Carcinogenesis

Reading File

Finding Sources

Reading File

Finding Sources

Reading File

Finding Sources

Reading File

I now have extensive material from Robbins Pathologic Basis of Disease and Robbins Basic Pathology to construct a thorough answer. Let me compile the full response.

Carcinogenesis

Carcinogenesis is the process by which normal cells undergo malignant transformation - acquiring the capacity for uncontrolled proliferation, tissue invasion, and metastasis. It is fundamentally a genetic disease driven by the stepwise accumulation of mutations and epigenetic alterations.

1. Fundamental Principles

Nonlethal genetic damage is the foundation of carcinogenesis. Initial damage may arise from:

Environmental exposures (chemicals, radiation, viruses)
Inherited germline mutations
Spontaneous/random mutations ("bad luck")

Key features of carcinogenesis:

Clonal origin: a tumor arises from a single precursor cell that acquired genetic damage; all daughter cells carry the same mutations
Multistep process: no single mutation is fully transforming; multiple complementary driver mutations must accumulate over time
Driver mutations: mutations that directly contribute to the cancer hallmarks; the first is called the initiating mutation, maintained in all cells; subsequent "hits" are required for full malignancy

- Robbins, Cotran & Kumar Pathologic Basis of Disease

2. Four Target Gene Classes

Four classes of genes are the principal targets of cancer-causing mutations:

Gene Class	Effect of Mutation	Genetic Behavior
Proto-oncogenes	Gain-of-function → excessive growth promotion	Dominant (one mutant allele sufficient)
Tumor suppressor genes	Loss-of-function → remove growth brakes	Recessive (both alleles usually needed)
Apoptosis-regulating genes	Loss-of-function → enhanced cell survival	Variable
DNA repair genes	Loss-of-function → accelerated mutation rate ("mutator phenotype")	Recessive; indirect effect

3. Oncogenes and Growth Promotion

Oncoproteins are constitutively active versions of normal growth-signaling molecules. They arise through:

Point mutations (e.g., RAS mutations - found in ~20-30% of all cancers)
Gene amplification (e.g., NMYC amplification in neuroblastoma, appearing as homogenous staining regions or double minutes)
Chromosomal translocations (e.g., BCR-ABL in CML)

NMYC amplification showing double minutes and homogeneous staining regions in neuroblastoma

Key oncoproteins by category:

Category	Example	Mechanism	Associated Cancer
Growth factors	TGF-α	Overexpression (autocrine loop)	Astrocytomas
Growth factor receptors	ERBB1 (EGFR)	Activating mutation	Lung adenocarcinoma
Growth factor receptors	ERBB2 (HER2)	Amplification	Breast carcinoma
Signal transducers	RAS	Point mutation	Many carcinomas
Transcription factors	MYC	Translocation/amplification	Burkitt lymphoma, neuroblastoma
Cell cycle proteins	Cyclin D, CDK4	Amplification	Many cancers

RAS acts as a key signaling node: growth factor receptors activate RAS, which normally cycles between active (GTP-bound) and inactive (GDP-bound) states. Mutant RAS is locked in the GTP-bound (active) state, continuously driving proliferation via MAP kinase and PI3K/AKT pathways.

4. Tumor Suppressor Genes

RB: "Governor of the Cell Cycle"

Normally, hypophosphorylated RB binds and sequesters E2F transcription factors, blocking G1→S transition
Growth signals → cyclin D/CDK4 → RB hyperphosphorylation → E2F release → cell cycle progression
RB is inactivated in cancers by:
- Loss-of-function RB mutations (retinoblastoma: "two-hit hypothesis")
- Amplification of cyclin D or CDK4
- Loss-of-function mutations in CDK inhibitors (e.g., p16/INK4a)
- Viral oncoproteins (e.g., HPV E7, polyomavirus large T antigen) that bind and inactivate RB

TP53: "Guardian of the Genome"

The most frequently mutated gene in human cancers
Activated by DNA damage, hypoxia, and oncogene activation
p53 induces:
- p21 (CDK inhibitor) → cell cycle arrest
- BAX → apoptosis
- Senescence programs
- Anti-angiogenic molecules (thrombospondin-1)
Loss of p53 removes all these safeguards and promotes angiogenesis

Other Key Tumor Suppressors

PTEN: Brake on PI3K/AKT signaling; mutated in Cowden syndrome and many sporadic cancers
VHL: Targets HIF-1α for degradation under normoxia. Loss → HIF-1α accumulates → VEGF/PDGF upregulation → angiogenesis and Warburg metabolism. Mutated in hereditary and sporadic renal cell carcinoma
TGF-β pathway: Normally activates CDK inhibitors and represses MYC; pathway lost in colorectal, stomach, endometrial, and pancreatic cancers
APC/β-catenin: Lost early in colorectal carcinogenesis

Cell cycle component mutations in cancer (summarized):

Component	Role	Cancer Relevance
CDK4/Cyclin D	Phosphorylate RB; allow G1/S transition	Amplified in many cancers
p16/INK4a (CDKN2A)	Blocks CDK4, preserves RB function	Deleted/silenced in many cancers
p21 (CDKN1A)	Induced by p53; arrests cell cycle	Lost in p53-mutant cancers
p27 (CDKN1B)	Responds to TGF-β; growth suppressor	Reduced in aggressive cancers
RB	G1/S checkpoint "pocket protein"	Lost in retinoblastoma, many cancers
p53	Stress sensor; DNA damage checkpoint	Most commonly mutated gene in cancer

5. Evasion of Apoptosis

Cancer cells circumvent programmed cell death through:

BCL-2 overexpression: t(14;18) in follicular lymphoma - BCL2 gene juxtaposed to IgH enhancer → overexpressed → blocks cytochrome c release → apoptosis inhibited
Loss of TP53: removes the apoptotic response to DNA damage
Activation of survival signals: PI3K/AKT axis phosphorylates and inactivates pro-apoptotic proteins like BAD

6. Growth-Promoting Metabolic Alterations (Warburg Effect / Oncometabolism)

Even with ample oxygen, cancer cells prefer aerobic glycolysis (Warburg effect), generating lactate rather than using oxidative phosphorylation. This supports:

Rapid ATP production
Biosynthetic intermediates for macromolecule synthesis

Oncometabolism (IDH mutations): Mutated isocitrate dehydrogenase (IDH1/IDH2) produces 2-hydroxyglutarate (2-HG), a prototypical oncometabolite. 2-HG inhibits TET2 and other α-ketoglutarate-dependent enzymes → abnormal DNA methylation → misexpression of cancer genes. IDH mutations occur in gliomas, AML, cholangiocarcinomas, and sarcomas. Targeted IDH inhibitors have been developed for clinical use.

Oncometabolite 2-HG pathway via mutated IDH

7. Limitless Replicative Potential (Telomeres and Cancer Stem Cells)

Normal cells have finite replication capacity due to telomere shortening with each division. Cancer cells escape this limit by:

Upregulating telomerase - an enzyme that rebuilds telomere ends
Acquiring cancer stem cell properties (self-renewal capacity), which can arise either from transformed tissue stem cells or from progenitors that acquire a mutation conferring "stemness"

8. Angiogenesis

Tumors >1-2 mm require new blood vessels (neoangiogenesis). The angiogenic switch tips the balance toward pro-angiogenic factors:

Hypoxia → HIF-1α stabilization → VEGF and bFGF upregulation
Loss of p53 → reduced thrombospondin-1 (anti-angiogenic)
RAS/MYC gain-of-function → VEGF upregulation
Proteases release ECM-stored bFGF; tumor-associated macrophages also secrete VEGF

This forms the rationale for anti-VEGF therapy (e.g., bevacizumab).

9. Invasion and Metastasis

Invasion and metastasis occur in sequential steps:

Loosening of cell-cell contacts: Loss of E-cadherin (CDH1) expression - by promoter methylation, mutations, or transcriptional repression by SNAIL/TWIST
Degradation of extracellular matrix: Matrix metalloproteinases (MMPs) and cathepsins degrade basement membrane and interstitial matrix; also release growth factors from ECM
Attachment to novel ECM components: Altered integrin expression allows adhesion to new stromal sites
Migration: Proteases and cytokines (TNF, EGF) stimulate motility

Epithelial-mesenchymal transition (EMT): Transcription factors TWIST and SNAIL drive loss of epithelial characteristics and gain of mesenchymal invasive properties.

Organ tropism of metastasis:

Many tumors arrest in the first capillary bed encountered (lung, liver most common)
Some show specific tropism via chemokine receptor-ligand matching with endothelial cells at target sites
Colon → liver (via portal blood); Breast → bone, lung, brain; Prostate → bone

10. Evasion of Immune Surveillance

The immune system normally recognizes and destroys nascent cancer cells. Tumors that persist have evolved immune evasion:

Downregulation of MHC class I molecules → invisible to CD8+ CTLs
Expression of PD-L1 → binds PD-1 on T cells → T cell anergy (checkpoint inhibitor drugs target this)
TGF-β secretion → immunosuppression; recruits regulatory T cells (Tregs)
Loss of tumor antigens → clonal selection removes immunogenic variants

Tumor antigens include: products of mutated oncogenes/TSGs, overexpressed proteins, cancer-testis antigens, and viral antigens (in virus-associated cancers).

11. Genomic Instability

Cancer cells accumulate mutations far faster than normal cells, due to:

DNA mismatch repair (MMR) defects: e.g., MLH1, MSH2 mutations in hereditary non-polyposis colorectal cancer (Lynch syndrome) → microsatellite instability (MSI)
Nucleotide excision repair (NER) defects: e.g., XPC, XPA mutations in xeroderma pigmentosum → sensitivity to UV-induced pyrimidine dimers
Chromosomal instability: Defects in spindle checkpoint genes → aneuploidy

12. Epigenetic Changes and Non-coding RNAs

Beyond DNA mutations:

DNA hypermethylation of CpG islands in promoters of tumor suppressor genes (e.g., silencing RB, BRCA1, MLH1) → gene silencing without mutation
Global DNA hypomethylation → activation of proto-oncogenes and genomic instability
Histone modifications: Altered acetylation/methylation patterns
microRNAs: Can act as oncogenes (OncomiRs, e.g., miR-21 suppresses PTEN) or tumor suppressors (e.g., miR-34a represses BCL2/MYC); frequently dysregulated in cancer

13. Multistep Carcinogenesis: The Colorectal Cancer Model

The classic illustration of stepwise carcinogenesis:

Normal epithelium → Hyperplasia → Early adenoma → Intermediate adenoma → Late adenoma → Carcinoma → Metastasis

Each step adds a new mutation:

Loss of APC (tumor suppressor; Wnt pathway) - early, initiating event
Activation of RAS - mid-progression
Loss of SMAD4 (TGF-β pathway)
Loss of TP53 - late event, associated with frank malignancy

This model demonstrates that individual mutations are insufficient - multiple complementary hits in different pathways are required for full malignant transformation.

14. Carcinogenic Agents

Chemical Carcinogenesis

Direct-acting carcinogens: Chemically reactive electrophiles that directly damage DNA without metabolic activation

Examples: alkylating agents (β-propiolactone, dimethyl sulfate), nitrogen mustards, anticancer drugs (cyclophosphamide, chlorambucil)

Indirect-acting carcinogens (procarcinogens): Require metabolic activation by cytochrome P-450 monooxygenases

Polycyclic aromatic hydrocarbons (benzo[a]pyrene from tobacco combustion) → epoxides → DNA adducts → mutations in RAS, TP53
Aromatic amines (β-naphthylamine) → bladder cancer in dye/rubber workers
Aflatoxin B1 (Aspergillus on stored grains) → characteristic TP53 codon 249 mutation → hepatocellular carcinoma
Nitrosamines (from food preservatives)

Initiation vs. Promotion:

Initiator: Causes permanent DNA mutation (single exposure sufficient); by itself insufficient for cancer
Promoter: Stimulates proliferation of initiated cells (not mutagenic alone; requires repeated exposure AFTER initiation)
Examples of promoters: phorbol esters, hormones, phenols
The key action of promoters = clonal expansion of initiated cells → accumulation of additional mutations

Radiation Carcinogenesis

UV rays (UVB, 280-320 nm): Form pyrimidine dimers (thymidine-thymidine); bypass by error-prone translesion polymerases → characteristic C→T transitions; cause skin cancers (BCC, SCC, melanoma)
Ionizing radiation (X-rays, gamma rays): Double-strand DNA breaks → chromosomal translocations, deletions → leukemias, thyroid, breast, lung cancer; risk is dose-dependent

Microbial Carcinogenesis

Oncogenic DNA Viruses:

Virus	Mechanism	Associated Cancer
HPV (high-risk: 16, 18)	E6 binds/degrades p53; E7 binds/inactivates RB	Cervical, oropharyngeal, anal cancer
EBV	LMP1 activates NF-κB; EBNA2 activates MYC	Burkitt lymphoma, Hodgkin lymphoma, nasopharyngeal carcinoma
HBV	HBx protein inhibits p53; insertional mutagenesis	Hepatocellular carcinoma
HHV-8 (KSHV)	Encodes viral cyclin, anti-apoptotic proteins	Kaposi sarcoma, PEL
Merkel cell polyomavirus	T antigen inactivates RB	Merkel cell carcinoma

Oncogenic RNA Viruses:

HTLV-1: Tax protein activates NF-κB, cyclins → Adult T-cell leukemia/lymphoma

Bacterial:

H. pylori: Chronic inflammation → IL-8, ROS → mutations; CagA oncoprotein activates RAS pathway → gastric adenocarcinoma and MALT lymphoma

15. Cancer-Enabling Inflammation

The tumor microenvironment is shaped by chronic inflammation:

Infiltrating macrophages, neutrophils, and stromal cells release EGF, VEGF, proteases, TGF-β
Effects: promote proliferation, remove growth suppressors, enhance angiogenesis, enable invasion, suppress anti-tumor immunity, cause EMT
This is why chronic inflammatory conditions predispose to cancer (e.g., Barrett esophagus → esophageal adenocarcinoma; ulcerative colitis → colorectal cancer)

Summary: The Hallmarks of Cancer (Hanahan & Weinberg)

Hallmark	Key Molecular Mechanism
Self-sufficiency in growth signals	Oncogene activation (RAS, EGFR, MYC)
Insensitivity to growth inhibition	Tumor suppressor loss (RB, TP53, PTEN)
Evasion of apoptosis	BCL-2 overexpression; p53 loss
Limitless replicative potential	Telomerase activation; cancer stem cells
Angiogenesis	VEGF upregulation; HIF-1α; loss of p53
Invasion and metastasis	E-cadherin loss; MMP activation; EMT
Altered cellular metabolism	Warburg effect; IDH mutations/oncometabolites
Evasion of immune surveillance	PD-L1; MHC loss; TGF-β; Tregs
Genomic instability	MMR defects; NER defects; chromosomal instability
Tumor-promoting inflammation	Macrophages, stromal cells in tumor microenvironment

Source: Robbins, Cotran & Kumar Pathologic Basis of Disease; Robbins & Kumar Basic Pathology

Carcinogenic agent

Finding Sources

Reading File

I now have comprehensive material from Robbins Pathologic Basis of Disease and Robbins Basic Pathology. Let me compile the full, detailed answer.

Carcinogenic Agents

Carcinogenic agents inflict genetic damage - the fundamental driver of carcinogenesis. Three major classes have been identified: (1) Chemical agents, (2) Radiant energy (Radiation), and (3) Microbial agents (viruses and bacteria). These may act alone or in concert, sequentially producing the multiple genetic abnormalities characteristic of neoplastic cells.

- Robbins & Kumar Basic Pathology

I. CHEMICAL CARCINOGENESIS

Historical Background

Over 200 years ago, the London surgeon Sir Percival Pott correctly attributed scrotal skin cancer in chimney sweeps to chronic exposure to soot. Following this, the Danish Chimney Sweeps Guild mandated daily bathing, and scrotal cancer disappeared - one of the earliest and most effective public health measures in oncology. Since then, hundreds of chemicals have been shown to be carcinogenic.

Mechanism of Chemical Carcinogens

All direct and ultimate chemical carcinogens contain highly reactive electrophile groups that form covalent adducts with DNA, RNA, and proteins. The resulting mutations in key cancer genes - particularly RAS and TP53 - drive malignant transformation. Specific "mutational signatures" exist for different carcinogens (e.g., aflatoxin B1 causes a characteristic TP53 codon 249 mutation), which can be used as forensic tools in epidemiologic studies.

Classification: Direct vs. Indirect-Acting

A. Direct-Acting Carcinogens

Do not require metabolic conversion to be carcinogenic
Generally weak carcinogens
Clinically important because some are used as chemotherapy drugs (e.g., alkylating agents used for Hodgkin lymphoma), which can later cause secondary leukemia

Class	Examples
Alkylating agents	β-Propiolactone, dimethyl sulfate, diepoxybutane, cyclophosphamide, chlorambucil, nitrosoureas
Acylating agents	1-Acetyl-imidazole, dimethylcarbamoyl chloride

B. Indirect-Acting Carcinogens (Procarcinogens)

Require metabolic activation (by endogenous enzymes, particularly cytochrome P-450-dependent monooxygenases) to produce the "ultimate carcinogen"
Genetic polymorphisms in P-450 isoforms affect individual cancer risk (e.g., different CYP isoforms convert benzo[a]pyrene to carcinogenic metabolites at different rates - affecting lung cancer risk in smokers)

Class	Key Examples	Mechanism	Associated Cancer
Polycyclic aromatic hydrocarbons	Benzo[a]pyrene (from tobacco combustion, broiled meats, smoked fish)	Metabolized to epoxides → DNA adducts	Lung cancer
Aromatic amines & azo dyes	β-Naphthylamine (aniline dye/rubber industry)	P-450 activation → bladder-specific carcinogens	Bladder cancer (50× increased risk)
Aflatoxin B1	Produced by Aspergillus on improperly stored grains/nuts	Characteristic TP53 codon 249 mutation	Hepatocellular carcinoma (Africa, SE Asia)
Nitrosamines	From nitrites (food preservatives) + amines in food	Nitrosylation → alkylating intermediates	GI cancers
Other occupational/environmental	Vinyl chloride, arsenic, nickel, chromium, PCBs, insecticides	Various DNA damage mechanisms	Liver angiosarcoma (vinyl chloride), lung cancers

Initiation-Promotion Sequence

A critical concept in chemical carcinogenesis is the two-stage model:

Initiation (carcinogen/mutagen):

Single exposure sufficient
Causes permanent, irreversible DNA mutation
By itself insufficient to produce cancer
Effect is heritable and cumulative

Promotion (promoter):

NOT mutagenic or tumorigenic alone
Requires repeated, sustained exposure AFTER initiation
Causes clonal expansion of initiated (mutated) cells
Stimulates proliferation → accumulation of additional mutations
Effect is potentially reversible if exposure stops

Examples of promoters: phorbol esters, hormones (e.g., estrogen), phenols, certain drugs, conditions of tissue repair

Mechanism of promotion: Induction of cell proliferation is the sine qua non of tumor promotion. If the initiator mutated RAS, promoters drive clonal expansion of that RAS-mutant clone, which then acquires additional mutations until full malignancy is achieved.

Initiation and promotion of cancer by chemical carcinogens

II. RADIATION CARCINOGENESIS

Radiant energy - whether UV rays of sunlight, x-rays, gamma rays, or particulate radiation - is mutagenic and carcinogenic. Radiation-induced cancers may arise decades after exposure, necessitating long observation periods.

A. Ultraviolet (UV) Radiation

Wavelength ranges:

UVA (320-400 nm): Less carcinogenic
UVB (280-320 nm): Most carcinogenic - causes skin cancers
UVC (200-280 nm): Potent mutagen but filtered by the ozone layer (hence ozone depletion concerns)

Mechanism:

UVB photons are absorbed by DNA
Covalent cross-linking of adjacent pyrimidine bases (especially thymidine-thymidine dimers) in the same DNA strand
DNA helix is distorted; proper base-pairing prevented
Normally repaired by nucleotide excision repair (NER) pathway (30+ proteins)
With excessive sun exposure, NER is overwhelmed → error-prone bypass mechanisms → characteristic C→T and CC→TT transitions (UV mutational signature)
Key mutations in TP53 and other genes → skin cancer

Associated cancers: Squamous cell carcinoma, basal cell carcinoma, melanoma of skin

Risk factors: Fair skin, intense/cumulative sun exposure, geographic proximity to equator (e.g., Queensland, Australia has highest skin cancer rate worldwide)

Nonmelanoma skin cancers: associated with total cumulative UV exposure
Melanomas: associated with intense intermittent exposure (sunbathing, tanning beds)

Xeroderma Pigmentosum: Inherited defect in NER → catastrophically increased risk of UV-induced skin cancers; illustrates the protective role of DNA repair in carcinogenesis

B. Ionizing Radiation

Types: Electromagnetic (x-rays, γ-rays) AND particulate (α particles, β particles, protons, neutrons) - all carcinogenic

Mechanism:

Causes single- and double-stranded DNA breaks (double-stranded breaks are most mutagenic)
DNA cross-linkage with itself and with proteins
Chromosome breakage, translocations, inversions, deletions
Less commonly: point mutations

Historical evidence:

X-ray pioneers developed skin cancers
Uranium/radioactive element miners: 10× higher incidence of lung cancer
Atomic bomb survivors (Hiroshima & Nagasaki): markedly increased leukemia (average latent period ~7 years), then thyroid, breast, colon, lung carcinomas after longer latency
Chernobyl nuclear accident: ongoing elevated cancer incidence in surrounding populations
Head and neck therapeutic irradiation → papillary thyroid cancers years later

Tissue vulnerability hierarchy (most to least sensitive):

Myeloid leukemias (most frequent)
Thyroid cancer (especially in young patients)
Intermediate: breast, lung, salivary gland cancers
Skin and GI epithelium (relatively resistant to radiation-induced cancer, though sensitive to acute cell killing)

CT scan risk: Children who receive 2-3 CT scans have a 3× higher risk of leukemia; 5-10 CT scans → 3× higher risk of brain tumors. Absolute risk remains low (~1 excess leukemia and 1 excess brain tumor per 10,000 CT scans over 10 years), but emphasizes minimizing unnecessary radiation exposure.

III. MICROBIAL CARCINOGENESIS

Collectively, viruses - particularly HPV, EBV, HBV, and HCV - are associated with 15-20% of cancers worldwide. A common theme: infection triggers initial polyclonal cell proliferation, which over time becomes monoclonal as driver mutations accumulate in rapidly dividing cells.

A. Oncogenic RNA Viruses

HTLV-1 (Human T-Cell Leukemia Virus Type 1)

Only human retrovirus firmly implicated in cancer
Endemic in: Japan, Caribbean basin, South America, Africa
Target cell: CD4+ T cells (tropism similar to HIV)
Transmission: Sexual intercourse, blood products, breastfeeding
Associated cancer: Adult T-cell leukemia/lymphoma (ATLL)
Only 3-5% of infected individuals develop ATLL, typically after 40-60 years of latency

Mechanism: HTLV-1 encodes Tax and HBZ proteins:

Tax stimulates viral RNA transcription; activates NF-κB and other signaling pathways
Together they promote cell proliferation, induce genomic instability, inhibit senescence
Does NOT contain a recognizable oncogene; does NOT integrate next to a proto-oncogene
Proviral integration is clonal, confirming infection preceded transformation

B. Oncogenic DNA Viruses

1. Human Papillomavirus (HPV)

Strains:

Low-risk (HPV 6, 11): Cause benign squamous papillomas (warts) - low malignant potential
High-risk (HPV 16, 18, 31): Cause cervical, anogenital, and oropharyngeal (tonsillar) cancers

Mechanism of integration: In benign warts, HPV exists as non-integrated episomal DNA. In cancers, the viral genome integrates into host chromosomes - always interrupting the E1/E2 open reading frame → loss of E2 repressor → overexpression of E6 and E7 oncoproteins

HPV E6 and E7 transforming activities diagram

Oncogenic activities of E6:

Binds and mediates ubiquitin-mediated degradation of p53 → abolishes DNA damage checkpoint and apoptosis
Stimulates expression of TERT (telomerase reverse transcriptase) → cell immortalization
High-risk HPV E6 has higher affinity for p53 than low-risk E6

Oncogenic activities of E7:

Binds RB → displaces E2F transcription factors → constitutive G1→S progression
Inactivates CDK inhibitors p21 and p27
Binds and activates cyclins A and E (high-risk types 16, 18, 31)
High-risk HPV E7 has higher affinity for RB than low-risk E7

Net effect of E6 + E7: Immortalize cells + remove restraints on proliferation + resist apoptosis → full oncogenic transformation (with additional host mutations in RAS, etc.)

Clinical implications: HPV vaccines (targeting HPV 16, 18) are highly effective in preventing cervical cancer. Women co-infected with high-risk HPV and HIV are at particularly high risk.

2. Epstein-Barr Virus (EBV)

First virus linked to a human tumor (Burkitt lymphoma)
Receptor: Uses complement receptor CD21 to infect B cells
In vitro: Causes polyclonal B-cell proliferation and immortalization of B lymphoblastoid cell lines

Oncogenic proteins:

LMP1 (Latent Membrane Protein 1): Acts as constitutively active CD40 mimic → activates NF-κB and JAK/STAT pathways → B-cell proliferation and survival
EBNA2: Activates cyclin D and SRC-family proto-oncogenes → promotes B-cell growth

In immunocompetent individuals: CTLs eliminate EBV-infected B cells, causing self-limited infectious mononucleosis. Residual B cells downregulate immunogenic proteins and persist as long-lived memory B cells.

How Burkitt lymphoma arises (endemic): Co-infections (malaria) impair immune competence → sustained EBV-driven B-cell proliferation → CTLs eliminate most cells → small residual population acquires t(8;14) MYC translocation → monoclonal Burkitt lymphoma

Cancer	Mechanism
Burkitt lymphoma (endemic)	EBV + malaria immunosuppression + MYC translocation
Nasopharyngeal carcinoma	EBV in virtually all cases
B-cell lymphomas (transplant/HIV patients)	EBV-driven polyclonal proliferation without T-cell control
Hodgkin lymphoma (subset)	EBV in Reed-Sternberg cells
Gastric carcinoma (subset)	EBV-positive

3. Hepatitis B Virus (HBV) and Hepatitis C Virus (HCV)

Together account for 70-85% of hepatocellular carcinomas worldwide
Dominant mechanism: Immunologically mediated chronic inflammation → hepatocellular injury → reparative hepatocyte proliferation → accumulation of driver mutations
HBx protein (HBV): Can inhibit p53; activates signal transduction pathways; promotes HIF-1α activity
HCV core protein: Activates signal transduction pathways contributing to carcinogenesis

4. Merkel Cell Polyomavirus

Associated with Merkel cell carcinoma (rare but aggressive skin cancer)
T antigen inactivates RB

5. Human Herpesvirus 8 (HHV-8 / KSHV - Kaposi Sarcoma Herpesvirus)

Encodes a viral cyclin (activates CDKs)
Encodes anti-apoptotic proteins
Associated with: Kaposi sarcoma, primary effusion lymphoma (PEL), Castleman disease

C. Bacterial Carcinogenesis

Helicobacter pylori

Associated cancers: Gastric adenocarcinoma AND gastric MALToma (mucosa-associated lymphoid tissue lymphoma)

Mechanism - Multifactorial:

Chronic inflammation → reactive oxygen species (ROS) → DNA damage in gastric epithelium
Reparative gastric cell proliferation → accumulation of mutations
CagA (Cytotoxin-Associated Gene A) oncoprotein: Activates RAS/MAP kinase growth factor pathways; disrupts E-cadherin
Polyclonal B-cell proliferation in gastric mucosa → with acquisition of driver mutations → monoclonal gastric B-cell lymphoma (MALToma)
Eradication of H. pylori with antibiotics can cause regression of low-grade MALToma (before genetic independence is acquired)

Summary Table: Carcinogenic Agents

Agent	Mechanism	Target Gene/Pathway	Associated Cancer
Alkylating agents	Direct DNA alkylation	Any gene	Secondary leukemia
Benzo[a]pyrene	Epoxide-DNA adducts	RAS, TP53	Lung cancer
β-Naphthylamine	P-450 activation → bladder carcinogens	-	Bladder cancer
Aflatoxin B1	TP53 codon 249 mutation	TP53	Hepatocellular carcinoma
UVB radiation	Pyrimidine dimers → NER overwhelmed	TP53	Skin cancers, melanoma
Ionizing radiation	DNA double-strand breaks, translocations	Multiple	Leukemia, thyroid, breast, lung
HTLV-1	Tax/HBZ → NF-κB, genomic instability	Multiple	Adult T-cell leukemia/lymphoma
HPV 16/18	E6 degrades p53; E7 inactivates RB	TP53, RB	Cervical, oropharyngeal cancer
EBV	LMP1 activates NF-κB; EBNA2 activates MYC	MYC, NF-κB	Burkitt lymphoma, nasopharyngeal carcinoma
HBV/HCV	Chronic inflammation + HBx signaling	Multiple	Hepatocellular carcinoma
HHV-8	Viral cyclin, anti-apoptotic proteins	CDKs, apoptosis	Kaposi sarcoma
H. pylori	Chronic inflammation + CagA → RAS	RAS, E-cadherin	Gastric adenocarcinoma, MALToma

Sources: Robbins, Cotran & Kumar Pathologic Basis of Disease; Robbins & Kumar Basic Pathology

This is a shared conversation. Sign in to Orris to start your own chat.