Cancer Biology: Hallmarks, Mechanisms, and Targeted Therapy

Introduction to Cancer Biology

Cancer is a collection of diseases characterised by uncontrolled cellular proliferation, invasion of surrounding tissues, and metastasis to distant sites. Cancer arises from the accumulation of somatic mutations—typically 2-8 driver mutations—in critical regulatory genes over years to decades. While each cancer is molecularly distinct, common principles organise cancer biology: Hanahan and Weinberg's 'Hallmarks of Cancer' framework identifies capabilities repeatedly acquired during tumour evolution including sustained proliferative signalling, evasion of growth suppressors, resistance to cell death, limitless replicative potential, induced angiogenesis, and activated invasion/metastasis.

The molecular revolution in cancer biology has transformed treatment. From cytotoxic chemotherapy targeting all dividing cells, oncology has moved toward targeted therapies exploiting specific molecular vulnerabilities of individual cancer types, immunotherapies harnessing the immune system's natural cancer recognition, and personalised medicine tailoring treatment to each tumour's unique genomic profile. Understanding cancer cell biology continues to reveal new vulnerabilities and therapeutic strategies with growing clinical impact.

Oncogenes and Tumour Suppressors

Oncogene Activation

Oncogenes are mutated, amplified, or translocated versions of normal growth-promoting proto-oncogenes. RAS point mutations (KRAS G12D, G12V) occur in 30% of all cancers, constitutively activating GTPase signalling through RAF-MEK-ERK and PI3K-AKT pathways promoting proliferation and survival. MYC amplification in breast, lung, and colorectal cancers drives widespread transcriptional activation of cell cycle, ribosome, and metabolic genes. BCR-ABL fusion in CML creates a constitutively active tyrosine kinase—the direct target of imatinib (Gleevec), the first molecularly targeted cancer therapy, achieving durable remissions in CML and validating oncogene addiction as a therapeutic concept.

Tumour Suppressor Loss

Tumour suppressors follow Knudson's two-hit hypothesis: both alleles must be inactivated for loss of function. Rb (retinoblastoma) loss removes the G1 brake; TP53 loss eliminates damage checkpoints and apoptosis induction. APC (adenomatous polyposis coli) mutations initiate colorectal cancer by constitutively activating Wnt signalling; germline APC mutations cause familial adenomatous polyposis with hundreds of colorectal polyps. BRCA1/2 inactivation impairs homologous recombination DNA repair; PTEN loss hyperactivates PI3K-AKT. Understanding tumour suppressor pathways reveals both cancer risk genes and downstream therapeutic vulnerabilities exploitable when suppressors are lost.

Tumour Microenvironment

Angiogenesis

Tumours beyond 1-2 mm require a blood supply to obtain oxygen and nutrients. Hypoxia activates HIF-1alpha, transcribing VEGF-A—a potent angiogenic signal. VEGF binds VEGFR2 on endothelial cells triggering sprouting angiogenesis forming disorganised, immature vessels with high permeability. Tumour vasculature is structurally and functionally abnormal: heterogeneous, prone to collapse, and highly permeable. Anti-angiogenic therapies (bevacizumab, anti-VEGF) are approved in colorectal, lung, and other cancers. Vessel normalisation—making tumour vessels more functional—may improve drug delivery and immune access to tumours, a conceptually important therapeutic goal.

Immune Evasion in the Tumour Microenvironment

Tumours evolve multiple mechanisms to evade immune destruction. PD-L1 upregulation on cancer cells engages PD-1 on T cells, suppressing their activity. Regulatory T cells and immunosuppressive macrophages (M2 phenotype) are recruited to tumours by cancer-derived cytokines. IDO (indoleamine 2,3-dioxygenase) depletes tryptophan in the tumour microenvironment, impairing T cell function. TGF-beta suppresses immune cell activity. Loss of MHC class I expression (through beta-2-microglobulin mutation or epigenetic silencing) prevents cytotoxic T cell recognition. Understanding these mechanisms drives development of combination immunotherapy strategies reversing multiple immune evasion layers simultaneously.

Metastasis

Metastasis—cancer spread to distant organs—is responsible for 90% of cancer deaths. The metastatic cascade involves local invasion (epithelial-mesenchymal transition, basement membrane degradation by matrix metalloproteinases), intravasation into vessels, survival in circulation as circulating tumour cells (CTCs), extravasation at distant sites, and colonisation of distant microenvironments. Organs colonised preferentially by specific cancers reflect both mechanical factors (blood flow patterns) and molecular compatibility between tumour cells and organ microenvironments (seed-and-soil hypothesis). Breast cancer metastasises preferentially to bone, lung, liver, and brain through specific molecular tropism mechanisms.

Examples and Applications

Example 1: KRAS Targeted Therapy Breakthrough

KRAS was long considered undruggable due to its smooth, featureless protein surface. AMG 510 (sotorasib) exploits a cryptic pocket near the GDP-binding site of KRAS G12C (cysteine allows covalent binding), irreversibly inhibiting this specific mutant. FDA approved in 2021 for KRAS G12C-mutant (12% of NSCLC), sotorasib achieved 37% response rate. This breakthrough validated covalent fragment-based drug discovery and prompted development of KRAS G12D and G12V inhibitors for pancreatic and colorectal cancers. Resistance mechanisms (RTK upregulation, KRAS amplification) are actively targeted in combination strategies.

Example 2: Checkpoint Immunotherapy Responses

Anti-PD-1 therapy achieves long-term responses in melanoma, NSCLC, and other cancers impossible with chemotherapy. Five-year survival in advanced melanoma reached 34% with nivolumab versus effectively 0% historically. Pembrolizumab (anti-PD-1) is approved in 15+ cancer types including tumour-agnostic approval for MSI-high/dMMR cancers based on molecular profile rather than histological origin—a paradigm shift in cancer classification and treatment. Combination of anti-CTLA-4 (ipilimumab) with anti-PD-1 improves response rates further with manageable added toxicity, demonstrating synergy between complementary checkpoint pathways.

Example 3: HER2-Targeted Therapy in Breast Cancer

HER2 (ERBB2) is amplified in 15-20% of breast cancers. Trastuzumab (Herceptin), a monoclonal antibody blocking HER2 signalling and mediating ADCC, transformed HER2+ breast cancer from the worst to the best-prognosis subtype, reducing mortality 30%. Pertuzumab (blocking HER2-HER3 dimerisation), T-DM1 (trastuzumab-emtansine antibody-drug conjugate), and T-DXd (trastuzumab-deruxtecan, more potent ADC) have progressively improved outcomes. HER2 targeting exemplifies the oncogene addiction paradigm—cancers driven by specific amplified kinases become dependent on them and respond dramatically to pathway inhibition.

Example 4: Clonal Evolution and Treatment Resistance

Most targeted cancer therapies eventually fail due to resistance evolution through clonal selection. EGFR-mutant NSCLC initially responds to erlotinib/gefitinib, but EGFR T790M secondary mutation in 50-60% of cases confers resistance; third-generation inhibitor osimertinib overcomes T790M. C797S mutation then emerges resisting osimertinib. Understanding this evolutionary sequence of mutations drives rational sequential drug design. Liquid biopsy monitoring ctDNA enables early resistance detection before radiological progression, allowing early treatment switching. Cancer evolution is thus a continuous arms race exploitable with molecular monitoring.

Example 5: Synthetic Lethality in Cancer

Synthetic lethality—when loss of two genes simultaneously is lethal while loss of either alone is not—exploits cancer-specific vulnerabilities. BRCA1/2-deficient cancers rely on PARP-mediated base excision repair because homologous recombination is unavailable; PARP inhibitors (olaparib) trap PARP on DNA breaks causing lethal replication fork collapse only in cancer cells lacking HR. This approach achieved unprecedented efficacy in BRCA-mutant ovarian, breast, prostate, and pancreatic cancers. Synthetic lethality screens across thousands of gene pairs are identifying new cancer vulnerabilities targeting the unique genomic and epigenetic state of individual cancer types.

Example 6: Liquid Biopsy

Liquid biopsy detects tumour-derived nucleic acids, cells, or protein in blood. Circulating tumour DNA (ctDNA) fragments from tumour cell apoptosis carry cancer-specific mutations detectable at sensitivity below 0.1% allele fraction. Approved clinical uses include detecting EGFR T790M resistance (guiding osimertinib selection), monitoring CLL with TP53 mutations, and early cancer detection through multi-cancer early detection tests (GRAIL Galleri). Circulating tumour cells (CTCs) enable culturing patient-specific cells ex vivo for drug sensitivity testing. Methylation-based cell-of-origin analysis of ctDNA localises cancer to tissue of origin—critical for multi-cancer detection utility.

Example 7: Cancer Stem Cells

Cancer stem cells (CSCs) are a subpopulation within tumours with elevated tumorigenic potential, self-renewal capacity, and resistance to conventional chemotherapy. CSCs in leukaemia (CD34+CD38- cells) and breast cancer (CD44+CD24-) were identified by functional assays showing tumour-initiating capacity in xenograft models. CSCs often express drug efflux pumps, activate autophagy, and are quiescent (reducing chemotherapy sensitivity). Eliminating CSCs may be necessary for durable responses; stem cell niche-targeting strategies and differentiation-inducing agents aim to eliminate this resistant subpopulation driving relapse.

Example 8: Epigenetic Dysregulation in Cancer

Cancer epigenomes are broadly altered: global DNA hypomethylation (activating oncogenes, retrotransposons) combined with focal hypermethylation silencing tumour suppressor promoters. Polycomb complex dysregulation silences differentiation genes maintaining cancer cells in a stem-like state. EZH2 gain-of-function mutations in lymphoma amplify H3K27me3 silencing; EZH2 inhibitors (tazemetostat) are approved for EZH2-mutant lymphoma and epithelioid sarcoma. BET bromodomain inhibitors displace BRD4 from super-enhancers governing oncogene (MYC) transcription—a novel approach targeting transcriptional addiction. Epigenetic therapy modifies cancer cell identity rather than directly killing cells, potentially reversing malignant phenotypes.

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