Etiopathogenesis of Squamous Intraepithelial Lesion (SIL)

1. Overview and Terminology

2. Etiology - The Role of HPV

HPV Biology

• HPV is a DNA virus. Strains are classified as high-risk (oncogenic) or low-risk (non-oncogenic) based on genotype.
• High-risk types: HPV 16 and 18 account for ~70% of all CIN and cervical carcinoma cases.
• Low-risk types (6, 11) cause condylomas (genital warts) but almost never cause cancer.

How Infection Occurs

• Transmission is primarily through sexual contact.
• Risk factors directly tied to HPV exposure include:
  • Early age at first intercourse
  • Multiple sexual partners (or a partner with multiple previous partners)
  • Persistent infection with high-risk HPV strains

Infection Outcomes

3. Molecular Pathogenesis - The E6 and E7 Oncoproteins

Site of Action

• HPV infects immature squamous cells (at the transformation zone/squamo-columnar junction).
• Viral replication occurs in maturing squamous cells, which are normally arrested in G1.
• E6 and E7 override this G1 arrest to allow replication of the viral genome.

E6 Protein - Inactivates p53

• E6 binds to and mediates degradation of p53 (the "guardian of the genome").
• Normally p53 halts the cell cycle in response to DNA damage and promotes apoptosis.
• E6 also upregulates telomerase, conferring replicative immortality.
• Result: cells with DNA damage continue dividing instead of undergoing apoptosis.

E7 Protein - Inactivates RB

• E7 binds to the retinoblastoma protein (RB), displacing the E2F transcription factors normally sequestered by RB.
• Free E2F drives cells through the cell cycle, promoting uncontrolled proliferation.
• Result: cells bypass the G1/S checkpoint and proliferate regardless of DNA integrity.

High-risk HPV E7 has much higher affinity for RB than low-risk HPV E7. Low-risk HPV E6 fails to bind p53 at all and instead dysregulates growth via the Notch signaling pathway. This mechanistic difference explains why low-risk types do not cause cancer.

4. Viral Integration - The Key Step Toward Malignancy

1. Disrupts the E2 gene - E2 normally represses transcription of E6 and E7. Loss of E2 leads to increased expression of E6 and E7 oncoproteins.
2. Causes genomic instability - Integration promotes accumulation of additional pro-oncogenic mutations in the host cell.

5. Co-factors and Host Factors

6. Natural History: LSIL vs. HSIL

• LSIL: High level of viral replication, mild host cell alteration, NOT considered premalignant. Koilocytic change (pathognomonic of HPV) is prominent.
• HSIL: Increased cell proliferation, arrested epithelial maturation, lower viral replication, HIGH risk of malignant transformation. Approximately 20% of HSIL develop de novo, without a detectable prior LSIL phase.

7. Histopathology of SIL

• Healthy: Orderly maturation from basal to superficial layers; abundant glycogenated cytoplasm in superficial cells.
• LSIL (CIN I): Dysplastic changes confined to the lower 1/3 of the epithelium + koilocytic change (perinuclear halos, nuclear enlargement) in superficial layers - reflecting active viral replication.
• HSIL (CIN II): Dysplastic immature cells extending to the lower 2/3 of the epithelium.
• HSIL (CIN III): Dysplastic changes involving full thickness of epithelium; marked nuclear pleomorphism, abnormal mitoses, absence of koilocytic change (viral replication is suppressed), and no surface maturation.

8. Summary of Pathogenetic Sequence

Sexual activity → HPV acquisition
       ↓
Infection of transformation zone (squamo-columnar junction)
       ↓
Transient infection → Immune clearance (most cases)
       ↓ (persistent high-risk HPV)
E6 destroys p53 + E7 inactivates RB
       ↓
Uncontrolled proliferation → LSIL (CIN I)
       ↓ (persistent infection + cofactors)
Viral integration → disrupts E2 → ↑E6/E7
       ↓
HSIL (CIN II/III) → genomic instability + mutations
       ↓ (10% of HSIL)
Invasive squamous cell carcinoma

• Robbins & Kumar Basic Pathology, pp. 685-688
• Robbins, Cotran & Kumar Pathologic Basis of Disease, Chapter 4 (Oncogenic DNA Viruses)