Uncontrolled Cell Growth: Molecular Drivers of Cancer and Risk Factors

Cancer arises from the accumulation of molecular alterations that disrupt the tightly regulated balance between cell proliferation, death, and differentiation. No single event causes cancer; it is a multi-step process (clonal evolution).

Core Molecular Drivers

1. Oncogene Activation (Gain-of-Function)

• Normal: Proto-oncogenes (growth factors, receptors, signal transducers, transcription factors).
• Activating mechanisms: Point mutations (RAS, BRAF, EGFR), gene amplification (HER2, MYC), chromosomal translocation (BCR-ABL, MYC-IGH), overexpression.
• Result: Constitutive pro-growth signaling independent of external stimuli.

2. Tumor Suppressor Gene Inactivation (Loss-of-Function)

• TP53 (p53): "Guardian of genome" — cell cycle arrest, apoptosis, DNA repair, senescence. Mutated in >50% of cancers.
• RB1: G1/S checkpoint control.
• PTEN: PI3K/AKT pathway brake.
• APC: Wnt/β-catenin regulation (colorectal).
• Mechanisms: Mutation, deletion, epigenetic silencing (promoter methylation).

3. DNA Repair Defects (Genomic Instability)

• Homologous recombination: BRCA1/2, PALB2, RAD51 → breast, ovarian, prostate, pancreatic cancer.
• Mismatch repair (MMR): MLH1, MSH2, MSH6, PMS2 → Lynch syndrome (colorectal, endometrial).
• Base excision / nucleotide excision: XRCC1, XP genes.
• Result: Mutator phenotype → accelerated accumulation of driver mutations.

4. Cell Cycle Checkpoint Dysregulation

• G1/S (p16/RB), intra-S (ATR/CHK1), G2/M (ATM/CHK2) checkpoints fail → damaged cells divide.
• Cyclin D/CDK4/6 overexpression, p16 loss, CDC25 overexpression.

5. Apoptosis Evasion

• Intrinsic: BCL2 overexpression, BAX/BAK loss, p53 loss.
• Extrinsic: FAS mutations, decoy receptors, FLIP overexpression.

6. Immortality & Metastasis Enablers

• Telomerase reactivation (TERT promoter mutations): 85–90% cancers.
• Angiogenesis: VEGF, FGF upregulation.
• Invasion/EMT: Loss of E-cadherin, MMP secretion, TWIST/SNAIL.
• Immune evasion: PD-L1, MHC-I loss, TGF-β, IDO.

Etiologic Factors (Exposures → Mutations)

• Tobacco (PAHs, nitrosamines → TP53, KRAS mutations)
• UV radiation (C→T transitions → TP53, PTEN)
• HPV (E6/E7 degrade p53/RB)
• Alcohol (acetaldehyde, ROS)
• Obesity/insulin resistance (IGF-1, inflammation)
• Inherited germline mutations (5–10% of cancers)

Precision oncology now targets specific drivers (EGFR inhibitors, PARP inhibitors for BRCA, immunotherapy for MMRd/MSI-H). Prevention remains paramount: tobacco avoidance, HPV vaccination, UV protection, healthy weight, alcohol moderation.