## Tumor Suppressor Gene Mechanisms ### Overview of Correct Functions **Key Point:** Tumor suppressor genes encode proteins that restrict cell proliferation, promote differentiation, or trigger apoptosis. Loss of function (usually biallelic) leads to malignant transformation. ### Analysis of Each Option | Gene | Function | Mechanism | Status | | --- | --- | --- | --- | | TP53 | Guardian of genome | Induces p21 → G1/S arrest; BAX → apoptosis | ✓ Correct | | RB | G1/S checkpoint | Binds E2F; prevents S phase entry | ✓ Correct | | PTEN | Phosphatase | **Inactivates** PI3K/AKT (not activates) | ✗ Wrong | | APC | Wnt regulator | Promotes β-catenin degradation | ✓ Correct | ### The Critical Error: PTEN Function **High-Yield:** PTEN is a **negative regulator** of the PI3K/AKT/mTOR pathway. It converts PIP~3~ back to PIP~2~, thereby **inhibiting** (not activating) AKT signaling. Loss of PTEN → unopposed AKT activation → increased cell survival and proliferation. **Clinical Pearl:** PTEN loss is common in: - Cowden syndrome (germline PTEN mutations) - Endometrial, prostate, and breast cancers - Glioblastoma **Warning:** Confusing PTEN as an activator of survival signaling is a classic trap. Remember: PTEN = **brake on growth**. ### Why the Other Options Are Correct 1. **TP53:** Canonical tumor suppressor; mutations in >50% of human cancers. 2. **RB:** Inactivation allows uncontrolled G1→S transition; mutated in retinoblastoma and many other cancers. 3. **APC:** Adenomatous polyposis coli; loss → Wnt pathway hyperactivation → colorectal cancer.
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