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).

Oncogenes are often activated by a dominant cellular alteration, whereas many tumour suppressors lose function through biallelic or multi-step events. This is a useful framework rather than an exception-free rule: dosage, dominant-negative effects, haploinsufficiency, epigenetic silencing and tumour context complicate it. Tumour genetics and inherited-risk counselling also answer different questions and should not be merged into one dominance rule.

Oncogene Addiction and Targeted Therapy

Oncogene addiction describes a disproportionate dependence on one driver or signalling state despite other alterations. Inhibition can produce apoptosis, growth arrest, differentiation or little response depending on lineage, drug exposure and co-alterations. A confirmed driver can support a treatment hypothesis without guaranteeing rapid regression.

Resistance can arise through an alteration in the target, bypass signalling, phenotypic or histological change, altered drug handling, microenvironmental support or persistence of pre-existing subclones. Secondary target variants such as EGFR T790M and ABL T315I are important examples, not an almost universal resistance template.

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 describes a relative dependency that can support therapeutic selectivity, but it does not guarantee the depth or duration of response.
• ·Acquired resistance can involve target variants, bypass signalling, cell-state change, pharmacology or microenvironmental support.
• ·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.
• ·Many oncogenic kinases are drug targets, but a molecular finding must match a disease-, variant-, assay- and regimen-specific evidence source.