HPV is a non-enveloped DNA virus with a genome organized into early (E) and late (L) genes. Which early gene product is responsible for inactivating the retinoblastoma (Rb) protein?

Explanation

## HPV Genome Organization and Oncogenic Mechanisms **Key Point:** HPV contains early (E) and late (L) genes. The E7 protein specifically targets the retinoblastoma (Rb) protein, while E6 targets p53. ### HPV Genome Structure The HPV genome is a circular, double-stranded DNA (~8 kb) organized into three functional regions: 1. **Early (E) genes**: E1, E2, E4, E5, E6, E7 — encode regulatory and transforming proteins 2. **Late (L) genes**: L1, L2 — encode structural capsid proteins 3. **Long control region (LCR)**: contains promoters and enhancers ### E6 and E7 Oncoproteins | Oncoprotein | Target | Mechanism | Consequence | |-------------|--------|-----------|-------------| | **E6** | p53 tumor suppressor | Ubiquitin-mediated degradation | Loss of cell cycle checkpoint, apoptosis resistance | | **E7** | Retinoblastoma (Rb) protein | Binding and inactivation | Uncontrolled G1/S transition, cell cycle progression | | **E5** | Growth factor signaling | Enhances EGFR and PDGFR | Increased proliferation | **High-Yield:** E7 inactivates Rb by binding to its hypophosphorylated form, preventing Rb from sequestering E2F transcription factors. This allows S-phase genes to be transcribed, driving cells into DNA synthesis. **Clinical Pearl:** The dual inactivation of p53 (by E6) and Rb (by E7) is essential for HPV-mediated malignant transformation. Either protein alone is insufficient for full transformation. **Mnemonic:** **E7-Rb, E6-p53** — E7 targets Rb (Retinoblastoma), E6 targets p53 (tumor protein 53). ### Late Genes L1 and L2 encode the major and minor capsid proteins, respectively. They are expressed only during the late phase of viral replication and are not involved in transformation.