cell lecture 15

What Are Oncogenes and Tumour Suppressor Genes?

• Oncogenes are mutated versions of normal genes called proto-oncogenes.

  • Proto-oncogenes normally help cells grow.

  • When mutated (gain-of-function), they become oncogenes, which cause unregulated cell growth and survival—a hallmark of cancer.

• Tumour suppressor genes do the opposite: they inhibit cell growth or cause damaged cells to die (apoptosis).

  • If these genes undergo loss-of-function mutations, they fail to stop cancerous growth.

  • Caretaker genes (a subtype) fix DNA damage. If lost, DNA mutations pile up.

Key idea: Cancer is often caused by too much cell growth (oncogenes activated) or too little control (tumour suppressors lost).

Effects of Mutations in Cancer Genes

How Proto-Oncogenes Become Oncogenes

These mutations can transform normal proto-oncogenes into dangerous oncogenes by:

1. Point mutation → protein becomes hyperactive.

2. Gene amplification → too much protein made.

3. Chromosomal translocation → abnormal expression or fusion proteins.

4. Insertional mutagenesis → viral DNA disrupts normal control.

5. Local DNA rearrangements → deletions/inversions that mess up normal protein function.

All of these lead to abnormal proteins or excess of normal proteins → unregulated cell growth.

Common Oncogenes

There are over 200 known oncogenes, many affecting the cell cycle and signaling pathways like:

• RAS, RAF, MEK, ERK → These are part of the MAPK cascade that tells cells to divide.

• MYC (transcription factor)

• BCR-ABL (in leukemia)

• Cyclins and Cdks (cell cycle regulators)

These mutations are gain-of-function, often causing proteins to be constantly "on".

Receptor Oncogenes

Receptors (like RTKs) can become overactive through:

• Mutations that make them active without a signal (ligand).

• Amplification that increases the number of receptors, making the cell over-responsive.

Result: Cell receives constant division signals, bypassing checkpoints.

Tumour Suppressor Genes

• Normally act as the brakes of the cell.

• Mutation (loss-of-function) removes those brakes.

• Examples: p53, Rb, BRCA1/2

You need multiple mutations (both oncogenes activated + tumour suppressors lost) to truly cause cancer. Cancer is usually a multi-step process.

Common Tumour Suppressor Pathways

Key idea: Tumour suppressor genes fall into two major groups:

1. Gatekeeper Genes – directly control cell growth and death

   • Example: p53 (mutated in ~50% of cancers), blocks division when DNA is damaged

   • TGFβ pathway also inhibits growth via SMAD proteins

   • Others: RB, APC, PTEN, etc.

2. Caretaker Genes – repair DNA to prevent mutations

   • Examples:

     • BRCA1/2 = repair double-stranded breaks

     • MSH2, MLH1, etc. = mismatch repair

     • XPA, XPB, XPC, etc. = excision repair

     • XPV = translesion synthesis (lets DNA polymerase pass lesions)

Why important?
If these genes are lost (mutated), DNA damage builds up, or growth isn’t stopped, which can lead to cancer.

Tumour Suppressor Genes & Two-Hit Hypothesis

Concept: We inherit two copies of each gene (one from each parent).

• Tumour suppressor genes follow the "two-hit hypothesis":

  • Both copies must be damaged/mutated for cancer to occur.

  • If you inherit one bad copy, you're already halfway there—only one more hit is needed.

This is why tumour suppressor mutations can run in families—you’re born with the first “hit.”

Two-Hit Hypothesis in Retinoblastoma (Mechanism)

Retinoblastoma (Rb) is a childhood cancer of the eye.

• The Rb protein normally blocks E2F, which is needed to enter S-phase (DNA replication).
  → This halts the cell cycle at the G1/S checkpoint.

• If Rb is mutated, it can’t block E2F → cell keeps dividing → cancer.

In hereditary cases:

• One copy of RB gene is already mutated (1st hit) from birth.

• A second mutation in a retinal cell causes cancer (2nd hit).

Hereditary vs Non-Hereditary Retinoblastoma

This diagram compares:

Hereditary retinoblastoma:

• One mutated RB gene is inherited in all body cells (first hit).

• A second random mutation in a retinal cell → cancer in both eyes (bilateral), early onset.

Non-hereditary (sporadic):

• Person inherits two normal copies of RB.

• Two random mutations must occur in the same cell.

• Much rarer → one eye affected, later onset.

Summary: Hereditary = higher risk because you're born halfway to cancer.

p53 and DNA Damage Checkpoints

p53 is called the “guardian of the genome” for a reason:

1. p53 is activated by DNA damage (via ATM/ATR and checkpoint kinases).

2. It stops the cell cycle at G1/S and G2/M checkpoints.

   • Activates p21, which blocks Cdk–cyclin → stops division.

3. If damage can’t be fixed, p53:

   • Activates Puma, which removes block on apoptosis → cell dies.

If p53 is mutated (which is very common in cancers):

• Cells keep dividing with damaged DNA.

• No apoptosis = cells become immortal and cancerous.

What Do BRCA1 and BRCA2 Do?

• BRCA1 and BRCA2 are tumour suppressor genes.

• They help maintain genome integrity by fixing double-strand DNA breaks using the homologous recombination repair (HRR) pathway.

• When mutated, DNA damage accumulates, increasing cancer risk.

• They also play roles in mitosis (cell division control).

Think of BRCA1/2 as DNA repair workers. If they’re not working (mutated), the cell’s DNA becomes unstable.

Homologous Recombination (HR) and BRCA

These slides show how BRCA1/2 help repair DNA double-strand breaks:

Step-by-step:

1. Double-strand break occurs.

2. End processing: Enzymes like Rad50/Mre11 prepare the DNA ends.

3. Strand invasion: Rad51 proteins help a good DNA strand invade the broken one to guide repair.

4. Repair synthesis and resolution: New DNA is synthesized using the undamaged strand as a template.

BRCA1 & BRCA2 roles:

• BRCA1: Recruits repair proteins to damage sites.

• ATM kinase helps activate BRCA1.

• BRCA2: Loads Rad51 onto DNA → essential for strand invasion.

If BRCA1 or BRCA2 is mutated, the repair is faulty or incomplete, leading to genomic instability, a key driver of cancer.

Treatments

Treatment strategies for cancers (especially with BRCA mutations) include:

1. Radiation therapy – Damages DNA to kill cancer cells (especially effective when BRCA is mutated because repair is poor).

2. Chemotherapy – Drugs that stop cell division.

3. Immunotherapy – Boosts the immune system to attack cancer.

4. Precision medicine – Targets cancer’s specific genetic mutations (e.g. PARP inhibitors work well for BRCA-mutant cancers).

Radiation Therapy

• Uses X-rays or ionizing radiation to cause massive DNA damage.

• Works best when p53 is functional, because DNA damage activates p53-induced apoptosis (cell death).

• Cancer cells are usually worse at repairing DNA than normal cells, so they die more easily.

  Key point: It's localized and relies on exploiting cancer cells' poor repair systems.

Chemotherapy

• Targets rapidly dividing cells, both cancerous and normal (e.g., hair, gut lining).

How it works (various mechanisms shown in the slides):

1. Inhibits nucleotide production → can't make DNA.

2. Crosslinks DNA (e.g., cisplatin) → strands can't separate → no replication.

3. Disrupts topoisomerases → DNA can't unwind (shown in DNA-doxorubicin complex).

4. Prevents mitotic spindle formation → cells can't divide (see next slide for details).

Examples:

• Vinblastine/Vincristine: Prevent microtubule elongation.

• Taxol: Stabilizes microtubules → prevents disassembly → mitosis is blocked.

Key point: Chemo is systemic, affecting the whole body, and targets cell division machinery.

Immunotherapy

• Boosts your own immune system to fight cancer.

How it works:

• CAR-T cells: Engineered T-cells that hunt cancer.

• Checkpoint inhibitors: Block "off switches" like PD-1/PD-L1, so T-cells can stay active.

• Monoclonal antibodies: Target specific markers on cancer cells.

• Some antibodies are conjugated with drugs or radioactive particles to kill cells directly.

Key point: Turns your immune system into a targeted cancer killer.

Precision Medicine

• Matches treatment to the specific mutation or subtype of the patient’s cancer.

Includes:

• Grading: how abnormal the cancer cells look under a microscope (Grade I = mild, Grade III = severe).

• Staging: how far the cancer has spread (Stage 0 = localized, Stage IV = metastasis).

• Subtyping: determining what receptors or genes are involved (see next slides).

Key point: Personalized approach that improves outcomes and reduces side effects.

Grading & Staging Diagrams

• Grading: Histology of tumor cells

  • Grade I: well-differentiated

  • Grade III: poorly differentiated

• Staging: Tumor size and lymph node involvement

  • Stage 0: In situ

  • Stage IV: Spread to distant organs

Important for deciding how aggressive treatment should be.

Subtypes of Breast Cancer

Shows three key subtypes:

1. Hormone receptor-positive (~40%)

   • ER+ or PR+

   • Treated with Tamoxifen (blocks estrogen receptor)

   • Best prognosis

2. HER2-enriched (~10–15%)

   • HER2 overexpressed

   • Fast-growing, treated with HER2 monoclonal antibodies (mAbs)

3. Triple-negative (~15–20%)

   • No ER, PR, or HER2

   • Most aggressive, common in BRCA1 carriers

   • No targeted therapy yet

Subtyping helps tailor treatment—especially useful for hormone blockers or HER2-targeted therapies.

Signaling Pathways

• HER2 Pathway: Activates downstream cascades like MAPK and PI3K-Akt, which promote cell survival and proliferation.

• Estrogen Pathway: Estrogen binds receptor → receptor dimerizes → binds DNA → turns on proliferation genes.

Both are common drug targets in breast cancer.

Cancer Survival Rates

• Shows huge improvements in 5-year survival for most cancers between 1970s and 2010s.

• Best improvements: testicular cancer, Hodgkin lymphoma, prostate, breast.

• Hardest to treat: pancreatic and lung cancer (due to late detection and aggressive nature).