Oncogenes in Cancer: Mechanisms and Targeted Therapy

Mechanisms of Oncogene Activation

Proto-oncogenes — normal growth-promoting genes — are converted to oncogenes through gain-of-function alterations that constitutively activate or overexpress the encoded protein. Three primary mechanisms operate: point mutations (KRAS G12C, BRAF V600E) that alter protein activity or conformation; gene amplification (HER2, MYC, CDK4) that increases protein expression proportionally to gene copy number; and chromosomal translocations creating fusion oncoproteins (BCR-ABL in CML, EML4-ALK in NSCLC) or placing a gene under strong transcriptional control (MYC in Burkitt lymphoma).

The critical distinction between oncogenes and tumour suppressors is dominance: a single allele with an activating mutation is sufficient to drive cancer (dominant gain-of-function), whereas tumour suppressors typically require biallelic inactivation (recessive loss-of-function). This distinction has major implications for cancer genetic testing and family counselling: a first-degree relative inheriting a BRCA1 mutation has ~50% chance of carrying it; the cancer risk from that mutation alone is high but not certain (requiring a somatic second hit).

Oncogene Addiction and Targeted Therapy

Oncogene addiction describes the phenomenon where cancer cells — despite having accumulated many oncogenic alterations — remain critically dependent on the continued activity of a specific driver oncogene for survival. Inhibiting that oncogene causes disproportionate harm to cancer cells compared to normal cells, explaining the therapeutic selectivity of oncogene-targeted agents. EGFR-mutant NSCLC cells undergo rapid apoptosis upon EGFR inhibition; BCR-ABL-positive CML cells are killed by imatinib; HER2-amplified breast cancer cells respond to trastuzumab.

The mechanistic basis for oncogene addiction remains incompletely understood but likely involves the rewiring of signalling networks around the dominant oncogene, creating quantitative rather than qualitative dependencies. Importantly, acquired resistance to oncogene-targeted therapy almost always involves secondary mutations in the targeted oncogene itself (gatekeeper mutations: EGFR T790M, ABL T315I) or bypass pathway activation (MET amplification escaping EGFR inhibition), rather than loss of oncogene signalling.

Historical Origins: From Rous Sarcoma to Cellular RAS

The oncogene concept has its origins in virology. In 1911, Peyton Rous demonstrated that a cell-free filtrate from a chicken sarcoma could transmit cancer to healthy chickens — the first evidence that an infectious agent could cause cancer, though the molecular basis was unknown for 65 years (Nobel Prize awarded 1966). The molecular breakthrough came when Bishop and Varmus demonstrated in 1976 that the v-src oncogene of Rous Sarcoma Virus was not a viral gene but a captured and mutated version of a normal cellular proto-oncogene (c-src) present in the genomes of healthy cells across species — establishing that cancer-causing genes are corrupted normal genes, not foreign sequences (Nobel Prize 1989).

The critical conceptual leap from viral to human oncology came in 1982 when three independent groups identified RAS as the first oncogene activated by point mutation in a human tumour (bladder carcinoma). This single discovery — that a G12V substitution in HRAS was sufficient to transform NIH 3T3 fibroblasts — established the point mutation paradigm for oncogene activation and made RAS the most intensively studied oncogene in cancer biology for the following 40 years. The subsequent identification of KRAS as the most commonly mutated human oncogene, and the 2021 approval of sotorasib for KRAS G12C, closed the loop from 1982 discovery to first approved direct KRAS inhibitor.

Key Takeaways

• ·Proto-oncogenes are converted to oncogenes through dominant gain-of-function alterations — point mutation (KRAS G12C), amplification (HER2, MYC), or translocation (BCR-ABL, EML4-ALK) — distinguishing them from tumour suppressors which require biallelic inactivation.
• ·Oncogene addiction — the disproportionate dependence of cancer cells on a single driver oncogene despite accumulated mutations — explains the therapeutic selectivity of targeted agents and predicts rapid tumour regression upon oncogene inhibition.
• ·Acquired resistance to oncogene-targeted therapy involves gatekeeper mutations (EGFR T790M, ABL T315I) in the targeted oncogene or bypass pathway activation (MET amplification escaping EGFR inhibition), rarely outright loss of oncogene expression.
• ·The first oncogene identified by mutation in human cancer was RAS in 1982 (HRAS G12V in bladder carcinoma), establishing the point mutation paradigm that underlies modern precision oncology.
• ·The kinome contains multiple oncogene-activated receptor and cytoplasmic kinases (EGFR, HER2, KIT, ABL, BRAF, JAK2) — each exploitable by matching targeted inhibitors when driver status is confirmed by molecular profiling.