DNA Oncogenic Viruses in Pathology

Overview

1. Human Papillomavirus (HPV)
2. Epstein-Barr Virus (EBV)
3. Hepatitis B Virus (HBV)
4. Merkel Cell Polyomavirus (MCPyV)
5. Human Herpesvirus 8 (HHV-8 / Kaposi Sarcoma Herpesvirus)

1. Human Papillomavirus (HPV)

• Cervical carcinoma (most important)
• Anal, vulvar, vaginal, penile carcinomas
• Oropharyngeal carcinoma (tonsil, base of tongue)

Mechanism of Transformation

• Binds to and mediates degradation of p53 (prevents DNA repair and apoptosis)
• Stimulates expression of TERT (telomerase reverse transcriptase) - promotes immortalization
• High-risk HPV E6 has much higher affinity for p53 than low-risk E6

• Binds RB protein and displaces E2F transcription factors, pushing cells through the G1/S checkpoint
• Inactivates CDK inhibitors p21 and p27
• Binds and activates cyclins A and E (in high-risk types 16, 18, 31)
• High-risk E7 has higher affinity for RB than low-risk E7

HPV infection alone is not sufficient for cancer - other genetic hits (e.g., mutant RAS), smoking, HIV co-infection, and immune failure are needed, typically over several decades.

2. Epstein-Barr Virus (EBV)

• Burkitt lymphoma (first human tumor linked to EBV)
• Hodgkin lymphoma
• Nasopharyngeal carcinoma (100% EBV-associated worldwide)
• Immunosuppression-related B-cell lymphomas
• EBV-positive gastric carcinoma, thymic carcinoma

Mechanism

• LMP1 (Latent Membrane Protein-1): Acts as a constitutively active CD40 receptor. Activates NF-κB and JAK/STAT signaling, promotes B-cell survival and proliferation, and prevents apoptosis via BCL2 activation. In nasopharyngeal carcinoma, LMP1 also upregulates VEGF, FGF2, MMP9, and COX2.
• EBNA2: Encodes a nuclear protein that activates expression of several cellular and viral genes, including the proto-oncogene MYC.

Burkitt Lymphoma

Nasopharyngeal Carcinoma

3. Hepatitis B Virus (HBV)

Mechanism

• Chronic liver injury + regenerative hyperplasia - repeated cycles of hepatocyte death and regeneration from a persistent inflammatory response increase the probability of accumulating mutations
• HBV X protein (HBx): Transactivates various growth-regulatory genes and may impair p53 function
• Viral DNA integrates randomly into the host genome, potentially causing insertional mutagenesis (though this is random and not the primary mechanism)
• Cirrhosis acts as a permissive background for neoplastic transformation

4. Merkel Cell Polyomavirus (MCPyV)

Mechanism

• Viral DNA integrates into the host genome in a clonal fashion (before tumor development)
• Expresses T antigens (large T antigen) that bind and inactivate RB and p53, similar to other polyomaviruses (e.g., SV40)
• Truncating mutations in the large T antigen prevent viral replication but preserve transforming capacity

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

• Kaposi sarcoma (classic, endemic, AIDS-related, and transplant-related forms)
• Primary effusion lymphoma
• Multicentric Castleman disease

Mechanism

• HHV-8 encodes viral homologs of cellular genes, including v-cyclin D, v-FLIP (inhibits apoptosis), and v-IL-6
• v-cyclin D binds CDK6 and drives cells through the G1/S checkpoint
• v-FLIP activates NF-κB, promoting cell survival
• Viral GPCR (vGPCR) activates VEGF and angiogenic pathways, contributing to the vascular nature of Kaposi sarcoma

Summary Table

Key Shared Concept

"The binding of viral oncoproteins to cellular tumor-suppressor proteins p53 and RB is fundamental to the carcinogenesis induced by most DNA viruses."

• Robbins, Cotran & Kumar - Pathologic Basis of Disease

Gangrene

• Bailey and Love's Short Practice of Surgery, 28th Ed.

Signs of Gangrene

1. Change of colour - pale → bluish-purple → finally black
2. Loss of temperature (cold to touch)
3. Loss of sensation (anaesthesia)
4. Loss of peripheral pulsation
5. Loss of function

Clinical Types

1. Dry Gangrene

• Cause: Gradual, slow occlusion of arterial circulation - most commonly atherosclerosis (hence "senile gangrene")
• The blood supply diminishes slowly, allowing the tissue to desiccate before infection sets in
• Features: Part becomes dry, shrivelled, hard, mummified, and discoloured
• A clear line of demarcation forms between viable and dead tissue within days
• Separation is clean with minimal infection; the stump tends to be conical when bone is involved
• No offensive smell (no putrefaction)

2. Wet (Moist) Gangrene

• Cause: Sudden arterial blockage (e.g., embolus) OR gangrene superimposed on inflammation
• The tissue does not have time to desiccate before bacteria invade
• Features: Oedema, blistering (blebs), foul smell from putrefaction, spreading infection
• The line of demarcation forms more proximally than in dry gangrene due to infection extending into adjacent living tissue
• A surgical emergency - requires urgent debridement or amputation

3. Gas Gangrene

• Cause: Clostridium perfringens (Gram-positive, anaerobic, spore-bearing bacillus)
• Spores are widespread in soil and faeces; thrive in anaerobic conditions (devascularised, necrotic tissue)
• Common in: war/military wounds (high-velocity missiles cause massive tissue devitalisation), immunocompromised patients, diabetics, patients with malignancy
• Features:
  • Severe local wound pain (early and disproportionate)
  • Crepitus on palpation (gas in tissues - may also be visible on plain X-ray)
  • Thin, brown, sweet-smelling exudate - Gram stain reveals organisms
  • Oedema and spreading gangrene from collagenase, hyaluronidase, other proteases, and alpha toxin
  • Rapid systemic deterioration: circulatory collapse, multi-organ failure
• Treatment: Large-dose IV penicillin + aggressive surgical debridement; early amputation may be life-saving; hyperbaric oxygen as adjunct

Specific Types by Cause

Diabetic Gangrene

1. Ischaemia - macrovascular atherosclerosis (typically crural vessels) + microvascular dysfunction (shunting)
2. Peripheral sensorimotor neuropathy (PSN) - sensory loss (stocking distribution) leads to unnoticed injury and neglect; motor involvement causes foot deformity and abnormal pressure loading → callosities
3. Immunosuppression - hyperglycaemia impairs immune function → predisposes to infection

Synergistic Gangrene (Necrotising Fasciitis)

• Cause: Synergistic action of non-haemolytic streptococci and staphylococci (classically); usually polymicrobial (mixed aerobic and anaerobic)
• Spreads rapidly through the deep fascia (hence "fasciitis")
• More common in diabetics, debilitated, or immunocompromised patients - but can occur in healthy individuals
• Features: Systemic septic shock, high fever, foul smell, crepitus (gas-producing organisms), subcutaneous necrosis spreading into chest/axilla/thigh/perineum
• Diagnosis: Gold standard is surgical exploration - easy dissection along fascial planes, grey/dusky/edematous fascia, brown stringy exudate
• Treatment: Immediate broad-spectrum IV antibiotics + early, aggressive surgical debridement (repeated until all necrotic tissue cleared) + skin grafting for survivors; hyperbaric oxygen advocated

Fournier's Gangrene

• A subtype of necrotising fasciitis localised to the genitalia, perineum, anus, and occasionally lower abdomen
• Peak age: 50-60 years; most have diabetes mellitus or immunocompromise
• Source: perirectal/perianal infection, untreated perirectal abscess, severe UTI, genital trauma
• Nearly always polymicrobial (mixed aerobic and anaerobic)
• CT scan to delineate extent; treatment is immediate surgical debridement ± diverting colostomy for rectal involvement

Causes of Gangrene - Summary

Separation of Gangrene

• In dry gangrene: line appears in days, separation is neat with minimal infection
• In moist gangrene: infection extends into adjacent living tissue, so the demarcation line is more proximal; separation is slower and messier
• If the proximal arterial supply is poor, no line forms at all - gangrene continues to spread unless revascularisation is achieved

Senile (Atherosclerotic) Gangrene - Clinical Picture

• Elderly patients (>50 years), lower limbs
• Progression: intermittent claudication → rest pain → trophic changes → dry gangrene
• Investigation: ABI (ankle-brachial index), oscillometry, Doppler, arteriography (shows site of block, calcification)
• Leriche's syndrome: bilateral internal iliac occlusion causing buttock claudication + impotence

Buerger's Disease (Thromboangiitis Obliterans)

• Young men <40 years; strong association with cigarette smoking
• Inflammatory reaction in small/medium arteries with involvement of adjacent vein and nerve → segmental thrombosis
• Starts in tibial arteries, foot vessels (foot claudication - at the arch, NOT the thigh/buttock)
• Characteristic arteriography: smooth normal larger arteries + extensive occlusion of smaller vessels with "corkscrew" collaterals
• Leads to digital ulceration and gangrene of toes/fingers

Gangrene in Pathology

What is Gangrenous Necrosis?

"Gangrenous necrosis is not a specific pattern of cell death, but the term is commonly used in clinical practice. It is usually applied to a limb, generally the lower leg, that has lost its blood supply and has undergone necrosis (typically coagulative necrosis) involving multiple tissue planes."

• Robbins, Cotran & Kumar Pathologic Basis of Disease

Gangrene in the Context of Patterns of Tissue Necrosis

Pathological Basis of the Two Main Types

Dry Gangrene = Predominantly Coagulative Necrosis

• Caused by gradual, slow arterial occlusion (e.g., atherosclerosis)
• Blood supply diminishes slowly - the tissue desiccates before bacterial invasion can occur
• Coagulative necrosis: the injury denatures structural proteins AND enzymes simultaneously, blocking proteolysis of dead cells
• Result: the architecture of dead tissue is preserved for days - intensely eosinophilic ghost cells with indistinct or "reddish" nuclei persist
• Gross appearance: dry, shrivelled, mummified, hard tissue; turns black (from iron sulphide formed by breakdown of haemoglobin)
• A clear line of demarcation forms between viable and dead tissue
• No offensive smell (no significant putrefaction)

Wet (Moist) Gangrene = Coagulative + Superimposed Liquefactive Necrosis

• Caused by sudden arterial occlusion (embolus) OR gangrene with bacterial superinfection
• When bacteria invade, liquefactive necrosis is superimposed:
  • Leukocytes are recruited → release lysosomal enzymes
  • Bacterial degradative enzymes (proteases, collagenase, hyaluronidase) destroy tissue
  • Dead cells are completely digested → tissue transforms into a viscous liquid
• Gross appearance: oedematous, blebs/bullae, foul smell (putrefaction), pus
• The line of demarcation is indistinct and forms more proximally because infection spreads into adjacent living tissue
• The necrotic material is creamy yellow from leukocytes (pus)
• No preserved tissue architecture remains - cellular outlines are lost

Gas Gangrene = Liquefactive Necrosis + Gas Production

• Clostridium perfringens (anaerobic organism) in devascularised tissue
• Produces alpha toxin (phospholipase C), collagenase, hyaluronidase → extensive tissue destruction
• Gas (CO₂, H₂, H₂S) produced by bacterial metabolism accumulates in tissues → crepitus
• Histology: widespread myonecrosis with relatively sparse inflammatory response (toxins kill leukocytes)
• Spreads rapidly through tissue planes

Why Does Gangrene Turn Black?

• Loss of blood supply → haemoglobin leaks from lysed red cells
• Haemoglobin is broken down by bacteria and tissue enzymes
• Haemoglobin → iron sulphide (FeS) - a black pigment
• This is not melanin and not carbon

Cellular Mechanism: Why Ischaemia Causes Necrosis

1. Ischaemia → ATP depletion → failure of Na⁺/K⁺-ATPase → cellular swelling
2. Mitochondrial dysfunction (irreversible) → failure of oxidative phosphorylation
3. Cytosolic Ca²⁺ rises → activates phospholipases, proteases, endonucleases → membrane damage
4. Lysosomal membrane rupture → enzymatic dissolution of the cell (necrosis)
5. Cellular contents leak out → local inflammatory response (this is why necrosis, unlike apoptosis, induces inflammation)

• Inability to reverse mitochondrial dysfunction even after restoring oxygen
• Profound disturbances in membrane function

Fate of Necrotic Tissue in Gangrene

1. Leukocyte infiltration → enzymatic digestion of necrotic cells → phagocytosis of debris
2. Zone of demarcation forms: granulation tissue develops between dead and living parts
3. If necrotic cells are not cleared promptly → calcium salts deposit → dystrophic calcification
4. In dry gangrene: autoamputation can occur as the demarcation line matures
5. In wet gangrene: infection spreads → the demarcation forms more proximally → requires surgical intervention

Summary Table: Pathological Comparison