic effectors — BAX frameshift mutations in MMR-deficient colon cancers.
• p53 inactivation — eliminates upstream BH3-only induction (PUMA, NOXA).
• Caspase suppression — XIAP and other IAPs block executioner caspases.

Therapeutic angle: BH3-mimetics — small molecules that occupy the BH3-binding groove of BCL-2 — release the brake. Venetoclax (a selective BCL-2 inhibitor) is approved for CLL and AML and represents a first-in-class success for protein-protein interface drug design.

5. Enabling Replicative Immortality

Normal somatic cells lose ~50–100 bp of telomeric repeat per division. After ~50 divisions (the Hayflick limit), critically short telomeres trigger senescence or apoptosis. Cancer cells bypass this clock by reactivating telomerase (the TERT reverse transcriptase + the TERC RNA template) or, in ~10% of cancers, the ALT pathway (alternative lengthening of telomeres via homologous recombination).

• TERT promoter mutations (C228T, C250T) — commonest non-coding cancer mutation; create new ETS transcription-factor binding sites.
• ALT phenotype — common in sarcomas and pediatric brain tumours, often associated with ATRX/DAXX loss.
• Crisis & bypass — brief telomere catastrophe generates the chromosome rearrangements that seed tumour heterogeneity.

6. Inducing Angiogenesis

Solid tumours larger than ~1–2 mm cannot survive on diffusion alone. The angiogenic switch — a shift in the local balance of pro- and anti-angiogenic factors — recruits new vasculature from the surrounding tissue. The dominant driver is VEGF-A, frequently induced by hypoxia via HIF-1α stabilisation.

• Tumour vasculature is abnormal — chaotic, leaky, mismatched perfusion. This both fuels growth and impedes drug delivery (the “perfusion paradox”).
• Anti-VEGF therapy — bevacizumab (anti-VEGF Ab), aflibercept (decoy receptor), VEGFR2 TKIs (sunitinib, sorafenib). Modest single-agent benefit; rationale for combinatorial use.
• Vascular normalisation hypothesis (Jain) — transient pruning of dysfunctional vessels improves perfusion and immunotherapy response.

7. Activating Invasion & Metastasis

Most cancer mortality is from metastasis, not the primary tumour. Carcinoma cells acquire migratory and invasive capabilities through partial or full epithelial-to-mesenchymal transition (EMT): loss of E-cadherin, gain of N-cadherin/vimentin, activation of EMT-TFs (Snail, Slug, Twist, ZEB1/2), and remodelling of the actin cytoskeleton into invadopodia.

The full metastatic cascade — intravasation, survival in circulation, extravasation, dormancy, and outgrowth at distant sites — is covered in Part VII.

8. Reprogramming Energy Metabolism

Otto Warburg observed in the 1920s that tumours consume glucose at a high rate and produce lactate even when oxygen is abundant — aerobic glycolysis, the Warburg effect. Modern interpretation: aerobic glycolysis is not a defect but a feature, providing biosynthetic precursors (ribose for nucleotides, glycerol-3-P for lipids, NADPH from PPP) and rapid ATP for proliferating cells.

• Glucose uptake via GLUT1 upregulation — the basis of ¹⁸F-FDG PET imaging.
• Lactate dehydrogenase A (LDHA) reduction of pyruvate maintains NAD⁺.
• Glutaminolysis — many cancers are addicted to glutamine for anaplerosis (MYC-driven), feeding TCA via α-ketoglutarate.
• Oncometabolites — mutant IDH1/2 produces 2-hydroxyglutarate; SDH/FH loss accumulates succinate/fumarate. All inhibit α-KG-dependent dioxygenases (TET, JmjC), reshaping the epigenome.
• Lipogenic switch — ACC, FAS, ATP-citrate lyase upregulated for membrane biosynthesis.

See Metabolism — Anabolism Integration for the underlying biosynthetic logic, and Part V for the cancer-specific deep dive.

9. Evading Immune Destruction

The immune system continuously surveils nascent tumours — the immunoediting framework (elimination → equilibrium → escape, Schreiber & Smyth). Tumours that escape have evolved mechanisms to dodge T-cell killing:

• Immune checkpoint upregulation — PD-L1 expression engaging PD-1 on T cells, dampening cytotoxic activity.
• Antigen presentation loss — β2-microglobulin or HLA-class-I loss, B2M frameshifts in MMR-deficient tumours.
• Recruitment of suppressive cells — Tregs, MDSCs, M2 macrophages.
• Cytokine milieu — TGF-β, IL-10 suppress effector T cells.

Therapeutic breakthrough: Allison & Honjo shared the 2018 Medicine Nobel for the discovery and clinical translation of checkpoint blockade (anti-CTLA-4 ipilimumab, anti-PD-1 nivolumab/pembrolizumab), which has produced durable responses in melanoma, NSCLC, RCC, MMR-deficient cancers, and others.

10. Enabling Characteristics

Beyond the eight functional hallmarks, two underlying conditions make their acquisition possible:

11. The 2022 Update — Four New Dimensions

In Cancer Discovery 12, 31 (2022), Hanahan proposed four candidate hallmarks and enabling characteristics emerging from the post-2011 literature:

12. Therapeutic Implications — Hallmark to Drug Class

The remaining seven parts of this course descend into each of these mechanistic and therapeutic axes in detail.