Hallmarks of Cancer

1. Self-Sufficiency in Growth Signals (Oncogenes)

• Growth factors (autocrine loops): Tumor cells secrete their own growth factors and co-express receptors (e.g., glioblastomas secrete PDGF and overexpress PDGF receptors; sarcomas secrete TGF-α and express EGF receptors).
• Growth factor receptors: Mutations cause constitutive signaling without ligand binding. Examples:
  • EGF receptor - point mutations in lung cancer
  • HER2/ERBB2 - gene amplification in breast cancer
  • JAK2 - point mutations in myeloproliferative neoplasms
• Downstream signal transducers:
  • RAS - most commonly mutated oncogene in human cancers; point mutations (especially codon 12) impair GTPase activity, causing constitutive RAS-GTP signaling through MAPK and PI3K/AKT pathways
  • BCR-ABL fusion in CML - constitutive ABL tyrosine kinase activity
  • BRAF and PI3K mutations in many cancers
• Transcription factors: MYC is a master transcription factor upregulated via chromosomal translocation (Burkitt lymphoma), gene amplification (neuroblastoma), or upstream signaling. MYC drives expression of cyclins, CDKs, and metabolic genes supporting rapid growth.
• Cyclins and CDKs: CDK4/cyclin D complex mutations accelerate G1-S progression.

2. Insensitivity to Growth-Inhibitory Signals (Tumor Suppressor Genes)

RB Gene - "Governor of the Cell Cycle"

• RB protein (pRB) normally keeps cells in G1 by binding and inhibiting the E2F family of transcription factors.
• In response to mitogenic signals, CDK4-6/cyclin D complexes phosphorylate RB, releasing E2F and permitting S-phase entry.
• In cancer: loss of RB function (by mutation, deletion, or hyperphosphorylation) keeps E2F constitutively active.
• Inactivation by CDK4 amplification, cyclin D overexpression, or loss of CDK inhibitor p16/INK4a all effectively disable the RB pathway.
• Germline loss-of-function mutation → familial retinoblastoma.

TP53 - "Guardian of the Genome"

• p53 is activated by DNA damage, hypoxia, oncogene activation, telomere shortening - essentially any cellular stress.
• Activated p53 acts as a transcription factor inducing:
  • p21 - CDK inhibitor causing G1 arrest (for DNA repair)
  • GADD45 - DNA repair gene
  • BAX - pro-apoptotic BCL2 family member (triggers apoptosis if damage irreparable)
• p53 is the most commonly mutated gene in human cancers (~50% of all cancers). In Li-Fraumeni syndrome, germline TP53 mutation confers increased risk of multiple tumors.
• MDM2 is the chief negative regulator of p53; MDM2 amplification (seen in many sarcomas) inactivates p53 without mutation.

Other Tumor Suppressors:

• CDKN2A (p16/INK4a): Inhibits CDK4/cyclin D; also encodes p14/ARF which stabilizes p53 by inhibiting MDM2. Mutated in melanoma, bladder cancer.
• TGF-β pathway: In normal cells, TGF-β activates SMAD proteins to arrest growth. In cancer, SMAD4 is frequently mutated (pancreatic cancer); TGF-β receptors are mutated in colon/endometrial cancers.
• E-cadherin (CDH1): Loss allows β-catenin release and activation of Wnt target genes driving proliferation; germline CDH1 mutations cause familial gastric carcinoma.
• NF2 (Merlin), VHL, APC, PTEN are other important tumor suppressor genes.

3. Altered Cellular Metabolism (Warburg Effect)

• Aerobic glycolysis is less efficient in ATP production but generates biosynthetic precursors (nucleotides, lipids, amino acids) needed for rapid cell growth.
• This is driven by oncogenic activation of HIF-1α (hypoxia-inducible factor), RAS, and MYC, which upregulate glucose transporters (GLUT1) and glycolytic enzymes.
• Autophagy - cancer cells under nutrient stress can activate autophagy to degrade organelles and macromolecules for energy. Autophagy may also help cancer cells survive initial growth in an avascular environment.
• Oncometabolism: Mutations in IDH1/IDH2 (in gliomas and AML) produce a neomorphic product, 2-hydroxyglutarate, which inhibits α-ketoglutarate-dependent enzymes, causing widespread epigenetic dysregulation.

4. Evasion of Apoptosis (Resisting Cell Death)

• Loss of TP53 function: p53 normally triggers apoptosis via BAX and PUMA; its loss in cancer prevents this. TP53 mutation frequency is even higher in relapsed tumors (after chemotherapy).
• BCL2 overexpression: BCL2 is an anti-apoptotic protein. Translocation t(14;18) in follicular lymphoma fuses BCL2 to the immunoglobulin heavy chain locus → BCL2 overexpression → cells resist apoptosis. These tumors are often indolent (low proliferation but low cell death). In CLL, loss of specific microRNAs leads to BCL2 upregulation.
• Overexpression of IAP (inhibitor of apoptosis) proteins prevents caspase activation.
• Apoptosis via the intrinsic (mitochondrial) pathway requires pro-apoptotic BH3-only proteins (PUMA, BIM, BAD) to overcome anti-apoptotic BCL2, BCLXL, and MCL1.
• Therapeutic targeting: Venetoclax (BCL2 inhibitor) is used in CLL; MDM2 inhibitors are in clinical trials.

5. Limitless Replicative Potential (Immortality)

• Telomeres are TTAGGG repeats at chromosome ends that protect chromosomal integrity.
• Cancer cells bypass this limit by upregulating telomerase, the ribonucleoprotein enzyme that adds telomeric repeats to chromosome ends.
• Telomerase is expressed at high levels in ~90% of human cancers but is silent in most somatic cells.
• Without telomerase reactivation, cancer cells in crisis show chromosomal end-to-end fusions, breakage-fusion-bridge cycles, and massive genomic instability.
• In addition, cancer cells may have stem cell-like properties allowing indefinite self-renewal.

6. Sustained Angiogenesis

• The balance between pro-angiogenic and anti-angiogenic factors is called the angiogenic switch.
• VEGF (Vascular Endothelial Growth Factor) is the chief pro-angiogenic factor; its expression is upregulated by hypoxia (via HIF-1α), RAS, and TP53 loss.
• VEGF-A, FGF-2 (basic FGF): Released from tumor cells and from the ECM by proteolytic enzymes.
• Anti-angiogenic factors (thrombospondin-1, endostatin, angiostatin) are downregulated in cancers. TP53 normally upregulates thrombospondin-1; loss of p53 shifts the balance toward angiogenesis.
• Tumor vasculature is often abnormal - leaky, tortuous, and inefficient, contributing to hypoxia.
• Anti-VEGF therapy (bevacizumab) is used clinically in many solid tumors.

7. Ability to Invade and Metastasize

Invasion of the Extracellular Matrix (ECM)

1. Loosening of cell-cell contacts: Loss of E-cadherin (by mutation, epigenetic silencing, or SNAIL/TWIST-mediated repression). E-cadherin maintains normal epithelial integrity; its loss promotes Epithelial-Mesenchymal Transition (EMT).
2. Degradation of ECM: By matrix metalloproteinases (MMPs) and cathepsins secreted by tumor and stromal cells. Proteolysis also releases growth factors (IGF, TGF-β) and angiogenic fragments from ECM.
3. Attachment to novel ECM components: Altered integrins allow cancer cells to bind type IV collagen and laminin.
4. Migration: Directed by chemotactic factors, proteolytic fragments, and actin cytoskeleton reorganization.

Vascular Dissemination and Metastasis

• Tumor cells enter lymphatics or blood vessels (intravasation), survive as circulating tumor cells (CTCs), and extravasate at distant sites.
• Metastatic patterns are often predictable: lung, liver, brain, and bone are common sites.
• Organ tropism: Some tumors preferentially metastasize to specific organs due to matching chemokine receptors on tumor cells and their ligands on target endothelium (e.g., CXCR4 on tumor cells/CXCL12 on bone marrow endothelium).
• Epithelial-Mesenchymal Transition (EMT): Transcription factors TWIST and SNAIL drive EMT, enabling cells to acquire mesenchymal, migratory properties. At the metastatic site, reverse MET (mesenchymal-epithelial transition) occurs for colonization.
• Tumor dormancy: Metastatic cells may fail to grow at distant sites for years due to immune suppression or unfavorable microenvironment, then suddenly reactivate.
• Breast cancer metastatic to bone: cancer cells secrete PTHRP → osteoblasts make RANKL → osteoclast activation → bone resorption → release of IGF and TGF-β → cancer cell growth.

8. Evasion of Immune Surveillance

Tumor Antigens (targets of immune recognition)

• Tumor-specific antigens (TSAs): Arise from mutated proteins (neoantigens) unique to tumor cells - driver mutations or passenger mutations.
• Tumor-associated antigens (TAAs): Overexpressed normal proteins (e.g., HER2), cancer-testis antigens (MAGE family), and viral antigens (HPV E6/E7).

Antitumor Immune Effectors

• CD8+ cytotoxic T lymphocytes (CTLs): Most important; recognize tumor peptide-MHC I complexes and kill cancer cells.
• NK cells: Kill cells with low MHC I expression.
• Macrophages, antibody-dependent cellular cytotoxicity (ADCC).

Mechanisms of Immune Evasion

1. Loss of MHC class I expression: Cancer cells downregulate MHC I or TAP (antigen processing), preventing CTL recognition.
2. Lack of co-stimulatory molecules: Without CD80/86 (B7-1/B7-2), T cells become anergic rather than activated.
3. Expression of immune checkpoint ligands: PD-L1 (programmed death ligand 1) on cancer cells binds PD-1 on T cells, inhibiting their cytotoxic activity. This is the basis of anti-PD-1/PD-L1 checkpoint immunotherapy (pembrolizumab, nivolumab, atezolizumab).
4. CTLA-4 pathway: CTLA-4 on T cells competes with CD28 for B7 binding, suppressing T cell activation. Anti-CTLA-4 (ipilimumab) blocks this.
5. Immunosuppressive tumor microenvironment: Tumor cells recruit regulatory T cells (Tregs) and M2 macrophages that suppress immune responses; secrete immunosuppressive cytokines (TGF-β, IL-10).
6. FAS/FAS-L: Tumor cells may express FAS-L, inducing apoptosis in infiltrating T cells.

Enabling Characteristics

A. Genomic Instability

• Cancer cells acquire mutations at a faster rate than normal cells due to impaired DNA repair.
• Mismatch repair deficiency: Inactivation of MLH1, MSH2, MSH6, PMS2 → microsatellite instability (MSI-high); associated with Lynch syndrome (hereditary non-polyposis colorectal cancer - HNPCC) and sporadic colorectal, endometrial cancers. MSI-high tumors are highly responsive to PD-1 checkpoint inhibitors.
• Nucleotide excision repair: Deficiency causes xeroderma pigmentosum; UV-induced skin cancers.
• BRCA1/BRCA2 mutations: Impair homologous recombination repair; associated with familial breast and ovarian cancers. BRCA-deficient tumors are susceptible to PARP inhibitors.
• Chromosomal instability (CIN): Defects in mitotic spindle checkpoint → aneuploidy and chromosomal rearrangements.
• Epigenetic changes: DNA methylation (gene silencing), histone modifications, and chromatin remodeling also drive cancer without mutating DNA sequences.
• Noncoding RNAs: MicroRNAs can act as oncogenes (miR-17-92 cluster) or tumor suppressors (miR-15a, miR-16-1 in CLL); long noncoding RNAs (lncRNAs) also play roles.

B. Tumor-Promoting Inflammation

• Chronic inflammation promotes carcinogenesis by:
  • Supplying bioactive molecules: growth factors, survival signals (EGF, TNF), pro-angiogenic factors (VEGF), ECM-remodeling enzymes (MMPs).
  • Generating reactive oxygen species (ROS) that damage DNA.
  • Providing chemokines and cytokines that attract immunosuppressive cells.
• Examples: H. pylori → gastric cancer; HCV → hepatocellular carcinoma; IBD → colorectal cancer.

Molecular Basis of Multistep Carcinogenesis

• Normal epithelium → adenoma (APC mutation) → intermediate adenoma (KRAS mutation) → late adenoma (SMAD4/TGF-β loss) → carcinoma (TP53 mutation) → metastasis.

Summary Table

Hallmarks of Cancer

Fig. 6.15 - Eight hallmarks and two enabling characteristics (Hanahan & Weinberg, Cell 2011)

INTRODUCTION

"It appears that all cancers display eight fundamental changes in cell physiology, which are considered the hallmarks of cancer." - Robbins Pathologic Basis of Disease, p. 268

HALLMARK 1 - Self-Sufficiency in Growth Signals

1. Growth factor binds its specific receptor
2. Transient activation of receptor tyrosine kinase
3. Signal transduction via second messengers (RAS, PI3K/AKT, MAPK)
4. Activation of transcription factors (MYC)
5. Cell cycle entry and division

• Tumor cells secrete growth factors AND express their own receptors
• Glioblastomas secrete PDGF + overexpress PDGF receptors
• Sarcomas secrete TGF-α + express EGF receptors

• EGFR - point mutations in lung cancer (targeted by gefitinib/erlotinib)
• HER2/ERBB2 - gene amplification in breast cancer (targeted by trastuzumab)
• JAK2 - point mutations in myeloproliferative neoplasms
• Mutant receptors signal constitutively without ligand

• RAS - most commonly mutated oncogene in human cancers (point mutations at codon 12 impair GTPase activity → constitutive RAS-GTP → MAPK + PI3K/AKT signaling)
• BCR-ABL fusion (t(9;22), Philadelphia chromosome) in CML - constitutive ABL tyrosine kinase activity (targeted by imatinib)
• BRAF V600E - melanoma, colorectal cancer
• PI3K mutations - many cancers

• MYC - master transcription factor; drives expression of cyclins, CDKs, metabolic genes
• Upregulated by: chromosomal translocation t(8;14) in Burkitt lymphoma, gene amplification in neuroblastoma, or upstream signaling (many cancers)

• CDK4/cyclin D complex phosphorylates RB → cell cycle progression
• CDK4 amplification or cyclin D overexpression bypasses normal G1 checkpoint

HALLMARK 2 - Insensitivity to Growth-Inhibitory Signals (Tumor Suppressor Genes)

RB Gene - "Governor of the Cell Cycle"

• pRB is the key brake on cell cycle progression at the G1/S checkpoint
• Normally, unphosphorylated pRB binds and inhibits E2F transcription factors, blocking S-phase entry
• Mitogenic signals → CDK4-6/cyclin D complexes → phosphorylate (inactivate) pRB → E2F released → S-phase genes transcribed
• In cancer: loss of RB function (mutation, deletion) keeps E2F constitutively active
• Same effect achieved by: CDK4 amplification, cyclin D overexpression, loss of p16/INK4a
• Germline RB mutation → familial retinoblastoma (autosomal dominant)
• Sporadic retinoblastoma: both alleles lost by somatic mutation (two hits)
• RB pathway disruption found in virtually all human cancers

TP53 - "Guardian of the Genome"

• p53 is activated by: DNA damage, hypoxia, oncogene activation, telomere shortening - any cellular stress
• Activated p53 (transcription factor) induces:
  • p21 (CDKN1A) - CDK inhibitor → G1/S arrest for DNA repair
  • GADD45 - DNA repair protein
  • BAX, PUMA - pro-apoptotic BCL2 family members → apoptosis if damage irreparable
• p53 is the most commonly mutated gene in human cancers (~50%)
• MDM2 is the chief negative regulator of p53 (ubiquitin E3 ligase that degrades p53); MDM2 amplification inactivates p53 without TP53 mutation (seen in many sarcomas)
• Germline TP53 mutation → Li-Fraumeni syndrome (sarcomas, breast cancer, brain tumors, adrenocortical carcinoma)
• TP53 mutations are even more common in relapsed/chemoresistant tumors (p53 also mediates drug-induced apoptosis)

Other Tumor Suppressors

• CDKN2A (p16/INK4a + p14/ARF):
  • p16 inhibits CDK4/cyclin D → keeps pRB active (growth arrest)
  • p14/ARF stabilizes p53 by inhibiting MDM2
  • Mutated in melanoma, bladder cancer, head and neck tumors, ALL
  • Often silenced by promoter hypermethylation in cervical cancer
• TGF-β / SMAD pathway:
  • In normal cells, TGF-β → SMAD2/3 phosphorylation → SMAD4 complex → transcription of CDK inhibitors; repression of MYC, cyclins
  • SMAD4 mutation common in pancreatic cancer
  • TGF-β receptor mutations in colon, stomach, endometrial cancers
• E-cadherin (CDH1): Loss allows β-catenin release, activating Wnt target genes; germline loss → familial gastric carcinoma
• APC, PTEN, NF2 (Merlin), VHL - other important tumor suppressors

HALLMARK 3 - Altered Cellular Metabolism (Warburg Effect)

• Less efficient in ATP production but generates biosynthetic intermediates (nucleotides, lipids, amino acids) needed for rapid cell growth
• Rapidly dividing cells need building blocks more than they need ATP

• HIF-1α (hypoxia inducible factor) upregulates GLUT1 glucose transporter and glycolytic enzymes
• RAS and MYC oncoproteins upregulate metabolic genes
• PET scanning exploits the Warburg effect (18F-FDG uptake)

• Under nutrient stress, cancer cells can degrade organelles and macromolecules for energy and biosynthetic building blocks
• Autophagy may be protective early (nutrient stress in avascular tumor) but may also paradoxically support cancer cell survival

• Mutations in IDH1/IDH2 genes (gliomas, AML) produce the neomorphic metabolite 2-hydroxyglutarate (2-HG)
• 2-HG inhibits α-ketoglutarate-dependent dioxygenases → widespread epigenetic dysregulation (DNA/histone hypermethylation)
• IDH inhibitors (ivosidenib, enasidenib) are FDA-approved for IDH-mutant AML

HALLMARK 4 - Evasion of Apoptosis (Resisting Cell Death)

• Cellular stress → pro-apoptotic BH3-only proteins (PUMA, BIM, BAD) activated
• These inhibit anti-apoptotic BCL2/BCLXL/MCL1
• BAX/BAK oligomerize on mitochondria → cytochrome c release → APAF-1 activation → caspase cascade → cell death

1. Loss of TP53 function - prevents upregulation of PUMA and BAX in response to DNA damage; most cancer cells with intact TP53 would undergo apoptosis when stressed
2. BCL2 overexpression:
   • Translocation t(14;18) in follicular lymphoma fuses BCL2 to immunoglobulin heavy chain locus → BCL2 overexpression
   • These tumors are indolent (low proliferation rate AND low cell death rate)
   • In CLL, loss of miR-15a and miR-16-1 allows BCL2 upregulation
3. Overexpression of IAP (inhibitor of apoptosis) proteins - directly inhibit caspases
4. MDM2 amplification - degrades p53, preventing apoptosis induction

• Venetoclax (BCL2 inhibitor) - approved for CLL, AML
• MDM2 inhibitors (nutlins) - clinical trials for MDM2-amplified sarcomas

HALLMARK 5 - Limitless Replicative Potential (Immortality)

• Telomeres are repetitive DNA sequences (TTAGGG)n at chromosome ends, maintained by telomerase (a ribonucleoprotein reverse transcriptase)
• With each cell division, telomeres shorten (lagging strand problem)
• Critically short telomeres trigger senescence (p53/p21 pathway) or crisis (mitotic catastrophe, apoptosis)

• Cancer cells reactivate telomerase (TERT gene) - expressed at high levels in ~90% of human cancers but silent in most somatic cells
• This maintains telomere length and enables unlimited replication
• Without telomerase, cells entering crisis show chromosome end-to-end fusions → breakage-fusion-bridge cycles → massive genomic instability - which paradoxically generates further mutations
• Some cancers use ALT (Alternative Lengthening of Telomeres) - a recombination-based mechanism

• Cancer stem cells have self-renewal capacity; they express stem cell transcription factors (OCT4, SOX2, NANOG) and are often therapy-resistant

HALLMARK 6 - Sustained Angiogenesis

• Normally, pro- and anti-angiogenic factors are in balance
• Tumors tip this balance toward angiogenesis

• VEGF-A - chief driver; binds VEGFR on endothelial cells → proliferation, migration, vessel formation
• FGF-2 (basic FGF) - released from ECM by MMP proteolysis
• PDGF - stabilizes new vessels

• Thrombospondin-1 - normally upregulated by p53; loss of p53 removes this brake
• Endostatin, angiostatin - fragments of larger ECM proteins

• HIF-1α (activated by hypoxia, RAS, VHL loss) is the master inducer of VEGF
• VHL tumor suppressor loss (von Hippel-Lindau syndrome, clear cell renal carcinoma) prevents HIF-1α degradation

• Bevacizumab (anti-VEGF monoclonal antibody) - colorectal, lung, ovarian, glioblastoma
• Sorafenib, sunitinib (VEGFR tyrosine kinase inhibitors) - renal cell carcinoma, HCC

HALLMARK 7 - Ability to Invade and Metastasize

Invasion of Extracellular Matrix (ECM) - 4 Steps:

• Loss of E-cadherin by mutation, promoter methylation, or SNAIL/TWIST-mediated transcriptional repression
• Epithelial-Mesenchymal Transition (EMT): cancer cells lose epithelial markers (E-cadherin) and gain mesenchymal markers (vimentin, N-cadherin), acquiring invasive, migratory properties

• Matrix metalloproteinases (MMPs) and cathepsins secreted by tumor cells and stromal cells degrade basement membrane and interstitial matrix
• Proteolysis also releases ECM-sequestered growth factors (VEGF, TGF-β, IGF)

• Changed integrin expression allows attachment to laminin and type IV collagen in degraded basement membrane

• Actin cytoskeleton reorganization, directed by chemotactic signals and growth factors

Vascular Dissemination and Metastasis:

• Intravasation: Tumor cells penetrate blood/lymphatic vessels
• Survival in circulation: As circulating tumor cells (CTCs), often in clusters (circulating tumor microemboli) resistant to anoikis
• Extravasation: At distant site, CTCs adhere to endothelium and penetrate vessel wall
• Colonization: Most cells die; few survive and proliferate to form metastasis

• Colon → liver (portal drainage)
• Lung → liver, brain, adrenals, bone
• Prostate → bone (osteoblastic metastases)
• Breast → bone (PTHRP → RANKL → osteoclast activation → bone resorption → IGF/TGF-β release → cancer cell growth), liver, lung, brain

HALLMARK 8 - Evasion of Immune Surveillance

Tumor Antigens:

• Arise from mutated proteins (driver or passenger mutations)
• Unique to tumor cells; strongest immunogenic targets
• Basis of neoantigen-based vaccines and adoptive T cell therapy

• Overexpressed normal proteins (HER2, EpCAM)
• Cancer-testis antigens (MAGE, SSX) - normally expressed only in testis/placenta, aberrantly expressed in cancer
• Viral antigens (HPV E6/E7 in cervical cancer; EBV proteins in Burkitt lymphoma)
• Oncofetal antigens (CEA, AFP)

Antitumor Immune Effectors:

• CD8+ CTLs - most important; recognize peptide-MHC I complexes and kill tumor cells via perforin/granzyme
• NK cells - kill cells with downregulated MHC I ("missing self")
• CD4+ Th1 cells - help CTL activation
• Macrophages (M1 phenotype) - phagocytosis and ADCC

Mechanisms of Immune Evasion:

1. Loss of MHC class I expression - downregulation of MHC I or TAP (transporter associated with antigen processing) prevents CTL recognition; also downregulates beta-2 microglobulin
2. Lack of co-stimulation - absence of CD80/CD86 (B7-1/B7-2) on tumor cells causes T cell anergy
3. PD-L1/PD-1 checkpoint:
   • Tumor cells express PD-L1 which binds PD-1 on CTLs → T cell exhaustion/apoptosis
   • Anti-PD-1 (pembrolizumab, nivolumab) and anti-PD-L1 (atezolizumab) antibodies restore anti-tumor immunity
   • Effective in melanoma, lung cancer, renal cell carcinoma, MSI-high tumors
4. CTLA-4 checkpoint:
   • CTLA-4 on T cells competes with CD28 for B7 ligands → inhibits T cell activation
   • Ipilimumab (anti-CTLA-4) - used in melanoma
5. Immunosuppressive tumor microenvironment:
   • Recruitment of Regulatory T cells (Tregs) - suppress CTL activity via IL-10, TGF-β
   • M2-polarized macrophages (tumor-associated macrophages) - promote angiogenesis and immune suppression
   • Secretion of immunosuppressive cytokines: TGF-β, IL-10, VEGF
6. FAS/FAS-L counterattack - tumor cells express FAS ligand, inducing apoptosis in infiltrating T cells ("tumor counterattack")

ENABLING CHARACTERISTICS

A. Genomic Instability ("Mutator Phenotype")

• Translocations (BCR-ABL, MYC fusions), deletions, amplifications, inversions
• Gene amplification manifests as double minutes or homogeneously staining regions (HSRs) on karyotype

• DNA methylation of CpG islands in promoters silences tumor suppressor genes (e.g., MLH1, CDKN2A, E-cadherin)
• Histone modifications (acetylation, methylation) by mutated HATs, HDACs, and KMTs alter gene expression
• IDH1/2 mutations produce 2-HG → inhibit TET demethylases and KDM histone demethylases → global hypermethylation (CpG island methylator phenotype, CIMP)

• MicroRNAs (miRNAs) as oncogenes: miR-17-92 cluster (B-cell lymphomas)
• MiRNAs as tumor suppressors: miR-15a/miR-16-1 (inhibit BCL2 in CLL; deleted in CLL)
• Long noncoding RNAs (lncRNAs): HOTAIR, MALAT1 - regulate chromatin and gene expression in cancer

B. Tumor-Promoting Inflammation

• Growth factors (EGF, FGF, HGF) promoting cell survival and proliferation
• Pro-angiogenic factors (VEGF) from inflammatory cells
• ECM-remodeling enzymes (MMPs, cathepsins) enabling invasion
• Reactive oxygen and nitrogen species causing mutagenic DNA damage
• Immunosuppressive mediators (IL-10, TGF-β, prostaglandins)

• H. pylori chronic gastritis → gastric adenocarcinoma and MALT lymphoma
• Hepatitis C virus → chronic hepatitis → hepatocellular carcinoma
• Ulcerative colitis → colorectal cancer
• Asbestosis → mesothelioma

MOLECULAR BASIS OF MULTISTEP CARCINOGENESIS

Normal epithelium 
    → APC mutation (adenoma initiation)
    → KRAS mutation (early adenoma growth)
    → SMAD4/TGF-β loss (intermediate adenoma)
    → TP53 mutation (late adenoma/carcinoma)
    → Additional mutations (metastasis)

SUMMARY TABLE

Hallmarks of Cancer

1. Self-Sufficiency in Growth Signals

• Autocrine loops: Tumor cells secrete their own growth factors and co-express receptors (e.g., glioblastoma - PDGF)
• Mutant receptors: EGFR mutations (lung cancer), HER2 amplification (breast cancer)
• RAS mutations: Most common oncogene mutation; impairs GTPase activity → constitutive MAPK/PI3K signaling
• MYC: Master transcription factor; upregulated in Burkitt lymphoma (t(8;14)), neuroblastoma
• BCR-ABL fusion (Philadelphia chromosome, t(9;22)) in CML - targeted by imatinib

2. Insensitivity to Growth-Inhibitory Signals

• Unphosphorylated pRB binds E2F, blocking G1→S transition
• Growth signals → CDK4/cyclin D phosphorylates RB → E2F released → cell cycle entry
• Loss of RB (retinoblastoma), or equivalent loss via p16 deletion or CDK4 amplification, drives unchecked proliferation

• Activated by DNA damage, hypoxia, oncogene stress
• Induces p21 (G1 arrest), GADD45 (repair), BAX/PUMA (apoptosis)
• Most commonly mutated gene in all cancers (~50%)
• MDM2 amplification inactivates p53 without mutation
• Germline loss → Li-Fraumeni syndrome

3. Altered Cellular Metabolism (Warburg Effect)

• IDH1/2 mutations (glioma, AML) produce 2-hydroxyglutarate → epigenetic dysregulation
• PET scanning exploits this (18F-FDG uptake)

4. Evasion of Apoptosis

• Loss of TP53 → no BAX/PUMA upregulation after DNA damage
• BCL2 overexpression: t(14;18) in follicular lymphoma → BCL2 fused to Ig heavy chain locus → anti-apoptotic protection → indolent tumor
• MDM2 amplification → p53 degradation
• Therapeutic targeting: venetoclax (BCL2 inhibitor) for CLL/AML

5. Limitless Replicative Potential (Immortality)

• Cancer cells reactivate telomerase (TERT) in ~90% of human cancers
• Telomerase adds TTAGGG repeats to chromosome ends, enabling unlimited division
• Without telomerase, cells enter crisis → chromosomal end-to-end fusions → genomic instability

6. Sustained Angiogenesis

• VEGF (chief driver) - upregulated by HIF-1α (hypoxia), RAS, loss of p53/thrombospondin-1
• FGF-2 released from ECM by MMPs
• Anti-angiogenic thrombospondin-1 is downregulated (normally induced by p53)
• Tumor vasculature is leaky and tortuous
• Targeted by bevacizumab (anti-VEGF)

7. Invasion and Metastasis

1. Loss of E-cadherin → Epithelial-Mesenchymal Transition (EMT) driven by TWIST/SNAIL transcription factors
2. Degradation of ECM by MMPs and cathepsins (also release sequestered growth factors)
3. Altered integrin expression → attachment to novel ECM components
4. Migration toward chemotactic signals

8. Evasion of Immune Surveillance

• Cancer cells express PD-L1 which binds PD-1 on CTLs → T cell exhaustion (targeted by pembrolizumab/nivolumab)
• Loss of MHC class I → CTLs cannot recognize tumor antigens
• Recruit Tregs and M2 macrophages → immunosuppressive microenvironment
• Secrete TGF-β and IL-10 to suppress immune responses
• CTLA-4 pathway exploited - targeted by ipilimumab

Enabling Characteristics

• Mismatch repair loss (MLH1, MSH2) → Lynch syndrome, MSI-high tumors (respond to anti-PD-1)
• BRCA1/2 loss → familial breast/ovarian cancer (respond to PARP inhibitors)