Regarding tumor suppressor genes and their mechanisms of action, all of the following statements are correct EXCEPT:

Question

Accepted Answer

B. PTEN functions as a phosphatase that activates the PI3K/AKT signaling pathway to promote cell survival. ## 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.

Answer

A. TP53 (p53) acts as the 'guardian of the genome' by inducing apoptosis or cell cycle arrest in response to DNA damage

Answer

C. APC protein regulates Wnt signaling by promoting degradation of β-catenin through the proteasome

Answer

D. RB (retinoblastoma) protein prevents progression from G1 to S phase by binding and sequestering E2F transcription factors

Regarding tumor suppressor genes and their mechanisms of action, all of the following statements are correct EXCEPT:

Explanation

Tumor Suppressor Gene Mechanisms

Overview of Correct Functions

Analysis of Each Option

The Critical Error: PTEN Function

Why the Other Options Are Correct

Practice similar questions

Join our NEET PG community

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