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Routes of Metastasis

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1. Seeding Within Body Cavities

• Ovarian carcinoma is the classic example: tumor cells shed into the peritoneal cavity and cover peritoneal surfaces widely, yet may not deeply invade underlying tissues. Here, the ability to implant is separable from invasive capacity.
• CNS tumors (e.g., medulloblastoma, ependymoma) penetrate the cerebral ventricles and are carried by cerebrospinal fluid to implant on meningeal surfaces around the brain or spinal cord.

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2. Lymphatic Spread

• The pattern of lymph node involvement follows local lymphatic drainage from the primary site:
  • Lung carcinomas → bronchial → tracheobronchial → hilar nodes
  • Breast carcinoma (upper outer quadrant) → axillary nodes; medial lesions drain to internal mammary nodes → then supraclavicular/infraclavicular nodes
• "Skip metastases": Cancer cells may bypass the immediately proximal nodes and be trapped in more distal nodes.
• Cells can traverse all nodes and reach the vascular compartment via the thoracic duct.
• Sentinel lymph node: The first node to receive lymph drainage from the primary tumor. Identified using dye or radiolabeled tracers, its biopsy determines extent of spread and guides therapy.
• Enlarged regional nodes do not always mean metastatic involvement — reactive hyperplasia from tumor antigens and necrotic cells can cause lymphadenitis. Biopsy is required for confirmation.

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3. Hematogenous Spread

• Cancer cells penetrate thin-walled veins preferentially over arteries.
• Bloodborne cells arrest in the first capillary bed encountered:
  • Portal drainage → liver (most common site)
  • Caval drainage → lungs (second most common)
• Tumors near the vertebral column (thyroid, prostate) embolize via the paravertebral venous plexus → preferential spread to the spine.
• Renal cell carcinoma and hepatocellular carcinoma characteristically grow within veins in a snakelike pattern, sometimes extending up the inferior vena cava to the right heart — yet this may not produce widespread dissemination.
• Organ-specific homing patterns:
  • Prostatic carcinoma → bone
  • Bronchogenic carcinoma → adrenal glands and brain
  • Neuroblastoma → liver and bones
  • Uveal melanoma → liver
  • Skeletal muscle, despite being rich in capillaries, is rarely a metastatic site

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Molecular Mechanisms Enabling Metastasis

Epithelial-to-Mesenchymal Transition (EMT)

• Downregulate epithelial markers (e.g., E-cadherin)
• Upregulate mesenchymal markers (e.g., vimentin, smooth muscle actin)
• Shift morphologically from polygonal epithelioid to spindly mesenchymal shape
• Increase production of proteolytic enzymes to degrade extracellular matrix

Deterministic vs. Probabilistic Models

• Probabilistic model: As tumor grows, cells randomly accumulate all mutations needed for metastasis — a function of cell number and time
• Deterministic model: Some tumors acquire metastatic mutations early in carcinogenesis, making metastasis an intrinsic tumor property

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Carcinogenic Agents and Their Cellular Interactions

The Multistage Model of Chemical Carcinogenesis

1. Initiation — an initiator causes gene mutations that increase proliferative potential and reduce apoptosis (permanent, irreversible)
2. Promotion — a promoter does not directly mutate genes but alters signaling pathways or the extracellular environment to increase survival, proliferation, or invasiveness of pre-cancerous cells (reversible if exposure stops)
3. Progression — chromosomal instability and accumulating mutations lead to full invasiveness and metastasis

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IARC Classification of Carcinogens

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Genotoxic Carcinogens — Direct DNA Damage

1. Metabolic activation: Most genotoxic carcinogens are procarcinogens — they undergo metabolism (usually by cytochrome P450 enzymes) in target tissues to form a reactive intermediate (the proximate carcinogen)
2. DNA adduct formation: The reactive intermediate covalently bonds to DNA
3. Alternatively: Reactive oxygen species (ROS) generated can oxidize DNA or form lipid peroxidation products that react with DNA
4. If unrepaired before DNA replication → mutation in a proto-oncogene, tumor suppressor gene, or DNA repair gene → selective growth advantage → cancer

Example: Benzo[a]pyrene (tobacco smoke)

• Oxidized by CYPs → 7,8-dihydrodiol (proximate carcinogen)
• Further oxidized → diol epoxide → reacts directly with DNA
• Or oxidized by aldo-keto reductases → catechol → redox cycles → generates ROS

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Nongenotoxic Carcinogens — Tumor Promotion Without DNA Damage

• Phorbol esters: Mimic diacylglycerol, activate protein kinase C (PKC) isoforms → stimulate MAPK pathways → proliferation, invasiveness, angiogenesis. Normal cells would undergo apoptosis with prolonged activation, but cells with prior mutations resist cell death.
• Estrogenic carcinogens: Activate estrogen receptor alpha (ERα) → stimulate proliferation and invasiveness of estrogen-responsive cells
• Chronic inflammation (e.g., asbestos, hepatotoxic chemicals): Inflammatory cytokines stimulate PKC signaling → proliferation and invasion; compensatory cell cycling during tissue repair increases mutation probability

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Epigenetic Mechanisms

• Hypermethylation of CpG islands in tumor suppressor gene promoters → silencing
• Demethylation of proto-oncogene promoters → overexpression
• Heavy metals (e.g., arsenic, cadmium): Do not directly damage DNA but interfere with proteins involved in DNA synthesis and repair → increased replication errors

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Complete Carcinogens and Cocarcinogens

• Complete carcinogens (e.g., benzo[a]pyrene, UV radiation): Can cause both initiation and promotion with repeated exposure
• Cocarcinogens: Enhance the carcinogenicity of other agents by:
  • Increasing absorption (e.g., ethanol acts as a solvent, increasing absorption of tobacco carcinogens in head/neck)
  • Depleting protective molecules (ethanol depletes glutathione, impairing detoxification of reactive metabolites)
  • Interfering with DNA repair, increasing mutation rates from a second genotoxic agent

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Summary Table

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• Robbins & Kumar Basic Pathology, Chapter 6 — Neoplasia
• Goodman & Gilman's The Pharmacological Basis of Therapeutics, Chapter 76 — Carcinogenesis

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PART 1: Role of Carcinogenic Agents and Their Cellular Interactions

The Multistage Model of Chemical Carcinogenesis

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IARC Classification of Carcinogens

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Category 1: Genotoxic Carcinogens — Direct DNA Damage

Cellular Interaction Sequence:

1. Absorption → carcinogen enters target tissue
2. Metabolic activation by cytochrome P450 (CYP) enzymes → proximate carcinogen (reactive intermediate)
3. DNA adduct formation — the reactive intermediate covalently bonds to DNA bases
4. Alternatively → reactive oxygen species (ROS) generated → oxidize DNA or form lipid peroxidation products that react with DNA
5. If unrepaired before DNA replication → permanent mutation in:
   • Proto-oncogene → gain of function, uncontrolled proliferation
   • Tumor suppressor gene → loss of brake on growth
   • DNA repair gene → increased future mutation rate
6. Mutated cell with selective growth advantage → clonal expansion → cancer

Prototype Example: Benzo[a]pyrene (tobacco smoke)

• Oxidized by CYPs → 7,8-dihydrodiol (proximate carcinogen)
• Second oxidation by CYP → diol epoxide → covalently reacts with DNA
• Alternatively, oxidized by aldo-keto reductases → catechol → redox cycles → generates ROS

Key Genotoxic Carcinogens Table

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Category 2: Nongenotoxic Carcinogens — Tumor Promotion Without DNA Damage

Mechanisms of Cellular Interaction:

• Phorbol esters mimic diacylglycerol (DAG), activating protein kinase C (PKC) isoforms
• PKC activates mitogen-activated protein kinase (MAPK) pathways → stimulates proliferation, invasiveness, and angiogenesis
• Normal cells would undergo apoptosis with prolonged MAPK activation, but cells with prior apoptosis-suppressing mutations are resistant

• Estrogenic carcinogens bind estrogen receptor alpha (ERα) → stimulate proliferation and invasiveness of estrogen-responsive cells (e.g., breast, endometrium)

• Inflammatory cytokines → stimulate PKC signaling → proliferation, invasion, angiogenesis
• Examples: asbestos (causes mesothelioma via persistent pleural inflammation), chronic hepatotoxic chemicals (stimulate compensatory liver cell proliferation → increases mutation probability)

• Heavy metals (arsenic, cadmium, nickel): do not directly damage DNA but interfere with DNA synthesis and repair proteins → increased replication errors
• Hypermethylation of CpG island promoters → silencing of tumor suppressor genes
• Demethylation of proto-oncogene promoters → overexpression
• These epigenetic changes are heritable through cell division

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Category 3: Complete Carcinogens and Cocarcinogens

• Complete carcinogens (e.g., benzo[a]pyrene, UV radiation): can drive both initiation and promotion with repeated exposure
• Cocarcinogens enhance carcinogenicity of other agents by:
  • Increasing absorption (ethanol acts as a solvent, increasing tobacco carcinogen absorption in head/neck mucosa)
  • Depleting protective molecules (ethanol depletes glutathione/GSH, impairing detoxification of reactive metabolites and ROS)
  • Interfering with DNA repair, amplifying mutations from a second genotoxic agent

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Summary: Genotoxic vs. Nongenotoxic

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PART 2: Types of Hereditary Disorders and Common Mendelian Disorders

Types of Hereditary Disorders

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Mendelian (Single-Gene) Disorders — Inheritance Patterns

A. Autosomal Dominant (AD)

• Manifested in the heterozygous state
• One affected parent → 50% chance of transmission to each child
• Both sexes affected and transmit equally
• New mutations can occur (no affected parent)
• Features: reduced penetrance (carrier is phenotypically normal), variable expressivity (same mutation, different severity)
• Involved genes usually encode regulatory proteins, receptors, structural proteins — not enzymes (since 50% enzyme activity is often sufficient)
• Age of onset often delayed (e.g., Huntington disease)

B. Autosomal Recessive (AR)

• Manifested only in the homozygous state
• Both parents are typically unaffected carriers
• Each child of two carriers has a 25% chance of being affected
• More often involve enzyme deficiencies — 50% enzyme activity from one normal allele is usually sufficient
• Often more severe and uniform in presentation than AD disorders
• Consanguinity increases risk

C. X-Linked Disorders

• Gene is on the X chromosome
• Males (hemizygous XY) are almost always affected
• Females (XX) are usually carriers with mild or no symptoms (one normal X provides protection)
• Father-to-son transmission does not occur
• Examples: Duchenne muscular dystrophy, Hemophilia A and B, Glucose-6-phosphate dehydrogenase deficiency

• A single gene mutation may have many phenotypic effects (pleiotropy) — e.g., Marfan syndrome affects skeleton, eyes, and cardiovascular system from one fibrillin gene mutation
• Different mutations can cause the same phenotype (genetic heterogeneity) — e.g., retinitis pigmentosa
• Modifier genes at other loci can alter severity (as seen in cystic fibrosis)

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Estimated Prevalence of Common Mendelian Disorders

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Common Mendelian Disorders — Brief Descriptions

1. Marfan Syndrome (AD)

• Gene/Protein: Mutation in FBN1 gene → defective fibrillin-1 (extracellular matrix glycoprotein)
• Fibrillin forms microfibrils that provide structural support in connective tissue and regulate TGF-β signaling
• Features (pleiotropy from one gene):
  • Skeletal: Tall stature, long extremities (arachnodactyly), pectus deformities, scoliosis, joint laxity
  • Ocular: Ectopia lentis (displaced lens), myopia
  • Cardiovascular: Aortic root dilation → aortic regurgitation, aortic dissection/rupture; mitral valve prolapse

2. Ehlers-Danlos Syndromes (EDS) (AD or AR depending on subtype)

• Gene/Protein: Mutations affecting collagen synthesis or processing
• Subtypes:
  • Vascular EDS: COL3A1 mutation → deficient type III collagen → arterial and bowel rupture (AD)
  • Kyphoscoliotic EDS: Lysyl hydroxylase deficiency → defective collagen crosslinks → congenital scoliosis + ocular fragility (AR)
  • Classical EDS: COL5A1/COL5A2 mutations → deficient type V collagen (AD)
• Features: Hyperflexible joints (predisposition to dislocation), hyperextensible and fragile skin, internal organ rupture

3. Familial Hypercholesterolemia (AD)

• Gene/Protein: Loss-of-function mutations in LDL receptor gene (80–85% of cases)
• Normal function: LDL receptors in hepatocytes bind and internalize LDL, providing feedback to suppress intracellular cholesterol synthesis (via HMG-CoA reductase suppression)
• Defect: Fewer/absent functional LDL receptors → LDL not cleared from plasma → elevated circulating LDL → loss of feedback control → continued cholesterol synthesis
• Features:
  • Premature atherosclerosis and myocardial infarction (MI may occur before age 20 in homozygotes)
  • Xanthomas (cholesterol deposits in tendons, skin)
  • Xanthelasmas (periorbital deposits)
  • Corneal arcus
• Heterozygotes: LDL ~2× normal; Homozygotes: LDL ~5× normal (severe, often fatal in childhood)

4. Cystic Fibrosis (AR)

• Gene/Protein: Mutations in CFTR gene → defective/absent cystic fibrosis transmembrane conductance regulator (a chloride ion channel)
• Most common: ΔF508 deletion (deletion of phenylalanine at position 508)
• Defect: CFTR cannot be properly folded → retained in ER → degraded; absent Cl⁻ secretion → thick, viscous secretions in exocrine glands
• Features:
  • Lungs: Chronic pulmonary infections (Pseudomonas aeruginosa, Staphylococcus aureus), bronchiectasis, progressive respiratory failure
  • Pancreas: Exocrine pancreatic insufficiency → malabsorption, steatorrhea
  • Reproductive: Male infertility (absent vas deferens); reduced female fertility
  • Sweat glands: Elevated sweat Cl⁻ (diagnostic: sweat chloride > 60 mEq/L)
• Modifier genes at other loci affect severity

5. Phenylketonuria — PKU (AR)

• Gene/Protein: Deficiency of phenylalanine hydroxylase (PAH) → inability to convert phenylalanine to tyrosine
• Defect: Phenylalanine accumulates → toxic to developing brain neurons
• Features:
  • Progressive intellectual disability (if untreated)
  • Seizures, behavioral issues
  • Fair skin and hair (tyrosine needed for melanin synthesis)
  • Musty odor (phenylketones in urine/sweat)
• Management: Phenylalanine-restricted diet from birth (newborn screening); detected by Guthrie test

6. Sickle Cell Anemia (AR)

• Gene/Protein: Point mutation in HBB gene → glutamic acid → valine substitution at position 6 of β-globin → hemoglobin S (HbS)
• Under low O₂, HbS polymerizes → sickle-shaped rigid red cells
• Features: Hemolytic anemia, vaso-occlusive crises (bone pain, acute chest syndrome), splenic sequestration, stroke, organ damage
• Carriers (sickle cell trait) are protected against severe malaria — explains high allele frequency in malaria-endemic regions

7. Familial Hypercholesterolemia vs. Related Enzyme Disorders (AR examples)

• Glucocerebrosidase deficiency → glucocerebroside accumulates in macrophages (liver, spleen, bone marrow)

• Hexosaminidase A deficiency → GM2 ganglioside accumulates in neurons → progressive neurodegeneration, death by age 2–5 (classic infantile form)

8. Huntington Disease (AD) — Atypical: Triplet Repeat

• Gene: CAG trinucleotide repeat expansion in HTT gene → toxic gain-of-function of huntingtin protein
• Normal: < 36 repeats; Disease: > 40 repeats
• Anticipation: Repeat expands in successive generations → earlier onset and greater severity in offspring
• Features: Progressive chorea, personality changes, dementia; onset typically 30–50 years; uniformly fatal
• Mechanism: Mutant huntingtin forms nuclear inclusions → disrupts transcription, causes neuronal apoptosis (particularly striatum and cortex)

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Key Distinctions Between Types

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• Robbins & Kumar Basic Pathology, Chapter 4 — Genetic and Pediatric Diseases
• Goodman & Gilman's The Pharmacological Basis of Therapeutics, Chapter 76 — Carcinogenesis