Oncogene-Induced Senescence: A Built-In Barrier to Cancer

An Unexpected Response to an Oncogene

Expressing an activated oncogene in a primary human cell typically causes a brief burst of proliferation followed by a stable arrest. The cell stays metabolically active but never divides again.

This was first characterised for oncogenic RAS and has since been seen with BRAF, MYC and others. It explains why single oncogenic events are usually not enough to start a tumour.

How the Arrest Is Enforced

Oncogenic signalling drives hyper-replication, which produces DNA-replication stress and activates the DNA-damage response. In parallel, the CDKN2A product p16INK4a accumulates and locks RB in its active state.

The p53-p21 axis contributes to establishing the arrest. Senescence therefore sits at the intersection of the two main tumour-suppressor pathways, RB and p53.

The Senescence-Associated Secretory Phenotype

Senescent cells secrete a mixture of cytokines, chemokines, growth factors and proteases known as the SASP. This can reinforce arrest in the same cell and spread senescence to neighbours, and it can recruit immune cells that clear senescent cells.

The SASP is double-edged: sustained over time it can also create a pro-inflammatory, tissue-remodelling environment that favours the growth of cells that have escaped arrest.

A Visible Example

Benign melanocytic naevi (moles) frequently carry the BRAF V600E mutation yet remain growth-arrested for decades. They are a clinical illustration of oncogene-induced senescence holding a mutant clone in check.

Progression to melanoma typically requires additional events, such as CDKN2A or TP53 pathway loss or TERT promoter mutation, that allow escape from the arrest.

Why This Matters

Finding a strong oncogenic driver in a lesion does not by itself indicate malignancy; the same mutation can be present in benign, senescent tissue.

Senescence is also being studied as a therapeutic state to induce deliberately, followed by drugs that kill senescent cells, but these approaches are experimental.

Key Takeaways

• ·An activated oncogene often triggers a permanent arrest rather than immediate growth.
• ·The arrest uses replication-stress signalling plus the p16/RB and p53/p21 pathways.
• ·The SASP can both reinforce and, over time, undermine tumour suppression.
• ·BRAF-mutant moles show senescence in action; escape requires further alterations.