Oncogenes and Tumor Suppressor Genes

Oncogenes and tumour suppressor genes are the two complementary classes of genes whose dysregulation drives cancer. Oncogenes are altered, hyperactive versions of normal proto-oncogenes that promote cell growth and survival, while tumour suppressor genes normally restrain proliferation or trigger cell death and contribute to cancer when they are lost or inactivated. Together they capture the central genetic logic of malignant transformation.

Definition

An oncogene is a gene whose product, when abnormally activated or overexpressed, promotes cancer; a tumour suppressor gene is a gene whose normal product restrains tumour development and whose loss or inactivation promotes cancer.

Scope

This topic covers the definitions and contrast between oncogenes and tumour suppressors, the dominant gain-of-function versus recessive loss-of-function nature of their alterations, the two-hit model for tumour suppressor inactivation, and representative examples such as RAS-family and kinase oncogenes and the TP53 and RB1 tumour suppressors. It treats these genes as molecular pathology concepts, not as a basis for individual testing decisions.

Core questions

• How does a normal proto-oncogene become a cancer-driving oncogene?
• Why are oncogene alterations typically dominant while tumour suppressor losses are typically recessive at the cellular level?
• What does the two-hit hypothesis explain about tumour suppressor inactivation?
• How do these gene classes map onto the hallmarks of cancer?

Key concepts

• Proto-oncogene activation
• Gain-of-function versus loss-of-function
• Dominant oncogene, recessive tumour suppressor
• Two-hit hypothesis and loss of heterozygosity
• Gatekeeper and caretaker genes
• Signal-transduction kinases as oncogene targets

Key theories

Two-hit hypothesis

Knudson's analysis of retinoblastoma proposed that two mutational events are required to inactivate both alleles of a tumour suppressor gene; an inherited first hit explains the earlier, bilateral disease seen in familial cases, and this model became the foundation for the concept of recessive tumour suppressor genes.

Driver-gene functional convergence

Cancer-driving alterations in oncogenes and tumour suppressors are interpreted as conferring selective growth advantage by enabling the core functional hallmarks of cancer; sequencing distinguishes these drivers from incidental passenger mutations.

Mechanisms

Proto-oncogenes can be converted to oncogenes by point mutation, gene amplification, chromosomal translocation, or insertional events that increase or constitutively activate the gene product; because a single altered allele suffices to drive proliferation, such changes act dominantly in the cell. Many oncogene products are components of growth-signalling pathways, including protein kinases that relay proliferative signals. Tumour suppressor genes, by contrast, normally brake the cell cycle or promote apoptosis, and typically both copies must be inactivated — by mutation, deletion, or epigenetic silencing — for their restraining function to be lost, as formalised in the two-hit model. The net effect of accumulating oncogene activation and tumour-suppressor loss is the acquisition of the functional capabilities that define cancer.

Clinical relevance

Oncogenes and tumour suppressors underpin the molecular classification of many tumours and the rationale for molecularly targeted agents and predictive biomarkers in oncologic pathology. This entry explains the underlying biology for reference and education; it does not direct testing or treatment for any individual.

History

The identification of cellular proto-oncogenes from retroviral oncogenes in the 1970s and 1980s and Knudson's 1971 statistical work on retinoblastoma established the two complementary gene classes. Subsequent molecular and genomic work mapped the pathways they control and, through large-scale sequencing, placed them within a landscape of driver and passenger alterations across cancers.

Key figures

• Alfred Knudson
• Robert Weinberg
• Bert Vogelstein
• Tony Hunter

Seminal works

• knudson-1971
• hanahan-weinberg-2011
• vogelstein-2013

Frequently asked questions

What is the difference between an oncogene and a proto-oncogene?

A proto-oncogene is a normal cellular gene that promotes regulated growth; it becomes an oncogene when a mutation or other alteration makes it abnormally or constitutively active in driving proliferation.

Why do both copies of a tumour suppressor gene usually need to be lost?

Tumour suppressors normally restrain cancer, so a single working copy is often enough to maintain that function; the two-hit hypothesis holds that both alleles must be inactivated before the restraining function is lost.