1.1 Accelerator vs Brake Analogy

• Oncogenes = accelerator
• Promote cell growth and proliferation.
• Tumor suppressor genes (TSGs) = brakes
• Apply “equal and opposite” force to restrain proliferation.

When tumor suppressor genes are disrupted:

• Cells become resistant to growth inhibition.
• Behavior starts to mimic oncogene activation.
• Result: uncontrolled proliferation and increased risk of malignant transformation.

1.2 Main Anti-Growth Programs Enforced by TSGs

Tumor suppressors can enforce several “anti-growth programs”:

1. G0 quiescence
• Temporary exit from the cell cycle.
2. Post-mitotic differentiation
• Cells become fully differentiated and stop dividing.
3. Non-replicative senescence
• Permanent cell-cycle arrest without cell death.
4. Apoptosis
• Programmed cell death to remove dangerous or damaged cells.

Why they are crucial:

• TSGs apply brakes at multiple levels:
• Cell-cycle arrest (checkpoints)
• Differentiation
• Senescence
• Apoptosis
• They maintain tissue architecture and genomic integrity, preventing inappropriate cell division.

2. RB (Retinoblastoma Protein) – “Governor of G1/S”

2.1 What RB Is

• RB gene:
• Classic tumor suppressor gene.
• RB protein:
• Central negative regulator of the G1/S checkpoint (restriction point).
• RB pathway is inactivated in most human cancers:
• Either RB itself is mutated/deleted
• Or components upstream/downstream of RB are altered.

2.2 Discovery and Knudson’s Two-Hit Hypothesis

• Discovered by studying retinoblastoma, a childhood eye tumor.
• Led to concept of the first tumor suppressor gene.

Knudson’s two-hit model:

• Both alleles of RB must be inactivated in a cell for tumor to arise.
• Familial retinoblastoma:
• Patient inherits one mutant allele (germline).
• Only one additional somatic hit needed in a retinal cell.
• Explains early onset and often bilateral tumors.
• Sporadic retinoblastoma:
• Both hits are somatic in the same cell.
• Occurs later, often unilateral.

Dominant clinically, recessive cellularly:

• At the organism level:
• Inheritance looks autosomal dominant (one mutant allele → high tumor risk).
• At the cell level:
• Both alleles must be lost → recessive at cellular level.
• Familial RB survivors have ↑ risk of osteosarcomas and some soft-tissue sarcomas.

2.3 RB and the G1/S Checkpoint

• RB controls the G1/S restriction point:
• Decision point where the cell commits to DNA synthesis (S phase).

Active RB (Hypophosphorylated) – Brake ON

• In early G1, RB is hypophosphorylated (active).
• Active RB:
• Binds E2F transcription factors.
• Recruits chromatin-modifying enzymes:
• Histone deacetylases (HDACs)
• Histone methyltransferases (HMTs)
• This complex represses transcription of:
• S-phase genes (e.g. cyclin E, DNA replication genes).
• Result: cell remains in G1 (no entry into S phase).

Inactivation of RB by Growth Signals – Brake OFF

• Mitogenic signals (growth factors):
• Increase cyclin D.
• Cyclin D + CDK4/6 complexes form.
• These complexes phosphorylate RB:
• RB becomes hyperphosphorylated = inactive.
• RB releases E2F.
• Free E2F:
• Induces transcription of cyclin E and other S-phase genes.
• Cyclin E + CDK2 further drives G1→S transition.

Result: cell commits to DNA replication.

Restraining the RB Axis

• p16 (CDKN2A):
• A CDK inhibitor (CDKI) that inhibits CDK4/6.
• Prevents RB phosphorylation → keeps RB active.
• TGF-β:
• Upregulates p16.
• Strengthens the RB brake by limiting RB phosphorylation.
• During M phase:
• Phosphatases remove phosphate groups from RB.
• RB returns to hypophosphorylated (active) state for the next G1.

Non-E2F Roles of RB

• RB also coordinates cell-cycle exit with differentiation:
• Binds lineage-specific transcription factors in:
• Muscle cells
• Fat cells
• Pigment cells
• Macrophages
• Helps enforce terminal differentiation.

Inactivating RB Without RB Mutation

• Many tumors have intact RB gene but RB pathway inactive because of:
• ↑ CDK4 activity
• ↑ cyclin D
• Loss of p16 (CDKN2A)
• These changes cause persistent RB phosphorylation:
• RB functionally inactivated without direct RB mutation.

Viral Inactivation of RB

• HPV E7 protein:
• Binds hypophosphorylated RB.
• Displaces E2F from RB.
• Leads to uncontrolled S-phase entry.
• This is a key mechanism in HPV-associated cancers (e.g. cervical carcinoma).

3. TP53 (p53) – “Guardian of the Genome”

3.1 What TP53/p53 Is

• TP53 gene encodes p53 protein.
• p53 is the most commonly mutated gene in human cancer.
• It is a central node in cellular stress responses that prevent malignant evolution.

3.2 Protective Outcomes Orchestrated by p53

p53 can direct the cell into one of three major fates:

1. Quiescence
• Temporary cell-cycle arrest to allow DNA repair.
2. Senescence
• Permanent cell-cycle arrest (non-dividing but alive).
3. Apoptosis
• Programmed cell death when damage is irreparable or stress is extreme.

3.3 Stresses That Activate p53

• DNA damage (e.g., radiation, chemicals).
• Hypoxia/anoxia (low or absent oxygen).
• Excessive oncogenic signalling:
• Overactive MYC, RAS, etc.

3.4 Regulation of p53 in Normal Cells

• In unstressed cells, p53 is kept low:
• MDM2:
• E3 ubiquitin ligase that ubiquitinates p53.
• Targets p53 for proteasomal degradation.
• p53 has a short half-life (~20 min).

3.5 Activation/ Stabilization of p53 Under Stress

• DNA damage and other stresses activate ATM/ATR and other kinases:
• These kinases phosphorylate p53 and MDM2.
• Phosphorylation disrupts p53–MDM2 binding.
• p53 accumulates and becomes activated as a transcription factor.
• Activated p53 transactivates target genes that mediate arrest, senescence, or apoptosis.

3.6 p53-Mediated G1 Arrest – Link to RB

• p53 induces transcription of p21 (CDKN1A).
• p21:
• Inhibits cyclin-CDK complexes.
• Prevents RB phosphorylation.
• RB stays hypophosphorylated (active).
• E2F remains blocked → G1/S arrest.

3.7 p53-Mediated Apoptosis

• When damage is irreparable or oncogenic stress is too high:
• p53 induces pro-apoptotic genes, including:
• BAX
• PUMA
• These activate the mitochondrial apoptosis pathway.
• This removes potentially malignant cells.

3.8 “Guardian of the Genome”

• p53 preserves genomic integrity by:
• Pausing the cell cycle for DNA repair.
• Forcing senescence in chronically damaged cells.
• Inducing apoptosis in severely damaged or oncogene-stressed cells.

3.9 Consequences of TP53 Loss

• Without functional p53:
• DNA damage is not repaired adequately.
• Mutations accumulate.
• Chromosomal instability increases.
• Malignant transformation accelerates.
• 70 % of cancers harbor p53 pathway defects:
• Direct TP53 mutations.
• Or MDM2 amplification / other pathway alterations.
• Common tumors with biallelic TP53 mutations:
• Lung carcinoma
• Colon carcinoma
• Breast carcinoma

3.10 Inherited TP53 Mutation – Li-Fraumeni Syndrome

• Li-Fraumeni syndrome:
• Germline TP53 mutation.
• ~25-fold increased cancer risk.
• Characterized by:
• Sarcomas
• Early-onset breast cancer
• Leukemias
• Brain tumors
• Adrenocortical carcinomas
• Often multiple primary tumors in the same individual.

3.11 Viral Neutralization of p53

• Oncogenic viruses (e.g. HPV, polyomaviruses, HBV) produce proteins that:
• Bind to p53.
• Inactivate or promote degradation of p53.
• Example: HPV E6 binds p53 and promotes its degradation.

4. TGF-β Pathway – Early Brake, Late Double-Agent

4.1 What TGF-β Is

• Transforming growth factor-β (TGF-β):
• A dimeric growth factor.
• TGF-β family includes:
• BMPs (bone morphogenetic proteins)
• Activins

4.2 Normal Anti-Proliferative Function

• TGF-β is a potent inhibitor of proliferation in:
• Epithelial cells
• Endothelial cells
• Hematopoietic (blood-forming) cells

4.3 Receptor and SMAD Signalling

• TGF-β binds to a receptor complex:
• TGF-βRII and TGF-βRI.
• Ligand binding:
• Causes receptor dimerization and activation.
• Activates SMAD signalling (SMAD2/3 and SMAD4).

4.4 Transcriptional Effects – How It Inhibits Growth

• TGF-β signalling leads to:
• ↑ CDK inhibitors (e.g. p21).
• Repression of growth promoters such as:
• MYC
• CDK4
• Overall effect: cell-cycle arrest and growth inhibition.

4.5 How Tumors Escape TGF-β Control

• Cancers can disable TGF-β growth-inhibitory effects by:
1. Loss-of-function mutations in:
• TGF-βRII
• SMADs (especially SMAD4)
2. Downstream resistance, such as:
• Decreased p21
• Increased MYC
• Cancers with common TGF-βRII mutations:
• Colon carcinoma
• Stomach carcinoma
• Endometrial carcinoma
• Cancer with frequent SMAD4 loss:
• Pancreatic carcinoma

4.6 Pro-Tumor Roles in Late Cancer

• In advanced tumors, TGF-β often becomes pro-tumorigenic:
• Promotes epithelial-to-mesenchymal transition (EMT).
• Stimulates angiogenesis.
• Creates immune suppression.
• Net result: increased invasion and metastasis.

4.7 EMT and TGF-β

• Epithelial-to-mesenchymal transition (EMT):
• Epithelial cells lose polarity and cell-cell contacts.
• Gain a more motile, invasive phenotype.
• TGF-β is a key EMT driver in advanced cancers:
• Upregulates EMT-related transcription factors (e.g. TWIST, SLUG).
• Contributes to metastatic spread.

5. Contact Inhibition, NF2 (Merlin), APC/β-Catenin/WNT

5.1 Contact Inhibition – Basic Concept

• Contact inhibition:
• Normal cells stop dividing when they achieve confluence and are fully surrounded by neighbors.
• Prevents overgrowth and “piling up”.
• Cancer cells:
• Lose contact inhibition.
• Continue proliferating and overgrow in layers.

5.2 E-Cadherin – Cell–Cell Adhesion

• E-cadherins:
• Key molecules that mediate epithelial cell–cell adhesion.
• Form homotypic interactions between adjacent cells.
• E-cadherin also binds β-catenin, linking adhesion to signalling pathways.

5.3 NF2 (Merlin) – Mechanism 1 of Contact Inhibition

• NF2 gene encodes Merlin.
• Functions:
• Acts downstream of E-cadherin to propagate “stop proliferation” signals.
• Crucial for proper contact inhibition in some tissues, especially neural.
• Loss of NF2:
• Homozygous loss leads to neural tumors.
• Germline NF2 mutation causes neurofibromatosis type 2:
• Characterized by vestibular schwannomas and other neural tumors.

5.4 E-Cadherin and β-Catenin – Link to WNT Pathway

• E-cadherin:
• Binds β-catenin at the cell membrane.
• Helps maintain epithelial organization and polarity.
• β-catenin:
• Also a key effector of the WNT signalling pathway.
• When free in cytoplasm and stabilized, it can move to nucleus and:
• Co-activate transcription of growth and EMT genes.

5.5 APC and β-Catenin in WNT Signalling

APC in the Absence of WNT

• APC:
• Tumor suppressor that forms a complex promoting β-catenin degradation when WNT is absent.
• Prevents β-catenin accumulation and restrains WNT target gene expression.

When WNT Is Present

• WNT signalling:
• Inhibits β-catenin destruction complex.
• β-catenin accumulates, moves to nucleus, and:
• Co-activates transcription of genes such as:
• MYC
• Cyclin D1
• EMT regulators (e.g. TWIST, SLUG) that repress E-cadherin.
• Consequences:
• Increased cell proliferation.
• Enhanced EMT and invasiveness.

5.6 APC and Colon Cancer

• Germline APC mutation:
• Causes familial adenomatous polyposis (FAP):
• Thousands of adenomatous polyps in the colon.
• Very high risk of colorectal carcinoma.
• Second hit in APC:
• Stabilizes β-catenin.
• Leads to increased MYC and cyclin D1, and decreased E-cadherin.
• Drives progression from polyp to carcinoma.
• Sporadic APC mutations:
• Seen in ~70–80 % of sporadic colon cancers.

5.7 Alternative WNT Activation with Intact APC

• If APC is intact, WNT signalling can still be constitutively activated by:
• Activating mutations in β-catenin:
• Make β-catenin resistant to APC-mediated degradation.
• β-catenin accumulates and drives continuous WNT target gene expression.

6. Integrated One-Page Summary

• Tumor suppressor genes act as brakes:
• Enforce quiescence, differentiation, senescence, apoptosis.
• Prevent inappropriate cell cycle progression.
• RB pathway:
• RB is the governor of G1/S.
• Active (hypophosphorylated) RB binds E2F and represses S-phase genes.
• Mitogens → cyclin D/CDK4/6 → RB phosphorylation → free E2F → S-phase entry.
• p16 and TGF-β restrain this axis; HPV E7 inactivates RB.
• p53 pathway:
• p53 is the guardian of the genome.
• Activated by DNA damage, hypoxia, or oncogene stress.
• Causes p21-mediated arrest, senescence, or apoptosis via BAX/PUMA.
• Kept low by MDM2; stress via ATM/ATR stabilizes it.
• TP53 loss or MDM2 amplification → genomic instability, high cancer risk.
• Li-Fraumeni = germline TP53 mutation with broad tumor spectrum.
• TGF-β pathway:
• In normal epithelium, strong anti-proliferative factor.
• Via SMADs, increases p21, represses MYC/CDK4.
• Cancers escape via TGF-βRII or SMAD (esp. SMAD4) loss or downstream override (↓p21, ↑MYC).
• Later in cancer, TGF-β can promote EMT, angiogenesis, immune suppression → metastasis.
• Contact inhibition & WNT:
• E-cadherin and NF2 (Merlin) mediate contact inhibition; NF2 loss → neural tumors (NF2 syndrome).
• APC keeps β-catenin low without WNT.
• WNT or β-catenin mutations → β-catenin accumulation → MYC, cyclin D1, EMT genes (TWIST/SLUG) up; E-cadherin down.
• Germline APC mutation → FAP; sporadic APC or β-catenin mutations common in colon carcinoma.

🔑 TUMOR SUPPRESSOR GENES — EXAM REFLEX BLOCK

🚗 Accelerator vs Brake

• Oncogenes = accelerator → push proliferation
• Tumor suppressor genes (TSGs) = brakes → restrain proliferation
• Loss of TSGs → resistance to growth inhibition → phenocopies oncogene activation

🧱 CORE ANTI-GROWTH PROGRAMS (What TSGs enforce)

TSGs stop cancer by enforcing one of FOUR fates:

1. G0 quiescence → temporary arrest
2. Terminal differentiation → permanent exit from cycle
3. Senescence → alive but non-replicative
4. Apoptosis → eliminate dangerous cells

👉 Exam line: TSGs act at checkpoints, differentiation, senescence, and apoptosis to preserve genomic integrity.

🧠 RB (RETINOBLASTOMA PROTEIN) — “G1/S GOVERNOR”

🔒 Core Function

• Negative regulator of G1/S restriction point
• Pathway inactivated in most cancers (directly or indirectly)

🧬 Knudson Two-Hit Hypothesis (Classic Viva Favorite)

• Both RB alleles must be lost in a cell
• Familial RB:
• 1 germline hit + 1 somatic hit
• Early onset, bilateral
• Sporadic RB:
• 2 somatic hits
• Late onset, unilateral
• Clinically dominant, cellularly recessive
• Survivors → ↑ osteosarcoma risk

⛔ Active RB = Brake ON

• Hypophosphorylated RB
• Binds E2F
• Recruits HDAC + HMT
• Represses S-phase genes (cyclin E, DNA replication genes)
• Cell stays in G1

▶️ Inactivated RB = Brake OFF

• Growth factors → ↑ cyclin D
• Cyclin D + CDK4/6 → RB phosphorylation
• E2F released
• ↑ cyclin E + CDK2
• G1 → S commitment

🧯 RB Restraints (Important Control Points)

• p16 (CDKN2A) inhibits CDK4/6
• TGF-β ↑ p16,inhibits CDK4/6
• M-phase phosphatases → dephosphorylate RB

👉 Exam trap: RB mutation NOT required if cyclin D↑ or p16↓

🧬 Non-E2F Role

• Coordinates cell-cycle exit + differentiation
• RB coordinates permanent cell-cycle exit with terminal differentiation
• Muscle, adipocytes, macrophages, melanocytes

🦠 Viral Inactivation

• HPV E7 binds hypophosphorylated RB → releases E2F
• Key in cervical carcinoma

🧿 TP53 (p53) — “GUARDIAN OF THE GENOME”

🎯 Core Role

• Most commonly mutated gene in cancer
• Central stress sensor

⚡ TP53 Activating Stresses

• DNA damage
• Hypoxia
• Oncogene overdrive (MYC, RAS)

🔄 Normal Regulation

• MDM2 ubiquitinates p53
• Short half-life (~20 min)
• MDM2 amplification functionally inactivates p53 without TP53 mutation (common in sarcomas)

🚨 Activation Under Stress

• ATM/ATR phosphorylate p53 + MDM2
• Disrupt p53–MDM2 binding
• p53 accumulates → transcription factor

⏸️ G1 Arrest (RB Link)

• p53 → ↑ p21
• p21 inhibits CDKs
• RB remains hypophosphorylated
• E2F blocked

☠️ Apoptosis Pathway

• p53 → BAX, PUMA
• Mitochondrial apoptosis
• Removes irreparable cells

🧬 Consequences of TP53 Loss

• No DNA repair pause
• Mutation accumulation
• Chromosomal instability
• ~70% cancers have p53 pathway defects
• Common: lung, colon, breast

🧬 Li-Fraumeni Syndrome

• Germline TP53 mutation
• 25× cancer risk
• Sarcomas, early breast ca, leukemia, brain tumors, adrenal carcinoma
• Multiple primaries

🦠 Viral Neutralization

• HPV E6 → p53 degradation

🔁 TGF-β PATHWAY — EARLY BRAKE, LATE TRAITOR

🛑 Normal Function

• Potent anti-proliferative
• by Epithelium, endothelium, hematopoietic cells

🔗 Signalling

• TGF-βRII + TGF-βRI
• SMAD2/3 + SMAD4

📉 Growth Inhibition

• ↑ p21
• ↓ MYC, CDK4
• Cell-cycle arrest

🚪 Tumor Escape

• Mutations in TGF-βRII
• Loss of SMAD4 (pancreatic cancer)
• ↓ p21 or ↑ MYC
• Seen in colon, stomach, endometrial ca

🧨 Late Cancer Role

• Promotes EMT(epithelial to mesenchyme transition)
• ↑ angiogenesis
• Immune suppression
• ↑ invasion & metastasis

🧱 CONTACT INHIBITION & WNT

🧲 Contact Inhibition

• Normal cells stop dividing at confluence(continous & single layer of cells)
• Cancer cells pile up-no confluence

🔗 E-Cadherin

• Cell–cell adhesion
• Sequesters β-catenin

🧠 NF2 (Merlin)

• Downstream of E-cadherin
• Enforces contact inhibition
• Loss → neurofibromatosis type 2
• Vestibular schwannomas

🔁 APC / β-Catenin / WNT

• No WNT → APC gene degrades β-catenin
• WNT present → β-catenin accumulates
• Nuclear β-catenin → ↑ MYC, cyclin D1, EMT genes
• ↓ E-cadherin
• WNT signalling stabilizes β-catenin by disabling APC gene-mediated degradation, allowing nuclear β-catenin to activate MYC, cyclin D1, and EMT programs while suppressing E-cadherin.

🧬 Colon Cancer Logic

• Germline APC loss → FAP
• Second hit → carcinoma
• 70–80% sporadic colon cancers → APC mutation
• Alternative: β-catenin activating mutation

🧠 ONE-LINE MASTER LOCK (Exam Gold)

Cancer results when tumor suppressor brakes (RB, p53, TGF-β, APC, NF2) fail, allowing unchecked cell-cycle entry, impaired DNA repair, loss of contact inhibition, EMT, and genomic instability.