1.Molecular Pathogenesis of Cancer

1. Why gene-by-gene description failed

• Large-scale sequencing shows:
• Each tumor contains hundreds of mutated genes.
• Mutations differ widely across tumors.
• Impossible to analyse cancers meaningfully by memorizing each gene individually.
• Requires a systems/phenotype-level framework, not isolated lists.

Key takeaway:

Cancer = complex network of mutations → must study shared biological behaviours.

2. Hallmark model = modern framework

All cancers acquire a shared set of phenotypic capabilities enabling malignant growth.

Eight fundamental hallmarks

1. Self-sufficiency in growth signals
• Oncogenes activated (RAS, MYC)
• Tumor makes or constitutively activates its own GF pathways
2. Insensitivity to growth-inhibitory signals
• Loss of tumor suppressors (RB, TP53, TGF-β pathway)
• Cell cycle checkpoints disabled
3. Altered cellular metabolism – Warburg effect
• Aerobic glycolysis (glycolysis despite O₂ availability)
• Provides ATP rapidly + biomass precursors for growth
4. Evasion of apoptosis
• Mutated TP53
• ↑ anti-apoptotic proteins: BCL-2 family
• Survival even with severe DNA damage
5. Limitless replicative potential
• Telomerase reactivation
• Avoid senescence/crisis
6. Sustained angiogenesis
• ↑ VEGF, FGF
• Hypoxia-inducible factor (HIF-1) stabilization
• Needed beyond ~1–2 mm thickness
7. Invasion & metastasis
• Loss of E-cadherin → loss of adhesion
• Matrix metalloproteinases (MMPs) degrade ECM
• EMT-like phenotype change
8. Evasion of immune surveillance
• ↓ MHC I
• ↑ PD-L1 or CTLA-4 signalling
• T-cell anergy/exhaustion

3. Enabling characteristics: accelerate hallmark acquisition

These are not hallmarks themselves, but create fertile ground for malignant evolution:

1. Genomic instability
• Defects in DNA repair pathways
• e.g., BRCA1/2 → HR repair defects
• MLH1/MSH2 → mismatch repair defects
• ↑ mutation rate → accelerates other hallmark acquisition
2. Cancer-promoting inflammation
• Chronic inflammation recruits immune cells releasing:
• cytokines
• ROS
• growth factors
• proteases
• Creates microenvironment supporting tumor survival and progression

4. Pathogenic gene classes linked to hallmarks

Cancer affects key genetic regulators:

A. Proto-oncogenes → Oncogenes

• Type of mutation: gain-of-function
• Promote hallmark:
• self-sufficient growth signals

Examples:

• RAS, MYC, ABL, EGFR

B. Tumor suppressor genes

• Type of mutation: loss-of-function
• Promote hallmark:
• insensitivity to inhibitory signals
• evasion of apoptosis

Examples:

• RB, TP53, APC, CDKN2A, PTEN

C. DNA repair genes

• Loss → genomic instability enabling mutations across genome

Examples:

• BRCA1/2, MLH1, MSH2

Bottom line:

Each tumor is a sum of accumulating hits in these 3 functional classes → produces malignant phenotype.

5. Hallmark logic in exam questions

When given:

• Oncogene activation → think growth independence
• Tumor suppressor loss → think cell cycle brake failure / apoptosis escape
• DNA repair defects → genomic instability
• PD-L1 signalling → immune evasion
• VEGF/HIF-1 → angiogenesis
• Telomerase activation → immortality

6. Gene/protein writing conventions

• Gene names italicized
• TP53, RB1, MYC
• Protein product plain text
• p53, RB protein, MYC protein

Mnemonic:

blueprint italicized, product non-italicized.

7. Clinical consequence of hallmark model

• Explains why cancer requires multiple mutations (“multi-hit hypothesis”).
• Informs targeted therapy development:
• anti-angiogenic therapy (anti-VEGF)
• immune checkpoint blockade (anti PD-1/PD-L1)
• PARP inhibitors for DNA repair-defective tumors

HALLMARKS OF CANCER — PROPER EXAM REFLEX BLOCK

Use this as a single-glance conversion tool in SBAs & MCQs.

STEM → IMMEDIATE THOUGHT → ANSWER

1. Oncogene activation

Stem clues

• RAS, MYC, EGFR, ABL
• proto-oncogenes → oncogenes (gain-of-function)
• Autocrine growth factor signaling
• Constitutive tyrosine kinase activity

Reflex

→ Self-sufficiency in growth signals

2. Tumor suppressor loss

Stem clues

• RB loss → unchecked G1→S
• TP53 mutation → no DNA damage arrest
• APC loss → unchecked Wnt/β-catenin signalling
• PTEN loss → uninhibited PI3K–AKT–mTOR survival/growth pathway

Reflex

→ Insensitivity to growth-inhibitory signals→ ± Evasion of apoptosis (especially TP53)

3. Glycolysis despite oxygen

Stem clues

• Aerobic glycolysis
• High glucose uptake
• Lactate production with normal O₂

Reflex

→ Altered cellular metabolism (Warburg effect)

4. Survival despite DNA damage

Stem clues

• TP53 mutation - Failure of G1/S arrest after DNA damage
• BCL-2 overexpression- Blocks mitochondrial (intrinsic) apoptosis
• Resistance to chemotherapy-induced apoptosis

Reflex

→ Evasion of apoptosis

5. Unlimited divisions

Stem clues

• Telomerase reactivation
• Avoidance of senescence/crisis
• Immortal cell lines

Reflex

→ Limitless replicative potential

6. Hypoxia response / vessel growth

Stem clues

• VEGF ↑
• HIF-1 stabilization
• Tumor growth beyond 1–2 mm

Reflex

→ Sustained angiogenesis

7. Local invasion or distant spread

Stem clues

• Loss of E-cadherin
• MMP secretion
• EMT-like phenotype

Reflex

→ Invasion & metastasis

8. Immune escape

Stem clues

• ↑ PD-L1 expression-Switches off cytotoxic T-cell killing
• ↑ CTLA-4 signaling-T-cell activation blocked
• ↓ MHC I on tumor cells-Cytotoxic CD8⁺ T cells cannot recognize tumor
• T-cell exhaustion-↓ cytokine production, ↓ cytotoxicity

Reflex

→ Evasion of immune surveillance

ENABLING CHARACTERISTICS — SEPARATE REFLEX

A. DNA repair defects

Stem clues

• BRCA1/2- Defective homologous recombination DNA repair
• MLH1 / MSH2 - Defective DNA mismatch repair
• Microsatellite instability - Accumulation of insertion–deletion mutations

Reflex

→ Genomic instability

→ Accelerates acquisition of all hallmarks

B. Chronic inflammation

Stem clues

• Long-standing inflammation
• Cytokines, ROS, proteases
• Tumor-supportive microenvironment

Reflex

→ Cancer-promoting inflammation

GENE CLASS → FUNCTION REFLEX

• Proto-oncogene → oncogene (gain-of-function)

→ Growth signal autonomy

• Tumor suppressor gene loss (loss-of-function)

→ Loss of brakes / apoptosis escape

• DNA repair gene loss

→ Genomic instability

ONE-LINE EXAM LOCK

Cancer is not a list of genes — it is a set of shared phenotypic behaviours produced by accumulated hits in oncogenes, tumor suppressors, and DNA repair genes