Tumour Suppressor Genes

Tumour Suppressor Genes

Inhibit cell division or promote DNA repair/apoptosis, helping to prevent cancer

How did Knudson contribute to the discovery of tumour suppression genes?

• Knudson (1971) studied retinoblastoma, a rare childhood cancer of the retina, and compared its hereditary and sporadic forms.

• Hereditary form: A child inherits one defective copy (allele) of the RB1 tumour suppressor gene, but remains unaffected. A second hit (mutation or change) affecting the remaining functional copy can lead to cancer.

• Sporadic form: Both copies must acquire hits in retinal cells during the child’s lifetime for the child to get cancer. If the child only has one mutated allele, they remain unaffected.

Two-hit hypothesis:

• Based on this, Knudson proposed that the mutation of 2 alleles of a tumour suppressor gene is involved in the development of the cancer - shows that it is a recessive mutation

• From this, he concluded that both copies of a tumour suppressor gene generally need to be inactivated for its protective function to be lost.

Clinical differences between the hereditary form and sporadic form of retinoblastoma

• Hereditary cases tend to develop cancer at a younger age, with multiple tumours that often affect both eyes (bilateral)

• Sporadic cases are more often later-onset and affect one eye (unilateral)

Why does hereditary retinoblastoma develop earlier?

• One hit is already inherited

• So retinal cells need only one additional hit to lose RB1 function (which then leads to uncontrolled cell division and cancer)

Why is hereditary retinoblastoma often bilateral?

• The inherited first hit is present in retinal cells throughout the body

• This makes the independent second hits more likely in both eyes

Why can a child inherit faulty RB1 gene without developing cancer?

• The inherited faulty allele increases risk

• But the remaining functional allele from the other parent may still protect cells until a second hit occurs

How are tumour suppressor genes different from oncogenes?

• Tumour suppressor genes behave recessively whereas oncogenes behave dominantly

• A single hit is enough to activate the oncogene = increase cell proliferation

• However, 2 hits (on both alleles) are required to inactivate the tumour suppressor gene = no longer slow down cell proliferation

Summary of the two-hit hypothesis in the context of retinoblastoma

• First hit: One copy of the RB1 gene becomes inactive.

• Second hit: The remaining functional copy becomes inactive.

• Result: The cell loses the protective function of RB1, allowing uncontrolled cell division and potentially leading to cancer.

Things to remember

• An allele is a version of a gene

• 2 alleles (version of the gene) are inherited - one from the mother and the father

• Hits can occur on either allele

How do tumour suppressor genes prevent cell proliferation

• Repress the cell cycle

• Negative feedback in signalling pathways

• Damage DNA and DNA repair

Examples of tumour suppressor genes

• RB1 gene (retinoblastoma)

• BRCA1 (breast cancer)

• APC (colorectal cancer)

What is an exception to the general recessive nature of tumour suppressor genes?

• TP53 gene

• A single mutation or hit of an allele is sufficient to disable the activity of TP53

Oncogenes and Tumour Suppressor Genes Summary

How is cell proliferation (growth and division) regulated?

• Controlled production and release: Cells regulate how much growth signal they produce and when they release it.

• Different levels in different tissues: Each tissue has different growth requirements, so the amount of signal varies.

• Bioavailability: The signal must be accessible to its target receptor to have an effect. Its availability depends on the surrounding environment.

• Sequestration: Signals can be held or stored in the surrounding extracellular matrix (ECM) and pericellular space, rather than being freely available.

• Localised release: Enzymes such as proteases and sulfatases can release or activate stored signals in specific locations.

These controls regulate signalling in both space (where it occurs) and time (when it occurs), preventing excessive cell growth

Which pathways are involved in regulating cell growth and proliferation?

1. MAPK pathway

2. PI3K-AKT pathway

Summary of the MAPK Pathway

The MAPK pathway is a cell-signalling pathway that regulates cell growth and proliferation. In cancer, mutations to this pathway lead to uncontrolled cell division.

1. Growth factor binds to the receptor e.g. EGFR and HER2 which are receptor tyrosine kinases

2. RAS is activated as RAS-GTP

3. Converts RAF to MEK to ERK

4. Promotes cell growth and division

Summary of the PI3K-AKT Pathway

The PI3K-AKT pathway is a cell-signalling pathway that regulates cell growth, proliferation and survival. In cancer, mutations to this pathway lead to uncontrolled cell division.

1. Growth factor binds to the receptor, activating PI3K e.g. EGFR and HER2 which are receptor tyrosine kinases

2. PI3K converts PIP2 to PIP3 which is a signalling molecule in the cell-membrane

3. PIP3 helps recruit and activate AKT (also called protein kinase B (PKB))

4. AKT activates signalling processes that promote cell growth, division and survival

How can receptor/ligand mechanisms cause excessive proliferative signalling in cancer cells?

• Autocrine loop: Cancer cells secrete growth factors that bind to receptors on their own surface, stimulating further growth.

• Paracrine loop: Nearby cells secrete growth factors that stimulate cancer cell proliferation.

• Increased receptor expression: Duplication of certain genes can increase receptor numbers (as more expression of these genes) on the cell surface, making cells more sensitive to growth signals.

• Structural alterations of the receptor: Mutations can cause receptors to become permanently active or hyper-responsive, allowing signalling even with low growth factor levels

How can receptor/ligand-independent mechanisms cause excessive proliferative signalling in cancer cells?

Constitutive activation of downstream pathways:

• AKA continuous activation of a signalling pathway

• Normally, a growth factor binds to the receptor activating the signalling pathway

• Somatic mutations (mutation acquired during a persons lifetime) permanently activate downstream signalling proteins, so a growth factor is not needed to initiate the signal.

• E.g. a mutation in BRAF (found in approximately 40–50% of melanomas) activates the MAPK pathway (RAF → MEK → ERK), promoting continuous cell proliferation.

• E.g. Mutations in the catalytic subunit of PI3K can also increase AKT/PKB signalling, promoting cell proliferation and survival

Disrupted negative feedback:

• Normally, negative-feedback mechanisms reduce signalling to prevent excessive cell growth

• Mutations can disrupt these controls.

• E.g. RAS normally swictches on when bound to GTP and switches off when GTP is hydrolysed to GDP. An oncogenic RAS mutation impairs GTP hydrolysis, leaving RAS persistently active, continuously stimulating downstream pathways such as RAF-MEK-ERK and PI3K-AKT

Loss of PTEN function:

• PTEN is a tumour suppressor phosphatase that converts PIP3 back to PIP2, opposing PI3K signalling

• Loss of PTEN function allows PIP3 to accumulate= increasing AKT/PKB signalling = promoting cell survival and proliferation.

• PTEN can be inactivated by mutations or reduced expression, including through promoter methylation.

Examples of drugs that interfere in certain places of the MAPK AND PI3K-AKT pathways to stop mutations from occurring and constitutive proliferation

Growth factors:

• Lapatinib inhibits the kinase of both HER2 and EGFR

• Erlotinib and gefitinib inhibit EGFR only

MAPK pathway:

• Tipifarnib acts at Ras

• Sorafenib acts at Raf

• AZD6244 acts at MEK

PI3K-AKT pathway:

• Perifosine acts at AKT

• Temsirolimus and everolimus acts at mTOR

Why are cell signalling pathways complex, and why do they involve cascades and multiple steps?

• Cell signalling is not linear: One signal can activate several pathways, and each pathway can influence multiple cellular processes.

• Transcription of every factor is regulated: Cells control which signalling proteins and other factors are produced, and how much, by regulating gene expression. This changes how strongly a cell can respond.

• Interactions are not simply binary: Signalling molecules do not always act as a straightforward on/off switch. Their effects can depend on concentration, duration, location and interactions with other proteins.

• Feedback loops: Some pathway components enhance their own signalling (positive feedback), while others inhibit it (negative feedback).

• Crosstalk between pathways: Different pathways communicate and influence one another. For example, the MAPK and PI3K–AKT pathways can both be activated by growth factor receptors.

• Pathways form networks: Because pathways branch, interact and contain feedback loops, they operate as interconnected networks rather than isolated chains